steam as transfer fluid for solar water heaters?

I've found many different ideas for transfer fluids that don't freeze in cold climates, and won't overpressurize, but steam is something we've been using safely since the 1800's and is fairly simple.  Not always safe, but handled properly it is.  It can over-pressurize, but we have systems that have handled that amount of pressure and successfully delivered heat from one place to another place for over a century, and most of us have survived.

In the city, you're looking at having solar water heater way up on a roof a story or two or three above the supplemental boiler (basement)--for better or for worse, this is the problem I need to solve.  The solutions available commercially seem to have a large embodied energy footprint, and the installer couldn't tell me the embodied energy of all that stuff (copper piping, stainless steel, plus the panels) and the glycol (I don't know how to make glycol.  Another thing to learn).  

Here's my hairbrained/brilliant idea I"ve never seen anyone discussing--heat steam up with the supplemental heater in your basement, so that it rises up to the panels on your 3rd story roof, gets much hotter and pushes back down again (it can fan out in either direction but once a certain pressure is reached a one-way valve encourages it to move in one direction?), and pushes back downstairs and  fills a big tank back in your basement (the one with your copper coil going through it running your actual water you're going to use, not the heat transferring agent) with very hot steam inside a radiator submerged in water in it (this water in the tank is for thermal mass; steam has very little of that).  Once the steam's got going it doesn't need help from the supplement heat source.  Again, a super-simple centuries-old (?) one-way valves help convince the steam to go mostly in one direction around this circuit.  The fact that the water will cool the steam in the radiator a lot also helps.  Then it gets sent back upstairs again for more heating.

(I guess putting the hot tank up in the attic might make more sense and then letting gravity help things but again that's not an option since  boilers were sent to the basement as punishment decades ago).

I haven't thought this through fully, please poke holes in the idea (before steam pokes holes in actual pipes).

Oh--some insulation to help the hot pipes deliver more of their heat to the tank rather than the neighborhood.  Simple insulation, dry dirt in a wool sleeve??

This can be done with salvaged, repurposed radiator parts.
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Note--don't anyone try this at home.  It is a classic, type 1, boom-squish setup.  


I realize the weakpoint pressure-wise is the solar collector itself, but I'm thinking that can also be a very simple, lame, inefficient thing that is safe.  

Pluses, minuses, and interesting points?

I think you will have issues getting a lame, inefficient, pressure-worthy, affordable solar heater to add useful amounts of heat to steam.

If you have to be absent or shut the system down for Reasons in winter, you may have issues with freezing after all, unless you have successfully gone to great lengths to design out any possible ways for water to remain after cooldown.

I don't know enough about steam to say for sure that getting the hotter steam to come back down will be hard... but it seems like it might be?

I also think that PV panels are a better value for the vast majority of cases at this point, and likely to keep getting better...

I don't think the one-way valves will convince the hot stream to down. You essentially have two vertical pipes with steam of different temperatures in them. The one with a higher temperature will rise and the other fall.

I love steam, but it is a tricky medium to work with.

It sounded like you wanted to get steam to thermos siphon, and it will not do that. You would have to put it under some sort of pressure. You would still have to put a high point vent in any spot that rises otherwise you will get a dead spot or steam hammer, but that is basic steam stuff. But once pressurized, it does not matter how high something is because its running in its own environment.

The other tricky part is that steam requires (3) systems; supply, return, and then condensate return. Every so many feet, x amount of steam is going to cool and drop out as water, so that water has to be collected, and returned to the boiler to be heated again. Any meaningful lengths of run, turns, etc is going to generate decent amounts of condensate, so that has to be dealt with.

But if you want to see something neat, look up Crowley's 6 stroke engine...that is a gasoline/steam engine hybrid! (Intake, compression, power (fuel oil), exhaust, 2nd intake, 2nd power (steam)...a true 6 stroke engine.

The other thing that might work better is a salt water brine water solution instead of steam???

