Solar Oven Design Engineering 102 Spring 2010

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

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

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

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

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

Solar Oven – Theory At equilibrium: At equilibrium: Energy out = Energy inEnergy out = Energy in Joules, BTUs, calories Joules, BTUs, calories Power out = Power absorbedPower out = Power absorbed Power is energy/time Power is energy/time Joules/sec, BTU/s, hp, Watts Joules/sec, BTU/s, hp, Watts Goal is to determine equations that predict P absorbed and P out and ultimately predict the internal Oven Temperature T io = “Model”Goal is to determine equations that predict P absorbed and P out and ultimately predict the internal Oven Temperature T io = “Model”

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

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

P 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  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 – angle of window with horizon Material properties of window, oven Material properties of window, oven ● a – absorptivity  – transmissivity  – transmissivity

P absorbed Sun Radiation, conduction and Convection Insulation

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

P out - details P = UAT P = UAT “U of A” eqn. “U of A” eqn. 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) P out = P absorbed P out = P absorbed

Rearranging for T io

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

Reflectors Energy Gain, “G” Energy Gain, “G” Extra solar energy is reflected by the mirrors INTO the oven, and P absorbed increases by the ratio G:Extra solar energy is reflected by the mirrors INTO the oven, and P absorbed increases by the ratio G: 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

Solar Oven with Reflectors

Vary M/L: