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re: a simple solar collector
3 aug 2004
gary   wrote:

>>>when the sun is shinning, raise the garage door about 10 inches...
>> 
>> it's simpler and more efficient to open the door all the way. i'm reluctant
>> to do that automatically because of burglars. 
>
>i agree that if i only want to heat the garage, it would be better to 
>just open the door all the way, but if i want to get the heat to the 
>house via the duct/fan arrangement described below it seems like i 
>have to do something like the partial door opening scheme above to 
>collect the heat for the fan intake -- or is there a better way?

warm air rises, but it seems to me you still need the screen to avoid
storing sun in the concrete floor, and maybe to reduce the amount of
light and radiant heat when you are working in the garage.
 
>i think that if i just opened the garage door all the way, and had 
>(say) a fan that would blow air into the house that the garage would 
>overheat, and also that i would not get as much heat to the house?

maybe not. the tradeoff is keeping the temperature and daytime loss
through the glazing low. in full sun, 0.9x250x130ft^2 = 29.3k btu/h
passes through the glazing. if the air near the glazing has temp t (f),
(t-40)130 is lost through the glazing. the 3.5"x18' slot has 5.25 ft^2
of area. with 70 f air in the bottom slot and a 7' height diff between
top and bottom slots, q = 16.6x5.25xsqrt(7(t-70)) cfm will flow into the
garage, carrying i = (t-70)q btu/h of heat. so we have something like
this, viewed in a fixed font: 

            t
     ---    |  1/130
|---|-->|---*---www--- 40
     ---    |
29.3k btu/h |  i -->
             ---www--- 70

29.3k btu/h = (t-40)130 + 231(t-70)^1.5, or t = 70 + ((265-t)/1.78)^0.666

plugging t = 100 f into the right side makes t = 182 on the left.
plugging t = 182 in on the right makes t = 82.9 on the left.
repeating makes t = 91.8, then 91.1... not too hot.
you might collect 22.4k btu/h of useful heat
with a 76% efficiency. 

>>>add a 12 inch tall vertical divider that runs east-west across the
>>>garage ceiling about 18 inches north of the plane of the garage door.
>>>the top of the divider is in continuous contact with the ceiling.
>> 
>> would that interfere with opening the door all the way?
>
>the 12 inches is as far down as the divider could go without 
>interfering with the door.  when the door is raised 10 inches, the top 
>of garage door matches up pretty well with the bottom of the divider 
>-- i think this will help to channel hot air into the cavity formed by 
>the divider, and then into the duct.

you may not need it, if the garage ceiling is insulated. you might add
thermal mass to the ceiling. 
 
>>>add a duct with an inline electric fan that connects from the cavity
>>>to the house space.

you may not need a duct, if the fan is near the ceiling.

>> how many cfm? 

>>>a manual override switch would allow the fan to be turned off when heat
>>>is not wanted... 
>> or a $15 line voltage thermostat.

if the house temp varies much, you might also use a differential thermostat.

>>>i suppose that the opening and closing of the garage door could be
>>>automated based on a sun sensor, but this seems like overkill...

>> i would definitely want to do that. perhaps duane johnson can design
>> and sell a kit that provides a pulse (like a finger on a push button)
>> in the morning and evening, with a door position sensor to make sure
>> the door is open during the day, vs vice-versa.

we might make this from standard parts, eg a thermostat in a glazed box.

>>>absorption = 0.95
>> 
>> as i recall, black aluminum window screen is about 0.5, so 25% of the sun
>> would find its way back out the glazing, with a reflective door... 0.95
>> would require a door reflectance rho, where 0.25rho = 0.05, ie rho = 0.2. 
>> green is 0.5. flat black is 0.04...
>
>i like the way that you can see through one layer of window screen -- 
>it seems to me it makes it look better from outside, and also gives 
>you a view from inside.
>if i am understanding what you are saying, half of the incoming light 
>goes through the screen, and part of that is reflected back by the 
>door, and half of the reflected part gets back through the screen -- 
>for the green this would be: (0.5)(0.5)(0.5) = 12%lost??  i think 
>something just a bit better than green at 0.5 would be ok with me.

if rho = 0.2 = 0.5g + 0.04(1-g), you might mix 35% green with 65% black. 
 
>>>collector loss = (acol)(ucol)(tcol - tamb)(hours door is open)
>>>               = (130 ft^2) (1.1 btu/ft^2-f-hr)(120f - 39.8f)(6hr) =
>>>               = 69k btu/collection period
>>>
>>>		tcol is a guess at the average collector temp
>> 
>> that depends on the upper door air gap and the fan cfm. 
>
>i did some hour by hour simulations for billings using the tmy2 data, 
>and the assumption of 120f for the collector temperature hurts the 
>performance quite a bit when outside temperatures are very low (high 
>losses), or solar gain is not high (eg partly cloudy) -- ie the model 
>calcs losses with a 120f collector temperature that would probably 
>never be reached on a partly cloudy day.

you might redo the calcs as above.

>i suppose a more complex control system and a variable speed fan would 
>also help?

maybe. you could put that into the simulation.

>>>net gain = 128k - 69k = 59k,
>> 
>> about 10k btu/h... 
>
>a 1000 cfm fan would have a 10 f temperature rise.
>?? not sure what this means ??

moving 10k btu/h from the garage to the house with
a 1000 cfm fan requires a 10k/1k = 10 f temp rise.

>-- the hourly simulation shows times during mid day on warm days when
>the supply exceeds the demand by quite a bit -- especially in the 
>warmer months.

more thermal mass in the house or the garage ceiling might help.

>> another pro: i really enjoyed the light with the door open.
>> much better than working under the single bulb in the garage. 
>
>yes, i open the big door sometimes in the summer now, but its not 
>something you want to do when it 10f outside :-)

should be fine, with the glazing and screen in place.

>>>i am a bit concerned about making this look ok from outside.  any ideas?
>> 
>> a 4'x8' flat polycarb panel probably needs wood on 2' centers to avoid
>> too much oilcanning. can the wood act as a spring to prevent it entirely? 
>
>i have rebuilt my barn collector so that the corrugated pc panels are 
>supported every 2.5 ft perpendicular to the corrugations -- this makes 
>an amazing difference -- there is almost no buckling or deflection 
>now.  this is true even thou the wood supports are only 3/4 by 3/4 wood.

you can see through dynaglas, with distortions... flat polycarbonate
(eg lexan rolls from ge) is more windowlike, but subject to scratching. 

nick




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