Review Analysis for the Energy Performance of Integrated Air-Conditioning Systems

Abstract

In response to the significant increase in cooling needs for the built environment due to climate change, hybrid air conditioning units can provide energy efficient alternatives to vapor compression systems. This paper reviews the reported energy performance of integrated air conditioning systems consisting of three types of hybrid options: direct expansion (DX) combined with evaporative cooling, DX with desiccant, and evaporative cooling combined with desiccant. In addition, the reported analyses of integrating these hybrid systems with phase change materials (PCMs) and/or photovoltaic (PV) systems are considered. The evaluated analyses generally confirm that integrated air conditioning systems offer substantial energy saving potential compared to traditional vapor compression cooling units, resulting in substantial economic and environmental benefits. Specifically, hybrid systems can reduce the annual energy consumption for space cooling by 87% compared to traditional air conditioning units. This review analysis indicates that hybrid systems can have a coefficient of performance (COP) ranging from 6 to 16 compared to merely 3 to 5 for conventional systems. Additionally, liquid desiccant cooling systems have reported notable improvements in dehumidification efficiency and energy savings, with payback periods as low as three years. Future work should focus more on real-building applications and on conducting more comprehensive cost–benefit analyses, especially when integrating more than two technologies together.

1. Introduction

People today spend most of their time indoors, with over 90% of the population spending more than 20 h daily in manufactured environments including homes, workplaces, and recreational areas. Therefore, the quality of the indoor environment is crucial for the well-being and emotional state of people [1]. The building sector is responsible for a significant share of the total energy consumption, especially for countries with hot climates and high air-conditioning energy demands. For instance, in Singapore, buildings use 57% of the country’s power. More than 75% of the electricity used in Saudi Arabia is consumed by the building sector, with 50% attributed to residential buildings, 15% to commercial buildings, and 12% to governmental buildings [2]. According to the DOE (U.S. Department of Energy) [3], in the United States, heating, ventilation, and air-conditioning (HVAC) systems account for up to 40% of the energy used by residential buildings and more than 50% of that used by commercial buildings. As demand for better indoor thermal comfort grows, building energy consumption is projected to increase [4]. Therefore, there is a growing interest in the need to reduce the energy use and costs associated with HVAC systems for buildings and their impact on greenhouse gas emissions and peak demands on electric grids [5]. This paper specifically focuses on hybrid HVAC systems due to their high energy efficiency performance compared to conventional systems [6,7]. Integrated systems can overcome the limitations of conventional air-conditioning units. Three types of integrated systems are commonly considered and evaluated to enhance the energy efficiency of air-conditioning systems. The first type of integrated system involves direct expansion systems and evaporative cooling units. The second type combines direct expansion systems with desiccant technology, while the last type integrates evaporative cooling with desiccant. In addition, these systems can be combined with PCMs and energy recovery technologies. PCMs can enhance the energy performance of HVAC systems by storing and releasing thermal energy during phase transitions, reducing energy consumption and peak loads. Moreover, PCMs can improve indoor temperature stability, lower compressor runtime, and boost energy efficiency [8].

This review analysis of the existing literature indicates that there is a significant need to evaluate the performance of hybrid HVAC systems under a wide range of climatic conditions. Specifically, the energy performance of hybrid HVAC systems integrating three technologies, DX systems, evaporative cooling, and desiccant dehumidification, is the main objective of the review study as it has not been addressed in the existing literature. Furthermore, integrating hybrid HVAC systems with other energy efficiency technologies has not been considered in other reviews. A critical analysis gap also exists in cost–benefit analysis for hybrid HVAC systems, particularly those integrating desiccant-based technologies. While desiccants are highly effective in reducing humidity and can significantly enhance the energy efficiency of HVAC systems, there are insufficient studies to evaluate their implementation costs and cost benefits. Addressing these gaps is essential to promote the adoption of hybrid HVAC solutions and to better understand the best conditions for their applications. This review analysis will provide an overview of recent studies on these integrated air-conditioning systems. First, the main sources and keywords used in the literature review are briefly described. Then, the applicable energy efficiency standards for HVAC systems are outlined in detail.

2. Literature Review Approach

For hot and humid climates, excess moisture levels can significantly increase the latent cooling thermal loads for ventilated and conditioned spaces, resulting in a deterioration in the energy efficiency of air-conditioning systems [9]. To identify some of the recent reported research studies in evaluating the performance of integrated cooling systems, specific keywords listed in

Table 1. Main keywords used for the literature search specific to each integrated air-conditioning system.

Direct Expansion System with Evaporative Cooling	Direct Expansion System with Desiccant	Evaporative Cooling with Desiccant
“Direct expansion system” “Evaporative cooling”	“Direct expansion system” “Desiccant”	“Evaporative cooling” “Desiccant”
“DX system” “Evaporative cooling”	“DX system” “Desiccant”

have been considered using various academic databases including Elsevier, Springer, Scopus, and MDPI [10,11,12]. Additionally, Google Scholar [13] was particularly useful for refining search results based on publication dates. The results of the literature review search are summarized in Figure 1 for the three types of integrated air-conditioning systems.

The selected keywords for each category are listed in Table 1, and their justifications are as follows:

• “Direct expansion system” and “DX system” are considered to cover studies on conventional refrigerant-based cooling systems.
• “Evaporative cooling” is combined with “DX system” and “direct expansion system” to identify research addressing hybrid cooling approaches aimed at enhancing energy efficiency of HVAC systems.
• “Desiccant” is paired with both DX and evaporative cooling technologies to identify studies that have evaluated dehumidification technologies used in conjunction with conventional and evaporative cooling strategies.

The retrieved studies have been screened for relevance based on their abstracts, and this has been followed by a full-text review of the screened papers. The key criteria for inclusion of the fully reviewed publications include:

• Experimental or simulation-based evaluation of integrated cooling systems.
• Performance assessment metrics for evaluating energy efficiency, thermal comfort, and humidity control of the hybrid systems.
• Applications of hybrid systems in hot and humid climatic conditions.

The number of research articles published on each system type over the last ten years is presented in Figure 1, which highlights the increasing interest in integrated HVAC technologies.

3. Energy Efficiency Standards

ASHRAE standards 90.1 and 90.2 [14,15], established and regularly updated by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), set the minimum energy efficiency ratings for heating and cooling systems in the US for, respectively, commercial and residential buildings. On the other hand, the Saudi Standards, Metrology, and Quality Organization (SASO) has developed SASO-2663 [16] for the same purpose and this is primarily applied to HVAC systems specified and installed in Saudi Arabia. Indeed, the Energy Efficiency Ratio (EER) and Seasonal Energy Efficiency Ratio (SEER) are used in the ASHRAE 90.1, ASHRAE 90.2, and SASO-2663 standards to indicate how efficiently air conditioners use energy. For heat pumps, ASHRAE 90.1 also uses the Heating Seasonal Performance Factor (HSPF) when the pump is operated in heating mode. However, the two standards cover various air-conditioning and heat pump systems. For example, ASHRAE 90.1 includes specific standards for single-package units, split systems, small-duct high-velocity systems, and space-constrained units. In contrast, SASO-2663 includes single-package window types (Category A and Category B), split type ducted and non-ducted air conditioners using air-cooled condensers, and heat pumps using air-cooled condensers.

Moreover, these were evaluated using the Air Conditioning, Heating, & Refrigeration Institute (AHRI) 210/240-2017 [17] testing procedure before 1/1/2023 and the updated AHRI 210/240-2023 procedure after that.

Table 2. ASHRAE 90.1 [14] Energy Efficiency Requirements for Air-Conditioning Systems in the US.

Product Class	Capacity Range	National Standards	Southeastern Region Standards	Southwestern Region Standards
Split-system air conditioners	<45,000 Btu/h single phase	SEER1 = 13.0 (COP1 = 3.8)	SEER1 = 14.0 (COP1 = 4.1)
		SEER2 = 13.4 (COP2 = 3.9)	SEER2 = 14.3 (COP2 = 4.2)
		PwOFF ≤ 30 W	PwOFF ≤ 30 W
Split-system air conditioners	>45,000 Btu/h and <65,000 Btu/h single phase	SEER1 = 13.0 (COP1 = 3.8)	SEER1 = 14.0 (COP1 = 4.1)
		SEER2 = 13.4 (COP2 = 3.9)	SEER2 = 13.8 (COP2 = 4.0)
		PwOFF ≤ 30 W	PwOFF ≤ 30 W
Split-system heat pumps	<65,000 Btu/h single phase	SEER1 = 14.0 (COP1 = 4.1) for cooling mode
		HSPF1 = 8.2 (COP1= 2.4) for heating mode
		SEER2 = 14.3 (COP2 = 4.2) for cooling mode
		HSPF2 = 7.5 (COP2 = 2.2) for heating mode
		PwOFF ≤ 33 W
Single-package air conditioners	<65,000 Btu/h single phase	SEER1 = 14.0 (COP1 = 4.1)
		SEER2 = 13.4 (COP = 3.9)
		PwOFF ≤ 30 W
Single-package heat pumps	<65,000 Btu/h single phase	SEER1 = 14.0 (COP1 = 4.1) for cooling mode
		HSPF1 = 8.0 (COP1 = 2.3) for heating mode
		SEER2 = 13.4 (COP2 = 3.9) for cooling mode
		HSPF2 = 6.7 (COP2 = 2.0) for heating mode
		PwOFF ≤ 33 W
Small-duct high-velocity systems	<65,000 Btu/h single phase	SEER1 = 12.0 (COP1 = 3.5) for cooling mode
		HSPF2 = 7.2 (COP1 = 2.1) for heating mode
		SEER2 = 12.0 (COP2 = 3.5) for cooling mode
		HSPF2 = 6.1 (COP2 = 1.8) for heating mode
		PwOFF ≤ 30 W
Space-constrained air conditioners	<65,000 Btu/h single phase	SEER1 = 12.0 (COP1 = 3.5)
		SEER2 = 11.7 (COP2 = 3.4)
		PwOFF ≤ 30 W
Space-constrained heat pumps	<65,000 Btu/h single phase	SEER1 = 12.0 (COP1 = 3.5) for cooling mode
		HSPF1 = 7.4 (COP1 = 2.2) for heating mode
		SEER2 = 11.9 (COP2 = 3.5) for cooling mode
		HSPF2 = 6.3 (COP2 = 1.9) for heating mode
		PwOFF ≤ 33 W

illustrates the energy efficiency requirements for different air conditioners in the southeastern and southwestern US regions, which are characterized by hot and humid climates, before and after 1 January 2023. The energy efficiency indicators for the air-conditioning systems are estimated before and after 1 January 2023, using, respectively, AHRI 210/240-2017 and AHRI 210/240-2023 protocols. As noted in Table 2, the requirements are slightly different before (referred to as “1”) and after (referred by “2”) 1 January 2023. For instance, “SEER1” stands for the Seasonal Energy Efficiency Ratio applicable before 2023, while “SEER2” corresponds to the standard effective from 2023 onwards. Thus, the SEER rating for split-system air conditioners with capacities of less than 45,000 Btu/h increased from 13.0 to 13.4 nationally and from 14.0 to 14.3 in both the southeastern and southwestern regions, before and after 1 January 2023, respectively. Larger split-system air conditioners, with capacities above 45,000 Btu/h but below 65,000 Btu/h, experienced a slight decrease in SEER ratings in both the southeast and southwest after 2023.

As an example, in Saudi Arabia, the minimum energy performance standards (MEPS) set out in SASO-2663 mandate that air conditioners have, at least, specified thresholds for EER values based on testing protocols under two conditions T1 and T3, as depicted in

Table 3. SASO-2663 [16] Energy Efficiency Requirements for Air Conditioners in Saudi Arabia.

Cooling Testing Conditions			Temperature T1				Temperature T3
			Indoor Section		Outdoor Section		Indoor Section		Outdoor Section
			Dry-Bulb (°C)	Wet-Bulb (°C)	Dry-Bulb (°C)	Wet-Bulb (°C)	Dry-Bulb (°C)	Wet-Bulb (°C)	Dry-Bulb (°C)	Wet-Bulb (°C)
			27	19	35	24	29	19	46	24
	Rated Cooling Capacity (CC) categories at test condition (T1) in Btu/h [W]		EER Values (Btu/h/W) [COP]
HVAC type	Single package of Window type—category A	CC ≤ 24,000 [7020]	9.8 [2.9]				7.0 [2.1]
	Single package of Window type—category B	24,000 [7020] < CC ≤ 65,000 [19,050]	9.0 [2.6]				6.2 [1.8]
	Split type ducted and non-ducted using air-cooled condensers, heat pumps using air-cooled condensers	CC ≤ 65,000 [19,050]	11.8 [3.5]				8.3 [2.4]

. For T1 conditions, the interior temperatures are set to 27.0 °C for dry-bulb and 19.0 °C for wet-bulb, and the outdoor temperatures are 35.0 °C and 24.0 °C for dry-bulb and wet-bulb temperatures, respectively. T3 has an interior temperature of 29.0 °C dry-bulb and a wet-bulb temperature of 19.0 °C and an outdoor temperature of 46.0 °C dry-bulb and a wet-bulb temperature of 24.0 °C. Air conditioners are classified into three categories: category A for single-package window types with cooling capacities up to 24,000 Btu/h (7020 W) requiring an EER of 9.80 (T1) and 7.00 (T3); category B for single-package window types with cooling capacities between 24,000 Btu/h (7020 W) and 65,000 Btu/h (19,050 W) requiring an EER of 9.00 (T1) and 6.20 (T3); and split type ducted and non-ducted air conditioners, as well as heat pumps using air-cooled condensers with cooling capacities up to 65,000 Btu/h (19,050 W), requiring an EER of 11.80 (T1) and 8.30 (T3).

The high energy consumption associated with the increasing use of HVAC systems has driven several countries worldwide to implement more stringent energy efficiency standards and regulations for heating and cooling equipment [18]. For example, Singapore has implemented regulations for air conditioners, setting a minimum COP of 3.7 for casement and window units and 4.04 for split systems, while, in Brazil, the minimum COP values are set at 2.84 for window systems and 3.02 for split units [19,20].

Moreover, several programs and initiatives have been established to deploy high energy efficiency HVAC systems worldwide. For instance, the initiative referred as the Mitigation Enabling Energy Transition in the Mediterranean area (meetMED) is supporting the energy transition in Algeria, Egypt, Jordan, Lebanon, Libya, Morocco, Palestine, and Tunisia [21]. The meetMED initiative highlights that the COP and the lifespan of air-conditioning systems are the most important factors that affect the energy performance of HVAC systems. Indeed, it is estimated that the average AC age is 10 years, and its typical rated COP is 3.0, underscoring the need for stricter regulations to eliminate the deployment of low efficiency AC systems [22]. Although these regulations are essential to improve the energy efficiency and sustainability of buildings, they may result in additional construction costs. For instance, several demonstration projects in Morocco have indicated that construction costs increase by 5–7% relative to conventional building practices due to the deployment of energy efficiency measures and renewable energy systems [23].

A recent study has found that, among the world’s highest carbon-emitting countries, China is the only country currently meeting the cooling seasonal performance factor (CSPF) targeted by U4E. This achievement is credited to China’s new policy to mitigate 470 Mt of CO₂ by 2030. Figure 2 compares the CSPF requirements for split systems with a capacity of 24,000 BTU/h (7 kW) for countries with high carbon emissions [24,25].

However, a study conducted by the European Commission estimated that the average cooling efficiency increased by 46% due to the enforcement of various eco-design and labeling initiatives. It is estimated that the enforcement efforts resulted in energy cost savings of €4.7 billion. The heating load reductions achieved through ventilation units accounted for 7% of these cost savings [26]. Several countries with various climatic characteristics have implemented strict minimum energy performance standards (MEPS) for HVAC systems [27,28,29,30,31,32]. A notable example of MEPS is the Saudi Arabian labeling program, governed by SASO 2663:2021, which provides a detailed framework for the SEER classification, a component of the energy efficiency label, and which is shown visually by color-coded bars. As the SEER calculation tool computed, the classification describes a specific model’s stated seasonal energy efficiency under rated conditions. In addition, depending on the operational stage (fixed, two-stage, multi-stage, or variable), the air conditioner’s performance is assessed under different temperature and humidity conditions, including whole, half, and minimum cooling capacities and power inputs at specific indoor and outdoor temperatures. The SEER values are the minimum required for each class.

Table 4. SEER Classification According to SASO 2663:2021 [16] Standard: Energy Class, Color Codes, and SEER Limits.

Bar Color	Energy Class	SEER Limits (Btu/W.h) [COP LiMITS]
Dark green	A	SEER ≥ 18.0 [COP ≥ 5.3]
Green	B	18.0 > SEER ≥ 15.0 [5. 3 > COP ≥ 4.4]
Light green	C	15.0 > SEER ≥ 12.5 [4.4 > COP ≥ 3.7]
Yellow	D	12.5 > SEER ≥ 10.0 [3.7 > COP ≥ 2.9]
Orange	E	10.0 > SEER ≥ 9.0 [2.9 > COP ≥ 2.6]
Red	F	9.0 > SEER ≥ 8.0 [2.6 > SEER ≥ 2.3]
Dark Red	G	8.0 > SEER [2.3 > COP]

, below, shows the SEER classification; every color and Arabic letter represents a different energy class.

4. Overview of Integrated Air-Conditioning Systems

To enhance the energy efficiency of air-conditioning systems, there is an increasing trend to combine several cooling technologies. In this review, reported studies for integrated systems involving two cooling systems are first discussed. Then, published analyses for heating and cooling systems integrating more than two technologies, including solar energy generation and thermal energy storage, are outlined.

4.1. Integration of Two Cooling Systems

4.1.1. Direct Expansion System with Evaporative Cooling

Direct expansion cooling units (DXs) can be coupled with evaporative cooling to enhance both the capacity and the energy efficiency of integrated air-conditioning systems. Several reported studies have indicated that this integrated system can achieve promising performance with significant energy efficiency, economic, and environmental benefits [33,34,35,36].

Table 5. Reported studies conducted on the combination of DX cooling with evaporative cooling.

No.	REF	Year	System	Methodology	Finding
1	[37]	2021	A hybrid indirect evaporative cooling-mechanical vapor compression (IEC–MVC)	Experimental and analytical	Tests showed that a cross-flow IEC could reduce the outdoor air temperature by 6–15 °C and humidity by 0.5–4 g/kg. Enthalpy recovery was 27–36% and increased with airflow, higher humidity, and lower temperatures. The IEC had a COP of 6–16 and could handle 34–77% of the cooling load. The overall COP was increased to 4.96–6.05 by the hybrid IEC–MVC system, exceeding standalone MVC by 19–135%.
2	[38]	2019	Evaporative cooling and condensation dehumidification	Experimental	The system’s COP increased by 8.5% due to the addition of a single-stage total heat recovery module. A single-stage sensible heat recovery module was added to improve the system further, increasing the COP by 3.3% and enabling direct room supply air without reheating. The system’s efficiency increased with higher humidity and fresh air temperature due to increased total heat recovery.
3	[39]	2022	Direct expansion system with indirect evaporative cooler	Experimental and analytical	As ambient temperature, humidity, and SHR increase, the COP decreases. The COP and indoor air quality may increase using the IEC ventilator in suitable outdoor conditions. When the ambient temperature and humidity are lower, a higher fresh air flow ratio should be used to achieve a larger COP.
4	[40]	2020	Direct expansion system with direct and indirect evaporative cooler	Simulation	Although hybrid systems use more power, they perform well in all climatic zones. The IEC–DEC–DX system balances wet-bulb effectiveness, thermal comfort, and cooling capacity. However, it consumes more water and power, has a lower COP, and has greater lifespan costs.
5	[41]	2019	Direct expansion system with evaporative cooler	Experimental	With the ‘triple-effect’ mode, significant energy savings were achieved, with a reduction of 60.4% when compared to DX cooling for ambient temperatures below 40 °C. However, for ambient temperatures greater than 40 °C, the energy savings decreased to 40.5%. The COP of the FAHU was doubled for ambient temperatures under 40 °C when using IEC combined with the DX cooling system. The ’triple-effect’ DX mode outperformed the single-effect DX mode for ambient temperatures over 40 °C, achieving 2.16 times better COP.
6	[42]	2019	Hybrid dew point evaporative cooler and vapor compression refrigerated	Simulation	Compared to the conventional DX and IEC/DX systems, the REC/DX hybrid system significantly improved energy savings, saving 38.2% more energy. The system also reduced GHG emissions by 28.5% compared to the DX and 4.3% compared to the IEC/DX systems.
7	[43]	2022	Gravity-assisted heat pipe (GAHP)-based indirect evaporative cooler and direct expansion	Mathematical model	When compared to a conventional hybrid HVAC system, the proposed system showed a 39.2% increase in the COP, along with reductions of 45% and 51.7% in energy consumption and total operating cost, respectively.
8	[44]	2024	Direct expansion photovoltaic-thermal evaporator assisted compression heat pump water heaters using a zeotropic mixture	Experimental	The study experimentally compared a direct expansion photovoltaic-thermal evaporator assisted a compression heat pump water heater using R134a and R290/R600a. The R290/R600a mixture showed better miscibility, system compatibility, lower charge requirement, improved panel cooling, lower pressure ratio, enhanced compressor efficiency, reduced power consumption, and 3.5–7.9% higher COP than R134a.
9	[45]	2024	Semi-Indirect Evaporative Cooler	Experimental	Explores the use of a ceramic Semi-Indirect Evaporative Cooler in a Decarbonised Evaporative-Based Air-Conditioning System to enhance HVAC energy efficiency. Experimental tests showed that the system, operating in both dry and wet modes, achieved 50–70% efficiency in dry mode and nearly 100% in wet mode, with a 30–50% efficiency boost.

summarizes the most recently reported analyses that evaluated the performance of a wide range of DX–evaporative cooling systems, with one study showing an integrated system COP exceeding 135% compared to a standalone DX system [37].

A study conducted by Chen et al. [37] indicated that the integration of the two systems can overcome some of their individual limitations. The authors investigated the performance of a hybrid process combining indirect evaporative cooling (IEC) with mechanical vapor compression (MVC) in avoiding some of the shortcomings of IEC including limited control over temperature and humidity levels and reduced cooling capacity during high humidity conditions. The hybrid process involves pre-cooling outdoor air through an IEC unit and then processing it to the desired conditions using MVC, as shown in Figure 3 and Figure 4. Combining the high energy efficiency levels of IEC with the temperature and humidity control capabilities of MVC results in a more effective and energy-efficient cooling process. The study tested the specific performance of a crossflow IEC unit using room air through wet channels using a prototype of a hybrid IEC–MVC unit. The laboratory experimental analysis showed that IEC could reduce the outdoor air dry-bulb temperature and humidity ratio by 6–15 °C and 0.5–4 g/kg, respectively. The effectiveness of enthalpy recovery was estimated to be in the ranges 27–36%, which was improved by lower outdoor temperatures, higher humidity ratios, and higher air flow rates. The energy efficiency of IEC, expressed by its COP, was in the range of 6–16, and the percentage of cooling load IEC could handle ranged from 34 to 77%, with COP obtained for hot and dry outdoor air conditions. The testing data showed that the hybrid IEC–MVC unit has an overall COP in the range of 4.96–6.05, which is 19–135% higher than that of the standalone MVC system. The hybrid unit shows excellent potential as a sustainable and energy-efficient alternative to existing mechanical vapor compression systems commonly used for the air conditioning of buildings.

To highlight the potential benefits of the hybrid units to improve indoor air quality and energy efficiency, a study by Yan et al. [39] evaluated the use of IEC as specialized ventilators when combined with traditional direct expansion air-conditioning systems. The analysis of the hybrid systems is carried out using the validated numerical models of IEC and DXAC used to maintain desired indoor conditions for a thermal zone under various ambient conditions. The modeling results showed that the IEC ventilator unit could provide some cooling load under moderately hot and dry conditions but may introduce additional moisture under humid conditions, increasing indoor humidity if the DXAC’s dehumidification capacity is exceeded. The COP of the DXAC–IEC hybrid system can be decreased when several factors increase, such as ambient temperature, humidity level, and sensible heat ratio (SHR). Compared to the standalone DXAC, the DXAC–IEC hybrid system has a higher COP, and better indoor air quality when the outdoor relative humidity is 60% or less and the dry-bulb outdoor air temperature is below 33 °C. Moreover, it was found that the DXAC–IEC system can effectively handle a wide range of SHR and achieve maximum COP using an optimal fresh air ratio of 49%, which is influenced by ambient conditions.

Al Horr et al. [41] also investigated the performance of a hybrid fresh-air-handling unit (FAHU) that combines vapor-compression cooling with IEC to reduce cooling demand in hot and humid climates. The reported analysis compared the performance of the hybrid system in different operation modes, including vapor compression only, direct expansion with mist, water shower, and mist and water shower, using Qatar’s harsh climatic conditions. The analysis results showed that the performance of the FAHU varied with outdoor air relative humidity and temperature. However, the best-performing operation mode of the FAHU was the “triple-effect” DX with mist and water shower, which achieved a significant reduction of 60.4% in energy consumption compared to cooling using DX only when outdoor air dry-bulb temperatures are below 40 °C. The study also discovered that when outdoor air dry-bulb temperatures exceed 40 °C, energy savings were reduced to 40.5%. Furthermore, the analysis indicated that IEC combined with DX increased the FAHU’s COP at outdoor air dry-bulb temperatures below 40 °C. For outdoor air dry-bulb temperatures above 40 °C, the “triple-effect” DX operation mode achieved a COP that is 2.16 times higher than that obtained using the single-effect DX operation mode. The analysis findings imply that the hybrid FAHU with IEC can save significant energy while maintaining acceptable indoor thermal comfort even for extremely hot and humid climates.

Furthermore, Zhang et al. [38] investigated a hybrid fresh-air-handling system that combines mechanical compression refrigeration with evaporative cooling. They found that higher humidity levels and fresh air temperatures improve the hybrid system’s energy efficiency by increasing the proportion of total heat recovery. Indeed, the basic system had high exergy destruction due to the heat and mass transfer process far from the saturation line. However, adding a single-stage total heat recovery module increases the COP by 8.5%. Moreover, a 3.3% increase in the COP and the ability to supply air directly into the room without reheating can be achieved by adding a single-stage sensible heat recovery module.

Using a cooling design day suitable for Poland, Rajski al. evaluated the performance of a novel gravity-assisted heat pipe (GAHP) for an IEC and DX hybrid system [43]. The results were compared to a conventional hybrid system, represented by a rotary heat exchanger and a DX cooling coil, and showed a 45% decrease in energy consumption. The authors highlight concerns about the number of components for the suggested hybrid system, noting that it may lead to reduced reliability and a potentially high initial cost. Nevertheless, the use of heat pipes as alternatives to traditional plate-type evaporative coolers offers significant advantages, including longer component lifespans and lower maintenance costs [43].

The potential energy savings that can be obtained in cooling residential buildings using hybrid evaporative–DX systems are assessed by several studies. For instance, Krarti et al. [46] have estimated the potential energy savings associated with retrofitting air-conditioning units in the entirety of existing Saudi residential buildings using the hybrid IEC/DX systems of Chen et al. [37]. The analysis was conducted using a validated bottom-up building-stock model to account for various Saudi climates and building types [47]. The analysis indicates that IEC/DX retrofits can be implemented in less than 1 year for most households and can reduce national electricity consumption by 51 TWh, resulting in a reduction of 38 million tons in carbon emissions [46]. Similar results are obtained when implementing IEC/DX systems for other climates [48] and other building types [49]. Moreover, Duan et al. [42] integrated dew point regenerative evaporative cooling (REC) with conventional DX cooling for a three-story residential building in Beijing, China. An hourly analysis of the hybrid system’s performance was conducted using a validated EnergyPlus-based model, considering four performance indicators, including energy consumption, thermal comfort levels, life-cycle costs, and greenhouse gas (GHG) emissions. The analysis indicated that the REC/DX system could save 38.2% in annual energy use compared to a conventional DX system and 8.5% relative to that achieved by an IEC/DX hybrid system. In addition, the REC/DX system can enhance environmental sustainability by reducing GHG emissions by 28.5% and 4.3% compared to DX and IEC/DX systems, respectively.

4.1.2. Direct Expansion System with Desiccant

Direct expansion coupled with desiccant units represents other categories of integrated HVAC systems. The integration of desiccant-assisted dehumidification with other HVAC systems, such as DX systems, VRC systems, and chilled water cooling, aims to minimize the equipment size and improve its COP, as discussed in numerous studies [50,51,52].

Table 6. Reported studies conducted on the combination of DX cooling with desiccant.

No.	REF	Year	System	Methodology	Finding
10	[53]	2021	A direct expansion unit and liquid desiccant	Simulation	Increases in building load and fresh air humidity led to an improvement in the LDAHU system’s performance. The LDAHU system had a seasonal system COP that was significantly higher than that of the ECB and ASHP systems, resulting in lower annual operating costs and a shorter payback period. Comparing the LDAHU system to the ECB and ASHP systems, the overall cost of operation over a 15-year lifecycle was much lower with the LDAHU system.
11	[54]	2018	Desiccant with DX	Simulation	The COP decreases when the regeneration air temperatures increase because of the rise in process air outlet temperatures and higher cooling demands on the enthalpy wheel and vapor compression cooling unit. Higher outdoor temperatures also reduce the efficiency of the MRR and dehumidifier, which increases the latent cooling load on the sensible cooling coil and lowers the COP.
12	[55]	2016	Liquid desiccant–vapor compression hybrid	Experimental	The performance enhancement of the conventional LDCH system is limited due to the experience of cooling capacity loss (over 10%) due to temperature and concentration differences in the system. The cooling capacity loss of the evaporator is reduced to approximately 1.5% by utilizing a novel LDCH system that uses an auxiliary regenerator. A larger concentration difference in the liquid desiccant enhances the cooling capacity of the evaporator and the dehumidification effect.
13	[56]	2011	Conventional liquid desiccant system and a vapour compression heat pump	Experiments and simulations	Increasing the temperature efficiency of the solution heat exchanger and the compressor’s isentropic efficiency can further improve the system’s COP. The system overall COPsys and the heat pump alone COPhp were 2.7 and 3.8, respectively
14	[57]	2018	Liquid desiccant heat pump (LD–HP)	Simulation	The improved LD–HP system used 33.2% less power over seven months than a conventional heat pump system, which decreased by 33.2% in annual CO2 emissions. The COP of the system was about 4.83 at the optimum point, which is much higher than the 2.74 COP of a conventional heat pump system with an electric heater. For the additional hardware required for the LD-HP system, a payback period of 3.04 years was predicted.
15	[58]	2020	Heat pump-driven liquid desiccant dehumidification system	Simulation	Modifications that created an improved cross-flow system decreased the uniformity coefficients, raising the system’s COPsys from 5.7 to 6 and its exergy efficiency from 20.1% to 21%. The system’s exergy efficiency increased to 25% and its COPsys to 7.4 due to further adjustments to lower the uniformity coefficients that further reduced exergy destruction.
16	[59]	2017	VAC system thin-multilayer activated alumina bed.	Experimental	The conventional vapor compression system with a thin-multilayer activated alumina bed was found to consume less power and operate more efficiently than the conventional system, with an average COP that is 7.52% higher. The system’s COP increased by 12.4% in extremely hot, humid environments, while the degree of superheating dropped by 14.5%. Longer desiccant cycles enhanced the COP and required less power from the compressor, while shorter cycles resulted in a more compact system.
17	[60]	2018	A heat pump-driven, internally cooled liquid desiccant dehumidification system	Simulation	The basic system’s cooling and heating capacities varied based on the properties of each fluid in the dehumidifier, limiting its overall COP, which was 5.96. An improved design decreased the mismatching ratios and increased COP by 12% to 6.68.
18	[61]	2022	Heat pump-driven liquid desiccant dehumidification system integrated with fresh air supply	Simulation	The use of residual heat in the regenerator for air heating, compared to processes without air heating, led to a decrease in condenser temperature and an increase in the COP of the air process by 3.19–10.57% The COP of the air process might be increased by 6.8% and 5.0% with the optimal air volume selection. The supply air’s absolute humidity greatly influences the system’s COP in the summer. In winter, the system’s energy-saving ratio could increase by 25.9% to 30.4%, directly proportionate to the work done by the compressor.
19	[62]	2018	Desiccant coated heat exchanger	Simulation	Exchangers coated with aluminum fumarate have an 11% greater regeneration capacity and an 8% greater dehumidification capacity compared to silica gel. However, silica gel-coated exchangers remove 17% more adsorbate in water-cooled conditions and 66.6% more in adiabatic conditions compared to aluminum fumarate-coated exchangers.
20	[63]	2024	Desiccant-coated heat exchanger	Experimental	The study indicates that deep dehumidification using desiccant-coated heat exchangers requires longer processing time due to slower adsorption kinetics. Lower cooling water temperature enhances performance in both normal and deep dehumidification modes, but the deep demands more cooling power. Higher inlet humidity improves dehumidification in deep mode, while air velocity has differing effects, boosting moisture removal and COP in the deep mode but having an optimal limit in the normal mode. The system achieves a dew point of −23.44 °C while maintaining a 23.11 °C dry bulb temperature, surpassing conventional dehumidification methods.
21	[64]	2024	Hybrid desiccant-dehumidification-absorption system, driven by external compound parabolic concentrators	Simulation	The hybrid desiccant-dehumidification-absorption system is found to achieve superior thermal comfort lowering dissatisfaction of people by 5.6%, a 49% average solar fraction, and a seasonal COP ranging from 0.46 to 1.38. While life cycle costs rise by $6824, each $1000 investment cuts 4619 kg of CO2.

outlines recently reported analyses that have specifically investigated the integration of DX systems with desiccant units.

In 2021, a study by Liang et al. [53] proposed a new air handling unit system, represented in Figure 5 and Figure 6, which is referred to as a direct expansion air handling unit assisted by liquid desiccant (LDAHU), and which uses liquid desiccant to assist air conditioning through direct expansion. A numerical model has been developed to analyze the hybrid system’s performance under different thermal load demands and weather conditions for an office building in Nanjing, China. The proposed system efficiently achieved the required air conditions during summer and winter, providing better thermal comfort than traditional HVAC systems. Additionally, it was found that the LDAHU system provides better thermal comfort performance than an air source heat pump (ASHP) system. The LDAHU’s seasonal COP was estimated to be significantly greater than that of an electric chiller and boiler (ECB) and of ASHP systems, resulting in lower annual operating costs and dynamic payback periods. Overall, the LDAHU significantly reduced the total HVAC operation costs during a 15-year lifecycle compared to both the ECB and ASHP systems. The study claims that the system’s performance improves as the building thermal loads and the humidity levels of the outdoor air increase.

Another study by Yinglin et al. [55] evaluates a conventional Liquid Desiccant–Vapor-Compression Hybrid (LDCH) air-conditioning system. In particular, the analysis involves a mathematical model for the LDCH to assess the effects of the concentrated solution branch within the solution-saturated heat exchanger (SSHE) on the evaporator’s cooling capacity. The LDCH air-conditioning system involves an auxiliary regenerator, which significantly reduces the temperature of the concentrated solution branch. With a concentration difference of 2.65%, the inlet temperature of the concentrated solution branch from the regeneration side of the SSHE decreases by over 6 °C. Consequently, the extra heat load entering the dehumidification process from the regeneration side is reduced, and the cooling capacity loss percentage drops from 8.2% to around 1.5%.

In a reported study by Jani et al. [54], the energy performance of a desiccant-assisted hybrid air-conditioning system is evaluated using a 49 m³ room. The considered room has sensible and latent cooling loads of 1.37 kW and 0.39 kW, respectively. The sensible heat ratio was 0.78, with airflow rates of 322.7 m³/h (process) and 196.8 m³/h (reactivation), and standard conditions at 55% RH and 25 °C dry-bulb temperature. Based on TRNSYS, the analysis has indicated that solid desiccant-assisted hybrid space cooling systems are energy efficient for hot, humid climates with the added benefits of significantly reducing humidity levels and enhancing indoor air quality. However, it is found that the hybrid system’s COP decreases with increasing outdoor and regeneration air temperatures due to increased process air outlet temperatures and higher cooling demands on the enthalpy wheel and vapor compression cooling unit. Higher ambient temperatures also lower the moisture removal rate (MRR) and dehumidifier effectiveness, resulting in a more significant latent cooling load and lower COP. Integrating solar energy or waste heat sources for rotary dehumidifier regeneration heat can enhance the performance of the solid desiccant-assisted hybrid space cooling system.

In 2011, Yamaguchi et al. [56] conducted a study to increase space cooling energy efficiency. They proposed and evaluated a hybrid liquid desiccant air-conditioning system made up of a conventional liquid desiccant system and a vapor compression heat pump. An aqueous lithium chloride solution is the liquid desiccant, and R407C is the heat pump’s refrigerant. The system cools air from 30 to 22.2 °C and dehumidifies it from 14 to 8.1 g/kg (DA). The reported COP values are 2.7 for the hybrid system and 3.8 for the heat pump. The study also finds out that the hybrid system’s COP can be improved by increasing the temperature efficiency of the solution heat exchanger and the compressor’s isentropic efficiency.

An analysis reported by Fatouh et al. [59] presents the experimental testing results for the performance of a hybrid air-conditioning system that combines a conventional vapor compression system with a thin-multilayer activated alumina bed. It is found that the hybrid system consumes less power, operates more efficiently, and provides a 7.52% higher average COP than the conventional system. In extremely hot and humid climates, the hybrid system exhibits a 12.4% increase in COP and a 14.5% decrease in superheating degrees. Additionally, the study indicates that the efficiency of the hybrid system is affected by the length of desiccant cycles. Indeed, shorter cycles result in a more compact system but a lower COP, while longer cycles improve the COP and use less compressor power.

Considered an energy-saving solution for dehumidification, Liu et al. [60] investigated a heat pump-driven internally cooled desiccant dehumidification system. Based on a numerical analysis, the study identified key issues and proposed performance improvement strategies for the hybrid system. Specifically, the basic design for the hybrid system has a COP of 5.96. The authors recommended lower solution flow rates and more efficient solution–solution heat exchangers to improve the integrated system performance. An improved design has been proposed to include an external pre-cooler and pre-heater to allow fluid matching. As a result, the mismatching ratios decreased, and the COP increased by 12% to 6.68 for the improved design hybrid system.

The exergy destruction associated with a hybrid air-conditioning unit using a liquid desiccant dehumidification system has been estimated by Zhang et al. [58]. Exergy is a concept that describes the portion of energy that can be infinitely converted into other forms of energy as a system reversibly reaches equilibrium with a predefined dead state. The analysis indicates that the exergy efficiency of the system can reach 21%, and its COP can be boosted to 7.4.

For optimizing, Sanaye and Taheri [57] presented an optimization-based analysis of a hybrid liquid desiccant–heat pump (LD–HP) system designed for cooling buildings located in hot and humid climates. The analysis was performed by assessing the energy, exergy, economic, and environmental impacts of the system. A multi-objective Genetic Algorithm (GA) method has been considered for the optimization analysis. The results indicate that the optimized LD–HP system can consume 33.2% less electricity than a conventional heat pump system. Along with this energy use reduction, annual CO₂ emissions decreased by 33.2%. The hybrid system’s COP at the optimum point is estimated to be 4.83, significantly higher than the 2.74 estimated for the COP of the conventional heat pump with an electrical heater. In addition, the payback period for the hybrid system has been determined to be 3.04 years.

