The Solar Energy Conversion Engineering Essay

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The suns energy can be converted to electricity to give us a constant source of energy that can be tapped into without fear of power cuts, excessively high electricity bills and to ensure a clean and pollution free environment. This can be done with the help of photovoltaic cells in solar power plants set up especially for this purpose. The solar power plants have solar panels that are made up of numerous solar cells. A solar cell is a small disk made of a semiconductor type of material like silicon chips. They are attached to a circuit by electrical wires. As the sunlight hits the semiconductor, sun's energy in the form of light gets converted into electricity that forms a circuit and flows through the wires. The solar cells can only produce power in the presence of strong sunlight. When there is no light the solar cells stop producing electricity.

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Solar batteries are fast becoming popular as a mode of conventional car fuel to help in the conservation of our environment and reduce carbon footprint. It minimizes the chances of being caught on the way with a dead battery and high bills for gasoline and other fuel. Green solar cars are being made popular by famous business house like Toyota, Panasonic, Venturi and others to promote awareness and create a favourable brand image. Federal and state governments also encourage the use of solar hybrid, eco friendly cars by initiating tax benefits.

Solar Energy Conversion

The theory of solar energy conversion is a modern science that came into existence in 1970s. In order to cater to our ever growing energy needs, various studies have been undertaken in recent times to explore means of developing efficient solar energy conversing techniques. The amount of energy that comes on earth from the Sun is of astonishing quantum-in one second the Sun provides around 1017 joules of energy to Earth also it is equally surprising to know that the Sun provides as much energy to the Earth in one hour that humans need annually. The rate at which the Earth receives solar energy from Sun is 1.2 Ã- 105 terawatts whereas the production rate of energy on earth by man-made techniques is merely 13TW. The quantum of solar energy though received by Earth is unprecedented but the same is not effectively used to cater the energy needs of the civilization. The non-renewable sources of energy like fossil fuels are still used as a major source to satisfy the energy requirements worldwide. Through the process of combustion fossil fuels are turned into useful energy but they tend to produce various greenhouse gases and other pollutants causing certain hazards to the environment. Various facts about solar energy cited below makes it more appealing than any other energy source:

wide availability

versatility

benign effect on the environment and climate

Figure 1: Solar Panel used as a source of alternate energy for everyday use.

The untapped potential of the solar energy could be harnessed by conversion of solar energy into electricity. Today various studies on energy conversion based on nano-materials focus on such conversion.

Solar Energy Conversion

Listed below are the three methods used for the conversion of solar energy into electricity:

1. Solar Energy Cell

2. Solar Energy Collectors

3. Solar Energy Concentrators

Fig 2: An efficient Solar Cell giving up to 40% efficiency developed by Spectrolab Inc.

Solar Energy Cells

The Solar Cells generally known as photovoltaic cells are used to convert sunlight into electricity directly, and the phenomenon is known as the photovoltaic effect. Photovoltaic batteries are made up of thin layers of semi-conducting material placed one above the other. Silicon is by far the most popularly used semi-conducting material that is used in photovoltaic cells. In recent times solar panels have proved their utility in both the residential solar power generation as well as for utility scale power plants. When the surface of the cells faces the sun, the electrons absorb the solar energy in two different semiconductors which in turn creates the electric current.

Modules is a term used to refer to an array of photovoltaic cells that are grouped together for the purpose of creating an energy flow and they are capable of holding around 40 cells. In the process of generating electricity for a building at least 10 such modules need to be mounted together, the number of modules needs to be increased for generating electricity for big constructions like power plant. The Sarnia Photovoltaic Power Station in Canada is the world's largest photovoltaic plant with a capacity of 80 MW.

Solar Energy Collectors

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This process is used to heat the buildings in winters. First of all solar panels are installed on the roof of the building. These panels along with heating up the building also heats up the water pipes being carried in it throughout thereby keeping the water heated up inside the building. The solar energy is therefore directly used to warm the water.

The two main components of a solar water heating system are: solar collector and a storage tank. Storage collector is a flat plated thin rectangular box facing the sun installed on the roof of the building. The solar energy heats up the absorber plate and in turn that heats up the water flowing through tubes inside the collector.

