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Can solar panels improve the efficiency of your ground source heat pump?

Posted by John Barker-Brown on 3 October 2011 at 5:05 am

There is a growing interest in using solar panels to deposit heat within the ground during the summer periods and then using a ground source heat pump to extract this deposited heat during the winter. These systems are commonly called inter-seasonal storage systems and can usually be classed as:-

* Passive or low temperature systems –these rely on highly insulated properties which are designed to use heat/energy naturally stored in the surrounding ground to heat the building.

* Warm temperature active systems – these use systems such as solar thermal to maximise the energy available from the sun and deposit this in the surrounding ground of the property for use in winter.

* High temperature active systems – this type of system also uses solar capture systems but deposits the energy into highly insulated storage devices which can result in high temperatures being stored.

Generally the systems currently being marketed domestically, fall with the classification of warm temperature active systems.

The effectiveness of these systems depends on a number of factors:-

* The conductivity of the ground – this dictates the speed at which the energy moves within the ground. Obviously if this is high any energy within the ground can more quickly out of the area.

* The moisture content – high moisture content will result again in high conductivity and can leech any deposited energy away.

* Types of ground array – arrays which have a smaller ground surface footprint are less affected by seasonal climate changes. Shorter boreholes placed close together have a greater ability to store energy; however their performance when heat is extracted can be compromised.

* The amount of solar panels available to be dedicated to heating the ground (as opposed to heating the DHW).

Warm temperature active systems generally will not raise the ground temperature above the normal ambient ground conditions as any heat deposited in this way will usually not remain within the ground collector area. Boreholes will tend to hold the heat deposited longer than horizontal systems.

Warm temperature active systems are really designed as an assist in recovering the ground back to its original temperature quicker, a bit like refilling a well with water. If the ground array is sized correctly then the ground should recover back to its original temperature for the start of the next heating season without any solar recharging.

One area where solar recharging might have a benefit is within the diurnal periods of the year, i.e. early spring and autumn. At these times energy is deposited within the ground during the day while the sun shines. This energy is deposited locally around the ground array pipes. As the sun goes down and the heating turns on, the deposited energy is still held locally and is absorbed back into the system improving the CoP and hence efficiency of the system.

However for domestic applications the amount of energy that is deposited by the solar panels and reclaimed by the Ground Source Heat Pump is relatively small and the cost of the additional equipment can be relatively high so currently it doesn’t seem a commercially viable system.

Where it can make a difference is large commercial projects which use the heat pump for cooling and actively deposit the removed heat into the ground. As these systems are on boreholes and cooling rates are high they ca, with careful design and modelling enable the designer to reduce the size of the borehole field.

Photo by mjmonty

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

6 comments - read them below or add one

FABECO

FABECOComment left on: 5 March 2012 at 4:45 pm

If you have a heat pump combined with a solar thermal system to top up your domestic hot water and the solar thermal can do it's job in 1 to 2 hours what is the point of turning it off for the rest of the day. What is the extra cost to install a small ground array, pump and valve to deposit the excess heat generated from your solar. I would imagine it could keep your heat pump working to its maximum cop for a much longer period of the year.

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John Barker-Brown

John Barker-Brown from Kensa Engineering LtdComment left on: 10 October 2011 at 9:59 am

Dear Mike,

There are lots of different systems marketed by various different companies, some are successful others are not so successful. The article was more on solar thermal systems which are being promoted as a means for recharging the ground and hence increasing the efficiency of the heat pump. Maybe I should have made the title clearer.

Buffer vessels are generally added to systems to avoid short cycling of the heat pump due to building not been able to accept the minimum run time of the heat pump and in fact can actually cause a drop in the overall system efficiency due to mixing and additional pumps. Adding a coil to the buffer vessel and linking the solar to this will increase the efficiency of the system as it will mean the heat pump will run less, however you need to ensure that the solar is adequately sized to provide the DHW plus the Space Heating assist and again it will only be in the Spring and Autumn when this will really make a contribution to the Space Heating. The system control is a little bit more complicated but it is possible.

The comments are valid for ground or air source

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mike's carbon revolution

mike's carbon revolutionComment left on: 7 October 2011 at 6:46 pm

John, is your answer to the question you pose in the title therefore 'no'? I'm surprised that you chose to focus your answer on the 'warm temperature active systems' given that, as you rightly point out, they are not really appropriate for residential properties. I have had an air source heat pump for nearly 2 years now. I'm no technical expert but have been looking into ways of improving the sub-optimal performance of my ASHP which simply feeds into my hot water cylinder. What I've discovered over the past year is that more and more system designers (and indeed manufacturers) are including a buffer tank in heat pump systems because they materially improve overall system performance. The reason I mention this is that I've had several designs and quotes recently presented to me that include solar thermal panels feeding into a buffer (or storage) tank. This helps prime the water before it goes into heat pump and also ensures a more constant supply of heat for the house. I'm guessing that this way of combining solar and heat pumps (whether air or ground source) falls into your 3rd type of inter-seasonal storage system, 'high temperature active'. Would you agree that, unlike 'warm temperature active systems', this option provides a resounding "yes" to the question in your title? Or does this not apply to groundsource? If the answer is 'yes', it is a shame they are not more widely marketed.

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John Barker-Brown

John Barker-Brown from Kensa Engineering LtdComment left on: 4 October 2011 at 1:21 pm

The article doesn't talk about if solar thermal reduces slinkies, but about its effect on the efficiency of the heat pump. 

With regards to efficiency there will be a small increase in efficiency in Spring and Autumn but the cost of installing equipment solely for this will outweigh any gains in efficiency.

If you have additional energy available from existing solar panels, you might increase your efficiency of the heat pump very slightly and whether it is worthwhile we depend on the cost of the modifications and the increase in efficiency.

Whether you can reduce slinkies is another question and personally I would say no, simply as the ground and hence slinkies are affected more by changes in ambient temperatures and water flow meaning any local held heat added by the solar can disappear to the surrounding ground. Nature always wants to be in equilibrium.

New MCS guidelines also now prescribe the minimum amount of slinky that can should be buried and makes no allowances for any solar heat addition to the ground.


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banjax

banjaxComment left on: 4 October 2011 at 12:25 pm

" However for domestic applications the amount of energy that is deposited by the solar panels and reclaimed by the Ground Source Heat Pump is relatively small and the cost of the additional equipment can be relatively high so currently it doesn’t seem a commercially viable system. "

This is confusing and potentially misleading and does not answer the question, do thermal panels reduce the required area for slinkies?

or 

Given that solar thermal is the most mainstream of renewables. When a householder already has thermal installation. Can the excess heat generated by the thermal panels be used to increase the COP of the GHSP ? 

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Green Directions

Green DirectionsComment left on: 3 October 2011 at 10:07 am

I think that the most certain way of improving the efficiency of heat pumps is to generate as much of the electricity needed to run them that you can.

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