Most solar hot water systems use solar collectors or panels to absorb energy from the sun. Water is heated by the sun as it passes through the collectors. It then flows into an insulated storage tank for later use.

     In passive systems, water flows due to a thermosiphon effect between the collectors and the tank. In active systems, water is pumped between the collectors and the tank.

     The storage tank is usually fitted with an electric, gas or solid fuel booster that heats the water when sunlight is insufficient. Some solar water heaters also have frost protection to prevent damage in frost prone areas.

     Solar hot water supply located in climate zones 4,5,6,7 and 8 is required to comply with Section 8 of AS/NZS 3500.4 2003 Heated Water Services (including amendments:1, 2 and 3.) For further information please refer to the Building Code of Australia (BCA) Volume Two, Part 3.12.5.
Solar collectors

     Solar collectors trap and use heat from the sun to raise the temperature of water. There are two main types of solar collector: flat-plate and evacuated tube collectors.

     Flat-plate solar collectors – These are the most common type. They are comprised of:

     An airtight box with a transparent cover.
A dark coloured, metallic absorbing plate containing water pipes.
Insulation to reduce heat loss from the back and sides of the absorber plate.

     One slight disadvantage of flat-plate collectors is that they only operate at maximum efficiency when the sun’s rays strike perpendicular to the flat plate. They also suffer some heat loss in cold weather.

     Evacuated tube solar collectors – This kind of collector consists of:

     A series of transparent outer glass tubes that allow light rays to pass through with minimal reflection.
Each tube contains an inner water pipe coated with a layer that absorbs the sun’s rays, generating heat. Water runs through this inner tube and is heated.
A vacuum (hence ‘evacuated’) exists between the outer tube and the water pipe, which acts as insulation, reducing heat loss.

     Evacuated tube systems are more efficient than flat-plate systems, particularly in the cooler months and on cloudy days. This is due partly to the vacuum insulation (which minimises heat loss) and partly to the fact that the curved surface of the tubes allows the sun’s rays to strike perpendicular to the water for a greater part of the day. Evacuated tube systems weigh much less than flat-plate systems but cost significantly more. Individual tubes can be replaced in the event of damage, making long term maintenance potentially less costly. In warmer climates, such as Darwin, the additional cost of evacuated tubes is usually not warranted as a flat plate solar collector will provide most of the energy needed for water heating

     Properly maintained solar thermal collectors should outlast the life of the storage tank. When the tank needs replacing, the existing collectors can be connected to the new tank.
Frost protection

     Frost protection for solar collectors is essential in frost prone areas. During a frost, water can freeze in the solar collector and damage it unless preventative measures are taken. Common types of frost protection include:

     Knock valves (mechanical drain down valves). These valves can be problematic as they often jam open and drain the tank, or fail to operate, causing severe damage.
Electric heating elements, which are vulnerable in the event of power failure.
Closed circuit systems, which separate the heating fluid from the water (see illustration below). Closed circuit systems are usually the best option in frost prone areas as they ensure that water does not flow through the solar collectors and therefore cannot freeze in the collectors.