Heat energy collection and storage device comprising a phase change material

A heat energy collection and storage device 6 for placing in a volume, such as the ground 4, forming a source or sink of heat energy. The device comprises a container 7 containing a phase change material 8, such as a substance based on paraffin. Pipework 10 may be provided for passing a heat transfer medium, such as a water and anti-freeze mixture, through the phase change material to transfer heat between the heat transfer medium and the phase change material. The phase change material is in a first, preferably liquid phase, at the ambient temperature of the volume and is in a second, preferably solid phase, at the temperature of the heat transfer medium when it enters the device. Heat may be transferred from the volume in contact with the container's surface to the phase change material, or vice versa. Preferably, the device comprises part of a cooling or heating system, such as a heat pump 2, where geothermal energy is used as a heat source. The ambient temperature surrounding the device may be increased by introducing additional solar heat energy.