Solar Water Heating

Solar Water Heating

Solar water heating can be effective in any part of Ireland.

Properly designed and installed, solar water heating can significantly reduce household energy bills. It is especially cost-effective if you live in a sunnier part of the country or use large amounts of water.

Solar panels on your roof (or some other sunny place) absorb heat from the sun and use it to heat water.

The water is stored in a cylinder, which might be placed on your roof, inside your roof space, or in the hot water cupboard.

On days when there isn't enough sun to fully heat your water, some heat will be supplied using electricity, gas or heat from your wetback (this backup energy source is called a 'booster system').

To make maximum use of the sun, the panels need to face north or fairly close to north, and be on the right angle.

In summer, it may be possible to heat all the water you need with solar energy. In winter, or on cold cloudy days, solar water heating will meet part of your hot water needs - you'll also need some supplementary heating from your booster system.

The exact amount you save from solar water heating will depend on a wide range of factors including how much hot water you use, the solar water heating system you install, and the quality of the installation. But it's been estimated that, for an average household, an effective system will:

  • provide between 50%-75% of annual hot water needs
  • reduce annual electricity use by about 2200 kWh
  • provide savings of €500-€600 a year (depending on the cost of your electricity or gas supply).

Value of your home

Developers are starting to build solar water heating systems into new housing projects. But at present there is no clear evidence about the extent to which solar water heating adds value to your home. However, as energy costs rise it's likely that solar water heating will help homes sell more quickly and for higher prices.

If you install solar water heating, you'll be helping to reduce Ireland's dependence on non-renewable energy sources. And you'll also be helping to cut greenhouse gas emissions as there will be less need for electricity to be manufactured by burning fossil fuels.

Every residential solar water heating system installed is estimated to save, on average, about 1.4 tonnes of carbon dioxide emissions a year (assuming a mix of coal and gas fired electricity generation is avoided).

If you're planning to have solar water heating installed, you can apply for government funding to offset some of the finance costs. To be eligible, you must choose a package system from a registered retailer, and have it installed by a registered installer.

Some of the materials in solar heating systems are recyclable and others can be disposed of harmlessly. However, some of the heat exchange fluids may be toxic. When today's systems start to be replaced in a few years time, there should be procedures for handling these fluids. Contact your local council or landfill for information.

The size of your solar water heating system will depend on your demand for hot water. The Energy Efficiency and Conservation Authority (EECA) recommends that a solar water heating system should have up to 100 litres of storage per person, compared to around 50 litres per person on a standard electric hot water system. Typically, solar panels should be around 1m2 of collectors per person in the house.

These numbers are a rough guide only. The actual cylinder size and collector area you need will depend on your individual household situation, the system you choose and the way it is configured. Discuss this with your supplier or installer to ensure you get a system that will meet your requirements.

It is not a good idea to skimp. Choose a system that will meet your home's needs in the foreseeable future. If your home has four bedrooms, it's a good idea to have a system big enough for 4-5 people even if only two people live there now.

You will need booster heating to keep the water hot when the sun isn't shining. The most common types of booster are electric, gas and wetback. It is also possible to use a diesel booster system.

With gas, the booster can either be in a hot water cylinder or in a separate instant hot water unit downstream from the cylinder.

Controls

A controller manages the use of booster gas or electric heating, and controls the pump in a pump system. It has a significant effect on the overall performance of your solar water heating system, so it's important that it is set up correctly.

There are two types of supplementary heating controllers: time trigger controllers and minimum temperature controllers.

  • A time trigger controller can be used to keep the booster heating turned off during the day, to ensure 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 supplementary heating before periods of high water use. This can greatly increase the system's efficiency.
  • Minimum temperature controllers simply trigger the supplementary heating whenever the cylinder temperature drops below a minimum pre-defined temperature.

The controller often comes as part of standard installation.

Note: to keep dangerous legionella bacteria in check, water must reach 60°C for at least one hour each day.

There are two main types of collector panels for solar water heating systems: flat plate panels and evacuated tube panels.

  • A flat plate panel looks similar to a skylight. It absorbs sunlight and transfers the heat into the water or fluid flowing through the collector panel. The common size for a house is one to three panels, between 2.5 square metres and 6 square metres in total.
  • An evacuated tube panel is made up of a series of glass tubes (between 1.5m and 2m long) sloping lengthwise up and down the roof.

