Solar Cooking and the Box Cooker: Technology and Development

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Mech425.jpg This page was part of a project for Mech425, a Queen's University class on Engineering for Sustainable Development.

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Status
This OSAT has been designed but not yet tested - use at own risk.
This OSAT has been modeled.
This OSAT has been prototyped.
This OSAT has been deployed by: NPOs, businesses, and individuals at least: 10,000s times in: countries all over the world.

See The solar cooking archive wiki for exhaustive details

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Contents

[edit] Abstract

The primary goal of this Appropriate Technology project is to consolidate existing information on solar cooking, as well as evaluate materials and design techniques utilized for the construction of widely used solar box cooker models.


Although hundreds of innovative solar cooker designs have been invented including the popular parabolic and panel cookers, the solar box cooker is given the widest consideration due to its widespread global usage, particularly in the developing world.

[edit] Introduction

[edit] Developmental Need

Cooking in developing countries is customarily done on open fires using biomass such as firewood, charcoal, and kerosene. This process inevitably leads to excessive deforestation in these rural regions as well as considerably increase CO2 emissions. Individuals in these developing regions also suffer from respiratory infections due to significant smoke inhalation.


About 5 million children in the developing world die each year from respiratory ailments and a further 5 million are estimated to die from diseases associated with contaminated drinking water. [1] These figures can hope to be partially reduced using solar cooking technology.


Solar cookers work on the basic principle of sunlight being converted to thermal energy that is retained and used for outdoor cooking purposes, and have the most positive impact in sunny, fuel-scarce regions of the world. An optimistic estimate states that solar cooking could cause a potential reduction of fuelwood use by 36% which corresponds to approximately 246 million metric tons of wood each year, thus resulting in a net greenhouse gas offset of nearly 140 million metric tons per year. [2]


Figure 1 below shows what processes occur over a temperature distribution of 135 degrees Celsius:


Temp SolarCookers.png


Figure 1: Temperature distribution for cooking with a solar cooker.[3]

[edit] Advantages

  1. In villages, women have to travel less often to forage for firewood, thus keeping them closer to home and safe as a result.
  2. Cheaper than cooking with firewood or charcoal.
  3. Reduced CO2 and methane emissions.
  4. Solar cookers can be easily constructed in a matter of hours after very basic training.
  5. Solar cookers are generally light-weight.

[edit] Limitations

  1. It can take very long to cook a meal.
  2. It is not always sunny.
  3. Solar cooking can only be performed when the sun is strong enough during mid-day, however hot food may be required much earlier on.
  4. Cooking has to be undertaken outside which may pose additional threats in developing countries.
  5. Will not perform well in windy areas.
  6. Can be inconsistent and may produce inedible food.

[edit] Alternate uses for solar cookers

  1. Water pasteurization
  2. To disinfect dry medical supplies
  3. To sanitize utensils


[edit] Scientific Principles

[edit] How it works

A solar box cooker is basically a large box with a glass lid that will function as an oven. However, the heat losses over a larger surface area will partially offset the additional gain through having a greater heat collecting surface. What is usually done to compensate for this is that a glazed surface cover and reflectors are used to increase the apparent collector area. These reflectors can be made from a variety of materials and their primary purpose is to reflect sunlight through the glazing material and into the cooking space inside of the box.

The box cooker consists of some type of heat trapping enclosure, which usually takes the form of a box made of insulating material with one face of the box fitted with a transparent medium, such as glass or plastic. This enables the cooker to utilize the greenhouse effect and incident solar radiation cooks the food within the box. The insulating material allows cooking temperatures to reach similar levels on cold and windy days as on hot days, as well as having an added benefit of blocking any leakages that could potentially seep through and damage the cooker. A dark cooking pot is recommended for cooking as it absorbs the maximum amount of heat and allows for higher cooking temperatures. [4]


A good rule of thumb that indicates when the sun is high enough in the sky to allow for efficient cooking is when the length of one's shadow on the ground is shorter than that individual's height.


