Applications of different types of heat pipes in solar desalinations: A comprehensive review Open Access

For human societies, supply of fresh water is vital due to the reliance of the societies on it for survival and development (Zheng 2017). Although the majority of the earth, more than 70%, is covered by water, there is fresh water scarcity in different regions of the world since more than 95% of water is saline or in brackish form that means it is useless for drinking and agricultural purposes and only less than 1% of water resources are reachable for human (Tiwari et al. 2003; Chauhan et al. 2021). Owing to some facts like uneven distribution of fresh water in the world, requirement of water for human activities and survival, and population growth, there is increasing trend in the importance of desalination (Alhuyi Nazari et al. 2021). It should be noted that desalination process is energy consuming and different systems and configurations have been proposed for this purpose by use of various energy technologies (Assad et al. 2020; Shakouri et al. 2021). This energy requirement can cause some problems and issues in supply of required energy and emission of harmful gases for the environment. Current technologies used for desalination consuming fossil fuels would likely to produce water with high emission of carbon and economic cost (Zheng et al. 2021). In this regard, it is crucial to apply clean and energy sources and alternative technologies in a sustainable way to have secure supply of fresh water for human beings' needs and requirements.

Interest in clean and alternative energy sources is sharply increasing in the world (Taner 2021). Renewable energies, as alternatives for fossil fuels, can be used for desalination process in direct or indirect way (Anand et al. 2021; Vaithilingam et al. 2021; Kumar et al. 2023; Mehtari et al. 2024). In direct cases, renewable energy sources are generally applied in thermal systems to supply required heat for evaporation of saline water and salt removal. These systems have been investigated by different scholars (El Haj Assad et al. 2022). In a study by Ahsan et al. (2014), parameters that affect the performance of a low cost solar still were evaluated. They reported that for their considered case, daily water productivity is inversely proportional to the water initial depth in the systems. Moreover, it was noted that productivity of the still was approximately proportional to the daily solar radiation. In some studies, effect of modifications on the performance of solar desalination systems has been investigated. For example, in a study by Bataineh & Abbas (2020), impact of adding internal reflectors to the sides of solar still was investigated. They reported that adding reflectors can improve the efficiency. Another technique for performance modification of solar desalination systems is making use of storage unit (Adamu et al. 2023). In addition to solar, other renewable energy sources with thermal content are applicable for desalination purpose in direct way. In a study by Hu et al. (2023), performance of a distillation desalination technology driven by ocean and solar thermal energy was investigated. They reported that in case of applying ocean thermal energy for driving the desalination system, up to 0.7 kg/h minimum freshwater productivity was achievable while coupling it with the solar thermal energy lead to improvement in the freshwater yield. Furthermore, they observed that the proposed desalination by use of both solar and ocean thermal energy leads to 30.1% more freshwater yield compared with the single solar desalination. Aside from the direct desalination by use of renewable energies, it is possible to indirectly use renewable sources. In these systems, renewable energies are applied for power generation to supply the electricity needed for the desalinations systems, e.g. Reverse Osmosis (RO) systems (Gorjian et al. 2020). Performance and feasibility of these systems have been investigated by different researchers in recent decades. For instance, Ghafoor et al. (2020) implemented a techno-economic feasibility study on the solar-based desalination system composed of RO systems. They reported that increment in the temperature of feed water leads to elevation in the membrane productivity. Moreover, they noted that payback period for their case study in Pakistan was 1.83 years. In a study by Shafaghat et al. (2023), performance of a RO desalination system equipped with PV/thermal collectors for Shiraz in Iran was investigated by consideration of stand-alone and grid-connected installation. They reported that in comparison with simple grid-connected RO system, proposed system decrements emission of CO₂ by 79% and 85.7 in stand-alone and grid-conned installation, respectively. In these systems, it is possible to use more than one renewable energy technologies, as hybrid systems, to produce power and use it in RO desalination (Mito et al. 2019). Performance of these systems have been studied in different research works (Jiang et al. 2022). For instance, Elmaadawy et al. (2020) implemented a study on the optimal sizing and feasibility assessment of a RO system driven by hybrid renewable energy systems by consideration of technical, environmental and economic criteria. They reported that CO₂ emission from the optimal system outperforms a diesel system by 81.5%.

