In our last post we expressed the need to find an energy resource which could solve our energy problems not only by providing power but being clean as well. And some say the obvious answer is the sun. And here is a good reason for this; if we can utilize 0.1% of the total energy absorbed by the earth over the course of a year from the sun, we can produce enough energy to eclipse the current energy generation by 10 times! The energy from the sun is mapped in the figure below.
Solar energy has been harnessed by humans since antient times and the technology is still evolving. Other renewable resources including wind, wave power, hydroelectricity and biomass wich are most of the renewable energy resources on the planet are secondary derivatives of the sun. A single post will definately not be enough for this, so here to a sunny summer.
Photovoltaic Cells
Photovoltaic cells translate solar energy to electrical energy because of the photovoltaic effect. The photons (packets of energy) in sunlight include energy corresponding to the diverse wavelengths of sunlight. When light strikes upon a photovoltaic cell these photons may pass through, be reflected or absorbed by the panel. On absorption of photons the energy contained in the photon is redirected to an electron contained in the atom of the semiconductor material the photovoltaic cell is made of.
General photovoltaic cells contain two layers of silicon wafers doped with phosphorous and boron. The layer doped with phosphorus (called the n-Layer) contains excess free electrons and the layer doped with boron has a tendency to attract electrons (called the p-Layer). Though neutral individually when stacked on top of each other a “p-n junction” is formed due to transfer of electron from the “n-Layer” to the “p-Layer” creating a barrier to prevent more electrons from moving between the layers and also reversing their polarity. This leads to poles, negative for the “n-Layer” and positive for the “p-Layer”. When placed under the sun photons in light strikes electrons in the “p-n junction”, energizing them and knocking out the atoms, thus attracting the electrons to the positive n-layer and repelled by the negative “p-layer” and hence generating electricity.
The simplest photovoltaic cell can produce much less energy as it does not fully utilize the entire spectrum of light. Photovoltaic cells are distinguished between each other by the type of the crystal used. As can be seen from the table below, Multi-junction cells boast the highest efficiencies in photovoltaic technology and are best with concentrators, however due to their expense Multi-junction are mostly used in aerospace operations.
Multi-junction photovoltaic cells
Multi-junction photovoltaic cells offer high-performance scientific development passageway for low cost electricity generated by concentrating sunlight. These photovoltaic cells consist of several thin films/layers that allow them to confine more solar spectrum to translate to electrical power. Semiconductors have a distinguishing band gap energy that allows absorption of light (electromagnetic radiation) very efficiently for a certain colour (spectrum portion). These semiconductors are needed to be cautiously selected to take up most of the spectrum of sunlight, leading to higher electricity generation.
Multi-junction PV cells use numerous layers of PV films, made of differing alloys of III–V semiconductor material. The band gap of each layer can be adjusted to allow the absorption of a specific band of electromagnetic radiation from the sun. However, it should be ensured that each layer is lattice matched to the other. The Layers are aligned optically in series with the largest band gap material on top. The top layer receives entire range, the photons with band gap higher than the layer are absorbed and the rest is channelled through to the lower layers.
The best know and available Multi-junction cells for terrestrial use are produced by “Spectrolab” called Ultra Triple Junction (UTJ) solar cells consisting of In0.56Ga0.44P / In0.08Ga0.92As / Ge layers shown in the figure above, on test have shown to provide efficiency of 40.7% with a concentration level of 240 suns and a general efficiency of 24.3 %.
Sources:
Wikipedia, need.org, Us Department of Energy, solarserver.de, Spectrolab
- Energy Engineers to the rescue
- OSSA
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