1. 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 billion years

2. 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 billion years

3. 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= × m2 kg / s)

4. 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.

5. 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.

6. 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.

7. 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

8. 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

9. Solar Spectrum

10. 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?).

11. How much makes it through the atmosphere

12. Why a seasonal variation?
First, why do we have seasons?
Earth’s axis is tilted 23.5° to the plane of its orbit

13. 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.

14. 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

15. 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

16. Active Solar heating Used for space and or water heating
Flat plate collector system