Cooling and heating potential of earth-air tunnel heat exchanger (EATHE) for non-air-conditioned building

A Review of Underground Soil and Night Sky as Passive Heat Sink: Design Configurations and Models

“…The reason behind such an interesting trend is the joint effect of low heat transfer coefficient and higher heat transfer area as the radius increases. In the analysis performed by Kumar et al [86,88], they observed that the outlet temperature reduced in the cooling mode when the pipe radius was increased from 0.41 m to 0.52 m. This suggests that the increasing surface area of the underground pipe was the dominating factor over the reducing heat transfer coefficient. However, further increase in the radius of the underground pipe (0.58 and 0.70 m) caused the outlet temperature to increase, suggesting that the increase in heat transfer area was not large enough to overcome the impact of reducing heat transfer coefficient.…”

“…Length: Different lengths of buried pipes have been used in experimental projects and theoretical analyses [1,2,15,16,36,[85][86][87][88]. Longer pipes typically resulted in better performance due to higher heat transfer with the soil.…”

“…However, an interesting trend has been observed by some researchers in which the outlet temperature first decreases with increasing radius but then increases. The point at which this reversal begins is called the "critical radius" [84,86,88]. The reason behind such an interesting trend is the joint effect of low heat transfer coefficient and higher heat transfer area as the radius increases.…”

“…Flow rate: Increase in flow velocity, and the mass flow rate causes higher outlet temperature when operating in cooling mode, and lower outlet temperature when operating in heating mode [2,6,15,36,44,45,55,85,86,88]. So, lower mass flow rate is typically considered a preferred situation, but may not result in the overall optimum performance.…”