How to design a off-grid electrical system (for dummies)
How to design a off-grid electrical system (for dummies) Manfred Amoureux – November 2013 http://manfred.amoureux.free.fr/ Work in progress – this document is not completed yet, Come back later to my web site and check out if a newer version is available, or email me ! So you want to live off-grid but you know nothing about electricity ? This document will explain you the basics you need to understand how an off-grid system works and how to design one, powered by solar modules and batteries with a generator as back-up. Table of Contents Scope of this document..........................................................................................................2 Audience............................................................................................................................2 Types of systems...............................................................................................................2 Limits of content.................................................................................................................2 Note about the language...................................................................................................3 Basics to understand electricity.............................................................................................4 The analogy with water flow..............................................................................................4 Power and energy..............................................................................................................4 Basic rules of electricity.....................................................................................................5 How to design an off-grid system...........................................................................................6 Thinking with functions......................................................................................................6 Types of systems...............................................................................................................8 Your off-grid system will most likely be 12 or 24 volts DC, or a combination with a part in DC (12, 24 or volts) and a part in AC.....................................................................................8 Components...........................................................................................................................9 Converters.........................................................................................................................9 Inverters.........................................................................................................................9 Rectifiers........................................................................................................................9 DC-DC converters.........................................................................................................9 Batteries.............................................................................................................................9 Choosing the battery voltage.......................................................................................10 Gasoline and diesel generators.......................................................................................10 Using the genset to power loads.................................................................................10 Using the genset to charge the batteries....................................................................10 Car battery chargers....................................................................................................11 Connecting to the genset.............................................................................................11 Connecting the battery charger...................................................................................11 Bad efficiency...............................................................................................................11 Charging through the island inverter...........................................................................12 Solar panels.....................................................................................................................12 Positioning and electricity yield of PV panels..............................................................12 Connecting solar panels..............................................................................................12 MPP tracker.................................................................................................................13 Choosing your solar panel...........................................................................................13 Wind turbines...................................................................................................................13 Choosing a spot for a wind turbine..............................................................................13 Connecting a wind turbine...........................................................................................14 Charge controllers an discharge controllers....................................................................14 Example...........................................................................................................................14 Sizing of the components.....................................................................................................16 Calculating the energy need............................................................................................16 Calculating the power need.............................................................................................16 Using