Foo - Evacuated tube heaters

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?

Solar space heaters? Wait..what? I don't understand.

i still don't get how it works. won't they break when it hails?

http://www.sssolar.com/collectors/heatpipetech.asp

i still don't get how it works. won't they break when it hails?

http://www.sssolar.com/collectors/heatpipetech.asp

Interesting point, goldfishin :D

Think of a tube containing a heat carrying fluid which cold be anything from helium to glycol. Center that tube inside another tube with a vacuum between the two tubes. Make the outer tube resistant to hail and transparent to heat radiation. Place the two tubes at the focus of a parabolic reflector much as the manufacturer puts a filament or H.I.D. emitter at the focus of your bike headlight. Aim the reflector at the sun. Light bounces of of the reflector, passes through the outer tube, and is absorbed by the inner tube. The vacuum keeps the inner tube from losing its precious heat energy. In winter the lower angle of the Sun to the horizon makes the light pass through more atmosphere so less heat gets to the collector. Not a problem at the Equator, a deal killer above the Arctic Circle. An active tracking system helps, how much I do not know. Maybe 20% to 40%? Some systems have compound, complex reflectors which reduce or eliminate the need for tracking. Check $$$ required per therm of heat energy gained to compare these systems with other set-ups.

The owner said that these are rated for impact of 1.5" hail. I've lived here all my life, and I've seen hail no more than 4 times.

Vacuum is a great insulator, imagine storm-windows with vacuum instead of air in that gap. The vacuum keeps the higher-heat concentrated in the center tube from being radiated out to the atmosphere.

well, it is a good insulator. 10^-5 mbars is pretty good. I suspect the guy is right, that these things will easily heat that tank to 150 degrees. Impressive

I am going to try and buy just one of these tubes. It's 100 bucks a tube, so it's something I can play with. I'd like to see how much heat I can get from just one tube, while the tube sits in cold weather.

Vacuum insulation kicks ass. Sounds like an interesting idea.

well I am mostly interested in the performance of these tubes in cold weather. The purpose is to generate heat. I suspect that one could get tubes to preheat the water that reaches ones furnace, so the furnace has to heat the water that much less. I am fascinated by these things though, and it's worth investigating.

Most importantly, I want quantifiable results.