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Water scarcity is a relevant issue whose impact can be mitigated through sustainable solutions. Humidification–dehumidification (HDH) cycles powered by photovoltaic thermal (PVT) modules enable pure water production in remote areas. In this study, models have been developed and validated for the main components of the system, the humidifier and the dehumidifier. A unique HDH-PVT prototype was built and experimentally tested at the SolarTech Lab of Politecnico di Milano in Milan, Italy. The experimental system is a Closed Air Closed Water—Water Heated (CACW-WH) that mimics a Closed Air Open Water—Water Heated (CAOW-WH) cycle through brine cooling, pure water mixing, and recirculation, avoiding a continuous waste of water. Tests were performed varying the mass flow ratio (MR) between 0.346 and 2.03 during summer and autumn in 2023 and 2024. The experimental results enabled the verification of the developed models. The optimal system performance was obtained for an MR close to 1 and a maximum cycle temperature of 44 °C, enabling a 0.51 gain output ratio (GOR) and 0.72% recovery ratio (RR). The electrical and thermal energy generation of the PVT modules satisfied the whole consumption of the system enabling pure water production exploiting only the solar resource available. The PVT-HDH system proved the viability of the proposed solution for a sustainable self-sufficient desalination system in remote areas, thus successfully addressing water scarcity issues exploiting a renewable energy source.

The desiccant air conditioning system has multiple advantages (e.g., no use of ozone-depleting refrigerants, highly efficient moisture control, easy regenerative integration) over traditional vapor-compression refrigeration systems, thus increasingly attracting more research interest. Recently, several studies have been conducted that primarily aimed to enhance the overall performance of desiccant air conditioners by innovating new desiccant materials, innovating new system configurations and improving system designs and controls, and integrating different hybrid energy sub-systems technologies. Therefore, this paper provides a comprehensive review of the studies mentioned earlier. The present comprehensive review dealt with several axes: first, an overview of the importance of using desiccant air conditioners and their operations, and performance indicators. Second, a summary statement for desiccant materials that includes: the new innovative desiccant materials and the most important composite desiccant materials. Third, detailed information on the newest innovative designs and configurations of desiccant air conditioning systems and their control systems. Fourth, a detailed statement on the most important hybrid energy sub-systems technologies integrated with desiccant air conditioners. Based on the latest developments in desiccant air conditioning systems, this study presents discussions of urgent issues and recommendations for future work that can help focus necessary efforts to find solutions to critical and pending problems, which lead to further improvements in the overall performance of desiccant air conditioners.

Solar desalination systems are a promising solution to the water scarcity problem since the majority of the earth’s water resources are salty. With the increasing focus on desalination research, many innovative methods are being developed to extract salts from saline water. Energy consumption is a significant concern in desalination, and renewable energy, particularly solar energy, is considered a viable alternative to fossil fuel energy. In this review, we will focus on direct and indirect solar desalination methods, specifically traditional direct solar desalination methods such as solar still and humidification dehumidification (HDH) desalination systems. We will also briefly discuss a recent advancement in the desalination method known as the fogging process, which is a development of the HDH desalination system.

Desalination has emerged as a vital solution to tackle global water scarcity, and among various desalination methods, the humidification–dehumidification cycle has garnered significant attention. This study provides a detailed review of performance enhancement techniques utilized in the humidification–dehumidification desalination systems. The review highlights previous studies conducted in this area. The humidification–dehumidification cycle, its working principles, and its advantages and drawbacks are then thoroughly discussed, establishing the groundwork for comprehending the significance of performance improvement strategies. The investigated techniques for multiple facets of the humidification–dehumidification cycle are multistage humidification–dehumidification, bubble columns, phase change materials, nanofluid application, variable pressure humidification–dehumidification, ultrasonic, and other techniques such as thermoelectric application, rotating belt, multiple insert, air saturator, fogging, and desiccant wheel. Through an extensive analysis of existing studies, the review summarizes the key findings of each technique, including improvements in system efficiency, freshwater production rates, heat and mass transfer rates, thermal energy storage, and reduced fouling. The findings provide valuable insights for researchers and practitioners, facilitating the selection and implementation of the most suitable techniques to achieve sustainable and efficient water desalination.

