Recently, the Water Research Institute, has revealed that 25 nations, accommodating 25% of the world population, encounter severe water stress annually, consistently depleting nearly all their accessible water resources ^[1]. Moreover, a minimum of 50% of the global population, approximately 4 billion individuals, reside in areas with intensely water-stressed circumstances for at least a single month per year ^[1]. The recently published 2023 Global Water Security Assessment reports that around the 70% of the world population face water insecurity, based on the UN definition of security as the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for any possible activity, from well-being and socio-economic development to preserving ecosystems ^[2]. This distressing situation encompasses the entire population of the African continent: notably, the report identifies 13 African countries as being in a state of severe water insecurity ^[2]. The cited reports also revitalize the discourse surrounding water security by clearly establishing connections between pivotal elements of the sixth Sustainable Development Goal (SDG) related to water and sanitation and various components of SDGs: 1 (no poverty), 3 (good health and well-being), 11 (sustainable cities and communities), and 13 (climate action) [1,2]^[1][2]. Water management will be crucial in determining whether the world achieves the SDGs and aspirations for reducing poverty and enhancing shared prosperity [2,3]^[2][3].

The need to develop sustainable desalination and water reuse processes is becoming increasingly urgent because of water scarcity and the energy crisis, which represent two of the main aspects of the overall climate crisis [2,3]^[2][3]. Currently, only a small percentage of total water production is linked to renewable and low-grade energy; the most used methods are based on reverse osmosis desalination connected to the electricity grid and, hence, responsible on average for 3–5 kg_CO2 m⁻³ of water produced ^[3]. The development of renewable and low-grade energy desalination is a key pathway for a sustainable water management system ^[4]. The availability of solar energy or waste heat sources is a marginally explored sector for water desalination technologies [5,6,7]^[5][6][7]. For instance, in the year 2018, 2437.3 TWh of waste heat was available in the EU below 100 °C. As thermal yields for energy generation are extremely low at low temperatures, the production of low-carbon and net zero water could represent a valid alternative application ^[8].

Humidification–Dehumidification (HDH) desalination is a suitable technology for exploiting solar energy and waste heat by virtue of several features. Basically HDH is an improved version of solar stills for small-scale and decentralized water production facilities that are not connected to the electricity grid or water distribution network. Solar stills have not been used for long time because, even under optimal operating conditions, their productivity is in the order of only a few liters per square meter per day of product water [9,10]^[9][10]. In HDH systems, air is humidified using a warm saline water source (e.g., seawater or brackish water) and then, the moist air, acting as a water carrier, is cooled and dehumidified by a cold saline water source. The latent heat of vaporization is exchanged between the humidifier and dehumidifier to minimize the external heat input and, consequently, maximize the gain output ratio (GOR) [9,10,11,12]^{[9][10][11][12]}.

As the process operates at atmospheric pressure and ambient temperature, it can be regarded as a low-tech imitation of the natural water cycle ^[10]. This leads to economic and compact installations with a small number of pieces of equipment, characterized by high flexibility in terms of distillate production, as well as remarkably low maintenance and ease of operation [10,11,12]^[10][11][12]. In addition, these systems can be powered by low-temperature energy sources, allowing the exploitation of low-grade waste heat where it is available, or renewable energy sources including solar and geothermal [5,13,14,15]^{[5][13][14][15]}.

The low operating costs can be coupled with affordable investment costs by selecting cost-effective construction materials, whose only requirement is to resist the corrosion from seawater. Hence, non-conventional low-cost materials such as resins and polyester have found wide use in the equipment in direct contact with the brine [7,10,13,14,15]^{[7][10][13][14][15]}. The work of Essa et al. ^[15] carried out a thorough survey on the different types of packing materials with various designs and configurations, including cellulose and honey-comb sheets. There are two main configurations for the HDH process, namely the water-heated and the air-heated cycles. Both cycles have advantages and disadvantages, and these have been reviewed in somewhat more comprehensive studies in the literature [10,12,15]^[10][12][15]. A further classification has been carried out on the basis of the water and air cycles, which can be closed or open cycles depending on the make-up and rejection of water and air. These conventional classifications are available in the cited literature [9,10,11]^[9][10][11].