In Ireland's climate, both types of panel are equally efficient.

Solar panels should be placed where they will get maximum exposure to the sun. This usually means they'll be on a roof. They should be sited so they aren't shaded by hills, trees or buildings.

Panels should ideally face north, although directions between north-east and north-west are usually satisfactory.

Panels should be tilted towards the sun. The optimum angle is equivalent to your latitude: in Auckland, for example, panels should be about 37% from horizontal and in Dunedin about 46%. If you tilt the panels up more than these figures, you will gain more heat during winter and less during summer. Tilting the panels down will give more summer heating and less in winter. If your roof doesn't slope correctly, your installer can supply a frame to give the right tilt. The frame will have to be well secured - you don't want it to take off in a strong wind.

Your roof and framing will have to be strong enough to support the weight of the cylinder if it is on the roof. Some strengthening may be needed.

In some systems, water is heated as it flows through pipes in the solar panels. These are known as 'open loop' systems.

With other systems, a fluid (usually a mixture of water and glycol) passes through the solar panels and absorbs the heat, which is then transferred to the water through a heat exchanger within the cylinder. These are known as 'closed loop' systems.

Discuss with your installer or supplier which option is best for you.

ClosedLoopSystem

Open loop system with pump circulation.

Closed Loop System

Closed loop system with pump circulation

The water or fluid can be circulated around the solar water heating system using a pump or it can be circulated naturally using a thermosiphon system.

  • Systems that use natural circulation by thermosiphon are often called 'passive' systems. In this type of system, the hot water cylinder has to be located above the collector panels which are usually on the roof. Cold water or fluid moves down from the cylinder into the collector panels. Then, once it is heated by the sun, it rises back up into the cylinder.
  • Systems that use pumps to circulate the water or fluid are often called 'active' systems.

Natural circulation by thermosiphon has the advantage that it does not require a pump and therefore is not dependent on electricity, whereas a pump system is.

With a pump system, the hot water cylinder can be located at a level below the collector panels. This can be helpful if you're installing a solar water heating system in an existing home and you want to use the existing cylinder, or if you would rather not have the cylinder on the roof. It is essential that the pump is used in conjunction with a controller to ensure the pump operates only when necessary.

The hot water cylinder can be part of the system on the roof or it can be mounted separately in another part of the house. In colder areas, it's a better idea to have the hot water cylinder inside the house - otherwise the heat losses from the cylinder can be very high.

You can use a conventional hot water cylinder or a specialist solar water heating cylinder. The key difference is that specialist cylinders are larger and they are specially designed to maximise the use of solar energy. If you choose a specialist cylinder, the system is likely to perform a lot better.

If you choose to use a conventional cylinder, you'll need to have a controller for the supplementary heating.

Conventional hot water cylinders in houses are usually 180 litres or less. This storage capacity is generally too small for a solar water heater to achieve good performance for a household of three or more people. 100 litres capacity per person is recommended. Performance can be increased with the use of a controller for the supplementary heating.

If you are planning to get a system that includes a cylinder, check the quotes carefully - a cheap price may mean the cylinder and other components aren't included.

StandardHotWaterCylinder

Standard hot water cylinder

SolarHotWaterCylinder

Check with your solar supplier that the cylinder you are already using is fit for purpose if you are adding solar panels. If you are installing a new system, check the expected lifespan of the cylinder first.

Location of the cylinder

For passive systems, the cylinder has to be located just above the collector panels. In this case, the roof will have to be strong enough to support the weight of a full cylinder. Some strengthening may be needed. In colder parts of Ireland, the heat losses from the hot water cylinders can be high, reducing their efficiency. It’s a trade-off between not needing a pump (and the system working even in a power cut) and winter-time heat losses.

For pumped systems, the cylinder may be in the roof space or in the hot water cupboard.

Insulation

To get the best performance from your system, all components will have to be insulated including the pipes. This is particularly important where there is a long distance between the cylinder and the hot water taps. It is critical in cold climates and for systems where the cylinder is located on the roof.

High temperatures

All pipes and cylinders will need to be able to withstand temperatures above 100°C.

Solar Water Heating

Finding the right installer is important. Make sure to get a written quotation form your intended Solar Heating Installer. Tell us about your experience with Solar Water Heating. Use the Contact Us form. We'd love to hear from you.
Solar Water Heating

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