Solar Heating system design help?

Kevin_in_Denver_2

Power failures are a little more common nowadays, and can last a couple hours in the middle of a summer day. Control, sensor, and valve failures all add up to a risky deal.

The heat diode is an elegant solution. When the collector fluid (the alcohol) reaches a high enough temperature, it stays completely boiled and almost completely stops transferring heat. So you really shouldn't need to reject any heat, because you're not collecting any more at the higher temps.
BTW I'm not trying be snarky.

bobbyg_2

I have ran the numbers and have a heat loss of 51,000 btu. I want to use a solar system with electric back-up. I have a great location for full access/view of sun. I have info on thermo-dynamics panels

http://www.thermo-dynamics.com/pdfiles/technical/G_Series_tech.pdf

and thermomax evacuated tube panels.
http://www.thermotechs.com/x_space.htm

One of my questions is about overheating? Is it possible to overheat either the closed loop if space heating and dhw is satisfied?

Or is there a need or way to dump any "extra" btu's gained via solar panels? AM I overthinking this!

bobbyg_2

A little more info about this job. 4" slab on grade (3) zones (shop, living area, sunroom)

115 degree water max for sunroom at design conditions. others lower (95)

Have SSU-45 SuperStor indirect to use for domestic hot water or buffer. Electric back-up with photo-cell and manual switch to disengage when sun is shining.

Brad White_83

Forgetting the solar (generation) aspects of it

it is the distribution/consumption side that needs attention.

The generation side has the usual aspects of storage/buffer volume and control inherent to gathering/producing heat, same as any harvested source.

Your question about overheating and storage is pertinent and common to any system, especially radiant.

Overheating can be avoided by proper use of controls, PID especially so you can gauge and adjust call and response over time. With a concrete slab this is more critical than an overlay or staple-up system. The controls have to anticipate the temperatures of the space, slab and rate of rise or loss.

Mixing valves, injection and other means of fluid temperature modulation will take you much farther in your quest for control than adjusting volume, so I would suggest that your consumption side use constant circulation/variable temperature.

When your DHW and space is satisfied, storage is the key. In fact it is on both counts, generation and consumption. How big a savings bank can you afford?

You are not over-thinking it, rather you are thinking intelligently.

My $0.02

Brad

Kevin_in_Denver_2

Sunny Climate?

You should study the amazing website ourcoolhouse.com

I get the impression that he's been disappointed with the solar performance because he gets so much cold, cloudy weather in the hills of Maryland. (His solar is passive, but the conclusions are the same)

Electric backup might be the most expensive backup heat, but you should be fine if you're in the Western US and your solar array is large enough. Since electric backup has the cheapest first cost, it often has the lowest life cycle cost in a sunny climate.

As for overheating, definitely get the manufacturer's recommendations, and report back. They've been in business quite a few years, and should have some knowledge on the best way to handle it. Don't forget you might take a 3 week vacation in the middle of summer.

bobbyg_2

Thanks for the feedback. I will look into everything you said and let you know how it's going.

paul thompson

im doing my own system right now, im using four 30 tube evac solar panels, and hoping to subsidise most (if not all) of my heating costs.
im in utah, and overheating is a big deal, if the system stagnates because nothing calls for heat, it gets way too hot, and you will have issues. the evac solar can make 300 degree water. and you dont want it!!!!
im wasting off my excess heat by sending the water through a serpentine of old baseboard heaters (the ones used for hydronic heat, they have a central tube, and a bunch of fins) i think thermomax makes a heat rejection panel, you need one per 30 tube panel, and they aint cheap. re-used baseboard heaters offers an attractive alternative. you can mount them under the eaves on the shady side of your house, or op by the panels themselves. just treat them as another zone, with an over temp sensor, to open the valve access to them when the system gets too hot.
by the way...im importing evac solar, identical to those on navitron.org.uk but without their name on them, $1200 each panel if you are interested, and i keep spare tubes. e-mail adddress is philandwifeyshouse@msn.com if interested.

Wayco Wayne_2

What about

using a system that drains down when not in use. I read that the new evac tubes can take not having fluid go through the header when it's sunny out and the system is not on. This is because of the fluid in the middle of the tube is filled with a solution that doesnt can take it. WW

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ALH_4

Drainback

Wayne is right. If stagnation is a concern, consider a drainback system before buying a bunch of heat rejection panels. If you use your solar as pre-heat, disabling the electric tank is no longer much of an issue. It's nice to keep everything as simple as possible.

-Andrew

Kevin_in_Denver_2

stagnation

What happens when you're on that vacation and the pump that rejects the heat fails?

A thermosiphon arrangement with the salvaged baseboard sitting higher than the solar panels would certainly be a lovely addition to your home!

Drainback is the best, but I'm not sure how or if the evacuated tubes can handle the thermal shock.

Paul Pollets

Vac tubes

Ther is no fluid in the Vaccuum tubes...just a vaccuum.

The collector header does the heat exchange.

Spend some time at www.thermomax.com

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paul thompson

yes drainback is great (for the dinosaur flat plate collectors you played with 25 years ago.) there is some thoroughly bogus info floating around. there is indeed a fluid inside the tubes. admitted, between the two layers of glass, there is nothing but vacuum. like a thermos flask. you can look into he top of the tube when disassembled. then, into the glass tube, is inserted a sealed skinny copper tube, containing a low boiling point fluid, like alcohol. this makes what is essentially a heat diode. they dont radiate back out. for any area where its gonna freeze, the best option is a closed loop with some type of glycol, or other antifreeze (like noburst). drainback was popular on ye olde dinosaur flat plates, where radiating heat back out at night was an issue. its old technology now.

paul thompson

and how long since pumps have been reasonably reliable???ive got 25 year old grundfos pumps, still going strong.
and for anyone with the slightest bit of imagination, it would be fairly easy to see that the slimline baseboard heaters could fit directly above the header (which is a box Approx 6" x 6" in cross section)
you wont see them from the road at all.

Bob Gagnon plumbing and heating

store the energy!

I too have 120 evac tubes and have no problems with overheating, simply by having a large storage tank. My 1500 gal. tank takes up only a 8'x 8' footprint which allows me to collect and use more solar energy, and also helps me get through long periods of rainy weather. A steep 65 degree angle tilt for the tubes, also limits the amount of collection in the summer and maximizes the solar energy I can collect in the winter with the low sun. As for controls what about a seperate set of controls and a seperate pump that would work off a battery for periods of no power, or control or pump failure. Solar works, simple solutions to simple problems. Bob Gagnon

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Kevin_in_Denver_2

Evac tube stagnation

Bob,

As long as we're on the subject, what do you think would happen to your system if you had no flow in the header (for whatever reason) during a sunny day?

ALH_4

Flat Plates

The pure simplicity and relatively low cost of flat plate collectors seems to make them a good choice at this point in time in my opinion. I realize discussing prices is taboo here, but in relative terms are the Thermomax evacuated tubes price competitive with flat plates? If so, I agree, flat plates are outdated. Do I understand correctly that the Viessmann Vitosol 300 collectors use the same (or similar) tubes as the Thermomax collectors with a different header? The price difference is very substantial between the Viessmann flat plate and evacuated tube collectors.

-Andrew

bobbyg_2

That is a serious issue I will have to research. I appreciate all of your comments.

Especially yours Bob Gagnon -- I like the idea of setting the angle for winter which minimizes summer gain (which I only need for domestic hot water). What 1500 gallon storage tank did you use Bob?

Right now, I am seriously looking at the thermomax evacuated tubes. But will have to carefully consider everything. (read large learning curve)

thanks again.

Bob Gagnon plumbing and heating

evac tube stagnation

if there is now flow the anti-freeze boils and blows off the relief valve, thats why I would like redundant controls. My 1500 gal tank is only framed 2x4s, 3/4" plywood, foam insulation, then lined with heavy duty roll rubber roofing with NO seams. Bob Gagnon

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Ericjeeper

Heat exchanger?

> if there is now flow the anti-freeze boils and

> blows off the relief valve, thats why I would

> like redundant controls. My 1500 gal tank is only

> framed 2x4s, 3/4" plywood, foam insulation, then

> lined with heavy duty roll rubber roofing with NO

> seams. Bob Gagnon

>

> _A

> HREF="http://www.heatinghelp.com/getListed.cfm?id=

> 331&Step=30"_To Learn More About This

> Professional, Click Here to Visit Their Ad in

> "Find A Professional"_/A_

Ericjeeper

Heat exchanger?

Bob what are you using for a heat exchanger? I am doing the same sort of setup. and am considering using a 1 inch supply to my tank and then going to manifolds and using four 50 foot loops of 1/2 inch pex.

hr

Seems like 120 tubes

could easily boil that 1500 gallon on low load conditions? Seems like a lot of horsepower you have at your disposal

Really don't want to over heat that glycol too many times. Keep an eye on the ph if it gets overheated to often. Acicic glycol will go after copper quite aggressively. I'd guess at 10- 12 psi it takes temperatures well over 250 to boil and pop a relief.

I also have heard there are various different evac tube designs and some do in fact circulate the fluid in the tube, others use an alcohol sealed fluid to heat the probe. I've installed a few Sunda evac tubes systems lately and they offer 4 or 5 different versions of evac tubes.

hot rod

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Unknown

as I understand it, in sunny climates regular flat panels work almost as well as evacuated tubes and open up drainback possibilities.

In cloudy or marginal solar climates like up here in the northeast, evacuated tubes are the only way to go.

So say our local solar guys (and viessmann) anyway.

Kevin_in_Denver_2

popping pressure relief valve

If I were a solar contractor putting in evac. tube systems for customers, this would be unacceptable. There's just too many ways that the no flow condition can happen. The customer could accidentally unplug it, etc., etc.

Doesn't the manufacturer have a recommendation?

mark thompson

IF you were an installer you would have something useful to say, a simple bit of research on your part would render your concerns irrelevant. Do you work for an oil company?

Kevin_in_Denver_2

I WAS an installer 25 yrs. ago

Popping the pressure relief was the number one biggest cause of callbacks. Stagnation in the collector loop was always the cause. I went to drainback exclusively because of it. I have researched the problem with evac. tubes, and frankly haven't figured it out yet. Maybe you can have a drainback header?

I'm NOT trying to be a troll, I'm just trying to find an explanation that makes sense to me. I'd seriously thank you for pointing me in the right direction for that simple bit of research.

bobbyg_2

Kevin,
I found thisinteresting link.

http://www.aetsolar.com/AET Literature/Performance - Flat Plate vs Vacuum Tube.pdf

It only gives more ammunition to the flate-plate side.

I'm still taking it all in and trying to learn more.

Kevin_in_Denver_2

Lessons Learned by Tom Lane

That book is the probably the best solar reference for operational results. He came to the same exact conclusions as the German study.
I'm still quite interested in Evac tubes because they're made of mostly glass, with very little copper. This gives them the potential of having the lowest manufacturing cost.

Bob Gagnon plumbing and heating

for stagnation the manufacturer

recommends an inverter that automatically swaps the power supply to a battery in the a power outage, like they have for computers. I think a more reliable solution may be to have a separate collector sensor, set near the boiling point, a separate controller and 2nd pump hooked to a battery to cover you if any of your equipment fails, or if there is a power outage.
Overheating is not an issue, I use more hot water in the summer for laundry and car washing, and my big tank hasn't got above 160 degrees. Maybe my tank losses balance that out? I have ONLY 3 1/2" foam insulation with the tank up against the foundation, and it bleeds heat through the foundation.
My in tank heat exchangers are made out of 5- 1/2"X 60' soft copper supplied with 1" to the collectors. Piped reverse return. I would be concerned with the heat transfer if you used pex. I hear the now have drop in exchangers with finned copper much smaller like a boiler tankless. Bob Gagnon

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Constantin

A couple of thoughts...

... a lot of good points have been brought up by all participants in this discussion.

Sustained high temperatures due to stagnation are a serious problem. Even flat-plate collector longevity ought to improve if the assembly/header is not subject to thermal shock-loading as a slug of relatively cold water hits stangation-temp metals. Thus, continuous circulation once the collector is gaining heat should be encouraged... perhaps ideally with a variable-speed circulator that runs off a ΔT + a differential.

Ensuring that circulation leaves us with a number of choices. In urban areas with very good power supplies, you can probably risk having nothing more than a grid-tied motor. In more rural areas, it may make more sense to have a DC-powered motor that is tied to a PV alongside the solar system. That way, whenever the sun is up, the pump will run. In this case, use the grid as a backup to the PV system.

Having an alarm alert you to stagnation conditions would probably be quite useful, particularly if you have a closed-loop system. I'm not aware of any solar controllers that have such "smart options", something that I think would be cheap and easy to implement (i.e. via some data processing and a dry contact interface for a central alarm).

Testing your HX fluid ph annually is also a really good idea. I plan to do it soon as I know some of the fluid in our system di experience stagnation.

I am not aware of whether evacuated tubes can or cannto be shock-loaded via drainback systems. Yet, given the instructions for the Vitosol 300 from Viessmann, I presume that drainback systems have issues with evac-tubes.

Then again, I don't quite follow the logic of the Viessmann pumping block either, where the PONPC is "downstream" of the grundfos pump, at least the way it seems to be installed in our system.

