Geothermal exchange heat pumps
A geothermal exchange heat pump, also known as a ground source heat pump or GSHP, is a heat pump that uses the Earth as either a heat source, when operating in heating mode, or a heat sink when operating in cooling mode. All geothermal heat pumps are characterised by an external loop containing water or a water/antifreeze mixture (propylene glycol, denatured alcohol or methanol), and a much smaller internal loop containing a refrigerant. Both loops pass through the heat exchanger. Air source heat pumps use the same principle but extract the heat from the air, rather than the ground. As such their installation is much simpler and cheaper.
There are three categories of geothermal heat pumps based on the type of external loop:
open loop system
closed loop vertical system
closed loop horizontal system.
In an open loop system the thermal transfer fluid (water) does not return and is a ‘once through’ type system. The open loop system draws water from a well or lake, passes it through a heat exchanger in the building, and then discharges it. The water can be discharged to a stream or lake, or injected into a second well. Deep lake water cooling uses a similar process with an open loop for air conditioning and cooling.
The pipe in vertical closed loop system uses a single well (or borehole) with the fluid in the pipe constantly recirculated to and from the well. If a borehole is used, it is commonly filled with a bentonite grout surrounding the pipe to provide a good thermal connection to the surrounding soil or rock.
The horizontal closed loop is placed below the frostline (1 to 2 m underground). In a horizontal closed loop system the pipe is often laid out as a helix (usually known in the UK as a slinky) to increase the contact area per length.
The amount of vertical or horizontal loop required is a function of the ground formation thermal conductivity, deep earth temperature, and heating and cooling power needed, and also depends on the balance between the amount of heat rejected to and absorbed from the ground during the course of the year. A rough approximation of the soil temperature is the average daily temperature for the region.
Geothermal heat pumps are also used in non-residential buildings, but the variety of loads and load patterns in these applications make it difficult to specify rules of thumb for capacity per unit of building area, or quantity of heat exchanger required per unit of heat pump capacity. In commercial applications a field of bore holes is drilled. Bore holes are spaced 5–6 m apart and are generally 15 m deep per kW of cooling. During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil. Reliable heat transfer models have been developed through sample bore holes as well as other tests.
Heat pumps are especially well matched to underfloor heating systems which do not require extremely high temperatures (as compared with wall-mounted radiators). Thus they are ideal for open plan offices. Using large surfaces such as floors, as opposed to radiators, distributes the heat more uniformly and allows for a lower temperature heat transfer fluid.
The Earth below the frost line remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location, so is usually 7-21 degrees Celsius (45-70 degrees Fahrenheit) depending on location. Because this temperature remains constant, geothermal heat pumps perform with far greater efficiency and in a far larger range of extreme temperatures than conventional air conditioners and furnaces.
To understand how a heat pump can heat during the winter and cool during the summer, let us consider each mode:
A diagram of a simple heat pump’s vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.
In heating mode, the external fluid is pumped from the well at 8-16 degrees Celsius and passes through the heat exchange unit. Within the heat exchanger, the refrigerant expands and changes from liquid into gas. This absorbs heat (latent heat of vaporization) from the external fluid, thereby cooling the external fluid. Meanwhile the refrigerant is pumped to the compressor where it is pressurized thereby becoming superheated. This ‘hot gas’ releases the heat and warms the air of the house. At the same time, the refrigerant gas loses heat to the air and changes back to a liquid. The external loop again provides the heat necessary to change the refrigerant back into a gas thereby cooling the external fluid. The external fluid absorbs heat from the soil and the process is repeated. Note the external fluid only changes temperature while the internal refrigerant changes both temperature and phase. There are some residential heat pumps that use refrigerant in the external loop.
The cooling cycle is very similar except a valve on the internal refrigerant loop reverses the direction of flow. Now the compressed refrigerant coming from the compressor heats the external fluid, before passing through the evaporator where it vaporizes taking up heat from the air in the house. The heated external fluid is pumped into the ground where it is cooled and recirculated. Alternatively, the superheated refrigerant may pass through a second heat exchanger allowing the water heater to absorb the waste heat. This means that in summer, the heat pump provides air conditioning and the hot water. The heat is being pumped from the air in the house to the water in the water heater.