Ground / Water Source Heat Pump
Overview
Ground / water source heat pumps differ from air source heat pumps primarily in using a different source / sink of heat. In both ground and water source heat pumps, heat is extracted from water rather than air, in the first case, the water circulates through pipes laid in trenches or boreholes so coupling the heat pump with the soil, whilst in the latter case, pipes are sunk in a stream, river, lake, or the sea so coupling with a body of water. Also in both cases, heat or cold is transferred to water which is used to deliver heating, hot water, and cooling as appropriate.
As a general rule, the COP of the heat pump under standard conditions will be very similar to an air source heat pump, however due to the fact that soil / suitably deep water temperature has a great deal of thermal inertia, there is a far lower temperature difference between summer and winter than with air. Given that the real world efficiency of a heat pump depends heavily on the temperature difference between source and load, this gives ground source heat pumps a significant advantage.
With an air source heat pump, it is likely in the UK that the source air will vary in temperature between around 0 centigrade and + 30 centigrade on the coldest and hottest days respectively, however with a ground source heat pump, temperature is much more constant averaging around 12.7 centigrade in Southern England, and 8.8 centigrade in Northern Scotland with a seasonal margin of only around 2 centigrade either side of this temperature.
For heating, even ignoring the need to defrost an air source heat pump from time to time, a ground source heat pump will be around 20 to 25% more efficient than an air source heat pump in mid winter, albeit with some loss of efficiency producing hot water in summer.
For cooling, it will be necessary to run the compressors of an air source heat pump in summer, whereas for ground source, only a heat exchanger is needed so delivering far more energy efficient cooling.
Ground source heat pump systems can deliver almost free summer cooling as the ground remains cool all summer, meaning that only a heat exchanger and not a the heat pump needs to be run.
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Installing a Ground Source Heat Pump
Installing a ground source heat pump can get expensive as a lot of pipe has to be put into the ground.
There are two main ways to install the pipe; trenching, and boring.
Where room allows, it is usually a lot more cost effective to lay pipes – especially where plant is on site and heat pumps are installed at the time of construction. Trenching is not suitable where room is constrained, or if trenching will disturb an established garden.
Around 50-80 metres of pipe, or 10 metres of slinky coiled pipe are required per kW of heat pipe capacity in a trench system, so a 20 kW system would need around 200 metres of trench to be dug with trenches at least 5 metres apart. For borehole systems around 400 metres combined borehole depth may be needed given that boreholes are narrower than trenches giving less opportunity for coiling the pipe.
For larger systems in areas with productive aquifers, an alternative approach is possible in which 2 or more boreholes are drilled, with at least one each for extracting and injecting water. Water extracted from the first borehole is used as a source of heat via the heat pump, or cooling via a heat exchanger, with the used water then being pumped down the second borehole some distance apart from the first. For a large commercial heat pump used in district heating or for commercial premises, this approach can be far more cost effective as the total amount of borehole drilling is much less, and the building becomes thermally coupled to an extensive aquifer rather than just the soil near the boreholes. Systems of hundreds of kW or even a few MW can be considered assuming enough extraction and injection boreholes can be placed on the site.
Please note that any system extracting and re-injecting water requires an environment agency abstraction licence.
Installing a Water Source Heat Pump
Water source heat pumps tend to be used for large premises requiring a lot of heat, or for community / district heating schemes. Typically, an extensive network of pipes is sunk into the sediment at the bottom of a lake or other body of water, with arrangements otherwise similar to a ground source heat pump.
Typically, both costs and efficiency are lower than a ground source heat pump, but higher than an air source heat pump.
In some cases, other bodies of water are used – such as water deep in flooded coal mines. These provide a great opportunity to cut capital costs as they tend to have extensive networks of tunnels containing very substantial bodies of water. What's more, deep mines obtain a geothermal advantage in the form of water that is sometimes significantly warmer than shallow soil. One such project is due to start construction in 2020 in South Wales near Bridgend where water 230 metres underground is at over 20 centigrade.
Another interesting project whilst not strictly a water source heat pump is the district cooling system serving 7,000,000 m2 of business premises in Stockholm. In this project, sea water heat exchangers are used to transfer cold from the sea which is then passed through 76 km of distributed district cooling pipes. The project is so cost effective that it has been achieved without subsidy.
It should be noted that ground source heat pumps require significantly less space than air source heat pumps or traditional central air conditioning systems, therefore conferring a significant advantage where space comes at a very high premium as a higher proportion of total space can be rented to paying clients.
Other interesting articles:
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Heat Pumps
Air Source Heat Pumps
Is the Heating System Working?
Air Conditioning Units
Thermal Storage
In many heat pump based heating systems, buffer stores are used to help cover peak demand. This can help reduce the required heat pump capacity, with the size of the buffer being an optimisation exercise.
For large systems, it may be worth considering sufficient buffer capacity to allow the heat pump to operate primarily in off peak hours when electricity is cheap, or where present, available from an on-site solar array. This may allow a significant reduction in electricity cost especially from spring to autumn when heat requirements are low.
A further possibility is to use smart controls on a smart tariff to support the power grid by switching the heat pump on and off to help stabilise local voltage or grid frequency, and to free up capacity for other users during demand spikes.
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