Initial Solar Cooker Paper by elfphabet3

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									                    Community Solar Cooker Investigation for Kenya
                                    Final Report:


                                           Submitted to


                              Dr. P.B. Ravikumar
                                   Professor
      ME 349, Engineering Design Projects, University of Wisconsin-Madison




                                                  by



          Paul Fraser, Matt Anderson, Josh Buser, Jun Mo Park, Erik Suomi



-------------------, -------------------, -------------------, -------------------, -------------------

                           Department of Mechanical Engineering
                                 1513 University Avenue
                             University of Wisconsin-Madison
                                        WI 53706
                                          8-10-06
Executive Summary
Solar cooking is a clean and inexpensive means to provide energy for cooking food and
pasteurizing water. Solar cookers have the potential to reduce health problems associated
with the inhalation of smoke from cooking fires that contribute to over four million
deaths each year. Solar cookers also have the potential to increase the productivity of
people in developing countries by reducing the time spent finding wood, reduce potential
conflict over energy resources, reduce pollution which damages the environment, and
reducing deforestation.

The primary components of a solar cooker include the pot, greenhouse dome, pot base
materials, reflectors, and an insulated box to keep food warm for solar cookers other than
box cookers. The pots used are typically those that can be found locally. The greenhouse
dome is typically a box cooker box, plastic bags, glass domes, or a hard plastic dome.
The pot base material is often soil, cardboard, clay, or wood and potentially stones or a
metal frame is used to lift the pot off the ground. The reflectors are either parabolic,
concentrated parabolic concentrator (CPC), or non-focusing flat reflectors and the
materials for reflectors are typically polished metal, glass mirrors, or aluminum foil.

The design process started with extensive research of current solar cookers.
Understanding the current situation in developing countries led to an objective of
designing an economical community scale solar cooker that can be made with local
materials. Not only does a cost effective design need to be made, but a thorough
implementation plan must be considered in the future. A functional analysis of solar
cookers and many solutions for these functions did not sufficiently point towards a final
design. Quantitative analysis of each component and an in-depth cost analysis will be
necessary for a more complete weighted objectives table. Testing to be completed
includes: comparing greenhouses, comparing the use of a reflector to no reflector, and
comparing the insulation of the pot’s bottom surface.

The data was collected using identical aluminum pots that were spray painted black and
sealed with half a liter of water and a thermocouple half an inch from the bottom surface.
The type T thermocouples were connected to a data acquisition system and recorded onto
a laptop during the experiments. The different parameters tested in the experiments
included comparing greenhouse domes, a reflector, propping the pots upon stones, and
testing indoors. The greenhouse domes compared were a box cooker, glass dome, clear
nylon bag, unclear polyethylene bag, and a hard polypropylene dome.

The results of the experiments are as follows:

      Adding one greenhouse adds 15-20ºC
      Adding one mirror to the solar cooker adds about 12..5°C
      A clear nylon plastic bag improves maximum temperatures over an unclear plastic
       bag around 4.5°C
      Adding a second greenhouse improves max temperature by 4-5°C


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      Adding stones as the base material may add 0.50-1.0°C to max temp
      Hard plastic bins perform better than unclear plastic bags by about 1°C but about
       3.0°C less than the clear plastic bags
      Solar cooker tests should not be conducted inside due to discrepancies in light
       wavelength

The solar cooker design suggestions were selected based off of the results of the
experiments. Both a reflector and a greenhouse dome are necessities for solar food
cooking since they significantly increase the temperature of the solar cooker pot. The
selection of the solar cooker components should be based primarily on whether the
materials are found locally for an inexpensive cost. If the materials are found locally, the
pot should be propped upon three stones, a hard polypropylene plastic dome is suggested
for the greenhouse and two if cost permits, and two glass mirrors with a support assembly
are suggested for the reflector. This combination of solar cooker materials should
optimize the cost, usability, and durability of the solar cooker.




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Table of Contents
Executive Summary                                          ............................................................................ 2
Introduction to Solar Cooking ........................................................................................ 8
Background Information                        ......................................................................................... 8
   Solar Cooking in Kenya                               ............................................................................... 8
   Existing Solar Cooker Designs                       ................................................................................ 9
Safety Considerations                            .................................................................................... 10
Solar Cooker Components                                     ......................................................................... 11
   Greenhouse Insulation                              ............................................................................... 11
   Reflector Designs                     ............................................................................................ 11
      Focusing Reflectors .................................................................................................. 12
      Indirect types ............................................................................................................. 12
   Reflector Materials                             .................................................................................. 13
   Pot Base Material                            ..................................................................................... 13
   Cooking Pot                        ............................................................................................... 14
   Maintaining Cooker Temperature                              ...................................................................... 15
      Calcium and sodium sulfate ...................................................................................... 15
      Concrete, stone, wood, clay, sheet metal, air ............................................................ 16
   Fireless Cooker (Insulated Box)                                  ................................................................. 16
Method of Investigation                       ....................................................................................... 17
      Clarifying Objectives ................................................................................................ 17
      Establishing Functions .............................................................................................. 18
      Setting Requirements and Determining Characteristics ........................................... 18
      Generating Alternatives ............................................................................................ 19
      Evaluating Alternatives ............................................................................................. 19
Experimental Data                         ........................................................................................... 21
   Experimental Procedure ................................................................................................ 22
   Solar Cooking Procedure .............................................................................................. 22
   Experimental Results .................................................................................................... 23
      Experiment #1: Outside Greenhouse Comparison .................................................... 23
      Experiment #1 Summary .......................................................................................... 25
      Experiment #2: Indoor Greenhouse Comparison ..................................................... 26
      Experiment #3: Outdoor Greenhouse and Base Surface Comparison ...................... 27
      Experiment #4: Outdoors Mirror Comparison .......................................................... 33
Analysis of Data ................................................................................................................ 35
   Results summary of data ............................................................................................... 35
   Error Analysis                     ............................................................................................... 35
Economic Analysis                       ............................................................................................. 36
   Cost Benefit of Solar Cookers ...................................................................................... 37
   Cost of Solar Cooker Materials..................................................................................... 38
      Greenhouse Dome ..................................................................................................... 39
      Reflector .................................................................................................................... 39
      Base Material ............................................................................................................ 40
   Community Sharing ...................................................................................................... 40
      Solar Cooker Components ........................................................................................ 40
      Time .......................................................................................................................... 41


                                                                                                                                       4
     Micro-finance ............................................................................................................ 41
Conclusions and Recommendations                           ........................................................................ 41
  Pot Selection                          ......................................................................................... 42
  Pot Base Material Selection                   .................................................................................. 42
  Greenhouse Selection                      ...................................................................................... 42
     Number of Greenhouse Domes ................................................................................. 42
     Greenhouse Dome Material ...................................................................................... 43
  Reflector Selection              ............................................................................................... 43
  Final Recommendation ................................................................................................. 43
Bibliography ..................................................................................................................... 44
Appendix A: Mirror Support Assembly Design ............................................................... 46
Appendix B: Social Dynamics of Solar Cooking ............................................................ 47
Appendix C: Other Materials ........................................................................................... 47




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List of Figures

Figure 1: Box Cooker ……………………………………………………………..…9
Figure 2: Panel Cooker        ……………………………………………………..…9
Figure 3: Parabolic Cooker ……………………………………………………..…10
Figure 4: In-Wall Cooker      ……………………………………………………..…10
Figure 5: Parabola Shape      ……………………………………………………..…12
Figure 6: Weighted Objectives Tree – Main Objectives       ……………………..…17
Figure 7: Four Equivalent Pots with High Temperature Silicone (orange) and K- type
Thermocouple ……………………………………………………………………..…23
Figure 8: Setting Up the Thermocouple to the Computer and Testing the Operation
of our System ……………………………………………………………………..…23
Figure 9: Greenhouse Testing with Open Air Condition       ……………………..…24
Figure 10: Greenhouse Testing with Unclear Plastic Bag with Steel
Wire Structures        ………………………………………………………………..24
Figure 11: Greenhouse Testing with Glass Dome ……………………………..…24
Figure 12: Greenhouse Testing with Box Cooker ……………………………..…24
Figure 13: Outdoor Greenhouse Comparison Tests ……………………………..…25
Figure 14: Indoor Greenhouse Comparison with 300W Light Bulbs ……………..…26
Figure 15: Indoor Comparison Test Setup ……………………………………..…27
Figure 16: Unclear Plastic Bag       ……………………………………………..…28
Figure 17: Clear Plastic Bag ……………………………………………………..…28
Figure 18: Hard Plastic Top with Pot on Stones      ……………………………..…28
Figure 19: Hard Plastic Top on Ground        ……………………………………..…28
Figure 20: Hard Plastic Top over Clear Plastic Bag ……………………………..…28
Figure 21: Larger Hard Plastic Top over Small Hard Plastic Top ……………..…28
Figure 22: Outdoor Greenhouse Comparison ……………………………………..…29
Figure 23: Ground Surface Comparison         ……………………………………..…30
Figure 24: Heating Dynamic Effects for a Double Walled Greenhouse       ……..…31
Figure 25: Examining of the Effectiveness of using a Mirror       ……………..…33
Figure 26: Solar Cooker comparison with a Mirror ……………………………..…34
Figure 27: Mirror Support Assembly with three Mirrors      ……………………..…46
Figure 28: Mirror Support Assembly during Testing ……………………………..…46




