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Solar Water Heating Systems

General Information

Solar Water Heating (SWH) systems are a mature technology and have been accepted in most countries for many years.

In Australia SWH systems are commonplace in the warmer states. In Western Australia, Queensland, South Australia and the Northern Territory SWH systems are installed as a matter of course. However, to encourage greater penetration of SWH systems rebates are available in most states. For information on solar water heater rebates and the eligibility of different systems see the web site of the Office of the Renewable Energy Regulator. Rebates are paid in each state by that state’s energy department.

The Green Plumber web site gives contact details for rebate information.

The are many types of SWH system. These include:

Close Coupled

The storage tank is horizontally mounted immediately above the collectors on the roof. No pumping is required as the hot water naturally rises into the tank (thermosiphon flow)

Pump Circulated

The storage tank is ground or floor mounted and is below the level of the collectors. This needs a circulating pump to circulate water or heat transfer fluid between the tank and the collectors.

Gravity Feed

These are low pressure units with the storage tank generally inside the roof space and above the level of the collectors. No pumping is required. If a low pressure unit is installed it is important to take the cold water supply to the shower from the header tank of the hot water system. This means that the hot and cold water pressure in the shower will be the same and there should be no interference when someone turns a tap on elsewhere in the house.

Collectors

Flat Plate Collectors
Copper risers bonded behind a flat metallic plate or a sandwich panel made up of two sheets of metal. These are installed inside a metal box with glass on the top side and insulation behind. The surface of the collector is either painted black or powder coated to maximise heat absorption. Once absorbed it is important to keep the heat in the collector and the insulation minimises heat loss from the rear of the collector.

Such collectors must be protected from freezing in frost prone areas otherwise the freezing water will expand and burst the collector tubes. This is done by the use of either:

Dump Valve – located at the bottom of the collectors and opens when temperatures close to freezing are detected. Water drains from the collectors. The dump valve closes and the collectors fill up again when the temperature rises above freezing.
 Or
Heat Transfer Fluid – Anti freeze solution is added to the water. In this case the “treated” water cannot be consumed, so a heat transfer system is used inside the storage tank to transfer heat from the “treated” water into potable water for use in the house.

Evacuated Tube Collectors

A heat pipe is located at the centre of an evacuated glass tube. These provide very good performance even in frosty climates. However, care must be taken to prevent freezing in the pipes leading to and from the storage tank.

Storage Tanks

These are fabricated from either marine grade stainless steel or use a fibreglass liner over a steel substrate.

The marine grade stainless steel tank will have a very long lifetime.

The fibreglass liner may deteriorate quickly and necessitate a new tank if proper precautions are not taken. Inside the tank is a sacrificial anode which is designed to corrode in place of the tank itself. Once this anode has been depleted electrolysis will attack the tank itself. Hence it is important to have a planned regime for replacement of the sacrificial anode “before” it is consumed. Such fibreglass tanks are generally cheaper than the marine grade stainless steel ones

Boosting

SWH systems are designed to deliver the optimum amount of hot water for most of the year. However, in the depths of winter there may not be sufficient solar heat gain to deliver sufficient hot water. In this case a booster is needed to heat the water.
The booster is powered by the consumption of energy generally derived from fossil fuels in the form of natural or LP gas, electricity or biomass (wood). The auxiliary heat can be delivered direct to the storage tank or may be used as an in-line heater. E.g. an instantaneous gas heater connected between the storage tank and the outlet taps will turn on of the water needs heating or let the water pass straight through if it is already hot enough.

Performance

The economic return on the use of a SWH system is dependent on two main issues:

1. the quantity of hot water that the household uses. If you have a large household and lots of hot water is sued then converting to solar will pay for itself much quicker, and
2. the pattern of use. If hot water is used predominantly in the morning then it will re-heat using heat from the sun and savings will be maximised. If hot water is used mainly in the evening the water in the tank will re-heat over night using the booster and saving will be minimal.

You should expect that a correctly sized and optimally operated SWH system will save enough in running costs to pay for itself within 3-5 years.

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