The application of exergy (second-law) analysis to aquifer thermal energy s
torage (ATES) systems is investigated in order to facilitate proper assessm
ents of overall system performances. An elementary ATES model is created, a
nd the corresponding expressions are developed for efficiencies and for the
quantities of energy and exergy that are injected and recovered. It is dem
onstrated that ATES performance measures based on exergy often are more use
ful and meaningful than those based on energy. Exergy efficiencies account
for the temperatures associated with energy transfers to and from an ATES,
as well as the quantities of energy transferred, and consequently provide a
measure of how nearly ATES systems approach ideal thermodynamic performanc
e. Energy efficiencies do not provide a measure of approach to ideal perfor
mance and, in fact, are often misleadingly high because some of the energy
recovered can beat too low a temperature to be available for a useful purpo
se. (C) 1999 Elsevier Science Ltd. All rights reserved.