Welcome to the Solar Collector
Brainstorming and Development Page!


 

Home

Hot Air Collector

Hot Water Project 1

Hot Water & Space Heating

Solar Electric

Solar Construction 101

FAQs

Best Collectors

Simply Solar
Register Calendar Latest Topics
 
 
 


Reply
  Author   Comment   Page 1 of 4      1   2   3   4   Next
TJTurner

Registered:
Posts: 16
Reply with quote  #1 
Afternoon everyone.

I've been lurking here for a while, and I'm quite impressed with all the work and projects everyone contributes. The detail here, and the volume of information has made me consider dipping my toe into the solar realm.

I've been thinking of starting with a 8'x4' collector to help heat my workshop, which is the back half of a 3-car garage (approximately 350 ft^2). I don't have to fully heat it, but any amount that I'm not running the heaters out there saves me big time. I'm finishing installing an insulated floor, then I'm hoping to focus my attention to a solar heater. I'm thinking of either an aluminum downspout or a screen absorber design, which will have to double-back (not sure what you guys call this) so the input and output ducts come from the same location more or less. To further constrain the problem, I know I'm limited to 4" input and output ducts (at least that's the best I can do without significant structural changes (i.e. drilling though the brick facade and the underlying cinder block). The collector will also have to be about 10 feet from the side of the building, so I was counting on having a blower fan (getting at least 3 CFM/ft^2...so like 100CFM or more) on the input side, and maybe getting fancy with a snap switch!

I do have one question for the experts out there. How do you guys go about calculating the volume of air that should be in the collector? I can see that I wouldn't want to have a smaller volume than the ducting so as to choke the collector. On the other hand, a huge volume would also likely be bad. But is there some rule for how much stagnation one seeks to have inside the collector? For instance, should the collector cross-section be some set ratio of the cross section of the ducting? Is there a rule for this...like make the collector cross section😃uct cross section be larger than 1 but less than X?

Thanks in advance...

gbwillson

Avatar / Picture

Registered:
Posts: 2,248
Reply with quote  #2 
Welcome TJ-

Interesting questions. The simple answer is to have a collector that captures the maximum amount of heat, and a fan that can extract that heat. Almost any heater is more efficient at higher fan volumes. A large amount of warm air is more efficient than a small amount of hot air. A fan speed control allows you to adjust the fan so that the output is to your preference. So make sure you have a fan a bit more powerful than you think you will need, and adjust accordingly. You might not like a lukewarm blast of air blowing on you, but it does maximize heat collection. Lower fan speeds will increase the temp of the output air, while a higher CFM will be more efficient and extract more BTU's from your heater. It takes very little air movement to ward off stagnation during the cold parts of winter. In fact, most of the winter, I doubt you would harm anything inside a collector should the fan not be running. Where stagnation is an issue is in the fall and spring during mild temps. The ambient air is much warmer and the sun is higher in the sky, so stagnation can be a real concern, especially if you live in an area with frequent power outages. 

I used to think the area of the ducting should match the area of the absorber. My early testing showed this to be true. But years later, I don't think you can put a specific ratio for any collector as the size and shape of the collector also have the variable of the absorber being used. I do think most collectors like an elongated profile rather than a square. This is simply because a long profile will keep air within the collector for a greater amount of time, which can lead to higher output temps. But if the air winds its way though a collector you can accomplish the same effect, at the cost of greater resistance to the airflow. So you have to be mindful of any choices you make. Compromise is always an issue. 

You have a size limitation, as well as a limit to the size of the ducting to be used. But for your first collector build, I wouldn't let either one slow or prevent your progress as any heater gives you more heat than the one you never end up building.

Greg in Minneapolis[wave]


TJTurner

Registered:
Posts: 16
Reply with quote  #3 
Greg...

Thanks for the response. I am defiantly going to use a speed control on the fan to find the optimal efficiency for the collector. I was thinking of an inline 4" duct fan with a 200 CFM max rating. That should be enough I figured, even with the pressure drop. I guess I want to give it the best shot at getting there by making it either deep enough (for a screen absorber), or have enough downspouts for the downspout absorber. 

Even with a fan on a speed controller, do you recommend a rule of thumb for that calculation? I definitely will make it such that I at least match the cross section of the 4" duct. I guess I was hoping even a rough guess at how you guys design them might get me started. Otherwise, I'll dive right in and report back what I do so I can contribute to the larger community here.

Thanks! 

Tj 
In Sunny Ohio.