softwares...............................................................................................................17 Electrical protections............................................................................................................18 Fuses and circuit breakers...............................................................................................18 Where to put the protections ?....................................................................................18 Sizing of fuses and circuit breakers.............................................................................18 Earthing............................................................................................................................19 Credits..................................................................................................................................20 Scope of this document Audience This document is obviously aimed at non-professionals who want to gain some understanding of what they will need to build and operate their own system for a single household. You should still seek some help, or at least complementary informations to design and build your system. Types of systems It would be impossible to write a document that covers all possible off-grid cases. This one is intended to help you understand, design, choose and size roughly your components for a small system, typically for a household with a maximum power demand of 1-2 kW. For electricity production, I will include solar panels, gasoline or diesel generators and to a lesser extend wind turbines. For electricity storage, I will only mention batteries, and focus on lead-acid ones. For consuming appliances, I will focus on DC and single phase systems, although once you understand single phase, 3 phase systems should not be hard to understand. For the examples, I use the values in usage in continental Europe (e.g.: for AC electricity, 230 volts, 50 Hz), but the principles stay the same in other parts of the world. Limits of content This document does not cover the installation procedure. I cannot explain everything in this single document. However, I can try to give you enough understanding of what you will need and how it works, so you can figure out by yourself what types of components you will need and how much it is going to cost you by looking at on-line catalogues. However, before you purchase any component, you should talk to professionals (ex: electricians, your retail salesman) to find out which exact component you should buy. Note about the language I am using some technical words. If your mother tongue is not English, when you are not sure of the meaning of a work, look it up on the internet. I advice looking on Wikipedia and using the menu that links to the similar articles in other languages to find out the translations and explanations in your mother tongue. Basics to understand electricity The analogy with water flow Electricity is a stream of electrons flowing through cables and components. I is similar to a stream of water flowing through pipes. To make things easier to understand and more tangible I will use this analogy. The main characteristics of electricity are : • Type : the stream can be direct current (DC) or alternate current (AC). DC is comparable to a stream of water that flows always in the same direction, while AC is comparable to waves or tides that flow back and forth, except it changes direction dozens of times per second. DC components have positive and negative terminals. AC components have also 2 terminals, one called the active or 'live' phase and the other the neutral. • When it is AC, it has a frequency, measured in Hertz. It is the frequency at which the stream cycles changing direction. • Voltage : measured in volts [V]. It is similar to the pressure of the water : the higher, the more easily they flow through the components, but also the more easily they escape as soon there is a fault in the insulation of the components. • Current : measured in amperes [A]. It is similar to the flow of the water (litres/second). For a given voltage, the current is determined by the characteristics of the appliance you are powering. • AC electricity can be either single phase or three phase. Three phase electricity is only used for important appliances, usually in the industry. For a single household, you almost always use single phase electricity (99.9% of the time). Power and energy The electricity you get at home in Europe is AC, 230 volts, 50 Hertz, single phase. A car's electrical system is 12 or 24 volts DC. • Power : measured in watts [W]. An appliance consumes power ; a generator or a solar panel produces some. The power is the mathematical product of the voltage and the current in the appliance. Example : for 230 volts, if the current is 10 amperes, the power is equal to 230x10 = 2300 watts. Since for a given voltage, the current depends on the appliance, the consumed power also does. • Energy : energy must not confused with electricity. Power is the rate at which energy is produced or consumed. The relationship between energy and power is similar to the one between distance and speed. In electricity, energy measured in watt-hours [Wh] and is calculated by the mathematical product of power (in watts) by the duration (in hours). So “1 Wh” means “a power of 1 watt during 1 hour”. Similarly, “10 Wh means “1 watt during 10 hours” or “10 watts during 1 hour” or “5 watts during 2 hours”, etc .. Wh are NOT “watt per hours”. There is no such unit and if anybody uses this pseudo-unit to talk of energy in front of you, it clearly shows they know nothing of the topic. For all these units, you may use the kilo- prefix to talk about thousands, like we do for kilometres which are thousands of meters. So kV are thousands of volts, kW thousands of watts, kWh thousands of watt-hours. Basic rules of