Chapter3: Solar energy

Question 1

Give Five characteristics of the sun:

Answer 1

• Radiative source
• Diameter dS = 1,392 ⋅ 106 km
• Surface temperature Ts = 5777 K
• Core temperature Tc = 15 Mio.K
• Spherical shape

Question 2

Give 4 characteristics of Earth:

Answer 2

• Spherical shape
• Diameter dE = 1,271 ⋅ 104 km
• Surface temperature Ts = 290 K (varies between -89.2 °C to 56.7 °C)
• Core temperature Tc = 7000 K

Question 3

Name all the angles that describe the positioning of the sun to the earth! draw a figure:

Answer 3

Question 4

When do stationary absorbers in Germany perform best?

Answer 4

Stationary absorbers in Germany perform best with an azimuth angle of 0° (South) and an inclination of 35°.

Question 5

Explain what a photon is and give some of its properties

Answer 5

Photon (derived from Greek: phōs - “Light”) is an elementary excitation of the quantized electromagnetic field

It exhibits properties of both waves and particles —> Wave-particle duality

Photons are massless –> m = 0 kg

Photons move at the speed of light –> In vacuum this is 𝑐0 ≈ 300 000 Km/s

Question 6

How does frequency and wavelength relate to each other?

Give also the relationship between energy and the freq of a photon!

Answer 6

All wavelengths have the same propagation speed, 𝑐₀ in vacuum so

𝑐₀ = 𝜆 ⋅ 𝑓 with 𝑓 = 1 /T

Planck’s constant describes the relationship between energy and frequency of a photon

Energy of an photon: 𝐸 = ℎ ⋅ 𝑓

Planck’s constant ℎ = 6.636 × 10−34 𝐽𝑠

–> The shorter the wavelength (or higher the frequency), the greater is the Energy of the photon

Question 7

Give the spectral range and name few of its ray types!

Answer 7

Question 8

What are the four interactions between electromagnetic wave and a material?

Answer 8

• Absorption: The absorption coefficient 𝛂 indicates how much radiation is absorbed
• Reflection: The reflectance ρ indicates the relationship between incident and reflected radiation
• Transmission: The degree of transmission 𝛕 indicates the relationship between incident and transmitted radiation
• Emission: The emissivity ϵ indicates how much radiation is emitted by the body

Question 9

The darker the body:

• The higher is the degree of absorption alpha ?
• The lower the degree of absorption alpha ?

Answer 9

• The higher is the degree of absorption alpha

Question 10

The emissivity for perfect mirrors is equal to ?

Answer 10

0

Question 11

Complete the following table

Question 12

State Planck’s Law and explain what does it state!

Answer 12

Planck’s law describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T.

• The wavelength of maximum intensity decreases with increasing temperature
• The higher the temperature, the higher is this maximum intensity

Question 13

What did Stefan-Boltzmann law add to the Plancks Law?

Answer 13

The law specifies the thermal power emitted by an ideal black body as a function of its temperature

So for a non black body: we add to the equation the emissivity variable _€(T)

Question 14

• Calculate the radian intensity at the earth orbit.
• Determine the total radiation reaching the earths surface.
• Compare this value with the annual energy demand.

Answer 14

Question 15

Transmission losses in the atmosphere: What factors influence attenuation of solar radiation:

Answer 15

𝜏_𝜆 = 𝜏_𝜆,𝑅 𝜏_𝜆,𝐴 𝜏_𝜆,𝑂3 𝜏_𝜆,𝑊 𝜏_{𝜆<em>,G</em>}

_{<em>Where: </em>}

1. 𝜏_𝜆,𝑅: Rayleigh Scattering by Molecules
2. 𝜏_𝜆,𝐴: Absorption by aerosols
3. 𝜏_𝜆,𝑂3: Absorption by ozone
4. 𝜏_𝜆,𝑊: Absorption by Water Vapor
5. 𝜏_{𝜆<em>,G</em>}: Absorption by air

Question 16

Write down which type of absorption figure in theabsorption gaps in the following figure:

Answer 16

Question 17

What types of solar irradiation exist?

Answer 17

1. Direct radiation: is reffered to a surface perpendicular to the incidence angle
2. Diffuse= Ideal diffuse+ Real diffuse

–> Global irradiation is the sum of direct and diffuse solar irradiation and referred to a horizontal surface.

Question 18

What measurement techniques are used to measure solat radiation, what is the difference between them?

Answer 18

1. Pyranometer
2. Pyrheliometer

Pyranometer vs Pyrheliometer

• Pyranometer is a dome like structure that measures diffused sun energy while Pyrheliometer is an instrument that measures direct sun’s energy.
• Both are often used in conjunction in meteorological research stations.
• While Pyranometer measures global solar radiation, Pyrheliometer measures direct solar irradiance.

Question 19

How does the actual use of photovoltaics look like in the world? For example Australia vs USA

Answer 19

Question 20

How is photon energy converted into electrical energy? Explain the semiconductor

Answer 20

A semiconductor consists of two bands with a gap in between. When an incoming photon has higher energy than the gap, it excites the electrons and frees them from their bonds - a charge is generated. Next, the electric field sweeps out the electrons, they move from full to empty band and therefore the charge is separated. Then, the charge is collected when electrons deposit their energy in the external load and the circuit is complete.

