Vacuum tube solar water heating

[tallslenderguy]

i did a quick search on this and didn't get any results, so apologies if this is a redundant question. i've been wanting to install a vacuum tube water heater on my suburban dwelling for a few years now and am ready to dive in. i'm pretty handy, restored 4 historical houses, installed a mini split system a year and a half ago and it's functioning nicely. i really like the efficiency of vacuum tube set ups and will probably get a ready made unit that's SRCC certified and take advantage of the 26% federal tax credit vs making my own. Looking for input and advice from anyone who is savvy or has done this. i really like the vacuum tube systems for their angle efficiency.
i live alone, so my hot water needs aren't high. I currently have a 50g electric hot water heater. I replaced the elements and one thermostat a couple of weeks ago (4500w x 2). The HW heater is in my garage and the roof right over it is very easily accessed from my back deck (pics attached). The roof is at a 10/12 angle (looks like?) and faces 240 degrees SW. i suppose i could angle/side mount the collector to face true south, but am wondering how necessary that is vs the ease and stability of just mounting perpendicular to the roof line? So placement input?

Second question is tank attached to collector or tankless? i'm a little surprised i'm not finding more collector choices in my search... but right now looking at Duda and Sunbank. The Duda is just a collector, the Sunbank has a 40gal tank attached and is double the cost. But that's not really apples to apples comparison, because the Duda has double the collectors at $1250 and 30 tubes vs the Sunbank having 15 collectors at $2500. Almost seems like a no brainer to go with the former? i cannot see any advantage of having the connected tank as they configure it to bypass their tank if you have an existing, otherwise double capacity would be an advantage? Also, having the added weight of a 40 gal tank on my roof might require shoring up the roof.

Sizing. Is 30 tubes overkill? The cost difference between the 20 and 30 tube Duda seems negligible to me in the long run. I live in the Willamette Valley in OR. Half the year its cloudy, the other half it isn't. I'd rather go with thirty tubes if i could just roof mount without getting creative about angles.

Any other sage input would be appreciated. i like to work a project over in my mind before undertaking it, seems to make the speed bumps more manageable once into the project.
thanks

 

[kernel]

We will generally use square feet of collector surface, btu or kilowatts to specify a collector. 30 tubes, 2-3" diameter and 6 or so feet long sounds about right for a conserving household. Most have a single bank of tubes or flat plate collector for dhw and 2-3 person home.

I would heat the tank via a counterflow heat exchanger. Requires differenti thermostat and two circulators. If you have heat exchange tubes in your tank (indirect or boiler storage) you would only need a collector loop circulator.

The tank should then feed your tankless if it can modulate its burner down low. Needs output temp control, if its ratio of inlet water dependant output temps could range wildly depending on storage temp.

I have installed these systems since 2004. Ill look in and help if i can.

 

[kernel]

Are the evacuated tube collectors heat pipe tech? Hope so unless your region does not freeze.

 

[tallslenderguy]

Hi Kernal,
Thanks for the reply, was beginning to feel lonely.
The tubes are 2.28"x 70" and 30 tubes. My understanding is they utilize glycol as a refrigerant in a sealed copper tube inside of the vacuum tube. The two collectors i am looking are either passive or active, that's about as sophisticated as i am getting. In both cases, heat exchanger design looks the same, i.e., the condenser portion of the heat exchanger is the top end of the glycol filled copper tube inserted into the vacuum tube. i don't get the impression that counterflow is a part of the type of systems i am looking at. Well, sort of maybe? With the passive system, tap water is fed into the bottom of the attached tank and heated water taken from the top.
The "active" system really looks similar to me with the exception of a remote tank, but the heat exchange is the same: water flowing over the condenser tip of a copper tube with glycol in it inserted into a vacuum tube. The tech seems the same though, colder water taken from the bottom of the remote tank and circulated through the collector with a circulating pump.
The only real difference i see is the direct contact of the condenser with the water storage tank in the passive system?

 

[kernel]

I can look. Active systems are 'actively' controlled and usually circulated.

Heatpipes can be water or acetone (or other non freezing working fluid, not glycol it becomes acidic and needs changing) filled as a heat transfer fluid.

Some tubes do not have a heat pipe and bulb and would be coupled to the tank with gaskets, grommet, etc.

If you want a tank inside, you generally would use a heat exchanger, shell and tube most common to transfer heat from a sealed collector loop fluid (antifreeze protected water) to the dhw in the tank without mixing fluids.

