DIY heat pipe (evacuated tube type solar collector)
 

This is a pretty cool article on how to make your own evacuated tube type collector heat pipes.

Fossil Freedom - Solar Thermal Capture Tube Manifolds

http://www.fossilfreedom.com/pics/TU...ADER_CENTE.jpg

Building your own tubes sounds cool. However, not very practical or inexpensive.

You can buy a pre made unit for nearly the same cost.

This unit comes with 18 tubes, stainless steel tank and a rack to hold it all together for
$1100 shipped.

Solar Water Heater NF Pressurized 200 liters by solarfox


This one is less, but no tank or rack.

solar water heater by solarfox


I hope to buy one of these sooner or later. (hopefully sooner)

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before going with the evacuated tube method you should figure out exactly what you need the heat for and do the math. They are far more efficient at getting to higher heat but since the collectors have so much empty space in them flat plate comes very close or matches the performance/sqft on clear days. On cloudy days tubes win but an extra 10-50% of not very much heat at all doesn't make any sense money wise in my case.

Strider makes a great point. The efficiency of flat plate collectors is a fair amount better when the delta T of the panel vs ambient is lower. Evacuated tube collectors work better in extremely cold climates, or where you want really hot water. For most house heating, flat plate seems to be the more cost effective solution.

There are solar installers who do not like the heat pipe collectors because they can over heat water as well, even with flat plate collectors you can get water hot enough to soften PEX tubing to the point that it will burst if you have a sensor that is slightly shaded or a solar powered pump that has a shadow in front of it for a few minutes while the rest of the system has full sun.
The other advantage of the heat pipe collectors is that you can reduce the amount of working fluid you have in your entire system.

For my solar hotwater system (which I should really start a thread on soon) I decided to go all copper/glass collectors just incase of high enough heat to melt pex. I intend to stagnate the collectors to protect my tank and limit it to 150 or so.

When I placed an all copper 1.5 sqft test collector in the back yard at about the same angle my big panels will be and I left it without water pumping the surface of the piping hit 218 F in 30 minutes. when I added water I got a bunch of steam coming out for the first few seconds. This was at about 65 degrees slope. I'm at the 49th parallel so that's a lot closer to a winter setting then summer and it still hit amazingly hot when water wasn't pumping.

Why do you use water ?

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In the collectors? Because water holds a lot of heat for it's volume. It's cheap. I am planning a drainback system so freezing isn't a concern. By using water I can get away with a single wall heat exchanger and not need to worry about poisoning the family should a leak develop( the inspector has yet to approve my system so this may have to change...)

Where you thinking of going with some form of antifreeze/water combination or something more exotic?

I suppose you could use hot air to heat water but I've yet to see a air system that would compete with water/water antifreeze systems.

Yeah, I was thinking a little more exotic.

Since evacuated tube collectors can achieve tempertures of 350 degrees, why not use some type of oil instead. In a thermal storage system, you would not have worry about it boiling off and or high pressures. The oil could be stored in a highly insulated tank then pumped through base board heat exchangers.

I realise that it is alot more expensive than using water but the ability to store 60% more btu's in a same sized system is attractive.

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true but you now need the collectors, tank, heat exchanger and all plumbing to be able to survive 350 degrees. That's going to force you into everything being copper and/or stainless steel. It might even require special valves for the turn offs.

Then there is the question of what to do with that heat?

You'll need some sort of control system that only pumps the oil into a heat exchanger only for short bursts to gradually heat whatever you want up since you can't just directly pump water through a heat exchanger in 350 degree oil. It would boil at first and if it did cool the exchanger enough to not be boiling it would still be amazingly hot and I'm not sure regular "inexpensive" tempering valves can handle beyond 160F or so.

If you where pumping the oil directly into radiators for space heating you have serious safety and fire risks if the rads are hitting 350.

you also have the issue of insulation on the tank and all plumbing. I had to track down polyisocyanurate for the back of my collectors and for around the tank since regular pink rigid insulation melts below 200. poyiso is only good to 300 or so is my understanding. If you can find insulation that will hold up due to the deltaT being so high compared to a 140F tank you're going to need a ton of insulation to make it effective.

Basically my point here is the cost of the materials will be insane vs a homebrew system using conventional materials like pex, off the shelf plumbing and the pond liner tanks. If you want to increase BTU storage go with a bigger tank...

Now if you where using that heat to turn a generator or something then oil is probably the way to go. Thats what the big solar heat power plants do. for space heating though you want about 180F for radiators, 90F for radiant and for domestic water 140F is just fine...

Does anyone know where one can get the glass tubes only? I want to try and build one from scratch?

If the oil will get up to 350* F the system could be down sized to not have as much heat capacity. To solve the problem of water boiling off right away I would look into using a Phase Change Material as the transfer medium so the system would be oil-PCM-water. The PCM will work as a buffer since once they reach the melting point the material stops getting hotter till the whole unit melts then the temp starts to rise again. With the water being heated it will slow the melting, and a side benefit is the PCM holds heat and slowly releases it allowing for more storage on cold cloudy days.

What will take a while to figure out is what to use as the PCM and as buffers for the temp. From a few quick searches I would go with a mix of paraffin wax, agar, and an equal mix of sodium carbonate and silicon dioxide (if you are worried about fire) for the PCM.

Why I would use those:
Stage 1
Paraffin with a typical melting point between about 46 and 68 °C (115 and 154 °F)

Stage 2
Agar melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F)
Agar chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium for microbiological work.

Stage 3 (This will never be reached)
Sodium carbonate and silicon dioxide react when molten to form sodium silicate and carbon dioxide: Na2CO3 + SiO2 → Na2SiO3 + CO2
Will smother the fire and the melting points are 851 °C (anhydrous) 100 °C (decomp, monohydrate) 34 °C (decomp, decahydrate) for sodium carbonate and 1600-1725 °C for silicon dioxide. (Something else might need to be used for these)

The last possible issue if heat gets to high could be helped by adding a filler to the container such as small rocks or glass. Glass will heat quicker but it will also cool quicker than rocks. Both could be used in taking that in to account see image below for container example.
Attachment 1731

Vacuum tubes can get to those high temps if the fluid is stagnant. As soon as you start pumping to take the heat out of the collectors, the temperature will go down. You would need a tremendous number of tubes to maintain that kind of temperature while pumping the fluid to a storage tank.

Have you ever done any actual experiments with this idea?

-AC_Hacker

No I have not done any experimenting on it yet I eventually plan on doing some but with the idea of running it in a car to keep the car warmer after shutdown. I know that PCMs work but most of the PCMs do not run in the temp range that I want or are flammable and I do not want that in a car if it bursts.

If anyone knows of a list of PCMs with the melting tempslet me know as I have not found a good one as most lists I have are salts that they super cool or heat past the temp a car will safely run.

There are very few materials that are not PCMs. Phase change is when a material goes from one state to another. Water for instance, have phase change temperatures of 0 degrees Celcius (freezing point) and 100 degrees Celcius, which is boiling, at sea level. A good PCM is one that needs a whole lot of energy for the transition from one state to another (solid, liquid, gas). So my suggestion is to just look for melting temperature of all materials, find those that match your needed temperature, and then figure out what the phase change energy demand is.