DEMOUNTABLE SOLAR STILLThis invention relates to a demountable solar distillation apparatus, or solar still.
Solar stills have existed since many years, and generally consisted of inclined large areas of glass planes held over shallow dark basins of sea water; the sun heats the water, the greenhouse effect being provided by the glass, so that the water evaporates, its saltless vapour condensing on the cooler glass into drops that run down the lower surface of the glass to collector channels, so providing saltless drinking water to coastal populations with no other sources of drinking water. During World War 2, an American physicist, Dr Maria Telkes, designed a near spherical inflatable solar still some 35 cm in diameter which provided sufficient drinking water for the survival of pilots who crashed into the sea.
This principle is very little used today, and it is the object of this invention to provide a compact transportable lightweight solar still that can provide sufficient drinking water, at family scale, from saline or polluted water sources.
According to the present invention in its simplest form, there is provided a flat bottomed square basin the bottom of which is of a dark colour, and the edge of which is formed by a perimeter U shaped channel at the outer edge of which rest four transparent triangular sections which, when brought edge to edge, form a pyramid. Polluted or saline water is poured into the basin through a hole in the pyramid with a funnel to form a thin layer of water which heats up rapidly because of the dark colour of the base and the greenhouse effect of the transparent pyramid and evaporates, so that its vapour, condensing on the underside of the pyramid, runs down the sloping triangular section and is collecte along the perimeter by the continuous U section channel, the condensate being generally pure distilled water In a more complex version of the invention, a flat plate solar water heater fitted with two folding reflectors, the panel being the same size as the still base, and so able to form its cover when packed, is placed below the still which can either benefit directly from the added energy to increase output, or to be fitted with a lower insulated hot water storage container linked to the panel and located below the evaporating tray, so that, accumulating energy as heat during the daytime, it permits the still to operate at night, when temperature differentials can make the overall system more efficient and more productive of clean water.
Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:Figure 1 shows a plan view of the simpler still, with an optional solar panel attached.
Figure 2 shows an elevation of the basic still with the distillate running into a collector vessel.
Figure 3 shows the evaporative tray edge detail, illustrating in section the pyramid component, the distilled water collector channel and the evaporative tray.
Figure 4 shows an elevation of the more complex still, with a lower water storage insulated container and a solar panel.
Figure 5 shows a plan view of the more complex still with sections at different levels to illustrate its functions.
Figure 6 shows a section through the more complex still, to illustrate the thermosyphon systems in operation, including that of the solar panel.
Figure 7 shows a perspective view of the whole assembly of the still, the solar panel and the distillate receiving vessel.
Figure 8 shows a plan view of the packed still.
Figure 9 shows a section through the packed still and the solar panel.
Figure 10 shows a perspective of the still and panel packed down to form a case with a handle for ease of transport and storage.
Referring to the drawings, the basic still comprises a transparent pyramid 1 made up of four collapsible triangular elements held together by angles at their contiguous edges, or possibly made from a collapsible light metal or plastic frame supporting a flexible transparent film membrane, this pyramid providing the greenhouse effect and constituting also the condenser. The transparent cover can also be of a small roof shaped geometry 38 as shown inFigure 2. It also comprises a shallow tray 2 with a perimeter edge channel 3 into which falls the condensate 36 which has run down the transparent sheet 1 cooled by the outside air. The perimeter channel is white and the transparent sheet 1 located by the base angle 11 is protected from the sun by a strip of aluminised tape 10, to avoid evaporation of the distillate. A thin layer of water 4 in the tray which is insulated at its base 9 evaporates as it is rapidly heated by direct or diffuse solar radiation, and its vapour condenses on the pyramid 1 running into the edge channel 3 which can be made to slope down to the distillate tubular exit point 5 to which is attached a flexible tube 6 taking the distillate to a clean water containing vessel.
With this basic still operating in sunlight, the vapour condenses on the pyramid 1 in small droplets which eventually create a white reflective layer which significantly reduces solar input and the consequent evaporation of water. To overcome this inherent problem, a flat plate solar water heating panel 7, made up of a top and bottom manifold linked by tubes or channels, insulated at the rear and glazed with glass or plastic at the front, is placed against one of the legs of the still such that the panel can be fixed at the top at various heights by a clip 5 held by a releasable plastic tie, to allow the panel to take up different inclinations to best face the sun, and to be rotated periodi! cally around the circular leg to follow the rotation of the sun, without having to move the still.
Flow and return tubes 20 from the still are linked to the panel 7, wound spirally round the leg to avoid airlocks and insulated by a flexible foam sleeve 21 fixed also by releasable plastic ties. A thermosyphon water circulation system is thus induced in the panel with the colder water being ducted to the base manifold of the panel 18 and the water rising in the panel to the top manifold 19, so heating the water in the still. The panel can be fitted with aluminised reflector flat sheets 8 held in position by struts 34 which increase radiation on the panel, but hinge back flat for packing.
In the more fully developed version as shown in Figures 5 and 6, below the pyramid 1 and the evaporative tray 27, which is designed to be removable for periodic cleaning of the whole basin, there is an insulated reservoir 35 of the same plan shape as the tray but of a deeper section that contains a significant amount of water, introduced with the help of a funnel into a hole in the pyramid 34, which has a round pivoting keyhole plate to close it when not in use.
Gravity forces the water first to fill the solar water heating panel 7 and its connecting tubes 20, then filling the reservoir 35, and then providing a thin sheet of water 4 over the evaporative tray itself, since they all intercommunicate.
In this version, the evaporating layer, as it cools, tends to flow through the pipe outlet 24, whilst hotter water from the panel 7 fed to the top layer of the reservoir 35 feeds the tray 4 through the inlet 23, establishing a thermosyphon flow constantly feeding the tray with hot water. In parallel, colder water from the base of the reservoir 35 falls through the tube 25 through the spirally wound connecting tubes 20, allowing height adjustment without airlocks, to the base manifold 18 whence it is heated in the panel and rises to the upper manifold 19 to be delivered to an inlet pipe 22 which is provided with a lightweight plastic non return flap to prevent nighttime reverse thermosyphon. In this way, a significant amount of heat is stored in the reservoir 35, constantly feeding heat to the evaporative tray, and allowing the distillation cycle to continue to operate after sunset, and also, with the help of the solar panel, to operate in higher latitudes.
As shown in Figures 8 and 9, the still is designed to pack down so that the tray can contain the elements forming the pyramid 32, whether plastic or glass, the removable legs 14 which can be provided at their base with screw height adjustor plates 37 to level the apparats, the level being given by graduations marked in the four corners of the evaporative tray, the insulation sleeve, pipe work, plastic clips, funnel and other components, the tray so containing all necessary elements, is closed by the flat solar panel and its folded reflector sheets, the whole so packing down to a flat rectangular case, provided with a handle, as shown inFigure 10When sources of clean water are becoming scarce, or when they are polluted by industry or natural disaster, a still using solar energy producing a few litres of distilled water per day can become vital, producing sufficient clean water for safe use in some specific cases, medical for instance, and, as drinking water, for sheer survival in others.