An uninterruptable electric supply is one of the crucial requirements to support overall development of a country. The solar power electricity generation has grown up to 227 GWe at the end of 2015. India is steadily venturing into renewable energy resources like wind and solar. Solar energy plays a major role in electricity generation. The Government of India expects non-conventional sources to account for 40% of total generation capacity by the year 2020. Government policies have supported for the development of solar markets by providing incentives to the solar sector. Also the advancement in new technologies, installation and regulatory incentives has made a rapid fall of the solar power generation cost. In general, solar technology is divided into passive and active categories. Passive solar energy exploits the heat or light of the sun directly, to provide natural light. Active solar technology consists of photovoltaic (PV) system and solar-thermal (CSP) system. Photovoltaic cell system generates electricity from sunlight using a semiconductor material that produces an electrical charge when solar photons excite its electrons. Concentrating Solar Power (CSP) technologies use mirrors to focus the sun’s light energy and convert it into heat to create steam to drive a turbine to generate electrical power. According to IRENA 2016 review, India is one among the top three countries in CSP installation (Refer Table I).
Figure 1 shows the globally installed solar capacity till 2015.
I. Solar Energy Technologies
India is endowed with large Solar Energy potential. Most parts of the country have about 300 sunny days. The average solar radiation is about 4-7 kWh per day. Hence, solar energy plays a major role in electricity generation. The solar power capacity installation as on December 2016 is 9012.69 MW. Government policies have supported for the development of solar markets by providing incentives to the solar sector. Also the advancement in new technologies, installation and regulatory incentives has made a rapid fall of the solar power generation cost.
By 2017 India’s total solar installation capacity is around 18 GW. This ranks India among top three global markets after China and USA. (According to Economic Times Energy World Report Jan, 2017).
Figure 1: Global installed solar power capacity till 2015
The key technologies in solar power generation are solar PV system and CSP system.
A. Photovoltaic (PV) System
PV system generates electricity from solar energy radiation through semiconductor materials called PV cells. These are made up of Silicon and semiconductor compounds such as GaAs (Gallium Arsenide) or CdTe (Cadmium Telluride). The basic principle for power generation is photovoltaic effect. During power generation, PV systems do not emit greenhouse gases.
The advantages of PV modules are minimum maintenance and easy expansion to meet the growing energy demands. This modularity permits users to mold PV system to the desired condition. High cost and the need for the application/load ratio to match with illumination of light output of Photovoltaic are the main disadvantages.
B. CSP System
Solar thermal power systems, also known as concentrating solar power systems (CSP) use concentrated solar radiation as a very high temperature energy source to produce electricity using the conventional heat route. Solar thermal technology is suitable for applications with direct, high intensity solar radiation. Much advanced research is already underway in India to upgrade and develop several types of solar water heating technologies. Forced circulation flat plate collectors and thermo-siphons are very popular. Even though they have already been installed in large commercial establishments, they are now being used in the domestic sector as well. Selectively coated and black paint absorbers on flat plate collectors are being manufactured and marketed in the country.
The complete blueprint for the flat plate collectors consisting of the black chrome selective coating for the absorber has been developed in India. Solar thermal power can occupy a unique position in the India’s energy arsenal. Its potential to absorb hybrid technologies and, coupled with storage, could pave way for unlocking the base load power, thereby, introducing a platform for large scale integration of renewable energy.
Figure 2: Top three countries in global solar market
To make solar thermal sustainable and feasible for the Indian conditions, much needs to be done on the ground level. Solar thermal is much more expensive than photovoltaic technology, because it has a long development period. In addition, the solar thermal plants need more water per unit of electricity produced. These factors present some challenges to solar thermal engineers; but, are not enough for the technicians to abandon the idea of solar thermal power generation on the whole; especially, when innovation and research can address some of the barriers.
To produce heat from solar radiation, solar collectors are used. There are three basic types of solar collectors used in the systems:
• Parabolic Trough Systems can reach temperatures as high as 400 degree Celsius heating water to steam, which powers turbines to produce electricity.
• Power Trough Systems use a receiver that is placed in such a way so that the reflected and converged rays of the sun are always aimed at it. This system can reach a temperature of nearly a 1000 degree Celsius.
• Parabolic Dish Systems can reach a temperature of 1000 degree Celsius at the receiver and thereby, achieve very high efficiency of energy conversion from solar to electrical.
An assessment published by the International Energy Agency suggested that PV and thermal energy systems could potentially constitute the largest source of global electricity by 2050. Compared with other non-renewable resources solar energy based resources are more sustainable and resilient. Solar energy is highly modular. It consists of many individual installations that can be linked together and can be implemented at many scales, from distributed generation. Because of the technological improvements, expanding markets and government subsidies and incentives, solar-technology costs have declined in recent years. Also number of new jobs is created in solar sector. Solar energy is non-polluting. The technology also has cut down on noise pollution associated with energy generation. They are considered safe for humans to operate and unlikely to produce dangerous amounts of radiation.
