Heat and the Light Spectrums

Light is energy http://en.wikipedia.org/wiki/Light

and the sun provides http://en.wikipedia.org/wiki/Insolation

In an ideal black coating, all the incoming light (the full visible spectrum plus most IR and UV) is absorbed and converted to thermal energy (heat). Some of this heat will re-radiate away according to the blackbody characteristics...

http://en.wikipedia.org/wiki/Black_body

Some more heat will be lost by conduction and convection to the surrounding air. Most of the heat (hopefully) will be carried away by the circulating fluid (hot water?).

For inexpensive home solar projects, a high temperature black paint is usually an acceptable choice. The are some other coatings (surface treatments/oxides) which absorb well but radiate poorly (this is a good thing). Research absorption and emissivity of various materials and coatings along with cost and availability before you decide.

Hope that helps.

I have read your references and they are informative. Actually the entire subject came about because I was looking at the evacuated tubes and wondering why they were more efficient than those with air. The answer was because the radiant heat was not dissipated as there was no air present to carry it away. I hope I am correct thus far. They I wondered if a gas could replace the vacuum in the tubes. One thing led to another and I found myself investigating gasses that do not transmit heat very well and wondered why they did not use something like Argon to replace the air in the tubes. Argon does not transmit heat very well and it seems that it would be easier to use Argon than to create and maintain a vacuum in a tube to improve its heat carrying efficiency. That gave birth to the question of what creates the heat in the copper tubes in the first place, or which spectrum of light was responsible?

You appear to understand why the evacuated tubes are more efficient. Any gas will reduce that efficiency, but some gases are better than others. Used in home windows, Argon is probably not a bad choice.

One additional link if you are interested in how the solar spectral energy is distributed.

http://en.wikipedia.org/wiki/File:Solar_Spectrum.png

If I recall correctly Argon does not impede Infrared and only affects UV radiation. According to the web site you recommended, the majority of our heating comes from Infrared then can I conclude that the use of Argon would be a less cumbersome way of improving panel function than evacuated tubes because of the potential loss of vacuum and cost of evacuating the tubes in the first place. Argon does limit the heat loss from the hot copper piping because it does not transmit heat well. Which gets me back to the original issue, is heat from the heated copper differ from the heat from the light striking the copper? And if it is not different, then Argon would also impede the light energy from reaching the piping in the first place. If it is different then Argon might well be a good replacement for air in the solar hot water panels. I feel like I am in deep mud and it is getting deeper.

RE windows:

Low-E is typically a glass coating that reduces IR transmission. This keeps IR OUT in the summer and IR IN in the winter. Low-E apparently stands for low-emissivity or low-emission, where low-transmission (IR) would be a more proper definition.

Glass coatings may also be used to block UV, but the IR reduction is the primary function.

Gas fill (Argon) is used because it is a relatively poor conductor. I don't think Argon has any "significant" impact on IR and UV transmission so it will not block light energy from getting through.

RE your question:

IMHO you may be over-thinking the issue if you are just planning to build a simple home system.

> Insulate the box for the collector pipes to minimize conduction/convection loss.
> I wouldn't use Low-E glass on front of box. While it reduces IR re-radiation from pipes, it also reflects away incoming solar IR.
> IF practical, use a special pipe coating (other than simple black paint) to minimize re-radiated losses.
> Proper fluid circulation is usually sufficient to minimize re-radiated losses.
> IF really necessary, seal and back-fill box with Argon to further reduce conduction/convection loss.

Unless there is a specific reason to eek out the last possible Watt falling on the solar collector, I'd use black paint, simple insulated box construction, and intelligent circulation of the working fluid. This will usually provide a very useful and cost effective system.

I'm inclined to agree with mjb1962853. I did some experimenting years ago and found that a lot of people were trying to break or set records for efficiency -- but at great expense. Problem with that is that the system never pays for itself! Reradiation can be made less with high-tech stuff, but how much does it cost?

The 30 sq ft panel I build with copper pipes, galvanized steel roofing and flat black paint cost about $10 (30 years ago) and it produced 140 degree F water from 55 deg water at 2 GPM.

I have heard from several sources of people using a coiled up length of black poly water pipe on the roof to heat water, under mains pressure. Is this successful?

I assume it would be a good idea to cover it with glass or plastic and reduce losses by air convection.

Is plastic sheeting OK to use rather than glass?

