testing conductivity service
EarthEnergy offers a comprehensive thermal performance testing service that predicts the thermal performance of a borehole based on thermal conductivity, thermal resistance and specific heat capacity, well beyond simple thermal conductivity measurements.
Closed loop ground source heat pump systems operate primarily by thermal conduction of heat through the bulk of the surrounding ground. Thus the average in situ thermal conductivity of the ground over the entire length of a borehole is the principle parameter required for the successful design of a ground loop array. In addition, the performance of a borehole based system is affected by the thermal resistance that occurs at the borehole itself due to the change in conductivity arising at the borehole wall interface, through the borehole fill material, and the U-tube material. The thermal resistance will vary depending on the type of borehole completion- ie borehole diameter, type and size of U-tube pipework, and grout conductivity. Thus the experimental procedure has to measure and distinguish between the borehole thermal resistance and the far field in-situ thermal conductivity.
Generally the procedure is to inject heat at a constant rate into the borehole, whilst measuring the change in inlet and outlet water temperatures with time. Provided that this is carried out for sufficient time, the effects of thermal storage in the U-tube and borehole, and the thermal resistance of the borehole completion are overcome, and the temperature rise is governed by the far field conductivity only. When the mean of the inlet and outlet temperatures is plotted against log time, the plot should become linear - at which point the slope of the graph can be used to estimate the conductivity. This is the most widely used approach to obtaining the in situ value.
In addition to carrying out the heat injection phase, EarthEnergy carries out a cooling phase in which the heaters are turned off, while the circulation pump continues to run. The temperature decay of the inlet and outlet temperatures is monitored. By using an iterative analytical model of borehole performance the thermal conductivity and borehole resistance can be determined with greater accuracy than with simple straight line matching of the semi-log analysis.
