Underground storage of heat in solar heating systems

Abstract The thermal interaction between underground heat storage and the surrounding ground is studied. The storage medium may be water in a tank, rocks, the ground itself, etc. For the case of hemispherical geometry, analytic steady-state solutions and numerical solutions to select time-dependent situations are presented and discussed. The quasi-steady-state average daily net heat loss into the surrounding ground from uninsulated underground heat-storage facilities is estimated to be only a few per cent of the heat storage capacity for typical home-size units in low-conductivity ground in the absence of ground-water flow. The economics of added insulation under these circumstances appears to be critically site and system specific. The time required for the average heat-loss rate of a new system to approach the steady-state value is on the order of a year. The ground surrounding a typical heat-storage facility contributes very little to the system's storage capacity either on a daily or a seasonal time scale.