A Comprehensive Review of the Applications of Hybrid Evaporative Cooling and Solar Energy Source Systems

Abstract

Recent advancements in single-stage evaporative cooling (EC) have showcased their effectiveness as an energy-efficient and sustainable air-conditioning (AC) solution. However, several challenges hinder the widespread adoption of EC in various applications. These challenges include climate sensitivity, substantial spatial requirements, and limitations in achieving desired output temperatures. To address these concerns, there has been a growing focus on integrating EC with solar energy (SE) systems. With traditional energy resources being depleted, the use of SE has gained prominence as a sustainable solution to meet future energy demands while mitigating environmental pollution. This paper presents a comprehensive review of hybrid EC–SE systems, aiming to elucidate the potential synergies, benefits, and challenges associated with this integration. The review explores the principles and mathematical approaches of various configurations of EC systems to assess their compatibility with SE sources. Furthermore, the review delves into the mathematical model of SE, encompassing both solar power generation and thermal collectors, with the aim of integrating it into the EC model. It delves into key aspects of energy consumption and performance, showcasing advancements in achieving higher efficiency and enhanced cooling capacity through the hybrid systems. Additionally, the review highlights research gaps in the existing literature, emphasizing the need for further exploration in this interdisciplinary field. In conclusion, this paper offers valuable insights into the potential of EC–SE systems to address energy and cooling requirements while promoting sustainable development.

1. Introduction

The excessive depletion of Earth’s resources has given rise to significant environmental and energy challenges. As depicted in Figure 1, global land and ocean temperatures from 1901 to 2020 show a consistent trend [1]. It is evident that global temperatures have surpassed pre-industrial levels by 0.9 °C since the 1980s. Notably, densely populated regions are expected to experience temperature increases exceeding 2 °C, in stark contrast to sparsely populated areas. In 2015, the United Nations Framework Convention on Climate Change (UNFCCC) recommended measures to prevent global temperatures from exceeding the pre-industrial level of 1.5 °C [2]. In this context, a corresponding increase in energy consumption is anticipated due to the growing global population and the expansion of manufacturing industries. This surge in electricity usage has led to a significant resurgence of global greenhouse gas emissions, particularly in regions heavily reliant on fossil-fuel-based power generation [3]. The consequences of this continued energy consumption manifest as climate change, the greenhouse effect, glacial melting, sea-level rise, and related environmental issues. It is clear that energy consumption remains dominated by oil, coal, and gas. While greenhouse gas emissions have reached alarming levels, they are gradually decreasing thanks to the adoption of renewable energy sources [4]. The International Energy Agency (IEA) predicts that renewable energy sources will constitute 90% of the global economic focus within the next three decades [5,6,7]. This underscores the pivotal role of renewable energy sources in addressing energy consumption challenges. Among the abundant varieties of renewable energy sources, solar energy (SE) often comes to the forefront during discussions. Renewable energy sources can be harnessed on a global scale, with exceptions in extreme day and night conditions near the North and South Poles. Jordan, for instance, has successfully utilized solar power to meet 10% of its electricity needs, contributing to energy conservation [8]. However, replicating such widespread utilization of solar power may not be feasible in all regions due to inherent challenges, such as the intermittent nature of SE and the need for substantial photovoltaic (PV) infrastructure [9]. As global energy and environmental issues demand urgent attention, there is a pressing need to promote and develop environmentally sustainable systems.

Data from the IEA highlight the crucial role played by the construction industry, responsible for 30% of global energy consumption and contributing to 28% of CO₂ emissions [10]. An important insight is that residential and commercial buildings worldwide account for 30–40% of annual primary energy consumption [11,12]. Advanced air-conditioning (AC) technology in contemporary times is categorized into several refrigeration methods, with the majority relying on traditional mechanical vapor compression (MVC) cooling technology [13]. The widespread use of MVC systems can be attributed to their well-established and cost-effective attributes, making them versatile across various scenarios. However, the prevalence of MVC systems in the construction sector raises concerns related to energy depletion and the environment [14]. Interestingly, evaporative cooling (EC) technology has not gained widespread adoption within society, despite its potential to address environmental and energy efficiency concerns [15]. In sharp contrast, EC technology emphasizes environmental preservation and energy efficiency. It employs water and air as working mediums, eliminating the need for environmentally harmful refrigerants, positioning itself as an environmentally conscious cooling method. Furthermore, the inherent heat and mass transfer processes in EC eliminate the need for compressors, efficiently harnessing renewable dry air energy from the surrounding environment [16,17]. Evidence suggests that the coefficient of performance (COP) of EC significantly exceeds that of MVC in various operational scenarios [18,19]. Moreover, when compared to other refrigeration systems in terms of energy consumption rates, EC demonstrates distinct advantages across various applications: (1) In the construction industry, EC is embraced due to its cost-effective retrofitting, broad efficiency spectrum, shorter maintenance cycles, and strong return on investment [20]; (2) While EC may not be as widely adopted as conventional MVC counterparts, its unique attributes, such as effective operation in high-humidity and low-temperature conditions, make it particularly suitable for preserving agricultural products and fruits [21]; (3) In regions characterized by hot and arid climates, EC finds extensive use in industrial processes and poultry farming [22] and (4) EC’s utility extends to high-temperature industries, spanning applications like water distillation, power generation, and pollution control [23].

EC, functioning as a self-contained cooling system, showcases exceptional versatility, serving as both a standalone solution and a component within integrated systems, thereby expanding its range of applications. EC has demonstrated compatibility with liquid-drying dehumidification systems [24] and solar-assisted drying dehumidification systems [25]. In regions characterized by high temperatures and humidity, it is common to integrate dehumidification systems upstream of EC units to facilitate air-drying [26]. Some researchers have proposed hybrid systems, such as the integration of EC with heat recovery devices, with the aim of reducing carbon emissions [27]. It is noteworthy that EC’s utility extends beyond thermal comfort and AC; it also provides efficient heat dissipation conditions for fuel vehicles [28]. Furthermore, EC has been effectively employed as a pre-cooling apparatus in conjunction with MVC systems to enhance their COP [29]. In general, these hybrid configurations have demonstrated superior cooling efficiency and energy consumption metrics compared to conventional refrigeration setups [30].

