Author Topic: Evacuated tube opinions - comments  (Read 7827 times)

0 Members and 1 Guest are viewing this topic.

Dave B

  • Hero Member
  • *****
  • Posts: 1000
    • DCB Energy Systems
Evacuated tube opinions - comments
« on: October 26, 2011, 10:25:41 PM »
  My next project will be solar hot water with the intent to supplement my home heating which is propane forced air. My location is Western NY and with limited sun I have been looking into most of the options and keep reading about evacuated tubes as being more efficient in areas with colder temps.

  Does anyone have experience with this type of system and if so can they be used as a drain back open system as well as the closed loop glycol type ? I seem to read conflicting reports as it seems that very high temps are reached at the manifold and a circulating pump failure of a drain back system during sun could cause overheating and possible failure ? The sealed system would always have liquid throughout the system even with a circulating pump failure.

   I am leaning toward an open drain back system as this seems to offer more options for water storage, heat exchangers etc. and seems a bit less complex. Any comments from those experienced with evacuated tube hot water and or vs. flat plate collectors is appreciated, thank you.  Dave B
DCB Energy Systems
http://dcbenergy.com/
Kuhns Bros. Log Homes Inc.
http://www.kuhnsbrosnypa.com/

rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #1 on: October 27, 2011, 03:00:49 AM »
  My next project will be solar hot water with the intent to supplement my home heating which is propane forced air. My location is Western NY and with limited sun I have been looking into most of the options and keep reading about evacuated tubes as being more efficient in areas with colder temps.

  Does anyone have experience with this type of system and if so can they be used as a drain back open system as well as the closed loop glycol type ? I seem to read conflicting reports as it seems that very high temps are reached at the manifold and a circulating pump failure of a drain back system during sun could cause overheating and possible failure ? The sealed system would always have liquid throughout the system even with a circulating pump failure.

   I am leaning toward an open drain back system as this seems to offer more options for water storage, heat exchangers etc. and seems a bit less complex. Any comments from those experienced with evacuated tube hot water and or vs. flat plate collectors is appreciated, thank you.  Dave B

I have a 60-tube system here.
There are two basic types: wet tube and heat-pipe.
I went with wet-tubes for the lower cost and slightly higher operating efficiency.
I don't live in your near-arctic conditions however, so my results may be quite different :)

You *CAN* reach very high manifold temperatures if you wish, or (like me) you can moderate them if you don't need it that hot. Lots of adjustables.

Got any specific questions?
Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

Dave B

  • Hero Member
  • *****
  • Posts: 1000
    • DCB Energy Systems
Re: Evacuated tube opinions - comments
« Reply #2 on: October 27, 2011, 11:35:53 AM »
Hi Ross,

  A couple questions. If you were to do it again would you still use the evacuated tube system or go with flat plate ? Why ? How long have you had the system and where are you located ? Do you know if either the wet tube or heat tube can be used as an open drain back system ? I use very little domestic hot water and plan to heat a large water bank for supplemental heating. With your experience with the evacuated tube do you have any suggestions or ideas for my plan ? If you don't mind (or maybe in an email) who is the manufacturer of your evacuted tubes ?  Thank you Ross,  Dave B.

  My next project will be solar hot water with the intent to supplement my home heating which is propane forced air. My location is Western NY and with limited sun I have been looking into most of the options and keep reading about evacuated tubes as being more efficient in areas with colder temps.

  Does anyone have experience with this type of system and if so can they be used as a drain back open system as well as the closed loop glycol type ? I seem to read conflicting reports as it seems that very high temps are reached at the manifold and a circulating pump failure of a drain back system during sun could cause overheating and possible failure ? The sealed system would always have liquid throughout the system even with a circulating pump failure.

   I am leaning toward an open drain back system as this seems to offer more options for water storage, heat exchangers etc. and seems a bit less complex. Any comments from those experienced with evacuated tube hot water and or vs. flat plate collectors is appreciated, thank you.  Dave B

I have a 60-tube system here.
There are two basic types: wet tube and heat-pipe.
I went with wet-tubes for the lower cost and slightly higher operating efficiency.
I don't live in your near-arctic conditions however, so my results may be quite different :)

You *CAN* reach very high manifold temperatures if you wish, or (like me) you can moderate them if you don't need it that hot. Lots of adjustables.

Got any specific questions?
DCB Energy Systems
http://dcbenergy.com/
Kuhns Bros. Log Homes Inc.
http://www.kuhnsbrosnypa.com/

rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #3 on: October 27, 2011, 03:05:05 PM »
Hi Ross,

  A couple questions. If you were to do it again would you still use the evacuated tube system or go with flat plate ? Why ? How long have you had the system and where are you located ? Do you know if either the wet tube or heat tube can be used as an open drain back system ? I use very little domestic hot water and plan to heat a large water bank for supplemental heating. With your experience with the evacuated tube do you have any suggestions or ideas for my plan ? If you don't mind (or maybe in an email) who is the manufacturer of your evacuted tubes ?  Thank you Ross,  Dave B.

Would I do it again? In a heartbeat.
Would I use evac tube or flat plate? Well, evac tubes are a little more expensive but they produce far "better" hot water I think. They are capable of producing VERY hot water.

The day I put mine together, I was still playing with it and perhaps not affording it the respect due. I carefully primed it before the sun came up (so I didn't get cold water against hot glass), and thought little more of it. Well, I went to have a look at it a couple of hours later and it was like a huge coffee percolator. Shooting a column of boiling water and steam about 10 feet in the air via the overflow vent I had (fortunately) put in "just in case".

After that, I did some considerable rethinking of how to tame it, and have a sensor at the "hot" end of the manifold, which directly controls a circulating pump. By adjusting the temperature the pump starts at means I can control the temperature the water comes out at. I currently have it set at 65 deg C (about 150F), and the pump just cycles as required. Over winter I had a lower setpoint because I was using the heat for hydronics as well as domestic hot water and the lower temperature suited my needs at the time.

I only got around to installing it in April this year (end of our summer - I'm 36 degrees south). I live underground, so the array sits "on the ground" which is really about 5 metres above the house "floor level".

Here's the thing during assembly. Sorry, nothing really as a size guide - but the stand is just over 6' high


Both systems can be used as drainback, although wet tubes obviously hold a lot more water so your drainback system will need a decent capacity. Also, the construction of most wet tubes don't really lend themselves well to drainback because only one end of the tube is open - and in order to prevent air pockets, that end is higher so they tend not to drain out well. (Only really a concern if you get freezing weather and don't want to pump warm water into the tubes to prevent damage).

I have two quite large hot water tanks - around 2,000 litres each (about 500 gallons each). Rough Plumbing diagram follows:

Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

Dave B

  • Hero Member
  • *****
  • Posts: 1000
    • DCB Energy Systems
Re: Evacuated tube opinions - comments
« Reply #4 on: October 27, 2011, 10:14:51 PM »
Wow,

  That's quite a system. Underground, your home is under ground ? Another subject altogether. I am torn between these systems as each has it's positives and negatives. Flat plate drain back I can build much myself if I want but the heat tube obviously is not a do it yourself as far as the panels go. My primary use will be to store hot water for supplemental heating. At about 42 deg. North and in the snow belt we can use heat here for 7 months of the year easily. Spring and Fall months is where this additional heat will really pay as well as of course providing a good bit of my DHW. Thank you for your information,  Dave B.
DCB Energy Systems
http://dcbenergy.com/
Kuhns Bros. Log Homes Inc.
http://www.kuhnsbrosnypa.com/

rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #5 on: October 27, 2011, 10:36:38 PM »
Wow,

  That's quite a system. Underground, your home is under ground ? Another subject altogether.

Aye, it is. Unusual for my area too. Actually we cleared a chunk off the top of the hill, built the house and then filled it all back in and over the top.
One wall is exposed. But as you say, another subject entirely :)

Quote
I am torn between these systems as each has it's positives and negatives. Flat plate drain back I can build much myself if I want but the heat tube obviously is not a do it yourself as far as the panels go.

The better evacuated tubes use borosilicate glass, selective-surface coating on the inner face, and are pulled down to a pretty decent vacuum. Almost impossible to DIY.

Quote
My primary use will be to store hot water for supplemental heating.

For clarification, neither evacuated tubes, nor flat plate collectors are good for storing hot water.


Quote
At about 42 deg. North and in the snow belt we can use heat here for 7 months of the year easily. Spring and Fall months is where this additional heat will really pay as well as of course providing a good bit of my DHW. Thank you for your information,  Dave B.

42 degrees seems entirely practical. You'd probably want to mount them more vertical than I have too, which would assist in collecting the maximum energy when the sun is lowest down.
The downside is, its harder to get to them to clean, should it be necessary, and they present more wind-loading. Of course, if you're putting them straight on your roof, that changes things somewhat.
Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

XeonPony

  • Hero Member
  • *****
  • Posts: 562
  • Country: ca
  • Sanity is over rated!
Re: Evacuated tube opinions - comments
« Reply #6 on: October 31, 2011, 10:38:05 AM »
You can get that vacuum diy, just need a good mechanicle vacuum pump that will go down to 1<Torr then a vapour difusion pump (Or turbomolecular pump) with a cryovapour trap!

And all can be had suprisingly cheap! I'd budget about 2 grand for the hard ware but now you can do stuff like vacuum freeze drier and so on ;)

Making heat pipes is easier don but tricky.
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

Nothing fails like prayer, Two hands clasped in work will achieve more in a minute then a billion will in a melenia in prayer. In other words go out and do some real good by helping!

rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #7 on: October 31, 2011, 06:29:11 PM »
You can get that vacuum diy, just need a good mechanicle vacuum pump that will go down to 1<Torr then a vapour difusion pump (Or turbomolecular pump) with a cryovapour trap!

And all can be had suprisingly cheap! I'd budget about 2 grand for the hard ware but now you can do stuff like vacuum freeze drier and so on ;)

Making heat pipes is easier don but tricky.

You seriously think the average DIY person can pump down a double borosilicate glass tube pair, then while it's still under vacuum heat it to molten and pinch it shut while maintaining the vacuum? And the same person will be able to make and vacuum-apply the base metal and selective-surface coating to the inner tube (only)?

There might be one or two individuals - but remember (and no disrespect intended here) - few can manage to sucessfully make a "simple" PV module from manufactured wafers.

And... given buying tubes already made and tested cost around $10 each.... why would you bother?

"you're dreaming!"
Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

Photon

  • Newbie
  • *
  • Posts: 18
  • Country: ie
Re: Evacuated tube opinions - comments
« Reply #8 on: October 31, 2011, 08:21:53 PM »
Wow! quite a system you got there. Looks like you got everything covered. What control strategy do you use?

Given you can store water at higher temps say 80deg C from the stove, CHP and solar does the ground source heat pump contribute much? i guess the solar contributes to heating in the shoulder months in spring autumn.
Looks like a really flexible system and itlooks like you are aiming for good stratification in the storage tanks. Also, are you storing hot water from Ground heat pump or is it piped straight to underfloor heating coils?
Sorry bout all the questions :)

rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #9 on: October 31, 2011, 08:34:24 PM »
Wow! quite a system you got there. Looks like you got everything covered. What control strategy do you use?

Several layered control systems actually. Mostly it's based on simply switching various pumps on and off, and the ability to dump more heat *somewhere* than I am able to create elsewhere.

Quote
Given you can store water at higher temps say 80deg C from the stove, CHP and solar does the ground source heat pump contribute much?

The GSHP hasn't been implemented yet. All I need is the condenser/compressor/evaporator, but it's been prohibitive to buy at this stage because it's still considered "fringe" or "green" and has the extra 0 on the pricetag. I'm not yet needing it, so will wait for either time and oportunity to make my own, or those who make them commercially to realise they'd do a lot better selling 200 times as many at 1/10 the price. Again with greed ruining things.

The GSHP coils are really only used at the moment as a last-ditch heat-sink - in the (rare) event I'm making too much heat and need to get rid of it quickly.

Quote
I guess the solar contributes to heating in the shoulder months in spring autumn.

And provides all the hot water (bathrooms, laundry, kitchen) in summer when the genset doesn't have to run at all.

Quote
Looks like a really flexible system and itlooks like you are aiming for good stratification in the storage tanks. Also, are you storing hot water from Ground heat pump or is it piped straight to
underfloor heating coils?

It's quite flexible - mostly by design, because we didn't know quite what we were going to have to deal with.
Basically, the tanks are the "core" of the system. Everything else is either a "source of heat" or a "consumer of heat". The hydronics is run from the tanks. In winter, I isolate the two tanks and use one purely for domestic hot water and the other purely for hydronics heating. I can handle the house getting cold, I can't handle cold showers :)

Quote
Sorry bout all the questions :)

Not at all. How else does one learn?
Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

XeonPony

  • Hero Member
  • *****
  • Posts: 562
  • Country: ca
  • Sanity is over rated!
Re: Evacuated tube opinions - comments
« Reply #10 on: November 02, 2011, 09:56:31 PM »
You can get that vacuum diy, just need a good mechanicle vacuum pump that will go down to 1<Torr then a vapour difusion pump (Or turbomolecular pump) with a cryovapour trap!

And all can be had suprisingly cheap! I'd budget about 2 grand for the hard ware but now you can do stuff like vacuum freeze drier and so on ;)

Making heat pipes is easier don but tricky.

You seriously think the average DIY person can pump down a double borosilicate glass tube pair, then while it's still under vacuum heat it to molten and pinch it shut while maintaining the vacuum? And the same person will be able to make and vacuum-apply the base metal and selective-surface coating to the inner tube (only)?

There might be one or two individuals - but remember (and no disrespect intended here) - few can manage to sucessfully make a "simple" PV module from manufactured wafers.

And... given buying tubes already made and tested cost around $10 each.... why would you bother?

"you're dreaming!"


you're talking to some one who used to make CO2 lasers for fun, so no I'm not dreaming I'm doing, all I said is one can do it easily if they invested in some basic tools, I used a murcury diffusion pump due to the lower back streaming durring opperation  and vacuum metalizing isn't rocket scince.

it just ins't cheap for the tools for a one off!  I used mine for a multitude of things, main one way my hoby of making lasers, which happend to lend it self to the same  principles as the vacuum colectors use. and besides you don't neet to sputter the metal reflector in, it isn't a laser a simple slide in sheet would do just as well.

and this damned scroll glitch is happening again!!!!!!!!!!!
« Last Edit: November 02, 2011, 10:00:41 PM by XeonPony »
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

Nothing fails like prayer, Two hands clasped in work will achieve more in a minute then a billion will in a melenia in prayer. In other words go out and do some real good by helping!

rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #11 on: November 02, 2011, 10:16:51 PM »
you're talking to some one who used to make CO2 lasers for fun, so no I'm not dreaming I'm doing, all I said is one can do it easily if they invested in some basic tools, I used a murcury diffusion pump due to the lower back streaming durring opperation  and vacuum metalizing isn't rocket scince.

I don't mean to be argumentative, but the inference in your original post was that "anyone" (interpretation: any of the readers here) would be able to do it "easily". Just because YOU can doesn't mean more than a handful (tops) of the users of this board have the space, power, time, materials, manual dexterity etc to be able to do it. And as I said - the cost of getting the stuff to make them.... probably far outweighs the cost of buying them pre-made.

Quote
it just ins't cheap for the tools for a one off!

QED.

