Household solar still with easy operation and maintenance and enhanced output

Abstract

The invention discloses the design of an improved household solar still with enhanced output and high recovery. The output and recovery are enhanced by (i) aligning the top glass and basin so as to maximize the input solar radiation, (ii) using North-South reflectors in V trough configuration to further raise the incident radiation, (iii) employing metallic condensers on sides to maximize condensation efficiency, (iv) using suitably heated and cooled saline water by taking advantage of ambient temperature differential during a day and using such water as feed water and condenser water respectively. Use of detachable top glass assembly and teflon cork at the bottom allows for easy cleaning of the interior of the still and discharge of concentrate, respectively. The still is also demonstrated to be useful for other applications where, besides producing distilled water, the concentrate is a more value added product than the feed.

Claims

1. A household solar still for desalination of brackish water comprising: a frame; a tilted water basin having five partitions for holding water in a tilted position; a tilted top glass assembly positioned over the tilted water basin, the tilted water basin and the tilted top glass assembly being tilted at an angle in the range of 18.9 to 20 to the horizontal with a top portion of the tilted top glass assembly pointing northwards and a bottom portion of the tilted top glass assembly pointing southwards, the tilted top glass assembly being fixed to the frame; a pair of a top reflector and a bottom reflector, the top reflector being fixed to a top edge of the solar still and the bottom reflector being fixed to a bottom edge of the solar still, each of the reflectors being in the form of anodized aluminium sheets, attached to a polymeric sheet; a first and a second metallic condenser being attached on eastward and westward sides of the solar still, the first metallic condenser being covered by a first tray, and the second metallic condenser being covered by a second tray; a third metallic condenser being attached to a back side of the solar still, the third metallic condenser being positioned so as to be substantially covered by the shadow of the reflector; each of the first and second metallic condensers comprise a lower steam chest part and an upper part; the lower steam chest part comprises a steam inlet and a condensed water outlet; and the upper part comprises a cold water inlet, a hot water outlet and a water holding/flow portion located between the cold water inlet and the hot water outlet, wherein the upper part is adapted to receive brackish water and to allow flow to the brackish water under gravity at a specified flow rate over the lower steam chest part to facilitate condensing of steam on an inner top surface of the lower steam chest part.

2. The solar still as claimed in claim 1, wherein the pair of the top and the bottom reflectors form a V-trough configuration and the angles of the two reflectors with respect to the vertical plane are optimized using geometric formula according to the position of the sun to have maximum solar energy on the tilted top glass assembly helping in maximizing evaporation.

3. The solar still as claimed in claim 1, wherein for a square meter of top glass assembly area, a tilted water basin volume is 18 L.

4. The solar still as claimed in claim 1, wherein the first, second and third metallic condensers are corrosion resistant.

5. The solar still as claimed in claim 1, wherein the first, second and third metallic condensers further comprise a sump and an insulated tank with a removable cover positioned behind the solar still, that cools the brackish water through night sky radiation to produce a cooled brackish water and the cooled brackish water flows over the first, second and third metallic condensers by gravity at an optimum rate and collects in the sump.

6. The solar still as claimed in claim 1, wherein the tilted top glass assembly is designed to be detachable to enable cleaning of a solar still interior and discarding of the concentrate every day prior to feeding of fresh stock of brackish water through leakproof plugs attached to a lower part of the tilted water basin.

7. The solar still as claimed in claim 1 comprising brackish water and an edible dye added to the brackish water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents the overall solar still assembly with condensers, reflectors.

(2) FIG. 2 represents the solar still basin.

(3) FIG. 3 represents the top glass assembly of the solar still.

(4) FIG. 4 represents the side condensers.

(5) FIG. 5 represents the condenser cover.

(6) FIG. 6 represents the reflectors.

(7) FIG. 7 represents the backside condenser.

(8) FIG. 8 represents the supporting structure.

(9) FIG. 9 represents variation of solar insolation, wind speed and ambient temperature on the days of experiment

(10) FIG. 10 represents the average productivity from the solar still for each month in a year

(11) FIG. 11 represents the variation of efficiency of the solar still for a year.

SUMMARY OF THE INVENTION

(12) Accordingly, present invention provides an improved household solar still for desalination of brackish and seawater with enhanced recovery and output, comprises a top glass assembly (02) fixed to a teakwood frame positioned on solar still over a rubber gasket strip and providing a water basin (01) with 5-partitions, reflectors (05) made of anodized aluminium sheets, attached to a PVC sheet fixed on the north-south edges of the still and metallic side condensers (03), and (06) being attached on both sides and back side of the said still. (04) represents the condenser cover above the side condensers (03).

