Solar Oven Design
Engineering 102
Spring 2010
Solar Ovens
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
Forms of Energy
Mechanical Energy Chemical Energy
Energy produced from Energy that is stored in
mechanical devices molecular bonds, the
forces that hold molecules
together
Forms of Energy
Thermal (Heat) Nuclear Energy
Energy Energy that is trapped inside
energy in the process of each atom
being transferred from one
object to another because
of the temperature
difference between them.
Forms of Energy
Solar/Light/Radiant Electrical Energy
Energy Energy as a result of the flow
Energy from the sun of charged particles called
electrons or ions
First Law of Thermodynamics
Energy can not be created or destroyed, but
changed from one form to another
Goal of a Solar Oven: change electromagnetic
(solar) energy into thermal energy, store as heat
Heat Transfer
Conduction - solids
Convection – gases and liquids
Radiation
• Trap heat/solar energy inside a
container
• Black surfaces adsorb and radiate
energy
• Shiny surfaces reflect light
Solar Oven – Theory
At equilibrium:
• Energy out = Energy in
Joules, BTUs, calories
• Powerout = Powerabsorbed
Power is energy/time
Joules/sec, BTU/s, hp, Watts
• Goal is to determine equations that predict
Pabsorbed and Pout and ultimately predict the
internal Oven Temperature Tio = “Model”
Handout Design
Horizontal top and bottom
• Not all sun necessarily gets in
Properly Aim the Oven
• Ideally, aimed directly at the sun
(90° to the sun)
Figure 2(handout) -Solar Oven Geometry (general)
Pabsorbed -- Factors
Sun
• I0 – incident solar power (W/m2)
• qS – angle of sun rays with horizon
Size or Area (Aw)
• W – width of glazing
• L – length of glazing
• b– angle of window with horizon
Material properties of window, oven
● a – absorptivity
• t – transmissivity
Pabsorbed
Pabsorbed I o Aw t a sin q s b
Sun
Insulation
Radiation,
conduction
and
Convection
Power Leaving ≡ Pout -- Factors
Radiation, Conduction, and
Convection
Factors
• A – Area through which energy flows
• DT – temperature gradient from inside
to outside
• Material
U – heat transfer coefficient (radiation,
conduction, and convection)
Pout - details
P = UADT Window/glazing
“U of A” eqn.
sb = Sides and
Bottom
w – Window
io – interior oven Sides/Bottom
Ambient – outside
oven
Pout Usb Asb U w A w Tio Tambient
Balancing Energy (out = in)
Pout = Pabsorbed
Pout U sb Asb U w Aw Tio Tam bient
Pabsorbed I o Aw t a sin q s b
Rearranging for Tio
I o Aw t a sin q s b
Tio Tam bient
U sb Asb U w Aw
Reflectors
Goal is to capture more light
Only FLAT mirrors may be used!!
(“no focusing”)
Reflectors
Energy Gain, “G”
• Extra solar energy is reflected by the mirrors INTO
the oven, and Pabsorbed increases by the ratio G:
• Pabsorbed with a reflector = G Pabsorbed without a reflector
r – reflectivity of reflector
M
G 1 N r sin 90 M – height of reflector
L a – angle of reflected light
N - # of reflectors
I o Aw G t a
Tio Tam bient
U sb Asb U w Aw
Solar Oven with Reflectors
Vary M/L: