Seasonal thermal energy storage in shallow geothermal systems: thermal equilibrium stage

Hossein Nowamooz,Cyrille Chazallon,Saeid Nikoosokhan

doi:10.1051/e3sconf/20160907003

Hossein Nowamooz, Cyrille Chazallon + Show 1 more

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https://doi.org/10.1051/e3sconf/20160907003

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Abstract

This paper is dedicated to the study of seasonal heat storage in shallow geothermal installations in unsaturated soils for which hydrothermal properties such as degree of saturation and thermal conductivity vary with time throughout the profile. In the model, a semi-analytical model which estimates time-spatial thermal conductivity is coupled with a 2D cylindrical heat transfer modeling using finite difference method. The variation of temperature was obtained after 3 heating and cooling cycles for the different types of loads with maximum thermal load of qmax = 15 W.m−1 with variable angular frequency (8 months of heating and 4 months of cooling).and constant angular frequency (6 months of heating and 6 months of cooling) to estimate the necessary number of cycles to reach the thermal equilibrium stage. The results show that we approach a thermal equilibrium stage where the same variation of temperature can be observed in soils after several heating and cooling cycles. Based on these simulations, the necessary number of cycles can be related to the total applied energy on the system and the minimum number of cycles is for a system with the total applied energy of 1.9qmax .

Highlights

Analysis on heat transfer in subsurface is important to size the Borehole Heat Exchanger (BHE) that optimum performance is achieved with minimum costs
We aim to study the thermal response of the multilayered subsurface soil around the Shallow Borehole Heat Exchangers (SBHE) installed in Hangenbieten of Alsace region
We perform a series of simulations to investigate the effect of variable time periodic thermal loads on the temperature variations around the SBHE

Summary

Introduction

Analysis on heat transfer in subsurface is important to size the BHE that optimum performance is achieved with minimum costs. To estimate thermal response of BHE accurately, knowledge of thermal properties of the ground is needed. The subsurface soil thermal properties such as thermal conductivity, and heat capacity play a vital role in heat transfer of BHE [1,2]. A few papers dealing with the influence of soil types and moisture conditions on ground heat pump performance have been published in the last decade [4, 5]. It was found that soil water content was a dominant factor responsible for seasonal thermal conductivity variations. Drown and Braven [7] monitored for several seasons the effect of soil conditions and thermal conductivity on heat transfer in ground heat storage. Increase of the soil thermal conductivity by 0.17 W.m-1.K-1 led to reduction of heat pump operating time by 1.3%

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