4. Analysis of factors affecting the production of solar stills

The important factors affecting the output of a solar still can be summarized in Figure 4.

cover layer. However, when the wind speed increased from 3 to 6 m/s, the distilled water output increased only 1.6%. As noted above, the wind speed is too high and leads to heat loss, so that the gain in water output is almost negligible. This result is consistent with Cooper's survey [16]. In his research, as Cooper increased wind speeds from 0 to 2.15 m/s, the output of the still rose by 11.5%; when wind speed was increased from 2.15 to 8.81 m/s, the output of

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The influence of ambient temperature on (i) insulated distillation devices and (ii) uninsulated distilling equipment was studied, and the results are shown in Figure 5. In case (i), the decrease in ambient temperature leads to a higher distilled water output, while in the case of (ii), the opposite is observed. This can be explained as follows: for the distillation equipment with good insulation, lower temperature will help cool the glass cover faster, thereby increasing the temperature difference between the water layer and cover sheet. However, when the distillation equipment is not insulated, low ambient temperature increases heat loss of the device, leading to a reduction in water temperature in the equipment. Low-temperature still cools the glass cover, but the results in Figure 5 show that the impact of increased heat loss is

The change of 5C in well-insulated distillation systems will make the average distilled water output change 4.5%. This result is consistent with the results of the Khalifa and Hamood [17]. Their research has shown that when the ambient temperature rose from 26.7 to 37.8C, the outputs may rise 11% and when temperatures are reduced from 26.7 to 15.6C, the outputs fall

The change of 5C ambient temperature for insulated distillation devices make the distilled

distilled water increased by only 1.5%.

more important than the impact of lower glass temperature.

water output 2.5% change, as a result of SOLSTILL [12].

Figure 5. Effect of temperature on the output of solar stills [12].

4.1.3. Effect of ambient temperature

14%.

#### 4.1. Effects of weather conditions

#### 4.1.1. Impact of solar radiation

Solar radiation is the main and the most important factor to yield distilled water. The greater the radiation received, the greater the volume of distilled water produced and vice versa. However, the greater the radiation, the greater the heat loss of the still. Therefore, the insulation of the still needs to be carefully considered.

#### 4.1.2. Effect of wind speed

Wind speed values were varied from 0 to 6 m/s when inputted into SOLSTILL [12]. Hourly solar radiation data and ambient temperature data are included in the software.

The results showed that as the wind speed increased from 0 to 3 m/s, the higher wind speeds gave greater water output. This can be explained by noting that high wind speeds will cool the glass cover faster, leading to an increased temperature difference between the water and the

Figure 4. Factors affecting the outputs of solar distillation systems.

cover layer. However, when the wind speed increased from 3 to 6 m/s, the distilled water output increased only 1.6%. As noted above, the wind speed is too high and leads to heat loss, so that the gain in water output is almost negligible. This result is consistent with Cooper's survey [16]. In his research, as Cooper increased wind speeds from 0 to 2.15 m/s, the output of the still rose by 11.5%; when wind speed was increased from 2.15 to 8.81 m/s, the output of distilled water increased by only 1.5%.

#### 4.1.3. Effect of ambient temperature

4. Analysis of factors affecting the production of solar stills

4.1. Effects of weather conditions

tion of the still needs to be carefully considered.

Figure 4. Factors affecting the outputs of solar distillation systems.

4.1.1. Impact of solar radiation

162 Desalination and Water Treatment

4.1.2. Effect of wind speed

The important factors affecting the output of a solar still can be summarized in Figure 4.

Solar radiation is the main and the most important factor to yield distilled water. The greater the radiation received, the greater the volume of distilled water produced and vice versa. However, the greater the radiation, the greater the heat loss of the still. Therefore, the insula-

Wind speed values were varied from 0 to 6 m/s when inputted into SOLSTILL [12]. Hourly

The results showed that as the wind speed increased from 0 to 3 m/s, the higher wind speeds gave greater water output. This can be explained by noting that high wind speeds will cool the glass cover faster, leading to an increased temperature difference between the water and the

solar radiation data and ambient temperature data are included in the software.

The influence of ambient temperature on (i) insulated distillation devices and (ii) uninsulated distilling equipment was studied, and the results are shown in Figure 5. In case (i), the decrease in ambient temperature leads to a higher distilled water output, while in the case of (ii), the opposite is observed. This can be explained as follows: for the distillation equipment with good insulation, lower temperature will help cool the glass cover faster, thereby increasing the temperature difference between the water layer and cover sheet. However, when the distillation equipment is not insulated, low ambient temperature increases heat loss of the device, leading to a reduction in water temperature in the equipment. Low-temperature still cools the glass cover, but the results in Figure 5 show that the impact of increased heat loss is more important than the impact of lower glass temperature.

The change of 5C in well-insulated distillation systems will make the average distilled water output change 4.5%. This result is consistent with the results of the Khalifa and Hamood [17]. Their research has shown that when the ambient temperature rose from 26.7 to 37.8C, the outputs may rise 11% and when temperatures are reduced from 26.7 to 15.6C, the outputs fall 14%.

The change of 5C ambient temperature for insulated distillation devices make the distilled water output 2.5% change, as a result of SOLSTILL [12].

Figure 5. Effect of temperature on the output of solar stills [12].

#### 4.1.4. Effect of the haze and dust

Solar stills, being placed outdoors to receive direct solar radiation, cannot avoid dust on the surface of coated glass. This reduces the coefficient of radiation incidents, thus reducing the efficiency of the distillation equipment. Additionally, if dust enters the inside surface of the glass, it can affect the condensation flow down to the collecting gutters with the distilled water dripping halfway down the glass. So, it is necessary to regularly check and clean the inside and outside of the cover to achieve the highest efficiency.

other seasons. This can be explained by the fact that in the late spring and summer, sunrise and sunset are to the south of the east-west axis. Therefore the kind of roof-type cover will benefit from having the second roof (that means a south heading) in the early morning and late afternoon. At other times of the year, the still with single-sloped cover will get all the available solar radiation and over a year; this still performs a little better. This is consistent with the

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Therefore, theoretically, one sloped cover should give a little more output than those having two slopes. In practice, however, the use of single slope introduces additional difficulties during system construction, requiring additional materials, and has more problems in terms of structural stability in high winds than the roof-type still. This may be the reason why the

Cooper, Khalifa and Hamood, Garg and Mann [16–18] showed that the glass covers can absorb approximately 4.75% of solar radiation energy, leading to an increase of the cover's temperature. On the other hand, the condensation of water below the glass surface creates a condensation water film, leading to a partially opaque glass surface and an increase in the glass' temperature. As a result, the temperature difference between the surface of the water and condensation glass cover will be reduced. Therefore, seeking to reduce the surface temperature of the glass is important in order to improve the water productivity (output) of the device [17]. To reduce the temperature of the glass cover, one can speed up the wind outside as mentioned in Section 4.1.2. However the wind is also a natural factor, and it is difficult to control this. So Husham [19] proposed a different approach—use a water cooling membrane on a piece of glass using tap sprays on the glass surface every 30 s with the time interval for the spray test of 10 and 20 min, respectively. Results of regular sprinkling helped increase productivity to 31.8

In the next section, the measures taken to reduce the temperature of covers and results

To take advantage of the latent heat of water vapor in the condensation, numerous studies have used distillation device models with two flat tanks (double basin) and three flat tanks (triple basin) [20]. This is a useful way to increase production of distilled water. However this device is complicated and costly. In Section 5 of this chapter, the results of experiments to take

The distance between the water surface coverings can affect the effectiveness of the solar distillation systems. As discussed in Section 4.2.1, if the distance between the water surface and coverings is small, the convective resistance of wet air flow inside the device is smaller, so that efficiency will be improved. But this gap is influenced by the inclination of coated glass. If

experimental results of Garg and Mann [18].

roof-type stills are still more favored.

and 15.7%, respectively.

achieved by this will be presented.

