A study on heat and mass transfer analysis of solar distillation system

The solar stills were developed to fulfil the freshwater need of the growing population. The paper presents the recent modifications made in still to improve their productivity like the application of phase change materials (PCM), connecting flat-plate collector (FPC), use of nanoparticles, stepped solar still, and attaching separate condenser in the still. Active solar stills are found more productive than passive ones and the thermal efficiency of active solar stills lie in the range of 50–70%, which is far better than passive still having 20–55% thermal efficiency. According to the literature studied in the paper, the maximum productivity of active solar still is 10 litres per day and in passive solar stills, it is 6 litres per day. The different approaches used to carry out the heat and mass transfer analysis of single and double slope active and passive solar stills are also discussed in the paper. VK , Gaur MK , MK , Tiwari GN . A study on heat and mass transfer analysis of solar distillation system. Ther Eng 2021;7( 5 ): 1184 – 1205 .


INTRODUCTION
Freshwater is an essential requirement of human life. 97.5% of saline water on earth is present in the form of seawater and only 2.5% of water is fresh [1,2]. erefore, researchers are making such devices that can convert impure water to potable water by using renewable resources. Solar still (SS) is a sustainable device that uses solar thermal energy to transform saline and dirty water into freshwater [3,4]. e solar stills are mainly categorised into two parts, passive SS and active SS. Passive SS completely relies on natural resources (Solar Energy) while external devices are used in active SS like at-plate collectors, PVT, electric water heaters, etc. ese are economical and can provide adequate fresh water to remote villages. Passive and active solar stills are further classi ed as single and double slope SS [4]. Single slope solar still has only one glass cover while two glass covers are placed on the double slope solar still. As per the study, the detailed classi cation of solar still is shown in Figure 1. In the last few years, researches in solar distillation systems are focused on minimising energy consumption, fabrication price, environmental impact and maximising productivity and thermal e ciency.
Erfan et al. [5] constructed a double slope SS with PCM and PV/T collector. PV/T collector was used for preheating the basin water and the use of PCM makes it possible to operate still at night. PV/T collector had also been used for electricity generation. ey study the e ect of PCM and PV/T collector on the productivity of double-slope solar still. e double step solar still is tested with four di erent modi cations by Kalita et al. [6] e rst modi cation is the designed single basin with double step, jute wick absorber plates as second, charcoaled jute wick absorber as third and the absorber with double glass cover as a fourth modi cation. Among all modi cations, the maximum productivity still was 3.94 l/m 2 in the case of the double glass cover, as it increases the condensation area.
Stepped solar stills are another method to boost the productivity of stills as less quantity of water is evaporated in more areas, so a faster evaporation rate is obtained [7]. Mu ah et al. [8] designed two di erent types of stepped SS, one is the conventional stepped SS and in another setup, an additional condenser is added in the solar basin. Due to large areas of condensation, the modi ed setup gives daily 2 kg/m 2 distilled water more than the simple stepped solar still. e application of thermal energy storage materials inside the stills increases the daily operation period that helps in producing more water. Bilal et al. [9] had used the two di erent masses (5 and 10 kg) of pumice stones in a still basins to store thermal energy as heat storage material. In the case of 10 kg, the productivity was less than as in the case of 5 kg by 130ml for the same water depth in both cases. is showed that too much use of thermal storage material may result in a decrease in productivity.
A Peltier-based active solar still (ASS) with PV/T system was developed by Pounraj et al. [10] e e ciency of the developed still is 30% higher and produces 6.5 times more water than conventional SS. It was further suggested that the use of PCMs improves productivity and also helps in the 2 or more hour's continuous operation of SS a er sunset. Shalaby et al. [11] tested the single basin SS having v-corrugated basin liner lled with para n wax. Due to the use of PCM, productivity increases by 72.7% during the night as compared to conventional SS. To absorb more solar radiation, dyes [12], and charcoal pieces [13] are placed inside the basin. By increasing the absorption capacity of the basin surface, the extra energy can be absorbed which can be used during cloudy weather and night time and this will increase water productivity.
Wick, nano uids, and various other types of energystoring materials had been also utilised in still to increase its productivity. A pin n wick-based solar still was made by Alaian et al. [14], in which 294 pin n wick elements were placed on the surface of the basin. Due to the ns, the radiation absorption capacity of the basin surface is increased, causing heat absorption and heat release through a large area and the basin water evaporates rapidly due to the capillary action of the n. rough this new modi cation, the productivity of solar still increased by 23%, compared to conventional still.