HDH processes usually operate at atmospheric pressure, with the humidifier and dehumidifier being a packed bed and a finned-tube heat exchanger, respectively. These set-ups have been extensively investigated [10,14,15]^[10][14][15]. All these peculiar elements and all the innovative configurations. Another relevant improvement (on which a lot of research is focused) concerns multi-staging, which has been widely covered both theoretically and experimentally. The concept of balanced multi-stage HDH systems was first introduced by Müller-Holst ^[9] for a water-heated HDH cycle, where air is naturally circulated between the humidifier and dehumidifier. Lienhard’s research group at MIT published pioneering works on the thermodynamic analysis of balanced, multi-stage HDH focusing on the minimization of entropy generation and the maximization of GOR [11,14,17,18,19,20]^{[11][14][16][17][18][19]}. The introduction of air extractions and injections makes the HDH operation curve follow the temperature derivative of the air enthalpy at saturation in the enthalpy–temperature plane, and results in a positive effect on the system performance. On the other hand, increasing the number of stages reduces the enthalpy pinch and leads to a consistent increase in the related heat and mass transfer area.

The thermodynamic limit of the process is given by the mild operating conditions, in particular, by the low top brine temperature (TBT) that has a strong influence on the GOR. As already mentioned, the main drawbacks of conventional HDH systems are the very low heat and mass transfer, resulting in large areas and high capital costs. For instance, finned-tube heat exchanger dehumidifiers are characterized by low condensation heat transfer coefficients due to the large amount of non-condensable gases. With the purpose of reducing both the energy footprint of HDH systems and the cost of product water, some improvements to the conventional process have been proposed.

2. Fundamentals of HDH System Enhancements

The key performance parameters by which to analyze a thermal separation process that are commonly adopted in the literature are defined below [4,10,14,15,16,17,18]^{[4][10][14][15][16][17][20]}. Commonly, the efficiency of HDH systems is evaluated determining the GOR and the performance ratio (PR) as defined by Equations (1) and (2), respectively. The PR is used when both thermal power and electrical power are inputted into the system. Δℎ𝑟𝑒𝑓 is the water latent heat of vaporization, assumed to be equal to 2400 kJ kg⁻¹. Often, and not quite rigorously, the denominator of PR presents the sum of different forms of energy. The recovery ratio (RR) and the specific energy consumption (SEC) are given by Equations (3) and (4), respectively. W_in and Q_in are the mechanical energy input and the thermal energy input of the separation process, respectively. If the distillate is indicated as a flow rate, the energy inputs must be reported as powers; therefore, the time derivative is indicated in the formulae. In Equation (4), the mechanical work is highlighted with an asterisk to indicate the equivalent work that takes into account the electrical power input plus the thermal power input converted into electricity by assuming a reference efficiency.