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List of Tables

Table 1: Thermal Conductivities of Various Local Materials         …………………14
Table 2: Heat Capacities for Various Materials      …………………………………16
Table 3: Design Processes and Methods       …………………………………………17
Table 4: Morphological Chart …………………………………………………………20
Table 5: Maximum Temperatures Achieved by Various Greenhouse Insulations …25
Table 6: Maximum Temperature of Comparisons and
Improvement of Temperatures          …………………………………………………32
Table 7: Thermocouple Error Table …………………………………………………36
Table 8: Monthly Fuel Costs for Regions in Kenya …………………………………37
Table 9: Material Costs       …………………………………………………………38
Table 10: Initial Cost and 20 Year Cost/Use Predictions for Solar Cookers and
Alternatives …………………………………………………………………………39
Table 11: Greenhouse Evaluation for Cost …………………………………………39
Table 12: Reflector Evaluation for cost     …………………………………………40
Table 13: Base Material Evaluation for Cost …………………………………………40




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Introduction to Solar Cooking
Solar cooking has provided people with clean and inexpensive meals around the world
for many decades and has been used in developed and developing countries for individual
families and entire communities. The UN, university researchers, and solar cooker
novelists throughout the world have tested and refined designs for enclosures, reflectors,
thermal storage, greenhouse effects, insulation, and durability. Solar cookers have
tremendous benefits including reducing pollution from traditional cooking which kills
millions of people each year, reducing environmental degradation and deforestation,
potentially reducing conflict over resources such as wood, and providing inexpensive and
thoroughly cooked meals. Solar cooking has improved conditions in many parts of the
world, and solar cookers will continue to become more influential in the future.


Background Information
Many solar cooker designs exist that range in size, cost and functionality. It is important
to analyze the local environmental, social and economic conditions when designing a
cooker for a given community.

Solar Cooking in Kenya

Using solar cookers in Kenya is attractive due to the shortage of convenient and
affordable cooking fuel for most of the population and the relatively good solar resource
available across the country. For several decades many international organizations have
promoted solar cookers in Kenya in an effort to reduce deforestation, improve the health
of cooks, and decrease money spent on cooking fuel.

Solar Cookers International (SCI) is one of the leading promoters of solar cooking in
Kenya. They have provided panel solar cookers and training to over 15,000 families in
Kenyan refugee camps. SCI focuses on increasing the acceptance of individual family
scale solar cookers in Kenya with the goal of increasing community acceptance with this
technology [1].

Trans World Radio (TWR) is a Christian organization that has radio programs on the
Kenya Broadcasting Channel that discuss public health and agricultural issues. TWR has
been making solar box cookers for several years. Their cookers have been used to cook a
maize porridge called “ugali,” vegetables, rice, corn, beans, cake, meat, fish, cookies,
cake, and bread. As of 2001, they had locally constructed and distributed over 2000 such
cookers [2].

AltEner is a solar energy technologies company based in Nairobi. They design custom
solar energy systems that are used for water heating, electricity generation (PV), crop
drying and solar cooking. In 1992 AltEner helped establish a solar hybrid kitchen at a




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boarding school in Kenya. The solar kitchen is capable of cooking two meals a day and
saves annually approximately 75% of cooking fuel costs [3].

Several factors prevent the widespread adoption of solar cooking in Kenya:
    Many solar cooker designs require materials that are not manufactured in Kenya.
       Current import tariffs are high on all of these materials. These tariffs can increase
       the cost of a family size solar box cooker by over 80 percent [4].
    A market for solar cookers has not been well established. Many of the solar
       cookers in use today in Kenya required financial support from international donor
       organizations. While in the long term most solar cookers are more cost effective
       than other technologies, the initial high cost, relative to alternatives, is inhibiting.
    The use of solar cookers requires a change from traditional cooking methods.
    The consistency and taste of meals cooked in a solar oven can change from those
       cooked in a device that uses fire.
    Because it is not always sunny, a backup cooking device is necessary.


Existing Solar Cooker Designs

Solar cooking has been around for a long time, and many people
have devoted much time to coming up with effective ways to
harness the sun’s energy. The three well known types of solar
cookers are box cookers, panel cookers, and parabolic cookers.
Another design that has been drawn up for larger scale use is the
in-wall cooker, designed by Joel H. Goodman.

The box cooker design, shown in Figure 1, is an insulated box with
a glass top. Either the glass top opens as a lid or there is a hinged
door on the side of the box. With the addition of a few reflective
panels, multiple pots can be cooked and kept warm. This
                                                                      Figure 1: Box Cooker
design can be made with local materials but will be a bit
expensive and will require much training.


                                  The panel cooker design is by far the cheapest and
                                  easiest to manufacture. The design in Figure 2 is
                                  basically a folded piece of cardboard with reflectors on
                                  it. The reflectors focus sunlight onto a pot that sits on
                                  the panels. The pot is often enclosed in a plastic bag to
                                  increase temperatures through use of the greenhouse
                                  effect. It cooks food evenly due to the comparatively
                                  lower temperatures.


  Figure 2: Panel Cooker




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The parabolic cooker design, shown in Figure 3, is a bowl-
like structure lined with reflective material. A pot sits at
the focal point of the bowl and is heated very quickly and
to higher temperatures than other designs. One downfall of
this cooker is that it requires sun tracking to work properly,
which adds to the cost and complexity of this design.




                                                                  Figure 3: Parabolic Cooker




                               The final design, the in-wall cooker, shown in Figure 4,
                               has been imagined by architect Joel H. Goodman. The in-
                               wall cooker would be a more costly permanent design that
                               focuses light onto the bottom of a pot that is inside a
                               building. The feasibility of this design, as far as cooking
                               is concerned, is yet to be determined. The concept,
                               though, is very feasible for larger scale projects that have
                               sufficient funding.




Figure 4: In-Wall Cooker [5]




Safety Considerations
Solar cooking presents itself with three safety concerns: the safety of the food or water
being cooked, the safety of the users and bystanders, and the safety the cooker and its
contents from theft. In order to cook most food and pasteurize water, temperatures must
reach above approximately 150°F or 66°C. All solar cookers, even in marginal sunlight,
can reach these temperatures. Safety concerns for people include burns to their skin and
in their eyes. Proper signage and structures to keep bystanders out of harm will be
necessary. Educating the people will also help keep them safe. Lastly is the concern of
theft. Food, cooking utensils, and the cooker need to be safe from theft. Solutions for


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this include hiring someone to monitor the area, fencing off the cooker and locking a
gate, or making an in-wall structure [6].



Solar Cooker Components
The first step of the design process for a solar cooker is to become familiar with all the
current solar cooking components and their functions. The important components that
are found in current solar cooker designs and that warrant in-depth research include
greenhouse insulation, reflectors, pot base material, pot selection, and thermal mass.


Greenhouse Insulation

The greenhouse effect refers to the process of trapping the energy from solar radiation in
an enclosed area, resulting in an increase in temperature. In a solar cooker, this is
accomplished by surrounding the cooking material (a pot and food) with a material that
allows low wavelength solar radiation to enter, and does not allow the higher wavelength
black body radiation emitted by the cooking material to leave. Solar box cookers often
use a sheet of glass or plastic as a lid. The rest of the box is insulated, reducing the loss
of energy due to heat transfer through the walls.

Panel cookers use a greenhouse material that surrounds the cooking material. A plastic
bag is an inexpensive and readily available option. Plastic materials degrade with
exposure to the sun and handling, however, so they must be replaced periodically.
Another option is glass that is low in iron. Silica, which is used to make glass, is often
found naturally with some iron. Therefore, low iron glass is more expensive than
common glass.



Reflector Designs

There are two main types of reflectors: the focusing reflector and the indirect reflector.
The focusing type concentrates solar energy to a focal point and heats the cooking pot
directly. The concentrated energy can cause the pot to reach very high temperatures in a
very short time compared with indirect reflectors. As a result, a cooker using focusing
reflectors can cook almost any type of food that a conventional stove can cook. The
indirect type reflects solar energy on an insulated pot or box, relying on the greenhouse
effect to reach high temperatures.




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Focusing Reflectors
Parabolic reflectors are the most popular type of
focusing reflector. The parabolic reflector, shown in
Figure 5, focuses the solar energy to a focal point,
allowing cookers using these reflectors to reach high
temperatures within short time compared with other
solar cookers. The parabolic reflector can reach more
than 500 F (depending on reflector size and
reflectivity of the materials used), and can cook in
almost any style that normal stoves can cook, such as
fry and boiling water [2]. Generally aluminum sheet             Figure 5: Parabola Shape
metal is used to make these reflectors, which is cheap,
lightweight, and has good reflectivity. Recently, many researchers are publishing easy
ways to make parabolic reflectors, helping to support solar cooking in developing
countries [7].

