Abstract
High fluid temperatures in ground-based heat exchangers (GHXs) during the cooling season may result in a decrease in thermal conductivity of adjacent backfill (λbackfill), potentially causing degradation in the performance efficiency of the GHX system. In this study, numerical modeling was performed using the SVOffice™ finite-element software to evaluate coupled heat and moisture transfer around two GHXs. Constant-temperature boundary conditions of 35 ºC were applied to the GHX surfaces, and thermal properties of the backfill were controlled for comparisons. For estimate typical hydraulic and thermal properties for the modeling, laboratory tests and literature review were performed. Modeling results indicate that coupled heat and moisture transfer occurs rapidly near the GHX involving a dry zone formation when λbackfill decreases. A boundary between dry and wet zones where soil thermal properties rapidly change was observed around 50% GHX temperature dissipated (T50), and accordingly T50 was used to optimize the pipe configuration. Coupled heat and moisture transfer increased when the GHX configurations were optimized with consideration of dry zone formation. These results imply that thermally enhanced, engineered backfill and optimized configurations can enhance GHX system efficiency.
Highlights
Ground-based heat exchangers (GHXs) in a groundsource heat pump (GSHP) system are components of cooling and/or heating units that beneficially use stored geothermal energy in the earth
Heat transfer gradually occurred through wet soil that has a relatively higher thermal conductivity while rapid heat transfer was observed in dry soil near the GHX due to a relatively lower thermal conductivity
In contrast to the rapid heat and moisture transfer around the GHX, more gradual gradients of temperature and volumetric water content (VWC) were observed at distance from the GHX
Summary
Ground-based heat exchangers (GHXs) in a groundsource heat pump (GSHP) system are components of cooling and/or heating units that beneficially use stored geothermal energy in the earth. Near-surface ground temperature fluctuates due to seasonal and/or meteorological effects (typically the upper 2–5 m), ground temperature becomes relatively constant at depth [1,2,3] Even within this shallow subsurface zone where ground temperature varies by the season, relative to air, ground has a relatively higher temperature during winter (i.e., heat source) and a relatively lower temperature during summer (i.e., heat sink). GSHP systems are cost-effective and environmentally friendly in terms of life-cycle emissions such as green-house gasses (GHG). Due to these environmental and economic benefits, GSHP systems have been widely used in many countries [4,5,6,7]
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