Heat transfer analysis of hybrid active solar still with water flowing over glass cover

The aim of this research is to carry out the heat transfer analysis of PVT hybrid active solar still (HASS) at different water depth to obtain maximum output. Experimentation is performed for validation of thermal modeling with and without flowing water, having water depth of 0.15m in the solar still basin. During experimentation, water flows above the glass cover. Theoreti-cally calculated values of basin water, basin liner, glass temperature and yield obtained using thermal modeling are very near to the experimental values having correlation coefficients 0.988, 0.981, 0.999 and 0.985 respectively. It is also found that thermal efficiency and daily exergy output increases by 4% and 8.2% respectively for this hybrid system whose glass cover is getting cooled by water flowing over it. Theoretical calculation for distillate output of the system was also calculated out for various climatic conditions of India using developed thermal modeling and it is found that proposed system gives higher annual yield of 2756.67 kg/m 2 for the climate of Mumbai. The experimental uncertainty of the HASS is obtained as 14.82%. Cite this article as: Gaur MK, Tiwari GN, Singh P, Kushwah A. Heat transfer analysis of hybrid active solar still with water flowing over glass cover. J Ther Eng 2021;7(6):1329–1343.


INTRODUCTION
Solar still consumes solar energy for producing fresh water from saline or impure water.As 97% of water is saline water available either in seas or oceans.Remaining water is locked in rivers, lakes and glaciers [1].There is need of such system which can produce fresh water for huge population growth.In an every field, the solar energy is used as an alternative energy source [2,3].For water purification, solar distillation system is found sustainable and it can be installed easily in remote/rural areas where availability of drinking water is poor [4].Basically the solar stills are classified into active and passive types [5].Active stills are better in terms of productivity than passive ones [6].
The innovative idea of utilizing thermal energy for evaporation of brine water and its consequent condensation for getting distilled water was traditionally in practice from very ancient times.Telkes firstly described the accurate methodical relationship between the transports phenomena involved in the distillation process [7].Digital simulation method was used by Cooper to analyze the solar still process [8].In another work, he presented the highest theoretical efficiency of ideal basin type solar still up to 60% [9].To evaluate the heat and mass transfer taking place inside the solar still, theoretical models and mathematical relations were firstly given by Dunkle (1961) [10].The Dunkle's model was found accurate up to some extent beyond the yield of 0.1 g/m 2 s, whereas the modified Dunkle's model gives over prediction of measurements at yield higher than 0.1 g/m 2 s, this was also lately confirmed by Rahbar and Esfahani's CFD numerical investigation [11].In early 1990's Malik et al. (1982) worked on solar still operating in passive mode [12].Tiwari and Tiwari (2008) studied about the active and passive type of solar distillation systems pertaining rigorous analysis on performance evaluation and enhancement [13].
The difference in water and glass cover temperature mainly affects the amount of distillate collected.This can be increased either by (i) Designing new system or using any external devices for increasing the temperature of water in basin.(ii) Utilizing latent heat of vaporization of distilled water by double effect distillation or Regenerative effect.
Theoretical model was developed by Kumar and Tiwari for calculating daily output of active type double effect solar still.The performances of active type single and doubleeffect solar stills was also tested with and without flowing water over the glass cover during the peak summer [14,15].