Performance on Grid
Performance of Grid Connected PV System
PV system generates power in a different manner than the way power has been generated in the past, and a power electronics interface is required to convert the native format of the PV generation so that it becomes grid compatible. Photovoltaic energy is one of the most scalable types of renewable energy generation because it can be produced in amounts from a few KW at the residential scale up to multiple MW at the utility scale. Due to the growing electricity demand, slightly reduction in PV system cost over the last years, the gateway and opportunities for PV smart grid system seem to be increasing.
Figure 3: Global solar employment strategy in 2016
Photovoltaic energy systems consist of arrays of solar cells which create electricity from solar radiation. The output of the PV (photovoltaic) system is primarily dependent on the intensity and duration of illumination. Solar electricity provides us with non-depleting, site-dependent and eco-friendly alternative energy option. PV offers clean, noise-free energy conversion, without involving any active mechanical system. Since this is all electric, it has a high span time for the work to be done to further enhance the efficiency of the PV system.
In this regard the focus mainly shifts to electro-physics, nanotechnology and materials domain. Some of the existing PVs and their efficiencies are:
• Crystalline and multi-crystalline solar cells having efficiencies of ~11 %.
• Thin-film Copper Indium Diselenide with an efficiency of ~12%.
Meanwhile, the connections of PV system to utility grids cause several issues on electrical networks. These issues mainly depend on percentage of PV penetration and the installation location of PV system. The possible issues are as follows:
1) Inrush Current
The small difference between grid system and PV system voltages may introduce flow of inrush current between the PV system and the utility grid at connection time, and deteriorate to zero at an exponential rate. This inrush current may cause annoyance trips, thermal stress, and other problems.
2) Reverse Power Flow
At high penetration level of PV systems, the net production is more than the net demand. As a result, the direction of power flow is reversed from low voltage side to medium voltage side. This results in excess power loss and overloading of distribution feeders. It also affects the automatic voltage regulator in distribution feeders and online tap changers in distribution transformers.
3) Overvoltage
The operation of PV systems is near to unity power factor for the full utilization of solar energy. So, only active power is injected to the grid by the PV systems. This leads to the change of reactive power flow of the system. Hence, the nearby bus voltage is increased to compensate the reactive power. This over voltage can have negative impacts on both grid and consumer end.
4) Power fluctuations
The reason for power fluctuations is variation of solar irradiance caused by the cloud movement, the size of passing clouds and the area covered by PV system. This power fluctuation may cause power swings, voltage flickers and voltage fluctuations.
5) Voltage Control Difficulty
The presence of more than one supply point leads to voltage control problems in power system with embedded generation. All the voltage regulating devices like capacitor banks are designed with unidirectional power flow system.
Figure 4: Employment strategy on PV System Installation in India
6) Harmonics
As inverters are used in PV systems, they cause power quality problems such as harmonic distortion. Due to this, series and parallel resonance, false operation of protective devices and overheating in capacitor banks are raised.
7) Electromagnetic Interference
The switching frequency of inverters in PV systems causes interference issues in capacitor banks, protection devices and DC links which lead to the malfunctions of these devices.
8) Islanding of PV systems
The PV systems should be disconnected once they lost the connection with utility supply. As per IEEE std. 929-2000 the PV system inverters should be disconnected within 6 cycles when islanding condition is detected. For this more islanding techniques are proposed such as active, passive and hybrid etc. While including these techniques, the overall cost of the PV systems is increased.
9) Frequency fluctuations
The imbalance between the produced and consumed power, leads to the frequency fluctuations. In small PV systems this issue can be negligible. But, this issue is more vulnerable when the penetration level of PV system is larger.
10) Phase imbalance
If there is no even distribution of inverters among different phases, it may cause phase imbalance which results in shifting of neutral voltage to unsafe values and increase the voltage unbalance.
11) Power Losses
In general, when the generation is closer to the load, the losses are reduced. If the increment of penetration level is above 5%, there may be excess power loss.
A. Performance of Grid Connected CSP System
In general, the CSP plants are considered as conventional thermal power plants with synchronous generators, governors, exciters etc. While integrating CSP system to the grid, the stability of the system should be examined. The stability studies are of two categories namely steady state analysis and Transient analysis.
Figure 5: Total Renewable Energy capacity (MW) Growth in India (According to IRENA Renewable Capacity Statistics 2017)
Steady state analysis involves with power flow calculation of the network as well as the design and planning of the connection with grid system. Transient events such as faults, rotor angle change and voltage oscillations which may cause failure of distribution system should be analyzed while integrating CSP with grid.
