Solar dealers have been arguing about which technology works better, flat plate or evacuated tubes? The sensible answer is that both collector types work quit well, but each has its own advantage. The real unbiased answer is found by ploting the performance graph of the collectors, shown below, and understanding what to look for if the purpose is heating domestic water in temperature differential rating Category C - where the air is 36 to 90F colder than the collector water.
The graph above shows the Solar Rating Certification Corp (SRCC) category B, C, and D graph, courtesy of The Heatspring Institute. Broadly, the graph shows that as the outdoor air temperature drops (to the left) the collector efficiency (ability to heat water) declines.
The key area to focus on is the light blue Category C zone in the middle, this is where the temperature range is for heating domestic water - when the air is 36 to 90F below the collector water temperature. At the right of the moderate DHW blue C zone, is where the collector temperature is 0 to 40F of the outdoort air temperature, this is Canada's summer time, where unglazed black plastic pool collectors work best. To the left is the the performance category D, where the air is about 100-120F colder than the collector, the range where Tubes have the advantage.
The SRCC graph shows the premium Selective Absorber flat plate (BluTec is the top Selective) is more efficient across the entire Category C area, and into D until a "cross over point" at 103F. This means that on a winter daytime temperature at freezing, 32F, the flat plate has the advantage up to a collector temperature of 135F, (32 + 103), which is about the top range for DHW. In other words, the Tube collector will marginally outperform on the few days that go about 10F below freezing, which may only be one week out of 52 on the BC coast. For all Provinces, the Selective flat plate produces substantially more hot water than Tubes on 40 to 50 weeks of 52, that is 80-98% of the time, and by a wider margin during that time.
Better performance is one of the reasons flat plates are 90% of European and North American sales. The Biggest reason is Reliability - the flat plate has tempered glass and is more rugged; whereas Tube glass is fragile, prone to breakage and leaking at the many glass to aluminum header joints, and made in China under expectations of 5-8 years before disposing of Tubes.
Better Price is another significant reason, consumers can buy more surface area of Flate Plate than Tube collectors per $1000. Even if 40 tubes was argued to outperform 2 Flat Plates, you could buy a 3rd Flat Plate, and that completely changes the comparison of BTU/dollar captured in the solar storage tank.
Appearance is a personal taste factor. Most people I have talked to prefer the smooth "skylight" look of the Flat Plate, over the shiny Chinese Tubes on their home roof.
The bottom line is Selective Flat Plates perform better and last longer in Canada for DHW heating (averaged over the year), until you live where there are more Average Daytime Highs below freezing - than above freezing, (183/365), which would be the North part of Alaska and North of Whitehorse , Yukon. - Weather Network
An excellent PDF report is available from the non-partisan Heatspring Institute, they trained over 5500 solar installers, and do not sell any collectors, so they are objective. Which Is Better - Solar Flat Plate or Tubes? Another wise summary comes from Dr Ben Gravely who has been a high level solar scientist since 1977, at his blog article: Evacuated Tube Versus Flat Plate Solar Hot Water Panels – Who Has The Upper Hand? Heat Spring also has a live side-by-side test where a premium Flat Plate and Tube collectors costing the same amount are operating on identical equipment, with the temperatures live or by periods over the past year, it is the ultimate showdown. Live Collector Data Feed
