Reviewing the relationship between thermal reservoir parameters and geothermal energy output

Abstract

This meta-study draws upon contemporary literature to examine parameters of thermal reservoirs and their relationships to geothermal power station output metrics. The objectives of the meta-study are to identify trends and quantify the influence of each parameter on the system as a whole. This study provides a framework for industry and researchers exploring new potential geothermal fields. Six reservoir parameters – well depth, temperature, enthalpy, mass flow rate, thermal gradient and crust thickness – were plotted against the net electrical output per production well (Enet/well) and exergy efficiency (ηB) of 64 geothermal facilities. The meta-study identified that reservoir temperature has the greatest proportionality to power output, with yields above 10MWe exhibited only for high enthalpy reservoirs exceeding 500K. Well depth has the greatest inverse proportionality to exergy efficiency, with upper limit values declining below 80% for wells deeper than 3000m. Well depth has a similar trend line, though lesser correlation, as reservoir temperature to power output. Crust thickness has an inverse correlation to exergy efficiency, with upper limit values dropping from 100% to 65% as thickness increased from 30 to 45km. There was significant clustering of data points in most trendless plots, suggesting a considerable degree of homogeneity between currently tapped reservoirs and turbine efficiencies. The low number of well-defined data trends implies a high degree of complexity arising from the relationships between reservoir parameters that make quantification problematic. Despite this difficulty, examination of the aforementioned parameters suggests that although hotter reservoirs are usually found at greater depths, the hottest and shallowest reservoirs should be prioritized for use in order to return maximal power outputs and reduce exergy losses that occur along large lengths of piping.