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
Sign up Calendar Latest Topics
 
 
 


Reply
  Author   Comment   Page 5 of 10     «   Prev   2   3   4   5   6   7   8   Next   »
stmbtwle

Avatar / Picture

Registered:
Posts: 2,826
Reply with quote  #41 
Well I have my materials together and I'm building a couple identical small collectors. Identical fans, pumps, heat exchangers, glazing, etc. The only difference will the absorber inserts. This way I think I'll be able to get some useful comparative data. I've also got an irradiance meter so I should be able to get a handle on efficiency.

I'm not too excited about just blowing air around inside the collector and hoping it will mix and pick up heat from the backplate. What experiments I've already done seem to bear this out, as do Scott's tests with the empty box collector. I think an absorber of some kind is necessary, either screen or metal. Nor am I too confident in the screen absorber, as it seems there is a possibility of a "short circuit" through the screen nearest the fan, without the air ever reaching the distant parts of the collector. As the collector gets larger I think this could be more of a problem.

This collector (with or without an absorber) is in essence a dual-pass collector; the air has to circulate from the fan to the far parts of the collector and back to the fan. You cannot simply pump air in at one end and out at the other. So I am leaning toward a metal absorber between the glazing and the back, with the air flowing between the glazing and the absorber, around the end/sides, and back underneath, picking up more heat from the back of the absorber. In the spirit of the project, I think corrugated roofing would be appropriate for the absorber. However I haven't had the opportunity to test this, yet.

Thoughts??

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

solardan1959

Avatar / Picture

Registered:
Posts: 1,845
Reply with quote  #42 
stmbtwle,
  Just to be clear, you are still talking about the same radiator concept but you want to now flow air through, (in and out), of the box.  I like the dual pass concept for an ordinary hot air collector and have been wanting to try a few ideas in that area also but just for air.  If I did have a heat exchanger in the box I would not have the circulating fans but it would let the rising heated air heat the exchanger as the hot air passed by it.

Dan
stmbtwle

Avatar / Picture

Registered:
Posts: 2,826
Reply with quote  #43 
Yes I am still talking about the radiator concept. From what little testing I've done, the fan is pretty much essential. While there is SOME heat transfer without a fan, it is much reduced. To get the heat into the water you have to circulate the hot air past the coils; the more air you move the better the transfer, and getting the heat into the water is what it's all about. It's really not that different from a regular air collector. You could take any air collector and duct the air to an external box with the radiator in it, then back to the collector. The principle is the same, just with the ARETHA the heat exchange is internal.

Not saying it won't work without a fan, but the air temperature will be much hotter (near stagnation) and you're going to need a really big radiator (or several) to get the heat transfer. You might as well build a water-tube collector.

The WATER flow seems to work the other way. When I was pumping a gallon a minute through the exchanger, the delta T was barely measurable. After all, the water was only in the tiny heat exchanger for a couple seconds. When I reduced the flow (smaller pump) the delta went up accordingly. In terms of actual heat transfer, though, I expect it was pretty much a wash.

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

Avatar / Picture

Registered:
Posts: 2,826
Reply with quote  #44 
No Dan I was NOT talking about an "in and out" box, I was making a comparison. Visualize an air collector (any type) where the hot air discharge is connected to the "cold" input thus recirculating the hot air. The collector will get hotter and hotter until something fails. Now visualize a heat exchanger in the airflow through which you pump water to cool the air and thus the collector, at the same time heating the water. The only difference is that the ARETHA has all the hardware inside the collector.

To be honest Dan I think you're playing with me. [smile]

I'm mildly surprised that one of our members who already has a hot-air collector installed but sitting idle (it's summer) hasn't jumped on the external radiator concept to get hot water. Yes you CAN get the high temperatures, by recirculating the air. Agreed it's not as "efficient" as running at low temperature, but remember that a collector that is NOT running is 0% efficient.