MECH425 SolarCooker.png
Figure 2: Schematics of a solar box cooker [5]


MECH425 ReflectorAngle.png
Figure 3: Correct position for reflector on box cooker [6]

[edit] Comparison factors between solar cookers

Several engineers and DIY enthusiasts have designed literally hundreds of different types of solar cookers. This expansive variety of options makes it difficult to standardize and evaluate solar cookers, however some critical factors must be met for a design to be successful, some of which are listed below:


  1. Cost - must be cheap enough to be viable for implementation into rural areas.
  2. Convenience - can be built by readily available local materials in a short period of time, as well as being light-weight.
  3. Safety - heated area must be well protected and no parts should be jutting out.
  4. Efficiency - how long will it take to cook the food?
  5. Wind resistance - must be sturdy enough to not be affected by light to moderate winds.
  6. Heating capacity - sufficient heating capacity based on it's use (water pasteurization vs. cooking food).
  7. Durability - repairs should be infrequent and easily performed.
  8. Simplicity of instructions.

[edit] Efficiency analysis of a solar cooker

[edit] Energy efficiency

Based on the 1st Law of Thermodynamics: [7]

Energy input = Energy output + Energy losses


Energy input to the solar cooker can be calculated as follows:


MECH425 EnergyInput.png

Where:
Ei is the energy input in W
It = total solar energy incident upon plane of the solar air being heated in W/m2
Asc is the surface area of the solar cooker in m2


Energy output from the solar cooker can be found as shown below:

MECH425 EnergyOutput.png

Where:
Eo is the energy output in W
mw is the mass of water in kg
cpw = specific heat of water in J/kgK
Asc is the surface area of the solar cooker in m2
Twi is the initial temperature of the water in K
Twf is the final temperature of the water in K
t is the time in seconds


Energy efficiency of the solar cooker can be found as shown below:


MECH425 EnergyEfficiency.png

[edit] Exergy efficiency

Based on the 2nd Law of Thermodynamics: [8]

Exergy input = Exergy output + Irreversibility


Exergy input to the solar cooker can be calculated as follows:


MECH425 ExergyInput.png

Where:
Exergy input in W/m2
cpw = specific heat of water in J/kgK
To is the outside temperature in K
Ts is the surface temperature of the sun in K


Exergy efficiency of the solar cooker can be calculated as follows:

MECH425 ExergyEfficiency.png

Where:
Asc is the incident area of the solar cooker

The ability of a solar cooker to collect sunlight is directly related to the projected area of the collector perpendicular to the incident radiation.

[edit] Cooking power

The primary reference measurement used by ASAE S580 [9] is the cooking power, which can be calculated as follows over 10 minute intervals:

MECH425 CookingPower.png

Where:
M is the mass of water in kg
C is the specific heat of water in J/kgK
T1 is the water temperature at the start of each interval in K
T2 is the water temperature at the end of each interval in K

This temperature change is detected over 10 minute intervals, thus the equation above is divided by 600 s.

[edit] Regional Considerations

Approximately 600,000 solar cookers are being used in the Andes, Tibet, Nepal, Mongolia and parts of China and an estimated 1.5 million around the entire world, although this figure is preliminary and has yet to be confirmed of it's accuracy. [10] [11] However, the biggest recent success story has been in villages in India.


Solar cooking may be suitable for a region if:

  1. Mostly-sunny days throughout several months of the year.
  2. Outdoor space available that remains sunlit for several hours and sheltered from high winds.
  3. Local cooking fuels are expensive or difficult to obtain.


Due to the abundance of sunlight in Asia, Africa, and Australia, these regions can greatly benefit from the use of solar cookers as illustrated in Figure 4 below:

MECH425 SunlightDistribution.png


Figure 4: Average annual sunlight distribution[12]

As well as sunnier climates benefiting more from solar cooking technologies, locations with less windier conditions will be able to utilize solar cookers much more efficiently. In windier areas, it may be more suitable to introduce heavier solar cookers, however this could lead to higher cost implications and longer cooking times if thicker bases are constructed, as additional raw material would be required. Another crude but cheaper option would be to place large stones or bricks around the cooker to help stabilize it in the wind.


[edit] Required Tools and Materials

A sturdy solar box cooker can be built from cheap and common materials in a matter of a few hours without the use of additional tools. To increase durability, outer, non-reflective surfaces can be painted, oiled or waxed to help protect from moisture. The surface that faces the cooking pot should be reflective or black.

Table 1: Material Requirements
Core materials Substitute Materials
Aluminum foil Metal or sheet metal
Black paint Wax or oil
2 large cardboard boxes Woven baskets, bricks, wood (last option)
Heat resistant plastic sheets Nylon bags, Polyester bags, Glass
Water-based polyvinyl acetate glues (diluted with water) Wheat or rice flower paste, acacia gum, casein glue *


'* Avoid using tape petroleum and rubber-based glues for inner cooking surfaces, as the inner box should be able to withstand high temperatures without releasing any fumes.