Six di erent types of heat-absorbing materials like quartzite rock, red brick pieces, cement concrete pieces, washed stones and iron scraps are used by Murugavel et al. [15] in solar still. ey found that quartzite rock had the highest productivity. Because the size of the quartzite rock was larger than the rest of the absorber stones, so it can store a higher amount of energy. Also, Murugavel et al. [16] had done a compressive study of the performance of double-slope solar stills with di erent heat storage materials such as black cotton cloth, waste cotton pieces, sponge sheets, and coir mate at di erent water depths. It was revealed that at 0.5 cm water depth, black cotton was getting the highest productivity. Productivity of the modi ed solar still increases due to low water depth, good capillary action and high radiation absorption properties of the black cotton cloth.
Sharshir et al. [17] used graphite and copper oxide nanoparticles in the basin water, a mixture of water and nanoparticles are increases the productivity of SS by 53.95% and 44.93% respectively than the conventional SS. External solar collectors were used by Tiris et al. [18] in simple basin type solar still, they achieve 5.18 l/m 2 distilled water in a day through modi ed still and 2.575 l/m 2 days by the conventional still and Sebaii et al. [19] used the solar pond for increasing the performance of SS, respectively by preheating water before the inlet to the still. e saline/brackish water is preheated in solar collectors and then it is supplied to the solar still. So, less amount of heat is needed to evaporate the saline water. Tanaka and Nakatake [20] attached an inclined and vertical re ectors to the solar still. e greater amount of solar radiations are incident on the inclined re ector as compare to the vertical re ector, hence the inclined re ector improved the output of the SS by around 16% as compared to the vertical re ector. For increasing absorption of radiation on the SS, Deniz [21] studied various parameters that a ect the productivity like the angle of the glass cover, the cooling system of the glass surface, and distance among the condensing cover and base uid surface.
Tiwari et al. [22] carried out the energy analysis of passive and active solar stills in a review paper. ey suggested that the double slope passive solar still provides superior yield than single slope, and PVT-FPC based single slope active solar still provides better yield than PVT-FPC integrated double slope active solar still. A review paper was made by Arun Kumar et al. [23], in which they considered the stills having productivity of more than 5 litres/m 2 /day. Omara et al. [24] studied the e ect of nanoparticles on heat and mass transfer of modi ed solar still in a review paper. It was found that the performance of nanoparticles based still was better than the without nanoparticles based still. ey reported that the thermal conductivity of the nanouids depends on the grain size, shape, and amount of the nanoparticles. Singh et al. [25] showed the performance of newly designed single basin passive solar still in terms of their e ciency, fabrication cost and daily productivity. e e ect of PCM on the productivity of di erent solar still had been studied by Shukla et al. [26]. Due to heat storage property, PCM based still provides continuous production even a er sunset. Hence the distilled water productivity of PCM based setup was higher than the conventional setup. Grewal et al. [27] reported in their review paper that the PCM based solar still gives 50-160% higher production than conventional still and PCM is more e ective at lower water depth. Kabeel et al. [28] make a review paper on the e ect of various heat exchange mechanisms on the productivity of solar still like PCM, other energy storage materials, glass cooling process, and water depth. It was found that all heat exchange mechanisms increases the heat transfer rate and productivity of the still. Recent developments and new techniques adopted in the 21 st century for productivity enhancement of solar still had been reported by Das et al. [29]. It was suggested that productivity can be improved either by increasing the basin water temperature or by decreasing the glass cover temperature. In the review paper of Pansal et al. [30], it was concluded PV based solar stills provide 25% more overall thermal eciency than the conventional one.
As no review paper reported till now showing the e ect of nanoparticles and PCM on the productivity and eciency of still along with consideration of heat and mass transfer analysis of active and passive single and double slope solar stills. e objective of this paper is shown below: • e paper encapsulates the di erent modi cations applied by various researchers to increase the eciency and productivity of SSs. • Application of PCMs, nanoparticles, separate condensers, modi cation in stepped solar still, and some other recent modi cations are reported in the paper. • e paper covers the energy balance established for the various parts of the still such as condensing cover, basin water, basin liner, and basin surface and also covers the procedure to carry out the heat and mass transfer study of single and double slope active and passive solar still done by di erent researchers.

