Experimental Investigations of Developed Solar Still for Increment in Efficiency and Rate of Distillate

2.1 Objectives and considerations

i. For high efficiency, the solar still should maintain

a) A high feed (impure) water temperature

b) Significant temperature gradient of feed water and condensing surface

c) Least possible vapour leakage.

ii. A high feed water temperature can be achieved if

a) Feed water absorbs higher proportion of solar radiation. For this, low absorptivity and high transmissivity glass is required.

b) Heat losses from the bottom and walls are kept low.

c) Proper ceramic wool insulation from all sides of basin.

iii. A large temperature difference can be achieved if

a) The condensing surface absorbs little or none of the incoming radiation.

b) Heat dissipated by condensing water on condensing surface is convected by another media.

2.2 Existing design approaches

i. Function Based 

Solar still may be classified into two types as follows

A) Passive solar still B) Active solar Still

A) Passive solar still

In passive solar stills the absorptivity of feed water is increased by using charcoal or other amorphous powders or dyes. The depth of water in the basin is decided on the basis of thermal capacity of the still. Also vapour tightness is assured by using rubber or gravel. Using reflective side walls and good insulation ensures minimum heat loss.

B) Active solar still

In active stills, to increase the rate of evaporation, the waste heat recovery or an additional collector is used to heat the water in basin. Also a vacuum may be maintained inside the still or glass cover may be cooled externally to increase the temperature gradient.

ii. Basin Types

1. Single-basin stills

A single basin of solar still has a thin layer of water, a transparent glass cover that covers the basin and channel for collecting the distillate water from solar still. The sunrays pass through high transmissivity glass cover and reach the basin surface through the feed water. The basin and the water get heated by this solar radiation. In a solar still, high temperature gradient between vapours and glass cover yields pure water at a higher rate. This enhances rate of evaporation from water surface in basin up to glass cover. Vapours travel towards glass surface due to pressure difference and condense on it. The distillate (condensate water) is collected through the adjoining channels. Efficiency of single basin type solar still is 15 % - 25 % typically.

2. Multiple basin stills

It has two or more compartments. The vapours in lower compartment condense on its upper compartment bottom surface. This heat is absorbed by upper compartment to vaporize the water further in that compartment, at higher rate. Efficiency of this still is higher about 35% than single basin still with increased cost and complexity of the system.

3. Wick still

In this type of still, feed water is passed through a porous pad which is capable of absorbing radiations, known as wick. These have a feature of tilting the wick so as to reduce reflections and avail larger effective area for solar insolation. Also water is heated at a faster rate due lower feed water depth in the still. Wick stills are more effective than the basin ones due to reduced cost for same output.

2.2.1 Remarks from approaches

From above approaches following can be concluded

1. The efficiency of single stage still is around 15 %.
2. The efficiency of multistage stills is higher than 20 %.
3. Most of the losses can be attributed to heat transfer losses

Thermal losses are mostly in form of conduction and convection and very little   by radiation- owing to low temperature. So we can assume radiation losses can be negligible.

2.2.2 Some problems with solar stills which would reduce their efficiency

• Leakage of cold air into the still due to improper/loose joints and fittings
• Reduction in transmissivity due to scratches, breakage or slight cracking on glass cover
• Un-cleaned glass surface due to accumulation of dust particles, formation of rust, scaling, etc. hampers performance, which requires regular cleaning.
• Basin absorbing surface may get damaged over time.
• Salts and Chlorides get accumulated at basin bottom which affects heat transfer and absorption rates.
• Possibility of PCM drain in latent phase change period through small holes or improper sealing.

Disturbance of reflector due heavy winds or rains causing angular misalignments

2.3 Aspects of manufacturing still

The cover can be either glass or plastic. Glass is preferable to plastic because most of the time in the long term due to ultra violet light from sunlight and because it is more difficult for water to condense onto it. Tempered low-iron glass is the best material to use because it is highly transparent and not easily damaged. To make it low cost affair, normal plain window glass of 4-6 mm thickness may be used. A high grade transparent polyethylene may be used for temporary application. Stills with a double sloping cover with the back made from an insulating material do not suffer from a very low angle cover plate at the back reflecting sunlight and thus reducing efficiency. It is important for greater efficiency that the water condenses on the channel as a film rather than as droplets, which tend to drop back into the saline water. For this reason the channel is set at an angle of 10° to 30°.

