Technology Review - Published By MIT

Log in to My.TechnologyReview.com | Register

click here...

Thursday, September 27, 2007

Storing Solar Power Efficiently

Continued from page 1

By Peter Fairley

Audio »

Share »

Favorite

Print

E-mail

Mills will not say what material his company's system will heat, although several recent solar-thermal plants by Ausra competitors--including one in Nevada that started up this summer and two under construction in Spain near Granada--plan to use molten-salt storage. Molten salts are inexpensive salt solutions that absorb considerable energy when they melt and give up that energy when they freeze.

What Mills can say for certain is that Ausra's storage system will lower its power-generation costs. That is a surprising statement since energy storage can as much as double the cost of electricity from photovoltaics or wind turbines.

Heat storage is more efficient than electricity storage: just 2 to 7 percent of the energy is lost when heat is banked in a storage system, compared with losses of at least 15 percent when energy is stored in a battery. More important, says Mills, is the fact that storage enables thermal plants to use cheaper turbines.

The bottom line is that Mills vows that adding storage plus savings from economies of scale and lower cost of capital (as banks become familiar with solar-thermal technology) will cut Ausra's current 10 to 11 cents per kilowatt-hour cost of power in half. By 2010, he expects solar thermal to provide California with baseline power cheaper than natural gas, currently set by the state at 9.2 cents per kilowatt-hour.

Why has solar-thermal power received little attention from the energy-storage community despite such promise? John Boyes, manager of the Energy Storage & Distributed Energy Resources at Sandia National Laboratories, in Albuquerque, NM, says that solar thermal is viable but inflexible compared with other means of storing energy, such as, say, coupling wind farms to large batteries, flywheels, and supercapacitors that can be placed almost anywhere on a power grid. "You can store energy anywhere you have electricity and a little bit of floor space," says Boyes.

The footprint of Ausra's planned 175-megawatt plant will be, in contrast, about one square mile.

Comments

	solution walt on 09/27/2007 at 2:12 AM
Posts: 13 Avg Rating:	The author does not seem to know what a 'solution' is. What background is required to write for this rag? Rate this comment: (Reply)

	Re: solution jpontin on 09/27/2007 at 11:04 AM		TR Staff Editor in Chief and Publisher
Posts: 13 Avg Rating:	What do you object to in the clause: "Solar-power startup Ausra, based in Palo Alto, thinks it has the solution..." Rate this comment: (Reply)

	Re: solution rhapsodyinglue on 12/30/2007 at 6:31 PM
Posts: 54 Avg Rating:	I believe he was referring to "salt solutions" which probably is a technical mistake. A salt solution would be a salt dissolved in a liquid. These systems use salts which are heated to melt them rather than being dissolved in a liquid. Rate this comment: (Reply)

	Re: solution tcawley on 09/27/2007 at 12:03 PM
Posts: 1 Avg Rating:	An intelligent reply would ignore semantics, grammar and incidental rhetoric to focus on the merit of the content. While certainly not the Holy Grail of energy solutions, this storage solution should be welcomed as one of many. When combined with other solutions such as PV and wind power. they will free this country from oil based energy dependence. Rate this comment: (Reply)

	How wude! dmm on 09/27/2007 at 3:07 PM
Posts: 135 Avg Rating:	What a troll you are! 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. Rate this comment: (Reply)

[no subject]
zhangtao on 09/27/2007 at 4:13 AM

Posts:
1

Rate this comment:

(Reply)
- [no subject]
  dmm on 09/27/2007 at 2:57 PM
  
  Posts:
  135
  Avg Rating:
  
  How thought-provoking! But what about the following:
  
  Rate this comment:
  
  (Reply)

	Interesting use of Fresnel Lenses mkogrady on 09/27/2007 at 12:05 PM
Posts: 72 Avg Rating:	Focusing heat to a collector is no differnent than frying ants with a magnifying glass. I believe the folks in New Mexico have similar designs, but don't store the heat per se. 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? Rate this comment: (Reply)

	Re: Interesting use of Fresnel Lenses davea0511 on 09/27/2007 at 5:32 PM
Posts: 11 Avg Rating:	Solar Thermal is only economical on a grid-level power plant basis. Distributed electricity generation is better done by Solar PV. There are efforts underway to use Solar Thermal on a much smaller scale, but it's generally for heating and cooling large buildings and complexes. Rate this comment: (Reply)

	Re: Interesting use of Fresnel Lenses eak on 09/29/2007 at 9:33 AM
Posts: 8 Avg Rating:	Here's one company attempting to make a residential scale solar thermal product: http://www.sunmachine.com/english/sm_solar.htm Rate this comment: (Reply)

