Thermal Feasibility of Ground Source Heat Pump Systems for Civil Defense Facilities

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Persistently high cooling loads and strict safety requirements make thermal regulation in civil defense facilities particularly challenging. Conventional cooling systems often struggle to meet long-term efficiency, reliability, and concealment demands. This study evaluates the thermal feasibility of a ground source heat pump (GSHP) system with high concealment for application in the Zhushan civil defense facility in Nanjing, China, aiming to assess its suitability for managing continuous and intensive cooling demands.

Numerical Modeling of Ground Heat Exchange

A three-dimensional numerical model was developed to simulate coupled flow and heat transfer processes within the ground heat exchanger (GHE). The model incorporates unsaturated porous soil conditions, time-varying fluid flow rates, and environmental disturbances, enabling realistic representation of heat transfer between the circulating fluid, grout, and surrounding soil. This modeling framework provides a robust basis for evaluating subsurface thermal behavior.

Coupling with Heat Pump Unit Performance

The ground heat exchanger model was coupled with a heat pump unit model through facility cooling load, fluid outlet temperature, and coefficient of performance (COP). This integrated approach allowed for a comprehensive assessment of overall system efficiency, capturing the interaction between subsurface heat exchange processes and surface-level heat pump performance under realistic operating conditions.

Cooling Load Characteristics and Energy Consumption

Results show that the civil defense facility experiences consistently high cooling loads due to continuous heat gains from equipment operation and occupant metabolic activity. System energy consumption is dominated by the compressor, accounting for approximately 80% of total power usage. A clear seasonal pattern was observed, with energy consumption increasing steadily and peaking in August, corresponding to the highest cooling demand.

Thermal Performance and System Stability

The average fluid outlet temperature of the GHE (Tf,o), heat pump COP (COPhp), and system COP (COPsys) were 24.51 °C, 5.35, and 4.42, respectively. Most values clustered within narrow ranges, indicating stable operation. Minimum values of Tf,o, COPhp, and COPsys were 18.78 °C, 5.17, and 4.22, while maximum values reached 26.43 °C, 5.78, and 4.71, demonstrating both robustness and efficiency across operating conditions.

Feasibility of GSHP for Civil Defense Applications

The tight clustering of thermal and performance indicators, combined with high COP values and controlled outlet temperatures, confirms that the GSHP system operates under stable and efficient conditions. These findings demonstrate the thermal feasibility of GSHP systems as a reliable and energy-efficient solution for managing persistently high cooling loads in civil defense facilities, supporting their broader adoption in similar high-demand, high-security infrastructures.

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