Bad experience with evacuated tubes.

[russ]

The simple fact that total heat collected by a similar foot print flat plate or a vac tube system is about the same means that for most applications it makes no sense to go for complexity.

[LucMan]

I am not sure why people say evacuated tubes are complicated...all the header is, is a pipe plane and simple, the beauty of the Solar Tubes is if any should break the system still functions and each tube can be replaced easily. Surely the assembly process is a little more complicated but the system itself is pretty basic.

The issues with Solar Tubes is frost and snow accumulation and the reason for that is because they are so efficient they don't lose enough heat to shed even the frost in the early mornings. I am looking into ways to remedy this possibly with the use of heat tape. Another advantage is a South facing unit captures sun as it rises and sets....I can still generate heat for my house till about 5pm after the tank has captured all it can. I am in a cold climate and using them for heat and hot water so maximizing energy capture is my main objective.

The reason flat plate collectors shed snow is because of heat loss which is not a good thing to maximize heat capture. I have read comparison that sound like they capture the same amount of energy but, I also noticed they compared from times when the Flat Panel is active which starts later in the day than evacuated tubes by at least an hour on either end of the day.

One design flaw I discovered in the basic set up of most systems is they do not optimize the heat from the collector as an example: Say your system heats your storage tank to 120 Degrees F and the differential is 10 Degrees which means it has to get to 130 Degrees+ to add additional heat but that water could be used to heat your house or basement. To solve this problem I added a 3 way bypass valve and Heat Exchanger with Fan to continue to circulate the water after the tank to extract that wasted heat. In order to get the system to continue circulating the water I take out the bottom tank sensor to trick it into believing the tank temp is cooler than it actually is. This way I can continue to circulate water even at temperatures around 100 Degrees which yields roughly 80 - 85 degree air coming through the exchanger heating my house while still maintaining the heat in the upper portion of the tank. Does anyone know if there are two stage pumping stations out there?

Evacuated tubes are prone to vacuum loss, heat pipe failure from freezing, and rubber seal failure at the header depending on the tube type.
Multiply that by the number of tubes in an array and you have many possible points of failure.
Here in NY state most pro installers won't install an evacuated tube system because NSERDA (the state subsidy authority) requires certified contractors to guarantee the complete system for a period of 5 yrs. That is along time to keep an evacuated tube system online with out any issues. Larger array's can cook the glycol because of the extreme temperatures generated during the summer month's. This requires glycol replacement as the glycol becomes degraded and loses it protection properties, and it's viscosity.

As far as a 2 stage pump, Grunfoss (Apha) and Taco make variable speed pumps that can be set to a specific delta T and may be what you are looking for.

[RJM]

I am not sure why people say evacuated tubes are complicated...all the header is, is a pipe plane and simple, the beauty of the Solar Tubes is if any should break the system still functions and each tube can be replaced easily. Surely the assembly process is a little more complicated but the system itself is pretty basic.

The issues with Solar Tubes is frost and snow accumulation and the reason for that is because they are so efficient they don't lose enough heat to shed even the frost in the early mornings. I am looking into ways to remedy this possibly with the use of heat tape. Another advantage is a South facing unit captures sun as it rises and sets....I can still generate heat for my house till about 5pm after the tank has captured all it can. I am in a cold climate and using them for heat and hot water so maximizing energy capture is my main objective.

The reason flat plate collectors shed snow is because of heat loss which is not a good thing to maximize heat capture. I have read comparison that sound like they capture the same amount of energy but, I also noticed they compared from times when the Flat Panel is active which starts later in the day than evacuated tubes by at least an hour on either end of the day.

One design flaw I discovered in the basic set up of most systems is they do not optimize the heat from the collector as an example: Say your system heats your storage tank to 120 Degrees F and the differential is 10 Degrees which means it has to get to 130 Degrees+ to add additional heat but that water could be used to heat your house or basement. To solve this problem I added a 3 way bypass valve and Heat Exchanger with Fan to continue to circulate the water after the tank to extract that wasted heat. In order to get the system to continue circulating the water I take out the bottom tank sensor to trick it into believing the tank temp is cooler than it actually is. This way I can continue to circulate water even at temperatures around 100 Degrees which yields roughly 80 - 85 degree air coming through the exchanger heating my house while still maintaining the heat in the upper portion of the tank. Does anyone know if there are two stage pumping stations out there?

And, collecting 50 % of the available energy of a 60% cloudy day with evac. tubes may not be worth the extra $$ over collecting 30% of the same 60% cloudy day's energy with a flat plate.

In cloudy climates, there is often not enough sunshine to justify the cost diff. - there just ain't enough energy to collect. In sunny climates, the high temp. performance due to lower loss coeff. is not needed, so paying for performance you don't need is cost ineffective.

[J.P.M.]

100% correct - for residential the vac tube types seem to do better in partly cloudy or colder situations as I remember.

The total heat collected by both types per day is really quite close.

And, collecting 50 % of the available energy of a 60% cloudy day with evac. tubes may not be worth the extra $$ over collecting 30% of the same 60% cloudy day's energy with a flat plate.

In cloudy climates, there is often not enough sunshine to justify the cost diff. - there just ain't enough energy to collect. In sunny climates, the high temp. performance due to lower loss coeff. is not needed, so paying for performance you don't need is cost ineffective.

[LucMan]

As stated above flat plate panels are the simplest most reliable way to heat residential domestic water. If you need more heat add an additional panel.
The foremost reason solar thermal underperforms in northern climates is not because of the type of panel used but the lack of sunshine on the panels.
The low path of the sun limits sun exposure at best to a few hours per day if there is no cloud cover.
Flat plates shed snow more easily than evacuated tubes.
Evacuated tubes with intact vacuums will frost in cold weather delaying heat output for several hours.
The research between flat plate and evacuated tube systems overall efficiencies are available online, as are the failure rates for both systems.

[russ]

To paraphrase your statement, evac. tubes have their place for sure, just not usually for domestic residential applications. Usually, When I see evac. tubes for DHW use, I have often found after speaking with the owners, it was done out of solar ignorance.

100% correct - for residential the vac tube types seem to do better in partly cloudy or colder situations as I remember.

The total heat collected by both types per day is really quite close.

[J.P.M.]

Well JPM I have to say with all the problems I have had and heard of I would think you are correct but, when you see the amount of energy you get out of Solar Evacuated Tubes it is impressive. Even with my system compromised I see 100 degree water at 9am. I am quite surprised they have not resolved this problem sooner. In my research I found that the flat plates lose a lot of energy compared to the EV tubes and don't get hot till later in the morning. My system is on my roof with a long run of pipe so heat loss from the panels would just be adding to the loss specially in the Northern climates. Flat Panels do have their place for sure but, I am trying to get heat to heat my house as well as my hot water and my system is doing that already just not as well as it could until the pipes are fixed.



00

The idea is to meet the needs of an application in the safest, most cost effective way, while making it as serviceable and robust as possible. All this with some regard for aesthetics. Except for safety, all the goals are somewhat interdependent.

One approach often used is to make things as simple as possible (or no more complicated than required). Another way to say that, particularly in the case of solar water heating, is to use appropriate technology.

Some solar water heating applications may require higher working temperatures and thus make the best all around use of the higher efficiencies that evacuated tube units can achieve at higher temperatures. Such applications usually have working temps. above, say, 60 deg. C. or so. Another application might be in severely cold climates where the lower (thermal) loss coeff. of evac. tube units will allow heat collection when/where (fluid temps.- amb. temps. ) commonly exceed 40 to 50 deg. C. or more.

All that however, comes at a price. At lower delta t's, a reasonably well designed flat plate collector may well outperform an evacuated tube unit. The overall loss coeff. will be higher than evac. tube, but the temp. diff. will also be lower. The small extra amount of heat gain from the evac. tube comes at a price premium, and due to added complexity, more maint. and increased probability of problems.

Usually, flat plate is a more appropriate technology (dumber) for heating water than evac. tube. Evac. tubes may be good (appropriate) for some process heating applications. DWH is low tech stuff.

Do not confuse temperature with quantity of heat. It's easier to heat 1000 kg of H2O from 20 deg. C to 60 deg. C. than it is to heat 500 kg. of H2O from 20 C. to 100 C. with either flat plate or evac. tube. The quantity of heat is the same, the quality of the energy is different. The evac. tube is probably better for the 2d application. The flat plate better for the first. Using evac. tubes for DHW applications is overkill.

You want 100 deg. (F., I assume) water in the A.M., insulate the crap out of any storage tanks. Long pipe runs that cannot be made shorter by design can be insulated more heavily. Each application has different design challenges. There are lots of tricks available before evac. tubes, most of them more cost effective.

To paraphrase your statement, evac. tubes have their place for sure, just not usually for domestic residential applications. Usually, When I see evac. tubes for DHW use, I have often found after speaking with the owners, it was done out of solar ignorance.

[RJM]

David,

Who is your supplier and will they sell heat tubes to others who may need replacements? Since this seems to be such a problem I would think someone would be able to supply them as long as they are better than the originals. Thanks.

Hi RJM / JPM,

My supplier has indicated that they will provide replacement copper, no cost to me (I'd still have a fair amount of work - but way less effort than what I've already been through). They claim that the tubes they get now don't have this problem. I don't have the replacement tubes and have not tested them, so right now it is just another of those projects I am hoping to get to in the next few months. I think it should be quite easy to test the bare tubes - measure the lower end, stand them up in a freezer through a few cycles - then remeasure. Obviously, they should not swell at all. On my system, the few tubes I had that were still good - were like new (had not bulged at all). As one would expect, they were either perfect, or split.

Regards, David

[RJM]

Well JPM I have to say with all the problems I have had and heard of I would think you are correct but, when you see the amount of energy you get out of Solar Evacuated Tubes it is impressive. Even with my system compromised I see 100 degree water at 9am. I am quite surprised they have not resolved this problem sooner. In my research I found that the flat plates lose a lot of energy compared to the EV tubes and don't get hot till later in the morning. My system is on my roof with a long run of pipe so heat loss from the panels would just be adding to the loss specially in the Northern climates. Flat Panels do have their place for sure but, I am trying to get heat to heat my house as well as my hot water and my system is doing that already just not as well as it could until the pipes are fixed.