2.4 Design methodology

The design methodology of the project involves designing symmetrical solar still as per the output requirements. The solar still is designed for an estimated output of around 2 litres. In addition to it, an external reflector and PCM has been used to increase the output efficiency. Figure 1 shows the actual setup of “Double Slope Solar Still”.

figure_1.png

Figure 1. Actual setup of double slope solar still

2.4.1 Base area

Estimation of base area is required for determining the quantum of incident solar radiation necessary for producing the desired output. With a desired output of 2 litres of water, the amount of solar energy required can be calculated as

$M_{w}=\frac{Q_{r e q}}{\text {Latent Heat of vaporiazation}}$              (1)

By calculation, $Q_{r e q}$ is 4520 $\mathrm{kJ}$

Now, in order to calculate the amount of incident solar energy $\left(Q_{\text {incident}}\right)$ it is needed to analyze the data of the average amount of solar energy incident in Solapur every month. The average solar energy received in Solapur throughout the year is 5.33 $\mathrm{kWhr} / \mathrm{m}^{2} / \mathrm{day}$

$\mathrm{I}=$ Average solar incident radiation for Solapur region

$\mathrm{I}=553 \mathrm{W} / \mathrm{m}^{2}$

Generally glass transmits 80 % of light through it. Let us consider 70 % of solar rays incident are utilized.

$Q_{\text {available}}=\tau_{\text {glass}} \cdot I$ (2)

By calculation, Q available is 387.1 $\mathrm{W} / \mathrm{m}^{2}$.

Thus, $Q_{\text {incident}}=Q_{\text {req}}$

Therefore, area required to get 4520 $\mathrm{kJ}$ is $Q_{\text {req}} / Q$ available.

Assuming that the system works 6 $\mathrm{hrs} / \mathrm{day}$

$Q_{\text {available}}$ is 7166.88 $\mathrm{kJ} / \mathrm{m}^{2}$;

Area required is 0.63 $\mathrm{m}^{2}$.

2.4.2 Reflector

In order to increase the incident solar radiation into the still, the external reflectors are incorporated. Due to this evaporation rate is increased. The additional amount of solar energy incident on the solar still, due to the external reflector can be calculated as follows.

Considering 4000 kJ of excess energy is available, area of reflector required to obtain this energy.

We get average solar incident radiation for Solapur region is 553 $\mathrm{W} / \mathrm{m}^{2} .$

$Q_{\text {req}}=\tau_{\text {glass}} \times A_{\text {reflector}} \times I$ (3)

By calculation, Qreq is 387.1 $\mathrm{W}$.

2.4.3 Phase Change Material (PCM)

For 1 kg pure water, additional 2600 kJ energy is needed. To store 2600 kJ energy in storage material i.e. paraffin wax we need

$Q=\left\{m \times C_{p}\left(T_{\text {melting}}-T_{\text {ambient}}\right)\right\}_{\text {solid}}+\left(m \times L_{h}\right)+\left\{m \times C_{p}\left(T_{\max }-T_{\text {melting}}\right)\right\}$ (4)

Then, we can carry the following calculation

$2600=\{m \times 2.14(55-30)\}+(m \times 215)+\{m \times 2.9(100-55)\}$

So, $m=6.5 \mathrm{kg}$

2.5 Specifications of solar still

Table 1. Specifications of solar still

4.1 Experimentations & results

Case-1: Still – Without reflector and without PCM tank

Sample 1 – Brackish Water (High contents of Sodium Carbonates – 40% soap solution)

Intensity of Solar Radiation I = 800 W/m²

Quantity of Sample = 6 kg

Atmospheric temperature = 30 °C

Table 3. Observations on 29^_th March 2017 from 10hrs to 16 hrs

figure_6.png

Figure 6. Evolution of the temperature with time (Case-1)

It is observed from Figure 6 that initially temperature was 25 °C and the maximum temperature achieved during this process is 71 °C. Also it is observed that temperature of basin increases from 31 °C to 72 °C and after the sunset it slowly decreases.

The graph (Figure 7) shows distillate output per hour and it is observed that during 12 hrs to 14 hrs distillate output is maximum and total collected distillate output is 1.07 litres in 6 hrs for 0.5 m²  incident area.

The efficiency of solar still is given by

$\eta=\frac{\sum M_{e w} \times L}{A_{g l a s s} \times I \times t} \times 100$ (5)

Therefore, $\eta$ is 27.98$\%$

figure_7.png

Figure 7. Graph of Time v/s Distillate Output (Case-1)

Case-2: Still – Without reflector and without PCM tank

Sample 2 – Waste water of RO plant

Intensity of Solar Radiation I = 800 W/m²

Quantity of Sample = 6 kg

Atmospheric temperature = 32 °C

Table 4. Observations on 30^th March 2017 from 10hrs to 16hrs

Time (hours)	Temperature of Basin water (°C)	Temperature of Basin (°C)	Glass Cover Temperature (°C)		Distillate output (ml)
			Inside	Outside
10	30	32	36	32	0
11	53	56	50	39	110
12	58	60	57	41	270
13	55	58	56	46	520
14	56	57	55	41	770
15	56	57	48	40	985
16	52	53	47	39	1135

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Figure 8. Evolution of the temperature with time (Case-2)

The graph (Figure 8) show that as time goes on increasing, the temperature of the water in the basin also increases. It is observed that initially temperature was 30 °C and after an hour temperature reached is 53 °C and the maximum temperature achieved during this process is 58 °C. Also it is observed that the temperature of basin is increasing from 30 °C to 60 °C and after the sunset it is slowly decreasing. From the graph, it is also observed that inside surface temperature of glass is maximum and that of outside surface of glass is minimum.

The graph (Figure 9) shows distillate output per hour and it is observed that during 12 hrs to 16 hrs distillate output is maximum and total collected distillate output is 1.135 litres in 6 hrs for 0.5 m² incident area.

The efficiency of solar still is given by

$\eta=\frac{\sum M_{e w} \times L}{A_{g l a s s} \times I \times t} \times 100$ (6)

By calculation, η is 29.38 %.

figure_9.png

Figure 9. Graph of Time v/s Distillate Output (Case-2)

Case-3: Still – With reflector and without PCM

Sample 1 – Brackish Water (High contents of Sodium Carbonates – 40 % soap solution)

Intensity of Solar Radiation I = 800 W/m²

Quantity of Sample = 6 kg

Atmospheric temperature = 32 °C

Table 5. Observations on 2^nd April 2017 from 10hrs to 16 hrs

Time (hours)	Temperature of Basin water (°C)	Temperature of Basin(°C)	Glass Cover Temperature (°C)		Distillate output (ml)
			Inside	Outside
10	30	32	30	32	0
11	61	64	57	36	56
12	68	70	69	47	371
13	68	72	61	46	610
14	69	74	64	42	859
15	61	63	60	42	1108
16	59	60	59	40	1260

figure_10.png

Figure 10. Evolution of the temperature with time (Case-3)

It is observed from Figure 10 that initially temperature is 30 °C and the maximum temperature achieved during this process is 69 °C. Also it is observed that temperature of basin is increasing from 32 °C to 74 °C. Because of mounting the reflector, the distillate output is maximum.

The graph (Figure 11) shows distillate output per hour and it is observed that during 13 hrs to 16 hrs distillate output is maximum and total collected distillate output is 1.26 litres in 6 hrs for 0.5 m² incident area.

