Fuel cells are analyzed theoretically using the carnot ratio which, it is explained, applies to both heat engines as well as fuel cells. A simple second law analysis shows where the loss of efficiency in different fuel cells occurs. Energy concepts are based on the web-book "Energy Science Made Simple".
Internal combustion engines are rated at % of Carnot achieved and as I said before low temperature engines such as powered by flat plate collectors will not be even vaguely close to what Carnot would lead you to believe. That is why there are no successful ones. Use Charles law and tell me what you get from 200F heater and 70F bottom end. Then tell me the relevance of Carnot re that same operating range. BTW my favorite thermo book is "Heat Engines" by John F Sandfort as he doesn't get carried away splitting hairs. Of course Feinman's 3 volume set on physics is what I recommend to students.
Kirk
Graeme Vagg <[EMAIL PROTECTED]> wrote:
Graeme Vagg <[EMAIL PROTECTED]> wrote:
Kirk,Jan's comments are closest to the real situation. 35% would be the average thermal efficiency of Internal Combustion Engines determined by the amount of work done (power output) divided by ther amount of energy available in the fuel. Current petrol engines have a thermal efficiency of about 30% and diesel engines about 40%.The Carnot Efficiency is the maximum efficiency possible for a heat engine operating between an upper and lower cycle temperature limit. It is a reference value that actual efficiency is compared with. The Carnot Efficiency for petrol engines operating on the Otto Cycle is about 70% so the efficiency ratio of an engine at 30% compared to the ideal Carnot cycle at 70% would be 42.8%.Fuel cells are not heat engines so there would be not Carnot Cycle reference value. As fuel is consumed you can still determine a thermal efficiency value by comparing the work output with the energy value of the fuel consumed.See relevant textbooks on heat engines and operating cycles for further clarification.Graeme
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