With all the hassle/cost/failures of evec. tube solar thermal collectors vs. the relatively simple, and it seems more reliable closed loop flat plate systems, or more simple yet open loop for non freezing climates, I'd opt and indeed have opted for the flat plate design. 7 yrs. and one expansion joint gasket replaced with no measureable performance deterioration. Solar DWH heating fraction > ~.95.

00

[Chipper5783]

Sadly I am one of the unlucky ones to get defective tubes on my array as well. After reading most of the posts here it is obvious this problem is wide spread which is sad for such great technology. I have come across a company not from China that manufactures replacement heat pipes they say they are good for -36 Degrees Fahrenheit. They also use water and an additive and stated this was the best combination they found after years of research, they too will not say what the additive is but said theirs have survived 10 years without incident in very harsh environments of -23 Degrees and tested them to -36. I mentioned using Acetone but they tried it and they did not hold up over an extended period of time...not sure why.

Since these are coming from overseas I wanted to see how many people might be interested in purchasing them....at this point I am guessing they will cost between $10-12 per pipe with all the shipping and customs etc. Please let me know if you are interested and how many so I can put an order together.

Hi RJM / JPM,

My supplier has indicated that they will provide replacement copper, no cost to me (I'd still have a fair amount of work - but way less effort than what I've already been through). They claim that the tubes they get now don't have this problem. I don't have the replacement tubes and have not tested them, so right now it is just another of those projects I am hoping to get to in the next few months. I think it should be quite easy to test the bare tubes - measure the lower end, stand them up in a freezer through a few cycles - then remeasure. Obviously, they should not swell at all. On my system, the few tubes I had that were still good - were like new (had not bulged at all). As one would expect, they were either perfect, or split.

Regards, David

[J.P.M.]

With all the hassle/cost/failures of evec. tube solar thermal collectors vs. the relatively simple, and it seems more reliable closed loop flat plate systems, or more simple yet open loop for non freezing climates, I'd opt and indeed have opted for the flat plate design. 7 yrs. and one expansion joint gasket replaced with no measureable performance deterioration. Solar DWH heating fraction > ~.95.

[RJM]

Replacement Solar Heat Pipes

Sadly I am one of the unlucky ones to get defective tubes on my array as well. After reading most of the posts here it is obvious this problem is wide spread which is sad for such great technology. I have come across a company not from China that manufactures replacement heat pipes they say they are good for -36 Degrees Fahrenheit. They also use water and an additive and stated this was the best combination they found after years of research, they too will not say what the additive is but said theirs have survived 10 years without incident in very harsh environments of -23 Degrees and tested them to -36. I mentioned using Acetone but they tried it and they did not hold up over an extended period of time...not sure why.

Since these are coming from overseas I wanted to see how many people might be interested in purchasing them....at this point I am guessing they will cost between $10-12 per pipe with all the shipping and customs etc. Please let me know if you are interested and how many so I can put an order together.

[MikeSolar]

Guys, there are over 1000 manufacturers of vacuum tubes in China, some in back yard garages and some in modern factories. Most importers here just buy a container of stuff which, in reality, comes without a warranty so most warrant them themselves (or not, haha). I am told by some Chinese solar people over there, that there is no warranty on tubes as they are so cheap. People just spend a $1 and get another one.

all that said, and as fun as it may be to play with brazing, you wouldn't have this issue with a flat collector. Just sayin...

[lark]

Purchasing help

Running out of what little part time steam I had when first got excited about these evacuated tubes. Seems like only 2 sellers for passive systems in states are Dudadiesel and Solarbank. The latter says goes to negative 35F cause uses copper dust/water in ht but have sold very few and hard to judge their rep. Lot pricier then Duda. Maybe worth it if quality superb but big if. No warranty on tubes. Will the vacuum leak? both use unknown china manufacturers.

Someone else also, can't remember but they cleverly hid fact didn't use heat rods just water in the tube.

Mention here of Apricus, kingman, thermomax but don't see any passive system at least with those tubes

Lots of problems and not sure I trust either of above. Asking people here to please chip in and tell me how, from whom to get a dependable machine won't leak or freeze. Otherwise maybe go flat panel and drain in winter. At least with FP won't have vacuum leaks. Haven't looked too much on those yet but seem pretty scarce also. Who there?

[Chipper5783]

Recharging heat pipes

I suppose there is more than one way to get'er done. I'm pretty low tech and try to work things out with what I've got on hand. The actual cash to complete the recharge is modest (fuel gas, S-solder, some power, 5 cents for acetone). The all in time to clean all the parts, recharge the heat pipe and reassemble the tube is about 1/2 hour - the time certainly adds up (that doesn't count the time to get it wrong, work out the steps, making fixtures/tools). I'd certainly like to hear of techniques that others have used.

I'm landing a bit better than 90% success rate on recharging the tube using the boil off method (using the "pot of boiling water test as the measure of success"). How long the S-solder plug lasts (or if the pinched tube now cracks) remains to be seen. I've been tweaking the procedure a bit with every batch of tubes that I work through.