When I was fiddling around with my solar panel, I read about (and saw pictures of) just a rig -- a coil of 1" black poly pipe which was placed under a round, plastic, skylight dome. Their claims were better than my 140 deg F at 2 GPM, but I don't remember the exact numbers.

1. yes 2.glass 3.no

Thank you for your response, every thing I have read has helped me design my collector. I will soon have a design that I will be able to share with you and soon after that I will have some empirical data to back up my design theories. I hope to be in production by October or soon there after. One thing I am sure of is that projected dates are seldom right. There is just soooo much to do.

I am aware that I need to have my design tested and this lab address is helpful, thank you again.

You wrote:-

The answer was because the radiant heat was not dissipated as there was no air present to carry it away. I hope I am correct thus far.

I do not know where you read this, but if it were true, the Sun's energy would never even reach the earth if it was true!!

The actual reason is actually is a combination of conduction and convection heat loss is severely curtailed when in a vacuum.....

Any company telling you that it stops radiant energy being dissipated is not a company to trust with your money either, they may make other erros too!

----------------------------------------------------------------------------

I personally find the evacuated tubes to be far too expensive for the extra efficiency they bring. Here for the price of a single tube system I could build probably 6 DIY arrays, and have a lot of fun at the same time! And still maybe save money!!!

The electronics needed would also be a splendid source of amusement while designing and programming them......

Plus the result not only being far cheaper, I will also know how to fix it myself in the event of an error/failure.....

If a severe lack of space in the Sun is a problem, then the tubes are probably a good idea.

Do yourself a favour too, do NOT mount them on your roof (unles space is severely limited elsewhere), as maintenance and repair are made harder working high up. The few extra meters the Sun's energy has to travel will not reduce the energy levels by much!!!

Best of luck

I am not inclined to mount anything on a roof. In trouble times, it is challenging to work on panels that are roof mounted. I would much rather mount them were they can be monitored. A nice ground mount, esthetically designed, is preferred. There will come a time when solar panels will just be accepted and no mention will be made of them at all, regardless of where they are placed.

<...the radiant heat was not dissipated as there was no air present to carry it away...>

By taking the air away, one is also taking away the conduction and convection mechanisms of losing the captured heat.

My understanding, for the configuration that you have mentioned: solar hot water panels - copper pipes covered by a black paint layer, is that the energy coming from the sun (mostly visible and IR) is absorbed in the black paint layer, converted to heat and transmitted to the copper pipe by conduction. A part of the heat is radiated, a part is lost by convection. This is why the solar panel has to be covered by a glass window.

The copper is the source and carrier of heat for the water inside, having (after silver) the lowest thermal Resistance. There are different of methods of covering the copper pipe, but I think that liver of sulphur gives a thiner black layer with good adhesion to copper. I soldered copper pipes on 0.021" copper sheet, to get a solar collector. That was more than one year ago. I put it on the back burner due to a more strict budgeting but, as soon as this recession is over (ten or twenty years from now), I will finish my project. For the other part of your questions, I haven't seen a solar collector with vacuum and, although interested, I still think that copper pipe is the best way to transmit heat to the water.

My issue is not how to get the heat to the water, it is how to keep it there. The hot copper wants to radiate the heat back out of the panel. That is why there are some vacuum tubes used to keep the heat from radiating out. see http://www.thermomax.com/ for a discussion of the product. My point is that rather than using a vacuum to keep the heat for radiating back out of the panel, why not use something like Argon which has a very low heat transmission value. It should not impede the incoming infrared rays so the water will still get hot and, if Argon is replacing the air in the solar panel, the heat will not radiate out. The efficiency of the panel should be very good unless I am missing something. If I am please tell me.

I think the problem is that, while Argon has a low heat transmission value it is not non-zero. Eventually the Argon gas will get hot, and once it is hot it will conduct and radiate heat back out of the panel.

I am not a specialist for these problems but I think that a qualitative, not quantitative analysis can be useful.

The sun, the one that radiates the energy has, at its surface, some 6000

degrees C. Part of the energy transformed into heat, into the paint/copper pipes, is transferred to the water. Let's say that the temperature that the system reaches is 60-70-80-90-maximum 100 C (water will be circulated before reaching 100 C). The loss of heat from the pipe-coating assembly is by convection, conduction and radiation. If your assembly is in a box with a glass window and some foam around its walls and bottom, convection and conduction losses will be minimized. The radiation will be that corresponding to a 100 C body, a fraction of a 6000 C body.

