Posted by *gary* on September 23, 2008, 7:06 pm

Hi Nick, ...

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

*> > 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.*

*> 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*

*> > 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 24, 2008, 2:24 am

Just to clear up the badly formatted table of results in the last post

-- 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

Posted by *schooner* on September 24, 2008, 10:15 am

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.

*> Just to clear up the badly formatted table of results in the last post*

*> -- 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 *

Posted by *gary* on September 24, 2008, 2:21 pm

Hi Schooner,

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

*> 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.*

*> > Just to clear up the badly formatted table of results in the last post*

*> > -- 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*

Posted by *schooner* on September 24, 2008, 2:49 pm

Gary thanks for the info. Ya that setup is certainly the best and most

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.

Hi Schooner,

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

*> 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.*

*> > Just to clear up the badly formatted table of results in the last post*

*> > -- 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*

> > 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.> 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> > 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