1. No category

The Exotics / Connecting to the Grid
Today I'll wrap up discussion of energy plants
Except for hyper-controversial nuclear, which we'll return to later
I will cover the "exotic" power generation technologies of:
- Tidal Barrage
- Tidal Stream
- Wave
- Geothermal
Which are really not so much exotic as limited in their likely impact
Then, to a challenge facing MANY new technologies: Connecting to the grid
That is, converting the natural form of their power output
Into a form that can be merged and shared
Tidal Power
Tidal power is really just a different form of hydro power
And as discussed in the earlier lecture on hydro and wind powers
Hydro is ultimately about gravitational potential energy:
D Egravity = M g Dh
Which for a continuous steady flow F (volume / second) gave us:
Phydro = 9.8 (kW-seconds / m4) x F x Dh
(kW = kilowatt)
(Nitpicking: salt water can be a few percent more dense than pure water)
However, big difference: For tides and waves, flows are NOT steady at all!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Alternative forms of tidal power generation
The simplest / oldest might have been some variation of this:
Floating boat / buoy tied via rope and pulleys to onshore counter weight
With movement of onshore weight or pulley used to do some sort of work
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
But can get a lot more power from a variation of a dam
Ocean:
Damned inlet or manmade basin:
Power generated when tide coming in
Power generated when tide going out
(a.k.a. "Tidal Barrage")
Also has potentially big benefits of moving power generation mechanism onshore
Or at least into dam which is connected to shore
And of concentrating / simplifying that mechanism (e.g. into single turbine)
Here recognizing the severe difficulty of keeping mechanisms working in saltwater
(Just ask Stephen Spielberg!)
How much power out?
With density of water ρ, reservoir area A, surface gravity of g:
Say tide raises sea level h, then lowers it h: net change in height = 2h
So full tidal rise => Gravitational energy of M g 2h. With mass of raised water:
M = density of water x its volume = ρ (2 h A) (Note: 2h enters again!)
Putting in values for water density and surface gravity:
Egravitational = ρ g (2 h A) 2h = (1000 kg/m3)(9.8 m/s2) 4 A h2
= (9800 kg m2/s2 x 1/m4) 4 A h2
= 39.2 kiloJoules /m4 x A h2
Tidal cycle is ~ 12 hours ~ 43,200 seconds, so cycle averaged power is:
Powertides = 0.91 Watts / m4 x (A h2)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
But it's actually 6 hours of rising tide + 6 hours falling:
But can extract power whichever direction tide is pushing water:
Get power when rising tide PUSHES water into reservoir
AND
Get power when falling reservoir PUSHES water back out to sea
So, it turns out that answer above is still about right
But because salt water is a little denser than fresh water, fair to round up to:
Powertides ~ 1 Watt / m4 x (Area h2)
where h = half tide
Of which we could recover a fraction εgenerator (efficiency of our hydro generator)
Note: Book gets 2X my number = Power out DURING falling tide (with 0 out during rising tide)
But there is also the "pumping trick"
As described in "Sustainable Energy without the Hot Air – David J.C. MacKay:"
Make your dam a bit TALLER than the high tide level, and add some pumps
At HIGH tide, pump extra water UP into reservoir (expending energy!)
At LOW tide that SAME water will fall LARGER DISTANCE = More energy back!
