SODB - Solar Oven Basics Approaches to Designing a Solar...

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Fall 2008 1 Approaches to Designing a Solar Oven All good engineering design starts with a clear understanding of the project’s needs, goals, budgetary constraints, material constraints, performance tolerances, and criteria (termed the “Performance Index” or P.I.) for judging the effectiveness and success of the final product. Once all of these design specifications are clearly understood, the process of actually designing the product may begin. (Note that in the “real world” any or all of these specifications usually change many times throughout the design process. For this project in ENGR102, they are clearly defined and are not likely to change during your work on the project.) One possible approach to producing a solar oven would be to design and build an actual oven, within the limits of the design criteria. You would then test the oven and measure the Performance Index that you achieved with your first design. Unless you were extremely lucky, it is highly unlikely that this first oven would achieve the desired P.I. You would then have to redesign and build many other ovens, hoping with each cycle of design and test to achieve the P.I. that the customer needs. Even with something as seemingly as simple as a cardboard solar oven, this could take weeks, if not months! Another approach would be to use Design of Experiments (DOE). Using DOE, one could (hopefully) identify the most important factors in a solar oven, and then make good decisions about the levels to test them at. Once these values were chosen, you could build all of the ovens needed to test a full-factorial combination of these factors and levels. Then, using the resulting DOE predictive model, you could build additional ovens and test them, until you achieved the desired Performance Index. Again, this could take a very long time. The solar oven project allows yet another approach to design, that of first building a mathematical model used to predict a value for the Performance Index. Based on knowing the physics of how a solar oven actually works, we are able to write down equations that predict the flow of energy into and out of the oven cavity. For the solar oven, this works because these equations are simple equations with closed-form solutions that are easily solved. This allows you to calculate and predict the internal temperature of the oven cavity. In turn, an equation for the Performance Index can then be written down and solved for, and used as the basis for the actual design of a specific oven. If a spreadsheet is used to solve these equations and to calculate the Performance Index, it is easy to iterate solutions by changing the input variables (in effect, changing the oven design), until the desired P.I. is achieved. (At this point, no time or money has yet been spent on actually building any ovens.) Finally, the oven that achieves the highest calculated P.I. can then be built and tested, using the values of the input variables in the spreadsheet. If the
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This note was uploaded on 04/07/2009 for the course ENGR 102 taught by Professor Likhatchev during the Spring '07 term at Arizona.

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SODB - Solar Oven Basics Approaches to Designing a Solar...

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