Solar (water/hydronics) Heating

[rossw]

After my PV ad battery upgrades last year, I went a couple of months without running the generator at all. Sounds good - but since the waste heat from the generator is what provides our domestic hot water, I was in the silly position of having to light a fire once a week just so we had hot water!

My cure to the problem was to spend up and get a cheap evacuated tube solar water heater. These tubes are made of borosilicate glass. Their construction is much ilke a thermos flask - 1.8m long by 57mm dia (about 6' x 2.5" dia). They're two tubes, one inside the other. The inner tube is coated with a "selective surface" that absorbs as much solar energy as it can, while reflecting and re-radiating as little as it can. The space between the two tubes is a (near) vacuum to prevent heat escape by conduction and convection.

deleted- get a life

A central manifold connects (and seals) the open end of all the tubes. The way I'm using it doesn't require a header tank. It would thermosyphon nicely, but I don't have a tank above it. I really should raise the assembly a bit to catch a few more watt-hours during winter but this will do for now.

This thing will boil water in a few minutes, so I've got a circulating pump to keep it under control. It heats my inside "storage tanks" which are 2x2000 litre (around 1000 us gallons total), so they hold a fair bit of heat. Primary purpose is for domestic hot water, but surplus heat is pumped through the concrete floor slab to provide heating.

My original layout used a simple breather tube to let steam out, and ensure the system couldn't pressurise much, but the water flow rate I needed to keep the water from boiling in midday sun made the riser pipe impractically high, so another option was required.

I tried to get a commercial TPR (temperature/pressure relief) valve, but the lowest pressure anyone had was 800kPa (about 100psi - or about 90psi more than the tubes are rated for). I found some low-pressure ones, but no way could I justify $594.53 on one!! So... the rest of this post is about a "DIY low-pressure relief valve"

I wandered around the plumbing supplies section of the hardware store and collected the bits I thought I could use, and headed home about $20 poorer.
Everything here is stock standard bits - the spring is from the junkbox.





The whole thing was tested and works great. Variable from virtually zero to about 20psi with the spring I used. Now, what do I buy with the $574 odd that I saved?

[ghurd]

Nice!
G-

[zap]

Pretty darn clever... and a nice rack of evacuated tubes too!

Think the steam might effect the spring at all?

[rossw]

Pretty darn clever... and a nice rack of evacuated tubes too!

Think the steam might effect the spring at all?

Thanks. The spring is stainless, so I'm hoping it won't suffer too much. It's a simple thing to replace though, should it cause problems. (unscrew the top cap, replace spring, do top cap back up).

That said, it's a safety device - I'm not anticipating it'll ever go off - just the consequences of it needing to (and not having a relief valve there) scare me!
I have a temperature sensor attached to the top of the manifold. It turns the circulating pump on as soon as it's hot enough to be useful (thats around 46 degrees C) - way before we have steam.

I'll be adding a second sensor to the manifold to measure bottom temperature too, and one to measure ambient temperature. The A/D system I have will handle 8 temperature sensors, has a bunch of extra I/O, and will do a few thousand lines of code - more than enough to make it smart enough to check levels, run the pump when it's useful - or short-cycle the pump to prevent damage when it's cold.
 

[oztules]

??
Nice valve, and cunningly done.........but I'm mystified by your description. Am I to understand that the water runs through your tubes?

The reason I ask, is that I was roped into helping a plumber to install a vacuum tube array for someone over here, and was surprised to find a copper tube was the only thing inside the glass tube. These ones actually boil the water in the copper tube (fully sealed at both ends) which is a tight fit into an end manifold. The steam rises up the tube, and into the Bulb on the end of it (fitted into the manifold press fit with silicon grease to help off load the heat from the bulb to the copper manifold). The steam condenses in the bulb, and transfers the heat to the manifold which has the water flowing in it..... no water at all goes into the tubes.

By your description, I see the water as going through the tubes...... necessitating the anti freeze cycle. These ones had antifreeze inside the copper transfer tube, and the manifold is of course insulated.

 In the static condition and in midday sun, the pressure inside the copper tube thinggy, is sufficient to stop the water from boiling dangerously at these temperatures.

Sounds to me there are several types of evacuated tubes...... neither of which I knew anything about a week ago.



Thanks




oztules

[rossw]

??
Nice valve, and cunningly done.........but I'm mystified by your description. Am I to understand that the water runs through your tubes?

The reason I ask, is that I was roped into helping a plumber to install a vacuum tube array for someone over here, and was surprised to find a copper tube was the only thing inside the glass tube. These ones actually boil the water in the copper tube (fully sealed at both ends) which is a tight fit into an end manifold. The steam rises up the tube, and into the Bulb on the end of it (fitted into the manifold press fit with silicon grease to help off load the heat from the bulb to the copper manifold). The steam condenses in the bulb, and transfers the heat to the manifold which has the water flowing in it..... no water at all goes into the tubes.

By your description, I see the water as going through the tubes...... necessitating the anti freeze cycle. These ones had antifreeze inside the copper transfer tube, and the manifold is of course insulated.

 In the static condition and in midday sun, the pressure inside the copper tube thinggy, is sufficient to stop the water from boiling dangerously at these temperatures.

Sounds to me there are several types of evacuated tubes...... neither of which I knew anything about a week ago.

Yes, you are right. There are at least two reasonably common styles.

Mine is what's called a "wet tube" - the water (or glycol or whatever) flows directly in the tube itself.
The other style - like you encountered - have a heat pipe up the middle.

The wet tube is quicker, easier and cheaper to make, and from all reoprts, marginally higher efficiency.
Normally they'd be installed as a "drainback" system to avoid the potential freezing issue, however due to a number of (probably unique) specifics of my place, it's not practical. (My house is basically an underground bunker - so the tank is 5 or 6 metres below the heater, I can't seal the tanks (they need to be open vented for other reasons), and I don't have a way to contain the 200 odd litres that'd drain back.

You should know by now - nothing I do seems to be normal (but none of it is done without a good reason either!)

[oztules]

"You should know by now - nothing I do seems to be normal (but none of it is done without a good reason either!)"


.......at least you have a unique and different set of problems to those that most surface dwelling folks face........ I'm trying to find a sensible reason for my abnormalities..... but I can't find a sane excuse to cling to.....

Island fever perhaps??

Nicely done anyway.



..............oztules

[MetricCook]

Wow, I never saw someone use a lathe as a mill, you must be an Army dude. How did the one tube break? And you probably have to shut down the panel if one breaks or is there a plug that can be put in?

[rossw]

Wow, I never saw someone use a lathe as a mill, you must be an Army dude.

No, just someone who uses what's available to do what's needed.

How did the one tube break?

In shipping.

And you probably have to shut down the panel if one breaks or is there a plug that can be put in?

Easy to plug. Easiest thing was to just turn the broken tube around and use the sealed end. (They have one open end end one closed end).

[ruddycrazy]

BUMP