alt.energy.renewable

> > > If cloudy days are like coin flips, a house that can store enough heat
> > > for N cloudy days can be 100(1-2^-N)% solar-heated, eg 97% for 5 days.

> > At 100% collection efficiency with no heat losses whatsoever in the
> > full desert summer sun, 240 ft^2 will collect 700,000 btus/day.
> > That's roughly 30,000 btus/hour, so we're sort of in the ballpark. In
> > reality, upstate NY gets half the average daily insolation (4 kwh/msq/
> > day) (480 ft^2), winter can reasonably be expected to be half of
> > summer (960 f2^2), and efficiency cannot be expected to exceed 50%
> > (2120 ft^2). That's quite a difference from the 64 square feet that
> > morris is claiming will heat a house.
>
> Umm - two 6'x8' panels works out to 96 ft^2 on my calculator, and
> I've only spoken about a farm shop building (not a house) because
> that's the only installation for which I can provide both photos
> and owner feedback.

Ah. I was using 4x8 panels (32 square feet) not 6x8. Reason
being, 4x8 is a commercially available sheet size in most materials
like plywood and sheet insulation whereas 6x8 is a custom cut. so
okay, my area was a little off. You're still in at less than half of
the extremely optimistic numbers provided by nick and less than 1/10
of my estimate based on optimistic conditions.

Thing with the workshop is that a) there is typically no
particular requirement that it be heated overnight (eliminating the
requirement for heat storage), and b) the heating standard is *far*
lower than that for a house. Most houses are heated to 75 degrees,
most workshops to 50. That's a 33% improvement on panel efficiency
due to smaller heat differential in the panel as well as a 33%
reduction in the btu requirement to maintain the temperature. That
makes it a pretty optimal application for passive solar, so while you
may not have cherry picked, you did pick quite a cherry!

> With vertical panel orientation, winter output is considerably
> more than double summer output - and when the ground in front of
> the panels is snow-covered, output can increase by more than 90%
> due to capture of snow-reflected energy.

Umm... dreaming. I'll buy that winter output for a properly
placed panel with snow cover for a few hundred feet between panel and
sun *may* match total summer output. Claiming double is insanity,
your incident angle to the sun is poor due to your vertical
orientation, your angle to the ground is poor for the same reason,
snow is not perfectly reflective, and is absolutely poor at reflecting
IR, and the day is half as long.

> I can't buy your 50% efficiency, but won't argue that until I've
> actually got measurements to back me up - but I'll warn that I
> think your number is much less than what I expect to measure.

Lemme know how that goes, I will be interested to see it.
Assuming that you can pull r6 average for the box front and back (R2
on the front and r10 on the back), at a 75 degree temperature
differential inside to outside, you should be losing 249 btu/hr/sq
foot and taking in a maximum of 307 btu/hr/sf (1 square foot at 1 kw/
m^2). That gives you an effective efficiency of under 18%. That's 75
degree water on a 0 degree day, not getting much heating value in a 75
degree house out of that. That's also assuming that ALL of the energy
that impinges on your panel is converted to heat, that NONE of it is
reflected by the surface, or simply lost to entropy. That can be
improved by removing the insulation on the back side and mounting it
against the side of the building. However, since the backside is the
well insulated side, the improvement is less than one might think.

> I'm glad to just take your word for lousy weather where you are.

On the plus side, I am WELL situated for wind energy :) That's
why I am working on the budget windfarm.