Renewable Energy in Mobile Phones

Transcription

1 Renewable Energy in Mobile Phones Report for 3NAB0 Applied Physical Sciences s Renee Noortman

2 Introduction In a world where renewable energy is becoming increasingly important, we are constantly seeking new ways to recycle our power. If we listen to climate gurus like Al Gore, saving the planet provides us with a daily challenge. The human race is trying to keep its head above the water while the icecaps slowly melt under its feet. Innovators on this suffering planet are coming up with methods to buy the entire civilisation some time before all hope is lost. Among these methods, the favourites seem to be solar power, wind energy and energy generated by humans. This report will focus on one of the applications of a combination of these three different forms of renewable energy: charging your mobile phone while on a bicycle. It will look at the physics behind this application such as the way in which the energy is collected and then passed on to the mobile device. The research question for this report is: Which kind of renewable energy that can be generated while on a bicycle is the most efficient for charging a mobile phone? The sub questions that will be answered in order to get to the final answer are: 2 How much energy is needed to fully charge the average smartphone? How much energy can be generated by solar power on a bicycle and how long would it take to charge a mobile phone with this type of energy? How much energy can be generated by the wind on a bicycle and how long would it take to charge a mobile phone with this type of energy? How much energy can be generated by biking with a dynamo and how long would it take to charge a mobile phone with this type of energy? 3

3 Types of Renewable Energy Three different types of renewable energy will be defined for this report: solar power, wind power and human power. Each of these types of power generation could be integrated in a bicycle to provide electricity to charge a phone or other mobile device while on the road. Solar power Solar energy is energy that is generated by the sun. It offers a clean, climate-friendly and inexhaustible energy resource for mankind. The costs of this type of energy generation have been decreasing significantly as it becomes more and more popular. Making use of solar energy is a good way to contribute to solving the big problems of this world: climate change and energy security. 1 When the earth absorbs energy from the sun, the temperature rises. Solar power is the transformation of sunlight into electricity. A common way of doing this is through Concentrating Solar Power (CSP) systems. These use lenses or mirrors to concentrate a large amount of sunlight in a small beam. The concentrated heat then can be used as a heat source for a normal power plant because a working fluid is heated. 2 Human power On a bicycle, one is in constant motion. This motion induced by pedalling can be used to generate electrical power. Research has shown that up to 60 Watts can be obtained in this manner. 5 This power can be generated by using a dynamo. A turning wheel causes a magnetised rotor to rotate within a coil and produce AC. 6 Wind power Another form of renewable energy is wind power. This is generated by wind turbines that convert energy contained by wind into electricity. These turbines have a high efficiency and therefore soon win back the energy that was used to build them. 3 The blades of a wind turbine catch the wind, resulting in a turning force. The blades then turn a shaft that goes into a gearbox which increases the rotation speed for the generator. The generator uses magnetic fields to convert the gathered rotational energy into electrical energy. The resulting power output goes to a transformer which converts the energy from 700 Volts to about Volts. This is the right voltage for the distribution system. This electrical energy can then be used like any other kind of electricity. 4 Wind power can also be used for generating power on a low scale, using for instance micro-windmills with a diameter of 4.2 cm International Energy Agency Solar Energy Perspectives: Executive Summary. 2 Müller-Steinhagen H., Trieb F Concentrating Solar Power: A review of the technology. Ingenia Slootweg J. G Wind Power: Modelling and Impact on Power System Dynamics. 4 The British Wind Energy Association BWEA Briefing Sheet: Wind Turbine Technology. 5 Starner T., Paradiso J. A Human Generated Power for Mobile Electronics. 6 Moore G. S. M The Modern Bicycle Dynamo. Physics Education 23.

4 Applications and Calculations By using information gathered on the various kinds of renewable energy, I did some calculations to look at the efficiency if these ways of generating power would be integrated in an everyday bicycle. Charging a mobile phone An average smartphone comes with a Lithium-ion battery with a capacity of 2100mAh and a voltage of 3.8V. This means that one of these batteries can hold = 8.0 Wh = kwh. It will take kwh to fully charge an empty battery. Solar power In the case of a bicycle, a small solar panel could be placed on the carrier on the back of the bicycle. The measurements of such a carrier are about 0.40m in length and 0.15m in width which provides an area of = m 2. The type of solar cell that is used most often is the crystalline silicon cell, which has an efficiency of 25.0 ± 0.5 %. 7 The formula for solar cell efficiency is η = P m /(E A c ) in which η is the efficiency, Pm is the cell s power output at its maximum power point in watts, E is the input light in W/m 2 and A c is the surface area of the solar cell in m 2. E is taken to be 1000 W/m 2 under standard circumstances (T = C). So, in this case: η η = 25.0 % E = 1000 W/m 2 A c = m 2 The formula can be rewritten as P m = η E Ac which gives P m = 15 W. If we presume that our user is on his bicycle for ten minutes then the energy generated can be calculated by using E = P t with t = 600 s. In this case the amount of energy generated would be E = J = kwh. A quick calculation shows that it would take (8.0/2.5) 10 = 32 minutes to fully charge the average smartphone. Wind power A micro wind turbine could be place on the front of a bicycle to generate power. In a windless environment, the incoming wind velocity for the wind turbine would be equal to the velocity at which the cyclist travels. Micro-windmills at the size of 2.0 cm in diameter could fulfil this task. If we were to place fourteen of these on the head tube in two rows of seven and the bicycle ride is for ten minutes at a constant speed of 15.5 km/h again, we can calculate the power generated by the wind. The formula that can be used to calculate the amount of power generated is P = ½ Av 3 C p, in which is the air density (1.23 kg/m 3 ), A is the swept area, v is the wind speed and Cp is the power coefficient (for which the values range from ). 8 The power coefficient describes the efficiency of a windmill and thus takes into consideration the friction of the windmill. In this case, we will take Cp to be 0.25 to calculate the minimal amount of power generated. The value of A can be calculated using the radius of the micro-windmill: A = r 2 = (0.010) 2 = m 2. The wind speed will be the speed of the cyclist: v = 15.5 km/h = 4.31 m/s. So, the values that we will use to calculate P are as follows: = 1.23 kg/m 3 ; A = m 2 ; v = 4.31 m/s; C p = 0.25 These values result in P = W for one micro-windmill and P = W for fourteen microwindmills. Using E = P t with t = 600 s, the energy that is produced in a 10-minute bicycle ride is E = 32.5 J = kwh. Taking into consideration the amount of energy that recharging a smartphone requires, it would take (8.0/ ) 10 = minutes on a bicycle to recharge a smartphone. Human power Most bicycles are supplied with a simple dynamo that could be used to generate electricity. The average velocity of a recreational cyclist lies around 15.5 km/h. 9 A normal hub dynamo generates about 14.2 V at this speed and has a current of 0.5A. 10 This means that the dynamo can generate P = V I = = 7.1 W. The energy that is collected in this way would be E = P t, with t at 600 s again. Then E = = J = kwh. In short, it would take (8.0/1.18) 10 = 68 minutes to completely charge a smartphone on a bicycle by using human power Green M. A., Emery K., Hishkawa Y., Warta W., Dunlop E. D Solar cell efficiency tables (version 39). Progress in Photovoltaics: Research and Applications; 20: The Royal Academy of Engineering. Wind Turbine Power Calculations. Retrieved 28 January Bicycle Statistics. City of Copenhagen website. City of Copenhagen. 13 June Retrieved 28 January Valerius C., Krupar J., Schwarz W. Electronics Power Management for Bicycles European Conference on Power Electronics and Applications.

