The power output of gas turbines (GT) reduces greatly with the increase of the inlet air temperature. This is a serious problem because gas turbines have been used traditionally to provide electricity during the peak power demands, and the peak power demands in many areas occur on summer afternoons. An aquifer thermal energy storage (ATES) was employed for cooling of the inlet air of the GT Water from a confined aquifer was cooled in winter and was injected back into the aquifer The stored chilled water was withdrawn in summer. to cool the GT inlet air. The heated water was then injected back into the aquifer A 20 MW GT power plant with 6 and 12 It of operation per day, along with a two-well aquifer, was considered for analysis. The purpose of this investigation was to estimate the GT performance improvement. The conventional inlet air cooling methods such as evaporative cooling, fogging and absorption refrigeration were studied and compared with the ATES system. It was shown that for 6 It of operation per day, the power output and efficiency of the GT on the warmest day of the year could be increased from 16.5 to 19.7 MW and from 31.8% to 34.2%, respectively. The performance of the ATES system was the best among the cooling methods considered on the warmest day of the year The use of ATES is a viable option for the increase of gas turbines power output and efficiency, provided that suitable confined aquifers are available at their sites. Air cooling in ATES is not dependent on the wet-bulb temperature and therefore can be used in humid areas. This system can also be used in combined cycle power plants.