Mankind’s survival is primarily dependent on a reliable energy source, and the quest for energy has been going on for centuries. Energy exploration involves a better understanding of nature. Though, on the one hand, technology has overcome a number of nature’s challenges in successfully extracting raw materials and generating energy, this widespread and large scale deployment of technology has also resulted in climate change. The dependency on fossil fuels is causing untold damage to the environment, with the survival of future generations at stake.
Tuesday, August 31, 2010: Energy shortage and climate change need to be addressed on an urgent basis. It is imperative that we accelerate the replacement of fossil fuels with renewable energies.
Without fundamental changes in how we generate and use energy, the challenges threaten our livelihood and our planet. Towards this end, renewable energy sources need to be utilised to its fullest extent. Solar power is one such unlimited energy source, and its supply is predictable, reliable and harnessable. The sun shines in abundance in India and energy tapped from just a fraction of the sunlight we receive is enough to meet the entire demand of the country. The best way to utilise this source of energy is to feed the electricity generated from it into the distribution grid, from where public utility service providers can channel it to meet the various needs of society.
The word photovoltaic (PV) is a combination of the Greek word ‘photo’ for light and ‘voltaic’, meaning voltage. It is the direct conversion of sunlight into energy by means of solar cells. The conversion process is based on the photoelectric effect discovered nearly two centuries ago, which involves the release of positive and negative charged molecules in a solid state when light strikes its surface. Solar cells are composed of semi conducting materials. To produce a solar cell, a semiconductor is contaminated by doping a positive charge carrying material or negative charge carrier to either obtain a surplus positive or negative charge. If two differently contaminated layers are combined, then a PN junction is formed. The usable voltage from solar cells depends on the semiconductor material. The current intensity depends on the light radiation. There are two different solar cell technologies—crystalline and thin film. The crystalline solar cells appear blue in colour and the white gridlines over it make the electrical connection. The thin layer of silicon film deposition on glass with another substrate material on the glass makes a thin film cell. While the thin film technology consumes less silicon material, the efficiencies are lower compared to crystalline. In fact, both the technologies have advantages of their own.
Grid interactive systems
The solar panel produces DC power. Multiple solar panels are coupled together as arrays, which produce higher DC voltage. The DC power is converted to grid compatible AC power. This power is fed to the grid utility. Grid interactive systems are of two types—for small scale applications (up to a few kilowatts) and for large scale power generation (of the mega watt scale). The small scale PV systems (mostly used at home) are of two types—grid interactive only (without any battery) and grid interactive with battery backup. The direct grid interactive system operates when the utility electricity is available. This is useful where the utility outages are rare. In the event of outages by utility service provider, the PV system shuts down till the utility power is restored. The grid interactive system with battery backup incorporates energy storage in the battery to keep critical circuits in the home operating during the utility outage. When the outage occurs, the unit disconnects from the utility and power specific circuits. During the availability of utility power, the power generated is fed to the home loads or to the grid. There can be a log maintained of the energy pumped into the grid—the energy consumed and the energy generated. This is called net metering, wherein, the net energy generated is assessed against the energy pumped or drawn from the grid and recorded.
System components and functions
A solar PV module is a packaged interconnected assembly of solar cells. Multiple units of this module make up a solar PV array. Electrical connections are made in series to achieve a desired voltage and/or in parallel to provide the desired amount of current. The modules are mounted in such a way that they receive maximum sunlight on the cells.
There are two ways of mounting the modules—on a fixed axis and tracked. In a fixed axis, the modules are mounted in one fixed tilt and do not move. Whereas, in a tracked system, they are mounted in such a way that they track the sun, thereby enabling sunlight to fall on it directly throughout the day and year. The tracking method can increase the power generation by up to 25 per cent but requires higher investments on the tracker. Tracking in a fixed axis configuration for an optimised tilt angle is done based on the latitude of the location. The solar PV modules are mounted on metal structures. Normally, they are designed to withstand the wind loads of the location. It is also required to build the support structure in such a way that the minimum clearance between the PV module and ground is maintained.
The DC power generated by the PV module is connected to the power conditioning unit. This consists of a maximum power point tracker (MPPT) and an inverter. The MPPT comprises an electronic system that operates the PV module in a manner that allows the module to produce all the power it is capable of. The MPPT is not a mechanical tracking system but an electronic system that varies the electrical operating point of the modules so that they can deliver the maximum available power. Additional power harvested from the modules is then made available as increased current. The MPPT has a high efficiency DC to DC converter, which uses special algorithms to extract the optimum power derived from the PV module. The harvested maximum DC power is then fed to an inverter for power conversion to AC, synchronising with grid frequency.
There are two kinds of inverters —the central and the string inverter. The central inverters are housed in an inverter station while the string inverter can be outdoors. A bank of arrays is connected to the string inverter and the AC output is then connected to the grid at lower voltages. String inverters are suitable for systems with mechanical tracking. The central inverter’s topology allows an AC side parallel connection of the inverter and voltage multiplication, thereby making the mega watt scale power production and pumping of energy to higher grid voltage, possible.
