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re: advice needed for cooling residential hydronic system...
2 jul 2001
al youngwert   wrote:

>nick@acadia.ee.vill.edu (nick pine) wrote in message

>> 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...
>
>...this would definitely work well in a few of the south-eas facing rooms..

removing the heat loss of 400 ft^2 of r4 windows while preserving most
of their solar gain (about 50% of 800 btu/ft^2 per day?) should reduce
the house heating requirement. you might turn the boards upside down in
summertime to make the windows air coolers. 

>> the reflector might be a 32' tall x 72' long x 32' deep lawn sculpture.
>
>it would be really tough to dig a 32' deep hole in a 35% slope...

how about a greenhouse with kerfed 2x4 bows on top of the ground?
or put the target above the reflector, like this...

   the most serious mistake was making the outer container of the receiver
   of plywood. we thought that the plywood would be sufficiently insulated
   from the copper panel which was the receiver proper, that it would not
   get too hot. the copper panel was separated from the plywood by 4" of
   fiberglass insulation. nevertheless, the plywood caught fire and the unit
   was completely destroyed. we suppose this is a success, of sorts...

   from "a solar collector with no convection losses," (a downward-facing 
         receiver over a 4:1 concentrating parabolic mirror) written by
	 h. hinterberger and j. o'meara of fermilab, ca 1976

might make a lovely skateboarding surface...

>the only other thing that will hurt us is our winter-time inversions. we
>average 2-4 weeks a year totally socked in a polluted fog, which is often the
>coldest time of year. we sometimes go 2 weeks without sunlight...

two totally cloudy weeks in a row seems unlikely, with a december average of
800 btu/ft^2 per day and a standard deviation of 87, and those 14 days would 
likely be close to the 30.1 f average temp, with a 22.5 average daily min
(1 sigma?), but say the house needs 14dx24h(70-30.1)1500 = 20 million btu
over 2 weeks (1500 btu/h-f still seems very high for the house conductance,
given your r50 ceilings and so on.)

that's 20million/(180f-80f) = 200k pounds or 25,000 gallons or 3125 ft^3
of water cooling from 180 to 80 f, something like a 4' deep x 32' diameter
swimming pool in the basement :-) or a smaller pool with a heat pump to
cool it further from 80 to 40... you might insulate the basement walls
and fill it with water...

>> you need about 45k/(69-60) = 5,000 lb/h or 10 gpm for 45k btu/h of cooling.
>
>this calculation doesn't seem right to me. if i need to move heat from
>the concrete, through the pex tubing and into the water it seems to me
>the equation would at least need to include the thermal conductivity of
>the pex tubing?

the main series thermal resistance is likely to be from the floor to the
room air, about r2/3, or more, with carpeting. that's where the 6 f delta
t and 69 f maximum usable cooling temp came from, for a 75 f house.

>where could i find the thermal capcitance of water...

it takes 1 btu to warm a pound of water 1 degree f. we've used that
twice now, to store 20 million btu and move 45k btu/h.

>> >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.
>
>24 f sky temp? do you mean 24 degrees farenheight?

yes. you deleted that calc.

>in the summer we get down to like 65 degrees farenheight.

the sky is cooler. outer space is about 4 k, ie about -453 f. satellites
use "deep space coolers" to regulate their temperatures. these resemble
$11 automatic foundation vents (except in price :-) with radiant surfaces
behind reflective louvers attached to bimetallic springs that open the
louvers as the satellite temperature rises.  

>i didn't think you could cool water to below the ambient temperature
>without some sort of heat pump/refigeration cycle.

radiating heat to a cool sky is one way to do that. the other is to
evaporate water. the "wet bulb temp" is less than the ambient temp
whenever the rh is less than 100%.

>i'd certainly like to learn more about how this could be done.

trickle water over the roof, flood a pond at night...

>the other thing i might have going for me is the radiant loop
>in the basement. i only insulated the perimeter... this could
>also be used as a heatsink.

it's hard to get heat to move downwards into soil, ie it has a large thermal
resistance which depends on the soil type and moisture and groundwater flow,
and the soil heats up over time as well... 

nick




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