### Solar Oven revised, cont

```Engineering 102
Lecture 7 – More Solar Oven
Today’s Agenda
Continue to explore the Solar Oven Project
and its requirements
Solar Oven Design, First Phase
Step 1: List the requirements (constraints)
Step 2: List the outputs of the model
Step 3: Sketch the system and write out the equations
Step 4: Model the system in Excel
Step 5: Solve graphically for Tio and Uw
Step 6: Change the design variables in the Excel spreadsheet to help
solve for the desired outputs
Requirements
Outputs
Sketch &
Equations
Excel Model
Solve for Tio &
Uw
variables
Step 1: Solar Oven Requirements
Chamber volume = 1,800 cm3
Chamber dimensions > 5 cm each
Chamber window is square (W = L)
Chamber has convenient access (food item can be
conveniently inserted and removed)
Chamber has a rack to support the food item from the
walls and bottom
Chamber has thermometer access with readout face
outside of the oven
Step 1: Solar Oven Requirements, cont.
Stand of some kind – backpacks, books, rocks
OK
Reflector(s):
M/L < 3
Zero to four panels
Minimum final oven temperature = 100
degrees C
No focusing lenses or parabolic reflectors
Step 1: Solar Oven Requirements, cont.
Why no parabolic lenses/reflectors?
Sources: http://solarcooking.wikia.com/wiki/Matt_West
http://www.lvrj.com/news/vdara-visitor---death-ray--scorched-hair103777559.html
Step 2: List the Model Outputs
Predict temperature from equilibrium condition
Show temperature effect from chamber dimensions
Give reflector design angle
Show improvement, if any, from double glazing
Calculate PI from Tambient, Tio, and Cost
Show temperature effects from layers of insulation
Show temperature effects on misalignment with the sun
(how sensitive is this to angle errors?)
Show effect of reflector length, M, on the gain, G
Step 2: List the Model Outputs, cont.
Notes:
You will not create the final list the first time
you work on it.
As you get into the design you will think of
other things and make changes.
However, it saves time and cuts down on the
number of iterations by making a list.
Step 3: Sketch the system and write out the equations
Design Variables:
Uw : heat transfer coefficient for window
Aw : the area of the window
τ : optical transmission coefficient of window
a : absorption coefficient of the cavity walls
β : angle of the window wrt the ground
Usb : heat transfer coefficient for sides & bottom of
cooking chamber
Asb : area of sides & bottom of cooking chamber
Step 3: Sketch the system and write out the equations
Given Constants:
Io: Incident solar power density (pp. 31)
τ : optical transmission coefficient of window (pp.
8 and 31); this could change if you use a different
window material.
a : absorption coefficient of the cavity walls (pp.
8 and 31) - this could change if you use a different
color.
Step 3: Sketch the system and write out the equations
At Equilibrium (Highest Temperature): Power in = Powerout
T = Tio - Tambient
Powerin = Io Aw tn a G
Powerout = Usb Asb
T
Powerout = Uw Aw
Io Aw tn a G = Uw Aw
Important!
T + Usb Asb
T = Tio – Tambient =
T
Io Aw tn a G
Uw Aw + Usb Asb
T
Step 3: Sketch the system and write out the equations
Tio  Tambient
I o Aw  G  n a

U sb  Asb  U w  Aw 
• We need to increase Tio
• Only 3 ways to do that…
1. Increase Tambient, which is possible if
you are close friends with sun.
2. Increase the numerator of the
equation.
3. Decrease the denominator of the
equation.
Step 4: Model the system in Excel
Note: This spreadsheet example for graphical illustration purposes only.
Step 5: Solve Uw / Temperature Variability Graphically
Note: This spreadsheet example for graphical illustration purposes only.
Step 6: Experiment with Excel Model to find Tio
• See Excel Spreadsheet Prediction Homework
Instructions for further information (pp. 30-33, Solar
Oven Document)
• Ultimately, try to find the combination of design
variables that predicts the highest internal oven
temperature
• From these design variable settings, build your oven!
• When your oven is actually tested, compare actual Tio
vs. predicted Tio
§ Last semester’s top team: 1 degree difference!
Solar Oven Project - Notes
Some design variables are specified for you in the
Solar Oven documents
Other design variables are your choice
Use the tools given to you in the Team homework and
Team Excel Predictions Spreadsheet to calculate
Don’t forget to factor in material costs, though
Solar Oven Project – Notes, cont.
Ultimate goal: Maximize Performance Index (PI)
See Solar Oven Document, pp. 34 – 37
PI = (Temperature inside oven – Temperature outside
oven)/Total costs of building oven
Two ways to increase PI:
Increase temperature difference
Decrease costs
Another goal: predicted Tio ≈ actual Tio
Solar Oven Team Homework Hints
As explained on page 11, Uw is a function of Tio
You are given 3 different ways to solve for this, I
suggest that you do it graphically.
You plot 2 lines:
One line: Using the given values for Uw and Tio from
Table 1
Second line: Using your Tio (based on all of your
design variables) and the given Uw values.
Where they intersect is your oven’s predicted
internal temperature at equilibrium.
Solar Oven Project Deliverables
Deliverables via D2L:
Solar Oven Comparisons Team Homework due Dec 12
UPDATED(pg. 29, Solar Oven Document)
Solar Oven Excel Prediction Model HW due Dec 16. (pp.
30-33, Solar Oven Document)
Team Presentation PPT, due TBD
Final Team Report, due TBD
Solar Oven Project
Other Activities:
Actual Oven Demos
Prizes?
First oven, Mar. 8 (Tuesday)
Final oven, Apr. 1 (Friday main lecture time)
Oral & PPT presentations in class, Mar. 22 &
Mar. 24
Based on initial oven test, other oven dissection,
Solar Oven Project - Sneak Peek
Friday: we will examine the Gain, G, from the