Enhanced Heat Transfer from Geothermal Systems Using Pressure Cycling

Abstract

Energy found in the earth’s crust as heat can be extracted from hot dry rock through enhanced geothermal systems (EGS). Harnessing that energy to produce electricity will require significant heat transfer from the rock to the working fluid. In this research, the possible enhancement of the heat transfer in a bench-top experiment simulating a single flow-path in hot rock was examined. The bench-top laboratory experiment consisted of a concrete cylinder with a small hole (6.35 mm) along the axis, with thermocouples imbedded at four radial locations. Heat transfer from the simulated hot dry rock (SHDR) to the liquid water flowing through the hole was first evaluated to assess the heat transfer similar to that found in flow through a connected fracture. Experiments were then rerun under conditions where depressurizing the water resulted in the production of steam. It was found that depressurization heat transfer rates were ten times that of low Peclet number steady flow experiments. The heat transfer limit for the flow experiment is the axial distance to the location where the fluid temperature matches the initial concrete temperature, whereas the limit for depressurization heat transfer can be either the mass of water in the tube or the thermal conduction from the SHHR. These results imply that the rate of production of energy from some geothermal systems might be dramatically improved if operated cyclically with respect to pressure.