We have plenty of solar energy available to us freely, which, when put to proper use, can provide innumerable benefits to mankind. It can be used to:
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Heat water-Solar Water Heaters
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Generate light and electricity-Solar lamps and solar generators
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Cook food-Solar cookers
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Store food-Solar refrigerators
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Increase Air circulation and keep the room cool-Solar fans and air conditioners
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Drying items-Solar dryers
The Solar water heater, in which water gets heated via solar energy, is dealt with here as it is popularly being used in many households. It is all the more prevalent in tropical regions as in India or China. Major players of Solar water heaters in the Indian market include Sunlit Solar Energy, Greentek India Pvt. Ltd. and Nuetech Solar Systems Ltd.
Working Principle and Brief History: The working of the solar water heater is based on two elementary principles- -->A black surface gets heated up when exposed to the sun’s rays. -->The inside of a car or a room, with closed glass windows, becomes hot due to the solar energy getting trapped inside. The above two principles were utilized in building the world’s first solar heating system- the “Climax” approximately around 1890. Around the year 1900, William.J. Bailey introduced the concept of a storage tank where the heated water could be stored and used later. He, thus, gave birth to the fist modern solar water heater. How does a basic Solar water heater work? A solar water heater consists of a collector or absorber panel to collect the solar energy and a storage tank to store the hot water. The glass panel is specially coated to maximize solar absorption. The energy from the sun’s rays which fall on the collector gets transferred to the pipes underneath. The water flowing in the pipes gets heated up and is then delivered to an insulated storage tank. The tank is generally made of copper insulated with materials like glass wool. Valves are present to control the supply of fresh cold water and the discharge of hot water into the home. Classification:
The solar water heating systems are broadly classified into:- Active Vs Passive
Active systems are those that require electricity-run or solar-run pumps for the heat transfer and for letting the water to the solar panel to get heated up when the sun shines. They also have controllers or valves for controlling the water flow. Passive systems do not have pump; instead they use convection and the pressure difference for water circulation. Here, the collector and the storage tank are usually mounted on the same chassis and there is continuous circulation of the water to the collectors and back to the tank due to the difference in the density between the hot water and the cold water. This is called “thermosyphoning”. 
Working of a Basic Solar Water Heater
Open Vs Closed systems
An open loop system has water flowing directly through the collectors into the tank to the home.
A closed loop system, on the other hand, has a non-toxic antifreeze which is used to pick up heat and transfer it to the water in the tank via a heat exchanger. This prevents excessive heat loss and damage in cold conditions and night time.
Integrated Collector and Storage Solar heater A special and simple type of solar water heater is the Batch heater or the ICS (Intergrated Collector and Storage) where the storage tank is also the “collector”. Here the principle of gravity is used for delivery of the water and this is well-suited for household with less people
Types of solar water heaters based on the collector:
♦ Flat-Plate Collector solar heater- The collector is made of an insulated box that contains a dark absorber plate, which is coated on its sun’s facing side. Metallic pipes to carry water, and glass sheets to absorb the solar radiation run underneath. At the surface is a glass plate.
♦ Evacuated Tube Collector solar heaters- Here, the collector is made of evacuated double layer borosilicate glass tubes. The outer wall of the inner tube is made of the absorbent material which absorbs the solar energy to be then transferred to the water which flows through the inner tube.
Flat plate collector Evacuated Tube collector
Benefits of Solar Water Heaters:
Proper Harnessing of Solar energy
A solar water heater is an efficient way of utilizing the solar energy. A sq. meter of collector area can be used to heat about 50 liters of water. However, for providing hot water on cloudy days, many water heaters have a built-in heating element. Wind or some gas is used to generate electricity; this heats up the heating up the water.
Cost-effectiveness
Although the initial outlay on a solar water heating system is high (roughly from Rs.18,000 onwards), in the long run, it saves substantially on electricity or other fuel used to obtain hot water. Typically, a 100-litre capacity solar water heater can save about 1500 units of electricity annually. Furthermore, there are loans available at nationalized banks for purchasing solar heaters.
Environment friendly
Solar water heaters are environment friendly in that they reduce the emissionof harmful gases into the atmosphere. It is estimated that a 100-ltr capacity Solar water heater can prevent emission of 1.5 tones of carbon dioxide per year as there is no more usage of fuel for heating water. Hence, they reduce pollution, and, in turn, save the earth from the harmful effects of global warming.