Quote
 I used mine for a multitude of things, main one way my hoby of making lasers, which happend to lend it self to the same  principles as the vacuum colectors use. and besides you don't neet to sputter the metal reflector in, it isn't a laser a simple slide in sheet would do just as well.

A slide-in sheet isn't going to have anything like the thermal transfer properties to the inner tube that a properly applied surface will.
And if you say "It'll only cost perhaps 10% efficiency" - surely that's much of the additional benefits of the evacuated tube down the gurgler before we even start?!

Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

joestue

  • Hero Member
  • *****
  • Posts: 943
Re: Evacuated tube opinions - comments
« Reply #12 on: November 02, 2011, 11:09:35 PM »
I used a murcury diffusion pump due to the lower back streaming durring opperation  and vacuum metalizing isn't rocket scince.

Most of us can't even find mercury for a reasonable price.
Furthermore its not that simple, the thickness of the coating has to be carefully controlled (selective destructive interference), and i'm sure some of that is trade secret.
Your best bet would be finding out what they use for the infrared reflecting coating on the inside of low pressure sodium bulbs, and try using that technology, but its probably going to reflect the IR right back at the sun.
I believe they use TiNxOy

Dave B

  • Hero Member
  • *****
  • Posts: 1000
    • DCB Energy Systems
Re: Evacuated tube opinions - comments
« Reply #13 on: November 03, 2011, 01:06:07 AM »
Let me try this again,

  If anyone has an evacuated tube hot water system and or has comparisons to a flat plate hot water system and would like to comment on this that would be great. Drain back as opposed to closed loop opinons from those using either would be appreciated also. Thank you,  Dave B.
DCB Energy Systems
http://dcbenergy.com/
Kuhns Bros. Log Homes Inc.
http://www.kuhnsbrosnypa.com/

DamonHD

  • Administrator
  • SuperHero Member
  • *****
  • Posts: 3035
  • Country: gb
    • Earth Notes
Re: Evacuated tube opinions - comments
« Reply #14 on: November 03, 2011, 04:26:46 AM »
FWIW, the UK's Energy Saving Trust recently (within the last month, IIRC) published a report showing little different between ET and FP systems that they were monitoring.  I only skimmed the report, but that was what I got from it.

Rgds

Damon

XeonPony

  • Hero Member
  • *****
  • Posts: 562
  • Country: ca
  • Sanity is over rated!
Re: Evacuated tube opinions - comments
« Reply #15 on: November 03, 2011, 08:41:32 PM »
I used a murcury diffusion pump due to the lower back streaming durring opperation  and vacuum metalizing isn't rocket scince.

Most of us can't even find mercury for a reasonable price.
Furthermore its not that simple, the thickness of the coating has to be carefully controlled (selective destructive interference), and i'm sure some of that is trade secret.
Your best bet would be finding out what they use for the infrared reflecting coating on the inside of low pressure sodium bulbs, and try using that technology, but its probably going to reflect the IR right back at the sun.
I believe they use TiNxOy
CO@ lasers opperate at the infared light spectrum, for that we use copper mirriors, for the full and aprtial, first phased 100% mirrior would do well for infared, but for full spectrum it'll most likely be a silver contianing coating, can't recall the best for UV which is the one I'd target as you get plenty of uv even during clowdy days
Ignorance is not bliss, You may not know there is a semie behind you but you'll still be a hood ornimant!

Nothing fails like prayer, Two hands clasped in work will achieve more in a minute then a billion will in a melenia in prayer. In other words go out and do some real good by helping!

frackers

  • Sr. Member
  • ****
  • Posts: 292
  • Is it finished yet?
Re: Evacuated tube opinions - comments
« Reply #16 on: November 03, 2011, 09:35:07 PM »
Let me try this again,

  If anyone has an evacuated tube hot water system and or has comparisons to a flat plate hot water system and would like to comment on this that would be great. Drain back as opposed to closed loop opinons from those using either would be appreciated also. Thank you,  Dave B.

