How Does A Solar Water Heater Owner Work?

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

In passive systems, water flows between the collector and tank via the thermosiphon effect. In active systems, water is pumped between the collector and tank.

The storage tank is usually equipped with an electric, gas, or solid fuel booster to heat the water when sunlight is scarce. Some solar hot water systems also have frost protection to prevent damage in frost-prone regions.

Solar hot water systems located in climate zones 4, 5, 6, 7, and 8 must comply with AS/NZS 3500.4 2003 Hot Water Services, Section 8 (including Addenda: 1, 2, 3). For more information, see the Building Code of Australia (BCA), Vol. 2, Section 3.12.5.

Solar Collectors

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

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

An airtight box with a transparent cover.

A dark metal absorber plate containing a water pipe.

Insulation to reduce heat loss from the back and sides of the absorber plate.

The only slight drawback of flat plate collectors is that they only operate at maximum efficiency when the sun's rays strike the plate perpendicularly. They also suffer some heat loss due to cold.

Evacuated tube solar collectors - This type 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 generates heat and absorbs solar rays. Water is heated through this inner tube.

A vacuum exists between the outer tube and the water tube, which reduces heat loss (hence the 'evacuation') and acts as an insulator.

Evacuated tube systems are more efficient than flat-plate systems, especially in cooler seasons and on cloudy days. This is partly due to vacuum insulation (which minimizes heat loss) and partly due to the curved surface of the tubes, which allows the sun's rays to strike the water perpendicularly for most of the day. Evacuated tube systems weigh significantly less than flat-plate systems. Individual tubes can be replaced if damaged, potentially making long-term maintenance less expensive. In warmer climates, such as Darwin, the additional cost of evacuated tubes typically provides most of the energy needed to heat the water, which is not guaranteed with flat-plate solar collectors.

Properly maintained solar collectors should extend the life of the storage tank. If the tank needs to be replaced, the existing collector can be connected to a new tank.

Frost Protection

Frost protection for solar collectors is essential in frost-prone regions. During 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 drains under the valves). These valves can be problematic, as they frequently jam open, drain the tank, or stop working, causing severe damage.

Electric heating elements are susceptible to power failures.

Closed-circuit systems separate the heating fluid from the water (see diagram below). Closed-circuit systems are usually the best option in frost-prone areas, as they ensure that water does not flow through the solar collector and therefore cannot freeze in the collector.