Active solar still with solar concentrating systems, Review

Clean water is quickly decreasing as the necessity for it rises year after year. Desalination of seawater is the only means to remedy this problem. The scientific community has focused on solar powered salty water desalination devices as a clean alternative throughout the last few decades. Solar stills convert solar energy into the heat required to generate clean water through the evaporation and condensation processes. The two very popular varieties of solar stills are active, and passive based on the heat source to evaporate the basin water, which can be either directly through the sun or indirectly using external heat sources such as concentrating collectors which are coupled with desalination units. This article provides an exhaustive analysis of active solar stills’ advancement with solar concentrating systems and techniques for improving performance, desalinated water production, efficiency, and maximum output. This work intends to keep researchers and the scientific community informed about developments in active solar stills equipped with solar concentrators.


Introduction
Clean water scarcity has lately been a challenge for several countries. Drinking water consumption has heightened in lockstep with people's growth and fast industrial expansion, resulting in a freshwater shortage in some countries [1]. Clean water is the essence of human life, and it is vital to a variety of sectors that support our everyday live applications. Several Middle Eastern and North African countries are experiencing water shortages [2]. In Egypt, the major source of clean water is the River Nile, despite the presence of this source, there is a deficiency of clean water, particularly in coastal areas, although there is a huge amount of seawater [3,4]. Desalination of seawater is the only means to remedy this problem [5]. Desalination processes are classified into various types based on the method for salt and freshwater separation from seawater. The first type is thermal desalination, and it is dependent on the phase change of water by evaporation and condensation phenomena for example multiple effective desalination (MED), and multiple stage effect desalination (MSED) [6,7]. The second type is powered desalination systems for example, electro dialysis (ED) and reverse osmosis (RO) [8]. These two solutions to seawater desalination require high energy consumption (electrical or mechanical) to extract the clean water from the seawater [7]. On the contrary, the third type is solar desalination, which uses solar energy to purify the seawater by evaporating the seawater. Solar energy is the most available and economical heat source [9]. Desalination processes can be classified as shown in Fig. 1. The scientific community has focused on solar powered saltwater desalination devices as a clean alternative throughout the last few decades [10]. Solar stills, transform solar energy into the heat necessary to generate freshwater by evaporating the seawater and condensing the vapor on a glass cover [11]. Broadly, the solar still was enhanced by several approaches and numerous research for example, altering the shape of the glass and basin [12,13], utilizing a sponge material [14], utilizing double basin area [15][16][17], the using of solar water heater [18], and changing still surface area as well as the basin water depth [19][20][21][22].
The two very popular solar still types are active and passive. Figure 2 depicts the classes of solar still distillation [23]. When it comes to passive stills, the saline water inside the solar still is directly heated by solar radiation, eliminating the requirement for exterior heating resources and allowing for internal desalination and heat collection [24]. The two types of passive solar stills are traditional and efficient design solar stills. Water in active solar basins is heated by the sun, but it also receives preheated water via an indirect channel that is heated externally, such as hot water from a solar collector or heater. This external heat raises the temperature of the water to raise the temperature in the basin, which increases evaporation rates. To overcome the challenge of reduced distillate yield output in passive solar stills, a number of active distillation techniques have been created [25].
There are three types of active solar stills: integrated solar collecting systems (concentrator collectors and flat plate collectors); waste heat-driven solar stills; and hybrid solar stills. There are numerous reviews that focus on active solar stills. Patel et al. [26] summarized solar powered water treatment systems with double inclination and evacuated tube collectors. Sampathkumar et al. [25] represented various kinds of active stills, while Singh et al. [27] verified published research based on solar still systems with concentrated and vacuum type tube collectors. As well as, Diab et al. [28] reviewed several studies on solar stills with revolving parts, which they found to be an effective and efficient design because rotating parts break the surface tension of the saline water, raise the rate of evaporation area, and improve distillation efficiency. The current study's goal is to provide a comprehensive analysis of the development of active solar still connected with solar concentrator systems and techniques to improve performance, desalinated water production, and efficiency to provide researchers and the scientific community informed about advancements in active solar still with solar concentrators. This article is divided into four main parts. First is an introduction to the methods of seawater desalination, especially desalination using solar distillers. Secondly, the different types of solar concentrators (parabolic trough and parabolic dish type). Thirdly is using of solar concentrators with solar distillers for the desalination of seawater. Fourthly, the conclusion and recommended future studies.

