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Photovoltaic Solar Energy

Solar energy is a vital building block for our future global sustainable energy scheme. Its potential is almost unlimited and it can be applied in sunny regions as well as in less favourable

conditions. Photovoltaic conversion is very versatile as it can be used from a MW to a GW scale.It can be used for consumer products, solar home systems, rural use, building integrated systemsand large-scale power plants. Photovoltaics is a field of technology and research in which devicesdirectly convert sunlight into DC electricity. Solar cells are the basic building blocks of  photovoltaic technology. Solar cells are made of semiconductor materials, such as silicon, whichare presently the most common. An important property of such semiconductors which makesthem very useful is that their conductivity may be easily altered by introducing impurities intotheir crystal lattice structures.

Figure ##1: Basic Photovoltaic Cell [##1]

When sunlight strikes the solar cell, the semiconductor material absorbs some of the light, photons. This energy triggers the release of electrons, allowing them to flow freely. As previouslystated, by introducing impurities (doping) such as phosphorus and boron atoms into the solar cell,the conductivity can be altered. Therefore the one side of the cell contains phosphorous atoms,with five valence electrons (one more than pure silicon), and the other side contains boron, withthree valence electrons (one less than pure silicon). The phosphorous atoms donate their valenceelectrons to the silicon thus creating negatively charged carriers, N-type. The boron atoms on theother side of the silicon have a greater affinity to attract electrons than the silicon material, P-type. Electrons diffuse from a region of high concentration, phosphorous, to a region of low

concentration, boron. When the electrons diffuse across the silicon, they combine again with freeholes on the boron side of the silicon. This imbalance of charges on either side of the siliconcreates an electric field which acts as a diode. This electric field encourages current to flow in acertain direction. Metal contacts are made to both sides of the solar cell, thus allowing the currentto flow to an external load. So in summary, photons from the sunlight strike the solar cellstransferring their energy to the charge carriers. The electric field across the solar cell separates the positively charged carriers from the negatively charged electrons, thus forming an electric currentwhen the circuit is closed and the field provides the voltage, thus power is produced. An anti-

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reflective coating covers the silicon because silicon is shiny material thus reflecting some of the photons from the sun light [##13].

 Figure ##2: Wafer based crystalline silicon [##4]

There are several different types of solar cells, but the most common consist of wafer-based

silicon cells, as seen in figure 3. These types of solar cells can either be cut from a single crystalrod, mono-crystalline, or from a block composing of many crystals, multi-crystalline. Single-crystal wafer cells are generally more expensive because they are cut from cylindrical ingots andthus do not completely cover a solar cell module without space being wasted. Multi-crystallinecells are less expensive to produce than single crystal silicon cells but are less efficient. Thethicknesses of the wafer-based silicon solar cells vary between 100 μm and 200 μm thick [##1].Solar modules made from wafers of crystalline silicon have been the dominant technology in land based photovoltaics for many years due to the massive resources and expertise available from themicro-electronics industry. Wafer-based silicon cells also have a record of being very reliable andthe cost is consistently dropping. Even though silicon is an extremely abundant raw material, the processing required to achieve the necessary purity of the silicon is very expensive. Significanteffort and money has been invested into reducing silicon consumption and developing new, less

energy-intensive techniques for silicon solar cells. Crystalline silicon devices are nowapproaching the theoretical limiting efficiency of 29% [##9].

Figure ##3: Thin Film Technology [##5]

Another type of solar cell technology is based on thin-films, figure 2, which range between 1 μmand 2 μm in thickness and therefore require significantly less semiconducting material [##5].Thin-film solar cells can be manufactured at a lower cost and in larger quantities and hence their market for them will likely increase in the future. However, they provide lower efficiencies thanwafer-based silicon solar cells, which means that a greater surface area is required and hence

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more material for the installation is required for a similar performance. Thin-film solar modulesare produced by depositing thin films directly onto large glass panels or long foils. Althoughefficiencies of thin-film materials are presently lower than those of crystalline silicon, thin-filmtechnology offers lower cost per watt and is thus growing rapidly and is expected to account for 31 percent of the global installed power by 2013 [##2].

In conjunction with the efforts to decrease the price of present day solar modules, more essentialresearch is being carried out with the aim of developing drastically lower cost or higher efficientmodules in the long term. Radical concepts incorporating and enabling technologies such asnanotechnology, (which aim to modify the active layer to better match the solar spectrum, or tomodify the incoming solar radiation before it strikes the active layer), are being researched .Closer to today’s market are the technologies of organic solar cells or even more so, concentrator  photovoltaics with have significant advances in recent years. This technology works byconcentrating direct sunlight onto a small but efficient material. Concentrator photovoltaics havethe possibility of reaching efficiencies of over 30 %, which have not been achieved by any other  photovoltaic system [##6].

