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Solar for Dark Climates

Continued from page 1

By Kevin Bullis

Wednesday, July 08, 2009
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A Stirling engine's efficiency is limited by the difference in temperature between the cool and hot side. Typically, reaching the necessary high temperatures using sunlight requires mirrors and lenses for concentrating the light and tracking systems for keeping the concentrators pointed at the sun. The concentrators require direct sunlight, so they don't work on overcast days, and they're too bulky to be mounted on the roof of a house.

To make a practical Stirling engine that runs at low temperatures and doesn't require concentrators, the engineers at Cool Energy addressed a problem with conventional engines that leads to wasted energy: heat leaks from the hot side of the system to the cool side, lowering the temperature difference between them. This happens because the materials required for high temperatures and pressures--typically metals--conduct heat. Working at lower temperatures, the engineers concluded, allows them to use materials such as plastics and certain ceramics that don't conduct heat, reducing these losses. These materials also help lower costs: they're cheaper than some of the metals typically used, and they don't require lubrication, improving the reliability of the engines and reducing maintenance costs.

Cool Energy's engineers are currently assembling the company's third prototype, which they say will allow them to reach their efficiency targets by the end of this summer, after which they plan to test pilot systems outside the lab. Within two years, they plan to manufacture enough systems to drive costs down and achieve their payback targets.

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solar power solar thermal  Stirling engine

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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.)
    Rate this comment: 12345

    killian
    07/08/2009
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    • 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
      Rate this comment: 12345

      spweaver
      07/08/2009
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      • Re: efficiency targets
        Thank you for the excellent reply. Which gas are using in the Stirling engine (e.g. hydrogen, nitrogen, air, ...)
        Rate this comment: 12345

        killian
        07/10/2009
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        • 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.
          Rate this comment: 12345

          spweaver
          07/13/2009
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  • 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%.
    Rate this comment: 12345

    gearss
    07/12/2009
    Posts:10
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  • Solar for Dark Climates
    this is an amazing technology.. This article explains the importance of the alternate energy. The traditional solar energy The traditional solar energy
    which we are using is not going to be adequate for our future needs and that too it reached its peak...going by the CEO's statment that the proposed solar is more economical than the existing solarthen undoubtedly it should be a great success in the market...
    Rate this comment: 12345

    raymason@gma...
    07/16/2009
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  • stirling symbiont
    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?
    Rate this comment: 12345

    rnojonson
    07/25/2009
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  • Updates to engine performance
    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
    Rate this comment: 12345

    spweaver
    11/02/2009
    Posts:3
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    5/5
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