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SunFun

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Reply with quote  #1 
I have been looking around online for material to use as the screen in a ZP collector. I thought insect mesh would be the thing to use but the cost of it made me gulp so I have been looking for alternatives. One option, coming in at less than 10% of the cost of insect mesh, is scaffold netting. I have never seen this in the flesh but from what I read it might be suitable. One thing that does concern me is that it is made of polyethylene. How well would this stand up to the temperatures in a panel?

What other alternatives do people use?

gbwillson

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Reply with quote  #2 
SunFun-

Where are you located? I'm guessing someplace other than North America unless your budget is very tight. Amazon ships all over the world, but shipping might offset any price savings. Regardless of price, the ZP needs to have a very open layer closest to the glazing so sun also gets to the back absorber layer. This back layer can be a bit more dense so little or even no sun gets to the back of the collector. The idea is to heat both the front and back layers while the air is heated as it passes between the layers. Both front and back layers should have a rough texture. The texture helps break up the laminar flow, it also helps heat up the air moving between the layers. A metal mesh could work, but I can't imagine it being less money than window screen. If metal, try to use aluminum or copper. How large will this collector be? I ask because there may be a solution that is based on the size of the collector. 

Another thought to keep costs down would be to use screen only on the front layer. This would allow sun to pass to the back layer and cut the screen costs half. Maybe the back layer could be a thin roll of batting or similar. As long as it's a rough texture and semi porous. Aluminum vented soffit sheeting could be used for the back layer, as it blocks far to much sunlight.

Greg in MN
SunFun

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Reply with quote  #3 
Greg, Thanks for your input. You are right about me not being in North America. I am in Northern Europe (Finland).

From Amazon UK insect mesh is over US$ 4 per sq yd whilst scaffold net is US$ 0.34 per sq yd (buying full rolls in both cases). That is quite a difference.

I had considered a metal mesh but as you suspected the cost is even higher, plus large pieces are hard to source.

The collector will be an aretha design about 25ft long and 4ft high. The exact size will be determined by the free glass I am collecting on Saturday - I will work to the size of the glass to simplify the build.

gbwillson

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Reply with quote  #4 
SunFun-

So you are saying Finland doesn't have bugs such as mosquitos, biting flies and gnats in the summer? Or has the EU taken control of window screen just to "bug" you guys? Sorry, couldn't resist...

If UPS, FedEx, Amazon, or some other delivery service covers your area, you might consider looking online. That, of course, assumes the delivery cost isn't so high as to offset any savings. If you could get a minimum of 25', you might be able to use only a single layer for the front layer, as long as you can find a suitable back layer. Common roll lengths here in the USA are 25' and 100'. Consider the larger roll as the price per square foot will be much lower. Any extra could be used for screens for the house windows, or you next solar heating unit. Nobody builds just one. In fact, I would guess most builders start thinking of new ideas and collector builds long before they finish their first build. This hobby is kind of addicting, but in a good way.

Also note that screen comes in different material such as aluminum and fiberglas, with fiberglas being half the cost of aluminum. The fiberglas performs just as well as aluminum, but may not last as long if the glazing has no UV coating. Without a UV coating, the fiberglas, paint and anything else inside the collector box will feel the effects of the sun after a few years. 

Here is a link to the USA Amazon site were you can see the cost per roll is quite inexpensive. While you won't be shipping this from the USA, it should tell you that Amazon does carry products like this. Otherwise, do a search for window screen or insect screen or other possible ways the product is defined. Somebody across the pond should carry this, if not locally, can delivery it at a reasonable cost. Look for screen that is either black or charcoal in color. Even if the screen is dark to start, I like to give it a very light spray coating of flat black paint to cut down the shine. Starting with the screen already dark mean I only have to apply the lightest of coats.

https://www.amazon.com/s?k=window+screen+roll+48%22+x+100&crid=VP8RDAWLD34L&sprefix=window+screen+roll%2Caps%2C184&ref=nb_sb_ss_i_8_18

What will you be using for the glazing and insulation? Oh, and polyethylene has a melting point that is far too low. It "might" work if you could be sure you would never allow the collector to get to hot. It only takes five minutes in the late spring or early fall to send the collector temps soaring without airflow. Winter stagnation is normally not fatal to a collector. We have examples of people trying poly-based mesh and the results were not pretty.

Let us know what you find in your search.

Greg in Minneapolis, MN
SunFun

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Reply with quote  #5 
Quote:
Originally Posted by gbwillson
SunFun-

So you are saying Finland doesn't have bugs such as mosquitos, biting flies and gnats in the summer?

Oh, I wish that were true. There is no shortage of bugs here but bug screen seems hard to find. I have only ever seen small packs intended to cover a single door or window and even that is not really common. I don't know why.

I have been looking online but up to now I had not found anything at a sensible price. However, using your search terms of "window screen roll" finds one product that my searches for variants of "insect mesh" did not find. I can get a 48" x 100' roll of fibreglass screen for the equivalent of $43, which is much better. Result! Thanks a lot for your help.

Quote:
What will you be using for the glazing and insulation?

Tomorrow I will collect a load of used windows removed from an apartment building and I intend to use them for glazing. I have not seen them yet and don't know their size, but I expect to get more than enough glass to make my collector. Obviously glass is heavy but it should perform well and the price (free) is right.

