Transcript SHPE Foundation Noche de Ciencias Hands

SHPE Foundation
Online SHPE Jr. Member
Activities
TeachEngineering Hands-on
Activity:
*Solar Ovens
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SHPE Foundation
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Solar Ovens
Reference Activity on TeachEngineering
•
Learn about heat transfer, then design, build, and test a solar
oven that can bake cookies!
• Engineering focus:
o Full Engineering Design Process
• Follow all the steps of the engineering design process to build, test, and
redesign a device, optimizing its performance.
• Learning objectives:
o Explain the concept of radiation and give examples of ways that the sun's
energy can be collected and stored for useful purposes.
o Explain that certain materials do not conduct heat well and are therefore
good for insulation.
o Give examples of different materials that can be used for collecting and
storing heat energy from the sun.
o Plot and analyze temperature vs. time data.
SHPE Foundation
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Solar Ovens
• Suggested time: 75-100 minutes (plus time for baking!)
• Materials
o cardboard boxes (at least 2), where one box fits inside the other
o 1 piece of Plexiglas (1/4-3/8 inch thickness) that will need to be cut to fit specific
dimensions of each oven (or the oven can be designed to fit the piece of Plexiglas
appropriately). Alternatively, plastic oven bag may also be used for this top layer and
is easier to work with. It does not provide as much insulation against heat loss by
conduction.
o scotch tape
o black duct tape
o 1 Granite Ware pan or regular black metallic cooking pan sized to fit inside your
oven.
o foam insulation (find at any home improvement store), enough to insulate your oven
box. Foam peanuts or newspaper are less expensive alternatives.
o 3-4 feet of aluminum foil
o utility knife or scissors; scissors must be strong enough to cut corrugated cardboard
o (optional) glue
o 3-4 pieces of black construction paper
o ruler
o food item to be cooked on metallic pan, pre-made cookie dough works best
o thermometer
SHPE Foundation
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Solar Ovens
Engineering Connection (Real World Application):
• The design, construction and testing of solar ovens is an engineering project
that combines materials science with mechanical engineering through
harnessing heat transfer mechanisms.
• Solar ovens are in use worldwide, providing fuel-free and smoke-free cooking,
baking and water decontamination especially helpful in remote and poor
regions.
Solar tea kettle in Tibet
Solar cooker used in a
village in Ghana
“Solar cake” in a solar oven
(GSO: Global Sun Oven) made
in developing countries and
USA
SHPE Foundation
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Solar Ovens
Engineering Design Process:
• In this activity, you will act as a real engineer by
following the full engineering design process.
• The engineering design process is a series of steps
that engineers use to guide them as they solve
problems.
• The design process is cyclical, meaning that
engineers repeat the steps as many times as
needed, making improvements along the way.
SHPE Foundation
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Solar Ovens
Vocabulary
Terms
Definitions
conduction
Heat flow due to the contact of two objects or within a solid object.
convection
Heat flow due to fluid movement such as water.
radiation
Energy transferred through the movement of electromagnetic waves; heat transfer not requiring a medium.
thermal
conductivity
The property of a material that determines how well it conducts/transmits heat. (Examples: Metal generally has
a high thermal conductivity, plastic generally has a low thermal conductivity.)
insulator/
insulation
A material that does not conduct heat very well and has a low thermal conductivity. (Examples: A good jacket,
fiberglass insulation, a sleeping bag, anything with air trapped in it.)
emissivity
A property of the surface of an object that determines how much electromagnetic energy is reflected and how
much is absorbed by an object in the form of heat. Emissivity is very dependent on color. (Examples: Aluminum
foil has a low emissivity because it reflects a majority of heat and a black surface has a high emissivity because it
absorbs a lot of heat; a black shirt gets hotter than a white one in the sun.).
solar radiation
The amount of power received on the Earth's surface per unit area. (watts per square meter in the SI unit
system)
renewable
resource
A resource that is inexhaustible or replaceable by new growth; limitless supply. (Example: Solar energy.)
nonrenewable
resource
A resource that is not replaceable after it has been used. (Examples: Fossil fuels such as oil or natural gas, iron
ore.)
SHPE Foundation
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Solar Ovens
Introduction/Background:
o How does heat move from one thing to another? There are three
fundamental mechanisms of heat transfer: conduction, convection,
and radiation.
• Conduction is heat flow due to the direct contact or two objects, or
within an object.
• Convection is the heat flow due to fluid movement, such as the
movement of air or water.
• Radiation is energy transferred through the movement of
electromagnetic waves, and does not require a medium.
o The design, construction and testing of solar ovens is an engineering
project that combines materials science with mechanical engineering
through harnessing heat transfer mechanisms.
SHPE Foundation
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Solar Ovens
Introduction/Background:
SHPE Foundation
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Solar Ovens
Procedure Overview
• Learn about ways that heat transfer
occurs and discuss ways to limit heat
transfer. Then, design an ‘oven’ that runs
off the sun’s power, build it, and see if you
can use it to bake a cookie!
Procedure
o Using the suggested materials, take the
freedom to choose your own designs.
o Clearly read the design considerations
on the following two slides prior to
building your solar oven.
