TECHNICAL PAPER # 36
UNDERSTANDING SOLAR COOKERS
AND OVENS
By
Thomas Bowman
Technical Reviewers
Mikos Fabersunne
Gary Flomenhoft
John Furber
John Yellot
Published By
VITA
1600 Wilson Boulevard, Suite
500
Arlington, Virginia 22209 USA
Tel: 703-276-1800 * Fax:
703/243-1865
Internet: pr-info@vita.org
Understanding Solar Cookers and Ovens
ISBN: 0-86619-247-6
[C] 1985,
Volunteers in Technical Assistance
PREFACE
This paper is one of a series published by Volunteers in
Technical
Assistance to provide an introduction to specific
state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
situations.
They are not intended to provide construction or
implementation
details. People are
urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
illustrated
almost entirely by VITA Volunteer technical experts on a
purely
voluntary basis.
Some 500 volunteers were involved in the production
of the first 100.titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Maria Giannuzzi
as editor, Suzanne Brooks handling typesetting and layout,
and
Margaret Crouch as project manager.
The author of this paper, VITA Volunteer Thomas E. Bowman,
is
Professor and Head of the Mechanical Engineering Department
at
the Florida Institute of Technology in Melbourne,
Florida. The
reviewers are also VITA volunteers.
Mikos Fabersunne is a mechanical
engineer employed with the Office of Energy Assessments for
the State of California in Sacramento.
Gary Flomenhoft is a
senior test engineer with TRW in Redondo Beach,
California. John
D. Furber is President of Pleasant Valley Software
Corporation
and Starlight Energy Technology in Aptos, California.
John Yellot
is Professor Emeritus of the College of Architecture at
Arizona
State University, and operates John Yellot Engineering
Associates,
a consulting engineering firm specializing in the use and
control of solar energy.
VITA is a private, nonprofit organization that supports
people
working on technical problems in developing countries.
VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
their
situations. VITA
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
of
volunteer technical consultants; manages long-term field
projects;
and publishes a variety of technical manuals and papers.
UNDERSTANDING SOLAR COOKERS
By
VITA Volunteer Thomas E. Bowman
I. INTRODUCTION
In many equatorial regions, native trees and bushes are
being cut
at an alarming rate to meet the growing demands for
agricultural
land, industry, and fuelwood and charcoal.
The environmental
consequences of this deforestation are severe, ranging from
soil
erosion and climate changes to flooding and the destruction
of
farmland. Moreover,
dwindling supplies of wood mean that people
must spend more time and energy in the search for cooking
fuel.
One alternative is to use kerosene, gasoline, or liquified
petroleum gas. But
these are often imported (hence expensive)
and transporting them to remote areas is usually difficult.
The next obvious alternative is to use the sun, particularly
in
equatorial regions where solar energy is abundant.
However,
although workable solar cookers have been developed, many
have
serious limitations.
For example, except for the indirect
cookers, most can only be used outdoors and during the day
(and
often only when the sun is high), most have to be adjusted
every
10-15 minutes to keep pace with the moving sun, and most are
suitable only for slow cooking and stewing.
In addition, some
designs can only accommodate one cooking pot.
Since even the best aren't as fast as stoves using
conventional
fuels, they are most suitable in conjunction with
traditional
methods, (i.e., using a solar cooker during the day and a
fuel-burning
stove at night, thereby conserving fuel and helping to
alleviate the deforestation problem).
HISTORY OF SOLAR COOKERS
Research and development of solar cookers began over 100
years
ago. W. Adams of
Bombay, India, designed one in 1876 that consisted
of an octagonal, glass-enclosed oven surrounded by glass
mirrors that collected the sunlight and directed it into the
enclosure. The span
of the reflector rim was 71 cm.
Adams reported that it cooked "the rations of seven
soldiers ... in
two hours, in January." (Sunshine Cooks Ltd., a
manufacturer of
solar cookers in the United States, is now marketing a
similar
cooker that has a somewhat smaller enclosure and larger
reflector
(94 cm).
One of the first indirect solar cookers (permitting cooking
at
night and indoors) was designed by Dr. Charles Greely Abbot
of
the Smithsonian Institute.
This used hot oil in narrow metal
tubing to take the heat from the solar collector to an
insulated
heat storage tank, and from there to the stove.
The cooking
element consisted of a tight spiral of copper tubing through
which the hot oil flowed before returning to the storage
tank.
Also, in 1981, Dr. Chang Sung-Ying of the Solar Energy
Research
Lab in Chen Chow, Hunan, reported that "several types
of solar
cookers have been developed" and thousands were
operating in
rural China. One
developed in his lab was a direct-focusing
cooker that was being sold outside of China in 1981 for
$150.00
(U.S.) Some were
also displayed in the China pavilion at the
1982 World's Fair in Knoxville, Tennessee.
During the late 1950s, two primary types were invented: (1)
the
direct-focusing cooker, in which a pot is supported at the
focal
point of a parabolic (or closely similar in shape)
reflector; and
(2) the Telkes oven, in which cooking pots are placed inside
an
insulated oven, one of whose walls consists of a window,
usually
double-glazed, surrounded by an array of plane reflectors.