Speaking of PONPC, I would make use of those Tyfocor containers to capture any spillage from the system. Simply attach a rubber hose to the PRV overflow pipe and run it into the 5-gallon container. For one, it gives visual feedback as to whether an even occurred, secondly it gives you a way to measure mass loss, lastly it allows you to save that previous fluid.

So here is how I would go about sizing your system:

1. Determine how much hot water you need every day. Consider ways to reduce the hot water needs, like GFX heat exchangers, as evac-tube solar collectors are not cheap.
   
2. If your hot water load is intermittent, size the storage tank to meet the peak load, then let the solar system do its magic over the course of the day to recharge it.
   
3. Once you know how many BTUs/kWh you need per day to keep the water flowing, I suggest using the free Excel files from Retscreen to determine what sized system you ought to buy, how to line it up, at what angles, etc.
   
4. Furthermore, Retscreen is a great tool for analysis if you want to do some space heating... that way you can plot output curves by time of year for different slopes and azimuths until you have a combination that works best.
   

Using solar for space heating is usually best achieved by designing good passive solar aspects into the house from the start (i.e. windows that admit winter sun with large overhangs to keep the summer sun out, etc.) Next, consider the ways in which you can design your system not to overheat during the summer. Bob Gagnons approach of using very steep angles is practiced widely. Retscreen will help here also. Lastly, besides using an emitter system that can take advantage of relatively cool water temps like RFH, you'll also probably have to up the storage capacity of your tank to the point where it can store enough BTUs to heat the home for a day or two without sunshine.

Good luck!

David Woycio

Solar drain back

I totally agree with Kevin. Drain back water systems with flat plate collectors (someone called them dinosaurs) IF installed correctly will give years of trouble free service. Always wondered if the tubes on a evac system would ever leak or if the vacume that is set in these is the same for ALL altitudes or ??? A well built collector will last for years and can be repaired if the neighbor kid gets out of hand with the BB gun. These collectors are tough to beat. I have turned systems on in the middle (scares the hell out of the homeowner) of the day from a dead cold and sure it will make a hell of a lot of noise for a while dealing with the 150 degree delta t then its back to normal. I don't like to rely on controls that have to "bleed off" excess heat because my collectors can't take it not to mention the wasted energy running a pump and whatever else.

THE only draw back (and this is really reaching) to a drain back system is they run a little noisier than a closed loop glycol system. Insulate your pipes and hang with antivibration hangers and problem sol-ved!

Flat plates also look better than the evacs. Evacs I think give more an industrial look to the installations. If you have the oportunity to design early with a builder and get all the roof angles correct a flat plate collector array will resemble a series of skylights.

We custom build our own stainless steel storage tanks, custom DHW coils, radiant floor tie-ins, pools and spas all drain back using flat plates I have looked at the evacs and personally haven't drank the cool aid yet. Haven't seen the performance out of these systems that has been advertised. Too many parts and too things to go wrong. We have lots of clients that leave in the middle of the summer and all they need to do is turn the system off. Since 1982 we haven't froze a system and we have installs in some of the worst climates in CO.

My company installs, designs, and services solar thermal heating systems and the number 1 common call we get is to recharge a closed loop system because od the pressure relief, expansion tank, or a leak, or fluid or ???? Very rarely do we get a call on a drain back.

My 2.5c

bobbyg_2

As usual for such a great forum, what great help everyone has been! Thanks everyone for the input. Especially yours, Constantin! I will be working on this in-depth for the next couple of weeks.

Bob Gagnon plumbing and heating

drainbacks' drawbacks

You mention with drainback the pumps are a lot noiser, aren't they also a LOT more expensive to run? A bigger concern in my area is the amount of minerals in the water, in my area, will build up on the inside of my collector manifold drastically reducing heat transfer. I imagine 1500 gal. of water in an open system will do that. Also won't my system wear out over time with all that fresh water being added. It gets down to 15 below here and an electrician I know abandoned his drainback system because it froze and dumped his tank water all over the roof and yard. Bob Gagnon

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Bob Gagnon plumbing and heating

drainbacks drawbacks

You mention that with drainback the pumps are a lot noiser, don't they also cost a LOT more to run? A bigger concern in my area, is the amount of minerals in the water, will coat the inside of my collector manifold, drastically reducing heat transfer. 1500 gallons of water in an open system will do that, and won't all that fresh water wear out my system over time. It gets down to 15 below in my area and an electrician I know abandoned his drainback system because it froze and dumped his tank all over his roof and yard. won't a serarate set of controls and another pump wired to a battery cover me in all situtations? Bob Gagnon