electricity Some rules you need to know : • You must never mix DC and AC in the same cables / components. They must always be on separate parts of the circuit. • When the current is higher, you need thicker cables, otherwise they will heat and might even burn. • When the voltage is higher, the cables must be better insulated or there may occur a short-circuit. How to design an off-grid system Thinking with functions When you look at the available devices on the market, you may get confused because there is plenty of them whose functions are overlapping each others. Instead of trying to understand you what devices you need, it will be easier for you to understand which functions you need to have in your system : • Generators : all devices producing electricity. in this category fall diesel or gasoline generators, but also solar photovoltaic panels and small wind turbines. You need at least one generator, but the more you have, the better. • Loads : all devices consuming electricity (lights, fridges, etc ..). • Charge controllers : are used to avoid overcharging your batteries, which would destroy them. They can also optimize the charging process. You need a charge controller to connect any generator to the batteries. You might be able to connect several generators to the batteries through the same charge controller, but you might also need several of them. • Discharge controllers : are used to avoid over-discharging the batteries, which would also kill them. When the batteries are too low, they will disconnect the loads. You usually need one discharge controller for the AC loads and one for the DC loads. You must never have an equipment that draws or feed energy connected directly to the battery without having some piece of equipment that controls you will not over-charge or over-discharge it. Inverters often do this type of control themselves but not always. Example of schematic diagram of a system, thinking with functions. See further down for a more advanced version. Types of systems You may roughly categorizes the type of the system by considering how the produced electricity is transmitted from a generator to the batteries and how it is transmitted to the loads. Transmission from production Comment to battery DC For many generators (solar panels, small wind turbine), this is natural since they produce DC current and it is what the batteries take in. This only allows short lengths of cables because the voltage is low (12 to 48 volts). Advice : to avoid if the length of cable from the generator to the batteries is longer than 10m. AC If the generator produces AC electricity, this is natural but a transformation into DC will be required to feed it to the battery. If the generator produces DC (solar, wind), transforming into AC and then back into DC implies additional costs and losses. It is only worth it if there is much power and/or there is a long distance to the batteries. Advice : not worth it if the power of the generator is less than 1 kW. Transmission from battery to Comment Loads DC Small systems. It only allows short lengths of cables / low amount of powers because the voltage is low (12 to 48 volts). Advice : only use it for loads consuming less than 100 W. AC Small to medium systems. Enables longer cables for the loads because the voltage is higher (230 volts). However, the transformation from DC to AC implies some losses and the cost of an inverter. Advice : you may consider using DC for loads less than 100W and distances between the batteries and the loads less than 25m. Your system may be a hybrid one : you may have a system in which some loads are in DC and others in AC ; you may also have some generators in DC and others in AC (typical case if you have a solar panel and a gasoline generator as back-up). Your off-grid system will most likely be 12 or 24 volts DC, or a combination with a part in DC (12, 24 or volts) and a part in AC. Components Converters As you have seen previously, electricity may come in different types (AC or DC, different voltages). In order to transform electricity from one type to another, you use converters. Inverters Inverters transform DC electricity into AC electricity. You must make sure that what you get is an island inverter, i.e. an inverter that works in off-grid and does not need to be connected to the electrical grid (like the regular photovoltaic inverters used to feed the current from the solar panels to the grid). Pseudo-sine inverters create AC electricity with a shape different from the one from the grid and do not fit for all appliances. Pure sine inverters create AC electricity with sine wave similar to the electricity from the grid and can be used to power any type of appliance but are more expensive. A pure sine inverter is required to power an appliances with an electric motor (ex: a food mixer, a power drill without a battery, an angle grinder). Appliances that use batteries (ex : laptop, cordless power drill) have their own internal converters and can be powered with any type of inverter. Rectifiers Rectifiers transform AC electricity into DC electricity. Their output voltage depends on the input voltage. You will most likely not need one. DC-DC converters These converters transform DC electricity from a certain voltage into DC electricity with a different voltage. You will most likely not need one. For the three types of converters, you should pay attention to the following characteristics : • the maximum power it can transform, • the voltage range it accepts in input, • the voltage it gives in output (and the frequency when the output is AC). Batteries The batteries are one of the most expensive component of your system. You have to replace them regularly and they will die quickly if they are not properly taken care of. We use lead-acid batteries. Lithium-ion batteries are not yet