Solar Energy Is The Energy Generated Engineering Essay

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Solar energy is one of the most important forms of renewable energy. Solar energy converted into electricity has various applications in residential, vehicular, aircraft, space, naval applications. This part of paper discusses about the introduction to solar energy, structure of PV arrays, sun tracking systems, maximum power point tracking techniques, shading effects on PV cells, power electronic interfaces, PV cell sizing, advantages, limitations, storage and other applications.

Introduction

Solar energy is the energy generated by the sun which is converted into electricity using 'photo electric effect'. This is accomplished by using PV cells. The light may be absorbed, reflected or pass through the PV cells. However, only the absorbed light is responsible for generation of electricity. PV cells absorb the packet of light from the sun known as photons which provides energy to the electron and thus creates mobility of electron in the cell thereby generating current. A special property of PV cell is known as "built in electric field" which provides the voltage required to drive the current through an external load. PV cell consists of two different layers of semiconductors which are in contact with each other. The two layers consist of a P-type semiconductor which has majority of holes and N-type semiconductor which has majority of electrons. This generates the required potential difference.

Types of Solar Cells [2]

Solar cell technology falls into three main categories:

Monocrystalline Photovoltaic Solar cell - These cells have highest conversion efficiency. The cells are manufactured from extremely pure silicon. This process is highly sensitive and expensive.

Polycrystalline Photovoltaic Solar Cells- These cells in comparison with the monocrystalline cells but are comparatively less expensive.

Amorphous Photovoltaic Solar Cells- These types of semiconductor materials are doped with impurities. The advantages of this technology are relatively simple manufacturing process, low cost, lower production energy conversion. With this technology has some disadvantages too which are larger installation surface, low efficiency and less life span.

Other technologies are thin film cell technology, Gallium arsenide cell technology, tandem cell technology.

Concentrators are lenses that focus sunlight onto the PV cells. Fresnel lenses generally used which have a concentration of 10 to 500 times [2] which are made up of cheap plastic materials.

A single PV cell typically produces 1-2 Watts of power. To have a greater output voltage we combine these PV cells together to form a PV module which further connected together forms a PV array.

Solar Harvesting is typically classified as passive solar system and active solar energy system. [1] Passive Solar energy system does not involve panel system or other moving mechanism to produce energy. This mechanism is generally used to capture sunlight with windows, tanks etc. This technology is generally used for heating, lighting, cooling or ventilation purposes. [2] This system is simple and cheaper. Active Solar energy system involves electrical or mechanical controlling components to orient the panel to maximum exposure to sunlight. This system converts the electricity to direct current which further with the help of power electronic converter is converted to alternating current which can be fed to the grid. This mechanism is complex and expensive. [1]

V-I and P-I Characteristics of Photovoltaic (PV) Systems

Current-voltage (I-V) relationships that measure the electrical characteristics

of PV devices are depicted by what we call "I-V curves"

􀂃 These I-V curves are obtained by exposing the cell to a constant level of

light, while maintaining a constant cell temperature, varying the resistance of

the load, and measuring the produced current

􀂃 On an I-V plot, the vertical axis refers to current and the horizontal axis

refers to voltage.

􀂃 The actual I-V curve typically passes through two significant points:

􀂃 The short-circuit current (Isc) is the current produced when the positive

and negative terminals of the cell are short-circuited, and the voltage

between the terminals is zero, which corresponds to a load resistance of

zero

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􀂃 The open-circuit voltage (Voc) is the voltage across the positive and

negative terminals under open-circuit conditions, and the current is zero,

which corresponds to a load resistance of infinity

􀂃 The cell may be operated over a wide range of voltages and currents

􀂃 By varying the load resistance from zero (a short circuit) to infinity (an open circuit),

we can determine the highest efficiency as the point where the cell delivers maximum

power

􀂃 On the I-V curve, the maximum-power point (Pm) occurs when the product of

current times voltage is maximum

􀂃 No power is produced at the short-circuit current with no voltage, or at open-circuit