The direct-focusing cookers are typically the least
expensive to
build and achieve the fastest cooking times (at least for
small
quantities of food).
Telkes ovens are much more expensive to
build aside from the large reflector area and two large pieces
of
glass for the window, the oven requires both inner and outer
sheet metal boxes, high-temperature insulation and a
door. Compared
to direct-focusing cookers, however, these ovens are easier
to use, cook greater amounts of food, protect it from blowing
dirt, and keep it warm.
They can also operate for long periods
without maintenance.
From 1957 until some time in the 1960s, over 200
direct-focusing
cookers were tested in Mexico.
Designed and developed at the
University of Wisconsin, they had rigid plastic reflectors
with
reflective films bonded to the front surfaces.
The mechanical
failures that appeared during the first tests were corrected
by
redesigning. But
although successful, those who used it regarded
it as just a novelty, and after a few months, went back to
their
traditional methods.
The cookers in the last tests were locally-built
with polyester shells, reinforced with muslin cloth and
burlap, and a number of small glass reflectors glued to the
front
surface. But those
were used only until the glass mirrors fell
off.
Attempts to introduce solar cookers in Mexico probably
failed for
a number of reasons:
o
The burdensome need to replace
periodically all or part
of the
solar cooker's reflective surface, whether plastic
foil or
glass pieces. Also these materials were
not
available locally and required a certain skill to
install.
o
Some operating characteristics of the
solar cookers
(i.e., the
need for frequent attention while cooking,
exposure of
the cooking pot to blowing dust and sand,
and lack of
heat storage) were unattractive.
o
Alternative cooking fuels, such as
kerosene and wood,
were
readily available at the time.
o
They had not been used traditionally and
they could not
be cleaned
with sand!
In the late 1950s and early 1960s, two companies in India
manufactured a large number of direct-focusing solar cookers
equipped with parabolic reflectors made of polished and
anodized
aluminum which meant they did not need the tubes replaced as
often. But many in
India refused to cook outdoors because of
health and religious regions.
Since the introduction of solar cookers in Mexico and India
over
20 years ago, solar cooking technology has not changed much
in
the case of direct-focusing cookers.
What has changed, however,
is that people in developing countries may no longer have
any
choice but to adopt them because traditional cooking fuels
are
disappearing. Indeed,
there are large areas where locally-available
combustible materials simply do not exist anymore, and
where petroleum-based fuels are not easily available.
II. DESIGN
VARIATIONS
Generally speaking, solar cookers fall into three
categories: (1)
direct-focusing cookers, (2) oven cookers, and (3) indirect
cookers.
DIRECT-FOCUSING COOKERS
A direct-focusing cooker uses a reflector to focus sunlight
directly onto a dark cooking pot which is either suspended
or set
on a stand at the focal point.
It consists of one or more
reflectors and a framework that supports both the reflector
and
the pot. Numerous
arrangements of this cooker have been devised
to allow the reflector to be tilted to always point toward
the
sun, with the pot remaining at the focal point.
Various types of reflectors can be used:
o
a plastic shell, parabolic or spherical in
cross-section,
lined with
a reflective plastic film such as
aluminized
Mylar, aluminized polyester, or reflective
acrylic
(e.g., "Scotchcal");
o
a plastic shell, as described above, with
a mosaic of
small glass
mirrors glued to the inside;
o
various types of clay or pottery shell,
lined with a
reflective
film or small glass mirrors;
o
a woven shell ("sun basket"),
smoothed on the inside
with papier
mache and lined with a reflective film;
o
various cardboard fabrications lined with
reflective
film;
o
various rigid foam shells, lined with
reflective films;
o
a Fresnel-type lens that consists of a
series of
concentric
hardboard rings, lined with a reflective
film (*);
o
a spun aluminum shell, polished and
anodized;
o
a folding fabric and metal device very
similar to a
large
umbrella, with a reflective film on the inside;
o
one or two fan-shaped arrays of individual
metal
reflectors,
designed to collapse into a compact package;
o
a fixed array of individual focusing
reflectors such as
shaving
mirrors.
(*) VITA can provide detailed plans for this system.
A number of schemes have been devised to make reflector
shells in
the field, but all still require highly-skilled artisans to
produce an accurate surface.
(Factory-produced shells, whether
injection-molded plastic, molded fiberglass, spun aluminum,
or
some other material, should be considered whenever
feasible.)
The reflector, if it is in one piece, should be about 1.0 to
1.5
meters in diameter to permit the cooking of moderately large
amounts of food. If
it is in several pieces, the total area
should be comparable.
Although cooking with much smaller reflectors
has reportedly been successful, small reflectors mean
less food can be cooked and sunlight conditions must be more
favorable.