Tide provided PART of energy to get
extra water up into reservoir
But YOU then get all the energy back
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Thereby expending/recovering additional power:
Say at (about) high tide, you pump water UP a further height b:
With pump efficiency = εpump and generator efficiency= εgenerator
That requires you to expend an energy:
Eexpended = (1/εpump) M g height =(1/εpump) (ρ A b) g b = ρ g A b2/εpump
But then, at low tide, that
:
Erecovered = εgenerator M g height = εgenerator (ρ A b) g (b + 2h)
Giving ratio of added power out to added power invested
Ratio out / in = (εgeneratorεpump) (b + 2h)/b
call εgeneratorεpump = εtotal
If efficiencies were 1, ratio would always be better than 1 => net gain
If efficiencies less than 1, ratio => 1 when b = 2h (εtotal)/(1- εtotal)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Can also pump water OUT near low tide
Putting this ALL together, "Sustainable Energy without the Hot Air" shows:
Net gain for pumping is a "boost factor" of (εtotal)/(1- εtotal)
For εtotal ~ 0.76 (corresponding to pump and generator efficiencies of ~ 87%)
Book generates table (averaged over tidal cycle):
Tidal Half
Amplitude (h)
Optimum
Boost Height (b)
Power
with pumping
Power
without pumping
1 meter
6.5 meter
3.5 W/m2
0.8 W/m2
2 meter
13 meter
14 W/m2
3.3 W/m2
3 meter
20 meter
31 W/m2
7.4 W/m2
4 meter
26 meter
56 W/m2
13 W/m2
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
However (paralleling conventional hydropower):
Above demands BUILDING those coastal reservoirs
By damming up bays or estuaries. Thereby modifying coasts with ecological value
E.G. water purification and animal rearing value of coastal marshes
And/or: visual / leisure time / vacation residence value
And/or: harbor / industrial value
"Worlds First" tidal power station
(1966) in Rance River estuary,
in Brittany France
62 MW average (240 MW peak)
~ 1/10 average U.S. power plant
http://en.wikipedia.org/wiki/Rance_Tidal_Power_Station
Final thoughts regarding tidal barrages:
It's worrying to note that while the above Rance tidal barrage claims to be oldest
Its output power level is cited by most sources as STILL being the largest
(Also, misleadingly, they mostly cite its peak rather than average power)
Suggesting, over fifty years, that a lot of people decided against this option
As relatively attractive, and relatively high power, as it appears
In addition, regarding the preceding pump enhancement trick:
That calculation assumes ALL the water is pumped up AT high tide
Or out AT low tide (i.e. all the extra water moved in ~ ½ hour)
But optimum "boost heights" were 5-7 times tidal height, making this unlikely
And pumping before or after peak tides => diminished energy gain
Leading to alternative of "tidal stream" power generation
Of which a few exist:
Strangford Loch, N. Ireland: 1.2 MW
~ 1/500 average U.S. Power Plant
Or, with some added artwork:
http://en.wikipedia.org/wiki/Strangf
ord_Lough
http://subseaworldnews.com/20
12/01/17/uk-seagen-tidalturbine-gets-all-clear-fromenvironmental-studies/
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
With a lot more contemplated, or at least imagined:
http://news.bbc.co.uk/2/shared/
spl/hi/pop_ups/07/uk_enl_11938
29329/html/1.stm
http://climatekids.nasa.gov/tidalenergy/
http://www.ecofriend.com/ecotech-nasa-s-jpl-develops-a-costeffective-way-to-harness-oceanenergy.html
http://www.esru.strath.ac.uk/Ea
ndE/Web_sites/1011/Tidal/tidal.html
But some of the radical new designs ARE being tested:
For Maine's Passamaquoddy and Cobscook bays:
http://www.pressherald.com/2012/07/21/maine-company-leading-way-as-tidal-energy-comes-of-age_2012-07-22/
Press Herald headline: "Maine company leading way as tidal energy comes of age:"
50 kW prototype ( ~ 1/10,000 average U.S. Power Plant)
"Much of the industry’s near-term expansion is expected to be in Nova Scotia . . .
(for units) that are community-owned"
All of which suggests limited overall role:
Which could, nevertheless, be very important for more remote/isolated locales
And IS a typical enabling factor for many/most of these "exotics"
Also could be practical in special locales where geography favors installations:
Rance River Barrage: Not much more than short bridge => low dam
Bay of Fundy (Nova Scotia): World's highest tidal range, up to 16 meters
Moreover, if costs and reliability COULD be improved . . .