Solar concentrating system
Solar energy is the cleanest and most inexpensive renewable energy source. However, extracting and applying hightemperature heat from a source of solar energy is a major challenge in the current period. Solar concentrator collectors have been investigated for some years as a means of focusing solar energy for high-temperature applications as well as gathering solar energy. Solar concentrators collect and concentrate light into a single point. Solar radiation intensity, angle of incidence, and the concentrator's position in regard to the sun and the heated reactor all have an impact on its performance [29,30]. Solar concentrators are classified as parabolic trough, dish, and heliostat field concentrators. As seen in Table 1, the temperature ranges of various concentrator types.

Solar parabolic trough concentrator (PTC)
Solar parabolic trough concentration systems are collectors that are formed in a parabolic shape and are composed of reflecting materials [30]. As shown in Fig. 3, solar energy is reflected by the collectors on their focal line to a receiver, which absorbs solar radiation and concentrates it to raise the temperature of the fluid within the focus line.

Solar parabolic dish concentrator (PDC)
As shown in Fig. 4, solar energy is concentrated by the parabolic dish shape and then directed to the receiver, which is located at the focal point of the dish collector to raise the temperature of the fluid within the focus point. The surface of the dish collector is made up of a set of mirrors in the shape of a parabolic dish [29,30].

Solar heliostat field concentrator (HFC)
To avoid shadows, HFCs use a colossal collection of mirrors called heliostats that are dispersed throughout a receiver. These heliostats reflect the direct rays of the sun to the  [30,31] receiver, which is located on the tower, as shown in Fig. 5, however, it is a very expensive technique compared with other concentration systems [31].

Solar still with solar concentrating systems
Solar stills are one of the simplest and cheapest methods for desalinating seawater [10]. The main issue with solar stills is their low freshwater productivity, which is limited to a range between 2.5 and 5 L m −2 day −1 [32]. Different concentrators have been utilized throughout the years, depending on the application. To generate a better output, the contractor is coupling with a solar still to enhance the saline water's temperature in the basin [25]. When a solar concentrator system and solar still are used together, the amount of desalinated water produced rises [23,33].

Solar still with solar parabolic trough collector (PTC)
Numerous studies investigated the solar still performance coupling with solar parabolic trough and parabolic dish concentrators in natural or forced modes of circulation. Singh et al. [23] reported an analytical formula for the temperature of water in an active solar still using flat plate collectors and a parabolic concentrator. Because the evaporative heat transfer coefficient in a concentrator is higher than that of a parabolic collector, system efficiency with a concentrator is higher than that of a parabolic collector. Kabeel and Abdelgaied [32] evaluated a parabolic trough concentrator in addition to solar still controlled by an oil heat exchanger, and PCM as shown in Figs. 6 and 7. The redesigned solar still has a 140.4% higher production than traditional solar still. The average efficiency for improved solar stills and traditional solar stills, respectively, is 25.73% and 46%. Mohamed Elashmawy [34], examined the tubular solar still's performance (TSS) in combination with a solar parabolic concentrator with a tracking system (PCST-TSS) as shown in Figs. 8 and 9. The experiments were carried out in Hail City (27.5N, 41.7E) in Saudi Arabia. The results showed that the PCST-TSS would be able to raise the daily TSS yield by about 676%. In another work, Arunkumar et al. [35] tested how well a solar tubular still with a compound parabolic concentrator performed.
On the other hand, Fathy et al. [36], in summer and winter, investigated the performance of a connecting parabolic trough collector (PTC) with double slope solar still, as depicted in Fig. 10. The findings revealed that when compared to traditional solar stills, the solar still with PTC has a greater temperature and production on the sill.
Kumar et al. [37] performed a single slope solar still performance with a parabolic trough collector as shown in Fig. 11. They examined the behavior of solar parabolic trough collector coupling with solar still for saline water depths inside the basin area of 50 mm, 100 mm, and 150 mm, and the yield was 4.1 L m −2 , 3.645 L m −2 , and 3.2 L m −2 , respectively. Moreover, efficiency was reported at 16.6%, 13.7%, and 12.2%, respectively.
Moreover, Madiouli et al. [38] investigated the traditional solar stills performances as well as the impacts of combining a flat plate collector (FPC) using water fluid circulation and a parabolic trough collector (PTC) using oil circulation with a packaged glass ball layer (PLGB) as a thermal storage medium for the systems as shown in Fig. 12, the incident solar energy is captured by the PTC-FPC through two independent loops of the finned tube that serve as heat exchangers in the still basin. During the winter and summer seasons, experiments were conducted with a saltwater depth of 50 mm. The solar still coupled with PTC, FPC, and PLGB, produces more freshwater at a rate of 6.036 kg m −2 during the summer and 2.775 kg m-2 during the winter, according to the results. Furthermore, when compared to conventional solar, FPC-PTC-PLGB solar increased productivity by about 172% in winter and 203% in summer.
Furthermore, Hamdy et al. [39] examined the freshwater production and efficiency of coupling a solar parabolic trough solar collector with a single slope solar still as shown in Fig. 13. As illustrated in Table 2, there are six solar still systems to evaluate. The experiments were conducted in both hot and cold weather situations in the city of Sohag, Egypt. The maximum freshwater production of 8.77 kg m −2 was attained in the summer condition of CSS + SD + PTC and represents a 102.1% increase over the winter case. The CSS + SD has the highest efficiency (40.7% during summertime and 35.82% during wintertime, while sand improves distillation daily efficiency by roughly 21.5% during summer and 10.2% during winter. In the case of CSS + PTC, the Fig. 6 A photo of cylindrical parabolic trough coupled with a modified solar still [32] distillation efficiency rose by approximately 14.3% in the summer and 9.8% in the winter. Subhedar et al. [40] evaluated the performance of a solar still with an integrated solar parabolic trough collector using a nanofluid. The experimental setup is shown in Fig. 14. They examined the parabolic trough collector under the climate conditions of Changa, Gujarat. They made an aluminum oxide-based water nanofluid and delivered various amounts (0.05 and 0.1% by volume fraction) to see their impact on the performance. For a 2.5 cm saltwater depth in a 1 m 2 basin, they found a maximum yield of 1741 mL using a 0.1% volume fraction of Al 2 O 3 /Water nanofluid.
