When exploring sustainable energy options for our Maidstone campus, we quickly realised that we had a central area on our estate that might be suitable for installation of a large ground source heat pump (GSHP) system. After thorough research and drilling of pilot bore holes in Spring 2023, we quickly established that a network of boreholes beneath our main car park would be an efficient method of supplying our buildings with heating and hot water, enabling us to remove the enormous gas boilers. 

GSHPs, also known as geothermal heat pumps, are energy-efficient heating and cooling systems that use the constant temperature of the earth to provide a comfortable indoor climate. They work based on the principle that the ground temperature remains relatively stable throughout the year, providing a consistent source of thermal energy.

Here's how ground source heat pumps work:

  1. Heat Exchange: GSHPs consist of two main components: the indoor unit (heat pump) and the outdoor loop system. The outdoor loop system is buried in the ground, vertically in our case. This loop contains a heat transfer fluid (usually a mixture of water and antifreeze) that circulates through pipes.
  2. Heat Absorption: During the heating season, the fluid in the loop absorbs heat from the relatively warmer ground. Even in cold climates, a few feet below the Earth's surface, the temperature remains relatively constant (typically between 50 to 60 degrees Fahrenheit or 10 to 15 degrees Celsius). This heat energy is naturally stored in the ground due to solar energy and geothermal heat.
  3. Heat Pump Process: The heat transfer fluid, now carrying the absorbed heat, is brought back into the building where the indoor unit (heat pump) is located. The heat pump uses a refrigeration cycle, similar to that of a refrigerator or air conditioner, but in reverse. The process involves the following steps:
    a. Evaporation: The heat transfer fluid evaporates at a low temperature and pressure, absorbing heat from the indoor air.
    b. Compression: The vaporized fluid is compressed, which increases its temperature.
    c. Condensation: The hot, compressed vapor releases its heat to the indoor air as it condenses back into a liquid.
    d. Expansion: The liquid refrigerant is expanded, causing it to evaporate again at low pressure, restarting the cycle.
  4. Distribution: The heat released indoors can be used for space heating or to provide hot water for domestic use. The heat pump system is equipped with a distribution system (such as underfloor heating, radiators, or forced air) to transfer the heat to different areas of the building.
  5. Cooling Mode: During the cooling season, the process is reversed. Heat from the indoor air is transferred to the heat transfer fluid, which is then sent to the ground loop system. The heat is absorbed by the cooler ground, and the cooled fluid returns to the building to help cool the indoor space.

The efficiency of ground source heat pumps lies in their ability to transfer heat rather than directly produce it. This makes them highly energy-efficient and environmentally friendly, as they rely on renewable geothermal energy sources and require less electricity to operate compared to traditional heating and cooling systems. However, the installation cost can be relatively high, and it's important to ensure proper design and sizing for optimal performance.