Plumbheat Article |
Ground source heat pumps (GSHP's) are one of the fastest growing, yet already long established, renewable energy sources. They are electrically powered systems, which utilise the largely unexploited resource of the earth's relatively constant temperature to make use of energy from the ground heat to provide space heating, cooling and hot water. As they become increasingly commonplace, it is of paramount importance that designers and installers are aware of the pertinent environmental and financial factors affecting system design and installation.
Most commonly GSHPs are indirect systems where a water/antifreeze solution circulates through the ground loop and energy is transferred to or from the heat pump refrigerant circuit via a heat exchanger, and work by applying some basic principles of physics
- Heat will always move from a warm surface to a colder one
- Liquid absorbs heat as it boils, and emits heat as it condenses.
- When a liquid or a vapour is compressed its temperature rises and when that pressure is released, its temperature falls.
A heat pump is essentially a vapour compression refrigeration cycle working in reverse, collecting heat instead of rejecting it. Low-pressure refrigerant is boiled at the evaporator by the heat capacity of the source. The saturated refrigerant vapour is then pressurised by the compressor. The mechanical work done by the compressor is transferred as heat energy that raises the temperature of the vapour. The high temperature, high pressure vapour is condensed to a liquid at the condenser, thus releasing its heat. The high-pressure liquid is finally passed through an expansion device that reduces the pressure and temperature to restart the cycle. The Coefficient of Performance (COP) is a measure of the efficiency of a heat pump.
The temperature difference between the ground and the fluid in the ground heat exchanger drives the heat transfer therefore it is important to determine the ground temperature. At depths of less than two metres the ground temperature will show marked seasonal variation above and below the annual average air temperature. As the depth increases the seasonal swing in temperature is reduced and the maximum and minimum soil temperatures begin to lag the temperature at the surface. At a depth of about one and a half metres the time lag is approximately one month. Below ten metres the ground temperature remains effectively constant at approximately the annual average air temperature (between 10-14 O c in the UK depending on local geology and soil conditions). Therefore, the design of the ground loop(s) is of paramount importance and if not designed and installed correctly could jeopardise the operation of the system.
Ground source heat pumps work most efficiently at lower temperatures (around 35°C) This makes heat pumps the ideal heat source match for underfloor heating, which also works on the principle of low temperature heating. The lower the temperature required to be delivered to the underfloor heating, the more efficient the heat pump is and the cheaper to run.
The underfloor heating system should be designed to operate at a mean temperature of 35 o C, within a very well insulated building. Nibe VIP installers and specialist system designers Anderson Floor Warming achieve this by installing the 16-20mm dia. PEX pipes at 100 to 200mm centres, giving a large volume of water within the screeded floor.
In a conventional radiator system the surface area of the radiator and the average temperature of the water flowing through it determines the heat output, therefore, for a given radiator size, the output can be varied by changing the mean water temperature. When this principle is applied to UFH where the size of the emitter, in this case the floor, is so large that the water temperature is reduced to well below that commonly used in radiators and still provide the required heat output. Therefore the UFH system operates at the temperature efficiently achieved by the GSHP.
Underfloor heating is not capable of meeting typical design criteria room temperatures within bathroom/en-suite areas, mainly due to the limited floor area and mechanical ventilation. Therefore, an additional source of supplementary heat, such as a towel rail is required. As previously described for radiators, the temperatures required are higher than a GSHP can efficiently produce. If a conventional wet towel rail were to be fitted with an electric immersion, to raise the temperature from 35°C to 60°C, there is a possibility that the immersion would run for longer periods, heating the entire system. The simple, cost effective solution is to use an electric towel rail controlled by a timer and electrical thermostat.
The generation of domestic hot water requires careful consideration. Domestic hot water should be delivered at a temperature of around 50 O c, with the facility to reach 60 O c to kill bacteria. Therefore, a supplementary heat source, usually an electric immersion heater utilising an economy mode, within the hot water cylinder, or heat pump is required to reduce running costs.
Heat pumps have many positive points such as high reliability, fewer moving parts and lack of exposure to weather. This also gives a higher level of security, as there are no visible external components to be damaged or vandalised. The lifetime of a heat pump is expected to be 20 to 25 years and up to 50 years for the ground coil, in comparison to a gas or oil system of 10 years. The maintenance cost of the heat pump is also expected to be around one third that of a conventional system. Also as a heat pump has no flue or fuel tank, there are no combustion or explosive gases within the building.
These benefits contribute to heat pumps being one of the fastest growing renewable energy technologies. Anderson Floor Warming Ltd welcome enquires from anyone interested in learning more about ground source or air source heat pumps and can be contacted on: 0141646 6716, e-mail: mail@andersonfloorwarming.co.uk web: www.andersonfloorwarming.co.uk
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