Energy and exergy analysis of waste heat recovery systems using organic ranking cycle

Abstract

Exergy analysis of thermodynamic systems along with the energy analysis is growing rapidly. Exergy quantifies the potential of available energy to do useful work. Exergy analysis is fruitful to identify processes where exergy destruction and losses occur and acts as a useful supplement to the energy analysis. Utilization of waste heat has attracted enormous attention in recent years due to environmental concerns. Organic Rankine Cycle (ORC) is a promising technology for conversion of low grade heat (at low temperature) into useful work. In the present study, detailed energy and exergy analysis of a bottoming ORC, utilising the waste heat, is performed. Suitable refrigerants (R141b, R124, R21, R1233zd(E), R1234ze(Z) and R245fa) having low Global Warming Potential (GWP) values and compatible with the thermal condition of the heat source are selected as the working fluids for the analysis. Effects of evaporator pressure on net work output, power consumption, mass flow rate, thermal and exergy efficiency, irreversibility of individual components as well as of the overall system is investigated. Both thermal and exergy efficiencies of the ORC increase with the evaporator pressure. Among the refrigerants considered, R141b results in the highest efficiency due to its highest pressure ratio, whereas R124 results in the lowest (pressure ratio, thermal efficiency and exergy efficiency for R141b are 45.1, 19.3% and 48.5% respectively at 60 bar evaporator pressure, whereas for R124, the corresponding values are 10.1, 12.0% and 30.2% respectively). Energy balance indicates that major portion of the input energy is rejected to condenser cooling water, although exergy balance suggests that its exergy content is very low (using R245fa at 10 bar evaporator pressure, heat rejection is 89% of total energy input but its exergy value is only 7% of total input exergy). However, heat rejection to condenser cooling water decreases with increasing evaporator pressure (the heat rejection value decreases by 9.1% when evaporator pressure is increased from 10 to 65 bar using R245fa). A major part of the total input exergy is destructed due to the irreversibility in various components of the system. Among the components, the highest exergy destruction occurs in the condenser because of higher temperature difference of heat exchanging fluids (using R245fa at 10 bar evaporator pressure, 70% of the total exergy destruction occurs in the condenser). Consequently, condenser has the lowest exergy efficiency among other components which is responsible for low overall efficiency of the system. The results also demonstrate that, commonly used refrigerant R245fa can be potentially replaced by low GWP refrigerants R1233zd(E) and R1234ze(Z) with slight variations of the performance of ORC.