[edit] Required Skills and Knowledge

There are no prerequisite skills or knowledge that are required to follow instructions on how to build a solar cooker. A few hours of basic training is needed to help educate masses living in rural areas on the basic construction and maintenance of solar cookers, after which they should be able to use them without any hassles, assuming that this training will include how to deal with all types of repairs pertaining to that particular solar cooker model.

[edit] Technical Specifications

[edit] Sample Dimensions

MECH425 SolarBoxCooker.png
Figure 5: Sample dimensions of a box and parabolic solar cooker[13]

[edit] Building a Solar Box Cooker

Several successful solar box cooker designs have been established, accompanied with simple building instructions as well as tips and tricks. Some are listed below:

  1. Minimum Solar Box Cooker. [14]
  2. Solar Box Cooker. [15]
  3. Collapsible Solar Box Cooker. [16]
  4. Inclined Solar Box Cooker. [17]
  5. Tracking Box Cooker. [18]
  6. The Tire Cooker. [19]

[edit] Cost Estimates

Approximately 2,400 square centimeters of scrap aluminum plate will make one 20 cm x 27.5 cm x 5.5 cm pan, with cover. The material cost is less than $0.30 per pan. [20]

This can be used as a reference to construct your own solar cooker with the aluminum plates available locally. The size of the cooker will be dependent on the size of the pan, and the overall unit size is subject to what is most economical in your particular geographic region.


Table 2: Cost Estimates
Materials Estimated Rates ($) Cost for project ($)
Aluminum sheet $1->100,833 cm3 [21] $0.30
Black paint $16.14/L [22] (~300 ml requirement) ~ $4.84
2 large cardboard boxes $1.4/m2 [23] ~$1.47
Glass $10.76/m2 [24] (~0.27m2 requirement) -> ~ $2.91
Acacia gum ~ $3.50/kg [25] (~0.25kg requirement) ~ $0.875
TOTAL COST = $10.39/unit


These prices were determined through mainstream North American stores such as Home Depot, and size estimations based on the dimensions in Figure 5. The assumption is that the total cost of $10.39 will be significantly lower in rural locations due to less mark-up of the prices as well as easier accessibility to low-grade materials in these areas.

[edit] Future Development Focus

In order to progress with this technology, more resilient efforts must be made to educate the masses in rural areas and measures should be put into place to continue motivating these individuals to use these solar cookers on a regular basis.

Wind-resistant models would prove to be of great benefit in making solar cookers viable in more regions around the world, where powerful winds across open areas pose a current challenge to this technology.

Mass production of some the solar box cooker models aforementioned can result in further cost reductions if pre-made products are introduced to areas of need, instead of training camps being set up to teach these individuals how to make their own.

[edit] Related Organizations

Several Non-Government Organizations (NGO's) and private organizations have taken it upon themselves to promote the benefits of solar cooking in developing rural areas, from creating DIY designs to setting up training camps to educate villagers on how to build and cook with these devices. Several of these organizations have successful cooker designs already posted online, and others are in various stages of design. Some of these groups are listed below:

  1. Solar Cookers World Network. http://solarcooking.wikia.com
  2. Solar Cookers International. http://www.solarcookers.org/
  3. Kyoto Twist Solar Cooking Society. http://www.kyototwist.org/
  4. Gadhia Solar Energy Systems Pvt. Ltd. http://solarcooking.wikia.com/wiki/Gadhia_Solar_Energy_Systems
  5. Solar Energy International. http://www.solarenergy.org/
  6. Centre for Rural Technology. http://www.panasia.org.sg/nepalnet/crt/crthome.htm
  7. The Central American Solar Energy Project. http://www.solaroven.org/
  8. SUN OVENS International, Inc. http://www.sunoven.com
  9. Lazola-Initiative. http://www.lazola.de

