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The Earth-Air Heat Exchanger: prediction of performance in constrained sites

Student Name: Mr V. Rangarajan
Guide: Prof. Atul Kumar
Year of completion: 2019

Abstract:

An Earth-Air Heat Exchanger (EAHE) is an underground heat exchanger that either captures heat from or dissipates heat to the ground resulting in the moderation of the temperature of the air that flows through it. Though a promising energy-efficient option for thermal comfort, the use of EAHE has not grown much in society. Apparently, one reason for its limited application, especially in urban areas, is space constraint that includes restrictions on the length and route of the EAHE pipe, positioning of the EAHE, the forced interaction between the civil foundations and the EAHE etc. Therefore, in this work an effort was made to investigate the effectiveness of such a ‗constrained‘ EAHE. In order to constrain the EAHE to a restricted length, around 50 m, i.e. more than 50% of the total length of EAHE was pushed under the building‘s footprint, a serpentine route of the EAHE was followed ( these are due to the limited plot size and priority of the building‘s civil foundations over EAHE route); a non-homogenous surrounding soil was used to back-fill the constructionexcavation, burial depth of EAHE was restricted to an average depth of 2.75 m (a factor decided by the cost of excavation and varying topography of the ground surface). The performance of this constrained EAHE was monitored, measured and predicted in this research work. Hence a predictive 3-dimensional numerical transient model was developed on the MATLAB© platform to predict the EAHE‘s performance under different scenarios. The parameter-rich model allows locationwise variation of material thermal properties, orthotropic thermal conductivity and varying topography of the ground surface along with a choice of climatic data. As a first step, the model was experimentally validated against measurements at the above ‗constrained‘ EAHE. To check the model‘s robustness it was subjected to two more ‗unconstrained‘ sites, each located in a different climatic zone. The model predictions were in good agreement with the measured values and the errors were always within 5% of the easured values. The simulation results revealed that the simplified assumption of uniform soil temperature is far from reality and the the surrounding soil (next to EAHE pipe) does not form an ideal heat sink; Therefore such simplistic assumption results in an over-prediction of the EAHE‘s performance.

Sensitivity analysis identified that the EAHE air velocity as a significant parameter and a little change in air velocity can signifacantly change the temperature of the outlet air. Outlet air temperature were also sensitive to the thermal properties of the surrounding soil and its moisture content.

The study also confirmed the consistent multi-year performance of the EAHE on account of adequate annual charging and discharging cycle.

An interesting finding of the study was that in summer, the constrained EAHE performance ( in terms of outlet air temperature) was better than an ideal (unconstrained) EAHE.

The data and the model predictions confirmed that in a semi-arid climatic zone if a constrained EAHE is placed strategically, its performance is not compromised, and it provides the desired cooling even when it is installed at a shallower depth.

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