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Depar tment of Applied Physics Modeling and optimization of urban energy systems for large-scale integration of variable renewable energy generation Jani Mikkola D O C TO R A L DISSERTATIONS Aalto University publication series DOCTORAL DISSERTATIONS 16/2017 Modeling and optimization of urban energy systems for large-scale integration of variable renewable energy generation Jani Mikkola A doctoral dissertation completed for the degree of Doctor of Science (Technology) to be defended, with the permission of the Aalto University School of Science, at a public examination held at the lecture hall K216 of the school on 24th of February 2017 at 12 noon. Aalto University School of Science Department of Applied Physics New Energy Technologies Supervising professor Professor Peter Lund, Aalto University, Finland Thesis advisor Professor Peter Lund, Aalto University, Finland Preliminary examiners Professor Lennart Söder, KTH Royal Institute of Technology, Sweden Professor Torjus Folsland Bolkesjø, NMBU Norwegian University of Life Sciences, Norway Opponent Professor Reinhard Haas, TU Wien, Austria Aalto University publication series DOCTORAL DISSERTATIONS 16/2017 © Jani Mikkola ISBN 978-952-60-7266-1 (printed) ISBN 978-952-60-7265-4 (pdf) ISSN-L 1799-4934 ISSN 1799-4934 (printed) ISSN 1799-4942 (pdf) http://urn.fi/URN:ISBN:978-952-60-7265-4 Unigrafia Oy Helsinki 2017 Finland Ab s t ra c t Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Jani Mikkola Name of the doctoral dissertation Modeling and optimization of urban energy systems for large-scale integration of variable renewable energy generation P u b l i s h e r School of Science U n i t Department of Applied Physics S e r i e s Aalto University publication series DOCTORAL DISSERTATIONS 16/2017 F i e l d o f r e s e a r c h Energy sciences M a n u s c r i p t s u b m i t t e d 12 September 2016 D a t e o f t h e d e f e n c e 24 February 2017 P e r m i s s i o n t o p u b l i s h g r a n t e d ( d a t e ) 1 November 2016 Monograph Article dissertation L a n g u a g e English Essay dissertation Abstract The rapid growth of variable renewable energy (VRE), such as wind and solar energy, will increase the unpredictability and fluctuations in energy systems in general, resulting in a need for increased fl exibility measures. Simultaneously, global urbanization will strengthen the importance of urban energy systems in the future. Therefore, it is highly important to fi nd solutions to enable largescale variable renewable energy generation in cities. This dissertation aims to fi nd ways to increase fl exibility and to enable higher shares of VRE in urban energy systems. For this purpose, three new computational models were developed in relation to both electricity and thermal energy (heating and cooling) in cities. The fi rst model simulates the spatiotemporal energy consumption in cities, the second focuses on distributed production and cooperation between different energy networks, and the third describes the energy system operation with existing power plants and large-scale VRE schemes. Results from Helsinki, Finland, show that power-to-heat (P2H) conversion with electric resistance or heat pumps can help to increase the wind power production in the city by 4–5 times (up to 60% of electricity demand) compared to the self-use limit of wind power, in which the focus is on the electric system only. Part of the increased wind power production would then be used to cover up to one third of the heat demand. Simultaneously, P2H makes coal very sensitive to price changes, which could lead to the removal of coal from the city's energy portfolio. In another case, in Concepción, Chile, the solar capacity of the city is, in theory, large enough to satisfy all energy demand, but the technical effi ciency and the power grid infrastructure limits the PV share in electricity to around 24%. With P2H, this could be increased to at least 34% without overloading the power grid; simultaneously a quarter of the heat demand would be fi lled. These results demonstrate the importance of system-wide consideration when planning integration of VRE. The thesis shows that, by viewing energy systems as a whole instead of focusing on only one energy form, the fl exibility of the system can be remarkably increased, even without massive investments in traditional network infrastructure. Through smart design and multi-energy thinking approaches, variable renewable energy sources can be turned into a main-stream option for energy production in urban areas. K e y w o r d s urban energy systems, energy system modeling, renewable energy, optimization, energy fl exibility I S B N ( p r i n t e d ) 978-952-60-7266-1 I S B N ( p d f ) 978-952-60-7265-4 I S S N - L 1799-4934 I S S N ( p r i n t e d ) 1799-4934 I S S N ( p d f ) 1799-4942 L o c a t i o n o f p u b l i s h e r Helsinki L o c a t i o n o f p r i n t i n g Helsinki Y e a r 2017 Pages 170 u r n http://urn.fi /URN:ISBN: 978-952-60-7265-4 Tiivistelmä Aalto-yliopisto, PL 11000, 00076 Aalto www.aalto.fi Tekijä Jani Mikkola Väitöskirjan nimi Kaupunkien energiajärjestelmien mallinnus ja optimointi laajamittaisen vaihtelevan uusiutuvan energian lisäämiseksi J u l k a i s i j a Perustieteiden korkeakoulu Y k s i k k ö Teknillisen fysiikan laitos S a r j a Aalto University publication series DOCTORAL DISSERTATIONS 16/2017 T u t k i m u s a l a Energiatieteet K ä s i k i r j o i t u k s e n p v m 12.09.2016 V ä i t ö s p ä i v ä 24.02.2017 J u l k a i s u l u v a n m y ö n t ä m i s p ä i v ä 01.11.2016 Monografia Artikkeliväitöskirja K i e l i Englanti Esseeväitöskirja Tiivistelmä Vaihtelevan uusiutuvan energian (VRE), esimerkiksi tuuli- ja aurinkoenergian, nopea yleistyminen kasvattaa maailmanlaajuisesti epävarmuutta ja tuotannon heilahduksia energiajärjestelmissä. Tämän vuoksi tarvitaan uusia keinoja järjestelmien joustavuuden lisäämiseksi. Samalla kaupunkien kasvu ja globaali urbanisaatio nostavat kaupunkiympäristöjen merkitystä energiakysymyksissä. Onkin erittäin tärkeää keksiä ratkaisuja, joilla laajan mittakaavan VREtuotanto kaupungeissa mahdollistetaan. Tässä väitöskirjassa etistään kaupunkeihin sopivia keinoja, joilla energiajärjestelmien joustavuutta voidaan lisätä ja uusiutuvan energian osuutta kasvattaa. Tarkoitusta varten luotiin kolme uutta tietokonemallia, jotka ottavat huomioon sekä sähkön että termisen energian (lämmityksen ja jäähdytyksen). Ensimmäinen malli simuloi kaupunkien ajallista ja paikallista energiankulutusta, toinen keskittyy hajautettuun tuotantoon ja eri energiaverkkojen yhteiskäyttöön, ja kolmannella mallinnetaan energiajärjestelmän toimintaa, kun mukana on vanhoja voimalaitoksia ja paljon vaihtelevaa uusiutuvaa tuotantoa. H e l s i ng is t ä s a ad u t t u l o ks e t o s o i t t ava t , e t t ä s äh kö – l ä mp ö - ko nve rs i o ( P2H ) j o ko s äh kö vas t u ks i l l a tai lämpöpumpuilla auttaa nostamaan tuulivoiman tuotannon määrää kaupungissa jopa nelin–viisinkertaiseksi (jopa 60 % sähköstä) verrattuna perinteiseen lähestymistapaan, jossa keskitytään yksinomaan sähköjärjestelmän toimintaan. Osa lisääntyneestä tuulituotannosta käytetään kattamaan jopa kolmannes kaupungin lämmöntarpeesta. Samalla P2H tekee hiilivoiman erittäin herkäksi hinnanvaihteluiden suhteen, mikä saattaa johtaa luonnostaan kokonaan hiilestä luopumiseen. Chilen Concepciónissa kaupungin aurinkoenergian potentiaali riittäisi kattamaan kaupungin koko energiantarpeen, mutta tekninen suorituskyky ja verkkoinfrastruktuuri rajoittavat aurinkosähkön osuuden noin 24 %:iin. P2H:n avulla osuus voidaan nostaa vähintään 34 %:iin ylikuormittamatta silti verkkoa. Samalla tuotettaisiin neljännes kaupungin lämmöntarpeesta. Tulokset osoittavat järjestelmätason tarkastelun tarpeellisuuden uusiutuvan energian suunnittelussa. Väitöskirja todistaa, että tarkastelemalla energiajärjestelmää kokonaisuutena (yksittäisen energiamuodon sijasta) sen joustavuutta voidaan lisätä ilman valtavia perinteisiä verkkoinvestointeja. Älykkäällä suunnittelulla ja monienergia-ajattelulla vaihtelevat uusiutuvat energiamuodot voidaan nostaa kaupunkiympäristöjen tärkeimmäksi energiamuodoksi. A v a i n s a n a t kaup unkie n e ne rgiajärje ste lmät, e ne rg iajärje ste lmie n mallinnus, uusiutuva e ne rg ia, optimointi, energiajoustavuus I S B N ( p a i n e t t u ) 978-952-60-7266-1 I S B N ( p d f ) 978-952-60-7265-4 I S S N - L 1799-4934 I S S N ( p a i n e t t u ) 1799-4934 I S S N ( p d f ) 1799-4942 J u l k a i s u p a i k k a Helsinki P a i n o p a i k k a Helsinki Sivumäärä 170 u r n http://urn.fi /URN:ISBN: 978-952-60-7265-4 V u o s i 2017 Preface This thesis is based on research carried out in the New Energy Technologies Group of the Department of Applied Physics, Aalto University School of Science, under the supervision of Professor Peter Lund. Funding for the work was provided by Fortum Foundation, Academy of Finland, and the Finnish Funding Agency for Innovation (Tekes). The financial support is greatly appreciated. I want to thank my instructor and supervisor, Professor Peter Lund for providing valuable support, guidance and inspiration through my whole academic career, and also for making it possible for me to work with a topic I found interesting and important. In the beginning of my research career, I was instructed by Dr. Mikko Mikkola and Rami Niemi, I want to express my gratitude also to them. I wish to thank the colleagues who helped me and took part in the research, Juuso Lindgren, Jyri Salpakari, and Joel Ypyä. I want to acknowledge also my Chilean colleagues Paulina Wegertseder and Rodrigo García. Furthermore, I want to thank our whole research group, everyone else I have worked with, and all my friends. Finally, I want to express my gratitude to my wife and my whole family for all the love and support throughout my life. Otaniemi, Espoo, January 17, 2017 Jani Mikkola 1 Contents Preface ...................................................................................................... 1 List of abbreviations and symbols ........................................................... 5 List of publications .................................................................................. 8 Author’s contribution .............................................................................. 9 1. Introduction................................................................................. 11 1.1 Background .............................................................................. 11 1.2 Objectives and scope ................................................................ 12 1.3 Thesis outline ........................................................................... 13 2. Urban energy systems ................................................................. 15 2.1 Electric power systems............................................................. 15 2.2 Heating and cooling ................................................................. 16 3. Energy system flexibility (Publication I) ..................................... 19 3.1 Flexibility challenge of renewable integration ......................... 19 3.2 Flexibility technologies ............................................................ 19 3.2.1 Supply-side flexibility and grid services............................... 19 3.2.2 Energy storage ..................................................................... 20 3.2.3 Power-to-X ........................................................................... 21 3.2.4 Demand-side management ................................................. 22 3.2.5 Novel grid infrastructure ..................................................... 22 4. Spatiotemporal consumption profiles (Publication II) .............. 25 4.1 Input data................................................................................ 25 4.2 Formation of spatiotemporal load profiles ............................. 26 4.2.1 Scaling base profiles to match the city demand .................. 26 4.2.2 Sub-class division ................................................................ 26 4.2.3 Spatial division .................................................................... 28 5. Multi-carrier energy networks (Publication III) ........................ 29 5.1 Structure of the multi-carrier energy systems model ............. 29 5.2 Energy balance in nodes and flows in network ....................... 31 5.3 Energy system operation in simulations ................................ 32 3 6. Optimal use of an energy system under large-scale VRE (Publication V) ............................................................................ 35 6.1 Model principles ...................................................................... 35 6.2 Input and output .....................................................................36 6.2.1 VRE production, consumption, and technical parameters .36 6.2.2 Economic parameters ......................................................... 38 6.2.3 Output ................................................................................. 38 6.3 Optimization of energy system operation ...............................39 6.3.1 Objective function ................................................................39 6.3.2 Operation principles of an energy system – boundary conditions.............................................................................39 7. Results for Helsinki ..................................................................... 41 7.1 Current energy system in Helsinki .......................................... 41 7.2 Spatiotemporal consumption profiles (Publication II) ...........43 7.2.1 Input data.............................................................................43 7.2.2 Spatiotemporal consumption profiles – results ..................44 7.3 P2H conversion enables more VRE (Publications III and IV) 46 7.3.1 Wind power with P2H and thermal storage ........................ 47 7.3.2 Effect of network structure ................................................. 48 7.4 Energy system operation with CHP plants and large-scale wind power (Publication V) .................................................... 50 7.4.1 Parameters for Helsinki energy system .............................. 50 7.4.2 Effects of wind power and P2H on plant operation............. 