There IS the fact (from Hydropower / Windpower lecture) that FOR flows:
Energy_Density_Waterkinetic = 0.5 (kg/liter) x v2
Energy_Density_Airkinetic = 0.59 (g/liter) x v2
Implying: Offshore hydro could be 1000X more power dense than offshore wind!
Wave power:
Name sort of says it all (and we have all experienced it)
Trick is HOW to capture it. Actually built:
http://www.biggreensmile.com/greenglossary/wave-power.aspx
http://www.bluebirdelectric.net/wave_power_energy_generation.
htm
Or extrapolated:
http://www.biggreensmile.com/green-glossary/wave-power.aspx
Common Theme:
Flexing at joints / pivot points => Pumps fluids => Drives generators
In other words, hydropower => hydraulic power => electric power
However, flaws (possibly fatal) that I perceive:
1) Water's power is ONLY collected from immediate vicinity of mechanism
That is why whole fleets of the units are envisaged
Vs. Tidal Barrage where turbine collected power from whole reservoir
2) (Red mechanism): All of mechanism is exposed to highly corrosive seawater
Multiple joints vs. single propeller shaft seal of Tidal Flow mechanism
3) (Red mechanism): Floating on surface, it completely obstructs navigation
4) (Yellow mechanism): Massive toilet bowl floats, from shore? (give me a break!)
What power outputs have actually been achieved?
Wikipedia identified a couple of dozen projects (http://en.wikipedia.org/wiki/Wave_power)
But cited power outputs for only a handful:
2.25 MW of Povao de Varzim, Portugal
3 MW off Scotland (exact location / ID not provided)
20 MW (expandable to 40 MW) off Cornwall UK
19 MW of Portland, Victoria, Australia
1.5 MW off Reedsport Oregon
Meaning that LARGEST was ~ 4% the size of single average US Power Plant
(of which we currently require ~ 5800)
So it's time to move on to: Geothermal Power
Which resurrects last lecture's theme of getting heat (from somewhere)
Using it to boil something
With fluid to vapor expansion then driving turbine generator
Source of heat: Earth's molten core (thought partly heated by radioactive decay)
So it gets hotter with depth = "Geothermal Gradient" ~ 25-30°C / km of depth
However, that's highly averaged number, applicable away from tectonic boundaries
NEAR tectonic boundaries (e.g. in Iceland) gradient can be much higher
Allowing Iceland to generate 25% of its power from geothermal1 OR
California's 15 geothermal plant "Geysers" system2 to reach 725 MW (!)
1) Orkustofnun – National Nower Authority: www.nea.is/geothermal/
2) www.geysers.com/geothermal.aspx
Geothermal energy is thus all about maps:
From the European commission: Extrapolated temperatures at 5 km depth
Conclusion? Not much - Turkey, a bit of Spain, plus the Balkans . . .
Source: http://ec.europa.eu/research/energy/eu/index_en.cfm?pg=research-geothermal-background
Or for the U.S.
U.S. National Renewable Energy Lab (NREL) map:
Conclusion? Build geothermal plants in the West/Northwest
Source: http://www.nrel.gov/gis/images/geothermal_resource2009-final.jpg
But how MUCH power?
Let's first try to read the fine print:
Black dots – "Identified hydrothermal site"
"Map does not include shallow EGS sources
located near hydrothermal sites"
Huh? Aren't those the best locations?
Is intent here to find only NEW power sites?
Of new "deep" class called out in title?
"Includes temperature at depth of 3 to 10 km"
"N/A regions have temperatures less than 150°C
at 10 km depth
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Clearly need better understanding to guess at likely power out
Coverage of geothermal in my textbook collection is very thin
But the best of them identifies three classes of geothermal:
Class 1: Shallow plants for sole purpose of heating surface buildings
Which would SAVE power but not produce it => Geothermal Heat Pump
Class 2: Systems using naturally produced steam (e.g. from geysers)
That is, minimal drilling and letting steam come to you
Occurring in very limited locales like Iceland, Geysers CA, Yellowstone
Class 3: Systems reaching depths deep enough / hot enough to boil piped in water
Called "Enhanced Geothermal Systems" or EGS
So we are mostly interested in EGS = What NREL map was also focusing on!
