Project 2

To develop & Study the Performance of a Solar Car

Chapter 1 Project Overview 1.1 Introduction "The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the Universe to do."

(Galileo Galilee) Solar power comes from the energy of our Sun; a yellow dwarf star located 93 million miles from the Earth. It is a middle-aged, mid-size star compared to the billions of other stars in the universe.

The interior of the Sun is a region very high in temperature and filled with dense gases. The Sun's core is estimated to be approximately 27 million degrees Fahrenheit. Heat and light from the Sun are produced through a process called nuclear fusion.

Sunlight is an excellent energy source and the future of using solar power is very exciting.

The Sun's energy can be used to heat and cool buildings, generate electricity, operate communication and navigation systems and even power solar cars.

Solar-powered cars all get their fuel from the same place - the Sun. The cars use hundreds of photovoltaic cells to convert sunlight into electricity. Each cell produces about one-half volt of electricity.

The Sun's energy powers the car's motor and charges a battery for use when the Sun is hidden by a cloud. If a car is designed to put all of its energy toward driving and keeps nothing in reserve, it will stop completely in cloudy weather. If too much energy is diverted to the battery, the engine runs too slowly to keep up in the race.

Engineers and scientists still have many questions and problems to tackle before solar power becomes an efficient and economical way to fuel vehicles. But as the demand on fossil fuel resources increases, research will continue to search for alternative energy sources, including harnessing the Sun's energy to drive a vehicle. The most exciting part of using solar power as an energy source is that it is pollution free and inexhaustible. If research continues, one day solar energy may replace today's combustion engine cars! [1]

1.2 Why Solar Energy: It has been realized recently that the world’s supply for fossil fuel may be depleted in the foreseeable future. As a result, fuel shortages, rationing and allocation programs, rapidly influencing fuel prices, economic and political instabilities and embargos has occurred in many Industrialized and fuel producing countries. As the standard of living improved the population slowly moved away from the renewable energy sources (primarily wood and other biomass) to the most concentrated fuel forms such as coal, gas, oil and fissionable isotopes. Potential substitutes and current fuel are presented in the table. These are represented in terms of the anticipation of U.S Energy requirement for the 25 year period from 1975 to 2000.

Table.1.1: U.S fuel reserve in Units of the estimated energy requirement for1975-2000.

Natural gas

0.3

Nuclear

fission 0.6

reactors Petroleum

0.4

Nuclear breeders

44.2

Oil shale

2.0

Nuclear fusion

10pow(6)

Coal

4.6

Solar energy(per year)

14.8

Inspection of this table reveals that the fuels presently used (gas, oil, coal, and nuclear power produced by fission reactors) will only last for another 200 years. A long term solution to the energy problem lies in the development of nuclear breeder reactors as well as nuclear fusion and solar energy. Of the three, breeder technology may not be acceptable to our society while fusion technology needs considerable development. Solar energy may be the best solution. It causer no environmental pollution and the only one for which the technology is available in many applications. Besides, solar energy is renewable and will not deplete within the next several billion years. [2] The growing energy crisis looms over us both locally and globally. Non renewable resources i.e. oil, coal, natural gas and Uranium are being rapidly depleted, compounded by consequent irreversible ecological as well as economic damages. Although, energy is a key factor for development, many of the people of poor countries have little access to grid electricity. About 70% of the people of Bangladesh [3] have not been yet provided with electricity.

Many developing countries are the worst victims of the global energy crisis. But, they are also especially in advantageous position to develop and benefit from energy systems and technologies utilizing solar resources as most of them are blessed with sunshine. Study of the solar and wind assessment data by SWERA project under UNEP shows that Bangladesh is richly endowed with solar energy [4]. Therefore, photovoltaic system can play a significant role in the energy sector for the overall development of Bangladesh.

1.3 Objectives •

Energy consumption through out the world is on increase, dependence on solar energy may have a significant effect on reducing the demand of petrol, diesel, octane etc.

•

Solar energy can be of great use in a country like ours where sun exposure is much longer and effective than most other countries. The roof of a car may be exposed to sunlight to produce energy.

•

Other fuels are not environment friendly, but use of solar energy can create a new concept.

1.4 Main Activity To build a custom-designed vehicle driven by photovoltaic cells.

Materials: •

8 small solar cells

•

Small motor

•

Wire

•

Rechargeable batteries

Procedure: 1. To place the solar cells side by side.

2. Connecting them in a series by twisting the negative and the positive wire of one cell to the next cell.

3. To attach the motor to the remaining positive and negative wires.

4. To attach the cells to a combination of rechargeable batteries.

5. The motor will be driven by the power from the batteries.

6. Designing a custom body for the car.

Fig1.1: Photograph of a solar car by the Pembina Institute.

1.5 Points to consider when designing a solar car: •

The car should be designed in order to maximize the area exposed to sun light in order to achieve maximum power.

•

The car shape should be so-called an aerodynamic shape in order to achieve minimum wind resistance, or the so-called drag force.

•

The car should be as light as possible, because the power expected from the solar cells is not that much. In addition, most of this power will be utilized to overcome friction and drag.

1.6 Solar Cars Solar cars combine technology typically used in the aerospace, bicycle, alternative energy and automotive industries. The design of a solar vehicle is severely limited by the energy input into the car (batteries and power from the sun). Virtually all solar cars ever built have been for the purpose of solar car races (with notable exceptions).