Although some of the direct-focusing cooker designs offer
good
performance at comparatively low initial cost, they have a
number
of shortcomings and special design considerations:
o
The cooking pot is exposed to many
hazards, including
blowing
sand and dirt, children playing, mishandling,
etc.
o
Direct-focusing cookers are much less
versatile than
ovens.
They cannot, for example, be used to cook
two
dishes at the same time, or to keep food
warm. Cooking
is done in
a single pot.
o
The wind load on the reflector of a
direct-focusing
cooker can
be quite high due to the large size of the
reflector. Therefore care must
be taken in the design
so that the
cooker will not tip over easily.
o
The materials used in some designs, such
as hardboard
or
cardboard, deteriorate if kept outdoors.
o
The older reflective films have limited
lifetimes when
left
outdoors, as do the adhesives used to hold them in
place.
The need for frequent replacement of these
reflective
films and adhesives increases the operating
cost and
makes it comparable to that of a kerosene
stove.
Thus far, "Scotchcal" seems to
work much better.
o
All reflective films scratch easily, and
consequently
must be
cleaned very carefully.
o
In some cases, direct-focusing cookers
with long focal
lengths may hurt the eyes.
o
Since direct-focusing cookers use only the
sun's direct
rays, they
work poorly on hazy days, and a cloud passing
overhead
will temporarily "turn off" the cooker.
Generally,
direct-focusing cookers need to be adjusted
every 10 to
20 minutes to face the sun.
o
Aluminum reflectors, the only really
permanent
solution,
should be electropolished (or, at the very
least,
machine buffed) to obtain good reflectivity, and
anodized to
keep their reflectivity. Solar cookers
equipped
with aluminum reflectors result in high initial
costs
compared to designs using reflective
films.
o
Applying a reflective film to a
doubly-curved (compound
curvature)
surface requires a high level of skill.
The
film must
he carefully cut into many small pieces,
fitted to
the shell, and bonded to avoid bubbles and
other
imperfections. Even applying a film to
a flat
surface
requires considerable practice and patience.
Figure 1 shows a direct-focusing cooker consisting of a
parabolic
26p06.gif (486x486)
dish.
OVEN COOKERS
A solar oven is an insulated box with a glazed cover that
cooks
food through the "greenhouse effect."
Sunlight enters the oven
through the glazing and heats the dark inside walls and
cooking
vessels. Since the
heat cannot escape through the glass, the
oven gets very hot.
Mirrors around the window send even more
sunlight into the oven.
<FIGURE 2>
26p07.gif (486x486)
Oven cookers are more versatile than direct-focusing cookers
because they can use either direct or diffused
sunlight. The
temperature, which may exceed 200 [degrees] C (328 [degrees]
F) when reflectors are
used, is hot enough for almost any kind of cooking except
frying.
In addition, many foods can be slow-cooked without adjusting
the
cooker to track the moving sun.
Solar ovens tend to be heavy and more stable in wind.
They also
retain heat longer than direct-focusing cookers.
For example, a
properly designed cooker with an insulated cover can retain
temperatures of 150 [degrees] C (238 [degrees] F) for an
hour after sunset. Bricks,
stones, and other heat storage media also may be placed in
the
oven to retain heat even longer, although they do cause the
oven
to heat up more slowly initially.
A wide variety of oven cookers have been developed over the
years. Those
described in this section are grouped into three
categories according to their concentration ratios:
(1) low-concentration
ovens, (2) medium-concentration ovens, and (3)
high-concentration ovens.
The concentration ratio is the ratio of the total
intercepted
area of incoming sunlight to the area of the oven
window. For
example, if the entire cooker, including reflectors, is seen
by
the sun as a 1.0 meter square object, and the window is a
square,
50 cm on a side, the concentration ratio would be four.
(The
definition assumes that the oven is designed and set up
properly,
so that all the sunlight hitting the reflectors is reflected
through the window.)
Low-Concentration Ovens
Low-concentration ovens are those with concentration ratios
of
one (no reflectors) or slightly greater, but less than
two. In
this category, the most important general types of oven cookers
are box cookers and pit cookers.
Box cookers. Box cookers are shallow boxes sitting flat on
the
ground, usually insulated, with either single-layer or
double-layer
glass covers.
Sometimes, a single-plane mirror is used to
increase the amount of sunlight entering the window.
The box is normally not tipped to face the sun, and hence a
reflector can greatly increase the amount of sun entering
the
cooker when the sun is low in the sky, even though the
maximum
increase is only about 60 percent with the sun directly
overhead.
To reduce heat loss from the sides, the box is normally
fairly
shallow, and the kinds of pots and other items that can be
placed
in the oven are limited as a result of the low concentration
ratio.
Unless a reflector is used and moved to follow the sun, a
box
cooker will only be effective when the sun is more than 60
[degrees] above
the horizon. Within
about 5 [degrees] to 10 [degrees] of the equator, it could be
during the middle of the day whenever the sun shines.