If the size of the pot being used is big, then the reflector needs to be big to have an
efficient cooker. In India there is a parabolic cooker with a radius of 25ft, which can cook
for more than 1000 people on sunny days [8]. However, this cooker uses a steam
generator at the focal point and moves the energy to a kitchen facility.

More problems with parabolic reflectors are that they require tracking of the sun, are
sensitive to weather conditions, static resistances of wind, heat emission by wind, and
safety concerns. Parabolic reflectors use direct energy from the sun, so this reflector is
required to track the orbit of sun every 15min to 30min depending on the reflectors [2].
Also this reflector is sensitive to weather conditions. Unlike other cooker designs, designs
using parabolic reflectors typically do not utilize the green house effect, but rather uses
direct energy from the sun. If the weather is partly cloudy, the efficiency of this reflector
decreases rapidly. On a windy day, the structural stability is low and the wind causes heat
loss. A safety concern is that the temperature of the pot is extremely high; the pot is
directly exposed to the air without any shield. Also, the reflection may cause eye injury,
so eye protection must be worn during cooking.

Indirect types
The indirect types of cookers use flat reflectors. Basically, these reflectors are not limited
in their size. From family size to community size cookers, there is no problem using
these types of reflectors. Depending on the energy requirement for cooking, the size of
the reflectors can change. However, for more efficient reflectance, large cookers use
many small reflectors set at various angles. This array of smaller reflectors gives not only
a wider angle for collecting diffuse radiation, i.e. more efficiency, but also reduces the
amount of solar tracking necessary. These types of reflectors are not as sensitive to
weather conditions as focal reflectors; cookers using indirect reflectors typically use the
greenhouse effect to aid in storing energy. Also, depending on the insulation used, it is
possible to keep the food warm until dinnertime. Cooking times are usually longer for


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this reflector setup than for focal reflectors. The styles of cooking one can do with a
cooker of this type are limited; boiling and baking are possible, but not frying or
browning [2]. If the temperature is not high enough, there is also the possibility of the
food spoiling. The food must be as dry as possible to prevent too high of humidity inside
of the box or in the insulation, which can cause condensation on the glass or plastic. Box
cookers and panel cookers make use of the indirect reflectors.

Reflector Materials

In searching for a reflective material that will address the needs of a low-income
populace, a number of factors must be considered. The reflector material must be able to
withstand the elements, unless the group is willing to bring the reflector system indoors
or under a shelter when not in use. The material should sufficiently concentrate the solar
energy to bring the cooker up to an acceptable temperature within a reasonable amount of
time.

Currently, a number of different materials are being used as reflectors for budget solar
cookers. Anything with a decent reflectivity is able to redirect solar radiation using
reflection. On the high end, mirrors do a great job reflecting solar radiation in a
predictable and efficient manner. A more budget oriented approach used by some
cookers is to use a cheap material such as aluminum foil to reflect the energy.

Aluminum foil is extremely cheap and is widely used in solar cookers, and is available
worldwide. A drawback to this foil is that it is not very durable on its own, and after a
while in the sun it degrades and loses much of its reflectivity.

Reflective materials from places such as 3M and ClearDome exist, and may make a great
reflector material depending on their availability in the part of the world the cooker is
being designed for. Sheet metal is a very strong material that would withstand a lot of
abuse, but it may oxidize. Aluminum sheet would have the same degradation problems
as aluminum foil.

Glass mirrors are a more permanent solution to reflecting the solar energy. Low grade
glass mirrors have better reflectivity and will not degrade in the sunlight as quickly as
aluminum foil. On the downside, glass is brittle and heavy. This may not be a problem if
the chosen reflector is stationary and safe from having heavy things such as pots dropped
on it.


Pot Base Material

The surface on which the pot will rest in a solar cooker will influence the temperatures
the pot can reach. The surface must not reflect sunlight away from the pot and it must
not absorb a large amount of energy from the pot or the incoming sunlight. The material
with the lowest conductivity should be the most effective base material. The thermal



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conductivities of various materials are in Table 1 below. The following are potential
surfaces the pot can rest on and comments about their effectiveness:
            Soil – Thermal conductivity changes with moisture and mineral content;
               avoiding soil would be good
            Clay – A great insulator, when combined with straw becomes even better
               [9]
            Sheet metal – Not a good insulator; it would absorb too much heat
            Rocks – Similar to clay.
            Aluminum foil – Could reflect a lot of sunlight away from pot.
            Air – If pot is propped up with three rocks at point contact, air over soil
               would be a cheap and effective method
            Styrofoam – The best possible material for base insulation, though
               degradation and cost are issues.

         Table 1: Thermal Conductivities of Various Local Materials [10]
Material            Conductivity         Material            Conductivity
                    (W/m*°C)                                 (W/m*°C)
Air                 0.024                Wood                0.12-0.16
Aluminum            250                  Plywood             0.13
Brick               0.69-1.31            Plastic             0.10-0.22
Cement              0.29-1.73            Sand                0.35
Cardboard           0.21                 Sawdust             0.06
Clay                0.15                 Steel               46
Concrete            0.9-2.0              Straw               0.09
Carbon Steel        54                   Styrofoam           0.01
Glass               1.05                 Water               0.58


Cooking Pot

A good pot for a solar cooker will have a large surface area to absorb the largest amount
of solar energy available. A large pot accounts for aiming errors in concentrating
reflectors, which may be essential considering the technologies being used to focus them.

A situation where the pot material will come into play more is if the cooker is functional
in all aspects other than it simply takes too long to heat. Food will cook faster the larger
and thinner the pot. A larger pan has more area to absorb solar radiation. Another thing
to consider is that some materials outgas, which needs to be examined to ensure a safe
cooking pot.




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Maintaining Cooker Temperature

In Kenya, and many parts of the world, the largest meal is consumed during dinner in the
evening. Dinner is often a cooked meal, so an effective solar cooker must be flexible
enough to keep the food warm until seven or eight in the evening. Current methods used
to keep food warm would be insulating the cooked food such as a box cooker that folds
the reflective panels over the top of the sealed off insulated region, or to use a straw
basket that is well insulated. The following section will discuss the various alternatives
to keep food warm after cooking it.

The other way to keep food warm is to use a thermal mass. A thermal mass is
advantageous to keep food warm by radiating heat after the sunlight isn’t strong enough,
but the drawback with a thermal mass is that it absorbs energy that could be absorbed by
the food if placed next to the solar cooker pot. This is why the thermal mass should be
separate of the solar cooker during the day because the pot is the thermal mass in the
solar cooker. One example for a thermal mass would be to place black stones out in the
sun and then in the evening they could be placed around the solar cooker pot.

One important material property to measure how well a thermal mass will retain heat is
the specific heat capacity of a material. A useful equation to determine how much energy
is required to heat a material and also the energy that is emitted by a material is in
equation 1 below [11]:

Q = m·c·ΔT                                      Equation 1

The term Q is the energy (Joules) to raise or lower a material a temperature change of
temperature ΔT (deg C) with a specific heat capacity c (J/kg*deg C), and a mass m (kg).

When viewing the best materials for their heat capacitance, the thermal conductivities of
the material must also be considered. Aluminum may have a slightly better heat
capacitance than stone, but aluminum will draw the heat out of the pot. Aluminum would
give heat back to the cooking pot once the pot became colder, yet the aluminum would
also transfer heat to the earth and air around it.



Calcium and sodium sulfate
The combination of calcium and sodium sulfate has significant potential for heat storage
and was investigated by Maria Telkes through the UN. What makes this chemical
combination unique is that it has a solid-solid phase change from a five sided crystalline
structure to a six sided structure [12]. The solid phase change allows for a more simple
solar cooker design and could be incorporated in community scale designs.

Another advantage to the calcium and sodium sulfate mixture is that the phase change
can be regulated between 350 to 500 degrees Fahrenheit by adjusting the concentration of
the two chemicals [12]. This allows for a constant cooking temperature to be determined


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based on the chemical composition added. The chemicals will absorb heat during the
early part of the day and go through the phase change; later in the day the cooking
temperature will remain constant at the phase change temperature set between 350 to 500
degrees Fahrenheit. Constant temperatures allow for bread and other food to be prepared
properly.

One drawback to this heat storage compound is that despite the chemicals being
inexpensive and somewhat common, it may still be difficult to find calcium and sodium
sulfate locally. The cost for the chemicals would most likely be prohibitive with family
sized solar cookers and adding these to the cooker would increase complexity. The
chemicals would have to be placed in the bottom of a box cooker, or would need an
enclosure around them. Plastic would be ideal, but would melt at these temperatures.

The specific heat for calcium sulfate is 732J/g*°C [13].



