How to design organic Rankine cycle system under fluctuating ambient temperature: A multi-objective approach

Affected by the fluctuating ambient temperature, the organic Rankine cycle (ORC) system deviates from the design point during operation, resulting in the deterioration of system performance. How to design the best ORC system considering the fluctuating ambient temperature is an important and challenging topic. This study proposes a multi-objective approach to determine the optimal design of the ORC system, based on hourly off-design operating conditions. The concept of optimal design ambient temperature (T_a,opt^d) is introduced to represent the optimal design of multiple system parameters. Its specific value, the relationship with the ambient characteristics, and the variation for three different target variables and two ORC configurations are revealed. The results of 15 cities in different climate zones indicate that, although the average value of annual ambient temperature (T_av) itself is not the best system design point, it significantly affects the value of T_a,opt^d. To maximize the lifetime electricity generation, T_a,opt^d should be designed to be 5 °C lower than T_av. In contrast, if the optimization objective is to maximize the net present value, T_a,opt^d should be 5 °C higher than T_av. Moreover, compared with the conventional approach where the design ambient temperature is 20 °C or T_av, using T_a,opt^d can increase the electricity production by 7% and 0.8%, respectively. Furthermore, the effects of electricity market parameters including the discount rate and electricity price on the optimal design are also revealed in the sensitivity analysis.