Experimental investigation of effectiveness variation of borehole heat exchangers for cooling mode operation

Abstract

• Parallel configuration shows higher effectiveness of 0.88 than series configuration with 0.81. • Borehole limiting ratio for parallel configuration is higher than that of series one. • Experimental study of space cooling operation using ground dource heat pump system. • Maximum rise in the tube-interface temperature is observed at 73 m depth of borehole. In the present research work, a novel technique is proposed to estimate real time variation of borehole temperature based on measured interface temperature along the depth of a double U-tube borehole heat exchanger (BHE). Experimentally determined transient variation of effectiveness of BHE is discussed in detail for experimental simulation of line source model and actual operation of a GSHP system for cooling mode operation. The real time borehole temperature computed using the present technique during actual operation of a GSHP system makes it possible to compute the actual variation of effectiveness of BHE. The above set of experiments were performed for both single and double U-tube BHEs considering series and parallel mode operations during peak summer conditions at a location situated closer to the Himalayan region in northern part of India. Effectiveness of BHEs was computed using varying borehole temperature obtained from line source model and the real time varying borehole temperature proposed in the present work. For the actual cooling mode GSHP experiments, the average value of thermal effectiveness of double U-tube BHE computed with constant borehole temperature is observed to be 0.72 for both series and parallel mode operations. However, with the use of real time varying borehole temperature, the effectiveness is observed to increase by 12% and 22% for series and parallel flow operations respectively.