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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.

What is it?

Also Wikipedia (2007) highlights that to heat water using solar energy, a collector is fastened to the roof of a building, or on a wall facing the sun. In some cases, the collector may be free-standing. The working fluid is either pumped (active system) or driven by natural convection (passive system) through it.

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.

Who uses it?

Flat-plate collectors for solar water heating were popular in Florida and Southern California in the 1920s. There was a flicker of resurgence of interest in them in North America in the 1970s. However, the main improvements now are occurring in other countries which have enabled effective use of solar heating in both private houses and large scale installations. Technical innovation has improved performance, life expectancy and ease of use of these systems, with Germany/Austria the clear leaders. Installation of solar hot water heating has become the norm in countries such as Israel and Greece, where there is an abundance of solar radiation, and Japan and Austria where there is considerably less.

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.

Advantages

Any time you use solar energy to offset the amount of fossil fuels that are burned, you contribute to everyone's health and welfare. Operating one solar water heater instead of an electric water heater saves the equivalent of nine barrels of oil every year and reduces carbon dioxide emissions (a greenhouse gas) by 1600 pounds and sulfur dioxide (contributes to acid rain) emissions by 12 pounds. The best savings in hot water come from no cost or low cost options.

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.

Disadvantages

The initial cost is the main disadvantage of installing a solar hot water systems, largely because of the high cost of the supplies. The efficiency of the system also relies on the location of the sun, although this problem can be overcome with the installation of certain components. The production of solar hot water is influenced by the presence of clouds or pollution in the air. Similarly, no solar hot water will be produced during night time although a backup system will solve this problem.

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.


http://www.cityenergy.org.za/resources/solar-water-heater
Last edited by Miriam Hansen .
Page last modified on Tuesday September 28, 2010 11:02:30 GMT.
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