The disparity between energy production and demand in many power plants has led to increased research on the long-term, large-scale storage of thermal energy in aquifers. Field experiments have been conducted in Switzerland, France, the United States, Japan, and the People's Republic of China to study various technical aspects of aquifer storage of both hot and cold water. Furthermore, feasibility studies now in progress include technical, economic, and environmental analyses, regional exploration to locate favorable storage sites, and evaluation and design of pilot plants. Several theoretical and modeling studies are also under way. Among the topics being studied using numerical models are fluid and heat flow, dispersion, land subsidence or uplift, the efficiency of different injection/withdrawal schemes, buoyancy tilting, numerical dispersion, the use of compensation wells to counter regional flow, steam injection, and storage in narrow glacial deposits of high permeability. Experiments to date illustrate the need for further research and development to ensure successful implementation of an aquifer storage system. Some of the areas identified for further research include shape and location of the hydrodynamic and thermal fronts, choice of appropriate aquifers, thermal dispersion, possibility of land subsidence or uplift, thermal pollution, water chemistry, wellbore plugging and heat exchangemore » efficiency, and control of corrosion.« less