[edit] References

  1. The Solar Cooking Archive, "Evaluating Solar Cookers", solarcooking.org/Evaluating-Solar-Cookers.doc, Accessed April 8, 2010
  2. The Solar Cooking Archive, "Evaluating Solar Cookers", solarcooking.org/Evaluating-Solar-Cookers.doc, Accessed April 8, 2010
  3. Solar Cookers International, "How to make a solar cooker", http://images3.wikia.nocookie.net/__cb20090108164302/solarcooking/images/5/57/CooKit_plans_detailed.pdf, Accessed April 6, 2010
  4. Wikipedia, "Solar cooker", http://en.wikipedia.org/wiki/Solar_cooker, Accessed April 3, 2010
  5. The Solar Cooking Archive, "Evaluating Solar Cookers", solarcooking.org/Evaluating-Solar-Cookers.doc, Accessed April 8, 2010
  6. Solar Cookers International, "How to make a solar cooker", http://images3.wikia.nocookie.net/__cb20090108164302/solarcooking/images/5/57/CooKit_plans_detailed.pdf, Accessed April 6, 2010
  7. Ozturk, H. "Second Law Analysis for Solar Cookers", http://www.informaworld.com/smpp/1138067100-85020668/content~db=all~content=a713635696, Accessed April 8, 2010
  8. Ozturk, H. "Second Law Analysis for Solar Cookers", http://www.informaworld.com/smpp/1138067100-85020668/content~db=all~content=a713635696, Accessed April 8, 2010
  9. ASABE Technical Library. "Testing and Reporting Solar Cooker Performance",http://asae.frymulti.com/abstract.asp?aid=24465&t=2, Accessed April 8, 2010
  10. The Solar Cooking Archive, "Evaluating Solar Cookers", solarcooking.org/Evaluating-Solar-Cookers.doc, Accessed April 8, 2010
  11. Deutsche Welle, "Cooking with the power of the sun", http://www.dw-world.de/dw/article/0,,5205895,00.html, Accessed April 3, 2010
  12. Solar Cookers International, "How to make a solar cooker", http://images3.wikia.nocookie.net/__cb20090108164302/solarcooking/images/5/57/CooKit_plans_detailed.pdf, Accessed April 6, 2010
  13. Ozturk, H. "Second Law Analysis for Solar Cookers", http://www.informaworld.com/smpp/1138067100-85020668/content~db=all~content=a713635696, Accessed April 8, 2010
  14. The Solar Cooking Archive, "Minimum Solar Box Cooker", http://solarcooking.wikia.com/wiki/Minimum_Solar_Box_Cooker, Accessed April 8, 2010
  15. Solar Cookers International, "How to make a solar cooker", http://images3.wikia.nocookie.net/__cb20090108164302/solarcooking/images/5/57/CooKit_plans_detailed.pdf, Accessed April 6, 2010
  16. The Solar Cooking Archive, "The Collapsible Solar Box Cooker", http://solarcooking.org/plans/collapsible-box.htm, Accessed April 8, 2010
  17. The Solar Cooking Archive, "The Inclined Box-Type Solar Cooker – A New Design", hhttp://solarcooking.org/plans/inclined-box-cooker.htm, Accessed April 8, 2010
  18. The Solar Cooking Archive, "The Tracking Solar Cooker", http://solarcooking.org/plans/Cookerbo.pdf, Accessed April 8, 2010
  19. The Solar Cooking Archive, "The Tire Cooker", http://solarcooking.org/plans/tire_eng.htm, Accessed April 8, 2010
  20. The Solar Cooking Archive, "SunPan Overview", http://www.sungravity.com/sunpan_overview.html, Accessed April 4, 2010
  21. The Solar Cooking Archive, "SunPan Overview", http://www.sungravity.com/sunpan_overview.html, Accessed April 4, 2010
  22. Home Depot, "Painter's Touch Multi-Purpose Paint", http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=980065&Ntt=980065&catalogId=10051&langId=-15&storeId=10051&Dx=mode+matchallpartial&Ntx=mode+matchall&recN=0&N=0&Ntk=P_PartNumber, Accessed April 8, 2010
  23. Shopping.com, "11" x 14" Corrugated Cardboard Sheets", http://www0.shopping.com/-cardboard+sheets, Accessed April 8, 2010
  24. GlassCages, "1/8 Plate Glass 3 mm Regular Plate Glass", http://www.glasscages.com/?sAction=ViewCat&lCatID=42, Accessed April 8, 2010
  25. Al-Mosawi. A, "Acacia gum supplementation of a low-protein diet in children with end-stage renal disease", http://www.springerlink.com/content/1rq1mafd2x233aa9/, Accessed April 8, 2010