Salt water--brilliant idea!  Probably could still freeze in new enlasiberia here, but worth checking into.

I dont think it needs to thermo siphon, just a) get up from the basement to the roof to start gathering heat better than air can, then b) get back downstairs to the basement to help heat the thermal mass.  

I guess it does not matter which direction anything goes.  It could just be a single pipe that backs up at the top.  

The hammer and high point vent part I didnt understand. I'll try a web search.

I figure all the condensation will just fall dripping back down to the bottom and get reheated.

What about aluminum as a transfer medium? A superhot aluminum pipe well insulated, no fluid.  

Thanks!

Travis Johnson wrote:I love steam, but it is a tricky medium to work with.

It sounded like you wanted to get steam to thermos siphon, and it will not do that. You would have to put it under some sort of pressure. You would still have to put a high point vent in any spot that rises otherwise you will get a dead spot or steam hammer, but that is basic steam stuff. But once pressurized, it does not matter how high something is because its running in its own environment.

The other tricky part is that steam requires (3) systems; supply, return, and then condensate return. Every so many feet, x amount of steam is going to cool and drop out as water, so that water has to be collected, and returned to the boiler to be heated again. Any meaningful lengths of run, turns, etc is going to generate decent amounts of condensate, so that has to be dealt with.

But if you want to see something neat, look up Crowley's 6 stroke engine...that is a gasoline/steam engine hybrid! (Intake, compression, power (fuel oil), exhaust, 2nd intake, 2nd power (steam)...a true 6 stroke engine.

The other thing that might work better is a salt water brine water solution instead of steam???

D Nikolls wrote:I think you will have issues getting a lame, inefficient, pressure-worthy, affordable solar heater to add useful amounts of heat to steam.

If you have to be absent or shut the system down for Reasons in winter, you may have issues with freezing after all, unless you have successfully gone to great lengths to design out any possible ways for water to remain after cooldown.

I don't know enough about steam to say for sure that getting the hotter steam to come back down will be hard... but it seems like it might be?

I also think that PV panels are a better value for the vast majority of cases at this point, and likely to keep getting better...

Pv is better value for money, but I'm looking for simple, buildable by hand too.

The cool water freezing thing--i dont know of steam pipes freezing in a regular radiator steam heating system that's turned off, doesnt the condenser water just drip back down to the basement and get slight geothermal heat? And are way lower pressure than a pipe that would possibly freeze to cracking anyway?

This system would have no water in it unless you turn it on.  That is, turn on the supplemental heat source to jumpstart the heat.  Unless air itself transferred a surprising amount of heat down through the pipes, starting some steam...if it could do that though then I think I would be able to just use air as the transfer medium, and I can't do that because air sucks at transferring heat, right?
Or did I miss what you're saying?

Sebastian Köln wrote:I don't think the one-way valves will convince the hot stream to down. You essentially have two vertical pipes with steam of different temperatures in them. The one with a higher temperature will rise and the other fall.

Right.  So it will essential rise and push the cooler part back down the other side, making things less efficient.   Maybe the single pipe of steaminess would make more sense.

Interesting thoughts.

Have you looked into "heat pipes"?  That's essentially what they do: use liquid to gas transition for better thermal transfer.

Usually rather than water, they use a lower boiling point liquid such as ammonia or acetone.  The gas flows freely, then when it turns back into a liquid, a wicking material takes the fluid back to the heat source.

In fact, a lot of evacuated tube solar collectors use a heat pipe in the collector itself, then use the "cool" end to transfer heat into the system's water.  Could that be used directly?  I don't know.

Also, since boiling point changes with pressure, maybe you could you pull a vacuum on the system and run the steam at a lower, potentially safer, heat?

Most of all, if you try something, let us know the results!!!

Freezing point of cottonseed oil: -55 f

Of fully saturated saltwater (25% salt): -5 f


It's a lot of cottonseed oil but it sure wont freeze!  Now, will it ignite?

