Solar water heating

A solar hot water system absorbs the energy from the sun in the collector panels located on the roof of your home.

The sun's energy is then transferred either directly or indirectly (depending on the system) to the water stored in the hot water cylinder. When there is not sufficient energy from the sun to heat the water in the cylinder a booster system (either electric, gas or wetback) is used to heat the water to the required temperature.

A solar water heating system consists of three main parts:

• the solar hot water collector - which are located on the roof; and
• the hot water tank which can be also located on the roof or on the ground (a split system)
• pumps and controller.

There are two main types of solar water heater collectors, either flat panel or evacuated tube.

Flat panel systems collect the suns rays via a metal plate with a dark coating. They work best with direct sunshine. Evacuated tube systems collect the suns rays in glass tubes with a vacuum inside. They are generally more efficient than flat panel systems because they trap heat even in low sunlight.

Geographic location, sunshine hours, home orientation and roof angle must be taken into account when choosing a suitable solar water heating system.

How solar heating works

Solar hot water systems are either closed loop or open-loop systems.

Open-loop system

An open-loop solar water heating system circulates the household’s water to the collector on the roof, where it is heated before it goes back to the cylinder inside the house. Because it’s the water itself being heated, the system is more efficient.

Closed-loop system

A closed-loop solar water heating system circulates a heat-conducting fluid (normally glycol) in a closed circuit from the collector to the hot water cylinder. The heated fluid then heats the water supply through a simple heat exchange. (The fluid never comes into direct contact with the water.) A closed loop system is generally more expensive than an open loop system.

There are other factors to consider, such as whether the system works by thermosiphon (the most environmentally efficient way) or electric pump; and whether the storage tank is located inside, outside or on the roof.

Controllers

Once you have decided on the type of system you need to ensure that you have the most appropriate controller. A controller manages the use of the boost system (electric or gas). A wide variety of controllers are available. However ensuring you have the best controller for your system and lifestyle is essential to ensure that you obtain the greatest efficiency from your system.

In simple terms, controllers use either a minimum temperature or a timer trigger. A minimum temperature controller simply triggers the boost whenever the cylinder system drops below a certain set temperature. A timer can be used to start the boost at specific times of the day such as before periods of high water use. If the boost system is either gas (or wetback) a controller is not required as the boost system automatically heats the solar hot water (if required).

Why they’re good

 

• Cut household water heating bills by up to 50–75%
• Reduced environmental impact
• System pays for itself in energy savings
• Makes an energy-efficiency statement

Are they safe?

Yes, solar water heaters are safe.

Things to consider

To get the maximum benefit from a solar hot water system, the homeowner is required to adapt their lifestyle and water usage to when the free hot water is generated.

It is possible to retrofit a solar hot water collector to an existing hot water cylinder. Undertaking a retrofit of a solar hot water collector to an existing hot water cylinder is a lower capital cost option but it does not deliver the same efficiencies. If your existing hot water cylinder is more than 10 years old then it will probably need to be replaced.

If you live in a frost-prone area you need to ensure that your solar water heating system has some form of frost protection. This will protect the water freezing in the collector panels and either damaging the panels, piping or causing water damage when it thaws. A closed loop solar hot water system which uses a glycol or glycol and water mixture provides appropriate protection. If you choose an open loop system then it requires another form of protection such as a temperature sensor system which runs hot water through the system to prevent frost damage or a frost valve which lets water run through the collector panels to avoid water damage. Another option on some evacuated tube systems is frost tubes which enables water to freeze in the collector panel without causing damage.

To maximise the benefit of a solar hot water system the solar collectors need to:

• face due north, or close to due north, up to 20° either side of north will still deliver an energy efficient outcome;
• be placed together which will require an area of between 2m² and 6m² (depending on the number of collectors);
• be located with good all day and year round exposure to the sun;
• at a pitch (angle) of between 30° and 45°. The optimal angle is an angle equal to the latitude of the location where the panels will be installed. New Zealand is between a latitude of 36° (Auckland) and 46° (Invercargill). In New Zealand most roofs are at an angle of between 20° and 30° so its likely that for optimal performance the collector panels will need to be elevated from the roof;
• Be located close to the hot water cylinder. The shorter the pipe from the collectors to the hot water cylinder, then the greater the efficiencies.

These siting considerations are a guide only and expert advice should be obtained to ensure your solar system is optimised.

Homeowners should also compare their options to consider whether another kind of energy-efficient water heating system, such as a hot water heat pump, is better for their needs.

How much will it cost?

Installed systems start at about $5,000 and range in price according to quality or size to about $10,000. At current energy prices, they can pay for themselves within 10 to 15 years.

The retrofit of a solar collector to an existing hot water cylinder will cost up to $3,000 depending on the location of the collector and cylinder location and type. You will need a building consent to install a solar hot water system. Contact your local council for the cost of this consent, which can be up to $500.

EECA subsidies of up to $1,000 are available for new solar hot water systems and your Right House Solution Consultant can assist you in claiming this rebate. For more information on the solar hot water system subsidy go to solar.energywise.govt.nz.

For a quote on a solar hot water system please contact a Right House consultant.

We are authorised suppliers and installers of:

 

• Rinnai (open loop, flat plate systems)
• Wagner (closed loop, flat plate system) 
• Skypower (evacuated tube system)

To find out more about the differences between an open loop and closed loop system and a flat plate and evacuated tube system download Fact Sheet: Comparison of Flat Plate and Evacuated Tube Solar Collectors or Fact Sheet: Open Loop and Closed Loop Solar Hot Water Systems.