Amiri et al. [41] investigated a new independent solar desalination technique, that comprises a solar parabolic trough collector, which serves not only as a reflector but also as a solar concentrator for solar radiation to a solar still. As shown in Fig. 15, the novel system comprises a parabolic trough collector placed beneath a standard solar still. The experimental setup was investigated and installed in Kerman, Iran for the four seasons. The findings showed that higher and minimum water productivity occurs during the summer and winter seasons, respectively. For a fixed  Fig. 8 Photograph of the PCST TSS [35] parabolic trough collector, their proposed solar still system generated 55% more clean water during the summertime than in wintertime in Kerman weather conditions. During the summer season, parabolic trough collector and tracking systems produced around 1.266 L m −2 day −1 , which is 70% more than in the winter.
Khairat Dawood et al. [42] investigated the solar still performance, which was connected with two solar parabolic collectors connected in series, with various oil flow rates. As illustrated in Fig. 16, phase change material was inserted under the area of the basin as well as in the inner tubes of the evacuated tubes. The traditional solar still produced 3.182 L m −2 day −1 . On the other hand, the system yields 4.7, 6.2, 8.8, and 11.1 L m −2 day −1 when utilizing 1.5, 1.0, and 0.5 L min −1 of oil flow rate and 0.5 L min −1 of nano-oil flow rate, respectively.
While Thakur et al. [43] evaluated the performance of the solar still integrated with a solar parabolic collector by adding activated carbon pellets as a storage energy material in order to increase the evaporation rate and freshwater productivity of the solar still. According to the results, the utilization of a parabolic collector and porous carbon augmented the yield of solar still by 85.2%. Also, Aqlan et al. [44] evaluated the solar still coupling with solar parabolic trough performance. The accumulated water productivity for the modified solar still rose by roughly 177% when compared to the traditional still, according to the findings of the field testing. Moreover, Madiouli et al. [45] presented an experimental comparison study between conventional solar still, solar still with connected with parabolic trough, and solar still with parabolic trough and packing layer of glass balls in the basin area. The analysis shows that the case of solar still with parabolic trough and packing layer of glass balls has the highest freshwater productivity followed by the cases and recorded maximum yield is equal to 2.4 kg m −2 . Sathyamurthy et al. [46] investigated the increase the contact time of water in the basin to enhance yield of fresh water by using a semicircular absorber solar still with baffles, and studied the influence of the number baffles and the water flow rate. The result showed that the yield of present solar still is higher than that for conventional still approximately by 16.66%. Otherwise, Arunkumar et al. [47] studied the performance of compound parabolic concentrator concentric tubular solar still the maximum yield is equal to 5 L m −2 . Hassan et al. [48] evaluated the performance of an coupled model of conventional solar still (CSS), flat plate collector (FPC) and parabolic trough collector (PTC) for the production of potable water using ZnO, Al 2 O 3 , TiO 2 and CNT nanomaterials. The result showed that the highest water production rate of 0.478 L m −2 h −1 was in case of integrated system consisting of CSS, FPC and PTC using CNT based nanofluid, which was 153% higher than that of CSS without nanoparticles. It is possible to summarize all these previous studies on the solar still with a solar parabolic trough concentrator (PTC), which were related to their enhancement techniques and the obtained data of their desalinated water production and daily efficiency in Table 3.

