Salud Partners, requested that a hot water system be designed to service the showers and kitchen. It was determined that liquid fuels (e.g. propane) would be too expensive to heat a hot water tank (boiler). The heating source chosen was a combination of wood and solar energy.
Solar Water Heater
A solar panel is comprised of pipes running back and forth inside of a box covered with glass or plastic. There are two general types: a passive system with many parallel pipes which circulates the water via thermal siphon and an active system with one pipe winding back and forth which circulates the water via a small water pump. Since the passive system usually requires the hot water tank to be located in the attic of the building, the active system was chosen as the best alternative. A common solar water heater can be seen in the following image.
A 20 gallon water tank was calculated to be a large enough tank to meet the hot water demands of the dormitory. Thermodynamic calculations were done to determine what size panel would be necessary to heat a 20 gallon hot water tank up to various temperatures. The average daily duration of direct light is about 5 hours. Since the people in the community are used to bathing in unheated water from the river, having temperatures of around 110 deg F as is typical in the U.S. was not essential. Also, the system will be complimented by a wood fired burner to heat the water further if and when desired. For these reasons, a temperature rise from 59 deg F to 95 deg F was determined to be acceptable. It was determined that 28 square feet would be sufficient to acheive this goal.
Wood Fired Boiler
To compliment the solar water heater, one or two wood fired heaters (and hot water tanks) should be used. They are simple to install and operate and heat the water through burning wood below them. The figure below shows a common wood fired boiler.
Within the unit, the gas (smoke) and heat travel upward through the center of the tank in a stack. The heat is transferred through the wall via conduction. Natural convection circulates the water in the tank to further distribute the heat. The water loop to and from the solar panel can be connected into these tanks. During the summer months, the solar panel will provide clean and free heating of the water to a suitable temperature for showers and cooking. During the winter months, due to the shorter duration of direct sun and colder outside temperatures, wood can be burned to heat the water to a higher temperature.