Solar charging

Solar Charging is a process that uses the earth as a thermal battery by collecting heat with solar panels, storing the heat in boreholes, 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.

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. This definition applies to 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 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.

A High temperature solar panel is not required. These are designed to deliver very high temperature to a water tank for domestic hot water. As the purpose of Solar Charging is to deliver heat to a mass of soil that is starting at 10-12ºC, an uninsulated Swimming Pool heater costing less than $200 can be used, mounted on a wall or roof. As it is uninsulated, it will pick up warmth from the air temperature in summer, even if the sun is not shining.

There will be an immediate benefit when used diurnally, eg a Warm day of collecting followed by an cool evening. 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.