CHAPTER ONE 1.0
INTRODUCTION Thee inno Th innova vati tion on of sola solarr char charge gers rs for for mo mobi bile le phone phoness as a prod product uct of rese resear arch ch and
development has been prompted by the challenge to uncover other possible means of charging mobile phones especially where and when power supply becomes erratic or totally inaccessible. This challenge has made solar charging which is one of the expedient alternative methods for charging mobile devices a necessity. Although this charging idea at present has not been widely known and accepted in this part of the world: specifically in Nigeria, it is the solution to the erratic and incessant interruption of power supply to technological equipments mobile phones being our focus. This fact is further substantiated by the simple fact that Nigeria is located in the tropics, which are areas that are typically known to have an abundant supply of sunlight all year round. The solar phone charger is inevitable in Nigeria as a case study, considering the facts that Nigeria is located in the tropics and at present, many parts of the country are suffering from an unstable, unreliable, erratic and severely unavailability electric power supply which poses a great deal of danger to electronic and electrical appliances and consequently shortens their life span, or incapacitates them at the most critical moments when they are needed to perform the functions why they were invented or manufactured in the first place. An electric phone charger !referred to as from now onwards as a "regular charger#$ is a device used to %force& current into the battery of a mobile phone by converting pulsating ac !alternating current$ from an ac supply outlet, to dc !direct current$ which is the type of current required by a mobile phone. 'n a solar mobile phone charger, the ac supply outlet is eliminated, since the required current and voltage is supplied by a dc cell known as a solar cell, which converts solar energy into electricity. A solar cell or photovoltaic cell is a large area electronic device that that conver converts ts solar solar energy energy into into electr electrici icity ty by the the pho photov tovol oltai taicc effect effect.. A solar solar charg charger er provid provides es an alternative source for charging mobile phones and furthermore harnesses the use of the abundant solar energy available for human use. There are many variations in the circuit design of regular electric
chargers and the circuit design of solar chargers. (or example, because a solar cell produces dc, which is what mobile phones generally require, if the solar cell ratings, as much as possible, closely matches the power requirements of the mobile phone, a transformer is not required,
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whereas a transformer is needed for a regular charger, since neither of the regular ))*+ or *+ can be supplied to a mobile phone even if it is dc. (urthermore, regular chargers have an ac input and a dc output which means, they definitely must have rectifier circuits and some sort of filter components to remove ripples, these requirements are somewhat eliminated in the design of solar chargers. These are some ma-or differences in the design of regular electric phone chargers and solar chargers, but generally, their mode of operation is the same. 't is worth noting that while regular chargers generally differ from solar chargers, regular chargers also differ one from another and solar chargers themselves have differences in construction and circuit requirements. These variations in their individual designs ma-orly depend on the level of efficiency required. A solar charger could be designed by simply using a + solar cell, connected in series with a suitable resistor at the positive side of the cell and practically charge a mobile phone, but for efficiency, it is better to use the solar cell to charge a battery pack which serves as a charge storage medium, which in turn is used to charge the mobile phone anytime. Another ma-or advantage of a solar charger is that, it is mobile and could be used anywhere, anytime as long as there#s enough sunlight to make the solar cell produce the power requirements of the phone being charged and this means that "on the move# charging is made possible by a solar charger, since it does not require a regular ac outlet electricity source. The ma-or disadvantage of a mobile charger, which has been innovatively eliminated in this pro-ect is that, a solar charger cannot be used anywhere or anytime there#s no available or sufficient sunlight, because, the solar cell requires sunlight to produce a considerable amount of current flow. This disadvantage can be innova innovativ tively ely minim minimi/e i/ed d by placin placing g the solar solar cell cell under under strong strong lights lights when when solar solar energy energy is insuffici insufficient ent or unavailable unavailable.. A better solution to this problem is to add a rechargeab rechargeable le battery battery to the circuit, which further makes our design complex. The solar cell is however used to charge this battery, while the battery in turn charges our mobile phone. 'n practice, solar cells only require a small amount of incident light to produce an output power, making it possible to charge round the clock with or without sunlight using a rechargeable battery.
1.1
SOLAR CELLS AT A GLANCE
A solar cell or photovoltai photovoltaicc cell is a large large area electronic electronic device that converts solar energy into electricity by the photovoltaic effect. Assemblies of cells are used to make solar modules, or 2
photovoltaic arrays which are used to supply either higher current or voltage which cannot be practically reali/able from single cells to loads requiring higher power. 0olar cells have many applications. 1ells are used for powering small devices such as electronic calculators, laptops, mp2 players etc. 3hotovoltaic 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 syste systems. ms. 0ilico 0ilicon n has som somee specia speciall chemica chemicall proper properti ties, es, especi especiall ally y in its its crysta crystalli lline ne form. form. An atom of silicon has 4 electrons, arranged in three different shells. The first two shells, those closest to the center, are completely full. The outer shell, however, is only half full, having only four electrons. A silicon atom will always look for ways to fill up its last shell !which would like to have eight electrons$. To do this, it will share electrons with four of its neighbor silicon atoms. 't5s like every atom holds hands with its neighbors, except that in this case, each atom has four hands -oined to four neighbors. That5s what forms the crystalline structure, and that structure turns out to be important to this type of 3+ cell. 6e5ve now described pure, crystalline silicon. 3ure silicon is a poor conductor of electricity because none of its electrons are free to move about, as electrons are in good g ood conductors such as copper. 'nstead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell. A solar cell has silicon with impurities 77 other atoms mixed in with the silicon atoms, changing the way things work a bit. 6e usually think of impurities as something undesirable, but in our case, our cell wouldn5t work without them. These impurities are actually put there on purpose. 1onsider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. 3hosphorous has five electrons in its outer shell, not four. 't still bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn5t have anyone to hold hands with. 't doesn5t form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place. 6hen energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons then wander randomly around the crystalline lattice looking for another hole to fall into. These electrons are called called free carriers , and can carry electrical current. There are so few of them in pure silicon, however, that they aren5t very useful. 8ur impure silicon with
3
phosphorous atoms mixed in is a different story. 't turns out that it takes a lot less energy to knock loose one of our 9extra9 phosphorous electrons because they aren5t tied up in a bond 77 their neighbors aren5t holding them back. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called dpi!", and when doped with phosphorous, the resulting silicon is called N#t$pe !9n9 for negative$ because of the prevalence of free electrons. N7type doped silicon is a much better conductor than pure silicon is. Actually, only part of our solar cell is N7type. The other part is doped with boron, which has only three electrons in its outer shell instead of four, to become P#t$pe silicon. 'nstead of having free electrons, 37type silicon !9p9 for positive$ has free holes. oles really are -ust the absence of electrons, so they carry the opposite !positive$ charge. They move around -ust like electrons do. The interesting part starts when you put N7type silicon together with 37type silicon. ;emember that every 3+ cell has at least one e%ectric fie%d . 6ithout an electric field, the cell wouldn5t work, and this field forms when the N7type and 37type silicon are in contact. 0uddenly, the free electrons in the N side, which have been looking all over for holes to fall into, see all the free holes on the 3 side, and there5s a mad rush to fill them in. There are high efficiency cells which are a class of solar cells that can generate electricity at higher efficiencies than conventional solar cells. 0olar cells are often electrically connected and encapsulated as a module. 3+ modules often have a sheet of glass on the front !sun up$ side, allowing light to pass while protecting the semiconductor wafers from the elements !rain, hail, etc.$. 0olar cells are also usually connected in series in modules, creating an additive voltage. 1onnecting cells in parallel will yield a higher current.
