Developing countries require simple low-technology methods to desalinate drinking water. Passive solar stills are an example of a simple low-technology innovation that can desalinate saline water for small populations. Compared to single-chamber solar stills, double-chamber solar stills have the potential of increasing the yield of solar stills due to an increased condensation surface area. An experiment was carried out to determine the optimal angle of double-chamber solar stills. The set-up comprised double-chamber solar stills with angles of 10°, 20°, 30° and 40°, with a control set-up of a 20° single-chamber solar still. The experiment was conducted in January 2022 at Juja in Kiambu County, Kenya. The double-chamber solar stills comprised an evaporation chamber and a condensation chamber. The dimensions of the chambers were 0.5 m × 0.5 m with a height of 0.25 m. The main assumptions were that there existed steady-state conditions and that the solar still was leakproof. The research found that the 40° double-chamber solar still had the highest yield of 3.756 l/m2/day and the 10° double-chamber solar still had the least yield of 1.644 l/m2/day. Comparing the 20° double-chamber still and the 20° single-chamber still (control), the double-chamber solar still had a higher external efficiency.

  • The configuration of the double-slope single-chamber has not been experimented on before.

  • The geographical location is unique (Kenya (1°north of the equator)).

Graphical Abstract

Graphical Abstract
Graphical Abstract

With 55% of groundwater being saline water and groundwater being a major source of water in arid and semi-arid lands (ASAL), saline water treatment is an important aspect of the water supply in these areas (Gleick 1996). The World Health Organization (WHO) recommends a total dissolved solids (TDS) upper limit of 600 mg/l for palatable water (WHO 2017). The available desalination techniques can be broadly divided into two: thermal techniques and membrane techniques.

For small communities in areas with sufficient hours of sunlight, solar stills offer a cheaper alternative to energy-intensive thermal and membrane desalination techniques. For communities in ASAL areas in Kenya, low-cost, low-maintenance systems are preferred due to the limited resource capacity of the local governments. The basin-type passive solar stills present the best option for achieving economy in construction and maintenance. Most research has been on single-chamber solar stills.

With the majority of passive solar stills having a single chamber, a double-chamber passive solar still was proposed in this research. Through natural convection, water evaporates in the first chamber and condensates in the second chamber. This research intends to increase the yield of solar stills by optimizing the roof inclination angle of a double-chamber basin-type passive solar still. Different glass inclination angles are evaluated.

Double-chamber passive solar stills improve the efficiency of single-slope solar stills by introducing a separate condensation chamber. A separate condensation chamber separated by a phase chamber can increase the production of distillate double fold, especially at night when there is no solar energy (Sathyamurthy et al. 2015). Double-chamber solar stills have been found to have lower surface temperatures, lower saline water temperatures and higher yields compared to single-chamber solar stills (Madhlopa & Johnstone 2009).

For single-chamber solar stills, the height between the basin and the roof is inversely proportional to the yield. A comparison of the relationship between the average height and the yield found an inverse relationship between the yield and the average height. An average height of 0.15 m resulted in a yield of 3.19 kg/m2/day and an average height of 0.45 m resulted in a yield of 1.12 kg/m2/day in the experiments (Jamil & Akhtar 2017; Rajaseenivasan et al. 2017).

The angle of inclination of the glass cover allows the condensate to flow to the collection gutter. An inadequate angle of inclination will result in some of the condensate falling back into the saline water before reaching the collection gutter. High angles of inclination will also lead to increased thermal losses from the glass cover. Some researchers have preferred using the latitude of the site location as being the angle of inclination of the glass cover. The latitude of a location corresponds to the inclination of the sun's locus in that particular area. The inclination of the sun's locus affects the inclination of the incident solar radiation on the surface of the solar still. Therefore, the use of a glass cover angle that corresponds to the latitude of an area holds ground. However, other researchers have found no correlation between the latitude of an area and the glass inclination angle (Khalifa 2011).