Drainback would provide freeze protection and stop thermal loss when the sun is not enough to heat. This cannot easily be done in a pressurized tank. Some are manufactured with a drainback tank inside a pressurized dhw tank. $$$.

Drainback can be done where water drains back to the tank and has a loop or several circuits of coils immersed in that heated water. These would be pressurized and contain domestic water to be heated.

If the system needs to have dhw delivery to backup water heater at near or at desired temp around 125ish or better, size it according to use and dont go too big for a single collector. A 40- 60 gallon tank would be plenty for a single collector in most regions in north america.

For taking the most heat on, output temp will be lower, but gain more energy and lose less heat all around with a larger tank. 80-100 gallon.

If you drive the circulators and controller with a pv module, solar domestic hot water has a co-efficient of performance of infinity!

 

[tallslenderguy]

Kernal,
Thank you for your input... i'm having to Google some of your terms that i am unfamiliar with, but i think i am getting a lot of what you are saying, but feel free to dumb it down for me a bit lol. Honestly, i'm just one person without a huge need for a complicated system. i live in a fairly temperate zone, so freezing isn't much of an issue and cannot see it being an issue at all with the passive system.

Your point about glycol becoming acidic is very valuable. I've seen it hinted at by some manufacturers who use glycol, but they minimize it or only mention it in passing, and usually in reference as a result of system overheating. Strange to me that these are SRCC certified systems that qualify for Federal Tax credit. i'm guessing they use food grade glycol to reduce liability in case of leaks, but it doesn't give longevity to the system. I haven't encountered systems like you identify with acetone or just using water (which would be safe), i guess i need to do a deeper search.

It seems drainback in a passive system where the tank is connected to the collector would be a fairly easy fix.

And your point about circulators and controllers powered by pv modules is duly noted. It's choosing and understanding the set up and then finding where to purchase said parts that i need to figure out.

 

[kernel]

A thermosyphon system would be considered passive. Tank above collectors, no automatic valves, circulators, limit switches, etc.

Then you need to get the water. In the third world and some homestead situations the water is bailed or pumped up to the tank and gravity fed to taps. I could see a manufacturer making a heat pipe type header in the side of a tank, where the tank is able to be pressurized, but i have not seen one, only ones where the tank is open to the evacuated tubes and at or open to atmospheric pressure.

A drainback system will pump water from a holding tank to the collector and then exchange its heat to a domestic hot water storage tank or tank heater. It is an active system and requires controller, higher head pump, sensors, etc.

They only seem complicated. Once you see the parts laid out and a thermal flow diagram you should have no issue. Its a tank, pipes, some valves and possibly something to control and drive circulation.

I would search home power magazine for past sdhw or solar domestic hot water how to and pro build articles. The best.

 

[kernel]

I re-read your post. Heating a 50gallon tank as storage with electric backup is closer to what you are looking. Dont know where i got tankless from.

You can heat the tank directly with an exchanger or feed it from a storage tank in series. Depends on your use pattern and budget.

The exchanger taps into the tank high(mid) and low. The tank water circulates (thermosyphon or circulator pump) into an upright cylinder maybe 4" diameter, thats the shell and makes a water jacket around tubes sometimes only one tube, usually 4. The tubes have the colletor fluid in them and would likely be pressurized and circulated counter to the flow of the shell contained water.

Shell and tube counter flow heat exchanger. This is highly likely and would even be used with some drainback setups. There are several types of exchanger that are practical. Most are diy friendly.

Duda has systems 30 tube collector, tank with exchanger, circulators valves control in one station 2,400$ and only needs some pipe or tubing.

Your collector could easily be at roof angle. Plenty of pitch for winter without erector set going on. Check for shade. That spot looked tricky.

Look for a roof pass through called Thermal Deck made by the maker of the Soladeck. Good stuff.

 

[kernel]

And now ive seen a pressurized tank type incorporated into a collector. Thanks.

This heater looks like a winner. 20 tubes, 40 or 53 gallon. Just connect it to your supply and then connect its output to your water heater cold inlet.

this HTML class. Value is https://www.dudadies

 

[kernel]

We have always built with components. Altestore in the past had good supply. Solar panel store i think has them. The duda site was refreshing. Plenty of stainless steel.

 

[kernel]

Link didnt work. Sus304 200 liter passive system. 20 tubes 200 liter.