The output of the inverter is filtered to reduce harmonics and is always synchronised to the grid. In the event of grid failure, the system switches off safely. When the grid resumes, the inverter reconnects and pumps energy to the grid as long as the PV modules generate DC power. The inverters are provided with galvanic isolation to significantly reduce safety risks. It is a complete physical separation between input and output, and is achieved by means of transformers. The output of the inverter can be single phase or three phase, depending on the amount of PV power. The AC power is boosted to the required high voltage by means of a power transformer and fed through the necessary safety switching modules. The whole system is protected from lightning hazards through the use of the necessary lightening arrestors. The entire power plant can be monitored and controlled by a monitoring system. Parameters like the amount of sunlight incident at the location, the ambient temperature and wind conditions are measured, besides the power generated by the PV system. This can enable the monitoring of the system’s performance and also provide fault diagnosis. There are ready made and custom built software based on computer aided supervisory controls and data acquisition mechanisms, which can not only provide us the information of these parameters but also control the whole system.
Even though solar power generation is generally possible everywhere, for large scale grid based power generation, there are a few specific requirements. Since the space required for the installation of PV modules is a minimum of 4 acres per megawatt of power generated, a shadow free area with grid evacuation potential is essential. It is important to evaluate the utility requirement before sizing the system components.
A PV system produces power in proportion to the intensity of sunlight striking the solar module surface. The intensity of light varies throughout the day, as well as from day to day, so that the actual output of a solar PV plant can vary substantially. There are other factors that affect the output of a solar PV system. These factors need to be understood. The DC output of solar modules is rated by manufacturers under standard test conditions (STC). These conditions are easily recreated in a factory and allow for consistent comparison of products but need to be modified to estimate the output under common outdoor operating conditions.
STC conditions specify that the module’s output should be measured at a solar spectrum as filtered by passing through 1.5 times thickness of atmosphere, with an intensity of 1000 watts per sq m of area and at an ambient temperature of 25 degree celsius. The crystalline modules also have production tolerance which need to be considered in system design. The amorphous module exhibits light induced degradation (LID).
The degradation of modules in the first 1000 hours of sun exposure results in an average of 15-20 per cent degradation in power output, and the power deliverable after such degradation is called rated stabilised power. It is essential to note the maximum day time temperature of the location while derating the PV module’s power generation capabilities. The nominal operating cell temperature (NOCT) data of a given manufacturer’s module can be useful in optimising system design.
Dirt and dust can accumulate on the solar module surface, blocking some of the sunlight and reducing the output. Even though this is cleaned off during rain, it is more realistic to estimate system output taking into account the reduction due to dust build-up in dry seasons. The maximum power output of the total PV array is always less than the sum of the maximum output of the individual modules. This difference is a result of some inconsistencies in performance from one module to the next and is called the module mismatch. Power is also lost to resistance in system wiring, as well as in the power conversion process. During the course of the day, the angle of sunlight striking the solar module will change, which will affect the power output. The output of PV modules will rise from zero, gradually during the dawn hours, increase with the sun angle to its peak output at midday and then gradually decrease throughout the afternoon and go back down to zero by night. While the variation is due, in part, to the changing intensity of the sun, the changing sun angle also has an effect.
The selection of module technology depends on the various factors. The crystalline module can function at efficiency levels of around 14-15 per cent. The amorphous model works at the approximate efficiency of 6-8 per cent, which makes the land requirement double that of the crystalline variety. While crystalline technology can require less space compared to amorphous technology, the latter has the ability to deliver higher yields for the same installed power of crystalline technology in a hot location. Due to the low thermal coefficient, the amorphous module delivers higher yields in diffuse
sunlight conditions compared to crystalline. With the high voltage and lower current, the cable costs can be lower in amorphous systems.
If the negative grounding requirements are ignored in amorphous technology, it can cause severe corrosion related issues in the long run. Hence, it is essential to adapt the technology depending on the location and the various other factors listed above. It is important to estimate the total energy generation possible at a particular location so that the pay back can be estimated. To do this, software tools like PVSYST or SolarPro can be used. Safety and risk mitigation must be evaluated along with the soil evaluation for installation suitability of the PV modules. Water conditions and rainfall data can also provide more inputs regarding PV module cleaning frequency and arrangements. Air and water can be used to clean the PV modules.
PV power generation systems can provide the most reliable output for a longer period. However, the modules also suffer a small degradation in power generation and this can amount to approximately 20 per cent over 20 years. The derating can be made up with additional modules after some time.
The regular cleaning of PV modules can ensure the reduction in power generation losses due to dust and dirt factor. Even though there are no moving parts in the power plant, a properly ventilated area with dust filters for the power conditioner unit can ensure the longer performance of the system. In short, what’s attractive about the PV system is its low maintenance.
India is blessed with abundant sunshine and our dreams of environment friendly energy generation can be best achieved with solar power farming. The technology costs are becoming more competitive, day by day. With volumes, the cost can be even more affordable. Solar farming can also lead to more meaningful green farming and harvest a bright future for the generations that follows.
The author is associate vice president—PV & Engg, Kotak Urja Pvt Ltd
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