	Re: Interesting use of Fresnel Lenses davea0511 on 09/29/2007 at 2:27 PM
Posts: 11 Avg Rating:	That looks to be more of a pellet stove on steroids. Interesting concept: combining the sun's heat with a combustionable material for greater heat to drive a stirling engine. That's got to be a first. Certainly is not renewable nor clean. 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. Rate this comment: (Reply)

	Re: Interesting use of Fresnel Lenses eak on 09/30/2007 at 3:21 PM
Posts: 8 Avg Rating:	No, it's not a first. This much larger version is being deployed in the Mojave right now: 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. Rate this comment: (Reply)

	makes huge sense ArtInvent on 09/27/2007 at 1:13 PM
Posts: 12 Avg Rating:	While batteries or super caps are easier to place, they've got to be vastly more expensive as a storage option than a pool of salt, in addition to the charging/discharging losses. I've seen the articles about wind generators installing banks of lithium batteries. I must say I have my doubts, how can that be affordable. Solar plants already take up lots of land area so this seems perfect. California already has a significant commitment to solar power in the Mojave, and capacity should double easily within the next few years. 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. Rate this comment: (Reply)

	Re: makes huge sense davea0511 on 09/27/2007 at 5:28 PM
Posts: 11 Avg Rating:	Well, it sounds low tech - but it's really the high-tech equipment design that makes it so cheap and attractive. 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. Rate this comment: (Reply)

	Re: makes huge sense eak on 09/29/2007 at 9:36 AM
Posts: 8 Avg Rating:	The most high-tech thing I can think of in their system is the steam turbine. The rest of the components David Mills has elsewhere said is like "farm machinery". Rate this comment: (Reply)

	Re: makes huge sense davea0511 on 09/29/2007 at 2:16 PM
Posts: 11 Avg Rating:	No. A steam turbine is a steam turbine. High tech, yes, but commonly in use, yes also. 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. Rate this comment: (Reply)

	Re: makes huge sense eak on 09/30/2007 at 3:30 PM
Posts: 8 Avg Rating:	By the way, there is a group working on cheap solar thermal at Caltech (e.g. Nathan Lewis). He is in Chemistry, and is looking for materials (e.g. paint) with which to lower the cost of things like reflectors. Here is a recent article of his: http://pr.caltech.edu/periodicals/EandS/articles/LXX2/powering.pdf Anything that decreases the $/kW is good... Rate this comment: (Reply)

	Re: makes huge sense Siphon on 10/09/2007 at 3:06 PM
Posts: 65 Avg Rating:	The trough plants in the Mojave also use evacuated tubes. They have shown 99% plant availability. Better than coal. Better than natural gas. Better than nuclear fission. Using incredibly advanced deductive logic, we can assert that using evacuated tubes is not a big liability for plant availability. Rate this comment: (Reply)

	Re: makes huge sense doteman on 10/02/2007 at 11:48 PM
Posts: 5 Avg Rating:	Regarding your comment on energy storage for Wind Farms, the industry is not serious about electrochem batteries, at least for the time being. Cost and reliability are the main issues. Compressed air energy storage (CAES) has had some recent interest here, and some historical success. A peaker plant in Alabama has been using CAES since the early 90's. Recently, an Iowa farm cooperative has decided to pursue a Wind-CAES project based on successful trials with natural salt cavity formation in Iowa, and even more recently General Compression, a start-up in MA is pursuing a turbine/farm technology that facilitates Wind-CAES. Rate this comment: (Reply)

	Re: makes huge sense Siphon on 10/09/2007 at 5:02 PM
Posts: 65 Avg Rating:	Natural gas is used for CAES turboexpanders. That's usually fossil methane. Which creates residual CO2 emissions. Biomass can be methanized with very high efficiency. Has anyone thought about using biomethane instead of fossil methane for the turboexpanders? Rate this comment: (Reply)

	Interesting, but... dmm on 09/27/2007 at 3:02 PM
Posts: 135 Avg Rating:	Thanks for the interesting article. But this all sounds very similar to things that were being written about in Popular Science in the 1970s. What has changed since then? Only the price of oil, or is there new tech that I am not noticing? Rate this comment: (Reply)

	Re: Interesting, but... davea0511 on 09/27/2007 at 5:25 PM
Posts: 11 Avg Rating:	New technology is why it's suddenly much cheaper and viable. We've had 30 years to learn from similar systems in the Mojave. Three things that really drive down costs: 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. Rate this comment: (Reply)