 

[tallslenderguy]

That's the one i had my eye on. Amazon has it here with "free" shipping (they just add it to the price) or can get it here direct from Duda. Here is the maintenance manual. The system does come with food grade propylene glycol as an antifreeze, but i haven't figured out if it goes inside the copper tube? If it goes in, that means it can be removed and refreshed with new stuff every couple of years to avoid acidity issues? i am wondering if the glycol goes in the copper tube how the tube is sealed? That seems like a potential for failure if it's treaded and exposed to repeated heating and cooling. Still, the system looks pretty cool to me, i really appreciate your looking at it and giving feed back. I like the simplicity of it.

My roof is 11 feet from top to bottom, and 9 feet wide (facing 240 degrees SW) so i think i can avoid any shade issues in the bottom right corner by placing it to the left. If i wanted to face directly south, i'd have to use Erector Set, resting one end on the peak and the other on stilts. Am hoping that the 240 SW facing will provide enough light. Summer is no issue. Winter.... Also, the existing HWH is right behind the orange wall in the pick so the water to and from the collector to the tank will be ~15 foot runs in the garage attic.

 

[tallslenderguy]

The more i learn i am having to decide between an active vs passive system. i am also looking at this active system. The advantage is see is having a storage tank indoors vs outside on the roof. I think there's a larger capacity also because the Duda indoor tank is stainless steel lined and can tolerate temps up to 200f. i doubt that's intended to be a constant, but storing at 150f for example would provide a shorter recovery time and give a much higher capacity. E.g., if your mixing valve is set at 120f, it takes less hot water from the storage tank to achieve that. A conventional HW heater stops heating at what ever temp it's set at, so if you have a 40 gal HWH set at 120f, that's all you have. This can be used for under the floor heating as well with two coils and have excess heat in winter, it could be utilized under a bathroom or kitchen. A lot less weight on the roof too not having the storage tank on the roof. It does require a circulation pump and "working station" to turn the pump on and off. Both require power, but seems could be accommodated by a small pv panel. They're both elegant systems in their own way.

 

[kernel]

The manual covers all of their different systems. Glycol is used in their divorced collector tank systems, cant remember what thay termed it.

The heat pipes come in non-freeze resistant, super freeze resistant and freeze resistant. You have to specify. It said regular is 18deg.F tolerant. And some bit more.

It will not matter for you so much, but at michigan we had a whole bunch of tubes come with some heat pipes containing pure water for areas that do not freeze. They exploded spectacularly and left 20 foot radius messes around every one that blew! Freezing caused them to not circulate properly and super heated the water.

Yes, i like solar storage at 180 deg. F. or so limit. The hotter everything is, there is more heat loss per hour through-out the system. 150 deg will likely be just fine and there will be a ceiling where you could easily make 300 deg water but the rate you remove heat from storage limits the average high to 150 anyway.

These systems require anti-scald of some sort. Any water heater really, but 150-180 deg. F is no joke when it comes from a household tap.

I like the tank inside for aesthetics and access. Plus it freezes hard here and is basically winter 5 months of the year. The environment strips heat off, windy, wet, etc.

This complicates the system, lowers efficiency and increases cost, so we do it anyway!
Just kidding. Ok, im not kidding.

Solar hydronics can be quite elegant indeed. Pretty too. Too bad we covered it all in insulation!

 

[kernel]

Old photographs for the evacuated tube stuff! These were my first couple installs.

 

[Steve777]

A few thoughts on your project:

1. You should think carefully about where you want to store your solar heated hot water. I found on my system that a separate HW tank that the solar heated, piped into the "conventional" HW heater cold input line was pretty inefficient. Typical small household hot water use does not move enough of the solar pre-heated water with typical use to keep the conventional hot water heater from cycling on quite a bit. Even when there is plenty of solar hot water in the tank. My choices were to add controls to circulate solar hot water to the conventional hot water tank as needed, or to unify the hot water storage by heating only one tank, with both solar and conventional backup. The second was the solution I went with, and it has been working well.

Your choices would be to get a solar HW tank with both heat exchanger for the solar and backup capability, or build an external heat exchanger with pumps to add the solar loop heating to an existing conventional hot water heater. Duda and others carry solar tanks with the necessary heat exchanger loops, or there are also heat exchangers which can be inserted into a conventional hot water tank if that has sufficient volume.

