The suns energy can be converted into electricity to give us a constant source of energy. This constant source of energy can be tapped into without fear of power cuts; excessively high electricity bills and ensures a clean and pollution free environment. This process can be accomplished 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 like structure made of semiconductors like silicon chips. They are attached to a circuit by electrical wires. Solar powered cars use the same technique to harness solar energy to drive the motor.
As the sunlight hits the semiconductor, sun's energy in the form of light gets converted into electricity. The free electrons that have been released as a result of the sun's rays hitting the semiconductor form an electrical circuit and the current thus produced 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 and necessitates the storage of excess electricity in batteries.
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 converting techniques. The amount of energy that comes from the Sun is of an astonishing quantum; i.e. in one second the Sun provides around 1017 joules of energy to the Earth. It is equally surprising to know that the Sun provides as much energy to the Earth in one hour that humans need annually. Figure 1: Solar panel produces alternate energy.
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 a mere 13TW. The quantum of solar energy received by the Earth is unprecedented; however, it is not effectively used to cater to the energy needs of the modern civilization. The non-renewable sources of energy like fossil fuels are still used as a major source to satisfy the world's energy requirements. Through the process of combustion, fossil fuels are made to produce useful energy. However, this tends to generate various greenhouse gases and other pollutants causing certain hazards to the environment and depleting the already scarce reserves of fossil fuels.
Solar-panelled and hybrid cars can provide an excellent and viable alternative to petroleum-based automobiles. These automobiles are environment friendly and do not cause any pollution. Solar cell panels do not consume gasoline and therefore, no exhaust fumes are produced. Some reasons why using solar energy is a more viable and effective alternative to conventional sources of energy are given below:
wide availability
versatility
causes minimum damage to the environment
Figure 2: Solar Cell with efficiency of 40% developed by Spectrolab Inc.The untapped potential of the solar energy can be harnessed by conversion of solar energy into electricity. Research is being conducted at various levels on using nano-materials to convert solar energy for use as anode in Lithium ion batteries. Photovoltaic textiles are also generating excitement as a novelty that has the potential to be used in a wide range of consumer products [1] . 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
Solar cells are used to convert sunlight into electricity directly, and the phenomenon is known as the photovoltaic effect. In 1839, Edmond Becquerel, a French physicist established the theory of solar conversion when he evolved a process of generating current by using solar energy. However, it took over a century and a lot of research to understand the practical implications of the process of solar conversion. In course of time, it was accepted that photovoltaic effect involved the conversion of solar energy into electric energy at the level of sub-atomic particles.
Solar Energy Cells
Solar cells or photovoltaic cells are made up of thin layers of semi-conducting material placed one above the other. Silicon is by far the most popular semi-conducting material that is used in photovoltaic cells. In recent times solar panels have proved their utility in residential solar power generation, utility scale power plants and solar vehicles. When the surfaces of the cells face the sun, the electrons absorb the solar energy in two different layers of semiconductors, p-type and n-type, which in turn creates the electric current. In a solar car, the solar panels are mounted on the car and as the sun's rays strike the solar panels, photons push the electrons on the silicon sheets, from the p-type or positive type layer to the deeper n-type or negative type layer. The movement of electrons across the solar panel creates an electric current within the sheet.
Figure 3: Solar powered car by ToyotaWhen solar energy is emitted by the sun on the photovoltaic cells, the entire energy is not absorbed right away. Certain amount of sunlight is absorbed and the rest is reflected back and/or passes right through the cells. The quantum of electricity generated depends upon the absorption level of the photovoltaic cells. The photons from the sun's radiation excite the free electrons present in the atoms of the semiconductors. The movement of these electrons create a flow of current in the electric circuit.
The photovoltaic cells are available in a wide variety of shapes and sizes. They may be as small as a postage stamp or may be of several inches in length. They are grouped together in order to create photovoltaic modules. Module 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 are capable of holding around 40 cells. The modules are often measured in feet. The photovoltaic system is a collection of modules or array and comprises of the following:
mounting hardware
various electrical connections
equipment for power-conditioning
batteries which store the excess energy produced
Practical devices:
These days various semiconductors are used in the manufacturing process of solar cells. They are explained below.
Crystalline silicon cells: They are the most common photovoltaic cells that are used for commercial purposes. The silicon atoms consisting of 14 electrons forms the basis of such cells, a crystal lattice is created with the junction of various silicon atoms. The solid material of the lattice works as semiconductors in the PV cells. Various cost effective versions of such cells are also available. Multi-crystalline material is used in such cells in place of the silicon crystals.
The usual lifetime of photovoltaic modules is approximately 20 years or more. The efficiency level of these batteries is about 18%.
Figure 4: Structure of the Si atom to explain photochemical activityAmorphous silicon solar cells: These solar cells arranged in the form of amorphous thin films. They cost less than the crystalline silicon cells but their efficiency is also low.
Cadmium telluride, gallium arsenide, indium phosphide and copper indium diselenide or their derivatives are also used to produce high efficiency solar cells. The applications of such cells is limited to high-end specific utility services like in power satellites. They require high-intensity concentrated sunlight for their application in any system.
Figure 5: Only the photons with energy equivalent to the energy band gaps in photovoltaic material are absorbed to free electrons and produce electricity. This energy band gap is different for different photovoltaic material.
Solar Energy Collectors
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.
The key component of the solar collector is the absorber consisting of various narrow strips. The heat carrying pipe is connected to such strip and the working fluid transfers heat through such pipe. Generally the absorbers are made of copper or aluminum. Absorbers used in swimming pools are made up of plastics.
The surface of absorbers is usually black coloured as it carries high degree of light absorption. Higher the quantity of solar energy getting absorbed, lower would be the chances of radiations being reflected away. When the temperature of the absorber reaches to a level higher than that of the atmospheric temperature, it radiates a large part of the stored solar energy in form of heat rays of longer wavelength.
For the purpose of monitoring the heat emission and to reduce the loss of energy due to such emission, surface coating is suggested. The process of coating enhances the absorption level to up to 90%. Such paints could be applied mechanically with brush or be galvanized with black chrome, nickel and aluminium oxide.
Figure 6: Structure of a flat plate collector
A flat plate collector usually includes absorber, insulator, a transparent covering and a frame to house the collector. In order to monitor the short-wavelength light spectrum a low iron tempered glass is used as the transparent cover. Also, such cover acts as a shield in protecting the heat collected so far from being carried away by wind and breeze. A frame made up of aluminum, galvanized steel or fiberglass-reinforced plastic is also used to protect the absorber from adverse weather conditions.
The back and side walls of the absorber are insulated with polyurethane foam or mineral wool in order to control the heat loss in the process of conduction. ( Solar 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
Figure 7: 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.
Figure 8: A solar concentrator [2]
The solar concentrators are typically based on either one axis tracking or 2 axis tracking systems. Systems with one axis tracking concentrate the sunlight onto an absorber tube in the focal line of the concentrator, whereas two-axis tracked system focus the rays of the sun onto an absorber at the focal point.
The merit of solar concentrators as a technique of solar conversion is its capacity to rule out the effects of heat stagnation. In conventional collector systems, in case of a failure of a component if the excess heat is not removed a standstill occurs. In those cases certain immediate actions were required like expansion of vessels etc so that temperature do not exceed the acceptable limits. Whereas a tracking system is used in the solar that make use of safety control routines that defocus the collector to avoid exceeding stagnation temperatures. However,it is suggested to install certain safety devices, such as flow indicator, in order to monitor the flow of heat transfer fluid through the absorber, and thus the removal of heat, before the collector is focused, in order to avoid overheating the system.
A CPS plant could be installed in either of the following versions:
Linear concentrator system
Dish/engine system
Power tower systems
Thermal storage systems
It has been suggested to install a small CPS directly at the place of power requirement. Systems are apt for distribution purposes . One dish system carries the capacity of producing 3-25 KW of power.
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.
Photovoltaic Cell Conversion Efficiency
The ratio of the quantum of solar energy absorbed by the PV device and the amount of electrical power generated out of it depicts the conversion efficiency of the photovoltaic (PV) cell, or solar cell. The key challenge of various research projects is to device measures to enhance this conversion efficiency.
