Numerical investigation on thermal performance of vertical U buried pipe heat exchanger with intermittent operation

The effect of intermittent operation on the heat transfer performance of vertical U buried pipe heat exchangers is studied by method of numerical simulation and analytical solution. The calculation area is divided into two parts by the boundary of the borehole wall. For the area in the borehole, the calculation method is steady analytical solution, and for the soil area, the method is finite volume method. In the borehole, steady heat transfer model is developed based on energy conservation, the two parts are coupled by the temperature of the borehole. According the actual operation mode of Office Building, Living Building and Marketplace three intermittent operation modes are determined: 10 + 14(operates 10 and stops 14 hours in one day), 12 + 12 and 14 + 10. The results show that by combining the analytical and numerical method, the method in current work provides a rapid and accurate solution. Under continuous operation, the unit heat flux decreases and the average temperature of borehole wall increases. During the intermittent operation, the heat flux and average temperature of borehole wall fluctuates, the heat flux increases after each intermittence, and the average temperature of borehole wall decreases. The restoration of 10 + 14 model is the best due to the biggest intermittence ratio (the ratio of intermittence time and operation time). With the operation proceeding, the enhancement of the heat flux caused by the intermittence becomes more obvious, and simultaneously the restoration ratio of average temperature of borehole decreases.