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 measure the electrical characteristics of PV cells 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 is of current and the horizontal axis is of 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 open-circuit voltage (Voc) is the voltage across the positive and negative terminals under open-circuit conditions, and the current is zero. 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.
α ideality or completion factor
I0 PV cell reverse saturation current [A]
IPV PV cell output current [A]
Isc short-circuit cell current (representing insolation level [A]
Np the number of parallel strings
Ns the number of series cells per string
q electron charge [C]
Rs series resistance of PV cell [Ω]
T PV cell temperature [K]
VMP PV cell voltage corresponding to maximum power [V]
OC open-circuit voltage [V]
VPV terminal voltage for PV cell [V]
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.
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, thus voltage boosting is not 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.
Courtesy: Dr. Khaligh' Lecture notes
Single Stage - Multi Module Topologies
This is simplest topology to connect to the grid. PWM inverter is connected to the utility through an LCL filter. The input voltage generated by the PV modules must be higher than the peak voltage of the load. The efficiency is about 97%. All the modules in this case are connected to the same MPPT device. This causes severe power losses during partial shading we need a large capacitor to decuple the power between PV modules and the load.
Courtesy: Dr. Khaligh' Lecture notes
Two Stages - 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 with grid-tie inverter and a simple dc/dc converter. The second topology consists of a dc/dc converter for each string and a common grid-connected inverter.
Courtesy: Dr. Khaligh' Lecture notes
Power Electronic Interfaces for Stand-Alone PV Systems
The stand alone PV systems composed of a storage device and its controller to meet the load power demands. The storage device with the controller provides the power difference when the available power from the PV panel is smaller than the required power at the load. When the available power from the PV panel is more than the required power, the PV panels supply the load power and the excess power and is used to charge the storage device. Based on the PV/Battery Connection, stand alone PV systems are divided to following categories:
PV/Battery Connection-Type 1
PV/Battery Connection-Type 2
PV/Battery Connection-Type 3
PV/Battery Connection Type 4
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- Average Ah/day production from the array is calculated by the product of number of sun hours and the module power current.
Load Data-The actual need of the load is essential for designing the PV array. With the product of load current to its daily duration we can get the equivalent load consumption.
Days of Autonomy- are the time till when the PV panels can deliver the power to the load without solar energy. For this we need a bigger battery pack to provide back up during the absence of sunlight. For charge storage for a long span of time we can use supercapacitors too.
Solar radiation is another factor to be taken under consideration if the radiation are variable we need to vary the array size and battery pack accordingly.
System Losses are to be taken under consideration including the conduction and switching losses in the conversion system.
Determination of Series connected PV module [1] is given by Ns=Vs/Vm
Ns = Number of series module
Vs=Voltage of system
Vm=Voltage of a module
Determination of Parallel Module[1]
Np= number of Parallel String
Lda= daily load average
SH= Sun hours
SL= System Losses
Imp=Module current at maximum power