In an experiment to investigate the relationship between the angle of reflectors and the glass cover angle carried out at a location with a latitude of 33.3°N, glass cover angles of 20°, 30° and 40° were considered. Solar still configurations that had glass cover inclinations of 30° and 40° had the highest yields, with the 40° solar still recording the highest yield of 3.9 l/m2/day (Khalifa & Ibrahim 2010)⁠. The angle of inclination was measured from the horizontal plane. A similar increase in the yield with an increase in the roof inclination angle was true for two-stepped single-chamber solar stills (Goshayeshi & Safaei 2020).

Sathyamurthy et al. (2015)⁠ showed that a double-chamber solar still with a phase change material separating the two chambers improved the yield two-fold. The majority of the work on double-chamber solar stills has been on the use of an external condenser that consumes energy to extract the water vapour from the evaporation chamber, making them active solar stills. To make the use of solar stills practical in ASAL areas in Africa, the cost of construction and maintenance should be minimized. Therefore, passive solar stills are preferred over active solar stills. The current research intended to improve the geometry of the passive double-chamber solar stills to increase the yield. The research gap is summarized in Table 1.

Table 1

Research gap summary

SourceFindingsResearch gap
Sathyamurthy et al. (2015)  A separate condensation chamber increases the yield of a double-chamber solar still Only one glass inclination angle used 
Rajaseenivasan et al. (2017)  An inverse relationship between the height of the solar still and the yield for single-chamber solar stills No similar research has been done for double-chamber double-slope solar stills 
Murugavel et al. (2010)  Single basin double-slope stills had similar theoretical and empirical yields The research did not include double basin (double-chamber) stills 
Dev et al. (2011)  Developed a characteristic equation for a single basin double-slope solar still The research did not include double basin (double-chamber) stills 
Azooz & Younis (2016)  Found the optimum glass inclination angle as 20–25° for single-chamber single-slope passive solar stills The paper did not research the optimum glass inclination angles for double-chamber double-slope passive solar stills. 
SourceFindingsResearch gap
Sathyamurthy et al. (2015)  A separate condensation chamber increases the yield of a double-chamber solar still Only one glass inclination angle used 
Rajaseenivasan et al. (2017)  An inverse relationship between the height of the solar still and the yield for single-chamber solar stills No similar research has been done for double-chamber double-slope solar stills 
Murugavel et al. (2010)  Single basin double-slope stills had similar theoretical and empirical yields The research did not include double basin (double-chamber) stills 
Dev et al. (2011)  Developed a characteristic equation for a single basin double-slope solar still The research did not include double basin (double-chamber) stills 
Azooz & Younis (2016)  Found the optimum glass inclination angle as 20–25° for single-chamber single-slope passive solar stills The paper did not research the optimum glass inclination angles for double-chamber double-slope passive solar stills. 

This paper focusses on double-chamber solar stills, which provide an increased surface area for condensation to occur. The objective of the research was to determine the best roof geometry of double-chamber basin-type passive solar stills to maximize yield. This research also tested whether the latitude corresponded to the glass cover inclination angle with the maximum yield for double-chamber solar stills.

Materials

Solar still materials affect the yield of solar stills. A summary of the materials used in various experiments on the solar still is provided in Table 2.