Like many race cars, the driver's cockpit usually only contains room for one person, although a few cars do contain room for a second passenger. They contain some of the features available to drivers of traditional vehicles such as brakes, accelerator, turn signals, rear view

mirrors (or camera), ventilation, and sometimes cruise control. A radio for communication with their support crews is almost always included.

Solar cars are often fitted with gauges as seen in conventional cars. Aside from keeping the car on the road, the driver's main priority is to keep an eye on these gauges to spot possible problems. Cars without gauges available for the driver will almost always feature wireless telemetry. Wireless telemetry allows the driver's team to monitor the car's energy consumption, solar energy capture and other parameters and free the driver to concentrate on just driving.[5]

1.6.1 Electrical and mechanical systems The electrical system is the most important part of the car's systems as it controls all of the power that comes into and leaves the system. The battery pack plays the same role in a solar car that a petrol tank plays in a normal car in storing power for future use. Solar cars use a range of batteries including lead-acid batteries, nickel-metal hydride batteries (NiMH), Nickel-Cadmium batteries (NiCd), Lithium ion batteries and Lithium polymer batteries.

Many solar race cars have complex data acquisition systems that monitor the whole electrical system while even the most basic cars have systems that provide information on battery voltage and current to the driver.

The mechanical systems of a solar car are designed to keep friction and weight to a minimum while maintaining strength. Designers normally use titanium and composites to ensure a good strength-to-weight ratio.

Solar cars usually have three wheels, but some have four. Three wheelers usually have two front wheels and one rear wheel: the front wheels steer and the rear wheel follows. Four wheel vehicles are set up like normal cars or similarly to three wheeled vehicles with the two rear wheels close together.

1.7 Our work:

A solar cell for our project was bought. It is a 10×10cm cell. The size permits us to make a division of 8 parts where each part will have a dimension of 5×2.5 cm. These small sections of cells will be connected in series. The series connected arrangement is to be placed to a small panel.

The

Motor

Mechanical interface

Car

Fig 1.2: Block diagram of energy production by means of solar cells in a solar car.

energy produced by the cells will be directed to a motor. The connection will further go to a mechanical interface. This interface will finally lead the energy to the driving wheel. In this way, in a solar car the motor’s mechanical power can be increased by means of solar cells.

Chapter 2 Theoritical Background 2.1 Solar Energy Energy from the sun travels to the earth in the form of electromagnetic radiation similar to radio waves, but in a different frequency range. Available solar energy is often expressed in units of energy per time per unit area, such as watts per square meter (W/m2). The amount of energy available from the sun outside the Earth’s atmosphere is approximately 1367 W/m2; that’s nearly the same as a high power hair drier for every square meter of sunlight! Some of the solar energy is absorbed as it passes through the Earth’s atmosphere. As a result, on a clear day the amount of solar energy available at the Earth’s

surface in the direction of the sun is typically 1000 W/m 2. At any particular time, the available solar energy is primarily dependent upon how high the sun is in the sky and current cloud conditions. On a monthly or annual basis, the amount of solar energy available also depends upon the location. Furthermore, useable solar energy depends upon available solar energy, other weather conditions, the technology used, and the application.

There are many ways that solar energy can be used effectively. Applications of solar energy use can be grouped. There are three primary categories: heating/cooling, electricity production, and chemical processes. The most widely used applications are for water and space heating. Ventilation solar air heating is also growing in popularity. Uptake of electricity producing solar technologies is increasing for the applications photovoltaic (primarily) and concentrating solar thermal-electric technologies. Due to recent advances in solar detoxification technologies for cleaning water and air, these applications hold promise to be competitive with conventional technologies.

2.1.1 The advantages of solar energy Solar energy has the following advantages over conventional energy: •

The energy from the sun is virtually free after the initial cost has been recovered.

•

Depending on the utilization of energy, paybacks can be very short when compared to the cost of common energy sources used.

•

Solar and other renewable energy systems can be stand-alone; thereby not requiring connection to a power or natural gas grid.

•

The sun provides a virtually unlimited supply of solar energy.

•

The use of solar energy displaces conventional energy; which usually results in a proportional decrease in green house gas emissions.

•

The use of solar energy is an untapped market.

2.2 A Solar Cell:

A solar cell or photovoltaic cell is a device that converts solar energy into electricity.

Photovoltaics is the field of technology and research related to the application of solar cells as solar energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified. Assemblies of cells are used to make solar modules, which may in turn be linked in photovoltaic arrays.

Solar cells have many applications. Individual cells are used for powering small devices such as electronic calculators. Photovoltaic arrays generate a form of renewable electricity, particularly useful in situations where electrical power from the grid is unavailable such as in remote area power systems, Earth-orbiting satellites and space probes, remote radiotelephones and water pumping applications. Photovoltaic electricity is also increasingly deployed in grid-tied electrical systems. Similar devices intended to capture energy radiated from other sources include thermophotovoltaic cells, betavoltaics cells, and optoelectric nuclear batteries.

Solar cells are the basic elements of a solar module (also known as a solar panel).

