Recent technological advancements in membrane distillation and solar stills: preheating techniques, heat storage materials, and nanomaterials — a detailed review

Freshwater and energy are critical components for the growth and progress of societies. The scarcity of freshwater and rapid population growth, especially in remote countries, has led to an urgent need to develop desalination technologies in order to raise its productivity and reduce its energy consumption rates. Membrane distillation is one of the effective methods characterized by its high productivity, but its disadvantage by higher electricity consumption. Also, solar stills are one of the sustainable and economical technologies, but the disadvantage by lower productivity. Accordingly, this manuscript dealt with a comprehensive review and detailed comparison of the most important modifications and innovations that were made to the design of the membrane distillation units, which aim to reduce electricity consumption rates, as well as the design of solar stills, which aims to maximize the productivity and efficiency. This was done by providing a detailed comparison of the most important three axes of modifications and innovations that were addressed by recent previous studies on the design of membrane distillation units and solar stills, and their statement as follows: preheating technology, use of the thermal storage materials, and nanomaterials technology. Finally, based on this review, the authors make some recommendations for future work in the field of solar and membrane desalination.


Introduction
Water is the most valuable and vital natural resource of all species. Fortunately, natural water is the most abundant compound on Earth, covering about 71% of the Earth's surface (USGS 2019). Despite this, natural freshwater resources around the world are very scarce (Mekonnen and Hoekstra 2016). This is because about 97% of the water on Earth is in the oceans with a salinity range of 3.5-5%. Only 2.5% of the world's total water resources are fresh; and only 31.4% of them can be accessed to support life on Earth (Powers 2017). Freshwater resources are available either as groundwater in the ground or as surface water in rivers, lakes, etc. In recent years, the stress on the available freshwater resources has been exacerbated as a result of many factors, the most important of which are: population growth, climatic changes, urban expansion, and changing lifestyles and prosperity. About 2.1 billion people during the current time period lack access to safely managed drinking water (UN-Water 2018; Xiao et al. 2021;Altarawneh et al. 2020). It is expected that by 2030, about 700 million people will be displaced worldwide due to the scarcity of fresh water (Wei et al. 2021;Al-Otoom and Al-Khalaileh 2020;Sharon et al. 2020). To overcome the problem of fresh water scarcity, some strategies have been developed that aim to reduce the demand for fresh water by implementing conservation practices or adjusting prices. However, previous strategies to mitigate fresh water demand do little to provide optimal solutions in highly affected regions (Gude 2017;Pourafshar et al. 2020;Siddiqui and Dincer 2018). On the other hand, the use of desalination technologies has enabled us to meet the increasing demand for fresh water by making use of the abundant reserves of brackish water and/or seawater.
Among the available desalination techniques, the solar distillation system is an effective procedure that relies on solar energy to produce fresh water, as it is built using locally available materials and also enjoys preserving depleted energy sources by using renewable resources (solar energy), which is clean and environmentally friendly energy (Manchanda and Kumar 2018;Velmurugan and Srithar 2011). Membrane-based desalination also has some advantages such as modularity, compactness, and sometimes the use of solar energy, thus preserving depleted energy sources by using renewable resources (solar energy), which is clean and environmentally friendly energy compared to multi-effect distillation (MED), thermal vapor compression (TVC), mechanical vapor compression (MVC), multi-stage flash distillation (MSFD), electro dialysis (ED), reverse osmosis (RO), freezing, and humidification and dehumidification (HDH) (Velmurugan and Srithar 2011). Despite the advantages of solar stills, the productivity is lower compared to the traditional desalination system. Membrane distillation (MD) has a lot of advantages such as it is operating with low-grade thermal energy and low pressures and large contact area; however, it also has some disadvantages especially high-power consumption which made it essential to reduce the power consumption and increase the efficiency of MD to make it even more practical. As a result of the high energy consumption of MD, it was found that it urgently needs renewable energy sources, heat storage mediums, and new configurations in membrane modules to reduce the required energy and improve the efficiency of MD (Gonzáleza et al. 2017). Ding et al. (2005) analyzed a solar-powered membrane distillation system; results have shown that the plant capacity in June can reach about 300 kg/day with solar energy. Another effective way of water desalination in remote areas is using solar stills. Solar stills use the radiation from the sun to evaporate water in a basin and then the water vapor condensate and accumulate as pure water. Solar stills productivity is affected by different factors like wind speed, solar rays, ambient temperature, glass-water temperature difference, absorber area, water-free surface area, water inlet temperature, basin water depth, and glass angle. The solar intensity, wind velocity, and ambient temperature cannot be controlled as they are meteorological parameters, whereas the remaining parameters can be controlled to improve productivity (Sivakumar and Sundaram 2013). Therefore, a great effort has been made in recent years to develop solar desalination systems, membrane-based desalination systems, as well as research into sustainable and energysaving methods for fresh water production.
The rates of total energy consumption of desalination units depend on the design of the plant, the type of desalination method, the salinity of the feed water, and the temperature of the feed water. Thermal methods of water desalination require thermal energy and electrical energy, which makes their energy consumption higher than membrane methods that require only electrical energy to desalinate water. Therefore, the rate of total Fig. 1 Heat transfer mechanisms in direct contact membrane distillation system energy consumption required to desalinate 1 m 3 of fresh water varies according to the desalination method used, where the total energy consumption required for MED system ranges between 14.45 and 21.35 kWh/m 3 , for MFS system between 19.58 and 27.25 kWh/m 3 , for TVC system 16.3 kWh/m 3 , for MVC system between 7 and 12 kWh/ m 3 , and for ED system it is between 0.7 and 5.5 kWh/m 3 (Maleki et al. 2016;Okampo and Nwulu 2021), while the total energy consumption for the RO system ranges between 2 and 4 kWh to produce one cubic meter of fresh water (Maleki et al. 2016;Okampo and Nwulu 2021).
The present manuscript dealt with two very important axes. The first axis dealt with the comprehensive review and a detailed comparison of the most important modifications and innovations that have been made to the design of membrane distillation units, which aims to reduce energy consumption rates as the following: feed water preheating technology, use of the thermal storage materials, and nanomaterials technology. Also, the second axis dealt with a comprehensive review and detailed comparison of the most important modifications and innovations that were made to the design of solar stills,  Finally, based on this review, the authors make some recommendations for future work in the field of solar and membrane desalination.