At 30 [degrees]
latitude it could only be used from about March 21 to
September
21 (or vice versa in the southern hemisphere), near the
beginning
or end of this period, the daily usable period would be very
short. As a
practical matter, the use of a horizontal box cooker
without a reflector would be limited to tropical latitudes,
and
even there other forms of cooking might still be necessary
during
a part of the year except for those living within about
1,000
kilometers (km) of the equator.
The farther from the equator,
the more useful a reflector.
In mainland China, this limitation has been overcome by
using box
cookers tipped at fixed angles.
The interior of such cookers is
evidently somewhat complicated, with tiers of small
horizontal
shelves. The fixed
tilt angle still means that the cooker will
perform well for some sun positions and not for others, just
as a
horizontal box would but without the latitude
limitations. Although
much cheaper to build than most others, box cookers should
be considered only where their limitations are not
critical. The
box itself can be made of wood, plywood, chipboard, etc.,
provided an "exterior grade" is used and the
exterior of the box
is well painted. The
bottom and sides of the box should be very
well insulated (insulating materials are discussed in
Section
III) and a black tray fitted above the insulation,
preferably
supported by clips or brackets on the sides of the box
rather
than resting directly on the insulation.
The glass cover should
probably be fitted with a frame and handles to make it
easier to
lift on and off. A
well-fitting latch to hold the cover and the
box together is highly desirable.
Pit cookers. Pit
cookers are pits dug into the ground and lined
with insulating material such as wood chips or rice husks.
The
oven itself can be a box or clay pot, etc., with a glass
cover.
These are even easier to build than box cookers because they
do
not require an insulated box.
However, the limitations that apply
to box cookers also apply to these, and they cannot be moved
around to follow the sun.
Medium-Concentration Ovens
Medium-concentration ovens are those with concentration
ratios
between two and five.
The two best-known general types of solar
ovens--Adams cookers and Telkes ovens--fall into this
category.
The oven enclosure in a Telkes oven is made up of insulated
walls
and a transparent front cover (like a box cooker except that
it
is three-dimensional), whereas in an Adams cooker, the oven
enclosure is transparent except for an insulated back
surface. In
both types, the transparent surface is surrounded by
reflectors.
Figure 3 is an illustration of the original Adams cooker
built around
26p10a.gif (486x486)
1878. Figure 4 shows
a Telkes oven built at the Florida
26p10b.gif (540x540)
Institute of Technology (FIT) in 1980.
For good performance with either type, the entire oven and
reflector must be kept pointing more or less in the
direction of
the sun. The oven
may need to be moved as much as once every 10
minutes or as little as once every 60 to 90 minutes,
depending on
the design of the oven and the position of the sun.
Other configurations could be devised that would produce
ovens
with concentration ratios of two to five.
(All existing medium-concentration
solar ovens can be described as either Adams
cookers or Telkes ovens.)
The Adams cooker, (although somewhat more expensive than a
box
cooker), can be used later (or earlier) in the day, and at
higher
latitudes, since it can be pointed at the sun.
Its relatively
high concentration ratio also means it reaches higher
temperatures and can be used on cool or cold days;
similarly, the
wind is less likely to keep it from reaching cooking
temperatures. It is
not as easy to build as a box cooker, but
much easier than a Telkes oven or a direct-focusing cooker,
and
should also have a lower material cost than a Telkes oven,
especially if the glass enclosure for the cooking space can
be
mass-produced. (If
the glass enclosure has to be made in the
field using pieces of flat glass, it might be best to
consider a
different type of cooker.)
It should also be kept in mind that
the glass enclosure gets quite hot, and must be handled
while
hot. The only access
to the food being cooked is by removing the
glass enclosure and then replacing it.
Like the Telkes oven, the Adams cooker is far easier to
use if
fitted with a swinging rack on which to place the food.
Adjustable racks that can be set up at various angles are
not
very satisfactory, especially if the tilt angle of the
cooker
keeps changing to follow the sun.
A swinging rack will keep the
food from spilling during reasonably minor movements of the
cooker, with no need for opening the heated enclosure.
The greatest disadvantage of the Adams cooker is the small
cooking space. Thus,
the Telkes oven may be preferable, despite
its higher cost.
With the Telkes oven, it is necessary to build
both an oven and a reflector array, and then fit them
together.
The oven should have heat resistant inner walls (i.e., sheet
metal) with insulation in between the inner and outer walls,
a
large window on one side, and a door on the other, making it
rather complex to build; the services of a sheet metal shop
are
almost essential.
The window is large, and needs to be strong
(two layers) or it will break easily.
Attaching individual
reflectors directly to the oven, so that they surround the
window,
is usually unsatisfactory due to both the wind loads on the
reflectors, and the normal daily bumps and knocks.
Instead, the
reflectors should be attached to a separate sheet metal
structure
with a stiffening outer ring.
Durability is important in a Telkes oven because one of its
most
attractive features is that it can be used all day, dragged
around to follow the sun, and tipped from horizontal at
sunrise
to vertical at noon and back to horizontal in the
evening. Because
it has a roomy interior, it tends to be big.
But it can
withstand a lot of abuse.
One of the main reasons the Telkes oven is perhaps the most
expensive type to build is because the total area of
reflective
material is much greater than in any other cooker for a
given
total amount of sunlight collected.
(This is because of the small
angle--30 [degrees]--between the incoming sunlight and the
reflector surface.)
On the other hand, it is an extremely versatile cooker
than can accommodate several large pots of food at one time,
can
heat a pot of cooking oil to 230 [degrees] C, can keep some
food warm while
others are cooking, and that can keep food warm well into
the
evening if a blanket is draped over the window.
When building a Telkes oven, there are two paths to failure:
(1) making the oven too large relative to the size of the
window
and reflectors; and (2) using materials, including
insulation
that cannot withstand high temperatures.
Regarding the size of
the oven, the primary concern is with total surface area
rather
than with volume. It
is therefore essential to design an oven
that makes the most efficient possible use of its interior
volume. At the same
time, though, it is helpful to be able to tip
the oven up and down to face the moving sun without spilling
the
food being cooked.
This can be done by installing a tray that
swings in and out of the oven.
Note, however, that it may be
difficult to satisfy both the need for minimum wall area and
the
need for a swinging rack.
High-Concentration Ovens
High-concentration ovens are those with concentration ratios
greater than five.
Such ratios are obtained by elevating the
ovens over focusing reflectors or arrays of reflectors.
Focusing
reflectors used in a solar oven normally have a simple
curvature,
resulting in a line focus, since the objective is to heat
the
inside of the oven rather than a specific target.
If an array of
plane reflectors is used, each reflector is usually about
the
same size as the window, and the theoretical concentration
ratio
is therefore equal to the number of reflectors.
(The
concentration ratios realized in practice are lower, because
when
the sun is high in the sky the oven casts a shadow on some
of the
reflectors, and when it is low the projected area of the
reflectors is lower than the actual area.)
The cooker shown in Figure 5, which was developed at FIT,
uses a
26p13.gif (600x600)
single parabolic reflector with its vertex near the rear
edge,
pivoted about the center of a circle passing through the
ends of
the parabola and the focal point.
Even at low sun angles, a
reflector designed along these lines stays closer to the
ground
and closer to a vertical line through the oven window than
would
be the case with a symmetric reflector or a higher pivot
axis, as
in Prata's design.
Hence, more energy enters the oven since the
window is long and narrow, and is at the bottom of the oven,
the
focus is a sharp line for one reflector position and a
broader
line for other positions.
In operation, the entire oven is turned periodically about a
vertical axis to face the sun, while the reflector alone is
tipped to follow changes in the sun's elevation.
Proper reflector
angle is indicated by the location of the bright focal line
on
the window, and is maintained by letting the reflector rest
against an adjustable support rod.
The window is wide enough to allow periodic rather than
continuous
adjustment of the reflector.
The reflector is typically
adjusted so that the focal line is at one edge of the window
(the
focus is on the back edge in the morning, and on the front
edge
in the afternoon).
As the sun rises or sets with the reflector
stationary between adjustments, the focal line, which is
very
bright and hence easily seen, moves across the window.
When it
reaches the opposite edge, the reflector should be adjusted
to
move the focus back to the original edge.
Adjustment is typically
needed every 10 to 15 minutes.
The reflector can be made in almost as many different ways
as the
reflectors used in the direct focusing cookers described
earlier.
Its curvature is simple, so a single sheet of metal will do.
Since both the area of window and the reflector are much
smaller
for a given oven size than in the Telkes oven, the material
cost
is less. The
materials for the oven itself are essentially the
same as in a Telkes oven, and the frame can be built fairly
cheaply in most developing countries, though the skills
needed to
build it are somewhat higher because of the need for an
accurately configured reflector.
Also, it is a little easier to
use than a Telkes oven, since the oven stays horizontal,
opens in
a "normal" fashion with a hinged, vertical door,
is located off
the ground and is heated from below rather than above.
Most of
the advantages of the Telkes oven relative to
direct-focusing
cookers also apply to this design.
However, it does not perform
as well as a Telkes oven of comparable size because the oven
shades part of the reflector.
Also, the size of the reflector is
limited because it must be able to clear the ground as it
swings.
The higher the oven relative to the ground, the larger the
reflector can be and the higher will be the temperature
reached
by the oven.
The solar oven shown in Figure 6, also designed at FIT, was
26p15.gif (600x600)
designed to overcome this limitation on reflector size.
Here,
individual reflectors pivot about their own central axis and
are
controlled through appropriate linkages by a single
operating
lever. To follow the
sun, the whole oven is rotated and the
operating lever is moved to shift the patch of reflected
light
from the oven shell onto the window.
If the individual reflectors
are plane, glass mirrors may be used; if they are curved,
fewer
reflectors are needed because they can be larger.
In the latter
case, each reflector should have a circular cross-section,
with a
radius twice the distance from the reflector to the
window. The
need for accurate reflector shape is much less critical than
in
the single-reflector case because the individual reflectors
are
smaller.
A comparison of the two FIT cooker designs indicates that
the
multi-reflector version has much higher performance, but at
the
cost of greater complexity and many more moving parts.
Total
material cost of the multi-reflector cooker is also somewhat
higher than the other FIT design, although still lower than that
of a comparable Telkes oven.
Both FIT versions are more difficult
to build than a Telkes oven.
Neither these nor Telkes ovens
can be constructed in the field.
Building FIT cookers also requires
a drill press and many careful measurements, which are not
needed for Telkes ovens.
Compared to direct-focusing cookers (and
except for the reflectors), FIT ovens can be sturdy and
require
little maintenance.
Of all the solar cookers, the FIT and Telkes ovens are
probably
the easiest to use and the most versatile.
The temperatures of
over 200 [degrees] C that a large Telkes oven is capable of
reaching have
not been achieved in FIT cookers of reasonable height, but
an
FIT cooker can reach about 150 [degrees] C, which is
adequate for most
purposes. FIT cookers
should be considered only if local workshop
facilities are available for their construction.
INDIRECT COOKERS
These are the only solar cookers that can be used
indoors. In an
indirect cooker, the enclosure where the food is cooked does
not
receive solar energy directly.
Instead, a solar collector,
separate from the cooking chamber, receives sunlight to heat
a
working fluid--usually either a vegetable oil or water.
The hot
fluid then travels through a pipe to heat the cooking
enclosure.
In 1964, Whillier of the Brace Research Institute, built an
indirect cooker consisting of a horizontal finned pipe at
the
focus of a stationary circular-cylindrical reflector,
connected
at one end to a double-boiler cooker.
Water in the pipe boils,
sending steam to the cooker, where it condenses on the
outside of
the cooking pot. The
aperture area of the reflector was 1.3
square meters.
One of their later designs consisted of a stationary flat
plate
solar collector inclined at a 45 [degrees] angle, with a
double-boiler
cooker mounted on top of the collector (see Figure 7).
In this
26p17.gif (437x437)
design, steam from the collector rises to the cooker, where
some
escapes and the rest condenses and runs back to the
collector.
However, when twenty of these were installed at a school in
Haiti
in the early 1970s, most were eventually dismantled when
they
failed to work properly.
(Dr. Erich Farber of the University of
Florida, however, reports that he is currently using several
of
these cookers with very good results.)
Indirect cookers have many disadvantages.
If, for example, an
indirect cooker uses atmospheric pressure steam as a working
fluid, the maximum possible cooking temperature is below the
boiling point of water, and thus is suitable only for slow
cooking and stewing.
Moreover, the insulated cooking box is
small. This limits
the size of the pot and the amount of food
that can be cooked.
And because most indirect solar cookers are
immovable units, they cannot be adjusted to track the
sun. As a
result, they are useful only part of the day.
Finally, efficient
flat plate solar collectors are difficult to build in the
field,
and some important components (e.g. low-iron glass) are not
readily available in small quantities.
These collectors also use
more glass than other solar cookers.
On the other hand, if the
flat plate solar collector is purchased as a unit, building
the
rest of the cooker is relatively simple and cheap.
III. DESIGNING THE
SOLAR COOKER RIGHT FOR YOU
SUMMARY COMPARISON OF SOLAR COOKERS
The main cooker types discussed previously have advantages
and
disadvantages that can be summarized as follows:
o
Direct-focusing cookers offer high cooking
temperatures
at a
relatively low cost, but have a number
of
disadvantages for everyday use as the primary means of
cooking
meals. They are also unsatisfactory in
windy
conditions,
or in hazy sunlight. Some designs are
suitable
for on-site
construction, but typically have high
maintenance
costs. Shop-built reflectors should be
considered
if at all
feasible. Workers building these
cookers
should have a fairly high level of skill.
o
Box and pit cookers are inexpensive and
easy to build,
and are
effective cookers within their limited range.
ey do not
accommodate large cooking pots, and cannot
be used in
areas far from the equator or in the early
morning or
late afternoon, although these latter two
problems
can be alleviated by modifying the basic
design. Worker skills need not
be as high as with other
types of
cookers.
o
The Adams cooker is probably adaptable to
on-site
construction if a tempered-glass oven enclosure can be
mass-produced (to reduce unit costs),
and distributed.
It offers
good performance and simplicity at a cost
somewhat
higher than a box cooker, but lower than most
other
oven-type cookers. Worker skills
required are
comparable to the box cooker, and less
than for other
oven-type
cookers or direct-focusing cookers. It
is
also is
suitable for a wide range of latitudes and
weather
conditions, and is less affected by windy
conditions
than some other types.
o
Telkes ovens generally offer the highest
performance of
any of the
oven cookers, but also have the highest
material
cost. Building one normally requires
the
services of
a sheet metal shop. They can be very
durable and
versatile, and work well in a variety of
wind and
weather conditions, latitudes, and times of
day.
