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Solar Thermal

How Solar Thermal and Photovoltaics Work

Have you ever felt warm water trickle out of a garden hose that’s been sitting in the sun? If so, then you’ve witnessed solar water heating in action.

Now imagine that same water moving slowly though a system specifically designed to heat and store water – that is the essence of solar thermal water heating. People have for centuries used water heated by the sun and stored it for bathing, hand washing, cleaning clothes, heating homes and much more. The solar thermal systems used today combine the most effecient techniques for capturing the sun’s heat with modern plumbing systems to produce cost effective hot water and reduce the need for gas or electricity to heat water.

There are a number of different solar thermal designs, but all are based on the same simple principle as the garden hose. Each has its pros and cons, and each is suitable for a specific application.

What are the different types of systems?

Passive vs. Active

The terms passive and active in solar thermal systems refer to whether the systems rely on pumps or only thermodynamics to circulate water through the systems.


The simplest systems are passive solar water heaters, also called batch or breadbox collectors, they are most common in regions that do not experience extensive periods of below freezing temperatures. The water in these solar collectors circulates without the aid of pumps or controls.


Active solar water heaters use pumps to circulate water or an antifreeze solution through heat-absorbing solar thermal collectors.

Direct vs. Indirect

An important distinction among solar thermal systems is whether they are of direct or indirect design. In a direct system, the water used by building occupants to wash their clothes or bathe is the same water that is pumped through the solar collector. In an indirect system, an antifreeze solution is pumped through the solar heat collector. This warm solution is then used to heat the water used by building occupants. In this case, water is indirectly heated.

Collecting the Sun

Solar thermal systems also differ by the type of collector used to gather and store the sun’s energy. Flat plate collectors are the simplest and most common type. Copper pipes wind back and forth through the flat plate collector, which is painted black to absorb heat and covered with glass, or “glazing,” to prevent heat from escaping. Often the pipes are painted black and bonded to the material of the flat plate collector to maximize heat absorption.