of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4.? k6h per day.
4
To make practical use of the solar7generated energy, the electricity is most often fed into the electricity grid using inverters !grid7connected 3+ systems$@ in stand7alone systems, batteries are used to store the energy that is not needed immediately.
1.&
APPLICATION AND I'PLE'ENTATION
Simp%e E(p%a!ati!
. 3hotons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. ). lectrons !negatively charged$ are knocked loose from their atoms, allowing them to flow through the material to produce electricity. =ue to the special composition of solar cells, the electrons are only allowed to move in a single direction. The complementary positive charges that are also created !like bubbles$ are called holes and flow in the direction opposite of the electrons in a silicon solar panel. 2. An array of solar cells converts solar energy into a usable amount of direct current !=1$ electricity.
Because solar cells are semiconductor devices, they share many of the same processing and manufacturing techniques as other semiconductor devices such as computer and memory chips. owever, the stringent requirements for cleanliness and quality control of semiconductor fabrication are a little more relaxed for solar cells.
5
dioxide as the antireflection coating of choice because of its excellent surface passivation qualities !i.e., it prevents carrier recombination at the surface of the solar cell$. 't is typically applied in a layer several hundred nanometers thick using plasma7enhanced chemical vapor deposition !31+=$. 0ome solar cells have textured front surfaces that, like antireflection coatings, serve to increase the amount of light coupled into the cell. 0uch surfaces can usually only be formed on single7crystal silicon, though in recent years methods of forming them on multicrystalline silicon have been developed. The wafer then has a full area metal contact made on the back surface, and a grid7like metal contact made up of fine 9fingers9 and larger 9busbars9 are screen7printed onto the front surface using a silver paste. The rear contact is also formed by screen7printing a metal paste, typically aluminium. Dsually this contact covers the entire rear side of the cell, though in some cell designs it is printed in a grid pattern. The paste is then fired at several hundred degrees 1elsius to form metal electrodes in ohmic contact with the silicon. After the metal contacts are made, the solar cells are interconnected in series !andEor parallel$ by flat wires or metal ribbons, and assembled into modules or 9solar panels9. 0olar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back. Tempered glass cannot be used with amorphous silicon cells because of the high temperatures during the deposition process.
1.)
CHARGERS AT A GLANCE
A *atter$ c+ar"er is a device used to put energy into a secondary cell or !rechargeable$ battery by forcing an electric current through it. The charge current depends upon the technology and capacity of the battery being charged. Battery chargers come in different physical shapes, si/es and various capacities. According to technological designs, chargers can be broadly classified into any of the following categories, although, some chargers may fall into more than one category. They are: 0imple 1hargers, Trickle 1hargers, Timer based 1hargers, 'ntelligent 1hargers, (ast 1hargers, 'nductive 1hargers, 3ulse 1hargers, 0olar 1hargers, D0B 1hargers etc.
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voltages, most of which are not compatible with other manufacturers5 phones or even different models of phones from a single manufacturer.
7
CHAPTER T,O ).* F'T;ATD; ;+'6 &.1
SOLAR CELLS: A solar cell or photovoltaic cell is a large area electronic device that
converts solar energy into electricity by the photovoltaic effect. 3hotovoltaics is the field of technology and research related to the application of solar cells for solar energy. 0ometimes 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, or photovoltaic arrays. 0olar cells have many applications. 1ells are used for powering small devices such as electronic calculators. 3hotovoltaic 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, arth7orbiting satellites and space probes, remote radiotelephones and water pumping applications. 3hotovoltaic electricity is also increasingly deployed in grid7tied electrical systems. 0imilar devices intended to capture energy from other sources include thermo7photovoltaic cells, betavoltaics cells, and optoelectric nuclear batteries. 1urrent research is targeting conversion efficiencies of 2*7*> while retaining low cost materials and manufacturing techniques. They can exceed the theoretical solar conversion efficiency limit for a single energy threshold material, that was calculated in G by 0hockley and Hueisser as 2> under sun illumination and 4*.?> under maximal concentration of sunlight !4,)** suns, which makes the latter limit more difficult to approach than the former$. 0olar cells are often electrically connected and encapsulated as a mdu%e. 3+ modules often have a sheet of glass on the front !sun up$ side, allowing light to pass while protecting the semiconductor wafers from the elements !rain, hail, etc.$. 0olar cells are also usually connected in series in modules, creating an additive voltage. 1onnecting cells in parallel will yield a higher current.
hours per day is often used. A common rule of thumb is that average power is equal to )*> of peak power, so that each peak kilowatt of solar array output power corresponds to energy production of 4.? k6h per day. To make practical use of the solar7generated energy, the electricity is most often fed into the electricity grid using inverters !grid7connected 3+ systems$@ in stand alone systems, batteries are used to store the energy that is not needed immediately. There are a few approaches to achieving these high efficiencies: •
•
•
Dse of excess thermal generation !caused by D+ light$ to enhance voltages or carrier collection.
