Understanding air flow in a solar collector

Discussion in 'Physics' started by praondevou, Mar 12, 2013.

1. praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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I'm starting doing simulations for a solar collector I would like to build (one day). I used Solidworks' FlowXpress for a first test, basically to see how the software works. (later on this requires a full version for heat transfer analysis)

I know that this design is very far from being efficient, just an initial test.
Apparently I don't know anything about fluid movement because I cannot understand the simulation results.

The picture below shows the airflow from one inlet to one outlet. The dimensions of the analyzed air flow space are: width 7.6m (about 25feet), total height 4.8m (about 16feet), depth about 3".
The inlet at lower left edge has an inflow of about 100cfm. The pressure at the outlet is mean atmospheric pressure (about 101kPa)

The velocity seems to be much faster near the edges. Why is that? I expected a more or less continuous airflow from inlet to outlet, widthening in the middel.

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2. praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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Answer: to much cfm at the inlet. When decreasing to 10cfm, it shows a result similar to what I expected.

Higher air velocity creates more turbulences.

We never stop learning.
Some interesting stuff: http://en.wikipedia.org/wiki/Reynolds_number

Thanks for looking anyway.

3. #12 Expert

Nov 30, 2010
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If it helps any, I see a stratification layer at the bottom based on the fact that hot air stays above cool air, and I see a chimney effect on the left edge.

Higher velocity will cause an entrainment function which means that the faster jet of air sucks surrounding air into its stream by the Venturi effect.

This might be helpful, and it might not.

As a guess, I would bet that this is not a solar-electric device. It only collects heat.

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4. THE_RB AAC Fanatic!

Feb 11, 2008
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Good catch! 100 CFM sounded a bit high, a good sized ceiling fan (kitchen extractor fan) runs about 12 to 15 CFM and is 12" to 14" across.

So 100 CFM through a small opening (you didn't say diameter) would be pretty severe!

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5. praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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Yes, just a heat collector. I forgot to mention that. Thanks for the hints.

And yes RB, absolutely right. If I put in 1cfm it does exactly what I thought it would do. (the opening is 6")

6. #12 Expert

Nov 30, 2010
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I would like to present these examples in the 190 to 250 CFM range and ask if this is what you had in mind. As an HVAC contractor, I find the idea of 10 to 20 CFM for a vent hood to be out of proportion to my experience.

http://www.ventahood.com/hoodmodels_specs.jsp?id=238

http://www.homedepot.com/p/t/202079...rizontal1-4-_-NA-_-202079988-_-N#.UUBRPTfdXGg

7. shortbus AAC Fanatic!

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Wow, this is the first time seeing a solar heating application in about 20 years. At one time solar heating, both air and water was the only "solar" stuff people talked about or built. Still a great way to use solar, just seems to have died out since solar electric came on the scene. Hope it makes a big come back.

8. praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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The goal is to slowly heat the air of a house, circulating it through the solar air heater. The air flow is probably really slow indeed, I have to research it. Passive devices use only the natural convection what I assume to be really low as well. Passive is not an option however.

9. praondevou Thread Starter AAC Fanatic!

Jul 9, 2011
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Soon efficiency of PV cells will increase. For now, for space heating it makes more sense to heat air directly or other fluids, like water which can store energy for some time.

All the incident solar energy on my south wall is simply lost and we have a long heating season. I expect to lower my heating cost in the long term.

10. shortbus AAC Fanatic!

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Back in my hippy days, almost every issue of Mother Earth News, had at least one solar heating article. Most of the air-air heaters had the same basic design. A box with a glass top, a metal divider separating the box into a to and bottom level. The cold air would come in on bottom level, getting pre-heated from the bottom of the metal divider and then transfer to the top section between the glass and divider.

Never made one because I lived in rental houses that landlords wouldn't allow that "New Age Crap" hanging off the side of their property.

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11. #12 Expert

Nov 30, 2010
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I would like to recommend that solar air and water heaters are very efficient (compared to solar-electric) and very cheap. My favorite design allows the chimney effect to send hot air into a high window. Less than a square foot of air duct is usually sufficient. A 4 inch (100 mm) computer fan can reduce the necessary openings to a 4 inch round duct.

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12. praondevou Thread Starter AAC Fanatic!

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Unfortunately between the space I want to heat and the solar heater there is a carport, that's why I need some ductwork and also a fan.

My very first simulation shows how little I understand of air flow. Fortunately computational fluid simulation can help a lot these days. (provided you have the computer power necessary or a lot of time )

I could build something that works more or less but since this is not some small a few square foot air heater I want to simulate it to get the maximum efficiency out of it. Especially interesting will be playing around with different collector materials/surfaces and materials transmittance/absorptivities etc.

13. THE_RB AAC Fanatic!

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Yeouch! Those are some humongous sized "pro extraction" fans!

Normal ceiling extraction fans as used in the ceiling of kitchen or bathroom etc are <20 CFM and 12" or so in (grille) diameter.

I was assuming of course that a "solar" powered air fan etc would be lowish power and low velocity. You don't need much air flow through a room to extract significant heat out of the top of the room.

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14. shortbus AAC Fanatic!

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Like#12 said, a computer fan made into one of these - http://www.grainger.com/Grainger/TJ...cm_sp=IO-_-IDP-_-RR_VTV70300505&cm_vc=IDPRRZ1

Another trick from the old days was to pump the heat from the collector into an insulated box with water jugs or if enough room water barrels. The water would heat up during the day and then release it after sun went down. A temp switch to control the fan after a set low temperature was reached. And one in the collector to do the same thing.

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15. #12 Expert

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A bit of math: Q=1.08 CFM dt
Translated: B.T.U.s = 1.08 Cubic feet per minute * temperature difference in F degrees.
The highest amount of heat you can get from sunlight is 270 BTU's per square foot per hour@ 100% efficiency.

One sheet of plywood (American) = 32 square feet.
270 BTU's * 32 ft^2 = 8640 BTU's per hour.
8640 BTU's per hour = 1.08 * 190cfm * dt
For temperature increase of the delivered air @ 100% efficiency
dt = BTU's/(1.08 * CFM)
dt = 8640/1.08 * 190
dt = 42F

Just providing a tool for your convenience, and never forget you can NOT achieve 100% efficiency.

Last edited: Mar 15, 2013
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16. praondevou Thread Starter AAC Fanatic!

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I get different values for maximum incident solar power on the internet for my region, but it's about in that range you said. The exact calculations seem to be very complex because I would have to take the heat loss of the building into account, which I don't know?

I guess if I can gain a couple of 100 W in winter out of my 30sqmeter wall I'll be happy. According to Ecotect analysis I would get not much more than 150W per square meter absorbed end of december (at noon).

17. #12 Expert

Nov 30, 2010
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I do this for my day job. I am inviting you to PM me.
(I rarely check the physics forum.)

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18. Metalmann Active Member

Dec 8, 2012
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I remember having a subscription to (Mother Earth News) back then. They always had great articles on these subjects.

Jump forward to last year...I asked the Wife to pick me up a copy when she went shopping. It was almost \$8.00 ...and was mostly ads.

I couldn't believe they published that many crazy ads. I'll never buy another copy.

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19. edwardholmes91 Member

Feb 25, 2013
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I saw a Grand Designs episode and someone used a very simple solar heating systems with louvers that you opened and closed manually to regular the air temperature inside the building. I think it was on an Australian episode and if I'm not confusing them... the gentleman also had an ingenious cantilevered designed that collected water in a massive tank in the center of the house.