5 Conclusion How much energy is needed to fully charge the average smartphone? For the average smartphone with a Lithium-ion battery, it will take kwh to fully charge this battery when it has completely discharged. How much power can be generated by solar energy on a bicycle and how long would it take to charge a mobile phone with this type of energy? In a 10-minute bicycle ride at a constant speed of 15.5 km/h, kwh of solar energy will be collected using solar cells. This means that, to fully charge a mobile phone, the ride should be at least 32 minutes long. This is a reasonable amount of time to spend on a bicycle ride, therefore this type of renewable energy is a good option for on a bicycle. How much power can be generated by the wind on a bicycle and how long would it take to charge a mobile phone with this type of energy? In a 10-minute bicycle ride at a constant speed of 15.5 km/h, kwh of wind energy will be collected using micro-windmills. This means that, to fully charge a mobile phone, the ride should be at least minutes long. This is too much time to spend on a bicycle ride. It would take about 150 hours to charge a mobile phone fully which is not worth it. The efficiency of this method is too low to provide the user with an advantage. 8 How much power can be generated by biking with a dynamo and how long would it take to charge a mobile phone with this type of energy? In a 10-minute bicycle ride at a constant speed of 15.5 km/h, kwh of energy supplied by the cyclist will be collected using a hub dynamo. This means that, to fully charge a mobile phone, the ride should be at least 68 minutes long. This is quite a lot of time to spend on a bicycle ride but it is still fairly reasonable. As a shorter ride would still partially charge the mobile device, this is a good option to be used on a bicycle. Overall conclusion In this study, I looked at different kinds of renewable energy that could be generated on a bike. I did this in order to answer the following question: Which kind of renewable energy that can be generated while on a bicycle is the most efficient for charging a mobile phone? Results show that solar energy is the most efficient type of energy as it takes the least time to generate the amount of energy needed to recharge a smartphone. Human power claims a good second place with about twice the amount of time needed. Wind power turns out not to be a good candidate as it would take days to charge a phone with this type of power. 9

6 Discussion References Although the results show that solar power is the most efficient option for charging a mobile phone on a bicycle, I doubt whether this is the best choice when looking from a design perspective. Solar panels are expensive and heavy to carry around on a bike. Thieves would probably try to steal the expensive panels from the bike which means the user would either risk getting his panel stolen or he would have to take it off and carry it with him at all times. Of course, this is not efficient in any way and would only create new problems. Because would it not be much easier to carry along a simple mobile charger? I do not think that anybody would go through the pain of lugging around a solar panel on his back just for the sake of the environment except maybe our beloved Al Gore. Not to mention that attaching and detaching the solar panel takes time and effort as well. I think human power would be a better choice because most bicycles are already supplied with a normal dynamo which can be integrated in a simple circuit to charge a phone. This option should not have to cost a lot of money because not many extra components are needed and a dynamo does not have to be too heavy. Last but not least, you do not need to attach and detach a dynamo because thieves would not bother to steal a small insignificant generator. Bicycle Statistics. City of Copenhagen website. City of Copenhagen. 13 June Retrieved 28 January Green M. A., Emery K., Hishkawa Y., Warta W., Dunlop E. D Solar cell efficiency tables (version 39). Progress in Photovoltaics: Research and Applications; 20: International Energy Agency Solar Energy Perspectives: Executive Summary. Moore G. S. M The Modern Bicycle Dynamo. Physics Education 23. Müller-Steinhagen H., Trieb F Concentrating Solar Power: A review of the technology. Ingenia 18. Slootweg J. G Wind Power: Modelling and Impact on Power System Dynamics. Starner T., Paradiso J. A Human Generated Power for Mobile Electronics. The British Wind Energy Association BWEA Briefing Sheet: Wind Turbine Technology. The Royal Academy of Engineering. Wind Turbine Power Calculations. Retrieved 28 January Valerius C., Krupar J., Schwarz W. Electronics Power Management for Bicycles European Conference on Power Electronics and Applications

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