Table 2

Solar still construction materials in various literature

S. No.BodyCoverInsulationReferences
Galvanized steel sheet – 0.7 mm thick Glass – 4 mm thick, transmissivity = 0.84 Polystyrene – 60 mm thick, thermal conductivity = 0.029 W/m °C Khalifa & Ibrahim (2010)  
Wood – 4 mm thick Transparent acrylic sheet – 3 mm thick, transmittance = 0.88 Underneath – sawdust – 150 mm
Sides of still – glass wool 
Arunkumar et al. (2012)  
Glass-reinforced plastic – 5 mm thick Glass – 4 mm thick Not specified Dev et al. (2011)  
Vinyl chloride sheet – 1 mm thick Vinyl chloride sheet – 0.5 mm thick Not specified Ahsan et al. (2012)  
Wood – 4 mm thick Glass – 4 mm thick Sawdust Arunkumar et al. (2016)  
Polymethyl methacrylate (acrylic glass) – 8 mm thick Glass – 2 mm thick Polystyrene – 20 mm thick Bhardwaj et al. (2015)  
Galvanized iron sheet – 1.5 mm thick Glass – 4 mm thick Polystyrene – 25 mm thick Kumar et al. (2016)  
Aluminium sheets Glass – 3 mm thick Wood Ozuomba et al. (2012)  
Glass-reinforced plastic (fibreglass) – 4 mm thick Toughed glass – 4 mm thick Not specified Panchal & Shah (2011)  
10 Galvanized iron sheet – 16 gauge Glass with silicon sealant Glass wool – 50 mm thick Vala et al. (2017)  
S. No.BodyCoverInsulationReferences
Galvanized steel sheet – 0.7 mm thick Glass – 4 mm thick, transmissivity = 0.84 Polystyrene – 60 mm thick, thermal conductivity = 0.029 W/m °C Khalifa & Ibrahim (2010)  
Wood – 4 mm thick Transparent acrylic sheet – 3 mm thick, transmittance = 0.88 Underneath – sawdust – 150 mm
Sides of still – glass wool 
Arunkumar et al. (2012)  
Glass-reinforced plastic – 5 mm thick Glass – 4 mm thick Not specified Dev et al. (2011)  
Vinyl chloride sheet – 1 mm thick Vinyl chloride sheet – 0.5 mm thick Not specified Ahsan et al. (2012)  
Wood – 4 mm thick Glass – 4 mm thick Sawdust Arunkumar et al. (2016)  
Polymethyl methacrylate (acrylic glass) – 8 mm thick Glass – 2 mm thick Polystyrene – 20 mm thick Bhardwaj et al. (2015)  
Galvanized iron sheet – 1.5 mm thick Glass – 4 mm thick Polystyrene – 25 mm thick Kumar et al. (2016)  
Aluminium sheets Glass – 3 mm thick Wood Ozuomba et al. (2012)  
Glass-reinforced plastic (fibreglass) – 4 mm thick Toughed glass – 4 mm thick Not specified Panchal & Shah (2011)  
10 Galvanized iron sheet – 16 gauge Glass with silicon sealant Glass wool – 50 mm thick Vala et al. (2017)  

To prevent vapour leaking, it is important to ensure that the solar still is air-tight. Silicon sealant can be used to connect the glass to the solar still basin (Vala et al. 2017).

The materials selected for the solar still were:

  • Body and cover in condensation chamber – galvanized iron sheet – 16 gauge thick

  • Cover in evaporation chamber – glass – 4 mm thick

  • Insulation – polystyrene – 50 mm thick

The solar still was leakproof. The galvanized iron sheet was adequately welded together. Similarly, epoxy resin was used at the glass–iron interface to ensure air tightness. All the inner surfaces of the solar still were painted black.

Methods

Five set-ups were running simultaneously, including one control set-up. There were four set-ups with double chambers with glass inclination angles of 10°, 20°, 30° and 40°, respectively. The experimental set-up is provided in Figure 1.
Figure 1

Solar still experimental set-up.

Figure 1

Solar still experimental set-up.

Close modal

The set-ups are listed in Table 3.

Table 3

Solar still set-ups

S. No.Glass inclination angle (°)
10 
20 
30 
40 
20 
S. No.Glass inclination angle (°)
10 
20 
30 
40 
20 

The control set-up was a single-slope solar still with a height H of 0.5 m and a glass inclination angle of 20°.

The flow of convection currents was idealized as shown in Figure 2.
Figure 2

Flow of convection currents in solar still.

Figure 2

Flow of convection currents in solar still.

Close modal

The solar stills were constructed in open ground with a free flow of air and direct sunlight. The solar stills were set up in Juja, Kiambu County, Kenya. Juja is an ASAL region located within 1°31′ and 2°59′ south of the equator and 37°8′ and 38°30′ east of the Greenwich meridian.

The water level in the evaporation chamber was maintained at a constant height of 20 mm.

