STEAM AND MIRRORS: A compact linear Fresnel reflector, like Ausra's plant in Australia pictured here, uses lines of mirrors to focus the sun's rays on an overhead trough, turning water into steam to generate electricity.
Image: COURTESY OF AUSRA
In the often cloudless American Southwest, the sun pours more than eight kilowatt-hours* per square meter of its energy onto the landscape. Vast parabolic mirrors in the heart of California's Mojave Desert concentrate this solar energy to heat special oil to around 750 degrees Fahrenheit (400 degrees Celsius). This hot oil transfers its heat to water, vaporizing it, and then that steam turns a turbine to produce electricity. All told, nine such mirror fields, known as concentrating solar power plants, supply 350 megawatts of electricity yearly.
In the face of mounting concern about climate change, alternatives to coal and natural gas combustion such as these never seemed more attractive. And with the bounty of the sun waiting to be captured near fast-growing major centers of electricity consumption—Las Vegas, Los Angeles and Phoenix, among others—interest in such solar thermal technology is on the rise. The first such plant to be built in decades started providing 64 megawatts of electricity to the neon lights of Vegas this summer.
But physicist David Mills, chief scientific officer and founder of Palo Alto, Calif.–based solar-thermal company Ausra, has bigger ideas: concentrating the sun's power to provide all of the electricity needs of the U.S., including a switch to electric cars feeding off the grid. "Within 18 months, with storage, we will not only reduce [the] cost of [solar-thermal] electricity but also satisfy the requirements for a modern society," Mills claims. "Supplying [electricity] 24 hours a day and effectively replacing the function of coal or gas."
The company insists it can do this at a cost of just 10 cents per kilowatt-hour, analogous to the price of electricity from burning natural gas in California if a cost was imposed for the emission of carbon dioxide, the leading greenhouse gas (as the state's Public Utilities Commission is considering).
Ausra will rely on a different type of concentrating solar power plant to deliver on this promise. French physicist Augustin Fresnel showed in the 19th century that a large lens, like the parabolic troughs of the existing solar-thermal plants, can be broken down into smaller sections that deliver the same focus. Applying this, Mills's design—a compact linear Fresnel reflector—allows for greater ground coverage, lower weight and greater durability than precision-shaped parabolic mirrors. "You can drop stones on it and they bounce off," Mills says. "We would be able to build these in Florida in the hurricane zone."
This Fresnel solar thermal plant also eliminates oil, directly heating water to a lower temperature of roughly 535 degrees F (280 degrees C) at a higher pressure, about 50 bars, or 50 times atmospheric pressure. Then, it uses the resultant steam to turn the same low-temperature turbines as those employed in nuclear reactors.
The amount of electricity produced is simply a function of the sun's bounty and the number of mirrors. "We're moving from 80- to 100-megawatt designs to 700 megawatts and above," says John O'Donnell, Ausra's executive vice president.
The key will be proving performance. Thus far, the company has exactly one solar array, hooked to a coal-fired power plant in Australia to provide extra steam that improves its efficiency at burning the dirty rock. At present, the Ausra mirrors produce just an additional 12 megawatts of extra heat, but there are plans to boost that as high as 38 megawatts thermal.
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11 Comments
Add CommentPlease note that power is measured in watts or kilo(mega)watts and energy in kilowatthours. So the statement the sun pours 8 kilowatthours is meaningless without a time. The statement 350 megawatts per year is equally inane. Please correct. Very sincerely cburckhardt@datacomm.ch
Reply | Report Abuse | Link to thisThe statement that "Thermal storage is generally considered to be quite a bit cheaper than electrical storage" is not a very relevant statement. Of course the statement is true because battery/capacitor electrical storage is not capable of large (5000 Gigawatthour) storage.
Reply | Report Abuse | Link to thisTo my knowledge, there are no large thermal energy storage systems in the U.S. However, there are over 20 Pumped Storage facilities which store the electrical energy produced in the form of potential energy for use at a later date. It is a proven technology with an overall efficiency (roundtrip) of greater than 72%. This is more efficient than the thermal energy storage proposed; and more practical as it is an existing technology.
Why would the author propose thermal storage?
Additional comments to this article now that I have time:
Reply | Report Abuse | Link to this1) When the author makes statements like "Within 18 months, with storage, we will not only reduce the cost of solar thermal electricity, but also satisfy the requirements for a modern society." and "The company insists it can do this for 10 cents/KwHr." make the author look ridiculous to any engineer who has worked in the energy field, especially when the company has "...exactly one solar array...hooked to a coal fired plant."
I have worked in the energy industry for 25 years, and can guarantee that this method of power cannot be produced for 10 cents/KwHr. My current Exelon rate is 9 cents/KwHr; and any country that forces "green" power makes its consumers pay the price. Electricity costs in Denmark (very green) are 29 cents/KwHr; Netherlands (very green)26 cents/KwHr; Germany (semi-green) 21 cents/KwHr.
This verifys that "green" power costs.
The author pitches thermal storage presumably because the costs of other storage technologies have been shown to be too expensive (including pumped hydro and compressed air energy storage). In 2003 Sandia National Labs estimated that these conventional storage methods could not reach a price below about 14 cents/kwh (with an input cost of 5 cent/kwh).
Reply | Report Abuse | Link to thisThe best known thermal storage demo was done by Sandia at the Solar Two power plant. It worked fine, but the main application is a very futuristic problem: if a region produced more solar electricy than it needed during the day, it could save some for the evening and night. That would be a nice problem to have, but we are not there yet.
> Sunny Outlook: Can Sunshine Provide All U.S.
Reply | Report Abuse | Link to this> Electricity?
Nathan2go
1. The last installation costs I had (I'm retired 6 years) for pumped storage were $1200/kw. What is the "projected" cost per KW (not Kwh as this is the sale price which does not break out the installation costs) for a 1000 MW thermal storage plant?
