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solar air heater review and a few questions

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Posted by Jersey John on September 17, 2008, 5:08 pm
 
Hello,

I intend on building a solar air heater in the coming weeks. I am
going with
a passive design with air flow thru an absorber screen made form a
double
layer of 80% open metal window screen. Dimensions of collector will be
24
inches wide x 12 foot tall. House is a spit level with no windows on
south
side exposure. Drawing air from lower level, returning to upper level.

Due to house construction I can not go with wide rectangular openings
- as
with Morris barn design. Ideally I would like to have two lower inlets
and
one return. The top of the collector would be peaked at perhaps a 15
degree,
left and right, to aid flow to a centrally located return.

Questions
1. Relationship of inlet and outlet size. Should they be equal in
terms of
square inches?

2. How do those sizes relate to the air passages widths illustrated as
a and
b below?
If my two inlet diameters are 3 inches = 7 sq inches x 2 = 14 sq
inches
Should my return be a bit greater than, less than or equal to 14 sq
inches?

3. I have light gauge Al sheet for the back as well as 5 mil Cu sheet
Which would be best for the backer sheet?

For example:
The absorber screen would be tilted like this    \
_____________
|__|      ____return
|a  : b  | 1 | 2 | 3 |
|    :     |   |    |    |        1 = al or cu black back (I have both
on
hand)
|    :     |                     2 = polyiso
|    :     |                     3 = Al sheet
|    :     |                     | = glazing
|    :     |                     : = absorber screen
|    :     |
|    :     |
|    :     |
|    :     |
|    :     |
|    :     |
|    :     |
|    :__ |
|______  inlet


Calc's - with inefficiencies not factored

  Collector Area: 24 (sqft)
  Collector Azimuth: 0 (deg) measured from South
  Collector Tilt: 90  (deg) measured from horiz
  Latitude: 39  (deg)
  Altitude above SL: 160  (ft) Above Sea Level

Date --  Sun -----     Collector --
Month Day Direct  Difuse  Total  Direct  Difuse  Total
1 15 2184 125 2309 40084 1500 41584
2 15 2527 150 2677 40049 1805 41854
3 15 2896 198 3095 34649 2379 37028
4 15 3075 284 3358 22363 3407 25770
5 15 3164 370 3533 13117 4436 17552
6 15 3183 418 3602 9000 5019 14019
7 15 3092 419 3512 10221 5032 15253
8 15 2965 369 3334 16979 4430 21409
9 15 2755 271 3026 28026 3255 31281
10 15 2547 194 2741 36799 2324 39122
11 15 2211 143 2354 39197 1715 40912
12 15 2040 119 2159 38767 1433 40200
Sum  32638 3061 35699 329252 36734 365986

I did not mention the back flow preventer.
It has been addressed.

Thank you for any and all comments.
John


Posted by gary on September 22, 2008, 1:18 am
 
Hi -- comments embedded below...


Yes, they should have about the same total area -- the larger the
better -- the idea is that the inlet and outlet vents should be as
large as possible to reduce airflow resistance, which increase
airflow, which removes heat better, which makes the collector more
efficient.
See below for sizing guidelines.




Not sure exactly what you mean, but the usual design rule is that the
depth of the collector should be 1/15 the height.  So, for a 12 ft
high collector, the 1/15 th rule would give you a depth of nearly 10
inches.  For such a tall collector, it may not be necessary to got his
deep.  If you went with (say) an 8 inch depth, than the cross section
area would be 24*8= 192 sq inches, and the inlet vent and outlet vent
area should each be about half this -- around 100 sq inches.

If you undersize them, the collector will still generate heat, but the
airflow will be slow, and the collector temperatures will be high,
which will cause it to lose more heat out the glazing and be less
efficient.


I think that is too small.  If this is the biggest vent you can
provide, you probably need a fan to get  to a reasonable amount of
airflow.


As big as you can make it, but, again, 14 sq inches is very small for
this size collector.


Either one should be fine.  If its right at the back of the collector
behind the screens, then painting black or a dark color will help
improve absorption.


