1. Why should someone use supercapacitors instead of a battery in a solar car?
2. Figures explaining the difference between the supercapacitors and shows why 12v supercapacitor is more suitable?
3. Discussion in what can the supercapacitor affect the solar car in an advantages and disadvantages way and what results could be happening if a supercapacitor is used?
With the realization of the nothing is an unlimited resource, conventional power system has been realized to be a very limited at the same time, expensive resource. So, future reliance on the alternative energy resources, like solar energy resources has been increasing. At the same time, energy conversion devices to convert the solar energy to the conventional usable energy play vital role in the power systems, especially, in the electric vehicles, like in solar cars. Rechargeable energy storage systems have become mandatory as part of the power systems with the electric or solar vehicles. Supercapacitors are used in niche applications so far, but very soon are going to replace the conventional batteries in the solar cars. This report explores the role of the supercapacitors in the solar cars, and its replacing capable features to the batteries, to increase the performance power system in these cars. Supercapacitors are described and explored for its effective application to be used in the solar cars, which are the future of the transportation.
Supercapacitors or ultracapacitors are electrochemical capacitors that have very high capacity of about 10,000 farads, according to Marin S. Halper, James C. Ellenbogen, Supercapacitors: A Brief Overview. These capacitors bridge the gap that exists in between the rechargeable batteries and electrolytic capacitors. Supercapacitors are preferred as because of the increased energy capacity, capability of accepting and delivering the charges faster. These are capable of tolerating increased number of cycles of charge and discharge. These capacitors are preferred as they are 10 times larger, when compared to the typical batteries. Supercapacitors are used in many applications, like in UPS systems, power generators, inverters, computer systems, automobile regenerative braking system, welders, cameras, power supplies, inverters and power conditioners, including the solar cars.
Supercapacitors are made with two strong sheets of graphene with an electrolyte sandwiched in between them, according to Marin S. Halper, James C. Ellenbogen, Supercapacitors: A Brief Overview. The benefits of these capacitors are because of the larger surface area electrode as well as the thin electrolytic dielectric. This feature gives increased energy density, at the same time high power density that is the usual property of the conventional capacitors is maintained.
Solar cars have been using the batteries presently for energy storage at many cases. Solar cars need batteries to store the energy. The size of the batteries are usually sized and configured according to the power and energy requirements of the solar car, according to Mazhar in Ultracapacitor/Battery Hybrid Energy Storage System. Another requirement is the life cycle performance. There are shortcomings of batteries in all these aspects and so the supercapacitors are used instead of the batteries.
The size and weight of the batteries has become a concern and so it has to be resized to the required levels. In hybrid electric vehicles, like solar car vehicles, good performance of the rechargeable energy storage systems is mandatory. At the same time the storage system must have longer lifecycle performance.
Electrical characterization of the rechargeable energy storage systems are usually depend on the significant electrical parameters of the rechargeable systems. The key concepts here are the power density, which is calculated in kilo watt per kilo gram and the energy density, which is measured in Watt hour per kilo gram. The other important electrical parameters needed are the efficiency of energy, charging rate and lifetime.
Solar cars have been using the conventional betteries, especially, lithium-ion batteries, according to A. Cooper in Advanced lead-acid – the battery system for hybrid electric vehicles. These batteries store the energy in the electrochemical energy. So electrical carrriers are released from the chemical reactions. Ions start traveling from the anode, high-energy material towards the cathode, low energy material, through a separator. The reverse do happen while charging. So, the processes of charging and discharging are very slow, degrading the overall performance of the batteries. Over a period of time, the chemical compounds present in the battery starts degrading over time. The result will be low power density of the batteries. Another result of the same will be losing the ability of energy retention in the lifetime of the battery, as the material gets damaged.
Unlike the batteries, supercapacitors use the storage mechanism that is different. Supercapacitors store the energy in the electrostatic and is stored on the surface of the material. So, there would not be any chemical reactions during its process. Because of the difference of this fundamental mechanism, the charging and discharging speed of these capacitors is very fast. The end result of the same is very high power density. The storage capability as well as the power density would not be degraded for longer period of time. Unlike the conventional batteries, supercapacitors do have charging and discharging cycles of around millions.