Diagram of an EGS system:
With detailed components given as:
1) (Surface) Reservoir
2) Pump house
3) Heat exchanger
4) Turbine Hall
5) Production Well
6) Injection Well
7) Hot Water to District Heating
8) Porous Sediments
9) Observation Well
10) Crystalline Bedrock
Source: http://en.wikipedia.org/wiki/Geothermal_electricity
We've been over this ground enough to figure out the rest:
Pump house (2) => To push supply water down into the
Injection Well (6) to then diffuse through the deep extremely hot
Porous Sediments (8) causing the water to boil, exiting as steam via the
Production Well (5) from where it is then routed to the
Heat exchanger (3) boiling clean mineral-free water with THAT steam going to
Turbine Hall (4) with small diversion to nearby shivering people via
Hot Water to District Heating (7) and rest of steam continuing on to
Surface Reservoir (1) where steam condenses (~ cooling tower/river/lake) with
Crystalline Bedrock (10) to keep most injected water from wandering away and
Observation Well (9) being the only thing still in need of explanation:
Which Wikipedia forgot to explain but I'd guess could monitor how much
plant is cooling earth (and thus be used to fine tune plant operation)
But what is Geothermal's potential?
Thermodynamics' Carnot cycle gives maximum "heat engine" efficiency of
Max efficiency (%) = (1 – Tlow / T
high)
x 100
For geothermal heat engines, Tlow ~ earth surface temperature ~ 300°K
And Thigh might be 200°C higher, e.g. 500°K giving theoretical limit of
Max geothermal efficiency ~ (1- 300 / 500) x 100 ~ 40%
Compared to wind's 40%, IGCC fossil fuel's 50% or hydroelectricity's almost 90%
But heck, with geothermal the "fuel" IS free!
So, 40% of WHAT? Of the thermal power flowing up through the earth's crust:
Wikipedia specs this as 65 mW / m2 on land (vs. 110 ocean bottom)
USGS and book "Hot Air" give about the same at ~ 50 mW / m2
From which:
Carnot limited extraction = (~ 40%) x (50 mW / m2) = 20 mW / m2
Total dry land area of world ~ 150 x 106 km2
Multiplying this land area by the capturable geothermal flow:
Powermax ~ (20 mW / m2) x (150 x 106 km2) ~ 3 x 1012 Watts
Divide this by world population of ~ 7 billion
Max personal geothermal power ~ 428 Watts
Which, while not trivial, is certainly not that impressive, especially when it requires
Geothermal power from TOTAL land area, at max efficiency possible
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Reality check?
US Energy Information Agency gives1 2013 geothermal total of 16,517 GW-hr
=> Total US Geo power of 1.88 GW (~ 4 average US power plants)
Out of total US renewable sourcing of 522,464 MW-hr (=> Geo ~ 3.16%)
But, from intro of Hydro / Wind Power lecture, US renewables ~ 9.1% of total
So Geothermal contributed about 0.28% of US power in 2013
What about new deep water injected EGS (Enhanced Geothermal Systems)?
Despite promise, the technology appears to be still in its infancy
With biggest experimental plant (Cooper Basin, Australia)
Only targeting 25 MW output
1) Source: http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_1_01_a
Takeaway message on Geothermal?