Adapting to evolving regional conditions, EC has shown potential for synergy with various SE technologies. The existing literature showcases several ways in which EC integrates with SE, including cooling on PV/thermal solar panels [31,32,33], solar sorption cooling systems [34,35], and solar thermal cooling (STC) [36]. Many STC systems incorporate EC as an indispensable component, while there is also ongoing research on using solar power generation to support EC systems, primarily for the air supply and water circulation systems. In the preparatory stages of an experiment, understanding the feasibility is crucial. As a result, the models for EC, solar power generation, and heat collectors are individually reviewed using a numerical simulation approach. Previous research has extensively explored EC’s prospects, primarily focusing on its utilization in thermal management systems, building environments, and automotive engines. However, a comprehensive review that delves into the integration of hybrid EC systems with SE, particularly in the context of energy efficiency, environment, and economics (3E), is notably absent. To address this research gap, this paper concentrates on examining the integration of EC with SE technologies, providing a thorough review of applications involving hybrid EC and SE systems.

This review offers a new perspective on the integration of EC with SE. Additionally, it represents a breakthrough in the current trajectory of EC research. Furthermore, a comprehensive summary of existing research on EC–SE systems has been presented. Lastly, the review presents objective insights into new research directions in the hybrid systems, thereby guiding future research endeavors. The structure of this review is outlined as follows: In the first section, we introduce various EC configurations and delve into the mathematical models of EC and SE, considering their distinct characteristics. These configurations serve as the foundation for subsequent discussions. The second section comprehensively analyzes studies related to EC–SE systems, encompassing aspects of energy consumption, system performance, and exergoeconomics. Finally, we discuss the limitations present in existing research, highlighting areas that warrant further exploration, and explore potential directions for future research.

2. Classifications and Principles of EC

EC can be categorized into three distinct types, as illustrated in Figure 2, each based on its operational principle: direct evaporative cooling (DEC), indirect evaporative cooling (IEC), and dew-point evaporative cooling (DPEC) [37]. Further classification reveals that DEC can be divided into active and passive types based on energy consumption, while IEC and DPEC can be categorized into counter-flow and cross-flow, distinguishing them by different methods of air inlet.

2.1. DEC

DEC represents the most straightforward form of EC technology, primarily focused on cooling supply air through direct contact with water, as illustrated in Figure 3a. It capitalizes on the vapor pressure difference between the water surface and the supply air, causing the supply air to cool along the isoenthalp line. This cooling process is facilitated by moisture absorption from the water source, as depicted in Figure 3b. In general, DEC can be classified into active and passive types. Active DEC relies on electricity to power the system and requires less power compared to conventional MVC systems. Active DEC has the potential to achieve energy savings of up to 90% under specific conditions [38]. Passive DEC, sharing a similar structural composition and operational principles with active DEC, relies on natural ventilation instead of auxiliary drive systems for its evaporation process. However, its effectiveness is significantly influenced by ambient environmental factors [39]. DEC achieves temperature reduction through the fundamental processes of heat and mass transfer between air and water. However, it is important to note that the accumulation of excessive humidity within enclosed spaces can be problematic. Prolonged exposure to overly humid conditions can lead to discomfort and potentially contribute to conditions associated with sick building syndrome.

2.2. IEC

IEC provides a solution to address the challenge posed by increased humidity levels that often accompany the air produced by DEC [40]. The fundamental operational principle of IEC closely parallels that of DEC, involving the reduction of air temperature through the transfer of heat and mass between water and air. However, IEC introduces a crucial distinction: it partitions air channels into dry and wet sections using thin plates, as depicted in Figure 4a. The dry channel serves as the path for air cooling, while the wet channel serves as the working zone with its surface covered in water. These two channels do not exchange moisture, ensuring that the humidity of the produced air remains unchanged. The airflow in the wet channel triggers the evaporation of water from the surface, leading to the absorption of heat from the supply air in the adjacent dry channel. The psychrometric chart of the IEC process is illustrated in Figure 4b. During this process, the temperature of the supply air stream decreases as its enthalpy is transferred to the working air stream. The working air, having absorbed a substantial amount of enthalpy, becomes nearly saturated and is then released into the ambient environment. IEC is typically categorized into two configurations: counter-flow and cross-flow, based on the directions of air intake [41,42]. In counter-flow IEC, the supply and working air flows move in opposing directions, as depicted in Figure 4a. In contrast, cross-flow IEC features perpendicular alignment of the supply and working air streams in adjacent channels, as shown in Figure 5. While IEC effectively addresses the humidity issue, it is important to note that its cooling efficiency is subject to certain limitations [43].

2.3. DPEC

The Maisotsenko cycle (M-cycle) [23,44] was introduced as an innovative approach to address the limitations of the conventional IEC technology. The M-cycle, depicted in Figure 6, aims to improve cooling efficiency by incorporating the concept of transferring a portion of air from a dry channel to a wet channel. Within the wet channel, this air undergoes EC, absorbing heat from the dry channel before being expelled as exhaust gases [45]. Consequently, the air stream generated by the M-cycle cools the surrounding air to its dew-point temperature. This cooling technology is also known as DPEC. The working principle of DPEC is illustrated in Figure 6a, along with its corresponding psychrometric chart in Figure 6b. DPEC can be classified into counter-flow and cross-flow configurations based on their air intake methods. However, DPEC introduces improvements that result in higher cooling efficiency and a more compact design compared to conventional IEC systems. In the counter-flow configuration (Figure 7a), the air flows in opposite directions in the dry and wet channels [46]. In the cross-flow configuration (Figure 7b), the dry and wet channels are positioned perpendicular to each other, creating a cross-shaped airflow pattern. The cross-flow configuration is generally favored, although it presents challenges in achieving a perfectly balanced flow state. Despite advancements, the cross-flow configuration is still under development regarding fabrication processes and material selection [47,48]. In comparison, the counter-flow DPEC holds the potential for superior cooling performance [49]. An early M-cycle structure, as depicted in Figure 7c, presents a modified version of the cross-flow configuration. Notably, DPEC systems can achieve supply air temperatures lower than their wet-bulb (WB) temperature, thereby enhancing cooling efficiency beyond that of conventional IEC systems [50].

