Experimental investigation of an active inclined solar panel absorber solar still—energy and exergy analysis

The objective of the current study is to investigate the performance of the inclined solar panel basin still (ISPBS) incorporated with a spiral tube collector (STC) for various mass flow rates of water (mf). The maximum potable water yield of 8.1, 6.9, and 6.1 kg is obtained for different mass flow rates of 1.8, 3.2, and 4.7 kg/h in each instance. Also, for mf of 1.8, 3.2, and 4.7 kg per hour, the daily average energy and exergy efficiency of the ISPBS is recorded to be 47.9, 39.3, and 31.02 % and 9.8, 7.9, and 5.6 %, in each instance. The average electrical, thermal, and exergy efficiency of the PV panel is noted to be 6.5, 7.1, and 7.5 %; 15.67, 17.1, and 18.04 %; and 20.03, 22.21, and 23.36 % for mf of 1.8, 3.2, and 4.7 kg/h in each instance. The rise in mf causes a drop in the fresh water production yield; thermal, exergy, and overall thermal effectiveness; and an enhancement in the power production of the panel, electrical, thermal, exergy, and overall exergy efficiency of the system. Also, the cost of yield production is noted to be low-cost in AISS at minimum mf of 1.8 kg per hour (0.019 $/l) when compared to the other two mf of 3.2 and 4.7 kg per hour (0.022 and 0.025 $/l).


Introduction
Our planet is globally marching toward ultimate progression in all aspects. This causes increasing demands for necessities of the human life , Manokar et al. 2018a. Greater desires and demands are causing the depletion of our natural resources. The needs of the industrial enterprises are fulfilled by the resources which would disappear in few years. This leads to the need for switching our concentration toward inexhaustible energy sources to meet our needs and protect our conventional sources of energy (Manokar 2019, Kabeel et al. 2019aManokar et al. 2018c). Water can be desalinated using the solar still, and the power requirements can be met with photovoltaic (PV) panel. These methods provide great hope to safeguard our future resource requirements. Commercially, PV panel is coupled with solar still to produce electricity and also fresh water at the same time. Scarcity of potable water and absence of electrical facilities can be tackled by this technique (Raj and Manokar 2017;Manokar and Winston 2017;Manokar et al. 2018b;El-Agouz et al. 2018). AlNimr et al. (2018) introduced a distillation unit model with photovoltaic/thermoelectric cooler (PV/TEC). The TEC is incorporated to enhance the condensation of water and prevent excessive heating in this model. The daily highest distillate yield of 4.2 kg was obtained at air temperature of 25°C, wind rate of 5.5 m/s, and solar intensity of 1000 W/m 2 . A total of 73 W of power output was reported with photovoltaic cell Responsible Editor: Philippe Garrigues efficiency of 12.32 %. Al-Nimr et al. (2019) introduced a novel PV/T distillation unit with photovoltaic cells that were submerged inside the basin water. Potable water yield of 6.8 L/m 2 /day was achieved. Al-Nimr and Dahdolan (2015) designed a novel PV/T unit which uses the eliminated heat energy for the purpose of distillation. The model is incorporated with a separate evaporator and a condenser chamber to augment its performance.
Layek (2018) investigated the yield and efficiency of basin type solar stills by making use of black toner, black ink, and black dye as absorptive substances. It was noted that the stills with black ink, dye, and toner showed highest energy efficiency of 41.3, 43.42, and 45.79% and exergy efficiency of 5.91, 6.34, and 7.10% in each instance. Panchal et al. (2018) did a relative study between a triangular pyramid still under active and passive mode to analyze the exergy and economic factors. The energy efficiency was increased at the time of sunset while the values fell during the day. Madhu et al. (2018) published the exergy analysis of conventional solar still (CSS) integrated with sand heat energy storage arrangement. The distillate yield from the still with energy storing substance was reported to be 3.3 kg/m 2 , and it showed 30 % greater exergy efficiency in relation to the CSS. Kumar et al. (2017) used a triangular pyramid solar still incorporated with inclined solar still (ISS) to enhance the distilled water production. The percentage rise in yield was recorded to be 79.05 % for the modified still at water depth of 0.02 m compared to the CSS. Kabeel et al. (2018) researched about the potable water production by the CSS combined with an ISS. The productivity of the modified still was reported to be 6.2 kg, and it showed 46.23 % greater yield production compared to the CSS at a water depth of 0.02 m. Tiwari et al. (2015) studied the developed still integrated with PVT-flat plate collector (FPC) to determine its yield production, exergy, and thermal efficiency.  examined the cost analysis by combining the energy payback period effect to the PVTcompound parabolic concentrator (CPC) combined solar still in active mode. The percentage rise in yearly yield was reported to be around 5 % higher for the double slope compared to that of the single slope still at a water depth of 0.14 m. The cost of freshwater production was also noted to be lesser for the double slope still. Singh and Tiwari (2017a, b, c) analyzed the PVT-CPC integrated solar still for various economic and production factors. The double slope still showed a percentage rise in distillate yield of 8.56 % in relation to the still with single slope structure at 0.14-m depth of water. Singh and Tiwari (2017a) experimented the double slope solar still combined with PVT-FPC. The energy producing factor and the life cycle conversion effectiveness were observed to be enhanced, while the time of energy payback and the potable water price were decreased for the modified still compared to the CSS. Singh and Tiwari (2017b) investigated the energy, exergy, and cost of producing distilled water in CSS integrated with tubular collectors. The percentage rise in energy and exergy values in the modified still was recorded to be 6.85 % and 12.30 % in each instance compared to single slope setup. Also the cost of obtaining the distillate yield from the developed still was 15.19 % lower compared to the CSS. Shmroukh and Ookawara (2020) studied the stepped solar still model incorporated with reflectors and copper fins. It was noted that the daily water production reached up to 8.28 L/day, and this model showed 129 % enhancement in yield when compared to the conventional model. Parsa et al. (2019Parsa et al. ( , 2020aParsa et al. ( , b, c, d, 2021 researched the SS performance at Mount Tochal. Recently, Hassan (2019a, b, 2020) used PCM in still. Attia et al. (2021a, b) used phosphate in still. Output of the ISS was improved by using wick materials (Hansen et al. 2015;Janarthanan et al. 2006;Mahdi et al. 2011;Sharon et al. 2017), stepped basin (El-Agouz 2014; El-Samadony and Kabeel 2014; El-Samadony et al. 2016) external condenser (Morad et al. 2017), external reflector (Tanaka 2011(Tanaka , 2013, active mode (Abdullah 2013;Kabeel et al. 2012), and PV/T integration (Manokar et al. 2019a, b, Sasikumar et al. 2020, Thalib et al. 2021, 22, Taamneh et al. 2020, Kabeel et al. 2019a. From the detailed reviews on latest manuscript and different ISS, it is found that only few manuscripts were published on active ISS. Here, this research is primarily aimed at the investigation of ISPBS integrated with STC at varied mass flow rate of water.

Diagram and fabrication of the suggested model
The illustrative diagram and the experimental organization of the ISPBS combined with STC (AISS) are depicted in Figs. 1 and 2. The STC is incorporated along with the ISPBS, and various parameters are analyzed for different m f . The measurements of the still covered by 0.4-cm-wide glass are 181 cm (length) × 92 cm (width) × 15 cm (height). The saline water at constant m f reaches the STC from the storage tank. The absorber tube in the heater heats up the water flowing through it and delivers it to the ISPBS. The construction of the STC solar water heater was done with a spiral tube, regulation valve. and a tank for storage. The solar collector has dimensions of 90 cm (L) × 60 cm (W) × 0.4 cm (H), and its construction was done using a wooden container (2-cm thick) along with a 0.4-cmwide cover glass. The heater includes a copper tube (0.1-cm thick) having three windings each of diameter 1 cm and separated from each other by 5 cm. A 5000-ml plastic columnar reservoir tank is used to store water.

Outcomes and analyses
Hourly changes in various parameters of the AISS The alterations in the solar radiation and air temperature of the AISS are depicted in Fig. 3a and b. The solar radiation rose to attain its highest value until 1 P.M. after which it started to fall. The highest values of solar radiation were recorded to be 880, 870, and 900 W/m 2 on 1.8.2017, 4.8.2017, and 6.8.2017 in each instance. The average solar radiation of 699 W/m 2 was observed on 1.8.2017 while on 4.8.2017, it was noted to be 696 W/m 2 and 719 W/m 2 on 6.8.2017. The highest air temperatures recorded were 33, 32, and 35°C on 1.8.2017, 4.8.2017, and 6.8.2017 in each instance. The average air temperature per day was noted to be about 30 to 33°C.
