1. Introduction

Evaporative cooling technology (ECT) is one of the most effective and sustainable alternatives to vapor-compressed cooling technology that could provide thermal comfort with less power consumption and installation cost ^[1][2]. In ECT, the refrigerant is water, which is environmentally benign, and there is no requirement for compressors, which consume a large amount of energy ^[2]. On the other hand, the demand for designing a more efficient and environmentally friendly cooling system has been increasing in recent years ^[3][4]. This is mainly because over 30% of the total energy usage in the world is consumed in buildings and mostly by air conditioning systems ^[5][6]. In addition, the global warming effect and high demands of cooling systems exacerbate energy consumption ^[7]. Among air conditioning systems, vapor-compressed cooling technology has dominated the market for more than 100 years ^[8]. This cooling approach has the two major drawbacks of high energy demand for operating mechanical compressors and global warming issues caused by typical refrigerants, such as R-134a and R-410a ^[3]. These factors have made ECT a hot topic for a long time. However, the humidity ratio of the inlet air significantly affects the system performance because ECT achieves cooling through water evaporation ^[9]. When the ambient air is humid, less water can be evaporated into the air, which means less cooling capacity is obtained. In this case, a dehumidification process is usually adopted to pre-dry the supply air before entering the evaporative cooling system ^[10]. In this hybrid system, the temperature and humidity ratio can be controlled separately, and the solid desiccant wheel is a typical dehumidification system that can be applied to remove the moisture, while the evaporative cooling unit is responsible for providing sensible cooling ^[11]. Therefore, the ECT integrated with a solid desiccant system attracts attention from engineers and researchers. Furthermore, low-grade heat, such as solar or waste heat, can be used to meet the regeneration temperature requirement of the desiccant unit, which makes such hybrid systems even more favorable ^[12].

Although some papers have conducted reviews for the evaporative cooling technology that could be found in the literature ^{[1][8][13][14][15][16][17][18]}, there is still a lack of reviews specifically on the ECT integrated with a solid desiccant system. In this paper, recent technical developments and evaluation methods of ECT and solid desiccant-assisted evaporative cooling technologies (SDECT) were comprehensively reviewed. Firstly, the development of ECT was reviewed. In this section, the history of ECT development was briefly introduced. Then, these two types of ECT were reviewed separately, direct evaporative cooling (DEC) and indirect evaporative cooling (IEC). Later, the solid desiccant material used in recent studies was also reviewed. After that, the recent development of conventional solid desiccant-based ECT was evaluated. Afterward, the enhancement techniques were discussed with respect to configuration optimization, desiccant unit improvement and integration of novel IEC technologies. Furthermore, experimental and numerical methods were used to evaluate the SDECT and report its advantages and limitations. The current research gap, challenges, opportunities and future research recommendations were discussed. This review provides useful information for researchers and engineers in the development of effective ECT technologies for wide community applications under different climate conditions.

2. Overview of Solid Desiccant Dehumidification

Desiccant dehumidification has been widely studied and applied in air-conditioning systems to control the room humidity. The desiccant dryer could be classified into liquid desiccant and solid desiccant according to the desiccant material, and each has its own characteristics. The solid desiccant has a higher water adsorption rate, simpler structure and no carry-over risk compared to the liquid desiccant ^[19]. The dehumidification process is to remove the moisture of air through the strong water vapor attraction property of the desiccant material. The water vapor pressure difference between the desiccant surface and flowing air is the main driving force ^[19]. As the process air is continuously dehumidified, the desiccant materials become saturated, and sorption ability decreases, which requires a regeneration process ^[20]. Thermal energy is usually used to regenerate the desiccant unit, and this could be achieved using solar energy, electrical heater, electro-osmotic and waste heat ^[21][22][23]. To improve the desiccant system performance, many studies have been conducted from the aspects of improving water adsorption capacity and reducing regeneration temperature ^[24]. Zheng et al. ^[24] reviewed the recent developments of solid desiccant material by considering both adsorption isotherms and regeneration ability, which covered composite desiccants, nanoporous inorganic materials and polymetric desiccants. The results showed that the dehumidification and regeneration capacity of composite desiccant could be enhanced via host matrix and immersed salts selection. For nanoporous inorganic materials, a good balance between regeneration and adsorption could be identified by modifying the textural properties. However, further investigation into advanced materials is needed to fulfill all the demands of solid desiccant cooling systems.

