Hybrid Solar Hot Water System

Mallacoota, Victoria

  • System Design: October 2009
  • System Installation: May to July 2010
  • System Commissioned: July 2010

Client: Methodist Ladies College

Renewable Energy Target (RET)

Project Goals

At the time of installation, the Margaret Williams Centre (MWC) at MLC's Marshmead Campus required an average of 450L per day of seventy five degree (C) hot water (when the campus is fully occupied). Peaks in consumption can see a large amount of hot water (of the order 1000L) being used over a short period.

Total energy load to heat 450L of water per day from 10 to 75 degrees (Celsius) for 40 weeks per year is 9530 kWh (kilowatt hours) or 34 GJ (gigajoules). The projected aim of this system is to provide 90% or more of the above load using solar energy.

The system designed to meet this load, with minimal gas boosting required, uses a combination of flat plate solar collectors and evacuated tube solar collectors on the MWC roof. Each collector type exhibits greater efficiency over the other, depending on the temperature of the fluid (the potable water in this case) being heated in the collectors at the time. To explain this further, the flat plate collectors are more efficient than the evacuated tube collectors when heating fluid from cold (approx 10 degrees C) to warm (approx 40 degrees C) in a temperate climate, whereas the evacuated tube collectors are more efficient when heating fluid from warm to hot. This design puts each collector type to work on the respective fluid temperatures where the collectors' solar efficiency is superior - to give a superior result to any system using the same area of either collector type. The solar heated water is transferred to a bank of three stainless steel cylinder tanks inside the building. These three tanks have a total capacity of 990L in total.


  • A hybrid system using both flat-plate and evacuated tube collectors. To our knowledge there is no other system like this in Australia
  • Three Australian-made stainless steel tanks with 20% more insulation on solar storage tank than required by Australian Standard for solar hot water tanks.
  • The special care taken in both design and installation to ensure an excellent job.

The system consists of:

  • 3 x 315L Rinnai stainless steel cylinder tanks;
  • 4 x Rinnai Excelsior FTC flat collectors;
  • 2 x 20 tube evacuated tube collectors;
  • 2 x Rinnai manifolded LP gas boosters. 


Indicative results from the first few years of system operation suggest that the system is close to achieving the high solar energy contribution intended by the design.