Every device we use in our day-to-day life such as mobile phone, computer, induction cookers, washing machines, vacuum cleaners, etc., requires electric power supply. Thus, the advancement in technology is increasing the electrical and electronic appliances usage – which, in turn – is increasing the power demand. Thus, to meet the load demand, different techniques are used for electric power generation. In the recent times, to avoid pollution and to conserve non-renewable energy resources like coal, petroleum, etc., renewable energy sources like solar, wind, etc., are being preferred for power generation. The combination of renewable energy sources can also be used for generating power called as hybrid power system. As a special case, we will discuss about the working of solar-wind hybrid system in this article.
What is Solar and Wind Hybrid Power Systems?
Solar and wind hybrid power systems are designed using solar panels and small wind turbine generators for generating electricity. Generally, these solar wind hybrid systems are capable of small capabilities. The typical power generation capacities of solar wind hybrid systems are in the range from 1 kW to 10 kW. Before discussing in brief about the solar and wind hybrid power system, we should know about solar power generation systems and wind-power generation systems.
Working of Solar Wind Hybrid System
To better understand the working of solar wind hybrid system, we must know the working of solar energy system and wind energy system. Solar power system can be defined as the system that uses solar energy for power generation with solar panels. The block diagram of solar wind hybrid system is shown in the figure in which the solar panels and wind turbine are used for power generation.
Working of Solar Power System
Solar energy is one of the major renewable energy resources that can be used for different applications, such as solar power generation, solar water heaters, solar calculators, solar chargers, solar lamps, and so on. There are various advantages of solar energy usage in electric power generation including low pollution, cost-effective power generation (neglecting installation cost), maintenance free power system, etc. Solar power system consists of three major blocks namely solar panels, solar photovoltaic cells, and batteries for storing energy. The electrical energy (DC power) generated using solar panels can be stored in batteries or can be used for supplying DC loads or can be used for inverter to feed AC loads.
Solar Panels’ Working Principle
A solar panel is made up of solar cells or solar photovoltaic cells, and is used for converting solar energy into electrical energy. The solar panels utilize Ohmic material for interconnections and external terminals. Thus, the electrons produced in the N-type material are passed to the battery through electrode and wire. From the battery, electrons reach p-type material, where these electrons and holes are combined. Hence, the solar panel connected to the battery behaves like another battery, and hence, is comparable to the two serially connected batteries.
The solar panel output is electric power and is measured in terms of Watts or Kilo watts. These solar panels are designed with different output ratings like 5 watts, 10 watts, 20 watts, 100 watts etc. So, based on the requirement of output power, we can choose appropriate solar panel.
But, in fact, the solar panels output is affected by number of factors like climate, panel orientation to the sun, sun light intensity, the presence of sunlight duration, and so on. During normal sunlight a 12 volt 15 watts solar panel produces around 1 Ampere current. Generally, solar panels maintained properly will work for 25 years. It is essential for designing the solar panel arrangement on the roof top for efficient usage and typically solar panels are arranged such that they face the East at an angle of 45 degree.
There are advanced technologies for improving the output of solar panels by using sun-tracking solar panel arrangement, that rotates the solar panel so as to get more light from the sun. The quality of wire must be good and have sufficient gauge for handling the current to ensure proper battery charging. If length of the wire increases, then the charging current reduces. So, the solar panels are arranged at a height of 10-20 feet from the ground level. It is recommended to properly clean the surface of the solar panels for removing dust, moisture and also to reconnect the terminals.
Solar Photovoltaic Cells Working
We must also know the working of the solar cells to understand how the solar panels convert solar energy into electrical energy. Solar cells or solar photovoltaic cells are the devices that are used for converting solar energy into electrical energy by utilizing the photovoltaic effect. These cells are used in many real-time applications such as railway signaling systems, street lighting systems, domestic lighting systems, and remote telecommunication systems.
