JECET, June – August-2013; Vol.2.No.3, 541-550.
Journal of Environmental Science, Computer Science and Engineering & Technology
An International Peer Review E-3 Journal of Sciences and Technology
Available online at www.jecet.org Engineering & Technology
Research Article
JECET, June – August 2013; Vol.2.No.3, 541-550. 541
Role of Solar Photovoltaic Technology (SPVT) in Meeting the Power Demand
I. Nawaz1, M. Emran Khan2 and M. Rafat3
1&2
Department of Mechanical Engineering, Jamia Millia Islamia, New Delhi,, India
3Department of Applied Sciences and Humanities Faculty of Engineering and Technology, Jamia Millia Islamia,New Delhi,, India
Received: 18 May 2013; Revised: 12 June 2013; Accepted: 22 June 2013
Abstract: For obvious reasons, recent research in the field of energy has been motivated by environmental considerations. Energy consumption and production are controlled by factors beyond individual priority. The realization has dawned on everyone that to meet the ever increasing energy demands without adverse impact on environment, renewable sources of energy are needed. A very promising source among them is solar energy. Efficient methods for utilizing solar energy are therefore needed. One efficient method is the use of photovoltaic (PV) technology. The current research interest in this area is motivated by a search for lower assembly costs. At the same time, lower maintenance cost and increased efficiency are obviously desirable.
Current research reports lead to optimism about the future of PV technology. In the present study, a survey has been attempted of the state-of-the art in this field.
Advantages and disadvantages of current practices are enumerated. Such a survey helps in identifying specific areas where further research is called for.
Key words: Electrical power generation, solar energy, solar photovoltaic cell, balance-of-system, efficiency of PV cell, environmental degradation
JECET, June – August 2013; Vol.2.No.3, 541-550. 542 INTRODUCTION
Solar Photovoltaic systems have been used for a variety of applications including lighting, water pumping, communication technology and others. The quality and performance level of these devices have improved steadily over the years; at the same time, the cost has come down considerably though not to the desired extent. We observe that single crystal and polycrystalline silicon cell technology have been developed and a number of industries round the world now produce solar PV panels of high quality and efficiency. The cost of this mode of electrical power generation, however, continues to be high. Except for remote areas and special applications, it is not considered cost effective. The cost of PV technology expected to come down significantly when thin film solar cells of comparable efficiency and performance produced. However, significant progress has been made in improving the efficiency of such cells; their relatively shorter life, lower efficiency and fast rate of degradation are major problems. They are yet to be overcome to make these cells commercially viable.
Another area that needs further improvement in this context is the subsidiary component of the system. It supports the main PV system leading to optimum utilization in any application. Subsidiary component includes electrical storage batteries, inverters, actual appliances for load matching and others. PV power is expensive so it must be used with the highest degree of efficiency and all sub- systems must satisfy the highest standard of quality.
Photovoltaics (PVs) method for generating electric power is by using solar cells. Photovoltaics (PVs) have emerged as crucial source of electricity among renewable energy sources. The inherent advantage of photovoltaics is that the input (solar) energy is available throughout. It does not involve fuel cost and transportation cost, modular system with same efficiency for a few W to MW system.
There is negligible operational & maintenance cost and environmental pollution. Long life has been amply proven over the years.
As the market for PV system is expending into rural as well as urban areas, further research is required in order to reduce the cost of PV system significantly. This can be achieved by designing better solar cells capable of giving higher conversion efficiency. Higher wattage modules needed for efficient balance of system (BOS). This will lead to better utilization of PV electricity. It would be possible to combine all the sub-systems at a low cost to generate electricity at present or near future utility rates. The cost-effectiveness of PV for remote or rural areas application has been well established; the experience of operating such systems has motivated the research for better systems with wider applications.
2. SOLAR PHOTOVOLTAIC TECHNOLOGY BASICS
Solar cells, also called photovoltaic (PV) cells by scientists, convert sunlight directly into electricity.
PV gets its name from the process of converting light (photons) to electricity (voltage), which called photovoltaic effect.
Traditionally i.e. first- generation of solar cells are made from silicon, are usually flat-plate and generally most efficient.