CSP system operation is entirely different from PV system. PV systems shall be used for individual residential as well as power grid demand. But CSP systems must be interconnected to power grid. While doing grid interconnection, it should be balanced by controllable generation units with effective demand management schemes. When the power demand is minimal, the generation, reserve and power exchange capability must be refined. Always there should be a controlled variation on both generation and demand side equation.
The installation of this type of generation should have an adequate energy resource. Therefore, it is important to determine the limits and best strategy of grid infrastructure to meet these limits. There is always a dependency on both grid infrastructure and CSP plant which leads to an uncertainty on development of both grid and plant.
Figure 6: Solar installation capacity in India (According to IRENA Renewable Capacity Statistics 2017)
B. Performance indicators of PV and CSP Systems
The entire performance of solar power plants can be defined by the Capacity Utilization Factor (CUF). It is the ratio of the actual electricity output from the plant to the maximum possible output during the year. The output from the solar power plant depends on the design parameters as well as several variables such as poor selection of panels, degrading of modules at high temperature, ohmic losses, atmospheric factors and radiation. In addition, the performance also depends on site location, solar insolation level; climatic conditions (especially temperature), technical losses in cabling, module mismatch, soiling losses, MPPT losses, transformer losses and the inverter losses.
The key performance indicators are:
• Radiation at the site
• Losses in PV system
• Temperature and climatic conditions
• Design parameters of the plants
• Inverter efficiency
• Module degradation due to aging
Hence, it would be desirable to monitor the solar plant installations and build up database for future work. For better performance the design parameters should avoid cable losses and inherent losses, so that efficiencies of inverters and transformers can be improved. To overcome the degradation of modules, it is required to utilize well qualified modules with improved technology and quality assurance. Hence, several manufacturers are proposing extended warranties with a safety of margin.
Sociopolitical Impacts
A. Job Creation
According to IRENA report 2017, in 2016, 9.8 million people are employed directly and indirectly in renewable energy sector. In that solar PV was the largest employer with 3.1 million jobs. China, Brazil, USA, India, Japan and Germany are the top countries accounted with more renewable jobs.
As per IRENA 2017 review, China is the top country in PV employment and The USA is the top country in CSP employment. Figure 3 shows the global employment strategy in solar energy sector in 2016.
In India PV installation employment should continue to expand drastically as, the installation capacity continues to increase. According to the Council on Energy Environment and Water (CEEW) and National Research Development Corporation (NRCD), 58000 direct jobs could be created by PV projects. Also CSP employment is still upcoming. Figure 4 depicts the PV installation capacity and PV employment growth in India in 2016 and 2017.
B. Government Plans
Emerging technology and economy of scale have created enough market force to drive renewable capacity in years to come. All over the world various countries are exploring the solar energy potential. This helped to develop the objectives and support plans provided by government. Nearly 164 countries have renewable energy goals for electricity production. Amongst them, 45 countries have plans to achieve the electricity generation through solar energy. So, the solar markets have benefits from both supply-side and demand-side drivers.
1) Solar Policy of Indian Government
The US withdrawal from the Paris Climate deal can possibly elevate India to a leadership role in the global fight against climate change which may happen due to massive building of renewable energy capacity
According to 18th Electricity Power Survey (EPS) conducted by the government of India, the peak demand of the country is about 285 GW (GigaWatt) by 2022. The present day Government of India is working to build 175 GW of renewable energy capacity by 2022. In that, 100 GW capacities are from solar energy.
Figure 5 shows the total renewable energy growth in India till 2016 as per the review by IRENA Renewable Capacity Statistics 2017.
Solar tariffs recently dropped to Rs. 2.44 per unit for a project in Rajasthan’s Bhadla Solar Park. This is 18% lower than the average tariff of Rs. 3/- per unit for power from NTPC’s coal-fired plants.
Also, because of the price slide of solar module imported from China, solar power generation cost is becoming 11% cheaper than a year ago. In addition, the government supports the solar industry by providing capital subsidies, interest free loans and tax breaks, a waiver of VAT and countervailing duties to promote domestic solar power installation. Hence, the solar energy industries have shown continuous installation growth for electricity generation.
Figure 6 indicates the solar installation capacity growth in India till 2016.
Conclusion
Both Solar PV and CSP systems are fast growing technologies in electricity generation in upcoming decades. The performances of these technologies have both positive and adverse effects on grid and society. Still the adverse effects are not assessed cautiously as they have more positive impacts globally. To overcome the issues and to improve the performance of these technologies further new research and policies should be deployed. The new ideas and deployment tactics are to be generated in a faster pace. Thus the increase in demand for electricity will be met effectively by the use of solar power generation in a better manner.
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