2. The reason to advocate pumped storage is because it has a round trip "average" efficiency of 72% (85% pumping/85% generating) which is much higher than Compressed Air Energy Storage (CAES) efficiency of 55% (maximum). The round trip efficiency for pumped storage could be increased to 81% if a higher efficiency generator/turbine is utilized. Please define what round trip efficiency the thermal storage can achieve. I'm guessing that it cannot be higher than 60%.
3. The main point is that there are over 16 "existing" pumped storage plants in the U.S.; many over 1000 MW; and more than 300 plants in the world. There are only "two" existing CAES plants in the world (110 & 290 MW); and no large thermal storage plants to my knowledge. Therefore, pumped storage plants are a tried and proven technology with zero emissions that works on a large (1000 MW plus) scale. What large advantage (efficiency/inexpensive/larger scale/etc.) does thermal storage have that makes it better than pumped storage?
Nitpicking:
Reply | Report Abuse | Link to thiswhen they say: "eight kilowatt-hours* per square meter" they mean 8 kilowatt-hours per day.
Mr David Clemen ... pumped storage is great, when and where it can be used, but reasons pumped storage isn't instantly recommended as ideal for these sorts of solar plants:
Reply | Report Abuse | Link to this1) water tends to evaporate in hot sunny areas, so efficiency of pumped storage in these sunny areas will be lower than in ideal locations.
2) solar power plants aren't likely to be located next to hydroelectric plants, and dry sunny areas like california don't have the water to use for this purpose anyway, they'd have to pump it in from elsewhere, which would again lower the net efficiency enormously.
3) if you use pumped storage a long distance away from the solar generator, then you have additional transmission losses.
Thats not to say pumped storage is not at all feasible for storing this solar power. Transmission of surplus solar power to a good pumped storage facility a long way away might still be quite efficient. But, maybe not as good a solution as storing power local to the solar generators - if steam storage can be improved. Thats all.
Mr. Ible Snover
Reply | Report Abuse | Link to this1. The efficiency of a pumped storage plant remains the same, whether it is located in a hot, sunny area or a cool area. The local temperature does not make any difference in the efficiency of the plant.
2. Evaporation is a concern in hotter climates; however, most pumped storage plants are situated in locales where they have approx. a 10% inflow, that is, the pumped storage cycle has additive water whether from a small stream, snowmelt, etc. that more than compensates for the evaporation in warm climates.
3. High voltage transmission of energy is very efficient. Transmission line losses should not exceed 5% of the total power transmitted, even for distances of 500 miles. If the transmission losses exceed this 5%, the transmission line was not designed properly.
4. Therefore, we come back to the crux of the matter:
a) Pumped storage is the most efficient energy storage system with an overall minimum efficiency of 72% versus the 50% efficiency of a thermal storage system (your "steam" system)
b) There are no large scale thermal storage systems in existence. Not a single one has been built.
c) There are over 400 pumped storage systems in existence, many larger than 2000 MW in size. This emphasizes that pumped storage is a mature technology vs. your "steam" system; and has design systems that have been perfected over many years.
Thermal storage is in fact extremely widespread in the form of hot water or hot oil used for domestic hot water and heating. Remember that most electricity is generated from steam turbines be it nuclear coal or gas power stations. Hot water is an efficient store of solar energy. In fact retrofitting solar preheating water before it enters existing power plants is the fastest and easiest way to get solar benefits and reduce fossil fuel use. Storing water at 80 to 95 degrees Centigrade (around 200 farenheit?) is much safer and cheaper than steam storage which is much more dangerous and costly. Steam solar power stations using gas turbines at night (why bother you have a grid?) would be far cheaper than fully 24/7/365 solar. The real point is to be pragmatic to optimize benefit and minimize cost.
Reply | Report Abuse | Link to thisAddendum: I should have added that hot water is an extremely cheap and dense energy store. To raise the temperature of water from soil temperature (around 20 degrees C) to just short of boiling (99 degrees) takes 79 x 4200J per kg.to heat a tonne of water to boiling point takes around 332MJ of heat energy. A tonne of water cost around 50c here in Australia and will be recycled. The cost of storage of the hot water for a solar power plant or a hybrid system would be very little as lagged underground tanks would be the cheapest and simplest method. I'm not an engineer but I suspect that storing hot water underground would be an easy retrofit for most existing power plants and much cheaper than steam or pumped storage. Recycled heat from steam turbines could be used to maintain and augment solar heating and increase efficiency. Saline solutions can improve heat densities and efficiency but increases costs and complexity.
Reply | Report Abuse | Link to thispspicerwensley
Reply | Report Abuse | Link to thisAlthough I do not dispute a number of your facts (water is a cheap and dense energy storage, etc.), I need to know the following to evaluate your plan:
1. Overall efficiency (from storage to electrical output) to be compared with the 85% overall efficiency of pumped storage, or 55% efficiency of Compressed Air Energy Storage (CAES). The efficiency of this scheme would be connected to how much energy must be added to the hot water to make steam for the inlet of the steam turbine; and the efficiency of this turbine.
2. Installed Cost of this scheme in dollars/kw to be compared to $1200/kw for pumped storage or approx. $2000/kw for CAES
3. Are there any existing plants in the Megawatt range utilizing this scheme?
4. Are there any plans for pilot plants utilizing this scheme?
If you don't have answers to the above questions, then the various engineering firms/studies must have decided the process is either a) inefficient or b) cost prohibitive on a large scale. That doesn't mean that it won't work on a household, or small scale; and more power to you if it is functional on a household basis. We all know that solar power heating is one third the cost of solar powered electrical systems; and significantly more efficient (50%) versus the solar electrical (15-20%)