On the other hand, to use a universal and rigorous approach from a thermodynamic point of view, many works in the literature adopt the “exergy” (or II law) efficiency, defined as the ratio between the least work of separation (minimum reversible work) and the actual work required [28,29,30]^[21][22][23]. Figure 1 depicts the conversion of heat into power through a Carnot Engine operating between the heat source at the hot temperature T_H and the sink at the lowest temperature of the system (or the reference environment temperature) T₀, as postulated by the MIT group [30]^[23]. The black box HDH receiving the separation work as mechanical (or electrical) energy and thermal energy that, according to the methodology proposed by Narayan et al. [30]^[23], can be converted into equivalent separation work through enthalpy and entropy, balances around the open system represented by the HDH separation block. This equivalence between thermal and mechanical energy enables people to compare the minimum work to the actual work of separation, leading to the definition of the II law efficiency as reported in Equation (5). The numerator represents the useful product (minimum separation work of the saline stream into a distillate and a concentrate), while the denominator represents the equivalent work, which includes the transformation of thermal energy into energy of the first kind. However, a research group recently implemented a modified equation (Equations (5) and (6)), including an additional term related to the sensible heat efficiency: T_H and T_C are the temperatures of the hot source and the cold source, respectively, and 𝜂𝑆𝐻 accounts for the exergy destruction in case of sensible heat sources [23,28]^[21][24]. The schematic shown in Figure 1 shows how to transform the thermal energy with variable temperature into equivalent work through a Carnot Engine, including the exergy destruction caused by the increasing temperature difference between the source and the receiver. The hot source at temperature T_H exchanges sensible heat and can cool down to a given temperature difference (Δ) with respect to the minimum temperature of the system T₀, i.e., the ambient temperature. The temperature of the hot source goes from T_H to T_C, which is higher than the minimum (ambient) temperature by a certain temperature Δ. To individuate the reference values for low-carbon HDH desalination, researchers realized a sensitivity analysis by varying the main external variables. Figure 2 shows the results of the sensitivity analysis on the exergy efficiency η as a function of the GOR and the temperatures T_C and T_H. T_C has been calculated in relation to the temperature difference Δ above the ambient temperature T₀ set to 25 °C. The results are reported for three assigned GORs and as a function of T_C. Since the processes are to be coupled with sensible waste heat at low temperatures, a range of T_H from 65 to 90 °C was chosen. The results shown in the graphs correspond to the  efficiency of the systems taking into account the "energy quality". With the same GOR, the processes that use low-temperature sources, represented by streams that exchange sensible heat and which can cool down to temperatures close to ambient temperature, obtain higher exergy efficiencies. The efficiency values are in line with the ranges identified by the MIT group and previous works [5,22,23,28,29,30]^{[5][21][22][23][24][25]}. With the same GOR, the higher the second order efficiency the lower the temperature of the hot source. Moreover, the efficiency increases as the temperature to which the hot stream can be cooled decreases: a high cooling corresponds to a high exergy destruction and, therefore, to a low quality of the energy used. Furthermore, it is possible to compare various desalination processes based on a survey carried out by Shazad et al. [32,33,34]^[26][27][28] in order to understand how the directions of HDH development can aim at more sustainable processes. In fact, Shazad et al. [32,33,34]^[26][27][28] have proposed a universal performance ratio based on the conversion of different forms of energy into work. They simulated a dual-purpose water–energy plant to extrapolate the correction factors to evaluate the work equivalent to a heat source at a certain temperature: basically, the work-value of a stream extracted from a turbine to export thermal energy is equal to the one that the stream would generate if free to expand at the lowest pressure in a Rankine cycle. Therefore, they reported the thermal and electrical expenditure of conventional large-scale desalination plants (reverse osmosis, multiflash, and multiple effect distillation) in the last 20 years [32,33,34]^[26][27][28]. Using their published data, researchers have applied the exergy efficiency proposed in Equation (5) extending the calculation to HDH processes, including those with high efficiencies obtained with sensible heat sources such as solar, geothermal, and waste thermal energy. Figure 3 reports the exergy-efficiency performance for conventional processes (from [32]^[26]), as well as the results for high-efficiency HDH systems with a heat source of 75 °C at different GOR values. The zone of sustainability pointed out by Shazad et al. [34]^[28] is also highlighted in Figure 3 (red area). By assuming higher GORs (6–10) at low temperatures, it can be noted that enhanced HDH processes can compete with more established processes (green area). Although the performances of high-efficiency RO are higher, HDH technology has two clear advantages: operating at mild conditions can increase the RR up to high brine concentrations, and it can reduce carbon dioxide emissions thanks to the use of low-grade waste heat.