In summary, DPEC has emerged as an innovative solution to overcome the limitations of traditional IEC technology. By incorporating concepts such as air extraction, evaporation, and heat absorption, the M-cycle enhances cooling efficiency and shows promise in both counter-flow and cross-flow configurations. This technology has the potential to provide more effective cooling solutions compared to conventional IEC systems, particularly in terms of cooling potential and efficiency.

2.4. Evaluation Indices

The assessment of the EC system performance can be based on the process air state and efficiency. The performance of the EC system depends on the cooling capacity (CC, kW) as expressed [51]:

$${\mathrm{CC}=\dot{\mathrm{m}}(\mathrm{e}}_{\mathrm{ai}} {- \mathrm{e}}_{\mathrm{ao}})$$

(1)

where $\dot{\mathrm{m}}$ is the mass flow rate (kg/s), ${\mathrm{e}}_{\mathrm{ai}}$ and ${\mathrm{e}}_{\mathrm{ao}}$ are the enthalpy of inlet and outlet air (kJ/kg), respectively.

The WB efficiency (${\mathsf{\epsilon }}_{\mathrm{wb}}$) is defined as the ratio of the actual humidity ratio variation of the process air to the theoretical maximum humidity ratio as given [52,53]:

$${\mathsf{\epsilon }}_{\mathrm{wb}}=\frac{{\mathrm{T}}_{\mathrm{a},\mathrm{i}} {- \mathrm{T}}_{\mathrm{a},\mathrm{o}}}{{\mathrm{T}}_{\mathrm{a},\mathrm{i}} {- \mathrm{T}}_{\mathrm{wb},\mathrm{i}}}$$

(2)

where ${\mathrm{T}}_{\mathrm{a},\mathrm{i}}$ and ${\mathrm{T}}_{\mathrm{a},\mathrm{o}}$ are the temperature (°C) of the process air at the inlet and outlet of the working air channel (g/kg), respectively. ${\mathrm{T}}_{\mathrm{wb},\mathrm{i}}$ is the WB temperature (°C) of the process air at the inlet.

The dew-point effectiveness (${\mathsf{\epsilon }}_{\mathrm{dew}}$) which, unlike IEC and DEC, is the ratio of air temperature drop over difference of incoming air dry-bulb (DB) temperature and WB, can be expressed as [54,55]:

$${\mathsf{\epsilon }}_{\mathrm{dew}}=\frac{{\mathrm{T}}_{\mathrm{a},\mathrm{i}} {- \mathrm{T}}_{\mathrm{a},\mathrm{o}}}{{\mathrm{T}}_{\mathrm{a},\mathrm{i}} {- \mathrm{T}}_{\mathrm{dew},\mathrm{i}}}$$

(3)

where ${\mathrm{T}}_{\mathrm{dew},\mathrm{i}}$ is the dew-point temperature (°C) of the process air at the inlet.

The COP is defined as the ratio of the CC to the input (fan and pump) power consumed as calculated:

$$\mathrm{COP}=\frac{\mathrm{CC}}{\mathrm{W}}$$

(4)

where W is the input power (W).

2.5. Mathematical Approach

The essence of various EC modeling approaches lies in their ability to model and analyze heat and mass transfer on wet surfaces. Initial research efforts laid the groundwork by establishing fundamental theories, outlining comprehensive general models, and exploring practical applications for heat exchangers involving wet surfaces [56,57,58]. While the underlying principles of these mathematical models are similar, the primary distinctions among them revolve around the boundary conditions and assumptions they incorporate. Researchers often opt to simplify their models by making certain assumptions, but this can come at the cost of accuracy, a trade-off that many researchers are hesitant to accept. Nonetheless, numerous mathematical models offer unique conditions and formulations, which we will delve into in more detail.

The first model introduced is DEC. For both hot and dry climates, as well as composite climates, the size of the cooler designed for a space is typically determined through experience. However, linear relationships can be employed to assess the cooler’s size relative to the space to be cooled [59]. The following are some basic numerical methods used to evaluate DEC parameters. Camargo et al. [60] proposed a numerical simulation for DEC using Computational Fluid Dynamics (CFD) methods. This simulation is based on the energy conservation equation for an elementary control volume, analyzing the heat and mass transfer between humid air and water. Experimental results indicated that higher saturation efficiency is achieved at high dry-bulb temperatures and low wind speeds. Fouda and Melikyan [61] developed a mathematical approach (CFD method) to examine the heat and mass transfer between air and water to validate DEC’s performance. Their results demonstrated that increasing the heat transfer area can enhance saturation efficiency. In addition to analyzing the structural aspects of the system through numerical simulations, the cooling medium itself should also be subjected to numerical analysis. Sellami et al. [62] proposed a mathematical model to study the effective parameters influencing the performance of direct evaporation of a porous layer, considering simultaneous heat and mass transfer characteristics. The solution of this mathematical model is based on the finite volume method. Furthermore, a wealth of theoretical and numerical studies on the heat and mass transfer processes of DEC have been documented, as summarized in

Table 1. Evaluation approach of DEC systems.

