In this era the high demand for energy has been on a constant rise which has led to the exhaustion of the limited fossil fuels available such as oil, gas and coal which account for most of the world’s energy production.
These fossil fuels have negative effect on the environment due the pollution that they emit upon usage or production. As a result of that scientist have been conducting research on renewable energy sources that can provide an unlimited clean source of energy.
One of these methods is the usage of photovoltaic (PV) energy which has the ability to convert solar energy into electricity. If this method is used in combination with a smart grid it allows the users to regulate and monitor their energy consumption while also having the option to sell their excess energy back to electric companies.
PV energy production can be regarded as into two main classification depending their required application[1]:
Grid – connected systems – These systems have a bi-directional relationship with the electrical grid, this allows them to feed back any surplus energy back to the grid when the user has sufficient energy and when there is a shortage energy it can be drown from the grid.
Stand-alone system – these kind of systems are self-reliant therefore they do not relay on the electrical grid and use a battery bank instead hence the name. These systems are commonly used for remote areas where grid connection isn’t available.
The reason for the increase on the demand for grid-connected PV systems is a result of the increase in energy prices along with consumer’s consumption of it and of the advances that are being made in this field from increases in its efficacy to the simplicity of installing them. The market for solar energy has seen an average of about 20% increase per year for the past 5 years. (http://solarcellcentral.com/) and in 2006 there was 36% increase in the US market (solar Energy industries association) due to the advances in the field from reduction in installation prices to better output efficiency.
Figure (1) shows a block diagram of the usual design for a PV system. The process starts with the PV cells that can be arranged depending on the desired output to form arrays. The number of arrays used depend on the desired output and the available space. Then the solar energy absorbed by the PV cells is converted to DC Voltage which its value is then increased by the boost converter that has a Maximum power point tracking (MPPT) to optimise its efficiency. Then between the converter and inverter a DC-Link is used to provide sufficient energy during power losses and to block and also to prevent transients from radiating back to the input that could damage the system.
After that a DC-AC inverter is used to enable the system to produce AC voltage, the voltage and frequency output can be adjusted to the user specified values.
Last of all the AC voltage produced is filtered to give a steady and stable waveform which can be either used or fed back to the electrical grid.
It should be noted that there are multiple topologies for each of these components where each one could have several advantages over the other in certain cases and depending on the components and values used it is possible to obtain any desired value.
One of the main aspects that will be discussed in this paper is the utilization of the micro-inverter (DC/AC inverter). When solar energy was first used it had a DC output but when the concept of AC module was introduced by implementing a micro-inverter in the system it received a wide appeal by its ease of use where it did not need DC wiring or DC-ground connected protection, where it could be directly connected to a building and could be sold as a complete power source package that is simple to implement.
Some of the other advantages of the AC PV module are:
Minimum and flexible systems sizes.
Each module has its own MPPT.
Cost has been reduced as a result of the mass production.
Enhanced safety over the conventional DC PV module.
Some of the drawback for this systems is that the power received depends on the location and climate where it is being used.
This project will concentrate on the design of a single-phase AC PV Module by using a DC-DC boost Converter that will utilise the perturb and observe (P&O) MPPT method with the use of a filtered H-Bridge inverter the produce a stable sinusoidal output that could be used directly or connected to the grid.
1.12 Aims and objective:
The focus of this project is to simulate a bi-directional grid connected single-phase PV system on Matlab/Simulink. The stated objectives are:
1. To simulate the ac PV module using Matlab/SIMULINK.
2. To build a small dc-dc converter for the maximum power point tracking stage of the ac module.
The simulation for the boost converter, H-bridge and filter have been constructed successfully. Perturb and observe has been implanted for the converter and a current control system had been used for the inverter.