voltage with no current

􀂃 We expect to find maximum power generated somewhere between these two

points

􀂃 Maximum power is generated at only one place on the power curve, at about the

"knee" of the curve. This point represents the maximum efficiency of the solar device

in converting sunlight into electricity

􀂃 A PV system consists of many cells connected in series and parallel to

provide the desired output terminal voltage and current

􀂃 This PV system exhibits a nonlinear I-V characteristic

􀂃 There are various models available to model the I-V characteristics of the

PV systems

􀂃 Masoum et. al., introduced a verified model for silicon solar PV panel

􀂃 The PV cell equivalent model, which represents the dynamic nonlinear I-V

characteristics of the PV system is described in the following equation;

PV Models and Equivalent Circuit

Single -diode and dual diode Model

Single- diode model without parallel resistance

Single diode model without resistances

Effects of Irradiance and temeprature [1]

As the radiance increases the short circuit current and open circuit voltage of the solar cell increase. The short circuit current is linearly proportional with the irradiance. On the other hand when the temperature increase the open circuit voltage decreases and short circuit current increases since the temperature is function of irradiance. The decrease in in open circuit voltage decreases the efficiency of cell while the short circuit current increases with the cell temperature. This is shown with the help of graph below:

Sun tracking system is a system that focuses the cells towards the sun since the direction of the sun changes with time. This increases the efficiency of the system and during hot summer afternoon we can tie the system to the grid in order to meet the peak demand. A passive system comparatively derives less power since it does not have the mechanism to move with the direction of the sun. One of the most common techniques in which sun is tracked with the relation between the angle of light source and differential current generated in two close photo diodes.

Maximum Power Point Tracking -The VI characteristic of PV cell is affected by condition of radiation and temperature as discussed above. We need to control voltage and current to track the maximum power. Maximum power point tracking system is used to track maximum power from the solar cell. The most common MPPT techniques are:

Incremental Conductance (INC) based MPPT

Perturb & Observe based Maximum Power Point Tracking

MPPT Controller based on Linearized I-V Characteristics

Fractional Open-Circuit Voltage based MPPT

Fractional Short-Circuit Current based MPPT

Fuzzy Logic Control based MPPT

Neural Network based MPPT

Ripple Correlation Control based MPPT

Current Sweep based MPPT

Dc Link Capacitor Droop Control based MPPT

Power Electronic Interfaces for PV Systems

Power electronic plays a vital role in converting dc energy from PV modules to ac loads or directly fed to the grid or to control the maximum power point tracking. [1] This also provides the features of wide operating range and provides different modes for various climatic conditions.

Power Electronic Interfaces for Grid Connected PV Systems

The power electronic interfaces for grid-connected PV systems can be classified into two main criteria:

Classification based on inverter utilization and converter stage and module configurations:[1]

Centralized inverter system

String inverters system

Multistring inverter system.

Based on number of converter stages and number of modules: [1]

two stage - single module,

single stage - multi module,

multi level single stage level,

Two stage - multi module.

Topologies based on Inverter Utilization

Centralized inverter topology In this topology, if enough number of PV panels are connected in series in each string, voltage boosting is not be required. Voltage can be stepped up by a dc/dc converter at the dc side or by a transformer embedded in a high frequency dc/dc converter. Separate MPPT can be applied to each string, to increase the overall efficiency of the system.

Multi-string inverter topology In this topology, several strings are interfaced with their own integrated dc/dc converter to a single common inverter. Individual PV strings can be turned on and off to use more or fewer modules. Further enlargements can be realized by adding integrated panel/converter groups. The outputs of the converters can be plugged into the existing platform, with all electrical connections in a single connector on the back plane. Therefore, this is a flexible design with high efficiency. [1]

Topologies based on Module and Stage Configurations

The power electronic conditioning circuits for solar energy systems can be transformerless, or they can utilize high-frequency transformers embedded in a dc/dc converter, which avoids bulky low-frequency transformers. The number of stages in these topologies refers to the number of cascaded converters/inverters in the system.

two stage - single module,

single stage - multi module,

two stage - multi module.

