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A Learning Report on PV System Part-1

By

Karthikeyan.S

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 2

List of Content

S.No Name of the Contents Page No

1 PV And its History 3

2 Basic Principles of Photovoltaic Effect 5 3 PV Cell / Module / Array & String 6

4 PV Cell Model 7

5 PV Module Structure 8

6 I-V Characteristics of a PV cell 9

7 I-V Characteristics of a PV module 11 8 Power Characteristics of a PV module 12 9 Parameters affects I-V Characteristics 13

10 PV Cell Connection 15

11 Types of PV Technology 17

12 Solar Energy 23

13 Types of PV Inverter 25

14 Battery 30

15 Battery Sizing 31

16 Solar Charge Controller Sizing 32

17 Grid Terms of PV System 33

18 Effect of shading on Solar PV Modules 34

19 Bypass Diode 35

20 Blocking Diode 37

21 Isolation Diode 39

22 List of References 40

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PV And its History Definition of Photovoltaics

· Photovoltaics (PV) is the field of Physics and Technology related to the direct conversion of Sunlight into Electricity.

· From the Greek wordφῶς (phōs) meaning "light", and"voltaic", meaning electric (from the name of the Italian physicist Alessandro Volta)

History of Photovoltaics

1839: Discovery of the Photovoltaic Effect

· French Physicist Edmond Becquerel discovered the photovoltaic effect

· While experimenting with metal electrodes he

discovered thatconductance rises with illumination.

1954: First silicon solar cell & Solar Battery

· Gerald Pearson, Daryl Chapin, and Calvin Fuller from AT&T Bell Labs develop the first silicon solar cell capable of converting light into Direct Current to run electrical devices.

· Efficiencies up to 4.5%.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 4

1958: Vanguard 1 Satellite, 0.5-Watt solar array

2009 International Space Station: 130,000W Solar Array

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Basic Principles of Photovoltaic Effect

· Solar Cells are devices, which convert solar energy directly into electricity.

· The most common form of solar cells is based on the photovoltaic (PV) effect in which light falling on a two-layer semi-conductor device produces a photo voltage or potential difference between the layers.

· This voltage is capable of driving a current through an external circuit and thereby producing useful work.

· To have a deeper understanding of PV effect, it is essential to become familiar with the principles of construction and operation of a two-layer semiconductor device popularly known as PN junction.

· There is another group of material whose conductivity (or say resistivity) lies between that of conductors and insulators.

· This group of materials are called semiconductor. These semiconductors are basic building blocks of all the electronic components and the solar cells.

· Silicon and Germanium are the examples of semiconductor materials.

· In pure silicon, the number of freed electrons is always equal to holes. Adding impurities in it can increase the conductivity of pure or intrinsic silicon.

· The impurity is referred to as dopant and the process of adding dopant is called doping.

· Depending upon the type of dopant used, the impure or extrinsic semiconductor is called P type or N type semiconductor.

· By joining these two types of semiconductors, it is possible to create internal electric field to sweep freed electrons out of the material and force them to produce usable current.

· The solar cell is nothing but a large area PN interface or junction. It is the internal electric field of the PN junction that sweeps electrons out of the cell.

· When light penetrates into the semiconductor material, knocking free electrons and giving them potential energy, the freed electrons wander until they are pushed by the electric field across the PN junction.

· They are forced out of the cell, and are available for useful work.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 6

PV Cell / Module / Array & String Solar Cell

· A PV / Solar Cell is a semiconductor device that can convert solar energy into DC electricity through the Photovoltaic Effect (Conversion of solar light energy into electrical energy).

· When light shines on a PV / Solar Cell, it may be reflected, absorbed, or passes right through. But only the absorbed light generates electricity.

Solar Modules

· To increase their utility, many individual PV cells are interconnected together in a sealed, weatherproof package called a Panel (Module).

· For example, a 12 V Panel (Module) will have 36 cells connected in series and a 24 V Panel (Module) will have 72 PV Cells connected in series.

