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re: advice needed for cooling residential hydronic system...
30 jun 2001
al youngwerth wrote:
>the house is nearly completed (roof on, painted inside...
>> >...this house has almost 800sqft of windows... and 85% of window
>> >area faces south-east).
>>
>> you might halve that, or build an insulated wall to the north, ie put
>> the windows over a sunspace vs 24-hour living space for more efficient
>> heating and cooling...
you might still push dark-colored foamboard panels into half of the
indoor window frames in the winter, with an air gap between the board
and the window and a slot at the top to make them air heaters which
contribute solar gain during the day but lose little heat at night...
>> about 400 btu per square foot of aperture per day. nrel says a 1-axis
>> ew concentrator can collect at most 507 btu/ft^2-day on an average
>> december day. you might lose 10% through a steep south attic glazing
>> and another 10% with 90% reflective mylar...
>if i'm designing for a 10 degree average temperature in the winter
>then my heat loss per day is 57k*24=1.368m btu/day.
with an average 30.1 f december temp and an average daily min of 22.5,
boise is unlikely to be 10 f for more than a day or two in a row...
you might figure on collecting (70-30)/(70-10)x1.368m = 912k btu of
solar heat on an average day, minus the heat from electrical usage and
the solar heat that shines in the windows.
>to cover that heat load with the parabolic reflectors would take
>1.368m/400=3420sqft of reflectors...
or maybe 912k/400 = 2280 ft^2 of solar aperture (the south facing area,
vs the area of the reflector itself. the reflector might be a 32' tall
x 72' long x 32' deep lawn sculpture.
>however, in the summer, i'll bet those reflectors could be pumping out
>some serious hot water. probably more than enough to cool our house
>with some sort of absorption chiller.
sure. and you might store some summer heat for winter in a licl solution.
>realistically, there's a spot where my wife probably wouldn't fuss to
>much that is about 1500sqft or so for parabolic collectors. anyone
>know how much these parabolic reflectors and absorption chillers cost,
maybe $3-4k if you build them yourself.
>> ...trickle water from a ridge pipe with holes over the north roof (and
>> maybe the south roof, too) at night and collect it in a gutter and store
>> the cool water in a large tank inside the house...
>trickling water over the roof will cool the water?
yes.
>...i would assume it would only cool it to the night time air temperature
>of 65 degrees?...
depends on the initial water temp and roof area and flow rate.
>i need an approximately 6 degree delta between floor and air for a 45k
>cooling load (from previous post) could i pump that much water in a
>day through the floor?
you need about 45k/(69-60) = 5,000 lb/h or 10 gpm for 45k btu/h of cooling.
>when you say .07 gpm with evaporation, how would i do that? build a
>cistern that holds x gallons of water, pump emough of that water
>through the floor to remove 250-300kbtu/day from the house to the
>water and then trickle it over the roof at night to remove the heat
>from the cistern water?
yes. the cooled water goes back into the cistern after it leaves the roof.
>...it still seems to me that with such small deltas between ideal slab
>temperature (say 68 degrees to keep house 74) and the water at night
>only getting down to 65 degrees you couldn't get enough thermal transfer
>to make this work.
>
>what am i missing here? will trickling the water over the roof cool it
>to less than 65 degrees?
sure, with a 43 f dew point and a 24 f sky temp.
nick
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