MODIFICATIONS ON PASSIVE SOLAR STILLS
e solar stills operating naturally without the aid of any external devices are called passive solar still. Single slope passive solar still (SSPSS) was constructed by Agarwal and Rana [31] in which V-shaped thermocol wrapped with black jute cloth is let to oat above the basin water of SS as shown in Figure 2. As Jute Cloth has good capillary properties so it continuously absorbs the basin water and the thermocol was used due to its lightweight and good thermal resistance. e V-shaped oating wick absorbs more radiation due to increased absorption area and results in an increased evaporation rate. e productivity of modi ed solar still (SS) increases by 3.20 litres/day as compared to conventional solar still with the same basin area.
Rattanpol et al. [32] had developed a mathematical model for predicting the quantity of water mass with the help of the Spalding theory of convection and Fick's law of di usion. Steel n was provided in the inner basin surface to increase radiation absorption capacity and ethanol solution was mixed in the basin water to increase convection and di usion capacities of the mixture even at low temperatures. e productivity of still was found 15.5% higher than the without ethanol-based stills and setup e ciency was increased up to 46% with an increase in the number of ns.
Nanoparticles act as a thermal storage material that increases the heat-absorbing capacity of still due to increased heat-absorbing surface area. As more heat is absorbed, so more heat is transferred to the water and due to this the evaporation rate gets increased and results in more distillate output than conventional still [33]. e e ect on productivity and e ciency of still when the nanoparticle is mixed with basin water is shown in Table 1.
Sahota and Tiwari [34] constructed a double slope passive solar still (DSPSS) in which aluminium oxide (Al 2 O 3 ) nanoparticles was used with di erent concentration. Nanoparticles based still gives 12.2% more productivity than the plain water-based still. e daily productivity of the setup was 2.77 l/m 2 . Gupta et al. [35] tested the solar still (SS) having white painted sidewalls and CuO nanoparticles have been mixed in the basin water. e white sidewall reduces the heat loss to ambient from the basin also the incident radiation on the sidewall is re ected the basin liner, which increases its temperature. e daily maximum productivity of the modi ed setup was 3.445 l/m 2 .
Kabeel et al. [36] constructed a solar still (SS) in which the inner basin surface and side walls were coated with nanoparticles mixed the black paint. e nanoparticles based black paint increase the heat transfer rate between the wall and basin water that will increase the temperature and evaporation rate of the saline water. e distilled output of the modi ed still was 16% greater than the conventional still with the same basin area.
Nijmesh et al. [37] had used KMnO 4 and K 2 Cr 2 O 7 nanoparticles in the single slope solar stills (SSSS). It was found that KMnO 4 based solar still gives the productivity of 4.7 l/m 2 while K 2 Cr 2 O 7 based solar still gives 4.1 l/m 2 .
e thermal conductivity of KMnO 4 is higher than that of K 2 Cr 2 O 7 , hence that KMnO 4 stores a superior quantity of thermal energy and keeps the basin water warmer for longer as compared to K 2 Cr 2 O 7 . Gupta et al. [38] constructed solar still (SS) attached with a water sprinkler as shown in El-sebaii [39] constructed a SS with a suspended absorber, which was made of mica, glass, and plastic and installed in the middle of basin water. Due to the low thermal conductivity of mica, radiation heat does not occur from the lower part. erefore, the entire radiation remains in the upper part and increases the water temperature. It was found that mica suspended absorbers give 23% higher productivity than the copper suspended absorber still. Previously, aluminium, steel, and copper materials had been used as suspended absorbers but these materials have the problem of corrosion, and mica is cheaper than the rest of the material and is rust free.
Arun Kumar et al. [44] constructed a hemispherical type of solar still (SS) with side walls lled with sawdust to reduce side heat loss. Water was owed over the glass cover of SS to rise the condensation rate of evaporated water and hence the productivity of still increases. e e ciency of the modi ed setup was greater by 8% than simple still. e e ect of water depth on the performance of inverted SS has been studied by Rahul Dev et al. [45] It was found that at minimum water depth the productivity or distilled output was high, because, in the minimum water depth, the basin water takes less time to heat due to which the water starts to evaporate in a very short time. Inverted absorber type systems have the advantage that setup does not have bottom heat loss, but rather gives extra heat from the bottom surface of the basin, which helps to raise the evaporation rate of water.
Nafey et al. [46] constructed a solar still in which a black painted perforated aluminium oats above the basin water. A skinny layer of saline water is formed above the perforated aluminium plate, which heated up in very little time, and due to the black surface of the plate the radiation absorption ability increases which enhance the evaporation rate of the basin water. Maximum radiation is absorbed by a black-painted aluminium plate so that the radiation does not reach to the bottom and thus no bottom heat loss.
e productivity of perforated aluminium plate based SS increases up to 40% as compared to simple SS.
Sarhaddi et al. [48] constructed a cascade type SS and studied the e ect of PCM on the productivity of solar still (SS). e daily e ciency was improved by 57% and the daily yield increases by 1.6 l/m 2 . Phadatare and Verma [47] studied the e ect of di erent water depths (2cm-12cm) on the internal heat transfer and productivity of the SS,made of plastic SS as shown in Figure 4. e transparent plastic body was insulated from all sides to reduce side and bottom loss. It was found that as the saline or basin water deepness increases, the productivity of distilled water decreases because due to the large water mass quantity, the water will take a longer time to heat up. But e ciency rises with an increase in water depth, as maximum e ciency was 37% at a maximum water depth of 12 cm.