There are many characteristic differences exist between the supercapacitors and batteries. The operating temperature range for the conventional batteries is around -20 to 50 degrees celcius, whereas for the supercapacitors it is incrased to -40 to 85 degrees centrigrade, according to Marin in Supercapacitors: A Brief Overview. Batteries need the charge time of few hours, whereas the supercapacitors have only a few minutes of time. The lifetime for charge and discharges for the battery is approximately from 500 to 1000 times, whereas for the supercapacitors is almost limitless. However, At the same time, batteries have restrictions on the charging and discharging, but the supercapacitors do not have any restriction on charging and discharging. Flow soldering is not applicable for the batteries, but applicable for the supercapacitors. Automatic mounting is not applicable for the batteries, but for applicable for the supercapacitors, especially, for the FM and FC series capacitors. When it comes to the safety risks there are more safety risks associated with the batteries rather than the supercapacitors. Batteries can be hazardous with explostion, combustion, leakage and ignition. But supercapacitors are hazardous with only gas emissions. However, it is important to operate these capacitors, within the limited conditions, like these should be operated within the normal operating voltages. When the temperature is raised beyond the standard limited levels, there will be certain consequences happening. The supercapacitor characteristics are deteriorated, especially the terms like, internal resistance and self discharge. The pressure existing inside these capacitors is also increased. When repetitive heating is occurred, can cause the premature aging of the contacts made from the metal can deteriate the terminal connections present in the supercapacitros rapidly. When the temperature exceeds the boiling point of the electrolyte liquid, which is 81.6 degrees celcius, electrolyte can be evaporated, decreasing the lifetime of these capacitors.
Because of the above differences both the supercapacitors and batteries are used in conjunction with each other. However, the recent technologies are combining both the supercapacitors and batteries to exploit the advantages of both.
Fig. Power Density Vs. Energy Density. ( From Wiki Commons)
Supercapacitors are usually divided in three classes. These three classes are pseudocapacitors, electrochemical double-layer capacitors and hybrid capacitors, according to Arbizzani, in New trend in electrochemial supercapacitors. Each of this class of the supercapacitors has its own characteristics defined by the unique mechanism utilized to store the charge. Electrochemical double layer makes use of the non-faradaic characteristics. So, does not make use the chemical mechanism. Instead of the chemical mechanism, physical procAresses do distribute the charges on the surface, but does not have the process of make or break of the chemical bonds. Psudocapacitors make use of the faradaic characteristics, in whihch the oxidation-reduction reactions do occur and the transfer of charges is involved in between the electrolyte and electrode. and finally, the hybrid one makes use of the combination of both faradic and non-faradiac.
Taxonomy of the supercapacitors
The three classes of capacitors along with the sub-classes of the same, are shown in the following figure below.
Fig. Taxonomy of the supercapacitors
For solar car and especially electric vehicles, 12V is needed for the operation, according to Marin in Supercapacitors: A Brief Overview. So, batteries are used with the operating voltage of 12V. to replace these batteries, supercapacitors used must also be of 12V. When this operating voltage is applied, these capacitors also must be of the same rating. When the supercapacitors are mentioned for the solar car, the operating voltage is of 12V.
Supercapacitors are deployed in many solar cars for the rapid accelerating requirements. The energy recovered through charging, while braking is performed. Supercapacitors are used in the solar cars because of the advantages that can be exploited from them. These capacitors use the carbon or nanotube technology, according to Arbizzani in New trends in electrochemical supercapacitors. Because of the carbon technology, they get very large surface area created at the same time, the separation distance is decreased to very small area.
Fig. Supercapacitors of Maxwell technologies
In some of the solar cars, supercapacitors are already employed to yield better energy storage benefits. When these capacitors are used in the cars, the electric cars accelerate with the help of the supercapaciors, which decrease the need of the conventional batteries. So, the weight of the solar car can be decreased as well as the range can be extended.