Silicon is by far the commonest of a variety of semiconductors from which solar cells are made. A typical modern solar cell is squared-shaped measuring 10 cm Ă— 10 cm. It is covered by a clear anti-reflection coating (ARC) that reduces the amount of light

lost

to

reflection

at

the

cell

surface.

Fig2.1:A Siemens solar cell

The term "photovoltaic" comes from the Greek (phos) meaning "light", and "voltaic", meaning electrical, from the name of the Italian physicist Volta, after whom the unit of electrical potential, the volt, is named.

The photovoltaic effect was first recognized in 1839 by French physicist A. E. Becquerel.

However, it was not until 1883 that the first solar cell was built, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient. Russell Ohl patented the modern solar

cell in 1946 (U.S. Patent 2,402,662 , "Light sensitive device"). Sven Ason Berglund had a prior patent concerning methods of increasing the capacity of photosensitive cells. The modern age of solar power technology arrived in 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light.

This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent. The first spacecraft to use solar panels was the US satellite Vanguard 1, launched in March 1958 with solar cells made by Hoffman Electronics. This milestone created interest in producing and launching a geostationary communications satellite, in which solar energy would provide a viable power supply. This was a crucial development which stimulated funding from several governments into research for improved solar cells.

In 1970 the first highly effective GaAs heterostructure solar cells were created by Zhores Alferov and his team in the USSR. Metal Organic Chemical Vapor Deposition (MOCVD, or OMCVD) production equipment was not developed until the early 1980s, limiting the ability of companies to manufacture the GaAs solar cell. In the United States, the first 17% efficient air mass zero (AM0) single-junction GaAs solar cells were manufactured in production quantities in 1988 by Applied Solar Energy Corporation (ASEC). The "dual junction" cell was accidentally produced in quantity by ASEC in 1989 as a result of the change from GaAs on GaAs substrates to GaAs on Germanium (Ge) substrates. The accidental doping of Ge with the GaAs buffer layer created higher open circuit voltages, demonstrating the potential of using the Ge substrate as another cell. As GaAs single-junction cells topped 19% AM0 production efficiency in 1993, ASEC developed the first dual junction cells for spacecraft use in the United States, with a starting efficiency of approximately 20%. These cells did not utilize the Ge as a second cell, but used another GaAs-based cell with different doping.

Eventually GaAs dual junction cells reached production efficiencies of about 22%. Triple Junction solar cells began with AM0 efficiencies of approximately 24% in 2000, 26% in 2002, 28% in 2005, and in 2007 have evolved to a 30% AM0 production efficiency, currently in qualification.

In 2007, two companies in the United States, Emcore Photovoltaics and Spectrolab, produce 95% of the world's Triple Junction solar cells which have a commercial efficiency of 38% [citation needed]. In 2006 Spectrolab's cells achieved 40.7% efficiency in lab testing.

Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date .

2.2.1

How

solar

cells

work

Like all semiconductor devices, solar cells work with a semiconductor that has been doped to produce two different regions separated by a p-n junction. Across this junction, the two types of charge carrier – electrons and holes – are able to cross. In doing so, they deplete the region from which they came and transfer their charge to the new region. This migration of charge results in a potential gradient or electrical slope, down which charge carriers tend to

slide

as

they

approach

the

junction.

When sunlight strikes a solar cell, atoms are bombarded with particles of light called photons, and give up electrons. When an electron is kicked out of an atom, it leaves behind a hole, which has an equal and opposite (positive) charge. If either carrier wanders across the junction, the field and the nature of the semiconductor material discourage it from recrossing. A proportion of carriers that cross the junction can be harvested by completing a circuit from a grid on the cell's surface to a collector on the backplane. In the cell, the light "pumps" electrons out one side of the cell, through the circuit, and back to the other side, energizing

any

electrical

device

that

is

connected

along

the

way.

Fig2.2: Simplified operation of a solar cell

The current generated in the semiconductor is extracted by contacts at the top and bottom of the cell. The top contact structure, which must allow light to pass through, is made of thin, widely-spaced metal strips (usually called fingers) that supply current to a larger bus bar. The cell is covered with a thin layer of dielectric material – the anti-reflection coating – to minimize light reflection from the top surface. [6]

2.2.2

Characteristics

of

a

solar

cell

The usable voltage that a solar cell produces depends on what semiconductor material it's made from. In the case of silicon-based cells, the output is approximately 0.5 V. Although the current increases with increasing luminosity, the terminal voltage is only weakly dependent on the amount of light falling on the cell.

The simplest solar cell model consists of diode and current source connected in parallel.

Current is directly proportional to the solar radiation. Diode represents PN junction of a solar cell. Equation of ideal solar cell, which represents the ideal solar cell model, is:

Where is: IPh - photocurrent (A), IS - reverse saturation current (A) (approximately range 10-8/m2), V - diode voltage (V), VT - thermal voltage (see equation below), V T = 25.7 mV at 25째C, m - diode ideality factor = 1...5 x VT (-) (m = 1 for ideal diode)

Vd

V

Fig2.3: Ideal solar cell model

Thermal voltage VT ( V ) can be calculated with the following equation:

Where is: k - Boltzmann constant = 1.38 x 10 -23 J/K, T - temperature ( K ), q - charge of

electron

=

1.6

x

10-19

As

Vd

V Fig2.4:Real

Solar cell model with serial and parallel resistance R s and Rp,

The consequences of resistances are voltage drop and parasitic currents Solar cell I-V characteristics

Solar cell Power

The working point of the solar cell depends on load and solar insolation.

