by Marios Cleovoulou
In January and February of 1997, in conjunctions with the British charity Salt of the Earth, the author visited the main campus of the aid organisation Social Change and Development (SCAD) near Cheranmahadevi, Tamil Nadu, India. While there SCAD requested him to see what could be done to provide hot water for the children at SCAD's school for handicapped (mostly polio affected) children.
The result was the solar water heater described below. The construction of this was viewed with bemusement by many of the locals, who questioned any need for hot water, rather wanting to know if cold water could be provided for the 40+°C hot season! However, the physiotherapists at the school felt that hot water would be advantageous for the childrens' therapy, and the kitchen staff saw that having hot water would mean less wood would be needed to get water to the boil for cooking.
As the school has over 50 children, the design had to be fairly large. A 250 litre water heater was built, mostly using materials found at SCAD's centre. It is not claimed that this is the most efficient solar water heater ever built, although it worked well. Nor would the heater, if it were built with new materials, be extremely cheap, although it was almost free for SCAD, the only item bought being the 250 litre plastic "Sintex" tank for Rs1250 (~£22 or ~US$36). The price of the tank alone puts the system out of the reach of most of the villagers served by SCAD, even if they could feel a need for hot water.
The Solar Collector Panel
The solar collector panel followed a standard design, being ad hoc only in the use of materials. A shallow wooden box frame, about 200 cms high by 160 cms wide, was built from available timbers (~2.5cms x 15 cms). The inside of the box was lined with 2.5cms thick polystyrene sheets. A sheet of tin, painted black using blackboard paint, was fitted over the polystyrene.
An interconnection of 15 vertical 2.5cms zinc pipes (see diagram) was constructed. As no welding equipment was available these were joined with "T" connectors and elbows at the ends and connected with unions in-between (to make screwing together everything possible, as all pipes and connectors were threaded in the same direction).
The pipework was fastened to the tin sheet by making small holes in the sheet and tying the pipes down with thick wire. Entry and exist pipes and holes in the frame were provided. All the pipework was painted black using blackboard paint.
Finally, the wooden frame was painted for protection against weather and insects, and a clear plastic sheet was fastened across the front of the collector to prevent heat loss to the wind.
The Tank and Connections
The tank used was a "Sintex" plastic 250 litre tank. It was sealed at the top (the tank comes with an openable lid). Four pipe connections were made. Two were on one side of the tank at the top and bottom for hot water outlet and cold water inlet respectively. The other two were on the other side of the tank for connecting to the solar collector panel, one at the bottom, the other about 80% of the way up the side.
The tank was insulated using hay-filled woven polyester bags (used cement sacks), these being lashed on and the whole lot covered in a plastic sheet for protection from the weather.
The connections to the solar collector were made using sections of garden hose, insulated from heat loss to the wind by plastic bottles. The bottles had holes made in the bottom and were then slid over the hoses, the neck of each bottle being fitted into the hole in the bottom of the one above. (The author saw this technique in use while attending the Sunseed Desert Technology Research Centre in Almeria, Spain.)
Two taps were provided, one for the hot water outlet and one on the cold water inlet side (see below). A stopcock was also fitted in the supply pipe.
Installation and Operation
The tank and collector were installed on the roof of the school, supported by concrete blocks to get them at the correct relative heights. The collector was inclined at about 22° and oriented south. As the site is at about 8°30' north of the equator this inclination was chosen to capture maximum sunlight during the cool season, reckoning that during the hot season the extra strength of the sun would be more than enough to offset the non-optimal angle for that time. As at this latitude the sun peaks in the north during the hot season care was taken to ensure that the collector would not then be in the shadow of the tank.
Initially there were some problems with air locks preventing water flow. This was solved by turning and inclining the panel slightly such that the horizontal pipes had a slight gradient upwards in the direction of the water flow, thereby allowing any trapped air to escape.
Regretably, thermometers with an adequate range were not available, so scientific measurements were not possible. However, the heater worked well, giving hot water even before it was lagged. The top part of the tank provided water too hot to the touch by mid-morning and the whole tank was hot by late afternoon.
As the effectiveness of the sealing of the top of the tank over time was unsure the system was designed to work in two modes.
Firstly, as a pressurised system, in which the cold water inlet stopcock would normally be left open and the hot water outlet tap would be used to control flow.
Secondly, as an overflow system, in which the hot water outlet tap would be left open and the cold water inlet stopcock, normally turned off, would be used to cause an overflow of hot water.
The tap on the cold water inlet side was found useful by the kitchen staff for providing a backup reservoir of water (cold through lukewarm to hot, depending upon the time of day) during the not infrequent losses of mainline water pressure. However, they did have to be trained to ensure the tank was refilled afterwards, by opening both the hot outlet tap and cold inlet stopcock, as the water heater stops working once the level of water falls below that of the top collector connection.