3. Integrated Evaporative Cooling and Solid Desiccant System

The temperature difference between the ambient dry-bulb temperature and dew-point temperature is an important indicator to determine the application eligibility of evaporative cooling technology. According to this criterion, evaporative cooling technology is not suggested in hot and humid areas ^[25]. In order to extend the application of evaporative cooling, integrated evaporative cooling with a solid desiccant unit gained lots of attention, and it is believed that the desiccant-based evaporative cooling system is one of the best alternatives to the vapor-compression system under wet and hot climates ^[26]. Compared with the traditional vapor-compression cooling system, the solid desiccant evaporative cooling system consumes less electricity, has no harmful refrigerant, controls air humidity effectively and maintains a high level of air quality ^[27]. The hybrid system usually includes two main processes: the dehumidification process and the evaporative cooling process. The function of the dehumidification process is to remove the moisture, and the evaporative cooling process is responsible for handling the sensible heat of the air. Some commonly used desiccant materials are silica gel, zeolites, alumina, hydratable salts and mixtures ^[12].

Figure 1. Schematic diagram of solid desiccant-based evaporative cooling system ^[28].

Figure 1 shows a typical solid desiccant-assisted evaporative cooling system configuration. From the configuration, the ambient air is dehumidified when flowing through the desiccant wheel. Then the dehumidified air flows through the sensible heat wheel, where it is cooled by the return air from the building zone. The cool, dry air then flows through the evaporative cooler and is further cooled and supplied to the building. The return air from the conditioned room flows through the evaporative cooler and is cooled. This cold air flows through the heat wheel, where it cools the supply air. Afterward, this return air is heated to the regenerative temperature in a heat exchanger by solar or electric energy. Then the high temperature returns airflow through the solid desiccant wheel, where the solid desiccant is regenerated. The exhaust air is discharged into the environment ^[28].

Solid desiccant-assisted evaporative cooling systems have been studied extensively from different aspects. Zadpoor and Golshan ^[29] investigated the performance enhancement of a gas turbine cycle with a desiccant-based evaporative cooling system; a comparative study was conducted between the proposed system and in combination with other types of evaporative cooling, such as direct evaporative and direct and indirect evaporative cooling. Numerical models were built based on the configuration and tested in three climatic conditions. The authors concluded that the performance enhancement obtained using this hybrid system was more obvious than the other evaporative cooling technologies for hot and humid climates. Khalid et al. ^[30] proposed four configurations of solar-assisted, pre-cooled hybrid desiccant cooling system and tested numerically and experimentally under Pakistan climates. The results showed that the fourth operation mode, which applied IEC for pre-cooling and DEC for post-cooling of the air, achieved the highest COP. However, none of these four operation modes could fully provide a cooling load without the assistance of the auxiliary cooling unit. For the city of Lahore’s climate, the hybrid system could only provide enough thermal comfort for three of the seven months of the cooling season. White et al. ^[31] used TRNSYS to study a solar-assisted desiccant two-stage evaporative cooling system without any backup heating equipment under Australian climates. The first stage of evaporative cooling was an IEC, and then the air passed into the second stage DEC for further cooling. The results revealed that the effectiveness of IEC, supply airflow rate and solar collector area were the main factors that would affect the thermal comfort of the building. The results also pointed out that the system was not suitable for tropical climates. Goldsworthy and White ^[32] numerically investigated the optimization of a desiccant cooling system design with IEC. The heat and mass transfer equations of the desiccant wheel and IEC were solved simultaneously. It was found that the hybrid system showed great potential to reduce energy consumption and greenhouse emission. The electrical COP could be greater than 20 when the regeneration temperature was 70 °C, supply and regeneration airflow ratio was 0.67 and the extraction air ratio of IEC was 0.3. Hatami et al. ^[33] conducted an optimization study of the solar collector surface requirement in a desiccant evaporative cooling cycle. They concluded that after considering the design parameters and operating conditions of the desiccant wheel, the compulsory solar collector areas could decrease by 45% compared to an empirical model. Parmar and Hindoliya ^[34] evaluated a desiccant cooling system performance under four types of climates in India (hot and dry, warm and humid, moderate and composite climates). From the research outcomes, the desiccant cooling system was suggested to use in warm and humid climates; the maximum system thermal COP achieved 4.98 when the flow rate ratio between the regeneration air and process air (R/P ratio) was 0.55. They also claimed that the increase of the R/P ratio led to an elevation of the regeneration heat load requirement and a reduction in system thermal COP. Angrisani et al. ^[35] compared three types of solar collectors applied in the solar desiccant cooling systems in terms of energy, environmental and economic performance with a reference system. A selection of 16 m 2 of the evacuated solar collectors was recommended to obtain a drop of 50.2% in energy consumption and 49.8% equivalent CO 2 emissions. But the economic payback period was over 20 years due to its high installation cost. Rafique et al. ^[27] developed a numerical model of desiccant evaporative cooling systems and conducted a feasibility analysis for a hot and humid city in Saudi Arabia. The results showed that this system was feasible to use in such weather conditions with optimum selection of the operating parameters. Ma and Guan ^[36] investigated the system performance of a solar desiccant evaporative cooling system for a commercial building under various climates in Australia. Performance parameters including thermal COP, annual primary energy consumption, annual energy saving and annual CO 2 emissions reduction were compared with a reference variable air volume system. The maximum energy saving and CO 2 emission reduction were obtained as 557 GJ and 121 tones per year, respectively, in Darwin. The highest values of 7 of thermal COP could be achieved during summertime in Darwin. They pointed out that the solar desiccant evaporative cooling technology was useful in Darwin, which has a tropical climate, while it was not a competitive approach for other climate regions. Ma et al. ^[37] evaluated several important performance parameters of a solar desiccant evaporative cooling system, which included solar fraction, electrical COP, electricity power consumption, CO 2 emissions reduction, payback period and net present value. A comparative study was also carried out between the proposed system and two other solar-assisted vapor-compression air cooling systems under Australian climates. The simulation results presented that the solar desiccant-evaporative cooling system is suitable for most Australian climates, especially for hot and humid places. A total of 82.1% of annual power saving, 178.45 tons of annual CO 2 emissions reduction, 3.9 years of payback period and AUD 466,199 net present value could be accomplished by applying this cooling system in Darwin. Ma et al. ^[38] conducted a parametric study on solar collector area, solar thermal storage tank volume and backup heater capacity for a cooling system under Australian climates. It was shown that both increasing solar collector areas or solar thermal storage tanks could lead to solar fraction and electrical COP increase, but the tank volume was more sensitive to affect the system. A humidity control set point was also given as 0.008 kg/kg by considering indoor design conditions and electricity usage of the backup heater. Narayanan et al. ^[39] evaluated a desiccant evaporative cooling system for a residential building in Brisbane by using TRNSYS software. They found that this type of system could provide thermal comfort for about 50% of the time required for cooling. They also concluded that the system performance was mainly affected by the evaporative cooling system, heat recovery system and desiccant regeneration process.

