List the advantages and disadvantages of using photovoltaic (PV) solar cells relative to forms of energy generation utilising fossil fuels.
AD – sustainable electricity no CO2
- Long life, very low maintenance
- noise and pollution free
- modular power source mW to MW
- high level of public acceptance
- only renewable that is suitable for urban environment
- energy payback of 0.5 to 4 years
DIS – PV cannot operate without light
- high initial costs that over shadow the low maintenance costs
- large area needed
- PV generated direct current so inverters are needed
- energy storage is usually needed in off grid applications
Three common types of PV solar cells are: crystalline-Si, Cd-Te and CIGS. Describe how these different types of PV solar cell material are produced.
- crystalline-Si: produced by Czochralski growth of float zone – both are expensive
- Cd-Te: thon film deposition techniques – uses toxic materials so need to consider end of life safety, efficiency has been increasing in the last 5 years
- CIGS: co-evaporation or sputtering process, expensive technique and same effeicey as Cd-Te
Define what is meant by the band gap of a material. Briefly describe the importance of the band gap, in relation to a PV solar cell material
- the band gap is the energy required to free an outer shell electron from its orbit to become a mobile charge carrier
- PV materials rely on illumination for added energy to cause electrons to jump the band-gap. When a photon is absorbed by the material and an electron jumps the fap to the conduction band, an electron-hole pair is formed
Explain how a PV “tandem” solar cell, may achieve a higher overall efficiency.
- efficiency of solar cells can be increased by stacking several cells with different bandgaps so that the gap energy decreases from the top
- the top cells absorbed high energy photons and bottom cells low energy photos so each cells converts part of solar spectrum at max efficiency
What are intrinsic and extrinsic semiconductors
- Intrinsic: thermal energy excites electrons from valence to conduction band
- Extrinsic: elements are added that create holes or electrons
Outline the major materials challenges associated with Concentrated Solar Power (CSP) systems.
- Thermochemical fatigue
- thermal shock if temporary blockage which damages some section of piping
- overheating
- creep
- hot corrosion
- erosion
- stress corrosion cracking
- solidification of molten salts
- combustion of oil
Bulk heterojunction organic PV solar cells can be formed by combining a C60-derivative, with a type of conducting polymer. Describe how these two components are usually mixed and discuss the important attributes of the resulting microstructure.
- 2 components are dissolved in a solvent and spin-coated onto a transparent conductive polymer which is on top of ITO – conducting glass
- if exciton reaches the interface with the fullerene, an electron is transferred to the fullerene phase creating an electron-hole pair
- electrons are collected at the ITO anode by transport through the fullerene phase and the holes are collected at the AI cathode by the transport through the polymer phase.
Describe the constituent components of a typical Dye Sensitized Solar Cell (DSSC). Outline the operating principles of a DSSC.
- thin porous layer of mesoporous titania coated with a monolayer of red ruthenium bipridyl complex which acts as a highly efficient light harvesting material
- electrons injected into the Titania film diffuse to the FTO contact where they are transferred to the external circuit
List 4 out of the main types of Concentrated Solar Power (CSP) system.
- parabolic trough: used to be most common
- solar dish: most efficient
- concentrating linear Fresnel reflector
- solar chimney
- solar power tower
