Geothermal energy refers to the thermal energy naturally stored in the hot rock formation buried deep beneath the Earth's surface. To harvest this source of energy, a fluid is pumped down in a set of injection wells, circulates through the hot rock and then is collected via a set of production wells for heating or other purposes. The selection of this fluid is an important component of a geothermal system. In this work, we review, evaluate, and develop the fundamental equations that can reliably predict the thermodynamic and the transport properties of air, CO2, and H2O. Using these equations, we characterize and compare the heat extraction and exergy carried by these fluids as they circulate through a hot rock reservoir to determine which fluid offers better functionality for geothermal heat harvesting applications. The results indicate that, in terms of heat extraction and exergy, air is a better fluid compared to CO2 and water. (C) 2018 Elsevier Ltd. All rights reserved.