My question to you is: do you believe that losses by radiation are that big? As I said, you minimize convection and conduction with the second glass layer if necessary (perhaps in Northern parts, 'cose in Houston area, the winter temperatures are seldom bellow 0. C.

And, yes, I think that you are right concerning argon. I believe that it is easier to maintain argon in a tube than vacuum, and I am not sure (as I already have mentioned) that losses through radiation of a 60-70 C body are that big.

I do not know what the loss via radiation will be, but the people who sell the vacuum tubes, at a high price I might add, tout the fact that they are the most efficient in the business. It must be worth while to go to the trouble to suck out as much air as the tube will tolerate. That is no simple process. I would think that replacing air with Argon would be a magnitude simpler and less costly. My questions are not academic in nature. In the next few months I expect to be manufacturing a new type of solar panel. It is radically different from what is now available and I hope it will be the most efficient and cost effective solar hot water panel on the planet. My mission is to manufacture a solar panel that is within reach of the working poor of the world, and thereby reduce their electric costs a fair amount. I will be making several panels and testing each so that I will have empirical data to rely on. The questions I have posed will then be answered.

I live in South Carolina, USA and we don't experience the extremes in weather that many other areas do. It has been near 100 for the last 15 days and that is as extreme as it gets. Last winter it got very cold and in to the low teens off and on for 60 days. I do not expect weather to be an issue that I will have a problem with. Thank you for your input.

Use quality car antifreeze in the water circuit.....never heat water directly for washing etc., always heat indirectly.

Andy is correct, but I'd suggest that you use RV-type anti-freeze instead because it's non-toxic and much more environmentally safe, especially if you have to drain the system and dispose of the coolant water.....

Just two more cents added, which isn't much after inflation...LOL

Have a sunny day!

I believe that is polyproplene glycol that you mention - the non-toxic antifreeze - much better choice though much more expensive and the tester is horribly expensive.

Car antifreeze is ethylene glycol which is toxic

Hey Guys, you should not start drinking the stuff!!!

Quality car antifreeze stops corrosion and keeps the pipes clean. I have used it my house heating system for over 20 years here and I have used it in previous houses as well.....

It also lubricates pump seals....

The only negative side (other than killing yourself if you start to drink it!!) is that the water picks up temperature ever so slightly slower. But as long as you err on the side of safety with a large water quatity system, you will never notice this tiny effect, ever.

Mitsubishi sent some cars to Germany in the early 80's with far too much antifreeze, the heads eventually got damaged.....they redesigned the heads and reduced the strength of the antifreeze to fix the problems.

If you get a leak in the system?

a) Safety first with a really good quality build and always test with only freh water.....

b) Troughs to catch any fluid loss inside a building, which is a very good idea with any system, even plain water!! You don't want any type of water coming through the ceiling now do you?

c) Outside it should not really matter to my mind, when it rains it should get diluted enough.

d) This part of the system should be 98% outside anyway....

I have never heard of it being bad for the environment, but of course that is possible.....

Andy & others regarding antifreeze--

Ethylene glycol is sweet tasting, so it is attractive to drink for many animals, including homo sapiens. However, the MSDS for it says that 100ml ingested is fatal to humans. It often causes rashes on skin contact and can be absorbed through the skin. Propylene glycol's MSDS says: "Relatively non-toxic. Ingestion of sizable amount (over 100ml) may cause some gastrointestinal upset and temporary central nervous system depression. Effects appear more severe in individuals with kidney problems."

For solar collectors, propylene glycol has a significantly higher viscosity than ethylene glycol, so a collector fluid that is treated with it for frost protection has a slower circulation rate or a higher pumping cost.

If you are using collectors with ethylene glycol the health codes will require the heat exchanger to be double-walled or made with a very special type of copper pipe that has leakage detection channels within the wall of the pipe. Double-walled heat exchangers are much less efficient thermally and the special pipe is much higher in price. The trade-off between these or higher pumping costs... is generally towards the higher pumping costs and the avoidance of the ethylene glycol.

--John M.

Diamond is a better heat conductor than Silver, but a poor choice for hot water systems.

My bad, I wrote without the tables in my hand, and my memory, sometimes, is not serving me as it used to.

using your idea of copper tube on a copper sheet is good . and better yet, imbedding half the pipe into the sheet then soldering very lightly, will create much more surface area conducting to the fluid ( water being the best heat x-fer, but with some mineralization). Copper is by far the most user friendly. and will last. worth its price in any age.

To optimize a solar roof, put it over a simple green house with double hung windows and screens, which has a twofold affect, starter plants are safe and get TLC, and the collectors are low for maintenance ( dust ). Keep it simple. CU is 99.9% pure.

