The Effect Of Temperature On The Extent
Soil Temperature
Virginia Tech has a website dealing with Geothermal Heat Pumps, which has much valuable information that is well worth having a look at Note: This link is currently not working, but here is a capture from the Wayback Machine Thanks to Peter for finding this. I've extracted a portion of one of the pages on their site dealing with ground temperature variations with season, location, and depth below the surface. The material below is from this page on the Virginia Tech website.
Soil The Effect Of Temperature On The Extent Soil temperature varies from month to month as a function of incident solar radiation, rainfall, seasonal swings in overlying air temperature, local vegetation cover, type of soil, and depth in the earth. Due to the much higher heat capacity of soil relative to air and the thermal insulation provided by vegetation and surface soil layers, seasonal changes in soil temperature deep in the ground are much less than and lag significantly behind seasonal changes in overlying air temperature.
Thus in spring, the soil naturally warms more slowly and to a lesser extent than the air, and by summer, it has become cooler than the overlying air and is a natural sink for removing heat from a building. Likewise in autumn, the soil cools more slowly and to a lesser extent than the air, and by winter it is warmer than the overlying air and a natural source for adding heat to a building.
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At soil depths greater than 30 feet below the surface, the soil temperature is relatively constant, and corresponds roughly to the water temperature measured in groundwater wells 30 to 50 feet deep. The amplitude of seasonal changes in soil temperature on either side of the mean earth temperature depends on the type of soil and depth below the ground surface.
At depths greater than about 30 feet below the surface, however, the soil temperature remains relatively constant throughout the year, as shown in Figure 3, below.
Vertical closed-loop earth heat exchangers are installed in boreholes to feet deep, where seasonal changes in soil temperature are completely damped continue reading. Well-based open-loop systems also extend to this depth or deeper. These ground loop configurations are thus exposed to a constant year-round temperature. On the other hand, horizontal-loop, spiral-loop, and horizontal direct-expansion DX loops are installed in trenches that usually are less than 10 feet deep. For these types of ground loops, it is important to accurately know the expected seasonal changes in the surrounding soil temperature.
The extra cost of installing such systems in deeper trenches may be outweighed by the gain in thermal performance, since deeper soils have less pronounced seasonal temperature changes and are eTmperature closer to room temperature, which reduces the work load of the heat pump units. Deeper soils not only experience less extreme seasonal variations in temperature, but the changes that do occur lag farther behind those of shallower soils. This shifts the soil temperature profile later in the year, such that it more closely matches the demand for heating and cooling. Referring to Figure 4 for example, the maximum soil temperature occurs Tekperature late August when cooling demand is high at a depth of 5 feet below the The Effect Of Temperature On The Extent surface, but occurs in late October after the heating season has begun at a depth of 12 feet below the surface.]
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