HOW SOLAR WATER HEATERS WORK
Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don’t.
Residential solar thermal installations fall into two groups: passive (also called “compact”) and active (also called “pumped”) systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) which is activated when the water in the tank falls below a minimum temperature setting such as 55 °C. Hence, hot water is always available. The combination of solar water heating and using the back-up heat from a wood stove chimney to heat water can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.
When a solar water heating and hot-water central heating system are used in conjunction, solar heat will either be concentrated in a pre-heating tank that feeds into the tank heated by the central heating, or the solar heat exchanger will replace the lower heating element and the upper element will remain in place to provide for any heating that solar cannot provide. However, the primary need for central heating is at night and in winter when solar gain is lower. Therefore, solar water heating for washing and bathing is often a better application than central heating because supply and demand are better matched. In many climates, a solar hot water system can provide up to 85% of domestic hot water energy. This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.
Interactive map of solar resources in the United States.
Active indirect systems: drainback and antifreeze
Pressurized antifreeze or pressurized glycol systems use a mix of antifreeze (almost always non-toxic propylene glycol) and water mix for HTF in order to prevent freeze damage.
Though effective at preventing freeze damage, antifreeze systems have many drawbacks:
- If the HTF gets too hot (for example, when the homeowner is on vacation,) the glycol degrades into acid. After degradation, the glycol not only fails to provide freeze protection, but also begins to eat away at the solar loop’s components: the collectors, the pipes, the pump, etc. Due to the acid and excessive heat, the longevity of parts within the solar loop is greatly reduced.
- Most do not feature drainback tanks, so the system must circulate the HTF – regardless of the temperature of the storage tank – in order to prevent the HTF from degrading. Excessive temperatures in the tank cause increased scale and sediment build-up, possible severe burns if a tempering valve is not installed, and, if a water heater is being used for storage, possible failure of the water heater’s thermostat.
- The glycol/water HTF must be replaced every 3–8 years, depending on the temperatures it has experienced.
- Some jurisdictions require double-walled heat exchangers even though propylene glycol is non-toxic.
- Even though the HTF contains glycol to prevent freezing, it will still circulate hot water from the storage tank into the collectors at low temperatures (e.g. below 40 degrees Fahrenheit), causing substantial heat loss.
A drainback system is an indirect active system where the HTF (almost always pure water) circulates through the collector, being driven by a pump. The collector piping is not pressurized and includes an open drainback reservoir that is contained in conditioned or semi-conditioned space. If the pump is switched off, the HTF drains into the drainback reservoir and none remains in the collector. Since the system relies upon being able to drain properly, all piping above the drainback tank, including the collectors, must slope downward in the direction of the drainback tank. Installed properly, the collector cannot be damaged by freezing or overheating. Drainback systems require no maintenance other than the replacement of failed system components.
A comparison of solar hot water systems
If you are interested in a solar water heater system, contact one of the pre-screened plumbing professionals at ®.