Photovoltaic Devices – AyjazHasan:
Photovoltaic devices are solar cells that convert sunlight into electricity, and the point of my presentation was to give an overview of how they work; as well as show that it is actually possible for them to potentially meet all of the world’s energy demands if they were made in a large enough supply.
Photovoltaic cells were first used in space applications to power satellites in the 1950s. The operation of a photovoltaic device results from how light interacts with electrons in a semiconducting material.
They are typically made of silicon because silicon has four valence electrons, making it a semiconducting material. As atoms feel more secure if they have 8 electrons in the third energy level (n =3), silicon actually forms bonds with other silicon atoms so that they end up with 8 electrons in their outermost shell.Although there are many other group 4 elements that could be used for photovoltaics, silicon is chosen in particular because it is easy to fabricate and is cheap.
Photovoltaic cells work on a p-n junction. This means taking silicon and on one side of the structure, doping it with a material that has 5 valence electrons, making an electron free to move around (as 4+5 = 9 electrons in outermost shell – so electron rich), whilst doping the other side with a material that has 3 valence electrons, which will create what is called an electron hole, also free to move around (4+3 = 7 electrons in outermost shell – so electron deficient).
The doped “electron rich” n-type semiconductor and doped “electron deficient” p-type semiconductor are then put into contact with each other; and so the electrons will naturally move from one side to the other and so will the holes, so that the electrons fill the holes. Whilst they do this, the electrons will leave behind static positive charges when they cross the middle, and negative charges will be left when the holes cross the middle going the other way.
This creates an electric field, which will act like a fence to stop the electrons going to the other side and vice versa for the holes.Then when the sunlight hits the conductor, the energy from its photons will make the electrons excited, and it is these electrons with the excitation energy that will make them free to move around and will cause them to want to leave the system.
This can then be connected to a device in a circuit that does work to allow the electrons to leave the system at one end; and the holes will also leave at the other end to counteract the imbalance of lost electrons in the system, creating an electric current which ishow the electricity is provided.
The use of photovoltaics is considered into two different categories:
- Off grid -Small units used to recharge batteries in portable electronic devices e.g. calculators.
- Medium-sized units used for camping, emergency battery charging, portable road signs, or power sources in remote areas.
- Larger installations used for residential electric power.
On grid – Exist in a number of different countries and provide power for distribution through the power grid.
The problem with photovoltaics is that different semiconducting materials are sensitive to different regions of the electromagnetic spectrum, which limits the efficiency of photovoltaic devices for converting energy from solar radiation into electrical energy.
This is because photons from the Sun correspond to different energies, depending on their wavelength. When the light is closer to the violet/blue end of the spectrum, the energy is higher. So some semiconducting materials are better at absorbing photons at lower wavelengths than others, which means it is the lower wavelength photons that are more favourable (as E = hc/ λ).
Traditional Si-based semiconducting photovoltaic cells can be close to 20% efficient, but there are other materials which yield a better efficiency e.g. GaAs, InP, CdTe. However, some of the elements in the other materials are toxic(As, Cd), and some are of limited availability (In, Ga).
At the moment, the other energy production methods are more economical than photovoltaics. This is because fossil fuels are relatively inexpensive in comparison to them. However, photovoltaic cells are improving in efficiency. Efficiency improvement techniques that are being made these days include concentrating the light and splitting the light into different spectral components that are incident on cells with specific spectral responses.
The total insolation on the outside of the atmosphere of the earth is 1.8×1017 W (Dunlap, 9.5). On average, about half of this is transmitted through the atmosphere, giving a total insolation at the surface of 9×1016 W. Considering a modest photovoltaic efficiency of 15%, this gives the potential for 1.3×1016 We from photovoltaic generation worldwide. The total primary energy use worldwide is 5.7×1020 J per year for an average power consumption of:
(5.7×1020 J/y)/ (3.15×107 s/y) = 1.8×1013 W. Thus, the utilization of only about 0.14% of the available solar energy would fulfil all of our energy needs.
This makes solar energy the only single renewable energy source capable of fulfilling all of our energy needs. Whilst the technology for potentially creating an abundance of photovoltaic powerplants is well within our means, there are a few drawbacks such as:
- Very expensive.
- Northern/Southern latitudes provide very low efficiency.
- Impractical to put photovoltaic arrays in oceans that are in high insolation areas.
- Arrays need to be in low population/remote areas that experience constant sunlight each day.
In conclusion,solar energy is not as desirable as other energy sources economically. However, anticipated advances in photovoltaic cell design and manufacturing; as well as diminishing fossil fuel resources, are likely to make photovoltaic solar energy an increasingly attractive resource.
References
Dunlap: Sustainable Energy, 9.3 – 9.6