Factors Affecting Conversion Efficiency
Major chunk of the solar energy that is absorbed by the photovoltaic cells is lost before the conversion process takes place. The quality of the material of the solar cell is also a major criteria which affects the efficiency of the conversion process as explained below:
Wavelength of Light
Around 55% of total solar energy delivered by the Sun on the photovoltaic cells remain unutilized. The reason being , the mismatch between the energy delivered and the bandgap of the material used in the PV cells.The term Bandgap is used to set a minimum criteria of energy required to set free electrons from its bond. Depending upon the materials used in the semiconductors, this energy level differs. When the energy from the sun is below this bandgap , no electricity is produced by PV cells. The radiations from the sun comprises of packets of energy , generally referred as photons, which are measured in wavelength. When photovoltaic cells come under direct contact of sunlight some photons are either reflected or just pass through the material but do not enter the cell. Out of the photons absorbed , some carry low levels of energy as well and they only contribute by setting the electrons free from their atomic bonds thereby producing electrons (negative) and holes(positive) that carry charge.
When the solar radiations fall on the PV cells, radiations would remain unabsorbed if the photon's energy level falls short of the band gap of the material of PV cell. This could lead to a probable loss of 25% of the energy so delivered from the sun on the PV cells. Also, when the photon carry energy level exceeding the band gap of the material, the excess energy is re-emitted as heat and leads to an additional 30% loss of the solar energy delivered. Thus, around 55% of the solar energy received by the photovoltaic cells is wasted due to ineffective interaction between material of the PV cell and sun light.
Recombination
In a solar cell, electrons and holes act as charge carriers. At times they both recombine abruptly prior to entering into the electric circuit and generating some current. In the process of direct recombination the light-generated electrons and holes encounter each other on random basis. This proves as a drawback for certain materials. Also, in the process of indirect recombination the charge carriers encounter issues relating to structure of the crystal which defective making the carriers prone to recombination. This also leads to efficiency deterioration of the material.
Natural Resistance
The efficiency of a cell is proportionately related its resistance to the flow of electrons. The conversion efficiency is affected due to natural resistance of the material at the following events:
Bulk of primary solar material
At the thin top layer of the PV device
Within the electric circuit at the interaction of cell with electrical contacts
Temperature
According to the properties of a solar cell, it has been predicted that the low temperatures are best suited for efficient working of the cell. The efficiency of the material in a cell is inversely related to the operating temperatures which imply that when the operating temperature is high, the efficiency of the solar cell deteriorates. The major part of the solar radiations falling on the photovoltaic cells turn into heat therefore it is suggested to keep the cell cool and match the material in the cell with the operating temperature.
Reflection
Out of the total solar energy delivered on the solar cells, merely 45% is absorbed by the cells. The rest of the energy is reflected away from the cell's surface and is not at all put to any use. For instance, around 30% of the solar radiations are reflected back in case of untreated silicon. A basic formula to maximize the efficiency of the cell is to minimize this reflection thereby increasing the absorption level of the cells. In order to maximize the absorption of the solar energy by the cells various antireflection technologies could be used explained below:
COATING: when the top layer of the cell is covered with coating, the absorption power is increased. The successive anti-reflective layer on the surface of the cell reduces the reflection at various wavelengths.
TEXTURING: The absorption power of the cell could also be increased by using texture at the top of the cell. The solar radiation so delivered on the cell strikes a second surface before it escapes. Using pyramids textures as anti-reflective techniques is very effective in this regard. All the energy radiating from the sun is bent through the textures and strike back on the cell. Within the cell the light gets reflected back and forth so that it gets absorbed completely.
Electrical Resistance
Generally when a cell is covered with a metallic contacts, to reduce the electric resistance , some amount of incoming light gets blocked. In order to maintain a balance between loss of energy due to resistance and shading effects various designs of top surface contacts are used like:
Grids: various thin fingers are scattered over the surface of the cell
Back surface contact
Thin layer of oxides across the cell's surface
Determining Conversion Efficiency
In order to compute the quantum of electrical energy a PV device could generate researchers use I-V curves. This curve reflects the performance of the photovoltaic device. I-V curve shows the relationship between the current (I) and the voltage (V). For the purpose of obtaining the I-V curve, the solar cell is exposed to constant level of solar energy while constraints such as cell temperature, load resistance are adjusted. In this process, the cell temperature is maintained constant, resistance of the load is varied and the current so generated is measured while keeping the solar cell in continuous light supply. The graph on which the I-V curve is plotted, the current is depicted on the vertical axis and the voltage is depicted on the horizontal axis. The two substantial points on the curve depicts the point of short-circuit current and the point of open circuit voltage. The former is attained when there is short circuit between the two terminals of the cell leading to zero voltage between them, also corresponding to zero load resistance. The latter is attained when there is an open circuit condition and there is zero current between the two terminals, corresponding to infinite load resistance.