Yousef et al. carried out the performance evaluation and modeling of regenerative type solar desalination unit [16].They have considered a regenerative unit with two basins, having an arrangement for circulating cold water in and out through the second basin.They have also found that the output in case of regenerative still was obtained higher than the conventional still by 20%.During experimentation in air regenerative solar stills, thermal efficiency of passive type was observed better than active type and also in a solar distillation system attached with wind-driven aspirator, the thermal efficiency rises with rise in flow velocity of air [17].The performance of regenerative type solar still was evaluated by Prakash and Kavathekar under ideal conditions [18].The conclusions reported after parametric study of regenerative type active solar still are (i) Thermal efficiency of Passive stills was better than active ones and (ii) with rise in velocity of water, the thermal efficiency also rises [19].Effect of thermal capacity on the performance of solar distillation system with water flowing above the glass cover was studied by Lawrence et al. [20].The result showed that the efficiency of the solar still gets rises with increase in water flow rate over the glass cover.Singh and Tiwari studied the thermal performance of a regenerative type active solar still working on thermosyphonic effect in climate of Delhi [21].It was observed that the overall performance improves significantly but regenerative effect had no impact on its yield.Performance of solar distillation system was enhanced by Bassam and Abu-Hijleh by cooling glass cover by using water film over it [22].They have reported that suitable usage of water film over glass cover increases the still efficiency, but if poor combinations of parameters were taken it will results in reductions of still efficiency.Wibulswa and Tadtiam have reported rise in daily output of the solar still with regenerative effect on vertical wall [23].Kudish have studied regenerative effect on glass cover of the solar distillation system [24].The idea of flowing water over glass cover and at the back wall of the still and then recirculating this hot water inside the still was given by Wibulswa and Suntrirat [25].
Gugulothu et al. suggested that sensible and latent heat storage system can be used as association to overcome the gap between the energy sources, the sun and the desalination unit [26].Effect of water depth was studied by Tiwari and Madhuri with transient analysis of a solar still [27].The result showed that dependency of yield at certain water depth depends on the brine temperature.The daily output of the still gets increased when the brine initial temperature lies above 40°C and below 45°C.With advancement in the field of solar distillation a new concept of incorporation of flat plate collector/collectors and a pump to circulate water in solar still was developed to raise output of solar distillation systems.Hybrid systems can be used in remote/rural and this makes the system self-sustainable.P.T. Tsilingiris derived a set of expressions for calculating mass transfer coefficient based on the Bowen's ratio definition for application in solar stills which include Dunkle's mass transfer models [28].Kumar and Tiwari estimated internal heat and mass transfer coefficients and analysed life cycle cost and exergy for HASS [29,30].Using energy and exergy analysis, the number of solar collectors in HASS was optimized by Gaur and Tiwari for water with different heat capacity [31].
Singh and Tiwari studied solar still integrated with N identical PV/T compound parabolic concentrator and found 7 collectors were optimum for active solar distillation system with mass flow rate of 0.04 kg/s.Kabeel and Abdelgaied modified the solar still by integrating it with cylindrical parabolic concentrator, oil serpentine loop and phase change material under basin [32].The mean daily efficiency of modified still was found 20.27% higher than conventional still.El-Sebaii and Naggar performed cost and annual performance analysis of single basin finned type solar distillation system using different fin materials [33].They have also evaluated the experimental model showing improvement ratio of productivity (IRP) around 19% and improvement ratio of efficiency (IRE) 24% during winter are attached in series to obtain maximum basin water temperature.A semi-transparent glass to glass PV module of 70 W p is attached on partial surface of first flat-plate collector to generate D.C. power for operating the pump which circulates water from solar still to flat-plate collector.A 1.2 m length and 0.0127m diameter fiber plastic pipe having small-small holes throughout entire length is provided at the top height of the solar still for flowing water over the cover.Flow of the water is maintained through control valve.A proper arrangement has been made for outlet of the water flowing above the glass cover.

THERMAL MODELING
The assumptions considered during the thermal modeling of the integrated HASS are:

