An introduction to water source heat pumps and how they work
Posted by John Barker-Brown on 12 July 2012 at 9:36 am
The majority of heat pumps sold within the UK are either ground source or air source. However, water is another source of energy which can sometimes be used for heat pumps.
Using water as your energy source has a number of advantages when compared to air or ground source:• The heat transfer rate from water is far higher than that in the ground or air. • The flow/circulation of the water source provides constant energy replacement.
• The use of a water source removes the need of digging large trenches, often reducing the cost of installation compared to a ground source.
• The return temperature to the heat pump is usually higher than either the ground or winter average air, increasing the CoP (coefficient of performance) of the heat pump.
Water sources can be lakes, ponds, rivers, springs, wells or boreholes and the systems are usually classed as either ‘open’ where water is extracted from the source, flowed around the heat pumps intermediate heat exchanger (or an open loop rated internal heat exchanger) and then discharged; or ‘closed’ loop where, similar to a ground source, pipes or heat exchange panels are placed within the water source and a water/antifreeze mixture is passed through the pipes/panels absorbing energy from the water.
Both systems have advantages and disadvantages:
Open loop water source heat pumps
An extraction licence is generally required from the Environmental Agency for any extraction above 20m3/day, this generally means anything above a 4kW water source heat pump needs an extraction license if the heat pump runs 24hrs a day. A discharge consent is also required and it is important to consider what happens to the colder water after it has flowed through the heat pump.
As there is no ground heat exchanger, i.e. coils of pipe that absorb the energy from the water, and hence no temperature drop across the pipe, open loop systems can be of a slightly higher efficiency than closed loop systems. However, care needs to be taken that this efficiency gain is not lost by any additional pumping costs if the water needs to be lifted higher by a pump, such as within a borehole.
There is a risk of freezing within the heat pump side exchanger. The outlet temperature must not be allowed to approach freezing point or ice will start to form. As a general rule the source water needs to be reliably above 8C for such systems to work well. If the heat exchanger freezes up, the heat pump stops working.
Water quality is also a concern with open loop systems as if the pH value of the water is not neutral, corrosion resistant pump, pipes and heat exchanger are required. Add to this the requirement for a filter and possible water treatment (to prevent algae deposits), maintenance requirements are increased.
Closed loop water source heat pumps
As stated above closed loop systems can be a slightly lower efficiency due to the ground (or lake) heat exchanger and any temperature losses in transferring this heat from the water into the ‘closed’ loop.
However there is no corrosion risk, and because no lifting is occurring, pumping energy loss can be lower. No water is extracted so no licence is required and as the fluid in the closed loop contains anti-freeze there is almost no risk of freezing.
Care has to be taken in the placement of the pipes (generally coiled on mats and called pond mats) or panels to avoid any boat traffic or debris which might float past but once submerged (usually at least a meter deep) they would normally not require any further attention.
With all water source heat pumps it must be remembered that if the water source dries up then without water there is no heat. This is particularly important if open loop boreholes are used. If they have a low replenish rate then the energy source the heat pump utilizes can be exhausted.Kensa has supplied a number of water source heat pumps within the UK, including systems ranging from Lee Valley White Water Centre (the Olympic slalom canoeing course) to sea water systems for the RNLI.
More information about heat pumps from YouGenFind an installer
About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.
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