Two Stage - Single Module Topologies

The two stage conversion systems may have many varieties. The most common two-stage topologies consist of a dc/ac grid-connected voltage source PWM inverter with a dc/dc PV connected converters, with its associated MPPT system.

Isolated Two Stage - Single Module Topologies

Isolated DC/DC converters consist of a transformer between the DC/AC and AC/DC conversion stage. The transformer provides isolation between the PV source and load. The two stage DC/DC converter consists of a DC/AC inverter, a high frequency transformer, and a rectifier.

A capacitor is also used at the transformer input, forming an LC resonant circuit with the equivalent inductance of the transformer. This resonance circuit reduces the switching losses of the inverter.

Isolated Two Stage - Single Module Topologies

In the push-pull converter topology, a middle terminal connection is required since the inverter has only one level with less number of switching elements.

Isolated Two Stage - Single Module Topologies

The push-pull converter is modeled and simulated using SimPowerSystems

Other Isolated Two Stage - Single Module Topologies

The fly-back current-fed (FBCF) inverter can be controlled to provide a rectified sine-wave output current into the inverter and to track the MPP. The current into the fly-back converter is discontinuous and hence a buffer Fly-back Inverter capacitor should be used to eliminate both low and high-frequency ripples.

Series resonant dc/dc converter and its grid connected-inverter

A series resonant dc/dc converter plus a full bridge grid-connected inverter that is modified by adding two additional diodes. The dc/dc converter is operating at 100 kHz and has a fixed voltage transfer ratio as a "dc-transformer." Switching losses are reduced by resonant tank through zero-voltage switching. The switching losses from the converter can be reduced in this way.

Single Stage - Multi Module Topologies

This is the simplest grid connection topology. The inverter is a standard voltage source PWM inverter, connected to the utility through an LCL filter. The input voltage, generated by the PV modules, should be higher than the peak voltage of the utility. The efficiency is about 97%.

On the other hand, all the modules are connected to the same MPPT device. This may cause severe power losses during partial shading. In addition, a large capacitor is required for power decoupling between PV modules and the utility.

Two Stage - Multi Module Topologies

In two-stage configurations, the connection of the modules and the inverter

can be classified into two categories:

one is that all modules are connected in series (a), which is similar to

the two stage single module topologies.

􀂃 A grid-tie inverter plus a simple dc/dc converter, such as boost,

buck or buck-boost can be used for the dc/dc conversion stage, if

isolation is not required.

􀂃 The second category consists of a dc/dc converter for each string and

a common grid-connected inverter, as shown in (b).

Power Electronic Interfaces for Stand-Alone PV Systems

􀂃 The stand alone PV systems composed of a storage device and its

controller for sustainable satisfaction of the load power demands.

􀂃 The storage device with the controller should provide the power difference

when the available power from the PV panel is smaller than the required

power at the load bus.

􀂃 When the available power from the PV panel is more than the required

power, the PV panel should supply the load power and the excess power

should be used to charge the storage device.

Based on the PV/Battery Connection, stand alone PV systems are divided to

five categories:

PV/Battery Connection-Type 1

PV/Battery Connection-Type 2

PV/Battery Connection-Type 3

PV/Battery Connection Type 4

PV/Battery Connection-Type 5

Sizing the PV panel and battery Pack for Stand-alone PV applications [1]- It is important to have a proper size of the battery and the PV array to provide high performance, better cost efficiency and increase the lifespan of the PV system. The vital reason for proper sizing of the PV panel is to define the requirement of the battery pack. During the sizing of the array we need to consider the practical issues such as losses too. [3] Other important criteria are the average daily load requirement. For proper sizing of the array we need to consider sun- hour, load data, days of autonomy and solar radiation [1].

Sun-hour

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