In the picture, I-V characteristics can be seen. Very important point in I-V

characteristics is Maximal Power Point - MPP. In practice we can seldom reach this point, because at higher solar insolation even the cell temperature increases, consequently decreasing the output power [7]. As a measure for solar cell quality fill-factor - FF is used. It can be calculated with the following equation:

Where is: Impp - MPP current ( A ), V mpp - MPP voltage ( V ), I sc - short circuit current ( A ), Voc - open circuit voltage ( V ) In the case of ideal solar cell fill-factor is a function of open circuit parameters and can be calculated as follows

Where is: voc - voltage calculated with equation below ( V )

Where is: k - Boltzmann constant = 1.38 x 10 -23 J/K, T - temperature ( K ), q - charge of electron = 1.6 x 10 -19 As, m - diode ideality factor ( - ), V oc - open circuit voltage (V)

2.2.3

Different

types

of

solar

cell

There are three main types of solar cells, which are distinguished by the type of crystal used in them. They are monocrystalline, polycrystalline, and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline rods are extracted from melted silicon and then sawed into thin plates. This production

process

guarantees

a

relatively

high

level

of

efficiency.

Efficiency in lab Efficiency of production cell

Material

(%)

(%)

about 24

14-17

polycrystalline silicon

about 18

13-15

amorphous silicon

about 13

5-7

monocrystalline silicon

The production of polycrystalline cells is more cost-efficient. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. As a

result

of

this

crystal

defect,

the

solar

cell

is

less

efficient.

If a silicon film is deposited on glass or another substrate material, the result is a so-called amorphous or thin-layer cell. The layer thickness amounts to less than 1µm – the thickness of a human hair for comparison is 50-100 µm. The production costs of this type are lower because of the lower material costs. However, the efficiency of amorphous cells is much lower than that of the other two cell types. As a result, they are used mainly in low power equipment, such as watches and pocket calculators, or as facade elements.

2.2.4

From

cells

to

modules

In order to provide suitable voltages and outputs for different applications, solar cells are connected together to form larger units. Cells connected in series have a higher voltage, while those connected in parallel produce more current. The interconnected solar cells are usually embedded in transparent ethylene vinyl acetate, fitted with an aluminum or stainless steel frame, and covered with transparent glass on the front side to make a solar module.

Typical peak power ratings of such solar modules range from 10 W to 100 W. The characteristic data refer to the standard test conditions of 1000 W/m 2 solar radiation at a cell temperature of 25° C (77° F). The manufacturer's standard warranty of 10 or more years

is quite long and shows the high quality standards and life expectancy of today's products.

2.2.5 Solar cell efficiency factors Energy conversion efficiency A solar cell's energy conversion efficiency (Ρ, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, Pm, divided by the input light irradiance(E, in W/m²) under standard test conditions (STC) and the surface area of the solar cell (Ac in m²).

Thermodynamic Efficiency Limit Solar cells operate as quantum energy conversion devices, and are therefore subject to the "Thermodynamic Efficiency Limit". Photons with an energy below the band gap of the absorber material cannot generate a hole-electron pair, and so their energy is not converted to useful output and only generates heat if absorbed. For photons with an energy above the band gap energy, only a fraction of the energy above the band gap can be converted to useful output. When a photon of greater energy is absorbed, the excess energy above the band gap is converted to kinetic energy of the carrier combination. The excess kinetic energy is converted to heat through phonon interactions as the kinetic energy of the carriers slows to equilibrium velocity.

Quantum efficiency Quantum efficiency refers to the percentage of photons that are converted to electric current (i.e., collected carriers) when the cell is operated under short circuit conditions.

External quantum efficiency is the fraction of incident photons that are converted to electrical current, while internal quantum efficiency is the fraction of absorbed photons that are converted to electrical current. Mathematically, internal quantum efficiency is related to

external quantum efficiency by the reflectance of the solar cell; given a perfect antireflection coating, they are the same.

Quantum efficiency should not be confused with energy conversion efficiency, as it does not convey information about the power collected from the solar cell. Furthermore, quantum efficiency is most usefully expressed as a spectral measurement (that is, as a function of photon wavelength or energy). Since some wavelengths are absorbed more effectively than others in most semiconductors, spectral measurements of quantum efficiency can yield information about which parts of a particular solar cell design are most in need of improvement.

2.2.6 Manufacturers Solar cells are manufactured primarily in Japan, China, Germany, Taiwan and the USA , though numerous other nations have or are acquiring significant solar cell production capacity. While technologies are constantly evolving toward higher efficiencies, the most effective cells for low cost electrical production are not necessarily those with the highest efficiency, but those with a balance between low-cost production and efficiency high enough to minimize area-related balance of systems cost. Those companies with large scale manufacturing technology for coating inexpensive substrates may, in fact, ultimately be the lowest cost net electricity producers, even with cell efficiencies that are lower than those of single-crystal technologies.

2.2.7 Solar array The solar array consists of hundreds of photovoltaic solar cells converting sunlight into electricity. The larger arrays in use can produce over 2 kilowatts (2.6 hp).

The solar array can be mounted in several ways: •

horizontal. This most common arrangement gives most overall power during most of the day in low latitudes or higher latitude summers and offers little interaction with the wind. Horizontal arrays can be integrated or be in the form of a free canopy.

•

vertical. This arrangement is sometimes found in free standing or integrated sails to harness wind energy.Useful solar power is limited to mornings, evenings, or winters and when the vehicle is pointing in the right direction.

•

adjustable. Free solar arrays can often be tilted around the axis of travel in order to increase power when the sun is low and well to the side. An alternative is to tilt the whole vehicle when parked. Two-axis adjustment is only found on marine vehicles, where the aerodynamic resistance is of less importance than with road vehicles.

•

integrated. Some vehicles cover every available surface with solar cells. Some of the cells will be at an optimal angle whereas others will be shaded.

•

trailer. Solar trailers are especially useful for retrofitting existing vehicles with little stability, e.g. bicycles. Some trailers also include the batteries and others also the drive motor.

•

remote. By mounting the solar array at a stationary location instead of the vehicle, power can be maximised and resistance minimized. The virtual grid-connection however involves more electrical losses than with true solar vehicles and the battery must be larger.

The choice of solar array geometry involves an optimization between power output, aerodynamic resistance and vehicle mass, as well as practical considerations. For example, a free horizontal canopy gives 2-3 times the surface area of a vehicle with integrated cells but offers better cooling of the cells and shading of the riders. There are also thin flexible solar arrays in development.

2.3 Rechargeable Battery A rechargeable battery, also known as a storage battery, is a group of two or more secondary cells. These batteries can be restored to full charge by the application of electrical energy. In other words, they are electrochemical cells in which the electrochemical reaction that releases energy is readily reversible. Rechargeable electrochemical cells are therefore a type of accumulator. They come in many different designs using different chemicals.

Commonly used secondary cell chemistries are lead and sulfuric acid, rechargeable alkaline battery (alkaline), nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium ion(Li-ion), and lithium ion polymer (Li-ion polymer).[8]

Rechargeable batteries can offer economic and environmental benefits compared to disposable batteries. Some rechargeable battery types are available in the same sizes as disposable types. While the rechargeable cells have a higher first cost than disposable batteries, rechargeable batteries can be discharged and recharged many times. Proper selection of a rechargeable battery system can reduce toxic materials sent to landfill disposal compared to an equivalent series of disposable batteries. Some manufacturers of NiMH type rechargeable batteries claim a service life up to 3000 charge cycles for their batteries.

2.3.1 Usage and applications Unlike nonrechargeable batteries (primary cells), secondary cells must be charged before use. Attempting to recharge nonrechargeable batteries has a small chance of causing a battery explosion.

Rechargeable batteries are susceptible to damage due to reverse charging if they are fully discharged. Fully integrated battery chargers that optimize the charging current are available.

Rechargeable batteries currently are used for applications such as automobile starters, portable consumer devices, tools, and uninterruptible power supplies. Emerging applications in Hybrid electric vehicles and electric vehicles are driving the technology to improve cost, reduce weight, and increase lifetime. Rechargeable batteries have been known since the lead acid battery was invented in 1859.

Grid energy storage applications use rechargeable batteries for load leveling, where they store electric energy for use during peak load periods, and for renewable energy uses, such as storing power generated from photovoltaic arrays during the day to be used at night. By charging batteries during periods of low demand and returning energy to the grid during periods of high electrical demand, load-leveling helps eliminate the need for expensive peaking power plants and helps amortize the cost of generators over more hours of operation.

The National Electrical Manufacturers Association has estimated that U.S. demand for rechargeables is growing twice as fast as demand for nonrechargeables.

2.3.2 Charging and discharging During charging, the positive active material is oxidized producing electrons, and the negative material is reduced, consuming electrons. These electrons constitute the current flow in the external circuit. The electrolyte may serve as a simple buffer for ion flow between the electrodes, as in lithium-ion and nickel-cadmium cells, or it may be an active participant in the electrochemical reaction, as in lead-acid cells.

The energy used to charge rechargeable batteries mostly comes from AC current (mains electricity) using an adapter unit. Most battery chargers can take several hours to charge a battery.

Fig2.5 : Battery charger

Reverse charging, which damages batteries, is when a rechargeable battery is recharged with its polarity reversed. Reverse charging can occur under a number of circumstances, the two most important being: •

When a battery is incorrectly inserted into a charger.

•

When multiple batteries are used in series in a device. When one battery completely charges ahead of the rest, the other batteries in series may force the charged battery to discharge to below zero voltage.

2.3.3 Common Rechargeable Battery Types

Nickel Cadmium Battery (NiCd) Created by Waldemar Jungner of Sweden in 1899 which was based on Thomas Edison's first alkaline battery. Using nickel oxide hydroxide and metallic cadmium as electrodes, NiCd batteries have longer life cycles and hold electrical charge longer. However, their voltage potential difference are often less than that of Nickel-metal Hydride's.

