Shared underground heat store for apartment blocks

Speculative concept
AI illustration · Illustrative scene, not a photograph of the specific device or experiment.

This is an idea, not a product. Nothing here has been built or tested unless a linked technology profile says so.

The problem

Residents of apartment blocks may need heat at times when it is expensive or high in carbon to make. Building owners may have no space or simple way to store heat made at another time.

The idea

A first version would be a small, insulated, below-ground store of sand or crushed rock, charged from an existing heat source and connected to one block's existing heating loop. It would supply space heating and hot water only, and would run alongside the existing boiler or heat pump rather than replace it until the operating data support that. Its size would come from a measured heat demand survey of the block, not from a reference design.

Enabling technology

  • Thermal energy storage: Thermal energy storage keeps heat in materials such as sand, bricks, rock or molten salt so that it can be used later for industrial processes or district heating. Operating systems now exist in Finland and at a steel plant in India, but most performance figures still come from the operators or from modelling.

What stands in the way

  • Heat losses depend on insulation quality. The profile's figure of about 1% of stored heat lost per day comes from NREL's insulated silo design, not from a buried unit under a residential building.
  • The efficiency figures are not like-for-like. Heat-in to heat-out figures above 95% over multiple days cannot be compared with the electricity-in to electricity-out figure above 50% from modelling, so a block's real benefit is not yet known.
  • The published designs run at very high temperatures, such as the 1200 °C NREL charging figure and the 1500 °C Rondo figure. The profile does not give the temperature a block's heating loop or hot water needs, so a suitable low-temperature store would need its own design and testing.
  • The sources in the thermal energy storage profile include no independently measured, multi-year operating data, and the cost figures are targets or estimates. The cost of a block-scale store is therefore not known.
  • Burial under or beside an occupied building needs site checks for space, ground conditions, water ingress and safety that the profiles do not cover.

A cheap way to test it

Bench test with no people involved. Build a small insulated box filled with sand or crushed rock, heat it with a thermostatically controlled electric heater that has a separate fixed over-temperature cut-out, and log temperatures at several depths for at least 72 hours with the heater off after charging. Work out the share of stored heat lost per day, and record the box's surface area and average temperature difference from ambient. Pass if the measured daily loss is at or below 1%, the figure reported for NREL's insulated silo. Fail if it exceeds 1% on two repeat runs. A small box may lose heat faster per unit volume than a large store, so a fail would mainly show that the insulation or logging method needs work, not that a buried store cannot work.