Study	Year	Approach	Major Feature
[63]	2002	CFD	A mathematical model, including the governing equations of liquid film and air phases as well as the interface conditions, was developed.
[64]	2015	Experiment	The efficiency of the internal two-stage evaporative cooler was higher than that of direct evaporative cooler, but it cannot be raised over 100%.
[65]	2006	CFD	The liquid film falling inside a vertical insulated tube in turbulent gas stream was studied.
[66]	2011	Experiment	Performance of pad thickness and inlet air velocity on the change of the system’s overall pressure drops, amount of evaporated water, effectiveness, humidity variation was analyzed.
[67]	2017	ANN statistical modelling	The prediction abilities of obtained models were analyzed, and a comprehensive error analysis was conducted through statistical modelling.
[68]	2009	CFD	The heat and mass transfer between air and water film in the DEC was theoretically analyzed.
[69]	2012	Experiment	Speed of frontal air, the DB of frontal air, and the temperature of the incoming water were tested for their impact on DEC cooling efficiency.
[70]	2016	Experiment	The exergetic performance indicators of DECs with wood charcoal, shredded foam latex, and jute fiber were presented.

.

In fact, there is little difference in the relationship between IEC and DPEC concerning analytical solutions and numerical models. To explore a method for achieving sub-wet bulb temperatures using IEC, Hasan developed an analytical model based on the effectiveness-NTU method (ε-NTU). A modified analytical model can also be established based on the ε-NTU method for sensible heat exchangers, provided proper adjustments are made by redefining potential gradients, transfer coefficients, heat capacity rate parameters, and assuming a linear saturation temperature-enthalpy relation of air [17]. In the context of numerical simulations for DPEC, Pandelidis [71] investigated a mathematical model for heat and mass transfer, considering two different cyclic ways (counter-flow and cross-flow) in heat and mass exchangers. This model was also based on the ε-NTU method. These numerical simulation methods reveal unique characteristics of the considered units, enabling accurate predictions of their performance and facilitating comparisons between the two types of heat exchangers in various AC applications to achieve optimal efficiency. Pakari and Ghani [72] proposed regression models that establish relationships between input parameters, including operational and geometrical factors, and selected output responses of counter-flow DPEC systems using numerical simulations and response surface methodology. The predicted outlet temperatures of the system, based on the regression model, closely match the predictions from the numerical model and experimental measurements, within margins of ±4% and ±10%, respectively. Moreover, the novel DPEC technology has garnered significant attention. Cui [73] validated the performance using a computational model through CFD methods for the cooler. The model demonstrated close agreement with experimental findings, with discrepancies within ±7.5%. In addition to the information provided, numerous theoretical and numerical studies on the heat and mass transfer processes of IEC and DPEC have been documented, as summarized in

Table 2. Evaluation approach of IEC and DPEC systems.

Study	Year	Type	Approach	Major Feature
[74]	2015	IEC	ε-NTU method	A sensitive analysis of heat and mass transfer process was performed on the base of the ε-NTU method to establish preferable operating conditions.
[75]	2014	DPEC	ε-NTU method	A modeling of heat and mass transfer in the DPEC was described.
[76]	2015	DPEC	ε-NTU method	A mathematical simulation of heat and mass transfer in eight different types of the DPEC was studied.
[77]	2023	IEC	ε-NTU method	The method was adopted for the application of a countercurrent evaporative heat exchanger featuring.
[78]	2016	DPEC	CFD; Experiment	A minimum parametric representation for evaluating the thermal performance of a counter-flow DPEC was determined.
[79]	2016	IEC	CFD	A computational model was developed and validated using experimental data.
[80]	2017	DPEC	CFD	A more realistic boundary condition on separating wall was obtained by simultaneous solving of momentum, energy, and mass transfer equations.
[81]	2017	DPEC	CFD	A numerical study of heat and mass transfer in counter-flow DPEC was presented using an approach proposed by previous work.
[82]	2015	IEC	Finite difference method	A modeling of an IEC with consideration of wall longitudinal heat conduction and effect of spray water temperature variation along the exchanger surface was presented.
[83]	2015	DPEC	Finite difference method	The numerical simulations of cross- and counter-flow DPEC were presented.

. Moreover, to illustrate the shared characteristics and distinctions between the IEC and DPEC models, Figure 8 serves as a valuable reference.

3. Mathematical Model of Solar Energy

SE is an indispensable source of energy worldwide. However, due to its intermittent nature, solar facilities often require extensive surface areas to meet the energy demands of various processes [84]. SE can be harnessed for two primary applications: power generation and heat storage.

3.1. Solar Power Generation

The principle of PV power generation lies in the PV effect, a reaction that occurs when PV materials come into contact with sunlight, leading to their conversion into electrical energy supplied to the system. Consequently, voltage and overvoltage margin emerge as crucial parameters in the modeling of solar power generation.

Solar power generation is converted into electricity by absorbing heat through PV panels. The voltage ($\Delta {\mathrm{U}}_{\mathrm{pv}}$) between the solar panel and the electrical appliance (%) is given as follows [85]:

$$\Delta {\mathrm{U}}_{\mathrm{pv}}=\frac{{\mathrm{rP}}_{\mathrm{pv}}}{{10\mathrm{U}}_{\mathrm{n}}^2}$$

(5)

where ${\mathrm{U}}_{\mathrm{n}}$ is the nominal voltage of the grid (kV), $\mathrm{r}$ is the resistance ($\mathsf{\Omega }$), ${\mathrm{P}}_{\mathrm{pv}}$ is the generating active power of solar (kW).

${\mathsf{\delta }}_{\max }$ is the “overvoltage margin” (kW). It means the difference between the upper limit of the grid voltage and the maximum voltage at the connection point before connecting the PV system, and it is defined as follows:

$${\mathsf{\delta }}_{\max }=\frac{{\mathrm{U}}_{\mathrm{upperlimit}}{\mathrm{U}}_{\max }}{{\mathrm{U}}_{\mathrm{n}}}$$

(6)

where ${\mathrm{U}}_{\mathrm{upperlimit}}$ is the upper limit of the grid voltage (kV), ${\mathrm{U}}_{\max }$ is the maximum voltage (kV).

