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Topic: Electric Cars: Batteries and Ultracapacitors
The goal of our research is to find the extent to which ultracapacitors can be utilized in the electric car industry. We intend on designing a 40 passenger hybrid bus using ultracapacitors and an electric motor. Currently, the term hybrid is used when defining an automobile which is powered by a combination of a traditional internal combustion engine as well as an electric motor. We would like to design a hybrid system that removes the combustion engine and replaces it with ultracapacitors to create a low emission hybrid bus -
Ultracapacitor / battery?
Different Ucap Buses
Variety of Maxwell Ultracapacitors
An ultracapacitor, also known as a supercapacitor, is a device that stores a large amount of charge. The typical capacitors that are used in either Electrcity and Magnetism or Electronics courses have a capacitance on the order of micro-Farads per capacitor. This relatively new device has a capacitance of hundreds to thousands of Farads per capacitor. Compared to other capacitors, the ultracapcitors has a high energy density, but when put in the ring against batteries or fuel cells, they simply don't stack up well. Ultracapacitors vary mainly in design from traditional capacitors because of the relationship between capacitance and surface area. Most traditional capacitors use a dielectric between the capacitors to enhance capacitance. However, there is a physical limit on how effective this can be. Ultracapacitors are made by coating two metal foil electrodes in carbon (used because of its high surface area-to-volume ratio), separating them with a thin piece of paper and then immersing these two coated metals in a liquid electrolyte. These are sometimes made by weaving carbon fiber threads. Regardless, this essentially functions as two capacitors in series which is why ultracapacitors have such a higher energy and power density than traditional capacitors and batteries, respectively. Because of their ability to pass large amounts of current in a short amount of time, they are extremely valuable when one is inneed of that extra boost. According to Maxwell Technologies, the time for an ultracapacitor to charge or discharge is .3 to 30 seconds. As we can see, this charge time is nearly 500 times faster than that of lead acid batteries. However, its discharge time is about 1000 times faster. According to the EPA, a bill passed in 1996 limited the gallons of gas pumped per minute to 10 gallons per minutes. Most transit buses hold approximately 125 gallons per tank. This means that it would take about 13 minutes to pump a tank. However, with ultracapacitors it would take approximately a minute, which would save time used to travel to a gas station and time spent waiting to refuel.
Traditional Electrostatic Capacitor on left compared to an Ultracapacitor on right
Another valuable feature of ultracapacitors is their long cyclability. Most lead acid batteries can maintain a life cycle from 500 to 1000 charges and discharges, but ultracapacitors can be cycled through at least 500,000 times. This reduces the times one would need to change the energy source which is costly and can be environmentally detrimental for batteries.
The big downside of relying heavily on ultracapacitors is that because their energy density is low, the range you can achieve on a single charge is not tremendous, which is why putting them in series is crucial to increase the amount of voltage they can output.
Comparison of lead acid batteries, Ultracapacitors, and conventional capacitors
Design Ideas and Practical Implementations
As mentioned above, one of the best features of ultracapacitors is how quickly their charge can be replenished. Of course, on the highway, stopping for even a short period of time is still quite inconvenient, but for city driving, our design is more realistic. For example, public transit vehicles make stops fairly often. When they do, it is usually for a duration of about a minute, which we now know, is plenty of time to charge an ultracapacitor. However, at every bus stop there would need to be some electrical outlet that would allow for the bus to be "plugged in". These would allow buses to access the energy required to power the bus for long enough to make it to the next bus stop.
Electric cars can either run on alternating current (AC) or direct current (DC). Ours will likely be DC. Based on their calculations, an absolute minimum of 96 volts (100 V for our bus) should be provided to the motor if one expects the automobile to move. The upper value that they have listed for a source is 192 volts. Our bus will run at 200 V.
In order for the car's electronics (such as the radio) to run smoothly, it is possible that some sort of step down voltage may be needed. While this might be able to be easily accomplished with an AC induction motor, the use of a DC motor forces us to use transformers or some similar technology to reduce voltage to appropriate levels.
Our bus will only use ultracapacitors, which are much more environmentally friendly than batteries; they are approximately 70% recyclable, last much longer than batteries, and do not contain any heavy metals that are detrimental to the environment. During deceleration, ultracapacitors can also store the energy from regenerative breaking. Due to the fact that electric vehicles do not directly emit carbon dioxide into the air while they are converting electrical energy to mechanical energy, they reduce the impact they have on the environment. Of course, one can say that electric cars emit zero greenhouse gases, but it takes energy to charge the batteries, and in this case, ultracapacitors which fuel it. To prove that internal combustion engines(ICEs) are far worse, in terms of emissions, a simple calculation can be made.
