Air Collector – Black Heat Paint vs Selective Paint?
Question:
For a solar air collector is there an advantage to using selective paint for the collector over normal heat paint? Does selective paint offer that much improvement and hold up better over time?
Response:
> For a solar air collector is there an advantage to using selective > paint for the collector over normal heat paint?
I’m no expert, but the higher the temperature the collector operates at the more heat that is reradiated and the more important a selective coating would be. So this would be less important for forced air solar than domestic hot water. Selective paints themselves reradiate more than a selective coating like black chrome. In other words, consider your application and look for a solution. I just wanted you to have something to mull over before a more experienced hand answers. This appears to be a low volume newsgroup. Cheers, Jeff Does selective paint – Hide quoted text — Show quoted text -> offer that much improvement and hold up better over time?
Response:
> For a solar air collector is there an advantage to using selective > paint for the collector over normal heat paint? Does selective paint > offer that much improvement and hold up better over time?
When you look at the science of selective coatings, it would *seem* like an easy choice. They re-radiate a lot less than they absorb, so should get much hotter. A simply black body absorbing sunlight will re-radiate more and more as it heats up until it radiates as much as it absorbes. For simply coatings that have an emissivity equal to their absorption, this max temperature is <200 F though. A high tech coating could improve on this to get higher temperatures. But, with all that said, those numbers and such *assume* a couple things. First, that no convective/conduction losses occur. Second, that there is no medium between the surface and the sunlight. But that isn’t typical collector construction. So putting a *glass* plate over a collector will reduce the incoming sunlight slightly, but reduce the re-radiated losses dramatically. So a simple black surface *under glass* should get much hotter simply because it can’t re-radiate as much energy back out. Air collectors don’t really need to get that warm. They can be kept relatively cool and maximize the amount of heat gained by absorbing as much sunlight as possible (flat-black surface) and high air flow rates. If the collector surface is kept cool with a high air flow rate, the re-radiation is minimized. Of course, you don’t want the air to lose heat to the outside through contact with cold glass either. So I’ve wondered if a second cheap plastic film to make a ‘double-glazing’ with only one layer of glass could work well. Lower conduction/convection losses due to the insulating air gap, and higher transmittance of light through just a single pane of glass. Sorry, got to rambling a bit there, not sure I answered your question really. daestrom
Response:
So selective paint really just allows you to up the max temperature of the collector correct? So if that is the case you would need to increase airflow to properly cool it.
– Hide quoted text — Show quoted text -> For a solar air collector is there an advantage to using selective > paint for the collector over normal heat paint? Does selective paint > offer that much improvement and hold up better over time? > When you look at the science of selective coatings, it would *seem* like > an easy choice. They re-radiate a lot less than they absorb, so should > get much hotter. A simply black body absorbing sunlight will re-radiate > more and more as it heats up until it radiates as much as it absorbes. > For simply coatings that have an emissivity equal to their absorption, > this max temperature is <200 F though. A high tech coating could improve > on this to get higher temperatures. > But, with all that said, those numbers and such *assume* a couple things. > First, that no convective/conduction losses occur. Second, that there is > no medium between the surface and the sunlight. But that isn’t typical > collector construction. So putting a *glass* plate over a collector will > reduce the incoming sunlight slightly, but reduce the re-radiated losses > dramatically. So a simple black surface *under glass* should get much > hotter simply because it can’t re-radiate as much energy back out. > Air collectors don’t really need to get that warm. They can be kept > relatively cool and maximize the amount of heat gained by absorbing as > much sunlight as possible (flat-black surface) and high air flow rates. > If the collector surface is kept cool with a high air flow rate, the > re-radiation is minimized. Of course, you don’t want the air to lose heat > to the outside through contact with cold glass either. > So I’ve wondered if a second cheap plastic film to make a ‘double-glazing’ > with only one layer of glass could work well. Lower conduction/convection > losses due to the insulating air gap, and higher transmittance of light > through just a single pane of glass. > Sorry, got to rambling a bit there, not sure I answered your question > really. > daestrom
Response:
- Hide quoted text — Show quoted text – > So selective paint really just allows you to up the max temperature of the > collector correct? So if that is the case you would need to increase > airflow to properly cool it. >>For a solar air collector is there an advantage to using selective >>paint for the collector over normal heat paint? Does selective paint >>offer that much improvement and hold up better over time? >When you look at the science of selective coatings, it would *seem* like >an easy choice. They re-radiate a lot less than they absorb, so should >get much hotter. A simply black body absorbing sunlight will re-radiate >more and more as it heats up until it radiates as much as it absorbes. >For simply coatings that have an emissivity equal to their absorption, >this max temperature is <200 F though. A high tech coating could improve >on this to get higher temperatures. >But, with all that said, those numbers and such *assume* a couple things. >First, that no convective/conduction losses occur. Second, that there is >no medium between the surface and the sunlight. But that isn’t typical >collector construction. So putting a *glass* plate over a collector will >reduce the incoming sunlight slightly, but reduce the re-radiated losses >dramatically. So a simple black surface *under glass* should get much >hotter simply because it can’t re-radiate as much energy back out. >Air collectors don’t really need to get that warm. They can be kept >relatively cool and maximize the amount of heat gained by absorbing as >much sunlight as possible (flat-black surface) and high air flow rates. >If the collector surface is kept cool with a high air flow rate, the >re-radiation is minimized. Of course, you don’t want the air to lose heat >to the outside through contact with cold glass either. >So I’ve wondered if a second cheap plastic film to make a ‘double-glazing’ >with only one layer of glass could work well. Lower conduction/convection >losses due to the insulating air gap, and higher transmittance of light >through just a single pane of glass.
The emisive/reradiated loss rises as the collector temp rises. Normal glazing (and I don’t know how you would make a one way IR reflective glazing) will simply let this escape back, no matter how many layers. It only helps with the convective/conductive loss. I think the interesting question is at what temp does the emissive loss excede the convective loss for single pane. I would think this would be well above 100 degree F (closer to the max temp limit mentioned above). Also, it looks like the selective paints must be applied very carefully. No real knowledge here, so take it like a "W" stay the course speech. Cheers, Jeff – Hide quoted text — Show quoted text ->Sorry, got to rambling a bit there, not sure I answered your question >really. >daestrom
Response:
> For a solar air collector is there an advantage to using selective > paint for the collector over normal heat paint? Does selective paint > offer that much improvement and hold up better over time?
Hi, The SRCC site ( http://www.solar-rating.org/ ) tests collectors and gives data on heat collected. You can compare collectors with selective coatings to ones without. For example, comparing 2 Radco brand collectors that appear to be identical except that one use a selective coating and the other uses black paint: Service ClearDay PrtlyCldy ClearDay PrtlyCldy C 38K 25K 45K 30K D 16K 6K 28K 15K Service "C" is for (Tinlet – Tambient) = 20C (36F) Service "D" is for (Tinlet – Tambient) = 50C (90F) The selective surface gain is moderate for service C — about 20%, but increases to around 100%(!) for service D. So, the difference between the collector fluid temperature (which is presumably related to the absorber temperature), and the ambient temperature seems to be a very big factor in how effective the selective coating is? These Radco collectors are flat plate water collectors, but I’m not clear on why they would be running at absorber temps that are that much different than air collectors (as has been suggested)? The air comes out of my air collector at around 120F under full sun conditions — the absorber is running a fair bit hotter than this — I’ll measure it next chance I get, but I’m guessing 160F ish. A water collector probably spends most of its time in the same general area — ie output temps around 120F depending on the storage tank temperature, and since there is excellent thermal contact between the absorber and the fluid, the absorber to fluid temperature difference is probably less than for an air collector. I’d be interested in hearing any further opinions on this, as I am shopping for absorber plates for my new project, and there is a whole lot of difference in price between some of the the selective coating absorbers and the black paint absorbers: For example: Sun-Ray: www.sunraysolar.com/absorber.html non-selective $4.50 sqft Solarenergy: http://www.solarenergy.com/ws400CS.cgi?category=sh_absorbers.html&car… selective $7.50 sqft I am leaning toward the selective coating even though its near twice the price. But, I’d be glad to be talked out of it 
Does anyone know a good (cheap) source of selective coating absorber plates? — Gary www.BuildItSolar.com "Build It Yourself" Solar Projects
Response:
Thanks Gary. How would you go about using those collectors in an air collector? They seem to be made for water based systems.
– Hide quoted text — Show quoted text -> For a solar air collector is there an advantage to using selective > paint for the collector over normal heat paint? Does selective paint > offer that much improvement and hold up better over time? > Hi, > The SRCC site ( http://www.solar-rating.org/ ) tests collectors and > gives data on heat collected. You can compare collectors with > selective coatings to ones without. For example, comparing 2 Radco > brand collectors that appear to be identical except that one use a > selective coating and the other uses black paint: > Service ClearDay PrtlyCldy ClearDay PrtlyCldy > C 38K 25K 45K 30K > D 16K 6K 28K 15K > Service "C" is for (Tinlet – Tambient) = 20C (36F) > Service "D" is for (Tinlet – Tambient) = 50C (90F) > The selective surface gain is moderate for service C — about 20%, > but increases to around 100%(!) for service D. So, the difference > between the collector fluid temperature (which is presumably related > to the absorber temperature), and the ambient temperature seems to be > a very big factor in how effective the selective coating is? > These Radco collectors are flat plate water collectors, but I’m not > clear on why they would be running at absorber temps that are that > much different than air collectors (as has been suggested)? The air > comes out of my air collector at around 120F under full sun conditions > — the absorber is running a fair bit hotter than this — I’ll measure > it next chance I get, but I’m guessing 160F ish. A water collector > probably spends most of its time in the same general area — ie output > temps around 120F depending on the storage tank temperature, and since > there is excellent thermal contact between the absorber and the fluid, > the absorber to fluid temperature difference is probably less than for > an air collector. > I’d be interested in hearing any further opinions on this, as I am > shopping for absorber plates for my new project, and there is a whole > lot of difference in price between some of the the selective coating > absorbers and the black paint absorbers: > For example: > Sun-Ray: > www.sunraysolar.com/absorber.html non-selective $4.50 sqft > Solarenergy: > http://www.solarenergy.com/ws400CS.cgi?category=sh_absorbers.html&car… > selective $7.50 sqft > I am leaning toward the selective coating even though its near twice > the price. But, I’d be glad to be talked out of it > Does anyone know a good (cheap) source of selective coating absorber > plates? > — > Gary > www.BuildItSolar.com > "Build It Yourself" Solar Projects
Response:
- Hide quoted text — Show quoted text -> For a solar air collector is there an advantage to using selective > paint for the collector over normal heat paint? Does selective paint > offer that much improvement and hold up better over time? > Hi, > The SRCC site ( http://www.solar-rating.org/ ) tests collectors and > gives data on heat collected. You can compare collectors with > selective coatings to ones without. For example, comparing 2 Radco > brand collectors that appear to be identical except that one use a > selective coating and the other uses black paint: > Service ClearDay PrtlyCldy ClearDay PrtlyCldy > C 38K 25K 45K 30K > D 16K 6K 28K 15K > Service "C" is for (Tinlet – Tambient) = 20C (36F) > Service "D" is for (Tinlet – Tambient) = 50C (90F) > The selective surface gain is moderate for service C — about 20%, > but increases to around 100%(!) for service D. So, the difference > between the collector fluid temperature (which is presumably related > to the absorber temperature), and the ambient temperature seems to be > a very big factor in how effective the selective coating is?
Look up Stefan-Boltzman law of radiation. The radiation is proportional to the 4th power of the absorber minus the 4th power of ambient. So, larger temperature differentials yield much higher losses. E=K(T 4th – To 4th). (Those are absolute temps, ie Kelvin) I’m sure that’s at least a large part of why solar air collectors are much more efficient at lower temperature differentials. Cheers, Jeff
Response:
> Thanks Gary. How would you go about using those collectors in an air > collector? They seem to be made for water based systems.
Hi, Sorry, I guess I confused things by talking about two things — Subject 1 The original question was whether the selective coatings help on an air collector. The info on the SRCC site says the selective coating makes a lot of difference on water collectors when there is a large temp difference between ambient and the collector. And, it seems to me, that this would also hold true for air collectors because air and water collectors operate at fairly similar temperatures. Subject 2 I’m looking for some water collector absorber plates, and was wondering if anyone has a good source (meaning cheap) of water collector absorber plates with selective coating? Or, whether anyone would like to argue that the selective coating is not worth the extra money? Gary – Hide quoted text — Show quoted text ->>For a solar air collector is there an advantage to using selective >>paint for the collector over normal heat paint? Does selective paint >>offer that much improvement and hold up better over time? >Hi, >The SRCC site ( http://www.solar-rating.org/ ) tests collectors and >gives data on heat collected. You can compare collectors with >selective coatings to ones without. For example, comparing 2 Radco >brand collectors that appear to be identical except that one use a >selective coating and the other uses black paint: >Service ClearDay PrtlyCldy ClearDay PrtlyCldy >C 38K 25K 45K 30K >D 16K 6K 28K 15K >Service "C" is for (Tinlet – Tambient) = 20C (36F) >Service "D" is for (Tinlet – Tambient) = 50C (90F) >The selective surface gain is moderate for service C — about 20%, >but increases to around 100%(!) for service D. So, the difference >between the collector fluid temperature (which is presumably related >to the absorber temperature), and the ambient temperature seems to be >a very big factor in how effective the selective coating is? >These Radco collectors are flat plate water collectors, but I’m not >clear on why they would be running at absorber temps that are that >much different than air collectors (as has been suggested)? The air >comes out of my air collector at around 120F under full sun conditions >– the absorber is running a fair bit hotter than this — I’ll measure >it next chance I get, but I’m guessing 160F ish. A water collector >probably spends most of its time in the same general area — ie output >temps around 120F depending on the storage tank temperature, and since >there is excellent thermal contact between the absorber and the fluid, >the absorber to fluid temperature difference is probably less than for >an air collector. >I’d be interested in hearing any further opinions on this, as I am >shopping for absorber plates for my new project, and there is a whole >lot of difference in price between some of the the selective coating >absorbers and the black paint absorbers: >For example: >Sun-Ray: >www.sunraysolar.com/absorber.html non-selective $4.50 sqft >Solarenergy: >http://www.solarenergy.com/ws400CS.cgi?category=sh_absorbers.html&car… > selective $7.50 sqft >I am leaning toward the selective coating even though its near twice >the price. But, I’d be glad to be talked out of it >Does anyone know a good (cheap) source of selective coating absorber >plates? >– >Gary >www.BuildItSolar.com >"Build It Yourself" Solar Projects
– Gary www.BuildItSolar.com "Build It Yourself" Solar Projects —-== Posted via Newsfeeds.Com – Unlimited-Uncensored-Secure Usenet News==—- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups —-= East and West-Coast Server Farms – Total Privacy via Encryption =—-
Response:
- Hide quoted text — Show quoted text ->Hi, >Subject 2 >I’m looking for some water collector absorber plates, and was >wondering if anyone has a good source (meaning cheap) of water >collector absorber plates with selective coating? Or, whether anyone >would like to argue that the selective coating is not worth the extra >money? >Gary > for a low-cost and highly-effective coating; that you may be able to > apply yourself (if suitably equipped with shop), look at: > http://lib.tkk.fi/Diss/2004/isbn951227003X/
Thanks — Looks promising — I’ll give it a good read — Gary > —-== Posted via Newsfeeds.Com – Unlimited-Unrestricted-Secure Usenet News==—- > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups > —-= East and West-Coast Server Farms – Total Privacy via Encryption =—-
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Response:
>> for a low-cost and highly-effective coating; that you may be able to > apply yourself (if suitably equipped with shop), look at:
http://lib.tkk.fi/Diss/2004/isbn951227003X/ >Thanks — Looks promising — I’ll give it a good read — Gary
You have a scanning electron microscope, and so on? Nick
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said: | You have a scanning electron microscope, and so on? Hmmm. You just stirred up an interesting question. If two smooth, flat metal absorber plates, one normal to the sun’s rays and one aslant, present the same (projected) area to the sun – will one absorb more energy than the other? — Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/solar.html
Response:
The one normal one will collect more heat as it has less surface to reradiate heat away.
– Hide quoted text — Show quoted text -> said: > | You have a scanning electron microscope, and so on? > Hmmm. You just stirred up an interesting question. If two smooth, flat > metal absorber plates, one normal to the sun’s rays and one aslant, > present the same (projected) area to the sun – will one absorb more > energy than the other? > — > Morris Dovey > DeSoto Solar > DeSoto, Iowa USA > http://www.iedu.com/DeSoto/solar.html
Response:
> The one normal one will collect more heat as it has > less surface to reradiate heat away.
An interesting point. Also, just about any surface reflects a different amount depending on the angle of incoming sunlight. More reflection, less absorption. daestrom
Response:
Yeah, I thought of that point, later, but I was asleep…LOL message > The one normal one will collect more heat as it has > less surface to reradiate heat away. > An interesting point. > Also, just about any surface reflects a different
amount depending on the – Hide quoted text — Show quoted text -> angle of incoming sunlight. More reflection, less absorption. > daestrom
Response:
|| The one normal one will collect more heat as it has || less surface to reradiate heat away. My server missed SolarFlaire’s post – I’m glad you quoted to I know to go to Google groups… Re-radiation wasn’t wasn’t my primary concern when I asked; but it certainly is part of the picture. This prompts me to inquire into the manner in which energy is re-radiated. Should it be considered as being emitted normal to the hot surface; or is it more correctly thought of as omnidirectional? | An interesting point. | | Also, just about any surface reflects a different amount depending | on the angle of incoming sunlight. More reflection, less | absorption. This comes closer to my original question. Remember that the surface is smooth and the metal opaque – are you saying that there is a surface effect analogous to critical angle behavior of glass? I’m having difficulty wrapping my head around this… — Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/solar.html
Response:
May not get this post but, there may be totally different reflective amounts of energy at the different spectrums. In other words if you are collecting heat it may a totally different picture than collecting PV electricity. I have not obtained information on this as yet. It may be needed at specific times, for specific interests so I haven’t bothered.
> daestrom (in
– Hide quoted text — Show quoted text – message > || The one normal one will collect more heat as it has > || less surface to reradiate heat away. > My server missed SolarFlaire’s post – I’m glad you quoted to I know to > go to Google groups… > Re-radiation wasn’t wasn’t my primary concern when I asked; but it > certainly is part of the picture. This prompts me to inquire into the > manner in which energy is re-radiated. Should it be considered as > being emitted normal to the hot surface; or is it more correctly > thought of as omnidirectional? > | An interesting point. > | > | Also, just about any surface reflects a different amount depending > | on the angle of incoming sunlight. More reflection, less > | absorption. > This comes closer to my original question. Remember that the surface > is smooth and the metal opaque – are you saying that there is a > surface effect analogous to critical angle behavior of glass? > I’m having difficulty wrapping my head around this… > — > Morris Dovey > DeSoto Solar > DeSoto, Iowa USA > http://www.iedu.com/DeSoto/solar.html
Response:
On Jan 14, 2:45 pm SolarFlaire wrote (captured from Google groups): << May not get this post but, there may be totally different reflective amounts of energy at the different spectrums. In other words if you are collecting heat it may a totally different picture than collecting PV electricity. I have not obtained information on this as yet. It may be needed at specific times, for specific interests so I haven’t bothered. >> Thanks for responding – even though news.qwest.net doesn’t seem to like you I’m collecting heat and my absorber consists of very thin black horizontal aluminum ribbons that have been curved so that any sunlight reflected from one ribbon is "focused" on the back of the ribbon directly above it. At present the ribbons are spaced such that (if all surfaces were mirrored) there would be no (reflective) return path to the glazing. [If you have difficulty visualizing from this description, one of the photos at the link below might clarify somewhat.] The heat is transfered to air circulating (bottom to top, as you’d expect) over both surfaces of these ribbons and this heated air is delivered into the space being heated. I’ve experimented with this approach until I’ve satisfied myself that it’s working well, now I’m trying to zero in on an optimal spacing for the ribbons… — Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/collectors.html