The heat comes from the entire spectrum but is strongest in the infrared range.

Actually for a 6000 Kelvin blackbody (i.e., the sun) the irradiance is about the same for the visible (380 to 780 nm) as for the infrared. For both the value is around 3.5 x10⁹ μW/cm². So with a black coating (that is black throughout the visible and infrared) the absorbed energy should also be roughly equal.

Hi,

gas filling is not as easy as you may think, you need nearly the same tightness as for a permanent vacuum.

If you get an inexpensive vacuum pump, you can easily pump down from time to time - tightness requirements are not as easy as without any Argon or vacuum environment but to be solved by gluing with Epoxi.

Radiation: total spectrum radiation can easily be calculated (if a black body is assumed, nearly existing with your black tubes).

Total radiated energy (all wavelengths) is P = σ * A * (T1⁴ - T2⁴)

where T1, T2 are absolute Temperatures (°C +273), A is the area in square meters, σ is a constant = 5.6*10^-8. P results in Watt.

It is easier to calculate with T/100, then σ₁₀₀= 5.6

If emissivity is not a black body but e1 and e2 of the 2 surfaces then σ is to be multiplied by E = 1/(1/e1+1/e2-1).

RHABE

Use Silicone (Silastic) not epoxy. Silicone is elastic, epoxy is not, and will crack. Silicone sticks to just about everything.

You can buy US Gov't surplus evacuated solar collector tubes on Ebay for a very cheap cost. I bought mine a few years ago for an average cost of around $23.50 apiece. Forget buying from the solar dealers as they'll hose you pretty good and empty your wallet faster than Jack Flash! I bought 52 of them to heat my house and provide domestic hot water. I'm now putting the entire system together.....you just need to hunt these things down and spend time doing it.

Don't forget that you can augment your evacuated tubes with curved linear Fresnel Lenses to increase your Sun-gathering ability, working like a magnifying glass. They are made of acrylic plastic. They aren't cheap but they'll boost the system typically up to around 15x Suns on a sunny clear day. Payback period for the them should be only a few additional years added to your system cost. I've found only 2 manufacturer's worldwide for these: one in the UK and the other in Japan. There may be a US manufacturer, but somehow I haven't found them yet during a tear long search.

Good Luck!

I question the ability of the Fresnel lens to boost the capacity of any solar panel. That lens if I understand it correctly, will focus the suns rays on a specific point and you can get a super heated surface. In pure btu terminology, you aren't getting any more btu's , you are just focusing them in one place. Unless of course your Fresnel lens is in addition to your surface area on your panel. I too had difficulty finding a Fresnel manufacturer.

Hi Mike,

Actually, Curved LINEAR Fresnel lenses will boost the IR collection values quite a bit, only if the width of the lens is large enough and the concentrated energy is focused linearly along the "copper hot-pipe" located inside the evacuated tube. My curved lenses are 12 inches wide x 48 inches long. BTW, there has been a ton of research done on this by NASA and the USDOE....in fact my tubes are NASA surplus and only a few years old. You may want to search both US Government websites or better yet, look into the "Mother-Earth" news website. There also a lot of other grass roots *.Org websites dealing with homegrown solar systems. You just have spend the time looking for all of them. If you need links I'll be happy to provide you with them later today. Also check out the various DIY sites and U-tube videos!

As for the curved linear Frensel lens manufacturer in the UK: www.microsharpsolar.com

There is a Japanese firm too, but their prices are sky high.

Nice thing about the UK firm is that they'll sell you the acrylic Fresnel lenses FILM to you in a roll without the acrylic plastic backing, which significant reduces the cost....then you can adhere the firm to you own bent pieces of acrylics plastic sheets.

Be advised if you do buy used/surplus evacuated tubes then your are going to have to build the copper heat pipes & bulbs, the copper heat transfer manifold and the copper piping with sufficient rock wool insulation wrap. Sweat all copper pipes with silver rich soldier (or braze) because of the higher operating temps.

Hope this info gets you on the right track....wish someone had been there for me when I was doing all my research and design way back when. I've been doing this for about 4 years now.

Let me ask a dumb question. It has been my understanding that for a square meter of area there is an upper limit of 1550 btu's/h that can be 'harvested' so to speak.

