Entropy and the Fallacy of Waste Heat Recovery

摘要

Retrofit waste heat recovery systems are usually justified only by the first law of thermodynamics neglecting the effects of the second law. This leads to the general misperception that irrespective of the conversion efficiency some benefit may be gained by the implementation of a heat conversion device at the exhaust stream. The second law which is as fundamental as the first limits the amount of energy recoverable from the exhaust. This study found that because of the second law, a net gain in useful work output may be achieved only when the efficiency of the downstream heat recovery system is higher than (η_0-η)/(1-η), where η_0 and η are the primary system efficiencies before and after addition of the heat recovery system, respectively. Lower efficiency recovery systems only result in the generation of more entropy or the destruction of exergy. Although thermoelectrics is often cited as a potential technology for waste heat recovery, it cannot meet this required efficiency. Quantum thermoelectrics may have the projected performance levels but the technology is at an early stage of development. This study further found that retrofit heat recovery systems are likely to be detrimental to the overall system performance unless its energy product is compatible, or may be made so, with the fundamental purpose or product of the primary system.