Solar Oven Comparisons Homework
Engineering 102B, Horner
This is an individual homework assignment, which means you need to create your own Excel file from scratch, but I strongly encourage you to work with your teammates to complete it. You will be creating an Excel template that you will use later to model your teamâ€™s actual solar oven. Please submit your Excel file by the due date to the D2L dropbox.
For both problems 1 and 2, use Solar Oven Comparisons Homework
the graphical solution approach to predict the internal oven temperature (Tio). Refer to the Solar Oven Excel Guide and the Solar Oven Basics documents for guidance. Please complete and submit this assignment as an Excel spreadsheet.
In this assignment, you are to compare the predicted internal oven temperature (Tio) of four different solar oven designs (two designs in Problem 1, and two more designs in Problem 2).
No reflectors will be used on these ovens. In addition, the Solar Oven Project oven volume constraint (1,000 cm3) does not apply to these oven examples.
Problem 1: The oven in a) has a window made of a single sheet of Mylar, and the oven in b) has a window with two layers of Mylar. In each case, the thickness of the insulation (wadded newspaper) is 150 mm, and the cardboard is 4 mm thick. The size of the window(W and L measurements) is 6 cm x 6 cm, and the depth of the oven cooking chamber is 10 cm.
Prepare graphs of Tio vs. Uw, assuming that ?s = 50Â°, ÃŸ = 40Â°, t= 0.92, a=0.9, Tambient = 21Â°C and Io=1000 watts/meter2. Be sure to show the intersection point on the graphs that you construct.
a) What is the predicted Tio of this oven with one layer of Mylar for the window?
b) What is the predicted Tio of this oven with two layers of Mylar for the window?
Problem 2: Now increase the size of the window to be 18 cm x 18 cm. The oven in c) has a window made of a single sheet of Mylar, and the oven in d) has a window with two layers of Mylar. All other parameters are the same as stated in Problem 1, including the cooking chamber depth of 10 cm. Prepare two more graphs of Tio vs. Uw. Be sure to show the intersection point on the graphs that you construct.
c) What is the predicted Tio of this oven with one layer of Mylar for the window?
d) What is the predicted Tio of this oven with two layers of Mylar for the window?
Additional questions: Answer the following questions in your Excel file. Type your answers into a text box for easy formatting. For questions 1 through 3, quantify your answers using the percent change in the predicted Tio value:
New ValueOld ValueOld ValueÃ—100%
1. What effect on the predicted Tio did adding the second layer of Mylar have?
2. What effect on the predicted Tio did increasing the size of the window have?
3. Which of these two changes had the larger impact on the predicted Tio?
4. Of the four oven combinations, which one yielded the highest internal oven temperature? Does this answer make sense to youintuitively? Explain why or why not this makes sense to you.
Solar Oven Excel Guide
Objective: To create an Excel spreadsheet that allows you to vary the input parameters for the solar oven project and to predict the inside oven temperature (Tio).
For reference, write the equation for the inside oven temperature (Tio) at the top of your worksheet (refer to the Solar Oven Basics for the derivation; final equation appears on pg. 11). Use the Insert > Equation feature of Excel to create the equation.
Create a table with the following fixed input parameters:
t = 0.92 (transmissivity for a single layer of Mylar)
a = 0.9 (absorption factor of oven chamber and contents)
Fill the table with a unique color.
Add the variable input parameters to the table. The variable input parameters are:
Tambient â€“ ambient temperature (Â°C)
Io â€“ solar power density (W/m2)
?s â€“ angle of the sunâ€™s rays with respect to the ground (Â°)
ÃŸ â€“ angle of the top surface of the oven with respect to the ground (Â°)
Fill this portion of the table with a different color and enter the values from the Solar Oven Comparisons assignment.
Add the design input parameters to the table. The design input parameters are:
L â€“ length of oven window (m)
W â€“ width of oven window (m)
h â€“ height of oven chamber (m)
n â€“ # of layers of mylar
For this section and the following section, you will want to create four columns to enter these values so that you can test the four different combinations of variables from the assignment on a single spreadsheet. Fill this portion of the table with a different color.
Add the following four calculated input parameters to the table (again, with four columns). Write Excel formulas with absolute and relative cell references (use $) to calculate these parameters.
Aw â€“ area of the window (m2)
Asb â€“ area of the sides and bottom of the cooking chamber (m2)
Vchamber â€“ volume of the oven chamber (m3)
Usb â€“ heat transfer coefficient for the sides and bottom of the cooking chamber
Fill this portion of the table with a different color.
The calculations for Aw, Asb, and Vchamber should be straightforward. The following information will help you calculate Usb.
Create the following table and fill it with the thermal conductivity values (k values) from Table 2 in the Solar Oven Basics document and the given thicknesses for the layers of cardboard and the insulation from the Solar Oven Comparisons assignment. The heat transfer coefficient for the sides and bottom (Usb) is dependent on the materials used and their thicknesses. Create four columns for this section as well. Even though we arenâ€™t changing these values for the Solar Oven Comparisons assignment, doing this now will make it easy to adapt this spreadsheet later to complete the Prediction Model for your oven. Fill the table with a different color.
If the side walls and the bottom are identical in terms of the materials and thicknesses of each layer, then we can calculate Usb for the side walls and bottom from the above data and the following equation:
(be very careful with the units)
Now that most of the input data have been entered and the necessary preliminary calculations have been completed, proceed with calculating the predicted temperature inside the oven. Create the following table and fill in the first three columns with the values found in Table 1 of the Solar Oven Basics document (pg. 9). Make sure you have read that section of the document carefully so you understand where these values came from.
Tio (Â°C)
Uw, single (W/m2â€¢Â°C) Uw, double (W/m2â€¢Â°C)* Empirical Tio Combo 1 Combo 2 Combo 3 Combo 4
*based on 1â€“2 cm gap between Mylar sheets for two layers of Mylar (Johnson, 2003).
Write an Excel formula to calculate Tio (Combo 1) based on the equation for Tio that you placed at the top of the spreadsheet. Create the formula using absolute and relative cell references ($ signs) so that it can be copied to the other cells. Copy the formula to the other cells in the table. Be sure you are referencing the correct cells for each combination and that you are using the appropriate column of Uw values (Uw, single for one layer of mylar and Uw, double for two layers of mylar).
Create graphs for each of the combinations in the table generated in step REF _Ref366969643 r ?6. You will be plotting two lines on each graph: Tio (Table) vs. Uw and Tio (Combo #) vs. Uw (again, be sure you are referencing the appropriate column of Uw values). Provide a chart title and label the axes. Include the units of measure in the axis labels.
Hint: Use a scatter chart with points connected by smoothed lines.
On each graph, locate the intersection of the two lines. The temperature at the intersection point is the predicted temperature inside the oven for that combination. Create a second copy of each graph that is zoomedin on the intersection so that you can read it more accurately. If you rightclick on the x or yaxis and choose â€œFormat Axisâ€, you can adjust the scale in order to â€œzoominâ€ on the intersection point. Determine the temperature value to the nearest tenth of a degree and report this value in a new section of the table you have created. Repeat the process for each combination.
Review the Solar Oven Comparisons assignment carefully and make sure you have entered all of the values accurately, clearly labeled your spreadsheet, and answered all of the questions. Submit the Excel file to D2L.
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