How do solar water heaters work?

Most of us are familiar with solar power. But, how do we use the sun to heat our water?
Most of us are at least somewhat familiar with solar power, which is derived by converting energy from the sun into thermal (heat) or electrical energy. We've all seen either solar panels or photos of them mounted on roofs, sparkling in the sunshine. But how do they work? And how can we use them to heat the water in our homes?

What is a Solar Water Heater?

A solar water heater system uses energy from the sun to heat water for everyday household use, such as for bathing and doing laundry. The solar water heater system, which consists of a collector and a storage tank, performs three functions: collect energy, transfer energy, and store energy. The collector is in the form of a panel installed on the roof or a wall with sun exposure. There are two main types of solar water heater systems — active and passive.

Active Water Heating Systems

Active systems use electric pumps to circulate water or other fluid from the collector to the storage tank. These systems can heat water either directly or indirectly.

Source: U.S. Department of Energy
  • Direct systems: Water circulated through the collector is heated by the sun. The hot water is then transferred to an insulated storage tank or a tankless water heater. Direct systems are not recommended for cold climates, as the water may freeze in the collector. In addition, because water is run directly through the collector, mineral deposits can build up in the pipes, requiring cleaning.

  • Indirect systems: A non-freezing liquid is heated in the collector, which then transfers the heat to the water in a storage tank. The non-freezing liquid (transfer fluid) is then re-circulated through the collector to start the process again. Indirect systems are a good choice for cold climates, as they eliminate the possibility of water freezing, which can damage the collector.

Passive Water Heating Systems

Source: U.S. Department of Energy
Passive systems rely on convection (the process by which heat transfer produces a current that causes hot liquid to rise and be replaced by cooler liquid) to circulate water or transfer fluid. These systems cost less and require almost no maintenance, but they are not as efficient as active systems. There are two types of passive systems: Integral collector storage (ICS) systems and thermosiphon systems.
  • Integrated collector storage systems: Also known as batch systems, ICS systems feature a single unit that contains both the collector and the tank. As with the direct systems discussed above, these are only appropriate for milder climates, as the water may freeze in the collector tubes.

  • Thermosiphon systems: Water is heated in the collector, where convection moves it into the separate storage tank. Because the storage tank must be above the collector (hot liquid rises), the roof design must be able to support the weight of the storage tank.
Almost all homes using solar water heaters will require an electric or gas backup water heater for cloudy days or when hot water usage exceeds the capacity of the system. Now you know the basics of how a solar hot water heater works.

If you are contemplating installing a solar heater, contact a professional installer who can help you decide what type of system is best for your home.

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