Multi-objective design of off-grid low-enthalpy geothermal generation systems considering partial-load operations

This work addresses the multi-objective design of an organic Rankine cycle system for low-enthalpy applications. The design is generated from end-user demand variations. The model uses the reference equations of state of four working fluids commonly used in the literature: R1234yf, R1234ze (E), R134a and R245fa. This approach allows computing the optimal sizing of each unit of the equipment that composes the cycle, obtaining a suitable configuration for each working fluid. That is, the comparison between configurations is linked to the thermodynamic properties of the fluid. In addition, it addresses the phase changes present in each operation and the operational adjustments associated with changes in demand. Therefore, the optimal design captures partial load operation through a multi-period model. Given the off-grid operating conditions, the effects of coupling with energy storage systems are analyzed. The considered objective functions are the total annual cost and energy efficiency of the system. Results indicate that the sizing of the heat exchangers is significantly affected by phase change effects and that the R1234yf and R1234ze (E) fluids are promising since their thermodynamic properties allow for trade-offs between the economic and energy performance of the system. System efficiencies, considering part-load operation and demand variability, can be increased by up to 2 % with the inclusion of energy storage, significantly improving economic performance and reducing costs by up to 10 %.