Solar Energy Concentrator

Solar power can also be converted into electricity indirectly through concentrated solar power (CPS).Under this method, mirror configurations are used to convert the solar energy into electricity. Various concentrating techniques are available which include the following:

Parabolic trough

Concentrating linear Fresnel reflector

Dish Stirling

Solar power tower

Fig 3: Parabolic Curved Mirrors

The parabolic trough technique is the most commonly used technique to collect the solar energy and use it to heat water. By using this technique the sunlight is focused onto a receiver pipe by using parabolic curved mirrors. The receiver pipe runs through the focal point of the curved surface. The working fluid in the pipes gets heated up and a conventional generator is used to produce electricity. The significance of this system lies in the fact that large area of sunlight is focused into a small beam by using lenses and mirrors. The troughs in the collector are aligned on a north-south axis to match the movement of the sun from east to west throughout the day.

The 354 MW SEGS Concentrated solar power plant is in California and is the largest power plant to harness solar energy in the world. Other CSP's include the Solnova Solar Power Station (150 MW) and the Andasol Solar Power Station (100 MW); both these power stations are in Spain.

In relevance to this project of designing a solar car solar energy conversion is useful for storage in batteries to be used to run a motor. For the components of a solar car to function optimally, the converted electric energy has to be distributed to the various loads from the motor. The source has to be connected to the loads such as the ventilating fan, headlights, radio, dashboard gauges, telemetry equipment, main disconnect relay and the horn. The current and voltage demands of the different loads require grounding, protection from overload and switching.

Storage of Electricity

After the conversion of the solar energy into electricity it becomes imperative to have proper means to store it to have continuous supply of electricity even when the sunlight is not available. Broadly speaking the solar electricity could be stored either through integration with the grid of the utility company or providing solar batteries to bank the electricity.

Grid Storage

This system of storage is used when electricity is being stored on a very large scale. The extra electricity generated in the peak hours get stored in the grid which can be withdrawn whenever required.

Battery Storage

The need for storing the additional energy produced by the solar panels for later use necessitates the use of solar batteries. The solar battery stores the excess charge and helps to power a solar driven motor on days when direct sunlight may not be available or even during the night time. Commonly used types of batteries are the Lithium polymer, Lithium ion, Nickel-Cadmium, Nickel-Metal Hydride and the lead-acid batteries. The most efficient of these, however, are the Lithium polymer batteries. They store their electrolyte in an organic solvent state and are non-inflammable and safe to use.

When long power outages from the grid are predicted then battery bank is used to store the electricity produced from the solar energy. This mode of storage is as easy as hooking up the batteries to the transmission grid and the excess solar power can then be stored in the batteries. This is one of the most efficient ways to store power, because rechargeable batteries can store the excess electricity for a longer duration of time. When the solar-electricity is produced, it is sent to the batteries where it gets converted into chemical energy and is stored in a liquid form. At the time of retrieving the electricity from the battery, an electric charge is produced to trigger a chemical process to convert energy back in the form of electrons. Various types of batteries are available to store solar-electric energy and are used in different application areas:

Vanadium Redox Flow Battery

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Under Vanadium Redox Flow battery electrical energy is stored in two tanks of electrolytes or fluids that conduct electricity. Such batteries could be used as storage backup for a time span of 12 hours. These batteries could also be used in integrating solar power in a residential neighbourhood or at several large industrial sites. At the time of energy requirement the liquid is pumped from one tank to another through a steady process after which the chemical energy from the electrolyte is transformed to electrical energy. During peak periods when there is maximum sunlight this process gets reversed and the excess energy gets stored in the battery. The size of the tank and its capacity to hold the electrolyte influences the quantum of energy that could be stored in the battery.

Sodium-beta alumina membrane battery

Under the sodium-beta alumina membrane battery sulphur and sodium are particularly used which serves the purpose of charging and discharging the electricity in/from the battery. The battery's core is made up of aluminium oxide consisting of sodium ions. The battery is built in tubular design and has the capacity to store lots of energy in a small space. This battery is best suited for powering electric vehicles because of its high energy density, rapid rate of charge and discharge and short, potent bursts of energy.