Array and String

· To achieve the desired voltage and current, Modules are wired in series and parallel into what is called aPV Array.

· Series combination of modules is also known as astring.

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PV Cell Model

Where is:

Iph – photocurrent (A),

Is – reverse saturation current (A) (approximately in the range of 10-8/m2 ), v- diode voltage in volt,

Vt– thermal voltage in volt (25.7 mV at 250C), m – diode factor.

The thermal voltage Vt for given temperature can be calculated with the following equation:

Where is:

k- Boltzmann constant = 1.38 x 10-23 J/K, T- temperature (K),

e - charge of electron = 1.6 x 10-19 Columbs.

Ideal Solar Cell Model Real Time Solar Cell Model

The real solar cell model consistsof serial resistance (Rs), parallel resistance (Rp) to

reflect the voltage drops and parasitic currents and the load resistance (Rl) to determine the operating point in the I-V curve.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 8

PV Module Structure

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I-V Characteristics of a PV Cell

1. It is the current and voltage ( I-V ) characteristics of a particular photovoltaic ( PV ) cell, module or array giving a detailed description of its solar energy conversion ability and efficiency.

Knowing the electrical I-V characteristics (more importantly Pmax) of a solar cell, or panel is critical in determining the device’s output performance and solar efficiency.

The Above graph shows current-voltage ( I-V ) characteristics of a typical silicon PV cell operating under normal conditions. The power delivered by a solar cell is the product of current and voltage (I x V). If the multiplication is done, point for point, for all voltages from

short-circuit to open-circuit conditions, the power curve is obtained for a given radiation level.

With the solar cell open-circuited, i.e. not connected to any load, the current will be at its minimum (zero) and the voltage across the cell is at its maximum, known as the solar cells open circuit voltage, or Voc.

At the other extreme, when the solar cell is short circuited, that is the positive and negative leads connected together, the voltage across the cell is at its minimum (zero) but the current flowing out of the cell reaches its maximum, known as the solar cells short circuit current, or Isc.

The point at which the cell generates maximum electrical power (shown at the top right area of the green rectangle) is the “maximum power point” or MPP. Therefore, the ideal operation of a photovoltaic cell (or panel) is defined to be at the maximum power point. The maximum power point (MPP) of a solar cell is positioned near the bend in the I-V characteristics curve.

The corresponding values of Vmp and Imp can be estimated from the open circuit voltage and

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 10 and the closer the fill factor is to 1 (unity), the more power the array can provide. Typical values are between 0.7 and 0.8.x

2. Short Circuit Current (Isc) – It is the maximum current (in A or mA) produced by the cell under given conditions of irradiance and surrounding temperature. Isc is the current when the load is short-circuited, i.e. the output voltage is zero. The output power at this point is essentially zero.

3. Open Circuit Voltage (Voc) – It is the maximum voltage generated by the cell under given conditions of light and temperature. Voc is the voltage when the load is open-circuited, i.e. the output current is zero. The output power at this point is again essentially zero.

4. Maximum Power (Pmax) – It is the maximum power that can be delivered from the cell under specific environmental conditions. The point at I-V curve at which the maximum power is attainable is called Maximum Power Point (MPP).

5. Current at Maximum Power (Imp) – It is the current that results in maximum power. Imp is also called the “Rated” current of the cell.

6. Voltage at Maximum Power (Vmp) – The voltage that results in maximum power output is called Voltage at maximum power. Vmp is also called “Rated” voltage of the cell.

7. Fill Factor (FF) – The fill factor is a figure of merit that indicates the “squareness” of the I-V curve. It is the ratio of the actual maximum power Pmax to the unattainable but ideal power that would result from operating at Isc and Voc.

8. Total Area Efficiency – It is the ratio of electrical power output (typically the Pmax) to the total light power incident on the entire cell area including frames (if Applicable), interconnects and pattern lines on the surface.

