Week 10

Question 1

Direct use of solar energy

Answer 1

heating, cooking, lighting (visible light)

Question 2

Indirect use of solar energy

Answer 2

Producing electricity:
- solar powered heat engines.
-Solar photovoltaic cells

Question 3

What is active technology

Answer 3

Uses pumps/fans (powered with electricity)

Question 4

What is passive technology

Answer 4

converts sunlight into usable heat/stores heat for future use

Question 5

Give examples of direct and indirect gain for passive building design

Answer 5

Direct: positioning of windows, skylights, shutters etc

Indirect gain: solar radiation captured by part of building- eg water tank

Question 6

What is direct solar radiation (GD)

Answer 6

Reaches earths surface without being scattered or absorbed by atmosphere.

Question 7

What is diffuse solar radiation (Gd)

Answer 7

Scattered radiation assumed to be uniform in all directions

Question 8

what are solar photovoltaic cells

Answer 8

directly converts light into electricity at atomic level.

Question 9

What is a semiconductor

Answer 9

Inbetween insulator and conductor.
Can display photovoltaic effect- causes them to absorb photons and release electrons- free electrons captured and creates current.

Question 10

What is the effect on superconductor conductivity with temp, light and adding impurities

Answer 10

Electrical conductivity increases with light, temp and addition of impurities (doping)

Question 11

What is doping

Answer 11

Intentional process of adding impurities to create more free electrons/holes

Question 12

What is a n-type material

Answer 12

more electrons than holes

Question 13

what is a p-type material

Answer 13

more holes than electrons

Question 14

what are the issues with silicon photovoltaic cells

Answer 14

1)only photons with energy greater than 1.1 eV can activate electrons
2) Excess energy wasted as heat
3) Max efficiency= 23%

Question 15

Define band gap (Eg)

Answer 15

Energy required to break a covalent bond and free an electron.
Metals have Eg= 0 (lots of free electrons already)

Question 16

Define electronvolt

Answer 16

Unit of energy. Equal to amount of kinetic energy gained by a single unbound electron when it accelerates through pd of 1V

Question 17

What is 1 eV equal to

Answer 17

1.6 x 10^-19

Question 18

what are fundamental power losses in solar cells

Answer 18

Heat losses+ inability to absorb below bandgap energy

Question 19

What are recombination losses in solar cells

Answer 19

electron-hole is annihilated and effects voltage

Question 20

what are collection losses in solar cells

Answer 20

loss in current

Question 21

Developments in solar cells

Answer 21

Antireflection surfaces- maximises absorption of radiation and minimises reflection
Passivating layers- protects surface against recombination
multijunction cells (eg tandem)- stacks of cells of different materials=higher efficiency

Question 22

Applications of photovoltaic solar cells

Answer 22

Low power devices- eg calculators, roadside telephones, powering orbiting satellites.