University of Minnesota aquifer thermal energy storage (ATES) project report on the second long-term cycle

Abstract

The technical feasibilityof high-temperature[>100°C (>212°F)]aquifer thermal energy storage (ATES) in a deep, confined aquifer was tested in a series of experimentalcycles at the Universityof Minnesota's St. Paul field test facility (FTF). ,_This report describesthe second long-termcycle (LT2), which was conducted from October 1986 through April 1987. Heat recovery_ operational experience;and thermal, chemical,hydrologic, and geologic effects are reported, Approximately614 of the 9.21 GWh of energy added to the 9.38 x 104 m3 of ground water stored during LT2 was recovered. Temperaturesof the water stored and recovered averaged 118°C (244°F) and 85°C (185°F),respectively. Results agreed with previous cycles conductedat the FTF. System operation during LT2 was nearly as planned. Operationalexperience from previous cycles at the FTF was extremely helpful. Ion-exchangesofteningof the heated and stored aquiferwater prevented scaling in the system heat exchangersand the storage weil, and changed the major-ion chemistry of the stored water. Sodium bicarbonatereplaced magnesium and calcium bicarbonate as primary ions in the softenedwater. Water recovered from storage was approximatelyat equilibriumwith respect to dissolved ions. Silica, calcium, and magnesiumwere significantlyhigher in recoveredwater than in injectedwater. Sodium was significantlylower in water recovered than in water stored. Temperaturesat Ironton-Galesville horizons in storage site monitoring wells reached~118°C (~244°F) during LT2. Following heat recovery, temperatureswere~40°C (~I04°F) at the same locations. Slow and slight thermal responseswere observed in low permeabilityzones. No thermal or chemical effects were observed at the remote monitoring site. The softener lowered hardness of the source water from 160 mg/L to less than I0 mg/L as calcium carbonate. Sodium concentrations in the source water increased from~44 mg/L to~122 mg/L in the water softener. 6. Groundwater chemistry results from LT2 suggest that the water from the source well was at equilibrium with respect to major ions. Water recovered from storage had reached equilibrium with respect to hardness and silica at the recovered water temperature. Mass balances calculated for different ports of LT2 show effects of ion-exchange softening, changes in equilibrium concentration or water temperature, and mixing. viii ACKNOWLEDGMENTS This work, supportedby the U.S. Departmentof Energy, was performed by the University of Minnesota as part of the UndergroundEnergy Storage Program at Pacific NorthwestLaboratory. Pacific Northwest Laboratoryis operated by Battelle Memorial Instituteunder contract DE-ACO6-76RLO1830. Special thanks go to program managers L.D. Kannberg and J.R. Raymond for their suggestions, encouragement,and guidance.

University of Minnesota aquifer thermal energy storage (ATES) project report on the second long-term cycle

“…Interruptionsand variations in flow and temperaturewere much less a feature of these experiments. The ion-exchange water softener permitted operation at a more constant temperatureand flow rate than was possible when the condenser was scaling up with calcium carbonate LTI was subject to some interruptionsuntil the water softener was operatingproperly (Hoyer et al 1991). bed.…”

“…Monitoring wells at the storage site recorded the aquifer response to the injection, storage, and recovery of the heated water during the cycle. As during the previotls cycles (Hoyer et al 1985(Hoyer et al , 1991Walton et al 1991) When injectionwas interrupted,temperaturesat some horizons dropped while others increased. Where shales predominated,temperatures increased throughoutthe storage phase so that several horizonswere warmer at the end of the stcrage phase than at the beginning.…”

“…Temperaturesin the porous and permeableportions of the aquifer declined during recovery (for example, see Walton et al (1991) and Hoyer et al (1991).…”

“…(samplingPort I) were influencedby LTI, which was completed 18 months before the initiationof LT2 (Perlingeret al 1987), and to a lesser extent by the four short-term test cycles (Holm et al 1987). The source water temperature ranged from 28°C to 38°C (82°F to tOO°F), reaching a maximum temperatureat a volume of approximately17,700 m3 (-76,000 m3, Figure 5.3), a result of returningwarmer-than-ambient water to well B during the recovery phase of LTI (Hoyer et al 1991; see also LT 2…”

Cyclic injection, storage, and withdrawal of heated water in a sandstone aquifer at St. Paul, Minnesota--Analysis of thermal data and nonisothermal modeling of long-term test cycles 1 and 2

“…Unlike the short-term test cycles, when injection was interrupted to clean the filters after 1 or 2 days, flow was maintained for many days at relatively constant temperatures during the long-term test cycles with few interruptions. The source water temperature varied smoothly and the temperature of the injected water was controlled by the temperature controller, or varied as a response to the capacity of the heat exchanger others, 1991a and1991b). Injection was interrupted eight times during long-term test cycle 1, and five times during long -term test cycle 2.…”