economically competitive for this kind of use. You must not use car batteries. They are designed for a different use and will die quickly. Batteries do not like cold or hot temperatures. Ideally, you should store them in a place where the temperature is always 20ºC. Choosing the battery voltage You basically have the choice between 12, 24 or 48 volts. Other voltages are technically possible but really uncommon. Many people choose to go for 12 volts because that it is what they are used to, because of cars, and because it is easier to find equipments for this voltage. It is true and it is a sensible reason. However, you should know that, as soon as your installation is bigger than a car or a truck, going for a higher voltage will likely make it cheaper : you will need less thick cables and PV panels suitable for 24 volts installations are sensibly cheaper than those for 12 volts installations. It will also enable you to charge the batteries faster with a genset. Gasoline and diesel generators A gasoline or diesel generator used to produce electricity is called a genset. A genset has much more power than a single solar panel. The smallest genset can produce already 2 kW (2 000 Watts) of electricity. It produces AC electricity. There are 2 ways you can use a genset : to charge the batteries or to power loads. It is possible to do both at the same time. Using the genset to power loads When used to power weak loads (less than half its nominal power), the genset consumes a lot of fuel for the little power it has to provide, so when you use the generator, you should better connect all the loads you can. So, when you are running the genset, I suggest charging at the same time all the appliances with batteries you can : mobile phones, power tools, torches and so on. However, you should never have more than a single genset or island inverter on the same AC bus, otherwise they will conflict. That means that you must either power an AC load with the genset or from the batteries with an inverter, but not both simultaneously. Some special inverters are capable of cohabiting with a generator (ex: SMA's Sunny Island) and are an exception to this rule. If the manufacturer's data does not explicitly show you can connect an inverter with a genset on the same AC bus, assume you cannot. Using the genset to charge the batteries First remember that the genset produces AC electricity at a high voltage and that the batteries require low voltage DC electricity. So it needs to be transformed. Secondly, you must not over-charge the batteries otherwise you will kill them. You need a charge-controlling appliance that will make sure this does not happen. Thirdly, good charging of batteries is achieved in a multi-steps procedure (named ”bulk, absorb, float and equalize”). You don't need to know what they do, but you need to understand that a proper battery charger uses this type of procedure. If your battery charger does not, then you should not plan on using it except for emergencies. You must also pay attention to the input and output voltages and frequencies, as well as the size of the battery bank. In particular, the battery manufacturer often recommends charging with a voltage which is higher than the nominal battery voltage (ex: 15 volts for a 12 volts battery). Also pay attention to the maximum current they can absorb, this will determine how fast you can charge the batteries and how much gasoline you will use. Indeed, usually your genset can provide much more power than what the battery charger can absorb and this will translate in gasoline costs. For example, a 20A charger on a 12 volts battery, will withdraw at the maximum 240 W, while your genset has a rated power of at least 2 kW, so you'll be actually wasting most of the gasoline you consume while charging the battery. An example of a good battery charger for off-grid applications is the Xantrex Truecharge series. CTEK also has a good range of such devices. Try to get a battery charger that will absorb between half and 2/3 of your genset's nominal power. Car battery chargers You may also find car battery chargers, such as : Profi Power car charger or Black & Decker BDSBC10A Battery Charger One problem with such battery chargers is that they are designed to charge car batteries and not solar batteries. Avoid them, except in case of emergency. Connecting to the genset Some gensets have outlets to produce 12 or 24 Vdc that can be used to charge batteries. Pay attention to several things : a/ what is the maximum current that can be drawn from there and b/ if is intended for battery charging or for a general purpose (powering 12 Vdc appliances). Most of them are intended for general purpose so they will not fill the role of controlling the charge of the battery. You might use a separate charge controller (such as the one connecting the solar panels to the battery), but I recommend simply not using the outlet and get a proper battery charger. Connecting the battery charger Whether your genset includes a battery charger or you get a separate one, when you have some DC loads, it could be tempting to connect the battery charger to the DC bus, so you could power DC loads at the same time. However this is error prone : the battery charger detects the state of charge of the battery based on its voltage and this latter fluctuates when current is being drawn (if loads are connected) or fed (if solar panels are connected). This may also cause the battery charge to fail. So don't do it and only connect the battery charger to the batteries. If you want to have power at the same time you charge the batteries, connect AC appliances in parallel to the genset. Bad efficiency Charging the batteries with a generator will be costly : naturally, fuel is expensive. In addition, generators have bad efficiencies, especially when running at