Low-Temperature Geothermal Energy: Systems Modeling, Reservoir Simulation, And Economic Analysis

Abstract

Performance of low-temperature geothermal energy systems has been investigated though systems modeling, reservoir simulation, and economic analysis. Both utilization of deep geothermal energy with focus on direct-use heat though Enhanced Geothermal Systems (EGS) and shallow geothermal energy exploited with hybrid heat pump systems have been studied. To assess power output and economic competitiveness of deep geothermal energy for production of heat and/or electricity, a computer tool GEOPHIRES has been developed which combines cost correlations and economic models with reservoir, wellbore, and surface plant models. Simulations show that low-grade EGS resources (with geothermal gradients of ~30? C/km) are unattractive for solely electricity production with estimated levelized costs of electricity between 20 and 60 ¢/kWhe . Utilizing low-grade resources instead for low-temperature (<120? C) direct-use heat applications, results in competitive levelized costs of heat (LCOH) between 6 and 14 $/MMBTU (2.0 and 4.8 ¢/kWhth ). Given that low-grade resources are widely available and the market for low-temperature heat is significant, geothermal energy becoming a major low-temperature heat supplier should be considered. To evaluate the energetic and economic performance of hybrid geothermal heat pump (GSHP) systems for cooling-dominated applications, a TRNSYS systems model has been developed and validated with data collected at a full-size experimental hybrid GSHP system providing cooling for a Verizon Wireless cellular tower shelter in Varna, NY with average continuous cooling load of 11 kWth . Simulations indicate that for the Varna Site weather and operational conditions in the base case scenario, GSHP-based systems allow the owner to save up to 30% of lifetime electricity consumption in comparison with airsource heat pump (ASHP)-based systems. However, mainly because of lower upfront capital costs, ASHP-based systems can have up to 10% lower total cost of ownership. A novel approach for simulating transient heat transfer with slender bodies in a conductive medium, e.g. geothermal wells and slinky-coil heat exchangers, using the slender-body theory (SBT) has been developed. An efficient numerical implementation is obtained based on a judicious choice of the discrete elements used to represent the body and implementation of the Fast Multipole Method (FMM). The SBT requires a onedimensional spatial discretization only along the axis of the body in contrast to the threedimensional discretization for finite element models. Two case studies, heat transfer from two parallel cylinders and heat transfer from a slinky-coil heat exchanger, are used to show the speed and accuracy of the SBT model and its ability to model interacting slender bodies of finite length and bodies with centerline curvature and internal advective heat flow.