Various combinations of voltages and currents could appear over the life time of the cell. In order to determine the highest efficiency point of the cell, the cell temperature is kept constant and the load resistance is varied from zero to infinity. At this point the cell delivers maximum power. The maximum power of the cell, Pm, as depicted on the I-V curve is the point where the product of the current and voltage is at a maximum. Pm denotes the maximum efficiency of the photovoltaic cell. No energy is generated at the points of short-circuit current with no voltage and open-circuit voltage with no current.
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
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 a large amount of energy in a compact space. This battery is best suited for powering electric vehicles because it has high energy density, displays quick rates of charging and discharging and provides 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 principle involved is the movement of the Li ions from negative electrode to the positive electrode during use and from the positive to the negative electrode while being charged. Lithium ion batteries have an intercalated lithium compound that acts as the electrode and also facilitates the rechargeable property of the batteries. Since these are rechargeable batteries they are energy efficient and cost effective.
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, the batteries undergo discharging when the sulphuric acid reacts with the lead electrodes to form lead sulphate. The process reverses during charging. 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 are the power trackers, motor controller and the data transmission system.
Solar Electricity and Motor
The solar electricity generated by the process of solar conversion can be used in various applications. One example of using such energy is in running the motors. When sunrays falling on the solar panel are tapped and converted into electrical energy then direct current is produced. The dc electrical power is then used to run motors. The power could be obtained by the motor either directly from the solar panel or it could be indirectly powered by the solar panel through a battery.
When a parallel circuit is set up consisting of a motor and a battery and a solar panel is installed through which the battery gets charged, dc electric energy is generated from the battery and the motor starts working.
In figure below panel a shows how motor works when dc electric energy is acquired directly from the solar panel and transferred to the motor through wires. Panel b of the figure shows the how the solar energy is tapped in the form of dc electric energy preserved in the battery. When the solar panel converts the energy from the sun into dc electric energy, the same is transferred to the batteries through wires. Also, a resistor and diode is installed in the circuit. The former controls the flow of electrical energy through the wires and the latter acts like a security valve and monitors the movement of the electrical energy from the battery. It ensures that the electrical energy would not move out of the battery to the solar panel in the absence of light.
Figure 5 panel a panel b source:www.makeitsolar.com
Figure 3 panel a panel b source:www.makeitsolar.com
Figure 2 below demonstrate working of the motor when battery is incorporated in the circuit and the motor acquires the dc electrical energy from the battery inspite of getting it directly from the solar panels.
Figure 2 source:www.makeitsolar.com
The solar energy is converted into dc electrical energy when the first switch is powered off and the second one is switched on also the same is transferred to the battery through the wires. Also, in order to run the motor , the second switch is powered on and the first one is switched off by which the dc electric power so stored in the battery gets transferred to the motor.
The recent development in the application of solar panels is deploying the energy so accumulated through solar conversion in running vehicles. The photovoltaic cells are used to transforms the solar energy into electrical power which run the vehicles. Although sun is the major source of energy that makes these cars work, the other major components includes solar array, batteries, power trackers, motor, controller. The role of each component in running the car is explained below:
Solar Array and Power Trackers The solar array is the basic equipment in solar cars, it absorbs the solar energy and generates electrical power. The weight of the solar array depends on the composition of the solar cells in the array. In order to get a lighter solar array individual solar cells are preferred over prefabricated solar panels. A solar cell consists of a film of very pure crystalline Si and works on an average 20% efficiency level which implies that out of the total solar energy which is absorbed by the panel 20% is converted into electricity. The quantum of cells to be used depends upon their size and the permissible solar area. The solar cells are grouped on the panel, wired across forming a series split into various zones depending upon their numbers. The advantages of forming such zones is that in case one zone fails, the motor would not breakdown as the remaining zones on the panel will continue to produce power. The peak power tracker monitors the energy generated by the solar arrays and transforms the excess energy into the system voltage after which it is stored in the batteries. The presence of power trackers in the car eliminates the necessity of matching the voltage of solar array with that of the system voltage as this task is done by the power trackers. In the absence of sunlight, the power trackers align the power to match with the system voltage ensuring the efficient working of the system.