low load, i.e. when they are used to produce only a fraction of their rated power. That means, to charge your batteries, compared to the amount of electricity you will be producing, you will use a lot of gasoline. So, it's better not to plan using this too often. Charging through the island inverter Some island inverters are bidirectionnal and can charge the batteries themselves, such as SMA's Sunny Island. It is simpler, more efficient and avoid errors. However, they are likely to be oversized for many applications (> 5kW). Solar panels Positioning and electricity yield of PV1 panels I will not explain the theory about photovoltaic. What you need to know is simply that the more light the panels receive, the more electricity they produce. You can use online tools to evaluate the yield of your panels.2 In order to achieve the maximum yield, the solar modules should be positioned where they will receive most sunlight : normally, that is facing the sun. The problem, obviously, is that the sun moves constantly. It moves from east to west during the day, and raises higher during the summer, and lower during the winter. Moreover, when the sky is cloudy, the daylight comes from at all directions simultaneously (objects have no shadows), so there is not really a point in orienting the panels toward the sun. They will receive more light if positioned horizontally, facing the sky. In most cases, you cannot move the panels constantly and need to have them in a fixed position. Usually, for photovoltaic systems, what is recommended in the northern hemisphere, is to put the panels facing south, with an inclination angle3 close to the latitude. However, in an off-grid system, your problem is to produce as much electricity as you can during winter because there is less sunlight but you need more electricity. In summer you will use less and the solar panels will produce much more than in winter. To achieve this, since the sun raises lower in winter than in summer, the module should be positioned in a more vertical position than in summer. A good rule-of-thumb is to incline the panel at an angle with the horizontal equal to the latitude plus 10º. In any case, you should always keep a minimal inclination (15º) for the panels, to evacuate naturally rain, dust and dirt. If there is snow, you will probably need more slope than that. Also, don't forget the shadows from objects in your environment. They really are a yield killer. Observe regularly the spot where you are considering installing your solar panels. Try to put the panels in a place where there will be no shadows. Connecting solar panels Solar modules produce DC electricity. They can be connected to a DC electrical system of a battery voltage inferior to theirs and will then use the system's voltage, but they will not function optimally. Solar panels are normally provided with a current-voltage curve, or at least the following characteristic values : maximum power point (MPP) voltage and current. If you do not intend to use a MMP tracker (see below) but connect the panel s to the battery4, this MPP voltage of the panel should be in a range between 1.25 and 1.75 times the battery voltage. Below this range, the panel will not be able to charge the battery. Above it, it will only be able to produce a minor fraction of its possible yield and you should consider another design (maybe a higher voltage). 1 2 3 PV = photovoltaic Example : PVGIS http://re.jrc.ec.europa.eu/pvgis/ i.e. the angle between the horizontal and the panel. 4 Through a charge controller, of course. You may use several panels connected together in series and parallel 5. However, you should not do this if they are not of the exact same type 6. Connecting solar panels of different types to the same charge controller is ok as long as they all have separate connection terminals and that the charge controller's manual say you can do it. MPP tracker In order to operate optimally, you may use a maximum power point tracker (abbreviated MPPT or MPP tracker), which is a special type of DC-DC converter that makes sure your solar module is operated optimally and transforms the produced electricity. Although it is not required, If you have enough solar panels, it's worth the investment because it will maximize your production (let's say about +30%). Be aware that most of the softwares that are available to evaluate the yield of your PV panel consider that you have a MPPT. If you don't, you'll have to reduce the yield of the panel. Choosing your solar panel Solar modules comes in different sizes and power and types. Pay attention to : • their output voltage (MPP and open-circuit). • Also to the input and output voltages of the MPPT if you are using one. Also, you should know that a good solar panel will last over 20 years. But a cheap one from an unknown brand may fail much sooner than that. It is a long-term investment, choose a reliable manufacturer. Wind turbines In the right location, wind turbines can produce more electricity than solar panels for a cheaper price. However, they are trickier to use. Wind turbines also require regular maintenance. Expect to have to take them down for maintenance once a year. Yet, they are an alternative worth considering if you live in a location where sun is not a reliable source of energy during part of the year. Choosing a spot for a wind turbine First of all, some rules to know : • The higher above the ground you locate the turbine, the faster the wind and the more the wind turbine will produce. • Wind turbines require a regular non-turbulent flow of air. If the spot you are considering is surrounded with obstacles such as buildings or trees, it will create turbulences and this will disturb the turbine, which will produce much less electricity and will also wear faster. It is often difficult to assess if and how much current a turbine can produce at a certain location, because it is difficult to measure how much wind there is. Knowing the average wind speed at the location where you think of positioning your turbine is the only way of being sure to reach a proper yield. If you cannot assess it with the proper measurement 5 If you don't know what this mean, just don't bother : use only one solar panel. 