In this video, you’ll learn how to identify the most practical setup for a solar hot water system based on the needs of the climate, building and the householders.
Solar Hot Water System
In passive systems- also known as thermosiphon systems- the tank sits above the collectors. Cold water sinks into the collectors whilst warmer water rises unassisted from the collectors into the tank. The tank might sit on the roof with the collector, a close coupled system or within the roof cavity above the collector.
A gravity feed system. With these systems, the roof support structures must be strong enough to handle weight of up to 800 kilograms.
Active systems use a small pump to move water between the collectors and the tank. The location of the tank can be within the roof or on the ground.
Each type of system has advantages. A passive system is space-saving and is easy to maintain with no moving parts. An active system is unobtrusive, lightweight, can use an existing tank and the tank can be located closer to where hot water is used reducing heating loss in pipes. But the pump uses some energy.
There are two collector types to choose from; flat plate and evacuated tubes. Evacuated tubes have the advantage of receiving perpendicular radiation for a greater part of the day.
This, combined with their vacuum insulation, means good winter performance. Flat plate collectors, particularly if purchased as part of a close coupled system, can be more affordable than evacuated tube collectors. However, the prices of evacuated tubes has fallen in recent years. Collectors should ideally face true north in a shade-free position.
Facing the collectors 45 degrees from the north will result in only minor reductions in performance. The ideal inclination is the angle of latitude +5-10 degrees. Sydney’s latitude is 33.5 degrees. So an ideal angle of inclination would be around 40. However, +/- 15 degrees would still provide adequate performance.
If your collectors face true east or west, the lower the angle of inclination the better. However, to allow thermosiphon flow, you must always keep the angle above 10 degrees. For the correct sizing of your collectors and tank, consult manufacturer’s guidelines to see what is best for the climate and household.
In places with minor frost, evacuated tubes may offer in-built protection. In climates with regular frosts, indirect or closed-loop systems are the most popular and reliable form of frost protection. Instead water an antifreeze fluid is heated in the collectors. The antifreeze passes heat onto the water via a heat exchanger.
The antifreeze does not come into contact with the household water supply. Other frost protection methods are available such as frost protection valves, temperature programmable pumps and drain back systems.
In an Australian summer, water and collectors can easily approach boiling point. This can cause excessive pressure buildup and can damage the lining of vitreous enamel tanks. The Australian standard states that solar hot water systems must provide over temperature protection without draining water.
So, a temperature pressure relieve valve is not enough. The most common way to prevent overheating in the tank is by automatically stopping the circulation between collector and tank when the tank water gets too hot.
This can be done using a solar control valve or a temperature-controlled pump. See the fact sheet on the trade secrets website for more information about how to control for frost and overheating.
The solar hot water system will require boosting at some times of the year. Electric boosters use a lot of power, so make sure the booster is controlled by a manual in-house switch. This switch should include a light to indicate when on.
Boosters which have no in-house switch or are permanently on are wasteful as they often heat water to 60 degrees, water which would otherwise be heated by the sun. When advising the householder on their energy tariffs options, explain the pros and cons of each option.
A regular tariff allows boosting at any time but it won’t lead to a reduction in their bills if their previous system was electric off-peak. Off-peak will be cheaper but may not fit the hot water usage patterns of all households. If available, extended off-peak or off-peak II is often a good compromise.
Natural gas boosters have the advantage of lower emissions and running costs compared to electric boosters. And also, on subject to time of use restrictions. Install a timer on gas storage boosters to ensure the gas does not heat water which would otherwise be heated by the sun.
So, select the system for the climate and use appropriate temperature controls. Locate the collector and tank to maximize performance. Install a manual boost switch and connect to a tariff which minimize emissions and costs.