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I-V Characteristics of a PV Module

· Photovoltaic panels can be wired or connected together in either series or parallel combinations, or both to increase the voltage or current capacity of the solar array. If the array panels are connected together in a series combination, then the voltage increases and if connected together in parallel then the current increases. The electrical power in Watts, generated by these different photovoltaic combinations will still be the product of the voltage times the current, ( P = V x I ). However, the solar panels are connected together, the upper right-hand corner will always be the maximum power point (MPP) of the array.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 12

Power Characteristics of a PV Module

The power generated by a solar cell will reach a maximum when the internal resistance of the cell is equal to the resistance of the load and is known as the maximum power point (MPP) or PMAX.

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Parameters affects I-V Characteristics.

Effect of Environment on Output of the Cell

The I-V curve of a solar cell is really just a “snap-shot” view of the potential out put under static environmental condition (solar radiation and temperature). If these parameters are changed, the output (voltage and current) of the device will change.

As the intensity of light changes, so does the number electrons release. So, the direct result of a change in light intensity is a change in the output current in all voltage levels.

The short circuit current Isc of a cell is directly proportional to the light intensity. The standard value of Isc is provided by manufacturer at the intensity of light of one sun or peak insolation that equals to 1000 W/sq.m (100 mW/sq.cm).

Isc (at given light intensity) = Isc (at STC) x (given light intensity/ 1000 w/sq.m)

The open circuit cell voltage (Voc), on the other hand, varies more slowly in a logarithmic relationship with light intensity.

When the cell temperature rises (due to rise in ambient temperature), the main effect is to reduce the voltage available at most currents. There is slight rise in current at very low voltage. The change in voltage is directly proportional to the rise in temperature. The proportionality coefficient is called temperature coefficient and measured in terms of +/- V per 0 C or +/- mA per cm.sq. per 0C. Sometimes the proportionality coefficient is expressed in terms of percent change per degree change in temperature.

The typical values of temperature coefficients for Voc and Isc for various cells are given in table

The fall in voltage and slight rise in current at very low voltage results in overall reduction in maximum power by 0.5% per deg.C in Cz cells and 0.3% per deg. C in amorphous cells.

With the increase in ambient temperature, Voc of the cell reduces, whereas Isc remains constant.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 14 With the increase in irradiance, Isc increases drastically, whereas Voc reduces by small amount (almost

constant).

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PV Cell Connection

Series connected modules

1. Achieved by connecting the positive at the front of one cell with the negative at the back of the second cell Voltage of solar cells adds together when these are in series

2. When we connect two solar cells with different currents in series the current in the series circuit corresponds to that of the weakest solar cell;

3. The same effect occurs when a solar cell is partially or completely covered due to shadow of a tree or a fallen leaf etc.

4. Same effect occurs if there is a break in the module or a solar cell

Parallel connected modules

1. Achieved by connecting all positives and all negatives together.

2. Current through individual cells is added together.

3. In practice, a module has all solar cells in series and modules are connected in parallel

PV Cell

Series Parallel Series -

Parallel

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 16

Series-parallel connected modules

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Types of PV Technology

Monocrystalline:

1. Nearly 80% of the solar cells manufactured all over the world are fabricated using crystalline silicon.

2. These have been used as semi–conductors almost over the last hundred years in diodes, ICs etc.

3. It is because of its wide availability as a raw material for the electronics industry.

4. Silicon ingot is pulled as a single crystal.

5. The internal crystalline structure is completely homogenous. Cells are sliced from ingot.

Types of PV Technology

First Generartion

Crystalline

Mono-

crystalline Poly-crystalline

Second Generation

Thin film

CdTe ClGS Amorphous-Si

Third Generation

Concentrating SP

Dyesensitized solar cells

(DSSC)

Organic solar cells

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 18 Some of the leading manufacturers of monocrystalline modules are:

JA Solar, China BHEL, India Sun Power, USA Canadian Solar, Canada

Polycrystalline:

1. As one does not require silicon with such purity levels, as required for manufacturing of

semiconductors for solar PV, many firms have developed methods to produce polycrystalline silicon blocks which can be used for cell manufacturing.

