Performance optimization of a new flash-binary geothermal cycle for power/hydrogen production with zeotropic fluid

In this study, the performance of a system consisting of an organic Rankine cycle (ORC) for generating power and an electrolyzer for producing hydrogen with a zeotropic mixture as working fluid to recover waste heat in a geothermal flash-binary cycle is investigated from energy and exergy point of view. The study also investigates the effect of using zeotropic mixtures with different compositions as the ORC's working fluid rather than pure fluids. Using the particle swarm optimization (PSO) algorithm, the optimization is performed to maximize the power production of the entire system. The results show that using the combination of pentane with other pure fluids as working fluid led to improved system performance in terms of power and hydrogen output. In view of ORC unit power output (31.51 kW), overall system power output (128.16 kW), and hydrogen output (0.39626 kg h(-1) per kilogram of geothermal water), the pentane (0.54)/butene (0.46) mixture produces the best results. Under optimal operating conditions, the rate of exergy destruction, exergy and energy efficiency for the whole system is equal to 95.81 kW, 37.25% and 20.17%, respectively. It is also found that the composition of the zeotropic mixture has a significant impact on the performance of the ORC as well as hydrogen production, which is associated with the extent of temperature glide in the mixture. When pentane (0.54)/butene (0.46) is used as the working fluid, the highest improvement related to use of zeotropic mixture instead of pure fluid is observed, which increases the power output of the ORC unit and the hydrogen output of the system by, respectively, 22.53% and 23.02%. In the end, the effect of flash chamber pressure on the total power and hydrogen output is also investigated. This investigation shows that as this pressure increases, the total power output decreases. However, although a lower chamber pressure is more desirable, given the impact on the hydrogen production and the turbine size, it is preferable to keep the flash chamber pressure moderate.

Automated summary

The study shows that using a combination of pentane with other pure fluids as working fluid can improve the system performance. The pentane (0.54)/butene (0.46) mixture produces the best results, with an overall system efficiency of 37.25% and an energy efficiency of 20.17%.