How a Photovoltaic Power Plant Works? Construction and Working of a Solar Power Plant
What is Solar Power Plant?
The solar power plant is also known as the Photovoltaic (PV) power plant. It is a large-scale PV plant designed to produce bulk electrical power from solar radiation. The solar power plant uses solar energy to produce electrical power. Therefore, it is a conventional power plant.
Solar energy can be used directly to produce electrical energy using solar PV panels. Or there is another way to produce electrical energy that is concentrated solar energy. In this type of plant, the radiation energy of solar first converted into heat (thermal energy) and this heat is used to drive a conventional generator. This method is difficult and not efficient to produce electrical power on a large scale.
Hence, to produce electrical power on a large scale, solar PV panels are used. In this article, we will explain details about solar PV plants and PV panels. Below is the layout plan of photovoltaic power plant.
Photo Voltaic (PV) Principle
Silicon is the most commonly used material in solar cells. Silicon is a semiconductor material. Several materials show photoelectric properties like; cadmium, gallium arsenide, etc.
Electron-holes pairs are created in solar cells. The PV materials have the property to absorb photons of sunlight. The valance band electrons of semiconductor material are at lower energy and the electrons of conduction band are at a higher energy level. The difference between this energy level is known as bandgap energy Eg.
When sunlight falls on solar cells, the difference between photon energy E and bandgap energy Eg is absorbed by the cell. And it excites some electrons to jump across the bandgap. These electrons move from the valance band to the conduction band and create holes in the valance band.
Therefore, if the potential difference exists within the cell, the electrons of the conduction band and holes of the valance band made the flow of current in the circuit.
According to Max Plant, the energy of photons is directly proportional to the frequency of radiation.
- EP = Energy of Photon
- h = Plank’s Constant = 6.62×10-34 J s = 4.135×10-15 eV s
- v = frequency of radiation (Hz)
- C = speed of light ≈ 3×108 m/s
- λ = Wavelength of radiation (μm)
substituting these values in the above equation;
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Components of Solar Power Plant
The major components of the solar photovoltaic system are listed below.
- Photovoltaic (PV) panel
- Energy storage devices
- Charge controller
- System balancing component
Photovoltaic (PV) Panel
PV panels or Photovoltaic panel is a most important component of a solar power plant. It is made up of small solar cells. This is a device that is used to convert solar photon energy into electrical energy.
Generally, silicon is used as a semiconductor material in solar cells. The typical rating of silicon solar cells is 0.5 V and 6 Amp. And it is equivalent to 3 W power. The number of cells is connected in series or parallel and makes a module. The number of modules forms a solar panel.
According to the capacity of power plants, a number of plates are mounted and a group of panels is also known as Photovoltaic (PV) array.
The output of the solar panel is in the form of DC. The most of load connected to the power system network is in the form of AC. Therefore, we need to convert DC output power into AC power. For that, an inverter is used in solar power plants.
For a large-scaled grid-tied power plant, the inverter is connected with special protective devices. And a transformer is also connected with the inverter to assures the output voltage and frequency as per the standard supply.
Energy storage devices
The batteries are used to store electrical energy generated by the solar power plants. The storage components are the most important component in a power plant to meet the demand and variation of the load. This component is used especially when the sunshine is not available for few days.
The capacity of a battery is that how much amount of electrical power it can store. The capacity of batteries is measured in Ampere-hours (AH) rating.
For example, a battery having 100 AH battery can supply 1 Amp current for 100 hours or 100 Amp current for 1 hour.
For a long life of a battery, never fully discharge a battery. And in case, if a battery is fully discharged, never keep fully discharged battery for a long time.
The capacity of a battery is affected by the temperature. There is a reduction of 0.6% of capacity for every degree Celsius rise in temperature more than 25˚ C.
There are two types of batteries used in the solar power plant;
- Lead-Acid battery
- Nickel-Cadmium battery
A charge controller is used to control the charging and discharging of the battery. The charge controller is used to avoid the overcharging of the battery. The overcharging of a battery may lead to corrosion and reduce plate growth. And in the worst condition, it may damage the electrolyte of the battery.
Sometimes, the charge controller is termed a solar battery charger. There are many technologies used to make a charge controller. For example, the most popular technique is the MPPT charge controller that is known as “Maximum Power Point Tracking”. This algorithm is used to optimize the production of PV cells.
System balancing component
It is a set of components used to control, protect and distribute power in the system. These devices ensure that the system working in proper condition and utilize energy in the proper direction. And it ensures maximum output and security of other components of a solar power plant.