MODIFICATIONS ON ACTIVE SOLAR STILL [ASS]
When external devices are attached to passive solar still, it is called active solar still. Various modi cations have been done in the active single and double slope SSs to improve their productivity and e ciency. Stepped solar stills are one such modi cation that increases the productivity of water by increasing the basin surface area in the form of steps. Omara et al. [49] constructed the stepped SS with a vertical re ector mirror as shown in Figure 5. e solar radiations were re ected the black stepped surface through the mirrors which increases the water surface temperature. It was found that the mirror integrated stepped still gives 75% higher productivity than conventional solar still while the e ciency of modi ed still was greater than conventional still by 21% [43]. e stepped solar still with storage tank and black painted cotton was constructed by El-Agouz [44]. Cotton absorbs a su cient amount of radiation and also water due to the capillary property, which increases the heat and mass transfer rate within the basin. e daily thermal e ciency of the modied setup was 70% while the thermal e ciency of the traditional still was 48%.
Modi ed stepped solar still has been constructed by Velmurugan et al. [56] using ns and sponges. e heat transfer area of the basin surface increases due to ns, causing a substantial amount of heat transfer to the water mass and due to capillary action, sponges sucked the brackish water and thereby increased the water exposure area. Modi ed stepped still gives 98% productivity as compare to simple stepped still. e maximum hourly productivity of the modi ed setup was 1.65 l/m 2 . El-Samadony and Kabeel [57] studied the e ect of lm thickness and ow rate of water owing over the condensing cover on the productivity of stepped still. ey found that, at constant lm thickness, with an increase in water ow rate, the lm cooling rate was increased. e use of a at-plate collector is another modi cation done on active solar stills. e saline/brackish water is preheated in solar collectors and then it is supplied to the solar still. So, less amount of heat is needed to be supplied to evaporate the saline water. us the evaporation rate increases, as more water is evaporated at the same time as compared to traditional still. Kabeel et al. [58] designed a stepped SS  in which wick was used in the vertical side and basin water had been preheated in the collector. For the same condition and area, the daily thermal e ciency of the modi ed setup gives 53%, and while the traditional still gives 33.5%.
Rajaseenivasan et al. [59] constructed a at-plate collector (FPC) based solar still (SS) and six separate compartments are made on the basin surface. e yield of the modi ed setup was 60% higher than the traditional SS with the same basin area while the e ciency of the modi ed SS and conventional SS is 60% and 37% respectively. FPC has preheated the saline water a er supplied to the basin and with the help of the separate compartment, the mass of the basin water is divided, which reduces the water mass in the compartment, but the water mass in the basin remains the same as before, so at the same water, depth gets higher evaporation rate.
A at-plate collector was attached to double slope solar still (DSSS) by Badran et al. [60] and its performance was evaluated with plain water and saline water. It was found that the yield of the setup was 231% with plain water while it was only 52% with saline water. Due to low density, the heat transfer rate in the plain water is very high, whereas the density of saline water is high, so it does not transfer the heat properly.
