Solar Oven Design

1. Solar Oven Design ENGR 102 Fall 2008 Class Notes

2. KINETIC ENERGY Energy in motion POTENTIAL ENERGY Stored energy General Categories of Energy What are some more forms of kinetic or potential energy?

3. Solar/Light/Radiant Energy Energy from the sun 1000 Watts/m2 at the earth’s surface !!!!! Electrical Energy Energy as a result of the flow of charged particles called electrons or ions Forms of Energy

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

5. 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 Forms of Energy

6. 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

7. solarcooking.org/plans.htm

8. 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

9. handout Design • Flat bottom, flat top • Not all sun gets in • Alternative:-Aimed Oven • Incident width = window width, W= L

10. Solar Oven – Theory • First law of thermodynamics • Energy in = Energy out • Joules, BTUs, calories • Power out = Power absorbed • Energy/time • Joules/sec, BTU/s, hp, Watts • Goal is to determine Power absorbed and Power out and ultimately to predict Oven Temperature Tio

11. Power Absorbed - 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 • t – transmissivity • a - absorptivity

12. Figure 3-Solar Oven Geometry (handout)

13. Power Absorbed Sun Insulation Radiation, conduction and Convection

14. Power out • 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)

15. Power out- details • Pout = UADT • sb = Sides and Bottom • w – Window • io – interior oven • ambient – outside oven Window/glazing Sides/Bottom

16. Balancing Energy (out = in) • Power out = Power absorbed

17. Rearranging for Tio

18. Predict Final Oven Temp TioUse an Excel Spreadsheet • I0 – fixed • Angle of sun – fixed • Position the oven • Window area • Bigger window but heat loss increase • Insulation • Reflectors Uw = f(Tio) Tio = f(Uw)

19. Reflectors • Goal is to capture more light and allow less heat to escape

20. Solar Oven with Reflectors

21. Reflectors • Energy Gain • Some solar energy reflected is adsorbed by reflector and more heat (energy) retained in oven • 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

22. M/L Ratio • Cannot merely make a Wide/Squat vs. Tall skinny Pyramid • Much of the sun’s rays would miss window