Disclaimer: All numbers used are conservative, meaning it is probably more beneficial to use ultracapacitors than the numbers stated here
average number of miles a bus is driven per year is 12,000 (US EPA)
mileage of the bus is 3mpg
it takes about 8.4MJ to drive 1 miles
there are 125MJ in a gallon of gas
electricity in CA is generated by NG and is 1/3 efficient
20gC/MJ for petroleum
15gC/MJ for natural gas
Internal Combustion Engine:
*Numbers do not account for carbon dioxide lost to the environment due to extraction of petroleum.
electrical to mechanical
*Numbers do not account for carbon dioxide lost to the environment due to extraction of natural gas.
Ratio of Carbon Dioxide Emissions = 37000kg/21000kg which equals ~ 2.
But California generates electricity from NG with better than 33% efficiency, no?
As you can see, ICE's are responsible for emitting 2 times as much Carbon Dioxide as our ultracapacitor bus.
Another important fact that we must not forget is there exists a way in which this amount of CO2 from the electric vehicle can be reduced even further. The idea of electrical energy is extremely powerful because it can be produced not only through the burning of coal and natural gas, but through nuclear , wind, solar, and geothermal power generation. Should policy makers and energy firms adhere to the set standards and goals aimed at creating a certain percentage of electrical energy through renewables, our hybrid vehicle of ultracapacitors have the potential to actually live up to its zero emissions (not in actuality, but once on the road it should) name. One feasible way of doing this is placing photovoltaics on the roof of our bus to gain energy from the sun to power itself.
The percentage shows how much energy is gained by regenerative breaking
Specifics of Design and Necessary Infrastructure:
Through our excel sheet we realized that we need approximately 8.4 MJ to power the bus for 1 mile, which is approximately how far a bus might need to travel to a bus stop to recharge. If we use 75 capacitors in one "pack" we would require about 11 packs because each pack holds .8 MJ of energy which is a total of 8.8 MJ. While this may seem like is may be cutting it short and look like it is not accounting for constant stopping and starting. This number does not account for regenerative breaking which would allow us to gain energy back every time we decelerated. Regenerative breaking occurs by switching the DC motor into a generator by changing the direction they spin, by doing this we create brake energy.
Photovoltaics in action!
While there is no grid infrastructure to allow us to charge buses at bus stops, the implementation of such infrastructure would not be incredibly costly or burdensome. Because our packs would run at 200 V because the individual capacitors in the pack are in series, if the packs were charged in parallel
, we would only need a voltage of 200V and this would also shorten the charge time and make it much more convenient to charge them as opposed to individually charging each pack for 200 V. Electrical lines would merely need to be converted to an outlet that a bus would be able to charge in at 200 V. This means that a transformer would need to be installed to increase the voltage to 200V and a full wave rectifier to convert the AC signal to a DC one. Best case scenario, we would install photovoltaics at the bus stop that would allow us to create an even lower emitting hybrid that would run purely off of solar, both on the bus and at the bus stops.
Another important part of our design is a computer that would allow the packs of ultracapacitors to be switched once a minimum voltage, which we determined to be 100V, to ensure that the bus would still be able to run.
If we use Maxwell ultracapacitors and buy in bulk over 100 ultracapacitors, each ultracapacitor would cost $80. We would need 825 ultracapacitors, that means that it would cost $66,000.
If we use a national gasoline average of $3.85
The cost of fuel for an ICE would be around $15,000.
If we use an average electrical price of 15 cents/kWh
The cost for our ultracapacitors would be around $5,000.
8.4MJ/ .85% efficiency
The Monetary Benefits of Turning to Ultracapacitors.
Internal Combustion Engine Emissions
electrical to mechanical
This is approximately 57% BAU(Business As Usual) emissions.
Summary of Design:
Our bus plans to hold 40 passengers not including the driver
A 200V electric DC motor that doubles as a generator
11 packs of ultracapacitors that would house 75 ultracapacitors in series per pack
Dimensions: 40 ft length; 8 ft width; and 11 ft height (based on a similar ultracapacitor bus)
A computer to control switching of ultracapacitor packs when voltage drops below a threshold value ~100V
A photovoltaic grid that would be implemented at bus stops and atop the bus to create a lower emitting hybrid bus.
Transformers and full wave rectifiers to step up the voltage to the 200V and convert the AC signal to a DC one
Step down voltage to power radio, computers, etc
Incorporate regenerative braking to gain lost energy
Curb weight~ 13 tons (based on a similar ultracapacitor bus)
Overall a very feasible design, however, it would be much more expensive and complicated to retro-fit a bus and so the cheapest way would be to construct buses as ultracapacitor buses. If this were done, these would significantly reduce the global carbon emissions and be cost effective too.
Schematic of a bus. Ultracapacitors would be held beneath the passengers between the wheels. The motor would be housed beneath the driver
Ultracapacitor Spreadsheet Model (1).xlsx
United States EPA
Dr. Thomas Bensky
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