How it Works
Many people have a negative view of solar water heating because of the susceptibility of some systems installed 15 to 20 years ago to have freeze damage. However, solar hot water systems installed today are immune from freeze damage and are designed for a long, useful life of 20 years or more.
There are two very different designs used to accomplish this. Both use an isolated, closed loop system that continually collects and recirculates heat from the solar collector on the roof to the domestic water heater through a heat exchanger.
Drainback system
The first design, a drainback system, uses gravity to drain the water at night from the components exposed to the elements — the solar collector and the pipes leading to and from it. The advantage of this system is that the heat exchange fluid is pure water. Pure water has a higher heat capacity than an antifreeze solution making it slightly more efficient. More importantly, this water does not break down over time due to high heat, so a drainback system requires less maintenance than a glycol system. A limitation in this system, however, is that the collector on the roof and the pipes to and from the solar collector must all be sloped down to the water heater so the system can drain when the pump turns off.
Glycol System
The other design, called a glycol system, uses a food-grade antifreeze solution in those same components to bring the freezing point of the solution down to a safe point. The benefit of this system is the lack of constraints on the collector placement and plumbing configuration. It can also use a simpler control system based on a pump powered directly by a small photovoltaic module. The disadvantage is that the glycol solution degrades over time because of the high temperature in the collector and so needs periodic replacement.
Both of these systems are active, meaning they depend on one or more pumps to move the heat transfer fluid. They also are both capable of meeting 50-90% of the hot water needs of a family.
Batch Heater
Another type of hot water system is called an integrated collector and storage (ICS), also referred to as a batch heater. This is a simple, passive system, meaning there are no pumps or controllers. This is the system DIY people most often make and install on their homes. The batch heater is simply a tank of 30 to 40 gallons that is under glass inside of an insulated case. The water in the tank heats all at once and when a hot tap is opened in the house, hot water leaving the batch heater is simultaneously displaced by incoming cold water. Batch heaters work extremely well when hot water demands for the household are low (1 or 2 people) and the hot water use is limited from mid-morning to evening. Expect no hot showers in the early morning during the winter months from batch heater systems without the backup electric or gas system turning on.