New approach to solar power with hybrid solar-thermoelectric systems

Oct 21, 2011 by David L. Chandler
This laboratory setup was used to test the principles behind Wang and Miljkovic's concept for a hybrid solar thermoelectric system that could deliver both electricity and heat. Photo: Dominick Reuter

Systems to harness the sun's energy typically generate either electricity or heat in the form of steam or hot water. But a new analysis by researchers at MIT shows that there could be significant advantages to systems that produce both electricity and heat simultaneously.

The new study incorporates thermoelectrics — devices that can produce an electric current from a temperature gradient — into a concentrating solar thermal system, also called a parabolic trough. Such systems use long, curved mirrors (the trough) to focus sunlight onto a glass tube running along the centerline of the trough. A liquid pumped through that tube gets heated by the , and then can be used to produce to drive a turbine, or used directly for space heating or industrial processes that require heat.

The new MIT study “shows a unique opportunity for thermoelectrics integrated within solar thermal systems,” says Evelyn Wang, associate professor of mechanical engineering at MIT, who was co-author of a paper describing the potential for such hybrid systems in the journal Solar Energy.

The novel arrangement proposed by Wang and graduate student Nenad Miljkovic embeds a thermoelectric system in the central tube of a parabolic-trough system so that it produces both hot water and at the same time. The key to making this work is a device called a thermosiphon that draws heat away from the “cold” part of a thermoelectric system, maintaining its temperature gradient.

Wang and Miljkovic’s system would modify a parabolic-trough system’s central tube into a series of concentric tubes: A narrower tube inside the first would contain the thermoelectric material, with an even narrower tube at the center of the apparatus housing the thermosiphon, passively transferring heat from the thermoelectric cold side and alleviating the need to pump cooling fluid as in a conventional parabolic-trough system. The heat carried away by the thermosiphon could then be used to heat water for space heating, industrial processes or .

One advantage such a system has over traditional photovoltaics (devices that generate electricity from sunlight), Wang says, is that “thermoelectrics can be much cheaper than photovoltaics.” Also, conventional solar cells do not operate well at high temperatures. But, she explains, thermoelectrics thrive in hot conditions, which allow them to build up a greater temperature gradient.

“There really is no solar system now to do combined electricity and heat production at high temperature,” Miljkovic says. But, he adds, “there are companies actively trying to pursue this.”

“There’s an opportunity for bringing together different technologies,” Wang says. The thermosiphon, which draws heat from one place to another just as a siphon draws liquid, is “a passive way to transfer heat … and can be low cost as well,” she says.

Thermosiphons are typically filled with materials that undergo a phase change (usually from liquid to vapor) as they heat up, and can achieve a thermal conductivity — a capacity for transferring heat from one place to another — “much higher than any solid material,” Wang says. “It’s an efficient way to carry away the heat, to whatever you want to deliver it to.”

Wang and Miljkovic devised a computer model to search for optimal combinations of existing materials for the thermoelectrics and the thermosiphon. This model allows different combinations to be tested at varying operating conditions to make the overall system as efficient as possible.

A system for a single house could provide both heat and electricity, Wang says. “In a house, you need a lot of heat, but you only need so much electricity,” she says. While the thermoelectric efficiency of such a system is relatively low, “in a household system you don’t need that much power” relative to , she says.

Abraham Kribus, a professor of mechanical engineering at Tel Aviv University in Israel who was not involved in this research, says this paper “describes a fresh approach to solar energy conversion, with optimistic results showing high theoretical conversion efficiency.”

Kribus adds that because this is still an early stage of analysis, it’s not yet clear how such a system would stack up to traditional solar systems on cost and reliability. But that’s not a criticism, he says: “This is the situation at early stage with every nonconventional idea. … Overall, the paper shows a nice start and a very capable team behind it.”

Wang agrees that it is likely to take a few years to develop a practical implementation of these ideas. She and Miljkovic are going ahead with “working on building a system to demonstrate” how the combination could work, she says.


This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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Eikka
3 / 5 (4) Oct 21, 2011
thermoelectrics thrive in hot conditions, which allow them to build up a greater temperature gradient.


Not really. The maximum temperature for the typical PN junction is around 150 degrees C, and pretty soon after that it stops working.

The major stumbling block in building these kinds of systems is the cooling loop, that has to have a low-enough thermal resistance to actually sink the heat.

If you have water at 80 degrees C on the "cold" side, and the thermosiphon necessarily impending heat flow in the middle, raising the temperature another 20 degrees at the thermocouple, and the solar concentrator is limited to 150 degrees maximum so as to not destroy the thermocouple, your dT is going to be just 50 degrees.

The ideal carnot efficiency over that range is about 12% but any practical thermoelectric device we have right now isn't going to give you half of that.

So what you really get is a lot of heat, and very little electricity for a high price of a complex system.
88HUX88
not rated yet Oct 21, 2011
amen to Eikka - and what about the temperature rise along the horizontal cooling pipe, fluid has to flow along the centre so it will form a gradient as heat is added. This is for a trough as described in the article, a single mirror wouldn't have that problem.
Isaacsname
not rated yet Oct 21, 2011
So would something like a vortex tube be useful in a system like this ?

http://en.wikiped...tex_tube

Type slowly, I'm still learning very basic science :)
dschlink
not rated yet Oct 21, 2011
Or run a low-temperature Stirling engine to produce additional electricity or power a heat-pump.

http://www.coolenergyinc.com/

Issac - probably not as a vortex tube uses the energy from the inlet pressure to separate the streams. Heat pipes generally run at zero over-pressure to keep the costs down.
Eikka
not rated yet Oct 21, 2011
Heatpipes generally operate on a wick-principle. That is, they have some sort of a porous surface that pulls a liquid in through capillary action and then evaporates it off like a wet sock on a bottle of water. The gas then condenses at the cold end of the pipe and returns to the hot end through the wick.

They are pretty effective at it, because the temperature at which the liquid boils keeps the temperature of the "sock" within a narrow range, but the problem still persists with the actual sink where the heat is being dumped.

A household water boiler will have a temperature of at least 55 degrees C to stop any bacteria from growing in the stagnant water in the pipes, and it will necessarily approach the boiling point of water because you can't have an infinitely large boiler to store all the heat.

So you really have two conflicting goals: store heat for later use, or generate electricity. A compromize between the two will just mean a high system price for little or no benefit.
nathj72
5 / 5 (2) Oct 22, 2011
@Eikka I assume you were looking at the commercially available bimuth telluride thermocouples. It is likely that the researchers are not looking at this material for the thermocouple. There is currently a significant amount of research being done on other materials that are great at high temperatures and have significantly better efficiency. There have been some great review articles on the subject, give them a read (if you have access to them, if not there may still be something in one of the publicly available journals).
jrsm
not rated yet Oct 23, 2011
Thermocouples are used to prove flame in gas furnaces and to measure high temperatures. They run at higher temperatures than 150C. Scaling up the process to provide usable power is one of the issues. When I was a kid there used to be a science project using a thermopile to get usable power from a candle flame. The date on the encyclopedia was 1964; this is very old technology and was being eplored in the 1960's. The limit to implementation was the material at hand at the time