The cooking enclosure is larger and easier
to use
than that
of an Adams cooker, but not as easy to use as
those found
in high-concentration cookers.
o
High-concentration solar ovens separate
the oven from
the
reflector, making the oven much easier to use.
Material
cost is also lower than a Telkes oven, but
higher than
most other types. The primary
disadvantages
are their
greater complexity relative to other solar
cookers,
and the need for a sheet metal shope and
machine
tools for construction. Their wide
range of
usefulness
parallels that of Telkes ovens, except that
they do not
make effective use of diffuse sunlight.
However,
further design improvements may alleviate some
of the
construction difficulties.
o
Indirect solar cookers have as their chief
advantage
the
possibility of indoor cooking. They are
also sturdy
and stable
and little affected by wind and cold
weather. But, they should
probably not be considered
unless good-quality, shop-built flat
plate solar
collectors
are available. Except for the
collector,
they are
easy to build and are low in material cost.
If
steam is
the heating media, they have very low maximum
temperatures (i.e., they cannot boil
water or fry
foods). Most designs can be used
only part of the day
since they
cannot be rotated to face the sun.
USE OF LOCAL RESOURCES
Most solar cooker designs require insulating material, and
either
glass mirrors or polished aluminum reflectors.
These items,
however, are seldom found in rural areas, and often must be
imported. Many
cooker designs also use steel or aluminum sheet
metal and/or structural steel or aluminum, which are usually
available only in larger towns and cities.
Hardware, such as nuts
and bolts, hinges, wheels, and door catches, are also often
required in many designs.
The skills needed to build most of these cookers include
basic
sheet metal bending, cutting and welding, drilling holes,
etc.
But even workers experienced by local standards may need
close
supervision to turn out an article that is unfamiliar to
them.
Some designs, especially those of direct-focusing cookers,
make
use of indigenous materials such as bamboo, and local crafts
such
as basket-making and pottery.
But these designs still require
imported reflective films and adhesives, since using
anything
less than the best will result in more maintenance and
decreased
performance. It is
certainly easier to import reflective film
than reflective aluminum, though importing the latter may be
cheaper in the long run if it requires fewer special skills
for
construction and less maintenance.
Although it may be possible to use indigenous materials for
insulation, (clay, sand, dirt, rock, bricks, etc.) most make
poor insulators.
Some inorganic materials that can be used to
include volcanic rock and tuff.
As a rule of thumb, any
lightweight non-metallic inorganic material is probably an
effective thermal insulation.
Typically, such material will
appear porous or foamy.
Organic insulating materials such as rice husks, shredded
paper,
cardboard, wood chips, or tree bark are better, but most
need to
be kept dry and this is often difficult.
And while cork is an
excellent insulating material that is impervious to
moisture, it
may not be able to withstand high cooking temperatures.
Fiberglass batts, one of the world's most readily available
commercial insulating materials, seems to be limited to a
maximum
temperature of about 150 [degrees] C, and it has little or
no resistance to
moisture. And while
the fibers themselves should be good to well
above 450 [degrees] C, and are not affected by moisture,
they are difficult
to work with in their natural state and their insulating
qualities
are disappointing.
Urethane foam, either in the form of boards or as a
foam-in-place
kit, is one of the best insulating materials commonly used
in the
United States because of its low thermal conductivity.
But
manufacturers of urethane foam point out it has an upper
temperature limit of about 150 [degrees] C, and at high
temperature it
gives off poisonous vapors.
It is also quite expensive,
especially in kit form.
Common asbestos board has a very high temperature limit, but
a
thermal conductivity five or six times higher than
fiberglass
board. It also costs
about five times as much and poses a
serious health hazard.
Firebrick, a lightweight porous brick, twice as good as
asbestos
board, is not much cheaper.
Calcium silicate is cheaper and a
better insulator, but is subject to water damage.
Compared to
fiberglass board, its conductivity is about 50 percent
higher and
its cost about twice as high, but its upper temperature
limit is
not a problem at all.
"Foamglas," another insulating material that can
be manufactured
from waste glass in a very simple shop, is much cheaper than
calcium silicate, with a slightly better (lower) thermal
conductivity and nearly as high a maximum temperature.
It is also
non-toxic, not at all affected by moisture, and has good
rigidity
and compressive strength.
Its rough, black surface is also a good
absorber of sunlight.
The chief disadvantage is that everything
that touches it rubs off rough, gritty particles.
An experimental
oven has been built entirely of foamglas, with no metal
parts.
SCALE
Thus far, no attempts have been made to design and test very
large solar cookers that could be used by several families
or an
entire village.
Until this work is undertaken, it is difficult to
speculate how the design of very large cookers would differ
from
smaller ovens.
Perhaps special platforms could be built to give
access to ovens far above the ground, etc.
However, unlike small
single-family units, large ovens would be hard to move
unless
they could be rotated by turntables resting on fixed
pedestals.
POSSIBLE PROBLEMS TO CONSIDER
So far we have limited our discussion to the technical
problems
and economics of various types of solar cookers.
