chp. 10 solar radiation

sun composition

80% is hydrogen, 19% is helium, remaining 100 plus observed elements makeup only a tiny fraction

comprised of many layers of gases, hotter towards its center

source of sun’s energy

hydrogen to helium thermonuclear reaction (unclear what the mechanism is)

temperature on the surface of the sun

~5760K

rate of energy emission from the sun

3.8 × 10²3 kW, about 1 billionth (1.7×10^14 kW) of its energy falls on earth

important for terrestrial users of sun’s radiation:

amount of energy, spectral and temporal distribution, and its variation with time and day of the year

solar energy is

the world’s most abundant source of energy

amount is 5000x greater than sum of all other inputs

of amount of energy intercepted by earth,

30% is reflected to space, 47% converted to low temp heat and reradiated to space, and 23% powers the evaporation/precipitation cycle, less than 0.5% is represented in the kinetic energy of wind and waves and in photosynthetic storage

solar radiation on land is

about 1/3 of total (1.5×10^17), which is 6000x total energy usage in the US

70% falls on oceans

thermal radiation fundamentals

wave lengths

visible light- the rainbow

index of refraction (n)

varies with wavelength linearly because different wavelengths interfere to different extents with the atoms of the medium

blue (shorter wavelength) is slowed down more by glass than red (longer wavelength)

thermal radiation

-one kind of electromagnetic energy

-all bodies emit thermal radiation by virtue of their temp

-when a body is heated- atoms, molecules, electrons raised to higher levels of activity (excited states), but tend to return to lower energy states- this process emits energy in the form of EM waves

changes in energy states

result from rearrangements in electronic, rotational, and vibrational states of atoms and molecules

electromagnetic energy spectrum

from high energy (cosmic waves) to low energy (radio waves)

high energy has short wavelength

thermal radiation- 0.1-100 micrometers

solar radiation- 0.1-3 micrometers

two ways to treat electromagentic energy

waves- c= lamda(v)

particles- Ep=hvp

h- Planck’s constant (6.625 × 10^-34 Jxs)

classical wave theory

may not work for photovoltaic or photochemical processes- use view from quantum mechanics- treating EM radiation as particles

energy transported by

particles or photons (energy units)

what is an electron volt (eV)

amount of kinetic energy gained by a single electron accelerating from rest through a vacuum

1 eV- 1.602 × 10^-19 J

blue light (shorter wavelength, higher frequency) has more energy than red light (longer wavelength, lower frequency)

radiator

something that radiates energy because its hot

perfect radiator- black body

does not exist in nature, but useful in engineering because its properties can be related to those of real bodies

energy density of radiation emitted at a given wavelength

total energy emitted by a black body (Eb)

integration over all wavelengths

stefan-boltzmann law

astronomers use

color of the stars to take their temperature, use simple concepts about blackbody radiation

total energy emitted varies

as the 4th power of surface T

energy density of radiation emitted

at a given wavelength varies as T varies

as peak wavelength emitted becomes shorter as it becomes hotter, photons have more eV

beer’s law

intensity of radiation of wavelength passing through transparent medium decreases as a function of distance travelled and extinction coefficient (K lamda)

extinction coefficient (K lamda)

combines effects of absorption, emissions, and scattering caused by atoms and molecules in that medium (air, clouds, dust)

concerned with transmission of solar radiation through the atmosphere

atmosphere consists of: molecules of gases in it, N2, O2, CO2, H2O, and aerosols such as dust particles, water droplets, and ice crystals

extinction processes of the atmosphere

1. absorption and emission by the molecules and aerosols

2. scattering by the molecules

3. scattering by aerosols

for solar energy, we want extinction coefficent

as low as possible

incoming solar radiation can be

reflected by clouds, absorbed and scattered by the atmosphere or allowed to reach the surface

primary gases responsible for most atmospheric absorption

water vapor, carbon dioxide, ozone

clouds are the enemy of solar energy

clouds absorb and reflect incident solar radiation, impeding ability of PV cells and solar thermal mirrors to capture light and generate energy

rain might

help solar panels as it cleans the surface and cleans dust off

solar panel soiling

soiling= shading= power less

any material which blocks, scatters or reflects sunlight

solar irradiance

power per unit area received from the sun in the form of electromagnetic radiation

west and south texas are good places for solar- cheap land, little cloud sover

distance between earth and sun changes through the year

minimum distance at winter solstice

maximum distance at summer solstice

average earth sun distance

1.496 × 10^11 m

amount of solar radiation interception varies throughout the year

maximum on december 21st (winter solstice)

minimum on june 21st (summer solstice)

earth’s rotational axis

23.45 degrees

this tilt is the major cause of seasonal variation- solar declination causes seasons

solar declination

angle between the earth sun line (through their centers) and the plane through the equator

declination varies between -23.45 on dec. 21 and +23.45 on june 21

tropics

tropic of cancer- 23.45 N

tropic of capricorn- 23.45 S

extreme latitudes where sun is overhead at least once a year

circles

arctic circle- 66.5 degrees N

antarctic circle- 66.5 degrees S

latitudes above which sun does not rise above horizon plane at least once a year

sun in ______ in the sky in the winter and ________ in the sky in the summer

lower, higher

*big implications for solar energy generation

days are ______ in the summer

longer

position of the sun can be described by two angles

solar altitude angle- angle between line collinear with sun rays and horizontal plane

azimuth- angle between due south line and projection of the site to sun line

positive west of south and negative east of south

solar zenith angle

angle between site to sun line and vertical at the site

solar hour angle

based on nominal time of 24 hour required for sun to move 360 degrees around earth or 15 degrees per hour

morning- negative

night- positive

information about solar radiation availability

at any location essential for solar energy system

long term available for many places and in other places climatic data is sued

solar energy is in the form of EM radiation

wavelengths ranging from 0.3 micrometers to over 3 microns corresponding to UV, visble, infrared

radiation wavelengths

UV- <0.4 microns

visible- 0.4-0.7 microns

infrared- 0.-3 microns

most solar energy is concentrated in

the visible and near infrared wavelength range (VNIR)

incident solar radiation (insolation)

irradiance or energy per unit area (energy/area-time)

units for solar radiation

watts per square meter, british thermal units, langleys per minute (calorie/cm²min)

solar constant (I0)

average irradiation falling on a surface normal to the rays of the sun outside the atmosphere of the earth at a mean earth-sun distance (D0)

nominal solar constant

I0- ~1366 W/m²

TSI

total solar irradiance

how does solar radiation for earth vary over the year

variation in seasonal solar radiation availability

can be understood from geometry of relative movement of the earth around the sun

since orbit is elliptical- earth sun distance varies- variation being ± 17% from the average

extraterrestrial solar radiation (I)

part is reflect back into space, part absorbed by air and water vapor, some scattered

direct or beam radiation

part that reaches the earth with essentially no change in direction

sky diffuse radiation

scattered diffuse radiation reaching surface from the sky

uncertainty of radiation levels on earth

extraterrestrial radiation can be predicted with certainty, but on earth subject to uncertainty (± 30%) from local climatic interactions

so most useful solar radiation data- based on long-term (>30 years), not available for for most locations

as solar radiation travels through atmosphere

attenuated due to absorption and scattering

solar radiation on clear days

total solar radiation on horizontal surface is the sum of the beam or direct radiation and sky diffuse radiation

solar radiation output: need collector area, azimuth, collect tilt, latitude, sun rise

clearness number

C= 1 on clear days

typical meteorological year (TMY) data to determine solar radiation

used to obtain what will be insolation on a solar collector at a certain site

gives average energy yield- reference

can be used to find total insolation- sum hourly values of lc

national solar radiation database

compiled by NREL

collection of hourly and half hourly values of 3 most common measurements of solar radiation- global horizontal, direct normal, and diffuse horizontal irradiance

collected at sufficient number of locations and temporal and spatial scales to represent solar radiation climates