Underground stored thermal energy, abbreviated as \(Q_s\), is when thermal energy (heat or cold) is stored in the subsurface, such as in geological formations, aquifers, bedrock, or caverns for later extraction and use.
Stored Thermal Energy Formula |
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\( Q_s \;=\; C_t \cdot V_r \cdot \left( T_{at} - T_r \right) \) (Underground Stored Thermal Energy) \( C_t \;=\; \dfrac{ Q_s }{ V_r \cdot \left( T_{at} - T_r \right) } \) \( V_r \;=\; \dfrac{ Q_s }{ C_t \cdot \left( T_{at} - T_r \right) } \) \( T_{at} \;=\; T_r + \dfrac{ Q_s }{ C_t \cdot V_r } \) \( T_r \;=\; T_{at} - \dfrac{ Q_s }{ C_t \cdot V_r } \) |
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| Symbol | English | Metric |
| \( Q_s \) = Underground Thermal Energy Stored | \(Btu\) | \(kJ\) |
| \( C_t \) = Volumetric Heat Capacity | \(Btu \;/\; ft^3 \cdot ^\circ F \) | \(J \;/\; m^3 \cdot ^\circ K \) |
| \( V_r \) = Reservoir Volume | \(ft^3\) | \(m^3\) |
| \( T_{at} \) = Average Reservoir Temperature | \(^\circ F \) | \(^\circ K \) |
| \( T_r \) = Reference (Cutoff) Temperature | \(^\circ F \) | \(^\circ K \) |
It takes advantage of the relatively constant temperatures, large storage capacity, and natural thermal insulation of underground environments to hold energy over periods ranging from days to seasons. Excess heat (for example from solar collectors, industrial waste heat, or summer ambient conditions) or cold can be injected into the ground and recovered when needed for heating, cooling, or industrial processes. This helps balance supply and demand, supports renewable energy integration, reduces peak electricity loads, and enables efficient seasonal storage.
Main Types Include
Aquifer Thermal Energy Storage - An open-loop system that circulates groundwater through natural aquifers.
Borehole Thermal Energy Storage - A closed-loop system using arrays of boreholes with heat-exchange pipes in soil or rock.
Reservoir or Cavern Thermal Energy Storage - Storage in engineered or existing underground cavities, flooded mines, or reservoirs.