The alterations in the wind speed and temperature of the cover glass of AISS are depicted in Fig. 4a and b. The average velocity of wind was recorded to be 1.1, 1.8, and 2 m/s on 1.8.2017, 4.8.2017, and 6.8.2017 on each instance. The highest temperature of the cover glass of 53°C was observed on 1.8.2017, while on 4.8.2017, it was noted to be 52°C and 47°C on 6.8.2017. The average temperature of the collector cover per day was recorded to be 47.1, 45, and 43.2°C on 1.8.2017, 4.8.2017, and 6.8.2017 on each instance. From the recorded data, wind speed and temperature of the cover glass of AISS are higher when the wind speed is maximum. On comparing 3-day readings, it was found that average temperature of the collector cover is minimum (43.2°C) when the average velocity of wind was maximum (2 m/s).
Influence on glass, basin, and brackish water temperature due to the mass flow rates The alterations in the basin temperature of the AISS with respect to the m f are depicted in Fig. 5a. The temperature of the basin rose with rise in solar irradiation to attain its highest value until 2 P.M. after which it started to fall. For m f of 1.8, 3.2, and 4.7 kg/h, the in each instance. The average temperature of the basin per day was noted to be 62.1, 59.1, and 55.2°C for m f of 1.8, 3.2, and 4.7 kg/h in each instance. The basin temperature fell by 4.8 % when m f rose from 1.8 to 3.2 kg/h and by 11.1 % when m f rose to 4.7 from 1.8 kg/h. So, from the noted and calculated values, it is seen that the basin temperature falls when the m f of water is raised as a large quantity of water tends to absorb the temperature from the basin.
The alterations in the temperature of brackish water of the AISS with respect to the m f of water are depicted in Fig. 5b. Similar to the basin temperature, the brackish water temperature also rose up to attain its highest value until 2 P.M. after which it started to fall.
For water m f values of 1.8, 3.2, and 4.7 kg/h, the highest temperature of saline water was observed to be 76, 73, and 70°C in each instance. The average temperature of saline water per day was noted to be 65.7, 63, and 60.7°C for m f values of 1.8, 3.2, and 4.7 kg/h in each instance. The average water temperature per day fell by 4.1 % when m f rose from 1.8 to 3.2 kg/h and by 7.6 % when m f rose to 4.7 from 1.8 kg/h. So, from the noted and calculated values, it is seen that the increased m f leads to the flow of increased quantity of water which in turn causes inefficient heating and ultimately reduces the brackish water temperature and therefore productivity of the still. Least m f rises the time of  Influence on the distillate yield, thermal, and exergy efficiency due to the mass flow rates The alterations in the distillate yield of the AISS with respect to the m f of water are depicted in Fig. 6. Highest daily productivity is achieved only when the water m f is the lowest as water temperature in the AISS can be enhanced only in this condition. The potable water yield was recorded to be 8.1, 6.9, and 6.1 kg for m f values of 1.8, 3.2, and 4.7 kg/h in each instance. The daily productivity fell by 14.68 % when m f rose from 1.8 to 3.2 kg/h and by 25.3 % when m f rose to 4.7 from 1.8 kg/h. During the testing, it was observed that m f plays an important role in ISS output. When the m f increases, the period of contact among the brackish water and absorber of AISS is declined which leads to ineffective heating and lower yield.
The alterations in the thermal efficiency of the AISS with respect to the m f of water are depicted in Fig. 7a. The highest thermal efficiency was recorded to be 68.3, 61, and 54.3 % for m f of 1.8, 3.2, and 4.7 kg/h in each instance. Also, the average thermal efficiency was observed to be 47.9, 39.3, and 31.02 % for the m f of 1.8, 3.2, and 4.7 kg/h in each instance. The thermal efficiency on average fell by 17.9 % when m f rose from 1.8 to 3.2 kg/h and by 35.2 % when m f rose to 4.7 from 1.8 kg/h. Thermal efficiency of the ISPBS decreases when m f increases. The reason is when m f roses from minimum to maximum, the amount of water flowing over the ISPBS was higher which leads to poor output.
The alterations in the exergy efficiency of the AISS with respect to the m f of water are depicted in Fig. 7b. The highest exergy efficiency per hour was recorded to be 15.5, 12.5, and 10.2 % for m f of 1.8, 3.2, and 4.7 kg/h in each instance. The average exergy efficiency was observed to be 9.8, 7.9, and 5.6 % for the m f of 1.8, 3.2, and 4.7 kg/h in each instance. Rise in m f led to the fall in exergy efficiency due to the above cited reason. The exergy efficiency fell by 19.3 % when m f rose from 1.8 to 3.2 kg/ h and by 43.7 % when m f rose to 4.7 from 1.8 kg/h. Thermal and exergy efficiency formulas are referred from Tiwari et al. (2009).