If that is actually true, then my comment about the Fresnel lens increasing the performance of any solar collector stands. Sure you can have a Fresnel lens focus the suns rays on a single collector pipe and it will increase the btu's/h on that pipe. Those btu's are coming from the area occupied by the lens and not the surface area of the pipe. If you add the two areas, then you should have the area that is collecting the energy. But is it beyond the 1550 btu's/h per square meters I have been lead to understand is the upper limit of energy that can be obtained?

Dear Cap'n,

I like your posts 99% of the time, but I am FASCINATED to hear why you wrote:-

Sweat all copper pipes with silver rich soldier (or braze) because of the higher operating temps.

What a waste of time and money.....also its called "solder" not "soldier"!!

How hot does your primary circuit get then?

Normal plumber's solder melts at around 250°C if I remember correctly, which is 2.5 times the boiling point of water!! Even if it was only 200°C then you still have a reserve factor of 100% - assuming you bring the water to the boil!!!! Most Solar systems do not get water past 100°C ever.......

Or are you making live steam for a turbine or steam engine.... Then you want something stronger than copper!!

Believe me, normal solder will work just fine, be easier to control and make better physically and better looking joints when done properly, with far less effort!

When I helped a RN colleague build small live steam model engines, with a copper boiler, we brazed and silver soldered and nothing ever broke, but I do forget exactly what pressure/temperature he was working with as its a long time ago!! Certainly far higher than any home solar hot water system (when working correctly!) will reach..........

It is going to be a silver-tin solder anyways if it is used for residential potable water systems, as lead based solders are illegal in US building codes, and I believe most 1st world european codes.

Andy, RCE is correct. We here in the USA cannot use lead solder anymore on domestic water systems. I wish I could use the old type of lead solder because it was easy to use and less costly, but with the temps I'm obtaining in my system I'm afraid the solder would become somewhat soft and pliable (but not melt)---something I was warned about that could happen. This was before I began designing and fabricating my solar thermal system..

During a test (beginning of May) of my prototype evacuated collector w/ curved Linear Fresnel lens attached, I was obtaining about 360 Degrees F at the top of the copper Heat Bulb....this bulb is attached to a copper heat pipe that's located inside the tube and is sealed at the base with a cap....it exists through a silicone gasket seal located at the top of the glass tube.....there's a small amount of MEK injected inside the heat bulb, no water whatsoever. The heat bulb slides into tight socket located along a copper pipe manifold that's water-filled and always circulating for heat transfer purposes. Water recirc flow rate can be adjusted to keep the water inside the manifold and downstream return piping from boiling. My greater concerning here in Upstate New York is freezing of the water inside the recirc and cold water supply system, hence I may add RV anti-freeze solution to my water. Thank goodness for multi-plate heat exchangers!

If no cooling agent (water?) is in contact with something being heated by the sun, yes you can get a high temp, but once the water of choice is circulating, then no way will any solder start to be affected......the temps for melting solder should never be reached except when something has gone very wrong!

Also, if you get a leak of the circulating water, there should be some sort of an emergency "shut down" system that prevents further sunlight getting onto empty tubing.......being confronted with boiling water/steam would scare the pants off me personally.....

Maybe THAT is why you need higher temperature solder to be used, for when things go wrong and no safety system......

It just struck me, of the few systems I have actually seen, I have not actually seen a commercial system where this type of safety system (or any other either!) was actually installed either.......so I guess with the expensive commercial systems, you climb on the roof with one of those reflective blankets for injured people and try and make them stay in place while you get some one in to fix it!!!

I would be most interested to hear how such safety systems are implemented....

The system I helped a friend in north Germany build some years ago had a "watchdog" timer, and if anything went wrong, or the timer timed out, a blind was released and gravity pulled it down and over the copper collectors....also, they were not on a roof, but in his (big) garden!!

If no cooling agent (water?) is in contact with something being heated by the sun, yes you can get a high temp, but once the water of choice is circulating, then no way will any solder start to be affected......the temps for melting solder should never be reached except when something has gone very wrong!

I am glad that you have mentioned that. It is a long time that I wanted to ask CR4:

Imagine a number of pipes, 10 cm spaced, connected at the end into a collecting pipe, perpendicular on the others. On the other end there is another collecting pipe. Water flows into one collecting pipe and circulates through the four or five 10 cm spaced pipes, to the other collector. All the system is on top of the flames of a large fire place. Solder is Ag based.