Heat pipes (HPs) are very efficient thermal devices operating based on a two-phase heat transfer mechanism. Effective thermal conductivity of HPs is much higher than conductive metals (Ramezanizadeh et al. 2019) which makes them as promising technology for heat transfer and attractive alternatives for conventional thermal mediums. HPs have been used in variety of clean and renewable energy systems for thermal management or as thermal medium to transfer energy from heat source to heat sink. As one of the most conventional application in renewable energy systems, HPs are usable in solar collectors to transfer absorbed thermal energy to the working fluid. In these systems, evaporator section of HPs are located in the absorber of collectors and heat is transferred to the condenser of the HP for the intended purpose (Alshukri et al. 2021, 2022). HPs are applicable for cooling of renewable energy systems need thermal management for performance enhancement. For instance, Alizadeh et al. (2018) proposed an approach for thermal management of solar PV cells by use of oscillating HPs to reach higher efficiency. In the proposed system, HPs were installed at the back of PV cell to transfer thermal energy from the cell in order to reduce its temperature and improve efficiency. Zhang et al. (2013) proposed a solar PV systems integrated with HP heat pump for water heating purpose. In their system, PV/looped HP was applied and the HP was used to transfer the absorbed thermal energy to the water tank. In another study (Mathioulakis & Belessiotis 2002), a solar domestic water heating system based on HP was introduced. In the proposed configuration, wickless loop HP was applied to transfer heat from the collector-evaporator to the tank via heat exchanger-condenser. In addition to the abovementioned applications, it is possible to use HPs to accelerate thermal energy transfer from the storage unit. Motahar & Khodabandeh (2016) applied HP in Phase Change Material (PCM) and reported dramatic enhancement in rate of both solidification and melting by use of HP. It is possible to use HPs in solar desalination systems both for transfer heat from collector or storage unit in order to modify the overall performance which is the topic of this article. HPs have been applied in different systems for desalination purposes. For instance, Zhang et al. (2018) proposed a thermal-driven desalination system based on low-grade waste heat with use of open loop HP.

As it was mentioned, it is necessary to develop desalination systems, particularly in regions water crisis. In regard to energy requirement for the desalination process and issues related to the fossil fuels, employment of renewable energy sources would be inevitable. Solar energy is one of the most appropriate renewable sources for water desalination and HPs are one of the most important components in some of the solar desalination systems. In this regard, it is useful to get deep insight into the applications of HPs in solar desalination systems and the factors affecting their performance. The main focus of the present review article is on the applications of HPs in solar thermal desalination systems. In the following section, different types of desalination systems are introduced and their classifications are represented. Afterwards, the use of HPs in solar desalination systems is reviewed. Finally, some suggestions are recommended for the future studies on the similar technologies and systems.

In general, desalination is divided into three main processes on the basis of the applied technology for freshwater production, as shown in Figure 1. In the evaporation and condensation technologies, thermal energy is employed for vaporizing water and then condensation of vaporized water to produce freshwater. The required energy for this purpose is provided by utilizing heat from a thermal process or via a mechanical process. In filtration technologies, a semipermeable membrane is used in general to separate salt from water. In the category known as crystallization, freshwater is extracted by producing ice as an intermediate product (Curto et al. 2021). There are some requirements for these processes such as feedwater pretreatment to decrease fouling, management of brine, water post-treatment production prior to its distribution and optimization of energy utilization and production of water (Kress 2019). The most important factors determining desalination process choice are the operation and capital cost in addition to the plant capacity (Fellows & Al-Hamzah 2015). The majority of the desalination plants in the world are RO type; however, in some regions of the world such as the Middle East, thermal technologies are used in larger scale compared with RO (Mahmoudi et al. 2023).

In this study, the main focus is on solar-based desalination systems, which are a subbranch of thermal processes. It should be noted that aside from the technical factors that are related to the specifications of the applied technology, quality of input water plays key role in the obtained water quality (Jalihal & Venkatesan 2019). Raw water classification is provided in Table 1.

