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Solar Oven Design. ENGR 102 Fall 2008 Class Notes. KINETIC ENERGY Energy in motion. POTENTIAL ENERGY Stored energy. General Categories of Energy. What are some more forms of kinetic or potential energy?. Solar/Light/Radiant Energy Energy from the sun

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Solar oven design

Solar Oven Design

ENGR 102

Fall 2008

Class Notes


General categories of energy

KINETIC ENERGY

Energy in motion

POTENTIAL ENERGY

Stored energy

General Categories of Energy

What are some more forms of kinetic or potential energy?


Forms of energy

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


Forms of energy1

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


Forms of energy2

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


Heat transfer
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



Solar ovens
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


Handout design
handout Design

  • Flat bottom, flat top

    • Not all sun gets in

  • Alternative:-Aimed Oven

    • Incident width = window width, W= L


Solar oven theory
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


Power absorbed factors
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



Power absorbed
Power Absorbed

Sun

Insulation

Radiation, conduction and Convection


Power out
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)


Power out details
Power out- details

  • Pout = UADT

  • sb = Sides and Bottom

  • w – Window

  • io – interior oven

  • ambient – outside oven

Window/glazing

Sides/Bottom


Balancing energy out in
Balancing Energy (out = in)

  • Power out = Power absorbed



Predict final oven temp t io use an excel spreadsheet
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)


Solar oven design

Reflectors

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



Reflectors
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


Solar oven design

M/L Ratio

  • Cannot merely make a Wide/Squat vs. Tall skinny Pyramid

    • Much of the sun’s rays would miss window