Evaluating cooling and thermal performances of a double pipe rectangular Earth-to-Air Heat Exchanger provided with a convection zone

Abstract

Earth-to-Air Heat Exchanger performances depend on many parameters, among them the pipe geometric shape which is also found be influential. On account of their large exchange surface, rectangular buried pipes are more performing compared to the circular ones. In this study, an innovative technique of improving heat transfer consisting in incorporating a convection zone within these systems is proposed. In view of its high thermal performance and harmless nature compared to organic liquids, water is used for filling the convection zone within a double pipe rectangular underground exchanger operating in cooling mode and submitted to the summery climatic conditions of the Algerian Saharan region. Moreover, a thorough investigation is conducted for assessing the response of the double pipe rectangular exchanger to the variation of its sizing and operating parameters in the presence of such a layer. The findings reveal that the presence of a convection zone makes the double pipe rectangular exchanger more performing in terms of the air temperature reduction and cooling energy production, even in the case where the basic system usually is ineffective due to using a larger pipe section, a smaller length, or higher air flow. For instance, the outlet temperature supplied by the exchanger with a convection zone at a pipe section of 1256 cm2 is found to dwindle by 2 °C compared to the basic system. Likewise, the average efficiency at a pipe length of 20 m rises to 83 % for the former, while drops to 72 % for the latter. It is also shown that at high air flow of 1780.4 m3/h, the cooling energy yielded by the improved exchanger rises to more than twice of that obtained with the unimproved one. Further, the integration of a convection zone is a cheaper and useful way for more enhancing the performance of commercial rectangular PVC pipes on account of their low aspect ratio. Thus, it follows that the novel technique of using rectangular buried pipes integrated with convection zones can to be considered as a powerful improving way that could be eventually combined with other techniques for further maximizing the profitability of these systems.