Domestic & Commercial Solar Water Heating

One of the most cost effective ways to incorporate renewable technologies (such as solar energy) into a home or building is by incorporating solar hot water. A typical system will reduce the need for conventional  water heating by about two-thirds, minimizing the expense of electricity or fossil fuel to heat the water and reducing the associated environmental impacts.

Most solar water heating systems for homes and buildings have two main parts: a solar collector and a storage tank. The most common collector used in solar hot water systems is the flat-plate collector. Solar water heaters use the sun to heat either water or a heat-transfer fluid in the collector. Heated water is then stored in the storage tank for use as needed, with a conventional system providing additional heating as necessary. The tank can be a modified standard water heater, but it is usually larger and very well insulated. Systems that use fluids other than water usually heat the water by passing it through a coil of tubing in the tank. The tubing is full of hot heat-transfer fluid. Solar water heating systems can be either active or passive, but the most common are active systems.

Active Systems

Active, indirect systems are often used in climates withfreezing temperatures.  Active systems use electric pumps, valves, and controllers to circulate water or other heat-transfer fluids through the collectors.

There are three types of active systems:

• Direct systems use pumps to circulate water through the collectors. These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water.
• Indirect systems pump heat-transfer fluids such as a mixture of glycol and water antifreeze through collectors. Heat exchangers transfer the heat from the fluid to the potable water stored in the tanks.
• Drainback systems, a type of indirect system, use pumps to circulate water through the collectors. The water in the collector loop drains into a reservoir tank when the pumps stop, this makes drainback systems a good choice in colder climates.

Passive Systems

Passive systems rely on gravity and the tendency for water to naturally circulate as it is heated, allowing water or heat-transfer fluid to move through the system without pumps. Because they contain no electric components, passive systems are generally more reliable, easier to maintain, and possibly longer- lasting than active systems.

Batch heaters or integral collector storage systems consist of one or more storage tanks placed in an insulated box with a glazed side facing the sun. During the winter, they must be drained or protected from freezing.

Thermosiphon systems rely on the natural convection of warm water rising to circulate water through the collectors and to the tank, which is located above the collector. As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the tank's cooler water below flows down pipes to the bottom of the collector, causing circulation throughout the system.