Solar Water Heating
According to Wikipedia (2007) solar hot water refers to water heated by solar energy. Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and a reservoir or tank for heat storage and subsequent use. The systems may be used to heat domestic hot water, swimming pool water, or for space heating.
Water heating accounts for about 30 percent of an average household's total greenhouse gas emissions and about the same proportion of total household energy use. Using solar energy to heat water produces no harmful greenhouse gas emissions. A solar water heater can provide between 50 percent and 90 percent of a household total hot water requirements, depending on the climate and the model of heater.
The upfront cost of a solar water heater (including installation) is higher than electric or gas water heaters but energy bill savings will compensate for this over time.
Besides, worldwide, about 1.3 billion people do not have access to safe drinking water. Yet, in many of the most severely affected regions, sunshine is an abundant source of energy that not only can cook food, but can also heat water to temperatures that kill harmful microbes, making water safe to drink. This procedure is called solar water pasteurization or disinfection (SODIS). Solar Disinfection of water, is a simple, cheap, efficient, useful, renewable and environmentally friendly practical technique that uses solar energy to inactivate and destroy pathogenic micro-organisms present in water. The process is a combination of heating and use of ultraviolet(UV) radiation from the sun which is sufficient to sterilise water.
The collector could be made of a simple glass topped insulated box with a flat solar absorber made of sheet metal attached to copper pipes and painted black, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In some cases, before the solar energy is absorbed, a parabolic mirror is used to concentrate sunlight on the tube.
A simple water heating system would pump cold water out to a collector to be heated, the heated water flows back to a collection tank. This type of collector can provide enough hot water for an entire family.
Heat is stored in a hot water tank. The volume of this tank will be larger with solar heating systems in order to allow for bad weather, and because the optimum final temperature for the absorber is lower than a typical immersion or combustion heater.
Regarding water disinfection, the treatment consists either filling transparent plastic (PET plastic), or glass bottles with water and exposing them to full sunlight for about five hours or two consecutive days under cloudy sky. To absorb more heat and raise the water temperature the bottle is painted black on one side and placed on a black surface in the sun, with the clear side facing the sun.
Also, solar hot water systems have become popular in China, where basic models start at around 1,500 yuan (US$190), much cheaper than in Western countries (around 80% cheaper for a given size of collector). It is claimed that at least 30 million Chinese households now have one, and that the popularity is due to the efficient evacuated tubes which allow the heaters to function even under gray skies and at temperatures well below freezing.
The SODIS technology for water treatment has been tested in may parts of the world including Kenya,
Tanzania, Ethiopia, Vietnam, and some countries in the Americas. China, Southern Africa, Brazil are some of the countries using SODIS in bigger scale.
The solar disinfection method is good for various reasons: chlorination costs money for tablets, filtration costs money for filters, and boiling is very laborious (and in some areas of developing countries the collection of wood is not allowed for environmental reasons). A plastic bottle is a sustainable resource with an insignificant cost.
In the case of SODIS if not left in strong sun for the proper length of time (due to carelessness, impatience or urgent thirst) the water may not be safe to drink and could cause illness. There is also concern over whether plastic drinking containers can leach chemicals or toxic components into water, a process possibly accelerated by heat.
A major limitation of solar heating is that only small volumes (around 10 litres) of water can be exposed
conveniently at one time per water container and solar reflector. Another important limitation is availability of sunlight, which varies greatly with season, daily weather (meteorological) conditions and geographic location. A third potential limitation of solar heating to disinfect water is the determination of water temperature.