51 8. Results for other cities................................................................. 55 8.1 Shanghai, China....................................................................... 55 8.1.1 Spatiotemporal profiles with limited input data (Publication II)..................................................................... 55 8.1.2 Large-scale photovoltaics together with microtrigeneration (Publication III) ............................................. 57 8.2 9. Concepción, Chile (Publication VI) .........................................58 8.2.1 Electricity consumption, city structure, and the power grid .......................................................................................58 8.2.2 Results.................................................................................. 59 Conclusions ................................................................................. 61 References .............................................................................................. 65 Publications ............................................................................................ 71 4 List of abbreviations and symbols Abbreviations CAES compressed-air energy storage CHP combined heat and power CO2 carbon dioxide COP coefficient of performance DH district heating DSM demand-side management GHG greenhouse gas HVDC high-voltage direct current MILP mixed-integer linear programming O&M operations and maintenance P2G power-to-gas P2H power-to-heat P2T power-to-thermal P2X power-to-X PHES pumped hydropower energy storage PV photovoltaics TSO transmission system operator V2G vehicle-to-grid VRE variable renewable energy Symbols ܣ floor area density (m2/km2) ܤ energy balance (MW) 5 ܾ relative base load profile (–) ܿ energy converted to other forms (MW) ݀ density of urban structure (unit/km2) ݀ maximum density (unit/km2) ds downstream nodes (–) ݂ energy power flow (MW) ܫ energy intensity (MWh/m2) ܭ constant minimum density of urban structure (unit/km2) ܲ energy production (MW or MWh) ܳ energy consumption (MW or MWh) ݍ energy allocated for conversion to other energy forms (MW) ݏ energy charged to or discharged from storage (MW) ݐ time, time step (min, h) ݔ spatial coordinate (km), or distance from the city center (km) ݔ distance of the maximum density from the city center (km) ݕ spatial coordinate (km) ݖ energy curtailment (MW) ߙ width parameter of density distribution (1/km2) ߟ efficiency (–) ߣ scaling factor (–) Subscripts and superscripts ܿ cooling city city total ݁ electricity ݂ fuel ݄ heat ݅ energy consumer class in incoming ݆ energy consumer subclass ݇ energy form 6 Introduction ݈ line of an energy network ݊ node out outgoing ݐ time step tot total ߢ other energy forms (than the one in question) 7 List of publications This doctoral dissertation consists of a summary and of the following publications which are referred to in the text by their Roman numerals. I. Lund, Peter; Lindgren, Juuso; Mikkola, Jani; Salpakari, Jyri. Review of energy system flexibility measures to enable high levels of variable renewable electricity. Renewable & Sustainable Energy Reviews 45, 785–807 (2015). II. Mikkola, Jani; Lund, Peter. Models for generating place and time dependent urban energy demand profiles. Applied Energy 130, 256–264 (2014). DOI: 10.1016/j.apenergy.2014.05.039 III. Niemi, Rami; Mikkola, Jani; Lund, Peter. 2012. Urban energy systems with smart multi-carrier energy networks and renewable energy generation. Renewable Energy 48, 524–536 (2012). DOI: 10.1016/j.renene.2012.05.017 IV. Lund, Peter; Mikkola, Jani; Ypyä, Joel. Smart energy system design for large clean power schemes in urban areas. Journal of Cleaner Production 103, 437–445 (2015). DOI: 10.1016/j.jclepro.2014.06.005 V. Mikkola, Jani; Lund, Peter. Modeling flexibility and optimal use of existing power plants with large-scale variable renewable power schemes. Energy 112, 364–375 (2016). DOI: 10.1016/j.energy.06.082 VI. Wegertseder, Paulina; Lund, Peter; Mikkola, Jani; García Alvarado, Rodrigo. Combining solar resource mapping and energy system integration methods for realistic valuation of urban solar energy potential. Solar Energy 135, 325–336 (2016). DOI: 10.1016/j.solener.2016.05.061 8 Author’s contribution Publication I: Review of energy system flexibility measures to enable high levels of variable renewable electricity The author was responsible for the Chapters 6 (Supply-side flexibility), 7 (Advanced technologies, except Subchapter 7.3) and 8 (Infrastructure). He conducted the literature review of these topics and was the main author of these chapters. Publication II: Models for generating place and time dependent urban energy demand profiles The author developed the model presented in the paper. He did the modeling work and calculations. The author was the main author of this publication. Publication III: Urban energy systems with smart multi-carrier energy networks and renewable energy generation The author developed the idea of the paper with the co-authors. He conducted the modeling work and calculations. He wrote part of the text. Publication IV: Smart energy system design for large clean power schemes in urban areas The author used the model he had developed to calculate spatial load profiles and conducted spatiotemporal analyses. He took part in writing the paper. Publication V: Modeling flexibility and optimal use of existing power plants with large-scale variable renewable power schemes The author developed the optimization model. He wrote the computational codes, and calculated and analyzed the results. He was the main author of the paper. Publication VI: Combining solar resource mapping and energy system integration methods for realistic valuation of urban solar energy potential The author developed a sub-model and run the simulations about energy system integration. He wrote the corresponding part of the text. 9 1. Introduction 1.1 Background The reduction of greenhouse gas (GHG) emissions is the main option for mitigating global climate change. Approximately 60–70% of the global GHG emissions stem either directly or indirectly from energy production [1,2]. Therefore, massive amounts of clean and renewable energy production are now being installed in the world. Several studies predict that a remarkable share of the new energy production will be based on variable renewable energy (VRE) sources, such as wind and solar power [3,4]. The potential of VRE, especially solar energy, is enormous. Each year, the sun radiates over 5,000 times more energy on the ground than mankind consumes [5]. The use of these technologies will gain increasing popularity as the technology develops and the price is reduced. The fluctuating and decentralized production of VRE sources creates new types of challenges to current energy systems, which are traditionally based in centralized and controllable combustion power plants and fossil fuels [6]. Local small VRE installations have not yet caused any major problems in the power systems, but large-scale VRE scenarios will require a different type of flexibility from the energy system to balance temporal and spatial differences between energy demand and supply. In electric systems, this balancing is especially critical, as the systems typically have little inherent inertia and low storage capacity. The transition from traditional to renewable energy production is a long process and will require the old and new energy systems to cooperate side by side for decades. Therefore, the traditional plants that may be designed for steady base-load operation will encounter totally different working conditions when VRE is introduced on a large scale. This scenario again emphasizes the need for power system flexibility, as increased fluctuations may decrease the lifetime of the plants [7] and increase the costs of power system operation [8]. In addition to increasing system flexibility, it is important to study the optimal operation of the resulting energy mix of new and old energy systems. A total of three fourths of all energy is currently consumed in urban areas, and the share of cities in total GHG emissions is approximately 80% [9]. Furthermore, the role of cities will only increase as urbanization continues in the future [10]. These facts highlight the importance of solving the challenge of energy system transition, especially in cities. Simultaneously, the progress has been rapid in distributed energy production technologies suitable for urban environments, especially solar energy, but also heat pumps, distributed bioenergy, small-scale wind power, and renewable district heating. Bringing energy production closer 11 Introduction to energy use and to consumers is an attractive option for the future, as it has several benefits, including avoiding large and costly energy infrastructures and involving end-users in energy investments [11]. In urban areas, thermal energy (i.e., heating and cooling) dominates the end use of energy over electricity. In the residential sector, the share of thermal energy in the end use varies between 50–80% depending on the location, whereas the share of electricity remains between 15–35% [12]. The difference is a little smaller in the service sector [13]. Thus, it is highly important to consider the energy system as a whole and not to limit the scope strictly to electricity alone. This is particularly important because thermal energy is often produced locally, as long-distance transportation of heat is not yet technically or economically viable [14]. 1.2 Objectives and scope The purpose of this thesis is to investigate ways of increasing the share of VRE in the energy system, especially in urban environments. Previous studies on energy system integration of VRE tend to focus on power only. In contrast, this thesis considers the energy system as a whole, taking into account thermal and electric energy side by side. Flexibility measures such as power-to-heat conversion (P2H) with thermal storage are given emphasis in this context. The purpose of looking at the system as a whole is to find new ways to increase the system flexibility and thus allow larger VRE installations to be done. Three main topics are covered in this work. First, it is important to recognize the characteristics of the current energy use; that is, to have data that is as detailed as possible about spatial and temporal energy consumption. Traditionally urban energy consumption is modeled either spatially or temporally. In this work, the aim was to combine these dimensions into a single model with a sufficiently detailed resolution. This type of data is needed in matching energy supply and demand but also in optimizing the energy system operation and identifying possibilities for systemic innovations and improvements. Second, the question of multiple energy carriers and their cooperation and mutual conversion is addressed. New ways to provide flexibility are reached for by viewing the urban energy on a system level with different energy forms. For this purpose a new comprehensive energy-carrier model was developed that includes control functions and network intelligence. Another improvement compared to the existing literature is the possibility of incorporating both macro and micro levels to the same model. The model makes use of the detailed data from the spatio-temporal consumption model (e.g., Helsinki, Finland and Shanghai, China). As an example of the use of this model, the limitations of urban energy infrastructures on the solar energy potential on a city-level (Concepción, Chile) was investigated. This was done in a novel way by combining highly-detailed solar potential mapping with the energy-carrier model developed in this thesis. Third and finally, the question of the combination of traditional energy systems with large-scale VRE integration is discussed. The thesis considers the 12 Introduction consequences of large-scale VRE for traditional power plants, as well as the optimal way to operate energy systems under large VRE schemes. There are several dispatch models available. However, many of these are non-linear, which often leads to long computation times. In addition, the existing models are often limited to one energy form or few power plants only. The aim here was to cover the whole urban energy system including both the electricity and thermal energy sectors, but still keeping the simulation model user friendly and quick. 1.3 Thesis outline The thesis is structured as follows. The first chapter presents a general background of the topic and presents the objectives of the study. Chapter 2 gives a concise presentation of energy systems in urban areas. Chapter 3 gives a short overview of different flexibility methods and technologies in energy systems (Publication I). In the three following chapters, three different models are presented: Chapter 4 handles the spatiotemporal load profile formation (Publication II), Chapter 5 presents the multi-energy carrier model together with spatial energy flows (Publication III), and Chapter 6 describes the optimization model created for energy system operation with large-scale renewable energy schemes (Publication V). The next two chapters (7 and 8) present the results from the simulations and optimizations; specifically Chapter 7 focuses on Helsinki, Finland, and Chapter 8 focuses on Shanghai, China and Concepción, Chile. The results are gathered from Publications II–VI. The model developed in Publication III is used, with minor appropriate modifications, in calculating the results for Publications IV and VI. Finally, Chapter 9 summarizes and concludes the thesis. 13 2. Urban energy systems The modern lifestyle is dependent on a constant and secure energy supply. Energy is needed in several forms, such as electricity, heating and cooling, and fuels. Commonly, electricity is the main focus of the discussion of energy, as it is needed in most of our modern appliances and in areas from communication to industrial activities, services and lighting. However, to keep societies working, other forms of energy are also important, and all forms must be considered in order to understand the whole energy scheme and the challenges posed by climate change and limited natural resources. Energy systems are ways of producing energy and delivering it to consumers. This chapter is a concise presentation of electric and thermal energy systems, focusing on urban areas. 