Question 21

Photovoltaic Device Fundamentals ?

Answer 21

1) Charge Generation:Light gives electrons enough energy to escape from their valence band to the conduction band
2) Charge Separation : the fact that we put a N-type next to the P-type, we induce an electric field that allows for a potential to build up within this PN region that acts as a barrier stopping the charges ( + / - ) .
3) charge collection : Electrons are freed by Photon energy in the edges of the N region , a short circuit is applied from N regions to P regions , electrons go through that , depositing their excess energy into an external load rejoining then the P region looking for a hole to fill

Question 22

Name the four types of Si mostly used in PV cells in Germany:

Answer 22

1. Thin Film
2. Mono-Si
3. Multi-Si
4. Ribbon-Si

Question 23

Compare the Single Crystalline Silicon with the multi-cristalline silicon:

Answer 23

Question 24

Maximum power point tracking, what’s this?

Answer 24

As the amount of sunlight varies, the load characteristic that gives the highest power transfer efficiency changes, so that the efficiency of the system is optimized when the load characteristic changes to keep the power transfer at highest efficiency. This load characteristic is called the maximum power point and MPPT is the process of finding this point and keeping the load characteristic there. Electrical circuits can be designed to present arbitrary loads to the photovoltaic cells and then convert the voltage, current, or frequency to suit other devices or systems, and MPPT solves the problem of choosing the best load to be presented to the cells in order to get the most usable power out

Question 25

Name 3 possibilities to further increase the efficiency of solar cells

Answer 25

1. Tandem cells: two PV cell semiconductors on top of each other, so that not-absorbed photons by one go to the other
2. Concentrator PV cells (CPV): use reflectors and filters to increase the light
3. Thermo-photovoltaic conversion: allow only light from a certain spectrum to reach the solar cell through a filter

Question 26

What are the commercial efficiencies of PV cells?

Answer 26

Question 27

What are the production steps of silicon solar cells?

Answer 27

1. Sand (SiO2) is reduced by adding coke, metallurgical or raw silicon is formed.
2. Purificiation by means of HCl.
3. Sawing of (round) silicon wavers.
4. Polishing and etching for texture.
5. Doping of p-n junction.
6. Antireflective coating to reduce losses.
7. Application of contact fingers.
8. Testing

Question 28

What are the detailed types of existing pv cells

Answer 28

Question 29

What are technical limitations of solar PV?

Answer 29

1. Light has to come into solar cell (avoid reflections on the surface)2. Optical losses: not all photons are absorbed3. Impurities4. Electrotechnical limitations

Question 30

Of which two components would an ideal solar heat engine consist of?

Answer 30

black absorber + carnot engine

Question 31

What is the absorbing efficiency of a Solar thermal power plant and how can it be improved?

Answer 31

efficiency = 1 - (Ta^4 / Ts^4)

–> the efficiency can be improved by keeping the temperature of the absorber as cold as possible

Question 32

What´s the maximum efficiency of solar energy conversion?

Answer 32

absorber efficiency * carnot efficiencyefficiency_max = (1 - (Ta^4 / Ts^4)) * (1 - T0/Ta)

Question 33

There are two types of solar thermal power plants?

Answer 33

• Using concentrated incident solar radiation on an absorber (CSP – Concentrated Solar Power)
• Without using concentrated solar radiation

Question 34

Where do we have heat losses in a solar thermal power plant?

Answer 34

Question 36

What are the types of concentrating solar heat plants?

Answer 36

1. Parabolic trough collector
2. Heliostat collector
3. Fresnel collector
4. Paraboloidal Collector

Question 37

Which types of concentrated solar thermal are still under development ?

Answer 37

Paraboloidal Collector: Temperature: 150-1500 °C Absorber efficiency: + Costs: – Concentration factor: 150-1500 Status: development Tracking: 2-Axis Note

Heliostat collector: Temperature: 300-2000 °C Absorber efficiency: + Costs: - Concentration factor: 150-1500 Status: development Tracking: 2-Axis

Question 38

What are the components of a parabolic through collectors:

Answer 38

1. • Absorber: Solar radiation is absorbed and converted into heat
2. • Receiver: Includes absorber and transfers heat to the heat transfer fluid
3. • Aperture: Area which receives solar radiation
4. • Reflector: Mirror which concentrates solar radiation to absorber
5. • Collector: Entity of receiver and reflector

Question 39

What types of reflectors exist in the market?

Answer 39

• Glass reflector:
  Composition: Acrylic glass layer, silver foil, copper plate, primary, intermediate and outer layer - Weight, high losses, brittleness + High chemical and thermal stability
• Alumina reflector
  Composition: Protection overcoats, Reflective layer and polished alumina as support structure - Little protection, short lifetime, expensive + Low weight, ρ upto 98 %
• Silver reflector
  Composition: Top coat and bonding layer, reflective layer, flexible polymer support structure - Little protection + Low weight and high flexibility, low costs, high lifetime, easy to mount.