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David Woycio

no drawback here

Bob,
The noise that I was referring to it the noise inherent to a non-pressurized loop being filled and emptied each time the pump is energized or de-energized by the differential controller. As far as the power consumption you are correct the solar pump that is needed for the solar loop for a drain-back system needs to have a higher head rating than a system that would be set up as a closed loop glycol with the exact same loop size(s). Also the pump on a drain back system would have to be bronze or stainless steel to resist rusting with the air that it would see. But the power consumption between say a TACO 009 (drain back) and a 008 (glycol) is 160 watts and 90 watts respectively. If your glycol system utilizes an external H.E. you will need another pump (say a 006) for the water side which will consume 60 watts. With drain back there is no need for a H.E. for the solar side. We are heating the storage tank to whatever temp the collectors are capable of putting out on a given day.

Minerals have not been a concern. If the water tests very high in mineral content we will treat when filling system. Usually it's a sediment problem and we will filter when filing. In 25 yrs I have never seen a collector or plumbing go south because of the mineral content in the water. I have however seen glycol turn to sludge because of repeated stagnation and/or mixing with water at the job site with suspect tap water.

Freezing a drain back solar system is a direct result of installation error. If the system is installed correctly it will drain back each and every time. A simple redundant freeze protection circuit is all that is required to protect the system electrically from freeze. We have systems in some of the coldest climates in the country (Winter Park and Crested Butte, CO) and never have a problem. Contrary to popular belief you can freeze a glycol system. If the glycol system has an external H.E. and the system runs all night (or day) in below freezing temps you will freeze.

Adding more controls to protect a solar system from itself is not what I would prefer to do. What is the power consumption of the battery charger and controls that you are running 24/7?

If you monitor your system closely none of these issues become problems with glycol systems. It's always that day in the middle of the summer that you are away from the house, the power fails, system stops circulating and now we have a problem. If a drain back system stops circulating is just drains back.

What can go wrong will go wrong and all on the wrong day.

ALH_4

Drainback

You could always run a heat exchanger in your tank and use clean water in the collector loop. A drainback system can can be a closed system. The glycol hurts the system performance because of the reduced specific heat of the fluid and the increased viscosity and therefore thickened boundary layer. Once a drainback system is filled it is essentially a closed loop. It just has a vacuum-breaking method to let air in the loop when there is no circulation. The additional head is required only to fill the collectors. Generally two pumps in series will be used and one will be shut off once the system is filled.

-Andrew

David Woycio

vacume breakers and drain-back

It has been our experience that if you have to use a vacuum breaker to get the system to drain back you're system is not designed correctly. As long as the system is sloped to drain back it will.

Vacuum breakers all leak. Mainly because there is not enough pressure to close them off. Worse yet if installed on the return of the collector they could freeze. Also on the return they will open allowing cold air to mix with the return line. You can tell when this is happening by listening for the gurgle.

Most of our system are open to the atmosphere via an overflow. This requires the homeowner to fill the system 3 - 4 times a year. May be too much to handle for some.

I would never shut off a pump on a drain back system. This is how you freeze a system. Timer do dad fails to turn both pumps on, middle of the winter, attic is -30 water won't make it the 130 deg collector and sits in the attic piping at the level the ONE pump will pump to, freeze, next time the system turns on, shower in the master bedroom.


Every tine a control is added to these system you need to think what will happen if it fails.

We have however installed many closed loop drain back water system that work great. Little stainless 15 gallon tank w/H.E. inside tied to the storage is nice system. No water level needs to be maintained.

If you stack pumps you need to be very careful. If one fails you could have problems.

ALH_4

?

I said a vacuum breaking method. You have to let air into the loop when the pump shuts off. It can be as simple as a hole in the stand pipe in the drainback tank. At that point the system is almost closed. I see your point wrt one pump failing.

-Andrew

paul thompson

drai back with evac

dont forget, you can not do a drain back with evac solar, on mine, the manufacturer says no more than 20 mins exposure to sun, without water in the headers.

David Woycio

evacy no drainy

Yep that's the spin on this thread. Check up a couple

Kevin_in_Denver_2

Service calls

I guess you have to do any glycol loop service at night.

hr

I've not had a lot of problems with glyol systems

I've had 4 of my own over the past 15 years Simple to install and maintain, plenty of off the shelf indirect tanks can be used as solar tanks. Many sizes available also. No high head or dual pumps required to start the circulation. No pitched panels or piping, no freeze concerns. No open tanks with legionella concerns

I've not installed a drain back system but I can see pros and cons to both systems.

I'm headed to Joplin today to do a first service ever on a 24 year old glycol solar. Should be an interesting call

hot rod

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