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Solar Oven Design

Solar Oven Design. Engineering 102 Spring 2010. Solar Ovens. Not just an Academic Exercise Water/milk pasteurization Cooking

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Solar Oven Design

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  1. Solar Oven Design Engineering 102 Spring 2010

  2. Solar Ovens • Not just an Academic Exercise • Water/milk pasteurization • Cooking Designed by solar engineers to be used in sun-rich but fuel-poor areas in the world to improve the quality of life and nutrition of some of the 2.4 billion people who lack adequate cooking fuel • Solar Oven Society

  3. Forms of Energy Mechanical Energy Energy produced from mechanical devices Chemical Energy Energy that is stored in molecular bonds, the forces that hold molecules together

  4. Forms of Energy Thermal (Heat) Energy energy in the process of being transferred from one object to another because of the temperature difference between them. Nuclear Energy Energy that is trapped inside each atom

  5. Forms of Energy Solar/Light/Radiant Energy Energy from the sun Electrical Energy Energy as a result of the flow of charged particles called electrons or ions

  6. First Law of Thermodynamics Energy can not be created or destroyed, but changed from one form to another Goal of a Solar Oven: change electromagnetic (solar) energy into thermal energy, store as heat

  7. Heat Transfer Conduction - solids Convection – gases and liquids Radiation Trap heat/solar energy inside a container Black surfaces adsorb and radiate energy Shiny surfaces reflect light

  8. Solar Oven – Theory At equilibrium: Energy out = Energy in Joules, BTUs, calories Powerout = Powerabsorbed Power is energy/time Joules/sec, BTU/s, hp, Watts Goal is to determine equations that predict Pabsorbed and Pout and ultimately predict the internal Oven Temperature Tio = “Model”

  9. Handout Design • Horizontal top and bottom • Not all sun necessarily gets in • Properly Aim the Oven • Ideally, aimed directly at the sun (90° to the sun)

  10. Figure 2(handout)-Solar Oven Geometry (general)

  11. Pabsorbed -- Factors Sun I0 – incident solar power (W/m2) qS – angle of sun rays with horizon Size or Area (Aw) W – width of glazing L – length of glazing b– angle of window with horizon Material properties of window, oven ● a – absorptivity t – transmissivity

  12. Pabsorbed Sun Insulation Radiation, conduction and Convection

  13. Power Leaving ≡ Pout -- Factors Radiation, Conduction, and Convection Factors A – Area through which energy flows DT – temperature gradient from inside to outside Material U – heat transfer coefficient (radiation, conduction, and convection)

  14. Pout - details P = UADT “U of A” eqn. sb = Sides and Bottom w – Window io – interior oven Ambient – outside oven Window/glazing Sides/Bottom

  15. Balancing Energy (out = in) Pout = Pabsorbed

  16. Rearranging for Tio

  17. Reflectors • Goal is to capture more light • Only FLAT mirrors may be used!! (“no focusing”)

  18. Reflectors • Energy Gain, “G” • Extra solar energy is reflected by the mirrors INTO the oven, and Pabsorbed increases by the ratio G: • Pabsorbed with a reflector = G Pabsorbed without a reflector r – reflectivity of reflector M – height of reflector a – angle of reflected light N - # of reflectors

  19. Solar Oven with Reflectors

  20. Vary M/L:

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