Borehole heat exchanger

The two most promising options are storage in aquifers (ATES) and storage through borehole heat exchangers (BTES). These concepts have already been introduced as commercial systems on the energy market in several countries. Another option is to use underground cavities (CTES), but this concept is so far rarely applied commercially. 

Keywords Geothermal response test, ground thermal conductivity, line source, GENOPT, TRNSYS, borehole heat exchanger... 

In the design of borehole heat exchangers accurate information on the soil thermal parameters, such as thermal conductivity, heat capacity and temperature, is essential for the design of an economically sized and well-functioning thermal energy store. Especially the soil thermal conductivity is critical, as... 

Figure 52. TRNSYS model with Excel data reader, simple tank (pipe volume) DST borehole heat exchanger model, and error function...

For longer boreholes the data may need to be synchronized by comparing the return temperature not with the current time step but with the time step— n, where n is the travel time. The error minimized is the sum square error of the difference between the calculated and measured borehole heat exchanger return temperature. We have set up the analyses procedure in such a way that it is easy to select discrete data-windows for the calibration. 

Figure 54. TRNSYS simulation, measured temperatures (red borehole-heat exchanger T-in, blue borehole-heat exchanger T-out) and purple modelled borehole-heat exchanger T-out), green line is the cumulative error... 

Figure 55. Geothermal response test with combined heat extraction/heat injection and different energy levels, shown are the borehole heat exchanger flow and return temperatures and the calculated heat flux. Borehole flow rate is not shown in the graph...

The Geothermal Response Test as developed by us and others has proven important to obtain accurate information on ground thermal properties for Borehole Heat Exchanger design. In addition to the classical line source approach used for the analysis of the response data, parameter estimation techniques employing a numerical model to calculate the temperature response of the borehole have been developed. The main use of these models has been to obtain estimates in the case of non-constant heat flux. Also, the parameter estimation approach allows the inclusion of additional parameters such as heat capacity or shank spacing, to be estimated as well. 

Keywords borehole heat exchangers, freezing problem, principal solution... 

How well does the borehole heat exchanger design match with the measured temperatures, and what impact do the measured loads have on the design ... 

Figure 74. Average daily ambient and borehole heat exchanger temperatures at Croydon (period 2001-2003)...

Figure 78. Comparison between TRNSYS model with and without horizontal connecting pipes, measured and modelled borehole heat exchanger temperatures (period 2001-2003). Also shown is the undisturbed ground temperature at depth of horizontal pipes...

---