The solar PV panels are connected with a battery. And these panels are used to charge the battery during sunlight is available. During charging of the battery, the current flows from panel to battery. But when the sunlight is not available, the current can be flow in a reverse direction and it may harm the solar panel. So, the blocking diode is a diode that is connected between the battery and panel to avoid reversal current from battery to panel.
The output of solar panels depends on sunlight. And the sunlight is not constantly available. It is continuously varying. Similarly, the output of the solar panel is also varying with respect to sunlight. This results in fluctuation in load current. The voltage regulators are used to maintain fluctuation within an acceptable range.
Performance of Solar Cell
A solar cell is nothing but a PN junction. The plot of short-circuit current (ISC) and open-circuit voltage (VOC) describes the performance of the solar cell. This plot is shown in the figure below.
As shown in the above graph, Initially, the short-circuit current remains constant with an increase in voltage. And a further increase in voltage results in a rapid decrease in current.
The power developed by the solar cell is calculated by multiplying current and voltage. And from that, we can draw a graph of power developed. As shown in the graph of developed power, at point P, the power is maximum. And we try to operate the panel at this point. This point is known as the maximum PowerPoint. And the algorithm used to track this point is known as maximum power point tracking (MPPT). The voltage at which the power is maximum is considered as maximum voltage (Vm) and maximum current (Im).
The factor which is used to describe the performance of the solar cell is known as the fill factor. The value of the fill factor remains between 0 to 1.
Fill Factor = Vm Im / VOC ISC
Vm Im = Fill Factor × VOC ISC
The equivalent circuit of solar cells is as shown in the figure below.
- Isc = Source current generated by the sunlight
- Ij = Junction current
- I = Current passes through the load
- RL = Load resistance
The relationship between current and voltage at the PN junction is given as the equation below.
- I0 = Saturation current
- V = Junction voltage
- e = electron charge = 1.602×10-19 J/V
- k = Boltzmann’s constant = 1.381×10-23 J/K
- T = Temperature (K)
Solar Cell Efficiency
The solar cells are a device that used to convert the photon energy into electrical energy. The efficiency of solar cells equates as below;
Quantum Efficiency QE,
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Factors affecting the efficiency of solar cells
Theoretically, solar cells are used to operate at maximum efficiency. The main factors affecting the efficiency of solar cells are listed below.
- Sun Intensity
- Solar Shading
Due to the intrinsic characteristic of the semiconductor material, the efficiency of solar cells is highly impacted by temperature. The solar cells cannot operate efficiently at a higher temperature. And the efficiency of solar cells is high with lower temperatures.
The sun’s intensity varies throughout the day. In the afternoon, the sun intensity is maximum. During this time, the efficiency of solar cells is maximum. During evening and morning time, the sun intensity is not at peak level. Hence, during this time, the efficiency is lower compared to around afternoon time.
The efficiency of solar cells is highly dependent on solar sheading. During a cloudy atmosphere, the solar cells are not capable to generate more energy. During the rainy season, the efficiency of solar cells decreases due to shading.
The solar cell collects photon energy. But the efficiency of cells will decrease if the cells reflect light away from the surface. Untreated silicon surface reflects light up to 30% of incident light. To avoid this situation, an anti-reflection coating is used on the surface of the solar cells. Due to this coating, the solar cells appear dark blue or black.
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Types of Solar Power Plant
The solar power plant is classified into two types according to the way load is connected.
- Standalone system
- Grid-connected system
The stand system is an independent power plant. It is not connected with a grid. It is directly connected with the load. This type of plant is used in a place where a grid is not available like forest, hilly area etc.
This type of plant can be used as a power backup plant when the power of the grid is not available, this plant is used to supply the load. A battery and charge controller is an optional part of this system. But in most cases, the battery and charge controller is used with this system to increase reliability.
DC loads can directly connect with this plant. But in the case of AC load, the inverter is required to convert DC power into AC power. Generally, this type of system is not used to generate electrical power in bulk amounts. This type of plant use to operate small loads or in emergency conditions only.
The block diagram of this system is shown in the figure below.
The standalone system can be categorized as below.
- Direct-coupled standalone system
- Standalone system with battery storage
- Standalone system with batteries and charge controller
- Standalone system with AC and DC loads
- Hybrid standalone system
Direct-coupled Standalone System
In this type of system, the solar panels are directly connected with the loads. This system is not suitable for AC load as this system does not have an inverter. So, DC loads are directly supplied by the solar panel.
This system cannot woks during the night or when sunlight is not available. Generally, this type of system is used for agriculture purposes to operate pump sets and other agriculture auxiliaries. The block diagram of this system is shown in the figure below.