2.1 Electric power systems Electric power systems are complex combinations of networks, production units, transformer stations, end-use connections, and market operators. Their job is to guarantee a secure and constant power supply to consumers. Traditionally, electricity is produced in power plants by burning fossil fuels. In 2013, more than 40% of all electricity in the world was produced by coal, while 22% was produced by natural gas. Hydropower is the largest renewable source, with a 16% share [15]. All these energy sources are controllable; thus, the traditional way of balancing supply with demand has been controlling the supply side. However, new variable renewable power sources, such as solar and wind power, are increasing rapidly; as such, it is obvious that new balancing methods are required [16]. Electrical grids deliver power from the generating plants to consumers. The plants are connected to energy systems through transformers that change the voltage of the generated power to the necessary level. The long-distance transmission is typically done by high-voltage lines with a voltage that is typically between 110–800 kV [17,18]. In cities, the local grid is typically connected to the national or other larger-area transmission network. A city network also typically consists of different voltage levels. For example, the main trunk lines could be high-voltage lines, like the national grid, whereas the actual distribution grid is typically a low-voltage grid, with voltages below 50 kV, and it is connected to high-voltage lines with transformer stations. Along 15 Urban energy systems the distribution grid, power stations that are close to actual end users convert the voltage to the 230 V or 110 V typically used in domestic power plugs [19]. The balance between supply and demand must be maintained in order to keep voltage and frequency stable in the grid. The voltage drops in a network line because of current flowing through loads and impedances along the line. To keep the power system running, the power stations have to regulate the voltage to stay within a tolerated margin [19]. Distributed generation (e.g., small-scale solar and wind power) compensates for the voltage drop in the lines, which can be utilized in power system design [20]. In the future, as distributed and variable generation increases, new kinds of technological solutions to power system operation will also probably arise. For example, the role of the urban power grid may change from a one-way channel of power delivery into a way of balancing spatial differences in supply and demand. This leads to different smart grid concepts, which enable all parties of a power system to participate actively in the power system operation and balancing [21,22]. Smart grids are discussed further in Chapter 3.2.5. 2.2 Heating and cooling The need for comfortable room temperatures forms one of the largest shares of total energy consumption. In urban areas, heating and cooling systems typically consume more energy than electrical appliances [12]. Heating energy is needed in two forms: space heating and domestic hot water. The most common form of heating in the world is local heat production. There are several options for this form of production, including furnaces or boilers burning wood, natural gas, propane, or fuel oil. Electricity can also be used to provide heat through electric resistance or heat pumps [23]. In sunny regions, solar thermal collectors are common, especially in water heating [24]. Lately, the role of architecture in heating demand for buildings has strengthened, as zero-energy buildings have become increasingly popular [25]. An alternative to local production is district heating (DH), which is now available globally. For example, in Europe, the share of district heating is about 10% of the heating market [26], and it is particularly widely used in Nordic countries [27]. DH systems are especially suitable for urban areas, whereas the losses of heat transfer would grow too large in sparsely-populated locations [28]. The idea of DH is to produce heat centrally, usually in large units or CHP plants, and then to deliver this heat to consumers through hot water or steam pipelines. These lines are built with two pipes; the first pipe carries the hot water (or steam) to the customers, where the heat is delivered directly or through heat exchangers to heating appliances or heat storage. Afterward, the cooled fluid returns to the heating station inside the other pipeline. The system is operated by pumps that pressurize the system and keep the fluid moving [28]. The demand for heating varies according to outside temperatures, but the heating systems must be able to deliver enough heat during the coldest months of the year. Thus, different types of heat production units are needed in the DH systems. The base load could be satisfied, for example, by CHP or nuclear power 16 Urban energy systems plants operating throughout the year, but a reserve capacity is needed for colder periods. Heat-only boiler stations are commonly used, but heat pumps [29] and central solar heating [30,31] may also be used to provide heat for DH network [32]. Easy storage of heat is one advantage of district heating systems. Large hot water tanks are relatively cheap to build and connect to the heating network, which also has its own storage capacity. The water temperature inside the pipes can be increased, thus giving short-term storage capacity for sudden variations in demand [28]. Similarly, cooling is also needed to keep room temperatures at a comfortable level. The most common cooling techniques are local small-scale techniques and appliances, such as ventilation [33], air conditioning [34] or absorption cooling [35,36]. Nowadays, district cooling systems are also becoming more popular. Their operation principle is quite similar to district heating, but in reverse: thermal energy is transmitted from the customer to the district cooling system [37,38]. 17 3. Energy system flexibility (Publication I) 3.1 Flexibility challenge of renewable integration A power system has to guarantee a continuous energy supply that matches the fluctuating demand. Traditionally, different types of controllable power plants have ensured this energy balance. However, with increasing amounts of variable renewable energy generation, new kinds of flexibility measures are required [39]. In addition to temporal balance, spatial flexibility is also needed, as the capacity of distribution and transmission networks may limit the balancing opportunities [40]. Especially in electric power systems, the demand and supply must be in balance at each time point, and the system is able, up to a certain point, to cope with variations and uncertainty. In electric systems, the flexibility relates close to grid frequency and voltage control, as well as the security of supply and power ramping rates [41]. For example, a portfolio of power plants with different response times has traditionally been used to provide the necessary flexibility [42]. As energy production is becoming more distributed and variable, the old system will need to continue to operate side-by-side with the new technologies for decades to come. In order to keep the demand and supply in balance in the future, a wide selection of different technologies will be of importance [40]. Publication I presents a broad review of current and future technologies that could fulfill the flexibility required. The article covers both supply- and demandside options as well as network infrastructure, storage, and energy market questions. Advanced technologies, such as power-to-X or vehicle-to-grid, are also included. 3.2 3.2.1 Flexibility technologies Supply-side flexibility and grid services Traditionally in power systems, the balance between supply and demand has been maintained by power plants and grid operators. Supply-side flexibility co- 19 Energy system flexibility (Publication I) vers the measures and technologies to modify the output from the power generation units in order to make the demand and supply match. Grid operators, such as transmission system operators (TSO), are responsible for keeping the power grid in balance. In addition to controlling the output of power plants, other measures, namely grid ancillary services, can also be used for this purpose. These ancillary services are generic measures to maintain the balance at different time scales, from power quality and regulation with, for example, flywheels and batteries (1 ms – 5 min), to spinning and power plant reserves (5 min – 1 h), to longer (1 h – months) load shifting with large storage facilities [43,44]. The ability of power plants to respond to demand fluctuations depends on the type of plants they are. The characteristics of different plant types are listed in Table 1. Typical examples of base-load power plants include nuclear and coal power plants. These are designed to operate mainly at constant (preferably nominal) power, and ramping and shut-down are avoided for economical or technical reasons. Load-following plants, such as hydropower or gas turbines, are run according to the balance situation in the grid. They are quick to start up and ramp the output (with response times ranging from seconds to minutes). Peaking power plants are used irregularly during high-demand hours [39]. Table 1. Key output and ramping parameters of different power plant technologies [39,45]. Indicator Typical unit size (MW) Efficiency (%) Steam Gas Combi-cy- turbine turbine cle turbine Engine Nuclear 600–900 10–300 60–400 1–20 800–1600 40–47 32–38 50–60 45–48 33–37 100–600 100–600 30–180 1–15 (i.e., up to Start-up and synchronize 120–3000 time (min) Ramp-up rate (%/min) Minimum output (%) 2 days) 3–6 5–20 3–6 20–25 40 50 40–50 30 30 One option to regulate VRE production is to curtail the output when necessary. Such situations can occur with limited transmission capacity, overly large and inflexible base-load generation, and oversupply of VRE [46]. Curtailment can quickly reduce the generation, thus dampening rapid changes in power output, but it can also be used to provide reserve power capacity through a rampup margin [47,48]. When curtailing, some electricity is always lost; however, this is acceptable if the small losses enable larger VRE installations. 3.2.2 Energy storage Storage of energy is a way to time-shift the energy supply, which makes temporal mismatches of supply and demand tolerable. Recently, interest in energy 20 Energy system flexibility (Publication I) storage has increased because of large-scale integration of renewable energy, which has led to advances in storage technologies [49]. Well-known examples of energy storage are hot water tanks (for storing heat) and fuel containers (for fuels), which are economical and technologically simple. Storing electricity is, however, more difficult due to high costs and technological challenges. Options for storage of electricity with different characteristics are numerous. Publication I considers pumped hydro power energy storage (PHES), compressed air (CAES), flywheels, batteries, hydrogen, superconducting magnets, and supercapacitors. Some technologies are more suitable for providing shortterm power, whereas others are more suitable for long-term storage. The chosen technology depends on the case, as none of the available systems can provide all of the desired features: low cost, high efficiency, long lifetime, and high power density [50,51]. PHES accounts for 99% of the total storage capacity in the world and is thus by far the most utilized technology, but CAES and lead-acid batteries are also considered as mature technologies [52]. In small appliances, lithium-ion batteries are currently the most widely used technology, due to their high efficiency, good energy density and low self-discharge rate. For large-scale grid services, however, they are still overly expensive [53,54]. The energy efficiency of the power system could be increased with energy storage, as base-load power plants would be able to continue their high-efficiency production even during periods of low demand. Also, the economic efficiency would be enhanced by using stored energy instead of peak-load power plants with high marginal costs [55]. 3.2.3 Power-to-X Power-to-X (P2X) refers to technologies through where electricity is converted to other energy forms, such as heating or cooling, gas (methane or hydrogen), and power for electric vehicles. This increases the flexibility of the system by providing new options (i.e., degrees of freedom) in the end use of energy. The electricity for P2X can be taken from surplus VRE production or from power markets when the price is low or even negative. These technologies make it possible to reduce or even avoid curtailment of renewable energy. In power-to-heat (P2H) or, more generally, power-to-thermal (P2T), electricity is converted into thermal energy (i.e., heating or cooling). The main technologies for this purpose are heat pumps and electric boilers connected to thermal storage or air-conditioning units. Typically, thermal energy dominates in final energy use, and it is easier and more economical to store. Advantages of P2H and P2T also include reductions in emissions of heat production from fossil fuels [56]. Excess electricity can also be used in power-to-gas conversion (P2G) – that is, production of high-value gas products, such as hydrogen or methane. Hydrogen can be produced by electrolysis or photoelectrolysis. Synthetic methane is typically produced through hydrogen and the Sabatier reaction [57,58]. 21 Energy system flexibility (Publication I) Electric vehicles (including plug-in electric vehicles) provide a way to store energy. Combining the batteries of the vehicles actively with the grid is called vehicle-to-grid strategy (V2G). The vehicles stand idle most of the time, with relatively long (8–12 h) charging windows but short (90 min) charging times, resulting in considerably flexible scheduling of storage [59,60]. 