Freshwater scarcity in remote and arid regions necessitates desalination systems that are not only productive but also lightweight, low-cost, and easy to deploy. In this study, a transient numerical model of a hybrid solar still–humidification–dehumidification (SS–HDH) desalination system is used to perform a comprehensive parametric analysis aimed at minimizing system weight while maintaining a target freshwater productivity. Key operational parameters (basin water depth and air mass flow rate) and design parameters (basin material and thickness, glass thickness and material, and absorber material) are systematically investigated under summer and winter climatic conditions. The results show that reducing basin water depth to 0.5 cm and air mass flow rate to 0.1 kg s⁻¹ increases total freshwater productivity by up to 15% compared to the baseline configuration while enabling operation under natural convection. Replacing a stainless-steel basin with black-coated cotton reduces the total system mass from 486.6 kg to 131.7 kg (≈ 73%) with negligible impact on productivity, while minimizing basin and glass thickness further decreases weight without affecting thermal performance. Using aluminum fins and glass covers remains preferable to preserve high productivity, whereas plastic covers and cotton absorbers cause productivity reductions of 10–15%. An optimized configuration increases freshwater production by 31% in winter and 26% in summer relative to the reference case. These results demonstrate that selecting optimal parameters can significantly enhance portability and efficiency, enabling practical deployment of hybrid solar desalination units in off-grid and resource-limited communities.

Water scarcity remains a pressing global issue, particularly in arid and coastal regions, necessitating the development of energy-efficient and sustainable desalination technologies. This study presents a novel humidification-dehumidification desalination system enhanced by the M-Cycle, which utilizes ambient thermal energy to minimize power consumption and eliminate reliance on fossil fuels. A coupled computational approach involving CFD simulations and PSO was employed to model and optimize system performance. The CFD analysis captured detailed temperature and humidity distributions across the heat and mass exchanger, while the PSO algorithm optimized three key parameters: air velocity, heat exchanger width, and dry air split ratio. Notably, the optimized configuration—an airflow velocity of 6.89 m/s, a device width of 0.63 m, and a 35.97% dry air split—resulted in a freshwater production rate of 3.04 kg per kWh of electricity consumed, 360.5 L per day, and an outlet relative humidity of 99%, with total power consumption of 4.94 kW. Compared to previous M-Cycle-based desalination systems, the proposed configuration demonstrates over a threefold improvement in energy efficiency, as measured by the GOR, highlighting its superior thermodynamic performance. Additionally, the study uniquely resolved the spatial distribution of relative humidity and evaporation rates, without relying on simplifications such as assuming saturated air at the outlet. The results confirm the potential of the proposed M-Cycle HDH system as a highly efficient, scalable, and eco-friendly desalination technology suitable for decentralized and off-grid applications.

A hybrid Multi-Stage Flash–Humidification Dehumidification (MSF-HDH) desalination system is investigated for energy recovery from an MSF system. The hybrid MSF-HDH system increases total productivity and performance ratio and reduces brine rejection. Hot condensed steam that leaves the MSF brine heater is used to warm the rejected pretreated brine from MSF to a higher temperature suitable for HDH system operation (about 60 °C). This allows us to increase the product (desalinated water) without additional “external” energy input to the hybrid system. Four different layouts of the integrated MSF-HDH system are presented and compared. The results show that an HDH system can utilize over 66% of an existing MSF brine blowdown, while the hybrid system can achieve a gained output ratio—GOR, water recovery ratio—RR, productivity and freshwater cost of 8.73, 44.86%, 30,549 m3/day and 1.068 $/m3 of freshwater, respectively. Utilizing 66.96% of MSF brine blowdown by the HDH system leads to a daily HDH productivity of about 670 m3 of drinking water, which is enough to support 134,000 persons considering a daily consumption of 5 L of drinking water per person.

