Drain down system with evacuated tubes ?

[liberti]

Does anyone have any experience of using a drain back configuration with an evacuated tube (especially the heat pipe types as sold by Navitron) Would the heat pipes be able to take the potentially  repeated thermal shock that may arise when the pump restarts and sends relatively cold water through the manifold which may happen under certain circumstances.

Out of interest no manufacturer has replied to this query, but two importers have and both say its ok, but I dont have much faith in their technical abilities. I will be setting up a test rig to explore this  but wanted the benefit of your experiences and opinions first.

I've played around by leaving a tube in the sun for until its very hot and then running cold water over the tip. The tube seems ok but you can feel the jolt through the tube which is unnerving!

[Antman]

I think you have answered your own question....

Haviing first-hand experience of the efficiency of the Navitron tubes, they are 'cooled' by the water flow in the manifold for good reason. Allowing them to heat up to temperatures in excess of 100 deg C and then blasting them with relatively cold water will result in thermal shock as you say. Fo long-term reliability this would not be good. Also the water would likely continually vapourise on contact with the hot heat-pipes and then recondense on it's way back down to the cylinder coil. I am not a physicist but I assume this would result in a loss of energy?

Also the manifold contains a thermal sensor to detect the tube (heat-pipe) temperature and control the solar pump - the heat being conducted via the water in the manifold. Take away the water and there is no means to detect the heat-pipe temperatures.

The drain-back principle  is used in some flat plate collector systems where the mass of the plate absorbs the heat and the temperature can  be sensed direct from the plate surface. The stated advantage is that antifreeze is not required because - in theory - the water drains out of the collector panel when the pump shuts off and therefore it should not freeze at night...

For what it's worth - it's not a method I would want to utilise on this design of evacuted tubes.

Ant

[liberti]

I agree. Its just I 'd come across a few web references to drainback with evacuated tubes which seemed to indicate that some are using it. A couple listed below.


http://www.btfsolar.com/dhw.htm

http://shop.solardirect.com/product_info.php?cPath=22_74_77_118&products_id=489

[Antman]

I agree. Its just I 'd come across a few web references to drainback with evacuated tubes which seemed to indicate that some are using it. A couple listed below.

http://www.btfsolar.com/dhw.htm

http://shop.solardirect.com/product_info.php?cPath=22_74_77_118&products_id=489

The BTF site shows a manifold/tube setup that looks very similar (near identical) to the Navitron system - maybe it is sourced from the same supplier? Unfortunately they do not include enough installation or technical info as far as I can see. The installation manual only shows panel assembly. I would still want to know how they sense the heat-pipe temperature using this setup.


Interesting contradiction on the Solar Direct website. The installation instructions for the Apricus panel referred to emphasise specifically in section 2.7.10  Air Purge how essential it is that the system must be fully purged of all air. In which case, how do they 'optionally' run it as a drainback system? The instructions appear to omit reference to this....
Notably, the Apricus AP manifold design shown, is different from the Navitron and appears to consist of a dual tube arrangement above and below the heat pipe sleeves. From an engineering point of view, this is probably cheaper to manufacture. However it must be less efficient at thermal transfer that the Navitron design since there is a reduced surface area of manifold in contact with the heat pipes.

Maybe the conducted heat in drain-back operation is sufficient to prevent excessive heat pipe temperatures forming during 'normal operating condiditons'. However it would surely cause a delay in the transfer of thermal energy?

Ant

[Ivan]

I cannot say that I have any experience of drain back systems, but I have a few comments:

1. Somehow you will need to a.)get the air out of the panel when refilling 2.)get the air into the panel to allow the water to drain out of the header.

2.The diagram in the link given above shows the drainback system having a significant volume of water and an extra indirect coil transferring the heat out of the drainback system. The extra coil and the extra stage in heat transfer will lower the efficiency and reduce temperatures.

3. Drainback will not prevent the panel from reaching very high temperatures (200C+) which if done regularly would shorten the lifespan.

If you really want a drainback system to prevent overheating, a far simpler method will be simply to allow the system to overheat. This will cause steam to be produced in the manifold, which will force the water out of the manifold. The increase in volume will cause more fluid to flow into the expansion vessel, and will increase the pressure in the system. A 20tube manifold contains around 1.5litres of water, but by the time enough water has boiled to produce 1.5litres of steam, it will force the water far enough away to stop further heating of the water. In practice, the water may not leave the manifold in its entirety, so some residual water may also boil, but this steam is likely to recondense as it is pushed further down the pipework. I have tested this theory on my own system, and the pressure increased from 1bar to 2.5bar, without any blow-off from the pressure relief valve. I have a standard Navitron 12litre expansion vessel, but I guess if you are designing the system with this in mind, it would be a good idea to increase the expansion vessel capacity to allow more margin for error - perhaps to 40litres (I think this is what some other manufacturers sometimes install)

Ivan
Navitron

[Ivan]

Here's a useful article on panel stagnation:


http://www.aee-intec.at/0uploads/dateien119.pdf


Ivan