Thermal effectiveness of the AISS is calculated by Exergy input to STC is determined using Useful heat gained by STC is calculated using Heat lost from STC is determined using Influence on power production, electrical, thermal, and exergy effectiveness of the PV panel due to mass flow rates Figure 8a, b, and c depict the alterations in the panel temperature, PV panel electrical, energy, and exergy effectiveness for an AISS at various m f . From Fig. 8a, b,  Increasing the m f of the AISS results in reduction in panel temperature. The average temperature of the panel per day reduces up to 7.02 and 14.42 % when the m f of water rises from 1.8 to 3.2 kg/h and 1.8 to 4.7 kg/h in each instance. The highest hourly PV panel power generation and efficiency of an AISS at 1.8, 3.2, and 4.7 kg/h is recorded to be 68.4, 71.4, and 74.1 W and 8.5, 8.7, and 9.2 % in each instance. The average power generation and efficiency in a day are 41.64, 46.14, and 50.44 W and 6.5, 7.1, and 7.5 % for the m f of 1.8, 3.2, and 4.7 kg/h in each instance. Power generation from an AISS increases with an increase of m f . The percentage increases in PV panel power production and efficiency are noted to be 9.78 and 9.1 % when the daily average PV panel temperature decreases up to 7.02 %. Similarly, 17.47 and 14 % rise in PV panel power production and efficiency are recorded when the daily average PV panel temperature decreases up to 14.42 %. From the studies on varying m f, it is found that performance of the PV panel was enhanced by using larger m f over the ISPBS surface. Panel output is inversely proportional to the temperature of the panel.
PV thermal effectiveness has the similar characteristics as the electrical efficiency of the solar panel, and it reaches its peak value of 20.45, 20.67, and 21.67 % at m f of 1.8, 3.2, and 4.7 kg/h, in each instance at 1 P.M. The average daily thermal efficiency of the solar panel is 15.67, 17.10, and 18.04% at m f of 1.8, 3.2, and 4.7 kg/h in each instance. The constant 0.38 is electric power production effectiveness for a conventional power From Fig. 8a, b, and c, it can be seen that the solar panel has higher exergy efficiency at least solar panel temperature. The average PV panel exergy efficiencies in a day are 20.03, 22.21, and 23.36 % at m f of 1.8, 3.2, and 4.7 kg/h in each instance. When the m f of water was increased from 1.8 to 3.2 kg/h and from 1.8 to 4.7 kg/h the exergy efficiency of a PV panel enhanced up to 9.8 and 14.3 % in each instance. The output of PV panel was improved by flowing higher m f so maximum m f (4.7 kg/h) has maximum exergy efficiency.
The performance of the PV panel is assessed by referring the work of .
The electrical efficiency of the solar panel is calculated using The thermal efficiency of the PV panel is determined using The exergy efficiency of the solar panel is calculated using

Hourly variations in the overall thermal and exergy efficiency of an AISS
The alterations in the overall thermal efficiency of an AISS (thermal efficiencies of an ISPB still and a PV panel) at various m f are depicted in Fig. 9a. The overall thermal efficiency of the AISS is the highest when the m f is the least. When the m f rises, the thermal efficiency of the ISPB still decreases, whereas there is an increase in thermal efficiency of the PV panel. The highest overall thermal efficiencies of the system per hour are 88.72, 76.76, and 76.01 % at m f at 1.8, 3.2, and It is noted that there is a 9.5% and 18.4 % reduction in the overall thermal efficiency of the system when the m f increases from 1.8 to 3.2 kg/h and from 1.8 to 4.7 kg/h in each instance. When the m f increases, thermal efficiency of the ISPBS considerably decreases and thermal effectiveness of the panel slightly increases. Figure 9b depicts the overall exergy efficiencies of an AISS at various m f . The average exergy efficiencies of the AISS in a day are recorded to be 9.8, 7.9, and 5.6 % and average PV panel exergy efficiencies of the model per day are about 20.03, 22.21, and 23.36 % and the overall average exergy efficiency of the AISS are 29. 97, 30.25, and 29.07 % at m f at 1.8, 3.2, and 4.7 kg/h in each instance. The daily average exergy efficiency of the AISS tends to reduce when the m f increases, whereas the daily average exergy efficiency of the PV panel is enhanced. The decrease in exergy efficiency of the AISS and increases in panel exergy efficiency will result in nearly equal overall exergy efficiency for all the m f .
