An approach to working fluid selection for absorption power cycle based on the sub-cycle coupling concept

摘要

As a kind of distributed energy system, absorption power cycle is an advanced low-grade heat/power conversion technology. This article aims at studying a new approach to working fluid selection for absorption power cycle. This study proposed that absorption power cycle is coupled by a Rankine heat engine sub-cycle and a chemical heat converter (CHC) sub-cycle, and developed a cycle performance model of absorption power cycle, which is constituted by the contributions of both sub-cycles. Using the method of weight factor, a new mathematic model has been proposed to correlate the contributions of the two sub-cycles and get the cycle performance. Based on the proposed approach, a power working fluid R245fa (1, 1, 1, 3, 3- Pentafluoropropane) has been chosen to combine with N, N-Dimethylformamide (DMF), dimethylether diethylene glycol (DMEDEG) and N-Methyl-2-Pyrrolidone (NMP). These three new working fluids for absorption power cycle have been simulated and analyzed with Aspen Plus computer software. The vapor liquid equilibrium parameters of the three systems were determined by authors' previous experimental works. The simulated efficiencies were 18.98%, 15.39% and 16.76%, respectively. The efficiencies of the Rankine heat engine sub-cycle of the three working fluids have been calculated at first, which are 20.51%, 15.76% and 17.74%, respectively. Then a general computer program of absorption refrigeration cycle has been built up to get the efficiencies of the CHC sub-cycle, adopting the three working fluids. The results are 13.68%, 13.87%, 13.43%, respectively. So using the new model, the efficiencies of absorption power cycle have been obtained, which are 18.77%, 15.19% and 16.60%, respectively. To validate the feasibility of the new model, the deviation between the efficiencies of new model and the direct simulation of three working fluids were calculated, which are 1.07%, 1.26% and 0.92%, respectively. The evaluation result proves that the new model can well predict efficiency of absorption power cycle based on the working performances of the two sub-cycles. In addition, the weight factor of the Rankine heat engine sub-cycle is greater than CHC sub-cycle, around 0.763 to 0.229, which means the performance of the Rankine heat engine sub-cycle should be considered firstly, in order to determine the appropriate working fluid for absorption power cycle.