I installed an evacuated tube hot water system that doesn't drain back as a result of the following factors
  • Evacuated tube is more efficient in winter when the sun average energy is lower
  • Limited roof space required high efficiency collector if I was to have any useful gain in winter
  • The controller that came with the system looks after electric (immersion) heating, frost protection, overheat protection, Legionnaires' disease cycling

A couple of the disadvantages I have found so far
  • in mid summer I have to dump hot water - if I turn off the circulating pump during the day and then turn it on again the boiling water causes pretty much every joint in the system to leak as the steam valve can't reduce the system pressure fast enough
  • Circulating pump not totally silent
  • Minimum winter temperature at roof level is about -5C so some protection needed (circulates hot tank water to the roof)

Overall I'm happy with the system - my hot water bill over the year is now about 30% of what it was :)
Robin Down Under (Or Are You Up Over)

tomtank

  • Newbie
  • *
  • Posts: 13
Re: Evacuated tube opinions - comments
« Reply #17 on: November 11, 2011, 06:21:43 PM »
We were Thermomax's largest distributor back in the mid '80's.
We did some side by side tests with flat plates and found that the performance of flat plates was close enough that the premium cost was not worth it.

There are several sites online that show this with real time monitoring.
Here is one:
http://chuck-wright.com/logger/tabs.php?deviceids[]=AF00000009F3281D_0_TP1&deviceids[]=AF00000009F3281D_0_TP2&timespec=getIntervalPrevNYears%281%29&action=plot&headerdir=.%2F&starttime=1237348800&endtime=1268971200&tab=3&loggerids[]=485E700680A6C389&plottype=auto&usef=true&aggin=AUTO

Whew, that's a long link.
There is no magic with vacuum tubes. The amount of sunlight square footage that is intercepted is what makes it work.
Flat plates in most areas equal or outperform Tubes.
And you can build flat plates easy and repair them easy.

Tom in Maine

rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #18 on: November 11, 2011, 06:40:47 PM »
We did some side by side tests with flat plates and found that the performance of flat plates was close enough that the premium cost was not worth it.
....
There is no magic with vacuum tubes. The amount of sunlight square footage that is intercepted is what makes it work.
Flat plates in most areas equal or outperform Tubes.

In an ideal world I'd agree that flat plates could intercept more surface area and therefore harness more energy for a given collector size.

However it's not an ideal world. The research I did before I got my tubes seemed to indicate that at *LOW* temperatures - ie, only "warm water", flat plates were probably the better bang-for-buck. However the vast majority of them have substantially higher losses at higher temperatures, and the higher the temperature differential from water temperature to outside, the greater the losses become.

The evacuated tubes on the other hand have far less losses, so if you want HOT water, the evacuated tubes do so much more efficiently.

My impression is also that the evac tubes work far better (than conventional plate collectors) in overcast and days of "poor solar". As usual, I doubt there's any "one size fits all" answer. Required temperatures, pressures, solar conditions, ambient temperatures, local conditions, price etc will all play a part as to which technology is most appropriate.

I know I'd do these evac tubes again without any hesitation in my application - the heat they were able to get on overcast winter days quite surprised me.
Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

GaryGary

  • Sr. Member
  • ****
  • Posts: 411
    • Build-It-Solar
Re: Evacuated tube opinions - comments
« Reply #19 on: December 12, 2011, 08:21:45 PM »
Hi,
Some sources of data on how flats and evac tubes compare:

This section has some comparisons -- some side by side:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Evacuated Tube

By the way, the first link in the section is Alan's large drain back evac tube system.  He switched to flats on his later systems.

You can look at all the common brands of flats and evac tubes at the SRCC site:
https://securedb.fsec.ucf.edu/srcc/collector_search_gl?action=search&msrcc_id=&mstatus=A&moptic_type=0&mstart_date=&mend_date=&results_per_page=400&submit=Summary
They test the performance of all SRCC certified collectors (which is nearly all commercial collectors) and publish the performance on each.
Just be sure to divide the collector output by the area of the collector so that you are comparing 1 sqft of each collector.

