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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 YouGen

Heat pumps information page

Heat pumps: 12 tips for people thinking of installing one

Air source heat pumps keep hotel guests warm, happy and intrigued

Five energy sources for heat pumps

Heat pump field trial: good or bad?

Find an installer

About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

8 comments - read them below or add one

vee-tail

vee-tailComment left on: 22 May 2015 at 10:43 pm

Somewhere in the UK there must be a company that can supply a simple water source heat pump?  Yet after nearly two years of searching I have yet to find one. My fast flowing river maintains +7'C or more thoughout the year. My nearby well insulated barn can be kept comfortable by a 3kw fan heater running continually. The same amount of electricity could produce rather more heat and keep the barn comfortable in winter.  But who can supply me with a water source heat pump that blows warm air for space heating? Not a particularly difficult design task surely?  Just supposing there is such a heat pump available it would be an even greater advantage to be able to run it from the 110volt 5kw dynamo that is driven by my main water wheel.  Perhaps someone on this website knows where I might find the heat pump I am looking for?

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John Barker-Brown

John Barker-Brown from Kensa Engineering LtdComment left on: 21 February 2013 at 11:08 am

Hi Jamie,

Sorry for the delay in getting back to you.

For any heat pump which is replacing mains gas the running costs are going to be roughly the same and we generally find are not sufficent for people to invest in the larger capital cost of a heat pump system.

To give you an idea, http://www.nottenergy.com/energy_cost_comparison/ is a good website for comparing costs of fuels.

For January it reports the best online price for mains gas was 5.05p/kWh, for a GSHP it was 4.12p/kWh, so less than 1p/kWh saving on running costs.

Having said all that, the introduction of the RHI will distort the picture somewhat as it will pay an additional amount depending on the heat demand (how this will be determined is still under discussion, i.e. metering, deeming or calculation). In fact we are not even sure that anyone with mains gas will be eligible or if they are whether it will be at a reduced tariff. Lots of questions on the RHI and not a lot of answers coming from DECC at the minute.

With regards to the question about hot water, a standard R407C heat pump will roughly provide 50C from the hot water cylinder, which is generally hot enough for showers etc. Although you do need to remember that the actual amount of available water for a same size tank is reduced as you use a lower amount of cold water to mix at the point of usage. We would recommend that at least once a week the temperature is raised by use of an immersion heater to above 60C for legionella pasteurisation and it is also important to remember that the tank needs to be designed for a heat pump which has a lower flow temperature than a fossil boiler.

If a lower flow temperature is a concern, there are high temperature units available on the market which use a different refrigerant such as R134a. These can produce flow temps of above 60C, however do generally have a higher capital cost.

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Jamieandthemagictorch

JamieandthemagictorchComment left on: 6 February 2013 at 12:17 am

Ps: would this generate a sufficiently high temperature to also supply hot water for taps and shower etc...?

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Jamieandthemagictorch

JamieandthemagictorchComment left on: 6 February 2013 at 12:05 am

Dear John, I have 30 metres of river running along my garden, which I have riperian rights on, so have been wondering about a closed loop WSHP.

The river is of medium speed flow, between 1.2 and 2m depth, width approx 8m. So I understand potentially good for a mat based WSHP.

We would refurb the house at the same time to make it as efficient as possible, as its currently single skin brick with rads......move to external insulation and UFH.

Question is, with us currently being on mains gas grid, will this actually make sense, or are my rosé tinted Eco glasses just getting in the way.

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John Barker-Brown

John Barker-Brown from Kensa Engineering LtdComment left on: 16 October 2012 at 1:34 pm

Currently the only other scheme which is considered equivalent to MCS is the CEN Solar Keymark Scheme. The CEN Solar Keymark covers the certification of Solar Thermal products only and, unlike MCS, does not cover installation company certification. Consumers are able to use a Solar Thermal product approved under Solar Keymark and will still be able to claim under the Government’s incentive schemes (RHI and RHPP), provided the installation has been completed by an MCS certified installation company.

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martini

martiniComment left on: 16 October 2012 at 1:28 pm

Thanks John.

The YouGen blog says that to be eligible for the RHI the equipment must be MCS (or equivalent scheme) certified. What is equivalent scheme?  I am still hoping that my Ciat heat pump will qualify under 'equivalent scheme'!

Martin. 

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John Barker-Brown

John Barker-Brown from Kensa Engineering LtdComment left on: 1 October 2012 at 11:23 am

Hi Martini,

From looking at the Microgeneration Certification website CIAT do not seem to have MCS approval for any heat pumps. To obtain the RHI you need to install a MCS accredited heat pump, so unfortunately you will not be eligible for the RHI. As far as I understand the scheme, even if CIAT obtained MCS product approval tomorrow, it is not retrospective so anything commissioned before the date of approval will still not be eligible for the RHI.

The running cost of the borehole pump is very important and can be, as you are finding, excessive. It's not much consolation to you but it is a requirement to quote running costs within any MCS quote and this should give the client some idea of expected costs, although it is only an estimate. It’s a lot more detailed now than it was in 2010, and should include the cost of running additional water pumps, however even in 2010 an MCS quote should have given you an estimate of basic running costs.

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martini

martiniComment left on: 29 September 2012 at 8:27 pm

I had a water source heat pump installed in 2010.  I was recommended a Ciat pump by the installers, Ecovision. It has worked extremely well but the electricity costs of running the pump in the borehole are very high.  I have offset this slightly by installing solar pv panels.  I am off the gas grid and previously had oil central heating which was costly but my current electricity bill is 7 or 8 times higher now than it was.  They don't tell you these things when they sell them!  Also, it appears that Ciat have not gained MCS accreditation in this country although they have several other respected international accreditations.  I am worried that this will prevent me from getting the RHI.  Can you comment? 

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