Auxiliary Heating of Geothermally Preheated Water or CO2 – A Potential Solution for Low- to Moderate-Temperature Geothermal Resources

Abstract:

Typically, low- to moderate-temperature geothermal resources are more effectively used for direct heat energy applications. However, due to high thermal losses during transport, direct use requires that the heat resource is located near the user. Alternatively, we show here that if such a low-temperature geothermal resource is combined with an additional or secondary energy resource, the power production is increased compared to the sum from two separate (geothermal and secondary fuel) power plants (DiPippo et al. 1978) and the thermal losses are minimized because the thermal energy is utilized where it is produced. Since Adams et al. (2015) found that using CO₂ as a subsurface working fluid produces more net power than brine at low- to moderate-temperature geothermal resource conditions, we compare over a range of parameters the net power and efficiencies of hybrid geothermal power plants that use brine or CO₂ as the subsurface working fluid, that are then heated further with a secondary energy source that is unspecified here. Parameters varied include the subsurface working fluid (brine vs. CO₂), geothermal reservoir depth (2.5-4.5 km), and turbine inlet temperature (200-600°C) after auxiliary heating. The hybrid power plant is numerically modeled using an iterative coupling approach of TOUGH2-ECO2N/ECO2H (Pruess, 2004) for simulation of the subsurface reservoir and Engineering Equation Solver for well bore fluid flow and surface power plant performance. We find that hybrid power plants that are CO₂-based (subsurface) systems have higher thermal efficiencies than the brine based systems at low turbine inlet temperatures. Specifically, our results indicate that geothermal hybrid plants that are CO₂-based are more efficient than brine-based systems when the contribution of the geothermal resource energy is higher than 48%.