Evacuated Tube Collectors (ETCs) are composed of a HP inside a tube that is vacuum sealed. Liquid-vapor phase change is used in ETCs in order to transfer thermal energy at high efficiency. The material of applied pipe which is made of copper is attached to a black copper fin, which fills the tube and creates the absorber plate of the collector. Each tube is ended at a tip made of metal that is connected to the sealed pope and works as the condenser. The thermal content of solar beams causes evaporation of the liquid, which rises due to the lower density and condensed there by heat dissipation. Afterwards, the condensed fluid is returned to the solar collector due to the gravity impact (Kalogirou 2016). Schematic of an ETC is illustrated in Figure 2. It is found that making use of HPs in ETC leads to higher temperature and efficiency output (Chopra et al. 2018). This cycle is repeated till there is solar energy as driving force in thermal format. In majority of the cases, water or water/ethylene glycol moves via the manifold to receive the thermal energy from the vapor condensation (Kalogirou 2016). In addition to the gravity assisted HPs, also known as thermosyphon, wick HPs can be used in these collectors (Kumar et al. 2017). This absorbed heat can be used as thermal source for different purposes, i.g. thermal desalination. It should be noted that in addition to direct use of produced thermal energy by ETCs for desalination, it is possible to apply the absorbed heat for driving heat-powered engine pump and use it in RO desalination (Takalkar & Bhosale 2020).

Operating fluid is among the most important factors in the performance of various kinds of HPs (Ramezanizadeh et al. 2018). In this regard, the overall performance of systems with HPs can be affected by the operating fluid. In a study by Fallahzadeh et al. (2020), performance of a Conventional Pyramid-shape Solar Still (CPSS) was compared with a Modified Pyramid-shape Solar Still (MPSS). In the MPSS, CPSS was integrated with HP solar collector in which five thermosyphons were installed in five glass evacuated tubes. For the MPSS, two operating fluids namely water and ethanol were tested in the HPs in three filling ratios including 30, 40 and 50%. They found that employment of HP solar collector leads to elevation of temperature difference between the basin water and glass cover by 30% and as a consequence, productivity improved by 62%. Furthermore, it was observed that the best performance for both working fluid is obtained in case of 40% filling ratio as shown in Figure 3. It was noted that making use of water in the HP can provide the best performance. In addition to productivity, exergy analysis was implemented and reported that using the collector increments the highest hourly exergy around twice that was attributed to the decreased thermal energy loss due to the existence of vacuum gap between the evacuated tubes two glass surfaces.

Solar has an intermittent nature and it is necessary to apply a storage unit to have active systems in night hours or daytimes with low radiation intensities. Thermal Energy Storage (TES) units can be applied in solar thermal desalination systems to extend its operating hours and overall yield. In these cases, it would be useful to employ HPs to improve heat transfer from the storage unit to water for being desalinated. In a study by Khalilmoghadam et al. (2021), a solar still was designed with latent heat storage unit. In the proposed system, the total solar radiation was used for heating the saline water and just the thermal energy from the vapor condensation was stored in the storage unit during the day. The dissipated thermal energy from the body of condenser was transferred to the storage and stored. After sunset, the stored thermal energy in the storage was transferred to the saline water by making use of HPs, oscillating type. Employment of this system causes prevention of water temperature drop that ensures the continue of desalination process after sunset. They reported that by using the proposed design, efficiency of the system increases to 48.5% while it is 23.7% for the conventional solar still. For their case study, in Iran, cost per liter for freshwater production was 0.0093$/l/m².

It is possible to have modified solar thermal desalination systems with storage unit by applying some modifications and adding some components. In a study by Faegh & Shafii (2017), a novel solar still was designed by use of external condenser that was filled with PCM (Figure 6). The system was designed in a way the contact between PCM and saline water was prevented to utilize the total radiated energy for evaporation of saline water during daytime. The produced vapor in this period, daytime, was conducted to a PCM-filled external condenser to be condensed. Vapor wasted heat in the condenser was absorbed by the PCM and stored. Consequently, PCM energy storage was just via the vapor and solar radiation was not directly used for storage. By installation of thermosyphon as thermal diode between the saline water at upper position and PCM tank at lower position, unidirectional heat transfer became possible. Consequently, in cases of this sort of solar radiation, in which saline water temperature is lower than PCM, heat was transferred from the PCM to water and it caused the desalination process to continue in the absence of solar radiation. They reported that by applying the external condenser in absence of PCM and insulating the solar still cover glass jointly results in 56% increase and utilization of PCM-filled external condenser leads to 86% increment in water yield.

