Solar Water Heating
As expectations of material comfort rise, the Ladakhis are increasingly using hot or warm water for washing. The trend is accelerated enormously by the presence every year of thousands of foreign tourists, who demand hot showers in their hotels. Too meet the new demand, fossil fuels (oils, coke and coal) are being imported from outside Ladakh in ever-larger quantities.
As a means of combating this costly dependence on imported fuels, we have designed a number of water heating systems using solar energy. The systems are constructed of locally available materials at affordable cost, using the skills of local craftsmen. They range in sophistication from a 'batch' system capable of heating 20 litres of water at a time to a thermosyphoninng system of 1,000 liters or more. Once again, we have confined our attention to 'passive' systems, since they are not only cheaper and less complex, since they are not only cheaper and less complex, but require no external energy source.
This is essentially a solar cooker which can also be used as a water heater. (For details of design etc. see under Solar Cookers) The solar cooker cabinet has been specifically designed to allow a 20-litre jerrycan (in which water is now commonly fetched and stored) to be placed inside. Water can be heated by this simple system in a few hours of direct sun. Since solar cookers are generally only used as cookers for a few hours of the day, they are available for most of the time for water heating. It is therefore quite possible to cook a midday meal as well as bringing at least 40 liters of water to a comfortable temperature for washing.
In this design, a rectangular tank with a large area /volume ratio is contained in an insulated box with a double-glazed cover. This collector is then mounted on the roof (roofs in Ladakh are flat), and inclined at a 45 degree angle to the horizontal, as a compromise between optimum summer and winter tilts. Cold water is piped into the top of the tank, and hot water drawn from the bottom.
The collector tank is constructed of 2mm welded GI sheet metal with internal support to prevent buckling. We generally make the tank about 10mm deep, with a surface of the tank is painted matte black.
It is perfectly possible to increase the above dimensions. One system which we installed, in the village of Shey, provides hot water at the village mosque for the whole Muslim community. Water is piped into a small sauna-like Trombe room, which is heavily insulated.
In a thermosyphoning system, natural heat flows are used to raise water from a solar collector into a remote storage tank, where hot and cold water stratifies. Cold water is fed to the bottom of the collector, and hot water can be drawn from the top of the tank without causing the water to mix. The thermosyphoning cycle continues until all the water in the tank is fed to the bottom of the collector until all the water in the tank is hot. Desired quantities of hot water can be drawn off at any time, and as the storage tank is insulated, hot water is available even after the sunsets.
Although they are somewhat more complicated the batch system, thermosyphoning systems are well-suited to restaurant or hotel use, where the demand for water is continuous.
The design which we have developed consists of two 1.7 m2 collectors plumbed in parallel to a 200-litre storage tank. Cold water is supplied to the system from a supply tank located about the solar storage tank. This keeps the solar tank filled, and provides water pressure when hot water is needed.
We have installed a large version of this same design on one of Leh's leading hotels. The system is sufficient to provide ten to twelve rooms with hot water. (It would easily cover twice as many rooms if tourists could be persuaded to be less wasteful). This installation uses five pairs of 1.7m2 collectors connected to five 200- litre insulated storage tanks.
The pipes that make up the collector are in the form of a grid of 15mm galvanized risers welded to 25mm headers. To maximize solar absorption, aluminum fins are attached to the risers, and secured with tensioned wire. Pipe and aluminum surfaces are painted matte black. The entire unit is set into a wooden cabinet, which is insulated with 10cm of lofted coconut fibre. The cabinet is single-glazed with 4mm glass. (Although double glazing would marginally increase efficiency, the extra cost greater maintenance demands make it impracticable. Storage tanks are made from standard 200-litre drums, coated inside and out with red oxide to prevent rust.
Each of the systems described above has its advantages and disadvantages.
The combination cooker/water heater is very cheap, and fulfils two functions. Water sufficiently hot for bathing is available after only a few hours, and temperatures in excess or 70 degrees centigrade can be reached. The main disadvantages are that the system is not provided with a tap, and that the heated water must be used nightfall.
In the drain-down system, water can be piped directly into the living-space where it will be used. At full capacity of 60 litres., water boils (89 degrees Centigrade in Ladakh) after three hours of direct sunlight As with the jerrycan system, however the drain-down system loses heat quickly at night.
Thermosyphoning system are the most versatile and efficient water heating systems available. The heated water is stored in an insulated tank, and is thus available 24 hours of the day. Water temperature in the 200- litre tank reach 55 degrees centigrade.
In three hours, or 80 degrees centigrade in six hours. The only real disadvantage of the thermosyphoning systems (apart from the greater cost) is that they are more complicated than bath units, and therefore that much more difficult to drain.
Copyright 1998-2005, Tibet Environmental Watch (TEW)