Thermodynamic Assessment of a Deep Geothermal Heat Pump System

Abstract

Current geothermal energy exploitation is confined to porous, water saturated sedimentary rock. Excluding a few small scale industrial and research complexes, little progress has been made to reach the underlying crystalline bedrock, called hot dry rock(HDR).Thepotentialofthisresourceisconsideredtobeabundantandfaroutweigh the standard geothermal source in sedimentary bedrock utilized today. The focusofresearchhassofarbeenin"engineered"HDR,meaninginsomewaycreating a fracture system, through which to circulate water, in the otherwise impermeable bedrock. However, creating this fractured network is a perilous enterprise in several ways. In experimental drilling there have been issues with fluid leaks, induced seismic activity and flow short circuits among others. It is of interest to investigate alternatives in extracting this energy. Thisthesishastheambitiontoinvestigateaclosedloopgeothermalsystem,consisting of two boreholes connected at a depth of 3500 m in the crystalline bedrock. A modelwasbuiltusingCOMSOLMultiphysics5.3. Therockpropertiesweredefined in a way common to Sweden and a heat pump is considered to be connected to the system, allowing for a constant temperature of the fluid returning to the surface. The purpose of the study was to discern which parameters that were affecting the energy extraction and how. With that goal in mind, fluid flow, incoming fluid temperature, borehole radius and thermal conductivity of the crystalline bedrock were alternated in a series of simulations. Fluid flow and incoming fluid temperature are operational phase values and are the only conditions which can be altered after construction of the system. Borehole radius has a strong impact on drilling cost. The thermal conductivity of the bedrock is a site specific value and changing this exposes the impact of more or less preferable subsurface conditions. The results shows that the convection in the boreholes were more than adequate to saturate the heat exchange with its surroundings. Limiting factors proved to be the temperature gradient between borehole and bedrock, together with rock thermal conductivity. The high thermal inertia of the bedrock, which signifies how slowly the temperature of a section of rock reaches that of its surroundings, and the relatively low thermal conductivity failed to supply adequate heat, resulting in a rapid decline in energy extraction, in the first few years of operation. Thus the heat penetration in the bedrock limits the possible thermal power output.