Concrete, stone, wood, clay, sheet metal, air

There are many local materials that could be used as a thermal mass to place next to the
solar cooker pot after the sun has gone down. The heat capacities of various local
materials are listed in Table 2.


                Table 2: Heat Capacities for Various Materials [10]
                Material                           Heat Capacity (J/g*°C)
                Concrete                                    800
                  Stone                                     840
                  Wood                                  1,340-2,000
                  Clay                                      920
               Aluminum                                     870
                   Iron                                     460
                    Air                                    1,060
             Calcium Sulfate                                732



Fireless Cooker (Insulated Box)
Once food is cooked by a solar cooker, fire, or other means, a device referred to as a
fireless cooker can hold the pot and keep it warm. A fireless cooker is basically an
insulated box consisting of a basket that can be bought or cheaply made with cloth or hay
for insulation and a straw basket for the frame. The food can continue to cook and will
hold its temperature for hours once placed in a fireless cooker. Using a fireless cooker
along with a small solar cooker could be very advantageous because it allows more than
one dish to be cooked per meal.



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Method of Investigation
The engineering design process involves many steps and both qualitative and quantitative
analysis. There were no objectives or guidelines laid out for a community scale solar
cooker project, consequently research was the first step in this design process. The
research was necessary to understand the problem that was at hand. The results of the
research are found in the previous section titled Solar Cooker Components. Once the
research was complete the following design processes and methods, shown in Figure 3,
were completed.

                       Table 3: Design Processes and Methods
                 Process                                    Method
          Clarifying Objectives                     Weighted Objectives Tree
          Establishing Functions                  Function Analysis Flow Chart
   Setting Requirements / Determining                   House of Quality
              Characteristics
         Generating Alternatives                             6-3-5
         Evaluating Alternatives               Morphological Chart and Weighted
                                                        Objectives Table


Clarifying Objectives
The research conducted included functional components of solar cookers and the social
aspects of implementing solar cookers in third world equatorial countries. After
individually listing objectives the group came together to place them into a completed
weighted objectives tree (see Appendix C).

                                                   O1
                   Design of a community scale solar cooker for a rural community in an
                                    equatorial region of the world
                                              1.0      1.0




        O11                      O12                           O13                         O14
      Safety                    Cost                  Cost Sharing Program            Functionality
   0.20     0.20            0.35     0.35                 0.10     0.10               0.35     0.35


                         Figure 6: Weighted Objectives Tree – Main Objectives

The four main objectives and their respective weights, as seen in Figure 6, are safety
(0.20), cost (0.35), cost sharing program (0.10), and functionality (0.35).



                                                                                                  17
       Safety – Food, theft, and user safety were all obvious objectives to have. Food
       safety is ultimately the limiting factor of any cooker design, but since so many
       solar cooker designs already exist that safely cook food, it presents itself as a
       minor issue. Protecting from theft and burns can be designed into a cooker near
       the end of its design and could change from case to case.

       Cost – Cost is a huge factor for third world countries. The materials need to be
       cheap and locally sourced, and the design needs to be simple enough to teach
       basic assembly and maintenance to locals.

       Cost Sharing Program – A cost sharing program is an objective that will have to
       be worked out when being implemented, not during the initial design.

       Functionality – This broad objective encompasses the solar cookers’ usability as
       the sun moves across the sky, its structural durability, cooking time, and social
       acceptance. Social acceptance is an objective that can only slightly be designed
       into the cooker. A cheap, safe, and quick cooker will be worthless unless the
       locals understand, embrace, and are willing to adapt to the concept of solar
       cooking.


Establishing Functions
The functions of this project can be split into two categories: solar cooking and solar
cooker building/implementation. A function analysis flowchart (FAF) was made for each
of these categories (see Appendix C).

       Solar Cooking FAF – This FAF includes the task of concentrating solar energy to
       heat up a pot to effectively cook food and pasteurize water. Designing for the
       function of the actual solar cooker is the first stage of this project is what will be
       focused on by this group.

       Solar Cooker Building/Implementation FAF – This FAF focuses on the teaching
       and integration necessary to convince the locals that this technology is
       advantageous and not out of their reach. The functions of this task will need to be
       carried out by a group such as Engineers Without Borders or a government run
       group.


Setting Requirements and Determining Characteristics
This step in the design process was attempted but left incomplete. As this solar cooker
project progressed it became clear that setting specific requirements was not feasible at
this time. The design process that is being followed will only lead this project to a
general design that meets the cost and basic functional needs of a solar cooker in third
world countries. Once this general design exists the design process can be reassessed
again in much greater detail at which time a House of Quality would help.



                                                                                          18
Generating Alternatives
Researching the components and understanding the functions of solar cookers is all that
has been done to this point. Brain-writing, otherwise known as the 6-3-5 method, is the
next step in getting input from all team members on how to solve the functions of the
solar cooker. The four sub-functions of the solar cooker that were chosen to use during
the 6-3-5 method are (1) Concentrate Energy, (2) Build Structure, (3) Monitor
Food/Safety, and (4) Store Energy. These are very broad sub-functions, each of which
could be broken down into more sub-functions for another analysis. Future analysis
undoubtedly needs to be done, but not until a basic design is chosen.

In the process of the 6-3-5 method, ideas overlapped from sub-function to sub-function,
but the process was a success in narrowing down solutions for each sub-function. For the
complete 6-3-5 sketch sheets see Appendix C.

       Concentrate Energy – Most of the ideas sprung from current designs that are
       explained in previous parts of this paper. A few of the solutions were flat panel
       reflectors, parabolic reflectors, lenses, combinations of reflectors and lenses, a
       steam heating system, and a non-imaging CPC. The ways of concentrating
       energy are fairly standard after 50 years of work has been done on solar cookers.

       Build Structure – This sub-function is very broad and each solution will have
       limited ways energy can be concentrated. The build structure, more precisely, is
       the structure that will hold the pot. The first solution for this is Joel Goodman’s
       in-wall design (Figure 4), which requires a non-imaging CPC to concentrate the
       energy. Other designs are insulated boxes, flat panels, hybrids that allow fire or
       coal to also heat a pot, or any combinations of these.

       Monitor Food/Safety – This sub-function’s solution will ultimately depend on the
       community it is used in. The solutions thought of were making it an in-wall
       design, fencing off the area, using a box cooker with a padlock, paying a person to
       monitor it, or keeping it within view of the schoolyard or other nearby
       establishment.

       Store Energy – The current energy storage device used is a straw basket, but that
       only stores energy after the cooking is done. Storing energy before the food is
       placed on the cooking surface and minimizing heat loss during cooking are
       included in this sub-function. Possible solutions for this include a thermal mass
       such as sodium sulfate with calcium sulfate (patent # 2808494), a thermal mass
       such as rocks or clay, an insulated box, and a greenhouse affect around the pot.


Evaluating Alternatives
Continuing with the generation of alternatives, but also part of evaluating alternatives is
the morphological chart. This method takes the best alternatives thought of during the 6-
3-5 method and mixes and matches sub-solutions to form alternative designs. Four
solutions for each sub-function were chosen for this method. Narrowing down from the


                                                                                        19
    6-3-5 to the morphological chart was based on cost, personal interest, and by combining
    some of the solutions. This step was quite easy because there were not much more than
    four solutions for each sub-function. See the original morphological chart in Appendix
    C.
                                 Table 4: Morphological Chart
                                        Morphological Chart
                                                          Sub-Solutions
    Sub-functions
                                    a                   b                  c               d
         Concentrate                                                non-imaging
A                         flat panel reflectors       lens                         parabolic reflector
           Energy                                                        CPC
                             J. Goodman's       closed box with
B       Build Structure                                               flat panel         hybrid
                                 in-wall            glass lid
           Monitor                                                                   within view or
C                           person present         pad-lock           in-wall
         Food/Safety                                                                closed off area
                                                   physical
                          sodium sulfate with
                                                thermal mass       greenhouse
D        Store Energy       calcium sulfate                                          insulated box
                                                 (rocks, clay,     affect on pot
                          (patent # 2808494)
                                                     etc.)

    Each Build Structure solution has great possibilities for being a community solar cooker.
    It was decided to start with each of these solutions and combine them with other sub-
    functions’ solutions to make the best solar cooker for each Build Structure. The
    combinations that made up our top four designs are as follows:

           AcBaCcDd – This is Goodman’s in-wall design with the only concentrator
           possible (non-imaging CPC). The safety of the food and cooker would be taken
           care of by being an in-wall structure. Having an insulated box around the pot
           would be our source of stored energy.

           AaBbCbDd – A closed box with a glass lid that is insulated would house the pot.
           Flat panel reflectors would concentrate energy into the box, and a padlock on the
           box would protect the food.

           AaBcCdDc – A flat panel to rest the pots on with flat panel reflectors to
           concentrate energy on the pot would be the basic structure. The energy would be
           stored with a greenhouse that would also assist in the cooking process. Keeping
           the cooker within view or in a closed off area would keep the food safe.