•
Dse of infrared spectrum to produce electricity at night.
Technologies include: •
0ilicon nanostructures
•
DpE=own converters
•
ot7carrier cells
•
Thermoelectric cells
&.1.1 Hi"+ efficie!c$ ce%%s Hi"+ efficie!c$ s%ar ce%%s are a class of s%ar ce%%s that can generate electricity at higher
efficiencies than conventional solar cells. 6hile high efficiency solar cells are more efficient in terms of electrical output per incident energy !wattEwatt$, much of the industry is focused on the most cost efficient technologies !cost7per7watt or IEwatt$. 0till, many businesses and academics are focused on increasing the electrical efficiency of cells, and much development is focused on high efficiency solar cells. An example of this is the Three7dimensional solar cells that capture nearly all of the light that strikes them and could boost the efficiency of photovoltaic !3+$
9
systems while reducing their si/e, weight and mechanical complexity. The new 2= solar cells capture photons from sunlight using an array of miniature %tower& structures that resemble high7 rise buildings in a city street grid . To increase efficiency of solar cells a system has been developed, known as concentrating photovoltaic systems which in practice are not cells but rather are methods that use a large area of lenses or mirrors to focus sunlight on a small area of photovoltaic cells. 'f these systems use single or dual7axis tracking to improve performance, they may be referred to as Heliostat Concentrator Photovoltaics !13+$. The primary attraction of 13+ systems is their reduced usage of semiconducting material which is expensive and currently in short supply. Additionally, increasing the concentration ratio improves the performance of general photovoltaic materials. =espite the advantages of 13+ technologies their application has been limited by the costs of focusing, tracking and cooling equipment
&.1.& T+e p#! -u!cti! The most commonly known solar cell is configured as a large7area p7n -unction made from silicon. As a simplification, one can imagine bringing a layer of n7type silicon into direct contact with a layer of p7type silicon. 'n practice, p7n -unctions of silicon solar cells are not made in this way, but rather, by diffusing an n7type dopant into one side of a p7type wafer !or vice versa$. 'f a piece of p7type silicon is placed in intimate contact with a piece of n7type silicon, then a diffusion of electrons occurs from the region of high electron concentration !the n7type side of the -unction$ into the region of low electron concentration !p7type side of the -unction$. 6hen the electrons diffuse across the p7n -unction, they recombine with holes on the p7type side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately on either side of the -unction which this diffusion creates. The electric field established across the p7n -unction creates a diode that promotes current in only one direction across the -unction. lectrons may pass from the n7type side into the p7type side, and holes may pass from the p7type side to the n7type side, but not the other way around. This region where electrons have diffused across the -unction is called the
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depletion region because it no longer contains any mobile charge carriers. 't is also known as the 9space charge region9.
(ig ).: The equivalent circuit of a solar cell
To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components whose behaviour is well known. An ideal solar cell may be modelled by a current source in parallel with a diode@ in practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The resulting equivalent circuit of a solar cell is shown
(ig ).): 0chematic symbol of a solar cell Characteristic equation
(rom the equivalent circuit it is evident that the current produced by the solar cell is equal to that produced by the current source, minus that which flows through the diode, minus that which flows through the shunt resistor: I J I L K I D K I SH
........................
where 11
quation ).
•
I J output current !amperes$
•
I L J photo generated current !amperes$
•
I D J diode current !amperes$
•
I SH J shunt current !amperes$
The current flowing through these elements governed by the voltage across them: Vj J V L IRS
...........................
Equation 2.2
where •
V J voltage across the output terminals !volts$
•
I J output current !amperes$
•
RS J series resistance !M$
By the 0hockley diode equation, the current diverted through the diode is:
.............................
where •
I * J reverse saturation current !amperes$
•
n J diode ideality factor ! for an ideal diode$
•
q J elementary charge
•
k J Bolt/mann5s constant
•
T J absolute temperature
•
(or silicon at )C1,
volts.
By 8hm5s law, the current diverted through the shunt resistor is:
12
quation ).2
...............................
quation ).4
where •
RSH J shunt resistance !M$
0ubstituting these into the first equation produces the characteristic equation of a solar cell, which relates solar cell parameters to the output current and voltage:
................
quation ).C
An alternative derivation produces an equation similar in appearance, but with V on the left7hand side. The two alternatives are identities@ that is, they yield precisely the same results. 'n principle, given a particular operating voltage V the equation may be solved to determine the operating current I at that voltage. owever, because the equation involves I on both sides in a transcendental function the equation has no general analytical solution. owever, even without a solution it is physically instructive. (urthermore, it is easily solved using numerical methods. !A general analytical solution to the equation is possible using Fambert5s 6 function, but since Fambert5s 6 generally itself must be solved numerically this is a technicality.$ 0ince the parameters I *, n, RS , and RSH cannot be measured directly, the most common application of the characteristic equation is nonlinear regression to extract the values of these parameters on the basis of their combined effect on solar cell behaviour. &.1.&a
Effect f p+$sica% sie
The values of I *, RS , and RSH are dependent upon the physical si/e of the solar cell. 'n comparing otherwise identical cells, a cell with twice the surface area of another will, in principle, have double the I * because it has twice the -unction area across which current can leak. 't will also have half the RS and RSH because it has twice the cross7sectional area through which current can
13
flow. (or this reason, the characteristic equation is frequently written in terms of current density, or current produced per unit cell area:
.................
quation ).
6here •
J current density !amperesEcm )$
•
L J reverse saturation current density !amperesEcm )$
•
r S J specific series resistance !M7cm )$
•
r SH J specific shunt resistance !M7cm )$
This formulation has several advantages. 8ne is that since cell characteristics are referenced to a common cross7sectional area they may be compared for cells of different physical dimensions. 6hile this is of limited benefit in a manufacturing setting, where all cells tend to be the same si/e, it is useful in research and in comparing cells between manufacturers. Another advantage is that the density equation naturally scales the parameter values to similar orders of magnitude, which can make numerical extraction of them simpler and more accurate even with naive solution methods. A practical limitation of this formulation is that as cell si/es shrink, certain parasitic effects grow in importance and can affect the extracted parameter values. (or example, recombination and contamination of the -unction tend to be greatest at the perimeter of the cell, so very small cells may exhibit higher values of * or lower values of r SH than larger cells that are otherwise identical. 'n such cases, comparisons between cells must be made cautiously and with these effects in mind.
14
&.1.&*
Ce%% temperature
(ig ).2: ffect of temperature on the current7voltage characteristics of a solar cell
Temperature affects the characteristic equation in two ways: directly, via T in the exponential term, and indirectly via its effect on I *. !0trictly speaking, temperature affects all of the terms, but these two far more significantly than the others.$ 6hile increasing T reduces the magnitude of the exponent in the characteristic equation, the value of I * increases in proportion to expT . The net effect is to reduce V !C linearly with increasing temperature. The magnitude of this reduction is inversely proportional to V !C @ that is, cells with higher values of V !C suffer smaller reductions in voltage with increasing temperature. (or most crystalline silicon solar cells the reduction is about *.C*>E1, though the rate for the highest7efficiency crystalline silicon cells is around *.2C>E1. By way of comparison, the rate for amorphous silicon solar cells is *.)*7*.2*>E1, depending on how the cell is made. The amount of photogenerated current I L increases slightly with increasing temperature because of an increase in the number of thermally generated carriers in the cell. This effect is slight, however: about *.*C>E1 for crystalline silicon cells and *.*G> for a morphous silicon cells. The overall effect of temperature on cell efficiency can be computed using these factors in combination with the characteristic equation. owever, since the change in voltage is much stronger than the change in current, the overall effect on efficiency tends to be similar to that on voltage. E1 and most amorphous cells decline by *.C7*.)C>E1. The figure below shows '7+ curves that might typically be seen for a crystalline silicon solar cell at various temperatures. &.1.)
Series resista!ce 15
(ig ).4: ffect of series resistance on the current7voltage characteristics of a solar cell
As series resistance increases, the voltage drop between the -unction voltage and the terminal voltage becomes greater for the same flow of current. The result is that the current7controlled portion of the '7+ curve begins to sag toward the origin, producing a significant decrease in the terminal voltage V and a slight reduction in I SC . +ery high values of RS will also produce a significant reduction in I SC @ in these regimes, series resistance dominates and the behaviour of the solar cell resembles that of a resistor. These effects are shown for crystalline silicon solar cells in the '7+ curves displayed in the figure below. &.1./
S+u!t resista!ce
(ig ).C: ffect of shunt resistance on the current7voltage characteristics of a solar cell
As shunt resistance decreases, the flow of current diverted through the shunt resistor increases for a given level of -unction voltage. The result is that the voltage7controlled portion of the '7+ curve begins to sag toward the origin, producing a significant decrease in the terminal current I and a slight reduction in V !C . +ery low values of RSH will produce a significant reduction in V !C .
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characteristics similar to those of a resistor. These effects are shown for crystalline silicon solar cells
in
&.1.
the
'7+
curves
displayed
in
the
figure
to
the
right.
Reerse saturati! curre!t
(ig ).: ffect of reverse saturation current on the current7voltage characteristics of a solar cell
'f one assumes infinite shunt resistance, the characteristic equation can be solved for V !C :
.........................
quation ).O
Thus, an increase in I * produces a reduction in V !C proportional to the inverse of the logarithm of the increase. This explains mathematically the reason for the reduction in V !C that accompanies increases in temperature described above. The effect of reverse saturation current on the '7+ curve of a crystalline silicon solar cell are shown in the figure to the right. 3hysically, reverse saturation current is a measure of the 9leakage9 of carriers across the p7n -unction in reverse bias. This leakage is a result of carrier recombination in the neutral regions on either side of the -unction.
17
&.1.2
Idea%it$ factr
(ig ).O: ffect of ideality factor on the current7voltage characteristics of a solar cell
The ideality factor !also called the emissivity factor$ is a fitting parameter that describes how closely the diode5s behaviour matches that predicted by theory, which assumes the p7n -unction of the diode is an infinite plane and no recombination occurs within the space7charge region. A perfect match to theory is indicated when n J . 6hen recombination in the space7charge region dominate other recombination, however, n J ). The effect of changing ideality factor independently of all other parameters is shown for a crystalline silicon solar cell in the '7+ curves displayed in the figure to the right.
. The latter tends to erode solar cell output voltage
while the former acts to increase it. The net effect, therefore, is a combination of the increase in voltage shown for increasing n in the figure to the right and the decrease in voltage shown for increasing I * in the figure above. Typically, I * is the more significant factor and the result is a reduction in voltage.
&.&
3ATTER4 CHARGERS
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A *atter$ c+ar"er is a device used to put energy into a secondary cell or !rechargeable$ battery by forcing an electric current through it. The charge current depends upon the technology and capacity of the battery being charged. (or example, the current that should be applied to recharge a ) + car battery will be very different from the current for a mobile phone battery.