3.2.4 Demand-side management Demand-side management covers a wide range of technologies that affect the end-use electricity consumption. Contrary to the traditional way of controlling the energy supply, its purpose is to modify the demand to better match the supply. It can be roughly divided into three categories: reducing, increasing, or rescheduling the demand. All of these can be used to even out the demand by clipping high peaks and filling valleys of low consumption [61]. Load shifting (i.e., rescheduling) typically requires intermediate storage. Examples of demandside management include controlling the electric heating in buildings and postponing the use of washing machines. An industry with high power consumption may reduce or postpone its production when power consumption is high and electricity is expensive. DSM is able to offer several benefits to power systems and markets. It balances the demand and supply in terms of both energy and power, and it operates in various time scales. Most DSM applications are used in the time scale of 1–12 h, where the largest fluctuations of VRE supply also occur [62]. DSM smooths the operation of power markets by reducing price peaks and the average spot price [63]. DSM can also lead to economic savings by removing or postponing the need for infrastructure expansions, by reducing transmission and distribution losses, and by facilitating energy savings [64–66]. 3.2.5 Novel grid infrastructure The flexibility of power systems requires sufficient transmission capacity [67]. Spatial differences in power supply and demand can be balanced by a robust and well-designed grid and appropriate grid codes [48]. Strong transmission lines are also essential for energy markets to function properly. Supergrids, smart grids, and microgrids are proposed novel technologies to help with the flexibility challenge. Supergrids refer to strong transmission networks that typically use high-voltage direct current (HVDC) in energy transfer. HVDC technology enables smaller electric losses and could be more economical in long-distance power transfer. Thus, it could be used to connect large remote VRE installations with demand [68,69]. Recently, smart grids have generally been considered as a key technology for efficient VRE integration. The goal of smart grids is to optimize the operation of the energy system and to enable new and more efficient energy market structures by connecting energy producers, consumers, network companies and storage facilities intelligently together. This requires automation, robust two-way communication technology and advanced metering. Smart grid development 22 Energy system flexibility (Publication I) aims at achieving improved power supply reliability and reduced ecological impact, while avoiding costly investments by finding new options for securing energy supply [21,22]. Microgrids are small local energy networks that can meet the demand of local consumers. These may include local generation (micro-CHP, VRE), storage, and demand-side management. They can be built in isolated locations, but also as a part of a larger grid system, where the microgrid significantly improves the local power reliability by offering a solution if the power supply from the larger system fails. Furthermore, microgrids could be used as a component to balance voltage fluctuations in larger systems [70,71]. 23 4. Spatiotemporal consumption profiles (Publication II) To better understand the operation of energy systems of cities, one must have detailed information about consumption in terms of both space and time. Detailed spatiotemporal data about consumption allows for better planning of large-scale renewable energy generation and smart-grid operations, as well as understanding their effects on energy networks and the overall system operation. Typically, energy consumption in cities is modeled either temporally or spatially. Publication II presents a model to combine these dimensions and to create so-called spatiotemporal load profiles. It is based on the spatial division of temporal profiles P(t) to P(x,y,t). The model guarantees that the original load profile P(t) stays unmodified. For spatial division, the model uses spatial data of urban structure, such as population and work-place distribution, but also building type and land-use data. The accuracy of the profiles produced depends on the available input data. The model can also be used with coarse data by utilizing different urban structure models, as demonstrated in Publication II with the Shanghai case. 4.1 Input data The model is based on classifying energy consumers into consumer classes. A typical rough division of consumer classes involves using classes: residential, services and manufacturing. These can be further divided into sub-classes: the ‘residential’ class may include single-family houses and apartment buildings, whereas the ‘services’ class may include offices, healthcare, educational buildings, restaurants and commercial buildings. This division depends on the data available in the city. As input data, the model requires the consumption time series of the whole city, and the consumer classes’ annual consumption numbers and temporal base profiles. In the absence of annual consumption numbers, floor areas (m2) can also be used with typical energy intensities (MWh/m2/year) of the classes. The base profiles are relative load profiles revealing how the consumption of a consumer class is distributed over time. Their yearly integral over time (or sum over discrete time steps) is equal to 1. 25 Spatiotemporal consumption profiles (Publication II) For spatiality, the data required are data about the spatial distribution of the classes in different districts of the city. These data sets include, for example, population densities and floor area data in the districts. If detailed spatial data about the urban structure and the building locations are not available, mathematical models for urban structure can be used. For example, in Publication II, the following exponential density model [72] was used: మ ݀ሺݔሻ ൌ ݀ ݁ ିఈሺ௫ି௫బ ሻ ܭ (1) where ݀ describes the maximum density of an urban structure, ݔis the distance from the city center, ݔ is the location of the maximum density, ߙ is the width parameter of the density distribution, and ܭis a constant describing the minimum density throughout the area. 4.2 Formation of spatiotemporal load profiles A visual illustration of the model is presented in Figure 1. For clarity, it shows a case with no sub-classification of consumer classes. If sub-classes are used, the process for them is similar to the one described here, with the total city load profile replaced by a load profile of the original consumer class. 4.2.1 Scaling base profiles to match the city demand The procedure begins by multiplying the base profiles ݂ ሺݐሻ with the corresponding annual consumption ܳ . Subscript ݅ refers to consumer classes. The multiplification forms a bottom-up type of preliminary load profiles for the classes. Their sum represents the total consumption of the city. If the original total city profile ܲୡ୧୲୷ ሺݐሻ is known, the summed profile (probably) does not match it. Therefore, independently at each time step, the summed profile is scaled to match the city profile ܲୡ୧୲୷ ሺݐሻ. The scaling factor is indicated by ߣሺݐሻ. The final load profiles ܲ ሺݐሻ for the consumer classes are, thus, of the form ܲ ሺݐሻ ൌ ߣሺݐሻܳ ݂ ሺ݅ݐሻ (2) where ߣሺݐሻ ൌ 4.2.2 ܲୡ୧୲୷ ሺݐሻ σ ܳ ݂ ሺݐሻ (3) Sub-class division If sub-class division is used for consumer classes, profiles ܲ ሺݐሻ must be further divided into ܲ ሺݐሻ, with ݆ indicating the sub-class. This is done similarly to forming class profiles ܲ ሺݐሻ. The sub-class base profiles ݂ ሺݐሻ are multiplied with annual consumption of the sub-class; that is, ܳ or ܣ୲୭୲ ܫ , where ܣ୲୭୲ is the total floor area of sub-class ݆ in the city, and ܫ is its typical energy intensity. These multiplied base profiles are again scaled up or down so that their sum profile corresponds to the load profile of the upper class ݅. That is 26 Spatiotemporal consumption profiles (Publication II) ܲ ሺݐሻ ൌ ߣ ሺݐሻܳ ݂ ሺݐሻ (4) where ߣ ሺݐሻ ൌ ܲ ሺݐሻ σ ܳ ݂ ሺݐሻ (5) Figure 1. Schematic representation of the model for creating spatiotemporal load profiles. Reproduced with permission from Publication II. Copyright 2014 Elsevier. 27 Spatiotemporal consumption profiles (Publication II) 4.2.3 Spatial division After temporal load profiles for consumer classes and sub-classes are formed, they are divided spatially according to any spatial data available. Here it is assumed that the division is made by floor area distribution ܣ ሺݔǡ ݕሻ, but any other density unit could be used (in Figure 1, general density݀ is used). The load profile of class ݅and sub-class ݆ for location ሺݔǡ ݕሻ is ܲ ሺݔǡ ݕǡ ݐሻ ൌ ܣ ሺݔǡ ݕሻ ܲ ሺݐሻ ܣ୲୭୲ (6) The final total load profile of location ሺݔǡ ݕሻ is formed by summing these classby-class load profiles of the location together: ܲሺݔǡ ݕǡ ݐሻ ൌ ܲ ሺݔǡ ݕǡ ݐሻ 28 (7) 5. Multi-carrier energy networks (Publication III) The energy system consists of several energy production, conversion, and distribution processes. Typical examples are conversion chains from primary energy into a utilizable form – in other words, energy commodities (or energy carriers), such as fuels, electricity and heat. These are transported and distributed to consumers by energy networks, such as power grids, gas distribution lines, and district heating networks. The commodities can also be converted into each other (e.g., gas into electricity and heat, or electricity into heat and cold). Note that low-exergy heat cannot be converted into high-value energy products. A multi-energy concept refers to technologies and practices in which different energy carriers are distributed, converted between each other, and stored optimally for the whole system. For example, the share of thermal energy in the final energy mix of built environments could be two to three times higher than that of electricity [12]. Thus, taking the thermal energy system into account when operating the electric system could provide significantly more flexibility and options for matching demand and supply. Storage of thermal energy is also significantly easier and cheaper. Another example is that of power-to-gas technologies, which convert renewable electricity into gas when excess production is available. Multi-energy thinking makes it possible to oversize the VRE production by utilizing the electricity that could not be fed into the electric distribution system, thus yielding notable VRE production during modest wind or solar conditions. Publication III introduces a concept for multi-energy simulations based on spatiotemporal data on urban energy consumption. In Publications IV and VI, the model is used for energy system analysis with more detailed input data. 5.1 Structure of the multi-carrier energy systems model The energy system is modeled here with a combination of nodes that are connected by lines forming networks, though not all energy networks will necessarily have the same spatial coverage. Depending on the case and the desired spatial accuracy, a single node can represent a neighborhood, a block or a single building of a city. Each node is typically equipped with time series of energy 29 Multi-carrier energy networks (Publication III) consumption (such as electricity ܳ , heat ܳ , and cold ܳ ). Furthermore, a generic node may contain different functionalities, such as energy production (ܲ), storage ()ݏ, and mutual conversion (ܿ) of energy carriers (electricity, fuel, heat, cold). In practice, the nodes may not include all the functionalities simultaneously but may contain some simple combination of them. Figure 2 shows an example of a generic node. Since the nodes are connected to each other by networks, each node also has incoming and outgoing energy flows (݂୧୬ and ݂୭୳୲ ). Figure 2. Representation of a generic node with different functionalities: energy flows (݂), production (ܲ), consumption (ܳ), storage ()ݏ, and mutual conversion of energy forms (ܿ). Subindex ݁ refers to electricity, ݄ to heat, ܿ to cold, ݂ to fuel, and ߢ to any of these. Figure 3 shows an example of two energy networks covering the area divided into nodes. The networks are connected to each other by intersecting nodes, in which the conversion occurs and the energy moves between the networks. These intersections are marked with dashed purple lines in the figure. The energy networks (power grid, district heating (DH) and cooling networks, and gas pipelines) may be connected to a higher-level transmission system. These nodes are called energy connectors and marked with circles in the figure. The connector, in a power grid, for example, is a transformer station, while in a district heating network it is a heat exchanger or a DH substation. Figure 3. Two energy networks (power grid and district heating network) connected to each other by intersections (purple dashed lines). Circles represent energy connector nodes (blue: transformer station; red: district heating station with heat exchangers). 30 Multi-carrier energy networks (Publication III) 5.2 Energy balance in nodes and flows in network ௧ Based on the conservation of energy, the energy balance ܤǡ (i.e., match of sup- ply and demand) is calculated for each node ݊ and each energy form ݇ at time point ݐ ௧ ௧ ௧ ௧ ௧ା ௧ି ௧ ܤǡ ൌ ܲǡ െ ܳǡ ݏǡ ܿǡ െ ܿǡ െ ݖǡ ௧ା ܿǡ (8) ௧ି ܿǡ where the terms and represent the conversion of other energy forms ௧ into form ݇ and vice versa, and the term ݖǡ is the curtailed energy production. ௧ The storage term ݏǡ can have either a positive or a negative value depending on whether the storage unit is discharging (+) or charging (−). It is assumed that the energy losses are included in the corresponding terms. Conversion terms may be defined with the efficiency of the conversion process between energy forms ௧ା ௧ ܿǡ ൌ ߟ՜ ݍǡ (9) ஷ ௧ where ݍǡ represents the amount of the other energy forms allocated for conversion into energy form ݇. The index ߢ covers all other energy types except ݇. In the opposite case, the energy form ݇ is converted into other forms ௧ି ௧ ܿǡ ൌ ߟ՜ ݍǡ (10) ஷ The efficiency is typically between 0 and 100%, but can also have values greater than 1, such as when using heat pumps in power-to-heat conversion, where ߟ corresponds to coefficient of performance (COP). Eq. (8) states how much of the energy consumption of the node can be satisfied by local production, discharging the local storage, and converting other energy carriers. On the other hand, the equation reveals if there is surplus production in the node. If the balance differs from zero, the energy flow in the network must compensate for that: ௧ ௧ ௧ ܤǡ ൌ ݂୧୬ǡǡ െ ݂୭୳୲ǡǡ (11) Flow terms heavily depend on the structure of the network and the operating principles of the whole energy system. With a simple non-looped network, the power flows in the network can be determined from the condition that a node must deliver all the power to the nodes downstream of it. For a line ݈ in the network, this yields a condition ௧ ௧ ݂ǡ ൌ ܤǡ (12) אୢୱሺሻ where notation ݊ אሺ݈ሻ refers to nodes that are downstream of line ݈. “Downstream” refers to the nodes that are network-wise further from the energy connector (e.g., transformer in a power grid). See Figure 4 for an explanation. For a looped network structure, more complicated iterative calculation methods are required, such as the Newton–Raphson method [73]. 