Solar generator: A prototype of a Stirling engine that's powered by a solar water heater.
Cool Energy
Solar technology that generates both heat and electricity could make solar energy practical in places that aren't sunny.
Cool Energy, a startup based in Boulder, CO, is developing a system that produces heat and electricity from the sun. It could help make solar energy competitive with conventional sources of energy in relatively dark and cold climates, such as the northern half of the United States and countries such as Canada and Germany.
The company's system combines a conventional solar water heater with a new Stirling-engine-based generator that it is developing. In cool months, the solar heater provides hot water and space heating. In warmer months, excess heat is used to drive the Stirling engine and generate electricity.
Samuel Weaver, the company's president and CEO, says that the system is more economical than solar water heaters alone because it makes use of heat that would otherwise be wasted during summer months. The system will also pay for itself about twice as quickly as conventional solar photovoltaics will, he says. That's in part because it can efficiently offset heating bills in the winter--something that photovoltaics can't do--and in part because the evacuated tubes used to collect heat from the sun make better use of diffuse light than conventional solar panels do.
The system is designed to provide almost all of a house's heating needs. But the generator, which will produce only 1.5 kilowatts of power, won't be enough to power a house on its own. The system is designed to work with power from the grid, although the power is enough to run a refrigerator and a few lights in the event of a power failure.
The company's key innovation is the Stirling engine, which is designed to work at temperatures much lower than ordinary Stirling engines. In these engines, a piston is driven by heating up one side of the engine while keeping the opposite side cool. Ordinarily, the engines require temperatures of above 500 °C, but Cool Energy's engine is designed to run at the 200 degrees that solar water heaters provide.
The success of the technology, however, hinges on achieving the efficiency targets, says Dean Kamen, the inventor of the Segway, who is developing high-temperature Stirling engines for other applications, including transportation. "We need data," he says. The company's second prototype was only 10 percent efficient at converting heat into electricity. Its engineers hope to reach 20 percent with a new prototype.
A simpler design could reduce the cost of solar power generated by concentrating sunlight on Stirling engines.
Nanomaterials could be used for lower-emission cars and solar panels.
These improvement for the engine may improve the efficiency by 100%
In heating chamber, there are two strokes:
First stroke: The gas media inside the heating chamber (HotGas) absorbs the heat from heat source (HeatSource)
and expands fast to drive the piston and the flying wheel. The cool gas flows to heating chmber from cool chamber.
During the period, HeatSource always heats up the HotGas.
Second stroke: The flying wheel drives the piston back and the piston pushes the HotGas to cool chamber from heating chamber.
During the period, HeatSource always heats up the HotGas.
We need the heat from HeatSource in first stroke but donot need HeatSource in second stroke, so the heat in second stroke is
wasted.
In order to utilize the wasted heat in second stroke, we use a pair of the sterling engines.
When the first engine just begins second stroke, there is a device (DirectDevice) to direct the sunlight to the second engine
which just begins the first stroke. when the first engine just begins first stroke, DirectDevice directs the sunlight to the
first
engine again.
In order to let the engine has enought time to abosorb heat from HeatSource in heating chamber and cool down the hot gas in
cool chamber,
try to use a flying wheel with bigger weight. The slower running speed of the piston can decrease the energy loss when
the gas flows between the two chambers.
These improvement for the engine may improve the efficiency by 100%.
I've have always like the Stirling engine idea. Seems each alternate energy source is focused on a segment of environmental influence. If we were to look at a home as a life sustaining unit and work on each part of the system, the whole thing becomes workable. I would throw in a buried fly-wheel generator in the system and a more efficient way to refrigerate/store food. Seems like we humans are always heating and cooling things for our benefit.
Could this Stirling engine be tied to geothermo?
About 5 weeks ago, Cool Energy was successful in the completion and operation of its third generation Stirling Engine platform. At 210C hot temperature, and 10C cold temperature, the engine generated 1,100 watts net electricity. Work is ongoing to improve the engine efficiency to generate power in the 1.5 kW range and get our thermal-to-electrical efficiency closer to 20%. We are also testing high-end evacuated tube solar collectors for our SolarFlow system.
In addition to the solar application, the Cool Energy engine can utilize waste heat (e.g. exhaust heat of a diesel generator or industrial facilities) to generate electricity and boost the total power generated by as much as 20%. This is a valuable increase for industry, the military and remote locations. We are also working on applications to capture waste heat from biomass systems such as pellet stoves and boilers.
Interest has been abundant from the northeast US, Canada and Germany. We are in the midst of qualifying several field trial sites and raising investment capital.
More info can be found at www.coolenergyinc.com.
Sam Weaver
CEO, Cool Energy
Manufacturing in the United States is in trouble. That's bad news not just for the country's economy but for the future of innovation.
On Twitter
Become a Fan on Facebook
Subscribe to the Feed
killian
74 Comments
efficiency targets
Kevin wrote, "The company's second prototype was only 10 percent efficient at converting heat into electricity. Its engineers hope to reach 20 percent with a new prototype."
Wow, 20% is an aggressive target. The Callen efficiency for a heat engine between 200C and 25C is 20.6%, so they are targeting looking to achieve near perfect conversion. (The Carnot efficiency limit for a reversible heat engine is 37%, but one never achieves the Carnot limit in practice.)
Kevin, any idea how this engine compares to waste heat engines previously covered in TR, e.g. Ener-G-Rotors? Also, did TR ever cover Cyclone Power or Clean Power Technologies? (I didn't see any results from a search.)
Reply
spweaver
3 Comments
Re: efficiency targets
killian writes:
"Wow, 20% is an aggressive target. The Callen efficiency for a heat engine between 200C and 25C is 20.6%, so they are targeting looking to achieve near perfect conversion. (The Carnot efficiency limit for a reversible heat engine is 37%, but one never achieves the Carnot limit in practice.)"
That's a great way of looking at the efficiency of our engine. The engine that we have developed is modeled in great detail with a couple of Stirling engine and multi-physics modeling packages, and the efficiency that we predict for the case that you pose (200C hot side, 25C cold side) is 21% for our current regenerator. This is very near the Callen (also known as the Curzon-Ahlborn) efficiency, as you point out. The Callen efficiency is a practicable limit, not a hard physical limit like the Carnot efficiency, and from our last prototype we believe we can come close to the Callen efficiency, and potentially surpass it with a superior regenerator design. The non-idealities of the second prototype that we built arose largely from mechanical and restricted gas flow causes, and when those measured losses were included in the engine models they predicted very near the 9% thermal efficiency that we achieved in that prototype. Our third generation prototype addresses these deficiencies, and initial testing on the new mechanism demonstrates significant improvement over the earlier prototypes.
Our SolarHeart Engine has an upper operating temperature limit of 250C, and has been shown to run at 100C to 210C with the second prototype. For waste heat applications such as increasing the power output of a diesel generator in which we can take advantage of these higher temperatures, we hope to approach 22% conversion efficiency. We will keep MIT TR up to date on our testing progress.
Sam Weaver
CEO, Cool Energy
Reply
killian
74 Comments
Re: efficiency targets
Thank you for the excellent reply. Which gas are using in the Stirling engine (e.g. hydrogen, nitrogen, air, ...)
Reply
spweaver
3 Comments
Re: efficiency targets
Our heat exchangers are optimized for nitrogen or air. The advantages of helium and hydrogen are more pronounced at higher temperature differentials, so their use is easier to justify for more conventional high-temperature Stirling engines. We have opted for simplicity because there are not significant performance reasons to use the other gases.
Sam Weaver
CEO, Cool Energy, Inc.
Reply