CIBSE Technical Symposium, Liverpool John Moores University, Liverpool, UK, 11-12 April 2013
The model hypothesizes a theoretical bulb of energy in and around the borehole, and it needs to consider three things: 1. The thermal energy extracted by the heat pump from the borehole, 2. The thermal energy injected into the borehole, (already known) 3. The amount of energy that the borehole is assumed to draw in from or lose out to the infinite surroundings. (This is the bit that moves into unknown territory.) When the author first considered the model in 2011, the shape considered was the same as the real ‘figure of eight’ twin borehole. The calculation of the changing volume and surface area was of a 3 dimensional complexity that defeated the more purposeful task of calculating the energy flows.
Figure 10 – 3D representation of the borehole: Left as a single deep cylinder than can expand and contract outwards but not downwards. Right, as the actual figure-of-eight twin interlocking boreholes. (Diagram and model by author) Figure 11 – Dialog box with some of the parameters that can be tweaked to assist the algorithm. See section 2.5.
If the Peveril house had a single borehole, the depth recommended by Earth Energy Designer [10] would be 85-100 metres. For this model, a simple single cylinder of that depth is good enough to establish the principle. The theoretical cylinder has a constant depth but of varying radius and surface area depending on its energy level (Fig.10). The depth is constant because the borehole cannot change depth significantly as the energy volume changes. Only the radius changes because the energy that may be lost or gained relative to the infinite surrounding is next to the borehole, not below it. Therefore the changes in volume are proportional to the square of the radius because the cross sectional The electrical energy used by the heat pump can be converted into an estimate of the thermal energy withdrawn from the earth. The algorithm of the model has to make an assumption about the COP of the heat pump. In reality, COP varies hour to hour, based on workload and temperature of the source, but for the model, one has Page 9 of 17