Modeling and experimental study of plate type indirect evaporative cooler (IEC) for energy recovery in hot and humid regions

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Modeling and experimental study of plate type indirect evaporative cooler (IEC) for energy recovery in hot and humid regions

 

Author: Chen, Yi
Title: Modeling and experimental study of plate type indirect evaporative cooler (IEC) for energy recovery in hot and humid regions
Degree: Ph.D.
Year: 2016
Subject: Evaporative cooling.
Air conditioning -- Efficiency.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxvii, 238 pages : color illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2925489
URI: http://theses.lib.polyu.edu.hk/handle/200/8735
Abstract: Indirect evaporative cooler (IEC) which uses water evaporation to produce cooling air without adding extra moisture has become increasingly attractive as a natural cooling technology. It has a number of advantages over the traditional mechanical vapor compression refrigeration (MVCR) system such as energy efficiency, environmental friendly, low energy consumption and easy maintenance. The IEC is widely used in hot and dry regions because larger cooling capacity can be achieved with low air humidity. In hot and humid regions, the IEC application is restricted to the high humidity in the past. In recent decant, however, the hybrid IEC and mechanical cooling system (also called IEC energy recovery system) has been proposed to break the regional limitation of IEC application and attracts great research interest for its high energy saving potential. In this hybrid system, IEC is used as an energy recovery device installed before an AHU or cooling coil in an air-conditioning system. The exhausted air from air-conditioned space is used as secondary air. Although IEC is being extensively used and related experimental and simulation research work has been carried out, current efforts focus on evaluating the performance in dry and moderate regions, where it was initially used. Little previous studies involving the modeling of IEC in hot and humid regions can be identified. Furthermore, no experimental study on evaluating IEC performance in humid regions can be found in open literatures. In dry or moderate regions, there would be no condensation from primary air, so it is understandable that only sensible cooling performance is discussed in previous studies. However, owing to the high humidity of fresh air in humid areas, condensation would occur on the primary air side. The heat and mass transfer process is much more complicated in this case compared with that of dry region cases. The condensation would greatly affect the overall performance of IEC, not only in sensible cooling but also newly brought latent cooling. The lack of modeling and experimental research of IEC with condensation provides obstacle for applying IEC energy recovery system in humid regions. Therefore, a programmed research work on modeling and experimental investigation of IEC, with an emphasis on IEC performances with condensation from high humidity primary air, has been carried out in this thesis. This thesis begins with establishing a short-cut analytical model for IEC considering condensation. The models for IEC under non-condensation, total condensation and partial condensation states were established and a method for judging the three states was proposed. The availability of the simplified IEC model would help facilitate annual performance prediction of IEC energy recovery system.
Secondly, the thesis presents a novel method for annual simulation of IEC energy recovery system based on the above IEC model. The simulation is achieved by incorporating IEC model into TRNSYS software, so that the annual performance can be precisely predicted by considering possible condensation of IEC, chiller operating state and dynamic cooling load. A detailed case study of a wet market in Hong Kong was presented and the simulation results were validated by a field measurement. The results shows that condensation in IEC takes up a large proportion (47.7%) of operation hours in hot and humid regions, especially in summer. Besides, the IEC shows a great potential in energy saving and peak load reduction. Thirdly, the development of a novel numerical IEC model considering condensation and wettability ratio is presented. A comprehensive parameter analysis under three operating states (non-condensation, partial condensation and total condensation) was firstly presented. The results show that the condensation lowers the wet-bulb efficiency, but improves the total heat transfer rate. Besides, the increase of secondary air velocity, decrease of channel gap and improvement of wettability under condensation state improves IEC cooling performance more effectively than in non-condensation state. Fourthly, the parameter sensitivity analysis of IEC by orthogonal test is presented based on the above numerical model. The influence rank of seven parameters was obtained under IEC condensation state. The optimization was conducted to the most influential parameters: channel gap and cooler height. The optimal channel gap is 2-3 mm and 3-4 mm under condensation and non-condensation state, respectively, while the optimal NTUp is 4-7 and 3-5, respectively. Finally, experimental study is conducted to evaluate the plate type air cooler performance under four operating modes. Under dry operating mode, the air cooler serves as a traditional air cooler, while under wet operating mode, it works as an IEC. Comparisons can be made between traditional air cooler and IEC under both non-condensation and condensation conditions. The cooler dynamic performance during different operating mode transition and steady performance under different parameter influence were investigated. The results shows condensation weakens sensible heat transfer but improves total heat transfer. The wet operating benefits both the sensible and latent heat transfer rates. The COP varies in a wide range under different operating mode and the highest COP achieved by IEC is 9.0.

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