SciAm Article: Solar Thermal Efficiency Only ~25%

[joe1347]

http://www.sciam.com/article.cfm?ar...ADDFDBEC8D41&chanId=sa013&modsrc=most_popular

September 19, 2007
Sunny Outlook: Can Sunshine Provide All U.S. Electricity?
Large amounts of solar-thermal electric supply may become a reality if steam storage technology works—and new transmission infrastructure is built
By David Biello

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.

"The maximum you can get into the grid is about 25 percent from solar," including photovoltaics

Scientific American has a recent article on Solar Thermal and besides the typical hype from the solar companies, I was surprised to read that solar thermal power plants (for generating electricity) are currently limited to about 25% efficiency.

I wonder what limits the overall efficiency in a solar thermal system? A quick look at figure from Schott on the evacuated glass solar thermal receivers indicates about 81% of incident (concentrated light) is converted to heat (95% absorbed - 14% emitted = 81% efficiency). I'm guessing that the concentrator optics are about 80 to 90% efficient, which then translates to about 70% conversion efficiency (to heat) for incident (unconcentrated) light. This seems to indicate that the thermal (hot oil or steam) conversion to electricity (heat exchanger/turbine) is about 33% (33% x 70% = 25% total system efficiency).

http://www.us.schott.com/solarthermal/english/products/receiver/details.html

The absorber tube (SCHOTT), which is made of steel and is located on the inside, must be capable of absorbing a lot of solar radiation without emitting significant amounts of heat. To achieve this, SCHOTT developed a coating that offers an absorption rate of 95%. At a temperature of approximately 400° Celsius, only a maximum of 14% of the total heat is emitted.

I believe that thermodynamic efficiency is increased if the temperature is increased, so wouldn't a larger parabolic or fresnel concentrator heat up the oil (or steam) to a higher temperature and thus improve efficiency? Desert land and mirrors seem to be fairly inexpensive. Or is the 400C temperature a maximum for some other reason. Such as, above 400C the heat emitted (lost) increases rapidly or does the downstream hardware fail in some way because it is unable to handle higher (>400C) temperatures. Alternatively, is there a law of diminishing returns associated with concentrator optics?

 

[Dymanic]

I was surprised to read that solar thermal power plants (for generating electricity) are currently limited to about 25% efficiency.

Well, I thought that was "including photovoltaics", which are gradually increasing in efficiency with technological advances (providing a possible spin opportunity here through the use of older efficiency numbers). I'm not sure if it was intended as a statement about fundamental limitations.

You raise some interesting questions, and I'll be watching for responses from those knowledgeable about these matters. One question I would add is: what is the energy return on investment for such a system (the amount of time required for such a system to produce an amount of energy equal to that required to manufacture and install it), and what is its expected serviceable lifetime?

 

[GodMark2]

Scientific American has a recent article on Solar Thermal and besides the typical hype from the solar companies, I was surprised to read that solar thermal power plants (for generating electricity) are currently limited to about 25% efficiency.

Well, in context, it becomes clear that the article was not saying that.

If those claims stand up, however, solar-thermal plants could provide a significant chunk of the Southwest's—and potentially the nation's—electricity. "The maximum you can get into the grid is about 25 percent from solar," including photovoltaics, Mills says.

The 25% figure is the amount of energy that can be supplied from solar sources compared to the total energy needs of the nation. Mills goes on to state that upcoming technology in electrical storage may increase this figure, which seems to indicate that the fact that the sun isn't shining everywhere all the time is the limiting factor. Electricity on the grid needs to be produced at the same time that it's consumed, so night is a problem for solar operations.

 

[joe1347]

Well, in context, it becomes clear that the article was not saying that.

The 25% figure is the amount of energy that can be supplied from solar sources compared to the total energy needs of the nation. Mills goes on to state that upcoming technology in electrical storage may increase this figure, which seems to indicate that the fact that the sun isn't shining everywhere all the time is the limiting factor. Electricity on the grid needs to be produced at the same time that it's consumed, so night is a problem for solar operations.

Possibly that is what the article meant, but a little more searching around turns up similar efficiency numbers (linked below) to what was originally posted. Converting the hot oil (or steam) to electricity seems to be the most inefficient part of the process. The original questions still appear to be valid:

1. I believe that thermodynamic efficiency is increased if the temperature is increased, so wouldn't a larger parabolic or fresnel concentrator heat up the oil (or steam) to a higher temperature and thus improve efficiency?


2. Desert land and mirrors seem to be fairly inexpensive. Or is the 400C temperature a maximum for some other reason. Such as, above 400C the heat emitted (lost) increases rapidly or does the downstream hardware fail in some way because it is unable to handle higher (>400C) temperatures.


3. Alternatively, is there a law of diminishing returns associated with concentrator optics?

http://www.volker-quaschning.de/articles/fundamentals2/index_e.html

Trough Power Plant Efficiencies

The efficiency of a solar thermal power plant is the product of the collector efficiency, field efficiency and steam-cycle efficiency. The collector efficiency depends on the angle of incidence of the sunlight and the temperature in the absorber tube, and can reach values up to 75%. Field losses are usually below 10%. Altogether, solar thermal trough power plants can reach annual efficiencies of about 15%; the steam-cycle efficiency of about 35% has the most significant influence. Central receiver systems such as solar thermal tower plants can reach higher temperatures and therefore achieve higher efficiencies.

 

[Dilb]

There are a number of problems at higher temperatures.

You need a collector which can withstand high temperatures, has a high thermal conductivity and will survive being left in the desert. Metals will have a tendency to oxidize (steel) or melt (aluminum). Nonmetals tend to be terrible conductors of heat, and even certain types of steel, especially stainless, aren't very good thermal conductors.

Second, you need a liquid to transfer heat to the steam. Directly heating the steam isn't going to work, as there won't be nearly enough surface area. Gases won't have enough heat capacity to be useful, so a liquid is needed. At several hundred degrees heavy oil can be used, but going higher means you need something else. Something else is generally either a liquid metal, or a liquid salt. Both of these are much harder to work with than oil. If I remember the problems correctly: salts tend to be extremely corrosive, requiring special alloys. Liquid metals are extremely reactive (sodium and potassium), or incredibly toxic (mercury).

Increased heat loss probably isn't an issue. Conduction and convection are both roughly linear with temperature, and radiation is fairly small below 1000C so long as the design is reasonable.

As to the gains, a rough estimate can be made with the Carnot efficiency
[latex]$1-\frac{ T_{cold}}{T_{hot}}[/latex]

The cold side is probably ~300 K, so at:
700 K (400 C) eff = 58%
1000 K (700 C) eff = 70%

And so on, with reducing gains for higher temperatures. Actual efficiencies tend to be more like a half of the Carnot efficiency, for good systems, and a third for not so great systems. Best-in-the-world power plants might be closer to two thirds the Carnot efficiency. Solar collectors, being rather small (compared to gigawatt power plants, for example), will be closer to the "not so great" level.