3. Advanced Heat and Mass Transfer Techniques

Since its first design in the early 1960s, several modifications of the conventional HDH process have been introduced to improve the heat and mass transport phenomena. In HDH systems the energy footprint (main voice of the operating costs) and the size of equipment (proportional to the investment costs) represent a clear trade-off: the lower the driving force and the entropy generation, the lower the energy consumption but the higher the heat and mass transfer areas, and thus the equipment costs. The conventional configuration includes a direct contact humidification in a packed bed and a dehumidification via the surface of a heat exchanger with seawater flowing inside finned tubes. In addition to the classic fouling problems (mitigated by low temperatures), these heat exchangers suffer from the prevailing presence of non-condensable gas compared to water vapor. This aspect makes the heat and mass transfer a limiting factor in the design of HDH systems.

3.1. Humidifier

The humidifier typically comprises a packed bed structure. Water is introduced through spraying at the upper section of the packing, while air is introduced in countercurrent from the bottom section of the bed. The analysis of this constituent can be accomplished using conventional methodologies originally developed for cooling towers [11,35,36]^[11][29][30]. While various types of packing materials can be used, it is commonly an economical polymeric material with an ample open area to minimize air pressure drop while offering a mass-transfer surface area for conducive evaporation. A notable advantage of the packed bed humidifier lies in its immunity to surface scaling or fouling, which would otherwise hinder the heat and mass exchange at the air–water interface. Furthermore, owing to its operation at atmospheric pressure and moderate temperatures, it is feasible to employ cost-effective structural materials, eliminating the need for expensive, corrosion-resistant metals. The abovementioned humidification columns have been widely investigated from the point of view of heat and mass transfer in process engineering, especially in relation to cooling towers operating at atmospheric pressure, where the packing heights are in the order of a few meters [35]^[29]. Along the axis of a packed bed, there two heat transfer mechanisms occurring simultaneously:

• a latent heat transfer mechanism supported by the transfer of mass in a stagnant medium (the inert air taking up the water vapor)
• a sensible heat transfer mechanism between the gas and liquid phases (the flow direction depends on the temperature of the two phases)

