Exergy Storage in the Ground

There are many systems in use that store heat in the ground. Most store the heat at a low temperature so they require heat pumps to raise the delivery temperature to a useful value, but that consumes power. Some, like the storage systems used for storing solar heat operate at a temperature that is high enough to avoid the necessity of using heat pumps. The latter are storing both the heat and the exergy that would otherwise be required to drive the heat pumps. Concentric ring heat stores can operate with a hot core that eliminates the need for heat pumps for space heating and DHW, and they can also be designed so that they also provide for space cooling, again without using a heat pump for delivering the cooling. That can be accomplished by using a heat pump to extract heat from the outermost ring and transferring that heat to a central ring. In such a design the average temperature of the outer ring over the year is the same as the ambient ground temperature so there is no net heat flow into or from the surrounding ground, but in the summer that temperature will be low enough to provide the cooling and in the winter it will be elevated so the heat pump can deliver a higher temperature at its output. In such a system, the heat pump operates only when excess power capacity is available (it can be operated directly by the grid operator), enabling the grid to handle power excesses from nuclear stations or wind turbines, and it does not consume any power (except a small amount for circulating pumps) during the peak demand periods. Such systems can use the summer air as the energy source, or they can employ waste heat from AC systems, or they can use solar heat, or any combination of these sources. A variant can also use the cold winter air as the source for large buildings that primarily require cooling. Such systems can be designed to achieve net zero energy objectives for both large and small buildings and in individual cases can even achieve net zero electricity consumption. The air is effectively an unlimited energy source so such systems can be scaled up to handle any likely future demands for heating, cooling and DHW. The physical size of the heat stores (and the resulting depth and cost of the boreholes) is quite small because such stores are very energy efficient and the ground between the rings swings through a wide temperature range.

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