Don't try it anywhere, do it where there is a lot more natural heat
USGS1: Yellowstone averaged 50X higher, and peaked 2000X higher
than typical earth surface location
For instance, target "ring of fire" tectonic plate boundary locations2:
But even then:
It's still very hard to estimate cost / potential
Because more site accommodating EGS tech
Has had only small-scale testing
And even less costing out
1) Source: http://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_sub_page_53.html
2) http://pubs.usgs.gov/gip/dynamic/fire.html
Moving on: Connecting to the Grid
As mentioned many times: Electrical power = Current x Voltage = I x V
But different applications lead us to select different combinations of I and V
In home appliances, lower voltages minimize bulk due to insulation
And the semiconductors used in electronics demand low, single digit voltages
But in transmission, problem is flowing electrons bumping into things
And thereby loosing energy to heat => Use of low currents / high voltage
All of which conspired to favor use of AC power (in the US at 110V and 60 Hz)
Because coil-based transformers made it SO easy to swap around I and V
But that means we transmit our power at a very precise combination of I and V
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Challenge of "synchronizing" new power source with grid:
All of the above means you can't just connect a new power source to the grid!
To merge, it has to be pushing and pulling just as hard, and in precise step
Voltage has to be within ~+/- 5%, frequency within ~ 0.1%
Outside of these limits you not only won't contribute, you can blow things up!
For instance, if you pull when grid pushes, you'll be blasted with power!
HOWEVER: Rotating machines naturally => Oscillating power
Via the motor/generator devices we discussed earlier
BIG/HEAVY rotating generators produce very well controlled oscillating power
Which is exactly what we want!
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
So big rotating generators => "High quality" power
The water driven turbine-generators of hydroelectric dams ARE big
The steam powered turbine-generators of fossil fuel power plants ARE big
And, for such turbines, efficiency/cost also favor keeping them BIG / MASSIVE
To that add fact that their speed is largely at our discretion, at our control:
Hydroelectric Dam: Open the water valve wider => More speed
Fossil Fuels: More fuel to the burners => More steam => More speed
That, plus help from the grid (i.e. motor/generator duality tending to synchronize)
=> Twentieth Century Power Grid
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Many modern alternatives do not alter this picture
If the technology's ultimate goal is to boil water to drive a turbine generator
Power generation results are pretty much the same
This includes: Nuclear Power, Solar Thermal, Geothermal power
As are economic incentives toward BIGNESS => LARGE POWER PLANTS
Leading to frequently cited US average power plant size ~ 500 MW
(Actually range of about 200-2000 MW)
But ALL renewable power sources do NOT follow this pattern:
Of either producing easily synchronized AC power OR
Of economics driving one to build BIG centralized plants
Indeed, neither of the currently most viable alternatives follow this pattern!
Sustainable energy's black sheep:
Wind turbines DO rotate and thus easily drive generators to produce AC
But they produce the MOST power when free to rotate at varying speeds
As determined by current wind speeds
=> AC of wildly varying frequencies
Early partial solution was to use weird "induction generators"
Resembling induction motors, these had normally unpowered rotor coil
With conventional stator electromagnets surrounding
Rotor coil was powered (by external power) ONLY during start up
Temporary electromagnet => Induced magnetism in stator coils
Which, with rotation => Induced magnetism in rotor coil
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
(continuing):
Meaning that once induction generator got rotating:
1) External input power to coil could be turned off (good)
2) Power was efficiently produced at variable rotation speeds (very good)
3) Output AC frequency changed with wind speed (TERRIBLE!)
At least as far as hooking into the grid was concerned!
