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Renewable energy sources have one thing in common; they all existed before man appeared on this planet. Wind, wave, hydro, solar, geothermal and tidal power are all forces of nature and are mostly intermittent energy sources, geothermal is the only consistent phenomenon. Geothermal renewable energy sources where probably the first to be fully utilised by man. Early civilisations tapped this heat to cook, fire clay pottery, create baths and spas and even heat their homes. Roman villas had under floor heating from natural hot springs over 2000 years ago.
Shallow geothermal resources (<400 m depth by governmental definition in several countries) are omnipresent. Below 15-20 m depth, everything is geothermal (Figure 1). The temperature field is governed by terrestrial heat flow and the local ground thermal conductivity structure (groundwater flow). In some countries, all energy stored in form of heat beneath the earth surface is per definition perceived as geothermal energy (VDI, 1998). The same approach is used in North America. The ubiquitous heat content of shallow resources can be made accessible either by extraction of groundwater or, more frequent, by artificial circulation like the borehole heat exchanger (BHE) system. This means, the heat extraction occurs–in most cases–by pure conduction, there are no formation fluids required. The most popular BHE heating system with one of more boreholes typically 50-200 m deep is a closed circuit, heat pump coupled system, ideally suited to supply heat to smaller, de-central objects like single family or multifamily dwellings (Figure 2). The heat exchangers (mostly double U-tube plastic pipes in grouted boreholes) work efficiently in nearly all kinds of geologic media (except in material with low thermal conductivity like dry sand or dry gravel). This means to tap the ground as a shallow heat source comprise:
Figure 1. Geothermal energy, comprising geothermal and mixed resources in the shallow subsurface (Knoblich, Sanner, & Klugescheid, 1993)
Figure 2. Typical application of a borehole heat exchanger (BHE) heat pump system in a central European home, typical BHE length = 100 m (Knoblich, Sanner, & Klugescheid, 1993)
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Groundwater wells (“open” systems),
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Borehole heat exchangers (BHE),
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Horizontal heat exchanger pipes (including compact systems with trenches, spirals, etc.), and
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“Geo-structures” (foundation piles equipped with heat exchangers).
A common feature of these ground-coupled systems is a heat pump, attached to a low-temperature heating system like floor panels/slab heating. They are all termed “ground-source heat pumps” (GSHP) systems. In general, these systems can be tailored in a highly flexible way to meet locally varying demands. Experimental and theoretical investigations (field measurement campaigns and numerical model simulations) have been conducted over several years to elaborate a solid base for the design and for performance evaluation of BHE systems (NASA, 2009; Rybach & Eugster, 1997). While in the 80s, theoretical thermal analysis of BHE systems prevailed in Sweden (Claesson & Eskilson, 1988; Eskilson & Claesson, 1998) monitoring and simulation was done in Switzerland (Gilby & Hopkirk, 1985; Hopkirk, Eugster, & Rybach, 1988), and measurements of heat transport in the ground were made on a test site in Germany (Sanner, 1986).