Cooling: Ground Source Heat Pumps
A ground-source heat pump (GSHP) comprises two main components: a heat pump and a ground-coupled heat exchanger.
A heat pump is a refrigerator. Usually a refrigerator is used to keep things cool, which means removing heat. The heat that is removed has to be rejected to somewhere and it is usually rejected to the atmosphere and lost. With a heat pump, instead of rejecting the heat to atmosphere, it is put to good use.
A characteristic of all heat pumps (and refrigerators) is that they “upgrade” the heat source. For instance, with a simple domestic refrigerator, air inside the refrigerator cabinet is transferred to the black coil at the back, and in the process the temperature of the transferred energy is increased to about 45° C (roughly the same energy content, but at a much higher temperature). Although this is the very purpose of a heat pump, the greater the temperature lift, the more energy that is used.
Ideally heat pumps would be used to provide simultaneously useful cooling and heating (e.g. cool an office and heat water for showers), but often the heating and cooling requirements are not simultaneous. When this occurs, a thermal store is needed. The most massive thermal store available is the ground.
In order to get thermal connection with the ground, heat exchangers have to be buried beneath the surface. Heat exchangers can be horizontal (slinkies) or vertical (piles).
Slinkies should usually be buried at least two metres below the ground surface (and sometimes deeper still). This is a lot of excavation and can be very expensive and disruptive. Alternatively, energy piles are bored into the ground – typically a hole around 400mm diameter is bored tens of metres (sometime a couple of hundred metres) into the ground into which a vertical U-tube (circa 80mm diameter) is inserted. The hole is then back-filled with a thermally-conductive material.
Once the ground-source heat exchangers are installed, there are a number of ways of using of them. One way would be to use the heat pumps to transfer heat from the ground to meet a heat demand (say domestic hot water). Because heat has been extracted from the ground, the ground will now be cooler. Using the same ground-coupled heat exchangers, but at a later time or date, it is possible to circulate water between the cool ground and a cooling load, which might be used to keep a building cool. Because the ground is a giant thermal store, the heat extracted earlier to heat the domestic hot water has created a heat sink that can be used to cool the building. The energy extracted from the building will be available later as a primary heat source for the domestic hot water.
Because of the huge thermal capacity of the ground, the heating and cooling phases can be weeks or even months apart. The main problem with GSHPs for Brazil is the absence of any significant heating loads. The load that does exist is domestic hot water, and domestic hot water should be stored above 60° C to prevent Legionella. At 60° C, heat pumps work quite inefficiently. Furthermore, domestic hot water loads are more easily satisfied by solar-thermal systems.
If the ground were to be used only as a heat-sink for cooling processes, it would become thermally saturated over time and then cease to be useful. Potentially evaporative or dry air coolers could be used at night and during the winter to restore the ground temperature, but this appears to be a very marginal benefit given the high cost of implementation: ideally there would be a balanced demand for heating and cooling over the annual cycle.
Barriers to Adoption of GSHP Technologies
There are a number of barriers to the widespread adoption of GSHP systems in Rio:
- Absence of a balanced annual demand for heating and cooling
- High natural ground temperature (about 20° C)
- Constructing the ground loop adds significant cost, complexity and risk to projects
- Cost estimations are difficult due to uncertainty – which may result in prices being inflated
- Space constraints in urban areas may restrict ground-loop construction
- Constructing GCHP systems increase project duration and create additional waste
- There are a limited number of qualified designers and installation engineers in Brazil, where the use of heat pump technology is not widespread