However, as the battery operates at high temperatures it has been suggested to modify the shape of the battery in order to fix the safety issues and also to improve the efficiency.

Lithium-ion battery

Generally Lithium ion, or Li-ion batteries are used in household gadgets and electric vehicles. These batteries are made up of different elements like lithium, manganese and cobalt. These are best suited for transportation applications because of their high energy and power capacity potential .The battery works when positively charged lithium ions migrate through a liquid electrolyte, while electrons flow through an external circuit, both moving back and forth from one side to the other. This movement creates and stores energy.

Lead-carbon battery

The Lead-carbon batteries are usually used as back-up generators and in automobiles. Various studies have shown that the lifespan of the traditional lead-acid batteries can be improved by adding carbon in it. Also, such lead-carbon batteries have high concentrated power which makes them suitable for source for solar power. In a normal lead-acid battery, sulphuric acid reacts with the lead anode and cathode to create lead sulphate in the process of discharge. The process reverses during charge. With time the battery's core gets filled up with lead sulphate due to crystallization. This process of crystallization can be prevented by adding carbon to the battery thereby enhancing the life of the battery.

The choice of using a particular battery from the above explained few depends upon the nature of application and the budget of the project.

A collection of connected 2-, 6-, or 12-volt batteries that supply power to the plant in case of outages or low production of electricity is known as a battery bank. In order to produce the current these batteries are wired together and a series is formed thereby producing 12-, 24-,or 48-volt strings. These strings are then connected together in parallel to make up the entire battery bank. The battery bank supplies DC power to an inverter, which produces AC power that can be used to run appliances. Factors like inverter's input, type of battery selected amount of energy storage required determines the size of the battery bank.

At the time of installation of new battery, it is suggested to check its life cycle and the number of deep discharges it will be able to provide in future. Also the thickness of lead plates need to be checked upon as the life of battery depends upon the thickness of the plates.

For purposes of running an electrical motor powerful enough to drive a solar car the battery and solar cells should be connected in parallel to the motor controller, so that both have the capability to supply power to the motor controller. The battery can also store the excess power from the solar array. The voltage from the battery bus is distributed equally between the battery, solar cells and motor controller. A low voltage circuit supplies power to the horn, cockpit fan, signal lights and back-up lights. DC-DC converters reduce the main bus voltage to lower voltages as per requirements by feeding two branch circuits. [i] 

Figure 4: MATLAB design of a DC electrical circuit

Maintenance of the solar battery

The normal life of batteries is around 10-15 years irrespective of the amount of their usage as the acid in the battery wears down the internal components of the battery. In order to keep the battery working over its entire life following practices must be undertaken:

1. Deep discharging of batteries in repeated intervals must be avoided. The life of a battery is negatively correlated with the number of times it is discharged i.e. the lifetime of a battery gets shortened by the number of times it has been discharged. Another way to fix this problem is by increasing the size of the battery bank .In order to support deep discharge of batteries every day the size of battery bank must be increased.

2. Batteries must be stored at controlled temperatures. Rating for battery life is done only for temperatures between 70°-75°. If batteries are kept in temperatures warmer than this it reduces their life significantly. An effective way to heat a battery storage unit is by using passive solar power, but the battery storage unit must also be well insulated. Maintaining the temperature of the battery storage unit below 70°-75° will not extend their lives to any significant degree but will tend to decrease their lifespan. The other hazard of discharged batteries is that they may freeze up and explode so it is important to maintain sufficient charge on the batteries in cold weather conditions.

3. Equal charge must be sustained in all the batteries. Though, generally, the battery pack may have an overall charge of say, 84 volts, some cells in the series may have more/ less voltage than its neighbouring cells.

4. Inspection of batteries at regular intervals is also required to keep a track of leakage as a result of swelling on the outside of the battery, appropriate levels of fluid for wet cell batteries, and for maintain equal voltage.

The solar battery should have a constant voltage of approximately a hundred volts to be able to power the solar car engine. The battery pack comprises of several modules wired together. The higher voltage corresponds to better efficiency even though it requires a more complex array. The electronics controlling the power to the car include peak power trackers, the motor controller and data transmission system.