The yield of the solar still was measured by a graduated cylinder connected to the condensing chamber. The yield was measured hourly from 8:00 a.m. to 5:00 p.m. The TDS test was done on both the saline water and the yield from the solar stills. To achieve saline water, table salt was added to tap water to achieve a TDS value of 1,000 mg/l. The solar radiation was measured using a solar power meter, and the wind speed was measured using a digital anemometer. The wind speed data were recorded every hour from 6:00 a.m. to 7:00 p.m. The temperatures were measured using K-Type thermocouples on an hourly basis. The temperatures of the following surfaces were obtained: glass cover, evaporation walls (evapwalls), evaporation base (evapbase), condensation roof (condroof), condensation walls (condwalls) and the condensation base (condbase). In addition, the internal ambient temperature and the external ambient temperature were recorded.

The surface temperatures were used to compute heat transfer equations. The heat transfer equations used were as outlined in Dunkle (1961) and Clark (1990). The condensing surfaces in single-chamber solar stills are two: the glass and the walls. Water condensing on the walls flows back to the basin and analysis of the heat transfer has primarily been on the heat transfer between the water in the basin and the glass surface. In contrast, the double-chamber solar still has five condensing surfaces: the glass, the walls on the evaporation side (evapwalls), the walls on the condensation side (condwalls), the sloping roof on the condensation side (condroof) and the base on the condensation side (condbase). Similar to the single-chamber still, water on the evaporation walls will flow back to the water basin and will not be harvested.

The convective heat transfer coefficient between the water and the condensing surface qc, w-cond (W/m2) can be given by Equation (1).
formula
(1)
The coefficient hc,w-cond is the convective heat transfer coefficient in W/m2 °C between water and the condensing surface. Tw is the temperature of the water in the basin in °C and Tcond is the temperature of the condensing surface in °C. The coefficient hc,w-cond was obtained using Equation (2).
formula
(2)
Pw is the partial pressure at the water temperature and Pcond is the partial pressure at the temperature of the condensing surface. Pw and Pcond were obtained using Equations (3) and (4), respectively.
formula
(3)
formula
(4)
The evaporative heat transfer coefficient between water and the condensing surface qe,w-cond (W/m2) can be given by Equation (5).
formula
(5)
where he,w-cond is the evaporative heat transfer coefficient in W/m2 °C. The coefficient was obtained using Equation (6).
formula
(6)
The radiative heat transfer coefficient between the water and the condensing surface qr,w-cond (W/m2) can be given by Equation (7).
formula
(7)
where hr,w-cond is the radiative heat transfer coefficient in W/m2 °C. The coefficient was obtained using Equation (8).
formula
(8)
In Equation (8), εeff is the effective emittance between water and the condensing surface and σ is the Stefan–Boltzmann's constant which is 5.67 × 10−8 W/m2·K4. εeff was computed using Equation (9) where εw is the emittance of the water and εcond is the emittance of the condensing surface. The emittances used for water, glass and steel surfaces were 0.95, 0.92 and 0.60, respectively.
formula
(9)
The total heat transfer coefficient qt,w-cond (W/m2) is the sum of the heat transfer coefficients due to convection, evaporation and radiation and was given by Equation (10).
formula
(10)
The external efficiency ηext of the solar stills was analysed using Equation (11).
formula
(11)

The parameter m is the mass of the condensate produced in kg, hvap is the heat of vaporization (J/kg), Io is the insolation flux (W/m2), Ag is the area of the glass (m2), and Δt is the time interval (s). The latent heat of vaporization at 25 °C was assumed to be 2.26 × 106 J/kg.

The internal efficiency ηint of the solar stills was computed using Equation (12).
formula
(12)

Yield

The yields per square metre obtained during the experiment are provided in Table 4.

Table 4

Yield production per square metre

DateYield/m2
Control40°30°20°10°
12/01/22 1,060 1,996 1,568 1,332 972 
13/01/22 928 1,932 1,428 1,032 800 
17/01/22 780 1,772 1,192 1,028 688 
18/01/22 640 1,628 1,020 864 576 
19/01/22 880 2,048 1,444 1,040 832 
20/01/22 984 2,848 2,064 1,384 908 
21/01/22 1,828 3,756 3,048 2,096 1,644 
DateYield/m2
Control40°30°20°10°
12/01/22 1,060 1,996 1,568 1,332 972 
13/01/22 928 1,932 1,428 1,032 800 
17/01/22 780 1,772 1,192 1,028 688 
18/01/22 640 1,628 1,020 864 576 
19/01/22 880 2,048 1,444 1,040 832 
20/01/22 984 2,848 2,064 1,384 908 
21/01/22 1,828 3,756 3,048 2,096 1,644 

The order of yield production from the highest to the lowest was as follows: 40°, 30°, 20°, control and 10°. The yield of the solar still is directly proportional to the glass inclination angle.

