The radiator-enhanced geothermal system: Benefits of emulating a natural hydrothermal system

We have developed a novel enhanced geothermal system (EGS) called radiator EGS (RAD-EGS). This system attempts to emulate naturally occurring hydrothermal systems by creating a vertically oriented heat exchanger or vane in the deep subsurface, mimicking a radiator in an internal combustion engine. Water is injected at the bottom of the vane and produced on the top. We propose to build the RAD-EGS in hot sedimentary aquifers (HSAs) with high-permeability vane(s) created in the plane defined by Shmax and S1 (vertical). We have evaluated 3D heat-transfer simulations to better understand the fluid and heat flows that may occur in RAD-EGSs. The simulations account for subsurface heterogeneity including the presence of underlying basement rock, an overlying confining layer, and an ambient hydraulic gradient, which causes background groundwater flow. Our simulations indicate that our induced upward flow in the vane significantly prolongs the lifetime of RAD-EGS when compared with downward flow because hydraulic short circuiting is avoided. Within the vane, convection may occur, and its onset is analyzed in terms of a characteristic Rayleigh number. A critical aspect of RAD-EGS, therefore, is that thermal recharge does not rely solely on heat conduction from the surrounding wall rock, which is typical for EGS built in hot dry rock (HDR). Instead, recharge is also due to heat advection through the surrounding water-saturated aquifer, substantially prolonging the lifetime of the thermal reservoir. Moreover, fluid losses as typical for EGS built in HDR do not occur. It is also possible that cold water injected at the bottom of the vane may sink into deeper rock layers, which displaces hot water from the surrounding aquifer into the RAD-EGS. We suggest that mimicking a natural hydrothermal system is a successful EGS strategy via RAD-EGS.