ENVIRONMENT AFFECTS SOLAR POWER GENERATION

Sun, the oldest source of energy, enriching life with all the basic amenities required to sustain life on earth from thousands of decades. The solar energy developments in this booming technological and commercially vigorous world has rendered engineers and scientists to harness it with a wide range of applications (lighting, heating, cooling, rural electrification, and many industrial applications). India being a tropical country is bestowed with ample solar energy with around 300 sunny days in a year. The total installed capacity of India as on March 31, 2016 is 298,060 MW in which solar along with other renewable sources contributes 38,822 MW. The solar radiation of about 5,000 trillion kWh per year is incident over its land mass with average daily solar power potential of 0.25 kWh per m2 of used land area with the commercially available. As on March 31, 2016, the total solar installed capacity of India is 6,762.85 MW. India expects to install an additional 10,000 MW by 2017 and a total of 100,000 MW by 2022.

A SOLAR PANEL PERFORMS BEST, GIVING THE MAXIMUM OUTPUT WHEN IT IS DIRECTED PERPENDICULAR TO THE SOLAR RAYS AND GETTING THE DIRECT RADIATIONS. WHEN A PANEL IS BLOCKED BY A SHADOW IT CAN REDUCE THE PERFORMANCE OF THE SYSTEM AS THE PANEL IS NO MORE EXPOSED TO DIRECT RADIATIONS.

Effect of High Temperature

Have you noticed how a liquid-crystal display (LCD), such as the calculator or cell phone screen, changes colour when exposed to extreme hot temperatures? Temperature affects how electricity flows through an electrical circuit by changing the speed of the flow of electrons. The collision of the electrons with each other leads to an increase in resistance of the circuit and ultimately increases the temperature. Likewise, resistance is decreased with decreasing temperatures. Imagine somebody running in a desert with a temperature around 47°C. How will his body react to such a hostile condition? Of course, it will be very tiring and exhausting. Now, imagine him running on a cool breezy evening. In the same way as a human body’s abilities change depending on the weather conditions, a solar panel’s output too depends on its working conditions. We generally believe that more the solar rays’ incident over the PV panels the greater will be its power output. It may seem to be counter-intuitive, but the solar panel efficiency is affected negatively by temperature increases. Photovoltaic modules are tested at a temperature of 25°C (77°F), and depending on their installed location, heat can reduce output efficiency by 10–25 per cent.

The best way to determine the panel’s tolerance to heat is to refer to the manufacturer’s data sheet. Temperature coefficient (Pmax.) is the maximum power temperature coefficient, it actually describes how much power the panel will lose when the temperature rises by 1°C above 25°C , STC (STC is the standard test condition temperature where the module’s nameplate power is determined). For example, if the temperature coefficient of a particular panel is –0.5 per cent, then for every 1°C rise, the panel’s maximum power will reduce by 0.5 per cent.

DUST ACCUMULATION IS THE CRUCIAL FACTOR WHICH DECREASES THE PRACTICAL EFFICIENCY OF PV PANELS AND TEND TO MAKE PV SYSTEMS AN UNATTRACTIVE ALTERNATIVE ENERGY SOURCE, PARTICULARLY FOR THE LARGER DOMESTIC MARKETS (AS THE DUST COLLECTED BLOCKS THE DIRECT RAYS TO STRIKE THE SOLAR PANELS).

So on a hot day, when panel temperatures may reach 45°C, a panel with a temperature coefficient of –0.5 per cent would result in a maximum power output reduction of 10 per cent. Conversely, if it is a sunny winter’s morning, the panels will actually be more efficient. It seems ironic but it is the fact that the more sunshine we get, the hotter the panels become and this in turns counteracts the benefit of the sun. The knowledge regarding temperature will help to improve the efficiency of solar panels that operate in non-optimal conditions. To maintain a favourable temperature of the panels, cooling systems are kept which may pass a cool liquid behind the panels to pull away heat and keep the panels cool. This is similar to how our body might sweat as a way to stay cool during the run at 47°C temperature.

Effect of Shading and Clouding

Solar modules or cells are designed to convert solar radiation (sunlight) into electricity which will then either be sold to a grid tie or used to charge batteries in an off-grid system. A solar panel performs best, giving the maximum output when it is directed perpendicular to the solar rays and getting the direct radiations. When a panel is blocked by a shadow either by a cloud, a chimney, tree branch, or neighbouring buildings, etc., it can reduce the performance of system as the panel is no more exposed to direct radiations and gets diffused radiations. Since the shadows are almost never evenly spread over every module in the array, mismatching outputs between modules in a string and strings in the array are induced. From the two different types of shades applied, two different effects occur.