The typical yield production during the day is shown in Figure 3.
Figure 3

Typical yield production per hour.

Figure 3

Typical yield production per hour.

Close modal

As shown in Figure 3, the yield production curves for all set-ups including the control follow the profile of the solar intensity from 8:00 a.m. to about 3:00 p.m. After 3:00 p.m., the yield per hour increases in the double-chamber solar stills, while the yield per hour in the control set-up continues to decrease up to 5:00 p.m.

There exist crossover points between the 10° still and the 20° and 30° stills at around 1:00 p.m. and again at around 4:00 p.m. When the sun's radiation was not at its maximum, the 10° solar still had a higher yield production rate compared to the 20° and the 30° stills. At non-peak sunshine hours, the lower inclined solar stills produced higher hourly yields as compared to the higher inclined solar stills. During peak sunshine hours, the higher glass inclination angle stills produced higher hourly yields (Tiwari et al. 1994).

From 5:00 p.m. to 8:00 a.m. the following day, the yields of the solar stills were higher compared to the yields between 8:00 a.m. and 5:00 p.m. The percentage of yield produced between 5:00 p.m. and 8:00 a.m. is detailed in Table 5.

Table 5

Percentage yield production at night

DateControl (%)40° (%)30° (%)20° (%)10° (%)
11/01/22 87.34 78.54 77.53 79.89 77.09 
12/01/22 80.38 69.34 78.32 79.88 81.48 
13/01/22 72.41 59.42 68.35 68.22 71.50 
14/01/22 86.95 79.28 76.91 79.53 83.48 
17/01/22 74.87 66.59 69.46 71.98 71.51 
18/01/22 71.25 60.20 65.49 67.13 61.81 
19/01/22 66.36 49.22 63.16 61.15 60.58 
20/01/22 42.68 42.42 45.35 38.73 42.73 
21/01/22 77.46 66.88 73.49 69.47 76.89 
DateControl (%)40° (%)30° (%)20° (%)10° (%)
11/01/22 87.34 78.54 77.53 79.89 77.09 
12/01/22 80.38 69.34 78.32 79.88 81.48 
13/01/22 72.41 59.42 68.35 68.22 71.50 
14/01/22 86.95 79.28 76.91 79.53 83.48 
17/01/22 74.87 66.59 69.46 71.98 71.51 
18/01/22 71.25 60.20 65.49 67.13 61.81 
19/01/22 66.36 49.22 63.16 61.15 60.58 
20/01/22 42.68 42.42 45.35 38.73 42.73 
21/01/22 77.46 66.88 73.49 69.47 76.89 

On all days except one, the percentage of yield production between 5:00 p.m. and 8:00 a.m. is higher than 60%.

The optimum glass inclination of 40° is consistent with the findings of Aybar & Assefi (2009) who found an optimum glass inclination angle of 35°. Different researchers had pointed to the correlation between the glass inclination angle and the latitude of an area (Goshayeshi & Safaei 2020). On the contrary, other researchers found no correlation between the latitude and the glass inclination angle (Khalifa 2011). For the test region with a latitude of 1°N, the optimum inclination angle of 40° is not equal to the latitude of the area.

Most experiments that support the proposition that the optimum inclination angle is equal to the latitude of the area were carried out in the northern hemisphere (Egypt, India, Pakistan and Cyprus). These areas have latitudes of between 13° and 35°.

Temperature

The temperatures of the different surfaces in the solar stills are presented in Figures 4 and 5. The surface temperatures of similar surfaces in the different solar stills are plotted together in Figure 7.
Figure 4

Solar still surface temperatures – evaporation zone at different angles: (a) 10°, (b) 20°, (c) 30° and (d) 40°.

Figure 4

Solar still surface temperatures – evaporation zone at different angles: (a) 10°, (b) 20°, (c) 30° and (d) 40°.