The efficiency of solar still is given by

$\eta=\frac{\sum M_{e w} \times L}{A_{g l a s s} \times I \times t} \times 100$ (7)

figure_11.png

Figure 11. Graph of Time v/s Distillate Output (Case-3)

Case-4: Still – With reflector and without PCM

Sample 2 – Waste water of RO plant

Intensity of Solar Radiation I = 800 W/m²

Quantity of Sample = 6 kg

Atmospheric temperature = 32°C

Table 6. Observations on 3^rd April 2017 from 10hrs to 16 hrs

Time (hours)	Temperature of Basin water (°C)	Temperature of Basin(°C)	Glass Cover Temperature (°C)		Distillate output (ml)
			Inside	Outside
10	30	31	30	33	0
11	58	62	55	37	100
12	65	69	67	45	330
13	74	74	68	46	615
14	69	70	61	44	880
15	62	64	59	43	1105
16	60	61	58	42	1255

It is observed in Figure 12 that initially temperature is 30 °C and the maximum temperature achieved during this process is 74 °C. Also it is observed that temperature of basin is increasing from 31 °C to 74 °C.

The graph (Figure 13) shows distillate output per hour and it is observed that during 13 hrs to 16 hrs distillate output is maximum and total collected distillate output is 1.25 litres in 6 hrs for 0.5 m² incident area.

The efficiency of solar still is given by

$\eta=\frac{\sum M_{e w} \times L}{A_{g l a s s} \times I \times t} \times 100$ (8)

By this formula, η is calculated to be 32.69 %.

Case-5: Still – With reflector and with PCM

Sample 1 – Brackish Water (High contents of Sodium Carbonates – 40 % soap solution)

Intensity of Solar Radiation I = 800 W/m²

Quantity of Sample = 6 kg

Atmospheric temperature = 32 °C

figure_12.png

Figure 12. Evolution of the temperature with time (Case-4)

figure_13.png

Figure 13. Graph of Time v/s Distillate Output (Case-4)

Table 7. Observations on 4^th April 2017 from 10hrs to 19:30 hrs

Time (hours)	Temperature of Basin water (°C)	Temperature of Basin(°C)	Glass Cover Temperature (°C)		Distillate output (ml)	Wax Temperature (°C)
			Inside	Outside
10.30	30	32	31	32	0	32
11.30	62	64	57	36	160	57
12.30	72	74	68	47	460	71
13.30	67	70	62	43	780	74
14.30	60	65	54	42	920	77
15.30	62	59	54	39	1130	60
16.30	61	65	53	42	1350	60
17.30	55	63	48	38	1480	59
18.30	51	52	40	37	1540	54
19.30	48	50	35	34	1580	52

It is observed in Figure 14 that initially temperature is 30 °C and the maximum temperature achieved during this process is 72 °C. Also it is observed that temperature of basin is increasing from 32 °C to 74 °C.

The graph (Figure 15) shows distillate output per hour and it is observed that during 9 hrs to 13 hrs distillate output is maximum and total collected distillate output is 1.58 litres in 9 hrs for 0.5 m² incident area. From 13 hrs to 17 hrs, the distillate output doubles to about 4 litres. Also in this case, the working time of solar still increases from 16 hrs to 19:30 hrs because of incorporating the PCM, which stores the excess energy available at peak time and discharges back to still water during sunset when solar intensity is low and thus maintains the efficient working of the still.

figure_14.png

Figure 14. Evolution of the temperature with time (Case-5)

figure_15.png

Figure 15. Graph of Time v/s Distillate Output (Case-5)

Actual efficiency

Maximum temperature reached by paraffin wax is 77 °C, so total energy stored in paraffin wax is given by

$Q=\left\{m \cdot C_{p} \cdot\left(T_{\text {melting}}-T_{\text {ambient}}\right)\right\}_{\text {solid}}+\left(m \cdot L_{h}\right)+\left\{m \cdot C_{p} \cdot\left(T_{\max }-\right.\right.T_{\text {melting}} ) \}$liquid (9)

Efficiency of solar still using energy storage material is given by

$\eta=\frac{\sum M_{e w} \times L}{\left(A_{g l a s s} \times I \times t\right)+Q} \times 100$ (10)

By calculation, Q = 6.5 × 2.14 × (55-30) + 6.5 × 215 + 6.5 × 2.9 × (77-55) = 2159.95 kJ; η is 41.31%.

