Oil in vacuum tubes

[Archetype-IS]

Thanks for replying. Is yours an open system? How do you keep the water in storage from over heating?

Not related to your questions above, but the below videos would give you ideas to alter or give insights on your house designs if you haven't done so.

You might want to look at Earthship designs? Here is the some videos with Earthship. Completely off grid. Not sure if you have the space in Maine to do so.

She has many videos I am including some that I have viewed and it could be applied to you on Earthship, Sunken Greenhouse, Build below the frost line, Greenhouse in Nebreaska in -20F(closer to Maine temp? or colder?), Greenhouse enclosed with a house. Great youtuber on documenting alternative designs that I have followed for years etc. It has always great to see people had done it and live in it to give what other issues could be improved.

Winter temperatures in Alliance, Nebraska can drop to -20°F (the record low is -40°F/C

house inside a greenhouse.

 

[WillyP]

It's open - i.e., not pressurized. There is a temperature sensor probe I use to cut off adding any solar if needed. With the wood/pellet burner, you kinds know how much fuel you use based on the current temperature, so that is used to stop feeding x amount of fuel (or in the case of wood, I just don't put more).

And of course, the tank itself is it's own closed system, and the floor heating as well. That all just taps from the large tank with heat exchangers.

I am trying to picture this. Does the water from the storage tanks ever mix with cold water? Or does the water from the heat source (wood stove or PV) separate and run through the exchanges? Where does the water go after it passes through the slab?

 

[upnorthandpersonal]

Where does the water go after it passes through the slab?

It goes back to where it came from, gets re-heated and sent back under the floor. There is some mixing going on in a manifold so that the water is cycled in the floor can have different temperatures based on preference (so called zones); this also allows control of the water temperature that gets sent into the pipes in the concrete floor. Something like this:

All the different systems that take heat out of, and put heat into, the tank are themselves sorta closed systems. In other words, no water leaves the tank, no water goes in - heat exchange coils are used for transfer. For example, there is water in the burner that gets heated, goes in a coil at the bottom of the tank where heat is exchanged to the water in the tank (at the bottom because heat rises, so bottom of the tank is coldest), and then goes back to the burner. Likewise, hot water for showering/etc. is generated by taking cold water from the well, sending it through a coil near the top of the tank and then sent to the shower or tap where it is mixed with cold water as needed.

 

[WillyP]

Yes, but most of those are not exactly practical at this time. You could use paraffin, but you have the disadvantage that it has a low thermal conductivity so getting energy into it is harder. For other reasons, molten salt would be excellent - but again, not exactly practical on the scale we're talking here...

The big problem with paraffin is it solidifies at 123°F The stored water will be at 95°F. That translates to the wax solidifying in the heater coils. But it does hold heat well.

Not related to your questions above, but the below videos would give you ideas to alter or give insights on your house designs if you haven't done so.

You might want to look at Earthship designs? Here is the some videos with Earthship. Completely off grid. Not sure if you have the space in Maine to do so.

She has many videos I am including some that I have viewed and it could be applied to you on Earthship, Sunken Greenhouse, Build below the frost line, Greenhouse in Nebreaska in -20F(closer to Maine temp? or colder?), Greenhouse enclosed with a house. Great youtuber on documenting alternative designs that I have followed for years etc. It has always great to see people had done it and live in it to give what other issues could be improved.

Winter temperatures in Alliance, Nebraska can drop to -20°F (the record low is -40°F/C

house inside a greenhouse.

Thanks for sharing this. There are some great ideas here. I really love the idea of using buried tires as a heat mass. That solves a couple of issues for me. One of the most expensive parts of my design is the underground heat barrier. The tire idea could remove the need for concrete forms.

 

[WillyP]

It goes back to where it came from, gets re-heated and sent back under the floor. There is some mixing going on in a manifold so that the water is cycled in the floor can have different temperatures based on preference (so called zones); this also allows control of the water temperature that gets sent into the pipes in the concrete floor. Something like this:

All the different systems that take heat out of, and put heat into, the tank are themselves sorta closed systems. In other words, no water leaves the tank, no water goes in - heat exchange coils are used for transfer. For example, there is water in the burner that gets heated, goes in a coil at the bottom of the tank where heat is exchanged to the water in the tank (at the bottom because heat rises, so bottom of the tank is coldest), and then goes back to the burner. Likewise, hot water for showering/etc. is generated by taking cold water from the well, sending it through a coil near the top of the tank and then sent to the shower or tap where it is mixed with cold water as needed.

