Week 10 Flashcards
Direct use of solar energy
heating, cooking, lighting (visible light)
Indirect use of solar energy
Producing electricity:
- solar powered heat engines.
-Solar photovoltaic cells
What is active technology
Uses pumps/fans (powered with electricity)
What is passive technology
converts sunlight into usable heat/stores heat for future use
Give examples of direct and indirect gain for passive building design
Direct: positioning of windows, skylights, shutters etc
Indirect gain: solar radiation captured by part of building- eg water tank
What is direct solar radiation (GD)
Reaches earths surface without being scattered or absorbed by atmosphere.
What is diffuse solar radiation (Gd)
Scattered radiation assumed to be uniform in all directions
what are solar photovoltaic cells
directly converts light into electricity at atomic level.
What is a semiconductor
Inbetween insulator and conductor.
Can display photovoltaic effect- causes them to absorb photons and release electrons- free electrons captured and creates current.
What is the effect on superconductor conductivity with temp, light and adding impurities
Electrical conductivity increases with light, temp and addition of impurities (doping)
What is doping
Intentional process of adding impurities to create more free electrons/holes
What is a n-type material
more electrons than holes
what is a p-type material
more holes than electrons
what are the issues with silicon photovoltaic cells
1)only photons with energy greater than 1.1 eV can activate electrons
2) Excess energy wasted as heat
3) Max efficiency= 23%
Define band gap (Eg)
Energy required to break a covalent bond and free an electron.
Metals have Eg= 0 (lots of free electrons already)
Define electronvolt
Unit of energy. Equal to amount of kinetic energy gained by a single unbound electron when it accelerates through pd of 1V
What is 1 eV equal to
1.6 x 10^-19
what are fundamental power losses in solar cells
Heat losses+ inability to absorb below bandgap energy
What are recombination losses in solar cells
electron-hole is annihilated and effects voltage
what are collection losses in solar cells
loss in current
Developments in solar cells
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
Applications of photovoltaic solar cells
Low power devices- eg calculators, roadside telephones, powering orbiting satellites.