31 Multi-carrier energy networks (Publication III) Figure 4. Simple network to explain the terms upstream and downstream. The node marked with a white circle is the energy connector (transformer, heat exchanger). The nodes with the red background are upstream of the node with the yellow star, while the nodes with the green background are downstream of it. 5.3 Energy system operation in simulations To run the energy system simulations, it is necessary to predetermine what to do in cases where the local energy production does not match the consumption, or in the event that the carrying capacity of the network limits the flows. Which actions are taken first? For this purpose, different strategies may be utilized depending on the case. A few examples are listed below: 1) No conversion–no storage: All excess energy production in the nodes is fed to the networks; if they get congested, the VRE production is curtailed. 2) Local use with conversion and storage: The target is to consume all locally produced energy in the same node. Energy is converted from one form to another to meet the demand of all energy forms. Nodes use their own storage units to avoid feeding energy into networks. 3) City-wide use: The purpose is to keep all produced energy within the city. Conversion may take place in several nodes, and it is applied when needed due to congested networks or a mismatch between supply and demand. Some energy forms may be stored in the nodes. 4) National transmission: Excess electricity or fuel is fed to national networks through energy connector nodes. Conversion is applied only if the network gets congested, preventing the use of national transmission. Heat is produced within the city or locally in the nodes. Typically, the energy balances are calculated first, and the calculation of power flows (as in Eq. (12)) requires balances as input data. Some of the above-mentioned strategies, however, require that the possible grid congestion is checked 32 Multi-carrier energy networks (Publication III) when determining the actions in the nodes and affecting the energy balance. In these situations, iterative calculations are needed, starting from the end points of the network (i.e., the nodes with no downstream nodes) and proceeding node-by-node towards the energy connector node. The energy balance is first calculated without any further actions. Next, the power flow in the upstream link is checked; in the case of congestion, the required amounts of flexibility methods such as conversion or storage are utilized. The power flow is then recalculated with an updated value of energy balance. 33 6. Optimal use of an energy system under large-scale VRE (Publication V) Energy production is gradually transforming from centralized, power plantbased generation into variable and distributed renewable generation. However, the old and new systems will still have to operate side-by-side for future decades. Thus, it is highly important to understand the effects of increasing variable renewable generation on the existing system, especially as the current power plant mix may not be designed for large-scale VRE. Publication V introduces a mixed-integer linear optimization model to find the most economical ways to operate the whole energy system (both electricity and heating) under large-scale VRE, consisting of traditional energy production plants (power plants, CHP plants, heat boilers), different flexibility technologies, energy spot markets, and emission costs. The model minimizes operating and fuel costs and maximizes revenues from energy sales, while the demand must always be satisfied. It can be used to model energy systems from single buildings to the city scale. Investment costs of new technologies are not included, as the focus is on the effect of VRE integration on existing power plants’ operation and economy, and thus on the operation of the whole system. 6.1 Model principles The model is based on mixed-integer linear programming (MILP). This formulation guarantees short calculation times and simple modifying of the model by easily adding or removing decision variables and boundary conditions. The model runs time step by time step, seeking the most economical solution that fulfils the requirement of always meeting the energy demand. The optimal solution is determined by considering several technical limitations in energy balance, energy production plants, and flexibility technologies. Figure 5 shows schematically how both internal and external factors affect the system operation and thus its optimization. 35 Optimal use of an energy system under large-scale VRE (Publication V) Figure 5. External and internal conditions affect the optimization problem. Reprinted with permission from Publication V. Copyright 2016 Elsevier. Figure 6 presents a schematic representation of the model principles. Time series of VRE production (e.g., wind and solar power) and energy consumption form the basis of the model (Point #1 in the figure). As input, properties of the existing energy system are also given (Point #2), including the technical limitations of energy production plants and energy networks, as well as storage and flexibility technologies. The mixed-integer linear optimization problem (Point #3) is solved at each time step for a given number of future time steps (i.e., optimization horizon). In forecasting the future consumption and VRE production, forecast errors should be considered, and thus this option is included in the model as well. As output (Point #4), the model produces optimal time series of power plant operation and flexibility technologies, as well as the costs and profits from satisfying the energy demand. Even though the optimization algorithm yields optimal time series for the whole optimization horizon at each time step, only the values of the first time step of the horizon are used in the output profiles, since the rest of the time steps are updated during the optimization of the following time steps. 6.2 6.2.1 Input and output VRE production, consumption, and technical parameters Energy consumption (electricity and heat) and VRE production time series are given as input. They are used in boundary conditions of the optimization problem to guarantee that the energy supply always matches the demand. The forecast errors can be included in the input time series by modifying the actual time series by error terms, which could be formed based on Wiener processes (i.e., random walks) [74]. The idea in doing so is to form separate Wiener processes starting from each time step. These processes are then cumulatively summed up so that the error for the next time step ( ݐ ͳ) is based on the Wiener process of that time step. The errors of additional time steps ( ݐ ʹǡ ݐ ͵ǡ ǥ) are based on the sum of Wiener processes of all time steps between the current and the forecasted time step. Finally, the forecast errors are scaled so that their expected values match the measured forecast errors. 36 Optimal use of an energy system under large-scale VRE (Publication V) Figure 6. Schematic illustration of the MILP model to find cost-optimal ways to operate energy systems under large-scale VRE. Reprinted with permission from Publication V. Copyright 2016 Elsevier. The technical parameters of power plants and energy systems are incorporated as boundary conditions of the optimization problem as well. These include characteristics of power plants, such as maximum and minimum power outputs, ramping rates, and start-up and shut-down times. For CHP plants, it is necessary to define how tightly power and heat outputs are connected to each other. Flexibility measures are defined as well: size of the heat storage, power transmission capacity to upper-level transmission system (e.g., national grid), coefficient of performance (COP) of P2H conversion, and upper limits for curtailment and P2H. 37 Optimal use of an energy system under large-scale VRE (Publication V) 6.2.2 Economic parameters Economic parameters are used as coefficients in the objective function of the optimization, since the target is to minimize the costs and maximize the revenues. The cost of producing energy in power plants is a combination of several factors, including the fuel price, operations and maintenance (O&M) costs, emission prices, and taxes. In this work, the production costs in CHP plants are divided into electricity and heat in relation to the nominal outputs of the energy forms. Ramping the power plant, especially if it is designed for base-load operation, has a degrading effect on the plant, and thus it is thought to have a cost. Thus, we have included a cost for ramping in the model, which is defined as euros per mega-watt change in the output. Income comes from selling energy to consumers within the grid area of the energy company. For district heating sales, seasonal pricing is commonly used, meaning that the price changes a few times per year. The selling price for electricity typically depends on the spot price of electricity, which is used in this work as the selling price. The price is different when electricity is sold to power markets and when it is bought from them. In this work, the spot price is used, but it is modified with a margin to take the transfer prices into account. The spot price is considered an external factor (Figure 5), which requires that the investigated area must be small enough compared to the whole energy market of which it is a part, so that the internal phenomena do not have significant effect on the market prices (e.g., the consumption of Helsinki city (Finland) is only about 1% of the whole Nordic–Baltic spot market). Operating the flexibility measures can also have costs. Thus, it is possible to define the costs of P2H conversion, curtailment, and storage of heat. The model also allows for the heat to be wasted, but typically the cost for this is set so high that it is avoided as long as possible. 6.2.3 Output During the simulations, the model produces several output time series. These include the power outputs from the energy production plants, the amounts of electricity and heat sold to consumers, and electricity sold to or bought from the spot markets. Additionally, the flexibility measures are recorded during the simulation: P2H conversion, the state of the heat storage, curtailed VRE production and wasted heat. All these actions together form the total costs and revenues from running the energy system. From those time series, additional results can be calculated, such as yearly energy balances, emissions, and capacity factors. 38 Optimal use of an energy system under large-scale VRE (Publication V) 6.3 6.3.1 Optimization of energy system operation Objective function During the simulations, the optimization is run at each time step considering the current time step and a number of future time steps (i.e., optimization horizon; see Figure 6). The complete mathematical formulation is presented in Publication V. The linear optimization problem follows the formulation ሺ ௧ െ ሻ subject to (13) a) Technical properties of energy production plants b) Technical properties of balancing methods c) Balance of energy demand and supply The costs from all time steps in the optimization horizon are summed up, and the revenues are subtracted from them to form the objective function. Production costs include fuel, O&M, and emission costs of energy production, as well as possible taxes. Furthermore, ramping costs from changing the output power of the plants are included here. Auxiliary variables are needed in the formulation for ramping to keep the model linear. Because the ramping may be positive or negative, absolute values would be needed in the objective function to keep the cost positive, which would break the required linearity. Balancing costs cover the O&M costs of the technologies, but the lost profits from non-sold electricity are excluded. The revenues from sales include the income from the energy sold to consumers in the studied area. The effect of the electricity that is produced in the city but used in balancing technologies instead of sold it, is shown as decreased sales in the revenue term of the function. O&M costs of the energy storage are included in the storage costs. 6.3.2 Operation principles of an energy system – boundary conditions The energy production plants typically have minimum and maximum values between which the output varies when the plant is on. In addition, start-up and shut-down procedures take time. These characteristics are included in the model by binary variables defining the plants’ current operational state (on, start-up, off, or shut-down), and, depending on the state, corresponding limits are set for output power. Separate boundary conditions are defined to guarantee that the operational state does not change too quickly (i.e., the start-up and shut-down times are obeyed). The plant operation is additionally restricted by ramping limits, or the maximum allowed change in output power between time steps. Some plants (e.g., heat boilers designed for peak-load operation) may have less strict conditions, such as only limiting the maximum output but allowing quick ramping. The boundary conditions for balancing methods are kept as simple as possible. Thus, their use is limited between zero and a maximum value. For curtailment, 39 Optimal use of an energy system under large-scale VRE (Publication V) the upper limit could be defined as an absolute limit or a relative amount of the momentary production. An important boundary condition is the balance between demand and supply. In this condition, one must include all possible options for energy. The production must be equal to the consumption, taking into account curtailment, P2H conversion, charging and discharging the storage units, and the electricity import from/export to the national transmission system. If the heat supply exceeds the demand after all the balancing measures are fully utilized, the extra heat needs to be wasted (but with high cost). 