Material for Solar Collector Cover

How about something like this:

http://www.sunsetter.com/

I am sure it would keep the sun off the coils, but I am just as unsure about the fabrics ability to withstand high temperatures for 10-20 years of so. It appears to be a cloth of some type.

If I knew how to upload a picture it would help. Used to be able to do that but it is not obvious anymore how it is done.

Take your picture and put it on your desktop. Reply to your OP. Click camera icon. For path click browse, and go to desktop. Go to picture and click open. Click submit on the CR4 box. Bingo.

Something like this may be promising.

The solar fabric is interesting and looks good. I have not been able to reach the company as yet, seems they are out of the office a good bit of the time. I will need to know the temperature that it distorts at. Seems a bit pricey at the get go, but that may change with quantity. Thank you for your suggestion.

US Patent D641,315S

This unit is only a prototype(ie. proof of concept) I have 8 of these being placed in conspicuous locations for testing; the production model will be 8 ft in diameter and the dome will be elliptical and not hemispherical. The cover will sit inside the dome. There is a cone floor that the copper sits on which supports it. The "fabric" will be pulled over the copper much like a dump truck pulls a cover over his load.

I would look into the material that they use in windshields to protect the insides of cars from the sun. I know those screens are pretty inexpensive and durable.

Surely an ordinary roller blind would do the job? They are routinely exposed to sunlight (OK through a sheet of glass).
Easy to obtain, easy to replace, cheap, srping loaded. What more do you want?
(No they won't do that)

Or build a swiming/paddling pool and dump the excess heat into that.... Probably simpler. Just operate a pump and/or solenoid when you hit the limit.
Del

Actually I was hoping for something that was more in keeping with a flexible cover. The inside of the dome is not flat as are window shades. 45point5 mentioned mylar and that may be the best material; it is pricey however.

I could just open a drain and dump the circulating fluid into the tank(there is a thermal storage tank just below the collector) to relieve the heat stress on the collector dome. Once dumped it could be restarted again once the heat emergency was resolved. That may be a good idea. Not everyone is going to have a swimming pool.

I assume "45point5" means me - moving on -

"mylar and that may be the best material; it is pricey however."

link

not that you need a 36" one, or colored, or printed: given all you have to do is shade maybe 25% of your coils

"45point5" Time grinds inexorably on for all of us... you've grown... hopefully that isn't your waist that grew?

Oops, sorry 'bout that, 34point5. I was working in the evening w/o lights and w/o my glasses. Now that is a setup for disaster. No insult intended. My apologies.

I like the idea if I could find a way to use it over and over again. I think I could mount it at the top of the cone which is truncated at a diameter of 3 inches. How would I put the mylar back in the "bottle" once it is activated? I would need to be out of sight and "completely cover the unit. I am going to have to submit my collector to the SRCC and they will put it through a stagnation test, which it would fail without a temp activated cover.

I just pull an old white (It was white when new!) cotton sheet over my collector whenever I leave on long trips. Did it a fortnight back when I left India for a month's visit to USA.

Do you know the flash point of a cotton sheet? One of the collectors will be getting quite hot and well beyone the boiling point of water. But that is the kind of material I was thinking of. easy to use, cheap, does the job.

The issue I am dealing with is that it will go inside the dome where it will get very hot. I would put it outside if it wasn't for vandalism and theft by curious kids.

Thank you

Cotton can stand quite high temperatures. The domestic iron maximum setting is for cotton. I have been doing this for years without problem.I am assuming that there is no concentrator like a parabolic mirror or Fresnel lens. A white sheet reflects heat and never gets hot enough to worry about.

Linen actually, not that 11 ⁰C is relevant here.

here is a cheap source of insulated mylar

http://www.quakekare.com/thermal-blanket-p-25.html?products_id=25&gclid=COXEvLCaqaoCFWsbQgodrn_QYw

do a search for

space blankets

for many more

Get a reflective fire blanket, the type wildland firefighters use to hunker down to avoid death while in the middle of a burn-over(I used to be one of those guys when I was still in shape) They are really tough, lightweight, completely reflective for both light and radiant heat.

