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Thursday, September 27, 2007 Storing Solar Power EfficientlyThermal-power plants that store heat for cloudy days could solve some of the problems with solar power. By Peter Fairley
Solar proponents love to boast that just a few hundred square kilometers' worth of photovoltaic solar panels installed in Southwestern deserts could power the United States. Their schemes come with a caveat, of course: without backup power plants or expensive investments in giant batteries, flywheels, or other energy-storage systems, this solar-power supply would fluctuate wildly with each passing cloud (not to mention with the sun's daily rise and fall and seasonal ebbs and flows). Solar-power startup Ausra, based in Palo Alto, thinks it has the solution: solar-thermal-power plants that turn sunlight into steam and efficiently store heat for cloudy days. "Fossil-fuel proponents often say that solar can't do the job, that solar can't run at night, solar can't run the economy," says David Mills, Ausra's founder and chairman. "That's true if you don't have storage." He says that solar-thermal plants are the solution because storing heat is much easier than storing electricity. Mills estimates that, thanks to that advantage, solar-thermal plants capable of storing 16 hours' worth of heat could provide more than 90 percent of current U.S. power demand at prices competitive with coal and natural gas. "There's almost no limit to how much you can put into the grid," he says. Major utilities are buying the idea. In July, the Pacific Gas and Electric Company (PG&E) signed a 25-year deal with Ausra competitor Solel Solar Systems of Beit Shemesh, Israel, to buy power from a 553-megawatt solar-thermal plant that Solel is developing in California's Mojave Desert. The plant will supply 400,000 homes in northern and central California when it is completed in 2011. Florida Power & Light, meanwhile, hired Solel to upgrade the 1980s-era solar-thermal plants it operates in the Mojave. Ausra, meanwhile, is negotiating with PG&E to supply power from a 175-megawatt plant that it plans to build in California, for which it secured $40 million in venture financing this month. What distinguishes Ausra's design is its relative simplicity. In conventional solar-thermal plants such as Solel's, a long trough of parabolic mirrors focuses sunlight on a tube filled with a heat-transfer fluid, often some sort of oil or brine. The fluid, in turn, produces steam to drive a turbine and produce electricity. Ausra's solar collectors employ mass-produced and thus cheaper flat mirrors, and they focus light onto tubes filled with water, thus directly producing steam. Ausra's collectors produce less power, but that power costs less to produce. One megawatt's worth of Ausra's solar collectors has been producing steam in New South Wales, Australia, since 2004; the steam is fed into the turbines of a primarily coal-fired power plant. The final piece of the system--a proprietary heat-energy-storage system--should be ready by 2009. |
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Comments
walt on 09/27/2007 at 2:12 AM
14
jpontin on 09/27/2007 at 11:04 AM
Editor in Chief and Publisher
18
rhapsodyinglue on 12/30/2007 at 6:31 PM
55
tcawley on 09/27/2007 at 12:03 PM
1
dmm on 09/27/2007 at 3:07 PM
136
Though I don't agree with everything printed here, I find this "rag" generally useful, informative, and interesting.
If you think you can do better, start your own technology emag. Put up or shut up.
zhangtao on 09/27/2007 at 4:13 AM
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dmm on 09/27/2007 at 2:57 PM
136
mkogrady on 09/27/2007 at 12:05 PM
92
Question to Ausra - can a collector system be designed to operate in colder climates, but instead of heating up a solution to drive a steam engine, can the heat be collected - stored - and used to heat a house via convection heating? How hot do the fluids get off these fresnel systems?
davea0511 on 09/27/2007 at 5:32 PM
16
eak on 09/29/2007 at 9:33 AM
8
http://www.sunmachine.com/english/sm_solar.htm
davea0511 on 09/29/2007 at 2:27 PM
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Even that one I am sure is geared toward a large complex or building.
When quiet 2kWh stirlings hit the $2000 barrier I believe then you'll start seeing solar thermal taking over the single family dwelling industry. I'd hate to be in PV at that point.
eak on 09/30/2007 at 3:21 PM
8
http://www.stirlingenergy.com/
I believe Southern California Edison has ordered 500-850MW, and San Diego Gas and Electric has ordered 300-900MW of the above. They have limited heat storage though. The claim is $1.40/Watt.
ArtInvent on 09/27/2007 at 1:13 PM
13
Innovations like this would make it quite possible for CA and much of the Southwest and certainly Australia to generate virtually all of its power day and night from solar. All from mirrors, pipes, salt, and steam generation. Beautifully simple, almost low-tech, really.