40 7. Results for Helsinki Helsinki, Finland, is a northern city in a global perspective with around 600,000 inhabitants. Its final energy use is dominated by heat, mainly due to its location (60 °N). Its current energy supply is almost completely dependent on fossil fuels, though CHP production and extensive district heating result in high fuel efficiency. The city, however, has a target of becoming carbon-neutral by 2050 [75], which requires drastic redesign of the energy system. For these reasons, the city was chosen as the main case study for the thesis. 7.1 Current energy system in Helsinki The current energy sources of the municipal energy company of Helsinki are shown in Figure 7. The energy supply of the city is based on three large CHP plants using natural gas or coal as fuel, providing both electricity and heat to consumers. These plants’ production capacity is around 1,000 MW of electricity and 1,300 MW of heat. For cold peak conditions during the winter, there are ten heat-only boilers in the city, with a total capacity of more than 2,000 MW. Additionally, there is a heat pump station able to produce 90 MW of heat to the district heating network. These energy generation plants are marked on the map in Figure 8. Annually, around 4 TWh of electricity is consumed in the city [76]. However, the total production capacity of the municipal energy company is 7 TWh, but all nuclear and almost all of the renewable production capacity (Figure 7 left) are located far from the city. The power plants in Helsinki typically produce somewhat more electricity than the city consumes. For example, in 2014, the CHP plants produced 4.7 TWh. The annual heat consumption in the city varies between 6–7.5 TWh depending on the weather [76]. Over 90% of the heat load is covered by a district heating system that has 1,300 kilometers of hot water pipelines [77,78]. In 2015, around 6.4 TWh of district heat was supplied in Helsinki, of which 88% was covered by CHP, 7% by heat pumps, and the rest mainly with heat boilers [79]. 41 Results for Helsinki 1% 13 % 20 % Natural gas 49 % 18 % 7% Coal 40 % 52 % Oil Nuclear Renewables Figure 7. Left: The energy sources of the electricity production of the municipal energy company in Helsinki in 2015, including the shares in the production capacity outside the city (nuclear + renewables). Right: The energy sources of the district heating in Helsinki in 2015. [76] Figure 8. Map of Helsinki city, with total floor area distribution. Large CHP plants are marked with A–C (A natural gas, B & C coal), heat-only boilers with D–F (D natural gas, E oil, F coal), and heat pump station with G. Reproduced with permission from Publication V. Copyright 2016 Elsevier. Examples of load profiles of electricity and district heating are shown in Figure 9 and Figure 10, respectively. The heat load includes both space heating and hot water. The space heating load depends almost completely on the outside temperature, whereas the hot water demand is fairly constant throughout the year. Due to space heating, the overall heat load is strongly concentrated in the winters only, as the consumption during the summers is mainly hot water consumption. In contrast to heating, the electricity consumption depends more on the actions of people than on weather conditions, even though weather also has a clear effect on the load profile. The electricity consumption is, however, distributed more evenly on a yearly scale, but there are clear weekly and daily cycles. 42 Results for Helsinki Figure 9. Temporal load profile of electricity consumption in Helsinki during one year. Reproduced with permission from Publication II. Copyright 2014 Elsevier. Figure 10. Temporal load profile of heat consumption in Helsinki during one year. Reproduced with permission from the Supplementary material of Publication V. Copyright 2016 Elsevier. 7.2 7.2.1 Spatiotemporal consumption profiles (Publication II) Input data The official district division (before 2013) of Helsinki city was used to divide the temporal load profiles (Figure 9 and Figure 10) spatially. Some slight modifications have been made since then to the district borders. The districts are shown in Figure 8. The consumers were divided into three main classes: households, services, and industry. These were further divided into sub-classes, which are listed in Table 2. The subclass division is the same one that is used in the official statistics of the city [80]. So-called standard load profiles based on German experiences [81] are used as the base profiles ݂ and ݂ required in Eqs. (2) and (4). The load profiles and their use with the consumer classes are explained in Publication II. The total 43 Results for Helsinki annual consumptions of the consumer classes and subclasses ܳ and ܳ are from the Helsinki Environment Centre [82]. In this work, values from 2011 were used; in that year in Helsinki, the household electricity consumption was 1.3 TWh, service sector consumption was 2.9 TWh, and industry consumption was 0.3 TWh. In 2015, the residential consumption increased by 10%, but service and industry consumption decreased by 8–11% compared to the 2011 values. The building distributions of different consumer types (i.e., subclasses) were obtained from the statistics of the city [83]. The total floor area distribution is shown in Figure 8. Two examples of building distributions of consumer classes are shown in Figure 11: a) households and b) office buildings. The residential buildings are located more evenly throughout the city, whereas the office buildings are mainly concentrated in the downtown area. Table 2. Consumer class division used in Helsinki. Reproduced from Publication II with permission. Copyright 2014 Elsevier. Households Services Industry Detached houses Commercial Industry Row houses Office Storage Apartment houses (less than 4 floors) Traffic Apartment houses (4 floors or more) Healthcare Recreational Education Figure 11. Examples of floor area distributions of consumer classes in Helsinki; a) households, b) office buildings. 7.2.2 Spatiotemporal consumption profiles – results Figure 12 illustrates how the electricity consumption behaves spatially during a typical working day in Helsinki. In the city center and other office areas, the demand peaks at noon or in the late morning hours, whereas in the residential areas further from the city center, the consumption peaks around 7–8 pm. A milder peak occurs during the morning hours of 6–8 am, before people leave for work. The highest peaks in electricity load in the neighborhoods over the whole year are found in the city center in early February. A level of 80 MW/km2 can be reached during the coldest times. In residential areas some kilometers away from the center, the peak load reaches only 3–6 MW/km2. 44 Results for Helsinki Figure 12. Spatial electricity load of an average working day in Helsinki between a) 2–3 am, b) 11–12 am, c) 7–8 pm. Reproduced with permission from Publication II. Copyright 2014 Elsevier. 45 Results for Helsinki Figure 13. Relative load profiles from three districts of Helsinki. Kluuvi is a district in the city center with a high share of office buildings; Punavuori has both apartment buildings and offices; and Puistola is an almost completely residential area with many detached houses. Reproduced with permission from Publication II. Copyright 2014 Elsevier. Relative load profiles of three selected districts are illustrated in Figure 13 (locations shown in Figure 12). Kluuvi is an office and commercial district in the city center, whereas Puistola is a neighborhood of mainly detached and row houses in the northern part of the city. Finally, Punavuori (in the immediate vicinity of the city center) is a mixture of these two, with a remarkable share of apartment buildings but also many offices and service buildings. In the downtown areas (Kluuvi), electricity consumption peaks during the working hours, whereas in the residential area (Puistola), the peaks occur in the morning and in the evening. The peak power consumption in these districts is more than two times higher than the off-peak consumption during the night hours. The profiles are also significantly different on the weekends in the different types of areas. The residential areas have high consumption numbers during weekends, whereas in the office areas, the consumption stays lower on weekends than weekdays. Validation of the results was difficult due to the lack of data from the districts. Therefore, a simple comparison was made with aggregated annual values. The consumption of the residential sector was 1,500 GWh according to the model, whereas the measured value was 1,300 GWh. The model showed a consumption of 2,800 GWh (with 2,900 GWh being the measured value) for the service sector, while it showed 260 GWh (290 GWh measured value) for the industrial sector. The modeled values can be considered satisfactory, with a maximum deviation of 15% in the results. 7.3 P2H conversion enables more VRE (Publications III and IV) As the city of Helsinki has a well-established district heating network that accounts for more than 90% of the city’s heat consumption, it was studied in this work whether the flexibility of the energy system could be increased through power-to-heat conversion, which could lead to larger VRE capacity. Converting 46 Results for Helsinki electricity into heat is associated with poor exergy efficiency (second law of thermodynamics), even though the first-law efficiency (energy quantity) could be as high as 300% when using heat pumps. There are, however, several factors in favor of P2H. Firstly, the heat demand in a built environment is large [12]. Secondly, the heat system has greater inherent inertia compared to the electric one, and the storage of heat is easier and cheaper. Furthermore, by using heat pumps, the thermal output could be increased with the same amount of electricity. Finally, P2H enables the use of more VRE in the electricity consumption by allowing the VRE production to be oversized, and as the capacity factor of VRE is clearly less than 1, the amount of converted energy in P2H is not proportional to the curtailment level of power but closer to the capacity factor of VRE (0.2–0.35 for wind) [56,84] (Publication IV). 7.3.1 Wind power with P2H and thermal storage Integration of large-scale wind power was spatiotemporally analyzed in detail, together with a large P2H scheme. The consumption values of 4.4 TWh for electricity and 6.6 TWh for heat were used in the analysis, and the spatial consumption profiles introduced in the previous chapter (Chapter 7.2) were used. The wind power scheme consists of three wind power farms that are connected to the grid close to the CHP plants (locations shown in Figure 8). The connection points were chosen because of the strong electric network in the locations. The amount of wind power was varied from zero to up to 4,000 MW with a unit output of 3.14 GWh/MW. P2H conversion was done with electric resistance (COP = 1) or heat pumps (COP = 3), and three different heat storage schemes were included (no storage, limited to 100 GWh, and unlimited). The self-use limit of VRE is the maximum amount of installed capacity where the energy production ܲ does not exceed the consumption ܳ on an hourly basis ( ܲ always ܳ ). With wind power, the self-use limit in Helsinki is around 400 MW, which covers 27% of the city’s electricity consumption. If the wind power installations are increased above this limit, part of the production needs to be curtailed or exported outside the city. Figure 14 shows the share of wind power in total electricity consumption and the share of P2H-produced heat in total heat demand with different amounts of installed wind power and different P2H strategies. The linear increase of the share of wind power in electricity consumption starts to slow down after the self-use limit (400 MW). Oversizing the wind power capacity, however, provides benefits on the electricity side as well, as the share of wind in electricity consumption continues to grow after the self-use limit. The surplus production is converted into heat. With 2,000 MW of wind power and 100 GWh of heat storage, more than 60% of the electricity and 70% of the heat that is needed could be produced with wind power. By comparison, without thermal storage, only slightly more than 30% of the heat could be delivered and more than half of the wind-based heat would be lost. 47 Results for Helsinki % of electricity consumption 80 no storage (COP=1) 60 no storage (COP=3) 40 limited storage (COP=1) 20 limited storage (COP=3) Share off heat Sh h Yearly share, % 100 unlimited storage (COP=1)) 0 0 2000 Wind power (MW) 4000 unlimited storage (COP=3)) Figure 14. Yearly share of wind power in electricity (black line) and heat consumption (purple, green and red lines) with different wind power amounts and operating strategies. Reproduced with permission from Publication IV. Copyright 2015 Elsevier. 