The energy balance around a control volume of section S and infinitesimal height dZ allows people to determine the packing height for the air–water system, as expressed by Equations (7) and (8): where the packing height Z can be calculated as the product of the height and the number of the gas-enthalpy transfer units 𝐻𝑡𝑂𝐺 and 𝑁𝑡𝑂𝐺, respectively [35,36]^[29][30]. The driving force is the difference between the enthalpy of the gas at saturation H_s and that at the operating conditions in each point of the bed. k_y is the mass transfer coefficient that is related to the Lewis number, and a is the specific surface area. Hence, the column height depends on the mass transfer coefficient as well as the enthalpy pinch between the saturation and the operating curve. Many works have analyzed the dichotomy between reduction in driving force (minimization of entropy generation) and increase in equipment size. The works by Lienhard’s group [11,29]^[11][22] presented a techno-economic analysis of HDH systems: the lower the terminal temperature difference (TTD) and the entropy generation, the higher the cooling-tower size factor, while, on the other hand, the lower the TTD, the lower the energy footprint of the process, as already discussed. However, despite an increase in investment costs, these devices are often built with multiple stages in order to reduce the driving force differences and the generation of entropy [17^{[16][22][31][32]},29,37,38], regardless of the geometric configuration, the direction of the flows, and the contact surface. Figure 4a shows the conventional packed bed humidifier along with other possible alternatives, some of them originated from theoretical works and others devised and tested in experimental works. Figure 4b represents a bubble column with a submerged coil to control the temperature and to modulate the evaporation rate. Figure 4c shows a direct contact evaporator with water spray (spray pond) and cross-current air flow. Figure 4d shows a similar configuration with a contactor made of hollow fiber membranes, which are the object of novel studies, as discussed in the following. Figure 4e shows a direct contact chamber with air and water flowing in co-current. This configuration is usually adapted from HVAC equipment; in fact, for completeness, the vertical finned tubes to control the temperature have also been reported. A first work coupling experiments and theory to evaluate the effect of process parameters on mass transfer is provided by Nawayseh et al. [39]^[33], who produced charts for estimating the mass transfer coefficients in humidifiers under both natural draft and forced air circulation. Zhao et al. [40]^[34] studied the performance of a four-stage cross-flow HDH system made of a polypropylene corrugated plate structured packing. They found an optimal air flowrate of around 300 m³ h⁻¹ was able to maximize the water productivity (≈50 kg h⁻¹ with GOR of 1.2 and TBT of 77 °C) and the minimize equipment cost. In fact, they claimed to achieve a remarkably high productivity (34.1 kg m⁻³ h⁻¹) and an affordable water production cost (USD 3.86 ton⁻¹). As mentioned, the realization of extractions, and therefore of multistage apparatuses, can improve the thermodynamic performances while the geometry and the materials of the filling can influence pressure drops and productivity per unit of surface. Instead of using metallic materials, the conventional tower packing can be constructed using alternative materials such as honeycomb cellulose or ceramic. Plastic materials are a reliable and cost-effective option, as they possess a hydrophobic surface with low surface free energy, making them water-repellent, lightweight, and easy to clean. Additionally, their self-cleaning and antifouling properties contribute to smooth water flow with minimal resistance [41]^[35]. A significant challenge is preventing brine droplets from becoming trapped in the humidified air, thus potentially contaminating the distilled water. In addition, conventional packed bed humidifiers can encounter various issues, including