Potential benefits of thermal energy storage in the proposed Twin Cities district heating-cogeneration system. Final report

Abstract

A new, large, cogeneration-district heating system has been proposed for the Twin Cities area, using hot water in a closed-loop system. The proposed system, as described by Studsvik Energiteknik AB of Sweden, does not employ thermal energy storage (TES). Four cases have been developed, describing system configurations which would employ TES, to evaluate the potential benefits of incorporating annual-cycle TES into the Twin Cities system. The potential benefits are found to be substantial, confirming results of earlier, generic studies of aquifer TES. The reference (Studsvik) system employs oil-fired boilers to supplement cogenerated heat, for handling peak loads and providing standby reserve. TES can serve the same function, with net energy savings in spite of heat losses during storage, by making it possible to operate the cogeneration equipment at higher capacity factors. Coal replaces oil as the fuel consumed. Energy savings of the reference system are impressive; energy savings with TES are 2 to 22% better. Capital cost requirements for boilers, cogeneration equipment, and pipelines are reduced by $66 to $258 million. The breakeven capital cost of TES is estimated to range from $43 to $76 per kilowatt peak thermal input to or withdrawal from aquifer TES. A factor in evaluating the breakeven operating cost of TES is the $14 to $31 million per year saving in cost of fuel. Abatement of air pollution and thermal pollution are concomitant benefits.