Solar water heaters work by harvesting thermal energy (heat) from sunlight and transferring it to water in a collector. Although it may sound a bit complicated, it’s really a simple process. In fact, some solar water heater types are simple enough for the average DIY enthusiast to build themselves at home.

There are essentially two types of solar water heaters, active and passive. Active heaters feature a pump system to circulate the water. Passive systems rely on positive or negative pressure or gravity to circulate the water.

In turn, each type is represented by different system variations which we’ll cover in detail later. Either way, solar water heating systems are far cheaper to run than comparable electric or gas systems. Solar water heating also represents a green alternative to heat water for household use.

The Main Types of Solar Water Heaters

As we have mentioned earlier, there are two main categories of solar water heating systems namely active and passive systems. To make the technicalities easier to understand, here are a couple of terms used throughout the article and their definitions.

  • Collector – Solar collectors are the components where water is exposed to sunlight which heats it via a process of radiation. There are two basic types of solar collectors – Flat plate collectors and batch collectors. In a flat plate collector, the water flows through a series of small tubes similar to the radiator of a car. In a batch collector, solar energy heats water in a single tank or series of tanks.
  • Storage tank/Backup heater – This is where the heated water is stored for later use. The storage tank may also be fitted with an electric or gas backup heating system.
  • Heat exchanger – This is typically a coiled tube that is placed in the water tank. Water heated in the collector is pumped through the coil, shedding heat into the storage tank.

How Does an Active Solar Water Heater Work?

An active system features a pump that circulates water through the collector and back to the home or a water tank. There are two main types of active solar water heaters.

  • Direct circulation systems – In these systems household water is pumped through the collector and straight back into the home’s hot water circuit. This type of system is suited to temperate and hot climates.
  • Indirect circulation systems – In an indirect circulation system, water treated with an anti-freeze solution is pumped through the collector. The heated water is then directed through a thermal exchanger placed in a tank. This arrangement “sheds” heat into the stored water for later use in the home. These systems are well suited to environments where freezing temperatures are common in winter. An example of this type of system is illustrated below in figure 1.
how does a closed loop solar water heater work

Image credit: Paul Scott

How Does a Passive Solar Water Heater Work?

Passive systems tend to be less expensive and more reliable than active systems. That said, they also tend to be less efficient than their active counterparts. Of course, that doesn’t mean they don’t have a valuable role to play in solar water heating.

The fundamental difference between the systems is the collector volume. Although they both use the sun’s energy to heat the collector water, active systems pump water through a series of thin tubes. So, the collector’s volume is small and constantly circulating.

In a passive system, the collector is typically a large volume tank or tanks which holds water on demand. The sun heats the water in the batch tank where it ready for later use

When hot water is and it flows into the water storage under gravity. When a faucet is opened, water flows out of the tank and into the hot water circuit.

Again, there are two main types of passive solar hot water systems.

  • Integral storage/batch collector systems – This type of passive heater is quite similar to active closed-loop systems. Water is heated in a batch collector and then passes to a separate tank. From there it is stored in a tank similar to an active closed-loop system. An example of an integral collector storage system is illustrated in Fig. 2 below.
integral storage batch collector solar water heater

Image credit: Paul Scott

  • Thermosyphon passive water heaters – These are the simplest type of passive solar water heating systems. Cold water is also heated by solar radiation in a collector, typically located on the home’s roof. When a hot water faucet is opened in the home, the hot water flows directly into the home plumbing system. This solar hot water system is illustrated in Fig 3. below.
Thermosyphon passive solar water heater

Image credit: Paul Scott

In Summary

As with most solar technology, a solar water heating system is effective while remaining environmentally friendly. In most cases, they can shoulder a household’s hot water needs while still maintaining the redundancy of conventional heating. This is due to the general inclusion of a conventional backup water heater.

If you have any comments or advice regarding this article, please post a comment below.

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