PVT Collectors Connected in Series
For PV module and collectors, useful heat output is given as [39]: Rate of useful thermal energy available at the outlet of the PV and PVT collectors = (Total solar thermal energy absorbed by both the collectors) -(Heat loss from absorber plates and solar cell to ambient) ( ) and summer and also shows finned-basin liner still are way better than conventional stills.Manokar et al. reviewed integrated PV/T solar still in comprehensive manner comparing performances of still in terms of yield along with considering testing place and latitude [34].Kumar et al. analysed the impact of water mass on triangulated pyramid solar still attached with inclined solar still [35].The study showed that maximum yield of 7.52 kg/m 2 was obtained from triangulated pyramidal solar still.The study on single slope active solar still coupled with two reflectors and PV/T system had also been done [36].The setup was tested in climate of Ksar Challala, Algeria and the productivity was found increased in all seasons especially 127.06% in winter.Monthly performance of active and passive solar still for diverse climate was studied by Singh and Tiwari [37].In order to get higher productivity from active solar still, absorber of porous nature with bubble wrap for insulation had been tested by Arunkumar et al. [38].
The present research focuses on the following objective: • The validation of the developed thermal modeling, analysis of thermal efficiency and exergy analysis of the developed HASS.• On the basis of developed thermal modeling, calculations of monthly and yearly yield for this modified solar distillation system is carried out in five zones of India namely New Delhi, Bangalore, Mumbai, Srinagar and Jodhpur having different climatic conditions.
Such kind of research has not been done before.The selection of these cities is made on the basis of divergence in their climatic conditions.The basically four types of weather conditions are shown in Table 1.
The data of average solar radiations in different climatic conditions has been taken from Indian Metrological Department (IMD), Pune.

EXPERIMENTAL SETUP
The HASS consists of one normal and one hybrid flatplate collectors inclined at 45º, an arrangement for flowing water over glass surface, water storage tank and conventional solar still with cover inclination of 30º.
Figure 1(a) and 1(b) illustrates the cross-sectional side and pictorial view of HASS respectively.Both the collectors  where, ( )

Energy Balance in HASS with Water Flowing over Glass Cover: (i) Basin liner
The expression of the T b can be written from the above equation as: Following assumptions are considered to get approximate solution of Eq. ( 6): • At any time interval Δt, a 1 is also constant.Now, the solution of Eq. 6 is gives by: ( ) Since the length of condensing cover is about 1m (small) and mass flow rate m ˙w2 over the glass cover is large (0.058 kg/sec) and hence there will be not much variation in T w2 with x.
After integrating above equation for the entire length of the cover, the Eq. ( 7) becomes: ( ) Here, T w2i = T a Or ( ) After putting the values of T g and T b in Eq. ( 3), we will get:

 (ii) Water mass
For water mass inside still, energy balance equation is given by: ) (iii) Glass cover The expression of T g can be written from the above equation as: (iv) Water mass flowing above the glass surface: Consider a small thickness (dx) of water film flowing over the glass cover as illustrated in Figure 2.
The energy balance for small area (bdx) of water film can be given as: Solving the above equation for the average water temperature.The Eq. ( 5) becomes: where, Following assumptions are considered to get solution of the equation ( 11): • f 2 (t) is considered constant at any time interval Δt i.e. f 2 (t) = f -2 (t).• At any time interval Δt, a 2 is also constant.Solution of Eq. ( 11) is written as: ( ) Where, T wo = Basin Temperature when t = 0 f -2 (t) = Mean value of f 2 (t) during time interval 0 and t.
In this experiment, t = 3600 sec.The rate of evaporative heat transfer is calculated using: The rate at which water evaporates in the still is given by:

Exergy Analysis
According to II nd law of thermodynamics, the exergy analysis of the solar still includes the total exergy inflow, outflow and destructed [40].
Where, ΣE ˙xin = exergy input, ΣE ˙xout = exergy output and ΣE ˙xdest = exergy destructed Exergy output from a solar still is given by: For solar still, Exergy input is summation of exergy input of solar still, PVT collector, collector and pump, and is given by: The exergy input to PVT collector and collector is given as [41]: Substituting the value of ( ) If we assume, ( ) then Eq. ( 10) becomes: The solution of above differential equation is given as  2.

