With the advancement of science and technology, people now pay more and
more attention to ecological environment protection and the application of green
energy. As a new type of energy storage component, supercapacitor has attracted
widespread attention and attention. Super capacitor is a new energy storage
device developed in the 1970s and 1980s between batteries and traditional
capacitors. Its emergence fills the gap between traditional electrostatic
capacitors and chemical power sources.
1. Principles and classification of supercapacitors
Supercapacitor is a physical secondary power source with super power
storage capacity and strong pulsating power. Supercapacitors are mainly divided
into three categories according to the energy storage mechanism [1]: ① Double
electric layer capacitance generated by charge separation at the interface
between carbon electrode and electrolyte; ② Using metal oxides as electrodes,
oxidation occurs on the electrode surface and bulk phase A Faradaic capacitor
that produces reversible chemical adsorption due to a reduction reaction; ③ A
capacitor that uses a conductive polymer as an electrode to cause a redox
reaction. Electric double layer supercapacitor is a new type of energy storage
device that relies on polarized electrolyte to store electrical energy. The
structure is shown in Figure 1.
Since the charging and discharging of electric double layer capacitors is
purely a physical process, it has a high number of cycles and a fast charging
process, so it is more suitable for use in electric vehicles. Electric double
layer supercapacitors are two inactive porous plates suspended in an
electrolyte, with a voltage applied to both plates. The potential applied to the
positive plate attracts negative ions in the electrolyte, and the negative plate
attracts positive ions, thereby forming an electric double layer capacitor on
the surfaces of the two electrodes. The capacitance of a supercapacitor unit is
as high as several farads to tens of thousands of farads [2]. Because this
structure adopts a special process, its equivalent resistance is very low, the
capacitance is large, and the internal resistance is small, making the
supercapacitor have a very high The peak current, so the supercapacitor has a
very high specific power. Its power density is 50 to 100 times that of the
battery, which can reach about 10×103W/kg. This feature makes the supercapacitor
very suitable for applications in short-term high-power situations. .
2.Characteristics of supercapacitor
There are no chemical reactions or mechanical movements such as high-speed
rotation during the use of supercapacitors. There is no pollution to the
environment and no noise. Its simple structure and small size make it an ideal
energy storage device. Supercapacitor products have the following technical
characteristics[3]:
(1) Charging speed is fast. It only takes 10 seconds to 10 minutes to fill
more than 95% of its rated capacity;
(2) Long cycle life. Deep charge and discharge cycles can reach 10,000 to
500,000 times. For example, the cycle life of the HCC250F/2.7V supercapacitor
produced by Beijing Hezhong Huineng Company and the series of capacitors
produced by Beijing Jixing Technology Company are both more than 500,000
times;
(3) High energy conversion efficiency. High current energy cycle efficiency
>90%;
(4) High power density. It can reach 300W/kg-50000W/kg, which is 5 to 10
times that of batteries;
(5) The production, use, storage and dismantling of raw materials are
pollution-free, making it an ideal green and environmentally friendly power
source; it has a high safety factor and is maintenance-free for long-term
use;
(6) High charge and discharge efficiency. Because the internal resistance
is very small, the charge and discharge losses are also very small, and the
charge and discharge efficiency is very high, reaching more than 90%.
(7) Wide temperature range. Reach -40～+70℃. The reaction rate of
supercapacitor electrode materials is not greatly affected by temperature;
(8) Convenient detection and control. The remaining power can be calculated
directly through the formula E=CV2/2. The stored energy can be determined by
simply detecting the terminal voltage. The calculation of the state of charge
(SOC) is simple and accurate, so it is easy to manage and control the
energy.
3. Problems with supercapacitors
Supercapacitors are used in situations with high energy density
requirements and long working cycles. Their main shortcomings are as
follows:
(1) Lower specific energy. The energy density of supercapacitors is about
20% that of lead-acid batteries; if the same energy is stored, the volume and
weight of supercapacitors are much larger than batteries.
(2) Low pressure resistance. The current withstand voltage of
supercapacitors is much lower than that of ordinary capacitors, with a voltage
of about 1-3V. If driven in series, the energy storage system will be relatively
large, which is not conducive to driving high-power equipment.
(3) The terminal voltage fluctuates seriously. During the use of a
supercapacitor, its terminal voltage changes exponentially. When the
supercapacitor releases 3/4D of energy, its terminal voltage will drop to 1/2 of
the original voltage.
(4) Voltage balance problem when connected in series. During the production
and manufacturing process of supercapacitors, there are problems with uneven
processes and materials. Capacitors of the same batch and specifications have
certain differences in parameters such as internal resistance and capacity.
Therefore, when using supercapacitor components, a series voltage equalizing
device is required to improve the energy utilization and safety of the
components.
4.Applications of supercapacitors
4.1 Application in electric vehicles
The main factors restricting the development of modern industry are
environmental pollution and energy shortage. Nowadays, the automobile industry
accounts for most of the energy consumption of modern industry. Therefore,
automobile energy-saving technology has become a key technology that must be
solved in the development of the automobile industry. Due to the superior
performance of supercapacitors, countries around the world are scrambling to
research and increasingly apply them to electric vehicles. Supercapacitors with
tens of thousands of farads can be used as short-term drive power sources for
electric vehicles. Under short-time, high-power working conditions such as
vehicle starting, acceleration and braking energy recovery, it can significantly
improve the power and economy of electric vehicles. properties and can
effectively improve battery performance. Supercapacitors have become a new trend
in the development of electric vehicle power supplies, and the composite power
supply system composed of supercapacitors and batteries is considered to be one
of the best ways to solve the power problem of electric vehicles in the
future.
