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SolarInterested

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Spam Stomper
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Reply with quote  #21 
Be Careful About Insulation (link)
"The common polystyrene insulation board (this is the "pink", "blue" and "white" stuff) will not hold up to much above 130F, and the batch enclosure temperatures will exceed this, and melt the insulation.
An example: http://www.builditsolar.com/Experimental/pinkfoam.htm
Polyisocyanurate insulation board works well, and comes with an alum reflective surface already installed."

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Both temperature rise and airflow are integral to comparing hot air collectors

TJTurner

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Reply with quote  #22 
Excellent, thank you guys. I figured there was a reason you weren't using it! I did find out that I have a Menards within a reasonable driving distance. They have it in stock...so I'll try that. The HD and Lowes have nothing...

Tj
gbwillson

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Reply with quote  #23 
Menards often has both the polyiso and the twin wall glazing on sale.
TJTurner

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Reply with quote  #24 
Gary...

Since you mentioned the twinwall, I've poked around and found your posts on the glazing comparison. Good stuff. But for the ZP, do you think you need the twinwall? If the airflow is not leaving the screen gap, could it be better to use a higher transmittance material, like the acrylic you used in your transmittance tests? I haven't compared costs, just thinking out loud.

Tj
gbwillson

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Reply with quote  #25 
Tj-

If you are looking for GaryBIS, he hasn't been here for a year so. If you meant me, I think the choice to use twin wall glazing has a few things to consider:

If you live in a cold northern climate, I think it's probably better to use twin wall. Yes, the ZP does a good job of limiting the air movement outside of the screen gap, but there is still the fact that the glazing is cold. So depending on the distance the screen layers are from the glazing, an insulated glazing probably does some good. The frigid glazing will draw heat from the screens due to the close proximity. Twinwall has an R-value of 1.6, whereas a single layer has about half that. And higher insulation value helps keep the heat inside. 

At least here in Minneapolis, the cost for a sheet of 6mm twin wall polycarbonate glazing can be purchased for as little as $30 for a 4x8 sheet when on sale. Whereas a 4x8 sheet of ⅛" clear polycarbonate is 2-3 times as much and never goes on sale.

6mm twin wall allows 82% of the desired light through to the inside of a collector. A single layer of polycarb allows about 92%. So you do gain 10% more light. But this may be more than offset depending on where you live.

And lastly, twinwall is semi opaque, so the view is mostly obscure from view. This can be good if you don't want someone to see the innards of your collector, especially if the quality of workmanship is less than stellar. It's always neighborly to have a nice looking collector.

I suggest you price out and check availability in your area as these factors may help make up your mind for you. 

Greg in MN
TJTurner

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Reply with quote  #26 
Sorry Greg...

You're right, I totally mixed up you and Gary. It's the "G"! Sorry about that.

Good points. I wonder where that break even is in terms of performance (not price). I'm in Ohio, so I'm not so far north as you. And we're pretty moderate in terms of temperature, but the R value is a great point. I'll likely have to depend on price anyway. I really don't care what my neighbors think. They can see some of my property, but we have no real rules out in the country where I live--kind of an advantage, huh!

Thanks! I appreciate the insight. I'm getting closer to building. In fact, my son and I may venture in on some smaller test collectors for a science experiment. Got to get him hooked, too!

Tj
gbwillson

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Reply with quote  #27 
Tj-

I sure wish I lived away from the city where you are not stifled by regulations and laws. My heater is close to the street. It's close enough that I had to clean the glazing last week from all of the salty road spray. It's also on a corner and the first house you see as you enter the neighborhood. So I kinda feel an obligation to have a nice looking house and yard.

Great idea getting your son involved. Kids learn about alternative energy in school, but having first hand experience using it will likely make for a lifetime of interest. Does your son's bedroom have a south window? Might be a great place for an experimental solar heater. 

