Solar Oven Design ENGR 102 Fall 2008 Class Notes

General Categories of Energy KINETIC ENERGY Energy in motion POTENTIAL ENERGY Stored energy

Forms of Energy Solar/Light/Radiant Energy Energy from the sun 1000 Watts/m 2 at the earth’s surface !!!!! Electrical Energy Energy as a result of the flow of charged particles called electrons or ions

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

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

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

solarcooking.org/plans.htm

Solar Ovens Not just an Academic Exercise Not just an Academic Exercise Water/milk pasteurizationWater/milk pasteurization CookingCooking 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 Solar Oven Society

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

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

Power Absorbed - factors Sun Sun I 0 – incident solar power (W/m 2 )I 0 – incident solar power (W/m 2 )  S – angle of sun rays with horizon Size or Area (A w ) Size or Area (A w ) W – width of glazingW – width of glazing L – length of glazingL – length of glazing – angle of window with horizon Material properties of window, oven Material properties of window, oven  – transmissivity a - absorptivitya - absorptivity

Figure 3-Solar Oven Geometry (handout)

Power Absorbed Sun Radiation, conduction and Convection Insulation

Power out Radiation, Conduction, and Convection Radiation, Conduction, and Convection Factors Factors A – Area through which energy flowsA – Area through which energy flows ● T – temperature gradient from inside to outside MaterialMaterial U – heat transfer coefficient (radiation, conduction, and convection) U – heat transfer coefficient (radiation, conduction, and convection)

Power out- details P out = UAT P out = UAT sb = Sides and Bottom sb = Sides and Bottom w – Window w – Window io – interior oven io – interior oven ambient – outside oven ambient – outside oven Sides/Bottom Window/glazing

Balancing Energy (out = in) Power out = Power absorbed Power out = Power absorbed

Rearranging for T io

Predict Final Oven Temp T io Predict Final Oven Temp T io Use an Excel Spreadsheet I 0 – fixed I 0 – fixed Angle of sun – fixed Angle of sun – fixed Position the oven Position the oven Window area Window area Bigger window but heat loss increaseBigger window but heat loss increase Insulation Insulation Reflectors Reflectors U w = f(T io ) T io = f(U w )

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

Solar Oven with Reflectors

Reflectors Energy Gain Energy Gain Some solar energy reflected is adsorbed by reflector and more heat (energy) retained in ovenSome solar energy reflected is adsorbed by reflector and more heat (energy) retained in oven P absorbed with a reflector = G P absorbed without a reflectorP absorbed with a reflector = G P absorbed without a reflector r – reflectivity of reflector M – height of reflector  – angle of reflected light N - # of reflectors

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