There are a lot of vacuum tube collectors (i.e. solar panels) available on the market these days, and there is definitely a lot of misinformation out there. I shall try to rectify this to some extent.
“Vacuum collectors are more efficient than flat collectors.” TRUE or FALSE?
False most of the time and true some of the time. A good flat collector will start off with a higher efficiency than almost any tube collector (with a few exceptions from Europe). During the summer, or whenever the collector is operating at less then 60C above the outside temperature, the flat collector will be superior to the vacuum collector (different collectors have different characteristics and efficiency curves so 60C is a bit of a generalization)
The time spent below a delta T (temperature difference) of 60C is actually the majority of the time for domestic water heating systems. It is only when it is -10C outdoors for example that the vacuum collector will outshine a flat collector, and due to the range of collectors out there this is not true of all collectors.
Here is an example: a Viessmann flat collector, at its most efficient (no heat loss through the enclosure of the collector) is about 79% efficient. When the outside temperature is 60 deg colder than the collector temperature, the efficiency drops to about 58%. This range is where the collector will spend 75% of the time. By contrast a good vacuum collector coming from China using the Sydney type double tube (the most used type of tube) has an initial efficiency of 60%. At 60C delta T the efficiency is 50%. The efficiencies of the collectors will meet when the temperature difference is closer to 80C. It is only in the dead of winter when this will happen and we don’t have a lot of energy in the sun at that time of year.
Another real issue is longevity. The Sydney tube is only about 20 years old and has only seen the light of day in North America and Europe for 6-8 years. As a matter of fact, of the hundreds of tube suppliers in China, only a small fraction will meet any certification. Most operate with no manufacturing standards controls. The market in China is much different than here. The tubes were designed to last 7-8 years and could be replaced one at a time if one broke. The economics of life in China dictated that the tubes needed to be cheap enough to grow the market. Few will last 20-25 years and not without a tube or two being replaced annually. The vacuum tube works under a lot more structural stress than a flat collector so it is natural to expect that it will not last as long.
Many flat collectors, by contrast, have been running for 25-30 years and show no sign of dying. More to come on this topic.
Here is an article from Germany printed a few years ago. There have been moderate improvements in both flat and tube collectors since then but nothing that would change the essence of the paper. Performance Test – Flat Plate and Vacuum Tube Collector