2. Some of the factors in favor of multi-crystalline modules are:

· Reduced wastage of silicon

· Reduced electricity consumption

· Efficiency at closer levels of Monocrystalline

3. However, they tend to be slightly less efficient, with average efficiencies of around 12%-16%. They have a speckled crystal reflective appearance, and again need to be mounted in a rigid frame.

Some of the leading manufacturers of polycrystalline modules are:

Vikram Solar, India Trina Solar, China Astronergy, China EMMVEE Solar, India Yingli Solar, China Canadian Solar, Canada

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Thin Film:

1. A thin-film solar cell (TFSC), also called a thin-film photovoltaic cell (TFPV), is a solar cell that is made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. The thickness range of such a layer is wide and varies from a few nanometres to tens of micrometres.

2. Amorphous Silicon Solar Cells

Unlike in crystalline cells, atoms are arranged in a haphazard manner in a–Si modules. It became a good candidate for solar cells after it was found that it can absorb almost 40 times more light than monocrystalline silicon.

Main advantages of a–Si modules are as follows:

High optical absorption Larger band gap

Less material consumption

Low energy consumption during manufacture

Possibility of automation of the manufacturing process Disadvantages are the following:

Low stabilized efficiency Light induced degradation

Some of the leading manufacturers of this modules are:

Sharp Solar, Japan NexPower, Taiwan

3. Cadmium telluride solar cell

A cadmium telluride solar cell uses a cadmium telluride (CdTe) thin film, a semiconductor layer to absorb and convert sunlight into electricity. The cadmium present in the cells would be toxic if released. However, release is impossible during normal operation of the cells and is unlikely during fires in residential roofs.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 20 4. Copper-Indium Selenide

CIS films (no Ga) achieved greater than 14% efficiency. However, manufacturing costs of CIS solar cells at present are high when compared with amorphous silicon solar cells but continuing work is leading to more cost-effective production processes.

Solar Frontier, of Japan, is the major manufacturer of CIS modules in the world.

PV/Thermal Energy:

1. Concentrating Solar Power (CSP)

Concentrating SP (CSP) systems utilize optical devices, such as lenses or mirrors, to concentrate direct solar radiation onto very small, highly efficient multi-junction solar cells made of a semiconductor material. The sunlight concentration factor ranges from 2 to 100 suns (low- to medium-concentration) up to 1000 suns (high concentration). To be effective, the lenses need to be permanently oriented towards the sun, using a single- or double-axis tracking system for low and high concentrations, respectively. Cooling systems (active or passive) are needed for some concentrating PV designs, while other novel approaches can get round this need. CSP

technology utilizes three alternative technological approaches:

· Trough systems

· Power tower systems

· Dish/engine system

2. Dye-sensitized solar cells

Dye sensitized solar cells use photo-electrochemical solar cells, which are based on semiconductor structures formed between a photo-sensitized anode and an electrolyte. In a typical DSSC, the semiconductor Nano crystals serve as antennae that harvest the sunlight (photons) and the dye molecule is responsible for the charge separation (photocurrent). These cells are attractive because they use low-cost materials and are simple to manufacture. They release electrons from, for example, titanium dioxide covered by a light absorbing pigment.

However, their performance can degrade over time with exposure to UV light and the use of a liquid electrolyte can be problematic when there is a risk of freezing.

3. Organic/polymer solar cells

Organic solar cells are composed of organic or polymer materials (such as organic polymers or small organic molecules). They are inexpensive, but not very efficient. They are emerging as a

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niche technology, but their future development is not clear. Their success in recent years has been due to many significant improvements that have led to higher efficiencies. Organic PV module efficiencies are now in the range 4% to 5% for commercial systems and 6% to 8% in the laboratory (OrgaPVnet, 2009).