Nickel-Metal Hydride Battery (NiMH) First developed around 1980's. The battery has a hydrogen-absorbing alloy for the negative electrode instead of cadmium. Even though NiMH batteries have higher voltage outputs, the batteries discharge quicker and have a limited service life compared to NiCd. Lithium-ion Battery The technology behind Lithium-ion battery has not yet fully reached maturity. However, the batteries are the type of choice in many consumer electronics and have one of the best energy-to-mass ratios, no memory effect, and a slow loss of charge when not in use. The popularity of Lithium-ion has spread as their technology continues to improve.

Less common types Lithium sulfur battery Claims superior energy to weight than current lithium technologies on the market.

Thin film battery (TFB) The developers claim a very large increase in recharge cycles, around 40,000 cycles.

Smart battery A smart battery has the voltage monitoring circuit built inside.

Carbon foam-based lead acid battery Firefly Energy has developed a carbon foam-based lead acid battery with a reported energy density of 30-40% more than their original 38 W路h/kg , with long life and very high power density.

2.3.4 Nickel-metal hydride battery Battery specifications

Fig 2.6: Modern, high capacity NiMH rechargeable batteries

Energy/size

140–300 Wh/L

Power/weight

250–1000 W/kg

Charge/discharge efficiency

66%

Energy/consumer-price

2.75 Wh/US$

Self-discharge rate

30%/month (temperature dependent)

Time durability

Citation Needed

Cycle durability

500–1000

Nominal Cell Voltage

1.2 V

A nickel-metal hydride battery, abbreviated NiMH, is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery but using a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd batteries, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH battery can have two to three times the capacity of an equivalent size NiCd. However, compared to the lithium-ion battery, the volumetric energy density is lower and self-discharge is higher.

Common AA batteries (penlight-size) have nominal charge capacities (C) ranging from 1100 mA·h to 2700 mA·h at 1.2 V, usually measured at a discharge rate of 0.2×C per hour.

Useful discharge capacity is a decreasing function of the discharge rate, but up to a rate of

around 1×C (full discharge in one hour), it does not differ significantly from the nominal capacity.

The specific energy density for NiMH material is approximately 70 W·h/kg (250 kJ/kg), with a volumetric energy density of about 300 W·h/L (360 MJ/m³).

It is common to refer to most NiMH products as Batteries, even though the word Battery refers to the grouping of multiple cells. As a result sizes AA, AAA, C and D are technically Cells while the 9V size is a real battery.[9]

2.4 Toy Motor

Fig2.7: Photograph of a toy motor.

Fig 2.7 shows a small motor, about as big around as a dime. From the outside the steel can that forms the body of the motor, an axle, a nylon end cap and two battery leads. If one hooks the battery leads of the motor up to a flashlight battery, the axle will spin. If the leads is reversed, it will spin in the opposite direction.

The nylon end cap is held in place by two tabs that are part of the steel can. By bending the tabs back, the end cap can be freed and removed. Inside the end cap are the motor's brushes. These brushes transfer power from the battery to the commutator as the motor spins.

The axle holds the armature and the commutator. The armature is a set of electromagnets, in this case three. The armature in this motor is a set of thin metal plates stacked together, with thin copper wire coiled around each of the three poles of the armature. The two ends

of each wire (one wire for each pole) are soldered onto a terminal, and then each of the three terminals is wired to one plate of the commutator. [10]

2.5 Single Pole Double Throw Switch An electronic SPDT switch comprising: a common port adapted to be switched between a first port in a first circuit arm and a second port in a second circuit arm.

2.6 Aerodynamics and Drag: A body immersed in a flowing fluid is acted on by both pressure and viscous forces from the flow. The sum of the forces (pressure, viscous, or both) that acts normal to the free-stream direction is the lift, and the sum of that acts parallel to the free-stream direction is defined as the drag. These definitions are perhaps one of the famous conclusions of the famous Bernoulli’s equation, which is one of the fundamental laws governing the motion of fluids.

It relates an increase in flow velocity to a decrease in pressure and vice versa. Bernoulli's principle is used in aerodynamics to explain the lift of an airplane wing in flight. A wing is so designed that air flows more rapidly over its upper surface than its lower one, leading to a decrease in pressure on the top surface as compared to the bottom. The resulting pressure difference provides the lift that sustains the aircraft in flight. The velocity of a wind that strikes the bluff surface of a building is close to zero near its wall. According to Bernoulli's principle, this would lead to a rise in pressure relative to the pressure away from the building, resulting in wind forces that the structures must be designed to withstand.

Another important aspect of aerodynamics is the drag, or resistance, acting on solid bodies moving through air. The drag forces exerted by the air flowing over the airplane, for example, must be overcome by the thrust force developed by either the jet engine or the propellers. These drag forces can be significantly reduced by streamlining the body. For bodies that are not fully streamlined, the drag force increases approximately with the square of the speed as they move rapidly through the air. The power required, for example,

to drive an automobile steadily at medium or high speeds is primarily absorbed in overcoming

air

resistance.

The following examples illustrate the importance of considering drag when designing a car

Fig2.8: Importance of drag designing a car

By comparing these three shapes, we notice that the shape of the airfoil is the one that shows minimum drag, because of its streamline shape. In addition, it shows less or almost no turbulence at the end.

Fig2.9: comparison of normal and designed car

The difference between the above and the other car is that the stream line design of the above gives it the minimum drag among all other cars. Such a design enables it to move at higher speeds and make good use of its solar power instead of wasting it in resisting drag.