Batteries: The batteries are the storage hub where all the power generated by the solar arrays is preserved and is used to run the motor. Various types of batteries are available including lead-acid batteries, lithium-ion and nickel-cadmium batteries. Out of the above the lead-acid batteries are most preferred because of their wide availability and low-cost range. Also, there is an option to choose between the wet or gel batteries. The former are filled with liquid sulphuric acid and are widely used in automotive purposes. Although they are bulky in weight as compared to the gel-cell batteries but the advantage they offer is that they do not carry the risk of being blown up in case they are overcharged. The gel-cell batteries are lighter and are sealed batteries but their voltage needs to be continuously monitored at the time of charging. The motor voltage is considered one of the key factors while deciding the number of batteries to be used. The batteries with higher amp hours have high storage capacity.
Motor & Controller: When the energy is stored in the batteries, the motor and motor controller make it available to run the car. The controller manages the energy flow to the motor to match the requirement of the throttle and that energy is utilized by the motor to move the wheels. Generally, the motor controllers are fitted on the wheel of the car and they are responsible for transforming the DC current from the solar panels/ batteries into AC current to power the motor. The speed of the motor is controlled by adjusting the frequency of the AC output.
Also, one could use DC brush magnet motor or a brushless motor. The DC brush magnet motor runs on an average of 80-90% efficiency and is cost effective. The brushless motor works on an average 94-99% efficiency level. It is advisable to use such motor and controller set up that incorporates regenerative braking system which provides the scope of putting back the energy into the batteries when driving the car downhill.
Instrumentation The major issue under instrumentation lies in calculating the precise estimate about the following:
System voltage
Amp draw
Battery energy
Remaining time
In order to get information about the above elements, an E-meter is put to use. It digitally displays the information about the number of amp-hours remaining in the battery. In addition to it, a speedometer could be used in this regard. In order to monitor the voltage of the battery a voltmeter could be installed for each battery. Depending upon the load on the battery, the voltage of the battery will be depicted on the voltmeter.
Steering & Suspension: In order to ensure the safety and stability of the car , the steering of the front wheels is suggested. Also suspension is required in order to protect the solar array from jolts and to ensure a stable ride.
Brakes: The mechanical brake systems are less preferred than disk brakes which are hydraulic lines and are easier to run to the wheels. However, when the pressure on the break is released the brake pads do not pull away from the brake rotors immediately and may exert some drag which can prove harmful for the functioning of the solar car as a whole.
Tires and Hubs: The distance travelling capacity of the solar car depends upon the rolling resistance of the tires of the car. Solar cars require high pressure thin tires that are fitted to customized wheels and require specially designed hubs.
Body of the Car
In order to maximize the efficiency of the solar car, following parameters must be considered:
Weight: To make optimum use of the power, it is advisable to maintain the car in as much light weight a condition as possible. Less bulky support components should be used while keeping in mind the comfort and safety of the occupants of the car.
Aerodynamic Drag: This helps in minimizing the loss of energy caused due to wind resistance and increases the efficiency
Surface area: a large flat area is required to mount the most number of solar panels in the car.
Conversion of Fuel Based Car into Solar Powered Car
The following steps incorporate the procedure of conversion of a fuel based car into a solar one. Six solar panels can be installed on the roof of the garage and the inverter and batteries can be housed in the garage.
Step 1: Remove the Engine
The initial step in converting the fuel car into solar is to remove the fuel engine and other components like radiator, tailpipe, muffler, etc
Step 2: Build Rack for Batteries
In order to build battery racks in the car, angle and square tubing are used. The batteries preserved in the rack at the rear put in plastic containers with the provision of running exhaust fans under the car while it is charging.
STEP 3: Install Electric Motor
With the help of a shaft coupler, electric motor is mounted to the adapter plate. In order to check the alignments try to run the motor on a very slow speed. In addition, it is required to install speed controller, throttle Box and contactor in the car.
