Heating and Cooling Potential of Earth-Air Heat Exchangers

ABSTRACT

Earth-to-Air Heat Exchanger (EAHX) comprises of lengths of pipe buried in the ground through which ventilation air is circulated. The performance of an EAHX system is determined by the percentage reduction in the amplitudes of the temperature variation of the ventilation air due to heat exchange between the circulating air and the soil. Identification of dimensionless parameters that determine the performance of EAHX is the aim of the present analysis. This paper deals with the heating and cooling potential of a single pipe buried in the soil for real climatic data. A complete analytical solution for the transient, coupled heat transfer system for an Earth-Air Heat Exchanger has been developed using Laplace transformation technique for arbitrary time dependent ambient air temperature. The solution contains three dimensionless parameters, namely the Biot Number, modified Stanton Number and the dimensionless soil diffusion length which depends on the time period of the ambient air temperature variations.

The air temperature at any location in the tunnel is obtained as the convolution integral of the time-derivative of the ambient air temperature and a convolution function. The convolution function models the effects of the physical and geometrical properties of the EAHX system and can be computed independent of the ambient air conditions. Analytical results are validated against a finite volume numerical simulation model for an ambient air temperature given as a periodic step function of time. The percentage reduction in the diurnal and yearly amplitudes of the outlet air temperature for a sinusoidally-varying ambient air temperature is calculated using the analytical solution as a function of Stanton number and Biot number.

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