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21 November 2018

Why and How to Choose Solar Hot Water Systems?

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Solar hot water (solar thermal) systems have been available in New Zealand for decades. In Detailed last month, Keith Huntington asked: Photovoltaic and Solar Water Panels: What to Choose? He raised a really interesting question of why the mature and highly efficient technology of solar thermal is not used more in New Zealand. One possible answer is a lack of understanding and therefore confidence in the technology.

What can solar thermal systems do? 

Solar thermal systems are exceptionally good at heating water. They harvest a high proportion of solar energy hitting their surface and store this heat energy in a cylinder for later use. Because heat energy is transferred directly, they operate at very high efficiency, maximising the solar energy harvest for every square meter of collector. For example, an Apricus ETC30 collector has 2kW peak output for 4.4m² collector, operating at 60% – 70% efficiency at domestic water temperatures.

Heating water is almost always the single largest user of energy in individual homes. In new homes, with improved insulation, energy-efficient appliances, heating and lighting, water heating uses an average of 46% of the home’s energy (electric lines company figures, 2014-16). Other buildings (accommodation, apartments, sports centres, hospitals) will also have relatively high energy use for water heating.

In these buildings and individual homes, solar thermal should be the first option to consider when specifying hot water systems and renewable energy technology.

A high quality, well designed and installed solar thermal system will reduce energy for water heating by around 75% in individual homes and by 40-70% in larger buildings.

How to choose between solar hot water systems?

The ultimate performance of a solar thermal system relies on four aspects:

  • Appropriate, full system design
  • Quality and longevity of collectors and other system components
  • Installation quality
  • Eliminated or minimised ongoing maintenance cost


Individual domestic systems have the most straightforward design and need to include careful consideration of booster heater control and ease of use by the homeowner.

Project-specific design and modelling are crucial for larger systems, undertaken by a suitably experienced solar professional. To increase your understanding of solar thermal design and specification, view the Engineering NZ webinar I ran recently. Internationally recognised modelling software for solar thermal are Polysun, TRNSys and T*Sol, not RETScreen which does not have adequate performance data or modelling capability.

Professional Report

System Quality and Longevity:

NZ does not have published, comparable performance data for solar thermal systems. Therefore, collector performance is best assessed using international certification — SRCC from USA, Solar Keymark from Europe. These provide comparable performance data for collectors plus rigorous manufacturing quality standards.

The other system components should be solar rated, high quality and from reputable suppliers and manufacturers.

Also check the length and comprehensiveness of system warranties, plus the history and reputation of the organisation and therefore likelihood they will be around to fulfil warranties.

Installation Quality:

A well designed, simple system with high-quality components, plus good technical support should be able to be installed by all good quality plumbers. As the complexity of the system increases so too does the potential for installation errors, especially if a “closed loop” solar circuit is involved. In NZ these issues can largely be avoided by choosing an appropriate use of technology: open loop for evacuated tubes and drain back for flat plates.

In larger systems, the solar supplier should attend site for commissioning and as-built sign-off.

Eliminated Maintenance:

Regular or costly maintenance should be avoided through consideration of the three aspects above. To cover all eventualities, ask the system supplier to provide a guaranteed and costed maintenance schedule.

Establishing a relationship with a supplier who can fulfil all these criteria should help ensure that all your clients can start to benefit from solar heated water. By specifying well-designed, long lasting and high performance solar thermal systems we can significantly reduce the energy use, environmental impact and running costs of heating water in our buildings.

If you are considering sustainable water heating for your next project, contact Marcus to discuss your project requirements, and discover the most effective hot water heating solution.

View more information on Apricus Eco Hotwater, including contact details.
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