Ian, you may get some useful data on what the vac tubes can withstand on www.spf.ch
I'll dig out some links on spacing out the vac tubes later for you. I have some links somewhere on my other PC. There's quite a few people using this, though possibly it needs direct flow (U tube) rather than heat pipe inside the vacuum flask?
If you are able to cool the glass quick enough with a suitable flow rate I see no reason why a tube can't be used to collect heat from an even wider area than you suggest. It's a really good idea and a nice low-tech way to get higher output from a high-tech vacuum tube.
Parabolic trough reflector?
I'd buy a few sq metres of this to experiment with - If there's any other takers we could get up to minimum order quantity.
An inexpensive way of procuring the vacuum flasks, last time i looked, was the navitron pool heater. These are 'direct flow' type of panel with vacuum flasks but no heat pipes (the water flows into the vacuum tube instead. Throw away the manifold and you were getting a bunch of 47mm (or 58mm?) vac tubes for about £5 apiece! Then build your own copper u-pipe to pump fluid inside the vac tube, using copper pot scrubber to achieve a good level of conductivity from the inside of the vac tube to the U pipe?
I'm really unsure how much a vacuum thermal tube adds in this case over a simple black tube - if you're using concentrators, and using it for water - and not something more exotic at - say - 500C or so.
The major minus for solar concentrated arrays is that they're almost useless in diffuse sun. Whereas a evacuated tube panel might put out 1/4 of the nominal output in diffuse light, the concentrated one will fall off to almost zero. (or more accurately about twice the output without the reflector)
This is simply because they only work when there is a nice bright disk of sun that you can reflect in one direction onto your collector. With diffuse sun - most of the light hitting the mirror is from the wrong direction, so misses the tube.
This gets a fair bit better if you are not trying for much concentration.
It doesn't take much thought to reveal that the back of the array is normally nearly totally dark, so if you could brighten it up a bit, you'd gain quite a lot. Existing panels have the tubes very closely spaced, so just putting a mirror behind them will do almost nothing.
I need to do some proper sums and raytracing to work out how tube spacing and mirror position works. I think it should be possible to fairly closely approach double in good sun, and maybe 1.5* in diffuse light.
The above site looks interesting, I need to read it more.
As to the mirror film - I'd be interested in around 4m^2 to play with. I haven't confirmed with 3M if that's the delivered price.
Last Edit: May 6, 2011 16:50:02 GMT -5 by speedevil
Ian, I think I see what you mean. Direct sun represents a distant 'parallel' light source which is focussed, but ambient is from all directions, most of which will miss the heat pipe where the reflector is parabolic and the pipe placed at the focus.
So what you're saying is that if you do not aim for fine concentration you will increase the chance of getting ambient hitting the underside of the tube? And do you achieve poor focus by using parabolic and not putting the collector pipe quite at the focus? I had read that average UK solar energy is 40% ambient, so there's a lot of energy there to capture if it can be encouraged towards the absorption pipe. If 'quality space' is tight, why not just install a vac tube collector on top of a flat panel - Flat panel to your warm store and tube panel to your hot store?
I don't quite understand why the vac tube is of little use when used with a concentrator mirror - Can you explain? Heat loss is proportional to temperature differential, so surely the insulation afforded by the flask will always be of benefit?
The chinese vac tube panels typically claim 5% extra with the reflector installed, but, as you say, the pipes are closely spaced. I've seen a (european?) brand called Ritter where the pipes are spaced much further apart. They use a double parabolic reflector under each pipe, much like the 'inverted bird wing' shaped reflectors you sometimes see in office flouro lighting. That must be to capture more of the ambient?
How much is polished SS? Is it cheaper than mirror film? SS is reputed to retain its reflectivity well if sited in a glazed box (cost of glass probably defeats the purpose!). What about reflective mylar as used in emergency heat blankets? This is about 50p per square metre. again probably only good in a glazed box. Glazing with UV resistant polythene?
Gentlemen, we provide 3M Solar Mirror Film 1100 in 5meter x 1.25 meter minimum. We also provide lamination services so all you need is a frame to support the mirror. This is a very cost effective, durable mirror at 95% specular reflectivity.