solar air heater review and a few questions
Jersey John
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
ga...@builditsolar.com
On Sep 17, 11:08 am, Jersey John <jerseyj...@comcast.net> wrote:
> 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?
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.
>
> 2. How do those sizes relate to the air passages widths illustrated as
> a and
> b below?
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.
> If my two inlet diameters are 3 inches = 7 sq inches x 2 = 14 sq
> inches
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.
> Should my return be a bit greater than, less than or equal to 14 sq
> inches?
As big as you can make it, but, again, 14 sq inches is very small for
this size collector.
>
> 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?
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.
>
> I did not mention the back flow preventer.
Good.
Gary
nicks...@ece.villanova.edu
>Jersey John <jerseyj...@comcast.net> wrote:
>>
>> I intend on building a solar air heater in the coming weeks...
>> Dimensions of collector will be 24 inches wide x 12 foot tall.
>
>airflow resistance, which increase airflow, which removes heat better,
>which makes the collector more efficient.
So the vents should be the same size as the glazing, as in a window? :-)
>... 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.
>... 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.
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=16.6*AV*SQR(12)'chimney constant
40 TI=100'initial temp
50 T=70+(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=18*(70-30)*2*AV'(Btu/day)
90 NET=GAIN-LOSS'(Btu/day)
100 EFF=100*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
ga...@builditsolar.com
Hi Nick,
On Sep 22, 4:55 am, nicksans...@ece.villanova.edu wrote:
> <g...@builditsolar.com> wrote:
> >Jersey John <jerseyj...@comcast.net> wrote:
>
> >> I intend on building a solar air heater in the coming weeks...
> >> Dimensions of collector will be 24 inches wide x 12 foot tall.
>
> >... 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.
>
> So the vents should be the same size as the glazing, as in a window? :-)
>
> >... 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.
> >... 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.
>
> Why half? Shouldn't the vent area depend on the glazing area?
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
Morris Dovey
> 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.
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/
ga...@builditsolar.com
We have a nice sunny day going today, so took some measurements of
collector output at various vent sizes.
Cross section area per half bay = (5.75)(23.25) = 133,7 sqinches
Vent when full open = (4)(18) = 72 sq inches -- just a touch above 50%
of cross section
Condition Vent velocity Col delta T Vent area Product
Relative
Full open 130 fpm 29F 72
271.4K base
Half blocked 158 fpm 34F 36 193.4K
71%
3/4 blocked 180 fpm 50.5F 18 163.6K
60.2%
The "Product" column is (vent velocity)(delta T from inlet to outlet)
(Vent area) -- so, proportional to collector heat output.
Temperatures in F, velocities in fpm, areas in sq inches.
Test started at 11:15am and went through 12:24 pm -- started with 4
measurements with full open vents at about 2 minute intervals, then 4
with half blocked vents at 2 minute intervals, then 3 at 3/4 blocked
at 2 minute intervals, then 3 back at full open.
Not a lot of variation from reading to reading -- pretty consistent.
Used the Kestrel wind meter for velocity and just did the center of
vent velocity (seems to me this favors the small vents with higher
perimeter to area ratio) -- checked readings against the Dwyer Vane
Meter.
For temperature, just used 2 hardware store alcohol thermometers with
the plastic cut away from bulb area so they would not block flow --
checked to make sure they read the same before starting, although it
would not make much dif for this test.
The azimuth on the collector was zero at 12:14.
Appeared to be very clear with some breeze.
Tambient was about 60F.
Note that collector output is still modest at this time of year
because sun is still fairly high (45 deg) and no snow field in front
of collector. On a mid winter sunny day, it routinely produces 50 to
60F temperature rise with 140fpm, and this is with a colder ambient.
Vertical collectors are the deal for space heating.
Seems to me the penalty for small vents is not so small -- wish it
were the other way round.
Seems like there is something that could be learned by studying this
vent thing in more depth -- for example, effect of nicely shaped
turning vanes at the vents and closer attention to a lower drag
configuration inside the collector. The numbers indicate to me that
not nearly all the flow resistance is due to the absorber?
Gary
ga...@builditsolar.com
-- here are the results from the three tests.
The first test is fully open vents, in the 2nd, half the vent area is
blocked with cardboard, and in the third 3/4 of the vent area is
blocked with cardboard.
Test 1: Vents full open
---------------------
Vent area = 72 sq inches
Vent velocity = 130 fpm
Delta T from lower vent to upper vent = 29F
Product of Area*dT*Velocity = 271.4K
Relative performance = base
Test 2: Vents half closed
------------------------
Vent area = 36 sq inches
Vent velocity = 158 fpm
Delta T from lower vent to upper vent = 34F
Product of Area*dT*Velocity = 193.4K
Relative performance = 71% of full open
Test 3: Vent 3/4 blocked
-------------------------
Vent area = 18 sq inches
Vent velocity = 180 fpm
Delta T from lower vent to upper vent = 50.5F
Product of Area*dT*Velocity = 163.6K
Relative performance = 60.2% of full open
Gary
schooner
Based on your numbers I wonder if moving more air through the unit would
further increase the performace by getting more air through the system and
in turn heated. Its really trying to find a balance between the temp
increase and the airflow to max out the overall heating performance.
<ga...@builditsolar.com> wrote in message
news:755778d5-a271-4fb3...@2g2000hsn.googlegroups.com...
ga...@builditsolar.com
I don't have a fan -- its designed as a thermosyphon collector.
I tested it last winter with pyranometer, temperature logger, and two
kinds of airflow monitoring, and its efficiency is quite good.
With the vents that are on it, it actually achieves the airflow that
is recommended for fan forced collectors.
I like the idea of not having to buy, power, and maintain a fan and
controller.
I really like the way it responds to low sun conditions -- as soon is
there is enough heat to warm the inside of the collector just a bit, a
very small airflow starts -- you can just barely see the back draft
damper move. As sun increases, the flow increases accordingly.
With a fan forced collector and controller, setting the controller is
always a compromise -- if you set it to respond to very low sun
conditions you may be using more energy running the fan that the
collector produces, and if you set it to wait for more sun, you are
not collecting the energy from low sun conditions (albeit small).
I am a "fan" of well designed thermosyphon collectors -- their
simplicity, low cost, no maintenance, and subtle control system are
amazing (to me).
Gary
On Sep 24, 4:15 am, "schooner" <schoo...@accesswave.ca> wrote:
> Gary - Do you have a fan in your unit?
>
> Based on your numbers I wonder if moving more air through the unit would
> further increase the performace by getting more air through the system and
> in turn heated. Its really trying to find a balance between the temp
> increase and the airflow to max out the overall heating performance.
>
> <g...@builditsolar.com> wrote in message
schooner
efficient.
I guess I'm just use to working with our designs for mounting on our house,
where we don't have the option to do such a wide inlet and outlet, which
forces us to use a single in/out from the collector boxes with the fan to do
the work to move the air, but overall it has worked well. For one of my
panels the inlet and outlet are both at the bottom due to the need to have
it heat the basement, no real other option when the access points need ot be
so low. Also handly if the panel needs to be mounted from the house a bit
and so on for best sun.
<ga...@builditsolar.com> wrote in message
news:9593825b-0b8a-4d04...@l64g2000hse.googlegroups.com...
Morris Dovey
> Just to clear up the badly formatted table of results in the last post
> -- here are the results from the three tests.
<snipped>
Thanks to Gary for posting his test results!
A couple of general observations...
The natural airflow appears to be directly related to the
cross-sectional area of the plenum, and inversely related to the surface
area of the plenum.
What this means is that the "ideal" plenum would be cylindrical. :-)
Given that we're practically constrained to a rectangular cross section,
the best we could do would be a square cross section, which still falls
short of what we want to build - but points us in the right direction:
to incorporate the deepest practical plenum.
