A numerical model for seasonal storage of solar heat in the ground by vertical pipes

We present a three-dimensional numerical model for seasonal heat storage in the ground using vertical heat exchanger pipes. The model also accounts for convective heat flows in the ground. The storage is employed in a district solar heating system with a heat pump. The effects of storage volume, storage medium, collector area, and collector type on system performances are studied for the Helsinki (60°N) climate. Economic optimization of the storage and collector installation is also briefly discussed. For a 500-house community, a collector area of 35 m² per house and a rock storage volume of 550 m³ per house would provide a solar fraction of 70%.