Poly-generation systems are an exciting new technology that provide an alternative to separating existing energy production methods in buildings. A poly-generation system enables the efficient simultaneous production of heating, cooling, fresh water, and electricity, resulting in many technological, economic, energy recovery, and environmental advantages. This study numerically investigates three proposed novel solar-driven poly-generation systems (BS, IS-I, and IS-II) integrated with organic Rankine cycle (ORC), humidification-dehumidification desalination system (HDH), and desiccant cooling system (DCS) with different heat recovery system arrangements. The suggested systems supply residential structures with energy, space conditioning, domestic heating, and fresh water. The effects of system operating circumstances on productivity and performance characteristics and several organic working fluid types (n-octane, R245fa, R113, isopentane, and toluene) on optimum system performance have been investigated. The results show that (i) the average enhancement percentage of TGOR using integrated poly-generation systems over the separated ones is 68.5%, 68.5%, and 95.5% for BS, IS-I, and IS-II systems, respectively; (ii) when comparing the three systems, the IS-I system outperforms the other systems (BS & IS-II); and (iii) the maximum values of W•net, m•fresh, Q•cooling, and Q•heating, obtained for different proposed systems using n-octane are 102 kW (all systems), 214.7 kg/h (IS-II), 29.94 kW (IS-II), and 225.6 kW (IS-I); (iv) R113 has the highest TGOR of 0.6924 (IS-I) compared to other organic fluids. (v) The improvements in Wnet•, mfresh•, Qcooling• and Qheating• with using toluene instead of R113 at tf1 = 40 °C are 177.5%, 105.8%, 389.25%, and 79%, respectively.

The humidifier plays a key role in a humidification–dehumidification (HDH) desalination system; it directly affects both the freshwater production efficiency and energy consumption ratio of the system. In this study, for a humidifier in an HDH system, a heat–mass coupled differential equation model of spray water and air on the surface of the packing material was established, and the effects of parameters such as the spray water temperature (tw), mass flow rate of spray water (mw), air temperature (ta), and air mass flow rate (ma) on the humidification performance of humidifiers composed of eight different types of packing materials were investigated. The results show the following: (1) Under the same inlet spray water and air conditions, the humidification performance of different packing materials from good to bad is as follows: cellulose paper, polypropylene, hackettes, saddles, snowflakes, wooden slats, polyvinyl chloride, gunny bag cloth. (2) Increasing the tw can significantly improve the humidification performance. To achieve higher humidification energy efficiency, it is recommended to increase the tw to above 80 °C. (3) With the increase in the mw, although the humidification efficiency (εhum) decreases slightly, the humidification rate (mhum) increases, and the specific humidification energy ratio (ηhum) decreases accordingly. To maintain a high mhum and a low ηhum, it is advisable to control the mw at not less than 0.5 kg/s. (4) Increasing the humidifier inlet ta can improve the mhum, εhum, and ηhum, although not as effectively as increasing tw. (5) Increasing the ma can improve mhum and εhum. However, it simultaneously increases the ηhum. The results of this study can provide theoretical guidance for the selection of efficient packing materials and the optimization of humidifier operating conditions in HDH desalination systems.

This study investigates the utilization of a sodium polyacrylate (SP)/CaCl2 composite as an adsorbent in a low-grade-heat desalination configuration designed for Saudi Arabian conditions. A dynamic system model was developed and validated for an adsorption desalination (AD) cycle integrated with a dual-ejector and a humidification–dehumidification (HDH) unit. Two operating modes were evaluated, including a production-oriented configuration that applies internal evaporator–condenser heat recovery (HR) when no cooling effect is required. Without HR, the AD–EJ–HDH system achieves 41–56 m3/ton·day SDWP and 2.6–2.9 GOR, with a freshwater cost of 1.8–2.4$/m3 under solar driving and 0.70–0.90 $ /m3 under waste heat. With HR, performance increases to SDWP 95–155 m3/ton·day and GOR 2.9–3.1, while costs decrease to about 1.34$/m3 (solar) and 0.38 $ /m3 (waste heat) in June. The SP/CaCl2 composite yields about 85% higher freshwater production than silica gel in the same system, highlighting the material’s potential for high-output hybrid adsorption desalination in hot-climate regions.