4.2.4. Utilizing the latent heat of evaporation

advantage of the latent heat of evaporation will also be shown.

4.2.5. Effect of the distance between the water level and covered glass

4.2.3. Effect of the temperatures of the covers

The simulation results of SOLSTILL show very clearly the dependence of distilled water output to the intensity of solar radiation that the distillation equipment receives [12].

### 4.2. Effects of cover properties

#### 4.2.1. Effect of glass cover's tilt

Distilled water output depends very much on the elements of the cover's angle and tilt direction. To ensure the distilled water will not drip while halfway down to the collecting gutters, the tilt of the covers must be more than 15. On the other hand, it is necessary to reduce the average distance between the water surface and the tilted covers; the tilt of the covers must be not more than 20 [17]. The SOLSTILL program also produces similar results, with the still output dropping rapidly when the cover slope angles are greater than 30 [12].

#### 4.2.2. The effects of single-sloped and two-sloped (roof type) covers

SOLSTILL can also be used to simulate the distillation equipment using one cover (single sloped) and two covers (double sloped, also known as roof type); the tilt is 15 in both cases. This assumes that the coverings the two types of devices have are the same and their axes lie along the east-west direction. The yields of the two distillation devices are shown in Figure 6.

The results show that the distillation device with double-sloped cover works better in late spring and summer while the distilling equipment with single-sloped cover works better in

Figure 6. The output of distilled water per month single roof and roof-type double.

other seasons. This can be explained by the fact that in the late spring and summer, sunrise and sunset are to the south of the east-west axis. Therefore the kind of roof-type cover will benefit from having the second roof (that means a south heading) in the early morning and late afternoon. At other times of the year, the still with single-sloped cover will get all the available solar radiation and over a year; this still performs a little better. This is consistent with the experimental results of Garg and Mann [18].

Therefore, theoretically, one sloped cover should give a little more output than those having two slopes. In practice, however, the use of single slope introduces additional difficulties during system construction, requiring additional materials, and has more problems in terms of structural stability in high winds than the roof-type still. This may be the reason why the roof-type stills are still more favored.

#### 4.2.3. Effect of the temperatures of the covers

4.1.4. Effect of the haze and dust

164 Desalination and Water Treatment

4.2. Effects of cover properties

4.2.1. Effect of glass cover's tilt

outside of the cover to achieve the highest efficiency.

Solar stills, being placed outdoors to receive direct solar radiation, cannot avoid dust on the surface of coated glass. This reduces the coefficient of radiation incidents, thus reducing the efficiency of the distillation equipment. Additionally, if dust enters the inside surface of the glass, it can affect the condensation flow down to the collecting gutters with the distilled water dripping halfway down the glass. So, it is necessary to regularly check and clean the inside and

The simulation results of SOLSTILL show very clearly the dependence of distilled water

Distilled water output depends very much on the elements of the cover's angle and tilt direction. To ensure the distilled water will not drip while halfway down to the collecting gutters, the tilt of the covers must be more than 15. On the other hand, it is necessary to reduce the average distance between the water surface and the tilted covers; the tilt of the covers must be not more than 20 [17]. The SOLSTILL program also produces similar results, with the still

SOLSTILL can also be used to simulate the distillation equipment using one cover (single sloped) and two covers (double sloped, also known as roof type); the tilt is 15 in both cases. This assumes that the coverings the two types of devices have are the same and their axes lie along the east-west direction. The yields of the two distillation devices are shown in Figure 6. The results show that the distillation device with double-sloped cover works better in late spring and summer while the distilling equipment with single-sloped cover works better in

output to the intensity of solar radiation that the distillation equipment receives [12].

output dropping rapidly when the cover slope angles are greater than 30 [12].

4.2.2. The effects of single-sloped and two-sloped (roof type) covers

Figure 6. The output of distilled water per month single roof and roof-type double.

Cooper, Khalifa and Hamood, Garg and Mann [16–18] showed that the glass covers can absorb approximately 4.75% of solar radiation energy, leading to an increase of the cover's temperature. On the other hand, the condensation of water below the glass surface creates a condensation water film, leading to a partially opaque glass surface and an increase in the glass' temperature. As a result, the temperature difference between the surface of the water and condensation glass cover will be reduced. Therefore, seeking to reduce the surface temperature of the glass is important in order to improve the water productivity (output) of the device [17].

To reduce the temperature of the glass cover, one can speed up the wind outside as mentioned in Section 4.1.2. However the wind is also a natural factor, and it is difficult to control this. So Husham [19] proposed a different approach—use a water cooling membrane on a piece of glass using tap sprays on the glass surface every 30 s with the time interval for the spray test of 10 and 20 min, respectively. Results of regular sprinkling helped increase productivity to 31.8 and 15.7%, respectively.

In the next section, the measures taken to reduce the temperature of covers and results achieved by this will be presented.

#### 4.2.4. Utilizing the latent heat of evaporation

To take advantage of the latent heat of water vapor in the condensation, numerous studies have used distillation device models with two flat tanks (double basin) and three flat tanks (triple basin) [20]. This is a useful way to increase production of distilled water. However this device is complicated and costly. In Section 5 of this chapter, the results of experiments to take advantage of the latent heat of evaporation will also be shown.

#### 4.2.5. Effect of the distance between the water level and covered glass

The distance between the water surface coverings can affect the effectiveness of the solar distillation systems. As discussed in Section 4.2.1, if the distance between the water surface and coverings is small, the convective resistance of wet air flow inside the device is smaller, so that efficiency will be improved. But this gap is influenced by the inclination of coated glass. If the tilt of the cover is increased, then the average distance between the water surface and coverings is widened, so the output of the still will decrease. In the latter part of this paper, the measures on a stepped solar still to achieve the smallest distance between the water level and the glass in order to achieve highest distilled water output will be presented.

4.4. Effects of other factors

surface.

4.4.1. Using the external condensing device

of the absorbing surface.

operation principles as follows:

of water

In traditional solar basin distillation systems, the glass covers are used to condense distilled water. This method enables the device to have a simple structure, but it also has two disadvantages:

Factors Affecting the Yield of Solar Distillation Systems and Measures to Improve Productivities

• Latent heat of vaporization released during condensation makes the temperature of the coated glass increase, resulting in increasing water vapor pressure near the coverings. This reduces the pressure difference between the water evaporative layer and the condensation

• The condensation of distilled water under the glass surface creates a film or droplet layer, reducing the ability of solar radiation penetrating the glass cover and reaching the bottom

The use of an external condenser and a recovery heat exchanger to take advantage of moist air stream with high temperature and humidity returned to the distillation system was also proposed and tested [12]. The results showed that the use of an external condenser could

of a recovery heat exchanger to circulate the moist air can increase by nearly 54% the output of

In Section 5 of this chapter, the results of theoretical and empirical research on the use of an

The process of heat and mass transfer inside a conventional solar still is a natural convective process. The low efficiencies of a conventional solar still may be overcome by changing the

• Using air as an intermediate medium and substituting forced convection for natural convection to increase the heat coefficients in the still, resulting in increased evaporation

• Replacing saturated air in the standard still by "drier" air to increase the potential for

• Circulating the air-vapor mixture from the standard still to external water-cooled condensers to gain efficiency from a lower condensing temperature. The cooler the cooling

• Recovering some of the heat extracted in the condensing process and using it to preheat

• Substituting the condensing area of the flat sheet covers in the standard still by the external condenser with much larger heat exchange areas to increase condensation efficiencies

) and the use

).

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167

increase output by 25% (average daily output of 3.87 l/m<sup>2</sup> compared to 3.10 l/m2

external condenser for a solar passive (or natural convective) still will be presented.

distilled water (average daily output of 4.76 l/m2 compared to 3.10 l/m2

4.4.2. The effect of the generation of forced convection inside the still

mass transfer in the still, leading to higher outputs

the air-vapor mixture entering the still

water available, the more effective this condensing process will be

In Section 5 of the chapter, this issue will be presented in greater detail.

#### 4.3. Effects of water properties

#### 4.3.1. The effect of water temperature in the still

The water temperatures in the equipment greatly affect the output of distilled water. As mentioned in Section 4.2.3, water on the cover as a thin film cools down the glass before running into the still. By using the latent heat of steam in the steam condensation under the glass covers, water can be heated and fed into the device. This approach can be applied to both passive and active solar stills.

In Section 5 of this chapter, the use of glass vacuum tubes to heat up the water in the basin of the still will be presented.

#### 4.3.2. The effects of water depth in the still

The depth of water in the device greatly affects the yield of distilled water. Due to thermal inertia, the deep water layers will make the absorption process of solar energy take longer, thus slowing the increase of water temperature and affecting the amount of distilled water. The experimental results of a single-basin solar still coupled with evacuated glass tubes [21] show that a test with a 1 cm depth of water in the basin produces 5.265 l/m<sup>2</sup> , which is 13.4% higher than a test using a 2 cm depth of water which produces only 4.555 l/m<sup>2</sup> , as shown in Figure 7. This agrees with the theoretical and experimental results in other researches [17, 22, 23].

Figure 7. Experimental distillate output with 1 and 2 cm water depth in the single-basin solar still coupled with evacuated glass tubes [21].

#### 4.4. Effects of other factors

the tilt of the cover is increased, then the average distance between the water surface and coverings is widened, so the output of the still will decrease. In the latter part of this paper, the measures on a stepped solar still to achieve the smallest distance between the water level

The water temperatures in the equipment greatly affect the output of distilled water. As mentioned in Section 4.2.3, water on the cover as a thin film cools down the glass before running into the still. By using the latent heat of steam in the steam condensation under the glass covers, water can be heated and fed into the device. This approach can be applied to both

In Section 5 of this chapter, the use of glass vacuum tubes to heat up the water in the basin of

The depth of water in the device greatly affects the yield of distilled water. Due to thermal inertia, the deep water layers will make the absorption process of solar energy take longer, thus slowing the increase of water temperature and affecting the amount of distilled water. The experimental results of a single-basin solar still coupled with evacuated glass tubes [21] show

This agrees with the theoretical and experimental results in other researches [17, 22, 23].

Figure 7. Experimental distillate output with 1 and 2 cm water depth in the single-basin solar still coupled with

, which is 13.4% higher

, as shown in Figure 7.

that a test with a 1 cm depth of water in the basin produces 5.265 l/m<sup>2</sup>

than a test using a 2 cm depth of water which produces only 4.555 l/m<sup>2</sup>

and the glass in order to achieve highest distilled water output will be presented.

4.3. Effects of water properties

166 Desalination and Water Treatment

passive and active solar stills.

4.3.2. The effects of water depth in the still

the still will be presented.

evacuated glass tubes [21].

4.3.1. The effect of water temperature in the still

#### 4.4.1. Using the external condensing device

In traditional solar basin distillation systems, the glass covers are used to condense distilled water. This method enables the device to have a simple structure, but it also has two disadvantages:


The use of an external condenser and a recovery heat exchanger to take advantage of moist air stream with high temperature and humidity returned to the distillation system was also proposed and tested [12]. The results showed that the use of an external condenser could increase output by 25% (average daily output of 3.87 l/m<sup>2</sup> compared to 3.10 l/m2 ) and the use of a recovery heat exchanger to circulate the moist air can increase by nearly 54% the output of distilled water (average daily output of 4.76 l/m2 compared to 3.10 l/m2 ).

In Section 5 of this chapter, the results of theoretical and empirical research on the use of an external condenser for a solar passive (or natural convective) still will be presented.

#### 4.4.2. The effect of the generation of forced convection inside the still

The process of heat and mass transfer inside a conventional solar still is a natural convective process. The low efficiencies of a conventional solar still may be overcome by changing the operation principles as follows:


In Section 5 of the chapter, this issue will be presented in greater detail.