Li et al. [61] proposed a new heat pump-driven liquid desiccant dehumidification system to improve energy utilization and efficiency for buildings with high moisture loads such as those prevalent in natatoriums. The energy efficiency of the hybrid system was investigated for eight different Chinese cities. It is found that lower ambient relative humidity levels led to better energy-saving ratios compared to supply air temperatures and the proportion of ambient air over the summer. During winter periods, it was found that the system’s energy-saving ratio could be increased by 25.9–30.4%. The evaluated system uses a two-stage dehumidification procedure involving a liquid desiccant dehumidifier and a surface condenser. The analysis results show that the use of residual heat in the regenerator reduces the condenser temperature and increases the COP by 3.19–10.57% compared to the case without air heating. The optimal air volume selection could further enhance the COP by up to 6.8%.

Using a year-round air-change test, Liu et al. [63] have evaluated the performance of both normal and deep dehumidification for desiccant-coated heat exchangers (DCHEs). The results show that deep dehumidification achieves lower inlet air humidity but consumes more energy than normal dehumidification. These kinds of systems have the potential to perform exceptionally well for specific applications that require a dry indoor environment such as soybean seed storage, lithium battery cathode material coating, and pharmaceutical processes. However, the performance of a desiccant cooling system largely depends on how effectively it is controlled, managed, and strategically operated [65]. Mohammad et al. [66] have classified control strategies into two categories: control performance and control optimization. Control performance focuses on methods and devices that enhance system performance, while control optimization involves identifying and optimizing variables to achieve the best possible outcomes. Other studies have also explored improved control strategies for desiccant systems [67,68]. For instance, Demir et al. [69] developed a proportional-based multi-mode control strategy that effectively maintained indoor thermal comfort while achieving significant energy savings. This approach resulted in total energy savings of over 40%, showing the importance of controllers in such systems.

4.1.3. Evaporative Cooling System with Desiccant

Several reported studies have documented the energy-saving potential of the hybrid IEC and desiccant system, with energy consumption reductions of more than 10% compared to conventional mechanical chillers [70]. However, due to the low COP of most dehumidifiers, they can be costly to operate [71] as found by the studies summarized in

Table 7. Reported studies conducted on the combination of evaporative cooling system with desiccant.

No.	REF	Year	System	Methodology	Finding
22	[4]	2017	A liquid desiccant dehumidifier and regenerative indirect evaporative cooling	Simulation-based	Inlet air temperature primarily affects the sensible cooling capacity, while air humidity affects both sensible and latent cooling capacities. The energy saving ratio and COP range from 22.4% to 53.2% and 4.3 to 7.1 under various inlet air conditions. The system’s energy efficiency is higher when FCU operates in dry-coil conditions.
23	[72]	2017	Liquid desiccant evaporative cooling system	Simulation	Increasing the hot water temperature, mass flow of hot water, and ambient air relative humidity improves system efficiency, but to varying degrees. However, cooling capacity exergy and exergy efficiency decrease with an increase in ambient air relative humidity. Increasing the hot water temperature results in more significant exergy destruction in each component; hence the recommended temperature is 75 °C. Increasing the gas-liquid ratio reduces exergy destruction, but the effect weakens when the ratio surpasses 1 kg·kg−1. Considering both exergy destruction and component efficacy, the system’s weak links are the regenerator, dehumidifier, and solution total heat recovery system. Furthermore, these components must perform better to increase the entire system’s performance.
24	[73]	2019	Desiccant evaporative cooling system	Simulation	The system has higher energy efficiency than the reference system. It uses more natural gas due to high regional humidity and emits 18.71% less CO2 Compared to the reference system. The system’s payback period is approximately three years, making it financially attractive for implementation.
25	[74]	2018	Solid desiccant system integrated with cross flow Maisotsenko cycle evaporative cooler	Experimental	Based on parametric testing done under the same conditions, the MC–DAC system has a 60–65% higher COP than the DAC system. The system works well in hot climates but is sensitive to the ratio of inlet ambient humidity.
26	[75]	2018	Desiccant wheels, DW, and indirect evaporative coolers	Simulation	The DW–IEC system showed lower annual energy consumption than the DX system across all studied climate zones, with notable energy savings in hot climate zones with high dehumidification demand. With a peak SCOP value of 2.8 in an extremely hot climate zone, the DW–IEC system consistently outperformed the DX system, which is always greater than a 25% difference om SCOPs. With a high demand for dehumidification in extremely hot climate zones, the DW–IEC system achieved up to 34.6% of energy savings. In cooler and colder climate zones with low dehumidification and high heating demand, energy savings were only 4.4% and 1.7%, respectively.
27	[76]	2020	Desiccant with evaporative cooling system	Simulation	In the summer, the DDS-ECW system significantly decreased the inner window temperature, especially in September when humid conditions reduced the efficiency of the ECW system alone. The integrated system decreased an 11% summer cooling load, compared to a 7% decrease in the system without dehumidification. The integrated system has a life-cycle cost Net Present Value (NPV) of $12,600, which is a 14% decrease over the conventional system’s $14,660 NPV over 20 years.
28	[77]	2021	Desiccant evaporative cooling system	-	The LDCS effectively controls latent air loads, collect low-grade energy sources, and significantly reduces wasted electric energy. LDCS can save up to 40% more energy than traditional VRF systems. Compared to alternative configurations, using an LD and M-cycle IEC provides greater dehumidification and supply air temperature performance.
29	[78]	2019	Desiccant enhanced indirect evaporative cooling system	-	Dynamic retrofit optimization improved LCC by 23.7%, ACOP by 50.5%, AWC by 153.2%, and ACE by 57.8% compared to static retrofit optimization, and the improvements in dynamic design optimization were 34.6%, 60.1%, 390.0%, and 69.9%, respectively.
30	[79]	2021	Evaporative cooling system integrated with liquid dehumidification	Experimental	The IECL system significantly enhanced heat transfer performance, improving overall efficiency. Despite the added complexity and larger size of the integrated system, it greatly reduces energy consumption in air treatment processes.
31	[80]	2017	Liquid desiccant-based evaporative cooling system	Experimental	The system had a dehumidification efficiency of 63.7% and decreased air temperature by 5.3 °C at 0.3 m/s air velocity. The system’s dehumidification effectiveness dropped to 56.1% when the air velocity reached 0.5 m/s. The system maintained a high COP range by utilizing external power derived from renewable sources, with an average COP of 5.5 at 0.3 m/s air velocity and 4.8 at 0.5 m/s.
32	[81]	2016	Desiccant and evaporative inter-cooler	Experimental	The COPth, adj was computed due to the moisture imbalance between the regenerator and dehumidifier, and the values reached 1.26 and an average of 0.72, indicating the need for additional work to address the imbalance issue. The electrical COPel averaged 2.5 and reached a peak of 3.67.
33	[82]	2015	Desiccant evaporative cooling system	Simulation	The study shows that the COP of three novel configurations of a desiccant evaporative cooling system is greatly affected by the ambient air humidity ratio more than the ambient air temperature for the considered ranges. Energetic analysis showed that the first design has the most significant cooling capacity. In contrast, the second configuration is easier to build and delivers a 28% higher system COP than the standard system. The third configuration provides the highest thermal and air handling COP. Moreover, its average exergetic efficiency is 54% higher compared to the conventional system.
34	[83]	2014	Solar-assisted liquid desiccant air-conditioning system	Simulation	The SLDAC system performs best in humid environments and is less efficient in regions with low outdoor humidity. In humid climates, the system consumes 40% less electricity and has a payback period of approximately seven years. In dry climates, an enhanced chiller COP can reduce energy use by up to 45%, but the cost payback period is around 22 years.
35	[84]	2019	Cascade liquid desiccant dehumidification in a desiccant and evaporative cooling-assisted system	Simulation	Findings indicated that a retrofitted system with a two-stage liquid desiccant dehumidifier (cascade liquid desiccant section) was more energy-efficient than a conventional liquid desiccant and an indirect/direct evaporative cooling-assisted outdoor air system with a single-stage liquid desiccant dehumidifier, using 12% less primary energy at peak load and 17.4% less during the cooling season. The retrofitted system had better total thermal and primary performance coefficients, 0.78 and 2.05, respectively, compared to the other system, 0.65 and 1.45.
36	[85]	2016	Liquid desiccant and evaporative cooling-assisted 100% outdoor air system	Simulation	Simulation results showed that Case 1, integrating a dew-point IEC, could increase cooling capacity by 41% over the base case. Because more energy was required to heat the desiccant solution, the thermal COP remained about the same. MEE was added to Case 1 to generate Case 2. The COP for Case 2 was double that of the other cases due to significant increases in cooling capacity and energy conservation from the regeneration process.
37	[86]	2014	Desiccant dehumidification system with two-stage evaporative cooling	Simulation	The system reduces water usage and keeps ideal humidity by cooling a portion of the dehumidified air, mixing it with bypassed air, and circulating it in the room. Then, uses a PEC to provide localized cooling. These help to reduce energy use by 16.15% and water use by 26.93% compared to a conventional single-stage evaporative cooling system.
38	[87]	2015	Solid desiccant evaporative cooling system	Simulation	The ventilated-dunkle cycle configuration provided the greatest results for Vienna, Sao Paulo, and Adelaide, representing continental, temperate, and dry-summer subtropical climates, with COPs of 0.405, 0.89, and 1.01, respectively. Contrarily, the ventilation cycle DEC was most effective in dry, semi-arid, and varied temperate regions, such as Karachi and Shanghai, with average COPs of 2.43 and 3.03, respectively.
39	[88]	2024	Desiccant dehumidification, indirect evaporative cooling and CO2 capture	Simulation	The system demonstrated improved energy efficiency under various conditions, achieving over 30% energy savings and a 53.3% improvement in COP. Efficiency increased with occupancy levels (2 to 22 occupants), ranging from 50.7% to 85.6%, while effectively addressing indoor air quality concerns through CO2 capture.
40	[89]	2024	Indirect Evaporative Cooling System Assisted by Liquid Desiccant	Simulation	The system operates through a two-stage process designed for extreme weather conditions in Hong Kong, addressing the high cooling demand of data centers. The case study demonstrated that the proposed system achieves a remarkable 72.7% improvement in temperature drop compared to a single IEC.
41	[6]	2024	A dew-point indirect evaporative cooler and a desiccant wheel	Experimental	This study experimentally evaluates a hybrid system combining a desiccant wheel and a dew-point indirect evaporative cooler for independent control of temperature, humidity, and CO2. The system achieves a high COP of up to 11.0 under extreme conditions, outperforming traditional systems. The results support the system’s potential for energy-efficient cooling, improved thermal comfort, and better indoor air quality, especially in heat waves and climate change scenarios.