4. Novel and emerging solar cell concepts

There are a number of novel solar cell technologies under development that rely on using quantum dots/wires, quantum wells, or super lattice technologies (Nozik, 2011 and Raffaelle, 2011). These technologies are likely to be used in concentrating PV technologies where they could achieve very high efficiencies by overcoming the thermodynamic limitations of conventional (crystalline) cells. However, these high efficiency approaches are in the fundamental materials research phase.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 22

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Solar Energy

Solar Energy

PV System

Grid Connected

Without Storage System

With Storage System

Stand-alone

Without

Battery With Battery Hybrid PV

system

Thermal System

Flate Plate ETC Parabolic

PV/Thermal System

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 24

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Types of PV Inverter

· Inverters play a crucial role in any solar energy system. An inverter’s basic function is to “invert” the direct current (DC) output of the PV modules into alternating current (AC). AC is the standard used by all commercial appliances, which is why many view inverters as the “gateway” between the photovoltaic (PV) system and the energy off-taker.

· Inverter technologies have advanced significantly, such that in addition to converting DC to AC, it provides a number of other capabilities and services to ensure that the inverter can operate at an optimal performance level, such as data monitoring, advanced utility controls, applications and system design engineering.

· Inverter manufacturers also provide post installation services that are integral to maintaining energy production and a high level of performance for the project, including preventative maintenance, O&M services and a quick mean time to repair (MTTR).

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 26

PV Inverter

Micro Inverter

String Inverter

Hybrid Inverter

Central

Inverter

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1. Micro-inverters:

A solar micro-inverter, or simply micro inverter, is a device used in photovoltaic that converts direct current (DC) generated by a single solar module to alternating current (AC). The output from several micro inverters is combined and often fed to the electrical grid.Micro inverters contrast with conventional string and central solar inverters, which are connected to multiple solar modules or panels of the PV system. Micro inverters have several advantages over conventional inverters. The main advantage is that small amounts of shading, debris or snow lines on any one solar module, or even a complete module failure, do not disproportionately reduce the output of the entire array. Each micro inverter harvests optimum power by performing maximum power point tracking (MPPT) for its connected module. Simplicity in system design, lower amperage wires, simplified stock management, and added safety are other factors introduced with the micro inverter solution.

· Advantages

(a) Resilience to partial shading effects as compared to the central and string inverters.

(b) MPPT at module level

(c) Highest system flexibility for future expansion (d) Minimum DC wiring costs

(e) Monitoring at module level

· Disadvantages

(a) High per Watt cost (b) High maintenance costs

(c) Difficult access for maintenance since the installation is under the PV modules

2. String-inverters

A string inverter is the type most commonly used in home and commercial solar PV power systems. It is a box that is often suspended on the module mounting structure. Depending on the size of installations, number of strings are connected to the inverter. The rated Maximum DC input power (Pdcmax) for these inverters will be in the range of 2 – 30kWp.

· Advantages

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 28 (e) Monitoring at string level

· Disadvantages

(a) The installation requires special racking for the inverter for each string (b) Poor flexibility at partial shading

(c) Higher per Watt cost than central inverter

3. Central Inverters

Central inverters are designed for applications such as large PV arrays installed on buildings, industrial facilities as well as ground mounted

· Advantages.

(a) The most traditional inverter topology (b) Easy system design and implementation (c) Low cost per Watt

(d) Easy accessibility for maintenance and troubleshooting

· Disadvantages

(a) High DC wiring costs and power loss due to Voltage Drop.

