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Converting Low-Grade Heat into Electrical Power

 Low-Grade Heat Sources--

Cogeneration

    Cogeneration  usually refers to the use of power generation system or a heat engine to simultaneously generate both electricity and useful heat. Cogeneration is rather an energy use strategy than a heat source. The reason it is listed here is because cogeneration system is directly related to our topic low-grade heat conversion, and the heat abandoned by the top power cycle is the heat source of the bottom one.

    Conventional steam Rankine power plants cannot efficiently convert heat below a certain temperature, while other thermodynamic cycles may work better in that temperature region. Then the idea becomes that combining a steam Rankine cycle with other thermodynamic cycles may yield higher efficiency.

    The figure that I drew below is an idea of the cogeneration. The cogeneration system consist of two power generation cycles.

Fig. 1 A cogeneration cycle

    The top power cycle is a conventional steam Rankine cycle, and the bottoming cycle could be a organic Rankine cycle, Goswami cycle, Ammonia refrigeration cycle, or a Kalina cycle. High-grade heat is partially converted to electrical power through the steam Rankine cycle, expelling low-grade heat. Provided that the low-grade heat could be not consumed directly, the bottoming power cycle could further convert part of the thermal energy into electricity.