![]() The COP actual as a measure of efficiency under actual operation conditions is evaluated by taking account of the dynamic behaviors of a given cycle. In order to evaluate the efficiency from COP, the second law thermodynamic efficiency, η II, as the ratio between the ideal (reversible) and actual conditions should be introduced: η II=COP actual/COP ideal. The COP based on the reversed Carnot cycle should not be used as the measure of thermodynamic efficiency of a given device because it is always larger than unity and is not applicable for evaluating efficiencies under different temperature conditions. However, the COP is strongly dependent on the temperature difference between high and low thermal energy sources, i.e., thermal energy source and sink, and particularly, it depends on the individual temperature levels of thermal energy sources. ![]() In the case of thermodynamic cycles such as refrigerators, air-conditioner units, and heat pumps, the COP has usually been introduced by supposing a reversed Carnot cycle, i.e., COP R=1–1/( T H/ T L–1) for a refrigerator and COP HP=1/(1– T L/ T H) for a heat pump where COP HP=COP R+1. It does not make any sense when the thermal efficiency or the coefficient of performance (COP) stated below is applied to different devices operated under different temperature levels. The thermal efficiency is always less than unity and it is difficult to evaluate the efficiency of a device by comparing the small differences. It is not adequate to use thermal efficiency based on a reversible process or cycle such as η th=1– T L/ T H where η th denotes the Carnot thermal efficiency of a reversible system that works between high temperature, T H, and low temperature, T L. The quality of the surrounding thermal energy sources, as well as the heat transmission characteristics of a unit during thermal energy intake and release, determines the thermodynamic efficiency. The rate of hydrogen transfer is directly regulated by the rate of thermal energy transfer. In a hydrogen storage unit, thermal energy is required in both cases of discharging and charging hydrogen from/to the unit. ![]() ![]() Suda, in Encyclopedia of Materials: Science and Technology, 2001 3.2 Thermodynamic Efficiency of Engineering Devices ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. Archives
December 2022
Categories |