channeling, foaming, flooding, and metal corrosion. To overcome these challenges, non-metallic polymer-based membrane contactors, such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polypropylene (PP), have been introduced [41,42]^[35][36]. These innovative contactors facilitate indirect contact air humidification by effectively separating the liquid feed and air using a porous membrane. Among the designs, hollow fiber membrane contactors (HFMC) are particularly preferred because of their simple liquid-sealing capabilities [42]^[36]. A few studies have investigated the performances of cross-flow HFMC applied to HDH systems. Zeng et al. [43]^[37] designed an innovative “light and compact” humidifier with a low pressure drop consisting of a dense array of vertical strings along which thin films of the heated liquid feed are allowed to flow under gravity and come into contact with the counter-current gas stream. In their work, they demonstrated a four-fold increase in the evaporation rates at a very low pressure drop, around few Pa m^–1. Another hollow fiber membrane-based humidifier in a membrane-based HDH has been tested by Li and Zhang [44]^[38], achieving a water productivity of 25.9 kg m⁻² d⁻¹ and an SEC of 19.2 kWh m⁻³. Their experimental apparatus, made of cylindrical fiber tubes, was used to validate their computational model to analyze forced convection heat and mass transfer operating under naturally occurring boundary conditions. Li et al. [42]^[36] presented a theoretical study on the performances of cross-flow HFMC, identifying three sets of operating conditions and performing sensitivity analyses on solution conditions (temperature, salinity, flowrate), air conditions (temperature, relative humidity, flowrate), and dimensionless parameters (number of heat transfer units, flowrate ratio of solution to air). Tariq et al. [45]^[39] designed a novel air saturator to maximize the water evaporation rate at the exit of a “Maisotsenko Cycle” wet channel, which had a geometric configuration similar to the one reported in Figure 4e. They demonstrated a maximum recovery ratio of 0.6. The mathematical model was solved for the flat-plate solar collector, brackish water storage tank, air saturator, and dehumidifier, obtaining a a reduction of carbon emission as compared to a direct contact humidifier-based desalination plant. Recent studies have also investigated the use of bubble columns as humidifiers. A coil is usually installed inside each bubble column to provide the heating or cooling load required during the experiments, as depicted in Figure 4b. El-Agouz and Abugderah presented an experimental investigation of a humidification process using air passing through brackish water. They have concluded the maximum vapor con at 75 °C for water and air [46,47]^[40][41]. Basically, these experimental plants are laboratory scale evaporators with low heated liquid heads and gas spargers for small air flows; the resulting water costs are still not competitive. As mentioned, the developed cooling tower configurations are still not outdated for large scales plants. A novel multi-stage bubble column humidifier has been assessed by Abdur-Rehman and Al-Sulaiman [48]^[42], who realized an optimal coupling of air and water stream temperatures and achieved a higher water vapor content compared to the previous studies. Nevertheless, these systems are more similar to water ponds than to humidification towers, and they allow less controllability for the benefit of great simplicity and economy. To intensify the mass transfer in the humidifier, Shehata et al. [49]^[43] introduced ultrasonic modules to promote the formation and oscillation of small vapor bubbles that can be rapidly transferred to the gas phase. A research group of mechanical engineers from Egypt [50,51]^[44][45] explored a solar HDH system incorporating a high-speed centrifugal humidifier; the rotating sprayer was utilized to create numerous small water droplets, enhancing their contact with the hot air stream and accelerating the evaporation process.