So wind turbines, operating most efficiently, cannot be tied directly to the grid
Leading to LOCAL (generally turbine by turbine) conversion of
Variable frequency AC => DC (i.e. ~ steady voltage and current)
Which then powers DC motor to turn AC generator at 60 Hz
Or DC is switched on and off ("alternated") to produce ~ 60 Hz AC
Modern semiconductor based "alternators" => Latter now favored
Required wind turbine power output conversion for grid compatibility:
Raw wind turbine generator output: 110 Volt-ish, 60 Hz-ish wobbly signal
"Rectified" and filtered to DC:
"Alternated" (chopped on and off) to square wave:
Smoothed and offset to form clean 110 VAC:
And (from traditional perspective) solar PV is even worse:
Solar "photovoltaic" cells naturally produce a trickle of ~ 1 Volt DC output
With varying sun => Big changes in current / small changes in voltage
They do NOT naturally produce AC
And, if coerced, would naturally produce ~ 1 Volt AC (rather than 110)
Leading to first requirement that DC voltage be stepped up by ~ 100X
Which can be done by wiring ~ 100 cells together, nose to tail
OR by using DC to DC "Universal Voltage" converter type circuits
As built around inductor coil spiking described in third lecture
~ 110 Volt DC then converted to 60 Hz AC by "alternator" schemes on last page
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Required solar cell power output conversion for grid compatibility:
Raw solar cell output: Low DC-ish voltage
"Stepped up" to higher voltage DC:
"Alternated" (chopped on and off) to square wave:
Smoothed and offset to form clean 110 VAC:
And bigger is not better
That is, it's generally NOT a good idea to do all of this conversion centrally
With wind turbines: They will naturally turn at different speeds
Meaning that their raw generator output AC's would NOT synchronize
So couldn't even hook TWO together without serious trouble
Solar cells are a bit more sociable
But wind's final AC => DC => AC conversion AND solar's final DC => DC => AC
Now employ semiconductor circuits which:
Are NOT cheaper in huge monolithic versions and which
DO produce waste heat (requiring cooling as in your PC)
So is better to keep small, and spread out for better air cooling
Thus, while we speak about wind or solar farms:
Fundamental, simplest, intrinsically most efficient unit is:
Single wind turbine with self-contained 110 VAC conversion
Bank of solar cells with self-contained (or local) 110 VAC conversion
If you WANT more power at that location, fine, install more units
But per unit installed cost will not then drop in proportion
So small farm can compete economically with large farm
CONTRASTING with fossil fuel / hydro / nuclear / geothermal / solar thermal
Where economics of steam generation, turbine generation, condensation
Tend to produce much greater efficiencies for large plants
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
Leading to brave new world of "Distributed Generation"
Where you COULD own a personal wind turbine
If costs were low enough
And neighbors didn't object (better talk them into doing same!)
And you COULD consider buying your own personal solar photovoltaic array
If costs were low enough
And neighbors didn't object
And have BIG properly oriented roof or HUGE yard
Because solar power is dilute! (will elaborate on this in Natural Resources lecture)
Could also consider "micro" versions steam-driven technologies
Where less well-behaved micro-turbines also have grid attachment issues
A "brave new world" because of new opportunities:
- Closer power sources => Less power lost in transmission
- Shorter transmission also mitigates voltage getting out of phase with current
I.E. "Reactive Losses / Limits" discussed in my third lecture
Also reducing need to add offsetting "capacitors"/"regulators" to grid
- When local source balances local consumption, it effectively vanishes from grid
- Some renewables tend to produce most power when power most needed
Winds tend to peak late afternoon ~ early evening consumption peak
Solar output increases in summer along with air-conditioning loads
- MANY distributed sources => Reduced opportunity for wholesale sabotage
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm
A "brave new world" because of new concerns:
- Potentially MUCH harder to balance electrical supply and demand
Which for grid is hyper important as involves power balancing
WHILE maintaining delicate AC synchronization
Loss of which (even locally!) has produced MASSIVE blackouts
- Engendering fear that local equipment failures could exacerbate instability
Because instead of local consumers
Would instead have local consumer / producers
With much tighter electrical bonds to the grid
- AND have to figure out how to properly credit consumers who also produce
=> Traditional power companies (for some valid reasons) viewing
"Distributed Generation" with significant apprehension!
Credits / Acknowledgements
Some materials used in this class were developed under a National Science Foundation "Research
Initiation Grant in Engineering Education" (RIGEE).
Other materials, including the "UVA Virtual Lab" science education website, were developed under even
earlier NSF "Course, Curriculum and Laboratory Improvement" (CCLI) and "Nanoscience Undergraduate
Education" (NUE) awards.
This set of notes was authored by John C. Bean who also created all figures not explicitly credited above.
Copyright John C. Bean (2014)
(However, permission is granted for use by individual instructors in non-profit academic institutions)
An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm