Transcript Solar Power - Astronomy at Western Kentucky University

Solar Power
• Power derived directly from sunlight
• Seen elsewhere in nature (plants)
• We are tapping electromagnetic energy and
want to use it for heating or convert it to a
useful form, usually electricity
• Renewable-we won’t run out of sunlight (in its
current form) for another 4-4.5 billion years
Solar Energy
• Sun derives its energy from nuclear fusion deep
in its core
• In the core Hydrogen atoms are combining
(fusing) to produce helium and energy.
• Physicists refer to this as Hydrogen burning,
though be careful, it is not burning in the usual
(chemical) sense.
• The supply of H in the sun’s core is sufficient to
sustain its current rate of H burning for another
4-4.5 billion years
Solar Energy
• The energy is released in the H burning deep in
the sun in the form of photons.
• Here we use the particle description of light,
where light is considered a packet of energy
called a photon.
• Photons have energy E=hν or E =hc/λ where ν is
the frequency of the light, λ is the wavelength of
the light, c is the speed of light (c=3.00x108m/s)
and h is Planck’s constant (h=6.626068 × 10-34
m2 kg / s)
Solar energy
• The photons take a long time to reach the surface of the sun, about
1 million years.
• Why? Deep in the sun, the density is very high. The photons travel a
very short distance before they are absorbed by an electron in an
atom.
• Normally in an atom, the electrons occupy specific positions
relative to the nucleus called energy levels.
• When the electrons are in the lowest energy levels possible, they
are said to be in the ground state.
• When an electron in an atom absorbs a photon, it gains more
energy and moves to a new (higher) energy level.
• It can only gain a photon with the correct energy to change energy
levels. The photon energy must equal the energy difference
between two energy levels in the atom.
Solar energy
• But electrons don’t like to be in these higher
energy states, so they will emit energy in the
form of a photon to drop to a lower energy
level.
Solar energy
• So in the sun, the photons emitted by the H
burning travel a short distance before they are
absorbed by an atom.
• The atom quickly re-emits the photon, but not
necessarily in the same direction it came from.
The atom can re-emit the photon in any
direction.
• The photon follows a random looking path on
its way out of the sun, called a random walk.
Random walk
• So the photons take this
random walk form the
core to the surface of the
sun.
• On average, it takes 1
million years before a
photon generated in the
core leave the surface of
the sun.
• It then takes another 9
minutes to reach the
Earth
Solar spectrum
• The photons emitted from the sun have a
range of energies, and therefore via Planck’s
law a range of frequencies and wavelengths.
• The distribution of the number of photons
(intensity) as a function of wavelength( or
frequency or energy) we call a spectrum.
• The maximum energy is at optical
wavelengths
Solar Spectrum
Energy from the sun
• We can measure the amount of incoming energy from the sun by
something called the solar constant
• 1,366 watts/m2 with fluctuations of almost 7% during the year.
• This measures the energy at all electromagnetic wavelengths at the top of
the atmosphere
• What reaches the ground (where a solar device would be ) is less
• By the time we take into account the effect of the Earth’s rotation, the
different angles of sunlight at different latitudes, we find that the average
intensity of sunlight is reduced by ¾.
• Then you have to consider how much is absorbed in the Earth’s
atmosphere, which reduces it further, so only 47% of the average makes it
to the surface of the earth, or about 160 watts/m2
• This is for a 24 hour day, averaging over an 8 hour day gets you about 600
Watts/m2 or 1520 BTU/ft2. This is often referred to to as the solar
insolation (varies from 300 in the winter months to 1000 in the summerwhy?).
How much makes it through the
atmosphere
Why a seasonal variation?
• First, why do we have seasons?
• Earth’s axis is tilted 23.5° to the plane of its
orbit
Why such a large seasonal variation
• In the Northern
hemisphere, the sun’s
rays fall more directly on
the earth than in the
winter.
• Heating is most efficient
when the suns rays strike
the surface ay 90°
(right)angles.
• So a solar energy device
should be oriented so
that the sun’s rays hit it at
right angles.
How is energy transferred
• Convection-Energy is carried by blobs of
material that are moving in a medium for
example -hot air rises, cold air sinks
• Conduction-energy transfer between two
objects that are in contact
• Radiative transfer-energy transferred through
the successive absorptions and emission of
photons
Types of solar heating and cooling
• Active
• Use a fluid forced
through a collector
• Need an external
energy source to drive a
pump
• Passive
• Design the structure to
make use of the
incident solar radiation
for heating and cooling
• No external energy
source
Active Solar heating
• Used for space and or water heating
• Flat plate collector system
Elements of a flat plate collector
• Cover (also called glazing) protects the system and
keeps heat in.
• Absorber plate-absorbs solar energy. Usually made of a
metal that is a good conductor of heat such as
aluminum or copper and painted with a coating that
helps absorb and retain the heat (black paint is the
lowest order of these types of coatings)
• Insulation on the bottom and sides to reduce heat
losses.
• Flow tubes –air or fluid to be heated flows though
these tubes
How does this work?
• Cover is transparent to sunlight, so the light passes through the
cover to the absorber.
• The absorber will absorb energy from the sunlight and then try to
re-emit it to come into thermal equilibrium with its surroundings.
But the absorber re-emits the energy at infrared wavelengths.
• Glass allows visible but not infrared radiation to pass through, so
the energy emitted by the absorber is absorbed by the glass.
• The glass re-emits this energy to the outside air and back into the
collector.
• The energy trapped in the collector heats the inside of the collector,
and this energy is transferred to the air or fluid in the tubes via
conduction
How does this work?
• The energy emitted from a hot surface is
described by Stefan’s Law:
P/A = εσT4
Where ε is the emissivity (describes the degree
to which a source emits radiation, ranges from
0 (no emission) to 1 (a perfect emitter) and σ
is the Stephan-Boltzman constant = 5.67 x 10-8
W/m2 K4. P/A is the power emitted per unit
area, T is the temperature in Kelvin.
How does this work?
• The wavelength at which this energy is
emitted from the surface is described by the
Wien Displacement Law:
λmax(μm)= 2898
T(K)
This gives the wavelength at which
an object emits the maximum
amount of energy
Types of flat plate collectors
• Liquid flat-plate
collectors heat liquid as
it flows through tubes
in or adjacent to the
absorber plate.
• Often unglazed
Types of Flat plate collectors
• Air flat-plate collectors – used for
solar space heating.
• The absorber plates in air
collectors can be metal sheets,
layers of screen, or non-metallic
materials.
• The air flows past the absorber by
using natural convection or a fan.
• air conducts heat much less
readily than liquid does, less heat
is transferred from an air
collector's absorber than from a
liquid collector's absorber, and air
collectors are typically less
efficient than liquid collectors
Types of Flat Plate Collectors
•
•
•
Evacuated Tube collectors -usually made of
parallel rows of transparent glass tubes. Each
tube contains a glass outer tube and metal
absorber tube attached to a fin. The fin is
covered with a coating that absorbs solar
energy well, but which inhibits radiative heat
loss.
Air is removed, or evacuated, from the space
between the two glass tubes to form a
vacuum, which eliminates conductive and
convective heat loss.
Evacuated-tube collectors can achieve
extremely high temperatures (170°F to
350°F), making them more appropriate for
cooling applications and commercial and
industrial application. However, evacuatedtube collectors are more expensive than flatplate collectors, with unit area costs about
twice that of flat-plate collectors.
Limitations
• Need a storage system for cloudy days and nights.
• Amount of solar energy that is usefully collected is
50%.
• To heat 100 gallons of water a day from a temperature
of 50° to 120° you need a collector with a surface
area of 112 square feet. That is one panel 9 ft x 14 ft.
This would fill a good portion of our classroom
• Where do you put it? In the back yard, on the roof?
• Are there structural, aesthetic considerations? (Al
Gore’s troubles with installing solar panels)