3.2. Dehumidifier

As the carrier gas is usually a non-condensable gas present in very high concentrations (60–95 mol%), a large additional resistance to heat transfer is present in dehumidifier columns. The convective heat transfer rate, defined in Equations (9) and (10), is limited by low values of the heat transfer coefficient h₀ (in the range of 18–30 W m⁻² K⁻¹) because of the non-negligible transfer of sensible heat via gas cooling [39,52]^[33][46]. On the other hand, the portion of heat transferred by vaporization is controlled by the mass transfer rate. In the traditional configuration, the condensation takes place on the metal surface of the tubes (often finned to obtain increased surfaces and higher exchange coefficients) in which cold water flows. To reduce the aforementioned thermal resistance, direct condensation of the vapor–gas mixture (rather than on cold wetted surfaces) has been studied [52]^[46]. This can occur on wetted surfaces where the cold liquid is sprayed (Figure 5a) or by letting the humid air cross a cold liquid column (Figure 5b). In the bubble column dehumidifier, moist air is sparged through a porous plate (or any other type of sparger) to form bubbles in the cold liquid column. Several factors affect the performance of direct contact condensers or bubble column dehumidifiers. These factors include the superficial velocity, the inlet mole fraction of vapor, the bubble diameter, the liquid column height, and the impact of bubbles on the coil. The main performance parameter of interest in the current investigation is the total heat flux exchanged between the coolant and the air-vapor mixture. Tow and Lienhard [52,53,54]^[46][47][48] investigated multi-stage bubble columns with different sizes of cooling coils and shallow trays, finding that, with a large volumetric interfacial area, the gas-side resistance is sufficiently low that it can be neglected in modeling their system. The experimental results focused on heat flux and effectiveness, shedding light on key parameters influencing the dehumidifier performance. The same research group developed a predictive model (validated against experimental data) demonstrating that bubble columns have an order of magnitude higher heat rates than existing state-of-the-art dehumidifiers if designed for high superficial velocity, low bubble diameter, and maximum bubble on coil impact. On the other hand, to minimize pressure drop, the liquid height can be minimized because the height has a slight effect on the performances [55]^[49]. Sharqawy and Liu [56]^[50] developed a model for the super-atmospheric pressure in a bubble column dehumidifier. They observed that the effectiveness of the bubble column was dependent on the number of transfer units and the pressure, impacting heat and mass transfer coefficients. Higher pressures improved the heat transfer, while the effectiveness was reduced. However, this reduction in effectiveness could be compensated for by using a large dehumidifier diameter. Huang [57]^[51] proposed the implementation of a multi-stage bubble column dehumidifier to fully extract water vapor from the carrier gas. The dehumidifier effectiveness was maximized by optimizing the accessible space through multi-staging dehumidification, thus recovering as much energy as possible for preheating the feed saline water [58]^[52]. Globally, bubble column dehumidifiers were found to be superior to the film condensation regime in indirect dehumidifiers, making them a suitable option for HDH systems. The air flowrate to create the bubbles and the inlet air temperature were crucial parameters in determining the effectiveness and the size of the dehumidifier. Eslamimanesh and Hatamipour [59]^[53] performed a numerical investigation on a direct contact HDH desalination process. Their mathematical model included the dependence on the temperature of inlet air and recycled freshwater, as well as the flowrate of feedwater and inlet air. The findings revealed that increasing the flowrate of inlet air and recycled water led to higher freshwater production. In addition, raising the temperature of the air entering the humidifier or decreasing the temperature of the water entering the dehumidifier enhanced the freshwater production. However, an increase in the ratio of water-to-air flowrates in the humidifier resulted in a decrease in the system productivity. It has also been shown that a direct contact dehumidifier utilizing spherical phase change material (PCM) elements as packing media led to enhanced condensation rates [60]^[54]. The PCM high heat capacity and melting temperature, similar to the dehumidifier operating temperature, contributed to this enhancement. A one-dimensional mass and heat transfer model was developed to explain the operation of PCM elements, yielding consistent results with experimental data. The system productivity was influenced by various factors such as the type, size, and thermal properties of the PCM packing, the ratio of air-to-water flowrate, and the geometrical dimensions of the dehumidifier. The study concluded that small-sized, inexpensive PCM packing with high heat conductivity provided the best results in terms of freshwater production rates. Heat and mass transfer across the direct contact dehumidifier have also been modeled using counter-current and co-current flow patterns [61]^[55]. An HDH system with a direct contact condenser containing spherical PCM packing and different flow regimes between cooling water and humid air was investigated. The PCM packing outperformed air capsules in freshwater production, and the counter-current flow showed better heat and mass transfer between water and air compared to the co-current flow. Another HDH system, where humid air was condensed by spraying cold water into the air stream, was developed by Niroomand et al. [62]^[56] and Agboola and Egelioglu [63]^[57]. While the first work provides a simplified mathematical model of direct contact condensation [62]^[56], the second focuses mainly on the experimental results of a solar water desalination system with spray jets, demonstrating that the incorporation of a wick onto the absorber plate yields a notable impact on system productivity [63]^[57]. Okati et al. [64]^[58] carried out a thermodynamic analysis of a solar HDH system with subsurface condensation technology, which showed promising potential for underground irrigation. In this system, humid air was directed into an underground condensation space, where its thermal energy was transferred to buried tubes, leading to condensation and producing freshwater. The results demonstrated a system productivity exceeding 265 L d⁻¹ of freshwater. In a follow-up study [65]^[59], the authors utilized a solar humidifier and a set of tubes as a subsurface condenser, achieving a freshwater production rate of 3.8 L h⁻¹ per meter of buried tube. Overall, combining a high concentrating solar collector, a phase change material, and a subsurface condenser in HDH systems can result in significantly improved performances. The choices of the carrier gas and the HDH geometrical configuration significantly affect heat and mass transfer coefficients [10,20,66]^[10][19][60]. In fact, HDH systems face challenges due to the large pressure drops and the low heat transfer coefficients in the gas side of the dehumidifier. To address these issues, the use of helium as a carrier gas instead of air offers a potential solution [67,68]^[61][62] thanks to superior thermophysical and psychrometric properties that can improve the heat transfer coefficients and reduce the pressure drop. This enhancement, which does not affect the thermodynamic performance (GOR) of water-heated HDH systems, leads to a substantial reduction in auxiliary power consumption and dehumidifier size. However, the challenge of limited availability of helium poses an obstacle to the commercialization of such systems [67,68,69]^[61][62][63]. Mousa et al. [68]^[62] investigated alternative carrier gases and employed modeling techniques to evaluate the system performance. Basically, low molecular weight gases can achieve efficient heat transfer rates, while gases with higher molecular weight, such as carbon dioxide, could be selected to obtain more effective mass transfer.