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- Hide quoted text — Show quoted text – > On Jan 14, 2:45 pm SolarFlaire wrote (captured from Google groups): > << May not get this post but, there may be totally > different reflective amounts of energy at the different > spectrums. In other words if you are collecting heat it > may a totally different picture than collecting PV > electricity. > I have not obtained information on this as yet. It may > be needed at specific times, for specific interests so > I haven’t bothered. >> > Thanks for responding – even though news.qwest.net doesn’t seem to > like you > I’m collecting heat and my absorber consists of very thin black > horizontal aluminum ribbons that have been curved so that any sunlight > reflected from one ribbon is "focused" on the back of the ribbon > directly above it. At present the ribbons are spaced such that (if all > surfaces were mirrored) there would be no (reflective) return path to > the glazing. [If you have difficulty visualizing from this > description, one of the photos at the link below might clarify > somewhat.] > The heat is transfered to air circulating (bottom to top, as you’d > expect) over both surfaces of these ribbons and this heated air is > delivered into the space being heated. > I’ve experimented with this approach until I’ve satisfied myself that > it’s working well, now I’m trying to zero in on an optimal spacing for > the ribbons… > — > Morris Dovey > DeSoto Solar > DeSoto, Iowa USA > http://www.iedu.com/DeSoto/collectors.html
It looks like you have a good idea to me. I have thought that the radiation is more omnidirectional and longer wave and is related to the temperature. In other words, the hotter it gets the more radiation given off. I think that is why selective coatings are used on trough concentrators that get well above 300f.
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<snip> | I have thought that | the radiation is more omnidirectional and longer wave and is | related to the temperature. Shucks – and thanks. That means that efficiency should go up if the ribbons are more closely spaced – which means it’ll take more aluminum. Not good since one of my objectives was to minimize the materials energy requirement. I think I try halving the spacing between ribbons and see how performance changes. I’m pretty sure there’s a point of diminishing returns here somewhere; but haven’t a clue where it might be. This business of building/measuring/discarding is beginning to seem frustratingly expensive. How come I can never find a physicist when I really need one? [I _still_ haven't figured out if photons are more like boogers or BBs.] :-/ — Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/
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Morris, Very dark surfaces, like charcoal, have convolutions so that for light to escape from the surface it must reflect off several surfaces. So if each individual surface has an absorptivity of, say, 90%, you get an overall absorptivity of 1 – (1-90%)^n, where n is 5-10. Accoustic absorbing surfaces work this way as well. Your curved vanes are accomplishing some of this convolution on a macroscopic scale, with better permeability to air, which is good. The overall effect is to increase the absorptivity coefficient. There is no change to the emissivity coefficient, nor any change to either the absorptivity or emissivity aperture. Why no change to emissivity aperture or coefficient? Emission is, so far as I understand, omnidirectional. You have more area, but much of your emissions will be absorbed by the other vanes, so there is no net gain of aperture. Now this could be a very interesting benefit. Look at the table (which I’m sure you know about) at http://www.redrok.com/concept.htm The best selective surface there, plated nickel oxide, has absorptivity = 0.92 emissivity = 0.08 ratio = 11.0 Now note that there are surfaces with better emissivity than that. Convolutions change one number but not the other, which gives the possibility of changing the ratio, and thus the stagnation temperature, and best of all… *doing it with a cheaper material than nickel oxide*. Specifically, polished aluminum is absorptivity = 0.09 emissivity = 0.03 native ratio = 3.0 absorptivity, 10 reflections = 0.61 convoluted ratio = 20.3 Aluminum foil is absorptivity = 0.15 emissivity = 0.05 absorptivity, 10 reflections = 0.80 convoluted ratio = 5.35 Aluminum sheet is absorptivity = 0.80 (seems really high, suppose 0.3 instead) emissivity = 0.12 absorptivity, 10 reflections = 0.97 convoluted ratio = 8.08 Chromium absorptivity = 0.30? emissivity = 0.10? absorptivity, 10 reflections = 0.97 convoluted ratio = 9.7 I’m sure you have the idea by now. You’re looking for something cheap, bright, tough, and moderately selective, say, emissivity < 0.12, and absorptivity > 0.2. One difficulty is that you must ensure actual reflection on the way in, not merely scattering (off dirt). That means your surfaces have to stay clean, which is difficult. Perhaps that means a cover glass. Another difficulty is that the convolutions require more material.