Close modal
Figure 5

Solar still surface temperatures – condensation zone at different angles: (a) 10°, (b) 20°, (c) 30° and (d) 40°.

Figure 5

Solar still surface temperatures – condensation zone at different angles: (a) 10°, (b) 20°, (c) 30° and (d) 40°.

Close modal
Figure 6

Control solar still surface temperatures.

Figure 6

Control solar still surface temperatures.

Close modal
Figure 7

Temperatures for similar surfaces in stills with different glass inclination angles.

Figure 7

Temperatures for similar surfaces in stills with different glass inclination angles.

Close modal

Variation of temperatures for different surfaces in the same still

For all four solar stills, the profiles of the temperature follow the profile of the solar intensity, except the evaporation base (evapbase).

From Figures 4 and 5, the temperatures followed the profile of the solar intensity during the day. The evaporation zone had higher temperatures compared to the condensation zone. The temperature gradient assisted in condensation occurring in the condensation zone.

In the control solar still (Figure 6), the temperature of the glass is not the highest during the day as in the other solar stills. The surface with the highest temperatures is the side wall. The surface with the lowest temperatures at 5:00 p.m. is the glass surface.

Variation of temperatures for similar surfaces in different stills

The variation of temperatures for similar surfaces in different stills is shown in Figure 7.

From Figure 7, the 10° solar still has the highest temperatures, followed by the control solar still. The 20° and 40° solar stills have similar temperature profiles.

The order of temperatures from the highest to the lowest is as follows: the control solar still, the 20° solar still, the 30° solar still, the 40° solar still and the 10° solar still.

Temperature gradients between evaporation and condensation zone surfaces

The temperature gradients between the evaporation and the condensation zone surfaces are illustrated in Figures 8 and 9.
Figure 8

Temperature difference between evaporation and condensation walls.

Figure 8

Temperature difference between evaporation and condensation walls.

Close modal
Figure 9

Temperature difference between evaporation and condensation base.

Figure 9

Temperature difference between evaporation and condensation base.

Close modal

Before mid-day, the condensation walls had higher temperatures than the evaporation walls. The 10° still set-up has the least variation in temperatures and may explain the low yields obtained in this set-up.

Maximum and average temperatures on surfaces

The highest and average temperatures in degrees Celsius recorded on each surface are detailed in Tables 6 and 7.

Table 6

Maximum temperatures on surfaces

SurfaceGlass inclination angle
10°20°30°40°Control
Condensation base 39 42 42 41 N/A 
Condensation wall 45 44 44 44 N/A 
Condensation roof 64 61 56 55 N/A 
Evaporation base 39 46 46 41 50 
Evaporation wall 49 60 58 54 61 
Glass 63 51 50 51 54 
Separator 43 55 54 46 N/A 
Ambient 46 55 57 41 64 
Grand maximum 64 61 58 55 64 
SurfaceGlass inclination angle
10°20°30°40°Control
Condensation base 39 42 42 41 N/A 
Condensation wall 45 44 44 44 N/A 
Condensation roof 64 61 56 55 N/A 
Evaporation base 39 46 46 41 50 
Evaporation wall 49 60 58 54 61 
Glass 63 51 50 51 54 
Separator 43 55 54 46 N/A 
Ambient 46 55 57 41 64 
Grand maximum 64 61 58 55 64 
Table 7

Average temperatures on surfaces

SurfaceGlass inclination angle
10°20°30°40°Control
Condensation base 26.27 27.21 27.73 27.62  
Condensation wall 29.67 30.21 29.75 30.38  
Condensation roof 33.75 33.35 32.00 32.19  
Evaporation base 26.00 29.25 28.65 26.85 31.40 
Evaporation wall 30.38 33.67 33.02 32.00 35.71 
Glass 34.06 29.52 29.24 30.65 32.17 
Separator 28.67 32.06 31.87 29.46  
Ambient 28.81 32.92 34.02 26.38 37.96 
Grand average 29.70 31.02 30.78 29.44 34.31 
SurfaceGlass inclination angle
10°20°30°40°Control
Condensation base 26.27 27.21 27.73 27.62  
Condensation wall 29.67 30.21 29.75 30.38  
Condensation roof 33.75 33.35 32.00 32.19  
Evaporation base 26.00 29.25 28.65 26.85 31.40 
Evaporation wall 30.38 33.67 33.02 32.00 35.71 
Glass 34.06 29.52 29.24 30.65 32.17 
Separator 28.67 32.06 31.87 29.46  
Ambient 28.81 32.92 34.02 26.38 37.96 
Grand average 29.70 31.02 30.78 29.44 34.31 