‘Soft shading’ can be described as simply lowering the intensity of the irradiance levels, without causing any form of visible separation of shaded and unshaded regions. A great example of soft shading would be due to clouds evenly blocking out some, but not all of the sunlight. Soft shade applied on some modules in a string and not evenly to others will cause an effect called ‘current mismatch’, where the current output of each module is varied. Since the laws of electricity dictate that all components connected in series must have the same current, what typically results is the string settling on the output of the lowest-performing module, reducing the output of the entire string to that of the most heavily-shaded cell in the string. This same effect occurs independently for all strings in an array, as strings are connected in parallel. However, despite being independent to each string, current imbalance in one string can still negatively affect other strings.

Hard shading is created when a physical object, such as a telephone pole, or tree is physically obstructing the sunlight, creating obvious visible regions of lit and unlit cells on the array. Hard shade, on the other hand, causes the output voltage of the shaded modules to drop. However, when two or more strings connected in parallel have shade unevenly applied to them, an effect called ‘voltage mismatch’ occurs. Voltage mismatch is the condition in which two parallel strings are giving different voltage output when measured independently. Hence, before installing we should have the knowledge and expertise in choosing the best areas of the roof to add or maximize return on investment. The areas with constant shade obstructions must be avoided. Some shadows may affect the panel in the morning, however, but may not affect the same panel for the rest of the day—resulting in overall good performance.

Effects of Dust and Pollution

Dust accumulation is the crucial factor which decreases the practical efficiency of PV panels and tend to make PV systems an unattractive alternative energy source, particularly for the larger domestic markets (as the dust collected blocks the direct rays to strike the solar panels). Dust is a term generally applied to minute solid particles with diameters less than 500 µm. It is a lesser acknowledged factor that significantly influences the performance of the PV installations and occurs in the atmosphere from various sources, such as dust lifted up by wind, pedestrian and vehicular movement, volcanic eruptions, and pollution. Dust would also refer to the minute pollens of fungi, bacteria, vegetation, and microfibres from fabrics, such as clothes, carpets, linen, etc., that are omnipresent and easily scattered in the atmosphere and consequently settle as dust. The characteristics of dust settlement on PV systems are dictated by two primary factors that influence each other, viz., the property of dust and the local environment. The local environment comprises site-specific factors influenced by the nature of prevailing (human) activities, built environment characteristics (surface finishes, orientation, and height of installation), environmental features (vegetation type and weather conditions). The property of dust (chemical, biological and electrostatic property, size, shape, and weight) is as important as its accumulation/aggregation. Likewise, the surface finish of the settling surface of PV also matters. A sticky surface (furry, rough, adhesive residues, electrostatic buildup) is more prone to accumulate dust than a less sticky and smoother one. It is also a well-known fact that dust promotes dust, i.e., with the initial onset of dust, it would tend to attract or promote further settlement; the surface becomes more vulnerable to dust collection. The effect of gravity, horizontal surfaces usually tend to accumulate more dust than inclined ones. This however is dependent on the prevalent wind movements. Generally, a lowspeed wind pattern promotes dust settlement while a high-speed wind regime would, on the contrary, dispel dust settlement and cleans it. However, the geometry of the PV system in relation to the direction of wind movements can either increase/decrease the prospects of dust settlement at specific locations of the PV system. After installing PV panels it is highly recommended to clean them on regular intervals of time for maximizing the power collection capacity and minimizing the losses.

SOLAR PANELS WORK BEST IN CERTAIN WEATHER CONDITIONS, BUT SINCE THE WEATHER IS CONSTANTLY CHANGING AND AS ENGINEERS ARE INSTALLING SOLAR PANELS AT VARIOUS PARTS OF THE WORLD IN DIFFERENT CLIMATE REGIONS, MOST PANELS DO NOT OPERATE UNDER IDEAL CONDITIONS.

Solar panels work best in certain weather conditions, but since the weather is constantly changing and as engineers are installing solar panels at various parts of the world in different climate regions, most panels do not operate under ideal conditions. That is why it is important for engineers to understand how panels react to different weather conditions before installing them. Thus, emphasis must be laid on a detailed study of the site considering the environmental factors as a need of the hour for captivating the maximum power output from these doped silicon panels along with the recommended measures, for a brighter and shiner future of the world.

Ms Avipsa Dey, Assistant Professor, Electrical Engineering Department, ITM Universe, Vadodara, Gujarat, India. Email: avipsadey@gmail.com