. However, in their review, Abd Manaf et al. [9] show that some studies indicate that up to 80% of energy can be saved using hybrid liquid desiccant cooling systems regenerated by solar energy. Desiccant evaporative cooling (DEC) improves the energy efficiency performance of the evaporative systems in hot and humid conditions. Moreover, the energy-efficient, cost-effective, and environmentally friendly hybrid system is an ideal alternative to traditional vapor compression systems. Indeed, the desiccant materials remove moisture from the air and can be regenerated by free energy sources, such as solar or waste heat. Various studies have investigated the energy saving potential of solar-driven desiccant cooling in different climates.

Heidari et al. [73] proposed a desiccant-based evaporative cooling system that combines desiccant wheels and evaporative coolers to provide cooling in hot and humid climates. The system recycles the moisture content of exhaust regeneration air to cover all the evaporative cooler water consumption, making it more environmentally friendly and energy efficient. The energy performance of the proposed system was simulated for a building in Bandar Abbas, Iran, which is characterized by a hot and humid climate. The simulation-based analysis results indicated that the integrated evaporative–desiccant system provided comfortable indoor temperatures and relative humidity levels. Moreover, the analysis simulation showed that the integrated system could provide comfortable indoor conditions during a summer week while producing excess water for domestic use. The integrated system consumes less electricity but uses more natural gas due to the region’s high humidity, and it emits less CO₂ compared to the vapor compression system. The economic evaluation proved that the proposed integrated system is feasible for the considered building with a payback period of around three years.

Shahzad et al. [74] investigated the performance of a solid desiccant dehumidifier integrated with a Maisotsenko cycle (M-cycle)-based crossflow heat and mass exchanger (i.e., IEC), as illustrated in Figure 7 and Figure 8. The study compared the efficiency of the desiccant air-conditioning system (DAC) with and without the M-cycle (MC) under identical operating conditions. The results revealed that the integrated MC–DAC system has a higher COP, resulting in 60–65% higher efficiency than the DAC system. The MC–DAC system achieves this high efficiency through its unique M-cycle indirect evaporative cooler, which provides comfortable indoor conditions even at low regeneration temperatures. Additionally, the MC–DAC system does not add moisture to the supply air, making it suitable for applications where humidity control is critical. However, the integrated system is more sensitive to inlet outdoor humidity ratios and performs optimally in hot climates. Furthermore, the study showed that the integrated MC–DAC system has a lower carbon footprint than the DAC due to its efficient energy usage and lower emissions. Its unique design also allows it to operate with renewable energy sources such as solar, geothermal, and waste heat, reducing its environmental impact.

Buker and Riffat [90] also evaluated liquid desiccant coupled with evaporative cooling to air condition indoor spaces. Specifically, the considered air-conditioning system combines a liquid desiccant dehumidifier and regenerative indirect evaporative cooling (LDD–RIEC). This system utilizes heat from solar collectors to regenerate the desiccant solution. It can provide a supply air temperature of 17.2 °C with a humidity level of 9.8 g/kg when the outdoor air is at 30 °C and 20 g/kg. The study found that the optimal extraction air ratio for the regenerative indirect evaporative cooling (RIEC) is 0.3 and that increasing the solar collector area can improve the system’s moisture removal rate and cooling capacity. The relative energy savings and COP of the integrated system range from 22.4% to 53.2% and 4.3 to 7.1, respectively, under various inlet-air conditions. The study discovered that the system’s energy efficiency is higher when the fan coil unit operates at a dry-coil condition. The study highlights the potential of the LDD–RIEC system as an efficient and sustainable air-conditioning solution. Its reliance on solar collectors for regeneration and its ability to handle high latent loads make LDD–RIEC an attractive alternative to traditional air-conditioning systems.

Kalpana et al. [77] have discussed the shortcomings of the vapor compression air-conditioning (VCAC) systems and propose a more eco-friendly and cost-effective liquid desiccant cooling system (LDCS) coupled with an evaporative cooler as an alternative. LDCS can efficiently control latent loads while utilizing low-grade energy sources and reducing electrical energy waste. LDCS system saves up to 40% of energy compared to variable refrigerant flow (VRF) systems. The author shows that liquid desiccant dehumidifiers coupled with evaporative cooling technologies can provide superior and more costly alternatives to other air-conditioning systems in hot and humid conditions. They address some challenges related to LDCS, such as desiccant carryovers with air and space limits, various technologies, and small devices to improve LD-based evaporative cooling systems’ cooling efficacy and energy efficiency. The results in the Kalpana et al. study also show that combining an LD dehumidifier and an M-cycle IEC leads to a better performance in terms of controlling dehumidification levels and supply air temperatures than conventional systems.

The challenge of improving the heat transfer rate of IEC systems in hot and humid climates has been considered by Zhang et al. [79]. They proposed integrated IEC and liquid dehumidification (LD) systems to enhance the heat and mass transfer performance of conventional IEC units. The analysis has indicated that indirect evaporative cooling combined with liquid dehumidification (IECL) exhibits a strong dehumidification performance in high-temperature and high-humidity conditions with significant heat transfer enhancements. However, the hybrid system has limitations, including complex components and their need for large dimensions, which can be problematic for restricted building spaces.

To overcome the challenges of the system size, complexity, desiccant leakage, and equipment corrosion in small-scale domestic applications, Elmer et al. (2016) [81] developed an Integrated Desiccant Cooling System (IDCS) by combining a regenerator, a dehumidifier, and an evaporative inter-cooler into a single unit. A moisture imbalance problem between the regenerator and dehumidifier was observed and led to an adjusted system thermal COP reaching up to 1.26 and averaging 0.72. However, the electrical COP of the hybrid system averages 2.5 and peaks at 3.67.

For extreme challenging climates, including those in Africa and the Middle East, Cuce [80] has conducted an experimental evaluation of a desiccant-based direct-contact evaporative cooling system. The analysis results have indicated that the dehumidification efficiency of the hybrid system is highly dependent on the air velocity. For example, at an air velocity of 0.3 m/s, the system decreased the air temperature by 5.3 °C and achieved a 63.7% dehumidification efficiency. When the air velocity is increased to 0.5 m/s, the dehumidification efficiency dropped to 56.1%. The COP of the hybrid system has been estimated to be 5.5 and 4.8 for 0.3 m/s and 0.5 m/s air velocities. The COP can be improved further by using renewable-based external power.

Ali et al. [87] analyzed the performance of five DEC systems for five different climate zones. The study uses an advanced equation-based object-oriented (EOO) modeling and simulation approach to identify the optimal system for each climate zone. The reported results indicated that the DEC system with the ventilated Dunkel cycle performed best for Vienna, Sao Paulo, and Adelaide, representing continental, temperate, and dry-summer subtropical climates, yielding COPs of 0.405, 0.89, and 1.01, respectively. On the other hand, the ventilation cycle DEC was ideal for arid and semi-arid and various temperate climates in Karachi and Shanghai, achieving average COPs of 2.43 and 3.03, respectively.

Across various climates, a study by Ronghui et al. [83] has analyzed the performance of a solar-assisted liquid desiccant air-conditioning (SLDAC) system when applied to commercial buildings. They found that the sensible total heat ratio significantly influences the system’s energy efficiency. Moreover, the best energy performance of the hybrid system has occurred in humid conditions, while the worst energy efficiency has been observed in climates with mild outdoor humidity levels. For instance, in the humid climates of Houston and Singapore, the SLDAC system has consumed 40% less electricity and has a payback period of about seven years. On the other hand, for the dry climates of Boulder, Colorado, improving chiller COP can save up to 45% of AC electricity. The author clarified that, by defining the dry operation fraction (DOF), the ratio of the whole operation time to the time liquid desiccant ventilation might eliminate excess air humidity, allowing the chiller to operate at a higher evaporation temperature and achieve a better COP. In addition, its cost payback period extends to around 22 years.

The performance of three novel configurations for a desiccant evaporative cooling system has been compared with that of a conventional system by Elgendy et al. [82]. A direct/indirect evaporative cooling unit is incorporated before the rotating heat exchanger in the first configuration, while it is inserted after in the second configuration. The third configuration adds an extra direct/indirect evaporative cooling unit. The analysis results have indicated that the performance of desiccant evaporative cooling systems is significantly impacted by the ambient air humidity ratio, and more than by the ambient air temperature. The first configuration has the highest cooling capacity, and the second configuration, which is simpler to construct, has a 28% higher system COP than the conventional system. The third configuration offers the highest thermal and air handling COP, when compared to a conventional system with an average exergetic efficiency higher by 54%.

For a spa building in different climatic conditions, Comino et al. [75] analyzed two air handling systems: desiccant wheels and indirect evaporative coolers (DW–IEC) and DX. Both systems’ energy performance and desiccant capacity have been estimated through detailed energy simulation. The analysis has indicated that the two systems have similar sensible and latent capacities. However, the IEC component of the DW–IEC system reduced the high air temperatures produced during the adsorption process of the desiccant wheel (DW). The hybrid DW–IEC system had lower annual energy consumption than the DX system for all climate zones considered in the study, with significant energy savings achieved for hot climates with high dehumidification demand. These energy savings resulted in better seasonal COP values for the DW–IEC system, reaching up to 2.8, which was obtained for a very hot climate zone. The difference in COP between the two systems has been estimated to be consistently higher than 25% for all climate zones. In particular, for climate zones with high dehumidification demands, the energy savings associated with the hybrid system have been estimated to be 34.6%. In contrast, for cool and cold climate zones, the energy use by the DW–IEC system is only 4.4% lower than that of the DX system due to low dehumidification demands and high heating needs.