2. If you get freezing temps where you are, you need to run antifreeze solution in the collector loop with a heat exchanger, or do a drain back system. Or plan on replacing your collectors after freezes. In most locales, non-toxic anti-freeze is required, and yes it does turn acidic and needs to be tested and replaced when needed (~5 years), just like auto antifreeze does. You might also need to use a dual wall heat exchanger to prevent any anti-freeze from contaminating drinking water, by your building code. There are other non-freezing liquids which could be used instead of non-toxic hydronic anti-freeze; a thin oil used to be used, and there are others. Problem with many of them is they don't have the heat transfer capacity of water or anti-freeze mix, and some are not very "kind" to the other plumbing (pumps, valves, etc).

Drain back systems can work, but they do fail too. They used to be scarcastically known as freeze back systems, so be sure to do it right if you go that route. Also I am not sure if the glass tube collectors are suited to drain back. Most of those get quite a rush of steam and knocking when the water is first pumped up to them in the morning; not sure if glass would take that.

3. You should find out what a typical sized collector area is for your locale. In my area, 8' x 12' of flat plate collectors are usually more than adequate for a average family. But this is sunny Colorado and it's likely diifferent in your area. Also keep in mind your storage tank needs to be sized appropriately too. Today, a fairly sunny 45F day here, my 85g tank was heated to ~155F by 2PM or so. In the summer, it can be at its max temp (about 160F) by a bit after noon. Of course it does not do nearly as well on cloudy days...

Good luck with the project.

 

[hugemoth]

I'm over on the sunny side of Oregon in Bend and have had a Solahart (made in Australia) solar water heater on the roof for about 35 years. When I installed it I got a tax credit from the state that paid for it, all but $56. The heater cost $1200. It's a passive thermosiphon system with an insulated tank at the top. Propolene glycol circulates through the collector and heats water in the tank. It also has an electric heating element in the tank that I've never used. Works well and has survived temps down to -18.

 

[Mr onetwo]

Guys...that thread has been dead for 4 years

 

[Bop]

My old place had a solar hot water system on the roof- as freezing isn't really an issue in northeast Australia, it just circulated the water from the 400l (88 gallons) hot water tank up to the roof collector with a tiny pump (30w) and back into the tank- it has a electric 'boost' element (it is actually a standard electric hot water heater, with the the original mains element switching on and off if required- we left its circuit breaker left off for most of the year (occasionally turned on if we had a week or two of really bad weather, or if we had visitors that overtaxed the small thermal panel we had)

That's it on the right in the left picture, with the eastern bank of PV panels on the far left (second identical set of PV panels facing west out of sight)- the overhead from google earth shows why it had to be a east/west split on the PV lol- the HWS tank is just out of sight bottom right (sits outside like most Aussie ones do)

(this was a shot of the before/after when we had the PV installed,back in 2016- you can see the SHWS boost turned off (orange MCB bottom right in the new fusebox, middle left in the old 'ceramic fuse' shot on the left)


As freezing isn't an issue here (although it sometimes gets down close to 0C, it rarely drops much below that, and not long enough to 'freeze water lines') most Australian installs just circulate the water directly- a T piece is connected to the hot water outlet and a pump runs it up to the rooftop collector, and back down to another T added to the cold water inlet of the pre-existing hot water tank...

 

[Warpspeed]

These types of solar thermal systems were popular thirty to forty years ago. They are not without their problems.

These days solar electric is the way to go, simply because solar panels are now so cheap, and solar panels do not leak, freeze, or corrode the way circulated "fluid" solar thermal systems can. Direct heat losses in very cold climates will be a lot higher with solar thermal.

With sufficient solar panels, and an electrically heated tank, its much easier to obtain and hold high temperatures even in cold cloudy weather. That is something direct solar thermal systems always struggle to do.

At least here in Australia, natural gas has almost ten times the heating value per dollar compared to direct off grid electrical power, even off peak power is relatively expensive compared to natural gas heated hot water.

The most cost effective and efficient system in Australia, is to run a solar electric water preheater, a large storage tank electrically heated by solar panels. For most of the year, the water in that tank will be hot enough, or almost hot enough to use directly.

Hot water output from the preheater, then flows into a normal natural gas powered storage hot water heater.
The natural gas then only has to keep the water at the required storage temperature.
With a well insulated storage tank, the amount of natural gas required to do that will be negligible in summer, and minimal in winter.