How Does a Solar Water Heater Work?

Impending depletion of the conventional sources of energy has emphasized on the urgent need to look for alternative resources. This has made the scientists march into unexplored domains.

Harnessing the non-conventional energy resources like the sun, wind and tides, is possible now. The solar energy is now used on a large-scale. Ambit of the application of solar energy ranges from heating water to facilitating the electricity requirements to a space shuttle.

The global energy needs for one year can be satisfied with just the amount of the solar energy received by the earth for one hour!

The solar water heater utilizes the radiation from the sun to heat water. The solar water heaters are widely used in countries like Australia and Israel. The solar water heater consists of 2 major parts. One is the collector  and the other is the water storage tank.

The collector is the part in which the solar energy is harnessed. It is generally a metal box topped by a glazed glass to avoid loss of heat. Inner section of the collector contains a heat absorbing material. It is often insulated and coated with a dark color from inside, for better heat retention. Collectors can be classified as:

• Batch: Here the storage tank is situated inside the collector. This minimizes the loss of heat from the water. The surface area of the collector influences the size of the tank. These are the simplest in terms of construction. They are also called Integrated Collector Storage (ICS) systems.

• Flat Plate: In this, the heat absorbing material is placed in a box, facing the direction of the sun. The collector is attached to two pipes. One at the top and the other at the bottom. They are called the top and bottom headers, respectively. Inside the collector, tubes called risers are located. 

Here the storage tank is situated below the collector. The water to be heated is sent through the bottom header. It gets heated and then rises through the risers and gets stored in the hot water tank through the top header.

• Evacuated Tube Collectors (ETC): This type of collector is evacuated. It generally contains double layered borosilicate glass tubes in it. The outer wall of the inner tube is coated with a heat absorbing material. These collectors are very efficient. They provide optimum heat retention.

They are usually double walled. The gap between the wall is filled with an insulating material. The storage tank needs to be big in size as the sunlight is available only for a few hours a day, so that maximum water is heated and stored in that time.

The solar water heaters are of two types, passive and active. Each of them have their sub-types. Although the main functionality of all of them is the same, depending on the type, their mechanisms change. I will be explaining each of the mechanisms.

These systems use electrical pumps to control the flow of water in the system. They are generally distinguished into:

• Direct: The movement of water into the collector and the flow of water from the collector to the tank is facilitated by the electric pumps.

• Indirect: In the direct mechanism, the water is directly heated. Whereas here, a heat absorption fluid is heated, which may be an antifreeze. An antifreeze is a substance that does not tend to freeze in cold climatic conditions. Non-poisonous propylene glycol is an antifreeze that may be used.

After heating this fluid, it is passed into a tube, which is surrounded by the water to be heated. This is called the heat exchange area. Here the water absorbs the heat from the fluid and in turn gets heated. It is important to take care that the fluid and the water do not get mixed.

In this case, no electric pumps are used for the flow of water. They can be differentiated into thermosiphon and batch. The only difference between these two is that one of them has the storage tank outside the collector while the other has it inside the collector.

By now you might have understood the variation in the working of the solar water heaters on the basis of collector classifications and the active and passive distinctions. To make it easy for you, I would be explaining the working of a basic solar water heater. This would help you to understand the common mechanisms in a solar water heater. The key mechanisms are as follows:

Heating: The collector is exposed to sunlight. The radiation from the sun passes from the glass top into the collector, here it is absorbed by the heat absorbing material. Thus, the radiation from the sun is converted into heat and the water inside the collector gets heated.

Movement: Now the water is heated and it needs to be transferred to the storage tank. Gravity or the convection can play a decisive role in the movement of the water.

• Gravity: If the storage tank is situated at a lower level than the collector, then the heated water will be transferred to the tank under the influence of gravity.

• Convection: It is the phenomenon by which the water tends to rise after heating. If the water tank is situated at a level higher than the collector, then the water rises to the tank through convection.

Storage and Use: Storage tanks are used to segregate the water from the collector. The tanks should ideally be kept in a conditioned environment to avoid the cooling of water due to external atmosphere. This water can be used by connecting pipes to the tanks. In a geographical region with ample sunlight, a solar water heater can meet 80-90% of the household requirements.

If we consider cost, then the solar water heaters are expensive. But they are economical in terms of maintenance. It is a great way by which you can heat water without consuming the conventional sources of energy. A passive solar water heater can be made at home. So, if you are planning to "Go Green", using a solar water heater would be a great idea and if you are planning to make one yourself, do let me know.