Solar still with dish concentrator (PDC)
The heat from the sun's solar radiation, which is represented as thermal energy, is converted into concentrated solar thermal energy using solar dish systems [49]. To attain the needed temperature, solar dish systems use a parabolic dish shape, which consists of mirrors collected in the support structure, to reflect and concentrate the solar energy to the focal point of the dish receiver. In the desalination processes, the contractor was connected with solar still to achieve higher freshwater production by increasing the saline water temperature in the basin [50]. To increase distillate productivity, a parabolic dish concentrator collector can be connected to the solar still with various methods, directly by using the same saline water circulation or indirectly by using  Fig. 13 Photo of solar parabolic trough collector coupling with solar still [40] another thermal fluid circulation [23,33]. Because of its low thermal loss, in comparison with other systems, the concentrator with point focusing offers an advantage, resulting in a high flow and output [51,52]. Kabeel et al. [53] tested a new, modified single solar still, which was equipped with two identical solar parabolic dishes and four PV modules with a total output of 1 kW. They also used a tracking system and control unit. Figure 17 is a schematic representation of their suggested improved single solar still. They evaluated the performance of a new modified solar still for three months Water inlet and outlet

Solar still
Parabolic trough collector  [42] under the parameters of Ismailia, Egypt's climate and they looked at the impact of 10-and 20-mm basin water depths and in three experimental scenarios, they performed a new modified single solar still, which represented the traditional solar still, one solar parabolic dish, and two solar parabolic dishes. The daily production with one solar dish was 8.8 and 5.45 kg m −2 day −1 at depths of 10 and 20 mm of water, respectively, according to the results. At the two water levels of 10 and 20 mm, respectively, the daily yield of two solar dishes equipped with solar still was around 13.63 and 7.69 kg m −2 day −1 . Gorjian et al. [54] manufactured a standalone solar parabolic solar still with a point focus. A solar parabolic dish concentrator, a two-axis sun tracker with programmable logic controllers (PLCs), and two plate heat exchangers (PHEs) for preheating the salt water before entering the absorber in the focal point and condensing the resulting steam were all part of the system. Figure 18 shows the independent parabolic solar still with a point focus. In October, they carried out the influence of environmental and operational parameters on the system under the weather conditions of Tehran. The highest level of freshwater production is 5.12 kg within 7 h a day (Table 4).
Abubakar et al. [56] studied the performance of coupling a solar dish collector with a solar still. The solar still is placed at the focal point. Figure 19 shows the solar still position.
Also, Srithar et al. Omara and Eltawil [50] investigated a solar dish concentrator and conventional solar still to see how well solar desalination systems work. The system contains a raw water tank, CSS (Solar Still, Single Slope Basin), SDC, boiler, condenser, control unit (tracking system, two-axis), photovoltaic system, solar heater, and modular programmable logic controller that comprises the developed solar thermal desalination system (MPLC). As shown in Fig. 21, a mini single slope-airtight solar still is built at the dish concentrator's focal point that serves as a boiler. The daily average of distilled water for SDC with preheating and CSS was 6.7 and 3 L m −2 day −1 , respectively. SDC and CSS had a day efficiency of 68 and 34%, respectively.
Bahrami et al. [58], investigated a solar parabolic dish collector system using a new solar still design mounted at its focal point for the desalination of salt water, both experimentally and theoretically, as shown in Fig. 22. The solar still is prepared to consider both the evaporator and the condenser.
On the absorber surface of the solar still, the incident solar irradiation on the surface dish is mirrored and gathered. The saltwater evaporates, followed by the condensation of the vapor produced by a plate condenser embedded in the top of the solar still. The results showed that with a 2 m aperture dish, as the reflectance of the absorber plate fell from 0.7 to 0.4 and the parabolic dish optimal efficiency increased from 0.5 to 0.8, the distilled water production increased by up to 120 and 80%, respectively. While the initial salinity, temperature, and volume of the water inside the evaporator had less than a 10% influence on distilled water production. Furthermore, a dish concentrator with a 3 m aperture diameter and specified parameters may generate roughly 75 kg of distilled water per day for an operating system period of 8:30 to 17:30. Arunkuumar et al. [59] investigated the efficiency as well as distillate yield of a concentrator-coupled hemispherical basin solar still with a phase change material (PCM). Figure 23 depicts a schematic representation of their proposed concentrator-coupled hemispherical basin single slope solar still. Experiments were carried out on two different types of operation: (1) single slope solar still with no PCM effect and (2) single slope solar still with  [60] optimized the augmentation of condense by enhanced desalination methodology by using a compound conical concentrator has been integrated with solar distillation systems. The maximum productivity was equal to 2912 mL day −1 . Otherwise, Nazari et al. [61] conducted a parabolic dish concentrator box solar still without a glass cover combined with a thermoelectric condensing duct. The maximum productivity was equal to 14.13 kg m −2 day −1 . On the other hand, Tawfik et al. [62] investigated the thermal performance of a new design of solar parabolic dish desalination system, consisting of the evaporating unit and external condenser. The maximum thermal efficiency was equal to 36%.

Conclusions
Throughout the literature survey on solar stills with solar concentrating systems, the following conclusions can be obtained: • The yield of conventional solar stills increases through integration with solar concentrating systems (parabolic trough concentrator or parabolic dish concentrator).

Solar dish
Foundation 1 3 • The integration of the parabolic trough concentrator with the solar still gave the highest yield output of about 11.14 L m −2 day −1 by utilizing a solar still. • Integrating the parabolic dish concentrator with the solar still gave the highest yield output of about 16.94 kg m −2 day −1 by utilizing a triple basin glass solar still with a photovoltaic panel and parabolic dish.
Researchers have studied the single slope solar still with a parabolic trough concentrator or parabolic dish concentrator, but it can be extended in many ways and presented in future work.
• The pyramid solar still can be coupled with a parabolic trough concentrator or parabolic dish concentrator instead of single slope solar still. • Sensible heat storage material like pottery clay can be used to enhance thermal energy. • Artificial neural networks (ANN) analysis of solar still is also unattended by researchers. Also, it would be excellent work to vary solar still parameters in the ANN and compare them to the experimental results.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). No funding used in this paper.

Conflict of interest
The authors declare that they have no conflict of interest.
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