Solar Cell Mobile Charger

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Table of Contents

Table of Contents .............................................................................Error! Bookmark not defined.

Introduction ................................................................................................................................... 2

Prototype Design ........................................................................................................................... 3

Hyper Magnifying Technology .................................................................................................... 7

Anatomy of the Solar Cell ............................................................................................................ 8

Solar Power Conversion ............................................................................................................... 9

Power Loss Information ............................................................................................................. 10

Advantages of Solar Power ........................................................................................................ 11

Quantitative Analysis.................................................................................................................. 13

Conclusion ........................................................................................Error! Bookmark not defined.


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Introduction

Cell phones are currently the most popular form of communication in almost all the

countries throughout the world. There are well over 5 billion mobile phones currently in uses

and the number is growing as technology gets better and the cost of production lowers.

However, the main problem is the average lifetime of a phone battery is less than 10

hours with moderate usage. This becomes very inconvenient for people on the road or occupied

with work. In order to recharge the phone, people must bring wall phone chargers. The newest

technology ofsolar phone chargers is a separate device that uses a small solar panel to absorb

light and then transfer to the phone. This process still forces customers to carry around another

device along with their cell phones.

Our project goal is to develop a miniature solar panel to be installed onto the cell

phone itself. This way, the phone can charge independently; independent of power outlets and

independent of wires. There wont be any need for electrical outlets or portable solar panels. The

mobile phone will be able to charge anywhere outside or where it is exposed to sun light. A

miniature solar cell will be built into the phone and able to absorb enough sunlight to charge the

device while in use.

Being liberated from wires and power outlets is just one of the many advantages of

having solar panels on cell phones. As the worlds resources are diminishing, governments are

encouraging for a green movementto help conserve the limited supply. Solar energy is

gaining popularity because of the free and abundant energy. This fact alone will save customers

money on their electric bills. The energy is also clean and produces no hazardous waste like

some of the other power generation resources.


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Prototype Design

For our design procedure, we used an IPhone 4 as the model mobile phone. There will be

two different solar panels used on the phone, one placed on each side. The back of the

mobile phone will be completely covered by a solar panel. Anytime users are outside and want

to charge their phone, they can just set it down with the back solar cell facing towards the sun

and the phone will charge. The front of the cell phone will also feature a solar cell. The plan is to

place an ultrathin film of solar cell within the layers of the cell phone screen.

Front Screen

Source: http://electronics.howstuffworks.com/iphone2.htm


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The layers of the touch screen on the front of the IPhone 4 are depicted in the image

above. Our plan is to place an ultrathin semi-transparent solar cell on beneath the glass substrate

layer. This will allow for solar energy to be absorbed while the phone is facing up towards the

sky. Another benefit to this layer placement is that the LED from the phone will also produce

energy for the solar cell so that the film will have power coming from both ways, but mostly

from the sun. As a replacement for the glass substrate layer, we have decided to add new,

innovative concentrated photovoltaic technology. This glass replacement is a new technology

focused on diverging the different energies and sending the specific photons absorbed by the

solar cell material. This process is done by mirrors and lenses within the magnifying film. The

solar cell layer on the front is only an option as a semi-transparent film may cause some

inconvenience in seeing the phone display.

The film used on the front will be new technology that was developed from different sources:

Ultrathin, transparent and flexible solar cells

A research group at the University of Illinois (Prof. John Rogers) has confirmed that

there are new etch methods for Silicon and Gallium Arsenide which allow slicing off ultrathin

wafer layers (2-20 microns) that can be formed into micro solar cells.

The thin geometry will not only reduce the cost but also make passive thermal dissipation

much more effective compared to the conventional bulk cells. This aspect is particularly

important for concentrator systems and simplifies the design of the focusing optics. Even the

optical transparency can be defined at the assembly stage as the spacing between individual

microcells can be controlled.


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Source: http://www.ecofriend.com/entry/eco-tech-semitransparent-solar-cells-to-begin-a-new-era-of-solar-

electronics/

Semprius co. has built working prototypes which are NOT flexible but are based on

similar technology.

The solar company known as Konarka also makes organic solar cells made from flexible

plastic and has developed a transparent solar cell that it hopes will be built onto electricity-

generating windows. It is energy-efficient and transparent with superior vertical performance

and a subtle red, blue or green aesthetic. With these features, BIPV (building-integrated

photovoltaics) will no longer need to be confined to spandrel or overhead applications, Arch

CEO Leon Silverstein said in a statement.

But these organic photovoltaics are NOT very efficient (only ~6%) at converting sunlight

to electricity and wont last as long as a rooftop solar panel, which is typically under warranty for

25 years. Although having this in addition to the solar panel on the back of the phone will

certainly help the charging process.


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Back Panel

The dimensions of the IPhone 4 are 110 mm high by 59 mm wide and 9.4 mm deep. For

this case, we would have a solar cell with thin, high-efficiency, non-transparent characteristics to

allow maximum solar energy absorption. The dimensions for this situation would be set at 100

mm by 50 mm to take over most of the phones back panel surface.