(13) In an embodiment of the present invention, the top glass and water basin are tilted at an angle in the range of 18.9 to 20 to the horizontal with the top pointing northwards and the bottom pointing southwards.

(14) In yet another embodiment of the present invention, reflectors are fitted on the top edge and bottom edge so as to form a V-trough configuration, with the angles of the two reflectors with respect to the vertical plane being adjusted suitably 12 times a year as per pre-defined geometric formula.

(15) In yet another embodiment of the present invention, for a 1 m.sup.2 of top glass area, the basin volume was 18 L and the spacing between the water surface and the glass was kept to a minimum.

(16) In yet another embodiment of the present invention, fresh stock of saline water is fed into the still each morning and such water is pre-heated the previous day and thereafter stored in an insulated tank to keep it warm.

(17) In yet another embodiment of the present invention, stainless steel condensers are fitted onto three sides of the still.

(18) In yet another embodiment of the present invention, saline water is allowed to flow over the condenser at the rate of 150 to 300 mL per minute, collected in a sump, cooled during the night and then poured into an insulated tank from which it could flow by gravity over the condenser during the day, especially during the period of the day when productivity is highest.

(19) In yet another embodiment of the present invention, the top glass assembly was designed to be detachable for easy cleaning of the still interior.

(20) The improved solar still as claimed in claim 1, wherein teflon corks attached to the lower part of the basin drains out the concentrate prior to feeding of fresh stock of saline water. The improved solar still as claimed in claim 1, wherein said solar still is effective as a household desalination device requiring no sophistication and using only seawater and solar energy to provide drinking water also utilized to concentrate juices, coconut water, etc. and also for recovering distillates of medicinal value such as distillate from cow's urine.

(21) The improved solar still as claimed in claim 1, wherein when seawater is used as feed water, such water is preferably clarified and deodorized by known methods prior to charging into the still.

(22) The improved solar still as claimed in claim 1, wherein a suitable dye can be optionally added into the feed water to enhance solar absorption, especially when the solar insolation is sub-optimal.

(23) In an embodiment of the present invention, said solar still give up a maximum of 7.265 L of distilled product water with 40.36% recovery from seawater feed, when the average insolation over 7 hours was 972.35 Wm.sup.2, the ambient temperature was 31.62 C. and the wind speed was 0.83 ms.sup.1.

(24) In another embodiment of the present invention, concentrate in the still was discarded every day.

(25) In an embodiment of the present invention, the improved solar still of FIGS. 1-8 has a capacity of holding 18 l of saline water.

(26) In another embodiment of the present invention, the absorber basin of the solar still of FIGS. 1-8 had an angle of 20 to the horizontal.

(27) In another embodiment of the present invention, reflectors attached to the solar still of FIGS. 1-8 raised the insolation on the collector by 40-50% as compared to the unit without reflectors.

(28) In another embodiment of the present invention, the solar still of FIGS. 1-8 had side and back metallic condensers attached to the basin such that they were an extended part of the basin forming steam chests.

(29) In another embodiment of the present invention, the solar still of FIGS. 1-8 the upper part of the condensers had arrangements to flow water under gravity at a specified flow rate to keep the condensers cool.

(30) In another embodiment of the present invention, the solar still of FIGS. 1-8, the cooling water from the condensers were collected in a sump and poured back to a tank placed at the back of the still.

(31) In another embodiment of the present invention, the 200 liter tank where to water was stored was insulated using hollow PVC sheets and thermocol strips were inserted within the hollow channels.

(32) In another embodiment of the present invention cover of the solar still was made using toughened glass to avoid breakage.

(33) In another embodiment of the present invention the cover glass was fitted to detachable teakwood frame positioned on the still over a rubber gasket strip so as to facilitate daily cleaning.

(34) In another embodiment of the present invention metal threaded teflon stoppers were attached on the bottom of the basin of the still to flow out the used water so as to facilitate daily cleaning.

(35) In another embodiment of the present invention, there are four outlets from solar still-three from the three condensers and the fourth from the still basin. This was to ascertain the volume of distillate from the condensers and the basin separately.

(36) In another embodiment of the present invention, the insulated tank is left open during night to obtain cooled water through night sky radiative cooling.

(37) In another embodiment of the present invention the next day's feed water is heated up by solar radiation and put it into a separate tank where it is kept warm.

DETAILED DESCRIPTION OF THE INVENTION

(38) The invention discloses the design of a household solar distillation unit or still with enhanced output and high recovery even with seawater as feed for drinking water production as per the FIGS. 1-8 (vide infra). Another useful element of design is easy maintenance of the unit. The output and recovery are enhanced by (i) aligning the top glass and basin so as to maximise the input solar radiation, (ii) using N-S reflectors in V trough configuration to further raise the incident radiation, (iii) minimising the gap between the water surface in the basin and the top glass cover, (iv) employing metallic condensers on three sides to maximise condensation efficiency, (v) using suitably heated and cooled saline water by taking advantage of ambient temperature differential during a day and using such water as feed water and condenser water respectively, and (vi) optionally adding a dye to further raise the feed water temperature. Use of detachable top glass assembly and teflon cork at the bottom allow for easy cleaning of the interior of the still and discharge of concentrate, respectively. The still is also demonstrated to be useful for other applications where, besides producing distilled water, the concentrate is a more value added product than the feed.

(39) The construction of the household solar distillation unit for drinking water production with its parts numbered from (1) to (7) is as shown in FIG. 2 to FIG. 8 laid out in sheet 01 to sheet 07. Reflectors (05), 1.2 m1.2 m made of anodized aluminum sheets (0.0005 m), were attached on 0.019 m PVC sheet and fixed on the North-South edges of the still. The angle of the reflectors could be changed according to the seasonal movement of the sun. The reflector angles throughout the year can be summarized in Table 1. The average declination and duration for a given average declination is given in Table 2.

(40) TABLE-US-00001 TABLE 1 Angle of the reflectors for solar still with respect to vertical plane Optimized Angle Angle day in the of of Number specific North South of time time side side interval From To period reflector reflector 1 November 6 February 2 Nov 19 or 9 60 Jan 21 2 February 3 February 23 Feb 14 4 56 3 February 24 March 12 Mar 5 +1 51 4 March 13 March 29 Mar 22 6 46 5 March 30 April 15 Apr 8 11 42 6 April 16 May 6 Apr 26 16 36 7 May 7 August 4 May 20 or 20 31 July 23 8 August 5 August 25 Aug 16 16 36 9 August 26 September 11 Sep 4 11 42 10 September 12 September 28 Sep 20 6 46 11 September 29 October 15 Oct 7 1 51 12 October 16 November 5 Oct 26 4 56

(41) TABLE-US-00002 TABLE 2 Duration for a given average declination in days Number Duration for a of given average time Average declination interval From To Declination (Days) 1 November 6 February 2 20.1 89 2 February 3 February 23 13.4 21 3 February 24 March 12 6.7 17 4 March 13 March 29 0 17 5 March 30 April 15 6.7 17 6 April 16 May 6 13.4 21 7 May 7 August 4 20.1 90 8 August 5 August 25 13.4 21 9 August 26 September 11 6.7 17 10 September 12 September 28 0 17 11 September 29 October 15 6.7 17 12 October 16 November 5 13.4 21

(42) The solar still assembly consisted of the frame, outer body, inner basin, product water line and the partitions. The frame of the solar still unit was made using solid teak wood. The outer body of the unit was made of 0.012 m marine plywood. The basin had dimensions 1.2 m0.85 m0.05 m. Product water line was made using PVC channel. The inner basin surfaces had black fibre re-enforced plastic to absorb the maximum amount of solar radiation incident and to make the basin completely leak-proof. The total basin had 5 partitions. The dimensions were 1.2 m0.012 m0.05 m. A jacket of 0.006 m plywood was positioned at the sides and bottom of the still and the annular space was filled with sawdust. The unit was oriented facing south and tilted at 20 to the horizontal. This inclination was to maximise solar irradiation on the glass cover. The cover material or glass assembly (02) was 0.006 m toughened commercial glass which was fixed to a teakwood frame positioned on the still over a rubber gasket strip. The cover could be removed easily for cleaning purposes. Metal threaded (hole of diameter 0.025 m) teflon stoppers were attached on the bottom of the basin of the still to flow out the used water.

