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solar cooking under clouds - Page 3

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Posted by dow on October 4, 2010, 3:07 am
 
:

Ummm... In the southern hemisphere, the summer solstice is on or about
December 22, pretty close to Christmas.

(My girlfriend happens to be in Australia right now. I have to wrap my
mind around the opposite seasons, as well as the 14-hour time
difference, when I talk with her.)

The basic problem with solar cooking under clouds is how to make dim,
diffuse light make something hot enough to cook food. Sure, PVs and
the like lose energy, but they *can* make an oven hot enough to cook
anything, or even to melt steel. It's just a matter of connecting
enough cells together. I can't think of any simple device using
mirrors or whatever that could do that, using the ambient light on a
cloudy day as its source of energy.

If anyone here can think of such a device, our problem would be
solved.

      dow

Posted by Morris Dovey on October 4, 2010, 4:27 am
 
On 10/3/2010 10:07 PM, dow wrote:


Ok - southern hemisphere it is. Local summer. Penguin for dinner. :)


PVs can - but remember that close to 90% of the energy falling on the
capture area is lost on that first conversion, so the 1 kW/m^2 is
immediately reduced to 100 W. On a day when clouds pass only 1% of the
light, you'll only realize 1 W/m^2 - and that's before taking any losses
associated with charge controller, battery, or inverter.

If we're only worried about Christmas dinner, that's probably not all
that large a drawback - but if we want to use 1 kWh each day, we might
need a fairly sizable array of PV panels.

Most people never think about it, but a flat panel can be used to
produce temperatures as high as the panel can survive. The amount of
energy harvested is determined by the capture area and efficiency, but
the operating temperature is dependent on factors other than area.


Today, I'm partial to the notion of a Stirling cycle engine like that
shown in the sketch at

   http://www.iedu.com/DeSoto/Misc/Fig3a.png

using water for the working fluid and operating bit below the critical
temperature. A low-viscosity, non-freezing working fluid would be more
appropriate for polar use, but efficiency would be improved by the cold
operating conditions. I like it because the radiation-to-electrical
conversion could be several times more efficient than current PV cells.

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E


Posted by Josepi on October 6, 2010, 1:54 am
 On the Stirling engine (I like it) vein consider this contraption.

We start with our input of a larger solar collector (parabolic reflector
etc..)

Focused sunlight runs a Stirling engine. (possibly via thermal storage mass
for short temp storage or heat dump in times of input overload = cooling fan
from PV??))

The Stirling engine shaft is connected to a compressor of a heat pump.

Heat pump removes heat from the Stirling engine "cold side" increasing the
differential for the Stirling engine.

The heat pump produces a usable high temperature for cooking, possibly more
controllable.

Sound like perpetual motion but it would only promoite efficiency.



Q1: Do Stirling engines require priming or do they self start?

Q2: Are there heat pump / A/C gasses that can function in high temperatures?






Today, I'm partial to the notion of a Stirling cycle engine like that
shown in the sketch at

   http://www.iedu.com/DeSoto/Misc/Fig3a.png

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E




Posted by Morris Dovey on October 6, 2010, 3:43 am
 On 10/5/2010 8:54 PM, Josepi wrote:

Given the lossy character of all heat engines (which includes IC
engines), it doesn't come close to perpetual motion - the only ways to
know how good this is are to either do a real test, or model the entire
set-up using known-good software and known-good parameter values.

The best I can offer on efficiency is Sadi Carnot's theoretical limit
equation:

   Emax = 1 - Tc / Th

where Tc is the cold head temperature in Kelvins, and Th is the hot head
temperature in Kelvins; which provides the maximum possible efficiency
of an ideal (no losses at all) engine.


The fluid-pistons I've seen all self started. I think those with free
pistons self-start, but I don't know that for sure; and that those with
crankshafts should self-start if the crank is in the right position
(although my toy-size crank-type Stirling always needs a nudge to start
no matter what).


I would guess so, but I'd also guess that there isn't a
one-size-fits-all answer.

I like the direction of your thinking, but I wonder if (assuming that a
concentrating collector can be used) whether it might not make more
sense to use high temperature storage and just bleed heat from storage
as needed to cook with...

If enough heat could be stored, it could possibly be tapped to run a
tiny generator (think transistor radio and/or LED study light) and heat
water on-demand.

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E


Posted by Josepi on October 4, 2010, 4:53 am
 Solar thermal loses most of it's energy because in a cooking application you
will only use it an hour or so per day.

The point was, PV applications can store energy and deliver power at a
faster rate than collected. Solar thermal systems have a hard time doing
this.  On a cloudy day you may not input enough thermal power (rate of
energy) to cook anything without a huge collector that is wasting (via time
of usage) most of it's capabilities, given a brighter sky.

 larger thermal mass may alleviate this problem given a low solar input day.
Collect heat until the temperature is suitable and then cook with the large
solar mass.


I have difficulty digesting this. Solar thermal has no conversion losses
(because there is no conversion).

PV loses most of the energy in the initial radiation-to-electrical
conversion, with additional losses in the electrical-to-chemical and the
chemical-to-electrical conversions at the battery. Charge controller and
inverter, if present, lose still more...

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E



On 10/3/2010 7:25 PM, Josepi wrote:



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