WASTE HEAT RECOVERY HEAT EXCHANGER USING SUPERCRITICAL CARBON DIOXIDE

Abstract

Common power cycles discard a large portion of useful energy into the environment via exhaust gases. Through the use of supercritical power cycle, this wasted energy may be utilized for power generation and hot water production. Heat transfer between cycles occurs through a heat exchanger. To maximize heat exchanger effectiveness, a supercritical working fluid is used in the supercritical power cycle to better match the heating curve of the sensible heat source. Carbon dioxide is selected as the working fluid because it possesses a relatively low critical temperature which makes it attractive for low temperature waste heat applications. In contrast to many other working fluids, carbon dioxide is inert, abundant, non-flammable, and presents negligible environmental impact. The purpose of this study is to do thermal design of helical coil heat exchanger, second law analysis and Thermo-Economic analysis of the same to gain insight for future research in the field of waste heat recovery. A program code is established using EES software to perform the calculations required for the waste heat recovery analysis considering real variation ranges of the main operating parameter such as length, diameter of the helical coil and temperature, mass flow rate of supercritical carbon dioxide at inlet condition. The effect of these parameters on system performance are investigated. By the second law analysis we are able to combine both the effects of heat transfer and pressure drop in a single close form equation by which we are able to know the exact irreversibility occurring in the system which can be used to improve the performance. Thermoeconomic cost will increase as usual with increase of tube bundle length and diameter of the tube but it decreases with increase with increase in the mass flow rate and inlet temperature of SCO2. The second law efficiency which is also known as rational efficiency is having a maximum value for varying the diameter of the tube which result in optimum value of the diameter. As mass flow rate increases rational efficiency decreases but there is increase in the rational efficiency as inlet temperature of SCO2 increases. In entropy generation number behaviour also we are able to obtain minimum value for varying the diameter which can obtain optimum diameter corresponding to lower irreversibility. As usual behaviour as the mass flow rate of SCO2 increases entropy generation number increases while entropy generation number decreases by increases in inlet temperature of SCO2.