(43) The condensers (03 & 06) on both the sides and the back were made using aluminum sheets of 0.001 m thickness. The size of each condenser was 0.9 m0.109 m0.05 m. A tray above the condenser (04) had a depth of 0.028 m. To facilitate steam condensing on the inner top surface of the condenser, cold water was allowed to flow over the outer surface. The water flowing out was collected in a sump during the day time and stored. In the evening the water was transferred to an insulated tank (1.01 m0.50 cm0.04 m) positioned behind the still. The tank was kept open to atmosphere at night to facilitate night sky radiative cooling.

(44) The main inventive steps are the following: 1. North-South reflectors with slotted arrangement for seasonal adjustment which enhances the incident radiation on the still over a day; 2. Covered side condensers and/or condenser positioned in the shadow of the reflector over which cooled water flows by gravity at a specified rate leading to marked lowering of glass temperature and concomitant rise in productivity. 3. Obtaining such cooled water through night sky radiative cooling especially in arid and semi arid locations and keeping the water cool during daytime by placing it in a raised insulated tank; 4. Detachable leak proof glass assembly and teflon corks at the bottom of the still enabling easy maintenance; 5. Daily changing of feed water to avoid scale formation especially when water is brackish/hard;

EXAMPLES

(45) The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

Examples

(46) In the examples, I(t) is the solar intensity, T.sub.g is the top glass cover temperature, T.sub.w represents the water temperature inside the solar still basin, T.sub.a represents the ambient temperature and W.S is the ambient wind speed.

Example 1

(47) Effect of Tilt

(48) Table 1 shows the enhancement of solar radiation on a surface tilted at an angle of 20 to horizontal and kept facing south.

(49) TABLE-US-00003 TABLE 3 The effect of tilt With reflectors Without (at optimized reflectors angle) Date of experiment 30 Nov. 2012 11 Dec. 2012 Solar energy falling on a surface tilted 19.62 29.72 at an angle of 20 to horizontal facing exact south for 7 sunny hrs, MJm.sup.2 Solar energy falling on horizontal sur- 15.67 15.19 face for 7 sunny hrs, MJm.sup.2

(50) The above example teaches us that the incident solar radiation is higher for a tilted surface facing south. The effect of the tilt is more pronounced with the use of North-South reflectors. Accordingly, the top glass surface is maintained in tilted position as optimum alignment.

Example 2

(51) Effect of Partitions/Steps

(52) Two stills were constructed in which in both cases the top glass surface was tilted 20 N-S. In one of the stills the black colored basin was kept horizontal while in the other still the basin was tilted at 20 N-S and 5 partitions used to hold the water in the tilted basin. The outer dimensions and material of construction were kept the same for both the stills. Also, similar insulation at the sides and bottom were applied to both. The horizontal still held 20 L of seawater whereas the still with tilted basin held 18 L. Data for two days are presented in Table 4. It can be seen that tilting the basin in desired orientation has a dramatic effect on the product water output from the still.

(53) TABLE-US-00004 TABLE 4 The effect of water in tilted basin with incorporation of steps on productivity of the still Horizontal Tilted basin basin Date of experiment 3 Dec. 2012 3 Dec. 2012 Average I(t) on glass surface 824.88 824.88 for 7 sunny hrs, Wm.sup.2 Max. T.sub.g, C. 43.7 55 Max. T.sub.w, C. 47.2 66 Avg. T.sub.a, C. 31.9 31.9 AvgW.S, ms.sup.1 0.4 0.4 Production, l 0.554 2.467

(54) This example shows us that the advantage of a stepped solar still as claimed in the prior art is in effect an outcome of proper alignment of the basin so that maximum radiation falls on it and while enabling the water to be retained uniformly on the basin.

Example 3

(55) Enhanced Output through North-South Reflectors in V-Trough Alignment

(56) Reflectors were added on the north-south edges of the still with tilted basin in proper alignment according to Table 1. Data were compared against another similar still without such reflectors.

(57) TABLE-US-00005 TABLE 5 The effect of North -South reflectors on still with tilted basin Without N-S With N-S reflector reflector Date of experiment 29 Nov. 2012 Average I(t) on horizontal surface 643.62 for 7 sunny hrs, Wm.sup.2 Average I(t) on glass surface for 792.62 1221.19 7 sunny hrs, Wm.sup.2 Max. T.sub.g, C. 54 78.8 Max. T.sub.w, C. 59 89 Avg. T.sub.a, C. 28 AvgW.S, ms.sup.1 0.7 Production, l 2.536 5.115

(58) This example teaches us that the output increases considerably upon incorporation of reflectors due to the increase in incident solar radiation.

Example 4

(59) Enhanced Output though Attachment of Condenser on the Still of Example 3

(60) Table below shows the variation of the maximum glass and water temperatures achieved and also the distilled water production from the stills due to the presence of condensers. Four days with different ambient conditions have been compared.

(61) TABLE-US-00006 TABLE 6 The effect of condensers on productivity Without With Without With Without With Without With condenser condenser condensers condensers condenser condenser condensers condenser Date of 14 Mar. 2012 21 Jun. 2012 12 Dec. 2012 19 Feb. 2013 experiment Average 1411.6 1422.39 1107.52 1102.15 1262.67 1280.64 1502.30 1502.30 I(t) on glass surface for 7 sunny hrs, Wm.sup.2 Max. T.sub.w, C. 93 77 87 73 93 74 81 77 T.sub.g, C. 87 58 78 62 84 64 75.2 66.6 T.sub.wmax Avg. T.sub.a, C. 30.5 39 30 25 Avg W .Math. S, 0.5 1 0.3 0.7 ms.sup.1 Production, l 4.717 7.062 4.421 5.306 5.440 6.272 4.067 6.750

(62) This example teaches us that although in all cases the output increases upon incorporation of condenser, the magnitude of the effect is most pronounced when the incident radiation is highest and, consequently, rate of evaporation is higher and for which use of condenser is most advantageous to minimize vapor loss. In the best result obtained, the product water recovery was 39.2% with respect to feed water taken.

Example 5

(63) Enhanced Output from the Still of Example 4 through Addition of Black Ink into Feed Water

(64) To check the effect of a black pigment on the productivity of the still, black ink @0.1% was added to the feed water. The data are summarized in the table below.

(65) TABLE-US-00007 TABLE 7 The effect of black ink in feed water on productivity Without With 0.1% Without With 0.1% black ink black ink black ink black ink Date of experi- 18 Feb. 2012 1 Mar. 2012 17 Feb. 2012 3 Mar. 2012 ment Average I(t) on 763.4 767.6 752.6 750.2 horizontal sur- face for 7 sun- ny hrs, Wm.sup.2 Max T.sub.w C. 66 70 62 66 Max T.sub.g C. 60 58 58 58 Avg T.sub.a C. 26.3 26.12 25.2 26.0 Avg W.S ms.sup.1 0.7 0.8 0.4 0.3 Production l 5.493 6.424 5.322 6.053

(66) The above example teaches us, that the output can be raised by ca. 10% through incorporation of black ink in the feed water. The product water had no trace of colour. The reject stream can be decolorized prior to discharge. The addition of ink is most useful when the solar insolation is sub-optimal.

Example 6

(67) Considering the year from January 2012 to 2013, the solar still with the condensers and North-South reflectors gave a maximum of 7.265 L of distilled product water with 40.36% recovery from seawater feed, when the average insolation over 7 hours was 972.35 Wm.sup.2, the ambient temperature was 31.62 C. and the wind speed was 0.83 ms.sup.1.

(68) TABLE-US-00008 Date of experiment 22 Mar. 2012 Average I(t) on horizontal surface 972.35 for 7 sunny hrs, Wm.sup.2 Average I(t) on glass surface for 7 1517.66 sunny hrs, Wm.sup.2 Max. T.sub.g, C. 66 Max. T.sub.w, C. 75 Avg. T.sub.a, C. 31.62 AvgW.S, ms.sup.1 0.83 Production, l 7.265

Advantages of the Invention

(69) The main advantage of the present invention is:

(70) 1. Systematic enhancement in solar still efficiency for desalination through incorporation of the features according to the present invention.

(71) 2. As the unit has north-south reflectors, the tracing of the reflectors can be done manually 12 times in a year.

(72) 3. The unit is most suitable for desalination of highly brackish water or even seawater which is less amenable through other means of desalination at household scale.

(73) 4. By pre-heating the feedwater on the day before and subsequently storing the hot water in a Styrofoam box, product output can be further enhanced.

(74) 5. Similarly, by allowing feedwater to cool during the night and thereafter filling it in manually in an insulated tank, one can have gravity fed cool water flowing over the condenser to aid the condensation process.

(75) 6. Up to 7.0-7.2 L of product water could be obtained under favorable conditions with 35-40% product water recovery from seawater.