2.2.1
TYPES OF BATTERY CHARGERS
Battery chargers are of various tyes a!" shaes# "ee!"i!g o! the ai$ or target of $a!ufacturers. They vary i! hysica% shae a!" si&e# circuit co$o!e!ts# chargi!g tech!i'ues# co$o!e!t rati!gs#
i!ut
re'uire$e!ts
a!"
their
outut.
Ge!era%%y#
chargers are c%assi(e" i!to the fo%%o)i!g categories* &.&.1a
Simp%e C+ar"ers
A simple charger works by connecting a constant =1 power source to the battery being charged. The simple charger does not alter its output based on time or the charge on the battery. This simplicity means that a simple charger is inexpensive, but there is a trade7off in quality. Typically, a simple charger takes longer to charge a battery to prevent severe over7charging. ven so, a battery left in a simple charger for too long will be weakened or destroyed due to over7 charging. These chargers can supply either a constant voltage or a constant current to the battery.
&.&.1*
Tric5%e C+ar"ers
A trickle charger is a kind of simple charger that charges the battery slowly, at the self7discharge rate. A trickle charger is the slowest kind of battery charger. A battery can be left in a trickle charger indefinitely. Feaving a battery in a trickle charger keeps the battery 9topped up9 but never over7charges. &.&.1c
Timer#*ased C+ar"ers
The output of a timer charger is terminated after a pre7determined time. Timer chargers were the most common type for high7capacity Ni71d cells in the late GG*s for example !low7capacity consumer Ni71d cells were typically charged with a simple charger$. 19
8ften a timer charger and set of batteries could be bought as a bundle and the charger time was set to suit those batteries. 'f batteries of lower capacity were charged then they would be overcharged, and if batteries of higher capacity were charged they would be only partly charged. 6ith the trend for battery technology to increase capacity year on year, an old timer charger would only partly charge the newer batteries. Timer based chargers also had the drawback that charging batteries that were not fully discharged, even if those batteries were of the correct capacity for the particular timed charger, would result in over7charging.
&.&.1d
I!te%%i"e!t C+ar"ers
8utput current depends upon the battery5s state. An intelligent charger may monitor the battery5s voltage, temperature andEor time under charge to determine the optimum charge current at that instant. 1harging is terminated when a combination of the voltage, temperature andEor time indicates that the battery is fully charged. (or Ni71d and Ni< batteries, the voltage across the battery increases slowly during the charging process, until the battery is fully charged. After that, the voltage "ecreases, which indicates to an intelligent charger that the battery is fully charged. 0uch chargers are often labelled as a P+, or 9delta7+,9 charger, indicating that they monitor the voltage change. The problem is, the magnitude of 9delta7+9 can become very small or even non7existent if !very$ high capacity rechargeable batteries are recharged. This can cause even an intelligent battery charger to not sense that the batteries are actually already fully charged, and continue charging. 8vercharging of the batteries will result in some cases. owever, many so called intelligent chargers employ a combination of cut off systems, which should prevent overcharging in the vast ma-ority of cases. A typical intelligent charger fast7charges a battery up to about ?C > of its maximum capacity in less than an hour, then switches to trickle charging, which takes several hours to top off the battery to its full capacity.
20
&.&.1e
6ast C+ar"ers
(ast chargers make use of control circuitry in the batteries being charged to rapidly charge the batteries without damaging the cells5 elements.
Pu%se C+ar"ers
0ome chargers use pulse technolo#$ in which a pulse is fed to the battery. This =1 pulse has a strictly controlled rise time, pulse width, pulse repetition rate !frequency$ and amplitude. This technology is said to work with any si/e, voltage, capacity or chemistry of batteries, including automotive and valve7regulated batteries. 6ith pulse charging, high instantaneous voltages can be applied without overheating the battery. 'n a Fead7acid battery, this breaks7down stubborn lead7sulphate crystals, thus greatly extending the battery service life. 0ome chargers use pulses to check the current battery state when the charger is first connected, then use constant current charging during fast charging, then use pulse charging as a kind of trickle charging to maintain the charge. 0ome chargers use 9negative pulse charging9, also called 9reflex charging9 or 9burp charging9. 0uch chargers use both positive and brief negative current pulses. 0uch chargers don5t work any better than pulse chargers that only use positive pulses. &.&.1"
I!ductie C+ar"ers
'nductive battery chargers use electromagnetic induction to charge batteries. A charging station sends electromagnetic energy through inductive coupling to an electrical device, which stores the energy in the batteries. This is achieved without the need for metal contacts between the charger and the battery. 't is commonly used in electric toothbrushes and other devices used in bathrooms. Because there are no open electrical contacts, there is no risk of electrocution.
21
&.&.1+
US3#*ased
0ince the Dniversal 0erial Bus specification provides for a five7volt power supply, it5s possible to use a D0B cable as a power source for recharging batteries. 3roducts based on this approach include chargers for cellular phones and portable digital audio players.
2.2.1i
So%ar chargers
0olar chargers employs solar energy in charging devices and are generally portable.
&.)