Park et al. [84] have evaluated the effect of dehumidification using a cascade liquid desiccant on primary energy usage for a liquid desiccant and evaporative cooling-assisted 100% outdoor air system. In the study, two system cases have been considered for air-conditioned office spaces. Case 1 consists of a conventional liquid desiccant and an indirect/direct evaporative cooling-assisted outdoor air system with a single-stage liquid desiccant dehumidifier, while Case 2 is a retrofitted system with a cascade liquid desiccant section made up of a two-stage liquid desiccant dehumidifier. The reported findings have indicated that Case 2 was more energy-efficient and used 12% less primary energy at peak load and 17.4% less during the cooling season. Additionally, Case 2 achieved a total thermal and primary COP of 0.78 and 2.05, respectively, compared to 0.65 and 1.45 for Case 1. As a result of better dehumidification and improved evaporative cooling, Case 2 also supplied a lower supply of air temperatures, improving the conditions in the space.

As part of an effort to improve the thermal and energy performance of liquid desiccant and evaporative cooling-assisted 100% outdoor air systems (LD-IDECOASs), Kim et al. [85] have proposed two retrofitting scenarios. The reported simulation results showed that Case 1, which incorporates a dew-point IEC, could increase the system cooling capacity by 41% compared to the base case, which consists of an indirect and direct evaporative cooling-assisted 100% outdoor air system. However, the thermal COP of both systems has been estimated to be similar because more energy was used to heat the desiccant solution. Another configuration of the hybrid system, Case 2, was considered by integrating a membrane enthalpy exchanger (MEE) into Case 1. The analysis results have indicated that the COP for Case 2 could be double that of Case 1 due to the significant improvements in cooling capacity and energy efficiency derived from the regeneration process.

To optimize its energy and water consumption while preserving occupant thermal comfort, El Hourani et al. [86] have evaluated a hybrid air-conditioning system that uses 100% fresh air and incorporates a solid desiccant dehumidification system with a two-stage evaporative cooling system. The first stage reduces water use and maintains ideal humidity levels by cooling a portion of the dehumidified air, mixing it with bypassed air, and circulating it throughout the room. A personalized evaporative cooler (PEC) is used as part of the second stage, which provides localized cooling. Compared to a conventional single-stage evaporative cooling system, the considered hybrid system reduced energy use by 16.15% and water use by 26.93%.

For optimizing purposes also, a study by Sohani et al. [78] investigated optimizing the performance of a Desiccant-Enhanced Indirect Evaporative Cooling (DEVap) system using a multi-objective optimization method combined with the Order Preference by Similarity to the Ideal Solution (TOPSIS) method. Static and dynamic optimizations were considered for the design and retrofit phases using four different optimization scenarios. The analysis was carried out for a 97 m² residential building using further optimization objective functions, including life-cycle cost (LCC), annual water consumption (AWC), the average yearly coefficient of performance (ACOP), and annual carbon dioxide emissions (ACE). The analysis showed that dynamic optimization led to better results for all cases. Compared to the static retrofit case, the dynamic retrofit optimization has increased the LCC by 23.7%, the ACOP by 50.5%, the AWC by 153.2%, and the ACE by 57.8%.

To enhance indoor environmental quality while improving energy performance, Luo et al. [88] integrated desiccant dehumidification, indirect evaporative cooling, and CO₂ capture into a single system. The study demonstrated promising results under various indoor and outdoor conditions, highlighting clearly the potential energy efficiency of the integrated HVAC system. The direct air capture for CO₂ mitigated challenges associated with outdoor air, improved indoor air quality, and achieved energy savings of over 30% for building operations. It is reported that the integrated system can provide a 53.3% improvement in the COP. Moreover, it is found that the energy efficiency of the integrated system increases with higher occupancy levels, primarily due to a reduced need for reheating, which lowers the evaporation temperature required to remove moisture. For instance, in indoor spaces with 2 to 22 occupants, energy efficiency improvements ranging from 50.7% to 85.6% can be achieved with higher levels of indoor air quality due to the effective CO₂ capture.

Beyond building applications, several studies have explored the performance of desiccant and evaporative cooling systems in other fields. For instance, Kashif et al. [91] evaluated the performance of such systems for improving the thermal comfort of livestock. Their findings highlighted the importance of mitigating heat stress, as it significantly affects animals’ fertility and milk production [92]. Experimental investigations revealed a wet-bulb effectiveness ranging from 46% to 78%, while theoretical studies of M-cycle-based desiccant air-conditioning systems demonstrated even higher wet-bulb effectiveness, ranging from 70% to 86%. Similarly, for greenhouse applications, Ashraf et al. [93] investigated the use of desiccant dehumidification and evaporative cooling systems to optimize temperature and humidity conditions. This optimization could potentially enhance production levels. The study concluded that the M-DAC system is a viable solution for greenhouse air conditioning, particularly under the climatic conditions in Multan, Pakistan.

One particularly challenging application is cooling data centers, given the enormous heat they generate due to the significant energy resources required for their operation [94]. A study conducted [89] in a hot and humid climate in Hong Kong investigated the performance of an indirect evaporative cooling system integrated with desiccant technology. The research focused on addressing the unique cooling and humidity challenges faced by data centers in such extreme climates. The system operates through a two-stage process, where the first stage incorporates a liquid desiccant-based IEC, and the second stage uses the air treated in the first stage as secondary air to enhance the cooling effect. The proposed system delivers a substantial 72.7% improvement in temperature reduction compared to a standalone IEC. Furthermore, the cooling performance increases as the mass flow ratio rises to 0.8 but declines when it reaches 0.9. This reflects the necessity of identifying the optimal value during the design process [89,95]. The authors suggest optimizing the system for ideal operating conditions and incorporating renewable energy solutions to enhance its sustainability and low-carbon footprint [89].

4.2. Integrated HVAC Systems with Photovoltaic and Thermal Energy Storage

Hybrid HVAC systems can also integrate clean energy resources and thermal storage technologies, as indicated by the recently reported analyses that are summarized in

Table 8. Reported studies conducted on the integrated systems with photovoltaic or thermal storage.

No.	REF	Year	System	Methodology	Finding
42	[96]	2021	Hybrid desiccant air-conditioning systems with thermal energy storage	Simulation	The proposed optimum system consumes 55.8 and 4.9% less electricity than CS conventional system and baseline system, respectively. The proposed optimum system enhanced thermal comfort by 13.9% and significantly lowered electricity and natural gas consumption compared to conventional systems. The proposed optimum system production of water from moisture harvesting exceeded its usage by 70% annually, providing water for the system and other residential uses.
43	[97]	2021	PCM-desiccant dehumidification and personal evaporative cooling	Simulation	The findings show an 87% decrease in overall energy costs compared to a conventional air conditioner. In addition, the Trombe wall was used in the system, resulting in energy savings of 55% compared to a system that just used the auxiliary heater.
44	[98]	2023	Hybrid indirect evaporative cooling and latent-heat thermal energy storage system	Experiments and modeling	The proposed system effectively achieves the required thermal condition by utilizing the peak load-shifting ability of the LHTES and the high cooling efficiency of the IEC. The study found that increasing the chilled water system’s temperature from 7 °C to 11 °C increased the chiller’s COP from 3.39 to 3.84, indicating possible energy savings with this approach.
45	[99]	2019	Desiccant cooling system coupled with a photovoltaic thermal-solar air heater	Simulation	The optimized system design for a commercial building in a hot and humid climate achieved a very high SF of 96.6%, meaning that solar energy provided most of the required energy. Additionally, the system had a high electrical COP of 19.8, significantly higher than the typical COP range for commercial buildings (2.6–3.0). It was calculated that 0.35 m2 of PVT-SAH was needed for every m2 of conditioned floor area to achieve this performance.
46	[100]	2022	Hybrid indirect evaporative cooling/thermoelectric cooling system	Experimental	Depending on the climate, the system’s COP can exceed 10, significantly higher than the typical COP of a vapor compression system (2–5). Even though the system’s COP slightly declines when spray water is used, dew point effectiveness increases.
47	[101]	2020	Solar-driven liquid desiccant air-conditioning system	Mathematical model	The system integrates photovoltaic and thermal solar power, dehumidification, and active cooling, aiming to optimize energy performance and the PV area ratio in very hot and humid conditions. Results show that the regeneration temperature is lowered by 2.8 °C, and the heat demand is reduced by 62.08 kW, with a 2% increase in indoor relative humidity. Conversely, a 10 °C rise in condensation temperature increases waste condensation heat by 173.7 kW. When the heat humidity ratio and cold load index are constant, the required PV/collector area ratio decreases.
48	[102]	2024	Liquid desiccant-based hybrid system integrated photovoltaic-thermal system and ground heat exchanger	TRNSYS-MATLAB co-simulator	The system demonstrates a payback period of 8 years with a 49% probability, achieving an efficiency of 45% and reducing energy consumption by 32%.
49	[103]	2025	Hybrid solar cooling systems for buildings: integrating PV and thermal energy storage in phase change materials	Review	This review highlights the potential of hybrid solar cooling systems, which integrate photovoltaic and thermal energy with phase change materials for efficient thermal energy storage. These systems utilize PV-generated energy to power adsorption chillers while storing excess thermal energy using PCMs for continuous cooling. When combined with PV panels and flat plate collectors, these advancements enhance energy efficiency, increasing PV output by up to 20%, and improving flat plate collector performance by 90%, offering a sustainable and effective cooling solution.

. As documented by the reported studies, the integration of thermal energy storage and renewable energy sources enhances the energy efficiency and sustainability of the integrated air-conditioning systems.