Overview
During the last few years, membrane desalination showed a high capability to produce distilled water. Membrane distillation is a dual technology that combined the advantage of thermal distillation and membrane separation (Anvari et al. 2020). Membrane distillation (MD) is classified according to its operational method into; direct contact membrane distillation (DCMD), sweeping gas membrane distillation (SGMD), air gap membrane distillation (AGMD), and vacuum membrane distillation (VMD) (Anvari et al. 2020). The advantages of membrane distillation over other desalination processes are concentration polarization, high rejection of non-volatile compounds, limited fouling, low operating temperature, and low operating pressure (Anvari et al. 2020). Therefore, membrane distillation technology has gained great attention for processes of desalination and treatment of wastewater (Anvari et al. 2020).

Heat transfer in MD process
The performance of membrane distillation technology depends on the temperature gradient between the feed side and the distillate side, which results in the transfer of heat from the hot side of the membrane to the cold side, and thus the water phase change from a liquid to a vapor state on the surface of the feed side of the membrane. As shown in Fig. 1, the heat transfer through the membrane occurs in three regions: the feed layer Q f , the membrane Q m , and the permeate layer Q p , calculated using Eqs. (1)-(3).
where; h f is the coefficient of heat transfer on feed side, h p is a coefficient of heat transfer on permeate side, T f is a feed temperature, T fm is a membrane surface temperature in feed side, T p is a permeate temperature, T pm is a membrane surface temperature in permeate side, k m is a membrane thermal conductivity, δ m is the membrane thickness, J is the membrane flux, and ΔH v is the evaporation enthalpy. The overall heat transfer through the three regions Q is calculated as follows: Coefficient of overall heat transfer H is calculated as follows: (1) Test rig of solar-powered membrane distillation with energy storage mediums  The membrane surface temperatures on feed and permeate sides T fm and T pm are calculated as follows:

Membrane distillation performance enhancement techniques
The most encountered problem in membrane desalination is that it requires high power consumption; a lot of research investigated the performance of membrane desalination with different techniques of improvement to reduce the power consumption and increase the productivity of the membrane distillation. Preheating the inlet water to the membrane is one of the most common methods. Preheating can be done by external heaters like electrical heaters, and it can take place by using solar energy to preheat the water inlet by using solar preheaters, which in turn can reduce the power consumption cost of the electric heater, and it is a renewable and cleaner source of energy . Another very effective way to improve the membrane performance is using thermal storage mediums especially, with solar-driven membranes as the thermal storage medium works as a heat supply for preheating the feed flow when the solar irradiance decreases . New configurations of membranes with nanomaterials can also be used to improve membrane performance (Elango et al. 2015).

Feed-water preheating technology
Different researches were done on preheating the feed water before its entering the membrane to increase the vapor pressure difference across the membrane sides which in turn increases the productivity. Shafieian and Khiadani (2019) studied experimentally and theoretically the behavior of the thermal-driven direct tubular contact membrane shown in Fig. 2 by using an electric heater to preheat the feed water before entering the membrane. They found that lower permeate temperature, as well as, higher feed water temperature results in higher freshwater production, and improves the freshwater production of the tubular DCMD unit. The system consists of three main loops including the solar heating loop, membrane feed loop, and membrane permeate loop. Three different operating cases were investigated (summer without  Water flux possessed by the CNT membranes tends to increase at first and then decrease with the increase of the CNT diameter in the investigating range cooling unit (Case I), summer with cooling unit (Case II), and winter without cooling unit (Case III)). They found that except for a few minutes in the morning, the solar collector was able to provide all required thermal energy to heat the feed water before entering the membrane unit. By adding permeate water cooling unit in the summer, the maximum production rate will be increased from 2.78 L/m 2 h in Case I to 3.81 L/m 2 h in Case II, as well as the overall efficiency of the system will be improved from 46.6% in Case I to 61.8% in Case II. Elzahaby et al. (2016) studied a direct contact membrane distillation system assisted by a cooling water tank and solar energy shown in Fig. 3. Effect of salt concentration, feed temperature, feed flow rate, the cooling temperature was investigated. They conducted that the water production rate increases with increasing the feed temperature; however, in this study, to avoid scale formation, the feed temperature is limited to 70 • C . Recently reported studies examining the effect of feed water preheating systems on the performance of membrane distillation systems are summarized in Table 1.

Membrane with thermal storage mediums
Thermal storage mediums are used in membrane desalination systems to store waste heat from any other process. It is often used with solar systems as it stores heat from the sun in daytime and use it as heat source for feed flow of membrane at night. Thermal storage mediums can be sensible like molten salt or latent like paraffin wax. Abdelgaied et al. (2020) built a test rig shown in Fig. 4 to improve a behavior of solar-assisted membrane distillation using the energy storage medium as paraffin wax. They conducted that the freshwater production rate is varying between 3.47 and 4.35 l/h at a feed flow rate of 16 l/min, respectively. Also, the gain output ratio reached 1.123 and 1.25 for 12 and 16 l/min feed flow rate, respectively. Recently reported studies examining the effect of thermal storage mediums on the performance of membrane distillation systems are summarized in Table 2.

Nanomaterials in membrane distillation
Nanomaterials can be described as a material that is in one dimension is less than 100 nm. At this size, material properties may change on the chemical and physical sides. Nanomaterials are characterized by a large surface area, more strength, and stability. Carbon-based nanomaterials and nano-zeolite will be discussed as they showed a remarkable improvement in water desalination.

