ASHRAE - American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc.
Scope
Ground source heat pump (GSHP) systems are commonly used in Sweden for both residential and commercial buildings. However@ there are several key differences compared with GSHP systems utilized in the USA. Scandinavian systems are often heating-only@ and instead of using grouted boreholes@ groundwater-filled boreholes are often used. These boreholes are cased from the ground surface to the usually shallow bedrock. A single or double U-tube is commonly suspended in the borehole. These boreholes are often deeper than those commonly used in the USA. The average borehole depth has increased over time@ and the average borehole depth for ground heat exchangers installed in 2013 in Sweden was 171 m (561 ft.) Boreholes as deep as 250-300 m (820-984 ft) are not uncommon and there is interest among installers of using even deeper boreholes. Incentives for deeper boreholes include limited area for drilling@ pre-existing boreholes on neighboring properties@ and deeper-than-usual layers of soil and unconsolidated rock. This paper reviews current Scandinavian practice for borehole design and discusses installations with boreholes 300 m (984 ft) deep or deeper. Aspects of the design include using larger pipe sizes or double U-tubes to keep pressure losses acceptable@ larger borehole diameters to accommodate the larger pipe sizes@ increased short-circuiting due to the long lengths@ and design temperatures for heating-dominant systems due to the geothermal gradient.