CIRCUIT CO'PONENTS
The following components were employed in the design of the solar mobile phone charger and since they come in various si/es and ratings, it is worthwhile to briefly discuss them. ).2.*
RESISTORS7 'n electrical and electronic circuits, there is a need for either
varying amounts of current of voltage to be applied at specific portions of the circuit and to various components. ;esistors, as their name implies, are used in such instances to ensure resistance to the flow of current to various portions of the circuit. Qenerally, these are materials with specific values of resistance in the range between that of a conductor and an insulator. Their values of resistance are expressed in ohms and their heat withstanding rating is expressed in watts. ;esistors are classified as either being fixed or variable. (ixed resistors have a constant value while variable resistors also known as potentiometers or pots have values that can be varied depending on requirements. Qenerally, resistors used in circuits are linear while non7linear resistors are used for special applications. The resistance of any material is given by: ; J RFEA 6here ; J ;esistance in ohms
!S$, R J ;esistivity of the material !SEcm$
F J Fength of material !cm$,
A J 1ross sectional area of material !cm)$
;esistors are connected either in series or parallel. (or resistors connected in series, the current that flows in each resistor is the same, and total resistance of the series connection is
22
equal to the sum of the individual resistors. 'f for example, n resistors of equal resistance are connected in series, the total equivalent resistance will be given as: ; T J n;. 'f n J 2 i.e. 2 resistors of same resistance connected in series, ; T J ; L ; ) L ; 2 (or resistors connected in parallel, voltage across each resistor is the same, but current across each differs. Therefore the total resistance ; T of say 2 resistors in parallel is given by: E; T J E; L E; ) L E; 2 (ixed ;esistors are generally manufactured in four basic types which are carbon composition, metal film, carbon film and wire wound. The carbon composition type is most commonly used in electronic circuits. Qenerally, resistors are colour coded and this means that to ascertain the value of a resistor, the colour codes must be understood. There are typically four colours on a resistor. The first two colours denote the first and second digits of the resistance value@ the third colour indicates the multiplier while the fourth colour indicates the tolerance value for the resistor. The table below gives the standard for colour coding resistors.
23
COLOUR
DIGIT
'ULTIPLIER
TOLERANCE
Black
*
7
Brown
*
>
;ed
)
*)
7
8range
2
*2
7
Uellow
4
*4
7
Qreen
C
*C
7
Blue
*
7
+iolet
O
*O
7
Qray
?
7
7
6hite
G
7
7
Qold
7
*.
C>
0ilver
7
7
*>
No 1olour
7
7
)*>
Table ). 1olour 1oding of ;esistors ).2.
CAPACITORS7 These are passive circuit components which are made of two
metal plates called electrodes separated by an insulator material called a dielectric. 1apacitors can be used for various purposes which include: (iltering, Tuning, Bypassing resistors, Qeneration of sinusoidal waveforms, nergy 0torage etc. The capacitance of a
24
parallel plate capacitor is directly proportional to the relative dielectric constant of the insulator and to the area of the plates. (urthermore, the capacitance is greater if the separation between plates is small and vice versa. These are expressed by the following equation: 1 J V ԐoAEd 6here 1 J 1apacitance in farads, ( Ԑo J 3ermittivity
V J ;elative dielectric constant
of free space !constant$ which is ?.?C W *7) (Em
A J Area of plates, m)
d J distance between parallel plates, m
't is seen from the equation that capacitance may be increased by increasing the area of the plates or the dielectric constant and by decreasing the separation between plates. Although, capacitors are available in various types, shapes and si/es, they can be generally grouped into four broad categories which are fixed, variable, chip and voltage variable. Types of capacitors available are silver mica, electrolytic, ceramic and trimmer capacitors. lectrolytic capacitors are generally employed in circuits where a large value of capacitance in a small volume is required and these capacitors can be either polarised or non polarised while silver mica capacitors generally have small capacitance and greater mechanical stability capacitance remaining constant at different temperatures with different voltages and does not easily wear with age of capacitor. They generally have uniform characteristics and will not break down at high voltages or high resistances. Trimmer capacitors are a type of variable capacitors operated by a screw driver instead of a knob. Their capacitance can be altered by pressing the plates tightly together which in turn alters the distance between the plates. ).2.)
INDUCTORS7 These are circuit components in which a magnetic field is created
when current passes through an integral wire core, which may be an air core armature and made of soft iron or some other ferromagnetic materials. 'f an applied e.m.f !electromotive force$ applied to the coil changes, a back e.m.f is induced which opposes the change !Fen/#s law$. The coil acts strongly against rapid changes in e.m.f and the strength of this effect is called the %i!ducta!ce & of the coil, measured in enry. 25
The voltage, + applied across an ideal inductor equals the value of the inductance F !in enry$ multiplied by the rate of change of current with respect to time !ampereEtime$. (or an indicator, the greater the voltage applied across the coil, the faster the current increases, ence: +JF
di
Edt
F J inductance of coil
where + J voltage across the coil di
Edt J rate of change of current in ampereEsec
't should be noted that inductance increases with permeability of core, increase in number of turns and with increase in area of core while increase in length for the same number of turns decreases inductance since magnetic field will be less concentrated. Also it is worthy to note that inductors are made of wires and the coil has dc resistance which is equal to the resistance of the wire used in the winding of the core. The amount of resistance however, is less with heavier wires and fewer turns. ).2.2
DIODES7 These are probably the simplest semiconductor devices.
are made from a host crystal of silicon !0i$ with appropriate impurity elements introduced to modify, in a controlled manner, the electrical characteristics of the device. They are formed when a p7type semiconductor is -oined to an n7type semiconductor and doped. The resulting component is a diode which allows flow of current in only one direction. To achieve this, the diode must be forward biased with a voltage higher than the threshold voltage which is *.+ for silicon and *.)C+ for germanium. The threshold voltage decreases at the rate of )m+ per degree rise in temperature. Another type of diode is a schottky !unipolar$ diode, manufactured by placing a metal layer directly across the semiconductor. =iodes are mainly used in rectification, which is the process of converting ac into dc. 3ower diodes are a type of diodes which are able to carry current of several amperes. Types of diode include: power diodes, /ener diodes, 0chottky diodes, Fight emitting diodes, photo diodes etc. ).2.4
TRANSISTORS7 They are three terminal devices !emitter, base and collector$
used as amplifiers and as switches. Qenerally, there are two types which are N3N and 3N3 transistors. A transistor is an active circuit component in that it is capable of modifying or amplifying the input signal. They are delicate and heat sensitive. Typically, a transistor is like two diodes with either their p7type or n7type materials -oined together at the ends. Transistors 26
come in different types and specifications, of which some are BXTs !Bipolar Xunction Transistors$, Dnipolar transistors, (Ts !(ield ffect Transistors$, X(Ts !Xunction (Ts$, <80(T !