           AdBdCdDc – A hybrid design would allow a fire or coals to heat up the pot on a
           cloudy day. A desire to further evaluate parabolic reflectors led us to select one
           for this design. The energy would be stored with a greenhouse that would also
           assist in the cooking process. Keeping the cooker within view or in a closed off
           area would keep the food safe.

    Some of the solutions were dropped completely during this process. Lenses were
    dropped due to cost and availability in third world countries. Having a person present as
    opposed to a pad-lock or fence can be changed later in the design process and is not too



                                                                                               20
critical for this or the next step of the design process. Thermal masses were dropped
because of the cost and complexity they would bring to this design.

It was apparent that a few small changes to these designs would produce 4 or 8 more very
good designs, but at this stage further analysis needed to be done. Further analysis and
reflection on the choices would help decide if some changes should be considered. The
next method to analyze these four designs is the Weighted Objectives Table (see
Appendix C).

The Weighted Objectives Table rates how well each design accomplished the objectives
laid out in the Weighted Objectives Tree. The results of this comparison were not
extremely clear. All four designs were within 10 points of each other. This is not
conclusive enough evidence to make a decision.

The next step to fix these unclear results would be to reevaluate the weights of each
objective and score each design more carefully.

Reassessment of Project
Before the design process was reevaluated, the group met with solar professors Klein and
Nellis at UW-Madison. The result of this meeting was that the design process had been
going well, but the time constraint of an eight week course required a new focus. No
longer was a full design and prototype testing going to take place.

The new objectives are to try to improve upon current solar cooker designs. The decision
was to focus on the greenhouse affect during cooking and heat loss through the bottom
surface of the pot, along with the affect of adding a reflector to the system. With this test
data and the previous research, a community scale cooker can be suggested for a future
design project.


Experimental Data
All tests and experimental data have been collected in accordance with the International
Standards for Testing Solar Cookers when possible and relevant. The current
international standards were developed during the Third World Conference on Solar
Cooking in January, 1997 in Coimbatore, Tamil Nadu, India. The international standards
provide a means to compare the performance of solar cookers irrespective of the location
the cookers are being used so that consumers can select the most appropriate cooker for
their application [14].

The following list is a summary of these standards as they apply to this design:
    Test method: Place a thermocouple 10 mm above the center of the pot
    Test range: Water temperature from 40 to 90°C
    Test conditions: Ambient air temperature between 20 and 35°C
    Test condition: Wind speed less than 2.5 m/s
    Test timeframe: 10:00 – 14:00 solar time
    Tracking frequency: once per hour


                                                                                          21
Although the solar conditions in Madison are significantly different than in Kenya due to
the difference in latitude, all tests measure the effectiveness of design parameters
compared to baseline parameters. For example, to test the effectiveness of greenhouse
materials, the water temperature in a pot with no greenhouse was recorded. The data
analysis compares how much a given greenhouse material increased the water
temperature over the testing period.

An important standard for testing solar cookers that is not addressed in this report is the
                         mTC P
cooking power, P: P              . The experimental analysis of this report focuses on
                            t
components of solar cookers and not the entire solar cooker. Cooking power should be
included in future design analyses.

Experimental Procedure

   1. Obtain four identical pots
   2. Spray paint the pots black
   3. Measure out 0.525L of water from the same source
   4. Place the water in the pot
   5. Immerse a type T thermocouple half inch from the pot bottom in the water
   6. Seal the pot with high temperature silicon to eliminate water evaporation
   7. Connect thermocouple to a data acquisition system and a laptop computer
   8. Place cardboard underneath the solar cooker
   9. Ensure equivalent sunlight and wind conditions between test pots
   10. Use one pot for a control
   11. Analyze greenhouse, pot base material, and reflectors

Solar Cooking Procedure

   1. Place pot in hard plastic dome on the way to the market in the morning
   2. Place stones in sun (to be placed in hay basket later)
   3. Have a person watch solar cooker to prevent theft
   4. Pay the person watching or be in a group of people that change off watching
   5. Person watching can also rent equipment (pots, reflectors, greenhouse dome etc.)
   6. Solar cooker heats pot until food is placed in it
   7. Place food in solar cooker once obtained food at the market
   8. Person continues to watch food (can also check food)
   9. Put hot stones into bottom of hay basket
   10. After sun goes down, transfer food to insulated hay basket




                                                                                        22
     Figure 7: Four equivalent Pots with High           Figure 8: Setting up the Thermocouple to the
    Temperature Silicone (orange) and K- type         Computer and Testing the operation of our System
                  Thermocouple




Experimental Results
Four experiments were performed that tested the effectiveness of various solar cooker
components by increasing the temperature of the water in the aluminum pots. The tests
analyzed the base material, greenhouse insulation, and reflectors. Three experiments
were performed using sunlight, while one of the experiments was performed inside with
300W light bulbs to determine if it is possible to test solar cookers in a controlled
environment indoors. The results of these experiments and the graphs of the data are in
the following section.


Experiment #1: Outside Greenhouse Comparison

Test conditions:
    Test start: 95.6°F, 44% relative humidity (RH)
    Cloud cover caused intermittent downfall of temps (see Figure 13 below)
    110 minutes from the start: 94.7°F, 45.9% RH

The first experiment tested the effectiveness of the various greenhouse domes. The
comparisons were of a box cooker, a glass dome, an unclear plastic bag, and one pot
without a greenhouse as shown in the figures below. The box cooker performed the best,
but it potentially performed better than the others due to aluminum foil reflecting more
energy into the pot. The open air pot performed about 15 degrees worse than the others
as shown in Figure 13 below which indicates adding a greenhouse is very beneficial for
solar cooking.




                                                                                       23
  Figure 9: Greenhouse Testing with Open        Figure 10: Greenhouse Testing with Unclear Plastic
               Air Condition                              Bag with Steel Wire Structures




Figure 11: Greenhouse Testing with Glass Dome
                                                 Figure 12: Greenhouse Testing with Box Cooker




                                                                                      24
                                        Outdoor Greenhouse Comparison Tests

                    75

                    70

                    65

                    60
 Temperature (°C)




                    55

                    50

                    45

                    40

                    35

                    30

                    25
                         0         20           40         60           80             100          120    140
                                                              Time (min)

                                          Box    Glass Dome      Unclear Plastic Bag     Open Air
                                        Figure 13: Outdoor Greenhouse Comparison Tests




Experiment #1 Summary

                        A greenhouse is a necessary component for a solar cooker
                        The glass greenhouse performed better than the plastic bag potentially due to
                         higher clarity and better insulation
                        The box cooker may have performed the best due to a clear glass plate, excellent
                         insulation in the walls, and aluminum foil reflecting energy into the pot

                Table 5: Maximum Temperatures Achieved by Various Greenhouse Insulations

                                                     Box        Glass        Plastic Bag       Open Air
                          Max
                                                 70.4           66.0            62.9                49.1
                     Temperature (°C)




                                                                                                             25
Experiment #2: Indoor Greenhouse Comparison

Test conditions:
    Test start: 72.6°F, 52.8% relative humidity (RH)
    1.2kW from test lights caused ambient air to become about 86°F and 34% RH
    300W light filament placed 15 inches from the ground

The four comparisons made in Experiment One were tested indoors with 300W lights to
determine if testing indoors under a more controlled environment would be comparable
to outdoors. The results shown in Figure 14 shows realistic curves for the open air pot
and the box cooker, however the curves for the plastic bag and glass dome are much
lower than what would be realistic outdoors, based on our first experiment. The plastic
bag and glass dome were less than 10 degrees above the open air pot and about 15
degrees below the box cooker. In the first experiment, the plastic bag and glass dome
were only about five degrees below the box cooker, and they were about 15 degrees
above the open air pot. This indicates that the wavelengths of the light from the 300W
light bulbs do not accurately simulate light from the sun. This experiment determined
tests should only be conducted outdoors.


                             Indoor Greenhouse Test with 300W Lights

              70




              60




              50
  Temp (°C)




              40




              30




              20
                   0         50         100             150           200           250   300
                                                     Time (min)

                                  Box     Open Air      Plastic Bag    Glass Dome

                       Figure 14: Indoor Greenhouse comparison with 300W Light Bulbs




                                                                                                26
                      Figure 15: Indoor comparison Test Setup



Experiment #2 Summary
    Solar cooker tests should not be conducted indoors due to discrepancies in light
      wavelength




Experiment #3: Outdoor Greenhouse and Base Surface Comparison

   A third experiment was conducted to compare the effectiveness of a clear bag and an
   unclear bag, placing stones under the pot to have better insulation, and having a
   second hard plastic greenhouse dome over the clear and unclear bags as shown in
   Figures 16-21 below.


