: Solar Photovoltaic Technologies and Systems
Table of Contents:
Global warming is a major concern as the demand for energy is ever increasing. Conventional energy resources, such as coal, natural gases and oil, are known to have high carbon emissions. For example, the greenhouse effect caused by CO, emission during coal plant operation is significant and a major concern. A typical coal-based thermal power plant can emit as much as 7000 Metric tons/MW CO, per annum. Notional damage to the environment for this is about 55 million US$ per annum. Besides this, the transport of coal is also responsible for additional GHG emissions. Disposal of end products, such as fly ash, is not environmentally friendly. The consumption of clean water, which is an ever-depleting resource, is also a major concern for coal-based power plants. A coal-based power plant typically requires about 2000 L/MWh of water. This requirement is much lower for alternative (renewable) energy generation, such as Solar Photovoltaic (Roy and Bose, 2018). Although continuous technological innovations, such as clean coal, are happening in these conventional technologies, concern remains as the demand for energy is also increasing. Particularly for the developing countries, the GDP growth rate can only be sustained by increasing the energy generation. The non-polluting conventional energy resources, such as nuclear, are no longer a preferred choice, due to some recent accidents associated with such plants. The unwanted environment damage due to radiation leaks in the atmosphere and the ocean have disastrous consequences.
The industry revolution originated due to the abundance of coal in western Europe. This was followed by the discovery of oil and gas in 1959. However, these energy resources are limited and are predicted to be exhausted by 2100, if not earlier (Roy and Bose, 2018; Gilbert, 2013). Global temperature rise is a major concern. Till 1960, the temperature rise was not noticeable, however it has risen by about +1 °C since then. Also the rapid global increase in CO, emission started from 1970 (280 ppm pa) and has gone up to 400 ppm pa at present (Boyle, 2004; Nelson, 2011). In the United Nations Framework Convention of Climate Change (UNFCCC, 2015), a roadmap for controlling GHG emissions has been developed. Goals have been set for several countries. India have pledged that by 2033, 40% of the electricity generation will be from renewable sources, including bio-oil (bio ethanol and bio diesel) and large hydro. Also, the total GHG emission divided by GDP would be 33%—35% less in 2033 as compared to 2005. All these are possible by widespread development of renewable energy resources. Renewable energy is therefore getting increasing attention all over the world. Apart from the reduction of GHG emissions, these technologies help energy access and energy security (Foster et al., 2010).
Renewable Energy Resources
A list of major renewable energy resources is given below:
Global energy scenario until 2017 is shown in Table 1. In 2018,178 GW renewable power was added globally. In India, the present capacity of renewable energy is 70 GW, with a Cumulative Average Growth Rate (CAGR) of 18%. The break-up is as follows:
Solar PV: 22 GW (21%); Bio: 9 GW (13%); Wind: 34 GW (49%); Small Hydro (< 2 MW): 5 GW (7%).
The projected capacity of renewable energy in India by 2022 is 200 GW, having 100 GW Solar PV, 60 GW Wind, 10 GW Bio and 5 GW Small Hydr o. The investment required would be about 125 billion
It is clear from the preceding discussion that Solar Photovoltaic is getting major attention. Although the total present capacity of wind energy is highest among renewable sources, the growth is slowing down. The highest growth, reflected by additional capacity being created, is happening for Solar PV.
Table 1. Global clean energy scenario until 2017.
Classification of Solar Photovoltaic (SPV) Technologies
Energy conversion of solar photovoltaic (Markvart and Castaner, 2005) is a direct process. The solar energy available in the form of irradiance is directly converted into electricity using solar cells. Mono- and multi-crystalline silicon solar cells (Green, 1995), which fall under first-generation technologies, have the highest market share (about 90%). High efficiency III-V compound based Multi Junction (MJ) solar cells (Geisza et al., 2008; Stan et al., 2003) also fall under first-generation technologies. These are mainly used in space applications. Ahnost all the satellites/spaceships use high-efficiency MJ solar cells. Limited use of such solar cells in terrestrial applications are found in Concentrated Photovoltaic (CPV) systems (Min et ah, 2009). Thin film solar cells, which are comparatively new, fall under second- generation technologies. This categoiy of solar cells has the rest of the terrestrial market share (about 10%). Amorphous silicon (a-Si) (Street, 2005; Ahmed et ah, 2017; Ahmed et ah, 2019), Cadmium Telluride (CdTe) (Cusano, 1963; Hamid and Fatima, 2013; Ikegami, 1988) and Copper Indium Gallium Selenide (CIGS) (Ohlsen et ah, 1993; Kronik et ah, 1998) are the main second-generation technologies and are used commercially. Some solar cells, such as Dye Sensitized Solar Cells (DSSC) (Gratzel, 2003), Organic and Polymer-based solar cells (Sam-Shajing and Serdar, 2005; Bose, 2012), which are yet to be commercially exploited, fall under third-generation solar cell technologies. Although the concepts have been proven and prototypes are built, some problems, such as stability, are yet to be solved so that these cells can be used commercially. Recently, a new type of solar cell, called perovskite solar cell (Nazeemsddin and Snaith, 2015), has been drawing a lot of attention. Technologies, mainly based on quantum mechanical principle, are under fundamental research (Chuang et ah, 2014). These are classified as fourth-generation technologies.
The sun emits about 3.9 x 1026 W of power and a small fraction of this, about 1370 W/m2, is intercepted by the earth, just outside its atmosphere (Gilbert, 2013). This is known as solar constant and its value is not the same during the entire year. As the distance between the sun and the earth varies throughout the year, the solar constant value also changes. However, the variation is small. The power at the surface of the earth is always less than the solar constant because of the attenuation of the power as the sunlight travels through the atmosphere, including clouds. This is known as irradiance and has different values at different locations depending on altitude and weather patterns. The irradiance value at a particular location varies during the day, lowest in the morning/evening and highest during noon. This also experiences seasonal variation. The irradiance variation during the day occurs as the distance travelled by sunrays from outside the atmosphere to a particular location varies during the day. This has been depicted in Figure 1. The distance is lowest when the sun is at zenith, i.e., directly overhead, as sunrays have to travel a minimum distance (Ц) or a minimum Atmospheric Mass (AM). This is referred to as AMI. At any other time of the day, the distance traveled by the sunrays is more than h[ and, therefore, has higher atmospheric mass. At a particular time (see Figure 1), if the distance is h,, then the atmospheric mass is defined as AM (h,/hj). AM 1.5 is approximated as the average irradiance received throughout the day for a particular location. Solar constant is applicable to outside Earth’s atmosphere and the atmospheric mass is zero (AMO).
The irradiance received by a collector at the earth surface has three main components. The major one is direct radiation, which manifests as “beam” or Direct Normal Incident (DNI). During clear sky, the perpendicular component of the irradiance received by the collector is denoted as beam or direct radiation. The incident sun rays are not perpendicular to the collector during the entire day, except when the sun is directly overhead. The power received by the collector, therefore, varies from the morning to the evening, depending on the altitude of the sun. This is given by “cosine law”, see equation (1).
where IDC is the effective direct irradiation receive by the collector. ID0 is the irradiance received if the sunrays are falling perpendicular to the collector plane. 0 is the angle between the sunrays and the normal to the collector plane.
Figure 1. During two different times of a particular day, sun rays travelling different distance through the atmosphere to
reach a particular point on Earth’s surface.
Another important component is the diffused radiation. Scattering of sunlight in the atmosphere, including clouds, causes some part of the irradiance reaching the collector as diffused radiation. During a cloudy day, the DNI component is zero and the entire radiation received by the collector is diffused radiation. Even in clear sky conditions, there is some diffused radiation. The diffused radiations are assumed to be coming from the sky uniformly in all directions.
Another minor component of the irradiance collected by the collector is the reflected radiations. The reflected radiation from the surface adjacent to the collector and the surrounding structures, such as snow-clad mountains. The amount of the reflected radiations is decided by the ground materials and the surrounding stractures/landscape. This component is very small and generally neglected.
At a particular' location, the total solar energy received by a collector during a year is made up of direct and diffused radiations. The relative contribution depends on the weather pattern, such as the number of cloudy days per year at that particular location. In a tropical country like India, where there are about 300 clear sky days, the share can be 75% direct and 25% diffused in a total year. The share of diffused radiation increases in areas where there are a greater number of cloudy days.
The energy emitted by the sun can be approximated by assuming that the sun is a black body with temperature of 5800K (Roy and Bose, 2018). The spectral distribution of the energy received at AMO has a peak at X = 0.5 pm and the total energy is distributed in Ultraviolet (UY: X < 0.4 pm) (7%), Visible (0.4 pm < X < 0.8 pm) (47%) and Infra-Red (IR: X > 0.8 pm) (46%). Some alteration of this spectral response happens due to the travel of sunrays through the atmosphere. The spectral distribution for AM 1.5 can be estimated as 2.0% UV, 54% visible and 44% IR. The peak shifts to the right marginally and remains close to 0.5 pm.