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What is a VSHE?
Below ground level, soil temperature is fairly constant, between 7°C and 13°C, due to the soil’s absorption and storage of solar radiation over the course of a year. This provides a good source of heat energy which we can tap into via ground source heat systems, such as the Vertical Soil Heat Exchanger (VSHE) planned at The Living Rainforest.
Commonly, ground source heat exchange systems use a closed circuit pipe, called a ground loop, set into the soil. The loop can be laid horizontally where local geology allows a 1metre deep trench to be laid and if there is space available to set this within a plot of roughly twice the area of the building to be heated. If this is not the case, as with the site at The Living Rainforest, a VSHE can be used. With this system, the piping is inserted into a series of one or more boreholes drilled 60-150m into the ground. Compare this depth with the 97 metre height of Big Ben!
Within the closed circuit loop is a fluid mix of water and glycol, which absorbs heat from the ground and consequently vaporises due to its low boiling point. An evaporator extracts this vapour and pipes it to a compressor, which raises the gas’ temperature to around 40°C. We sometimes experience this process when we apply a lot of force to a bicycle pump, compressing the air inside, leading the nozzle to become hot. Finally, a condenser transfers this heat to the building’s heat distribution system, often under-floor heating, before the cooled gas liquefies once again and re-enters the ground loop to be warmed by heat transfer from the soil.
In the case of The Living Rainforest’s VSHE system, fan driven devices will be suspended to distribute heat as necessary but the system will also extract and store excess solar heat collected within the new Green Greenhouse on hot days by reversing the heat exchange process, thereby cooling the glasshouse to an appropriate temperature and allowing soil heat around the bore-holes to be replenished. Ideally, such a seasonal storage system, would use a pair of wells drilled into natural underground water stores, known as an open-loop subsoil aquifer. During summer, water would be pumped from the cold well for cooling purposes and then returned as heated water to the warm well. In colder periods this process would be reversed, enabling the extraction of stored heat. Unfortunately, the deep soil beneath The Living Rainforest is chalkstone and unsuitable for an underground water store. Hence, we are using the VSHE system, based on conduction of heat through the soil.
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