Closed-loop BTES systems depend less on site-specific hydrogeologic conditions than ATES systems and are better suited for areas where relatively high well yields are not obtainable.
In general, two types of UTES for combined heating and cooling applications can be distinguished: ATES and BTES (Nordell 2000).
A BTES system consists of a number of closely spaced boreholes; the principle is shown in Figure 5.
The lowest costs can be reached with ATES and BTES.
For BTES, the experiences with CSHPSS built in Neckarsulm (Nussbicker et al.
Ford and Wong (2010) studied the above-mentioned phenomena and presented regional models for screening potential underground areas for ATES and BTES systems implementation.
In BTES, thermal energy is directly stored in the ground.
BTES has a horizontal rather than vertical temperature stratification from the center to the borders.
For solar thermal applications, due to the need for seasonal storage of solar heat, minimized heat exchange between the BTES and surrounding ground is desired.
General demands and recommendations for the design of ATES and BTES systems can be found in the Verein Deutscher Ingenieure's (VDI [Association of German Engineers] n.
Additional information for ground-source cooling systems with TES, utilizing ATES and BTES concepts, is given in a pre-design guide developed by Hummelshoj (2004).
Table 2 shows typical general values for BTES systems.