This side would be the most efficient energy absorbing cell of the two films used on this

device. In order to increase efficiency, an ultrathin magnifying layer would also be used

instead of glass substrate to both protect the solar cells and magnify the photonic absorption.

There would be an array of modules working cooperatively to absorb energy and feed the cell

phone.

Source: http://www.skldintl.com/solar-cell-phone.htm


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Hyper Magnifying Technology

Modern day solar panels average around 12% to 18% efficiency when absorbing solar

energy. This illustrates a great loss of power and exposes the poor technology that is currently

around. The power losses are explained by the wide variety of photonic energy transmitted

through solar rays. Only a fraction of the photon energy can be absorbed effectively by the

impure silicon material.

New developments in concentrated photovoltaic technology have potentially quintupled

the efficiency of the solar panels. This innovative technology employs optical equipment such as

mirrors and lenses to magnify solar energy. This thin magnifying film that is being created has

the ability to separate the different spectrums and route the necessary energy to exactly where

they are needed on the solar cells. This would the solar cells highly efficient.

HyperSolar is one of the leading companies in Concentrated Photovoltaic Technology

Source: http://www.hypersolar.com/technology.html


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Anatomy of the Solar Cell

Photovoltaic Cells

The driving force behind solar panels begins with the photovoltaic cells. These cells are

responsible for converting photons from the solar light directly into electrons. The name itself

originates from Greek words and can be broken down to photo which means light and voltaic

which translates to electricity. Photovoltaic cells are fabricated from special material known as

semiconductors which fall right in between conductors and insulators when it comes to the

magnitude of electron flow. Normally, the most commonly used semiconductor is Silicon.

Silicon, Semiconductor Material

Silicon is the most common semiconductor used in solar panels because of its ability to

remain a semiconductor at very high temperatures under the sun. However, the silicon material

used in solar cells must be doped and made impure because pure silicon crystalline serves as a

very poor conductor of electrons. Once the silicon material is doped, a lot less energy is needed

to knock the electrons out of their connections into a free flowing current.

Solar Panels

Modules or groups of photovoltaic cells electrically connected together are placed into

frames where energy absorption can be concentrated. These casings are placed next to each other

over a relatively large surface area to be as efficient as possible when absorbing the light. An

anti-reflective coating is added to the solar panels to reduce power losses and obtain maximum

absorption ability. Above that layer, a glass cover plate is used to create durability and protect

against erosion.


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Source: http://www.hightechscience.org/solar_express.htm

Conversion of Photons to Electrons

When solar rays collide with the photovoltaic cells, the photonic energy is absorbed and

transferred within the semiconductor material. Once the photons are absorbed, the heat causes

electrons to leave their respective shells. The photonic energy collides with the electrons freeing

them to flow without restrictions. This free flowing energy can be moved into any direction by

the natural electric fields produced by the photovoltaic cells. The electric fields force the

electrons into a current flowing in a single direction. Another form of a support to directing the

current is artificially made. Metallic contacts are placed above and below the solar cells, serving

as a guide to the free flowing energy, directing it in one direction. Once the flowing current is

combined with the voltage from the cell, the power wattage is produced.


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Source: http://micro.magnet.fsu.edu/primer/java/solarcell/

Power Losses

According to the U.S. Department of Energy, the average solar cells efficiency is only

about 12% to 18%. The power losses are substantial and can be explained by analyzing the

electromagnetic spectrum. When solar rays are shined down, light is the only visible part of the

spectrum. However, there is a broad range of wavelengths that all come within the solar rays

which can be a problem when absorbing energy efficiently. In order for the electrons in the

doped silicon to be knocked free, a specific amount of energy known as the band gap energy is

needed. Photons with too much energy or not enough energy will not disconnect the electron

groups. The most optimal choice for band gap energy is calculated to be about 1.4 eV.

Another situation where power is lost is when the electrons are knocked freely and

transferred across the solar cell in currents created by the electric fields and conducting

materials. During travel, several electrons are blocked off by the grid or fall out of the current.


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Advantages of Solar Power

Currently, solar power has the smallest market in the United States coming in at 0.01% of

all power generation, yet there is an unlimited supply. However, the industry is gaining

popularity each year. It is estimated that in 2025, of the entire worlds power generation, 10

percent will be solar power.

There are many advantages to solar power generation. Firstly, the cost for obtaining solar

power is essentially free. As long as the human race will be around, the sun will always be

shining so this is the most abundant energy source. Solar energy is also clean and best for

conserving the environment among forms of energy generation. According to a solar energy web

page, In one hour more sunlight falls on the earth than what is used by the entire population in

one year. This energy needs to be harvested.

Incorporating solar panels into the structure of residential homes or commercial buildings

can save lots of money. The building can generate its own power to run sufficiently and any

excess energy can be sold back to power companies for profit. Solar panels have relatively long

life spans of 30-40 years and rarely need to be replaced for being faulty. They also produce as

much energy over their lifetime as nuclear fuel rods without hurting the environment. Solar

panels work with no moving parts which results in silence as well as a miniscule requirement for

maintenance. Through innovative technology in concentrated photovoltaic, the efficiency of

solar panels is due to double or triple.