Test 1 Conditions:
    Test start 10:55am: 80.2°F, 54.7% relative humidity (RH)
    11:45pm: 81.9°F, 50.9% relative humidity (RH)
    2:20pm: 90.1°F, 32.8% relative humidity (RH)
    3:22pm: 88.0°F, 33.2% relative humidity (RH)
               Figure 16: Unclear Plastic Bag                        Figure 17: Clear Plastic Bag




   Figure 18: Hard Plastic Top with Pot on Stones           Figure 19: Hard Plastic Top on Ground




Figure 20: Hard Plastic Top over Clear Plastic Bag   Figure 21: Larger Hard Plastic Top over Small Hard Plastic Top


                                                                                                    28
                                    Outdoor Greenhouse Comparision


              80



              70



              60
  Temp (°C)




              50                                               Hard Pastic 2 On Rocks

                                                            Hard Plastic 1 Off Rocks
              40

                                                         Hard plastic 1 On Rocks

              30
                                                         Hard Plastic 2 Off Rocks

              20
                   0       50           100           150           200            250           300
                                                  Time (min)
              Clear Bag   Unclear Bag         Hard Plastic 1      Hard Plastic 2         Crossover rocks to ground

                                Figure 22: Outdoor Greenhouse Comparison



In Figure 22, the clear nylon bag and unclear polyethylene bags were compared in
addition to the base material being compared by placing one pot on the cardboard
ground and one on top of stones. To eliminate thermocouple error, the pot initially on
the stones was switched to the ground at 135 minutes in, and the pot initially on the
ground was switched onto the stones. This allowed for the improvement by placing
stones under the pot to be measured by subtracting the difference in the two signals at
135 minutes in with the two signals at 215 minutes in. The cooling rates of a pot on
and off stones was also tested as shown in the Figure 22 above, however, the methods
to block the sun from the pot were different for each pot, so that test should be
repeated to ensure accuracy.




                                                                                                             29
                              Ground Surface Comparison for Heating Using Stones
                         73

                         72

                         71
      Temperature (°C)




                         70

                         69

                         68

                         67

                         66

                         65

                         64
                           130    140     150      160      170      180       190    200        210
                                                           Time(min)

                                    Hard top switched off rocks     Hard top switched on rocks
                                        Figure 23: Ground Surface Comparison

The ground surface of a pot has the potential to conduct energy away from the cooking
pot. Stones were placed under one pot, and another pot was placed on the cardboard
ground. It appeared that the pot on the stones was performing around two degrees better,
however, the error from the thermocouples is up to ±0.83°C, so the pot on the rocks was
switched off the rocks, and the pot on the cardboard was placed on the rocks after 135
minutes into the experiment. In Figure 23 above, the starting point was when the pot on
the rocks was removed from the rocks. The temperature difference at the start was 2.1°C
and at the end of the experiment, 1.4°C. This indicates that placing a pot on stones may
potentially increase the temperature by 0.70°C. More tests would be needed to ensure
this was an accurate conclusion.




                                                                                                       30
                                     Heating Dynamics for Double Greenhouse

                     70




                     65
  Temperature (°C)




                     60




                     55




                     50




                     45
                          0         20            40            60            80           100    120
                                                            Time (min)

                                             Hard Top    Hard Top + Larger Hard Top
                              Figure 24: Heating Dynamic Effects for a Double Walled Greenhouse

A second plastic greenhouse dome (larger hard top) was placed over the smaller hard top
to view the effectiveness of a second greenhouse dome as shown in the figure above.
The double greenhouse dome started below the single greenhouse yet surpassed it after
an hour and eventually by over three degrees Celsius. This test, in addition to the
comparison of the second greenhouse domes placed over the plastic bags, indicates that a
second greenhouse dome increases the maximum temperature in the solar cooker.




                                                                                                    31
Experiment #3 Summary

     Placing a second greenhouse to the solar cooker contributes another 4-5°C to the
      maximum temperature
     Hard plastic bins perform better than unclear plastic bags by about 1°C but about
      3.0°C less than the clear plastic bags
     Placing pots on stones may improve the maximum temperature by 0.5-1.0°C
     A clear nylon plastic bag improves maximum temperatures over an unclear plastic
      bag around 4.5°C


      Table 6: Maximum Temperature of Comparisons and Improvement of
                             Temperatures

                                     Max Temp (°C)        Improvement (°C)
          Clear bag                      74.9
   Clear bag with hard top               79.7                      4.8
         Unclear bag                      71
  Unclear bag with hard top              75.2                      4.2
  Hard plastic top off rocks             71.3
  Hard plastic top on rocks               72                       0.7




                                                                                    32
Experiment #4: Outdoors Mirror Comparison


A comparison was made to determine how effective a mirror is to increase the
temperature of the water within the pots. One comparison was made between a pot with
no greenhouse and a mirror to a pot with no greenhouse nor a mirror. The other
comparison was made with a greenhouse and mirror, and a greenhouse without a mirror.
These can be viewed in Figure 25 below.

Experiment #4 Test Conditions:

Test Start: 83.3°F, 44.2% RH
1hr 25min elapsed: 83.3°F, 44.2% RH
3hr 12min elapsed: 86.3°F, 41.1% RH




                    Figure 25: Examining of the Effectiveness of using a Mirror




                                                                                  33
                                     Comparison with one mirror


                        100
                         90
     Temperature (°C)




                         80
                         70
                         60
                         50
                         40
                         30
                         20
                              0               50                    100              150
                                                         Time( min)

                                  Hard Plastic with Mirror
                                  Open Air with Mirror
                                  Hard Plastic Top without Mirror
                                  Open Air without Mirror
                                  Starting Point Second Greenhouse on Hard Plastic With Mirror




The four different test parameters can be viewed in Figure 26 above. The vertical line
indicates the point where a second hard plastic greenhouse dome was added to the hard
plastic dome with mirror. Viewing the data, adding one mirror provided approximately
the same performance as adding one greenhouse dome.


Experiment #4 Summary
    The heating rate appears to temporarily decrease after placing a second
      greenhouse on but eventually increases the maximum temperature
    Adding one mirror to the solar cooker adds about 12.5°C
    The maximum temperature of the double greenhouse with one mirror was 91.4°C




                                                                                                 34
Analysis of Data
The data for each experiment was collected on four separate days. Many of the
comparisons were analyzed only one to three times, so the accuracy and projections made
with the data is limited. Additional testing is recommended to strengthen the
relationships that have been identified. However, the data collected provides useful
evidence correlating the tested solar cooker components and the pot water temperature.


Results summary of data
Performance summary:
    Adding one greenhouse adds 15-20ºC
    Adding one mirror to the solar cooker adds about 12.5°C
    A clear nylon plastic bag improves maximum temperatures over an unclear plastic
       bag around 4.5°C
    Adding a second greenhouse improves max temperature by 4-5°C
    Adding stones as the base material may add 0.50-1.0°C to max temp
    Hard plastic bins perform better than unclear plastic bags by about 1°C but about
       3.0°C less than the clear plastic bags

Test Suggestions:
    Solar cooker tests should not be conducted inside due to discrepancies in light
       wavelength
    All solar cooker tests must be performed outside in sunlight to obtain accurate
       data


Error Analysis

Within our experiments, several sources of error are present.     A few of the major
contributors are highlighted below.

      Error in thermocouple reading ( ±0.83°C) (See Table 7 below)
      Angle of incidence of radiation to greenhouse
      Amount of wind affecting convection
      Ambient temperature differences between pots
      Variance in solar energy between days
      Variance in pots themselves (silicon seal, spray paint, differences from
       manufacturing)




                                                                                    35
               Table 7: Thermocouple error table [ME368 Lab manual]
.




    The thermocouple error is the most significant contributor to error. The differences in
    temperature between the different solar cooker pots in experiments was often less than
    4°C, so the maximum error range of 1.7°C (±0.83°C ) will significantly affect our data.

    Differences in the angle of incidence are another potential source of error. The amount of
    radiation entering and leaving our system is directly correlated with the surface area of
    the greenhouse, which was different from test to test (i.e. plastic bag vs. glass dome)

    The difference in solar radiation available between days could also cause error in our
    experiments. The first day of this experiment, the sunlight was strong enough to heat up
    the pots to maximum temperature within two hours. However, other days it took up to
    three or four hours to reach maximum temperature.

    During the outside experiment, different sources of wind were present. Natural wind with
    varying velocities and cold winds coming from an open door in a nearby building may
    have introduced errors into our data. The cold wind from the open garage door was
    around 72°F (atmospheric temperature was around 88 to 93°F during our experiments).

    Some other sources of error include the possibility of the seal surface of the pots not
    being completely sealed and differences in paint thicknesses.

    The fourth experiment testing the mirror could also have error from the angle adjusted for
    the mirror. The angle a mirror is tilted can change experimental results if it is tested on
    another day with a different angle.



    Economic Analysis
    It is very important to consider cost while comparing the different types of solar cooker
    designs. A final solar cooker design must be a balance between minimizing cost while
    keeping the performance high enough for people to consistently use it. The two most
    costly components in a solar cooker are the greenhouse and the reflector, so optimizing
    these will ensure the best design possible. In addition to reducing the cost of the solar
    cooker while maintaining or improving the performance, there are several other things
    reduce solar cooker costs. These would include a community sharing program for solar


                                                                                            36
cooker materials, having a group of people switch off watching the solar cookers, and
starting micro-finance programs within the community. The following section will
discuss these topics.



