The recommended thermal energy storage capacity for geothermal-based district heating systems is considered. Storage is estimated using several different methods. First, the maximum capacity is evaluated based on the worst case weather event at each locality. Two different relationships for heating demand versus ambient temperature are investigated, one linear and the other nonlinear which has lower heating loads at colder temperatures. For the linear case, maximum recommended storage capacities occurred at a system load ratio (system design heating load; resource production rate) of 1.25, while the nonlinear case showed peak storage sizes for a load ratio of approximately 1.6.

Since the use of only a single worst case weather event ignores the significance of event spacing and may be overly conservative, the operation of the storage tank was simulated over a 20-year period and the storage capacity reevaluated. A final evaluation of storage capacity was done from an economic perspective to find the size that gives the lowest cost peaking system. A strong disparity in storage size requirements among the cities became apparent. District heating systems located in cities characterized by maritime climate were found to be better suited for the use of storage tanks than colder inland climates. The results of the economic analysis model indicate that the maritime cities have a minimum peaking system cost at storage size close to the maximum storage capacities previously predicted, while the results for the two colder inland cities suggest that no storage tanks be included there.

Units: Dual