Energy conversion of heat from abandoned oil wells to mechanical refrigeration - Transient analysis and optimization

Abstract

This study investigates the potential of using the geothermal energy from abandoned oils in a novel three-loop system; geothermal, power and cooling loops to produce cooling effect. The geothermal loop drives the power and cooling loops of a thermo-mechanical refrigeration (TMR) system consisting of expander-compressor units (ECUs). The system advantages lie in its simple, flexible, and low-cost design as well as in its ability to be driven by a low-temperature heat source (as low as 60 °C). To evaluate the performance of the system, comprehensive models are developed including transient model for the abandoned oil well (geothermal loop) in spatial and time domains and thermodynamic and optimization models for the entire three-loop system. The effects of the temperature variation of the geofluid over operation time, the working fluids, the high pressure and the temperatures of the heat source and sink are investigated. Results show that at realistic and conservative conditions, the geofluid temperature considerably decreases for the first four months of operation (by an average of 30 °C) and tends to be constant after half a year of operation. However, the geofluid temperature still high enough to drive the proposed geothermal TMR system over the full operation period. Among 43 investigated refrigerants, R1234ze(E) has higher efficiency, lower Pumping Work Ratio (PWR), and requires a smaller size of the heat exchangers. Using the genetic algorithm optimization method with R1234ze(E) as working fluid in both power and cooling loops, a maximum power loop efficiency of 6.3% and COP of 5.3 were obtained at a high pressure of 29 bar (in the power loop) with minimal expander diameter of 64, compressor diameter of 171 mm, and 18 expander-compressor units.