Faegh and Sha i [50] constructed a SS with an external condenser with PCM and a heat pipe placed inside it, as presented in Figure 6. Generally, the latent heat released by water vapour is lost in SS, but in this setup, the latent heat is immersed in the PCM and transmitted back to the basin water through the heat pipe. e use of PCM increases the operating time of still per day, due to which the productivity increases. e yield of the modi ed setup was 86% higher    Figure 7. (a) Evacuated tube collector (ETC) coupled system working on natural circulation mode (b) ermo-syphon performance in each evacuated tube [61].
than the without PCM based SS. e e ect on productivity and e ciency of stills by using PCMs is shown in Table 2.
Singh et al. [61] carried out the thermal analysis of SS with an evacuated tube attached to it as shown in Figure 7. A er analysing the experimental data, it was found that the energy e ciency of the system increases with a decrease in water depth. 33% energy e ciency and 2.5% exergy eciency was achieved by the evacuated tube collector (ETC) based still.Evacuated tube collector helps to preheat the basin water, so less amount of energy is needed to heat the water inside the basin, due to ETC basin water started to evaporate at very less time.
e energy and exergy analysis was carried out by Kumar et al. [62] inactive solar still (ASS) with ETC. e mass ow rate of evacuated tube collectors to solar basins and basin water depths was optimised. At 0.01 m and 0.03 m, water depth the daily productivity was 3.47 l/m 2 and 3.9 l/m 2 and e ciency was 33.8% and 2.6%, respectively at 0.006 l/s water ow rate. It was observed that at the same con guration, the annual performance of the forced mode system was better than the natural mode system.
Mohamed et al. [63] evaluated the e ect of di erent sizes (1, 1.5, and 2 cm) of ne black stone on the heat and mass transfer rate occurring inside the solar still (SS). e e ciency of SS was increases maximum by 123% for 2 cm stone size as compared to conventional solar still. External and internal re ectors had been installed in simple SS by Tanaka [64] as shown in Figure 8. e heat and mass transfer rate inside the basin was increased with the help of internal and external re ectors. e daily productivity of the setup was increased by 70% to 100%.
Radiations that are falling outside of the solar still can be transferred inside the basin with the help of re ectors. Re ectors also increase the intensity of solar radiation and at the same time collect the radiation at one place and more radiation transfer to the basin liner [65].
Monowe et al. [66] constructed a solar still in which an external re ecting booster and an external condenser were attached to utilise the latent heat, as shown in Figure 9. e re ectors attached to the still transmit more solar radiation inside the still and hence the more heat is available inside the still to evaporate water. e latent heat released during the condensation of evaporated water in the outer condenser is utilised to preheat the saline water. E ciency can be up to 85% when preheated salty water was used at night and water productivity was 10 l/day.  . ermal electrical solar still with external condenser [66].