	Boyes is full of baloney davea0511 on 09/27/2007 at 5:15 PM
Posts: 11 Avg Rating:	Boyes says energy from windmills and solar PV is easier, but there's no reason in the world why they couldn't build a larger turbine and generate all the electricity real-time and then store excess electricity the same way windmills and PV does. Solar thermal simply allows you to have an additional, and much cheaper, means of storage. Besides, energy needs never spike on a national scale, so distributing the nightime energy from stored heat is just as easy and effective as doing so from a battery or flywheel full of energy. Rate this comment: (Reply)

	why heat storage makes it cheaper davea0511 on 09/27/2007 at 5:20 PM
Posts: 11 Avg Rating:	The author seemed confused as to why heat storage makes the whole system cheaper. The answer is simple: 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. Rate this comment: (Reply)

	Re: why heat storage makes it cheaper ChuckInReno on 10/01/2007 at 12:23 PM
Posts: 10 Avg Rating:	Maybe cheaper, but less efficient. You need some way to convert the energy from heat to mechanical energy. You'll get substantial second-law conversion loss, however you do that conversion. I guess that's the attraction of molten salts; high temperature, higher efficiencies. 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. Rate this comment: (Reply)

	Re: why heat storage makes it cheaper Siphon on 10/09/2007 at 3:58 PM
Posts: 65 Avg Rating:	What Ausra is working on, is simply storing the heat transfer fluid - that is, H2O - in an underground cavern. No need for heat exchangers, which lowers capital costs and system efficiency as well. Supposedly they can use passive direct boiling which could also lower pumping losses. Combining this with an inexpensive 240 MW nuclear turbine, where the water is flashed directly into steam, could be a particularly cost effective combination of technologies. If they pull it off that is. 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. Rate this comment: (Reply)

	Nice trick jrc55 on 09/27/2007 at 7:22 PM
Posts: 1	It must be quite a trick to keep the pipes fed with just the right amount of water to heat-up, and not allow it to boil away to the point where the pipes run dry and maybe crack at the seams. Probably couldn't have been done 20 years ago because it would need the advances in materials and computers that we have today. Rate this comment: (Reply)

	cost is what makes this important eak on 09/29/2007 at 9:42 AM
Posts: 8 Avg Rating:	What is important to understand is that any power generation technology lives or dies based on its cost effectiveness. The cheap always pushes out the good. Sometimes that means using low tech solutions instead of high tech ones. For example, they give up some efficiency in their solar concentrators to save cost. What is important about Ausra is their claims of near coal power prices when their designs are deployed on a massive scale. Thermal storage is part of that: as another comment pointed out, being able to store energy from the day and generate at night means you need half as many steam turbine for the same annual GWh, and that reduces $/GWh. Rate this comment: (Reply)

	This is not a solution ....Vegetation is a solution DJTal on 09/30/2007 at 6:36 AM
Posts: 109 Avg Rating:	i get tired of hearing how wonderful all these solar power collectors are ..... vegetation has been using the sun's energy for billions of years , very efficiently , quietly , attractivly , in a perpetual state of motion , in a way which removes CO2 from the atmosphere and converts the energy into a form which can be STORED for millions of years . Lets hear it for plants . We would do far better to invest all this money in building up biomass production per acre and building up the amount of carbon stored in the soil . Rate this comment: (Reply)

	Re: This is not a solution ....Vegetation is a solution eak on 09/30/2007 at 3:39 PM
Posts: 8 Avg Rating:	See Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass by 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. Rate this comment: (Reply)

	Re: This is not a solution ....Vegetation is a solution DJTal on 10/01/2007 at 4:57 AM
Posts: 109 Avg Rating:	Photosynthesis may have a fairly low efficiency however , direct sunlight isn't the only renewable energy which plants make use of . Fact of the matter is , all of the renewable energies play a role in creating life on earth , geothermal , wind , wave , tidal , nuclear radiation . Mother nature got there before us , and she can use all of these energies and others that that we aren't normally aware of . If you add on the cost of reducing CO2 levels to the cost of this new solar tech or any other which doesn't have the ability to reduce CO2 levels the cost of the energy increases . Unless this new technology is a means of taking carbon out of the atmosphere and turning it back into solid material which can be stored long term in the soil , it will not answer the biggest problem we face . There is also the issue of energy reserve . Biofuel is the best means that we have of building large reserves of energy which we WILL need to maintain economic stability , in order to cope with fluctuations in supply and demand or to cope with some future unforseen emergency situation . What are the priorities for us ?.... : the need to reduce CO2 levels , the need for sustainable renewable energy , the need for a reliable source of energy , the need for energy reserve .... bioenergy meets all of these demands and many more in a wider context of worldwide development . Rate this comment: (Reply)

Re: This is not a solution ....Vegetation is a solution