The overall thermal effectiveness of an AISS is calculated by The overall exergy effectiveness of an AISS is determined using Relative study between the productivity of various PV/T solar stills A relative study on the amount of distillate yield from various hybrid PV/T solar still was done and noted in Table 1. Dev and Tiwari (2010) analyzed the performance of the CSS integrated with FPC and PV module and obtained a yield of 7.223 kg/m 2 . Tiwari (2009, 2010) recorded productivity in the range of 6-10 kg/m 2 from their hybrid active solar still. A yield of 7.9 kg/m 2 was noted in Gaur and Tiwari (2010) study involving hybrid solar still incorporated with collectors. Saeedi et al. (2015) obtained an enhanced yield of 8.37 kg/m 2 from the active solar still model. Eltawil and  Table 2. From the yield mentioned in Table 2, it is found maximum output of distilled water of 14.7 kg was achieved by using ISS with PCM (Kabeel et al. 2019a, b), and the present research produced the maximum output water of 8.1 kg at minimum flow rate of brackish water. The maximum thermal efficiency of 66.7% was obtained by ISS incorporated to exterior condenser by Morad et al. (2017), and the present system has maximum thermal efficiency of 47.9%.

Economic analysis
In this division, in-depth cost-effective study of the AISS at various m f has been presented. The payback period of the AISS and the cost of output water generation are determined using the following parameters (Kabeel et al. 2010).
The principle amount (capital) recovery factor (PARF) is calculated as follows: where n is considered to be 10 years and I is assumed to be 12% every year.
The annual first cost (AFC) is determined using the following formula: Annual maintenance cost (AMC) is given by Sinking fund factor (SFF) calculated as follows: The annual salvage cost (ASC) is calculated as follows: Annual cost (AC) is calculated as follows: The cost of producing fresh water is determined by Cost of fresh water =lÞ ¼ AC=Annual ð ð yieldyieldÞ The annual yield is found by multiplying the yield from a single sunny day by the number of clear sunny days (270).
The payback term of AISS (in months) is given by where CF = yearly yield * selling price of the output water The selling price of distilled water is assumed to be Rs 5 per liter in this computation.
In Table 3, the PARF, AFC, AMC, SFF, ASC, AC, cost of output water, and the payback term of the AISS at various m f are compared. The fabrication cost of the AISS is $289.2. The production cost of yield was seen to be cheaper in the case of AISS at minimum m f of 1.8 kg per hour as compared to other two m f (3.2 and 4.7 kg per hour). The payback term was also reduced for the AISS at minimum m f of 1.8 kg per hour as compared to other two m f (3.2 and 4.7 kg per hour).

Conclusions
This study is based on the influence of different m f in the ISPBS incorporated with the STC unit.
The following results have been reported: 1. The panel temperature and solar radiation determined the power production from the PV panel. Rise in m f led to the enhancement in the performance of the PV panel but the still performance declined. 2. The daily distillate water output, energy, exergy, and overall thermal efficiency of the arrangement falls by 14.68, 17.9, 19.3, and 9.5 % in each instance when m f is changed to 3.2 from 1.8 kg/h. 3. The power production of the panel, electrical efficiency, and exergy efficiency of the system is improved by 9.78, 9.1, and 9.8 % in each instance when m f is changed to 3.2 from 1.8 kg/h. 4. The daily distillate productivity, thermal, exergy, and overall thermal efficiency of the arrangement falls by 25.3, 35.2, 43.7, and 18.4 % in each instance when water m f is changed to 4.7 kg/h from 1.8 kg/h. 5. The power production of the panel, electrical efficiency, and exergy efficiency of the system is improved by 17.47, 14, and 14.3 % in each instance when m f is changed to 3.2 from 1.8 kg/h. still; STC, spiral tube collector; AISS, ISPB still combined with STC; m f , mass flow rate of water; PSS, pyramid solar still; ISS, inclined solar still; FPC, flat plate collector; CPC, compound parabolic concentrator; A, area (m2); h, heat transfer coefficient (W/m 2 K); I, current (A); I(t), solar intensity (W/m 2 ); L, latent heat of vaporization (kJ/kg K); M, hourly productivity from solar still (kg/m 2 h); P, partial vapor pressure (N/m 2 ); T, temperature ( o C); V, voltage (V); η, efficiency (%); a, ambient; c, convective; d, daily; e, evaporative; g, glass; gi, inner glass; pv, photovoltaic; s, surface area of condensing cover; w, water Author's contribution Mohamed Thalib Mohamed Rafeek-Writingreview and editing Vimala Muthu-Writing-review and editing Muthu Manokar Athikesavan-Writing, formal analysis, review and editing Ravishankar Sathyamurthy-Project administration, software, review and editing Abd Elnaby Kabeel-Data curation, review and editing Data availability All data are given in the manuscript.

Declarations
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Competing interests The authors declare no competing interests.