Comparative performance of ‘U-tube’ and ‘coaxial’ loop designs for use with a ground source heat pump

The installation cost and coefficient of performance (COP) of a ground source heat pump (GSHP) system can be greatly affected by the design of its ground loop. An experimental investigation was conducted to compare the performance of two loop designs for use with the ground source heat pump of an ‘energy pile’ installation or a conventional borehole system. A coaxial loop (a 40 mm OD coaxial tube with a 20 mm OD interior pipe) and a U-tube (20 mm OD pipes) of a length of 72 m were constructed, tested and analysed in terms of ground heat extraction capability and hydraulic flow characteristics. The use of the coaxial loop design could potentially reduce the cost of drilling boreholes and make the installation easier on site, as the effective diameter would be smaller than a comparable U-tube with the same mass flow rate, considering that the U-tube requires a minimum 10 mm spacer between the legs. Hydraulic performance tests have shown that the U-tube achieved the transitional-turbulent flow at a glycol flow rate of approximately 0.14L/s, whereas the flow in the coaxial remained laminar across the glycol flow rate range of 0.05–0.25L/s. Heat pump performance tests have shown that the U-tube achieved a COP of at least 0.08 greater than the coaxial loop across the range of glycol flow rates investigated, while the heat output was at least 12% greater when using the U-tube. Therefore, it can be concluded that the coaxial loop in its current form is seen not to add any performance benefit against the U-tube.

► A coaxial loop could reduce cost of drilling and installation comparing with U-tube.
► A coaxial loop and a U-tube of a length of 72 m constructed and tested with a GSHP.
► The U-tube’s COP and heat output were 0.08 and 12% greater than the coaxial loop.
► The coaxial loop in its current form has no performance benefit against the U-tube.