How Do Solar Water Heaters Work?

Heating water accounts for a good portion of a home’s energy usage. This has made people take to solar water heating options. Solar water heating caters to a wide market and has been hugely popular because of its proven technology.

Let’s look at how water heaters work.

Before getting into the working of a solar water heater, we need to know the components in it.

The major components of a solar water heater are listed below:

  • Solar collectors
  • Water storage tank
  • Circulation system

Let’s look at how each of the component contributes to the working of the system

1) Collectors: Where the sun’s energy is absorbed

Solar water heaters use the sun to heat either water or a heat-transfer (usually an anti-freeze) fluid in the collector. These collectors are usually of two types.

  • Flat plate collectors: These consist of insulated outer metallic box which is covered on top with a glass sheet. Inside, there are blackened absorbers sheets that absorb solar radiation and transfers the heat to the water or working fluid that flows through it.
  • Evacuated Tube Collectors: These consist of double layered glass tubes in order to provide insulation to trap heat. The tubes are coated with absorbing material and heat from solar radiation is transferred to water or the working fluid that flows through it.

2) Circulation systems: Where fluid in the collectors is transferred to the storage tank.

The circulation system circulates the heated liquid/hot water from the collectors to the storage. Solar heating systems are classified into active and passive systems based on the mechanism used for the circulation of hot fluid.

In active systems, pumps are used to circulate household water or the antifreeze liquid. They also have electronic controllers for regulating the overall heating process. These controllers make sure that the pump is on only when the circulating fluid is warmer than the water stored by around 8 degrees Celsius. This way overheating of water can be avoided.

In passive systems, circulation depends purely on natural convection forces. Since warmer air rises, the storage tanks are placed above the solar thermal collectors. They are cheaper than active systems but aren’t really reliable.

Now, if water itself is used as the working fluid, it is a direct heating system with no need for heat exchangers. In cold countries, the direct heating systems wouldn’t be practical.

Antifreeze liquids like glycol are employed as the working fluid in these systems that come under indirect solar heating systems.In these, heat energy from this liquid is passed to the water kept in a storage tank using heat exchangers.

3) Finally the storage tank: Where the hot water is stored

The storage tank is well insulated and therefore doesn’t lose heat. The households can use hot water from the storage tanks as and when required. Most storage tanks are designed in a way that the water stored stays hot for a day.


Solar water heating is now a mature technology and the system has technically evolved with time. Different combinations ranging from flat-plate based passive direct water heaters to evacuated tube based active indirect water heaters are available in the market. Wide spread utilization of solar water heaters can reduce a significant portion of the conventional energy being used for heating water in homes, factories and other commercial and institutional establishments. Internationally the market for solar water heaters has expanded significantly during the last decade.

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India Renewable Energy Expert