6 There are some work-around solutions but this is beyond the scope of this document. equipment, you will be taking a risky guess. Connecting a wind turbine Small wind turbines normally produce DC electricity (actually, internally, they produce 3 phase AC, but it is rectified into DC). This DC electricity fluctuates in voltage with the spinning speed of the turbine. You must use a charge controller to charge a battery. You could also use some MPP tracker device, but there is only a very limited range of such devices commercially available. Such a small wind turbine, when connected to a battery7, may be spinning fast without actually producing any electricity. It only starts to produce electricity when the wind becomes strong enough so that the turbine can produce power at a voltage higher than the one of the battery. Otherwise, it spins but does not produce any electricity. A model of small wind turbine is designed to charge batteries of a given voltage, under a range of wind speed conditions. If the wind at the spot you have chosen is lower than this range, your turbine will not produce electricity. If the wind is too strong, this will usually trigger a mechanism to avoid the destruction of the wind turbine (called 'furling' or 'stall' or 'pitch' depending on the type of mechanism) and it will also not produce any electricity. One possible trick, if the wind turns out to be too weak or too strong at the spot where you have located your turbine, is to change the mast's height and position it higher or lower. Check out any possible urbanism laws in your area regarding the allowed maximal height for a mast. If your system transmits power from the turbine to the battery by AC, which is only worth it for turbines of a certain size (several kW), the inverter transforming the produced electricity into AC will probably have a MPP tracker which will make your life easier. Charge controllers an discharge controllers The function of controlling the charge of the batteries is often provided by a device that also fills another role. For example, charge controllers for solar panels or wind turbines often also serve as MPPT. This is nevertheless not always true and some charge controllers only fill this role. It is also possible to find devices that can be used as charge controllers or discharge controllers. As already seen in the paragraph about using a genset to charge batteries, good charging of batteries is achieved in a multi-steps procedure (named ”bulk, absorb, float and equalize”). A proper battery charger should provide such functionality. Discharge control is usually either provided by a specific device (for DC loads) or included in the island inverter (for AC loads). Example This is the same example as previously, but this time, you can see the components. 7 Without a MPP tracker. Sizing of the components Once you have an idea of what components you will have in your system, the question is what size will hey have (power, voltage, current, ..). To find this out, you first need to answer the following questions : • How much energy do you need ? • What is the maximum power you need ? How much energy you need will tell you how big you solar modules need be. How much power will tell you how big your generator and inverters need be. Calculating the energy need Professionals usually estimate the energy demand by establishing a demand profile for typical days. Depending on your climate and the variations of your activity, you may want to establish more or less daily profiles : • A single profile for the whole year • 4 profiles for each one of the 4 seasons (winter, spring, summer, autumn) • A profile for every month of the year You may also want to distinguish between working and non-working days if the occupancy of the household is not the same. For a given profile, proceed as following : for each appliance, estimate how many hours per day it will be used. From this, calculate the energy demand for this appliance (power x duration) and sum up all the energy demands from each appliance to find out the total daily energy demand. Example : - 3 fluocompact light bulbs of 18 W, used 4 hours a day each : 3 x 18 W x 4 h = 216 Wh - One radio of 30 W, used 3 hours per day : 30 W x 3 = 90 Wh - 2 cell phones charger consuming about 8 W each, used 5 hours per day each : 2 x 8 W x 5 h = 80 Wh Total daily demand = 216 + 90 + 80 = 386 Wh / day Calculating the power need The power need is basically the maximum amount of power you will need to draw from the system at once. You could sum up the rated power of all your appliances but you would probably end up oversizing your system, as in fact you are unlikely to use all these appliances simultaneously. For example, you are unlikely to have to power all the lights of the household (that only happens at night) and at the same time your electrical tools (you usually use them during the day). You'd better ask yourself which appliances you might need to power simultaneously and sum up their power. If you have among your appliances some that have electrical engines, you have to be aware that during their first seconds of start-up, they draw much higher currents that during normal functioning. Most electrical tools, such as drills, can be started progressively (“variable speed drives”), which avoids such phenomena. Alternatively, many island inverters are capable of delivering higher amounts of power for short amounts of time (30 s.). If this is