Promoting best practice with CIBSE
The Code of Practice for surface water source heat pumps published by the Chartered Institution of Building Services Engineers (CIBSE), has affirmed the many opportunities afforded by the UK’s water sources when used as a heat source for ground source heat pumps. In this blog Kensa’s Stephanie Gregory explains how installers can maximise the potential of ground source heat pumps by tapping into water sources, now with the added support of the new Code to set best practice and raise industry standards.
Water sources have long been regarded as a means to produce affordable and secure heating from a low carbon source, yet their low profile to date is not reflective of their scale of opportunities.
Especially common in cases of larger, more rural domestic and commercial properties, those in close proximity to a water course – including seas, canals, rivers, lakes and estuaries – can access and extract heat energy from this body of water at a much lower cost than traditional ground collectors and alternative forms of heating.
A Kensa project featured in the CIBSE Code demonstrates the use of an open loop ground source heat pump harnessing heat energy from sea water at the Baltimore lifeboat station in Ireland.
The submerged pipework in the body of water absorbs the heat energy from its surroundings in the same manner as it would if buried in ground, however, due to water being an excellent conductor of heat, it is especially efficient for heat extraction, making ground source heat pumps with water sources an exceptionally efficient form of heating. This also makes them especially lucrative when it comes to the Renewable Heat Incentive as payments are based on the renewable portion of heat produced by the system; the more efficient, the better the RHI payment.
What makes water a good heat source for ground source heat pumps?
- The heat transfer rate from water is higher than that in the ground;
- The water is in close contact with all of the pipe at all times enhancing its efficiency;
- The flow/circulation of water provides constant energy replacement;
- The use of a water source removes the need for digging or drilling, reducing the cost and duration of the installation;
- The return temperature to the heat pump is generally 5-6°C higher than ground collectors, increasing the efficiency of the heat pump.
Case Study: Grade II listed property with lake water source, Cambridge
River House has a large lake on the grounds which is fed by a natural spring. Measurements of its flow rate along with its area and depth, confirmed that more than sufficient heat energy from the lake could be extracted to provide heat and hot water to the Grade II listed property via a ground source heat pump.
Specially created pond mats consisting of slinky pipes attached to corrosion resistant stainless steel frames were sunk into the lake on a closed loop system. The pipes were connected up to the ground source heat pump unit in an annex building via pipes running under the lawn.
Owner of River House, Keith Clarke, said: “I knew that ground source was a great technology for extracting the natural heat that’s available from the environment to provide a completely green heating system. The good thing about the pipes being in the lake is that water has great conductivity meaning we are able to extract heat sustainably throughout the winter to keep the house perfectly warm.
“I know that there’s a big investment involved in getting a ground source heat pump, however we realised that we could get the whole thing paid for with the Renewable Heat Incentive, including the installation. Plus, we’ve ended up with a system where the yearly cost of electricity is less than we would have been paying for gas.”
Mr Clarke’s project is featured in a Kensa video available to view here:
Different water sources offer different considerations for the end user:
If your client has a lake on their property this could be ideal for pond mats. The number of mats you need will depend on the size of the heat pump as well as the size of the body of water; and importantly, how that body of water is fed.
If it is stagnant water then the only real heat gains are from the sun and rain so you will need to space your mats correctly to avoid over-extracting energy.
If the lake or stream is spring fed, try and check the average temperature over a couple of seasons.
Pond mats can be placed right next to each other if there’s really good water flow across them; as a general rule, allow around 3 litres per minute per kilowatt of heat pump size.
Streams and rivers:
If your client has a stream or river, then this can be a suitable heat source if of sufficient depth.
If you find that it is too small or too shallow for pond mats diverting the course of smaller rivers and streams is possible, providing you with a better location for placing mats in.
Again, following the 3 l/min rule will help you check that you’ve got the required flow rate.
Where a more considerable water source is available an open loop system could be considered. Often this is thought of as drawing water directly from the river and putting it through the heat pump’s evaporator heat exchanger. To overcome risks of corrosion and filtration Kensa generally suggest using a second plate heat exchanger as an intermediary, so that only a clean water/glycol solution passes through the heat pump itself protecting the components in the unit.
With this kind of system bear in mind that there is a potential maintenance cost because you may have to clean the open loop pumping equipment and filters from time to time; how often will depend on the quality of the water.
Remember that there are abstraction limits in place, which prevent the removal of too much river water; you are allowed to take 20 cubic metres of water per day from an open loop water source without any licence requirement. If you plan on using more than the limit, you will generally need to apply for permission from the relevant authorities.
Open loop systems utilising aquifers or water ways are also an excellent means to extract heat energy. Consulting a hydrologist will help to determine the existence and suitability of any subterranean water source, with particular consideration again over the sources yield. Many farms and rural commercial premises have access to a potable water boreholes, which can be an excellent source of heat provided it is deep enough and provides enough yield.
Both open and closed loop systems are possible. Due to sea water’s corrosive properties, specialist titanium heat exchangers can be used, or pond mats fixed to piers or pontoons is an alternative option.
Further advice on the use of water sources is available via CIBSE’s Code of Practice for surface water source heat pumps, available to purchase here: http://www.cibse.org/knowledge/cibse-other-publications/cp2-surface-water-source-heat-pumps-cop-for-the-uk (free to CIBSE members).