Assumptions
e thermal modelling of solar stills (SSs) can be understood through the energy balance of various parts of SSs such as water mass, basin liners, basin absorbers, and glass cover [67]. Some important assumptions considered for the energy balance in solar still (SS) are: • e water quantity inside still was considered constant throughout its operating time. • Heat loss due to evaporation of water inside the SS is neglected. • e temperature of the water is considered uniform throughout its depth. • e heat absorption ability of glass cover and insulated material is neglected. • e area of the glass cover, basin water, and basin are considered the same. • Solar still should be completely airtight and there should not be any water vapour leakage.
Generally, for all solar still, the basin water is found by solving rst-order di erential equations, in the form as shown below [68]: Heat transfer from the exterior of the SSs takes place through convection, conduction, and radiation modes, whereas heat and mass transfer inside the SS takes place through convection, radiation and evaporation modes [68]. Figure 10 shows the heat transfers occurring through different modes in single slope solar still (SSSS). e thermal modelling of solar stills (SSs) can be understood through energy balance on various parts of SSs such as water mass, basin liner, basin absorber, and glass cover [69]. e energy balance equations were developed considering the unit area of the SS component.

ENERGY BALANCE ON OUTER GLASS COVER
Heat energy conducted to the internal glass cover is equal to the heat lost by the radiation and convection from the outer glass cover to the ambient, following energy balance is given by Dev et al. [70] for the glass cover.
All unknown factors are given in the appendix. Using the energy balance equation given by Eq. (1) and Eq. (2), the following relation for outer glass surface temperature (T co ) can be obtained as [70]:

ENERGY BALANCE ON INNER GLASS COVER
e total rate of energy lost by the outer glass surface is equal to the addition of total energy absorbed by the glass from solar radiation and the total energy received on the internal surface of the glass by convection, radiation, and evaporation. Energy balance on the inner glass surface is developed by Ferna et al. [72], given as: Dev and Tiwari [75] assumed the heat absorptivity of the glass as negligible (α c =0) and then the inner glass temperature is given as:

ENERGY BALANCE ON BASIN LINER OR ABSORBER
Total radiation absorbed by the basin liner is equal to the total rate of energy lost by the basin liner through convection and conduction to basin water and ambient, respectively [70].
For phase change material (PCM) charging and discharging mode, El-Sebaii et al. [76] give the following equation for basin liner (absorber).
For PCM charging mode: All unknown value is given in appendix A of El-Sebaii et al. [76].

ENERGY BALANCE ON BASIN WATER
Energy received by basin water is equal to the energy loss from the basin water. Solar radiation absorbed by basin water + Heat energy received from basin liner = Energy stored in the basin water + Total heat transfer from the water surface to the inner glass surface. e energy balance for basin water is shown below, given by Velmurugan et al.
e temperature of basin water (T bw ) can be calculated through the following equation [70]: Where T bw0 is the initial temperature of the water, when the initial temperature of the water is zero, i: e t = 0, (T bw/t=0 = T bw0 ).
All unknown values are given in the appendix. e following equation is given by El-Sebaii et al. [76] to calculate the mixture of PCM and basin water temperature. e temperature of water in terms of PCM charging mode: Where ft () is the average value of f(t) between time intervals 0 and t. [77].

HOURLY YIELD
Hourly distilled water productivity can be obtained by the following equation which is given by Sahota et al. [78]: Where L is the latent heat of the vapour

THERMAL EFFICIENCY OF THE SINGLE SLOPE PASSIVE SOLAR STILL
e ratio of thermal energy used to evaporate water to the total energy incident over the solar still (SS) is termed as thermal e ciency.
e thermal e ciency of passive single slope SS (PSSSS) can be obtained by the following relation, given by Sahota and Tiwari [79]:

DOUBLE SLOPE PASSIVE SOLAR STILL (DSPSS)
Double slope SS (DSSS) has two slopes, which permits more solar insolation inside the basin than a single slope but it has extra thermal losses. e schematic diagram with the representation of heat transfer occurring from DSSS is shown in Figure 11.