Solar photovoltaic cell consists of a P-type of silicon layer that is placed in contact with an N-type silicon layer. The electrons diffuse from the N-type material to the P-type material. The holes in the P-type material accept the electrons but there are more electrons in the N-type material. So, with the influence of the solar energy, these electrons in the N-type material moves from N-type to P-type.
Thus, these electrons and holes combine in the P-N junction. Due, to this combination a charge on either side of the P-N junction is created and this charge creates an electric field. This formation of electric field results in developing a diode like system that promotes the charge flow. This is called as drift current and the diffusion of electrons and holes is balanced by drift current. This drift current occurs in an area where mobile charge carriers are lacking and is called as the depletion zone or space charge region. Thus, during night time or in the darkness, these solar photovoltaic cells behave like reverse bias diodes.
Generally solar panel open circuit voltage (voltage when battery is not connected) is higher than solar panel rated voltage. For example, consider a 12 volt solar panel giving an output voltage of around 20 volts in bright sun light- but, whenever a battery is connected to the solar panel, then the voltage drops to 14-15 volts. Solar cells are made of most frequently used semiconductor materials such as silicon.
Solar photovoltaic (SPV) effect is a process to convert solar energy into DC electricity using an array of solar panels. This, DC electricity can be stored in batteries shown in the figure or can be used to feed DC loads directly or can be used to feed AC loads using an inverter that turns DC electricity into 120-volt AC electricity.
Working of Wind Power System
Wind energy is also one of the renewable energy resources that can be used for generating electrical energy with wind turbines coupled with generators. There are various advantages of wind energy, such as wind turbines power generation, for mechanical power with windmills, for pumping water using wind pumps, and so on.
Large wind turbines are made to rotate with the blowing wind and accordingly electricity can be generated. The minimum wind speed required for connecting the generator to the power grid is called as cut in speed and maximum wind speed required for the generator for disconnecting the generator from the power grid is called as cut off speed. Generally, wind turbines work in the range of speed between cut in and cut off speeds.
Wind turbine can be defined as a fan consisting of 3 blades that rotate due to blowing wind such that the axis of rotation must be aligned with the direction of blowing wind. A gear box is used for converting energy from one device to another device using mechanical method; hence, it is termed as a high-precision mechanical system. There are different types of wind turbines, but the frequently used wind turbines are horizontal axis turbines and vertical axis turbines. The figure shows different blocks of the wind turbine generator system.
Wind Turbine Generator
An electrical generator is coupled with wind turbine; hence, it is named as wind turbine generator. There are different types of wind turbine generators and these wind turbine generators can be directly connected to the power grid or loads or batteries based on different criteria. In general, there are of four types:
- Squirrel cage induction generator is directly connected to the power grid or to feed AC loads or DC loads using appropriate converters.
- A generator along with an AC to DC to AC converter is connected to power grid.
- A wound rotor induction generator, which is connected to power grid or batteries whose speed can be adjusted using rheostats for maintaining required outputs.
- A double fed induction generator, which is connected to power grid whose speed can be controlled using back-to-back converters.
Consider DFIG double fed induction generator with 3-phase wound rotor and 3-phase wound stator. An AC current is induced in the rotor windings due to three phase AC signal fed to rotor windings. Due to mechanical force produced from wind energy the rotor starts rotation and produces a magnetic field. The speed of the rotor and frequency of AC signal applied to rotor windings are proportional to each other. This result of constant magnetic flux passing through stator windings produces AC current in the stator winding. Due to variation of speed in wind speed there is chance of getting AC signal output with varying frequency. But, the AC signal with constant frequency is desired. So, by varying the frequency of input AC signal given to the rotor windings we can obtain AC output signal with constant frequency. Grid side converter can be used for providing regulated DC voltage to charge batteries. Rotor side converter can be used for providing controlled AC voltage to the rotor.
Thus, as shown in the above solar wind hybrid system figure the electric power generated from solar energy system and wind energy system can be used for charging the batteries or for feeding DC loads or we can use the entire power for feeding AC loads. Hybrid solar wind charger is a practical project in which the electric power generated from solar energy and wind energy are used for charging the batteries.
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