Response:
Iain McClatchie (in Iain… Thank you! Your response was/is *most* helpful. | Very dark surfaces, like charcoal, have convolutions so that for | light to escape from the surface it must reflect off several | surfaces. So if each individual surface has an absorptivity of, | say, 90%, you get an overall absorptivity of 1 – (1-90%)^n, where n | is 5-10. Accoustic absorbing surfaces work this way as well. | | Your curved vanes are accomplishing some of this convolution on | a macroscopic scale, with better permeability to air, which is good. | The overall effect is to increase the absorptivity coefficient. | There is no change to the emissivity coefficient, nor any change to | either the absorptivity or emissivity aperture. Yuppers – this is the observed behavior. I had a choice of bright metal or powder-coated in my choice of colors. I opted for the thinnest (opaque) black the factory could produce with the intention of spraying the absorber vanes flat black (my first real goof in the design since all of the coatings I could add actually /degraded/ the absorber performance). | Why no change to emissivity aperture or coefficient? Emission | is, so far as I understand, omnidirectional. You have more area, | but much of your emissions will be absorbed by the other vanes, | so there is no net gain of aperture. Exactly so – this was my original "aha!". However, it would appear that the emissivity aperture can be improved (reduced) by spacing the vanes closer together. There’re a couple of downsides to this: it raises the panel costs and it’ll impede airflow within the panel. At the moment I’m inclined to say: "It’ll cost what it costs," but it’s still bothersome. The airflow trade-off is a PIA and will need to be determined experimentally: I’ll build a pair of collectors and do the analog of a binary search by altering the vane spacing of the least efficient panel each trial until I can’t observe improvement in performance… | Now this could be a very interesting benefit. It has been – and is. [useful info quoted from http://www.redrok.com/concept.htm snipped] | I’m sure you have the idea by now. You’re looking for something | cheap, bright, tough, and moderately selective, say, emissivity | < 0.12, and absorptivity > 0.2. I think I don’t want ‘bright’ and I’m not sure I want ’selective’; but I definitely want to minimize _losses_ resulting from emissivity and maximize the other characteristics. | One difficulty is that you must ensure actual reflection on the | way in, not merely scattering (off dirt). That means your surfaces | have to stay clean, which is difficult. Perhaps that means a | cover glass. To my amazement, dirt hasn’t been a significant degrading influence. The 6′x12′ panel in my shop has sucked up a _lot_ of powder-fine sawdust (although it isn’t very visible to anyone outside) without significant performance drop-off. I suspect that effective aperture plays a much bigger role in this design than I’d imagined… | Another difficulty is that the convolutions require more material. I’m not sure about that. I don’t think I want to attempt those convolutions in the extrusion process because of die wear considerations; but it may be possible to improve the surface with a rolling process that’d net out with the same amount of material. Iain, thank you again. You’ve been more help than you could even begin to guess. — Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto
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Morris> However, it would appear that the emissivity aperture can Morris> be improved (reduced) by spacing the vanes closer Morris> together. Can you elaborate on this? Is the emission from a surface a bit anisotropic? I see a possible way to reduce emissivity slightly, detailed below, but it has nothing to do with aperture. Morris> I think I don’t want ‘bright’ and I’m not sure I want Morris> ’selective’ If you want to get high utilization of the solar energy crossing your aperture, you’ll need absorptivity > 0.2. If you want to get high stagnation temperatures, you’ll need a ratio better than 3-5. I haven’t done any math on this, but if you want to match the stagnation temperature of black chrome, you’ll need an emissivity < 0.10, since after reflections the best your absorptivity could be is 1.0. Since 0.2 > 0.1, you’re looking for a surface that is at least moderately selective. However, you don’t need super-high selectivity. It seems like the selectivity of aluminum or chrome will do fine, and both are known to weather well and are widely available. Anything that’s not a high cost highly selective surface, with an emissivity < 0.10, is going to have an absorptivity < 0.30. It’s also got to be highly reflective to make the multiple-bounce trick work. Any such surface will appear bright. So, I stand by my point: you’re looking for something bright, not black. Morris> I suspect that effective aperture plays a much bigger role Morris> in this design than I’d imagined… What’s "effective aperture"? I suspect your absorptivity/emissivity ratio is not dramatically better than 3 yet, and you are not yet relying on multiple reflections. My guess is that when you get multiple reflections actually working in your favor, you will see a noticeable boost in performance. Maybe you don’t need that boost if you’re happy with what you see now. Morris> it may be possible to improve the surface with a rolling Morris> process that’d net out with the same amount of material. You aren’t going to get more than 2-3 reflections (average) with an extruded surface, and there is no way you’ll get the material thickness down. Rolled aluminum sheet, or chrome-plated steel sheet, would appear to be your best bet. I know you don’t yet buy the idea of using a bright shiny surface to capture sunlight. Give it a try, please, and tell me if I’m wrong. Oh, one other thing: how to get 10 reflections. It’s a lot. – Imagine a "V" pointed at the sun. Imagine a ray of sunshine coming straight in. It’s first reflection (most obviously), and every reflection after (less obviously), will be turned by the interior angle of the V. If you want 10 reflections before that ray goes out, you’ll need an interior angle of 18 degrees. That’s a depth 3.2 times the aperture opening. – But you want to get 10 reflections even when the sun does not come straight in. Let’s say it comes in (and subsequently leaves) at a 45 degree angle. Then you need 10 reflections to turn it 90 degrees and an interior angle of 9 degrees. That’s a depth 6.4 times the aperture opening. – I guess, but haven’t worked through the geometry, that your curved vanes are nearly the same as the straight-sided V above. In particular, the vane length will have to be the same. So if you have a vane every inch, then you’ll need a six-inch wide sheet of metal, curved through nearly 90 degrees. The resulting structure is 3.8 inches deep. If the curved vanes are attached in any way where they lap over one another, they’ll be quite strong. If they are soldered onto a copper tube where they lap, you’ll have an interesting liquid- cooled absorber. Final note: I think you can get your emissivity down further. If you draw a V with a bunch of reflected light rays, you’ll notice that most of the reflections happen deep in the V. So that’s where most of the absorption happens. But most of the emission happens at the outer part of the V. If there is a temperature difference between these two surfaces of the V, you will alter the absorption/emission ratio. Since emission goes as the 4th power of temperature, a difference of even 30 F (say, 170 F vs 200 F) would change your relative point emission by 20%. You might see an overall 10% difference then, which would lower an emissivity coefficient of 0.1 to 0.09. This is a small effect compared to the V thing, but it’s still something. If you draw air from outside in, you’ll get some sort of temperature delta. It will be larger in a less conductive material like chromed steel, and less in aluminum. If you solder a copper tube to the back side of the vanes and absorb the heat with water, you’ll get a reverse temperature gradient, and see more emission than you’d like. I don’t see a way around this yet. Perhaps having the tubing run perpendicular to the vanes, as in a heat pipe CPU heatsink, and midway through the vanes might help. But this gets into fin-tube construction, which is already expensive without adding curves.
