Comparative study and multi-objective optimization of the use of volumetric expanders in a series double cascade-evaporator organic Rankine cycle.

The choice of refrigerant and expander is always one of the main concerns of researchers using organic Rankine cycles. The problem of using the organic Rankine cycle becomes more complicated when there are two waste heat sources with different temperatures in a system, such as exhaust gas and jacket water heat sources in an internal combustion engine. Hence, in this paper, a thermoeconomic program is developed using four types of volumetric expanders, including piston, screw, scroll and root, and eight different organic fluids in the series double cascade-evaporator organic Rankine cycle. The studied organic fluids were selected to cover a wide range of pressure levels in the series double cascade-evaporator organic Rankine cycle, including R245 FA, R123, n-pentane, acetone, n-heptane, benzene, n-octane and toluene. In the present study, 16 different pairs of expanders were examined for a cohort of 8 dissimilar fluids, resulting in an aggregate of 128 distinctive cases. In each of the 128 different cases, the important parameters, including power production, exergetic efficiency, total investment cost, net present value and levelized cost of electricity generation, are calculated. Also, the Pareto method is used to select the best organic fluid and expander pair. The results show the fluids with lower critical temperatures performed better due to the pressure ratio limit. The lowest cost of the expander compared to the total cost was 4.98%, related to the pair of root–scroll expanders, and the highest cost of 48.21% was related to the piston–screw expander pair. The lowest total investment cost was 377,478$ for the n-pentane and the piston–scroll expander pair. The screw expander costs more than other expanders, especially when in the low-pressure cycle. According to the optimization results, the optimal pair of expanders for the series double cascade-evaporator organic Rankine cycle system is scroll–scroll with n-pentane as the working fluid. Additionally, the exergetic efficiency, payback period and total investment cost values in this case are 0.3945, 4.089 years and 385,786$, respectively. [ABSTRACT FROM AUTHOR]