__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
solardan1959

Avatar / Picture

Registered:
Posts: 1,845
Reply with quote  #45 
stmbtwle,
   Not playing at all, maybe just to stupid to understand.  When you threw in the dual pass it kind of confused me, not sure how it would help with this concept.  I actually was not sure you were still talking about a radiator collector and was going to suggest you start a new thread.  Early on in this thread people were talking about flowing air through but that would reduce the heat in the box too much and drop your water temp.
  I still think two radiator as exchangers would gather more of the heat rather than blowing air around but it is your project and interesting so far.

Dan
stmbtwle

Avatar / Picture

Registered:
Posts: 2,826
Reply with quote  #46 
I don't doubt that two radiators would be better than one, but I'm convinced that you DO have to blow the air THROUGH the radiators to get the best heat transfer (for the same reason your car has a fan). Whether it has to be blown "around" is yet to be determined.

I did a new test today with the first of the new collectors, and I was NOT impressed. Over 30 minutes I had an average of 992 w/sq m or 335 wh or 1142 btu input, and I think I captured 348 btu. IF my calculations are correct (and I'm not sure they are) that gives me only about 30% efficiency. I don't expect it to be terrific but I think it should be better than that. Back to the drawing board...

"Flowing air through" DOES work if you're flowing air you've already heated. Suppose ambient is say 30F and your collector can raise the temperature another 30, to 60F. If you recirculate this air you now have a 60F input. Raise that another 30 and you're up to 90F. Another "pass" and you have 120F, and so on. This is what the ARETHA does (only the air never leaves the collector). Trust me, it gets REALLY hot REALLY fast. Hot enough that it can heat water to 140F or so, no kidding (done that). Without the water circulating through the radiator (which actually cools the unit) I'm convinced it would overheat in short order.

__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
solardan1959

Avatar / Picture

Registered:
Posts: 1,845
Reply with quote  #47 
stmbtwle,
That's kind of my point, If you don't blow air around it will get very hot which should transfer to the radiator.  If it gets hot speed up the flow of the water to cool it down and with two radiators it takes even more heat away from the air.  Does blowing air transfer better than very hot not flowing air?  I wouldn't thinks so but maybe the cooler radiator cools the air right around it so the transfer does not work as well.  The blowing air would remove this colder air layer right around the metal and replace it with hotter air.

One smaller fan that circulates the air through the through the collector such as the dual pass you mentioned.  The fan would pull air down the back section of the dual pass and this flow and natural thermo syphoning up the front past the radiator would be the transfer method.  It may run hotter than your set up but again, an increased flow should keep it in control.

But you are dealing with a lot higher temps than I normally see.  Our high today was about 70 though we did see eighty yesterday.  If I was where you are I would be building a batch heater.

Dan
stmbtwle

Avatar / Picture

Registered:
Posts: 2,826
Reply with quote  #48 
When I get the second collector built I'll set it up with no fan at all, so it can have free air flow to the heat exchanger. I don't have any faith in the approach but it's worth a try, and at least we'll know.

My tests so far seem to indicate that the fan is necessary to get heat transfer through the heat exchanger. Output temperature can be controlled with the pump speed. But when you're pumping into a large reservoir, it's not the output temp that matters, it's the total heat. As the storage heats up so does the collector, creating a feedback loop, and eventually the desired temperature is reached. This is my experience so far.

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

Avatar / Picture

Registered:
Posts: 2,826
Reply with quote  #49 
Dan read my tests in posts # 28 & 29. In both of them I observed that without them the heat transfer didn't work as well. I'll try it one more time with a comparative test. It's a good idea, 'cause fans add complexity and cost, but so far it just hasn't worked as well.
__________________
Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
solardan1959

Avatar / Picture

Registered:
Posts: 1,845
Reply with quote  #50 
stmbtwle
I have read all your post and did see what you posted, you may be right and I was trying to explain above why it may work better that way.  It just goes against my "common sense" since it's the opposite reaction to a car radiator in use.  Blowing fast air just seems like it would have a cooling effect and a much lower flow seems like it would work better than a fast moving fan.

Dan.
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.

 

web statistics