How solar water heating systems work
Solar water heating uses the thermal energy of the sun to heat water that's stored in a tank. On less sunny days, the system may need back up generation, usually provided by electricity or gas.
Getting optimum performance out of a solar water heating system means maximising the energy provided by the sun. This page describes how the different components work together to store the right amount of sun-heated water for your needs, while minimising the need for back up generation.
On this page:
Components of a solar water heating system
How the components work together
Components of a solar water heating system
The main components of a typical solar water heating system include:
- The solar collector
- The storage tank (cylinder)
- The circulation system
- Controller or timer.
In a packaged system all the components are chosen for you to ensure your solar water system is as efficient as possible.
It's important to look at how a whole packaged system performs rather than just the individual components. EECA publishes the performance modelling data for all solar water heating systems qualifying for a $1000 grant. This is a useful way to compare the performance of systems eligible for a grant.
How the components work together
The collector absorbs the sun's energy and this heat is transferred to the water in the storage tank (or cylinder) by the circulation system. How efficiently this is carried out depends on the perfomance of each component and how well they are matched together as a system.
The size of the hot water tank in your system needs to be matched to your hot water usage and the size of solar collectors on your roof. If your tank is too small or too big you will be paying extra to boost the temperature of the water.
A properly insulated system (pipes and tank) is also very important. This helps to reduce the amount of heat lost while the hot water is being circulated around your house.
Timers, thermostats and controllers are other important components that contribute to the efficiency of your system. These, for example, will ensure that the backup heating is not heating water that would otherwise be heated by the solar heating system. While doing this, they'll also ensure there's enough hot water avaliable when it's needed.
Solar collectors
There are two main types of solar collectors for solar water heating systems: flat plate panels and evacuated tubes.
A flat plate panel looks similar to a skylight. It absorbs sunlight and transfers the heat into the water (or heat transfer fluid) flowing through the collector panel. A typical house would have around 1m2 of collector per person in the house.
An evacuated tube system is made of a series of glass tubes between 1.5m and 2m long that slope lengthwise up and down the roof.
Usually solar collectors are positioned on your roof. However, an in-roof installation option is also available. In-roof solar collectors are put into the roof in a similar way to a skylight. It is also possible to mount solar collectors at ground level, which can reduce the installation costs.
Storage tanks
In solar water heating systems, the storage tank can be part of the system on the roof or set up in another area of the house.
You can use a conventional hot water tank or a specialist solar water heating tank. Specialist tanks have extra water and thermostat connections. This type of tank is specially designed to maximise the use of solar energy. Your system is likely to perform better with one of these.
Historically, domestic hot water tanks were around 180 litre capacity. This is generally too small for a solar water heater to achieve optimal performance for a household of three or more people.
Open or closed loop
Open loop systems circulate water through the collector panels on your roof, heat it and return it to your hot water tank.
With closed loop systems, a heat transfer fluid (usually a mixture of water and glycol) circulates through the collector panels. This fluid passes through a heat exchanger in your hot water tank, heating up the water. The heat transfer fluid and the water in your tank don't mix.
A closed loop system could be a good option if you have hard water. Check your kettle - if it has a build up of minierals on the element, you have hard water. In an open loop system these mineral deposits can build up in the collector over time and reduce the performance of the system.
Talk to your supplier or installer about which system is best for you.
Circulation system
Water or fluid can be circulated around a solar water heating system using a pump. Or it can be circulated naturally, in what's called a 'thermosiphon' or 'passive' system.
In these systems, the storage tank is located above the collector panels, usually on the roof. Cold water or fluid moves down from the tank into the bottom of the collector panels. Once it's heated by the sun it naturally rises to the top of the collector panel and back up into the tank.
Systems that use pumps to circulate the water or fluid are often called 'active systems'.
Thermosiphon circulation doesn't require a pump and isn't dependent on electricity. A pump system does require electricity to work.
With a pump system, the hot water tank can be positioned below the collector panels. This can be helpful if you want to use an existing hot water tank, or if you don't want to see the tank on the roof.
Controllers must be used to turn the pump on when there is enough solar energy avaliable and off when there is not.
Controllers
A controller manages the use of supplementary or 'booster' gas or electricity in solar water heating systems. If there's a pump, it controls this also. The control of the supplementary heating can greatly affect overall system performance, so it's important to have the controller set up right.
There are two types of heating controllers: time trigger controllers and minimum temperature controllers.
- Time trigger controllers are set to turn off the booster heating during the day. This is so you're not paying for electricity or gas to heat your water when the sun could be doing the job for free. The timer can also be set to turn on the booster heating before periods of high water use. This can greatly increase your system's efficiency
- Minimum temperature controllers trigger the booster heating whenever the tank temperature drops below a minimum pre-defined temperature
- Some sophisticated controllers have both these functions.
It's also worth reviewing your electricity tariff when you install a solar water heating system. This is to make sure you're paying the best rate for supplementary heating.
A controller will ensure that you'll never be without hot water, regardless of the time of day. Talk to your installer or supplier about the type of controller you need.
Frost protection
In frost-prone areas you'll need to ensure your system has frost protection. This is important as the water or fluid could freeze in the collector panels otherwise.
In open loop pump systems, temperature sensors can turn the pump on to run water through the system before the collector panels freeze. This won't work during a power outage, however.
Some open loop thermosiphon systems have frost valves. These let the water flow through the collector panels when the temperature is close to freezing.
Frost tubes could also be considered on some systems. These enable the water in the collector panels to freeze without causing damage. The tubes absorb any pressure build-up created by the water expanding or freezing.
The water/glycol mixture used in most closed loop systems has anti-freeze properties.
Talk to your supplier or installer about what method is appropriate for the system you choose.