Output from my program! :)  Remember that this is sunny day solar
input to the collector, so if you do a good job designing and building
the collector, you may get about 60% of this heat into the room.




Posted by nicksanspam on September 22, 2008, 10:55 am
  

So the vents should be the same size as the glazing, as in a window? :-)


Why half? Shouldn't the vent area depend on the glazing area?

In full sun (250 Btu/h-ft^2), a 70 F room on a 30 F day with R1 glazing
with 90% solar transmission might look like this, viewed in a fixed font:

    0.9x250x24ft^2 = 5400 Btu/h    I = 16.6(100/144)sqrt(12)(T-70)^1.5

           ---                         ---        = 40(T-70)^1.5 Btu/h
|---------|-->|-----------------------|-->|---->
           ---    |                    ---
                  |
          1/24    |
30 F ------www-------- T

with T = 30 + (5400-40(T-70)^1.5)/24 = 70 + (153-0.6T)^(2/3).

Plugging in T = 100 on the right makes T = 90.5 on the left.
Repeating makes T = 91.4, then 91.3, with I = 3927 Btu/h and
a 65% collection efficiency.

If 100 in^2 vents collect 23,562 Btu on a 6-hour sunny December day and
lose 18h(70-30)2x100/144/R1 = 1000 at "night," would larger or smaller
vents be more efficient?

10 FOR A = 25 TO 250 STEP 25'vent area (in^2)
20 AV=A/144'vent area (ft^2)
30 K.6*AV*SQR(12)'chimney constant
40 TI0'initial temp
50 Tp+(255*24/K-24*TI/K)^(2/3)
60 IF ABS(T-TI)>.1 THEN TI=T:GOTO 50'iterate
70 GAIN=6*K*(T-70)^1.5'(Btu/day)
80 LOSS*(70-30)*2*AV'(Btu/day)
90 NET=GAIN-LOSS'(Btu/day)
100 EFF0*NET/(250*24)/6'(%)
110 PRINT 1000+A;"'",T,NET,EFF
120 NEXT A

in^2      T (F)         net gain      eff %

025       117.761       19521.39      54.22607
050       102.3038      21495.75      59.71041 <-- 1.4% of the glazing
075       95.38025      22226.95      61.74154
100       91.30493      22561.75      62.67152
125       88.56468      22706.79      63.07442
150       86.57139      22744.71      63.17974
175       85.04369      22715.67      63.09907
200       83.82823      22641.51      62.89307
225       82.83368      22535.45      62.59846
250       82.00187      22405.81      62.23837

It looks like the vent area doesn't affect the efficiency much.
For more efficient heat storage (eg in a shiny ceiling mass),
we might use a smaller vent area with a higher outlet temp.

Then again, smaller vents work as well on average vs sunny days, and
vents also lose heat on cloudy days... 1% might work well above.
 
Nick


Posted by gary on September 23, 2008, 1:38 pm
 
Hi Nick,

On Sep 22, 4:55am, nicksans...@ece.villanova.edu wrote:

It seems like the glazed area does come in through the cross sectional
area -- as you increase the width of the glazing, the cross section
also goes up, and as you increase the height, the cross section also
goes up because of the 1/15 guideline??




I find that very surprising -- I'll take some measurements for a some
different vent sizes on my collector and see how it comes out.
It would be nice if one could get away with such small vents, but I
have my doubts.  Have to wait for a good sunny day.

Gary






Posted by Morris Dovey on September 23, 2008, 2:25 pm
 gary@builditsolar.com wrote:


I didn't see any indication of whether this is a passive or active
collector, and that'll probably be important.

With a blower of sufficient horsepower/cfm you could probably get away
with postage stamp size vents...

...but for a passive (non-blown) collector, vent size definitely /is/
important. At some point, a smaller vent size will slow airflow, raise
the temperature in the absorber chamber, and increase losses due to
re-radiation, and conduction.

In addition, allowing the temperature at the surface of the absorber to
rise is detrimental to efficient removal of heat from an air heater - an
aspect of which Nick (along with dow and daestrom) first made me aware.
This effect can be particularly significant in passive collectors, and
raises the amount of blower power needed for active collectors.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/

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