Case-6: Still – With reflector and with PCM

Sample 2 – Waste water of RO plant

Intensity of Solar Radiation I = 800 W/m²

Quantity of Sample = 6 kg

Atmospheric temperature = 32 °C

It is observed in Fig.16 that initially temperature is 32 °C and after an hour temperature is 52 °C and the maximum temperature achieved during this process is 73 °C. Also it is observed that temperature of basin is increasing from 33 °C to 71 °C.

The graph (Figure 17) shows distillate output per hour and it is observed that during 9 hrs to 13 hrs distillate output is maximum and total collected distillate output is 1.61 litres in 9 hrs for 0.5 m² incident area.

From 13 hrs to 17 hrs, the distillate output doubles to about 3.3 litres.

Table 8. Observations on 5^th April 2017 from 10hrs to 19:30 hrs

Time (hours)	Temperature of Basin water (°C)	Temperature of Basin (°C)	Glass Cover Temperature (°C)		Distillate output (ml)	Wax Temperature °C)
			Inside	Outside
10.30	32	33	30	32	0	32
11.30	63	64	58	35	165	58
12.30	71	73	68	45	482	72
13.30	73	71	63	44	800	74
14.30	65	68	58	42	980	77
15.30	62	64	54	39	1120	76
16.30	65	70	56	42	1365	61
17.30	55	64	50	41	1490	60
18.30	54	52	40	35	1560	59
19.30	51	50	38	34	1610	55

 

figure_16.png

Figure 16. Evolution of the temperature with time (Case-6)

figure_17.png

Figure 17. Graph of Time v/s Distillate output (Case-6)

Actual efficiency

Maximum temperature reached by paraffin wax is 77°C, so total energy stored in paraffin wax is given by

Q = {m·C_p·(T_melting- T_ambient)}_solid+ (m·L_h) + {m·C_p·(T_max- T_melting)}_liquid TERGENT n Waxll  Title  Page No.  (11)

$\eta=\frac{\sum M_{e w} \times L}{\left(A_{g l a s s} \times I \times t\right)+Q} \times 100$ (12)

By calculation, Q = 6.5 × 2.14 × (55-30) + 6.5 × 215 + 6.5 × 2.9 × (77-55) = 2159.95 kJ; η is 42.10%.

4.2 Testing reports of distilled water

The water samples selected for testing purposes i.e. Sample 1 and Sample 2 above cases1 to 6, are based on using very high TDS water which is obtained from waste water of washing clothes, utensils and that of RO plant, which is to be recycled. Distillation is the only method in which such waste water can be recycled with greatest efficiency i.e. the distilled water will not contain any TDS as only pure water is evaporated from the brackish water.

4.2.1 Results of Sample 1 – Brackish Water (High contents of Sodium Carbonates – 40 % soap solution)

Table 9. Results of distilled water analysis (sample 1)

Sr. No.	Test	Brackish Water	Result of Distillate Brackish Water	Acceptable limit
1	pH	8.3	7.1	6.5-8.5
2	Alkalinity  (mg/l)	172	120	200
3	Hardness (mg/l)	236	160	200
4	Chlorides  (mg/l)	278	206	250
5	Turbidity  (NTU)	3.8	0.1	1
6	Total Dissolved Solids (mg/l)	780	250	500

 

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Figure 18. Water filled in Still (Sample 1), Before Purification

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Figure 19. Water filled in Still (Sample 1), After Purification

4.2.2 Results of Sample 2 – Waste water of RO plant

Table 10. Results of distilled water analysis (Sample 2)

Sr. No.	Test	RO plant waste water	Result of Distillate RO plant waste water	Acceptable limit
1	pH	7.82	7.3	6.5-8.5
2	Alkalinity (mg/l)	166	168	200
3	Hardness (mg/l)	178	166	200
4	Chlorides (mg/l)	235	228	250
5	Turbidity (NTU)	1.1	0.6	1
6	Total Dissolved Solids (mg/l)	420	260	500

 

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 Figure 20. Water filled in Still (Sample 2), Before Purification

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Figure 21. Water filled in Still (Sample 2), After Purification