Starting from the flame, there is the pipe wall, about 1.2 mm of Cu. Than is the (previously melted and solidified) solder, followed by the connected pipe, another 1.2 mm of Cu. Than is water that flows insides the pipes. Water cannot go higher that 100 C (a circulation pump controlled by temperature). There is a gradient of temperature from (say) 100 C in the water, to 600 C in the fireplace fire. What is the temperature of the solder? Will it melt (250-300 C) or the energy is transmitted to the water, with little losses, so the junction (I mean solder) stays bellow 200 C?

I wish I knew the answer! Maybe you or other smart guys will know it.

If the speed of the water is so slow that before it comes out of the collector it is boiling (or worse!), then the water speed needs to be increased dramatically.

Efficiency drops if the water temperature is too high.

If the pipe is too small to get the required amounts of water through, then it was badly designed....

If you say that you get on average 10 hours of sunlight (summer days could be more!), then you need to heat up an approximate amount of water to supply baths, showers, washing etc for 24 hours...

Too much hot water can and will be a problem.......therefore you need a method of cutting down on the amount of heat gathered.....blinds or covers or similar....usually only in summer....

I personally believe that in a properly designed system that you will never get water above 80°C, and then only in summer. In winter you will be happy if you get 35°C water!! But a heat pump can be used to increase its temperature easily as long as the "amount" of heat is enough.

Even if you are only able to supply the hot water system with pre-warmed water in winter, you will still be saving a lot of money!! Better than heating from icy cold!!

Safety features also need to be included to bring this to a halt in the event of a major malfunction...

I hope this helps you further.....

Just reading your post, I remembered the old trick to boil water into a paper conteiner. I think that the answer to my question is, no the solder will not melt as long as there is water inside the pipes.

INDEL YOU ARE BRILLIANT!!!!!

What a PERFECT example!!!!

GA!!

Of course many people here may not know or remember this, you may need to explain it in detail for them - maybe!!

This link below is not a perfect example, but close enough I feel. Check it out and let me know!!

http://www.youtube.com/watch?v=xGseQk_r9EI

http://www.youtube.com/watch?v=IEovFDaAZtk&feature=related

In fact it is you who found the "proof".

Thank you

You have made some good interesting posts recently, I thank you most kindly.

I have given some thought to the "stagnation" problem and I agree fully that the system must handle this and if the heat transport fluid actually leaves the collector for any reason, then I am sure that some systems on a sunny day get hot enough to melt softer solder.

There is no question in my mind that the scenario could happen.

My problem is twofold because commercial units have seemingly no "Safety" sun shield:-

1) Why are professional systems seemingly (here at least!) not given some sort of emergency system to shield the units from the sun if this "stagnation" happens?

2) Because I cannot see just how a repair man can work on this unit by day, probably on a roof, when it is glowing almost white hot!!! Any water, by implication of possible melting solder, is not water but steam!!!

So a form of sun shield would a) keep temperatures well down and b) allow day time repairs!!

I have also given some further thought to the anti freeze problem and a method I use to stop the windscreen wiper fluid from freezing, is to add a proportion of Methyl Alcohol (denatured alcohol) to the water. It also seems to assist the washing up liquid and water that I use, to clean even better......

I have no information as to what amounts would be needed for a particular temperature to prevent freezing, nor have I any idea if the fluid will degrade over time with the higher temperatures in use.

It will make the water "more fluid" and probably find places that were not "leaks" with just water, as does car antifreeze. But that should not be a problem for a properly built system I feel!

It is relatively cheap, available and as long as you don't try drinking it, otherwise not particularly poisonous or dangerous as the gas/volatile part rises quickly away from the earth as it is lighter than air, and will get oxidised by the sun's rays in the atmosphere......

What are your thoughts?

methanol is volatile and a dangerous gas to inhale, just as dangerous as it is to ingest (just not as toxic as Ethylene glycol). It would be better to use ethanol since it could afford the same benefits with respect to depressing the freezing point, and has similar properties as water, and is fairly non-toxic (unless you slam a liter of tequila). Both ethanol and methanol biodegrade fairly readily, and would likely react with any organics in the presence of heat. Methanol is also a fairly strong oragnic solvent, much m,ore so than ethanol or water, so rubber and other seals could have substantially reduced life expectancies.

You mean the stuff in the windshield washer of your car? That methanol.

The toxicity is true but no big deal. With vacuum tube designs I have seen chances of it coming into contact with the water side are zero.

For some reason all manufactures seem to use methanol in heat pipes for vacuum tube collectors.