${\mathrm{P}}_{\mathrm{pv}}$ is defined as Equation (3):

$${\mathrm{P}}_{\mathrm{pv}}=\frac{{\mathrm{U}}_{\mathrm{n}}^2}{r}{\mathsf{\delta }}_{\max }$$

(7)

3.2. Solar Thermal Collector

The following assumptions are usually employed in the solar thermal collector: (1) The air flow in the evacuated tube is constant; (2) There is no heat loss in the entire solar collector; and (3) The heat conduction and heat transfer convection in the vacuum jacket are neglected.

The total valuable energy collected through the collector is written by [86]:

$${\mathrm{Q}}_{\mathrm{s}}{=\mathrm{F}}_{\mathrm{R}}{[\mathrm{I}}_{\mathrm{t}}\left(\mathsf{\tau }\mathsf{\alpha }\right){\mathrm{A}}_{\mathrm{e}} {- \mathrm{A}}_{\mathrm{a}}{\mathrm{U}}_{\mathrm{L}}{(\mathrm{T}}_{\mathrm{f},\mathrm{i}} {- \mathrm{T}}_{\mathrm{o}}\left)\right]$$

(8)

where ${\mathrm{Q}}_{\mathrm{s}}$ is the solar heat transfer rate (W), ${\mathrm{I}}_{\mathrm{t}}$ is the solar radiation (W/m²), ${\mathrm{F}}_{\mathrm{R}}$ is the heat removal factor of the collector, ${\mathrm{A}}_{\mathrm{e}}$ is the area of effective heat absorption of absorber tubes (m²), ${\mathrm{A}}_{\mathrm{a}}$ is the outer surface of absorber tubes (m²), ${\mathrm{U}}_{\mathrm{L}}$ is the total heat loss factor of collector, $\mathsf{\tau }\mathsf{\alpha }$ is the effective absorptance–transmittance product, ${\mathrm{T}}_{\mathrm{f},\mathrm{i}}$ is the temperature of inlet flow (K), ${\mathrm{T}}_{\mathrm{o}}$ is the temperature of outlet (K).

The heat removal factor of the collector is calculated by:

$${\mathrm{F}}_{\mathrm{R}}=\frac{{\dot{\mathrm{m}}\mathrm{C}}_{\mathrm{p},\mathrm{air}}}{{\mathrm{A}}_{\mathrm{a}}{\mathrm{U}}_{\mathrm{L}}}[1-\exp (-\frac{{\mathrm{A}}_{\mathrm{a}}{\mathrm{U}}_{\mathrm{L}}{\mathrm{F}}^{ }}{{\dot{\mathrm{m}}\mathrm{C}}_{\mathrm{p}}}\left)\right]$$

(9)

where ${\mathrm{C}}_{\mathrm{p},\mathrm{air}}$ is the specific heat of air (kJ/kg∙K), $\dot{\mathrm{m}}$ is the mass flow rate (kg/s), $\mathrm{F}$ is the collector efficiency factor.

${\mathrm{U}}_{\mathrm{L}}$ is the collector total heat loss factor as evaluated by:

$${\mathrm{U}}_{\mathrm{L}}=0.003\left({\mathrm{T}}_{\mathrm{f},\mathrm{i}}{-\mathrm{T}}_{\mathrm{o}}\right)+0.89$$

(10)

4. Hybrid EC and SE Systems

Solar technologies harness energy directly from solar radiation for various purposes, including heating and electricity generation. One common method of storing solar thermal energy is through the use of heated water, often referred to as sensible heat. The efficiency of solar thermal energy systems is highly dependent on the efficiency of the storage technology. This dependence arises from two main factors: (1) the unpredictable nature of solar radiation exposure; and (2) the time-dependent characteristics of energy storage. Alternatively, solar thermal energy can also be stored using a suitable phase change material (PCM), which stores energy in the form of latent heat. PCM offers a high heat storage capacity per unit volume-to-mass ratio. In contrast, solar photovoltaic (SPV) systems convert solar radiation directly into electrical energy. The generated electrical energy can either be stored for later use or used immediately. In some cases, surplus power can be integrated into the main grid or combined with other renewable energy sources. SPV systems provide clean and reliable power for a wide range of applications, including agriculture, livestock farming, industrial processes, and residential usage [87]. The utilization of SE not only contributes to a reduction in carbon emissions but also enhances the longevity of energy systems [88]. One limitation of solar air heating panels is their heat transfer efficiency, despite an increase in panel surface area [89]. However, SE offers specific advantages over conventional AC systems in terms of heat transfer. Conventional AC systems, which heavily rely on compressors and refrigerants, have a significant ecological impact. Compressors in direct-expansion AC systems impose a substantial electrical load due to their operational role [90]. Riahi and Shafii [91] incorporated PCM into conventional evaporative-compression cooling systems, resulting in significant improvements in CC and efficiency, albeit with a slight increase in electrical energy consumption. Achieving a fully carbon-neutral and energy-neutral refrigeration system, even with the integration of solar power, remains a challenge. To move closer to the goals of near-zero carbon emissions and energy consumption, EC systems play a crucial role in refrigeration technology. Additionally, the prudent utilization of SE resources is a vital aspect. Alahmer and Ajib [92] discuss the latest technology related to energy consumption in cooling and AC systems, as well as the feasibility of solar-driven cooling systems compared to conventional ones. Bilgili [93] introduced a solar combined condenser and compressor refrigeration technology as a step toward a hybrid system. Lai et al. [94] evaluated the performance of a solar-assisted solid desiccant M-cycle system with different recirculation air ratios. Tripathi and Kumar [95] conducted research on the performance of SE in IEC systems with novel wet channels, focusing on factors such as air inlet flow variables. Abed et al. [96] introduced a novel hybrid evaporative solar cooling system powered solely by a transpired solar collector. When the cooling system is powered only with a transpired solar collector, indoor air temperature reduces 5 °C less and relative humidity 15% higher than ambient. Gao et al. [97] investigated a hybrid system whereby passive EC enhanced the electricity production of a PV-solar thermoelectric generator (PV-STEG). The findings revealed that EC significantly reduced the operating temperature of the PV-STEG. Mekonen et al. [98] aimed to optimize an evaporative air cooler using a CFD-based model, considering parameters such as airspeed, porosity, and thickness of the bundle. This subsection will highlight research related to the integration of EC with SE systems.