ENERGY BALANCE ON DIFFERENT PARTS OF DSPSS
More radiations are travel inside the double slope SS compare to the single slope because in the double slope, solar radiations are incident on two facings. Double slope SS is placed in the East-West orientation, due to which solar radiations are fall on the surface of setup in the early morning and for a long time till the sunset [81].
Energy balance on East side glass surface is developed by Sahota and Tiwari [78]:  Taylor et al. [82] gives the Energy balance on West side glass surface: Energy balance on basin water mass [82]:

TEMPERATURES EQUATIONS FOR VARIOUS PARTS OF THE DSPSS ARE GIVEN BELOW
Outer glass surface temperature for east and west direction can be calculated using Eq. 28 and Eq. 29, an equation for calculation of east and west side inner glass temperature is developed by Sahota and Tiwari [79]: Inner glass surface temperature which located in the east direction (T ci,E ) and in west direction (T ci,W ) can be calculated through the following equation, given by Sahota and Tiwari [79] and Taylor et al. [82], respectively: e hourly yield of double slope SS can be calculated through the following equation [83]: Where L is the latent heat of vapourisation can be obtained through the following relation [78]: T v is the temperature of the vapour.

THERMAL EFFICIENCY OF THE DOUBLE SLOPE PASSIVE SOLAR STILL (DSPSS)
e ratio of thermal energy used to evaporate water to the total energy incident over the solar still (SS) is termed as thermal e ciency.
e ermal e ciency of DSPSS obtained through the following equation, which was given by Singh et al. [84]:

INSTANTANEOUS EFFICIENCY
e amount of energy absorbed and lost by solar still during the operating period is called gain and loss of Instantaneous e ciency respectively. Sahota and Tiwari [85] developed the Instantaneous gain e ciency equation: And also the Instantaneous loss e ciencyequation is given by Sahota and Tiwari [85]:

SINGLE SLOPE ACTIVE SOLAR STILL (SSASS)
e productivity of the active SS depends on the performance of the external device. Many factors a ect the performance of solar stills (SSs) that we cannot control such as solar insolation, pressure, and temperature of the atmosphere and air ow direction. But some parameters can be controlled to boost up the yield of SSs like water depth, base uid temperature, angle of the glass, fabrication materials, location of setup and thickness of insulation etc. [22].

ENERGY BALANCE ON DIFFERENT PARTS OF THE SSASS:
e energy balance of SSASS for glass cover is done in the same way as in the case of single slope passive SS given by Eq. 1 and 4. In the case of active SS, the energy balances of additional components are needed to be carried out [51,55].
Kumar et al. [86] develop energy balance on basin water: All unknown value of heat transfer rate can be calculated through the equations which are given in the appendix.
In the case of passive SS there is no need of external device: Energy balance on basin liner: Due to small contact surface area, heat loss from the sidewall is neglected, energy balance on basin liner is given by Singh et al. [61] and Kumar et al. [61]: Energy balance of evacuated tube water mass can be expressed as [62]: Where dT w is varied from small time interval dt (0 < t < dt) Energy balance of water mass inside the still with evacuated tube collector (ETC) [62]:

TEMPERATURE OF VARIOUS PARTS OF THE SSASS
A er rearranging the above equations 41-43, the equation of basin liner temperature, the temperature of basin water and the evacuated tube water temperature has been calculated which is given below, following equations is given by Singh et al. [61] and Kumar et al. [61]: e Temperature of inner and outer glass cover: For inner glass: Sampathkumar et al. [87] give the outer glass temperature: Basin water temperature: Kumar et al. [62] developed the expression of water temperature for evacuated tube collector (ETC) integrated active solar still: Where, T w0,ETC = Initial water temperature of evacuated tube collector at t = 0 and T bw0 = Initial basin water temperature at t = 0.
Kumar et al. [62] gave the equation for calculating the water temperature of the evacuated tube collector (ETC), which is written below: All unknown factors of the above equations are given in Appendix ' A' of Singh et al. [61] and Kumar et al. [61] Singh et al. [88] developed the expression of water temperature for PVT integrated at-plate collector (FPC) based active solar still:

THERMAL EFFICIENCY OF ACTIVE SINGLE SLOPE SOLAR STILL:
e ratio of thermal energy used to evaporate water to the total energy incident over the solar still (SS) is termed as thermal e ciency. e hourly thermal e ciency of single slope active SS can be calculated as [88]: Where A Ex is the area of the additional device, I Ex is the solar intensity on external device and m˙b w is the hourly distilled out-put (kg/h) Tiwari and Sahota [89] have been developed the equation for calculation of overall thermal e ciency for PVT base single slope active solar still.
Where FF is the Fill factor of the module, V oc is the open circuit voltage, S sc is the short circuit current, I(t) is the solar intensity on the collector and A m is the area of the module.

