To obtain exact efficiency data on self-made solar water heaters is impossible. Data is extracted from experience and therefore very subjective. I do not think, that this is a disadvantage, but rather a possibility for further enquiry.
Since the start of solar technology in the late 70´, we developed methods of efficiency estimation for solar collectors. In the beginning, the estimates were based on experience, and one would say for example, that a 1,5 m flat-plate solar collector will warm-up, depending on the season, 180 to 225 litres of water to a temperature between 46 to 79 0C (central Californian presumptions).
Today, we know a lot of boundary conditions, and before we build, we try to estimate the efficiency of a solar device by calculation. As these calculations become more and more complicated, we rely on the manufacturers of these devices to support us with the relevant data for their products. If one, like me, tries to plan some very simple devices, which can be build for little money, out of, whenever possible recycled material, one goes of into the dark and estimation calculation becomes very vague. Nevertheless, I collect the data available and combine them with the experiences other people made and try to apply them for Sitio Joaninha, where the development of a solar collector and storage tank, especially designed for the local climate and peri-urban setting could be part of the adult education program and become a source of income for the village.
Source: http://www.eeca.govt.nz/content/pdf/!REN1_K8.pdf
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As we have quite a good global radiation value of 1.613,3 kWh/m2/a in Sitio Joaninha, hot water provision for a family of 4 to 6 should be achieved with simple and cheap means. So lets presume, that we build a flat-plate collector, with a normal efficiency, which lies at 60%. The efficiency depends on the absorption capacity and the amount of losses provoked by reflection, heat radiation and heat losses trough the pipe system and insufficient insulation of the collector itself and the water storage. |
 Source: http://solar1.mech.unsw.edu.au/glm/papers/ISES99%20Plenary.PDF |
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With the right orientation of the collector, which is at an angle of 35° and with a maximum deviation of 15° in either direction of the absolute north, the global radiation value can be raised to 1.740,0 kWh/m2/a. In Europe, one would presume, 1 - 1,3 m2 collector area per person and 60 - 80 litres of storage capacity per m2 collector area. Lets think, that in the newly build 55m2 houses lives a family of four. They have a demand of 120 litres of warm water per day. Due to the high global radiation in Sitio Joaninha, we can reduce the collector area to 0,8 m2 per person, but raise the storage capacity to 100 litres per m2 collector area. Thus the hot water demand can be satisfied throughout the year. |
 Source: http://solar1.mech.unsw.edu.au/glm/papers/ISES99%20Plenary.PDF |
It could be quite a challenge to develop an appropriate solar water heater system, which fulfills the hot water requirements, and which is cheap enough to be bought by many low-income families. One example, altough I think it can be done better, can be found here: www.redetec.org.br/mostraenergia/PDFs/ufrgs.pdf
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