From Tables 6 and 7, there is a correlation between the maximum and average temperatures and yield. The higher the grand maximum and the grand average temperatures, the lower the solar still yields. This correlation is illustrated in Figure 10. The findings in Figure 10 are supported by the temperature gradients between the evaporation and condensation surfaces as illustrated in Figures 8 and 9.
Figure 10

Yield and temperatures against glass inclination angles.

Figure 10

Yield and temperatures against glass inclination angles.

Close modal

To investigate whether there was a significant variation in temperatures for the different surfaces for the different glass inclination angles, the single-factor F test was used. The null and alternate hypotheses were as follows:

  • H0: There is no significant difference in the temperatures of similar surfaces for different glass angles, i.e., x1 = x2.

  • H1: There is a significant difference in the temperatures of similar surfaces for different glass angles, i.e., x1x2.

The level of significance used in the test was 0.05. The single-factor one-way ANOVA analysis results are summarized in Table 8.

Table 8

ANOVA analysis results

Source of variationSSdfMSFP-valueFcrit
Between groups 16.536 5.512 1.024 0.395 2.901 
Within groups 172.175 32 5.380    
Total 188.711 35     
Source of variationSSdfMSFP-valueFcrit
Between groups 16.536 5.512 1.024 0.395 2.901 
Within groups 172.175 32 5.380    
Total 188.711 35     

The Fcalc value of 1.024 is less than the Fcrit value of 2.901. Therefore, we do not reject the null hypothesis. There is no significant difference between the temperatures of similar surfaces for different glass inclination angles. The differences in yield among the solar stills are not attributed to the differences in the temperature gradients of the solar still surfaces.

Figure 10 presents a graph of yield, maximum temperatures and average temperatures against the glass inclination angles.

Experiments carried out on single-chamber pyramid-type passive solar stills showed that the solar stills with the highest yields also had the highest surface temperatures (Kabeel et al. 2016). Goshayeshi & Safaei (2020) also found similar results for convex-shaped vs. flat glass covers. The convex-shaped single-chamber stills have higher inclination angles. The convex-shaped stills had higher yields and also higher surface temperatures compared to the flat glass covers. The findings in this research do not conform to the findings of other researchers. For double-chamber solar stills, the higher yields are associated with lower average surface temperatures. The correlation coefficient between average temperatures and yield for the data illustrated in Figure 10 is −0.49. The correlation coefficient indicates a moderate negative correlation between the average temperatures and the yield.

Heat transfer coefficients and efficiency

The convective heat transfer coefficients (qc,w-cond) and the evaporative heat transfer coefficients (qe,w-cond) for the four condensing surfaces, i.e., glass, condroof, condbase and condwall, are illustrated in Figures 11 and 12, respectively.
Figure 11

Convective heat transfer coefficients – (a) water to glass, (b) 20 water to condroof, (c) water to condbase and (d) water to condwalls.

Figure 11

Convective heat transfer coefficients – (a) water to glass, (b) 20 water to condroof, (c) water to condbase and (d) water to condwalls.

Close modal
Figure 12

Evaporative heat transfer coefficients – (a) water to glass, (b) 20 water to condroof, (c) water to condbase and (d) water to condwalls.

Figure 12

Evaporative heat transfer coefficients – (a) water to glass, (b) 20 water to condroof, (c) water to condbase and (d) water to condwalls.

Close modal

Characteristically, the evaporative heat transfer coefficients were higher than the convective heat transfer coefficients in the order of 10. The 10° double-chamber solar still had the highest heat transfer coefficients between water and glass and between water and the condensation roof.

The internal efficiencies computed using Equation (12) are summarized in Table 9.

