Deep Boreholes for Ground Source Heat Pump Systems – Scandinavian Experience and Future Prospects
|Publication Date:||1 January 2016|
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.