Re: Storing the summer heat
From: N. Thornton (bigcat_at_meeow.co.uk)
Date: 12/01/04
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Date: 1 Dec 2004 11:06:46 -0800
The Ghost In The Machine <ewill@sirius.athghost7038suus.net> wrote in message news:<is4082-156.ln1@sirius.athghost7038suus.net>...
> In sci.physics, jimp@specsol-spam-sux.com <jimp@specsol-spam-sux.com> wrote
> on Wed, 1 Dec 2004 00:49:09 +0000 (UTC) <coj4e5$drn$1@mail.specsol.com>:
> > In sci.physics N. Thornton <bigcat@meeow.co.uk> wrote:
> >> "Fred McGalliard" <frederick.b.mcgalliard@boeing.com> wrote in message news:<I801Kn.9LB@news.boeing.com>...
> >> I see no advantage in suing such low temps, requiring a expensive
> >> pumping, when one can solar heat it for free to above house temp and
> >> let simple air or water circulation heat the house. No heat pump
> >> installation, no heat pump run cost. A real big curved trough
> >> reflector is miles cheaper than a heatpump system.
> > If there is any solar energy worth capturing in the first place.
Theres loads, at least for US and here in Europe. Admittedly it might
not work in Greenland.
> > Once you get past the implementation costs of such systems, your're
> > still left with the undependability of weather systems.
> >
> > Will there be enough hot/cool/sunny days for any "nature" dependant
> > system to work?
I dont see what you base that on. The sun shines most days, there are
only so many overcast days a year, one can count on that. There will
be enough direct sun every single summer to heat your heat store up
every season, as long as its designed right.
Lets be clear: I am talking about habshi's huge seasonal store idea,
but using rocks not water, and solarthermal heating of the rock, using
a big trough mirror to get low concentration. Concentration keeps the
system efficient and allows higher temps to be used, thus allowing
simple ventilation for heat recovery. It also allowws lower levels of
sun to heat the rock, thus using sun more hours per day.
> There is also the little problem of density-packing.
> Say one wanted a constant insolation on a location, oh,
> 28 degrees 35 minutes north of the equator (the latitude,
> as it turns out, of New Delhi),
I thought New Delhi wants cooling, not heating. And I'm not clear why
you'd want constant insolation, I'm puzzled on that point.
> and assume a variance of
> 23 degrees 30 minutes and a maximum insolation
> of 1350 W/m^2 -- during the daytime, of course, when
> the sun's directly overhead.
>
> If we wanted 2000 W as a minimum year-round we'd need
> to cover 2000 / (1350 * cos(pi/180 * ( (28+35/60+23.5)))) m^2
> = 2.41 m^2
It would be significantly more than that in US or Europe, insolation
is lower and heat energy requirement higher.
> -- and that's assuming perfect efficiency and
> nice blue skies, which we wouldn't be able to guarantee anyway.
> I'm not sure how much one can get, really -- solar cells have
> been quoted at 20%, if not 10%, though that was awhile ago.
I think you might be thinking of solar PV cells, which are a totally
different animal.
With a shallow trough reflector our losses would be apx:
5% reflector loss
10% dirt on reflector and glass front loss
absorber efficiency 80-90% depending on concentration level and
absorber material - note the concentration seriously boosts
efficiency, ditto a mesh absorber.
This gives an overall efficiency of 77% for a 90% efficient plate
collector. With multilayer gauze this is achievable. For other types
of absorber the efficiency would be lower.
> (Note that 2.41 m^2 = 26 ft^2.)
>
> If we assume 20% capture efficiency the figure ups to 12.05 m^2,
> or 130 ft^2 -- the size of a US master bedroom, if not larger.
2.4/.77 = 3.1m^2
800w insolation, 5kW requierement, 77% efficiency, would give
5k/.8/.77 = 8.1m^2
> This might work for single-family ranch-style houses (if
> India has that concept; I don't know what they'd call such)
> but 2.41 m^2 would be woefully inadequate for Manhattan-style
> apartment complexes of height more than 3 stories or so.
> It's a variant of the square-cube problem.
A house has a lot of wall space, thats where I'm expecting the
reflector would go, with a ground based collector, which would simply
be a large glass panel over the rock store, with either absorber and
fan, or else a metal grid lined trough shaped cavity in the rocks for
better absorption.
> And then there's the nighttime factor. I've compensated for
> that to some extent by doubling the power requirements, but
> I'm not sure it's enough during the winter when the nights
> are longer than 12 hours,
its not meant to be, we're talking an interseasonal store here. It
charges up in summer. Winter sun would only provide a small percentage
of additional heat to extend the use time.
> though in India it's less of a problem
> than, say, in Montreal, Canada. Cloud cover makes life mildly
> interesting;
just another factor to calculate in. In US cloud cover of sun is a
small percentage of time, in UK its substantial. 5% cloud cover for US
would make little diffrence to collector size, but in the UK it would.
> And then there's the monsoons, which tend to blow things over,
> if not drown them.
>
> Not the prettiest of pictures.
We dont get many of those here in Europe. In monsoon and hurricane
prone areas, the reflector would have to be thoroughly secured to the
building. The ground panel would be flush with the ground, needing
only guard wires to keep heavy objects off.
I'm not saying this is all practical, just offering it as a more
workable version of Habshi's original plan, and seeing what falls out.
NT
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