Greg in MN
JC77

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Reply with quote  #28 
I realize I'm a little late to the party here but I found an article over at BuildItSolar.com that said a good rule of thumb for collectors less than 200 square feet in size is to plan for 2.5 to 3 cfm per square foot of collector and fine tune after that to achieve roughly 50 degrees of temperature rise across the collector. A higher temperature rise reduces collector efficiency and lower temperature rise likely means too much airflow. Obviously, the higher the airflow needed to achieve a 50 degree temperature rise means your absorber material is doing well. I'm not sure why they chose a 50 degree temp rise but it had to do with providing a similar discharge temperature to that of a gas furnace. Personally, I'd be ok with a smaller rise in temp as long as it didn't make an enormous increase in the work the fan had to do. If you could get an outlet temp of even 80 or 90 degrees F, you'll probably be ok.  You will have restriction to airflow throughout your collector/duct system and efforts should be made to reduce this but your larger fan should be able to overcome it. The less restriction the better. Round duct work is usually better than square. You could build your collector and still use your fan. After you get it all done and running, perform a bag test to see what kind of cfm you actually had and what kind of temperature rise you achieved. Then maybe build some kind of damper so you can adjust airflow. Generally speaking, placing the damper on the inlet side of the fan is more efficient than placing it on the outlet as this will reduce motor amp draw at the same time airflow is reduced. The most efficient would be to have the proper size fan for the application so no damper is needed or use a rheostat if possible but you can still make use of the fan you have. You can place the fan on the inlet or outlet side of your collector and there are arguments for and against either. Since your fan appears to be a bit oversized, you could put it on the outlet side or your collector and the collector itself will provide some restriction to suction flow. The downside is that your collector will be under vacuum so any air leaks will pull unheated, outside air into the collector (and possibly moisture). Putting the fan on the inlet side means your collector will be under positive pressure and air will leak out. Putting the fan on the outlet means less restriction to the discharge flow so it could possibly push air farther out into your shop but will expose the fan to the higher discharge temps coming out of the collector (probably not an issue for your smaller collector/larger fan setup). We could probably start a small discussion topic on best fan placement as there are numerous arguments for and against each but ultimately, it all comes down to what works best for your setup. I see a lot of collectors with fans on the outlet but these have no flow control dampers so I'm guessing the fan is properly sized. Since you mentioned that this will be in a shop area, I'd make provisions for some way to filter the air before it enters the collector depending on what kind of work you do in your shop. A fan's CFM rating is sometimes listed with no flow restriction. Some fans will give flow charts based on amount of flow restriction (inches of water column). There are charts available online that list flow restriction for a given duct type, size and length plus how much restriction is added by using standard elbows, tee's and such. You will have to experiment with your setup and tweak it to your liking. Anyway, hope this helps. Regards, Jason
stmbtwle

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Reply with quote  #29 
As the solar input varies with the weather, time of day, and location, I don't see how you can possibly "calculate optimal flow rates". You can calculate the flow rate for the maximum expected input, but any other time it will be too much. If you shoot for an average, there will be times when it is not enough and others when it is too much. "Optimal" my @$$! It looks to me like an exercise in futility, as evidenced by the number of two-speed and adjustable-speed setups.

The old reliable snap switch attempts to accommodate this by cycling, thus achieving an AVERAGE flow close to what is needed, but the cycling may be annoying to some. Also, most snap switches aren't adjustable and the ones that are have a pretty wide dead band. You end up with "good enough" instead of "optimal".


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Willie, Tampa Bay
JC77

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Reply with quote  #30 
I agree completely. Maybe in a perfect world you could have a temperature controlled vfd fan where you could set the outlet temp at a temperature of your choosing then when the collector reached the setpoint, it would start the fan on its lowest possible speed. If after a certain period of time went by and the collector outlet temp was still above setpoint, it would incrementally increase fan speed til collector output cooled down to setpoint and continue doing this as the sunny day progressed thru it's peak. Then as collector outlet temp began to fall, it would slowly lower fan speed to maintain a set outlet temp. Once the fan reached minimum speed and collector outlet temp stayed a few degrees below outlet temp for a set period of time, the fan would shut off. Maybe such a controller already exists. But I agree that a single speed fan running throughout the day and season and so forth is definitely not optimal. Solar radiation varies from hour to hour, day to day and season to season. For the average DIY solar build, as long as the builder understands the basic principals and concepts, the only thing one can do is experiment and see what works. That's what makes sites like these so great. It's nice to find a few "rules of thumb" but that's all they are. A starting point for planning your build. Get it built and start tweaking it is what I say. One thing I read was that the collector sq ft should be roughly 20 to 30% of the size of the area to be heated. I've never found much on collector volume like the original post asked. I'm not aware of any rule of thumb ratio other than the 20 to 30% thing. But I think that's kind of bogus too because every collector is different just like every space that's being heated by one is different. Like the original post said, I think a smaller collector is a great place to start. He won't wrap up a ton of money in it and he'll get a feel for what kind of heat he gets out of his collector. Almost all of us have a "lessons learned" or things we'd do different on a future setup. He'll live and work around his new small collector and constantly be thinking of ways to try to make it better or how the heat could be better applied to his shop. Then when he's ready to build something bigger and more permanent, he'll have a pretty good idea of what he wants to do. He's on the right track.
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