Abstract
Borehole thermal energy storage (BTES) exploits the high volumetric heat capacity of rock-forming minerals and pore water to store large quantities of heat (or cold) on a seasonal basis in the geological environment. The BTES is a volume of rock or sediment accessed via an array of borehole heat exchangers (BHE). Even well-designed BTES arrays will lose a significant quantity of heat to the adjacent and subjacent rocks/sediments and to the surface; both theoretical calculations and empirical observations suggest that seasonal thermal recovery factors in excess of 50% are difficult to obtain. Storage efficiency may be dramatically reduced in cases where (i) natural groundwater advection through the BTES removes stored heat, (ii) extensive free convection cells (thermosiphons) are allowed to form, and (iii) poor BTES design results in a high surface area/volume ratio of the array shape, allowing high conductive heat losses. The most efficient array shape will typically be a cylinder with similar dimensions of diameter and depth, preferably with an insulated top surface. Despite the potential for moderate thermal recovery, the sheer volume of thermal storage that the natural geological environment offers can still make BTES a very attractive strategy for seasonal thermal energy storage within a “smart” district heat network, especially when coupled with more efficient surficial engineered dynamic thermal energy stores (DTES).
Highlights
Introduction and TerminologyThe term “thermogeology” [1, 2] has been applied to the science of the occurrence, movement, and exploitation of heat in the earth’s subsurface
underground thermal energy storage (UTES) can be subdivided into two categories: (1) Borehole thermal energy storage (BTES), where a field of borehole heat exchangers (BHE) exchanges heat with the surrounding rock or sediment mass, predominantly by processes of conduction
The fluid is initially “prewarmed” by heat acquired from the BHEs in the outer zone of rock and is heated by the core of the BTES array to its full temperature
Summary
The term “thermogeology” [1, 2] has been applied to the science of the occurrence, movement, and exploitation of heat in the earth’s subsurface. (ii) A similar district heating system, relying on summer harvesting of solar heat and storage in a BTES array comprising 100 boreholes to 65 m depth, spaced at 3 m, (volume 65,000 m3) in crystalline bedrock at Anneberg, near Stockholm, Sweden [21]. (iii) Another district heating system at Neckarsulm, Germany, storing summer solar thermal energy at temperatures of up to 80°C in a rock mass, via a BTES system of volume 63,360 m3 comprising (as of 2006) 528 borehole heat exchangers to depth 30 m [22]. In such low-permeability rocks, in a wet climate, the water table is often close to the surface and the boreholes are naturally groundwaterfilled It is this groundwater that provides the thermal contact between the heat exchange pipe and the rocks in the borehole walls. The borehole structure itself possesses a “thermal resistance” (Rb) to heat flow
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