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known those depth numbers were way too good. Assume V width is 2 inches. Assume straight in, 10 reflections to turn 180 degrees, internal angle 18 degrees, then depth = 6.3 inches. Assume 45 degrees in and out, 10 reflections to turn 90 degrees, internal angle 9 degrees, then depth = 12.7 inches. That’s more like it. Now since the noon sun swings through 47 vertical degrees, (half of that in the colder six months of the year) and the United States is just 31 N – 47 N (another 16 degrees), and in the three hours on either side of noon the sun swings up/down perhaps another 20 degrees, you really only need to have a "sweet spot" for this collector that’s 50 degrees wide. Furthermore, if your material actually has an absorbtivity of 30%, 8 reflections will do. 8 reflections turning 130 degrees is an internal angle of 16 degrees, depth = 7.1 inches, which is not so costly. Finally, even if the panel is always mounted vertically, you can bias the entry angle of the vanes to point up from the horizon a bit. I think I’m done now.
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Iain McClatchie (in | Morris> However, it would appear that the emissivity aperture can | Morris> be improved (reduced) by spacing the vanes closer | Morris> together. | | Can you elaborate on this? Is the emission from a surface a bit | anisotropic? | | I see a possible way to reduce emissivity slightly, detailed below, | but it has nothing to do with aperture. I’ll try – keep in mind that I’m not a physics cat and that it’s been four decades since I took my last thermodynamics course. Consider me "vocabulary impaired". I’d like to inexpensively capture all energy that makes it past the glazing. My objective is not to make the box (or any part of it) hot – but rather to transfer as much of that energy to the air in the box as quickly and efficiently as I can; and to facilitate the natural convective flow of air through the box. The aluminum vanes that constitute the absorber are fairly thin (only a few thousandths of an inch thick) and seem to warm and cool quite quickly as the incident energy varies in intensity. My gross interpretation of this is that they’re functioning well as both absorbers and exchangers. That interpretation is reinforced by observing that even in the brightest sun, the individual vanes are nearly impossible to discern from the front of the panel. For the photo on my web page, it was necessary to set the absorber in an unpainted, unglazed panel and aim the camera up through the vanes to see anything other than just a black area in the photo. This has led me to consider the entire absorber area as an "input aperture"; which eventually led to consideration of an "output aperture" – and to consider the quality of both of these and how their qualities might be optimized. Obviously, I’d like to maximize the ability of the input aperture to accept energy – and minimize the ability of the output aperture to provide a return path. I think that spacing the vanes more closely will not have a significant effect on the quality of the input aperture. I could be wrong but don’t think so. I think that spacing the vanes more closely may "close the window" at least somewhat on the return path – not so much for the reflected energy which already seems adequately trapped – as for the re-radiated energy. I could be wrong here, too, but in this case I’m not quite so sure. Thus the urge to conduct the experiment. | | Morris> I think I don’t want ‘bright’ and I’m not sure I want | Morris> ’selective’ | | If you want to get high utilization of the solar energy crossing | your aperture, you’ll need absorptivity > 0.2. Ok. I’ll buy that. What I’m attempting to do is catch what’s pitched at me, and to catch what I miss on the first (second, third, …) bounce. My hope is to have an effective absorbtivity that is the sum of all of the "catches" and minimizes re-radiation back to the outside. | If you want to get high stagnation temperatures, you’ll need a ratio | better than 3-5. I haven’t done any math on this, but if you want | to match the stagnation temperature of black chrome, you’ll need an | emissivity < 0.10, since after reflections the best your | absorptivity could be is 1.0. | | Since 0.2 > 0.1, you’re looking for a surface that is at least | moderately selective. However, you don’t need super-high | selectivity. It seems like the selectivity of aluminum or chrome | will do fine, and both are known to weather well and are widely | available. I have a problem here. I don’t understand what you mean by "stagnation" and "selectivity" – and my dictionary doesn’t provide definitions that make sense to me in the current context. I know that I don’t want to block the removal of heat from the panel and I know that I don’t want to exclude any of the incident energy from the capture process. Would you please clarify? | Anything that’s not a high cost highly selective surface, with an | emissivity < 0.10, is going to have an absorptivity < 0.30. It’s | also got to be highly reflective to make the multiple-bounce | trick work. Any such surface will appear bright. So, I stand by | my point: you’re looking for something bright, not black. Aha! This