4.1. EC Integrated with Solar Power Generation

The choice of PV materials plays a crucial role in solar power generation. Silicon-based PV panels currently account for approximately 3% of the global annual electricity production [99]. Technological advancements have led to the emergence of new generations of PV panels. Graphene, in particular, stands out as a superior alternative to traditional silicon PV due to its improved efficiency, reduced land requirements, and lower costs [100]. Moreover, addressing the challenges associated with the traditional PV value chain, which involves lengthy processes and results in bulky and heavy panels, perovskite, a semiconductor material, has emerged as a promising option. It has the potential to decrease manufacturing costs [101]. Kiyaninia et al. [102] presented a hybrid system in which SE served as an auxiliary power source for DEC, as shown in Figure 9. The core purpose of this system was to produce cooling through active DEC. The findings revealed that the system’s output temperature was notably affected by factors such as water temperature, air flow rate, and inlet air temperature. Furthermore, when subjected to exergoeconomic analysis, the system exhibited a significant reduction of approximately 38% in the exergoeconomic factor over a 15-year system lifespan.

Alktranee and Péter [103] conducted a study to investigate the impact of utilizing DEC on the performance and thermal characteristics of PV modules. They achieved this by incorporating cotton wicks immersed in water. The results demonstrated a significant improvement in DEC performance, resulting in a 22.3% reduction in the temperature of the SPV modules and a remarkable 73% increase in output power. In a similar vein, Žižak et al. [104] conducted an assessment of the efficiency and sustainability of EC applied to PV cells. The EC applied to PV cells in the whole system is illustrated in Figure 10a. Figure 10b depicts the experimental setup of the system, with DEC integrated behind the PV modules. This passive DEC system aimed to cool the modules and, consequently, enhance their efficiency. The principle of mass and heat transfer of the passive DEC as depicted in Figure 10c. The findings revealed that urban areas without cooled PV systems exhibited a power generation range of 162.5–201.6 kWh/m² per year. The implementation of DEC led to an additional power generation increase of 5.9–9.4 kWh/m².

In research on integrating EC technology with solar power, the sun not only supplies power to EC but also contributes to radiant heating within the room. When the cooling demand is less than the thermal radiation, EC may not generate sufficient cooling to effectively cool the entire room. Consequently, the primary focus of this research is to explore the correlation between solar exposure and building characteristics, aiming to identify the optimal relationship between EC and CC for effective climate control.

4.2. EC Integrated with Solar Thermal Energy

4.2.1. Integration of EC with Solar Collector

The typical applications of solar thermal energy revolve around the use of solar collectors. A solar collector is a device designed to convert the radiant energy from the sun into thermal energy. Tariq et al. [105] introduced a comprehensive mathematical model for a solar collector system, which is integrated with an M-cycle air saturator, a brackish water storage tank, and a dehumidifier. The solar collector plays a crucial role as an auxiliary heating element to reduce the brackish water concentration. Notably, the condensation process in seawater desalination employs a counter-flow DPEC method to enhance heat transfer efficiency, as illustrated in Figure 11. The research findings emphasize that the HDH system achieves a maximum recovery rate of 0.6 when the relative humidity is at its lowest point. This system results in a 30% higher freshwater production rate, a 46% higher recovery ratio, and an 11% increase in overall output.

The concept of solar-desiccant EC proves to be particularly advantageous in challenging climatic conditions, offering efficient operation with minimal energy consumption. Kim et al. [106] introduced an innovative approach by proposing the integration of a liquid desiccant system into an EC-assisted 100% outdoor air system, as depicted in Figure 12. Their findings demonstrated that the integrated system, which combines IEC and DEC (I-DEC), achieved an impressive 51% reduction in cooling energy consumption compared to the conventional variable air volume system. Notably, the inclusion of a solar water heating system for desiccant solution regeneration significantly contributed to the reduction in operational energy. Elsarrag et al. [107] conducted an extensive review of key components in liquid desiccant EC systems. The authors suggested the possibility of utilizing energy from solar thermal collectors to power the regenerator. Hussain et al. [108] proposed a refrigeration system structure for a solar-assisted desiccant cooling system, as illustrated in Figure 13. This system integrates a solar-assisted heat exchanger, vapor-compression cooling line, desiccant wheel, fans, and a refrigeration unit. Combining IEC for delivering cold air to the room and DEC for cooling the desiccant wheel, it proves to be a robust alternative to conventional AC. The study demonstrated the effectiveness of IEC under various environmental conditions, achieving an indoor temperature of 23 °C with an ambient air temperature of 40 °C. Ditta et al. [109] explored a hybrid configuration involving solar thermal collectors and desiccant IEC, utilizing counter-flow operation to provide thermal comfort conditions. The findings showcased a maximum monitored CC of up to 4.6 kW, indicating the feasibility of this approach. Ghosh and Bhattacharya [110] introduced a system designed to ensure thermal comfort in large office buildings. This system integrated a solar regenerator, liquid desiccant, and a cooling unit. By employing counter-flow DPEC in the refrigeration process, the system effectively enhanced cooling efficiency.