I have not decided about insulation yet. Locally I can get what I think is polyiso, foil-covered. I am leaning towards that, 30mm thick, possibly with a layer of something cheaper like styrofoam behind it.

Quote:
It only takes five minutes in the late spring or early fall to send the collector temps soaring without airflow.

That is a concern for me as power outages are not unknown. I have some evacuated tubes and have seen the effects of a power outage in sunny conditions. I am pondering using solar electric panels to keep something happening when the mains goes out, but I don't know if I can realistically do enough to make a difference. Especially as I plan an aretha design so I have to keep both air and water flowing. Any other bright ideas for back-up solutions?
gbwillson

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Reply with quote  #6 
I'm glad I could help SunFun!

You seem to have a very clear idea about your build, so you must have been paying attention in class. And building so large for a first collector I can't help button be either very impressed, or very scared!!! It's not that the concepts are different between the large and small builds, but without actual experience, you might assemble or process something out of order. And with your build being so large, it could take a lot of time(and money), to make the correction. Anyway, hats off to you for the ambition. 

It sounds like the insulation you will be using is the right stuff. And depending on the exposure to the prevailing winter winds, added insulation on the back of the collector sounds like a good idea. I have a question about the windows you will be using in that I wanted to know if these were single of multi-paned windows. Multi-pane windows, even older ones may have a Low-E coating. You need to check that as it will affect how it is installed. Do you have a photo you could share of where you planned to install this collector? 

As far as a safety valve or protection for an overheating collector, there have been discussions and more than a few ideas tossed around. But as every build is different, every solution can be different as well. Some sort of solar PV panel or battery holding a charge from a solar panel seems the most common answer. But with a unit so large as yours, the safety solution gat get large too. You need larger solar panels, fans, pumps etcetera to keep the temps down until repairs can be made. I'd also consider some sort of temperature alarm that could alert you via your phone should the system fail. That may be all you need to ask a neighbor to stop over and pull off a vent or something until you get home. In any case, there is no settled solution. So if you have an idea, toss it out there in the experimental section and let the brainstorming begin! 

Greg in MN

stmbtwle

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Reply with quote  #7 
PV panels work pretty well IF they aren't accidentally shaded. That can be a disaster.

As arethas are recirculating collectors, they can get really hot really fast if the water flow is cut off. I'd consider some way to dump the heated air and bring in cool fresh air when needed to prevent overheating.

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Willie, Tampa Bay
gbwillson

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Reply with quote  #8 
It's hard to think of heat when the heat index is a bit over 100˚F!!!

The biggest problem I see with PV as a backup for any large solar heater is the watts needed to keep things cool enough until help arrives. A hundred watts isn't going to power anything running on AC, but it sure could run a few DC fans. Perhaps PV that keeps a battery fully charged to run auxiliary fans or pumps. Or maybe operate a servo which pops open a valve to cool things off should stumps reach a certain level. 

On an Aretha design, are two backups needed, one for air and one for water? I would think any pipes, even if PVC or PEX, could handle any stagnation temps. So backup fans or a way to dump excess heat would be the priority.


stmbtwle

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Reply with quote  #9 
A solar powered DC fan and a servo to open a pair of vent doors could cool the collector, if the main fan or pump failed.

Aftermarket auto radiator fans require from 30 to 80w, depending on size (they seem to have the same motor) and will run off a solar panel. A "buck" converter can reduce the high solar voltage to a safe level without the need for a battery and charge controller.

One could also use a battery with a small AC battery charger.

If you have fans to protect the collector, that will protect your plumbing as well. It will get no hotter than the collector itself.

__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
SunFun

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Reply with quote  #10 
Wow, some excellent suggestions which are changing my thinking. I particularly like the idea of automatically opening vents if the mains power fails. I reckon I can do something along those lines. The major issue is to make sure the vents don't freeze in position, closed or open. With a bit of thought I should be able to work around that problem.

I doubt my windows have a Low-E coating but I do not know that for sure and do not know how to check it. There is a style of window that has a sealed 2-pane outer casement with a separate single pane casement mounted on the inside of that. I have the inside casements from these, so single-pane rather than multi-pane. Apparently these windows have sat in a shed since being removed from an apartment block in 1991. I have 22 of them and they are about 4'9" x 4'5". Should be fine for making a panel.

My panel will be placed in the area I have attempted to show in yellow in this photo:PanelPos.jpg This is not an ideal position but it is the best compromise I can come up with. The panel will face south.
Panel height is set by the height of the windows. Panel length is arbitrary. Extending to the left in this photo I have space for up to 60' but I am aiming for a panel about 25' long.

Inside the house, behind the ladder in the photo, is the original boiler room. The hot water tanks are in there and that is where I plan to store heat for hot water and also put heat into the heating system. There is an oil burner in there which I do not use. Nowadays the house is heated by a GSHP located in the basement below the old boiler room. House heating uses hot water radiators. The radiators never (and never need to) get much over about 50 Celsius (120F).

My panel will have a manifold at each end and my thinking is to mount PV panels on the manifolds to power the back-up safety system. I will only get about 200W so have to devise a system that can work with this power level. I realise that this will be difficult so the ideas of opening vents and not needing to run water pumps to remove heat are very interesting to me.
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