Wireless Solar Mobile Phone Charger Honda idream Contest Gilburt L Chiang Sanyam Bajaj Advisor: Steve Bibyk
Page 1 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.
Page 2 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 of solar 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 won t 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 world s resources are diminishing, governments are encouraging for a green movement to 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.
Page 3 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
Page 4 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.
Page 5 Source: http://www.ecofriend.com/entry/eco-tech-semitransparent-solar-cells-to-begin-a-new-era-of-solarelectronics/ 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 electricitygenerating 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 won t 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.
Page 6 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 phone s 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
Page 7 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
Page 8 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.
Page 9 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.
Page 10 Source: http://micro.magnet.fsu.edu/primer/java/solarcell/ Power Losses According to the U.S. Department of Energy, the average solar cell s 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.
Page 11 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 world s 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
Page 12 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.
Page 13 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 designed to 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.
Page 14 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 endothermic process [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, P m represents the maximum power point while E is the input light irradiance and A c is the surface area of the cell (or module or array, as applicable). The solar cell s 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
Page 15 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 sun s 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.
Page 16 Flow Chart Solar Cells Charge Controller Batteries DC Loads Inverter AC Loads
Page 17 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.
Page 18 BIBLIOGRPHY 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 discharge http://www.micro-power.com/userfiles/file/mp_tempcharge-1250026530.pdf