Seasonal thermal energy storage (STES) involves storing thermal energy, such as winter chill, summer heat, and industrial waste heat for future use in heating and cooling buildings or for industrial processes. Widespread development and implementation of STES would significantly reduce the need to generate primary energy in the U.S. In fact, 1980 data indicate that STES is technically suitable for providing 5 to 10% of the nation's energy with major contributions in the commercial, industrial,

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... nd residential sectors. Aquifer thermal energy storage (ATES) is predicted to be the most costeffective technology for seasonal storage of low-grade thermal energy. Approximately 60% of the U.S. is underlain with aquifers potentially suitable for underground energy storage. Under sponsorship of the U.S. Department of Energy, Pacific Northwest laboratory (operated by Battelle Memorial Institute) has managed numerical modeling, laboratory studies, evaluation of environmental and institutional issues, and field testing of ATES at several sites. This report describes the computer program UCATES, developed by Dr. H. Haitejema as a first step toward developing a comprehensive screening tool for ATES systems in unconfined aquifers. The program is capable of predicting the relative effects of regional flow on the efficiency of ATES systems, as well as predicting the effects of water table variation due to ATES pumping and injection. The code can be applied in its present form to virtually any unconfined ATES analysis. The model described here uses a somewhat unrealistic treatment of heat loss to overburden and underlaying soil. Inclusion of a more realistic heat loss mechanism within UCATES would improve the code making more reliable predictions of absolute ATES efficiencies possible. However, the predictions of the code described in this document should be adequate for preliminary design, evaluation, and optimization of unconfined ATES systems. Landis D. Kannberg, Manager Underground Energy Storage Program Convective heat transport in unconfined aquifers is modeled in a semi-analytic way. The transient groundwater flow is modeled by superposition of analytic fUJlCtions, whereby changes in the aquifer storage are represented by a network of triangles, each with a linearly varying sink distribution. This analytic formulation incorporates the nonlinearity of the differential equation for unconfined flow and eliminates nlDDerical dispersion in modeling heat convection. The thermal losses through the aquifer base and vadose zone are modeled rather crudely. Only vertical heat conduction is considered in these boundaries, whereby a linearly varying temperature is assumed at all times. The latter assumption appears reasonable for thin aquifer boundaries. However, asslDDing such thin aquifer boundaries may lead to an overestimation of the thermal losses when the aquifer base is regarded as infinitely thick in reality. The approach, indicated above, is ~lemented in the computer program UCATES, which serves as a first step toward the development of a comprehensive screening tool for Am5 systems in unconfined aquifers. In its present form, the program is capable of predicting the relative effects of regional flow on the efficiency of A'lES systems. However, only after a more realistic heatless mechanism is incorporated in UCAn:S will reliable predictions of absolute A'IES efficiencies be possible. v