Simulation-based performance analysis of residential direct evaporative coolers in four climate regions of Iran

Highlights

• The effect of DEC design parameters on its performance was evaluated.

• Increasing saturation efficiency decreases electricity and water consumption.

• Increasing flowrate decreases electricity while increases water consumption.

• High thermostat set-point leads to saving in electricity and water consumption.

• The largest impact was in Esfahan which has the lowest wet-bulb temperature.

Abstract

In this paper, the effect of direct evaporative cooling (DEC) system design parameters, including the cooling pad saturation efficiency, the airflow rate, and the thermostat set-points on the system electricity and water consumption, is evaluated in four different climates of Iran according to Köppen-Geiger classification. EnergyPlus is used to perform the simulations for a typical residential apartment. The results show that using pads with a 30% higher saturation efficiency can significantly decrease the electricity and water consumption by up to 50.0% and 29.6%, respectively, because of the working time reduction. The maximum impact is in Esfahan city in cold desert climate as its wet-bulb temperature is lower than other investigated climates. Besides, increasing the airflow rate from 3000 m³/h to 6000 m³/h leads to 43.4%–55.2% reduction in electricity consumption in different climates, although the water consumption increases by 20.5%–58.8%. According to the results, by using the DEC system with 80% saturation efficiency and 6000 m³/h airflow rate, the average indoor relative humidity and CO₂ concentration is 42.3%–48.5%, and 450 ppm–470 ppm, respectively, in different climates, which can provide acceptable indoor air quality. Increasing the saturation efficiency or decreasing the airflow rate increases the indoor relative humidity and CO₂ concentration. The results are also compared with a typical packaged terminal air conditioner (PTAC). The total hours when the indoor relative humidity is out of comfort zone in the DEC system, with 60% saturation efficiency, are about 92% lower than the PTAC system, on average. Moreover, with 24/26 °C thermostat set-points, the PTAC system consumes 234%–265% more electricity than the DEC one with 80% saturation efficiency in different climates.

Introduction

One of the proper cooling systems to use in warm-dry climates are evaporative coolers. Evaporative cooling is a process in which the evaporation phenomenon is used as a natural heat absorber. In this process, the sensible heat of the air is absorbed and used as the latent heat required to evaporate water. The amount of absorbed sensible heat depends on the amount of evaporated water. Evaporative cooling can be used directly or indirectly. In the direct evaporative cooling (DEC) systems, the amount of water in the supply cooling air increases. These systems use no synthetic refrigerant; they use full fresh air and consume much less power than conventional air conditioners. Since the high evaporation rate increases the air relative humidity and may cause discomfort conditions, the DEC systems should be used in areas where the relative humidity is low. In these regions, the relative humidity is relatively below the comfort zone, which is usually between 30% to 60% [1,2]; and the direct evaporative coolers can help to close the indoor relative humidity to the comfort zone by increasing the supply air moisture content.

Wu et al. [3] performed a theoretical analysis, which shows that the inlet air velocity and the thickness of cooling pads are two important factors influencing the cooling effectiveness of a DEC. The optimal input air velocity should be about 2.5 m/s, which can be used to select the cooling pad size for a DEC. Farahani and Heidarinejad [4] studied the potential of a multi-step cooling system, which included a night sky cooling, a cooling coil, and a two-stage evaporative cooling. The proposed system was evaluated for four cities with different climatic conditions. According to their results, the system efficiency was almost 9% more than the stand-alone IEC. In another research work, Sheng and Nnanna [5] investigated the role of three parameters, i.e., the inlet air velocity, the inlet air dry-bulb temperature, and the make-up water temperature, on the cooling performance of DEC systems. The results demonstrated that within certain ranges, the cooling efficiency was directly related to inlet air dry-bulb temperature and was inversely proportional to the inlet air velocity and the make-up water temperature. Chiesa et al. [6] studied the effect of using DEC in reducing the discomfort conditions in 20 urban locations with different climatic conditions in Southern Europe and the Mediterranean. In each climate, a sample building was simulated using EnergyPlus, and the cooling degree hours and the virtual climatic discomfort hours were evaluated for free running and DEC systems. The results showed that DEC could considerably reduce the discomfort hours, especially in hot-dry climates. In 2017, Kovacevic and Sourbron [7] presented a numerical model to solve the heat and mass transfer in a metallic-compact DEC by MATLAB. The model was based on the assumption that air flowed between parallel water walls whose temperature was equal to the input air wet-bulb temperature. The results showed that when the thickness of the pad is 90 mm, the system had the highest efficiency. Kabeel and Bassuoni [8] introduced a model to reduce the water consumption rate of DEC systems. The saline water at various concentrations was used as cooler feed water. It was found that at 200,000 ppm salinity ratio, the water consumption decreased about 1.5 L per hour, while the outlet air temperature increased by 8.6%. Besides, Bishoyi and Sudhakar [9] had research work on the importance of cooling pads in evaporative coolers. Based on actual climatic data, they evaluated two types of cooling pads, including Honeycomb and Aspen, in a DEC system. Considering the cooling capacity, the power consumption, and the energy efficiency ratio as the performance parameters, their experimental results indicated that the energy efficiency ratio and the cooling capacity of the evaporative cooler with the Honeycomb pad were better than the cooler with the Aspen pad. Yunran et al. [10] presented simplified models to estimate the annual energy-saving potential of IECs in hot and humid regions. It was concluded that in the humid climatic zones, the IEC could bring out 43.8%–56.4% energy saving. Recently, Meng et al. [11] presented a cross-flow IEC with a transparent cover plate to observe condensation in primary air channels. Also, the outlet's primary air temperature, the wet-bulb effectiveness, the water consumption rate, the heat transfer rate, and the condensation rate were analyzed under three conditions of non-condensation, partial condensation, and total condensation. The results showed that condensation improved the outlet's primary air temperature, the heat transfer rate, and the water consumption rate; while it decreased the wet-bulb effectiveness. Besides, because of a thick water film, it was impossible to ignore the thermal resistance of the condensate film. Sohani et al. [12] modeled the performance of a dew-point IEC throughout a year and evaluated its performance in four various climatic conditions in the world where IEC is a proper cooling system option. Considering the operation cost, the coefficient of performance, and the water consumption as the objective functions, a multi-objective optimization was done hour-by-hour. According to this study, it can be concluded that the hourly optimization strategy has the potential to improve the abovementioned objective functions by 17.8%, 19.6%, and 36.2%, respectively. In 2019, Lizhi et al. [13] suggested covering the wet channel surface of a small size counter-cross-flow cooler with nylon fibers to improve the evaporative cooler performance. After analyzing the effect of the operating parameters on its thermal performance, the obtained results indicated that the cooler coated with nylon fibers provided a better efficiency. Based on the results, the dew-point effectiveness of the polystyrene + nylon fiber (PS + NL) media enhanced from 46.7% to 78.6%. Tewari et al. [14] studied the performance of office buildings in terms of thermal comfort using DECs in the Indian climate during the summer. They used Energyplus as simulation software and experimental results to calibrate the thermal models. Using multiple linear regression (MLR) technique, they applied the output results to predict supply and indoor air temperatures as functions of control factors. They also established the statistical significance of temperature correlations using variance analysis. They finally assessed the potential of DEC application in the Indian climate and found that about 50% of discomfort hours could be eliminated using DECs. Duan et al. [15] proposed a hybrid dew-point regenerative evaporative cooling (REC) and direct expansion (DX) system. They simulated a dynamic model of the hybrid REC/DX system applied to a residential building in Beijing using EnergyPlus. In addition to positive effects that the proposed system had on the thermal comfort, the life cycle cost and the greenhouse gas emission, the results indicated that the seasonal energy saving by the proposed system is 38.2% and 29.7% in comparison with a typical hybrid indirect evaporative cooling and a typical direct expansion (IEC/DX) system, respectively. Chiesa et al. [16] studied the geo-climatic possibility of using passive DEC to increase thermal comfort in the Mediterranean region. They used three different methods, including the comparison of two new key performance indicators with an existing method, simulations in 60 different regions, and comparing different building specifications for implementing DECs. They concluded that improving thermal comfort strongly depends on the parameters which affect the cooling load. They also showed that DECs could operate optimally as low-energy systems, in the Mediterranean region.

In this research, the effect of three main design parameters of DEC systems, including the cooling pad saturation efficiency, the airflow rate, and the thermostat set-point on the system electricity and water consumption, is evaluated for a typical residential building. The results are also compared with a typical packaged terminal air conditioner system. The research work is performed in four main climatic regions of Iran to investigate the effect of climatic conditions on the system performance.