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  • 2012 Jim Dunlop Solar

    Chapter 2

    Solar Radiation

    Terminology & Definitions Geometric & Atmospheric Effects Solar Power & Energy

    Measurements & Data

    PresenterPresentation NotesSolar energy is the fuel that creates and sustains life on earth. The nature and characteristics of the solar radiation resource are of fundamental importance in understanding how solar PV systems are designed and perform.

    References: Photovoltaic Systems, Chap. 2National Renewable Energy Laboratory - Renewable Resource Data Center: www.nrel.gov/rredc

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 2

    Overview

    Defining basic terminology associated with solar radiation, including solar irradiance (power), solar irradiation (energy) and peak sun hours.

    Identifying the instruments used for measuring solar radiation.

    Understanding the effects of the earths movements and atmospheric conditions on the solar energy received on the earths surface.

    Locating the suns position using sun path diagrams and defining the solar window.

    Accessing solar radiation data resources and quantifying the effects of

    collector orientation on the amount of solar energy received.

    PresenterPresentation NotesReference: Photovoltaic Systems, p. 25

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 3

    Sun - Earth Relationships

    Sun: Diameter: 865,000 miles (1,392,000 km, 109 times earth) Mass: 2 x 1030 kg (330,000 times earth) Density: 1.41 g/cm3 Gravity: 274 m/s2 (28 g) Surface Temperature: 10,000 F (5800 K)

    Earth: Diameter: 7,930 miles (12,756 km) Mass: 5.97 x 1024 kg Density: 5.52 kg/cm3 Gravity: 9.81 m/s2 (1 g) Typical Surface Temperature: 68 F (300K) Earths Orbit Around Sun: 1 year Earths Rotation about its Polar Axis: 1 day

    93 million miles, average (1.5 x 108 km)

    1 Astronomical Unit (Distance traveled in 8.31 minutes at the Speed of Light)

    PresenterPresentation NotesA comparison of the physical properties for the sun and earth illustrate the vast differences.

    Reference: Photovoltaic Systems, p. 26-27

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 4

    Solar Radiation

    Solar radiation is electromagnetic radiation ranging from about 0.25 to 4.5 m in wavelength, including the near ultraviolet (UV), visible light, and near infrared (IR) radiation.

    Common units of measure for electromagnetic radiation wavelengths: 1 Angstrom () = 10-10 meter (m) 1 nanometer (nm) = 10-9 meter 1 micrometer (m) = 10-6 meter 1 millimeter (mm) = 10-3 meter 1 kilometer (km) = 1000 meters

    NASA

    PresenterPresentation NotesThe sun produces immense quantities of electromagnetic radiation. The tiny fraction reaching the earths surface amounts to approximately 170 million gigawatts (GW), many thousands of times greater that the electrical power used on earth. Note that the United States has just over 1000 GW of electrical power generation.

    Reference: Photovoltaic Systems, p. 26-36

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 5

    Electromagnetic Spectrum

    Wavelength (m)

    near infra-red

    near ultra-violet

    Visible light

    0.3 0.5 0.7

    Wavelength

    Gam

    ma

    rays

    1

    10

    100

    0.1 m

    1 m

    m

    1 m

    10

    m

    100 m

    10 m

    m

    100

    mm

    1 m

    10 m

    100

    m

    103 m

    104 m

    105 m

    X ra

    ys

    Ultr

    avio

    let R

    adia

    tion

    Visi

    ble

    Ligh

    t

    Infr

    ared

    Rad

    iatio

    n

    Mic

    row

    aves

    Shor

    t Rad

    io

    Wav

    es (F

    M/T

    V)

    AM

    Rad

    io

    Long

    Rad

    io W

    aves

    Solar spectrum

    0.25 m 4.5 m

    PresenterPresentation NotesThe electromagnetic spectrum ranges from short wavelength gamma rays and x-rays, to long wavelength radio waves. The solar spectrum includes visible light and near infrared radiation.

    Reference: Photovoltaic Systems, p. 30

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 6

    Solar Irradiance (Solar Power)

    Solar irradiance is the suns radiant power, represented in units of W/m2 or kW/m2.

    The Solar Constant is the average value of solar irradiance outside the earths atmosphere, about 1366 W/m2.

    Typical peak value is 1000 W/m2 on a terrestrial surface facing the sun on a clear day around solar noon at sea level, and used as a rating condition for PV modules and arrays.

    1 m

    1 m One

    Square Meter

    Typical peak value per m2

    1000 watts = 1 kilowatt

    PresenterPresentation NotesSolar irradiance is suns radiant power incident on a surface of unit area, commonly expressed in units of kW/m2 or W/m2. The Solar Constant is the value of solar irradiance outside the earths atmosphere on a surface facing the suns rays, which averages about 1366 W/m2. Due to atmospheric effects, typical peak values of terrestrial solar irradiance are on the order of 1000 W/m2 on surfaces at sea level facing the suns rays under clear sky conditions around solar noon. Consequently, 1000 W/m2 is used as a solar irradiance reference condition for rating the peak output for PV modules and arrays.

    Reference: Photovoltaic Systems, p. 26-29

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 7

    Solar Irradiance

    Time of Day

    Sola

    r Irr

    adia

    nce

    (W/m

    2 )

    Sunrise Noon Sunset

    For south-facing fixed surfaces, solar power varies over the day, peaking at solar noon.

    PresenterPresentation NotesFor south-facing fixed (non-tracking) surfaces on a clear day, the incident solar irradiance varies over the day in a bell-shaped curve, peaking at solar noon.

    Reference: Photovoltaic Systems, p. 28

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 8

    Solar Irradiation (Solar Energy)

    Solar irradiation is the suns radiant energy incident on a surface of unit area, expressed in units of kWh/m2. Typically expressed on an average daily basis for a given month. Also referred to as solar insolation or peak sun hours.

    Solar irradiation (energy) is equal to the average solar irradiance (power) multiplied by time.

    Peak sun hours (PSH) is the average daily amount of solar energy received on a surface. PSH are equivalent to: The number of hours that the solar irradiance would be at a peak level of

    1 kW/m2. Also the equivalent number of hours per day that a PV array will operate

    at peak rated output levels at rated temperature.

    PresenterPresentation NotesSolar irradiation is the suns radiant energy incident on a surface of unit area, commonly expressed in units of kWh/m2/day for monthly averages and a given surface orientation. Insolation is also a term used to represent solar irradiation. Solar energy data are used to estimate the performance of buildings and solar energy utilization systems.

    Reference: Photovoltaic Systems, p. 28-35

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 9

    Solar Power and Solar Energy

    Time of Day

    Sola

    r Irr

    adia

    nce

    (W/m

    2 )

    Sunrise Noon Sunset

    Solar irradiation (energy) is the area under the solar irradiance (power) curve

    Solar irradiance (power)

    PresenterPresentation NotesSimilar to electrical power and energy, solar power and solar energy are related by time. The amount of solar energy received on a surface over a given period of time is equal to the average solar power over the period multiplied by the time. Graphically, solar irradiation (energy) is the area under the solar irradiance (power) curve.

    Reference: Photovoltaic Systems, p. 33

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 10

    Peak Sun Hours

    Time of Day (hrs)

    Sola

    r Irr

    adia

    nce

    (W/m

    2 )

    1000 W/m2

    Sunrise Noon Sunset

    Peak Sun Hours

    Solar Insolation

    Solar Irradiance Area of box equals

    area under curve

    PresenterPresentation NotesPeak sun hours (PSH) represents the average daily amount of solar energy received on a surface, and equivalent to the number of hours that the solar irradiance would be at a peak level of 1 kW/m2 to receive the same amount of energy. It is also the equivalent number of hours per day that a PV array will operate at its peak rated output levels at rated temperature.

    Peak Sun Hours = # equivalent hours @ 1 kW/m2 irradiance.

    Worldwide, the solar energy resource averages around 5 PSH (5 kWh/m2/day) on optimally oriented fixed surfaces facing the sun.

    Reference: Photovoltaic Systems, p. 33

  • 2012 Jim Dunlop Solar Solar Radiation: 2 - 11

    Solar Power and Energy: Examples

    The solar power incident on a surface averages 400 W/m2 for 12 hours. How much solar energy is received? 400 W/m2 x 12 hours = 4800 Wh/m2 = 4.8 kWh/m2 = 4.8 PSH

    The amount of solar energy collected on a surface over 8 hours is

    4 kWh/m2. What is the average solar power received over this period? 4 kWh/m2 / 8 hours = 0.5 kW/m2 = 500 W/m2

    A PV system produces 6 kW AC output at peak sun and average

    operating temperatures. How much energy is produced from this system per day if the solar energy received on the array averages 4.5 peak sun hours? 6 kW x 4.5 hours/day = 27 kWh/day

    PresenterPresentation NotesAverage solar power multiplied by time in hours equals solar energy. Solar power defines the instantaneous output of a PV system, while the amount of solar energy accumulated defines the amount of energy produced and the equivalent n

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