Results of the Third Long-Term Cycle at the University of Minnesota Aquifer Thermal Energy Storage (ATES) Field Test Facility 929051

The third long-term ATES cycle (LT3) was conducted between October 1989 and March 1990. Objectives of LT3 were to demonstrate that high-temperature ATES could supply a real heating load and to simplify the water chemistry modeling. For LT3 the Field Test Facility (FTF) was connected to a nearby campus building to demonstrate the FTF's ability to meet a real heating load. For LT3 the wells were modified so that only the most permeable portions of the Ironton-Galesville aquifer were used to simplify water chemistry comparisons and modeling. The campus steam plant was the source for heat stored during LT3.

A total volume of 63.2 x 10³ m³ of water was injected at a rate of 54.95 m³/hr into the storage well at a mean temperature of 104.7°C from October through December 1989. Tie-in to the Animal Sciences Veterinary Medicine (ASVM) building was not completed until late December. A total volume of 66.0 x 10³ m³ of water was recovered at a rate of 44.83 m³/hr from the storage well at a mean temperature of 76.5°C from January through March 1990. Highest and lowest temperatures of recovered water were 100.0 and 47.8°C, respectively.

Approximately 66% of the energy added to the aquifer was recovered. The useful minimum temperature for recovered water was 49°C; approximately 50% of the energy added to the aquifer above 49°C (33% of the total energy stored) was delivered to the ASVM building. Approximately 15% of the usable (10% of the total) energy stored was actually used in the ASVM building. Operations during heat recovery with the ASVM building's reheat system were troublefree. Integration into more of the ASVM (or other) building's mechanical system would have resulted in significantly increasing the proportion of energy used during heat recovery. The cost to connect to other ASVM building systems for this experimental cycle was the main reason for not incorporating other building systems into the FTF.

The FTF ion-exchange water softener reduced hardness of the source water to <5 mg/L as CaCO₃ prior to heating, allowing successful operation. Water chemistry is critical to the operation of the University of Minnesota FTF, or any ATES facility.

This work was conducted for the U.S. Department of Energy through its Pacific Northwest Laboratory.