Road map
This dissertation …
LITERATURE REVIEW
PHOTOVOLTAIC CHARECTRISTICS
The processes that a PV cell convert’s sunlight into electricity is by absorbing the sunlight photons that it is composed of, the absorbed photons contain different amounts of energy depending on the size of the irradiance wavelength. When this energy is absorbed the electrons from the semiconductor atoms are knocked out of position to for an electrical circuit which can be used to supply power to a load. [2]
There are several factors that affect the overall power produced by the system that need to be taken into consideration while the system is being designed, these factors are [3]:
Solar irradiance. This factor depends on the location of the system. This could lead to a decrease of the system output by about 25% or could lead to an increase by 30%.
Theoretical power at test conditions. Most systems will have similar expected output vales within a 5% calculation error.
Operating temperature. This could result in the reduction of the power produced from 2-10% depending on the mounting technique, design and ambient cell temperature.
MPPT. This method is utilised to get the most out of the system and will be discussed in more detail at….
Soiling. This may lead to a decrease of the annual energy produced by up to 10%.
Optical losses. This depends on the suns irradiance angle. Due to the reflectance of the systems glass small losses could accoutre to have an overall reduction up to 10%.
The ideal PV cell is represented by a parallel connected current source with a rectifying diode as can be seen in figure (2)
Figure (2)
One of the most common method of representing a solar module is by using its I-V characteristics that is described by its Shockley solar cell equation that can be seen below:
I=I_ph-I_o (e^(qV/(K_B T))-1)
K_bis the Boltzman constant, T is the absolute temperature, V is the terminal voltage of the cell, I_o is the saturation current and I_ph is the photogenrated current which is related to the absorbed photon flux that depends on the lights wavelength. [2]
Figure (3) shows the I-V and P-V curve for the idol solar cell. I_sc The short circuit current is equal to the photogenerated current. The maximum power is found by determining V_m and I_m of the system which can be found by the fill factor (FF) that represents the comparison by the max power and theoretical powerP_T at both the open circuit voltage and short circuit current. This can be represented by the following equation:
FF=P_max/P_T =(I_M V_M)/(I_sc V_oc )
The IV curve is used to find multiple combinations for the output depending on the desired power.
The P-V curve will be discussed in more details in the MPPT section of the litruiter review.
Figure (3)
Also the variation of the factors irradiance and temperature affect the P-V curve as can be seen in figure (4).
It can be seen from the graph above the irradiance is directly proportional with the system output as for the temperature it’s inversely proportional with the power produced.
Figure (4)
REVIEW OF DC-DC CONVERETS
A DC-DC converter is an electric power converter that can either decrease or increase the voltage level. The basic structures of these converters are classified as second order or forth order converters. In addition by the number of switches they are categorised into two groups: single input single output (SISO) and multiple input multiple outputs (MIMO) [4].
The most commonly used converters are the SISO second order converters and the most popular are:
Boost (step-up) converter
this type of converter boostes the inputed voltage to get a higher output hence the name. for example if a battery is used in a system and its voltage is less then what is needed to power the device. A boost converter is constructed of an inductor, a diode and a switching device.
The way a step up converter works is by utilising the duty cycle of the switch. When the switch is closed current is stored in the inductor in the form of a magnetic field hence energy is stored in the inductor. After the switch is opened the energy of the inductor is transferred out to increase the voltage, this is because current in the inductor cannot change instantaneously therefore the current has to flow through diode and into the output
.
Buck (step-down) converter
The buck converter reduces the input-voltage to give a lower output. This type of converter is a type of switching mode power supply which usually consist of an inductor, diode and a switching a device however a capacitor is
Sometimes used as filter with it to rescue the output voltage-ripple.
An example of the step down converter is when a 12v batteries is used in a 5v device, in this case the buck converter would adjust the value depending on the device needs to avoid damage to it. The way buck converter works is by controlling the duty cycle of the switch to either open or close the gate to adjust the voltage value.
Buck-boost converter
The buck boost converter is a combination of both the step-up and step-down converter, this allows it to use both of their applications from increasing or decreasing the voltage. This can be achieved as a result both converters have the same components.
An example use for this converter is a battery powered supplies device. At the start of the battery’s life its voltage output could be greater than the device that could result in damage to it therefore the step down application would be utilised however as its life decays it would produce less voltage then the device requires to operate, at this time the boost method would be used.
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