Flat plate solar collectors are other type of collectors that have been applied for variety of solar thermal applications. It would be possible to apply flat plate solar collector with HPs as additional unit to improve performance of solar still. In a study by Kargar Sharif Abad et al. (2013), a novel configuration composed of two sections, solar still and flat plate collector with oscillating HP was proposed. Although Cost Per Liter (CPL) of the active system, with use of HP, was just 8% higher than the passive system without HP, its production rate was much more than passive type by around 75%.

Aside for the mentioned configurations, it is possible to integrate solar desalination systems with other thermal components and equipment to have improved production. In a study by Rastegar et al. (2020), two solar desalinations, namely passive and active were compared. In the active configuration, thermosyphon HPs were used to recover heat from hot exhaust gases. Hourly variation in the distilled water production rate at various day time revealed that active system is around 2.06 times higher than the passive systems. The mean exergy and thermal efficiency of the solar still in active mode was improved by 41% and 65.5%, respectively, in comparison with the passive systems. According to the economic analysis, mean CPL value for the active system is 32% higher than the passive system. At the base case, exergoeoconomic parameters for the active and passive types were 0.25 kWh/$ and 0.32 kWh/S, respectively, while at the worst case, these values were 0.11 kWh/S and 0.12 kwh/$, respectively.

In Table 2, the reviewed works are summarized.

In section 3, the research works focus on the desalination systems utilizing various types of HPs were reviewed. There are some points and ideas that can be considered in the future studies. For instance, owing to the significant role of working fluids on the performance of HPs and consequently the overall performance of the solar desalination systems, it would be useful to evaluate more operating fluids. Studies have shown that there is high potential for nanofluids to improve performance of various HPs (Su et al. 2015; Nazari et al. 2019). In this regard, nanofluidic HPs can be applied in solar desalination systems to have higher productivity. In addition to the operating fluid, structure of HPs, i.g. channels geometry and connections in oscillating HPs (Ebrahimi et al. 2015), are influential on the performance. It is suggested to apply HPs with modified structures in the solar desalination systems in upcoming works. Wick HPs, due to the assistance of capillary force in the fluid movement, are less sensitive to the orientation and could be attractive options for the solar desalination with collectors installed in near horizontal orientation. It should be noted that there are various structures for wick of HPs (Khalili & Shafii 2016) and it is possible to use HPs with modified wick to have better overall performance. Aside from the mentioned factors, dimensions of the HPs and lengths of different sections, i.g. adiabatic, condenser and evaporator, are involved in the performance of HPs. It can be a useful idea to consider different lengths and dimensions to reach the optimal case by consideration of technical and economic factors. Moreover, regarding the applicability of HPs for heat recovery systems (Abdelkareem et al. 2022), more cases can be considered for heat recovery to use the recovered energy in the solar thermal desalination systems. In cases of applying HPs in thermal energy storage unit of solar desalinations, it is possible to consider making use of fins for heat transfer improvement. It should be mentioned that PCMs' heat transfer with finned HPs can be affected by different factors such as fins' length and spacing (Tiari et al. 2015) that should be considered in the design process to have the most efficient system.

Benefits in use of renewable energy sources instead of fossil fuels are motivator for transition towards use of these sources in different energy consuming processes such as desalination. HPs are among the efficient thermal devices with applicability in solar thermal desalination systems. In this study, solar desalination systems with HPs are reviewed and the key findings can be outlined as follows:

• HPs could be used in the collectors, heat recovery units and thermal energy storage components of the solar desalination systems.

• Overall performance of solar desalination systems is affected by the characteristics of applied HPs and their thermal performance.

• Gravity-assisted or thermosyphon HPs are proper devices for being used in the storage unit of solar thermal desalination regarding their ability in provision of unidirectional heat transfer.

• Employment of HPs with collectors in a solar still can lead to improvement in the productivity.

• Adding external condenser for the solar desalination systems could provide much more productivity.

• Improvement of heat transfer between the collector and HPs by using some techniques such as adding oil in the gaps between the tube and glass would improve the overall performance.

This study was supported by the Prince Sattam Bin Abdulaziz University through project number 2023/RV/19.

H.B. Bacha: Writing original draft, analysis, supervision. M.A. Nazari: Writing original draft, finding references. Ullah and Shah: Revision, data gathering.

All relevant data are included in the article.

The authors declare there is no conflict.