SHPE Foundation
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Solar Ovens
Design Considerations
o Design should ideally be independent of position of sun. For this
purpose, make use of reflectors and consider designs that would
not require you to continually reposition your solar oven.
o Heat loss, heat gain and heat storage: The best insulation is a
natural substance such as newspaper or Styrofoam. Another
option is to use an inner cardboard box and leave a small empty
space between the two boxes. It is also important to consider the
color of the inside box. Lining the sides with reflective material such
as aluminum foil will reflect solar radiation towards the food to be
cooked. If the inside is painted, it is important that the paint be
non-toxic.
(Note: Do not use materials in the oven that are likely to melt at 200 to
250 °F. For example, a black garbage bag used to line the inside of
the box will melt at high temperatures.)
SHPE Foundation
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Solar Ovens
Design Considerations
o The solar oven collects heat through the heat transfer mechanism
of radiation. Reflect as much sunlight as possible towards the food
that is being cooked. Make the pot or cooking container a dark
color so that it absorbs as much solar radiation as possible. Make
sure that the box is large enough to hold a dark, lightweight,
shallow metallic cooking container. Graniteware, which can be
found in any chef store, is a good material to use.
o The solar oven combats heat transfer through conduction through
the use of insulation to maintain its temperature. Heat is lost
through conduction through the sides of the oven. Insulation slows
this heat loss mechanism.
o Convection is generally only a significant heat loss mechanism for
a solar oven if it is in a particularly windy environment. Heat loss
through convection can be reduced by shielding the solar oven
from the wind.
SHPE Foundation
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Solar Ovens
Procedure (continued)
A detailed explanation of how to construct an example solar oven can
be found at
http://www.backwoodshome.com/articles/radabaugh30.html.
o Upon completion of constructing your solar oven, test the effectiveness
of your oven on a sunny day by baking cookies in the oven to
qualitatively determine how effective your oven is.
o While cookies are baking, attach a thermometer to measure the
temperature inside the oven at a location where you can read the
temperature on the thermometer without having to open and remove
the thermometer.
o Measure and record the temperature every 10 minutes for 1 hour or for
the length of time it takes to fully bake your cookies. Use your data to
plot temperature vs. time on a graph. Take notes about any
environmental changes, such as cloud cover, etc.
o Eat your baked goods!
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Solar Ovens
Safety Issues
o Be extremely careful when cutting cardboard boxes with scissors or
utility knives.
o Do not cut the Plexiglas yourself. Have someone at the home
improvement store when the material is purchased cut it, or use fit
your design to the shape of the Plexiglas or use an oven bag as an
alternative material to replace the Plexiglas.
o Remember to use oven mitts to remove cookies from the oven.
o Be careful when opening your oven after it has been sitting in the
sun as it may be very hot and may release steam, which can burn
skin.
SHPE Foundation
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Solar Ovens
Troubleshooting Tips
o If the oven is not reaching the desired temperatures, make sure that all seals are
tightly closed and air is not leaking out. Make sure any cracks in the box are
glued or taped and well insulated and that the lid forms a tight seal with the box
and where the glass meets the box. Adding foam around the rim where the lid
meets the box can help improve this seal.
o If all seals have been insulated and the box is still not reaching 185 °F, make sure
that the box is facing toward the sun for at least 20 minutes every hour. This
project should only be done during the spring or summer months. If the box still
will not heat adequately, add more insulation by placing the box inside an even
larger container and adding insulation. Finally, the box may simply be too large.
Consider making the area that needs to be heated smaller by placing another
box within the original space or allow more time for heating.
o To make the reflectors, cover the flaps of the box with aluminum foil.
o Get plenty of black duct tape, as it tends to run out.
o Foam insulation tends to work better than newspaper.
o The reflectors are hard to attach and may cause the oven to tip over. Stabilize
your oven in case of mild breezes.
SHPE Foundation
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Solar Ovens
Investigating Questions
o Why is it important to understand heat transfer and storage?
o What advantages does solar energy provide over other types of
energy?
o The Sun's corona has a temperature of millions of degrees. Why
does it not incinerate us?
o Why and for whom could solar ovens be important?
o What are the similarities between the greenhouse effect and the
solar ovens?
SHPE Foundation
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Solar Ovens
Assessing Your Understanding!
Presentations:
o Present to a friend or family member about your solar oven. Describe how your oven
works and why you made certain material/design choices, such as reflector types and
insulation used. In addition, describe the three basic forms of heat transfer—
conduction, convection and radiation – and the role each type of heat transfer plays
in the baking a cookie in your solar oven.
Questions:
o How does the oven work and why did you choose those materials?
o What parts of the solar cooker have high thermal conductivity? Low thermal
conductivity?
o How is radiation used?
o Do reflector panels have high or low emissivity? Why?
o How is convective cooling prevented?
o Where are your insulators? What kind of insulation did you choose and why?
Analyze your temperature versus time plots:
o At what time did your oven get the hottest?
o Was your oven temperature affected by clouds?
SHPE Foundation
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Solar Ovens
Activity Extensions
o Visit a nearby solar house that uses solar hot water or passive solar
heating.
o Determine how well the oven performs in the winter. How
important is the season? How important is the time of day? How
important is the outside temperature?
o Modify your design to determine the importance of individual
aspects, such as the color of the box or how different insulators
affect the temperature.
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