not the case or if you are not sure, I recommend doing the calculation considering that the power of the biggest of these tools is the triple from its rated value. Example : Your house contains 8 light bulbs of 20 W each, a 10 W mobile phone charger, a small HiFi consuming 80 W, a food blender using 100 W a fridge at 120 W. You think you will not need simultaneously more than : the fridge, the mobile phone charger, half of the lights, the HiFi or the food blender (but not both at the same time since the noise of the food blender makes listening to HiFi uncomfortable). So the maximum power you will be using is : • mobile phone charger : 10 W • the HiFi or the food blender : 100 W (the maximum between 80 and 100) • 4 light bulbs at 20 W each : 80 W • the fridge : 120 W. So, this inventory would make in total 310 W. But, if that specific food blender can be started progressively, this is not the case of the compressors of fridges. Thus it is necessary to triple the consumption of the fridge to evaluate the peak power demand, which is in this case : 10 + 100 + 80 + 3x120 = 550 W. You may find some appliances designed especially for off-grid uses (mostly fridges 8). They are worth having a look at what makes them specially suitable for off-grid. Some of them can be powered with DC. Ask the retailer. Using softwares The method described in this document is a simple one and you only need a sheet of paper and a pen to use it (although a spreadsheet will make your life easier). You may find design softwares that will enable you to make much more precise and detailed simulations.9However, more detailed simulations will require you to input much more data and in the end, the results will still only be as accurate as the data you entered. Given the uncertainty of many parameters (such as the energy demand or he solar radiation in your place), the method used here is often as accurate as using these softwares. 8 Example : http://www.watteo.fr/product_info.php? n=R_frig_rateur_cong_lateur_solaire_Steca_PF_166_12V_24V&products_id=3078%26osCsid %3Dc78e4ad9b15d08410ba05474c07c5667&language=en 9 One such software is HOMER (http://homerenergy.com/). It is available as a 2-weeks trial free of charge. You may also use the old 2.68 version which is available for free and will largely suffice for your needs. HOMER regrettably only works on Windows platforms (but on Linux platforms, you may try out with the Wine emulator). Electrical protections Fuses and circuit breakers Electrical protections come in three types. Their role is to open the circuit to stop the current flow when something wrong is happening. • Fuses : they are simply small pieces of metal that will melt when the current running through them is too high (when a short-circuit occurs). Cheap and simple but you must replace them every time they are blown. • Circuit breakers : they are switches that are triggered open when the current rises too high. They serve the small role as fuses. They are more expensive but you do not need replacing them every time. Beware : they may be specifically designed for AC or DC electricity. They are often difficult to find and expensive as soon as you do not want standard sizes (10A, 16A and above). For this reason, I recommend using fuses for • Differential circuit breakers for AC electrical systems: differential circuit breakers are some kind of watchdog that constantly compare the currents running in the active and the neutral phases. If there is a difference, it means there is a current leak somewhere and it will trigger itself open. In most of Europe, the rating of differential circuit breakers for a household is 30 mA. You normally only need one differential circuit breaker for one household. Where to put the protections ? The purpose of fuses and circuit breakers is to protect against excessively high currents. In an off-grid system, there is often only one component that can give such high currents : the battery. You must absolutely put a fuse at one of the battery's terminals, unless specified otherwise in the manuals of equipment you are buying. To a lesser extent, a generator may produce high currents and you may want to put a fuse or a circuit breaker to the cable connecting it to the rest of the system but this is often superfluous. You can put additional fuses at other locations in order to protect specific components, but this is usually unnecessary. If it is needed, the technical manual of the equipment will tell you. You may use car fuses with specific fuseholders : they are easily available and cheap. However, only use them for low voltages (12 or 24 volts). You connect the differential circuit breaker on the AC cable connecting the inverter / generator to the electrical loads. Sizing of fuses and circuit breakers To size the battery's fuse, consider the maximum power that will flow in or out the battery. It is given by the highest value between a/ the sum of the power that can be produced by all solar modules and the generator, and b/ the sum of the power from all the appliances consuming electricity (if you have an inverter, simply use its power value). Once you have that maximum power flow that needs to go to the battery, divide by its voltage to know the maximum current and add some 25% for safety. Earthing You must earth your system, at least on the AC side. Some components on the DC side may also require earthing (see the manufacturers' manuals). To earth your system, connect the earth cable to a steel rod stuck into the ground. If there is lightning protection rod in your house, don't connect your earth cable to it : the electromagnetic shock wave sent in the system in case of lightning would destroy your system. Stuck a separate rod, several meters away from the one connected to the lightning rod.
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