STEP 4: MAIN WIRING
Welding cables are used to connect batteries in series and complete the circuit from contactor to speed controller to the motor. The throttle box is to be connected to the cable from the gas pedal and is then wired to the speed controller.
STEP 5: ADDITIONAL WIRING
A DC Converter is attached to a 12 volt car battery which has the capacity to transform 72volts to 12volts. This is used to to power headlights and other loads. The rear of the car is fitted with a 120 volt battery charger that is connected to the main battery bank along with volt meter and ammeter.
STEP 6: Solar Panel Set Up
Half inch mounts for solar panels need to be built. The six panels should be wired together in the fused electric box. The large gauge wire should go through the box into the garage.
STEP 7: Garage Set Up
ConclusionThe charge controller transfers the power generated by the solar panels into the batteries. It also keeps the batteries from overcharging. On an average 30 amps of power is generated through the solar panels on a clear day. An inverter with a 1000 W capacity should be connected to the battery using the large gauge wire. The 120 V charger in the car is then connected to the inverter. The step -by-step procedure explained above has proved very useful in conducting pilot projects for the conversion of fuel based car into solar ones.
In order to replace fossil fuels with the solar energy as a possible substitute of the source of energy, emphasis should be given in developing better conversion techniques. The widespread potential solar energy is still untapped because of the reason of cost and conversion capacity. The fossil fuels are concentrated on specific areas therefore using them to satisfy the energy needs is cost effective. Also, the efficiency of the conversion techniques lags far behind the minimum acceptable limits. The best crystal silicon solar cells are merely 18% efficient. The remaining solar cells based on other organic matters like dye sensitization of oxide semiconductors etc are maximum 10%efficient. This is a great technological challenge that requires investment of larger financial and intellectual resources to find innovative solutions.
Understanding and exploring the practical implications of the nanoscale phenomena has become imperative in order to develop cost effective , high efficiency conversion techniques.
PERFORMANCE
The figure depicts the three generations of solar cells. First-generation cells are made up of the expensive silicon wafers and make up 85% of the current commercial market. Various organic matters like amorphous silicon, nanocrystalline silicon, cadmium telluride, or copper indium selenide are used in manufacturing the second generation cells. The cost of such material is however lower than the first generation cells but their efficiency is also less. Third-generation cells depicts a dramatic increase in efficiency that maintains the cost advantage of second-generation materials. Their design may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlight concentration, or new materials. Developing such cells have become the goal of various research projects. The horizontal axis represents the cost of the solar module only; it must be approximately doubled to include the costs of packaging and mounting. Dotted lines indicate the cost per watt of peak power (Wp).
Solar electricity costs 5-10 times more than the cost of producing electricity from the fossil fuels and accounts for satisfying merely 0.015% of the world's electricity demand.
POTENTIAL OF SOLAR ENERGY
Deployment: As a generic estimate the potential of solar energy exceeds
5 fold the current global energy consumption. The aggregate electricity demand in US in 2002 was 418GW .This energy demand could have been satisfied by covering
a land surface of 180km square with photovoltaics. The contribution of solar energy as a source of power in satisfying the demand of energy worldwide is still minimal. Recent data shows that current electricity generation from PVs is only of the order of 2.6GW3 compared to 36.3GW for all renewable energies leaving only hydroelectric power .
The high capital cost per kW installed as compared to fossil fuels and the intermittent nature of the energy source, makes the power generated from the solar energy as less appealing alternative of power generation.
Cost of electricity: In order to reduce the cost of solar energy, it is suggested to replace the p-n junction photovoltaic devices with the organic-based photovoltaics. Also there is a margin to reduce the manufacturing and installation cost if lightweight, flexible and low-cost plastic substrates are used. Certain studies show that cost efficiency could be attained through manufacturing scale. In addition, Design and technological innovations could decrease the storage cost of the energy which include : roll-to-rollmanufacturing technology,using printable semiconductors, plastic substrates and polymer alternatives etc.
Environmental aspects: Solar energy is promoted as a sustainable energy supply technology because of the renewable nature of solar radiation and the ability of solar energy conversion systems to generate greenhouse gas-free electricity during their lifetime.
References for Images
