Modelling of Optical Parameters for Optimum Collection of Concentrated Solar Radiation - Dissertation Example

? Modelling of optical parameters for optimum collection of concentrated solar radiation for photovoltaic devices Chapter1 Introduction Traditionally, concentrators were intended for large power plants to provide great amounts of electricity to replace fossil fuel-generated power to help solve the problem of climate change and global warming. Innovative ideas have come out to save the earth from destruction of its ozone layer. World population is expected to increase by about 10 billion by year 2050 and if electricity demands are not programmed by now, there will surely be insurmountable problems for the people and the deterioration of the environment will exacerbate. Much has been utilized from our important ecosystems such as the forests, fisheries, wetland, fresh water resources, and that we have to move fast and change our course of actions to save the environment and the planet as a whole. The world’s program of action has not improved these past decades because we still depend on fossil fuel; specifically eighty per cent of energy comes from fossil fuels. The threat posed by climate change and global warming is pushing mother Earth to its near end. Industrialisation, consumerism in America and globalisation, among other drivers of change, have pushed and motivated scientists to find ways to address environmental problems which have continued to plague humanity. Constructions of buildings, new inventions and new technologies, have become a race for supremacy among big international organisations. Environmental problems of the world today have been worsened by technology. But we can use technology to save the environment. Fossil fuels and coal can be depleted. There will come a time that we’ll lack resources for these forms of energy source. In other words, there must be a way to produce a substitute for fossil fuels and coal, something that is not harmful to the ozone layers and the environment as a whole. Solar energy and other forms of environmentally friendly energy using the wind, the tidal waves, hydro, and other similar forms, must be tapped. The DESERTEC website says that a few hours the earth receives power from the sun is equivalent to a year it can use for its requirements to run factories, industries, businesses, homes, and offices. The deserts of Africa and Arabia can absorb much heat from the sun using mirrors to heat water to produce steam and convert this into energy. With this technology, solar energy is convertible into High Voltage DC current; the technology uses AC current converted into Direct Current using gargantuan transformers and components that store electric currents like capacitors. (Desertec Foundation, 2009) This paper aims to provide studies and research on solar radiation and how to ‘catch’ the rays of the sun to provide electricity. With this, we focus on what seems to be an emerging, but is quite becoming popular, technology – photovoltaic concentrators. Concentrated solar radiation is one of the sources of electricity that is environment friendly. Photovoltaic concentrators are the result of studies of solar cells whose primary aim is to generate large amounts of non-polluting renewable energy. Photovoltaic concentrators are used instead of flat panels of solar cells. This is composed of optical elements that concentrate the heating power of the sun and ‘feed’ into solar cells to produce electricity. A photovoltaic concentrator has two elements: a collector whose main function is to redirect the rays of the sun towards a smaller area, the solar cell. Usually, flat mirrors are used to receive the intense heat of the sun. An example of a photovoltaic concentrator is shown in the Figure 1. Figure 1a shows a lens with a rotational symmetry which redirects the rays of the sun onto a point focus, the receiving solar cells. Figure 1b shows a parabolic cylinder concentrator which forms the linear image of the sun and passes it on to the receiving solar cells. Figure 1 (a and b) A photovoltaic concentrator SOURCE: Handbook of Photovoltaic Science and Engineering (Luque & Hegedus, 2011) 1.1 Definition and evolution of Photovoltaic Concentrators Concentration photovoltaic technology and terrestrial photovoltaic came about almost at the same time. The first to emerge in the scene was the PV cells but since they were so expensive to produce and maintain, the idea of using optical elements became popular to substitute the PV cells. A program led by the Sandia National Laboratories (DOE) was launched in the United States by 1975 purposely to source ideas on photovoltaic prototypes. But then, even with the fast development of CPV technologies during the years 1975 to 1976, no company or entity wanted to provide large facilities for the development of CPV, since it had little modularity of less than 1MW. To attain a modularity of several MW required large facilities. With several experiments and introduction of new concepts in the Sandia Labs, what prevailed were high-efficiency multijunction cells which can operate at several hundreds of suns. (Luque & Hegedus, 2011, p. 76) A photovoltaic concentrator is made up of lenses or mirrors that collect and direct solar radiation to smaller areas of solar cells. Large areas of solar cells can therefore be minimised through the application of low-cost concentrators. PV concentrators first collect solar radiation with the use of lenses and mirrors, and then lead them to smaller areas of solar cells for electricity converstion. (Boes & Luque, 1993, p. 361) Before the popularity of photovoltaic concentration, flat panels were used. Flat panels are solar cells with optical elements which increase the sun’s luminous power and direct and it to special cells. (Sala & Anton, 2011) A concentrator photovoltaic (CPVs) is made of solar cells, optics, module, and sun tracker. Components should have the characteristics that include: efficient solar cells, high-acceptance angle optics, efficient heat extraction module, and a precise-pointing sun tracker. (Agora & Rey-Stolle, 2012, p. 25) Most concentrators now use silicon semiconductor cells but other manufacturers have used compound semiconductor and multijunction concentrator cells which have relatively higher efficiencies. Cost analyses for PV concentrators have been provided by some well-meaning researchers. For example, electricity for PV concentrators may drop below U.S. $0.15 per kWh but this can be lowered to $0.05 per kWh within 10 to 15 years from now. Presently PV concentrators have been manufactured and installed to provide few kilowatts of power due to the complexity in installation. Besides, PV concentrators can be installed in regions where the sun is available or where there is direct solar radiation, but in cloudier regions, they are not so effective. PV concentrators can be useful for centralized village power systems. It can also be distributed to supply homes. PV concentrators can be very useful in sunny regions such as Africa. (Boes & Luque, 1993, p. 361) 1.2 Ideal Concentrators Ideal and good concentrators collect all available rays or light coming from the source to reach the entrance of the concentrator, making sure that not too much rays escape from the concentrator. PV concentrators use efficient solar cells. So, this is one of the advantages in addition to the reduction of costs (flat-plate modules are more expensive). Solar cells also provide higher conversion efficiencies. Present day photovoltaic modules are made up of crystalline silicon (C-Si) wafers. This material provides high-module efficiency (12-19%), efficient solar cells, robust, and so on. Wafer-based silicon is also used in integrated circuits (IC). Crystalline silicon is also well researched and is very much in use in the electronics industry. (Luque & Hegedus, 2011, p. 78) 1.3 Types of Concentrators Concentrators are classified according to the optical means devised in concentrating the light, the number of axes that move in tracking the sun, the mechanical device used in tracking, and so on. The value for a total incident power for a concentrator to achieve is expressed in the equation: This equation is used to make calculation for the light that comes from a remote disk source. This has an angular extension ±?s, with a uniform brightness Bs; the concentrator entry area As, and can achieve a total incident power as expressed in the equation As:Ps = ?AsBssin2?s. The concentration of the power in the optical system is expressed in this first law. There is an increase in the light beam section, which is the area, and compensated by an increase of the angular width, which is expressed in sin2?s. Most concentrators have tracking device to track the sun’s apparent daytime motion. The tracking device or structure is integrated into the system and can automatically position the concentrator optics so that the cells are directly focused to the sunlight. The sun tracker consists of a structure with collecting surface and where the concentrator module can maintain an optimum reception of the sun’s rays. (Luque-Heredia et al., 2012, p. 61) Commercially available PV concentrators are installed with two-axis trackers, such as the azimuth-elevation axes and a tilt-roll tracker. These trackers are equipped with sun sensing, but with the advent of semi-conductors used in microcontrollers and other new technology available in the market, sun sensing is not anymore required but the process is based on digital computation ‘of precise analytic sun ephemeris equations’ (Luque-Heredia et al., 2012, p. 62). Two-axis trackers are of three kinds. The pedestal form has a pedestal that supports a flat tracking array structure. Tracking is usually done with a gearbox, which tracks the rays with a vertical axis and a horizontal elevation rotation. This device is simple to install. But there is disadvantage in that the winds put weight on the central gear drive onto a large torque, which requires large capacity gears. (Swanson, 2003, p. 456) Figure 2 (a & b)The ‘two-axis tracker’ using a pedestal There is another kind of two-axis tracker which is the roll-tilt structure. The roll-tilt two-axis tracker has its wind loads considerable reduced, but it requires more rotating bearings and linkages. It also requires a large-section horizontal support. The structure necessitates multiple foundations that are strictly aligned. Installation for this kind of structure is rather complicated. Figure 3 (a & b) Figure 3 Roll-tilt tracker with central torque The main objective of a photovoltaic concentrator sun tracker is to perfectly align the pointing axis with the local sun vector so that maximum power can be produced. It may not be perfect but the point of any CPV is to permanently align the pointing axis and in doing so, errors cannot be avoided. A so-called off-tracking tolerance can be provided. A certain acceptance angle of the concentration system may be provided. (Luque-Heredia et al., 2012, p. 63) There are two reasons for the decrease of sun tracking performance of photovoltaic concentrators. First, there is the factor on the precise pointing of the tracker to the source of light, and second, there can be reasons for the acceptance angle to deviate or shrink, thus the tracking accuracy is affected. Errors can be had in the mounting and alignment of the concentrator system. This kind of problem is connected with the design of mechanical fixtures and such factors like assembly of the fixtures, the module levelling, and following the mounting protocols in installing the tracking system. There are also factors like the stiffness of the tracker and the bending angle allowed in the different elements of the structure. (Luque-Heredia et al., 2012, p. 63) 1.3.1 Types of Optics 1.3.1.1 Fresnel lenses Concentrators use lenses that are either refractive or reflective and are in the form of dishes and troughs. Fresnel lenses usually do not exceed 5 cm in diameter, thus they are preferable in considering low cost concentrators. These are considered standard plano-convex lens, which have either flat or curved lens surface. Fresnel lenses may also have point-focus with a linear symmetry in their axis, or linear focus, which has a ‘constant cross section with a transverse axis’ (Swanson, 2003, p. 453). This type of lens focuses the light in a single line. The difference between point-focus and line-focus lenses is that in the former one cell is used behind each lens, whereas in the latter the lenses have a linear array of cells. Experience proved that the domed Fresnel lens has been a successful linear configuration citing its greater rigidity. But domed point-focus cannot be left behind as researchers are also developing this type. Figure 4 (a, b, & c) Figure 4 (a,b, & c) shows the Fresnel lens configuration. Fresnel lenses are contained in modules consisting of the lens, or a parquet of several lenses, a housing that protects the back portion of the lenses, and the cells. The cell may have another part called secondary optical element (SOE) to provide more concentration of light. 1.3.1.2 Reflective lenses Reflective mirrors are also used as reflective lenses forming a shape of a parabola which focus the light parallel to the parabola’s axis and at the parabola’s focus. Parabolas can be used for point focus and line focus configurations. Point focus can be obtained by rotating the parabola around its axis to create a paraboloid, while line focus is obtained by turning the parabola perpendicular to its axis. The photovoltaic static concentrator that use parabolic trough was first developed by the All-Russian Research Institute for Electrification of Agriculture for large-scale projects. This has some advantages. The cost of electrical energy is not dictated by the capacity installed; and a big amount of solar radiation can be collected. (Kowalik, Gorski, & Sachenko, 2004, p. 88) Reflective concentrators, such as the line focus using troughs, require a single axis tracking to allow the PV receiver within the focus line. But due to the daily variations of the sun’s directions, sunlight incidence in the tracker is not correctly maintained, the result is an oblique ray reception, and there are losses in the received energy. On the other hand, the line focus refractive concentrators, using the Fresnel lenses, experience optical aberrations when light is not normal. This type needs a two-axis sun tracking device. (Luque-Heredia, 2012, p. 62) A compound parabolic concentrator is formed with parabolas installed at the sides of the concentrator, but ‘the focus for each side is at the opposite side of the cell and the axis of parabola a is along the direction of maximum acceptance angle, Omax’ (Swanson, 2003, p. 454). Figure 5 shows the relation of the angular width of the light source which is the sun and the concentration gain. SOURCE: Adapted from Handbook of Photovoltaic Science and Engineering, by A. Luque & S. Hegedus (2011) Reflectors follow a simple optical law which states that the angle of incidence is equal to the angle of reflection and the ray of light is kept within a plane presumed normal before and after reflection. This concentration will also assume that the rotational parabolic disk mirror is perfect in shape and reflectivity. (Luque & Hegedus, 2011) 1.3.2 Static Concentrators What are previously discussed above are concentrators with sun-tracking devices but there are some concentrators which do not use the tracking principle. The reason for this is that the sun does not appear in all areas of the sky but only at particular portions at a time. It is not therefore possible to receive light from every direction. What maximum light the concentrator optics receives depends on the angular regions where concentrators accept light. It is however possible for cells to receive light from both sides; this is what we call bifacial cells with a factor of 2 using bifacial cells. Static concentrator designs have been proposed and made by researchers. These have a concentration ratios ranging from 2 to 12, which use compound parabolic concentrator (CPC). (Swanson, 2003, p. 460) Static mounts are usually feasible for low concentration factors, which is below 5x. Static can also use luminescence and photonic crystals. (Luque-Heredia, 2012, p. 61) The interest on static concentration in photovoltaic is due to several factors. Static concentration reduces the solar cell in terms of area which makes it cost-effective because the solar cell is the most expensive component of a flat PV module. The cost of module components is distributed among the different parts of the system. Moreover, the static system does not need higher maintenance compared to the flat modules. (Winston, Minano, & Benitez, 2005, p. 353) 1.4 Ultra-High Concentration Ultra-high concentrations (those above 1,000 suns) can provide cost reduction, increased output and efficiency, and availability of raw materials. Ultra-high concentration devices have been made available with multi-junction solar cells. According to Algora and Rey-Stolle (2012, p. 22), they have developed and made into fruition GaInP/GaAs dual-junction concentrators which had an efficiency of 32.% at a concentration ranging from 499 to 1,026 suns. They further boasted that it is the world’s record efficiency holder for a dual-junction cell. An efficiency of 40% at 1,000 suns is also achievable, according to the researchers. (Algora & Rey-Stolle, 2012, p. 22) Multijunction solar cells (MJSCs) have been seen by the researchers to have achieved the highest conversion efficiency of PV devices. MSCs are expensive to manufacture and thus commercially available are rather expensive too. But its performance and the fact that it can be further improved provide some experimenters to bite the bait; meaning they don’t care of the high cost. Some users and researchers experimented by using concentrator optical systems to provide illuminate on the solar cells. This new method requires smaller solar cells which have low cost optics. (Algora & Rey-Stolle, 2012, p. 22) 1.5 Concentration Ratio The most common concentration ratio is known as “geometric concentration ratio” and the formula is ‘the area of the primary lens or mirror divided by the active cell area’. The active cell area receives the illumination but it is not the entire cell area that needs to be illuminated. The nonilluminated part is connected with buss bars for electrical connection. One other method of concentration is known as intensity concentration or “suns”. The formula for this is: the ‘ratio of the average intensity of the focused light on the cell active area divided by 0.1 W/cm2’. The standard peak solar irradiance has been set at 0.1 W/cm2. (Swanson, 2003, p. 455) 1.6 The Solar Spectrum The sun is the source of all power; it is so hot that hydrogen and helium are converted into ions. The stars derive their light from the sun through fusion. The sun’s interior records a temperature of billions of degrees while the outer surface is a bit cooler. Energy is converted with Einstein’s formula, E = mc2. The sun’s properties that are significant in this discussion include the nuclear fusion, the color temperature emanating from the light, and the geometric properties of Earth and the sun which form the solar disk. In solar cell studies, two parameters are of paramount importance: the irradiance which is ‘the amount of power incident on a surface per unit area … and the spectral characteristics of the light’ (Smestad, 2002, p. 15). The earth receives heat from the sun in the form of radiation. As it receives heat, it also gives up heat or radiation. This is known as black body radiation. But the earth has to give up radiation in the same amount it receives, otherwise something happens. If the earth receives more energy than it releases, the temperature will rise; likewise, if it receives less than what it releases, the temperature decreases. But this is made balance since the sun’s radiation is absorbed by only half of the earth while the earth radiates in all directions. (Scofield, 2009, p. 5) The solar radiation is similar to that of a black body with temperature reaching 6000 K. Solar radiation is measured at the earth’s surface but it has to pass through the earth’s atmosphere before it can be measured which, in a sense, has also reduced its intensity. 1.6.1 The Stefan-Boltzmann Law This law is stated in the following equation: P = a?AT4 This pertains to electromagnetic waves or the energy radiated from a surface, which is called “blackbody radiation” (Schofield, 2009, p. 1). The Stefan-Boltzmann law states that the total power radiated by a surface P is equal to ? = 5.67 times 10-8 Wm-2K-4; ? is known as the emissivity which is also called the “fudge factor” whose value ranges from 0 to 1. The Stefan-Boltzmann law is applicable to any body irrespective of temperature. The perfect absorber/emitter is expressed in ? = 1. (Scofield, 2009, p. 1) 1.6.2 The Planck Distribution Electromagnetic waves are released by hot objects or bodies through the entire electromagnetic spectrum and the amount of energy released depends on the temperature of the objects or bodies. The hotter the bodies, the more they release electromagnetic waves which become shorter in wavelengths, while cooler bodies release longer wavelengths. The sun’s radiation belongs to the visible spectrum which peaks in the wavelength of yellow light. The radiation is termed as “blackbody radiation”. This is so termed because of the principle that the radiation emitted by an object is close or equal to the radiation it absorbs. A “black” object absorbs any radiation ‘which is incident upon it,’ no matter how long its wavelength. Objects that absorb radiation which is related to the radiation they emit are called perfect emitters. (Scofield, 2009, p. 1) The density of blackbody radiation is expressed in the following equation: According to this formula, h = 6.63 x 10-34, Js is Planck’s constant, k = 1.38 x 10-23, while the wavelength-dependent emissivity ?=1, applicable to all wavelengths. A wavelength with a narrow range is expressed as ?? while the energy flux with a radiation wavelength is expressed in S???. The Planck spectrum for a perfect radiator is expressed with a temperature, T1 = 6000 K, and the other which is lower is T2 = 5000 K. Figure 6 The Planck spectrum charted for two temperatures, with a temperature, T1 = 6000 K, and the lower temperature, T2 = 5000 K. SOURCE: Adapted from Photovoltaic concentrators, by J. Scofield (2009, p. 2). 1.6.3 Wein’s Displacement Law In Figure 6, the distribution does not change at any temperature except the height of the curve and the peak’s location. The height is situated under the curve, and given by the Stefan-Boltzmann formula. The curve therefore gets higher as the temperature rises. The position of the peak also changes along with the temperature change. This principle is known as the Wein’s Displacement. The purpose of using PV concentrators is to reduce the cost of electricity since PV converters are more expensive than optical material. Another major significance is the acquisition of higher performance PV cells. Concentration modules may provide 20% more energy conversion efficiency. But installation of concentrator modules is not easy since these require ‘high heat flux and electrical current density’ and the need for suitable tracking systems and module designs. (Swanson, 2003, p. 449) Concentrators are needed and useful in areas that need solar power for applications that need heat and the temperatures are above 80°C. Homes and industries need temperatures within the range of 300°C, for example in space cooling, cooking desalination, or in operating turbines. There are many instances where solar power is a necessity and there may come a time that it’s all what we want. References Algora, C. & Rey-Stolle, I. (2012). Chapter 2: The interest and potential of ultra-high concentration. In A. C. Lopez, A. Vega, & A. L. Lopez (Eds.), Next generation of photovoltaics: new concepts (pp. 23-52). Heidelberg; London; New York: Springer. Boes, E. & Luque, A. (1993). Photovoltaic concentrator technology. In T. Johansson, H. Kelly, A. Reddy, & R. Williams (Eds.), Renewable energy: sources for fuels and electricity (pp. 361-402). Washington D.C.: Island Press. Desertec Foundation: Clean power from deserts. (2009). Retrieved 10 July 2012 from http://www.desertec.org/fileadmin/downloads/DESERTEC-WhiteBook_en_small.pdf Kowalik, J., Gorski, J., & Sachenko, A. eds. (2004). Cyberspace security and defense: research issues. The Netherlands: Springer. Luque, A. & Hegedus, S., eds. (2011). Handbook of photovoltaic science and engineering. West Sussex, United Kingdom: John Wiley & Sons, Ltd. Luque-Heredia, I., Quemere, G., Cervantes, R., Laurent, O., Chiappori, E., & Chong, J. (2012). The sun tracker in concentrator photovoltaics. In A. C. Lopez, A. Vega, & A. L. Lopez (Eds.), Next generation of photovoltaics: new concepts (pp. 61-91). Heidelberg; London; New York: Springer. Sala, G. & Anton, I. (2011). Chapter 10: Photovoltaic concentrators. In A. Luque & S. Hegedus (Eds.), Hand book of photovoltaic science and engineering (pp. 402-449). West Sussex, United Kingdom: John Wiley & Sons, Ltd. Scofield, J. (2009). Chapter 3: the Solar Spectrum. Dept. Physics & Astronomy, Oberline College. Smestad, G. (2002). Optoelectronics of solar cells. Washington, USA: SPIE – The International Society for Optical Engineering. Swanson, R. (2003). Photovoltaic concentrators. In A. Luque & S. Hegedus (Eds.), Handbook of Photovoltaic Science and Engineering (pp. 449-501). UK: John Wiley & Sons, Ltd. Winston, R., Minano, J., & Benitez, P. (2005). Nonimaging optics. London: Elsevier Academic Press. Read More