I understand. But I don’t really /want/ this stuff to reflect. I want it to stick (be absorbed) without bouncing. Unfortunately, in the real world some of it is going to bounce, no matter what my preferences. What I decided was that I could live with the reflectivity so long as there was a really poor quality exit path (to the outside world). | Morris> I suspect that effective aperture plays a much bigger role | Morris> in this design than I’d imagined… | | What’s "effective aperture"? Good question. I wish I had a nice, crisp answer. In sloppy terms I think that if emissivity is omnidirectional, then it will be reduced with a closer vane spacing which may cause energy that might have been unproductively radiated to be absorbed by a neighboring vane. | I suspect your absorptivity/emissivity ratio is not dramatically | better than 3 yet, and you are not yet relying on multiple | reflections. My guess is that when you get multiple reflections | actually working | in your favor, you will see a noticeable boost in performance. | Maybe you don’t need that boost if you’re happy with what you see | now. Somehow I suspect that I’ll always be looking for "just a little more," no matter how well it performs. | Morris> it may be possible to improve the surface with a rolling | Morris> process that’d net out with the same amount of material. | | You aren’t going to get more than 2-3 reflections (average) with an | extruded surface, and there is no way you’ll get the material | thickness down. Rolled aluminum sheet, or chrome-plated steel | sheet, would appear to be your best bet. | | I know you don’t yet buy the idea of using a bright shiny surface to | capture sunlight. Give it a try, please, and tell me if I’m wrong. | | Oh, one other thing: how to get 10 reflections. It’s a lot. | – Imagine a "V" pointed at the sun. Imagine a ray of sunshine | coming straight in. It’s first reflection (most obviously), and | every reflection after (less obviously), will be turned by the | interior angle of the V. If you want 10 reflections before that | ray goes out, you’ll need an interior angle of 18 degrees. | That’s a depth 3.2 times the aperture opening. | – But you want to get 10 reflections even when the sun does not | come straight in. Let’s say it comes in (and subsequently | leaves) at a 45 degree angle. Then you need 10 reflections to | turn it 90 degrees and an interior angle of 9 degrees. That’s a | depth 6.4 times the aperture opening. | – I guess, but haven’t worked through the geometry, that your | curved vanes are nearly the same as the straight-sided V | above. In particular, the vane length will have to be the same. | So if you have a vane every inch, then you’ll need a six-inch | wide sheet of metal, curved through nearly 90 degrees. The | resulting structure is 3.8 inches deep. | | If the curved vanes are attached in any way where they lap over | one another, they’ll be quite strong. If they are soldered onto a | copper tube where they lap, you’ll have an interesting liquid- | cooled absorber. Indeed, but we’re talking about serious cost increase here. Ultimately the product must be inexpensive enough for ordinary people to purchase as a practical means of buying down current heating costs without having to mortgage the farm. | Final note: I think you can get your emissivity down further. | If you draw a V with a bunch of reflected light rays, you’ll | notice that most of the reflections happen deep in the V. So | that’s where most of the absorption happens. But most of the | emission happens at the outer part of the V. This is the effect I’m after, even though I’m not using the V geometry. | If there is a temperature difference between these two surfaces | of the V, you will alter the absorption/emission ratio. Since | emission goes as the 4th power of temperature, a difference of | even 30 F (say, 170 F vs 200 F) would change your relative point | emission by 20%. You might see an overall 10% difference then, | which would lower an emissivity coefficient of 0.1 to 0.09. This is | a small effect compared to the V thing, but it’s still something. From where I am now, I don’t think anything affordable is going to produce an overall 10% difference. FWIW, it took a year to get the maximum operating temperature down from 180F to 165F. | If you draw air from outside in, you’ll get some sort of | temperature delta. It will be larger in a less conductive material | like chromed steel, and less in aluminum. | | If you solder a copper tube to the back side of the vanes and | absorb the heat with water, you’ll get a reverse temperature | gradient, and see more emission than you’d like. I don’t see a | way around this yet. Perhaps having the tubing run | perpendicular to the vanes, as in a heat pipe CPU heatsink, | and midway through the vanes might help. But this gets into | fin-tube construction, which is already expensive without adding | curves. I did a bit of experimenting in this direction – and yes, the fin tube isn’t cheap. Iain, thank you agin. You’ve give me much to think about and provided some much-needed "handles" to hang onto. — Morris Dovey DeSoto Solar DeSoto, Iowa USA http://www.iedu.com/DeSoto/solar.html
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