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re: solar greenhouse
8 mar 1999
dave gower   wrote:
 
>> you might get about 11x4x8x0.8x311btu/h = 88k btu/h of peak sun
>> through those 11 panels...

>   my original calculations were close to that except that i think this
>assumes they are all facing due south. in fact, 10 of them face 15 deg
>east of south and one 15 deg south of west. (the greenhouse is attached
>to my house so the house orientation dictated the pane orientation).
>also allowing for less than perfect transittance (about 90% according to
>the ge sales brochure for the lexan) and we're down to about 70k btu/hr.

same ball park. i figured 10% transmittance per layer (the 0.8 above.) 

>> seems like 700 cfm would allow an empty greenhouse temp to rise about
>> 88k/700 = 125 f above the rock bed temperature...

>   these are key points, so let's look at them. the fans are in series,
>so the air flow is probably close to 350cfm. my objective was to be able
>to drop the air temp by at least 35 deg f as it flowed through the rocks
>(peak value). this would absorb 40k btu/hr...

it seems to me that 1 btu heats about 55 ft^3 of air 1 f, so 350 cfm
of air that flows with a temperature difference of 35 f would carry
closer to 350x35x60/55 = 13.3k btu/h of heat power, about 20% of the
peak solar power, by your calculation.

>   the peak temperature in the greenhouse stabilizes out at about 85 deg
>f (with a air temp exiting from the rockbed of 45-50). this extra heat
>isn't wasted since it gets absorbed in the top of the soil, the
>greenhouse structure and the concrete paver floor...

well, yes, you have the other objective of keeping the greenhouse warm
at night, beyond simply heating the attached house.

>   my concern is when the rock bed warms up more, that peak temperatures
>in the greenhouse may get high enough to damage plants. if this appears
>to be a problem i may need to look into supplementary cooling (such as
>an extra fan connected to a regular thermostat).

or add more storage, or maybe use a passive thermofor sash vent actuator
like the $54 fc115 classic from jade mountain (800) 442-1972, which
"opens any hinged window up to 15 pounds a full 15"" at a selectable
temperature between 55 and 85 f.

>   the fans are rated by the manufacturer at 100 w each. seems amazingly
>low but it's on the sticker. they sure blow well.

i wonder who make's 'em? grainger's 4c688 moves 560 cfm with 36 watts,
at about 1/8" h20 of static pressure. their $110 48" 4c853 ceiling fan
moves 21k cfm with 86 watts. i could see using two of them to blow warm
air down from the top of a sunspace into an 8' insulated cube with 32
55 gallon water drums inside and a 4'x8' motorized foamboard damper above
the fans that's hinged on the north side and reflective underneath with
some passive plastic film one-way exhaust damper skirts below to collect
56 peak horsepower of sun from a housewarmer with a $25 16x32' piece of
plastic film glazing and a 3 f temperature rise, and a cop of 244.

nick




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