I am curious of the performance of Stirling engine incoporated with a generator used to recover the waste heat energy. I found a company "cool energy" claimed there's a product available for this. But no further information on the web.
Hi Weidong,
I will advise a bit of caution in this. It is true that there are some claims on such products if you go through the web, but in many cases they turn out to be small table-top demonstration setups of little practical or commercial value.
If I were you, I would always check with any claimant about the authenticity of an advertised Stirling system, as well as the actual performance details. I have burnt my fingers a couple of times !!
Cheers !
-Sanjay
The thermal efficiency of an ideal Sterling cycle/ engine is equal to that of Carnot
cycle between the same source and sink temperatures. The thermal efficiency can be calculated as follows:
Thermal efficiency = 1 - (T_cold)/(T_hot)
Hence, to calculate the thermal efficiency of the Sterling engine, you need to know the hot side temperature (T_hot) of the waste heat and the heat sink temperature (T_Cold). The heat sink temperature can be the ambient air temperature. Remember to use absolute temperatures in the equation I cited above.
Hope this helps answer youor question.
Professor Yehia Khalil
Yale University
USA
While Prof Khalil is quite correct, you won't find any real Sterling cycle engine running at low temperatures to be close to the ideal. The heat from most modern generators is too low-grade to turn into electricity efficiently. Just use it to generate hot water.
As noted by Yehia F. Kahlil, the maximum possible efficiency is the Carnot efficiency, which is 1 - Tcold/Thot. But most engines operate closer to a speed that maximizes power output. The efficiency at maximum power output, assuming heat fluxes being proportional to temperature differences, is the Curzon-Ahlborn efficiency 1 - SQRT(Tcold/Thot).
the company selling the stirling heat pump is as follow:
IMP PROMONT d.o.o.
Pot k sejmišču 30a, p.p.4957
1231 Ljubljana Črnuče, SLOVENIJA
T: +386 (0)59 07 39 12, 59 07 39 60
GSM: +386 (0)51 380 558
F: +386 (0)1 53 73 726
WEB: www.imp-pro-mont.si
A lot of companies use nitrogen gas in their processes (i.e. to fill up food cans), which is delivered in liquid phase (-190 C) ) and has to be heated before use. In the same factories there are other (bigger) processes that require cooling, using water as heat medium. One factory uses say 60 m3 liquid N2 a day, the waste heat is several orders bigger. On this scale it can be interesting to use the waste heat to heat up the N2, or better, to use a stirling motor to generate electrical power. But the problem here is that these big companies pay low energy prices, say 0,07€/kWh, so any investment like the stirling motor has a long payback time. Plus it adds complexity to the process. Even though these temperatures are ideal for a useful stirling motor application, I have not heard of such installation in operation somewhere yet. (Please let me know if there are)
Your thermal efficiency will be directly affected by the heat source and cold sink temperatures of your application. Optimally, you will get the Carnot efficiency, but in practice you are far from it (the Curzon-Ahlborn efficiency proposed hereabove will give you a better idea of what you can get). But personnaly, I don't have knowledge of any stirling engines commercially developed for waste heat recovery applications. In practice, low-grade waste heat is preferably valorized into power by means of organic Rankine cycles. These systems show better scalibity for waste heat applications (up to MW units, which is not the case of more compact stirling engines). Furthermore, the large variety of working fluids that can be used in ORCs permits to properly design your thermal engine over a wide range of heat source conditions (from 100°C to 300°C).
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