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Solar Thermal Power May Make Sun-Powered Grid a Reality

It's solar's new dawn. For five decades solar technologies have delivered more promises than power. Now, new Breakthrough Award–winning innovations are exiting the lab and plugging into the grid—turning sunlight into serious energy.
Published in the November 2008 issue.

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Parabolic Solar Trough: The long mirrors in parabolic trough plants are designed to focus incoming sunlight onto a narrow, liquid-filled tube that runs parallel to the array. At the Nevada Solar One plant, 180,000 mirrors help heat a mineral-oil transfer fluid to 735 F.

Modularity has other benefits, too. Since each 25-kw SunCatcher has its own Stirling engine producing electricity, there’s no single point of failure. “If something goes wrong with one dish, it doesn’t matter,” Osborn says. In contrast, the thousands of mirrors in a parabolic trough plant all feed a central turbine, so when the turbine is down for maintenance, power production stops. The SunCatcher design also shortens the wait for power during construction: Electricity will flow once the first 40 are built—a “solar group” that can churn out 1 Mw.

The breakthrough efficiency of the dish results from focusing the sun’s rays on a single spot instead of on a long pipe, which allows temperatures to reach 1450 F, compared to 750 F for parabolic troughs. In addition, the Stirling engine has a relatively flat effi­ciency curve: It produces close to maximum output even when the sun is obscured or low in the sky. So while the record 1-hour effi­ciency achieved earlier this year was 31.25 percent, the SunCatcher’s full-year, sunrise-to-­sunset efficiency is still a respectable 24 to 25 percent, roughly double that of parabolic trough systems.

Another twist on CST designs confronts the challenge that dogs every solar power scheme: “When the sun sets, that’s it for the day,” as Tisdale puts it. “But in Arizona in midsummer, it’s hot as hades, so people have their a/c cranked until 9 or 10 in the evening.” A hot liquid can be stored more efficiently than electricity; the analogy used by one industry executive is that a $5 thermos can hold as much energy in the form of heat as a $150 laptop battery can store electrochemically. Two 50-Mw plants that should begin operations by the end of this year in Spain will operate on this principle, using what amounts to a giant thermos filled with molten salt.

In the U.S., a thermal storage facility is scheduled for completion in Gila Bend, Ariz., in 2011. The 280-Mw Solana plant, being built by Spanish company Abengoa Solar, will use a parabolic trough design, but will incorporate a thermal storage tank that can keep the plant running for 6 hours with no sun. “We could design a plant that runs 24 hours a day,” says Fred Morse, an adviser for Abengoa who was formerly the Department of Energy’s solar czar, “but that would make no economic sense.” Instead, the plant is designed to cover Arizona’s peak energy-use periods, when power is most expensive.

A Matter of Scale

The enormous scale of the Abengoa and Stirling Energy plants provides an answer to skeptics who doubt whether a few rooftop panels here and there can ever play a meaningful role in the world’s energy portfolio. But size also creates its own set of problems. For one thing, the power has to be transmitted to where it’s needed, and the empty deserts best suited for sprawling CST plants tend to be in the middle of nowhere. The site of Stirling Energy’s future plant for the San Diego market currently has enough transmission capacity for 300 Mw, or 12,000 dishes. The remaining 24,000 dishes will be built only if San Diego Gas & Electric is able to complete a proposed 150-mile transmission line between the plant and the city.

Water use is another issue. CST plants with steam turbines can require hundreds of millions of gallons of water to cool their con­densers—a challenge in regions where water is already at a premium. In this respect, Stirling Energy’s hydrogen­-based system has a significant advantage, since it only uses water to rinse the mirrors every few weeks. Osborn estimates that the San Diego plant, when producing power for 500,000 households, would use the same amount of water as 33 average homes.

Utility-scale solar power also requires enormous capital, which keeps it out of reach of people in the developing world, where such solutions are desperately needed. That’s a challenge RawSolar, an MIT spinoff, is trying to meet with a dish that is just 12 ft. wide, and simple and cheap enough to make for stand-alone operation. The nonprofit Solar Turbine Group, another MIT spinoff, built an even more bare-bones mini-CST system in Lesotho last summer, using spare car parts for the heat engine.

The most natural fit for small-scale solar, though, is the good old photovoltaic cell. It takes in sunlight and spits out electricity with no moving parts, requires no water and can be situated wherever electricity is needed, to avoid transmission losses. PV panels can generate useful amounts of electricity even in the weaker sunlight of northern states where big CST plants aren’t practical. Also, they’re ideal for homeowners, since they are simple to install and maintain—in fact, integrated building materials like PV roof tiles will make new homes even easier to connect.



Reader Comments
32. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
This article ignores the contribution that solar-thermal can make to individual buildings in the form of hot water and space heating, which are responsible for up to a third of an individual's carbon emissions. These systems are extremely reliable when designed and installed by professionals, and will easily last 20 to 30 years. However, they have the downfall that one really only gets full use from the solar field for half the year (more if you live in the far north), because you don't require nearly as much thermal energy during the summer. Therefore, your large investment in solar equipment does not generate income as often as a similar investment in PV, though in today’s era (well maybe yesterday’s, we’ll see) of high fuel prices for home heating, the solar-thermal system pays off very quickly when it is producing energy at full capacity. The other issue is that a PV system can also produce heat, albeit at low sun to heat efficiency. However, if one can build a Stirling engine that operates at a relatively small temperature difference, with much lower temperatures then what the Suncatcher engine uses, it becomes possible to operate a small (1kW range) engine from a home solar field. There are two types of domestic solar heating panels in wide use: flat panels and evacuated tubes. The evacuated tube collectors, while more expensive than flat plates, offer very high efficiency for conversion of solar energy to thermal energy (70% efficient by aperture, or actual illuminated area), and can reach temperatures of nearly 550F during the summer if provided with hot fluid or allowed to ‘stagnate’ (IE no fluid moving in the collector). They will also boil water comfortably on a windy, 0F day if there is reasonably good sunshine. This is more than hot enough to drive what is called a ‘low-delta-T’ Stirling engine. The efficiency of a heat engine is theoretically determined by the temperature difference between the hot and cold areas of the engine, and with a temperature difference of roughly 400F, one could get an engine that is 38% efficient. The best Stirling engines that I know about have run at 60% or so of the ideal maximum, due to losses in the drive-train, from gas drag inside the engine, and thermal conduction from the hot to the cold end of the engine. So, if one creates an engine that runs at ½, or 50% of the theoretical limit of efficiency, one gets about 20% conversion of heat energy to electricity. Ultimately, the system can generate electricity at between 10 and 14% efficiency, which is very similar to existing PV systems. However, we now get to have a system on our house that does two things, namely heat the home and domestic water supply at very high efficiency, and also generate electricity with for personal use or net-metering back into the grid at reasonable efficiency for a solar system. The system I talked about above is under development at a Boulder, CO company where I work, Cool Energy, Inc. We have proven that it is very much possible to get domestic solar-thermal panels to generate temperatures in excess of 400F, and also that a Stirling engine can run and produce power (over 200W so far) at this temperature. Go check out our website, www.coolenergyinc.com, to learn more about home combined heat and power systems.

31. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
This article ignores the contribution that solar-thermal can make to individual buildings in the form of hot water and space heating, which are responsible for up to a third of an individual's carbon emissions. These systems are extremely reliable when designed and installed by professionals, and will easily last 20 to 30 years. However, they have the downfall that one really only gets full use from the solar field for half the year (more if you live in the far north), because you don't require nearly as much thermal energy during the summer. Therefore, your large investment in solar equipment does not generate income as often as a similar investment in PV, though in today’s era (well maybe yesterday’s, we’ll see) of high fuel prices for home heating, the solar-thermal system pays off very quickly when it is producing energy at full capacity. The other issue is that a PV system can also produce heat, albeit at low sun to heat efficiency. However, if one can build a Stirling engine that operates at a relatively small temperature difference, with much lower temperatures then what the Suncatcher engine uses, it becomes possible to operate a small (1kW range) engine from a home solar field. There are two types of domestic solar heating panels in wide use: flat panels and evacuated tubes. The evacuated tube collectors, while more expensive than flat plates, offer very high efficiency for conversion of solar energy to thermal energy (70% efficient by aperture, or actual illuminated area), and can reach temperatures of nearly 550F during the summer if provided with hot fluid or allowed to ‘stagnate’ (IE no fluid moving in the collector). They will also boil water comfortably on a windy, 0F day if there is reasonably good sunshine. This is more than hot enough to drive what is called a ‘low-delta-T’ Stirling engine. The efficiency of a heat engine is theoretically determined by the temperature difference between the hot and cold areas of the engine, and with a temperature difference of roughly 400F, one could get an engine that is 38% efficient. The best Stirling engines that I know about have run at 60% or so of the ideal maximum, due to losses in the drive-train, from gas drag inside the engine, and thermal conduction from the hot to the cold end of the engine. So, if one creates an engine that runs at ½, or 50% of the theoretical limit of efficiency, one gets about 20% conversion of heat energy to electricity. Ultimately, the system can generate electricity at between 10 and 14% efficiency, which is very similar to existing PV systems. However, we now get to have a system on our house that does two things, namely heat the home and domestic water supply at very high efficiency, and also generate electricity with for personal use or net-metering back into the grid at reasonable efficiency for a solar system. The system I talked about above is under development at a Boulder, CO company where I work, Cool Energy, Inc. We have proven that it is very much possible to get domestic solar-thermal panels to generate temperatures in excess of 400F, and also that a Stirling engine can run and produce power (over 200W so far) at this temperature. Go check out our website, www.coolenergyinc.com, to learn more about home combined heat and power systems.

30. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Website: http://blog.mapawatt.com
To all readers new to clean tech, make sure you realize that solar thermal power generation, is much different that residential solar thermal, which heats hot water for your showers: http://blog.mapawatt.com/2009/02/24/solar-thermal-intro/ While stirling engines are great, I dont expect to see one of these in anyone's backyard anytime soon!

29. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
How many times do we have to say it?!! We don't need a solar grid! We need 5-10 new megawatt nuclear reactors with a modernized distribution grid. Nuclear is the cheapest power source that provides the highest level of energy. This administration is simply anti-nuclear, anti-clean coal and anti-fossil fuel with scrubbers at the expense of the economy and our nations growning demand.

28. How much does it cost?
How much does one of those things cost? I would love to install one in my backyard. Considering that to put solar panels on my house would cost 40k, these sound like a better deal

27. Poor siting
This plan is to put all 36,000 units in one place and build a powerline thru a wilderness reserve. Why not distribute these units and reduce the impact?

26. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
The idea of parabolic reflectors has been around since the 50s. I remember a unit that was around 25kw in the 50s or60s. Concentration is fine, however a storage system is needed to have 24-7 service. We also need transmission of the power . Think of the wind and waves, power generated that travels 1000s of miles naturally, this is the answer. Think of the broad picture one system is not the answer, in nature everything interacts. An one system approach is not going to work. If we work together we can solve this energy crisis.

25. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
RE:20. The Federal Government is the primary funder of research and we all benefit from this. The States governments have different policies regarding tax-breaks for going solar. It is real money. A person who can afford to pay for solar usually has to pay taxes too. Going solar in most states results in credits that are virtually money-in-the-pocket.

24. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
RE:22 A simple concentrating collector in the form of a trough designed to reflect the sunlight on a tubular reciever supported along its length is far easier to manufacture than a parabola and the temperature range generated be the smaller ratio of collector-to-reciever area in the trough collector is much more ameanable to heating water. A simple black flat plate collector will raise the water to pool temps, a trough collector will give real hot hot-tub temps, and a parabolic array can boil too easily and cannot be designed as a gravity driven passive circulation heating system.

23. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
RE: 1. "Control of RPM of solar stirling engine..." The operating RPM range is designed into the unit with the engine sized to accomodate the relatively narrow power band of the generating array of mirrors; the torque load can also be increased to limit RPM by the action of a control unit programmed to require more current from the actual electrical generator. RE:4. A flat panel array that tracks in two axes is actually the most efficient method of capture. If PV technology had 30% efficiency it would have the advantage, at 50% it would solve the energy needs of tomorrow. Parabolic collectors are by their nature concentrators. They are far more effective at stepping up the thermal energy to a level that can be utilized to produce mechanical energy, or work. A parabolic solar array requires constant tracking adjustment and the systems are more complicated in their manufacture than a flat plate thermal collector. RE:7. A home system using a concentrating collector is most efficient when used to heat the home in cold seasons in the Northern regions of our hemisphere. It can be manufactured to power AC units with ammonia cycle refrigeration as can the ref-freezer appliances by using a holding plate system and good insulation. A block sized approach is also practical in that a few homeowners can invest in never having to pay another bill for power and reap the economies of scale. RE:8 Physics states that a body will share its thermal energy with its environment and that if that immediate environment is shared, the bodies will eventually reach thermal equilibrium. The factor here is eventually, a timed event that has many varibles influencing duration of time till full equibrilium is reached. While the solar energy is feeding a concentrator, it is being captured and multiplied. The focal point of all that energy is a ratio of the exposed collector area and the area of the receiver, less losses due to mirror efficiency and angle, and reflection from the reciever material and etc. The collector is both receiving and radiating energy. If this was an equal process it would generate no gain in heat at all. But while there is input into a concentrating array, it will multiply the input and concentrate it in a smaller area. A "perfect" parabolic mirror of sufficient size aimed at the sun on a clear day can generate more heat than the surface of the sun because of this multiplying effect. A flat plate collector of sinitered tungsten on steel will reach maybe 150 F here on earth and more above the atmosphere, as it travels toward the sun the inverse of the law of diminishing light waves says that the received light per unit will increase as the plate gets closer with it reaching almost to the 10,000 surface of the sun before it is burnt to atoms by the much hotter chromosphere above the surface. So suffice to say that our source of radiation for the concentrating collector is far above the surface temperature of the sun itself. RE:9. Silkscreen printed solar film has yet to reach anywhere the efficiency per square unit of area or long term durability. Their advantage is in their economy. Maybe one day paint that generates power will be put everywhere. RE:10. Do some research of your own. Just do it. Search for company names and write them using your key terms. Shop around. Steam is corrosive and the systems are far from user friendly and require much maintenance. PV and storage is a great investment of capital at 30 KW. My personal advice to you...Think of every way you can conserve and minimize your actual consumption and size your system accordingly. If you use an actual 30 KW you need much more capacity in the array. The sun is on for less than half a day and its intensity is graphically like a bell-shaped curve from sunrise to sunset. This info is on the web. Go with two-axis tracking! Good luck.

22. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Is is possible to use something like a dish to heat a swimming pool up in cooler months? It might sound silly but lots of people spend a small fortune on heating an outdoor pool. Why not aim a big magnifying glass at the water - as we did with bugs as a kid? :)

21. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Damn, pretty much had this exact idea running through my head for at least a year now.

20. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Here's a thought. since the government likes to bail out company's. would'nt it be nice if they would help home owner's pay for solar panels on there house.I'm sure this would help create jobs, cut down on burning fuel, gases. this would be very helpful to cut down on greenhouse gases.

19. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
I'm glad to see work that started at Ford Aeronutronics by Hal Haskins in Ca in the 80's and funded by Jimmy Carters DOE is getting closer to the market. This is a very difficult problem having it freeze every nite and go to 12,000 degrees during operation is High tech. Plus at that high of a temperature almost every thing either reacts or is transparent to the Hydrogen that runs the cycle. My hats off to them for solving problems that have stumped others.

18. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
For more information on the solar thermal power plants I found very interesting the studies of Italian Physics Nobel prize Carlo Rubbia and its practical applications in Italy, Spain and in other Mediterranean countries (called Project Archimedes after the Greek Mathematician who set on fire the enemy boats with large concentrating mirrors during the siege of Syracuse in Sicily around 212 BC) As a rough estimation, 1 sq meter of mirror surface produces in one year the equivalent energy of a barrel of oil.

17. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Since this revolutionary development is the breakthrough in the production of low cost power, will it become available in New Zealand? We could certainly use it as power charges are spiraling out of control here!

16. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Its great technology, but I doubt it will make PV obsolete. PV works better in cloudy light, CSP or CST does not. i.e. is requires direct sunlight. CSP / CST can store thermal energy more cheaply than PV. PV allows greater decentralisation of power and more domestic application. They need a balanced approach to solar, which is what we have and that's great.

15. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Solar energy is the most reliable one,I would like to set up in our country with least investment for selling to villagers as affordable price

14. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Follow the MONEY. Like the ill fated electric cars of a decade ago. There will be no buyers and manufacturers will cease research, development, production and go out of business unless large numbers of buyers , using their own money, will purchase a cost effective, economically practical product. A few greenies whooping and hollering about the environment and their great ideas for the rest of us are not going to get anywhere.

13. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
There is no doubt that the current unstable oil prices and definite reduction in volume due largely to increased demand will make solar/photovoltaic film technology unavoidable.Please link me up with companies with financial info for comparison and those ready to investn in Africa

12. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
I disagree with comment #6. The 1'st law of thermodynamics states that the change in the systems internal energy (the dish) is the outcome of the energy intake (from the sun) and the work done by the system (dish) on its surroundings. There WILL be a thermal equilibrium between the dish and the sun but not necessarily a temperature equilibrium because thermal energy taken in here is being converted to work. By the way, I think these innovations will really pave the way for efficient energy use in the next few years.

11. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
please use mirrors to enrich sun light in photovoltaic panels

10. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
I have money to invest in a 30 MW system; is there a web site featuring a cost comparison study regarding land use requirements, total cost of equipment, installation cost estimates with annual operational expenses inclusive of debt service with return on investment for the top 5 or 10 solar generated electricity via either Photo Voltaic or Steam generation?

9. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
How does this compare with Nanosolar in San Jose, CA, which rolls off thin sheets of photovoltaic cells at a fraction of the cost of regular PV panels?

8. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Uhm...basic physics demands that the maximum "concentrated" temperature cannot exceed the temperature of the surface of the sun--which ever is hotter will radiate its heat back to the other. I.e. the concentrated point will, very efficiently, radiate excess heat back through the mirror back out toward the sun. So the maximum theoretical limit is 1 sun, not 13,000. On the other hand, a source of 4000 to 6000 degree heat is nice!

7. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
I have been wanting to build one of these in my yard. Though, my neighbors might not like it when a bird flies through the concentrated sunlight and lights on fire then lands on their house.

6. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Website: http://www.iedu.com/DeSoto/solar.html
Good stuff! It's about time solar thermal got serious attention!

5. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Why don't you consider testing this equipment in the Atacama desert of Antofagasta, Chile, which has the highest solar radiation rate on Earth ( 1 KWH/m2 at sea level and 2KWH/m2 at 13,000 feet elevation? Energy potential is enormous compared to other places in the world. Please think about it. We would be happy to cooperate. Thank you.

4. Very hot solar power systems
It would be nice to compare solar panels with this parabola approach. That way its clear if there is an advantage over flat panels. from tony

3. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
"Two 50-Mw plants that should begin operations by the end of this year in Spain". The plants mentioned here are the Andasol plants in Granada in Spain. Actually first plant to start autumn 2008, second plant is in spring 2009.

2. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
Website: http://thealternativeenergyinvestor.blogspot.com
I have been saying for months that technology like this will make poly-silicon solar obsolete. Great Post!

1. RE: Solar Thermal Power May Make Sun-Powered Grid a Reality
For the stirling engine how do they control the rpm of it?

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