Second-generation solar cells called thin-film solar cells because they e made from amorphous silicon or non-silicon materials such as cadmium telluride.
Third-generation, solar cells being made from a variety of new materials silicon, including solar inks using conventional printing press technologies, solar dyes and conductive plastics.
JECET, June – August 2013; Vol.2.No.3, 541-550. 543 3. PHOTOVOLTAIC MATERIALS
Materials presently used for photovoltaics include monocrystaline silicon, polycrystalline silicon, amorphous silicon, and cadmium telluride and copper gallium selenide. Although crystalline silicon cells are the most common type, photovoltaic (PV) solar cells, can be made from many semiconductor materials. Each material has unique strengths and characteristics that influence its suitability for specific applications. For instance, PV cells materials may differ from one another in crystalline, band-gap, absorption and manufacturing complexity. Since solar cells are the major components of PV system, major thrust of research has been towards development of photovoltaic materials and cells to obtain high conversion efficiency. The goal is to approach the theoretical limits as far as possible.
The different types of solar cells can be classified according to the microstructure of the active material:
• Single crystal or multi crystalline cells
• Poly crystalline cells
• Amorphous cells.
3.1- Single crystal or multi-crystalline cells:The materials used in these solar cells are Silicon (Si), Gallium Arsenide (GaAs) and Indium antimonide (InSb) . Silicon (Si) is the best-known PV material.
It is abundantly available, chemically stable, non-toxic and technologically its use is very advanced (due to microelectronic industry). Almost 80 % of the total PV output utilizes Si cells based on the low cost, high throughput; moderate efficiency is about (13%) screen-printing devices [1]. More sophisticated device design utilizing light trapping structures (PERL, inversion layer etc.) have demonstrated efficiencies greater than 25%.
The cells use single crystal made by czochralski technique (Cz-Si) or multi-crystalline (HEM or cast) Si as starting material which is cut into thin of thickness range between 300-500µm. The wafer cost contributes almost one third to the total cell manufacturing cost. In order to reduce this, large (>100 kg) ingot, pseudo square large wafers for maximum module power output are being developed as shown in figure (1) and a typical components of a crystalline solar cell is also shown in figure (2).The wafer thickness < 250µm has been obtained using wire-saw technology with high mechanical yield and high manufacturing rates. Wafer as thin as 100µm is possible but not at present manufacturing level.
Light trapping schemes allow thin Si to be used without losing efficiency (21.5% for 47 µm thick Si).
An optical path length enhancement upto 58 times has been achieved which allows absorption of long wavelength photons close to the depletion region. It may be noted that light trapping requires surface texturing. Anisotropic etching achieved by NaOH solution from random pyramids with sides reflecting the light towards the substrate. This is not effective for multi-crystalline Si since the reactivity of the etchant is high at grain boundaries leading to a non-optimum texturization. Other techniques like mechanical scribing, laser scribing and photolithography are therefore, used for this purpose. The other advantages of these techniques are that structures other than the simple pyramids and groves can be made. The facet orientation is not restricted to crystallographic planes. Though such cell structures reported, incorporating these methods in a conventional process line may prove difficult.
The rapid progress in the lab scale Si solar cell efficiency has to be translated into manufacturing level efficiency, in order to reduce the module cost. Automatic and large volume productions are necessary for making this a technological reality. More efficient use of human and equipment resources, without
JECET, June – August 2013; Vol.2.No.3, 541-550. 544 significant increase in operation cost, can effectively reduce the module price for a few tens of MW/year production, even without taking advantage of the technological developments in cell design.
Fig.1: pseudo square solar cell
Fig.2: Typical components of a crystalline solar cell module
3.2 Polycrystalline Cells: Thin film polycrystalline solar cells are major contenders for replacing single crystal Si devices for large-scale PV application at a relatively low cost. This stems from the advantage offered by thin film technology-less material requirements, use of different types of substrates, low process temperatures, possibility of integrated manufacturing process etc. The efficiency of these cells for small areas has already reached >15% in labs. The current research effort is now to adopt all process steps for production of these cells over large area.
JECET, June – August 2013; Vol.2.No.3, 541-550. 545 The major materials for this category of cells are:
• Copper Indium Diselenide (CIS)
• Cadmium Telluride (CdTe)
• Thin film Si
Active area cell efficiency for Copper indium gallium selemide, CuInGaSe2 (CIGS) solar cells has reached 19.9% for an area of 0.41 Cm2 with81.2percentage fill factor, which is a record for all thin film solar cells2. Ga alloying has been a key development in achieving such efficiencies. The effective band gap increases, thereby increasing the open-circuit voltage (Voc) with a slight reduction in the short circuit current. Making such an alloy film with required photovoltaics has been achieved by systematically studying the film properties with varying Ga alloying. The window layer of CdS is normally deposited by solution growth technique. Attempts have been made to replace this layer by another n-type films eliminating Cd from the cell structure. In facts, by varying the doping levels in CIGS, CdS and conducting oxides, it expected that more efficient cell structures could be created.
CdTe with its band-gap of 1.5eV has been thought to be the best semiconductor for PV applications.
This has been proven by the fact that the efficiency in the range of 12-14% have been reported by several groups preparing CdTe by a variety of techniques (closed space sublimation, electroplating, spray pyrolysis etc.). The recent result of 17% cell efficiency by spray pyrolysis suggests that CdTe cells are keeping pace with CIS cells. However, the cell design (TCO/CdS/CdTe/contact) is simple to fabricate, optimization of deposition and annealing conditions is the key to achieving the high efficiency. Thin CdS window layer is necessary in order to have an increased blue response of the cells. Use of dual SnO2 improves Voc for cells using thin CdS. CdCl2 treatment has been the main reason for CdTe cells crossing the 10% barrier in the first place, which is due to the improved structural and electronic properties. One of the major problems facing these cells is the rear contact to CdTe. CdTe cannot be doped easily due to self-compensation effect (and hence the layer is highly resistive) and has a high work function. No metal can, therefore, make proper ohmic contact with CdTe. Graphite paste doped with copper has been shown to be one of the best contact materials.
Another way to achieve contact is to etch the CdTe surface before contact metal (Au) deposition. The etching makes the surface Te rich (and hence P+). However, both graphite contact and etching/contact deposition are not well understood and are still subject of active research.
Toxicity related issues are of great importance in CdTe cells, because of the use of Cd in cell structure. The industry manufacturing these cells have considered issues related to this and have explored ways of limiting the exposure to workers in the factories, disposal of broken modules etc.
Use of compound semiconductors in CdTe and CIS solar cells can lead to degradation over the required period of operation (15 years or more). Unlike the Si solar cells, Si thin film cells on an inexpensive ceramic substrate combine the advantages of Si with that of thin film technology. With efficiency of about 15% for 1Cm2 area, such cells can be manufactured at a lower cost than any other commercial cell. The cell design can be improved by increasing the diffusion lengths to 100µm (for 50µm Si film) with use of light trapping, little grid coverage and interconnected sub-module. The projected efficiency could be 20%, which will make the module, cost less than 0.64 US dollar per Watt3.
3.3 Amorphous Si Cells: Amorphous Si solar cells have utilized the better optical absorption characteristics of amorphous Si and the possibility of doping/ alloying hydrogenated amorphous Si.
The plasma CVD techniques used for deposition have been successfully developed for large area
JECET, June – August 2013; Vol.2.No.3, 541-550. 546 deposition, making a Si:H a major macro-electronic material with applications in order areas also. The device structure utilizes a p-i-n structure with light absorption in i layer and then the internal field causing drift for the photo generated carrier separation. Such single junction solar cells have yield efficiencies >10% for small areas4. For as deposited devices, however, photo-degradation due to Staebler-Wronski (SW) effect, results in a rapid decrease in cell efficiency initially and then it stabilizes. The established module efficiency for single junction cells is about 5%. Though the physical mechanism causing SW effect is not very clear, defect pool model has established a relationship between the presence the weak bonds which interact with the photo generated carriers to give rise to dangling bonds (dbs) or mild gap states. These db states cause recombination losses resulting in photo degradation. Recent results show that use of deuterium instead of hydrogen results in less photo-degradation. This has opened the possibility of making 10% stabilized efficiency single junction cells. The other approach (engineering one) is to have thin absorber layer so that the high electric field present removes the photo-generated carriers before the db states created. However, in order to efficiently absorb the solar spectrum, such thin absorber layers have to be transmitted into tandem structure. The reported efficiency of a triple junction solar cell is 13.7% for small areas and 10.2% for modules5. The low production cost involved even for such complicated device structures suggests an energy payback time of less than 1 year. There, amorphous Si holds a great promise to provide an alternative to crystalline Si even for PV power generation. The common solar cell materials summarized in Table 1.
Table-1: Common Solar Cell Materials Single or multicrystalline Polycrystalline (Thin film) Silicon (Si)
Single crystalline Multicrstalline silicon
Amorphous silicon (Non-crystalline Si for higher light absorption)
Gallium Arsenide (GaAs)
Cadmium telluride (CdTe) Copper indium diselenide (CIS)
4. PHOTOVOLTAIC SYSTEMS
PV cells are the building blocks of all PV systems because they are the devices that convert sunlight to electricity. Performance of photovoltaic systems depends on the solar radiation and temperature.
Therefore, it is very site specific and variable. Proper sizing of PV system is necessary for a reliable performance for long periods. Several software packages based on techniques like fuzzy logic, expert systems have been reported to simulate the performance of PV module and balance-of-system (BOS) components under any given operating conditions. Such software tools allow a better (in terms of cost and performance) and reliable PV system design.
Apart from having better system design, it is also important to monitor the performance of PV system in field conditions. Thus, the monitoring and diagnostics on a real time basis for photovoltaic system is an important requirement to establish the reliable operation of these systems. The monitoring of different PV system parameters like power generation by PV system, Environmental parameters like solar radiation, sunshine hours, temperature, etc. Battery operation (charging, discharging, protection against over-charging and deep discharging), load management (load on/off, duration load operation,
JECET, June – August 2013; Vol.2.No.3, 541-550. 547 inverter operation etc.). On day-to-day and long-term basis, this evaluation needs to be done in order to understand the system performance in field conditions.
Distributed grid-connected PV electricity generation has enormous potential for wide spread usage of PV technologies. Such systems can be used for meeting peak demand, and reducing the overloading of transmission lines and transformers. PV plant output is, in general, highly correlated with the local distribution system peak load. Thus, PV system may reduce the thermal over-loads of transformers and conductors. The current and power associated with these devices may be limited to their rated capacity. This obviates the need of reconductoring the line, upgrading the transformer bank or addition of new circuitry. Further, such PV systems will reduce electrical loss, provide kilovolt- ampere reactive (KVAR) support and increase reliability. Photovoltaic for Utility Systems Applications (PVUSA) studies have shown that PV can be an economic alternative, taking into account all the benefits accrued. The need is to serve the utility supply and meet demand side requirements. Further research will be able to identify the economic and technological advantages.
An AC module is a PV module with integrated DC to AC inverter, which generates grid connected AC power. AC module can be an interesting alternative for conventional grid connected PV systems.
It offers possibilities to overcome problems associated with high DC voltage levels, safety, ohmic losses, risk of DC arcs, fire hazards and protection. PV systems based on AC modules are highly modular which allows easy system expansion with ranges of about 100-200 watts. The BOS costs are definitely lower for AC modules because the inverter cost per kWp are equal while any other system can either be omitted (connection and junction box, string and bypass diode, isolation detector) or be replaced by lower cost materials (cabling), engineering and installation costs are lower.
5. BALANCE-OF-SYSTEM (BOS)
A solar PV balance-of-system refers to the components and equipment that move DC power produced by solar panels through the conversion system, which in turn produces AC electricity. Balance of system encompasses all components for a PV system other than PV panel. This includes wiring/cables, fuses, switches, battery, inverter, charge controller, connectors, etc in the case of off- grid system.
Batteries are a critical component of photovoltaic systems, since solar cells cannot store the energy themselves. These also contribute to the cost of the total system both in terms of the capital cost as well as maintenance cost due to limited life (1000 charge and discharge cycles typically). An optimum system design involves proper battery sizing considering both these costs and proper charging and discharging to prevent discharge below maximum depth of discharge or overcharging.
In fact, charge controllers are key components of PV system ensuring proper battery utilization to increase the battery life to the maximum. Pb- acid batteries are most popular for PV applications, with sealed Pb-acid with AGM for PV lighting and vented tubular for high power applications. In general, undercharging causes major battery damage and over sizing of PV array can be better for stand-alone systems. Charge equalization is necessary to extend battery life to bring the low capacity cells to full charge in large voltage system (large No. of batteries connected in the bank). Otherwise, voltage reversal occurs during discharge, which can be detrimental to the entire battery bank. When the battery is connected to an AC load through inverters, a pulse discharge current may be produced. The performance of the battery in such a situation is poor as compared to steady discharge current occurs.
Stratification of electrolyte also affects the battery life and artificial stirring of the acid by means of air bubbling seems to be required as a part of the battery sub system.
JECET, June – August 2013; Vol.2.No.3, 541-550. 548 6. PV CELL EFFICIENCY
Tables 2 &3 showing an extensive listing of the highest independently confirmed efficiencies for PV cell module presented.
Table -2: The highest efficiency of solar cells6
Material Efficiency (%) Area (Cm2) Voc (V) Jsc (mA/Cm2) Fill Factor Si
Perl 25 4.0 0.706 42.7 82.8
Multi-crystal 20.4 1.002 0.664 38.0 80.9
Thin 21.5 4.017 0.494 29.7 79.2
GaAs
Single 27.6 0.126 1.107 29.6 84.1
Si Sub. 21.3 0.126
Multi-Junction
GaSb 32.6 0.053
Si 29.6 0.317
Thin Film
CdTe 15.8 1.05 0.843 25.1 74.5
CIGS 16.4 1.025 0.678 32.0 75.8
Thin Si 14.9 1.02 0.600 37.9 81.1
a-Si
Single 12.7 1.0 0.887 19.4 74.1
Triple 12.4 0.27 2.541 7.0 70.0
TiO2 7.4 0.53 0.808 12.5 73.0
Table-3: The highest efficiency of solar cells6
Material Highest Efficiency (%)
Mono-Crystal Silicon 25.0
Poly- Crystal Silicon 20.4
HIT(Hetero junction with Intrinsic Thin layer) 26.8
Amorphous Silicon 12.8
Poly- Crystal Silicon thin film 16.6
CuInSe 19.5
Cds/CdTe 16.5
GaAs/InGaP 30.28
InGaP/GaAs.3-Junction concentrator 36.5
Nano-Crystal Silicon 10.1
TiO2 (Titanium dioxide) 11.0
JECET, June – August 2013; Vol.2.No.3, 541-550. 549 7. ADVANTAGES AND DISADVANTAGES OF SOLAR PV
Advantages and disadvantages of Solar Photovoltaic – Quick Pros and Cons of Solar PV: Solar Photovoltaic (PV) Panels are undoubtedly, what comes to peoples’ minds when they talk about solar energy. Considering that in an hour, the sun radiates solar energy enough to cover for human energy consumption for a year then going green with solar Photovoltaic (PV) panels is perhaps in the right direction! However, with solar energy systems’ technology, we are still behind in capturing this naturally free vast amount of energy provided by nature.
Herein one may review some basic advantages and disadvantages of solar energy panels (PV panels).
7.1 Advantages of solar PV – briefly:
PV panels provide clean – green energy. During electricity generation with PV panels there are no harmful greenhouse gas emissions thus solar PV is environmentally friendly.
Solar energy is energy supplied by nature – it is thus free and abundant!
Solar energy can be made available almost anywhere as long as there is sunlight
Solar energy is especially appropriate for smart energy networks with distributed power generation – DPG is indeed the next generation power network structure!
Solar Panels cost is currently on a fast reducing track and the reduction is expected to continue for the next several years – consequently solar PV panels indeed have a highly promising future both for economical viability and environmental sustainability.
Photovoltaic panels, through photoelectric phenomenon, produce electricity in a direct electricity generation way
Operating and maintenance costs for PV panels are considerably low, almost negligible, compared to costs of other renewable energy systems
PV panels have no mechanically moving parts, except in case of –sun-tracking mechanical bases;
consequently they have far less breakages or require less maintenance than other renewable energy systems (e.g. wind turbines)
PV panels are very silent, producing no noise at all; consequently, they are a perfect alternative for urban areas and for residential applications.
Because solar energy coincides with energy needs for cooling PV panels can provide an effective solution to energy demand peaks – especially in hot summer months where energy demand is high.
Though solar energy panels’ costs have seen a drastic reduction in the past years, and are still falling, nonetheless, solar photovoltaic panels are one of major renewable energy systems that promoted through government subsidy funding (FITs, tax credits etc.); thus, financial incentive for PV panels make solar energy panels an attractive investment alternative.
Residential solar panels are easy to install on rooftops or on the ground without any adverse impact on residential lifestyle.
7.2 Disadvantages of Solar PV – briefly:
As in all renewable energy sources, solar energy has intermittency issues; the sun is not shining at night and during daytime there may be cloudy or rainy weather.
Consequently, intermittency and unpredictability of solar energy makes solar energy panels less reliable a solution.
Solar energy panels require additional equipment (solar inverters) to convert direct electricity (DC) to alternating electricity (AC) in order to be used on the power network.
JECET, June – August 2013; Vol.2.No.3, 541-550. 550 For a continuous supply of electric power, especially for on-grid connections, Photovoltaic panels require not only Inverters but also storage batteries; thus increasing the investment cost for PV panels considerably
In case of land-mounted PV panel, they require relatively large areas needed for deployment;
usually the land space committed for this purpose is for a period of 15-20 years – or even longer.
Solar panels efficiency levels are relatively low (between 14%-25%) compared to the efficiency levels of other renewable energy systems.
Though PV panels have no significant maintenance or operating costs, they are fragile and can be damaged relatively easily; additional insurance costs are therefore of ultimate importance to safeguard a PV investment.
8. CONCLUSION
Tables 1& 2 show the highest efficiency of solar cells. Different types of photovoltaic devices are the subject of active research with focus on efficiency improvement and technological improvement. In all cases, a large production level in tens of MW/yr will lead to reduction in the cost, which will make PV system more cost effective and will give a further boost to PV technologies. The cost effectiveness of photovoltaics for rural electrification has been demonstrated beyond doubt. The major reason for this in comparison to diesel generator or grid expansion is the fact that the requirement in such area is relatively small (say a few kWh/day). Interestingly, the same is true if one considers the backup electrical generation for emergency purposes. We have seen that PV systems can be inexpensive in comparison to diesel generators for such applications in urban areas. Distributed PV power generation supporting the utility has also shown that PV technology can contribute to the existing electric power generation.
These developments and innovations in PV devices are expanding PV applications. As the cost of PV cells and components decreases over the years, photovoltaics are poised to become a major source of electrical energy. Solar photovoltaics are one of the key programmes, particularly for decentralized applications in rural and remote areas. It is a simple, reliable and environmental friendly technology, which is immediately available to provide electricity to widely dispersed households.
REFERENCES
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2. New World record efficiency for Cu(in, Ga) se2 thin-film solar Onlinelibrary. wiley. com/ doi/10.
1002/pip.1078full
3. Photovoltaics-wikipedia, http://en.wikipedia.org/wiki/photovoltaics
4. National Renewable Energy Laboratory (NREL) nrel.gov/learning_photovoltaic.html
5. Tiwari D.N. and Mishra R.K. Advanced Renewable Energy Sources. RSC Publication, Cambridge CB40WF, UK, 2012
6. Solar cell efficiency table (version 37)- Electrical Engineering www.eecs.berkeley.edu/news/SC efficiencytsables V37.pdf
*Corresponding Author: I. Nawaz;Department of Mechanical Engineering, Jamia Millia Islamia, New Delhi-110025, India
