Hybrid Car – More Fun with Less Gas

solar cooking under clouds - Page 4

register ::  Login Password  :: Lost Password?
Posted by Morris Dovey on October 4, 2010, 6:55 am
On 10/3/2010 11:53 PM, Josepi wrote:

Agreed - but let's do some really simple arithmetic and assume six hours
of useful sun per day...

[1] Solar thermal without storage discards 5/6 (83%) of incoming energy
because we only use it 1/6 of the time, leaving us with a 17% efficiency.

[2] An efficient solar-powered Stirling engine can (in theory - I've
been working my way toward practical testing) convert heat energy to
mechanical energy with an efficiency greater than 30%, and that
mechanical energy used to drive an alternator to produce electricity. If
the alternator produces losses of 30%, then the overall generating
efficiency is slightly in excess of 20%.

[3] PV with or without storage loses approx 90% of delivered energy to
losses in the radiant->electrical conversion, leaving us with an approx
10% generating efficiency.

All three approaches allow storing energy for use later.

I'll agree with the first sentence for the comparison of [1] and [3].
The entire motivation for investigating approach [2] is that it should
deliver at least double the performance of [3], possibly at a much lower

There's a huge amount of unexplored territory here. I'm confident that I
could build a flat panel to operate above 400F/200C, but I don't have
a good handle on storage at that temperature (phase change materials,

We have heat pumps that extract heat from the ground to warm homes to
comfortable temperatures. We've built Stirling cycle heat pumps and
we've built solar-powered Stirling engines. Could a combination of the
two be used to deliver cooking temperatures from low-grade stored heat?
Probably. Would it be cost-effective? Dunno - I suspect it's never been

All systems now in use already waste most of most of their capability,
so that shouldn't be a show-stopper. At least with solar we don't waste
something that's increasingly difficult to find or expensive to replace
- and it's not likely to trash our environment. :)

Morris Dovey

Posted by Josepi on October 4, 2010, 12:24 pm
From my experience those temperatures would be hard to achieve or extreme
with a flat panel. The main issue there, even more so, would be the cloudy
sky factor.

I figure some concentration for usuable cooking temperature and storage to
gather enough energy to deliver that temperature range for the few hours
needed to cook would be in order.

This, probably, implies some active mechanisms.... maybe a tracker and smart
mechanism for capture / delivery switch over. Perhaps a multiple day capture
would be in order in an extreme climate (Antarctica)

<much snipped>
 I'm confident that I
could build a flat panel to operate above 400F/200C, but I don't have
a good handle on storage at that temperature (phase change materials,

Morris Dovey

Posted by Morris Dovey on October 4, 2010, 2:09 pm
 On 10/4/2010 7:24 AM, Josepi wrote:

For typical wood-construction panels, they /would/ be pretty extreme -
altogether too close to Bradbury's 451F (There's an interesting article
at http://en.wikipedia.org/wiki/Autoignition_temperature ) - but the
maximum temperature is determined by the point at which the panel's
losses equal the energy input. It really does look like a materials issue.

The cloudy sky factor is important. If you reduce the input to 50%, then
it's going to take double the capture area to acquire the same energy.
It looks like the panel area would need to be adjusted by the reciprocal
of the "sky factor". (But notice that the starting point is always going
to be the 'clear sky' performance).

Agreed, although it hadn't occurred to me until last night that there
might be an option to apply the concentration (with a heat pump) _after_
the energy has been collected.

Maybe so. I don't have much passion for providing Antarcticans with
solar cooking - and see that part of the issue as dow feeling the need
to occupy himself while his partner is away in Australia. ;-)

OTOH, there are plenty of other places where a small amount of
technological improvement could leverage large improvements in quality
of life for large numbers of people - which is why I tend to advocate
for picking the low-hanging fruit first...

Morris Dovey

Posted by dow on October 4, 2010, 2:14 pm
Geez! I sleep for a few hours, and come back to find all these new

I started this thread in response to an e-mail I got some months ago
from someone who lives on the west coast of Canada. He mentioned that
he had not been able to test his solar cooker designs because the sun
had been obscured by cloud almost all the time for the previous
several months. There are plenty of places in the world with cloudy
climates, not just Antarctica. If we want solar cooking to be globally
useful, we must try to make it work under clouds.

We should assume that the amount of direct sunshine is *zero*. Cookers
that capture heat when the sun is shining and use it later may be
useful in some situations, but not in really cloudy conditions. We
need something that will reach a useful cooking temperature under an
overcast sky. Will a flat plate do that? I've never seen one that

The PV idea doesn't imply that devices such as inverters and batteries
must be used. The PVs could be connected directly to a resistive
heating element, or to a Peltier-effect heat pump. Of course the
impedances would have to be reasonably well matched, but that
shouldn't be difficult.

The vacuum tube idea apparently works in diffuse sunlight, but I don't
know about complete overcast.

I don't really like the idea of using an electric cooker, but so far
it seems to be the only one that would definitely work.


Posted by Morris Dovey on October 4, 2010, 3:18 pm
 On 10/4/2010 9:14 AM, dow wrote:

Seems reasonable to me. It does seem that there's overmuch emphasis on
dealing with extremely poor conditions when we've barely gotten started
with solar applications in regions where it's relatively easy to do.

"Globally useful" doesn't imply (to me) that a single solution is
appropriate for all locations (or all menus).

Based on my own limited panel-building experience, I think it's possible
- but I can't say much more than that until you define "a useful cooking

My experience is centered about producing maximum airflow at minimum
temperature, and I scrapped a fair number of test panels that ran too
hot with too little airflow - which is what you're now asking for.

I'll mention again that the only burn I've ever gotten from a flat panel
happened on an overcast April day in upstate New York. I regret now that
I didn't have any way to quantify the conditions.

There's a great deal to be said in favor of using ideas that work. :)

Morris Dovey

This Thread
Bookmark this thread:
  • Subject
  • Author
  • Date
please rate this thread