I don't remember ever hearing of vapor picking up appreciable heat from a solar collector. Liquid, yes, vapor no. "Wet" steam carries water droplets, but not that many per unit volume - most of the volume is vapor. At a guess, it won't "catch" any solar heat.

Other problems occur to me, but that looks like a dirt simple show stopper: No heat gain.


Regards,
Rufus

Why not the simple solution: Draining the pipes at night?

EDIT: Saltwater sounds like a nightmare if any metals are involved (corrosion!)

Sebastian Köln wrote:Why not the simple solution: Draining the pipes at night?

EDIT: Saltwater sounds like a nightmare if any metals are involved (corrosion!)

I just assumed they used stainless steel? (I do not know, that is a guess).

I saw this being used on some solar set-up where they had a parabolic reflector shining at a tube that was heated by the reflected light. To deal with the freeze/extreme heat issue, they pumped brine through it which I loved because that is so do it yourself. We have done brine for years by pumping brine into the tires of our tractors. Just buy a few bags of Calcium Chloride at $5 a bag, mix it with water and pump it into the tire. It is honestly that easy, and works.

What I liked about the parabolic solar array that I saw was, there was no moving parts. It was welded so that as the sun arced across the sky, the solar array curved with it. I think you had to manually change it from summer to winter mode, but otherwise was fixed. No electronic controllers and motors to track the sun. I liked that simplicity.

Thanks for posts.

I think it does conduct heat pretty well, but has a low low thermal mass, so it would need to circulate someway, or conduct its heat into the target somehow (maybe go through a radiator submerged in the heat tank).  

I'm not sure how draining the pipes at night would work, or what that means, can you say more?  you mean that water would condense when it's cool and drain back down again?

Here's a picture from a nuclear power website about thermal conductivity of steam, I'm not sure what it means.

So steam is great when you have excess heat at the heat source - think a giant gas burner.  In Hot Water solar systems you work very hard to ensure that the system never approaches the boiling point - sure a release valve works, until it doesn't and then you blow out a panel.  I also think that the pressures you can get with steam may exceed those of standard plumbing (about 70 psi), which is why the old steam radiators always seemed to be connected with steel pipes with something that looks like a 1/4" wall.

In that same vein ... we had a 2 panel (~100 sq ft) system in Wisconsin.  In the summer we could boil the water tank, but in the winter ... if we could just preheat water to 50 F that was a victory.  Maybe on the equator you could get steam in the winter?  Adding more panels probably wouldn't work as the Delta Ts are too low.

Stay away from salt water.  You've got to think about the whole system - even if you get pipes that won't corrode (and I think that's pretty much just PVC!  yay, PVC...) then you need a panel and peripherals as well.  There's a reason the panels are made with copper - it transfers the heat to the fluid! - so using another material isn't as efficient.  Pumps for this application are not expecting salt water  - some of the preferred pumps are cast iron and salt water will eat that quickly.

There is a solar water design called "Drain Back" - and it relies on a clever temperature sensor and a valve.  Too cold? dump the water.  Again, works great until it doesn't and everything on the roof freezes.    The advantage of drain back is that it can be an open system - the water just comes from your main feed and gets heated until it wanders down a pipe to your shower.  A glycol or closed system requires a fancier heat-exchange tank - but my installers were convinced that glycol was the superior solution - its easy to measure the glycol in the water and calculate the freezing point, it lasts a long time ( I think at least 5 years), and after install it doesn't require anything to work properly to prevent damage.  And you don't need much glycol - maybe two gallons?

Of course, I wouldn't install solar water now.  PV is so much less expensive now, and paired with a heat-pump water heater you get a solution that: heats your water, produces electricity for the rest of your life, and dumps cold air into your house (in the summer... pipe it outdoors in the winter).  Our WI system worked best in spring and fall ... in the winter we had to rely on the in-line gas heater and in the summer we had so much heat that we had to upgrade the controller to bleed off heat at night!  The 80 gallon storage tank would approach 200 F, and there's really no way to do enough dishes, laundry and showers to deal with that much heat.  PVs don't care about the cold winter and merrily produce power all year.