Like these right here:

http://www.thefirestore.com/store/product.cfm/pid_7073_anchor_industries_new_generation_fire_shelter/

http://www.adlinsulflex.com/fire-blankets/pyreflect-blanket

thank you for that. I didn't know such existed. do you get in them like a sleeping bag?

How about miniature Venetian blinds beneath the dome, actuated by a thermal bulb?

Draining the coil is a better option as this also caters for a winter situation where the pipe coil may burst through freezing. Ensure that the coil is vented after draining. A build up of steam pressure from any residual water would be explosive. Is your pump self priming?

I am using an el cid circulating pump. I would need an other valve to release the pressure and the fluid(not water) back into the tank.

We have what are called in Germany, Rolladen. They are a bit like the shop window covers of UK 50 years or more ago, but are usually plastic nowadays, reflect sunlight, are rolled on a motor driven roll, last at least 40 years, can be cut to width, and are easily linked to each other to make any length/width you want.

Here is a crazy chick who visited Germany and was interested in some:-

http://www.youtube.com/watch?v=BMjoJKjGEE8

If you lived here you could easily buy them, but I am sure such products exist in most countries.

We also have blinds, fitted outside of the windows that can be rolled up and down and also open or closed. They look like normal internal blinds but are much stronger so that no damage occurs with high winds, they are generally made of painted Aluminium.

There is a demo here:-

http://www.youtube.com/watch?v=qQLk32i91iI

They could be sandwiched between two glass panels, but solar losses may be higher....

You can "open" and "close" these without rolling them up and down, Just like any blinds, could also be between two panes of glass......though modern plastics lose less sun energy I am told.

Or you could move a reflective sheet of say Aluminium across the collector, sliding across mini ball bearings, or have the bearings mounted on the sheet, and/or sitting in a plastic/metal rail.

As white reflects sun energy very well, you do not have to have a silvered reflector of any material, quality white paint will do just as well, just make sure that the sun shield has air cooling it from both sides when in operation.

Make sure that you get some sort of warning when the shield is supposed to operate but doesn't.

My thoughts were always for a gravity system, held in the open position with a pin, which can be retracted when overheating is noticed or automatically, or no need of solar power. But would need resetting by hand. Naturally it could be completely automatic as well.....

There are special struts, which open green house windows when it is too hot, maybe you could get them to put a cover over the solar collector when too hot....

I like stepper motors with limit switches, but that's just me!!! The motors do not need to be steppers....small AC/DC motors with a gearbox would work just fine, speed is not of the essence!

You can drive long threaded rods to open and close heavy things with small motors.....

good thinkin Andy. thanks for the rolladen link.. loved that, especially the use of the web tape for acutation, without putting the actuator outdoors. genius.

Chris

If I choose to use a shade of mylar of a similiar product, then an ac/dc motor with limit switches is my choice; adding a reversing switch makes it complete. This could run off a companion PV module that powers the circulating pump and temp probes.

I always try Rochford Supply ,they come out with new and improved Fabrics alot.

How about using one of those highly reflective "Space Blankets" or a length of reflective Mylar attached to a spring-tensioned roller like those use in old fashion window treatments? Both materials are cheaply purchased almost anywhere, including eBay.

If there was a way to do that with a curved surface and base, this idea would be perfect. Remember I have a circular base and a hemispherical top. Makes it more difficult. But a good idea, and easily obtainable.

The mylar idea and the spring tension roller suggestion have started me thinking. I wonder if I could fabricate something like an umbrella type spring where the cover would come out of slot at one end of the base and expand like an umbrella to cover the unit and then re-collapse as it is pulled back inside its housing when not needed. Can anyone expand on that?

Alminized Mylar springs to mind. Developed for the NACA (later NASA) it was used in stratospheric research and tacky supermarket novelty ballons. Very reflective, very tough. Don't know about longevity, though.

For the dome, use the same material as in Photo-Gray or Photo-Sun sunglasses?

Photocromic coating?