Ehsan et al. [96] explored a solar-assisted desiccant air-conditioning system integrated with thermal energy storage (TES) using PCM capsules that were designed particularly for residential applications. The study also considered the possibility of producing water from the high moisture content in exhaust air. The reported study utilized the response surface methodology (RSM) to determine the best combination of critical design characteristics for hot and humid conditions using energy, economic, and environmental (3E) metrics. Specifically, the RSM involves using statistical methods to evaluate how various factors interact with specific responses. These variables are essential in system design, and the reactions are system performance indicators influenced by these factors. This RSM allows building experiments to evaluate these reactions. A set of correlations has been developed to predict the system’s 3E energy performance. The study provided an in-depth analysis of the hybrid air-conditioning system’s abilities to cool, heat, and deliver water simultaneously. When optimized, the proposed hybrid system can have higher solar fractions during peak demands compared to the baseline system. The hybrid system can also improve indoor thermal comfort by 13.9% and significantly reduce electricity and natural gas use compared to the conventional systems. Furthermore, the annual water produced by the hybrid system from moisture harvesting can be 70% greater than its needs, allowing it to supply water for its operation as well as other uses. The findings of the study confirm the economic feasibility and environmental benefits of replacing traditional vapor-compression air conditioners with hybrid desiccant air-cooling systems. Figure 9 and Figure 10 outline the basic operation of a solar and TES integrated air-conditioning system during cooling and heating modes, respectively. Moreover, Figure 11 illustrates the state points for various processes of the integrated system during cooling mode process using a psychrometric chart. For further clarification of Figure 9, Figure 10 and Figure 11, For further clarification of Figure 9, the arrows indicate airflow direction, while letters (A–H) mark key thermal exchange paths, where A–B–C–D–E–F–A represents the Charging Path of PCM capsules, and A–G–D–E–F–A represents the Discharging Path of PCM capsules. The system components are: (1) Air-to-air heat recovery, (2) DW, (3) Air fan, (4) Heating coil, (5) HW, (6) DEC, (7) Domestic water storage tank, (8) Evacuated tube solar collectors, (9) Pump, (10) 3-way valve, (11) Hot water storage tank, (12) Auxiliary boiler, (13) TES, (14) Condensate water, (15) Water path to evaporative coolers, (16) Water path for domestic use. For the state numbering, ambient air enters DW, where moisture absorption increases its temperature (1–2). It is precooled in HW (2–3) and cooled in DEC (3–4) before supply. Mixed ambient and return air cool in the second DEC (5–6), then enter HW for preheating (6–7). The air is further heated in a coil (7–8) before regenerating DW, removing moisture (8–9). The humid regeneration air passes through an air-to-air heat exchanger, condensing moisture for water harvesting (9–10).

An investigation by El Loubani et al. [97] evaluated the energy performance of a hybrid cooling system, integrating a PCM storage layer, a photovoltaic evaporative cooler (PEC), and a solid desiccant wheel regenerated using an auxiliary heater and Trombe wall, suitable for hot and humid regions including the Mediterranean and Gulf coastal areas. For unoccupied office spaces, the hybrid system enables efficient thermal energy storage using PCM during nighttime. The hybrid system is found to maintain acceptable thermal comfort levels while reducing energy costs by 87% compared to conventional air conditioning (without PV and PCM). Moreover, the hybrid system can achieve a reduction of 55% in annual energy consumption when integrated with a Trombe wall, effectively addressing the challenges of cooling office spaces in hot and humid climates.

Wan et al. [98] proposed an IEC system that pre-cools ambient air and integrates a latent-heat thermal energy storage (LHTES) system encapsulating PCM. This innovative process utilizes the high cooling efficiency of the IEC and the peak load-shifting ability of the LHTES. An experimental analysis using operating conditions for commercial buildings could reduce air temperatures by 6–10 °C and humidity ratios by 2–11 g/kg of dry air. Additionally, the study found that increasing the chilled water system’s temperature from 7 °C to 11 °C increased the chiller’s COP from 3.39 to 3.84, resulting in cooling energy savings.

Fan et al. [99] developed an integrated model to assess the performance of a desiccant cooling system paired with a hybrid Photovoltaic Thermal Collector-Solar Air Heater (PVT-SAH). The model evaluates the energy performance of the hybrid system using two indicators, including Solar Fraction (SF) and COP. For a commercial building in a hot and humid climate, the optimized design of the hybrid system can achieve an SF of 96.6% and a COP of 19.8. Moreover, it is determined that 0.35 m² of PVT-SAH would be needed per m² of conditioned floor area in order to exceed typical COP (2.6–3.0) for commercial buildings.

A new hybrid air-conditioning system combining indirect evaporative cooling and thermoelectric cooling technologies has been proposed and evaluated by Zhou et al. [100]. The hybrid system, comprising of a crossflow RIEC and a water-cooled thermoelectric cooler (TEC), cools incoming air in two stages to reach desired indoor temperatures and humidity levels. However, it is determined that the hybrid system can cool air to lower temperatures than traditional evaporative cooling systems, reaching even below the dew point temperatures, while maintaining higher COP values than those specific to vapor compression refrigeration systems. In addition, the analysis results show that while the hybrid system’s COP can be slightly lowered by spraying water, its dew point efficiency is higher. However, the hybrid system’s COP can exceed 10 for some climatic conditions, which is higher than the typical COP values for vapor compression systems (ranging from 2 to 5).

Other studies have investigated and estimated the environmental and economic benefits of integrating renewable energy with hybrid HVAC systems [64,104,105,106,107,108,109,110]. For instance, Beigi et al. [102] have proposed and evaluated a system integrating a liquid desiccant-based hybrid system with a photovoltaic-thermal system and a ground heat exchanger. This system was found to achieve a 32% reduction in the annual energy consumption of a building located in a hot and humid climate.

5. Discussion

Hybrid HVAC systems have demonstrated significant potential for improving energy efficiency and reducing environmental impacts. Findings from various experimental, simulation-based, and analytical studies indicate that these hybrid cooling approaches can effectively enhance HVAC energy performance, particularly in hot and humid climates, where conventional vapor compression systems often face energy efficiency challenges.

Table 9. Comparative summary of performance metrics for hybrid cooling systems.

System Type	Best Performance	Energy Efficiency	Cost–Benefit Ratio	Climate Suitability
DX + Evaporative Cooling	IEC–MVC hybrid, [37]	COP increased by 19–135%	Moderate, might consumes more water and power	Best for hot-dry and mixed climates
DX + Desiccant	Heat pump-driven desiccant, [57]	Seasonal COP improved, CO2 reduced by 33.2%	High; reduces operational costs significantly	Best for hot-humid climates
Evaporative Cooling + Desiccant	DW–IEC system, [75]	Up to 34.6% energy savings	High; short payback period	Effective in humid and mixed climates
DX + Renewable Energy (PV/Thermal Storage)	PCM-desiccant dehumidification and personal evaporative cooling, [97]	87% reduction in energy demand	High; long-term sustainability	Suitable for extreme climates with high cooling demand

compares different integrated HVAC systems. Hybrid cooling systems that integrate DX cooling with evaporative, desiccant, and solar-assisted technologies offer substantial energy savings, particularly in hot and humid climates. Combining indirect and direct evaporative cooling with DX has been shown to boost COP by 19–135%, significantly reducing compressor load, while integrated systems using gravity-assisted heat pipes and semi-indirect cooling lowers energy consumption by 45% and costs by 51.7%. However, water consumption and high initial costs remain key challenges. Desiccant-assisted DX systems further improve dehumidification, enhancing seasonal COP and cutting CO₂ emissions by 33.2%, with heat pump-driven liquid desiccants increasing COP by 12.4%. Yet, efficiency drops at high regeneration temperatures, necessitating better regenerator design and control strategies. Evaporative cooling with desiccants is especially effective in humid climates. Additionally, integrating DX, desiccant, and evaporative cooling with solar and thermal storage has shown energy reductions of up to 87%, with PCM storage and solar-driven liquid desiccants enhancing their sustainability. However, high upfront costs remain a barrier, requiring efforts to improve affordability and scalability for wider adoption of these integrated HVAC systems. Despite the promising results of the hybrid cooling strategies, several challenges require further investigation, including economic feasibility, optimization of control strategies, water management, and field validation for different applications.

6. Summary and Concussions

All the reviewed integrated systems have indicated, through experimental and modeling analyses, promising potential to enhance energy efficiency as well as improve the indoor air quality of air-conditioning buildings located in different climatic conditions. For instance, integrating the DX with evaporative cooling increased the overall COP by 19–135% compared to the conventional DX systems. In addition, these DX systems with evaporative cooling systems can reduce GHG emissions by 28.5%, making them excellent alternatives for sustainable cooling solutions. Similarly, systems combining DX with desiccant have been reported to reduce annual CO₂ emissions by 33.2%. Moreover, the integration of liquid desiccant within an air-handling unit system can significantly enhance seasonal COP rated values compared to those of currently available air source heat pumps and electric chillers. A hybrid DX and desiccant system was reported to increase the rated COP by as much as 12.4% in extremely hot and humid climates. However, evaporative cooling with desiccant shows a high potential for increasing the energy efficiency of air-conditioning systems in hot and humid climates, with up to 50% of energy savings compared to conventional systems. This type of integrated air-conditioning system has been found to also be suitable for dry and temperate climates, offering substantial energy savings, lower emissions, reduced operational costs, and short payback periods.

In addition, the review analysis has indicated that integrating hybrid air systems with solar energy on-site generation and or thermal energy storage can provide further energy saving benefits, with as much as 87% reduction in annual energy demand compared to conventional systems. In addition, these systems enhance the sustainability and the resiliency of air conditioning, especially under extreme climate conditions.

Overall, the reported studies and configurations of integrated air-conditioning systems can offer not only more energy efficient but more resilient, sustainable, and cost-effective alternatives to conventional cooling systems commonly used in both residential commercial buildings. Suggested topics for future research include integrating hybrid cooling systems with renewable energy sources to enhance the sustainability and the resiliency of the built environment. For desiccant systems, research should focus on improving the performance of internally heated regenerators, refining control strategies, and optimizing overall system design to ensure greater reliability and effectiveness of integrated air-conditioning equipment. Further analyses are needed to evaluate integrated air-conditioning systems in real buildings to verify and confirm their superior performance to both building owners and professionals. Future research should focus on enhancing system design, implementing advanced control strategies, and expanding applications beyond residential use to commercial and industrial applications. Several key challenges require further investigation, including economic feasibility, optimization of control methods, water management, and field validation across various applications. Additionally, exploring the integration of more than two systems could further improve the efficiency and adaptability of integrated HVAC systems.