TJTurner

Registered:
Posts: 16
Reply with quote  #4 
And here is like what I was thinking for the basic collector... SolarBox_ISO.png  SolarBox_front.jpg 
gbwillson

Avatar / Picture

Registered:
Posts: 2,248
Reply with quote  #5 
I was going to suggest about 200CFM, but until you build the absorber, you won't really know just how much you need. If in doubt, use a more powerful fan. It's very rare to have too much flow with a good collector design. You can always control the speed. If you already have a fan of pretty much any size, you could use that until you get the unit up and running. Then you would have a better idea as to the airflow needs of your design. You could also look at 6" fans as they don't cost much more than a 4" fan, but their output is much higher. All you would need is a 4" to 6" adapter. 

If the drawing is to scale, you may want to open up the wall at the return. This point will cause a lot of airflow resistance. 

Greg in MN
TJTurner

Registered:
Posts: 16
Reply with quote  #6 
Thanks Greg. Would you recommend a screen absorber over a downspout design? And if so...how deep? And I agree, I need to open up that top turn around part.

Thanks!

Tj
gbwillson

Avatar / Picture

Registered:
Posts: 2,248
Reply with quote  #7 
TJ-

Either design will work fine. But unless you already have the downspouts, screen material is less expensive, especially fiberglas. If you use screen there are different ways to configure the screen. Most common is a double or triple layer of angled screen. Another way is to have the screens parallel with the glazing, where the air passes between the two layers screen. 

The depth of a collector is usually not a huge issue. Although a deeper collector box contains more air, so the box may take slightly longer to warm up in the morning. But if the box is too shallow, the air passes close to the cold glazing.

Greg in MN
TJTurner

Registered:
Posts: 16
Reply with quote  #8 
Thanks Greg.

I thought I read somewhere that some of you guys were experiencing some out-gassing with the fiberglass screen. Is that still true? Has anyone tried the pure sun screen variety? I supposed the mesh may be too tight on that product though for effective passage through the material. Maybe for the ZP though?

Tj
gbwillson

Avatar / Picture

Registered:
Posts: 2,248
Reply with quote  #9 
TJ-

I've never had a problem with outgassing fiberglas screen. I gave my large roll of fiberglas screen the "smell test" before purchase. I suppose different brands might be different. But since both fiberglas and aluminum screen are often painted, what you would smell would be the paint. If in doubt, you could always contact the manufacturer directly about possible off-gassing or ask for an MSDS. Another option would be to get a mill finish screen and apply paint of your choice. I have yet to find an aerosol can of no VOC spray paint. But you could thin down the appropriate paint and apply it via a spray gun. I do give the sun-facing side of all my screens a light coating of flat black paint to cut back on the sheen. These are enamel, and have no noticeable smell once dry. But, I suppose anything can outgas if the conditions are right, such as if a collector stagnates. A newly finished collector will also have that certain "smell" for a few days until it airs out a bit. Paints, adhesives, silicone, plastic glazing all have a certain smell when new. But exhausting the collector outdoors until any smell dissipates is certainly recommended.

On my ZP, I have been doubling up the back screen layer to make it a bit denser and keep the heat in the screen gap, while still allowing some air to move around the screen mesh. I also picked up a couple of small rolls of solar screen which is Roughly 2-3 times as dense as regular screen. So I will use that as the back layer on the next build. I think you would have no problem using sun screen for the back layer. But it has to have enough openings to allow some air movement through it. Being a lot thicker than regular screen, it would also be very difficult to use traditional screen frame material, as it would be nearly impossible to stuff in the groove with spline. And of course, don't use a dense screen layer for the front-most screen as this will block too much of the sun from passing into the screen gap where it can be the most effective.

Greg in MN
 
TJTurner

Registered:
Posts: 16
Reply with quote  #10 
Thanks Greg!

I've been reading up on your ZP design, and the ones others have built. They look impressive. After your advice here, I have have found a way to get 6" ducts into my workshop after all, so I'm thinking of going that route.

It seems like all the ZP designs are running horizontal. Is that right? And if so, is there a reason for it? Could a ZP design run horizontal on a 8'x4' panel, for instance? 

I'm getting more inclined to go with the ZP design. Looks like you settled on something with just over a 1" gap between the screens? I would think that the solar screen would be a great back screen layer. I guess I am confused through, why would air have to go through it? Don't we want the flow to stay between the screen layers? 

My workshop is almost finished up--insulating the floors. Then I'm going to move on to building the collector!

Thanks!

Tj
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.

 

web statistics