A ground source heat pump or GSHP is a heating & cooling system for buildings that uses a water-to-water heat pump to transfer heat to, or from a ground heat exchanger. The system takes advantage of the relative consistency of the Earth's temperatures through the seasons. GSHP are among the most energy-efficient HVAC systems, using far less energy than traditional fossil fuel burning boilers and other not heat pump source of heat. Efficiency is given as a coefficient of performance (COP) can reach a range of 4 to 6, meaning that the devices provide 4 to 6 units of heat for each unit of electricity used.
Installation costs can be higher than other HVAC systems due to the requirement to install ground loops by drilling boreholes. However, the significant operational cost savings and low maintenance cost make GSHP a viable option for many projects.
Fig 1. Illustration of ground source heat pump system
In loadmodeling.tool, GSHP can be configured on the Mechanical Plants page. A fully defined system is divided into three main plant loops: Condenser, Heating, and Cooling.
Fig 2. Line diagram of condenser and heating/cooling plant loop from EnergyPlus
The default Vertical Borehole Ground Heat Exchanger - Condensing Loop contains a variable speed pump, a vertical borehole ground heat exchanger on the supply side, and a water-to-water heat pump on the demand side. There can be more than one heat pump connected to a condenser loop, each connected to its cooling or heating plant loops.
The Water to Water Heat Pump Plant - Cooling Loop connects to the Condenser Loop via the heat pump component. This loop contains a variable speed pump, a cooling water-to-water heat pump on the supply side, and a series of connected hydronic cooling coils on the demand side.
The Water to Water Heat Pump Plant - Heating Loop connects to the Condenser Loop via the heat pump component. This loop contains a variable speed pump, a heating water-to-water heat pump on the supply side, and a series of connected hydronic heating coils on the demand side.
For a ground source heat pump system to be complete, a Condenser Loop must be paired with a Heating or Cooling, or both plant loops via the Heating Rejection Source input under the Water to Water Heat Pump properties, shown in Figure 3 below. Adding a Condenser Loop alone will not provide heating or cooling, since there must be a connecting water source heat pump to facilitate the transfer of thermal energy between the ground heat exchanger and the heating or cooling coils of a distribution system.
Fig 3. Heat Rejection Source input under the Heating/Cooling Loop properties
Adding a Water to Water Heat Pump Plant Loop to the project automatically adds a paired Condenser loop. Adding any additional Heating or Cooling plant loops will connect them to the first condenser plant loop that was added unless an additional Condenser loop is added. Then, the user can use the Heat Rejection Source input to specify how their GSHP plants are linked.
Note on Autosizing
Unlike other mechanical plant equipment, the Ground Heat Exchanger is not autosized by the EnergyPlus engine. In order to run a simulation with ground heat exchangers the required capacity will need to be determined first. This can be done by running a simulation with placeholder heating and cooling plants, such as the District Connections. Once the required capacity is know, modifying the bore hole size and flow rate will unlock the full simulation.
On indication that the Ground Heat Exchanger is undersized is the Sever warning of plant temperatures going out of range. If the EnergyPlus warnings include a similar message to figure 4, please increase the capacity of the Ground Heat Exchanger and re-run the simulation.
Fig 4. Example temperature out of control warning
More details on the ground heat exchanger can be found in the EnergyPlus Documentation.