evaculated tube types?

[Mladen]

can someone compare (explain) to me two types of solar (evacuated) tubes? First with water-flow going through them and other with no water (just copper core)? is there big difference in efficiency, since prices are quite different...

thanks,

[Ryland]

I've never seen the vacuum tube solar collectors that have water flowing in the center of them, I have however seen a hand full of brands that have the fin in the center with a very small tube that has a few spoon fulls of water in it and a condenser at the top that it clamped on to the pipe of water that you want to heat.
The advantage of this type of solar hot water collector is that there is very little heat loss, the copper tube in side of the glass tube is a form of heat pipe, giving near instant heat transfer to the top end, because it's in a vacuum there is no air to conduct heat so it's a near perfect insulator, the coating tends to be blue instead of black, this color absorbs other spectrum's of light but does not radiate heat back out the same way that black does so the heat that it absorbs it holds, transferring to the tiny bit of water in the tube, turning it to steam and moving that heat to the water in your house and if you wanted to, you could have the domestic water flowing nearly all the time without risking heat loss.

[Mladen]

I thought there were two types of tubes.



one with circulating water in it from boiler, and another with copper core and no water. I was afraid that during long cold nights, water in first kind could froze and damage the pipes.

M.

[Ryland]

I'm not sure what the first drawing is, but I've never seen a working example of an evacuated tube solar collector that looked like that, or that circulated the water in the tube.
The 2nd image you posted is exactly what I was talking about, with a heat pipe in the center, the water in the heat pipe has enough space that freezing is not an issue, after all it's going to be looking for the coldest spot and condense there and freeze there in droplets instead of freezing in a pool at the bottom.
We have long cold winters here and I've never heard of an issue with the heat pipes freezing up and bursting, I started seeing those types of collectors about 15 years ago and every new company that comes out is more or less using the same design and I don't think a solid copper bar in the center would work because heat doesn't travel in copper very fast and heat will travel in steam nearly instantly.

[Mladen]

Thanks for the explanation. First picture is describing the cheap version of solar boiler which is nor under high pressure. It is common here (at least at merchants). Solar is popular, but very few people have it here. Price for first version (just pipes with double wall-evacuated... and black painted glass-wall inside) cost about $500 here with insulated boiler attached, while this other kind(you were talking about it) is much expensive (3-4 times). I now understand that for quality usage, only this other kind is good, with no worry for freezing over nights...

M.

[AC_Hacker]

Originally Posted by Mladen

can someone compare (explain) to me two types of solar (evacuated) tubes?

The first type, that has water flowing through the tube, was the first to be developed. It has a serious potential problem in that if any one of the evacuated tubes gets broken the whole heating system loses heating water, and is unable to function until that tube is replaced.

The second evacuated tube system, with the copper fins and the heat pipe, etc., can withstand damage to a tube or more tubes. It will function with somewhat reduced effectiveness, but it will continue to function.

I had the opportunity to get 70+ tubes and manifolds that were of the first type, for a very, very cheap price, but I passed on it when I realized the broken-tube shortcoming.

-AC_Hacker

[randen]

I've often wondered about the Evac tube with heat pipe. Do we really have an efficent method of extracting heat from the Evac. tube with the copper heat pipe assembly?? What about a copper loop with copper sheet web between like that in a flat plate. After all supposely the only draw back to a flat plate collector is it lack of better insulation.

Randen

Anyone up for an experiment??

[Ryland]

I think the heat pipes are the best way to extract heat from the tube, the heat is moved nearly instantly and without the use of any pumps, the only real draw back that I have heard of is that they can get the water to hot if the flow rate in the heat exchanger is to slow, but that does tend to be the case with pretty much any solar hot water panel, but most solar hot water panels self regulate to a point as well by having more losses.

[frankwest12345]

The manifold looks like it is quite easy to make out of standard copper pipe and evacuated tubes can be ordered separately (eBay) so it would not be that hard to custom build something to fit an available area, the big problem is you have to design it to work in winter so in summer there will be excessive heat running thru the system just when you don't want it, which would have to be dumped to a radiator placed where the heat will do no harm. Something potentially simple becomes more complex... I can't quite get my heat around it, but wonder how you could integrate it with an air-conditioner type heat pump?

[AC_Hacker]

Originally Posted by frankwest12345

... how you could integrate it with an air-conditioner type heat pump?

You would need a heat storage tank to store the water that would be heated by the solar collector. The water from this tank would be circulated into the house for heating, when required.

Then you need a monitoring circuit to measure the temperature in the tank. If it gets too hot, you need to stop heating the water in the tank, and re-direct the heat to where it can be more useful.

If the water in the tank gets too cold, you need to switch on the axillary heat, like a heat pump, to supply extra heat, until the tank reaches the proper temperature.

-AC_Hacker