Well if toxicity of ethylene glycol was the concern, not a great improvement moving to methanol, especially since it is more volatile (though it does absorb into water very well). I guess the toxicity is relative.

as far as why manufacturers use it, well it is a lower viscosity fluid, not super toxic, extremely common, available and very cheap (and something overlooked frequently, it doesn't scare people to tell them you use alcohol). While it is a great solvent for organic, it doesn't corrode metal parts like water does.

Thanks for the reply.

Andy,

My memory of the upper limit for a flat plate collector's stagnation temperature is around 350 °F, or about 175 °C. Probably some components may be a little hotter. Although this is hot enough for serious burns, it is well below the true white hot. I just haven't read on this topic in a couple decades!

Problems with a collector system fall into internal collector failure (fairly low incidence), plumbing connection leaks (moderate incidence), and control or circulation failure (higher incidence). Therefore, most repairs do not require direct contact with the collectors themselves and often do not require anything more than an inspection of the collector array. For a sun shade, one can use a sheet of foil-faced foam insulation board, which is low-cost and usually readily available. Because collector arrays are exposed to all the types of weather in their locality, I do not envision a cost-effective system having deployable sun shades. Such would be too easily damaged by wind, hail, wind-borne debris, ultra-violet rays, etc.

Because methanol is toxic, it cannot be used in single-wall heat exchange systems. This is a health law issue. Also, many model building codes limit the volume of flammable fluids that can be used if the fluid is circulated into residential occupancies. This is for purposes of safety in the event of system rupture during a fire, and explosions or acceleration of the fire from it.

Another person (Guest, I believe) mentioned the use of methanol in evacuated glass tube collectors. I believe he is correct. However, in these systems, the collector tube is a replaceable component, complete with the sealed heat transfer medium in its metal pipe. The tube closely connects to a header that then has a circulating water or antifreeze solution to move the collected energy to the storage tank. This approach, with the use of an intervening heat transfer fluid satisfies the health code concerns that would arise with the single-wall separation of methanol and potable water. Although the probability of failure in a single-wall system is low, they do happen. The probability is much lower with a double-wall or two-step system, because there would have to be two separate connected failures.

Regards--John M.

That heat transfer pipe you refer to is called a 'heat pipe'. There should be a second heat exchanger in the collector tank downstream as well. Unless you have very clean water (soft etc) you don't want it going through the system.

The automatic heat shades are a non-starter for many reasons. I have seen people cover part of a collector system in the hottest months.

I have never heard of using a system full of methanol - makes no sense and provides no advantage.

Heat pipes - I would not worry the toxicity of methanol a lot - it will be diluted rapidly. There is also very very little chance of fire with heat pipes - the amount of methanol in each tube is small. It refluxes in the heat pipe to transfer the heat - even the small pipe is not full of it. www.apricus.com has decent information on vacuum tube systems.

Thanks for the reply.

I said white hot (thats what your fingers might tell you!), well temperature wise it would be well above what anyone could touch, we both agree on that! Any water in that area would be steam, maybe under pressure - very dangerous!

I tend to agree with some here that with a good system with say Fresnel lenses, soft solder will melt. Which is generally higher that 175°C....

Remember that the French government has had a Sun collection system under test for many years where temperatures well above 1000°C are generated daily.......

As regards a working shade when things go wrong, we have window blinds here on most house windows, made of interlinking plastic that withstand everything that the elements can throw at them from UV to huge pieces of ICE/hail. Thats their job - to protect the windows.

They last longer than you can imagine, a lifetime of usage and they still look great!!! Also easy to clean. If you need a photograph just let me know.

I replaced all or ours a few years ago, they were over 50 years old, still working really well, but the new windows came with modern units already installed!!

They are relatively cheap to buy new, or to find on the scrapheap......

I envisaged a roll with the blind rolled up on it at the top of a box. A weight is arranged to pull it down with the aid of gravity, but a temperature sensitive component (Bimetal strip?), will only release it if the tubes upper area exceed say 110°C....or thereabouts as a starting point. Or it could be electrically released say from the house if a hail storm appears....

If hail is expected, they could/should be deployed to stop damage to the collectors!!

So I do not understand why in the USA (or here) you do not have such systems on commercial units.....even if the system hung up occasionally (there is no reason for that, but I expect that to be a point open for criticism!), you are still better off when it does deploy and its probably quite easy to free up the hung ones ands wait for things to cool down....I would do just that!!!

With regards to the antifreeze, a simple one we used many years ago in the RN for cooling systems was simply salt!!! In a solution with water, its called Brine. Its relatively easy to make such a solution that will keep water liquid down to around -20°C. Which is quite cold and probably good enough for many areas of the world....colder than that and I feel that the system should be better switched off and drained till spring comes......