For a given design/construction, the operating temperature is directly
related to the height of the panel.
For a given design/construction/height, the amount of energy captured
and the amount of heat delivered will be directly related to the width,
but the temperature is not, in general, affected by the width.
The "ideal" intake vent matches the cross-sectional area of the plenum.
If the intake vent is larger than the plenum, then the plenum becomes
the bottleneck. If the intake vent is smaller than the plenum, then the
intake vent becomes the bottleneck.
The "ideal" discharge vent is likely to be larger than the intake vent
in order to accommodate the same natural flow of expanded air, but I'm
unsure how to generalize this be cause design dependencies may come into
play allowing (or not allowing) the rate of flow to increase through the
discharge vent.
HTH
Morris Dovey
<snipped>
> HTH
Hmm - no feedback so I don't know if I managed to communicate anything
helpful or not.
On the off chance that the info might be useful to someone, I've
incorporated it into a(nother) web page at the link below...
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
Bill Kreamer
Bill K here. Sorry to jump in so late in the discussion. You have a
nice project going. I would make several observations.
1. The whole idea of an absorber is to let as little light through as
possible. A metal screen absorber, if you are bent on using that
material, should have at least 5 layers. Rather than 80% open, you
want the composite to be 80-90% closed.
Better yet, use a thick black material with fine fibers. You can try
weed control landscaping cloth. Caveat: “shade control” material is
generally not very good at stopping light. Try black polyester felt is
available at fabric stores. Get a thickness that stops a good amount
of light. Test by looking carefully through the candidate material at
the sun, with that "80-90% closed" in mind. I have used black
polyester felt as a solar collector absorber material for 20 years
without observing significant material degradation. The material is
best mounted at an angle, as you show above, silicone-glued onto 1"x1"
el "rails" of aluminum flashing.
A felt absorber's finely divided and dispersed mass gives many
benefits: reliance on internal heat-transfer is reduced; the heat-
transfer path (moving along the fiber from a sun-lit fiber site to an
adjacent shaded site) is very short; and the surface area is very
great, as much as fifty times the surface area as 5 layers of screen.
Surface area is the name of the game in air-cooled solar absorbers.
And the micro-turbulence of the air flow is good.
There are advantages of felt that accrue because of the small diameter
of the fibers. A sun-lit fiber transfers its heat in several ways.
First, through outer-surface heat exchange. Unlike liquid-cooled
absorbers, a goodly portion of the total heat transfer occurs directly
at the site at which the heat is generated, i.e. without moving
anywhere by means of internal conduction. Second, heat is conducted
internally along the fibers, but only a very short distance. Because
the bulk of sun-lit interception sites are very short due to the
random crossing and shading of the fibers in the material matrix, a
virtual tripling of the initial sunlit surface area is available (for
increased heat exchange) within one fiber diameter of distance in
either direction along the fiber. The increased surface area being "a
fiber diameter away" means that smaller fibers are better.
Another advantage of the short conduction path is that using a
material that is less conductive than metal does not hurt
performance.
2. The planar back wall of the collector has a poor heat exchange
capability compared with the absorber. Ideally, it should reflective,
and serve as a mirror to turn un-intercepted light back to the
absorber, which has the qualities to do the job.
3. It's not the vent cross section area per se that matters, but the
flow rate per glazing area. Efficiency is better at higher flow rates.
The flow rate should range from 70 to 120 cfm for a 4'x8' collector.
You will probably need a fan to do this.
Best of luck on your very interesting project - Bill Kreamer
Jersey John
around the rationale for the " 1/15th rule of thumb". Care to
elaborate?
