I have built a heat pipe, from scratch, to transfer captured Solar Energy out of my Evacuated (vacuum)Solar Thermal Energy Capture Tubes.
I soldered a sealed tube with a condenser bulb (made from a standard ½ inch copper "Stub-Out" fitting and a ½ to ¼ inch reducer fitting) at the top and a 42 inch long X ¼ inch ID copper boiler tube that extended down the inside of the Evacuated Solar Thermal Energy Capture Tube. Before sealing the tube I had added 0.078 fluid ounces of acetone as a heat pipe heat transfer working fluid.
The idea was to boil the acetone into acetone vapor, thereby cooling the inside of the Solar Thermal Energy Capture Tube by using the large heat-of-vaporization of the acetone working fluid.
The design looks like this (NOTE: Selective Coating rendered transparent for clarity):

I wrapped two stretched out pads of "Brawny" Stainless Steel Pot Scrubbers (total cost$1.98) around the boiler tube, and inserted them into the bore of the Evacuated Solar Thermal Energy Capture Tube, so that the stainless Steel wool was in thermal contact with both the acetone heat pipe boiler tube and the solar heated inner wall of the energy collector tube. The purpose of the stainless steel wool, was to form a permanent, intamate heat transfer medium, that is both rugged, and cheaper than the brass or copper wools, that would not rust or corrode, at continuous high operating temperatures.

Then, I placed the heat pipe Solar Thermal Energy in the sunlight, and slipped a piece of thermal foam pipe insulation, over the condenser bulb, and slipped a 60 - 250 F meat thermometer into the foam, along side the condenser bulb. After 15 to 20 minutes, I returned, and was stunned, to find that the thermometer read off-scale at 250 F, and the thermal foam had melted due to the very high temperatures being given off by the heat pipe condenser bulb, as the vaporized acetone condensed, and gave up is heat of condensation, to became a liquid once again, and was returned to the boiler, by gravity to re-vaporize again, and begin another rapid heat energy transfer cycle.

Here is a picture of the heat pipe condenser after the thermal insulation foam was melted by the hot Solar Energy Capture Tube Heat Pipe Condenser Bulb!

This type of thermal pipe insulation foam is in common use, on hydronic (radiators or hot-water floor or baseboard) heating systems world-wide that operate continuously at 180-200 F.
So to melt such a high temperature foam, the condenser bulb had to be well above the 250 F scale limit on the meat thermometer.

My home brew DIY heat pipe worked very well, the first time I tried it!

The DIY heat pipe exceeded my initial expectations for both ease of fabrication, and long term operating performance.

The fabrication was very simple and straightforward, and I plan to make a LOT more of these easy to assemble and use heat pipes, not only to use with my solar thermal energy collection tubes, but also, to modify some ammonia propane boilers in RV refrigerators, to operate on captured Solar Thermal Energy instead of a propane flame.
Speople have asked for more information and updates, so here it is:

I used copper throughout, since acetone will not attack copper, but
stainless should work OK.

Certainly an ammonia based heat pipe would need stainless.
The first version I built had a huge 6 inch long x ½ inch diameter
condenser bulb, but empirically, I found that when I cooled the first
1½ inches with a water jacketed cooling manifold, then, the top 4.5
inches immediately dropped to the water cooling ambient.
So only a small portion of the 6 inch long condenser bulb was actually condensing any acetone under these conditions.
So, I just reduced the size of the copper bulb to 1.25, need far less costly copper, in the heat transfer region.
That makes the manifolds much lighter and thus far less costly in copper content.
Thermally they are just as efficient as the old larger one.
So, this copper size reduction, was just an optimization, rather than a cost compromise.

The important lesson learned, is that the heat transfer is so fast and efficient, that much smaller manifold sizes are possible, compared to our intuitive experience, based on moving large amounts of heat around by weakly vibrating molecules, hopelessly confined, in
a solid phase metal crystal lattice.
Thus, a tiny amount of acetone vapor, can move heat around, thousands
of times faster, and far more efficiently, than even an expensive
pure silver bar or rod!

Frankly, I was astonished how easily the heat pipes can be designed and built by a DIY who can sweat solder copper.
Some care is required, if using flammable working fluids like acetone, so this sweat soldering should be done outdoors, with a water source nearby.
Cotton clothing with no polyester content should be worn. With a water
hose running nearby.
Eye protection should also be worn but that is true any time you ever
sweat solder!
The amount of working fluid used is so tiny, that there is really
little danger involved.
I purposely lit off some of the acetone vapor with my propane torch just after I had purged out the oxygen/nitrogen/ during the purging step, and it just burned off like a small blow torch jet.
So, the average DIY experimenter can make simple heat pipes even using flammable working fluids safely if simple precautions are taken, and good construction practices are used.

Actually, when I think about it, the uses for inexpensive DIY heat-pipes, are almost unlimited!

Photos of manifold construction

Applying Flux to fittings with brush

©2006
Patrick Ward
11 Dec. 2006

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