Experimental Uncertainty
The accuracy of the measured data is validated by calculating the uncertainty in the measured data.Uncertainty analysis provides the error possible in the values measured by using a certain instrument.The accuracy and standard uncertainty are given by Table 3.The standard uncertainty is calculated by Eq. 22 given as, Experimental uncertainty has been calculated by taking the square root of the summation of individual standard uncertainties divided by square of number of samples taken.The internal uncertainty (U) is given by Eq. ( 23) as follows [42], The exergy input of the pump is given as: The wattage of the pump (in this case the wattage of pump is 40W for 8 feet height) For solar still the exergy input given by: ( ) Overall Efficiencies of Regenerative HASS Overall thermal efficiency of the HASS is given by: The overall exergy efficiency of still is calculated by: Exergy destruction (E ˙xdest ) inside a solar still is given by:

Experimental Procedure and Observations
The experiments were conducted on November 19 and 20, 2018, with and without water flowing above the glass cover of the HASS having 0.15 m water depth in the basin.To maintain the quasi-steady state condition, the solar still is to be filled with 150 liters of water in 24 hours before the commencement of the experiment.Water is allowed to flow  The total experimental uncertainty for hybrid active solar still has been calculated as 14.82%.

Theoretical Observations
Data collected from IMD (Indian meteorological department) Pune have been used for the theoretical calculations.Following steps are used for computing the hourly values various temperature i.e. basin liner, basin water temperature, glass surface temperature and yield: i) Initially when t = 0, the temperature of basin water (T wo ) and inner glass (T gi ) are observed.Total internal heat transfer coefficient was calculated for different temperature, design and other climatic parameters.After calculating the value of total internal heat transfer coefficient, the value of still water temperature (T w ) is also calculated at time t using Eq.12. Using T w , the values of T g and T b are evaluated and then hourly yield is obtained using Eqs.13-14.
ii) Now by using Eq. 8, the value of average water temperature (T -w2 ) has been calculated.After getting the value of (T -w2 ) the new value of water temperature (T w ) and hourly yield at time t is calculated.
iii) The step I and II are repeated to calculate all heat transfer coefficients and yield on hourly basis for a day.iv) Thermal efficiency and exergy analysis for HASS with and without water flowing over the glass cover were calculated using Eqs.(19)(20).
The value of T -w2 is calculated from 9 am to 5 pm only.The value of T -w2 used in all calculations between 6 pm to morning 8 am has been taken as zero (i.e.no water flow above the glass cover).Algorithm MATLAB 7 is used for numerical computation.

RESULTS AND DISCUSSION
The hourly variation in solar intensity with ambient air temperature on a typical day of Nov.19 is illustrated in Figure 3.
Figure 4a-c show the hourly variation of calculated and observed values of basin liner, basin water and glass temperature of HASS with water flowing over glass cover for the typical day of November month for the composite condition of Delhi climate.All Figures show good agreement of theoretical results with the experimental results.
Variation in theoretical and experimental values of yield per hour is given in Figure 5. Figure shows close agreement of theoretical value of yield with the experimental value with correlation factor of 0.985.Figure 6 illustrates the hourly change in yield with and without water flowing over the glass cover of the HASS.The higher yield is obtained in case of HASS is due to the fact that with flow of water above the    glass surface the liberation of latent heat of condensation of water vapour becomes faster and hence a higher yield.The hourly yield curves for with and without water flow overlaps each other at night.This is because of the fact that the water flowing above the glass surface is stopped at 5:00 p.m.
Variation of daily efficiency has been shown in Figure 7. Figure shows that the daily efficiency of HASS is higher in case of water flowing over the glass surface.It is found that the efficiency for the HASS with water flow is 4% higher than the without water flow.The variation of hourly exergy and analysis of daily exergy with and without water flow for the HASS is shown in Figures 8a-b.Higher exergy output was reported for the HASS with water flowing over the glass surface and this is due to higher output of daily yield.Figures 9a-e shows the monthly variation of yield of four different climatic conditions for five climatic zones (i.e.Bangalore, Jodhpur, Mumbai, New Delhi and Srinagar) of India.Figure 9a shows that Bangalore city generates maximum yield for the b climatic condition throughout the year except three months (Mar, Apr and Sept.).It is due to the fact that maximum total number of days is for the b climatic condition and hence total maximum radiations.
The Figures 9b-c shows that for the climatic condition of Jodhpur and Mumbai the monthly yield is maximum for the b climatic condition throughout the year.It is due to the fact that in both cities the b climatic condition prevails almost for half the year, and hence receives maximum radiation.For the New Delhi climatic conditions the monthly yield is maximum for all four climatic conditions but in different months of the year as shown in Figure 9d.It is because of the fact that New Delhi has composite climate and there is large variation in the ambient air temperature throughout the year.Figure 9e shows that at least for six month, monthly yield for the 'a' and 'b' climatic condition is maximum for the Srinagar.It is due the fact that Srinagar has maximum number of days of the 'a' and the 'b' climatic condition (101 and 173 respectively).Srinagar generates the minimum monthly yield in the month of October, November, December and January because of low radiations and minimum ambient air temperature.
Figure 10a shows that for the a and b climatic condition the calculated value of yearly yield is maximum for the Bangalore and Jodhpur respectively but for both the c and the d climatic conditions the value of yearly yield is maximum for the New Delhi climatic zone.From the Figure 10b it is clear that the total annual yield is maximum for the Mumbai climatic condition and minimum for Srinagar climatic condition.The maximum annual yield (2756.67) for the Mumbai climatic condition is due to maximum total annual radiation and the moderate overall ambient air temperature throughout the year which enables solar distillation unit to generate higher temperature difference between basin water and glass surface and hence higher distillate output.The lowest yield for the Srinagar is due to minimum annual radiation and low ambient air temperature throughout the year.