Solar panels can also be used anywhere there is sun light. The desert which is most

uninhabitable can be used to for collecting solar energy. This would allow many of the

undeveloped countries to produce their own energy at very cheap rate. A solar energy system has


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the ability to constant generate energy nonstop with the backup help of a battery. The energy

from the sun can be converted into AC power to charge devices, can be stored in batteries,

insulated, or reflected.

The potential for the solar energy industry is huge. As more and more people begin to

realize the great benefits of solar power generation, they will start to shift towards using it. The

introduction of the Smart grid power system also encourages customers to produce their own

energy because power companies are willing to buy excess energy produced from residential

homes.


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Quantitative Analysis

Battery Charge

This section discusses quantitatively the charging of phone batteries using our unique

design. Lithium-ion batteries are the most widely used due to several advantages such as being

light, portable and able to operate for long periods of time. An Apple iphone4 (our prototype)

uses a Lithium-ion battery with 3.7V and 5.25 Whr specifications. The solar cells would be

custom designedto conveniently satisfy these requirements and be able to charge from 0 to

100% in usual recharge period (about 1 hr).

Special attention would be paid to sustain battery life. In the process of charging a

Lithium-ion battery, electricity moves through the cell (i.e. voltage is applied) and the lithium

ions migrate from the negative cathode to the positive anode, where they wait for the circuit to be

closed and return back to the graphite cathode. Unlike Ni-Cd batteries, Li-ion batteries should be

charged early and often. These should not be fully discharged and then recharged as these

cycles reduce life (1 cycle per month recommended by apple.com/batteries). Our design will take

advantage of this characteristic and start charging as soon as there is enough energy

produced. A simple transistor circuit would ensure that there is enough voltage (3.7V)

produced before the recharging circuit closes between the battery and the two solar cells, while a

regulator circuit (another transistor with diodes, capacitors and resistors as shown in the

picture) would help in supplying a constant voltage.


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A regulator circuit (soldered). Source:http://jhau.maliwi.de/mot/voltreg.html

Finally, we would also give the user a choice to enable or disable this innovative

feature. Although there is no harm in constantly charging the Li-ion battery as mentioned

earlier, some users may not like the feature turned on at all times. In fact, recharging of a Li-ion

battery is an endothermicprocess [2], which means the battery absorbs heat when it recharges

and thus there is no additional heat produced in the process.

Solar Panel Output

The overall energy conversion efficiency of a solar cell is the percentage of

electric power converted from the incident light. This is calculated by using the following

formula:

In this formula, Pm represents the maximum power point while E is the input light irradiance and

Ac is the surface area of the cell (or module or array, as applicable).

The solar cells output is dependent on the temperature of the solar cell. A high

temperature of the solar cell results a lower output as well as lower efficiency. However, it is


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harmful for the battery to overheat. A lithium ion battery that is responsible for power an IPhone

4 should remain at 92 degrees Fahrenheit or lower. We are adding a feature that would alert the

user to put the phone in their pocket and away from the sun if the temperature of the battery

reaches over 95 degrees Fahrenheit due to the suns heat. We are going to install a mini

thermometer and sensor within the phone to detect overheating. This should be one of the safety

features to protect the phone.


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FlowChart

Solar Cells

Charge

Controller

Batteries DC Loads

Inverter

AC Loads


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Conclusion The development of the wireless solar cell mobile phone charger project has borne an

innovative way of charging phones. There are plethora advantages and benefits to using a

wireless process as well as using solar energy to charge the battery. However, there are always

ways for improvement in the future. Firstly, researchers all over are working towards increasing

and maximizing the efficiency of solar cells. The first step was mentioned above in the form of

magnifying concentrated photovoltaic in solar cells. This will help bring solar cell efficiency to

the goal of 100% one day. Another improvement that can be worked on is decreasing the

thickness of solar cells without losing efficiency. This will also benefit the phone in having a

slimmer, sleeker shape. An improvement to solar cell film across the front of the phone is to

fabricate a fully transparent solar cell as to not hinder the touch screen or the display. There is

always room for improvement, but for now, our prototype is new and revolutionary.


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1. J. Yoon, A.J. Baca, S.-I. Park, P. Elvikis, J.B. Geddes, L. Li, R.H. Kim, J. Xiao, S. Wang, T.H.Kim, M.J. Motala, B.Y. Ahn, E.B. Duoss, J.A. Lewis, R.G. Nuzzo, P.M. Ferreira, Y. Huang, A.

Rockett and J.A. Rogers, "Ultrathin Silicon Solar Microcells for Semitransparent, Mechanically

Flexible and Microconcentrator Module Designs,"Nature Materials 7, 907-915 (2008).

2. David Gunderson, Li-ion battery temperature trends during charge and dischargehttp://www.micro-power.com/userfiles/file/mp_tempcharge-1250026530.pdf

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Solar Cell Mobile Charger