Regeneration of borehole fields using residential waste heat from ventilation and sewage

Ground-source heating systems in Finland face challenges in their long-term sustainability and feasibility in densely built urban environments. Enough space must be left between the boreholes used for heat collection to avoid premature cooling of the ground due to heat extraction. Ventilation heat recovery for preheating supply air is mandated in buildings, but much of this heat is lost during summer, when there is no need for heating. Sewage heat recovery on the other hand is not required at all. This simulation study examines how injecting residential waste heat into borehole fields can prevent the rapid temperature decline that follows from reducing the spacing between boreholes. Dynamic energy simulations were performed using IDA-ICE and TRNSYS. The results show that storing waste heat in the ground can sustain stable temperature levels in tightly spaced borehole fields indefinitely. This is especially important in large borehole fields meant to service a building cluster. • Tight borehole spacing accelerates temperature decline in a borehole field. • Low surface-to-volume ratio of large borehole fields reduces natural regeneration. • Temperature decline cannot be stopped by direct utilization of apartment waste heat. • Seasonal storage of waste heat stabilizes the temperature of a tight borehole field. • Waste heat storage allows over 80% reduction in borehole field space requirements.