6USES7 The fuse is a simple and reliable safety device. 't is second to none in its
ease of application and its ability to protect people and equipment. The fuse is a current7 sensitive device. 't has a conductor with a reduced cross section !element$ normally surrounded by an arc7quenching and heat7conducting material !filler$. The entire unit is enclosed in a body fitted with end contacts.
CHAPTER THREE 2.* =0'QN AN= 1';1D'T ANAFU0'0
27
The design and construction requirements for this solar mobile phone charger are given below. The circuit diagram is divided into two ma-or parts. Dsing the battery as our point of division, every component towards the left hand side !the solar cell, capacitors 1, 1), 12, resistors ;7;, =iode =, 'nductor F and transistors H and H)$ are for trickle charging the battery pack while the remaining components to the right of the battery including the battery itself actually supply the voltage and current requirements that charge the mobile phone. The addition of a battery to the circuit ensures that with or without sunlight, the circuit can charge the mobile phone round the clock. The reason for the addition of a battery pack is to ensure as might practically occur, that days of very little or no sunshine do not incapacitate the charger. 8n days when there is sufficient sunlight as required by the solar cell, the battery pack is kept charging and whether or not there is sufficient sunlight on other days, as long as the battery is present and has been charged to a level, any connected mobile phone can be charged, without totally depending on the intensity of sunlight available at that instance. 'n the actual construction of the circuit, the following optional components were added for the sake of flexibility: ) switches 06 and 06), added to each upper diagonal extreme of the circuit and a )A fuse between switch 06) and the output -ack X. This means that there is a switch 06 immediately after the photocell 31 !the positive terminal of the cell$ to stop the cell from supplying current indefinitely to the battery at all times, thereby avoiding overcharging of battery. Another switch 06) is placed between resistor ; and the output -ack X to avoid indefinite supply of current as long as a phone is connected to the output terminal. A )A fuse is further added between switch 06) and the output -ack X to disconnect the load from the circuit in case of excess current exceeding )A. Additionally, a light emitting diode may be installed close to the positive terminal of the output to light up when 06) is in the 8N position.
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Fig 3.1 – Schematic of a solar mobile phone charger !ith batter"#
The circuit shown above uses a small 2 volt solar cell to charge a volt Ni1adENi< battery pack which, in turn, may be used to charge many models of cell phones and other portable devices. The circuit 9scavenges9 energy from the solar cell by keeping it loaded near .C volts !maximum energy transfer value$ and trickle charges the internal battery pack with current pulses. The simple circuit isn5t the most efficient possible but it manages a respectable O*> at ** mA from the cell and 2*> when the cell is providing only )C mA which is actually pretty good without going to a lot more trouble or using more exotic components.
2.
<8= 8( 83;AT'8N:
6hen the voltage on the emitter of H rises a little over .C volts, both transistors turn on quickly, snapping on due to the positive feedback through ;C and 1). The current increases in F through H) until the voltage across the cell drops somewhat below .C volts. The circuit then switches off quickly and the voltage on the collector of H) -umps up, turning on =, allowing the inductor current to flow into the battery. 8nce the inductor has discharged into the battery, the process starts over. The circuit can charge higher voltage batteries without any circuit changes since the voltage will -ump up quite high on the collector when the transistors turn off. The circuit should !t be operated without a battery attached. (or a little more efficiency,
29
increase ;C in proportion to the voltage increase on the battery. !(or example, double ;C for charging a ) volt battery.$ A Ni1ad instead of a Ni< battery is preferable because they are particularly forgiving of overcharging, simply converting the excess current into heat.
LIST O6 RE8UIRED CIRCUIT CO'PONENTS $ef. %escription &'1 3 (olt solar cell from a si)e!al* solar light '1
22 +F, 10 (olt (al-es not critical#
'2
100 pF, an" (oltage or t"pe, t"picall" ceramic
'3
10 +F, 16 (olt or more for higher (oltage batter"
$1
1.5 *, an" t"pe
$2
3.9*, an" t"pe
$3
10*, an" t"pe
$4
180 ohm, an" t"pe
$5
4.7*, an" t"pe
$6
10 ohm &' see te/t#.
1
50 to 300 + see te/t#
%1
15818 schott*" rectier, -st abo-t an" !ill )o.
1
24403, or similar
2
24401, or similar
1 1
o-tp-t ac* 6 (olt i'a)i batter" !f-se
(ig 2.) Fist of required circuit components
The photocell can be salvaged from an inexpensive solar sidewalk illuminator, gotten from other solar products or bought in the market and it should have an open7circuit voltage of about 2 volts and supplies about ** mA in bright sunlight. The circuit can handle more current but cells that supply more than )C* mA should be avoided. The inductor has a low resistance winding but a 30
surprising number of cores will work fairly well. The value of inductance isn5t critical, perhaps between 4* and 2** Y and during proper operation there will be a pulse waveform on the collector of H) with several *s of microseconds period. This prototype operates at about 4* Y0 as shown and the inductance measures about C* Y. (or experimenting with cores or other circuit values, the Ni1adENi< battery can be replaced with a /ener diode of the same voltage and replace the solar cell with a 2 volt power supply with a series resistor, about )) ohms to simulate moderate sun.
their life, if ' understand correctly. The tradeoff is between fast charging the cell phone battery and possibly shortening its overall life span or charging it more slowly !which increases the time it takes to fully charge the cell phone especially when the phone battery is highly discharged$ and increasing the lifespan.
