An alternative energy store opened up in my hometown. Their focus however, is solar. Not necessarily solar panels, but solar heating. They sell solar space heaters, and solar water heaters. I had a good chat with the owner, and the product he seems most proud about is the evacuated tubes, which can be used for domestic water heating, or even space heating (through water). Apparently, one rack of 8 can heat 40 gallons of water. Each tube is probably 4" OD and 6' long. He says that in the summer, it can bring the water past a boil if allowed to.
Then I asked about winter performance.
He said obviously the performance is decreased when it's below freezing outside, but that these tubes will still bring the water to well over temperature for domestic water use. I am a bit intrigued by this. I guess the actual heat pipe system has that glass tube shrouding it so that there is a vacuum inside. The brochure of the tube says that the pressure is approximately 10^-5mBar, so that's quite a vacuum indeed. The owner said that this vacuum allows the unit to still function well when it otherwise wouldn't. I was just curious to see if anybody here has experience with evacuated tube solar heaters; what is your experience in regards to cold weather performance?