Ok, makes sense.   Salt is out, but cottonseed oil?


What I'm looking for is a salvaged piece solution,  so assuming the pipes can be taken from a radiator system, and I take two radiators too, one to collect heat (painted black), one to disperse it into the tank down at the bottom,  

How do solar turbine generators work? A web search says they pump the heat from the solar collector to the turbine--this seems strange if the steam is supposed to have enough pressure to _turn_ the turbine.  Is it true the (glass?) collector would burst if it were holding all the pressure?

I'm thinking PV is the best solution for the city situation, but I still want to understand all the options and factors.  Thanks!

Eliot Mason wrote:So steam is great when you have excess heat at the heat source - think a giant gas burner.  In Hot Water solar systems you work very hard to ensure that the system never approaches the boiling point - sure a release valve works, until it doesn't and then you blow out a panel.  I also think that the pressures you can get with steam may exceed those of standard plumbing (about 70 psi), which is why the old steam radiators always seemed to be connected with steel pipes with something that looks like a 1/4" wall.

In that same vein ... we had a 2 panel (~100 sq ft) system in Wisconsin.  In the summer we could boil the water tank, but in the winter ... if we could just preheat water to 50 F that was a victory.  Maybe on the equator you could get steam in the winter?  Adding more panels probably wouldn't work as the Delta Ts are too low.

Stay away from salt water.  You've got to think about the whole system - even if you get pipes that won't corrode (and I think that's pretty much just PVC!  yay, PVC...) then you need a panel and peripherals as well.  There's a reason the panels are made with copper - it transfers the heat to the fluid! - so using another material isn't as efficient.  Pumps for this application are not expecting salt water  - some of the preferred pumps are cast iron and salt water will eat that quickly.

There is a solar water design called "Drain Back" - and it relies on a clever temperature sensor and a valve.  Too cold? dump the water.  Again, works great until it doesn't and everything on the roof freezes.    The advantage of drain back is that it can be an open system - the water just comes from your main feed and gets heated until it wanders down a pipe to your shower.  A glycol or closed system requires a fancier heat-exchange tank - but my installers were convinced that glycol was the superior solution - its easy to measure the glycol in the water and calculate the freezing point, it lasts a long time ( I think at least 5 years), and after install it doesn't require anything to work properly to prevent damage.  And you don't need much glycol - maybe two gallons?

Of course, I wouldn't install solar water now.  PV is so much less expensive now, and paired with a heat-pump water heater you get a solution that: heats your water, produces electricity for the rest of your life, and dumps cold air into your house (in the summer... pipe it outdoors in the winter).  Our WI system worked best in spring and fall ... in the winter we had to rely on the in-line gas heater and in the summer we had so much heat that we had to upgrade the controller to bleed off heat at night!  The 80 gallon storage tank would approach 200 F, and there's really no way to do enough dishes, laundry and showers to deal with that much heat.  PVs don't care about the cold winter and merrily produce power all year.

I am not so sure you are far off Joshua, we are in the Northeast so we spend a ton of money on heating things, whether it be to heat our radiant floor heating systems for our home, or our domestic hot water, etc,, I have always concluded as cheap as grid electric is, it would be a better use of my time/energy to heat water than the current way that I do, which is in using propane. Electricity to heat is expensive, and yet while propane is cheaper than electricity, it still is costly, so I have always wondered, what other way could I heat water?

Obviously I do not know if that is your situation though, but did not want you to think your conclusions were too far off.

I always recommend against the use of glycol in any heating system though because it makes the system instantly 10% less efficient over straight water. To prevent freezing, just keep water moving, flowing water does not freeze so having a back up generator is a very easy solution, and gets your heating efficiency way up.

