Now that you have familiarized yourself with the types of solar collectors in use today, it is time to explore solar hot water systems with separate collection and storage. We’ll begin with the simplest of systems.

Convection drives the heat transfer fluid in this thermosiphon system, eliminating the need for a pump and electricity. Direct systems use water as the heat transfer fluid. Indirect systems use antifreeze as the heat transfer medium. Note the absence of a heat exchanger in this system.

The most basic solar hot water system is the thermosiphon system, shown in the diagram below. As you can see, this system consists of a collector, a storage tank, and pipes connecting the two. Notice, however, that there are no pumps or sensors or controls in the system. There’s no need for them. Water flows through the system by convection.

How does this work?

Convection is the movement of a hot fluid such as air or water. As you may recall from high school physics, air and water both expand when heated. When they expand, they become less dense. This causes them to rise. In the process, they carry heat with them.

But that’s not the end of the story.

A liquid rising by convection creates a vacuum that draws fluid in; the result is a natural pump called a thermosiphon. Consider an example: sunlight striking the Earth heats its surface. Heat is transferred to the air above it, causing it to expand and rise. Hot air rising, however, creates a vacuum that draws cooler air in from neighboring areas to fill the void.

In a thermosiphon solar hot water system, like that shown in the diagram, water rises in the pipes inside the solar panels when it is heated, drawing cooler fluid in from elsewhere. The result is a natural pumping action, a convective loop. The convective loop propels the hot water into the house to a solar storage tank, where the heat is deposited. Cooler water from the bottom of the tank then flows back into the collector to be reheated.

The convective flow of liquid in this system is a simple, non-mechanical pump that operates throughout the daylight hours, stripping heat from the solar panels and depositing it in the storage tank in the house.

Thermosiphon systems are simple and elegant, and less expensive than more complicated pump-driven systems, discussed shortly. Like solar batch water heaters, thermosiphon systems are not, technically speaking, active systems.

They have no mechanical pumps.

Before you race out to buy such a thermosiphon system, let’s examine some crucial factors you’ll need to know before laying down your hard-earned money.  ....read more