CHAPTER 6OUR 32
4.* 18N0T;D1T'8N, T0T'NQ AN= ='01D00'8N Before the actual soldering of circuit board components was done, some other tasks were carried out to ensure success and proper operation of the components. All components !resistors, capacitors, transistors, fuse and the diode$ were tested, and measured values compared with ratings to ensure they were as closely matched as possible. =uring this exercise, it was discovered that some of the components were either not working at all or were not even the required specifications. This timely discovery helped in conserving the time that might have been wasted if they had been directly soldered onto the board without carrying out these initial test and confirmation. Qenerally, it was a very tedious task getting the solar cell and the )N44*2 transistor in the market as they proved to be very scarce products that are not yet readily available. The construction started by first deciding how big the final design would be and this made it necessary to choose what part of the +ero board would be used, how close to one another the components would be and where the output -ack would be located. 't was thought that having a general idea on the final outlook of the circuit and the component overlay would make it easier to decide where and how a component should be installed and soldered to avoid removing already soldered circuit components, which in turn would lead to an untidy construction. After having a mental idea of the final circuit, capacitor 1 was installed, followed by resistors ; and ;), then transistor H. The component installation proceeded gradually, while constantly making reference to the circuit diagram, watching out for short circuit, open and wrong connections, till the output -ack was installed. The output -ack used was the output -ack of Nokia phones. 't was thought that since this brand of phones was a very popular brand, it would be wise to use a -ack that would offer the possibility of reaching a wider audience. Although, the output pins where the -ack was connected were designed in such a way that they could accommodate other types of phone models, by simply pulling out the -ack in use, a new -ack for a different phone model can be connected, plugged in, and charged !polarities should be carefully noted$. The output -ack design was made to be easily detachable by making it possible to pull out the -ack connector, replacing it with a desired phone model -ack and plugging it back in place. 't was later thought wise to install two switches 06 and 06). 06 was installed between the positive terminal of
33
the solar cell and the positive terminal of capacitor 1 to avoid the solar cell from charging the battery pack indefinitely and 06) was installed between resistor ; and the positive terminal of the output -ack, to make it possible to disconnect the phone from charging even when physically plugged in. A light emitting diode was further added to indicate when 06) is in the open or close state. 'nstalling capacitor 1) and ;C which formed out positive feedback path was not so easy, but after so much thoughtful efforts, success was achieved. The type of battery used was a + Ni< !Nickel
/.1
CONSTRUCTION O6 THE CASING
The casing of the solar mobile phone charger was salvaged from the transparent plastic7 like casing of slightly flexible material. By cutting it into the required si/es to serve as an enclosure for the circuit board, the different si/es were glued together and taped at the sides to form a firm transparent casing. The inductor F used for the circuit was sealed and cylindrical in shape. This initially served as a setback in the design of the casing but the top part of the casing was later cut open to allow only the upper part of the inductor shoot out a little above the case. To allow easy access to switches 06 and 06), the sides of the casing, restricting access to these switches were cut open. The whole assembly was made compact and rigid, bearing in mind that making the assembly too heavy would be a turn off when considering its mobility.
/.&
CIRCUIT ASSE'3L4 TESTING
34
After installing every component, it was time to test the assembly. A Nokia phone was connected to the output -ack and nothing happened. The switches were confirmed to be closed and the output voltage of the solar cell was measured and found to be within required range. 't was later discovered that the Ni< battery had naturally discharged over time and the measured voltage was -ust .)+ which was far too low to charge a mobile phone. The assembly was later left in the sun for a very long period of time to ensure that the battery stored enough charge to charge the mobile phone. A + speaker was tested and it was found to produce signs of current flow, but when the Nokia phone was plugged, it simply displayed %battery not charging&. 't was later discovered that some models of Nokia phones generally in practice do not like a very low impedance battery as a charging source and the * ohm 3T1 was added in series with the output not only to limit the available power but also to allow the unit to charge those special models of Nokia phone that don#t like a very low impedance charging source. !The phone simply displays 9battery not charging After the tests, it was reali/ed that what determined if the unit would charge a phone or not was not the amount of sunlight or power output of the solar cell, but rather, the output power of the attached Ni< battery. 0o if the battery becomes too low, the phone will not charge.
CHAPTER 6I9E 35
C.* 0D<
36
'n conclusion, it has been a worthwhile experience and the effort and time invested into this design and construction has really paid off. 't has revealed that solar energy is in abundance and can be harnessed for use in a lot of ways, even to the point of charging mobile devices which removes total dependence on frequently unavailable and highly erratic electric power supply from utility grids. 't has also showed that mobile charging is possible and by mobile charging, what is meant is "charging while on the move#.
RECO''ENDATIONS ' want to recommend that solar mobile phone chargers should be designed and constructed indigenously by both students !for educational purposes$ and corporate bodies !for commercial purposes$ Also, ' want to recommend that more research effort should focus on harnessing the abundant energy from the sun into various useful means. ' want to recommend that practical hands on electronic circuit design and construction be introduced early enough in schools to stir up early interest.
;(;N10
37
. <. A. Qreen, 91onsolidation of Thin7film 3hotovoltaic Technology: The 1oming =ecade of 8pportunity,9 3rogress in 3hotovoltaics: ;esearch and Applications, vol. 4, pp. 2?27 2G), August )**. ). 3. A. Basore, 910Q7:
9ow 0olar 1ells 6ork.9
* April )***.
ow0tuff6orks.com.
[http:EEscience.howstuffworks.comEsolar7cell.htm\ 4 Xanuary )**G. . Battery 1harger *2 Xanuary, )**G. 6ikipedia.com. http:EEwww.wikipedia.comEBattery 1harger.html O. 9ANG2: 0witch7
).Bergstrom, 0ven. 9Nickel71admium Batteries _ 3ocket Type9. Xournal of the lectrochemical 0ociety, 0eptember GC). GC) The lectrochemical 0ociety. 2.www.-ohn6hock.com ).* ) + solar charger.doc7
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