Transcript Solar Oven - Arizona MESA

Solar Oven Design
ENGR 102
Fall 2008
Class Notes
General Categories of Energy
KINETIC ENERGY
Energy in motion
POTENTIAL ENERGY
Stored energy
Forms of Energy
Solar/Light/Radiant
Energy
Energy from the sun
1000 Watts/m2 at the
earth’s surface !!!!!
Electrical Energy
Energy as a result of the flow
of charged particles called
electrons or ions
Forms of Energy
Mechanical Energy
Chemical Energy
Energy produced from
Energy that is stored in
molecular bonds, the
forces that hold molecules
together
mechanical devices
Forms of Energy
Thermal (Heat)
Energy
energy in the process of
being transferred from one
object to another because
of the temperature
difference between them.
Nuclear Energy
Energy that is trapped inside
each atom
Heat Transfer
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Conduction - solids
Convection – gases and liquids
Radiation
• Trap heat/solar energy inside a
container
• Black surfaces adsorb and radiate
energy
• Shiny surfaces reflect light
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solarcooking.org/plans.htm
Solar Ovens
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Not just an Academic Exercise
• Water/milk pasteurization
• Cooking
Designed by solar engineers to be
used in sun rich but fuel poor areas
in the world to improve the quality of
life and nutrition of some of the 2.4
billion people who lack adequate
cooking fuel
 Solar Oven Society
handout Design
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Flat bottom, flat top
• Not all sun gets in
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Alternative:-Aimed Oven
• Incident width = window
width, W= L
Solar Oven – Theory
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First law of thermodynamics
• Energy in = Energy out
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Joules, BTUs, calories
• Power out = Power absorbed
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Energy/time
Joules/sec, BTU/s, hp, Watts
• Goal is to determine Power absorbed
and Power out and ultimately to predict
Oven Temperature Tio
Power Absorbed - factors
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Sun
• I0 – incident solar power (W/m2)
 qS – angle of sun rays with horizon
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Size or Area (Aw)
• W – width of glazing
• L – length of glazing
 b– angle of window with horizon
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Material properties of window, oven
 t – transmissivity
• a - absorptivity
Figure 3-Solar Oven Geometry (handout)
Power Absorbed
Pabsorbed  I o Aw t  a  sinqs  b 
Sun
Radiation,
conduction
and
Convection
Insulation
Power out
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Radiation, Conduction, and
Convection
Factors
• A – Area through which energy flows
● DT – temperature gradient from inside
to outside
• Material
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U – heat transfer coefficient (radiation,
conduction, and convection)
Power out- details
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Pout = UADT
sb = Sides and
Bottom
w – Window
io – interior oven
ambient – outside
oven
Window/glazing
Sides/Bottom
Pout  U sb  Asb  U w  Aw Tio  Tambient 
Balancing Energy (out = in)

Power out = Power absorbed
Pabsorbed  I o Aw t  a  sinqs  b 
Pout  U sb  Asb  U w  Aw Tio  Tambient 
Rearranging for Tio
Tio  Tambient
I o Aw t  a  sin q s  b 

U sb  Asb  U w  Aw 
Predict Final Oven Temp Tio
Use an Excel Spreadsheet
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I0 – fixed
Angle of sun – fixed
Position the oven
Window area
Uw = f(Tio)
• Bigger window but
heat loss increase
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Insulation
Reflectors
Tio = f(Uw)
Reflectors
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Goal is to capture more light and
allow less heat to escape
Solar Oven with Reflectors
Reflectors
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Energy Gain
• Some solar energy reflected is adsorbed by reflector
and more heat (energy) retained in oven
• Pabsorbed with a reflector = G Pabsorbed without a reflector
M 
G  1  N  r    sin   90
 L
r – reflectivity of reflector
M – height of reflector
a – angle of reflected light
N - # of reflectors
Tio  Tambient
I o Aw  G  t  a

U sb  Asb  U w  Aw 
M/L Ratio
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Cannot merely make a Wide/Squat
vs. Tall skinny Pyramid
• Much of the sun’s rays would miss
window