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.
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