CONCLUSIONS
The following conclusions have been reported on experimental and theoretical results for the given parameters of HASS: • Calculated values of yield and temperature of basin liner, basin water and glass using developed thermal modeling shows good correlation with the experimental data.

Figure 1 .
Figure 1.(a) Schematic diagram of Side view of hybrid solar still with water flow over the glass surface.(b) Developed Hybrid solar still with water flow over the glass surface.

Figure 2 .
Figure 2. Cross-sectional view of elementary length of flowing water over the glass surface.

2 ,
. Then Eq. (9) becomes: above the glass surface of the still by using control valve from 9 am to 5 pm.The experiment was carried out for 24 hours i.e. from 8 am of19 Nov to 8 am of 20 Nov 2018 (local time of New Delhi).Various parameters were measured hourly for different water depth like Temperature of Basin liner (T b ), Inner glass (T gi ), Outer glass (T go ), Vapour temperature (T v ), Water Temperature (T w ) at different depth, Ambient temperature (T a ), Total incident radiation, Hourly distillate yield, Wind velocity (V a ), etc.The various design parameters of the experimental setup are shown in Table

Figure 3 .
Figure 3. Hourly variation of solar intensity on horizontal plane, flat plate collector, and solar still and ambient temperature.

Figure 4 .
Figure 4. (a) Hourly variation of theoretical and experimental values of basin liner temperature (With water flow).(b) Hourly variation of theoretical and experimental values of basin water temperature (With water flow).(c) Hourly variation of theoretical and experimental values of glass temperature (With water flow).

Figure 5 .
Figure 5. Hourly variations of theoretical and experimental values of yield (With water flow).

Figure 6 .
Figure 6.Hourly variation of yield with and without water flow.

Figure 7 .Figure 8 .
Figure 7. Variation of daily efficiency with and without water flow.

Figure 9 .
Figure 9. (a) Monthly variation of calculated values of yield for different climatic conditions of Bangalore city.(b) Monthly variation of calculated values of yield for different climatic conditions of Jodhpur city.(c) Monthly variation of calculated values of yield for different climatic conditions of Mumbai city.(d) Monthly variation of calculated values of yield for different climatic conditions of New Delhi city.(e) Monthly variation of calculated values of yield for different climatic conditions of Srinagar city.

Figure 10 .
Figure 10.(a) Calculated values of Yearly yield for different climatic conditions for different climatic zones.(b) Calculated values of Yearly yield for different climatic zones in the various climatic conditions.(a)

Table 1 .
Different weather conditions with basis on which they are classified IVCloudy (fully) More than 0.75 Less than 5 hours

Table 2 .
Design parameters of photovoltaic module, flatplate collector and solar still

Table 3 .
Technical specification of the instruments used in the experiments 5.6697×10 -8 W/m 2 K 4 ) ε