Standalone System with Battery Storage
This type of system can be operating while sunlight is not available. During the daytime when sunlight is available, the solar panel is used to charge the battery. And the battery is used to supply power during the night. This system is cheap as it is not using a charge controller. But, in this system, the battery may overcharge or fully discharge and it reduces the life of the battery. The block diagram of this system is shown in the figure below.
Standalone System with Battery and Charge Controller
The charge controller is used to control the charging and discharging of the battery. The cost of this system is high. But, the life of this system is high. Due to the charge controller, the battery works efficiently compared to the standalone system without a charge controller. The block diagram of this system is shown in the figure below.
Standalone System with AC and DC Loads
The output of the solar panel is in the form of DC power. Hence, DC load can directly connect with the solar system. But if you need to connect the AC load, the inverter is necessary to convert the DC power into AC power. Generally, this plant is connected with other AC sources also. And this source is used to charge a battery during sunlight is not available. The block diagram of this system is shown in the figure below.
Hybrid Standalone System
In this type of system, more than one source is connected with the load. These sources may be a diesel generator, small water turbines, fuel cells, etc. This will increase the reliability of the system and reduce the battery capacity. The block diagram of the hybrid standalone system is as shown in the figure below.
This type of system is used to generate bulk power and transmit it to the load by a grid. Hence, this plant is known as a grid-connected power plant. In this system, a greater number of solar panels are used to generate more power. And it requires a large area to build a power plant.
The grid power is in the form of AC. And if we need to supply power to the grid, we need the output of solar plants similar to the power of the grid. In this system, the most important condition is that the output frequency and voltage must be matched with the grid’s frequency and voltage. And also, the power quality maintains the grid standard. The block diagram of this system is shown in the figure below.
Types of Solar Panels
- Monocrystalline Solar Panels
- Polycrystalline Solar Panels
- Thin-film Solar Panels
Monocrystalline Solar Panels
This is the oldest type of solar panel. The monocrystalline solar panel is the most developed and very efficient type of panel. The efficiency of the latest monocrystalline panel reaches up to 20%.
The cells are made of pure silicone and it is the purest form of solar panel. These panels look uniform in dark color. The shape of the cells of this panel is a round corner (oval shape). And it recognizes by appearance. This type of panel has high power output and occupies less space compared to a polycrystalline panel. But the cost of these panels is high.
The main advantage of this panel is that it slightly less reacts at high temperatures compared to a polycrystalline panel.
Polycrystalline Solar Panels
Polycrystalline panels use melted silicon. This process is faster and cheaper compared to the monocrystalline panels. The shape of the solar cell is rectangular with a sharp corner. Generally, this panel looks blue color because of the impurities added to the silicon.
The efficiency of this type of panel is slightly less compared to the monocrystalline panel. The efficiency is around 15%. And the life span of this panel is also less compared to the monocrystalline panel.
Thin-film Solar Panels
This type of solar panel is manufactured with one or more films of photovoltaic material. The polycrystalline panel is less expensive as the process to make this panel is easy. The major advantage of this panel is that it is a flexible panel. As the name suggests, thin-film panels, this panel is approximately 350 times thinner compared to the monocrystalline and polycrystalline panel.
The main disadvantage of this pane is that it requires more space. And this issue makes this panel unsuitable for residential applications. The life span of this panel is short compared to monocrystalline and polycrystalline panels.
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Advantages and Disadvantages of Solar Power Plant
The advantages of solar power plants are listed below.
- Solar energy is a clean and renewable source of energy which is an unexhausted source of energy.
- After installation, the solar power plant produces electrical energy at almost zero cost.
- The life of a solar plant is very high. The solar panels can work up to 25 years.
- This plant is not causing pollution.
- There are no moving parts in solar cells. So, maintenance is not needed to keep a solar plant running.
- It does not produce any noise.
- For a bulk generation, this plant can be installed in any land. So, there are no specific site selection criteria like thermal and hydropower plants.
- The solar plant can be installed on the house or flat. So, it reduces the transmission cost as it generates energy near the load center.
- In a grid-tied power plant, the electrical generate power can directly transfer to the grid and this will reduce the burden of conventional power plants.
The disadvantages of solar power plants are listed below.
- The initial cost of a solar panel is very high.
- It requires large land to produce electrical power in bulk amounts.
- The solar plant is only installed in countries where sunline is available efficiently.
- During a cloudy atmosphere, the solar plant cannot operate efficiently.
- The efficiency of a solar panel is very less.
- This plant generates electrical energy when sunlight is available. During the night, this plant cannot generate electrical power. Hence, if you need to use electrical power at night, you have to install a battery and charge controller. That increases the cost and maintenance of the plant as the life of a battery is very short.
- In a grid-tide power plant, the inverter is required, which is costly and needs skilled manpower and new technology to make sync with grid power quality.
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