DIY heat pipe (evacuated tube type solar collector) - Page 2

cdwiggi · 09-12-11, 03:01 PM

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

Phantom · 09-13-11, 01:07 PM

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.

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solarman · 10-13-11, 10:11 PM

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.

AC_Hacker · 10-14-11, 12:42 AM

Quote:

Originally Posted by Phantom

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.

Have you ever done any actual experiments with this idea?

-AC_Hacker

Phantom · 10-15-11, 10:30 AM

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.

Ko_deZ · 10-22-11, 06:22 PM

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.

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09-12-11, 03:01 PM	#11
cdwiggi Lurking Renovator Join Date: Sep 2011 Location: Missouri Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts	Does anyone know where one can get the glass tubes only? I want to try and build one from scratch?

09-13-11, 01:07 PM	#12
Phantom Apprentice EcoRenovator Join Date: Sep 2010 Location: Louisville, KY Posts: 101 Thanks: 13 Thanked 8 Times in 7 Posts	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.

10-13-11, 10:11 PM	#13
solarman Lurking Renovator Join Date: Oct 2011 Location: MA Posts: 2 Thanks: 0 Thanked 0 Times in 0 Posts	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.

10-15-11, 10:30 AM	#15
Phantom Apprentice EcoRenovator Join Date: Sep 2010 Location: Louisville, KY Posts: 101 Thanks: 13 Thanked 8 Times in 7 Posts	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.

10-22-11, 06:22 PM	#16
Ko_deZ Helper EcoRenovator Join Date: Jun 2011 Location: Norway Posts: 63 Thanks: 3 Thanked 10 Times in 10 Posts	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.