As long as plastic or copper fittings are used and no mixture of metals, there should be no corrosion problems....brass is not a good idea as the salt could leach out the Zinc (I believe its Zinc!) component.

Sacrificial anodes could be easily added if there was seen to be a need (also zinc if I remember correctly!!)

I think that most problems are fixable once the circumstances are understood a little.....

Hi Andy. Megasun, No 1 under the sun does not apply any solder at all. The fins are Omega shaped and pressed over the piping. Check their TUV ratings. I have big questions marks how to focus a Fresnel lens on a pipe. In cooler places I first should check about parabolic mirrors with the pipe in the center. Better go with solar ideas from experienced sources. Some in Europe have built solar for 35 years and many are still original. Tin- lead solders might be forbidden in some states- it must have something to do with the water composition pH? - lead is more dangerous through a chimney than as solder for a drinking water pipe- although in a closed solar system there is no consumable water involved in that part - but asbestos is still not on the black list.

Actually, it is down from EPA, it has to do with the accumulated high lead concentrations in the inital flush after a valve is turned on and drinking water standards. It is only applied to new facilities building since the early 1990s. they just dont want lead in the potable water system. however, it is pretty much impossible to by lead solders from Us distributors for plumbing systems anymore, since no high volume sales reason to use it for plumbing and other solders are just as effective. As long as the system is closed to potable water, air gaps for refilling stuff such as that. I guess it could use lead based solder, not sure if any plumbing suppliers still sell it though.

BTW, Asbestos is illegal too for new building, and if it is friable.

I use what is termed "Plumber's Solder", it is supposedly OK for any water system, potable or heating.

Also we have a lot of calcium, it seals everything up after a few months anyway!!

I prefer a silver alloy solder Sn95Ag5 for my own home plumbing applications. It is usually available next to the general plumbing alloy Sn95Sb5 in most stores here at a slightly higher cost.

Higher temperature solder suggestion was made in a paragraph discussing solar collector evacuated tubes and may be a consideration because

"Evacuated-tube collectors can get very hot, exceeding the boiling point of water and can cause significant issues in an existing domestic solar water system."

I personally consider it a prudent choice to use polypropylene glycol(non-toxic) instead of ethylene glycol(toxic) as the working fluid in any home "potable" water heating application. Leaks are inevitable and unpredictable. Cheap health insurance when compared to our other health-care costs (ouch!)

I do not know much about the "spectrum", but it seems to me that since light is traveling at 186, at the point it hits the dark copper painted black, the energy is absorbed instead of reflected. (consider the copper pipe being mirror like and reflecting the energy, how much heat you would not get). If you put out a piece of clear plastic, and a piece of black plastic, the clear plastic is completely disintegrated because at the "point of attack" the energy is changing direction 180 degrees in a fraction of a fraction of a second. conifer green is the best absorber. Also, since copper is 99.9 % pure mass, (no oxygen to de grade it from the inside) it transfers energy very well.

Sir,

I believe you are attributing the observed effects on clear plastic to the wrong causes. The UV light that is a significant part of sunlight is quite damaging to the chemical bonds in plastics. To avoid this, chemicals (often called stabilizers or plasticizers) are added that block the UV light--such as carbon black. Clear plastics are much harder to protect against UV degradation than colored or black plastics, therefore they are much more likely to disintegrate (as you correctly mention).

Regarding the "energy is changing direction 180 degrees in a fraction of a fraction of a second," only a small portion of the light is reflected (typically less than 10%). The vast majority is transmitted through the clear plastic and a small portion (proportional to the thickness) is absorbed. With black plastic of the same chemical family and surface finish, the same amount is reflected but the balance is absorbed.

--JMM

Hi,

using Silicone as glue in vacuum systems is a bad choice as Silicone is not tight at all (in vacuum's sense) and transmitting and containing a lot of water!

Using a linear Fresnel is a good idea as it may concentrate the sunlight (VIS and IR) to a much smaller black tube that is easily encapsulated by a glass tube that is evecuated.

The smaller copper-tube will get much hotter (if not adequately cooled) so it may be much easier to get hot water in noit so sunny days.

And total cost should go down.

RHABE

Many things in this thread are a prefect example of why one needs to know at least the basics of what they are working on in order to innovate.

Many unusable and commercially rejected ideas are presented as being just what is needed.