HI-85-26-1 -- Ground-Coupled Heat Pumps with Low-Temperature Heat Storage: Some Swedish Experiences PDF

A large research program in the field of solar-coupled heat pumps with seasonal storage in the ground, using vertical pipes to a depth of 10-40 m, has been going on for several years in Sweden. In this paper, some main results from two experimental plants are presented. As well, results from a technical-economical optimization study are discussed. In these calculations comparisons are made among systems using ambient-air natural convection heat exchangers, lowtemperature solar collectors, and "conventional" ambient-air heat pumps without storage.

One of the experimental plants heats a school and consists of low-temperature solar collectors on the roof, four diesel-driven heat pumps, and a storage system with vertical pipes in clay, depth 35 m. Seasonal performance factors (SPF's) are 3.6 (heat pumps) and 1.7 (total system). The other plant is an experimental one for a single-family house with an electrically driven heat pump, natural convection heat exchangers on the roof, and a storage system in clay, depth 10 m. Annual SPF's are 3.0 (heat pump) and 2.5 (including all auxiliary electricity consumption).

Systems evaluation calculations indicate optimal sizes of the three main components, although the optimas are flat. The differences in economy between systems with solar collectors and with natural convection heat exchangers are small. However, systems with solar or ambientair heat collectors and storage systems cannot today compete economically with ambient-air heat pumps without storage systems, at least if Swedish conditions are assumed.

Units: SI