But they must
also be socially acceptable.
In one sense, the problems of
social acceptance are often overstated, because if the need
is
great enough and the solar cookers are good enough, they
will be
accepted. On the
other hand, though, solar cookers still have
many technical imperfections that require correction if they
are
to be accepted in any culture.
One problem with solar cookers is that they don't work when
the
sun doesn't shine.
There is a lot of difference between our
perceptions of the weather in any particular location, and
what
the weather actually is.
Many areas are not as sunny as they may
seem.
Another problem with solar cookers--or, for that matter, any
new
piece of equipment--is that in many parts of the world,
especially the rural areas of developing countries, a solar
cooker tends to be taken out of service for long periods,
even
though it may have suffered only minor problems.
Solar cookers
will probably not be successfully introduced in large
numbers in
any of these countries unless a team of solar cooker
technicians
can be formed and maintained, even if they do nothing more
than
tighten screws, oil pivots, and clean windows and
reflectors.
Ideally, these technicians should know where all the solar
cookers in any given district are, inspect each one
regularly,
put it back in service if it is not being used, service it
even
if it is being used, and remind the user of some of the
basics of
using and maintaining it.
MARKETING/DISSEMINATION
There are many problems blocking the widespread introduction
of
solar cookers. To
overcome these problems, a solar cooker
marketing and dissemination program should adopt several
strategies.
First, promoters of solar cookers must make potential users
aware
of the limitations of these devices, and emphasize that they
are
meant to complement, not replace, traditional fuel-burning
stoves. At the same
time, solar cookers should be introduced with
fuel-efficient varieties of stoves that burn wood, charcoal,
or
other traditional fuels as part of an overall plan to reduce
fuel
consumption.
Portable fired-clay stoves and other alternatives
might be incorporated into such a plan.
The important point is to
emphasize the complementary nature of these technologies,
and
conduct training accordingly.
Second, promoters must pay more attention to providing good
designs and better materials.
Designs should be technically
sound, incorporating research findings of recent
decades. They
also must be appropriate for the local geography, market
conditions, cooking traditions, etc.
A single change in design or
materials can affect performance significantly.
For example, one
solar oven project in Lesotho ran into problems because
exposure
to the sun caused the black paint on the interior to give
food a
strange taste. The
problem could have been avoided if another
type of paint had been used.
This example illustrates the third consideration, which is
to
make sure there are enough quality materials to cover both
original construction needs and spare parts.
To do this,
promoters may have to buy materials at bulk rates to achieve
the
necessary economies of scale.
Long-lasting black enamel, for
example, might have to be custom made and imported, and
would
have to be brought in quantity to reduce unit costs.
Fourth, a solar cooker that is both effective and durable is
probably far too expensive for those people who need it
most. So,
promoters must develop better systems or designs to enable
solar
ovens to become more competitive economically, and to help
users
make the capital investment needed to obtain them.
On the other
hand, solar cookers that are given away often tend to be
regarded
as worthless. This
seems to be a problem that sociologists,
economists, political scientists, and other should be
tackling,
not just with respect to solar cookers but all new technologies.
Solar cookers would make a valuable test case in dealing
with
this problem, since they are so closely involved with one of
life's most basic necessities, yet they are usually far
beyond
the scope of what the individual user could make for himself,
even with outside help.
And finally, those promoting solar ovens,
whether expatriates or local citizens, should use them to
cook
their own daily meals.
It is important to create a popular image
that solar cooking is modern and prestigious.
IV. FUTURE OF THE
TECHNOLOGY
Currently, many of the most promising solar cooker designs
exist
only as prototypes, usually built in university laboratories
for
experimental purposes.
As a result, they were not designed to be
produced efficiently in large numbers.
The next major
technological step that must be taken is totally redesigning
these cookers from the standpoint of manufacturability.
Improvements in terms of simplicity, cooking efficiency, and
ease
of use should also be made.
SUPPLIERS
AND MANUFACTURERS OF SOLAR COOKERS
SUPPLIERS
House of Russcar
3908 West Franklin Street
Richmond, Virginia 23221 USA
(Product: Swiss
design solar concentrator)
Solar Usage Now, Inc.
Box 306
420 East Tiffin Street
Bascom, Ohio 44809 USA
(Product: Direct-focusing
and oven cookers)
MANUFACTURERS
Clevlab, Inc.
Box 2647
Littleton, Colorado 80161 USA
(Products: Solar and
hybrid ovens)
Etampes et Mecanique
L. Serafini
9 CH du Centurion
CH-1227 Carouge-Geneve
SWITZERLAND
(Product: Swiss
design solar concentrator)
Kerr Enterprises, inc.
P.O. Box 27417
Tempa, Arizona 85281 USA
(Product: Solar Box
Ovens)
Sunshine Cooks, Ltd.
11806 North Tower Drive
Fountain Hills, Arizona 85268 USA
(Product: Adams-type
solar cookers)
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