(b) Single MPPT for the entire PV system

(c) System output can be drastically reduced in case of partial shading and string mismatch

(d) Difficult to add strings or arrays for future expansion (e) Single failure point for the entire system

(f) Monitoring at array level (g) Huge size

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4. Hybrid inverter

The function of a smart grid is to enable selection and orientation of renewable energy, the energy extracted from thegrid and energy stored based on consumption. This system also allows choosing whether electricity from photovoltaic panels should be stored or consumed through an internal intelligent apparatus control unit. This is possible through a technique that adds different energy sources (phase coupling: on-grid or grid-tie techniques) and the

management of stored electricity in the battery (off grid technology). Hybrid inverters therefore operate on grid tie as well as off-grid, bothon-grid and off-grid at the same time and Backup.

· Advantage

(a) All-in-one inverter solution for grid-connected solar-plus-storage systems (b) Frequently intelligent and programmable for maximizing overall system

efficiency and savings

(c) Can usually be installed without batteries for future expansion

· Disadvantage

(a) Less design flexibility than modular solutions which use separate PV and battery inverters.

(b) Generally, less efficient than dedicated solar-only or battery-only inverters.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 30

Battery

1. Battery:

a. A device that stores electrical energy in the form of chemical energy. The rechargeable battery can convert it back to electrical energy and vice-versa.

b. Batteries have three parts, an anode (-), a cathode (+), and the electrolyte.

2. Capacity

a. Amount of electrical energy the battery stores (in AH) b. Amps x Hours = Amp-hours (Ah)

c. 100 Amp-hours = (100 amps for 1 hour / 1 amp for 100 hours / 10 amps for 10 hours) d. Capacity changes with Discharge Rate

e. The higher the discharge rate the lower the capacity and vice versa.

3. State of Charge (SOC)

a. Energy available in battery (in %) 4. Depth of Discharge (DOD)

a. Energy drawn out of the battery (in %) 5. Efficiency

a. Energy out/Energy in (typically 80-85%)

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Battery Sizing

1. Recommended battery type deep cycle battery 2. Sizing

a. Calculate total kilo Watt-hours per day used by appliances.

b. Multiply the total kilo Watt-hours per day used by 0.85 for battery and system losses.

c. Divide the answer obtained in item 4.2 by the nominal battery voltage.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 32

Solar Charge Controller Sizing

1. The solar charge controller is typically rated against Amperage and Voltage capacities.

2. Select the solar charge controller to match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your application.

3. For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration).

4. According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array.

5. Solar charge controller rating = Total short circuit current of PV array

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Grid Terms of Solar PV System

1. Capacity Utilization Factor (CUF):

a. CUF is Ratio of actual output from a solar plant over the year (in KWh) and the maximum possible output from it for year (KWh) under ideal conditions.

b. CUF = (Actual Plant Output in KWh over the year/ 24*365*Installed Plant Capacity in KWp)

2. Performance Ratio (PR)

a. Ratio of plant output versus installed plant capacity at any instance with respect to the radiation measured.

b. PR = [{(Measured output in kW)/ (Installed Plant capacity in kWp)} x {(1000 W/m2) / (Measured radiation intensity in W/m2)}]

3. Grid Parity

a. It occurs when Solar Energy can generate power at a levelized cost of electricity (LCOE) that is less than or equal to the price of purchasing power from the electricity grid.

4. Levelized Cost of Electricity (LCOE)

a. It is the net present value of the unit-cost of electricity over the lifetime of a generating asset.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 34

Effect of shading on Solar PV Modules

1. Performance of a series connected string of solar cells is adversely affected if all its cells are not equally illuminated (partially shaded). In a solar array spread over vast area, it is likely that shadow may fall over some of its cells due to tree leaves falling over it, birds or bird litters on the array, shade of a neighboring construction etc.

2. In a series connected string of cells, all the cells carry the same current. Even though a few cells under shade produce less photon current but these cells are also forced to carry the same current as the other fully illuminated cells.

3. The shaded cells may get reverse biased, acting as loads, draining power from fully illuminated cells.

If the system is not appropriately protected, hot-spot problem can arise and, in several cases, the system can be irreversibly damaged.

4. A shadow falling on a group of cells will reduce the total output by two mechanisms:

a. By reducing the energy input to the cell

b. By increasing energy losses in the shaded cells.