Chapter 3 Working on the Project 3.1 Panel design To design the panel a solar cell of 10×10 cm dimension was bought. To cut that into some convenient number of units was decided. It was cut into eight equal pieces. Each of the pieces had a dimension of 5×2.5cm.The cells had two sides, negative and positive. One cell’s positive side was connected to the negative of the other. In this way in a manner of series connection the whole panel was made. It was anticipated that one single cell would generate 0.5volt. Then a total of 4 volt was hoped to be found from the panel. +

-

+

+

+

+ -

-

==

-

+

+ -

-

+

Fig3.1: Block diagram of panel connecting eight equal sized cells.

To cut the cell into eight equal parts was performed first by marking it with perfect measurement. Then with a glass cutter the cell was cut. Much care was needed in doing the work.

The panel was placed on stable foam where it doesn’t have the possibility of being destructed. The connection of the wire was made carefully, but much handling of the panel may cause any of the connection disconnected. So, a stable foundation was needed to place the cell arrangements. Then, the panel was formed.

Fig3.2: Photograph of the designed Solar panel.

3.2 Motor: The aim was to build a toy car that can be run by solar power. So the motor from a toy car was enough to be used for our solar car. But whether the panel voltage was sufficient to drive the motor must be investigated. A motor that can be driven by a voltage of approximately 3 volt was selected. The motor was collected from a toy car.

3.3 Rechargeable Batteries: Rechargeable battery is needed for the design to drive the car. When a car roof is exposed to sunlight in the day, power to drive the car can easily be got from the solar panel. But when the day is cloudy or in night solar panel can’t produce enough power to drive the car.

So, by means of a single pole double throw switch our panel was connected to the motor as well as rechargeable battery. When the switch is connected from the battery to the panel, the battery gets recharged; when the switch is connected from battery to motor the motor can work. The later one is the working mode of the car. When a car is parked, the solar panel on its roof may absorb the energy and charge the battery.

There are many kinds of rechargeable batteries. They have different features. For the purpose to drive just a toy motor a pair of NiMH batteries was chosen. If they get fully charged, they can supply almost 2.4 Volts (1.2V each).

Fig3.3: Photograph of NiMH Rechargeable batteries

To place the rechargeable batteries a battery holder was needed. For convenience of use while charging as well as supplying voltage the battery holder was required. It was managed from a toy.

Fig 3.4: Battery holder collected from a toy

3.4 Car:

To carry on the designing procedure, a toy car is necessary where we would place the solar panel and the whole mechanical systems. The motor has to be connected to the wheel of the car by means of a ribbon. When the motor rotates it will also cause the wheel to rotate.

And thus the car will begin to move. We selected a toy car in which we could properly utilize its mechanical inertia.

Fig3.5: Car on which the mechanical system to be placed.

3.5 Working on the Project Design & Analysis: The circuit was designed as the project requirements. The motor and the panel are placed in parallel with their one end connected and the others open. The open ends can be connected to the switch as necessary, when to recharge the batteries the switch goes to the panel, and when the car is to run the switch is connected to the motor. The port of the switch is connected to the rechargeable batteries. The batteries are connected in series.

The whole mechanical system is to be placed on the car.

switch +

-

Rechargeable batteries

+

+

motor

panel

-

-

Fig 3.6: Circuit diagram of the mechanical system of the solar car.

Before making the circuit arrangement, the circuit components were examined separately.

Suppose an experiment on the solar panel whether we have an I-V curve like the normal solar cell was made.

After completing the making of the panel, the voltage with a multimeter from the panel was measured. It was in a closed room with insufficient light. So, a negligible amount of voltage was showed on the digital multimeter. A table lamp was used then. The lamp was lit upon the panel and then the multimeter showed a voltage nearly 2.5 volt.

The voltage of the panel in the mid day in open sunlight was measured. It was a sunny day.

There a satisfactory result was found. The voltage was nearly 3.8volts. Voltage in various time of a day was measured. The result varied with the intensity of sunlight. With a good intensity a good voltage was found, but as the intensity decreases at other times of the day the voltage also decreases. Again cloud was a factor that influences the voltage. As cloud covers the sun the voltage comes down instantly to nearly the half it was previously showing.

Experiments to know the I-V characteristic curve of our solar panel was made. The experiment was done under a table lamp that has a 200W tungsten bulb.

Table 3.1: Data table for I-V curve:

Current, I (mA)

Voltage, V (volt)

81

0.34

80

0.50

74

0.86

65

1.20

45

1.95

40

2.05

20

2.52

07

2.75

100 80 60

Series1

40

Current (mA)

20 0 0

1

2

3

Voltage(V) Voltage (V) Fig 3.7: I-V characteristics measured under a 200W tungsten bulb at a height of 4.5 inch over the panel.

The I-V characteristic curve was measured for two different heights of the lamp over the panel. The data were taken from the digital multimeter as correctly as possible.

Table 3.2: Data table for I-V curve:

Current, I (mA)

Voltage, V (volt)

140 125 112 86 50 42 10

0.28 0.89 1.22 1.57 2.11 2.25 2.7

150 100 Series1 50

Current (mA)

0 0

1

2

3

Voltage(V) Fig 3.8: I-V characteristics measured under a 200W tungsten bulb at a height of 3 inch over the panel.

