Solar charging

Solar Charging is a concept of using the earth as a thermal battery by collecting heat with solar panels, storing the heat in a borehole, and then retrieving it later using a Ground Source Heat Pump (GSHP). The GSHP is a well established technology, but the ground deep down does get cooler with time, reducing the efficiency of the GSHP.
To avoid confusion with electric charging of cars etc from solar panels, it might also be termed Solar Geocharging but this is a longer mouthful.
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The deep subsoil remains at one temperature all the year round - broadly it receives and stores solar heat, but only the top 5 metres (and somewhat less in temperate climate) seems to be affected by the seasons. More than 5m down and the ground is perpetually 10-12 º Celcius, and solar heat could take many decades to get deeper. (We are not talking about volcanic heat here.) The context is that of thermal boreholes 10 to 80metres below the building. ) The assumption of a GSHP installation is that the thousands of tons of soil below contain enough mass to heat a house for many many years. Solar Charging is a process to ensure that it does.
This process can be done using the same glycol (antifreeze) circuit that is already being used by the GSHP. By adding to the existing loop a simple Solar panel on the roof or wall of the house, and providing a temperature activated pump, the panel will send heat deep into the ground throughout the summer months. The ground will not get hot, it may only get warm, but it will be stopped from getting very cold in winter. The efficiency (Coefficient of Performance - CoP) of the GSHP will be retained throughout the winter, instead of declining as the ground gets colder.
The process already exists in a proven form, but presently as a by-product of heat reclaim or fan coil cooling. In heat recovery a MVHR (Mechanically Ventilated Heat Recovery as required in the Passive House concept) unit in the loft can extract heat from warm extract house air, and return this to the glycol loop, and thence to the ground. In 'Comfort cooling', a fan coil unit cooler can pass hot house air through the unit, and heat is passed to the glycol loop, and thence to the ground. The use of solar collectors externally is the same principle, but one is cooling the atmosphere and catching solar heat and moving this heat down to the ground. The process can be modeled on computer, test rigs exist, but has not yet been applied to a full size house.
There is no doubt it will work in a diurnal way, balancing daytime and night time, eg a warm sunny day of collecting followed by an cool evening in Equinox periods. There is still a question mark over how long the solar charged heat will remain down below before dissipating an infinite distance away from the borehole. The chances are that in a half year of the summer and autumn, the heat does not go much further than 5m from the borehole pipes, and thus during the cold 6 months of the year (winter and spring), it will be close enough to return when the GSHP sends cold glycol down.
The idea only works with deep dense soil such as a clay or marl or rock. Soil with large airpockets, deep layers of gravel and ground water will not work, as the water will simply carry the heat away. Nobody can actually go down there, but the process can be modelled on a computer. For testing with a real house lived in by a real family, assumptions about the effectiveness of Solar Charging can be derived by running the process for at least two full years, and monitoring the flow rates, input and output temperatures and performance indicators of the GSHP, including, all importantly, its electrical consumption.

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