As a simple and direct heating method, SE can enhance the cooling efficiency without affecting the system’s operation. Preisler and Brychta [111] highlighted the use of SE in DEC and energy storage through solar thermal means. The solar-driven DEC systems incorporate a sorption rotor, spray humidifier, and solar thermal storage. The cooling method involves active DEC, as illustrated in Figure 14. Solar thermal energy is used to heat the sorption rotor and the heat recovery wheel for reusability. The results indicated that the system could save 50% of primary energy at an ambient temperature of 24 °C and 60% relative humidity. However, the sorption rotor spends most of its time on heating and humidity recovery and less time on the desiccant rotor for dehumidification.

Kabeel et al. [112] introduced an innovative hybrid system that combines IEC and humidification–dehumidification (HDH) desalination, as depicted in Figure 15. The primary objective of this system was to optimize the utilization of waste heat generated by the IEC system with the assistance of solar collectors to power the HDH desalination process. This integration of SE and EC helped save overall system space. The part of the hybrid system used counter-flow IEC. Research findings revealed that the cooling load rate varied between 253.26 and 417.4 W for a 70 m³/h air flow rate during 8.00 a.m. to 6.00 p.m. The COP of this system ranged between 0.214 and 3.49 at a 70 m³/h air flow rate.

In the exploration of integrating EC with solar collector, the solar collector typically serves as an auxiliary system, offering dehumidification or heated air for an evaporative cooler to facilitate the reutilization of the entire system. However, in this research context, emphasis is placed on the significance of sealing and insulation to prevent heat loss during heat and mass transfer. Therefore, it is advisable to contemplate the incorporation of insulation cloth into the ventilation ducts.

4.2.2. Integration of EC with Solar Chimney

Maerefat and Haghighi [113] introduced the innovative concept of integrating SE and EC to create more comfortable thermal conditions within living spaces. Their system combined a solar chimney (SC) with a cooling cavity, as illustrated in Figure 16. This setup included an SC, a water storage tank, and an EC cavity. The cooling cavity utilized passive DEC, with careful attention to ventilation and cavity length to enhance its effectiveness. The study revealed that with a heat flux density of 200 W/m² in the SC, outdoor air temperature and relative humidity set at 34 °C and 30%, respectively, indoor temperature and relative humidity could reach 25.49 °C and 86.65%, creating a more comfortable indoor environment. In a similar vein, Miyazaki et al. [114] proposed a passive refrigeration unit designed to reduce the cooling load of AC in hot and humid climates. This passive system, depicted in Figure 17, incorporated an SC heated by solar radiation to induce upward airflow. The cooling air of the system was generated by the DPEC process. The results demonstrated that the room temperature could effectively be maintained around 25 °C, successfully managing a cooling load of 40–50 W/m² without raising the ceiling temperature. Implementing this system had the potential to further reduce the cooling load by approximately 10%. For a comprehensive overview of the studies mentioned above,

Table 3. Representative studies of hybrid EC and SE systems.

Study	Year	Type	Major Feature	Role of EC
[102]	2019	Active DEC	Performance of the exergoeconomic analysis of a solar PV-DEC was assessed. The effects of inlet air rate, humidity, and thickness of cooling pad on the system were analyzed.	Producing cold air for the indoors by using SE.
[103]	2023	Passive DEC	The impact of rectangular aluminum fins and DEC on the performance of PV module was analyzed.	Cooling PV module and increasing system efficiency.
[104]	2022	Passive DEC	The efficiency and sustainability of EC of PV mode were assessed. With sustainable EC 5.9 to 11.3 kWh/m2, more energy could be produced.	Improving PV module performance.
[105]	2018	Counter-flow DPEC	An innovative and novel integrated M-cycle-based air saturator was proposed. The performance of exergy and efficiency was analyzed.	Decreasing the air temperature to provide conditions for desalination.
[106]	2013	I-DEC	The integration of a liquid desiccant system into an EC-assisted 100% outdoor air system was suggested. The performance of the liquid desiccant system in I-DEC operations was evaluated.	Maintaining absorber temperature of liquid desiccant system.
[108]	2020	I-DEC	Numerical investigations of the solar-assisted hybrid desiccant EC were introduced. The solar-assisted hybrid desiccant EC unit hybrid air collectors could achieve an average COP of 0.85.	Coping with cooling load, particularly sensible load.
[109]	2022	Counter-flow IEC	The hybrid configuration of solar thermal collectors was analyzed for efficiency of solar collectors and solar fraction.	Manufacturing CC.
[110]	2021	Counter-flow DPEC	The thermal COP of the system for diurnal operation in the most humid month of a calendar year varied between 0.40 and 0.96.	Preventing overheating of conditioned space.
[111]	2012	Active DEC	The performance of the solar-driven DEC system was evaluated. The primary energy-saving potential was analyzed.	Exhaust air can dry sorption rotor and wheel while providing cooling air.
[112]	2018	Counter-flow IEC	An experimental investigation on a hybrid system of IEC, HDH, and SE was proposed. The performance of this system was evaluated.	Offering the freshwater for the HDH desalination subsystem.
[113]	2010	Passive DEC	A great performance was achieved using DEC. The effects of geometric parameters on the system performance were discussed.	Reducing ambient warm air temperature and increasing indoor humidity.
[114]	2011	Counter-flow DPEC	An SC-driven DPEC system for buildings was proposed. The effects of the channel width of the evaporative cooler were synthetically discussed.	Effectively reducing the cooling load of main AC in buildings.
[115]	2023	Counter-flow DPEC	A novel solid desiccant DPEC system integrated with SE and HDH was proposed. Performance of the system was numerically evaluated.	The air can be dried by the solar desiccant wheel and producing cold indoor air.
[116]	2021	Passive DEC	A system combining SC and EC was proposed. Increasing the thermal efficiency of SC led to improving the natural cooling.	Increasing the relative humidity in the house.
[117]	2021	Active DEC	A performance of the solar-desiccant-assisted EC system was analyzed. The system had the potential to save up to 62.9% energy cost compared to the conventional.	Regulating air quality and providing a good environment.

summarizes the key features and roles of EC within hybrid EC and SE systems.

