Because-- Solar water heaters save electricity and thus money; electricity is becoming more and more expensive; they could even turnout to be more reliable than electric power supply (at least in many parts of our country); they are clean and green and thus reflect one's commitment for preservation of environment; they are safer than electric geysers as they are located on the roof; and, if well designed, may even look good on the house top.
What is a flat plate Collector?
It is the heart of a solar water heating system. It consists of an absorber plate which is coated on its sun facing surface with an absorbent coating, also called selective coating. The absorber consists of a grid of metallic tubes and sheets. Water flows through the tubes. Sheet absorbs the solar radiation falling on it and transfers it to water. The absorber plate is placed in a top open box to protect it from weather. The space between back and sides of the absorber and the box is filled with insulation to reduce heat losses. The front of the box is covered with a high transmittance glass plate. Flat plate collectors are specified on the basis of their area and are of commonly 1x2 m size.
What should one look for in a good Flat plate Collector?
Bureau of Indian standards has laid down specifications of flat plate collectors for use in solar water heating systems and thus the IS mark is an assurance on use of proper materials. The important characteristics are, the materials of construction of the absorber plate, the type of absorbent coating on it, the quality of the glass plate used, the material of the box, the insulation thickness, etc.
What are the desirable Characteristics of hot water Storage tank?
The hot water storage tank in domestic solar water heating systems is typically a double walled tank. The space between the inner and the outer tanks is filled with insulation to prevent heat losses. The inner tank is generally made of copper or stainless steel to ensure long life. The outer tank could be made of stainless steel sheet, painted steel sheet or aluminium. Electrical heating elements controlled by thermostats can be provided as an option in the tank itself to take care of those days when sun is not there or demand of water has gone up. The capacity of the tank should be in proportion to the collector area used in the system. A commonly used thumb rule is to provide 50 litres of storage for every sq. m of collector area. Too large or too small tanks are both detrimental to efficiency.
How big a system should one purchase?
The fundamental rule is that it is better to buy a system smaller than your requirement and use back up when you fall short of hot water, rather than buy a system much bigger than your requirement (because you can afford it).This will lead to inefficiencies and may even cause operational problems. The best is to make an actual estimate of daily demand of hot water by measurements on the main use points. Do remember that the solar system is capable of heating only an approximately fixed quantity of water and is designed for typical sunny days. Thus in this characteristic it is unlike an electric geyser which can supply widely varying quantities of hot water in a day. Also remember that the temperature of water in the solar system is determined by the combination of collector area and the tank capacity. Typically it would be 50 - 60°C, which is much hotter than the bathing water temperature (around 40°C). Estimation of your demand can also be made with the help of the table given below. As a typical example on sizing of solar systems, it may be mentioned that a 100 litres system is considered generally optimum for family of 3 adult members.
How much electricity and money can be saved?
This may be roughly Rs. 120 per person per month.
What is the expected life of a solar water heater?
Typical solar water heaters made using materials as per BIS specifications could last for 15 -20 years depending upon the general upkeep, etc.