The power voltage curve was also drawn for the height of 4.5 inch of the lamp over the panel.

Table 3.3: Data table for P-V curve:

Power, W (watt)

Voltage, V (volt)

0.03 0.04 0.064 0.078 0.09 0.082 0.06 0.04

0.34 0.50 0.86 1.20 1.95 2.05 2.15 2.31

0.1 0.08 0.06

Series1

0.04

Power (W)

0.02 0 0

1

Voltage(V)

2

3

Fig 3.9: P-V characteristics measured under a 200W tungsten bulb at a height of 4.5 inch over the panel.

Power is one of the ways to measure the capability of the solar panel. For the solar panel it was measured also. Power was not constant all through the day. It differs from time to time.

At midday when the sun is at top, a power close to 0.6 watt was found. At Other times, the power decreased. Intensity of the sun at different times was collected from the ‘Renewable Energy Research Center’ (RERC) of our university.

Table 3.3: values of power in different times:

Date 28.10.2008 28.10.2008 29.10.2008 29.10.2008 29.10.2008

Time 11.00 a.m 11.30 a.m 12.15 p.m 01.00 p.m 02.00 p.m

Intensity(W/m2) 832 838 838 723 800

Power(W) 0.57 0.59 0.59 0.49 0.54

Table 3.4: values of power in different times:

Power(W) .005 .01 .015 .02 .036 .07 .09 .13 .26 .38

Intensity(W/m2) 13 26 39 52 77 129 150 180 284 361

0.4 0.3 0.2

Power (W)

Series1

0.1 0 0

100

Intensity (W/m2)

200

300

400

Fig 3.10: Power- Intensity characteristics measured in different time.

It had to be measured also if the batteries are charged up properly from the panel. From the current direction connecting the panel and the batteries it was found that the panel can charge the batteries. After charging for sometime, battery voltage was measured and found the proof of the batteries being charged.

When the batteries were charged to sufficient voltage, driving the motor with the voltage obtained from the batteries was tried. The motor axle rotated.

Chapter 4 Result & Discussion 4.1 Achievement of the Work: •

The panel I-V characteristics were quite similar to that of an practical solar cell.

•

The panel can supply voltage as well as power close to our anticipation, which proves a good cutting, wiring and maintenance of the whole panel.

•

The power vs solar intensity curve was linear to certain extent.

•

The rechargeable batteries were charged successfully to their highest capacity from the panel.

•

The motor axle rotated when the motor was connected to the charged batteries.

•

Above all, by means of a belt joining the motor axle and the wheel of the car, the car can move.

4.2 Discussion on result: •

From the figure 3.7 it is seen that the short circuit measurement could not be taken.

Measuring it may burn the potentiometer.

The figure also reveals that the series resistance of the panel is very high due to the smaller size of the cells. •

Figure 3.8 shows a better curve than the previous one. It is seen from this curve the series resistance is smaller here.

•

Figure 3.10 shows the power-intensity curve. This curve is not an exact linear one.

The output power increases when the intensity increases. This characteristic clearly indicates that the panel works better in higher insolation level.

Another reason behind the curve’s being non linear may be the slight inaccuracy of perfect measurement, though the the panel and the pyranometer (by which radiation was measured) was placed horizontally on the same plane.

4.3 Future: There is 10,000 times more sunlight than we need to meet 100 percent of our energy needs, Meeting the energy challenge and the other grand challenges of the 21st century is simply imperative to our survival on the planet. Solar and wind power currently supply about 1 percent of the world's energy needs, but advances in technology are about to expand with the introduction of nano-engineered materials for solar panels, making them far more efficient, lighter and easier to install.

Regardless of any one technology, many scientists think that we are not that far away from a tipping point where energy from solar will be [economically] competitive with fossil fuels.

Other technologies that will help are solar concentrators made of parabolic mirrors that focus very large areas of sunlight onto a small collector or a small efficient steam turbine.

Use of solar energy may have a good aspect if it is cost effective. But comparatively it is not yet so cost effective. The making of mechanical system and the rechargeable battery as well as the solar cells are the prime factors to increase the cost.

Though a solar car in our country may not have as much good aspect as in other countries considering the cost, solar cell power has some good aspects. We can use solar cells in the remote parts of Bangladesh where electricity is not available yet. With solar power not a

large number of electrical home appliances can be run but in those regions, a bulb of fewer watts can easily be lit. That will be of great use as much as the power requirements of Bangladesh are concerned.

At present, Bangladesh is going through a severe electricity crisis. In this situation, solar power can be a good alternative small scale power source that does not need any fuel. The impact of the system on the environment issue should be taken into account as the system does not pollute the environment at all.

Reference: 1. www.reachoutmichigan.org/funexperiments/agesubject/lessons/newton/solarcar 2. Fundamentals of solar energy conversion 3. Bangladesh Economic Review, 2006.

4. SWERA-Bandgladesh Report, UNED/GEF, February, 2007 5. www.re-energy.ca/t_solarelectricity 6.www.daviddarling.info/encyclopedia/S/AE_solar_cell.htm 7. http://www.pvresources.com/en/solarcells.php 8. en.wikipedia.org/wiki/Rechargeable_batteries 9. http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery 10. http://electronics.howstuffworks.com/motor2.htm