WOW! That is a great design. How hot does the wood stove get the water? How do you prevent boiling? I originally started out my design with a wood stove as my main source of heat. But over the years it evolved into this passive solar heater. I think this was a result of having far too much time on my hands.

 

[upnorthandpersonal]

How do you prevent boiling?

This is kinda automatic, as long as you don't keep adding more fuel. However even if it should boil, because it's an non-pressurized system the steam will just escape through a safety vent. But in the end, you just learn how much fuel is needed based on the temperature of the water in the tank and you won't boil the water. You keep the water around 80C, maybe 85C at times, so that still gives you quite some margin before you boil the water in the tank (especially a large 3000L one).

 

[Archetype-IS]

The big problem with paraffin is it solidifies at 123°F The stored water will be at 95°F. That translates to the wax solidifying in the heater coils. But it does hold heat well.

Thanks for sharing this. There are some great ideas here. I really love the idea of using buried tires as a heat mass. That solves a couple of issues for me. One of the most expensive parts of my design is the underground heat barrier. The tire idea could remove the need for concrete forms.

Another great site is https://www.builditsolar.com/ You are welcome. Keep learning and see what fits your design up in Maine.

You might want to look into Geothermal pump design for house. Good luck.

 

[svetz]

... the disadvantage [of isocane is] that it has a low thermal conductivity so getting energy into it is harder....

Agreed that isocane has a thermal conductivity a third that of water, so to be water's equal it would need 3x the heat exchange area, or 3x the ΔT, or additives. Also agree that it (and other PCMs) are not ready to drop into everyone's home.
But, in terms of "nothing beats water as a storage system", that was just too cavalier as PCMs are a very promising branch of technology. For example, each kg of of Isocane that changes phase you get 246 Kj of storage. Whereas each kg of water from 128-68 = 60 x 75.4 = 4.5 kJ. In this example the PCM stores ~55x as much energy per kg. Easy to see why there's interest in PCMs. Okay... off my soap box! ;-)

 

[WillyP]

So as a follow up. It sounded good in theory. But the good people on this and a couple of other boards pointed out that oil lacks the ability to hold heat. Even though it does heat up quickly. Water with antifreeze seems to be a far better solution. As for price, oil is actually cheaper and would require less replacement. Oil would last for many years where as antifreeze needs to be replaced every couple of years. That is one of the reasons I wanted to go with oil.

but at the end of the day, the inability to hold the heat is why oil would not be efficient. Oil is used to cool systems for this very reason. The heated oil is run through tubes that quickly cool, this cools engines, and machines that make plastic... I'll tell ya, you learn some interesting things, when you ask the right questions.

 

[Mart Hale]

For the solar scientist fanatics this site is cutting edge -> https://www.builditsolar.com

I also considered using oil to store heat heated via a rocket stove. Another option is salts they also can store heat. I have a vacuum solar tube I use it to cook with, at least I did till I got my large solar array up now my insta pot does about all of my cooking

 

[Mart Hale]

The big problem I have seen with oil that is heated is keeping it hot.... I have considered using zeolite instead which store the heat in water chemically, to extract the heat you just add water ->

 

[Hedges]

It wasn't the wheel they reinvented. It was the availability of inexpensive rubber and the additional speed from the technology of the automobile that made the steel rimmed wheels obsolete.

And so now we can drive easily to our favorite fishing stream.
Unfortunately, there weren't any salmon there.

University Research Links Salmon Death to Tire Dust Chemical

Salmon are as important to West Coast ecosystems as they are delicious. So what do tires have to do with it? A tire dust constituent called 6PPD is key.

www.motortrend.com

 

[bengelha]

Yes, but most of those are not exactly practical at this time. You could use paraffin, but you have the disadvantage that it has a low thermal conductivity so getting energy into it is harder. For other reasons, molten salt would be excellent - but again, not exactly practical on the scale we're talking her

Is there any reason why you could NOT use molten salts in a solar vacuum tube?

 

[Hedges]

Is there any reason why you could NOT use molten salts in a solar vacuum tube?

Yes, if you mean common salts like NaCl

Evacuated tubes stagnate around 250 to 350 degrees F.
NaCl melts at 1470 degrees F.

You need a salt (or other crystalline material) with melting point above the temperature you can produce.
There are some exotic ($$$) materials that melt at lower temperatures in the right range.

With concentrators you could melt materials of high temperature. They will require active tracking to keep the sun focused.
Also need materials that can contain the melted substance. Likely melted salt will be quite corrosive.

 

[bengelha]

Yes, if you mean common salts like NaCl

Evacuated tubes stagnate around 250 to 350 degrees F.
NaCl melts at 1470 degrees F.

You need a salt (or other crystalline material) with melting point above the temperature you can produce.
There are some exotic ($$$) materials that melt at lower temperatures in the right range.

With concentrators you could melt materials of high temperature. They will require active tracking to keep the sun focused.
Also need materials that can contain the melted substance. Likely melted salt will be quite corrosive.

I was thinking 60% sodium nitrate and 40% potassium nitrate with a melting temperature nearer to 300 C. Also, as you suggest, it will need concentrated solar.

I think the key question is the maximum temperature of the selective surface in the vacuum tube. Do you know what the over the counter max temps are? I've seen 300/350C, which is maybe the binding constraint.

 

[Hedges]

May I suggest you don't also order fuel oil?
You might hear a knock on the door ...

My knowledge of chemistry is limited to blowing up the junior high school chem lab, and managing to get a passing grade in JC Chem 1A.
I've absorbed some published literature on solar-thermal and evacuated tube collectors, don't know the particular temperature limits except I saw a stagnation temperature for the heat pipe.
As for materials, got to watch the TCE because higher temperatures would cause more expansion and stress on glass. Mostly I saw double-wall glass thermos tubes. Alternate construction could have an inner metal tube and an accordion fold metal seal for more compliance. might need a radiating surface (heat sink) to limit what temperature ends of the glass reaches.

You could probably fabricate a horizontal trough reflector, glass tube, inner metal tube, some thermally insulating seal (like silicone or teflon gasket between two discs.) A vacuum pump could restore vacuum as air leaks in.

By the way, amorphous glass block UV but quartz passes it, so you might want quartz to capture that end of the spectrum.

 

[WillyP]

I was thinking 60% sodium nitrate and 40% potassium nitrate with a melting temperature nearer to 300 C. Also, as you suggest, it will need concentrated solar.

I think the key question is the maximum temperature of the selective surface in the vacuum tube. Do you know what the over the counter max temps are? I've seen 300/350C, which is maybe the binding constraint.

everything I have read say 185°F

 

[renewablejohn]

Yes it can quite easily be done using themal oil. I would not recommend unless you are used to dealing with liquids at high temperatures in excess of 200C. Commercial systems using thermal oil are well developed with the bakery industry using thermal oil ovens. For storage of hot thermal oil insulated shipping containers used in the Asphalt industry are widely available. From a domestic point of view hot thermal oil at 200C and water in excess of 100C do not mix it is a recipe for a steam bomb and I would not recommend. Again commercially thermal oil steam evaporators are available to do the heat exchange safely but the systems are not cheap.

 

[acdoctor]

This is the biggest problem. There are so many variables, it is almost impossible to simply plug in a mathematical formula and just calculate the needs. One of the biggest unknowns is; what is the normal discharge temperature of the water in a slab with pex pipes. Of course computing that requires knowing all sorts of other unknowns, like the slab temperature, the length of the pipes,

Depending upon the floor coverings 80 to 85 degF entering the slab. Most use a mixing valve at the zone manifold. Or course wood is the most sensitive.

 

[Zil]

We last heard from WillyP near the end of May 2021.
Wondering how they did with solar heat storage.