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 equations Step 4: Model the system in Excel Step 5: Solve graphically for T io and U w Step 6: Change the design variables in the Excel spreadsheet to help solve for the desired outputs RequirementsOutputs Sketch & Equations Excel Model Solve for Tio & Uw Adjust design variables

Step 1: Solar Oven Requirements Chamber volume = 1,800 cm 3 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: 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 T ambient, T io, 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: U w : heat transfer coefficient for window A w : 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 U sb : heat transfer coefficient for sides & bottom of cooking chamber A sb : area of sides & bottom of cooking chamber

Step 3: Sketch the system and write out the equations Given Constants: I o : 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 = Power out Power in = I o A w t n a G Power out = U w A w  T Power out = U sb A sb  T  T = T io - T ambient I o A w t n a G = U w A w  T + U sb A sb  T I o A w t n a G U w A w + U sb A sb  T = T io – T ambient = Important!

Step 3: Sketch the system and write out the equations We need to increase T io Only 3 ways to do that… 1.Increase T ambient, 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 U w / Temperature Variability Graphically Note: This spreadsheet example for graphical illustration purposes only.

Step 6: Experiment with Excel Model to find T io See Excel Spreadsheet Prediction Homework Instructions for further information (pp , 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 T io vs. predicted T io § 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 T io ≈ actual T io

Solar Oven Team Homework Hints As explained on page 11, U w is a function of T io 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 U w and T io from Table 1 Second line: Using your T io (based on all of your design variables) and the given U w 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 , 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 12 Solar Oven Team Prediction Model due Dec 16 Read pp. 13 – 20 in Solar Oven Doc. For Friday Remainder of class: