|
|
re: minnesota solar power
25 mar 1999
jim wrote:
>i was thinking about doing something like this in nw montana.
great falls isn't bad for solar house heating, but kallispell's awful,
with an average dec temp of 22.7 f, an average daily max of 29.9, and
only 490 btu/ft^2 of sun that falls on a south wall and 310 that falls
on a horizontal surface on an average day. like seattle, but colder.
>what about collecting that heat and pumping it into a 1000 to 2000 gal
>insulated tank?
a large water container like that with a small surface to volume ratio
wants some sort of solar water heater, eg a few automobile radiators
with fans near the top of a sunspace.
>that way your losses are down and you can control the rate of release into
>the heated spaces.
sure. altho heating a space with low temperature water requires lots of
heat transfer surface, eg walls o' radiators, or a water source heat pump.
you might make the tank a $500 24'x4' deep 15,000 gallon circular swimming
pool with a floating styrofoam cover over a large piece of plastic film
and lots of thin board insulation tightly wrapped around the outside.
this might make a nice hot tub.
>also, i plan on putting in some sort of insulation on the glazing
>during dim days and at night.
people tend to tire quickly of that activity, vs more automatic systems.
a 1' slice of a long 8' tall quarter-cylindrical housewarming sunspace with
2 layers of r1 polycarbonate glazing with 90% solar transmission and a
reflector in front would collect about 1.3x8ft^2x490x0.9x0.9 = 4,100 btu/d
of kalispell's southern solar component and 8ft^2x310x0.9x0.9 = 2,000 btu
of overhead sun. with a sunspace temperature t and a 6 hour december solar
collection day, it would lose about 6h(t-26f)12ft^2/r2, eg 2,900 btu/day
for t = 106 f. if we make the sunspace too warm, it loses too much heat to
the outdoors, and if we make it too cold, the water stores too little
useful heat. what's the optimum temperature?
magicaire's copper $150 2'x2' shw 2347 duct heat exchanger with a fan
(or a old auto radiator) might transfer 45k btu/hour between 125 f water
and 68 f air at 1400 cfm, with a 0.1" h20 air pressure drop, for an
effective conductance of about 45k/(125-68) = 800 btu/h-f. with one of
these for every 8 linear feet of sunspace storing heat in 100 f water,
we'd have an equivalent dc circuit like this (use courier font)
t
rg | 1/800
26 f -----www-----*-----www-----100 f
|
isun |
|--| --> |---
where rg is r2/(8x12ft^2) = 0.021 "ohms" and isun is 8(4100+2000)/6h =
8100 "amps," with a simpler thevenin equivalent circuit
isun t rt
|--| --> |---*----www------tt
where the parallel combination rt = 0.00118, and t with the current
source removed is tt = 95.8 "volts," so t = tt + isun x rt = 105.4 f.
nick
|
|