Whaaa? C'mon guys photochromic my a$$... D'uh... a solar collector that covers itself when the sun comes out???...
Just make the damn thing smaller or don't build it.
It strickes me that the big design problem is having it domes, it seems to be creating problems...dunno what point there is to a clear dome, it's not much different to a clear flat sheet.
Del
<click...Grumpy mode off>

This thing has to go into a householder's garden. Maybe the dome is an artistic statement.

Hi Del,

grumpy mode now off? Claws sheathed? Good!!

Actually, anyone building some form of solar heat collector must take several things into account, and if he doesn't, he will get problems.

1) Taking into account the amount of heat differences between summer and winter, so one needs to design the system for the heat available in winter, which will oversize it for summer.

2) When installing or repairing such a system, you don't want to have to work at night, just to avoid near boiling water spraying me from head to foot.

3) In the case of a failure, you need an automatic shut down, till you are able to start working (you are at work, a pump fails and the system temperature goes way over safe for example),or someone will need to be in the house 24 x 7......Also what about holidays away?

With regard to point 1) above, many people build a complicated system that follows the sun in some way, its usually a waste of time completely, just angle it so that winter sun will hit at midday (so some hours before and after will still get energy) and make sure that your hot water storage is properly insulated to keep the water hot for at least 24 hours. Though my personal guess is that a practical system will need some help from anther form of energy at that time, be it electrical, wood, pellets, gas, oil or coal, plan for it!!

These few points alone are enough reason to have some sort of shading installed when needed and I am sure I did not cover them all either!!!

maybe a retractable cover that decorated like an eyelid

I was just helping editor crankshaft out, he was having trouble remembering the name of the technology

an 8 foot dome with a retractable cover would seem unnecessarily complicated

as AG points out the usual solution would be increasing the storage or in an extreme case some form of load shedding

but how many of us have designed an elaborate solution, only to fall back on a much more troublefree design during the prototyping process

or.. if one is making a 'sandwich' for the dome, then why not liquid crystals?

Not fond of UV, cold, or heat. Otherwise - perfect

thanks

Oh contrare grumpy. With a dome I can place my collectors up in the dome and get the advantage of the morning and evening sun. If the sun is above the horizon it will see my collector; that will give me a greater btu accumulation for the day on the whole. And the dome shape is very strong.

I shall be very interested to see how your numbers compare with one of these units.

As I'm sure will 'grumpy' Del

Michael,

I am thinking of a cover made of sections of aluminized mylar with a curved wire between each section. The ends of each wire are on two pivots so the cover opens and closes in a semi-spherical shape. One side of the cover is fixed to the side of your collector dome and the other side of the cover is a movable flat metal strip. Attach this flat metal strip to a bimetallic actuator with a spring in its movement. As the actuator heats up from the night-time cold until you approach the stagnation temperature, the spring is taking-up its movement. But when the temperature continues to rise, the actuator then causes the cover to open--proportionally to the temperature rise.

This method uses no outside energy sources and should have good longevity.

Another method--use the same type of bimetallic actuator as above, but have it swing up hinged sections from the inner surface of your "cone". When down, they are part of the cone, but when up, they then are reflecting the light back out. This approach again uses no outside energy sources and is fully self-storing.

--JMM

Thank you for your input. I was able to visualize the entire mechanism as you described it. Using no input is a real plus. I will take this idea with me when I go to see the engineer I have hired to resolve the issue.

Since the copper tubing sits on a truncated cone and the truncated diameter is near 4 inches, I considered fabricating a "umbrella type of device" that would spring up out of the center of the cone at the truncated part. And it would retreat again as it cooled. But this takes energy to do; or at least a co2 cartridge.

Thanks again

Michael,

Your umbrella idea is also adaptable to an energy input from the bimetallic coil. Since it unfurls and furls in an area protected from wind and other problems, it does not have to push against a spring. Perhaps use the moving arm of the bimetallic actuator to directly push the "umbrella" ribs open and have a weight to hold them in the closed position. If I understand your design correctly, you do not need to block more than around 75 or 80% of the incoming energy, so there is a lot of play to provide clearances, etc.

--JMM

Seems like an idea that could be efficiently constructed and would work over and over again. I could use something as simple as cloth or as esoteric as mylar. I would trust the mylar and I think SRCC would be more impressed as well.