Standing column wells (SCWs) have the potential to deliver much higher rates of heat transfer to geothermal heating and cooling systems in buildings via heat pumps than conventional vertical borehole heat exchange arrays. Its open-end column design with porous casing along the borehole (depending on the formation) encourages the flow of groundwater from the rock’s porous matrix into the well or the opposite way according to the hydraulic gradients. This approach induces a further heat transfer mechanism in addition to the conduction: it is advection. Advection induced by the groundwater movement due to the hydraulic gradient and the action of the well pump causes warmer water (in winter) and cooler (in summer) to be drawn into the well thus increasing heat transfer capacity. This is beneficial for SCWs to offer much higher heat transfer performance than other conventional approaches. The development of a numerical model for clusters of standing column wells is described in this thesis. The model is three-dimensional, dynamic and solves the governing equations using a finite volume discretisation scheme with a fully implicit algorithm. The slower acting field equations are solved using a wider time interval than that used for the faster acting well equations and the two sets of equations are coupled through the field equation source terms. A groundwater bleed feature is incorporated. The model has been validated thermally and hydraulically using existing field data. Two test cases have been applied to reveal the advantages of using SCWs in UK conditions, competing with the conventional closed-loop system of vertical borehole heat exchangers. The results of the applications suggest that SCWs can deliver substantially higher rates of heat transfer than conventional closed-loop borehole heat exchanger arrays, typically up to 250Wm-1, especially when groundwater bleed is operational. The results also confirm that a bleeding operation can offer up to 2.2K improvement (reduction) in the outlet well water temperature in summer and (increase) in the well water temperature in winter. Investigation results on borehole diameter confirm that a larger well borehole diameter would offer improved heat transfer performance in some cases, according to the relative change of the heat transfer coefficient. Analysis of borehole to borehole spacing seems to suggest that 5m is the most effective spacing of the three different spacing choices for this type of application. The results also show that SCW installation in London Clay performs less well than Magnesian Limestone and Old Red Sandstone; the latter two seem to be appropriate formation types to work with this type of application. The advantage of adopting multiple well arrangements (SCW clusters) over the use of single wells has also been confirmed. The important practical consequence of this is that far less geotechnical drilling is needed as the required borehole depth reduces substantially under multiple well arrangements. The results gathered from three different buildings also reveal that the balance between heating and cooling demands appears to have less impact on the mean formation temperature change than the large cooling application, which is beneficial to maintain a steady system performance over a long period of time. The results also suggest that the impact on the rock formation was very dominant in the first few years but it declined towards the end of the 5 year analysis period used in this work. The results from the CO2 emission analysis demonstrate that an annual carbon emission reduction of up to 46% can be achieved by using the geothermal system with SCWs instead of the conventional system consisting of a gas-fired condensing boiler and a conventional aircooled chiller. They also confirm that the balance between heating and cooling demands has a substantial impact on the carbon saving delivered by this technology.
|Publication status||In preparation - Oct 2011|