My sketch calls for a panel roughly two inches thick, a plenum cross-
section of one inch (the other inch being polyiso). 1 x 24 x 144 = 24
cubic feet With the plenum cross-section of 24 sq in, related to the
inlet/outlet of 14 sq in each, the inlet/outlets are 58% of plenum
cross-section, granted, perhaps not as efficient but passive. As Gary
noted, I will pay a 25-30 % penalty but it will be passive not active.
Will be taking pictures along the way and I may even attempt to make a
web page showing it all.
Did I miss something?
Regards,
John
Steve O'Hara-Smith
Morris Dovey <mrd...@iedu.com> wrote:
Strangely this did not arrive at my news server until Saturday.
> Thanks to Gary for posting his test results!
Seconded.
> A couple of general observations...
<snip> Interesting and at the same time unsurprising observations -
it's very nice to see expectations confirmed by experimental evidence.
> The natural airflow appears to be directly related to the
> cross-sectional area of the plenum, and inversely related to the surface
> area of the plenum.
>
> What this means is that the "ideal" plenum would be cylindrical. :-)
Hmm - contemplates a hemicylinder - nah rectangular it is.
> Given that we're practically constrained to a rectangular cross section,
> the best we could do would be a square cross section, which still falls
> short of what we want to build - but points us in the right direction:
> to incorporate the deepest practical plenum.
Yep somewhere diminishing returns will bring a sensible limit to
depth.
> For a given design/construction, the operating temperature is directly
> related to the height of the panel.
>
> For a given design/construction/height, the amount of energy captured
> and the amount of heat delivered will be directly related to the width,
Height too of course - but adding width is more efficient than
adding height since ...
> but the temperature is not, in general, affected by the width.
...
> The "ideal" discharge vent is likely to be larger than the intake vent
> in order to accommodate the same natural flow of expanded air, but I'm
> unsure how to generalize this be cause design dependencies may come into
> play allowing (or not allowing) the rate of flow to increase through the
> discharge vent.
My inclination is that it's not going to be terribly critical once
the discharge vent is big enough not to be the bottleneck. Expansion is
proportional to temperature (all other things being equal) so the ratio
of discharge vent size to plenum CSA is proportional to height.
<brain engages - whirrr>
Hmm - ideally the plenum should be expanding in CSA with height to
accomodate the expansion of the warmed air. Anyone care to take a stab at
the appropriate rate ?
--
C:>WIN | Directable Mirror Arrays
The computer obeys and wins. | A better way to focus the sun
You lose and Bill collects. | licences available see
| http://www.sohara.org/
Morris Dovey
> On Wed, 24 Sep 2008 11:57:38 -0500
> Morris Dovey <mrd...@iedu.com> wrote:
>> The "ideal" discharge vent is likely to be larger than the intake vent
>> in order to accommodate the same natural flow of expanded air, but I'm
>> unsure how to generalize this be cause design dependencies may come into
>> play allowing (or not allowing) the rate of flow to increase through the
>> discharge vent.
>
> My inclination is that it's not going to be terribly critical once
> the discharge vent is big enough not to be the bottleneck. Expansion is
> proportional to temperature (all other things being equal) so the ratio
> of discharge vent size to plenum CSA is proportional to height.
>
> <brain engages - whirrr>
That may be the most pleasant sound I've heard all week... :-)
If we assume unimpeded/frictionless airflow and constant viscosity with
equal intake, plenum, and discharge vent sizes, then we should see a
higher linear velocity at the discharge vent than at the intake vent
because the discharge volume is greater than the intake volume.
If we choose to design the absorber plenum in such a way that the intake
and discharge linear flow rates are equal, then we need to examine the
ratio of the absolute temperatures at the two vents - and apply that
ratio to determine the reverse taper of the plenum and the vent sizes so
that the discharge vent is as large as necessary, but no larger. :-]
> Hmm - ideally the plenum should be expanding in CSA with height to
> accomodate the expansion of the warmed air. Anyone care to take a stab at
> the appropriate rate ?
