How do they work?

Domestic solar hot water systems can be divided into two groups - flat plate collectors and batch water heaters.

1. Flat plate collectors
Flat plate collectors are the most commonly used solar hot water systems in Australian households. They act as sophisticated “greenhouses” which absorb and use the sunlight to raise the temperature of water up to about 70 degrees Celsius. It is important to make sure the collectors are located in the best position to absorb sunlight and they are not excessively shaded by trees or nearby buildings. Flat plate collectors are generally placed on top of the roofs of households and they face north.

There are four kinds of flat plate collectors. The two traditional collector systems include the “Serpatine” and “Parallel” systems. They consist of a number of copper tubes that are placed in contact with a dark coloured metal plate. Insulation is placed between the metal plate and the external wall of the pipes to prevent heat losses.

Then there are two flooded plate collector systems. These systems have two metal absorbing plates that are sandwiched together. The copper tubes are arranged between the two metal plates.

Three types of storage tanks are used with flat plate collectors. The most common tank is the close-coupled system. These storage tank systems are installed on the home roofs, above the collectors. The storage tank and the solar collectors are positioned close together to reduce the length of connecting pipes. This system makes use of the principle of “thermosyphoning”, that is, water becomes less dense when it is heated. As the sun becomes hotter, water is drawn from the mains pressure system and it flows through the solar collector. The water becomes hotter and lighter and rises into the storage tank above the collector. The hot water then flows through a pipe into the home.

During this time, the cooler water that was left over in the storage tank is now forced down to the bottom of the collector. The cooler water is warmed up and the cycle repeats itself while the sun continues to shine during the day.

The other two types of storage tanks also use the principle of thermosyphoning. Gravity feed storage tanks are installed inside the roof cavity of a home. These tanks are the cheapest to purchase, but household plumbing must be suitable for the gravity feed system.

The rarest storage tank is the forced circulation system. This kind of tank is installed on the ground, with the collector on the top of the roof. A pump is activated when the sun shines and cold water is pushed through the collector and then the thermosyphoning process begins. Forced circulation systems require a pump that uses electricity, but they are not expensive to run.

Different coatings can be applied to the surface of the collector plates to ensure more effective heat absorption.


Above - Solar hot water testing facilities.

Batch water heaters
Batch water heaters are also known as “breadboxes” and they are very simple systems that have been used since the early 1900s. Batch systems consist of a black storage tank that is contained within an insulated box that has a transparent cover. Cold water is added to the hot water in the storage tank, whenever hot water is removed. To retain the absorbed heat within the water, you need to place an insulated covering over the glazing at night.

For more information, go to:
http://www.rise.org.au/info/Tech/lowtemp/hotwatersys.html

An auxiliary heater (or “booster”) is used to increase the water temperature on cooler days. This booster can be powered by electricity, gas or wood fuel. Electric boosting is the most expensive way to run your solar hot water system and it will result in more greenhouse gas emissions than using a gas-boosted system. The booster control needs to be in an accessible location with an indicator light and a timer switch. A thermostatic tempering valve is sometimes used with a Western Australian solar hot water system. The valve helps to control the temperature of hot water during the summer months.

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