Technical Evaluation of Zeotropic Fluid Mixtures in Geothermal ORC Applications

orc2019 Tracking Number 95

The ORC technology demonstrated to be a sustainable and reliable technology to exploit low-temperature sources, such as low-enthalpy geothermal reservoirs. In the last years, numerous studies focused on fluid mixtures in ORC applications. Heberle and Brüggemann (2015) investigated fluid mixtures as working fluid in geothermal ORC applications according to boundary conditions available in Molasse Basin, Southern Germany. The combination isobutane/isopentane appeared as the most promising one. The corresponding thermodynamic analysis revealed how the 90/10 (mole-fraction isobutane/mole-fraction isopentane) composition provides the highest turbine power output. In this work, an extended technical evaluation is provided with regard to the use of isobutane/isopentane mixtures in geothermal ORC power systems. On-design simulations are firstly performed in order to maximize the turbine power output for several mole fraction compositions. The 95/5 composition provides the highest turbine power output, 6.78 % more than pure isobutane. Later, yearly simulations considering the ORC behaviour are performed according to real ambient temperature data in Southern Germany. A comparison between on-design and average annual results is proposed. For the 95/5 composition, the annual average net power is 103 kWel lower than the on-design value. Next to technical criteria also selected economic parameters are calculated: the 70/30 composition provides +7.66 % more in net cash flow than the 95/5. In principal, even though the 95/5 mixture provides the highest annual power production, the 70/30 appears more economically feasible under consideration of yearly ambient temperature profile and the corresponding ORC off-design performance.