Thermal response test : method development and evaluation
Abstract: Since the first introduction of thermal response tests with mobile measurement devices in Sweden and USA in 1995, the method has developed and spread in North America and Europe. Thermal response tests have so far been used primarily for in situ determination of design data for BHE (borehole heat exchanger) systems, but also for evaluation of grout material, heat exchanger types and groundwater effects. A variety of analytical and numerical data analysis models have been developed. Various applications of the line source theory is the most commonly used model for evaluation of the response test data because of its simplicity and speed, and is dominant in Europe. The use of the cylinder source model and numerical models coupled with parameter-estimation techniques are common in USA. The Swedish response test apparatus TED has been used at a number of tests since 1996. The main purpose has been to determine in situ values of effective ground thermal conductivity, including the effect of groundwater flow and natural convection in the boreholes. The tests indicate that convective heat transfer may play an important role for the thermal behaviour of groundwater-filled BHE, which is the typical BHE design in Sweden. The magnitude of the induced natural convection depends on the heat transfer rate and the temperature level. The influence is small on grouted boreholes. To shed light on the influence of groundwater flow on thermal response testing, simulation models for estimating the heat transfer effect of groundwater flowing near a borehole heat exchanger were developed. The groundwater flow was represented as 1) a flow through an equivalent porous medium (continuum), 2) a flow through an impermeable medium with a porous zone, and 3) a flow through an impermeable medium with a thin vertical fracture. The three cases result in significantly different temperature field patterns around the borehole and all three cause lower borehole temperatures. The fracture flow model results in higher effective thermal conductivity than the continuum and porous zone models within a certain flow rate interval. This illustrates the efficiency of the high flow velocity in the fracture and the large temperature gradient between the borehole and the fracture flow. The effect of the flow in the fracture or porous zone decreases with the distance from the borehole, but even at distances of half a meter or more the porous zone or fracture may result in significantly enhanced heat transfer. Even a relatively narrow fracture close to a borehole may result in greater effective thermal conductivity, although estimations made with a continuum approach may indicate otherwise. A thermal response test is likely to induce a thermosiphon flow due to the temperature difference between borehole and surroundings, resulting in an enhanced effective thermal conductivity estimation. The enhancement of the effective thermal conductivity of the BHE depends on injected power rate and flow resistance in fractures. The fracture flow resistance may be quantified in terms of hydraulic condcutivity. The findings from the groundwater flow and thermosiphon simulation are encouraging for further studies, both as simulations and in field experi- ments. The author suggests further studies of the possibility to develop models for estimating and investigating the influence of groundwater from drilling data and hydraulic testing. A future aim should be to gain enough knowledge of fracture flow and thermosiphon effects that hydraulic well test and drilling data may be used in borehole thermal energy storage design.
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