Cost Benefit of Solar Cookers

 There are many cost advantages of solar cookers including reducing firewood fuel costs
for cooking by up to 60%, reducing smoke inhalation which causes chronic illness and
millions of deaths each year, reduces the time for people searching for firewood, reduces
pollution that damages the environment, reduces deforestation, and reduces potential
conflicts over limited wood fuel resources. There are many cost benefits to solar cookers
and the monetary benefits they would provide can be realized by viewing Table 8 below.

                    Table 8: Monthly Fuel Costs for Regions in Kenya
                                              Monthly Fuel
                               Region            Cost
                                                ($U.S.)
                              Central              50
                               Coast               20
                              Eastern              15
                              Nyanza               15
                             Rift Valley           25
                              Western              15
                              Nairobi              75
                             Mombasa               60
                              Kisumu              145
                              Nakuru              165
                            Other Urban           130




                                                                                      37
Cost of Solar Cooker Materials


Table 9 below is a list of raw materials used to construct solar cookers. The prices given
are the cost of materials in the U.S. unless specified.


                                    Table 9: Material Costs
Item                                                                                cost     SA or Vol

Reflector (KENYAN PRICE...cardboard reflector used in the cookit)                    $2.50   ~2 ft^2
Mirror (Glass mirror U.S. price)                                                     $1.50   1 ft^2
Mirror support wood, wire and nails (2 mirrors…lighter weight, portable, cheaper)    $1.00   1ft^2
Mirror support wood, wire and nails (3 mirrors)                                      $1.50   1 ft^2
Plywood                                                                              $0.31   1 ft^2
Hard plastic flat sheet (1/8" PC)                                                    $2.80   1 ft^2
Glass sheet                                                                         $8.00?   1 ft^2
Aluminum foil (put value to $0.06 to see how costly it is!!!!!)                      $0.00   1 ft^2
Glass dome                                                                           $5.50   ~0.5ft^3
Hard plastic dome                                                                    $3.00   ~0.75ft^3
Hard plastic dome (approx cost for one just larger)                                  $4.00   1ft^3
Hard plastic dome (larger to fit over first)                                         $6.00   ~3.0ft^3
Plastic bag (with metal wire)…KENYAN PRICE given $0.30/10-20uses                     $0.30   1 ft^2
box cooker expenses (nails, paint, stands, sheet metal, glue, screws, labor,
handles)                                                                             $2.50   1ft^3




The raw materials in Table 9 were used to predict the initial cost of a solar cooker in
Kenya, and also the cost of the solar cooker per use over a 20 year life span as viewed in
Table 10 below. The assumptions made in the initial cost were that materials cost 40%
more in Kenya than the U.S. and a solar cooker with a one cubic foot volume was
constructed. The 20 year cost per use analysis assumed a 40% more material cost in
Kenya, that there are 216 days with sufficient sun to cook, a 50% utilization rate (they
only use the solar cooker 108 days per year), a hard plastic dome lasts 3 years, plywood
and glass last 6 years, and the plastic bags last 2 months. This analysis is very sensitive
to the longevity of the materials and predicting the cost of materials in Kenya. Durability
tests and obtaining local material prices would enable the cost analysis to be much more
accurate.

 An analysis was also made for alternatives to solar cookers which involved the Jiko
cooker, which is the most common wood stove, and using PV panels to use electricity for
cooking. The PV panels could be cost competitive against higher end solar cookers and
solar cookers are most cost effective for the majority of Jiko users. People with low
incomes tend to spend hours finding fuel wood whereas others pay significantly more
money to fuel the Jiko compared with a solar cooker.




                                                                                             38
Table 10: Initial Cost and 20 Year Cost/Use Predictions for Solar Cookers and Alternatives
                                                                  Upfront
                Kenya cost of solar cookers:                       Cost        Cost per Use

 Box cooker with 3 mirror reflector                               $17.99             $0.031
 Hard plastic greenhouse with 3 mirror reflector                  $12.60             0.029
 Plastic bag design with 3 mirrors                                $8.82              0.039
 Hard plastic double greenhouse with 3 mirrors                    $18.20             0.056

                Alternatives to solar cookers

 Community PV solar panels with electric range/microwaves         $9,000             $0.08
 Jiko firewood stove                                                $3               $0.50



Greenhouse Dome

A more specific cost analysis was conducted on the greenhouse dome. The CooKit
plastic bags used in Kenya refugee camps for solar cooking have an expected life of 10-
20 uses and cost $0.30 per bag [2]. A hard plastic bin like the one used in the
experiments above would also cost approximately the same as a plastic bag. The
performance of each would have to be analyzed to see how it is affected with UV
degradation and handling, and the life expectancy should also be tested for better
predictions. The cost of a box cooker and a glass dome should be more expensive than
the prior options. The box cooker often has higher performance and insulates the food
after it is cooked. Overall, the hard plastic bin may be the best greenhouse dome solution
because it is much more user friendly than the plastic bag, and it is less costly than the
box cooker. A drawback to the hard plastic bin is if it is stolen, it is more valuable than
the plastic bag.

                        Table 11: Greenhouse Evaluation for Cost
                                                         Life (# of
            Greenhouse Cost Analysis:           Cost       uses)        Cost/use

                  CooKit plastic bag            $0.30        20             $0.015
                   Hard plastic bin             $3.00       200             $0.015
                   box cooker box               $15.00      600             $0.025
                    Glass dome                  $8.00       200             $0.040



Reflector

A reflector is the other significant contributor to cost for the solar cooker. A very
common reflector in Kenya is the CooKit reflector which involves gluing aluminum foil
to cardboard. The cost in Kenya is about $2.50 and a rough life estimate for it is 100 uses
although this number would have to be investigated. A potentially cheaper and better
performance alternative would be to mount two 1x4 foot mirrors onto a wooden frame


                                                                                              39
which is mentioned in the Mirror Support Assembly Design section in Appendix A.
Further research would be required to determine which of the materials would be a better
reflector in Kenya.

                           Table 12: Reflector Evaluation for cost
                                                                 Life (# of
             Reflector Cost Analysis:                    Cost      uses)      Cost/use

                   CooKit Reflector                      $2.50       100       $0.025
 Mirror (Glass mirror + support wood, wire and nails)    $3.00       200       $0.015
                ClearDome Solarflex                      $3.75       100       $0.038



Base Material

The base material was not fully tested yet, so it will not be possible to suggest the best
material for it. The best option for a base material would obviously not cost anything,
such as propping the pot off the ground using three stones. The cost of various base
materials is given in the table below.

                        Table 13: Base Material Evaluation for Cost
                                                                 Life (# of
          Base Material Cost Analysis:                  Cost       uses)      Cost/use

                       clay                               0                       0
                      sticks                              0                       0
                      stones                              0                       0
                   aluminum foil                        $0.06        10        $0.006




Community Sharing

Another method to reduce the cost per use of a solar cooker would be to begin
community sharing programs. These would include but not limited to sharing solar
cooker components such as reflectors, greenhouse domes, and pots, sharing time to have
a group of people rotate watching the solar cookers so food or the cookers are not stolen,
and beginning micro-finance programs. These concepts are explained in further detail
below.



Solar Cooker Components

There are two things that will reduce the use of solar cookers from a cost standpoint,
being the initial cost of a solar cooker and the cost of the solar cooker for each use. Many
people in developing countries make under $2 a day and many make less than $1 a day.


                                                                                         40
The CooKit panel cooker initial cost in Kenya is $8 and the Trans World Radio (TWR)
box cooker in Kenya costs $25. It would be possible for a school or church to purchase
these solar cookers, or construct their own solar cooker and rent them to people in the
community. Solar cooking components could either be rented to customers daily,
weekly, or monthly where they would bring the solar cooker back to their home, or they
could be rented out daily and kept in the vicinity of the church, school, or entrepreneur
who rented them.


Time

Time is a very valuable commodity and it is often wasted in developing countries by
tasks such as collecting firewood for cooking. Women are often the ones who spend
upwards of four hours walking in search of firewood for cooking. Solar cooking would
reduce the need of wood by up to 60% (assuming 216 sunny days per year [3]) which
would cut down on wasted time. Solar cooking still involves spending time to watch the
solar cooker and potentially alter the angle of the reflector, but wasted time can be
reduced by having a time sharing program.

If solar cookers are kept in one’s home, a group of four or more people can alternate
watching over the cookers each day. The time spent watching the solar cookers with a
group of people trading off watching them should be less than the time spent collecting
firewood in most regions. If solar cooker components are rented and kept in the vicinity
of the church or school, the person who rents them to the customer can watch over them
to prevent theft and also to rotate the reflectors to maximize cooking performance. A
time sharing program that is suited for a community could be the difference of solar
cookers being a socially accepted means of cooking food.



