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re: nick, nick, nick
13 jun 1996
william r stewart wrote:
>[followups to alt.solar.thermal, for the umpteenth time...
feeling uncomfortable, will? :-)
>nick pine wrote:
>> william r stewart wrote:
>> >nick pine wrote:
>> >> >see solar today sept/oct 1995.
>
>a publication of the american solar engineering society.
my copy says "published by the american solar energy society."
>> how many cs graduates are willing to understand simple physics, involving
>> ashrae-type r-value calculations?
>
>those who also have electro-mechanical engineering degrees.
i have yet to see one... these calculations are just hvac craftsmanship, not
"thermodynamics." (is "electro-mechanical engineering" pinball machines? :-)
>you fail to realize that the thermal store can be much warmer than the ambient
>room temperature. since the water wall will be painted (or papered) a light
>color, most of the heat transfer will be from convection. do you understand how
>to calculate convection?
sure. i just use an r-value of 2/3. i don't add anything for paint or paper.
other people go on about nusselt and grashof numbers, but the results are
similar. r-values are actual physical measurements that include convection.
>> >the thermal store can be far above the ambient temperature of the room.
not for long.
>>the heat goes in one side of those window-blocking water walls and out
>>both sides, almost immediately, heating the room air.
>by what transfer mechanism, convection? please show me the calculation.
my nrel data book says about 1050 btu/ft^2 would shine in your double-glazed
windows over a 6 hour winter day, of which they estimate 710 btu/ft^2 would
get through the window, and shine on the water wall in your 70 f room, with
a still air film r-value of 2/3 for each side (see ashrae hof, fig 1, p 22.1),
heating it up to a temperature t, where 710 = 6(t-70)2/(2/3), so t = 109 f.
if the wall contains 4" of water, ie about 21 pounds per square foot of wall,
the rc time constant of the wall is about 2/3/2x21 = 7 hours. these walls
might keep your house warm for several hours, not just a half-hour...
if you close your curtains at 3 pm, and keep your solar house at 70 f all
night by burning wood, by 9 am the next morning, the water walls might have
a temperature
t(t) = 70 + (109-70)exp(-18/7) = 73 f.
if the next day is cloudy, and you leave your curtains closed, and keep
your solar house at 70 f by burning wood for the next 24 hours, the next
morning your water walls might have a temperature of
t(t) = 70 + (109-70)exp(-42/7) = 70.1 f.
suppose you didn't burn wood the first night. as i recall, your house had
an overall u value of about 150 btu/hr-f. so over 18 night hours, at 30 f
outside, it would need about 18(70-36)150 = 92k btu to stay warm. the 3300
lb of water walls and 1000 btu/f (?) of house would need to cool 92k/4.3k
= 21 degrees f to do that, so in the morning, your solar house and water
walls would have a temperature of about 70-21 = 49 f.
over the next 24 hours, if it's 30 f outside, and you keep all of the thermal
curtains drawn all day, and they insulate as well as you say they will (who
makes these curtains, anyhow, and how much do they cost?) your house would
need less than 24(49-30)150 = 68k btu to stay at 49 f, so after another day
your solar house and water walls would be at least 49 - 68k/4.3k = 33 f...
you could do these calculations in more detail, if you desired.
the results wouldn't change much.
nick
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