7.3.2 Effect of network structure The importance of network structure and the location of the connection point are described with a simplified model of the city of Helsinki (Publication III). Figure 15 shows the spatial consumption profiles and simplified non-looped energy networks used in the simulations. The spatial distribution of consumption is based on a radial consumption profile of Eq. (1). Different connection points for a large-scale wind power plant were studied (A–C in Figure 15). Table 3 summarizes the different cases and their results. The first option considered for the connection point was on the shore side at the end of a major trunk line of the power grid (point A). At this point, the maximum power channel was around 100 MW, while the heat network could deliver up to 35 MW of thermal energy by P2H at the same point. If P2H were also used in the downstream locations of point A, the wind power capacity could be increased by 50% from the non-P2H case to 150 MW. Next, point B was considered. It is closer to the main junction of the electric trunk lines, but it is also along the strong line in the heat network. The superior location in the power grid already triples the wind capacity that is seen without P2H, but P2H could further increase the wind capacity to more than 1 GW, while simultaneously producing 11% of the heat demand of the city by P2H. Additionally to point B, point C is a similar node along the main lines of both networks. Without P2H, these points together would allow for a capacity of more than 600 MW of wind power. With the increased flexibility offered by P2H, the wind power capacity could be increased to nearly 2 GW. If heat pumps (COP = 3) were used instead of electric resistance, the wind capacity could be reduced while maintaining the same share of P2H in heat demand. In the case of point A, the wind capacity would decrease by 8–23 MW from the values in Table 3. 48 Results for Helsinki Figure 15. Spatial distribution of electricity (top) and heat (bottom) demands in MW/km 2 for simplified Helsinki simulations. Left: average demand, right: peak demand. Spatial resolution: 1 km × 1 km. Reproduced with permission from Publication III. Copyright 2012 Elsevier. Table 3. Effects of P2H conversion and the location of the wind power connection on the accessible wind power capacity. A COP of 1 was used in these results. Reproduced from Publication III with permission. Copyright 2012 Elsevier. Conversion strategy and grid connection point of wind power Wind power at Point A No P2H conversion P2H at Point A P2H downstream of A Wind power at Point B No P2H conversion P2H at Point B P2H downstream of B Wind power at Points B & C No P2H conversion P2H at Points B & C Max. wind (MW) Heat by P2H, peak (MW) / total annual (GWh) Share of wind in electricity (%/a) Share of wind in heat (%/a) 101 135 153 35/0.27 53/9.6 5.3 7.0 7.7 0.037 0.15 319 1070 1080 769/754 779/770 16.7 38.5 38.6 11.4 11.8 609 1943 1367/1293 31.7 71.6 19.6 49 Results for Helsinki These results demonstrate that the capacity for VRE could be increased when moving from the city periphery toward the strong grid points and optimal junction points for P2H conversion. P2H offers a major increase in possible VRE production. Point A could deliver around 8% of the yearly electricity demand in Helsinki; point B, 39%; and points B and C together, 72%. In all final energy use (excluding the transport sector), the percentages are 3%, 22% and 40%, respectively. 7.4 Energy system operation with CHP plants and large-scale wind power (Publication V) To study the effects of large-scale VRE integration on the existing power plants of Helsinki, the optimization model of Publication V (Chapter 6 in this thesis) was used with large wind power schemes. Scenarios of up to 1,500 MW of wind power were considered, an amount that roughly corresponds to the yearly electricity demand of the city. The option of linking wind power to the heat demand through a power-to-heat (P2H) conversion was also studied, as doing so increases the flexibility of the system, thus solving the possible mismatch between power supply and demand. Three specific cases were analyzed: 1) the reference case with wind power integrated into the power system without any flexibility measures; 2) wind power with large-scale P2H conversion connected to the district heating system through electric resistance heating (COP=1); and 3) wind power with P2H connection to the district heating through heat pumps (COP=3). 7.4.1 Parameters for Helsinki energy system The power plants and their main characteristics are listed in Table 4. Detailed lists of all the values for technical and economic input parameters are included in Publication V, but a short summary is given here. Table 4. Main power plant characteristics used in the Helsinki case. The letters A–F refer to symbols in Figure 8. NG refers to natural gas. Partly reproduced from Publication V with permission. Copyright 2016 Elsevier. Plant Electr. Heat Min. output Max ramping Fuel (MW) (MW) (% of max.) (%/min) A Vuosaari CHP NG 630 580 50 4 B Hanasaari CHP Coal 220 420 40 3 C Salmisaari CHP Coal 160 300 40 3 D Heat boilers NG – 360 0 4 E Heat boilers Oil – 1900 0 5 F Heat boilers Coal – 180 0 3 G Katri Vala heat pump Electr. – 90 0 – 50 Results for Helsinki An overall efficiency of 90% was assumed for the CHP plants, and a constant ratio of power to heat outputs was used. Furthermore, the coefficient of performance of the heat pump plant was also assumed to be constant (COP = 3). In the P2H cases, the storage capacity of the district heating network is utilized. A capacity of 5 GWh was assumed, roughly corresponding to a temperature difference of 50 °C in the lines. The hourly spot price from Nord Pool [85] was used as a selling price for electricity, and the district heating price was taken from the local energy company Helen [86]. Production costs include fuel costs, operations and maintenance (O&M) costs, and costs from carbon emissions based on the European emissions trading system (ETS). Fuel costs are from Statistics Finland [87]; for natural gas, a price of 51 €/MWh was used, and for coal, 36 €/MWh. Investment costs are excluded, since the optimization is done only for the energy system operation. 7.4.2 Effects of wind power and P2H on plant operation Figure 16 illustrates the effects of increasing wind power installations on the traditional CHP production in Helsinki. Rising shares of wind power decrease the CHP production, as expected. However, in the reference case without P2H, only gas CHP is being replaced by wind, as the current price ratio of coal and gas favors the coal-based production over gas CHP. Interestingly, the decrease in CHP production levels out at high shares of wind. This leveling stems from the large need of locally produced heat in the city and results in high amounts of exported electricity (Figure 17). The coal plants have lower power-to-heat ratio and lower fuel price, which makes them preferable in heat production. Furthermore, because of the need for heat, wind power alone would not be able to decrease the emissions of the city. When adding flexibility to the system through large-scale heat pump installations, the gas CHP gains an advantage over coal. The P2H “electrifies” the heat production, which favors the higher power-to-heat ratio of combined-cycle gas CHP. In addition, the more electric energy system prefers gas CHP due to its higher flexibility in ramping the output power. The flexible P2H case with heat pumps thus results in a clearly smaller share of coal CHP, which also positively affects the CO2 emissions. Using heat pumps alone (even with no wind power) would decrease the emissions by roughly 30%. Moreover, decreasing shares of fossil CHP with increasing shares of wind power would reduce the emissions even further. For example, 1,500 MW of wind together with P2H would drop the emissions by almost 60%. Additionally, the export of electricity would be reduced and the import increased, resulting in a better balance. 51 Power production from exixsting plants (% of consupmtion) Results for Helsinki 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 200 400 600 800 1000 1200 1400 1600 Wind power (MW) Gas – Reference Coal – Reference Gas – COP 1 Coal – COP 1 Gas – COP 3 Coal – COP 3 Figure 16. Electricity produced by CHP plants with wind power and different P2H schemes. Reference: no P2H; COP 1: large-scale P2H with electric resistance heating; COP 3: large-scale P2H with heat pumps. Reproduced with permission from Publication V. Copyright 2016 Elsevier. 100% Electricity import/export (% of consupmtion) 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0 200 400 600 800 1000 1200 1400 1600 Wind power (MW) Import – Ref Export – Ref Import – COP 1 Export – COP 1 Import – COP 3 Export – COP 3 Figure 17. Import and export of electricity with wind power and different P2H schemes. Reference: no P2H; COP 1: large-scale P2H with electric resistance heating; COP 3: large-scale P2H with heat pumps. Reproduced with permission from Publication V. Copyright 2016 Elsevier. 52 Results for Helsinki Power production (% of consupmtion) The use of heat boilers would also be reduced drastically by using P2H. Already, resistance heating (COP=1) and 1,500 MW of wind power would reduce boiler use by 76–88%. With heat pumps (COP=3), all the boilers could, in theory, be removed. It might, however, be reasonable to leave some boiler capacity in the city for extreme peak conditions. Reasons for doing so might include a requirement of higher water temperature in the heat network than can be achieved with heat pumps, or of a higher capacity factor for P2H (in the modeled cases between 18–33%). Increasing the heat pump capacity above the modeled 500 MW (electric power) would not further decrease the CHP or boiler operation; rather, it would just lead to increased electricity imports from the spot market during the low-price hours. The Emissions Trading System (ETS) includes all large power plants in the European Union. Thus, the effect of the price of CO2 on the results was analyzed in this work. Currently, the price is around 10 €/tCO2, and it was compared against two other prices (0 €/tCO2 and 40 €/tCO2). Figure 18 shows how increasing the emission price shifts production from coal to gas, and this shift occurs much more radically with a higher emission price (40 €/tCO2). However, at large shares of wind power, the gas plant alone is not able to cope with the increasing variability in the net demand, and thus the shift from coal to gas is significantly smaller. With heat pumps, the share of gas CHP decreases with increasing emission price. A large-scale heat pump scheme with an emission price of 40 €/tCO2 would eliminate almost all coal use in Helsinki. 100% 80% 60% 40% 20% 0% 0 500 1000 Wind power (MW) Gas – ETS 0 Gas – ETS 10 Gas – ETS 40 1500 0 500 1000 1500 Wind power (MW) Coal – ETS 0 Coal – ETS 10 Coal – ETS 40 Figure 18. Electricity produced by CHP plants with wind power and emission prices (ETS, €/tCO2). Left: reference case without P2H; right: P2H with heat pumps (COP=3). Reproduced with permission from Publication V. Copyright 2016 Elsevier. 53 8. Results for other cities 8.1 Shanghai, China Shanghai is a megacity in a developing country with rapidly increasing energy consumption [88]; thus, it is highly important to study this city’s energy system structure and possibilities for renewable integration in order to avoid further investments in fossil energy and possibly to reduce its use. 8.1.1 Spatiotemporal profiles with limited input data (Publication II) The model presented in Chapter 4 and Publication II was also applied to the case of Shanghai, which offered limited spatial input data. In 2010, the electricity consumption in the city was 130 TWh, of which 55% was the share of industrial loads and 13% was the share of residential loads [89]. The electric load profile of the whole city is shown in Figure 19. It reaches its maximum values during summers, when the cooling load is high. Radial density distributions were used in this case. The city is divided into 16 districts, of which the densities of population, industrial employees, and office floor area are known [90]. Eq. (1) is fitted to these values, and the resulting density profiles are used as an input for the model to produce the spatiotemporal load profiles. Figure 19. Electricity load profile of Shanghai, China. Reproduced with permission from Publication III. Copyright Elsevier 2012. 55 Results for other cities Figure 20. Simulated spatial consumption in Shanghai. Left: average load; right: peak load. Spatial resolution in the figure is 1 km x 1 km. Reproduced with permission from Publication III. Copyright 2012 Elsevier. Figure 21. Simulated temporal electricity load profiles at different distances from the Shanghai city center. Reproduced with permission from Publication III. Copyright 2012 Elsevier. The resulting spatiotemporal load profiles are shown in Figure 20 and Figure 21. The temporal profiles resemble each other at different distances, because the consumer classes were assumed to spread throughout the city, and no clear division was made between industrial, residential and office areas. The high share of industrial load is another reason, as it was assumed to spread even more evenly than offices (which are concentrated in the city center). The dominance of industry also explains the lack of clear evening peaks, which are characteristic of residential areas. The peak power in the city center (slightly over 60 MW/km2) remains lower than in the highest district in Helsinki (80 MW/km2, Chapter 7.2.2). This is, however, due to the coarse resolution of input data. The densest district in the city center has an area of 23 km2. If the source data were, for example, from 1 km2 areas, the peak power would probably exceed 500 MW/km2 in the most high-rise business areas. 