4. Evaluation Methods of the Solid Desiccant Evaporative Cooling System

The main experimental investigations on solid desiccant-based evaporative cooling systems are illustrated in Table 1. It is noted that only a few experimental studies existed in the literature based on the literature survey. This is mainly caused by high initial construction cost and large geometric size. As it can be seen from Table 1, previous experimental studies mainly explore the following considerations.

Table 1. Experimental study of solid desiccant evaporative cooling system.

Although each experiment has a different arrangement of configurations, evaporative cooler and desiccant unit, the experimental results are similar. It proves that the hybrid system can provide thermal cooling in hot and humid areas without the need for the backup cooling equipment. Most efforts have been made to improve dehumidification performance at low regeneration temperatures, to use solar energy to reduce regeneration energy usage and to use advanced evaporative cooling technology.

In comparison with experimental approaches, numerical methods are more popular because they can provide accurate results within a short time and at a low cost. Among the numerical methods, the transient system simulation program (TRNSYS) was commonly adopted to simulate solid desiccant-based evaporative cooling systems ^[46]. TRNSYS is a flexible and component-based software package that mainly focuses on assessing the performance of thermal and electrical energy systems. Around 150 models are included in the TRNSYS library, which are written in Fortran, such as HVAC equipment, solar components and weather date processors. It is possible to easily connect and modify the existing components or even create your own models when necessary ^[47]. MATLAB, EnergyPlus and EES have also been used to simulate the hybrid system. ε -NTU and finite element/difference/volume methods are often applied to solve the heat and mass transfer equations of the system. For different numerical models, the major difference between each model is the assumptions adopted for building the models. The commonly used assumptions in the literature are listed below. In fact, most of the assumptions are made for simplification of the numerical model, which will result in sacrificing accuracy. When a system is working at a steady-state, most assumptions are suitable to use. However, when it comes to a dynamic situation, some assumptions are not appropriate anymore. For example, the cooling performance of the evaporative cooler relies on the inlet air condition heavily, if the cooling effectiveness of the evaporative cooler is still assumed as a constant when the inlet air condition varies, a huge error will be obtained in the result. Table 2 shows the assumptions that are commonly used in numerical simulations.