Solar Thermal Systems
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. Consult with us to determine which is best for your situation.
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.
Here is a simple schematic showing a Dual-Coil Solar Tank with Boiler back-up
click on the image below for a better view
In an indirect system, also known as “closed loop,” a simple pump moves the antifreeze solution through a loop into the solar collector, through the collector’s pipes, and out of the solar collector. Then, the sun-warmed antifreeze solution flows into a heat-transfer unit where it warms the cool water heading into a conventional hot water tank. The antifreeze solution then returns to the pump and again flows into the solar collector without ever mixing with the building’s water. Indirect systems are encouraged in climates with extended periods of below-freezing temperatures. We primarily install these types of systems. We find they produce the greatest amount of energy, more consistently, with a greater service life than other 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.
A direct system, also known as “open loop,” is a little simpler. There is no antifreeze solution; the water heated directly by the sun is the same water used by building occupants. A thermometer and controller sense when the solar collector is warm and ready to heat water. The controller starts a pump that moves cold water into the solar collector, where it is heated. The solar heated water is then stored in a conventional hot water tank. It is typical, especially during high use or periods of little sun for the water to be kept warm through supplemental gas or electricity. This type of system, because it circulates pure, potable water through an outdoor collector, is susceptible to freezing in many climates, unless additional safeguards are added.
In reality, both direct and indirect systems are somewhat more complicated than presented here. Differential thermostats, pumps, sensors, and controls are used so the simple systems illustrated work effectively and safely. However solar thermal technology is mature and proven with few maintenance requirements from the installed systems.
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.
Solar pool heating systems use a similar design, but sometimes glazing is removed to save money and to prevent the pool water from becoming super-heated. Some non-glazed systems look like flat black mats. Inside the mats is a network of headers through which the water slowly passes.
More advanced systems like evacuated tube collectors and parabolic trough collectors can heat water or other fluids to much higher temperatures appropriate for industrial needs.