56 Results for other cities 8.1.2 Large-scale photovoltaics together with micro-trigeneration (Publication III) For Shanghai, large-scale distributed photovoltaics (PV) and micro-trigeneration schemes were considered. The detailed analysis and input data are given in Publication III. The calculations were made with the multi-carrier model of Publication III (Chapter 5 in this thesis). In Shanghai, the only network in the model was a power grid, but spatiotemporal consumption profiles were created for heating and cooling as well. As a starting point, electricity covers 44% of the heating and 82% of the cooling load. Results from the simulations are presented in Table 5. In the reference case, with no PV, electricity must be imported from the large central power plants close to the city. Case A (in the table) shows the self-use limit of PV-produced electricity in Shanghai to be 19 GW, which could produce 21% of the electricity demand in the city. If the PV installations were increased to 150% of the self-use limit (case B), 31% of the demand could be covered, and, simultaneously, surplus electricity would be produced (in an amount equivalent to 1% of the yearly electricity load). The surplus could be used in local electric heating systems. The spatial analysis, however, suggests that the power carrying capacity of the power grid could be easily exceeded in the periphery areas of the city in this case. The problems could be solved by rearranging a small portion of the distributed PV panels closer to the city center with stronger power lines. Cases C and D include gas-driven trigeneration schemes, which were installed in the city center area (5 km × 5 km). Trigeneration technology combines gasdriven solid-oxide fuel-cell cogeneration (gas to electricity and heat) and absorption cooling technology (heat to cooling). Assumed efficiencies were 40% for the gas-to-electricity and gas-to-heat processes and 70% for the heat-tocooling process [91,92]. The trigeneration is operated in such a way that heating and cooling loads are satisfied first, and then electricity production is adjusted accordingly. PV installations need to be reduced in order to avoid excess electricity production. Table 5. Shanghai cases with different PV and trigeneration schemes. DEGS refers to distributed energy generation systems (i.e., PV + trigeneration). Reproduced from Publication III with permission. Copyright 2012 Elsevier. Scheme A. PV (self-use limit) B. PV (150% self-use limit) C. PV + Trigeneration covering 25 % of cooling and heating in city center D. PV + Tri-generation 50% PV GWp 19.3 28.6 PV share in electricity+, % 21 31 Trigeneration GW(el) – – DEGS share in electricity, % 21 31 DEGS share in energy, % 18 27 16.0 18 3.7 28 28 9.3 11 7.3 31 32 57 Results for other cities In case D, where 50% of the thermal loads in the city center are satisfied by trigeneration, it is possible to produce one third of all energy consumption by distributed means. In addition, the CO2 emissions of the city could be reduced by 38% assuming a reference system based on coal power. This reduction is due to the high fuel efficiency of trigeneration technologies. 8.2 Concepción, Chile (Publication VI) Chile is an interesting country in which to study the integration of solar photovoltaics. There, PV has reached grid parity, and thus the solar PV market is growing rapidly, especially in urban areas [93]. The city of Concepción was chosen as the case city to study the potential of solar energy as well as the possible limitations that the power grid infrastructure may impose on the large-scale PV installations. Such may arise in many developing countries with a high solar resource. Detailed spatial data about the solar potential in Concepción were presented in Publication VI. Solar radiation and the urban structure were taken into account in the assessment of the solar potential. With respect to urban structure, spatial details of the city, such as building typologies and shading due to topography and neighboring buildings, were included in the assessment models. 8.2.1 Electricity consumption, city structure, and the power grid Energy consumption patterns of the buildings were gathered simultaneously with the spatial research on building typologies. As the total electricity consumption of the city was also known, it was possible to produce a spatiotemporal load profile for the city. For this purpose, all the buildings in the city were grouped into 5500 building combinations. Each of these was then assigned a load profile depending on the building type (household, service, hospital) and typology (low, mid, high-rise sparse, and high-rise dense). The electricity consumption profile is shown in Figure 22, along with the PV production curve. Figure 23 reveals the spatial distribution of the consumption. Figure 22. Electricity load profile of residential and service sector in Concepción and the PV production curve at the self-use limit of 73 MW. Reproduced with permission from Publication VI. Copyright 2016 Elsevier. 58 Results for other cities Accurate data about the power grid were not available, and thus some assumptions were made. The focus was set on the mid-voltage distribution and transmission lines, omitting the low-voltage distribution to customers. The city was divided into 1 km × 1 km squares, which were connected by the mid-voltage lines. Load profiles of single building combinations were summed up to form a single load profile for each square. The squares and assumed power lines are shown in Figure 23. Present power demand (without PV) is used to define the power-carrying capacity of the grid. This means that the maximum allowed power flow in a power line is the flow that would occur in the current situation. This conservative assumption guarantees that no extra stress, compared to the current situation, is placed on the grid. Figure 23. Left: Peak power demand in Concepción. Right: Annual power demand with assumed power grid. Light green shows high-voltage connections between the power stations (light green dots), and dark green shows the mid-voltage lines. Building combinations are marked with small blue crosses. Reproduced with permission from Publication VI. Copyright 2016 Elsevier. 8.2.2 Results The total yearly solar radiation on the buildings of Concepción was found to be around 3,900 GWh, which exceeds the total energy demand of the city by 22%. The total solar capacity, however, is severely limited by technical factors (e.g., the efficiency of appliances and the capacity of the power grid). The self-use limit of the PV production in Concepción was found to be 73 MW, which would cover 13% (95 GWh) of the power consumption. The corresponding production curve is shown in Figure 22. By doubling the amount of PV, 24% (170 GWh) of the consumption could be satisfied, but 17 GWh of surplus electricity would be produced. PV production throughout the city decreases the consumer’s need to buy power from the grid. Therefore, with relatively low amounts of PV (1–2 × selfuse limit) the average power flows and, thus, the stress in the grid decrease (Figure 24). No overflow problems to the grid are caused by 150 MW of PV, but the momentary oversupply of PV production turns the flow from downstream to upstream. The upstream flow, however, stays below the maximum downstream flow of the reference case. In the case of increasing the PV installations closer to the total potential of the city, overflow problems will occur. For example, PV of five times the self-use 59 Results for other cities limit (i.e., 370 MW) corresponds to over 80% of the total solar capacity (assuming 15% efficiency for PV), and would produce 34% of the electricity demand, but the capacity of the grid would be exceeded by up to 80% in some lines. This is shown in Figure 25. In practice, all parts of the grid would be overstressed. The problem could be solved, for example, by local P2H appliances. If all excess solar PV production were converted to heat, about 28% of the heat demand could be satisfied. The high-PV case, also demonstrates how the role of the grid changes from simply supplying power into a way of balancing fluctuating PV production. The average flows in Figure 25 (right) are in the same range as in the reference case with no PV (Figure 24, left), but the maximum peaks are clearly stronger (Figure 25, left). Figure 24. Average power flows in the grid lines in Concepción as a percentage of the power line size. Left: reference case with no PV; right: 150 MW of PV (2 × self-use limit). Reproduced with permission from Publication VI. Copyright 2016 Elsevier. Figure 25. Power flows in the grid lines in Concepción with 370 MW of PV (5 × self-use limit). Left: maximum flows; right: average flows as a percentage of the power line size. The color bars have different scales. Reproduced with permission from Publication VI. Copyright 2016 Elsevier. 60 9. Conclusions The use of variable renewable electricity, such as wind and solar power, is increasing rapidly around the world. The use of such renewable energy sources will increase the unpredictability and fluctuations in the net load patterns. Energy systems need to match the energy supply to the demand. Traditionally, controllable combustion power plants have handled the problem, but with growing amounts of VRE, new flexibility measures are required. On the other hand, the role of urban areas in climate change mitigation will increase due to the global urbanization. The aim of this work was to study the characteristics of urban energy systems and their readiness to incorporate large-scale VRE schemes. For the purpose, three novel energy system models were created. The first model focused on the spatiotemporal patterns of energy consumption in cities. This type of data is seldom available with sufficient spatial resolution, and previous models rarely consider both the space and time dimensions. The developed model, which is based on readily available geographical data, is able to reach satisfactory spatial and temporal resolutions, thus enabling detailed analyses of the urban energy systems. The second model analyzes the possible advantages of viewing the energy system as a whole, taking all final energy carriers into account. Traditionally, energy carriers are handled one at a time, thus omitting several degrees of freedom in energy system flexibility. In the model, a city is divided into nodes of distributed energy production, consumption, and mutual conversion between energy carriers. The nodes are connected with different energy carrier networks. This work focused on electric power grid and district heating, but the model allows generic multi-carrier analyses. Another advantage over the existing literature is the comprehensiveness of the model, which simultaneously considers both macro and micro levels, and includes control functions and network intelligence. The third question addressed by this work involved the role of the existing power plants in the new situation with increased VRE generation. A new optimization model is thus presented, which simulates the most economical use of energy system assets. It is based on a mixed-integer linear problem, and it can handle many types of energy systems, although this work focuses on the city scale. Different flexibility measures are included in the model, such as powerto-heat conversion, thermal energy storage, and electricity spot markets. Compared to most of the existing models, the model is kept simple and is fast to run and easy to modify, despite the fact that it covers the whole energy system, from 61 Conclusions different energy carriers to several flexibility technologies and spot markets. Another advantage of VRE dynamics is the shorter time step of the model (e.g., 10 minutes), instead of the traditional hourly or even longer time step. In the case of Helsinki, Finland, the analysis focused on how large-scale wind power schemes could be integrated into a mid-sized northern city with a high heating load and a well-established district heating network that covers the whole city area. The self-use limit of wind power was found to be around 400 MW, which could satisfy up to 27% of the electricity consumption of the city. With power-to-heat conversion and thermal storage, however, this number could be dramatically increased. For example, 2000 MW of wind power would be able to cover more than 60% of the electricity consumption and 20% of the heat consumption of the city if P2H with electric resistance were used. With heat pumps, the share in heat could theoretically be increased to close to 70%, but this increase could be limited by the heating network capacity, and thus requiring large-scale heat storage. The optimization results clearly indicate the need for system flexibility in the wind power integration in Helsinki. Without any flexibility measures, wind power alone would not be able to replace the fossil-based CHP production. Most of the wind power production would thus be exported outside the city, mainly due to the high demand for heating and the need to produce it locally. P2H with heat pumps would change the situation by electrifying the heat production, thus resulting in decreasing coal use when wind power is increased. Sensitivity analysis against fuel and emission (European emissions trading system) prices show that P2H makes the coal-based CHP more sensitive with respect to price increases, thus offering considerable help in eliminating the use of coal in the energy production of Helsinki. The co-operation of different energy carriers was also studied in the southern megacity of Shanghai, China. The self-use limit for solar PV integration was found to be below 20 GW (accounting for 20% of the power demand). In Shanghai, a distributed trigeneration scheme was studied with fuel cells and absorption chillers to produce electricity, heat, and cooling. Trigeneration, together with PV, was able to cover more than 30% of all electricity and energy consumption in the city, yielding a savings of almost 30 Mt of CO2. The city of Concepción, Chile, is a good example of the importance of studying limitations rising from the energy system. Even though the overall solar potential, in theory, exceeds the energy demand of the buildings, technical efficiency and power grid infrastructure limits the PV share in electricity to about 24%. However, with a local P2H strategy, this could be increased to at least 34%, and, simultaneously, 28% of heat would be produced without overloading the electric power grid. Thus, to form a realistic estimate of the solar energy possibilities, both the pure VRE potential and the power system infrastructure need to be assessed. The presented results show the importance of system-wide thinking when defining policies for energy system transition and carbon neutrality. Energy systems must be considered as a whole when flexibility is sought. It was observed that inherent features in energy systems, such as smart use of district heating 62 Conclusions networks, could offer much of the needed flexibility for large-scale renewable energy integration. Therefore, massive investments in traditional network infrastructure could largely be avoided. For example, viewing electric and thermal systems together instead of as two separate systems could reveal remarkable new options for VRE integration. P2H technologies are able to at least double the VRE intake of energy systems, but with district heating networks and thermal storage the increase could be up to fivefold. 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