Evacuated tube and flat plate collectors are the two main types of solar collectors available to homeowners for solar water heating applications. Both perform well in Canada when installed correctly. That said, there are significant differences between the technologies and our experience has led us install flat plate collectors almost exclusively. Below you will find details behind this decision that may be useful in making yours.

Durability

Evacuated tubes are constructed with annealed glass, flat plate with tempered glass. Annealed glass being weaker than tempered glass makes evacuated tube collectors susceptible to damage. The potential for hail damage can be mitigated to some degree by mounting them at a greater angle although this will limit performance as well.

Photo: Hail damage to evacuated tube collectors. Banff, Alberta

In addition, the vacuum that evacuated tube collectors rely on to minimize heat losses often fails. The seal at the opening of the tube near the header is under incredible pressure to maintain the vacuum. The extreme conditions of the Canadian climate (-30C to +30C) make this a difficult design criteria.

Performance

Evacuated tube collectors are designed to minimize heat losses during the colder winter months. Unfortunately this can work to their disadvantage as they lack the ability to shed snow and frost during the winter months. Flat plate collectors will remain covered in snow during cloudy weather; however, when the sun comes out, the collector will warm up slightly creating a film of water between the glass and the snow. Gravity does the rest as the snow will slide off.

Photo: Collectors with Snow (left) and Frost (right) during Winter Time Operation (Photo from Performance of Vacuum Tube and Flate Plate Collectors Concerning Domestic Hot Water Preparation and Room Heating - Trinkl, Zorner, Alt, Stadler, 2005)

Although the above is a German study, it outlines well the problems associated with the use of evacuated tubes in a winter climate. The study was presented at the 2nd European Solar Thermal Energy Conference by the Centre of Excellence For Solar Engineering from Ingolstadt University of Applied Sciences.

Additional 3rd party sources for comparison of flat plate versus evacuated tube:

• NRCan (Natural Resources of Canada) has made a comparison of CSA certified systems here. The performance ratings for different brands and models can be seen.
• SRCC (Solar Rating and Certification Corporation) provides independent test data of solar collectors here.
• Enerworks’ comparison of flat plate versus evacuated tube collectors can be downloaded here. Analysis includes a graph comparing the efficiency curves of Enerworks flat plate and Apricus evacuated tube collectors
• Home Power magazine published an excellent article comparing flat plate and evacuated tube solar thermal collectors in their most recent August/September 2009 issue. Search for the author “Mehalic” using their advanced searching tool at www.homepower.com. Article is titled: “Flat-Plate & Evacuated-Tube Solar Thermal Collectors” – The pros and cons of flat-plate and evacuated-tube collectors. Plus, how to choose the right one for your solar hot water application
• See a LIVE SIDE BY SIDE COMPARISON of an evacuated tube system and flat plate system.