Micro-finance

Another program that can help solar cookers be implemented would be to have micro-
financing to provide loans for buying solar cookers. The initial cost of solar cookers can
be prohibitive and churches or schools may even want to use micro-finance to purchase
enough solar cooking components to rent to the community. The Kenyan National
Federation of Co-operatives Ltd. has five million members and has a program set up for
micro-financing projects [2].



Conclusions and Recommendations
There are many conclusions and recommendations that can be deduced from the
experimental data, and several hypotheses that would have to be tested further to
determine if they are accurate. The recommendations are for the selection of pots, base


                                                                                       41
materials, greenhouse, and reflectors. The suggestions made for having a greenhouse and
reflector are obvious based on our experimental data, however, many of the other
suggestions made will require further testing to ensure accuracy. These suggestions are
derived from the analysis of the data and are made out of the interest of communities in
developing countries. The following section contains recommendations and further
research for the various solar cooker components.


Pot Selection

In determining which pot to use for the cooker, an essential variable is what types of pots
the people currently use. The surface can be altered in a way that helps it absorb solar
energy (i.e. painting it black), but constructing new pots specifically for the solar cookers
when suitable pots already exist might be the added cost that prevents a design from
being economically feasible. Also, a pot with a lower thermal conductivity (such as the
cast-iron pots already used in developing countries) will retain heat better and continue to
absorb energy, whereas a pot made of a material like aluminum will transfer heat back to
the solar cooker if it is not insulated well.


Pot Base Material Selection

The pot base material was tested to determine whether it is advantageous to place a pot
upon stones. The test conducted to determine this indicated that propping the pot upon
three stones will increase the temperature by about 0.7°C. Based on this, it would be
recommended to prop the pot up on stones, but further research will be needed. The
other base materials that still need to be tested would be to compare a black ground
surface against aluminum foil and cardboard. Once these tests are performed, it will be
possible to confidently suggest the base material and whether to prop the solar cooker
pots up on stones.


Greenhouse Selection

The selection of a greenhouse for the solar cooker is the most important component of a
solar cooker to increase temperatures and contributes to approximately fifty percent of
the solar cooker cost. A greenhouse is integral to a solar cooker and should never be left
out because it can raise water temperatures by over 20 degrees Celsius. The selection of
a greenhouse must be dependent on availability of local materials and the income level of
the people using the cooker.

Number of Greenhouse Domes

Adding one greenhouse dome to a solar cooker is extremely important and will increase
the cooking temperatures by 15 to 20 degrees Celsius. An additional greenhouse will
increase the rate the food cooks and increase the maximum temperature of water by over


                                                                                          42
four degrees. Maria Telkes from the UN conducted extensive research on the number of
greenhouse domes and she determined that two domes maximized the power potential in
a solar cooker [12]. The best solar cooker design will incorporate one greenhouse and if
money permits, two greenhouse insulators would be the most effective.


Greenhouse Dome Material

Most of the greenhouse dome materials had fairly similar performance. Clarity and how
well a greenhouse was insulated were the most important factors that determined the
performance of a greenhouse material. The most economical solution for a greenhouse
material may be the hard polypropylene plastic bins. The hard plastic domes are also
much more durable and user friendly than plastic bags which are the most common
greenhouse used in panel cookers. If cost is not an issue, a box cooker would be the best
greenhouse dome insulator; otherwise the best solution appears to be hard plastic domes.


Reflector Selection
The selection of a reflector is equally important as the greenhouse selection since it can
raise temperatures by approximately 12 degrees Celsius and the reflector contributes to
about fifty percent of the solar cooker cost. The economic analysis determined the most
cost effective reflector may be two glass mirrors mounted on a support assembly similar
to the one used in this experiment. Having two glass mirrors instead of aluminum foil
mounted on cardboard should also provide better performance. Potentially having better
performance at a lower cost indicates two glass mirrors to be the best reflector
suggestion.



Final Recommendation

Based on the evaluation of various components of the solar cooker, the final
recommendation for the solar cooker would be:

       Use two greenhouse domes that are durable, inexpensive, and obtained locally
       Use at least one reflector that is durable, inexpensive and obtained locally
       Use pots that are obtained locally and painted black
       Prop the pot upon stones to reduce conduction between the bottom of the pot and
        base of the solar cooker




                                                                                       43
Bibliography
[1] Owino, Margaret. Email Correspondance. July 3, 2006.

[2] Baptista, T.L. et.al. “Solar Household Energy, Incorporated: A Market-Based
       Strategy for Introducing Passive Solar Ovens in Kenya.” Michigan Business
       School. May, 2003.

[3] AltEner: Energy Technologies, Definitive Energy Solutions. Retrieved July 14, 2006
       from http://www.ecoterra.org.uk/altener.htm

[4] Munson, Paul. Telephone conversation. June 29, 2006.

[5] Goodman, Joel H. “CPC – Earthen Vaults Community Solar Cooker,” Retrieved June
       29, 2006, from http://www.solarcooking.org/earthen1.htm#new

[6] Kerr, Barbara. “Food Safety and Solar Cooking” excerpt from “The Expanding
       World of Solar Box Cooking,” Retrieved June 29, 2006, from
       http://www.solarcooking.org/foodsafety.htm

[7] “Lecture Notes on Solar Cooking” Arba Minch Solar Initiative. Retrieved July 12
       2006 from http://home.arcor.de/Ernst.Willand/amsi-contents.html

[8] “The Solar Bowl”. Auroville. Retrieved July 12 2006 from
       http://www.auroville.org/research/ren_energy/solar_bowl.htm

[9] Austin, R. “Natural Insulation: Regulating Heat and Cold.” The Institute for
       Planetary Renewal. Retrieved July 20, 2006 from
       http://www.planetaryrenewal.org/ipr/insulation.html

[10] The Engineering Toolbox. Retrieved August 5, 2006 from
       http;//www.engineeringtoolbox.com

[11] Specific Heat Capacity. Wikipedia, the free encyclopedia. Retrieved July 15, 2006
       from http://en.wikipedia.org/wiki/Specific_Heat_Capacity

[12] Radabaugh, J: Heaven’s Flame, A Guide to Solar Cooking. 2nd Edition.
       Home Power Publishing. 1998.

[13] Calcium Sulfate. Wikipedia, the free encyclopedia. Retrieved July 15, 2006 from
       http://en.wikipedia.org/wiki/Calcium_sulfate

[14] Funk, P.A. “International Standards for Testing Solar Cookers.” Solar Cooker
       Review. April, 2000. Retrieved August 3, 2006 from http://solarcooking.org




                                                                                       44
[15] Kerr, Barbara. “Food Safety and Solar Cooking” excerpt from “The Expanding
       World of Solar Box Cooking,” Retrieved June 29, 2006, from
       http://www.solarcooking.org/foodsafety.htm

[16] Harrison, J. “Investigation of Reflective Materials for the Solar Cooker.” Florida
       Solar Energy Center. December 21, 2001. Retrieved on July 12, 2006 from
       http://www.fsec.ucf.edu/Solar/projects/solarcooker/cooker.htm


[17] Kammen, D. “Introducing Solar Ovens in Kenya.” Retrieved on July 7, 2006 from
       http://solarcooking.org/kammen93.htm




                                                                                          45
Appendix A: Mirror Support Assembly Design
A mirror support assembly was designed to test out the effectiveness of reflectors as
shown in the figures below. Depending on the availability of local materials, using
mirrors with supports may be less expensive and give solar cookers greater performance.
The mirror support was designed with two triangular sections that are connected with one
piece of wood in the back. Wire is used to quickly adjust the angle of the mirror and also
to tie down the mirror so the angle is retained in higher winds. The triangular side
supports allows for nails in the front and back of the mirror to be used as tie downs and
angle adjustment. This mirror support assembly is very low-cost and simplistic which is
necessary for developing countries.




                     Figure 27: Mirror Support Assembly with three Mirrors




                       Figure 28: Mirror Support Assembly during Testing
                                                                                       46
Appendix B: Social Dynamics of Solar Cooking
The following list contains important social factors in Kenya that influence the
dissemination and adoption of solar cookers in communities.
     It is customary to cook extra food for dinner in case friends or relatives stop by
       without notice. Due to the limited cooking capacity of many solar cooker
       designs, it is difficult to cook extra food.
     For some, the process of cooking using solar energy is not understood and
       therefore associated with negative spiritual effects.
     Box cookers can be viewed like coffins, so it is best to paint them green or blue.
     Some men expect a hot meal to be ready when they return home from work.
       Because solar cooking occurs during the day, this can be difficult.
     Since men generally control the finances in a home and women are in charge of
       cooking, men are sometimes reluctant to invest in tools that make cooking easier,
       even if it means spending less time and money collecting fuel.
     According to Solar Cooking International, a leading solar cooking promoter in
       Kenya, solar cooking technology must be accepted and utilized in Kenyan
       families before it will be accepted at a community scale [m1].




Appendix C: Other Materials




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