Techno-economic optimization of regenerative indirect evaporative cooling systems with/without middle air flow return paths

Evaporative cooling is an energy-efficient and environmentally friendly method for cooling indoor spaces by utilizing the process of water evaporation to create cooling effect. When the supply air in a dry channel is cooled by airflow passing over water film within the adjacent channel (wet channel), the system is named Indirect Evaporative Cooling (IEC). This study models and optimizes regenerative type IEC systems with two or three channel compact heat exchangers. Two cases of without middle airflow return paths (IEC-R) and with middle airflow return paths (IEC-R-MAP) are investigated. The results of this investigation are used for selecting appropriate IEC systems with two or three channels and with/without middle airflow return paths for various case studies (including various operational and environmental conditions). Furthermore, two series and parallel configurations of these compact heat exchangers are investigated. The modeling results of IEC systems with four configurations (IEC-R / IEC-R-MAP with two/three channels) are obtained and validated. Multi-objective techno-economic optimization was performed with the coefficient of performance and total annual cost (comprising investment and operational costs) as objective functions. Optimization results under all analyzed case studies for two climatic conditions of Yazd (hot and dry) and Tehran (moderate and dry), indicated that three-channel systems at the fixed specified cooling capacity and supply air volumetric flow rate into the room space, had advantages over two-channel systems. For a case study with a supply air volumetric flow rate of 3000 CFM, IEC-R with three channels had 94% / 86% higher coefficients of performance and 225% / 94% lower total annual costs in Tehran/Yazd compared to IEC-R with two channels. The presence of middle airflow returns paths in IEC-R-MAP with two/three channels reduced the coefficient of performance in comparison with that for IEC-R with two/three channels by 65% / 5% in Yazd and 12.5% / 4% in Tehran, respectively. This increased the size and total annual cost of these systems by 25% / 0.5% for Yazd and 7% / 2% for Tehran, respectively. Furthermore, the study examined the use of series/parallel configuration in the IEC-R system with three channels. To achieve the lowest energy consumption and total annual costs, a series configuration is not recommended.