Hmm back at you. :-) If we examine the original assumptions of
unimpeded/frictionless airflow and constant viscosity and consider the
effects of holding more and warmer air in the absorber plenum for a
longer time, I think the results will be heavily dependent on, of all
things, the absorber - since it is at the absorber/heat exchanger
surface that the effects of those factors I was so happy to make
assumptions about will be most pronounced.
ga...@builditsolar.com
...
> The "ideal" intake vent matches the cross-sectional area of the plenum.
> If the intake vent is larger than the plenum, then the plenum becomes
> the bottleneck. If the intake vent is smaller than the plenum, then the
> intake vent becomes the bottleneck.
I agree that it would be nice to be able to make the vents the same
size and area as the collector cross section. And, further to have
nice rounded aerodynamic corners to help the air make the corner.
It would be nice to find out how much this actually gains from the
vents that are half the cross section area.
Just not sure I want to cut holes that big in my wall :)
Gary
ga...@builditsolar.com
Hi Bill,
>
> Bill K here. Sorry to jump in so late in the discussion. You have a
> nice project going. I would make several observations.
>
> 1. The whole idea of an absorber is to let as little light through as
> possible. A metal screen absorber, if you are bent on using that
> material, should have at least 5 layers. Rather than 80% open, you
> want the composite to be 80-90% closed.
I think the 2 layers of screen actually works out quite well for a
thermosyphon collector.
When you do the numbers, and take into account absorption at layer 1
on the way in, layer 2 on the way in, the back wall of the collector
absorbing, then layer 2 on the way out, and layer 1 on the way, very
little light escapes.
The big advantage of screen is that it has little flow resistance and
still has a lot of area, so you can get both good absorption and good
airflow, where the airflow is just as important to the heat output as
the temperature rise.
My collector has 5 identical bays, and I tried a side by side test
with 1 layer of screen, 2 layers of screen, and 3 layers of screen.
The heat output for 2 layers was more than 1 layer, but 2 layers and 3
layers were nearly tied. So, it seems to me that while the 3rd layer
increased absorption a bit (a very small bit I think), it increased
flow resistance enough that the net result was no more heat output.
Gary
Morris Dovey
> Hi Morris,
>
>
> ...
>> The "ideal" intake vent matches the cross-sectional area of the plenum.
>> If the intake vent is larger than the plenum, then the plenum becomes
>> the bottleneck. If the intake vent is smaller than the plenum, then the
>> intake vent becomes the bottleneck.
>
> I agree that it would be nice to be able to make the vents the same
> size and area as the collector cross section. And, further to have
> nice rounded aerodynamic corners to help the air make the corner.
> It would be nice to find out how much this actually gains from the
> vents that are half the cross section area.
The wider vents are worthwhile - the rounded corners (which would
subtract from the height of the glazed area) do not seem worthwhile.
> Just not sure I want to cut holes that big in my wall :)
I understand the feeling. It's not just a matter of cutting holes
because a wide opening calls for a pair of jack studs to support a
header over the opening to support the structure above the panel. I have
a construction drawing somewhere here, and I'll put it up on a web page
so people can see what's needed. It's not rocket science, but it does
need to be done right.
I guess it's less a matter of feeling than doing what's needed in order
to have the full benefit to be derived from paying the costs.
You can get an idea what the wide openings look like from the photos on
the web page at the link below.
If you did cut holes that big in your wall, you'd be glad. :-)
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
ga...@builditsolar.com
Hi John,
The 1/15 th "rule" is for thermosyphon collectors -- ones that operate
on natural convection and don't have a fan.
I believe that it came out of work done by Steve Baer years ago, and
is based on experiments with actual collectors.
The idea is that if you use the groundrule, you will have enough air
flow through the collector to make it relatively efficient.
It says that if the height of your collector is 8 ft (96 inches), than
the depth of the collector airflow passage should be 1/15th of that or
about 6 inches -- this is the depth of just the air flow passage, and
does not include insulation.
The depth of the airflow passage increases as the collector gets
taller because the collector glazed area goes up with height, and the
greater glazed area means that collector generates more heat, which
requires more airflow to carry the heat away and more collector depth
to handle that airflow.
So, your 1 inch deep air flow passage is way way to thin to meet this
guideline, and (I think) the airflow will be very low, and the
collector not very efficient. You would get a large temperature rise,
but very little airflow, and the losses out the glazing would be
high. But, the depth of the airflow passage is closer if you are
going to use a fan to move air through the collector.
Gary
Morris Dovey
> I have
> a construction drawing somewhere here, and I'll put it up on a web page
> so people can see what's needed. It's not rocket science, but it does
> need to be done right.
Found it! There's a quick and dirty web page at the link below.
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
Bill Kreamer
Hey there Gary,
Your are right, of course, the increase in resistance caused by a
denser absorber does reduce the flow in a passive system. But you will
see that problem go away, and efficiency and output increase, if you
bite the bullet and decide to use a small quiet fan. Incidentally, as
soon as you do, the back wall makes a relatively small heat exchange
contribution (the denser absorber becomes the major contributor) and
so in this case (fan, w/denser absorber) it should be made reflective.
Please keep up the good work. Your website (builditsolar.com) is a
pleasure!
-Bill
Steve O'Hara-Smith
Morris Dovey <mrd...@iedu.com> wrote:
> Steve O'Hara-Smith wrote:
> > On Wed, 24 Sep 2008 11:57:38 -0500
> > Hmm - ideally the plenum should be expanding in CSA with height
> > to accomodate the expansion of the warmed air. Anyone care to take a
> > stab at the appropriate rate ?
>
> Hmm back at you. :-) If we examine the original assumptions of
> unimpeded/frictionless airflow and constant viscosity and consider the
> effects of holding more and warmer air in the absorber plenum for a
> longer time, I think the results will be heavily dependent on, of all
> things, the absorber - since it is at the absorber/heat exchanger
> surface that the effects of those factors I was so happy to make
> assumptions about will be most pronounced.
I rather suspect you're right there, worse yet it's going to depend
on the absolute temperatures of intake and outflow which are not constant.
It might be interesting to make a type 2 collector (your terminology) with
an adjustable sloping middle panel and experiment to see if there's any
appreciable impact on efficiency. My gut feeling is that it's probably not
great.
Morris Dovey
> On Sat, 27 Sep 2008 18:04:45 -0500
> Morris Dovey <mrd...@iedu.com> wrote:
>
>> Steve O'Hara-Smith wrote:
>>> On Wed, 24 Sep 2008 11:57:38 -0500
>
>>> Hmm - ideally the plenum should be expanding in CSA with height
>>> to accomodate the expansion of the warmed air. Anyone care to take a
>>> stab at the appropriate rate ?
>> Hmm back at you. :-) If we examine the original assumptions of
>> unimpeded/frictionless airflow and constant viscosity and consider the
>> effects of holding more and warmer air in the absorber plenum for a
>> longer time, I think the results will be heavily dependent on, of all
>> things, the absorber - since it is at the absorber/heat exchanger
>> surface that the effects of those factors I was so happy to make
>> assumptions about will be most pronounced.
>
> I rather suspect you're right there, worse yet it's going to depend
> on the absolute temperatures of intake and outflow which are not constant.
> It might be interesting to make a type 2 collector (your terminology) with
> an adjustable sloping middle panel and experiment to see if there's any
> appreciable impact on efficiency. My gut feeling is that it's probably not
> great.
I think your gut feel is accurate. With a straight (untapered) plenum,
we end up with the cool air moving more slowly than the warm air which,
when you think about it, is pretty much what we want.
Although the temperatures aren't constant, we know that we want the
discharge temperature to be as close to the intake temperature as we can
manage - which means that the ratio of the absolute temperatures will be
(we hope) fairly close to unity...