The only problem in the integration of EC and SC is the limitation of the application. This type of combined system is usually applied in certain stand-alone buildings and needs to consider local ambient weather. Therefore, when used in the Americas or Africa, they can significantly reduce the regional energy consumption for electricity.

5. Current Research Gaps and Potential Future Research Opportunities

5.1. Current Research Gaps

The synthesis presented above delves into the domain of EC–SE systems, emphasizing their critical significance in terms of performance, sustainability, and feasibility. Energy consumption within EC primarily concerns power and cooling medium requirements. Conversely, SE offers not only substantial power generation potential but also the capacity to serve as an efficient heat transfer medium for EC applications. In various combined refrigeration systems, EC can either function as a cooling component within a larger system or operate independently. While existing hybrid technologies demonstrate commendable efficiency and rational system structures, challenges persist concerning the study of these systems and the practical applicability of EC–SE technology. Addressing these challenges opens avenues for future research, as outlined below:

(1) Configurations optimization of EC in coupled SE systems:

The efficiency of EC is influenced by variables such as flow configuration type, air velocity, channel size, and other operating parameters [118]. In the context of EC–SE systems, design schemes must strike a careful balance between efficiency and cost, representing a pivotal factor in achieving low energy consumption and high output.

(2) Adapting to challenging climates:

In regions with challenging climates like savannas or tropical rainforests, the effectiveness of solar-coupled EC may be diminished. Incorporating energy storage systems, such as solar heat storage tanks, can help mitigate this challenge. It also enhances the utilization of the EC–SE system.

(3) Repeated utilization of hybrid system:

In hybrid systems, SE, owing to its intermittency, cannot consistently deliver optimal power. Hence, consideration can be given to day and night radiation, another manifestation of SE known as night radiation cooling. This addition enhances the sustainability of the hybrid system.

(4) Optimal choice of EC in hybrid system:

Selecting the appropriate EC system is one approach to improve the efficiency of hybrid systems, while optimizing the blending process presents another challenge. Consequently, mathematical simulation experiments are necessary before undertaking the experimental study to establish a foundation for selecting a suitable EC system. This can effectively reduce time and cost in the experimentation phase.

(5) Lack of 3E impact analysis and evaluation metrics:

Numerous studies in the existing literature focus on the 3E impact analysis of EC, offering a comprehensive examination of various parameters and performance evaluations. However, there is a scarcity of 3E impact analyses for EC–SE systems. Beyond the 3E impact, evaluation metrics should also encompass the energy consumption of the entire system.

5.2. Potential Future Research Opportunities

The following section explores potential future research opportunities in the context of EC–SE systems, highlighting the need for further development and analysis in various areas:

(1) Product and facility development

The efficiency of EC is affected by various parameters, and it is worth noting that these parameters are influenced by the structure of the facility. Given the relatively short history of EC–SE systems, continued development of suitable products and production facilities is necessary.

(2) Addressing humidity and temperature variation effects

EC–SE systems are sensitive to fluctuations in ambient air humidity [119]. Subsequent research should explore the impact of these variations on the efficiency and performance of EC–SE systems and consider strategies for mitigating their effects.

(3) Periodicity research of hybrid system

In the realm of green and renewable energy sources, the periodic nature of hybrid systems holds significant importance. Although the cooling efficiency in EC combined with other systems is generally assured, subsequent maintenance can become cumbersome. Hence, the investigation of EC–SE systems should also encompass an examination of the system’s operational lifespan.

(4) 3E impact analysis

In contrast to other non-renewable energy systems [120,121], the 3E impacts of EC–SE systems have received limited attention in the existing literature. Subsequent research should prioritize a thorough analysis and understanding of these impacts to offer a comprehensive assessment of the benefits and drawbacks associated with EC–SE systems.

(5) Evaluation metrics

Various metrics are available to evaluate the performance of EC–SE systems, including temperature deviation, efficiency, sustainability, cooling performance, exergy analysis, exergoeconomic analysis, and COP. Each metric has its strengths and limitations within the context of EC–SE systems. Subsequent research should strive to establish a comprehensive evaluation framework that collectively considers these metrics, enabling an overall assessment of the effectiveness of EC–SE systems.

6. Conclusions

This paper offers a comprehensive review of existing research, findings, and future research directions in the realm of EC–SE hybrid systems. The primary objective is to explore the potential for energy conservation within hybrid EC systems, characterized by low energy consumption and high output. Furthermore, the secondary objective is to present established foundational concepts for the development of innovative hybrid EC systems. Last but not least, SE is a straightforward and universally available renewable energy source, offering limitless possibilities for research on the hybridization of EC with other renewable energy sources. EC–SE systems exhibit distinct attributes, including energy efficiency, environmental sustainability, and commendable cooling performance when compared to conventional AC systems. Historically, concerns have arisen regarding the applicability and performance of various EC variations across diverse environmental contexts. This review introduces a transformative synergy between EC and SE sources, significantly enhancing the viability and real-world applicability of EC systems. Key conclusions from this review include:

(1)

EC–SE systems encompass various types. SC systems are primarily used in buildings. Studies on PV power generation and solar drying should consider the 3E problem;

(2)

Among various EC–SE systems, the majority utilize the DEC configuration, followed closely by the IEC configuration. These configurations are primarily employed to generate cool air and maintain thermal comfort conditions;

(3)

Studies on solar–EC systems have matured, demonstrating the potential to achieve human comfort conditions in hybrid systems. Augmenting the SPV capacity of EC systems has been identified as a method to enhance both cooling performance and power output;

(4)

There are two main approaches to heat convection and solar power generation in SE. These approaches result in minor differences in the system’s output temperature. However, efficiency and exergoeconomic considerations need to be taken into account; and

(5)

While EC–SE systems may not be as efficient as traditional AC due to solar intermittency, their long-term operating life and lower economic cost make them an attractive choice.