Table 9

Internal efficiencies

Condensing surface10° (%)20° (%)30° (%)40° (%)
Glass 87.85 26.08 49.93 39.79 
Condensation roof 88.08 51.93 40.77 53.04 
Condensation base 18.93 27.31 28.18 24.90 
Condensation walls 32.75 28.67 27.48 34.81 
Average 56.90 33.50 36.59 38.13 
Condensing surface10° (%)20° (%)30° (%)40° (%)
Glass 87.85 26.08 49.93 39.79 
Condensation roof 88.08 51.93 40.77 53.04 
Condensation base 18.93 27.31 28.18 24.90 
Condensation walls 32.75 28.67 27.48 34.81 
Average 56.90 33.50 36.59 38.13 

The external efficiencies computed using Equation (11) are summarized in Table 10.

Table 10

External efficiencies

Solar still angle10° (%)20° (%)30° (%)40° (%)Control
Efficiency 58.67 68.34 91.43 92.67 58.30 
Solar still angle10° (%)20° (%)30° (%)40° (%)Control
Efficiency 58.67 68.34 91.43 92.67 58.30 

From Table 9, the 10° solar still had the highest internal efficiencies. The high efficiency in the 10° solar still did not translate to higher yields. For the 20°, 30° and 40° stills, the order of their efficiencies corresponded to the order of their yields. The external efficiencies in Table 10 are directly proportional to the yields obtained from the stills. Comparing the 20° still and the control still, the double-chamber solar still had a higher external efficiency. The difference between the external efficiencies of these stills was 10.04%.

Correlation of yield with solar intensity, wind and ambient temperature

The correlation of yield with solar intensity, wind and ambient temperature is provided in Table 11.

Table 11

Correlation of yield with solar intensity, wind and ambient temperature

Set-upControl40°30°20°10°
Solar intensity 0.19 0.60 0.55 0.57 0.44 
Wind 0.53 0.47 0.47 0.54 0.89 
Ambient Temperature 0.08 0.64 0.49 0.51 0.50 
Set-upControl40°30°20°10°
Solar intensity 0.19 0.60 0.55 0.57 0.44 
Wind 0.53 0.47 0.47 0.54 0.89 
Ambient Temperature 0.08 0.64 0.49 0.51 0.50 

The 40° set-up had the highest correlations between yield and solar intensity and between yield and ambient temperature. The 10° set-up had the highest correlation between yield and wind. The positive correlations in Table 11 indicate that the three factors: ambient (external) temperatures, wind speeds and solar intensities are directly proportional to solar still yields. The findings in this research are consistent with the findings of Ahsan et al. (2014), Prakash & Velmurugan (2015) and Andrew Jones et al. (2014). The control set-up had very low correlations between the yield and solar intensity and between yield and ambient temperature. The low correlations may account for the low yields experienced in the control set-up.

The 40° inclined solar still produced the highest yield and the 10° inclined solar still produced the least yield. The order of yield production from the highest to the lowest was as follows: 40°, 30°, 20°, control and 10°. Comparing the conventional single-chamber solar still to the double-chamber solar still, the double-chamber solar still produced higher yields. The 20° inclined double-chamber solar still produced 31.5% more yield on average compared to the 20° inclined single-chamber solar still.

The yield production at night accounted for more than 60% of all the yield produced by the stills.

There was no correlation between the latitude of the area and the optimum glass inclination angle of the solar still for the region (1°N of the equator). The tests were carried out in an area whose latitude was 1°N and the optimum glass inclination was 40°. A single-factor one-way ANOVA analysis showed that there is no significant difference between the temperatures of similar surfaces for different glass inclination angles.

The 10° solar still had the highest internal efficiencies. However, the high efficiency in the 10° solar still did not translate to higher yields. For the 20°, 30° and 40° stills, the order of their efficiencies corresponds to the order of their yields. Comparing the 20° still and the control still, the double-chamber solar still had a higher external efficiency. The difference between the external efficiencies of these stills was 10.04%.

There were positive correlations of yield with solar intensity, wind and ambient temperature. The 40° set-up had the highest correlations between yield and solar intensity and between yield and ambient temperature. The 10° set-up had the highest correlation between yield and wind.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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