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 summer-why?).

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, evacuated-tube collectors are more expensive than flat-plate 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)

Cost effectiveness• Assume a $5000 system• Pays itself off in 27 years if replacing a natural gas

or oil hot water heating system• 14 years if replacing or supplementing electric

hot water heating• Between 1980 and 1985 there were tax credits

for installing these systems. You could install one up to $10,000 at no personal cost.

• Similar credits have been reinstated in 2005 and in the stimulus package