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Help with Solar Oven article by sci

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Posted by sgotr on March 11, 2006, 7:35 am
 
The Solar Cookers International has an article (in pdf), "Solar Baking
Under the Sonoran Sun" by Laurie Stone, in which describes the building
of a big solar oven. As the article was written in 1997 I have some
questions about the construction of the oven.

1. I do not think the high temperature for the oven was given and I
would like to know if the solar oven reached 400 degrees or above. If
the solar oven does not reach 400 degrees what could be done in it's
construction so that it would reach the 400 degrees.

2. The article mentions using Everbrite for the reflectors. Are glass
mirrors better for reflectors than the Everbrite. Since 1997 are there
newer materials that would make better reflectors than the Everbrite?

3. The article says that 3/4 inch plywood was used in constructing the
solar oven's frame. Would a higher cooking temperature be obtained if
the frame was built more like a box solar cooker in that one frame
construction is constructed and then another one constructed about 2 or
3 inches bigger than the first one which would sit inside the second
one which is about 2 or 3 inches bigger?

4. The article mentions using PV fans (I think these are solar cells to
run the fans) -"The fans are run by a 6 Volt, 5 Amp module." - if these
are solar cell powered fans information on the types of solar cells to
use and the type of fans. And do you run the fans on all the different
types of meals or just the cookies and pasteries - how about bread

A few questions about cooking with the solar oven described in the
article.

1. Can a 16 inch pizza be cooked with the solar oven?

2. Can frozen foods be cooked with the solar oven such as: frozen pot
pies - like Banquet and other type brands. Frozen TV type dinners.
frozen fish breaded portions - like Gorton's and other types of brands.
Frozen pizzas. Frozen egg rolls. Frozen Boritoes. Frozen fries like
oreda fries.

3. Breads - sence the sun comes in through the glass do you move the
bread after it has cooked for so long so the backside of the bread gets
some sun

Thanks


Posted by sgotr on March 11, 2006, 8:09 pm
 
here's the link to the pdf article

<http://www.solarenergy.org/resources/docs/stone_hp59.pdf>


Posted by ghostwriter on March 13, 2006, 7:56 pm
 


Really depends on where you live and the season. In Mexico I do not
imagine that they have a lot of trouble getting it up to 400F.
Especially given the size of the reflectors that oven was using.  The
further north you go the larger the mirrors would have to get to that
kind of tempature. 250F is a more commom working temp, think crockpot.


Glass mirrors are a lot better reflecter than Everbrite or any other
polished aluminum, but also more expensive.  I suspect the most cost
efficient system would use larger amounts of cheaper material, with
reflective mylar film being the cheapest.


Yes, the air space serves as insulation.


It is important to mention that the sun isnt for the most part doing
the cooking, its the floor of the oven and the air in the oven that
actually transfers the energy. So the path for the majority of the
energy is
       SUN--FLOOR OF OVEN--COOKIE SHEET/AIR IN OVEN--COOKIES.

Cookies and pasteries are the most picky. They can burn on the bottom
before the tops are done.  A fan pulls heat out of the cookie sheet and
into the air and makes that less likley.


Very easily


Yes, although the average working temp is generally in the 250F range
for most solar ovens and so mositure content becomes important.  The
surface of food tends to get very dry since they do not seal as quickly
in the lower temp. Wrap things in foil to converse the heat and
moisture whenever possible.  The crusts of pot pies will be tricky, you
might need to wrap in foil for part of the cooking and then unwrap to
allow the crust to harden and dry.


No, since the sun is not doing the cooking directly.

Ghostwriter


Posted by sgotr on March 14, 2006, 12:33 am
 
Really depends on where you live and the season. In Mexico I do not
imagine that they have a lot of trouble getting it up to 400F.
Especially given the size of the reflectors that oven was using.

Solar zone 5
NOTE: the article is for a 72" long
solar oven - I am thinking more like about
half that size





Glass mirrors are a lot better reflecter than Everbrite or any other
polished aluminum, but also more expensive.  I suspect the most cost
efficient system would use larger amounts of cheaper material, with
reflective mylar film being the cheapest.

What kind of glass mirrors should be used -
in that the mirrors would be getting hot I
would think and working on getting to the 280c (thinking pizza here:))
heating solar oven - which ones are best at not getting to hot
to break or crack?
and
Would a photographers lighting kind of set up
work - that is have some refecltors on stands
and place them into the two areas that the
built onto the solar cookers leave between
the top lift up reflector (portable construction
so they can be brought inside after using)






Yes, the air space serves as insulation.

The article mentions using ductboard
if you have viewed the article can you
tell where they are putting the ductboard
and
using the double box construction would you
put insulation in the air space?
and
since 1997 what newer insulation materials
would work for approaching the 280c  for the
solar oven
and
approaching the 280c (over 400F) for the
solar cooker should some other material be
used for the box construction instead of wood
- like some sort of metal or other material?






Yes, although the average working temp is generally in the 250F range
for most solar ovens and so mositure content becomes important.  The
surface of food tends to get very dry since they do not seal as quickly
in the lower temp. Wrap things in foil to converse the heat and
moisture whenever possible.  The crusts of pot pies will be tricky, you
might need to wrap in foil for part of the cooking and then unwrap to
allow the crust to harden and dry.

Would the wrap be needed if at the 400F


Posted by ghostwriter on March 14, 2006, 2:07 pm
 With controling your tempature, I think the best way to do it is build
a modular mirror system.  Serveral mirrors with hinged legs on the
back, use a length of rope to set the angle of the mirror. Adjust the
length of the rope to change the angle to get the best sun.  Point as
many of these modular units at the oven as necessary to get the
tempature you are interested in.  A good themometer is essential to
this, but with enough mirrors you can get as high a temp as you could
want.


The mirrors reflect 90% of all of the energy that hits them, the last
10% is really not that big of a deal.  Nobody worrys about overheating
the mirrors on their cars on a sunny day.  Its a non-issue.


Thats basically what I imagine is the best set up, although I would
just take a piece of thin plywood, staple mylar to the surface, then
attach a V-shaped leg made of two 2*4 to the back with a hinge.  Screw
two eyes into the legs and two into the plywood and tie some string
between the eyes to make a tripod.


The ductboard is used to insulate the cooking chamber from the frame of
the oven. I assume they used black sheet metal to line the cooking
chamber, the ductboard is sandwiched between the metal and the frame.


That what I think they did, one box of metal, then ductboard, then the
wooden frame.


Ductboard may of gotten better but easily available materials havent
changed that much.


The lining of the cooking chamber would benifit from being metal, I
assume they used it in the article but maybe not.  If you dont use
metal in the cook chamber I strongly suggest a good fan.  The flash
point of wood is only 454F. The frame is actually best built out of a
material with poor thermal transfer, like wood.


No, just cook as per the normal recipe if at 400F.  The modular mirror
set up also has the advantage of allowing you to adjust the angle of
indivual mirrors and really home in on the 400F to a lot higher
percision than other systems.  A lot more work and material involved
but the end system should do what you want.

Ghostwriter


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