Getting a little more complex, the heat can also be used to drive Stirling heat engines rather than steam generators. A lot of potential.
davea0511 on 09/27/2007 at 5:28 PM
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This is the perfect example of why it's important to invest development in promising technologies - the only way they've been able to bring down the costs is through the existing Mojave desert system which they've used as a test bed for the last 30 years.
eak on 09/29/2007 at 9:36 AM
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davea0511 on 09/29/2007 at 2:16 PM
16
The unique technology (ie expensive) here is the unusually robust dewar tubes with the integrated back reflector, and the suber heat absorbant black stainless steel pipe, plus the anti-reflective coating, as well as the mirrors. These aren't regular mirrors. In many cases these are cast ceramic parabolic mirrors. Even the cheaper aluminum ones have special coatings to prevent oxidation, shed water, and maximize reflectivity. The single most expensive consumable however are the dewar tubes, as when they break the whole plant has to be shut down for a replacement.
eak on 09/30/2007 at 3:30 PM
8
http://pr.caltech.edu/periodicals/EandS/articles/LXX2/powering.pdf
Anything that decreases the $/kW is good...
Siphon on 10/09/2007 at 3:06 PM
93
Using incredibly advanced deductive logic, we can assert that using evacuated tubes is not a big liability for plant availability.
doteman on 10/02/2007 at 11:48 PM
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Siphon on 10/09/2007 at 5:02 PM
93
Biomass can be methanized with very high efficiency. Has anyone thought about using biomethane instead of fossil methane for the turboexpanders?
dmm on 09/27/2007 at 3:02 PM
136
davea0511 on 09/27/2007 at 5:25 PM
16
1) Dewar tubes that contain the working fluid are much more robust and reliable.
2) Heat Storage technology has improved dramatically.
3) Using water for the working fluid and generating the steam directly. Until now they used salts or oil, then used the salts and oil in a heat x-fer to generate water steam inside the plant. They've bypassed that step in this system.
davea0511 on 09/27/2007 at 5:15 PM
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davea0511 on 09/27/2007 at 5:20 PM
16
You convert half of the steam to energy in the daytime That makes your energy conversion plant requirements half of what would be required if all the heat was immediately converted to electricity. That results in much lower costs.
ChuckInReno on 10/01/2007 at 12:23 PM
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None of these "heat storage" devices is anywhere near as efficient as storing potential energy through gravity, something like pushing water uphill to a reservoir.
Siphon on 10/09/2007 at 3:58 PM
93
Until they've got this cavern storage worked out, they will likely use a proven thermal storage method such as Caloria oil storage.
You are incorrect about the efficiency of pumped hydro. This is 80 ish percent efficiency at best, a bit lower than thermal energy storage.
Of further relevance is the fact that hydro-electric requires much more land per unit of energy produced than all of the array field, turbine, and thermal energy storage systems. Especially if (or, more likely, when as it's not rocket science) Ausra's underground cavern storage plan is commercialised.
Then there's flow batteries. Unfortunately, these use large amounts of rare materials, resulting in high cost and ruining scalability. And flow batteries aren't quite as efficient as direct hot H2O storage. If flow batteries could be designed using common inexpensive materials, and operate at high efficiency, then things might change. Otherwise, it's niche.
Flywheels are very promising, and even more efficient than thermal storage (not that it gets that much better from 97 percent though), but also require more development to enter mainstream energy storage markets.
Thermal energy storage, on the other hand, is already commercially proven to be very cost-effective and efficient as well. Of all the other storage technologies, only pumped hydro can make this claim. And, considering that pumped hydro is geographically limited, thermal energy storage is the only scalable inexpensive energy storage method we have right now.
jrc55 on 09/27/2007 at 7:22 PM
1
eak on 09/29/2007 at 9:42 AM
8
DJTal on 09/30/2007 at 6:36 AM
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eak on 09/30/2007 at 3:39 PM
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David Tilman:
http://www.sciencemag.org/cgi/content/abstract/314/5805/1598
Yes, this is good stuff. However, photosynthesis is usually 1-7% efficient, and conversion of biomass to electricity or liquid fuel reduces that further. In comparison, solar thermal is 30% efficient, one needs a lot less land to power the U.S. There is likely a place for LIHD biomass, but I think most of our energy will come more directly from the sun than taking it through plants.
DJTal on 10/01/2007 at 4:57 AM
114