Carbon-based nanomaterials
Carbon-based nanomaterials such as carbon nanotubes and graphene have been widely used for their important features like high surface area, high thermal conductivity, have a low thermal expansion coefficient, and high mechanical strength. These properties, especially the high thermal conductivity, tend to improve the performance of membrane productivity and salt rejection. Bhadra et al. (2013) utilized the carboxylated nanotubes which are more polar to improve the productivity of membrane distillation (MD), a sweep gas membrane with 1 L/min dry air was used and results showed a flux reaching 19.2 kg/m 2 with salt rejection 99%. Recently reported studies examining the effect of carbon-based nanomaterials on the performance of membrane distillation systems are summarized in Table 3.

Nano-zeolite in membrane distillation
Nanosized zeolites are crystalline microporous solids with physicochemical characteristics like those of micron-sized crystals (Mintova et al. 2016). Nanozeolite is characterized by a large surface area and easy shaping. Using nano-zeolite in desalination showed a promising future in increasing the productivity of   Table 4.

Overview
The use of solar distillers is one of the appropriate ways to address the problem of freshwater scarcity, but the main problem of solar distillers is the low rate of daily water production. The intensity of solar radiation has a direct impact on the productivity of solar distillers since the hot and dry climatic conditions characterize most of the remote regions that suffer from water scarcity and have a high solar intensity. The use of solar distillers can help these countries save drinking water. The performance of solar distillers is mostly affected by the rates of water evaporation and condensation on the glass surface, where the increase in the temperature difference between the basin water and the glass cover (condensing surface) helps to increase the water productivity of the solar distillers.

Theoretical analysis of solar stills
Thermal analysis of the solar distillers was found to be dependent on the co-efficient of internal heat transfer and efficiency. The formula of these parameters was discussed below (Baskaran and Saravanane 2021): The thermal efficiency η th of the solar distiller depends on the entirety of hourly productivity ṁ dw , latent heat of vaporization h fg , solar intensity I(t), and the projected area A of solar distillers which calculated as follows: Latent heat of vaporization h fg and entirety of hourly productivity ṁ dw calculated by the following Eqs. (10), (11): The co-efficient of evaporative heat transfer h ew calculated by: The co-efficient of convective heat transfer h cw calculated by: The co-efficient of radiative heat transfer h rw calculated by: where, ε w = ε g = 0.9 , T w is basin water temperature, T g is glass cover temperature, and P w and P g are partial vapor pressures at free water surface and glass cover, respectively.
h rw =

Solar still performance improvement
Solar stills use the heat from the sun's irradiance to evaporate the water and then condense it on a glass sheet to produce pure water. The main goal to improve the distillers performance is to increase the amount of heat absorbed by the solar still, this can be achieved by using thermal storage materials with high thermal conductivity to increase the absorbed heat and even store it; also preheating the feed flow to the solar still can improve the performance as it makes the process faster and requiring less sun irradiance; another way is to use the nanofluids due to high thermal conductivity of these materials.
Preheating the feed flow of solar still Badran and Al-Tahaineh (2005) investigated the effect of using solar heaters on the production of the solar stills, Fig. 5; they found that coupling of a solar collector with distiller improved the production by 36%, as the productivity with solar collector 3510 mL and without the solar heater was 2240 mL. Mahmoud et al. (2018) studied the effect of adding a solar concentrator to increase the heat directed to the solar still integrated with humidification-dehumidification desalination system shown in Fig. 6; results showed that with a concentration ratio of 4 and a Basin water height of 0.01 m, the system yield was about 16.3 kg/m 2 . Recently reported studies examining the effect of feed water preheating technologies on the performance of solar distillers are summarized in Table 5.

Solar still with thermal storage mediums
Thermal storage mediums are integrated with solar stills to increase the heat absorbed by the basin of the still and to work as a heat source at night. Dhivagar and Mohanraj   Fig. 7. The results conducted that the yield, energy, and exergy efficiency of graphite plate fins and magnet solar still (GPF-MSS) were increased by 19.6, 21.4, and 18.1%, respectively, compared with conventional solar still. Recently reported studies examining the effect of thermal storage mediums on the performance of solar distillers are summarized in Table 6.