I have radiant floor heat and the cost of glycol would have been prohibitive, not so much for the cost of glycol, but because of the 10% inefficiency. In the first year alone I would have easily paid for a back up generator just in what extra propane would have cost me. So there a lot of costs and situations to think about.

About draining the water at night:

When the pump is running, it pushes water into the small reservoir at the top (and sending some down the air pipe). The hot water then runs down the other water pipe under gravity. Everything works as expected.
When then pump stops running, the air pipe allows the air from the big reservoir to move up and replace the water between the small reservoir, the solar collector and the big reservoir. Unfortunately not on the other side, so that needs to be kept from freezing (or a drain valve added that opens when the pump doesn't run.

I always recommend against the use of glycol in any heating system though because it makes the system instantly 10% less efficient over straight water.

WOW - I had no idea that glycol did this.  I guess it makes sense - water is just about the perfect substance for holding heat, so adding anything to it is likely to reduce its effectiveness.  Alternately, we don't get to BOTH change the boiling point AND maintain efficiency.  I wonder if this matters in a solar system ... any heat that isn't taken from the panel remains there - its not like it goes up the flue.

Alas, I know nothing about the use of oils in such systems.  I will point out that oil generally doesn't want to remain in its virgin state - it wants to become a more complex and thicker substance, combining with oxygen to turn rancid and eventually a gel.  How quickly this happens almost certainly depends on the presence of oxygen, time, and temperature.  Since you're heating it, Id be very concerned about how that heat is going to accelerate the process (old style Boiled Linseed Oil was literally boiled ... and then it would harden when exposed to air.  Now its "chemically boiled", but the concern is the same ... you don't want to start pumping goo!)

Joshua I really appreciate your desire to rethink this and see how you can salvage it up.  That's very cool - but you're also doing this in a space that a lot of people have thought about for the last 50 years so I'm doubt there's a lot to gain (but please, prove me wrong!).  Re-reading your initial post, I can't see how you could ever create steam and then super-heat it using solar.  Well, not in a domestic setting anyway.  Assuming your steam is just barely there at 225 degrees as it reaches the roof, you'd need to have a solar arrangement that exceeds 225 degrees - and the greater the temperature above 225 degrees the more effective it is.  So yes, if you can build a giant solar oven on the roof and get it to 300 degrees this might work.  But now you're talking about a LOT more than some spare radiators chucked onto the roof.  And of course, if you don't get those temperatures all you're doing is using the radiators to cool the steam....

A note on radiators.  Radiators are meant to lose heat, not gain it.  Right?  Solar panels are giant because you need a LOT of surface area to get meaningful heat from the sun. Radiators have a lot of surface area, but its not solar-available because of the shape.  This makes them VERY inefficient at capturing the sun's energy.  Placing a radiator inside another tank to exchange heat might work although most heat exchangers are simply loops of pipe that don't add substantial friction/turbulence and also avoid stratification inside the pipe - I don't know the flow pattern in a radiator but if its essentially a big box (with fins)  that the fluid/steam goes into and then comes out colder I fear your exchange efficiency will suffer.

Solar turbines are complicated things - and they generally involve having lots of mirrors to concentrate the sun and superheat (generally indirectly) water to force the steam through a turbine.  One design actually uses salt as a thermal battery - apparently you can get salt to phase change to a liquid if you add a LOT of energy.  So yes, solar can be used to create steam, but in the presence of some really expensive mirrors, tracking mechanisms, computer controls, etc.

Back to thinking about PV vs Solar Water ... don't discount the cost of actually running this stuff!  If you're running steel pipe, that's HEAVY and needs a lot of support and its going to be very expensive/difficult to insulate as well.  Comparatively some copper wire is child's play.  PV panels are also very light vs water panels (or radiators!) as they aren't insulated.  PV is in many ways more difficult to understand while solar water is pretty easy to think about (at least at a basic level).  But if you've got pipe and radiators vs k$ for PV, or if you're going to need an electrician to re-wire the house then maybe water makes more sense.