Lecture 7 – More Solar Oven

Engineering 102

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 equationsStep 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 Mode

l

Solve for Tio & Uw

Adjust design

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_Westhttp://www.lvrj.com/news/vdara-visitor---death-ray--scorched-hair-103777559.html

Step 2: List the Model Outputs

Predict temperature from equilibrium conditionShow temperature effect from chamber dimensions

Give reflector design angleShow improvement, if any, from double glazing

Calculate PI from Tambient, Tio, and CostShow 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 groundUsb : heat transfer coefficient for sides & bottom of

cooking chamberAsb : 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

Powerin = Io Aw tn a G

Powerout = Uw Aw TPowerout = Usb Asb T

T = Tio - Tambient

Io Aw tn a G = Uw Aw T + Usb Asb T

Io Aw tn a G Uw Aw + Usb Asb

T = Tio – Tambient =Important!

Step 3: Sketch the system and write out the equations

wwsbsb

woambientio AUAU

aGAITT

• 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.

n

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 optimal values for your oven

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 1Second 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

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, and your spreadsheet analysis

Prizes?

Solar Oven Project - Sneak Peek

Friday: we will examine the Gain, G, from the addition of reflectors

Read pp. 13 – 20 in Solar Oven Document for Friday

This example has 4 reflectors

In closing…

Solar Oven Team Comparisons HW due Dec 12Solar Oven Team Prediction Model due Dec 16Read pp. 13 – 20 in Solar Oven Doc. For Friday

Remainder of class: