The heat content of shallow or deep aquifers can be used for space heating. Two innovative systems are described below in detail: a geothermal heat pump system based on a single well in China (= shallow), and a cascading use of tunnel waters (= deep) in Switzerland. The “Single Well System” (HYY SWS) was invented and developed by Beijing Ever Source Science & Technology Development Co., Ltd (HYY) to provide buildings with heating and cooling as well as with domestic hot water. The powerful system operates at about 500 kWth capacity. Unlike traditional groundwater heat pump systems, in which two wells are used (one for pumping groundwater out and the other to dispose of cooled water), the HYY SWS uses only one, specially designed well for production and reinjection. A borehole with a depth of about 70–80 m and a diameter of 0.5 m is drilled for HYY Single Well Systems. The necessary local geologic site condition is to have a shallow aquifer with a hydraulic conductivity of 10 m/sec or higher. Many such systems operate now in China, several of which, for instance, serve the 2008 Summer Olympic Facilities in Beijing. Switzerland has, in its mountainous parts, hundreds of deep tunnels. Tunnels drain the rock overburden and, depending on its thickness, water temperatures up to 50 °C can be encountered and utilized. The most straightforward and cheapest form of tunnel heat usage is to collect and transport inflowing waters via ducts to the portals, with as little temperature drop as possible. The thermal power depends on flow rate and temperature. At or near the portals the heat content of the waters can be used for various applications. When the temperature level of the tunnel water outflows is too low for direct applications (e.g. for district heating), heat pumps are employed. From Switzerland a whole suite of uses can be reported: space heating, greenhouses, balneology and wellness, fish farming. At the northern portal of the 35 km long Loetschberg base tunnel at Frutigen, the tunnel water is used subsequently (“cascading”) for space heating, greenhouse, and fish farming (incl. caviar production).
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