5. Problems become more serious when shaded cells get reverse biased. Power dissipation in the shaded cell may be substantial leading to increase in its temperature.

6. Due to increased temperature, the cell current gets concentrated in an increasingly small region of the cell, producing the hot spot.

7. This can damage the cell encapsulation and eventually produce module failure

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Bypass Diode

If many cells are connected in series, shading of individual cells can lead to the destruction of the shaded cell or of the lamination material, so the Panel (Module) may blister and burst. To avoid such an

operational condition, Bypass Diodes are connected anti-parallel to the solar cells as in the above figure.

Consequently, larger voltage differences cannot arise in the reverse-current direction of the solar cells. In practice, it is sufficient to connect one bypass diode for every 15-20 cells. Bypass diodes also allow current to flow through the PV module when it is partially shaded, even if at a reduced voltage and power. Bypass diodes do not cause any losses, because under normal operation, current does not flow through them.

The by-pass diode for the current below Imp' becomes forward biased (as the polarity of the cell reverses for the current above Imp'). In this case the diode will pass the difference current to other cells, thus reducing the heating effect to the shaded cell and increasing the module power even with shaded cell in series with other cells. In normal condition (unshaded cell) the cell is in its normal polarity and therefore the diode is reverse biased. All the cell current passes through the cell itself. The price paid for adding a by-pass diode is the forward voltage drop of about 0.7 V in it

It is impractical to add a diode to each cell. Instead, the cells in the module are divided into three strings of equal cells and by-pass diodes (two numbers) are inserted between each string.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 36 If a module is constructed by placing cells in series, what happens if one cell fails or is shaded. In theory, the whole module would fail, as would the string it was part of. In practice, bypass diodes are used to separate out part of each module. Thus, current can be routed around failure, just losing voltage.

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Blocking Diode

The blocking diode is a low forward voltage drop semiconductor device that blocks the flow of current in reverse direction. During the day when there is sufficient sunlight, the solar module acts as a generator of electricity. It means during the day the current flow from the module to the storage battery. But during nights the module IV curve is shifted downwards to zero current level.

At this moment the battery voltage will see the module as a drain (i.e., the module will be seen as a load) and current will start flowing from the battery to the module, thus losing precious energy gained during the daytime. The amount of reverse leakage current will depend upon the battery voltage and the shape of IV curve. Poorer the IV curve, higher will be the reverse leakage current. Though the leaking current will not harm the module but if precautions are not taken, the current stored in the battery will leak to the module and converted into heat during dark hours. Therefore, to reduce the leaking current a diode is connected in the path between the module and the battery in such a way that during the day it is forward biased from module point of view and during the night it remains reverse biased from battery point of view.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 38 With the blocking diode, the leakage current is limited to the reverse saturation current of the diode (normally in the range of micro-Amperes). The penalty for using the blocking diode is the forward voltage drop in it during the daytime charging of the battery. This drop is about 0.7 V for silicon diodes and 0.3 V for low forward drop Schottky diodes or germanium diodes. Even if the module does not incorporate a blocking diode, it has to be inserted during the wiring of the system. In most cases a charge regulator has the blocking diode in it. If the load of a PV module is not a storage battery, then blocking diode can be omitted.

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Isolation Diode

If one string becomes severely shaded, or if there is short circuit in one of the modules, the diode connected in series with each string prevents the other strings from losing current backwards down the shaded or damaged string. By use of these diodes the shaded or damaged string is "isolated" from the others, and more current is sent to the load. These diodes perform the same function as the blocking diodes, but because they isolate the damaged or shaded string, they are also called isolation diodes.

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Karthikeyan SK https://www.linkedin.com/in/simplykarthik/ Page No: 40

List of References

1. Introduction to Photovoltaics – SMA 2. Basics of Solar – SK Sangal

3. Training manual on solar PV System – ESAP 4. Power Project PVSyst Training

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