Solar still with nanomaterials
Nano Al 2 O 3 Xia et al. (2016) conducted the influences of adding nano Al 2 O 3 and Ti O 2 on heat transfer rate; they found that that the thermal conductivity and dynamic viscosity of Al 2 O 3 and Ti O 2 nanofluids are both improved with the increase of  Table 9.

Conclusion
In addition to the importance of both membrane desalination plants and solar stills in the field of water desalination, this paper aims to provide a comprehensive review of the most important recent studies aimed at improving performance that was conducted on both membrane desalination plants and solar stills. The improvement axes that were carried out on both the membrane desalination plants and the solar stills were categorized according to three axes that are very effective and have a direct impact on the performance of the membrane desalination plants and the solar stills, which are as follows: feed water preheating technologies, thermal storage materials, and nanoparticles. This survey focuses on showing the impact of the previous improvement axes on pure water productivity, energy efficiency, and the cost of producing pure water. The most important results can be outlined as follows: • The contribution of solar heat and waste heat used in the operation of the process leads to a lower cost of water production as well as making the desalination system more competitive, sustainable, and economically viable for small and remote applications.
• Permeate pure water through the membrane was increased by 52 g/m 2 /min for increasing the feed water temperature from 30 to 60 °C. • Using waste heat and solar thermal energy reduced the cost of pure water produced from the membrane distillation plant from 6.80 $/m 3 to 1.6 $/m 3 compared to a membrane distillation plant that operated with standalone grid electricity. • The overall system efficiency of the membrane distillation plant improved from 46.6% to 61.8% for utilizing the pre-cooling unit on the permeate flow loop before entering the membrane unit.
• The pure water productivity of the membrane distillation system will be improved by a rate varying between 33.11 and 43.18% compared to cases without PCM.
• Use of thermal storage materials improved the cumulative yield and the gain output ratio of membrane distillation units by a rate up to 43.2% and 34.4%, respectively.
• With an increase in the amount of MWNTs in the membrane, the water flux increased from 14.86 to 28.05 L/m 2 .h, while the salt rejection decreased slightly.
• For utilizing 0.5 wt.% nano-zeolite, the salt rejection reached 99.52% and the water flux increased by 34.2%.
• Using the solar collector as a feed water preheating unit is the effective choice that increases the pure water productivity and energy efficiency with rates reaching 40.98% and 57.4%, respectively, and reduces the cost of producing pure water to a rate reached 0.0102 $/L. • The utilization of nanofluid improved the cumulative productivity of solar stills by a rate up to 116%, and also the use of thermal storage materials (PCM) improved the cumulative productivity of solar stills at a rate of 105.5%.

Recommendation and future scope
In the processes of reducing the electricity consumption rates in membrane distillation units, various modifications were shown in many of the recent studies that were conducted, all of which aim to reduce the electrical power consumption rates, but there is still room for brainstorming. Also in the processes of enhancing the cumulative yield and raising the efficiency of solar stills, various modifications were presented in many of the researches that were conducted, all of which aim to enhance the cumulative yield and raise the efficiency of solar stills, but there is still room for a brainstorm. The following are the most important recommendations that could be useful for further study, correction, and modification of membrane distillation units and solar stills.
• The heat released from the waste outlets in industries and exhaust of the engine is the most effective choice as a feed water preheating unit to improve the performance of membrane distillation units.
• Development of the new combination of the membrane distillation and solar still to achieve the highest performance and lower cost.
• Incorporating high-performance nanomaterials, thermal storage materials, and feed water preheating units in the water treatment process are good choices to achieve the highest performance and lower cost.
• Future studies are also required to be focusing more on the effects of novel combinations of effective modifications in order to determine the best designs of the solar stills that achieve the highest performance and lower cost.

Consent for publication Not applicable
Competing interests The authors declare no competing interests.
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