Figure ##4: Solar Module [##7]

The problem with using a single solar cell is that it does not provide a lot of electricity, so byconnecting a number of solar cells together and mounting them in a single frame to form a photovoltaic module, a higher more usable amount of electricity can be extracted, as seen infigure 4. These modules have been designed to supply a set amount of voltage such as 12V. Thecurrent on the other hand is directly proportional to the intensity of the light striking the cells. To produce even more electricity, these modules can be wired together to form an array which can beconnected in both parallel and series depending on the required voltage and current combination.The solar cells in the modules and arrays produce direct current electricity which can be useddirectly to charge a battery or run certain equipment, but this is not necessarily a good thing because the integrated grids of many countries supply the public with AC electricity. There aretwo main types of photovoltaic systems namely Grid-connected systems and autonomoussystems. Grid-connected systems are connected to the grid and provide electricity straight into the

grid and thus the direct current produced by the solar modules has to be converted into AC. Solar inverters are used to change the DC electricity produced by the cells into AC current which canthe be used by the public once passed into the integrated grid system. Autonomous systems on theother hand can be operated without the need of the integrated grid system. The majority of  photovoltaic systems worldwide are currently implemented as grid-connected systems. Trackersand sensors are used to optimise the performance of solar cells. Tracking systems can increase

feasible output by up to 100% [##8].

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Figure ##5: Solar Panels on a Satellite [##5]

Advantages of harnessing solar power in the form of photovoltaic systems are as follows; solar  power is pollution free and safe, photovoltaic installations can operate for a long time with littlemaintenance (ideal for satellites, figure 5), isolated locations do not need power from gridsystems therefore saving money on power lines, solar power is near limitless and of high density

in most places around the world, there is very little research done on this technology which meansthat there is room for improvement in the near future. Disadvantages of this technology include; photovoltaics are expensive to install (roughly $10 per Watt in the U.S.)[##4], unless programssuch as “feed-in tariffs”[##3] are implemented it becomes very expensive, solar energy is not produced at night and is reduced with cloudy conditions, cells produce DC which needs to beconverted into AC which incurs a small amount of energy loss.

In Conclusion, in order for photovoltaic to be employed on a significant scale, the cost of solar electricity needs to be reduced. This cost is mainly derived from the raw material, (high puritycrystalline silicon), and the low density of energy gained from the photovoltaic collectors. Theonly way forward is by creating thinner and more efficient silicon wafers or by introducing newtechnologies which are more efficient. The newer thin film cells are cheaper but less efficientthan the wafer type cells therefore more material is required to achieve similar efficiencies.However solar array power plants have been successful and can produce 40-60kW of power at$5/W at efficiencies as high as 27%, but the area required for such power plants is very large[##7].

Photovoltaic concentrators are commonly used to produce energy for stand alone geysers. Byconsidering the amount of solar radiation hitting the earth in a certain region and the concentrator yield, a formula can be used to calculate the area required of photovoltaic material. 

Cy = Sr × ηc × ηsys; [##11]

• Cy is the concentrator yield in kWh/m²

•Sr is Solar radiation in kWh/m².day

• ηc is the concentrator efficiency

• ηsys is the system efficiency

The formula used to calculate the amount of energy produced by photovoltaic cells is as follows;

E = Gd × Apv × η [##12]

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• E is the energy produced in kWh/day

• Gd is the global radiation per day in kWh/m².day

• Apv is the area of the photovoltaic cell

• η is the efficiency of the solar cell material used

References:

1. http://ec.europa.eu/energy/publications/doc/2009_report-solar-energy.pdf ,retrieved on 03/03/11

2. http://www.renewableenergyworld.com/rea/news/article/2009/11/thin-films-share-of- solar-panel-market-to-double-by-2013, retrieved on 03/03/11

3. http://www.evoenergy.co.uk/evoenergy-news/helping-our-customers-make-informed-decisions-on-pv-finance-our-mds-thoughts/, retrieved on 03/03/11

4. http://www.nrel.gov/docs/fy09osti/43844.pdf  pg 11, retrieved on 03/03/11

5. http://www.zoomers.ca/profiles/blogs/solar-people-become, retrieved on 03/03/11

6. "A New Invention To Harness The Sun" Popular Science, retrieved on 03/03/11

7. http://www.concentrix-solar.de/power-plants/?L=1 , retrieved on 03/03/11

8. "Small Photovoltaic Arrays". Research Institute for Sustainable Energy (RISE),Murdoch University.

http://www.rise.org.au/info/Applic/Array/index.html , retrieved on 03/03/11

9. "Photovoltaics: Thin-film technology about to make its breakthrough".http://www.solarserver.de/solarmagazin/index-e.html, retrieved on 03/03/11

10. PHOTOVOLTAIC SOLAR ENERGY: Development and current research, 2009, ISBN978-92-79-10644-6.

11. Concen

12. PV

13. University of the Witwatersrand, School of Electrical and Information Engineering,Johannesburg. Elen2000 Electrical Engineering Course Notes.

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