From markh@ORA.COM Sat May 14 09:45:21 1994
Date: Fri, 13 May 1994 22:51:10 -0700
From: Mark Harrington <markh@ORA.COM>
To: Multiple recipients of list INDKNOW <INDKNOW@UWAVM.BITNET>
Subject: Solar oven design

On May 9,  9:39am, Eng-Leong Foo mtc wrote:
} Subject: ET-012: The Solar Oven A Tool Of Sustenance
>The Solar Oven A Tool Of Sustenance
>
>Mark Harrington <markh@ora.com>
>phone: (H) 707-526-0867
>       (W) 707-576-2434
>
>===================================================================
>
>        Our greatest example of sustainability is nature herself. By
>emulating her we have the possibility of designing and creating tools
>of sustenance. The Solar oven is an elegant example of sustainable
>design.  The intention of this paper is to provide design ideas,
>practical information, rules of thumb, and support for the
>dissemination and implementation of this elemental ecotechnology.
>
>        Passive Solar Ovens are a very simple, inexpensive, yet
>effective tool which can be used for cooking a wide array of
>foodstuffs, sanitizing medical instruments and bandages, and water
>pasteurization (1).   These devices work effectively in Northern and
>Southern latitudes during spring to fall seasons and in equatorial
>zones they have the potential to function very efficiently throughout
>the year.  Solar Ovens generally require relatively clear days for
>operation although they can utilize some diffuse light. I have heard
>of complicated, high tech parabolic cookers which store high
>temperature oil and can even operate at night.  Parabolic cookers are
>well suited to high temperature cooking such as stir frying which
>can't be accomplished in an oven.  Although, overall they are less
>versatile then the oven.  Parabolic designs will not be evaluated in
>this conference.
>
>        Solar energy has been bestowed in a compensatory fashion.
>Often the areas with the greatest poverty, deforestation, and
>shortages of costly cooking fuel are the equatorial zone, the area
>with the greatest solar resource.  Besides providing an economically
>advantageous alternative to other renewable or non renewable cooking
>fuels, many of the worlds poor, women in particular, die prematurely
>due to respiratory diseases from exposure to smoke from burning dung,
>wood, coal and other fuels.  Significantly increasing time and energy
>are needed to forage for fire wood.  This is especially a burden for
>pregnant and nursing mothers.  Deforestation continues unabated,
>especially in developing countries, with an ever increasing loss of
>agricultural potential.  In the hot season residences may also be
>rendered more comfortable when foods can be cooked outside with solar
>energy. Where air conditioning is utilized energy can also be
>conserved by reducing waste heat from within the dwelling.
>
>        In affluent countries cooking fuel is taken for granted. In
>developing countries heirloom furniture and clothing has been used as
>cooking fuel!  In desperation, mature, bearing fruit trees have been
>cut down to provide fire wood! The optimal operational temperature
>range of a solar oven is from 2500  to  4000 F (1210-2500 C) A wide
>variety of foods can be prepared in the solar oven: legumes, grains,
>pasta, meat, fish, tea, coffee, vegetables, breads etc. "Solar cooking
>can best be described as a combination of mild broiling and oven
>baking" (2).  Higher temperatures are not necessarily desirable,
>unless time is essential. Irregardless, solar cooking requires advance
>planning, yet less effort.  You don't need to stir the pot.
>
>sustainable design it is important to use locally available, and
>preferrably renewable materials whenever possible.   Use of synthetic
>construction materials may result in the emission of chemical vapors,
>or gases, used in their manufacture, for example formaldehyde resins
>in wood products, or materials such as styrofoam or plastics may emit
>hazardous decomposition by-products at elevated temperatures such as
>cyanide. These emissions may result in the contamination of food-
>stuffs cooked in the oven.  My preferred construction materials are:
>glass, soil, bamboo, grass, solid-wood, metal, etc. These materials do
>not typically present off-gassing problems at design temperatures.
>
>        I strive for a minimalists design, this is my bias.  I ask the
>question how can a design be simplified and sustained, so that it can
>be replicated by nomads in the Niger, Quechua indians in the Peruvian
>Andes, or suburban North American earthquake survivors.  With "space-
>age" materials you may be able to squeeze more performance out of an
>oven, but you can quite capably do the job with very basic materials.
>
>        The body, or outer shell of a Solar oven can be constructed of
>cardboard, wood or wood products, adobe bricks, rammed earth, wattle
>and daub, wicker, thatch, reed, sheetmetal, fiberglass, composites
>etc.  Optimally, the body of the oven should be non conductive and
>resistant to ultra-violet light and water.  If the body is a good
>conductive material, such as galvanized steel, design for additional
>insulation as there will likely be greater heat loss.
>
>        Some designs will have an inner liner made of sheet metal, or
>other conductive or non-conductive materials, with insulating material
>filling the void between the inner and outer shells. If an inner shell
>is used optimum performance will come from a good conductor that is
>light weight, such as thin, blackened sheet metal.  The inner surfaces
>of the oven should be of low thermal mass (light-weight) construction.
>Excessive mass will not allow the oven to achieve design temperatures.
>Insulating materials can vary widely: fiberglass, synthetic foams,
>perlite, cardboard, rice hulls, straw, sawdust, wood chips, dry dung,
>hair, paper, corrugated plastic (3), etc.   Dead air spaces and aluminum
>foil (4) (shiny side facing in) are other strategies which may be used
>either by themselves or in partnership with other insulating
>materials.  Typical wall insulation thicknesses range from 20 to 150
>mm.
>
>        Some form of caulk or weather seal is desirable and can make
>an immense difference in the performance of an oven.  It should
>ideally be non conductive, soft and resilient, resistant to
>degradation by ultra-violet light, non toxic, and able to withstand
>peak design temperatures. This is especially critical for the mating
>surfaces of the solar aperature (glazing) with the body.  And also the
>access port, which is often the glazing. If this is the case it is
>important that the caulk does not harden, or adhere to the glazing.
>Some possibilities for the caulk are: rubber, latex, tar, silicone
>glue, felt, plastiline, leather,grease, pitch, gum resins etc.
>
>          The solar ovens I am describing here can be grouped into two
>categories, Portable and Monolithic ovens. The portable oven is
>versatile due to it's relative light weight, mobility and enhanced
>performance potential due to it's tracking capacities.  The most
>common configuration for a portable oven is a rectangular, or wedge
>shaped box with insulated sides and bottom.  The rectangular box form
>is easier to construct and  the angle of the glazing makes this design
>well suited for the tropics. In more extreme lattitudes the wedge
>shape design will be more efficient.  The optimal glazing angle will
>be perpendicular to incoming solar radiation. In practice this usually
>means manually moving the oven in increments along a horizontal plane,
>tracking the sun to optimize solar gains.  Some designs also have
>provisions for simultaneously pivoting the oven along a vertical axis,
>in order to optimize efficiency.  The latter design element, in my
>opinion, is usually superfluous, unless the oven is used in extreme
>lattitudes.  This design element requires a more complex oven design,
>with an inner suspension, so that the contents of the cooking
>vessel(s) do not spill. Portable ovens have the potential for much
>greater efficiency than their monolithic counterparts, although they
>are often more fragile, and a durable design is usually more expensive
>and complicated to build.  Portable ovens are more versatile.  They
>can be moved and transported and you can typically prepare two meals a
>day in a single oven, with an effective working range of 6-8 hrs. per
>day.
>
>         Monolithic ovens are usually fixed in position.  When earth
>is used as the structural body of the oven we create the alchemical
>marriage of the elements of earth, fire and water (food). They can be
>dug into a hillside, mound, or pit, where the earth itself, becomes
>the crucible, or they can be built above ground from adobe bricks,
>rammed earth etc.  The beauty of this design is that it is the most
>elegant, materially accessible, inexpensive and potentially one of the
>most enduring.forms.    The dug-in designs are well suited to desert
>areas, or dry season use.  The above ground designs should have a
>thatch or other protection from the erosive affects of rain.  A fixed
>oven's working range may only be 2-4 hrs. per day, usually one meal
>per day. It is feasible to have several fixed ovens, with different
>solar orientations, which share a common glazing and reflector.  In
>this manner we can compensate for the monoliths lack of tracking
>ability.
>
>        Single or multiple layers of glass or synthetic glazing
>material enclose the top and comprise the solar aperature.  Multiple
>glazings will have an air gap of 3 to 40 mm. between layers. Dead air
>spaces increase the insulative properties of the aperature.  However,
>consider that increasing layers of glazing will also reduce the amount
>of transmitted solar energy.  In most cases a single layer of standard
>window glass is quite adequate.  As a rule of thumb the thinner the
>glass the greater the transmittance of ultra-violet radiation, our
>workhorse.
>
>         A supplemental reflector, to increase the effective solar
>aperature, is usually required.  In equatorial regions the reflector
>may be optional if the oven is tight, well insulated, and oriented for
>maximum solar gain. Reflectors are strategically oriented and usually
>consist of glass mirrors, polished sheet metal, aluminum foil, or
>aluminized mylar (a type of plastic film).  Cardboard or plywood
>sheets painted white, or taut-stretched white fabric may also be
>effective, though less efficient.
>
>        You are welcome to join this conference with your questions
>and ideas.  Feel free to disseminate this paper.  In Sustenance...
>
>References:
>1. The Construction, Theory, and Use of the PAX Solar Water
>   Pasteurizer with Flat Heat Recuperator (Dale Andreatta, P.E. and
>   Derek Yegian)  Note: I understand that Derek will be presenting
>   this as a seperate paper for Ecotechnology Conference '94
>2. SBCI, Solar Box Cooker International Newsletter Spring. 1993
>3. The Solar Cookbook by Stella Andrassy
>4. SunWorld, Volume 14, Number 1, 1990
>
>
>************************************************************************
>              Please remember to refer to paper "ET-012"
>               in the "Subject" title of your message.
>Pls e-mail your questions, comments and answers to ET-LOKE@SEARN.SUNET.SE
>************************************************************************
}-- End of excerpt from Eng-Leong Foo mtc


I have had numerous requests to post this, while it is long, I hope
you will find it useful.

--
Mark Harrington

email: markh@ora.com
phone: (H) 707-526-0867
       (W) 707-576-2434