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re: solar heating for barn
8 feb 2004
gary  wrote:

>the estimated heat loss (after insulating) is 160 btu/hr-f...

maybe more, with some thermal bridging?

>the climate is sw montana -- avg jan high 29f, avg jan low 6f...

nrel data indicate december is the worst-case month for solar
house heating in helena, when 390 btu/ft^2 of sun falls on the
ground and 810 falls on a south wall on an average 21.2 f day
with an average daily high and low of 11.2 and 31.3, for an 
average daytime temp of about (21.2+31.3)/2 = 26.3. 

>the collector design i am leaning toward is as follows...
>size: aprox 7 ft high by 16 ft wide (about 103 ft^2 net).

nrel's data assumes a 0.2 ground reflectance. their solar
radiation data manual for buildings suggests adding iadj
= 0.5(rd-0.2)ih(1-cos(b)), where rd is a larger ground
reflectance (about 0.6 for snow) and b is the (90 degree)
collector tilt and ih (390) falls on the ground, so the
collector might see 810+0.5(0.6-0.2)390(1-cos(90)) = 888
btu/ft^2/day, or 0.9x888x103 = 82.3k btu/day through (clear
vs tinted?) polycarbonate glazing with 90% solar transmission.

if the collector works for 6 hours on an average december day,
you might have 82.2k = 6h(t-26.3)103ft^2/r1 + 24h(t-21.2)160,
approximately, which makes the average barn temp t = 40.4 f.

>the glazing panels are to be mounted 4 inches off the barn
>siding...

steve baer might suggest h/15 = 7x12/15 = 5.6"...

>black, metal window screening will be hung between the barn
>siding and the glazing as an absorber.

sounds good.

>the flow through the collector is to be by natural convection from the barn
>space, into the bottom of the collector, up through the collector, and out
>the top of collector back into the barn.  the vents through the barn siding
>into the collector are as follows:
>one set of 8 vents along the bottom of collector, each 4 inches by 20 inches
>(aprox 4% of collector area)
>a similar set of 8 vents will be provided along the top of collector.
>flapper valves at the top set of vents will prevent reverse flow at night.
>
>i plan to block some or all of the vents in the summer to prevent
>overheating of the barn.

sounds good.
 
>i plan to provide some form of outside ventilation for the collector so it
>does not overheat when the vents to the barn are blocked off (not sure what
>form this takes).

some kind of plugabble holes or closable doors at the top and bottom.
you want them to close tight in wintertime, and keep wasps out...

>one reference (principles of solar engineering) estimates the sunny day
>solar gain per sq ft at 44nlat for a vertical collector at 1500
>btu/day-ft^2.

it's better to use data based on longitude too, ie historical data for
the actual location, for average vs clear days, including clouds and fog.

>to get some idea of the daily temperature fluctuations, i modeled the barn
>contents and the internal barn structure as 4000lb of water.

good!

>so, when the heat gain exceeds the loss, the barn's (modeled as water)
>temperature goes up, and vice-versa when heat loss exceeds the heat gain.
>for any given hour in this simulation:
>
>    heat loss for the hour  = (160 btu/hr -f) (toutside - tinside)
>
>    heat gain for the hour = (hourly gain) (103 ft^2) (45%) (1.5)
>
>    change in barn temperature = (heat gain - heat loss) /(4000 lb)...

there will be some thermal resistance between the mass and the air.
this might not matter much, if the mass has a large area.

>   time           toutside      tinside        (heat gain - heat loss)
>    6am               6 f            44 f             -4600 btu
>    8am              16               38                +7100
>   10am              24              44                 +14400
>   12 noon         29               51                 +14500
>     2pm             28               57                 +10100
>     4pm             20               58                 -2600
>     6pm             10               54                 -7400
>     12 midnight    6               46                 -6600
>     4am               6                38                -5400
>
>so, if you believe this, the minimum garage temperature (7am) is 35f, and
>the mid afternoon temperature gets up to the high 50's -- the solar heater
>is keeping the inside of barn around 20 to 40f warmer than the outside
>temperature.  this would be great (if true).

it might be closer to 20 than 40 f warmer.

>are the performance calculations shown above likely to be in the ballpark?
>does the collector seem to be about the right size to meet the objectives?

larger seems better.

>will the natural convection vent scheme dramatically reduce the efficiency
>of the collector?

a fan or blower might double it, and allow better air temp control, and let
you make the collector taller.

>are the vents large enough (4% of collector area) to provide good circulation?

i'd say so.

>is the black metal window screen an ok absorber?

sure.

>should i arrange it such that the air has to flow through the screen in
>getting from bottom vents to top vents?

yes, with barn air next to the glazing and heated air on the north side
of the glazing.

>flapper valves (to prevent reverse flow through collector):
>i have seen references to the flapper valves in this group, but i don't know
>what they are made from, what the geometry is, or how well they work??

dry cleaner bags hinged at the top, hung over holes in the wall. they
work pretty well. they can get stuck or folded or torn. it's good to
inspect them every week or so. the holes can be covered with chicken
wire to keep the bags from sucking through in the wrong direction.
 
>summer collector vents (to prevent overheating):
>how much summer collector ventilation area should be provided to prevent
>overheating of the collector?

same as winter? polycarbonate plastic can take very high temps, 150 f air
on the inside, or more... 

>any suggestions to improve the geometry, construction, etc. of the system?

you might think about making your "sunspace" deeper, eg 4' or 8' out from
the barn, to decrease the airflow resistance and to better separate barn and
solar-warmed air and to slow down the airflow near the glazing and to allow
more summer venting and shading with an overhang, and to make more usable
floorspace, which might include a couple of chairs or a clothesline.

nick




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