Solar cars do require more energy to accelerate. This is the major application and deployment of the supercapacitors. They can deliver the required amount of energy for acceleration very quickly. It makes it to be a perfect complement to the conventional high weighted and space occupying storage batteries, according to Marco Notarianni, who is a researcher in Queenslant University of Technology. The acceleration is possible, as the supercapacitors can deliver high power output in very short period of time, which gives quick acceleration rate for the solar car. Usually, at room temperature, the lifetime of the supercapacitors expected can be 10 to 15 years and sometimes even more time.
Supercapacitors have the advantage of the longer lifetime, when compared to the conventional batteries used for the solar cars. it is because these do not rely on the chemical reactions. The lifetime is a factor of the evaporation rate of the electrolyte that is in the form of liquid. Various factors that influence the rate of evaporation of the electrolyte are the load, temperature, voltage and current cycle frequency.
Supercapacitors give better fuel economy when compared to the batteries, up to 20%, for shorter trips, rather than the longer trips, according to Arbizzani in New trends in electrochemical supercapacitors. After the lifetime of the supercapacitors, they will not become hazardous to the environment, as these are environment friendly. These capacitors are safer compared to the batteries, so while operating them there are almost no hazardous situations arise while using in the solar cars.
Unlike the usual and conventional batteries used in the solar cars, supercapacitors are of very small size and weigh much lesser than them. The supercacitors have the capability of reversibility, which is not possible with the conventional batteries. The material used for these capacitors is of low toxic materials. The cycle efficiency of these capacitors is much higher and is about 95%.
When compared to the batteries, supercapacitors have extremely low internal resistance. So, this important electrical parameter yields the supercapacitor to gain with high output power, improved safety, higher efficiency and low heating levels.
Apart from these basic advantages of the supercapacitors in the solar cars, there are many other potential advantages that could improve the overall performance of these capacitors to be better suitable to be used in the solar cars.
Supercapactors have stable electric properties, enabling to give better performance consistently at all occasions and at all times. Another important advantage is that they have broader temperature range. So, the performance and function of these capacitors do not easily vary on the broader range of temperature.
Supercapacitors are for sure good alternatives to the high powered and high weighted conventional batteries for many of the advantages. However, there are a few shortcomings associated also with the super capacitors.the major shortcoming of these supercapacitors is that they have very low energy density. Because of this low energy density, these capacitors can store only small amount of energy can be stored per unit weight, according to Yonghua in Assessment of Energy Capacity and Energy Losses of supercapacitors in fast charging-discharging cycles. This is the major shortcoming, when compared to the batteries, for which high amount of energy can be stored per unit weight in batteries.
Another shortcoming associated with these capacitors is that the cost of them is more. It is because the materials used for manufacturing these capacitors are often high. Usually, graphene is used as the material. At present the supercapacitors in market have the energy density of about 28 Wh per kg, but the Li-ion battery has more energy density compared, i.e. 200 Wh per kg. It shows major gap in between these energy sources.
However there is considerable research is happening, where the material production methods can be improved. When the research becomes successful in its objective, the gap between the energy density between the supercapacitors and batteries can be bridged. Later very soon, supercapacitors can be used as conventional energy storage systems in the solar cars.
Supercapacitors do have smaller charging and discharging time, when compared to the conventional batteries used in the solar cars. When these capacitors have a few minutes of time, batteries long for several hours. Overall weight, cost and volume are the major disadvantages of them that usually mitigate the advantages of them.
The present storage technology makes use of combined efforts of the batteries and supercapacitors to yield combined advantageous results of both. According to the Journal of Power Sources and Nanotechnology, the energy densities of the supercapacitors have demonstrated from 8 to 14 watt-hour per kg, and the power densities are increased from 250 to 450 KW per kg. the research is done in Queensland University of Technology in Australia. During the research, electrodes have used the graphene film material and current collectors used carbon nanotube films.
Various research projects like tailored-pore-size electrodes, nanostructured electrodes, doping material, pseudocapacitive coating and improved electrolytes. There is a lot of research is going on to replace the batteries completely with the supercapacitors, because of their potential benefits.
Conclusion
The solar car rechargeable energy storage systems in terms of the batteries, which are conventionally used as well as the supercapacitors, which are known to be the best alternatives to these batteries are discussed. Both these storage systems are discussed while comparing and differentiating with each other.
References
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