This is a calculator that allows you to plug in sun, collector temp, and tank temp and get the efficiency of the collector -- its based on data from the SRCC site:
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

I'll admit to being a bit prejudiced on this myself, but my conclusion would be that for most applications, flats do as well or better than evac tubes.  If you are buying commercial collectors and the prices are about the same than its kind of a toss up unless the clearing snow from evacs is a problem for your installation. 
If you are building the collectors, then commercial flats or evacs are in the $30 a sqft area (plus shipping), and good, carefully made DIY flats  are in the $6 or $7 per sqft area -- this makes it a pretty easy decision if you don't mind the work of making the collector.

I use this system: http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm
Its my 3 rd generation homemade collector design, and I think its performance is good.
http://www.builditsolar.com/Experimental/PEXCollector/SmallPanelTests.htm


One thing to bear in mind is that any solar heating system will benefit from a heat distribution system that does not require really hot water.  If you have a system that needs to have tank temps up in the (say) 160F area to work, its not going to be very efficient whether its a flat or an evac tube -- both suffer from high tank temps, although evacs handle it better.  You are better off with either kind of collector to make a system that can use tank temps from (say) 90F up to 120F ish.

Gary












rossw

  • Hero Member
  • *****
  • Posts: 845
  • Country: au
Re: Evacuated tube opinions - comments
« Reply #20 on: December 12, 2011, 10:11:05 PM »
One thing to bear in mind is that any solar heating system will benefit from a heat distribution system that does not require really hot water.  If you have a system that needs to have tank temps up in the (say) 160F area to work, its not going to be very efficient whether its a flat or an evac tube -- both suffer from high tank temps, although evacs handle it better.  You are better off with either kind of collector to make a system that can use tank temps from (say) 90F up to 120F ish.

90-120F = 32 to 49 degrees C.  At the upper end, it might ok for heating - but certainly where I am, it would be illegal for domestic hot water, which is *REQUIRED* (last I know) to be heated over 70 deg C (158 deg F)  to ensure legionella is killed.

Those of us using the same tank for DHW and hydronic heating, without the option of boost or using the solar as "preheat only" are somewhat restricted in our choices.
Another abused ex-moderator who's made the move to http://www.otherpower.com.au where DNS works, admins do stuff apart from randomly deleting other peoples posts and sigs, and people are nice.
JW: QUIT EDITING MY SIG!

PaulJ

  • Jr. Member
  • **
  • Posts: 92
Re: Evacuated tube opinions - comments
« Reply #21 on: December 13, 2011, 03:12:05 AM »
"90-120F = 32 to 49 degrees C.  At the upper end, it might ok for heating - but certainly where I am, it would be illegal for domestic hot water, which is *REQUIRED* (last I know) to be heated over 70 deg C (158 deg F)  to ensure legionella is killed."

Interesting, here in Vic Australia  the "cold" side of my solar hot water shuts down at 65C;
the LPG booster we use in winter heats to 60C (the standard instantaneous LPG hot water systems heat to 50C, I understand the extra 10C on the solar boosters is to sterilize the water).
This is a commercially purchased system, so I assume it meets some sort of standard.

Despite that, the thought of a tank full of lukewarm water in winter (rainwater collected from the roof at that) breeding who knows what and then only briefly passing through the gas booster worries me a bit.
Does 60C kill legionella? How long does it take?

Paul.

Larsmartinxt

  • Newbie
  • *
  • Posts: 45
  • Country: no
Re: Evacuated tube opinions - comments
« Reply #22 on: December 13, 2011, 09:50:49 AM »

Does 60C kill legionella? How long does it take?

Paul.

at 70C legionella bacteria dies instantly
at 60C 90% dies after 2 min
at 50C 90% dies after 80-124min
48-50C legionella can survive

So yeah it dies at 60C. They set it to 70C to be 100% sure that it's safe.

source: http://en.wikipedia.org/wiki/Legionella