ENERGY BALANCE ON DOUBLE SLOPE ACTIVE SOLAR STILL (DSASS)
Based on the rst law of thermodynamic, the energy balance of DSSS is shown below. e equation of glass temperature, basin water temperature, and basin liner temperature is given based on heat and mass transfer analysis [37,40].
For inner and outer glass surface: Energy balance on the inner and outer glass surface of the DSASS is the same as the double slope passive SS as shown by Eq. (23) to (27).
Energy balance on basin liner: Total radiation absorbed by the basin water is equal to the total rate of energy lost from the basin water through convection and conduction [74]. Due to the black surface, the maximum solar radiation is absorbed by the basin liner; the absorbed radiation is converted into the heat and transfers into the basin water through the convection and loss to the atmosphere through conduction. e total rate of energy loss to the ambient from the basin water and basin liner is given by Kumar et al. [62], which is given below: A er rearranging the equation (56), temperature of the water can be expressed through the following equation which is obtained by Tiwari and Sahota [22] and Sampathkumar et al. [87]: T bw0 = Initial basin water temperature at t equal to 0 All unknown factors of the above equations are given in the Appendix of Sampathkumar et al. [87].

TEMPERATURES EQUATIONS FOR VARIOUS PARTS OF THE DSASS
When an external device is added to the double slope passive SS, then it is called double slope active SS. External devices such as at-plate collector, heater, fan, PVT, external condenser and glass cooling system, etc. [90]. Temperature equations of the di erent parts of the SS are written below: Dwivedi et al. [91] developed the temperature equation for DSASS which is given below: Some changes had been done in the thermal e ciency of the DSASS by Singh et al. [92], external device parameters are added in the thermal equation, the thermal eciency of double slope active solar still is given below: Where, A pv is the area of the PV module.

CONCLUSIONS
e productivity of solar stills are found still low nearly about 10 litres/day and various researches are putting their e ort to further improve the output of solar stills by implementing various modi cation on solar stills. e productivity of any solar still depends on the internal and external heat transfer rates. e study on the heat transfer analysis and modi cations in still are concluded as follows: • To increase the internal heat transfer rate of passive solar stills, heat-absorbing materials such as PCM, nanoparticles, and ne stones were used in the setup. • Based on literature review it is found that the thermal e ciency of nanoparticles and PCM based solar still lies between 17-62% and 17.93-59.14%, respectively. • Due to heat storage properties, PCM based still provides 3-4 hours more production a er sunset. • Re ectors, at-plate collectors, and external condensers are used to increase the evaporation rate of water inside the active solar stills. • e productivity of passive solar still can be increased by 50-85% if an external device is used. • e productivity of modi ed passive and active solar still was found maximum up to 6 litres and 10 litres, respectively. • Stepped solar stills can also raise the evaporation rate inside the setup as there is a thin layer of basin water in each step which does not take much time to evaporate and rapidly converts into vapour. • e productivity of the solar still has also been enhanced by the ow of cold water over the glass cover. rough this method the temperature di erence between the glass and water surface (T w -T g ) is increases, this increases the evaporation and condensing rate of water. • e present work is limited to the modi cations made in passive and active solar still while an extensive review can be made by considering all the modications in still like single slope, double slope, multi basin and multi-stage etc. along with heat and mass transfer analysis done on hybrid active solar stills. e exergy analysis can also be included to make a more exhaustive review of solar stills. Instantaneous thermal e ciencies η itL Instantaneous thermal loss e ciencies APPENDIX Absorptivity of di erent component of solar still can be obtained through following equations, given in the paper of Sampathkumar et al. [87]and Tiwari et al. [93]:

DATA AVAILABILITY STATEMENT
No new data were created in this study. e published publication includes all graphics collected or developed during the study.

CONFLICT OF INTEREST
e author declared no potential con icts of interest with respect to the research, authorship, and/or publication of this article.

ETHICS
ere are no ethical issues with the publication of this manuscript.