The Technology

The operational aspects of a solar water heating system are elementary. One or more panels, called collectors are mounted in a position where they will get the most direct sunlight, or solar gain. The collectors are designed to absorb and retain as much of the energy of incoming sunlight as possible. Dark coatings are applied to the surface for absorption, while measures such as glazing on the glass (radiation loss) and insulation within the panel (conduction loss) keep the heat in. The figure at right illustrates the most common collector type, the flat-plate collector.

Another fairly common collector type is the evacuated tube collector, also called a solar-diode (because of its ability to allow heat flow in one direction only). This technology is based on the latent heat of evaporation of certain salts and is a very efficient heat transfer mechanism, however the cost of such systems is a fair bit high, putting it beyond reach of most homeowners.

Ideally, collector orientation is due south, although solar thermal systems are very forgiving and 15° east of west will make little difference. Angle is typically a compromise to optimise solar gain thoughout the year, as the height of the sun in the sky changes from season to season. In Canada, the general rule of thumb is that collectors be mounted at an angle equal to the location's latitude (See our Canadian Latitude Map if you are unsure about your location's details).A steeper pitch can be used to augment winter gain.

Pumps are used to circulate a fluid through common copper tubing, into the collector panel. As the fluid flows through the panel, heat transfer takes place between the hot panel (collector temperature can easily reach 90°C) and the fluid, thereby increasing fluid temperature. We say "fluid" because there are several options depending on system use and type. Freeze protection is an important consideration given our climate. Potable water may be used in seasonal installations (a cottage or other recreational, summer only use) where winter freezing isn't a problem, however for year round use some type of anti-freeze solution is the most common solution. There are other methods such as drainback and recirculating systems that can be used in mild freeze applications, however they are not particularly suited to most Canadian climates.

Most of our systems use a 50/50 solution of food grade propylene glycol and water, a mixture which provides freeze protection down to -40°C. This setup requires a heat exchanger whereby heat from the glycol solution can be transferred to a domestic supply tank. The components of such a system, called an active, closed-loop system are shown below. The heat exchanger/storage tank shown in the diagram is connected in series with your existing water heater. The solar heated water acts as a preheat for the conventional heater's supply. When solar heated water temperature is adequate, the conventional water heater will not fire up.

©2004 Nexus Solar Corporation.