Japan's Honda's FCX fuel cell-supercapacitor hybrid vehicle is the world's
first commercialized fuel cell car. The vehicle was launched in Japan and
California in the United States in 2002. Nissan Corporation of Japan has also
launched a natural gas-supercapacitor hybrid bus, as shown in Figure 2. The
vehicle's fuel economy is 214 times that of traditional natural gas
vehicles.
Switzerland's PSI Institute installed a supercapacitor bank with 360Wh of
energy storage on a 48kW fuel cell vehicle. The supercapacitor assists the fuel
cell with a 15s rated pulse power of 50kW, taking on all the transients during
deceleration and starting of the drive system. state power. In 1996, the Russian
Eltran company developed an electric vehicle using supercapacitors as power
source. It used 300 capacitors in series and could travel 12km on one charge at
a speed of 25km/h. The hybrid bus developed by the NASA Lewis Research Center in
the United States uses supercapacitors as the main energy storage system; the
zinc-air fuel cell electric vehicle designed and developed by the American
Electric Fuel Company (EFC) also uses supercapacitors as auxiliary energy and is
equipped with supercapacitors. Its driving range has been increased by nearly
25%.
After my country's 11th Five-Year Plan and 863 electric vehicle projects
were launched, domestic companies that develop supercapacitors have also
increased their development efforts. my country's first capacitor-storage
variable-frequency drive trolleybus was put into trial operation in Zhangjiang,
Shanghai in July 2004. When the trolleybus is at the stop, it can be charged
quickly within 30 seconds, reaching a speed of 44km/h, and can continuously
provide electric energy. This trolley bus makes full use of the characteristics
of supercapacitor's high specific power and fixed parking of public
transportation. The supercapacitor electric bus developed by Harbin Institute of
Technology and Jurong Group has a maximum speed of 20km/h and can accommodate up
to 50 passengers. A total of 1,147 energy-saving and new energy vehicles were
invested in the 2010 Shanghai World Expo: supercapacitor vehicles, fuel cell
vehicles, pure electric vehicles and hybrid vehicles. The new energy vehicles in
the park accounted for 66% of the transportation capacity in the park during
actual operations. , achieving the goal of zero emissions from public
transportation within the park and low emissions from public transportation
around the park. In addition, Shanghai Aowei Company, Beijing Jixing Company,
Jinzhou Baina Electric Company and Harbin Jurong Company have all launched their
own supercapacitor products for HEV (hybrid power devices) or EV (electric
vehicles). However, the current domestic design and control of supercapacitors
and battery composite power supply electric vehicles are basically still in
their infancy, and the supercapacitors produced by domestic companies are still
far behind foreign products.
4.2 Working principle diagram of composite power supply electric
vehicle
Pure supercapacitor electric vehicles use supercapacitors as the only
energy source for electric vehicles. This method has a simple structure, low
cost, practicality, and zero emissions. Therefore, it is more suitable for use
in areas with short distances and fixed lines, such as in schools and
kindergartens. Food delivery trucks can be used, tractors at train stations or
airports can be used, and tour buses and electric buses in parks can also be
used. Electric vehicles that use supercapacitors and batteries or fuel cells to
form a composite power system are more flexible and have a broader application
space.
The working principle of the supercapacitor-battery composite power supply
system for electric vehicles is shown in Figure 3. First, under normal driving
conditions of an electric vehicle, the battery provides energy to the electric
motor through the power converter. When the vehicle is driving with a light
load, the battery charges the supercapacitor, so that the supercapacitor has
high power output capability; when the vehicle accelerates or climbs a hill, the
supercapacitor and the battery provide energy to the motor at the same time;
when the vehicle brakes or travels downhill, When the motor is in generator
mode, the regenerated energy charges the supercapacitor through the power
converter. If the supercapacitor cannot accept all the regenerated energy, the
remaining part is absorbed by the battery.
4.3 Application in UPS system
Most of today's USP (uninterruptible power supply) systems use lead-acid
batteries as electrical energy storage devices. If used under frequent power
outages, the batteries will be sulfated due to long-term insufficient charging,
and the service life will be greatly shortened. Supercapacitors are not affected
by frequent power outages and can be fully charged in a short period of time.
Supercapacitors are good emergency power sources due to their high power density
output characteristics. For example, the blast furnace cooling water process in
a steelmaking plant is not allowed to be interrupted. In the event of a power
outage, the supercapacitor can immediately provide high output power to start
the diesel generator set and supply power to the blast furnace and water pumps
to ensure safe production of the blast furnace.
4.4 Application in special systems
The U.S. military uses supercapacitors in armored personnel carriers, heavy
trucks and tanks. The power used by Oshkosh Automobile Company to manufacture
the HEMTT LMS concept vehicle for the U.S. military is the ProPulse hybrid
electric propulsion system produced by Maxwell Company, which uses PowerCache
super capacitor. PEMFC power generation technology has extremely broad
application prospects in civil air defense command projects due to its
advantages of high efficiency, cleanliness, light weight, small size, and low
operating temperature. However, no matter which power supply method is adopted,
the unstable DC power generated by the PEMFC generator must be converted into
stable DC power before it can be supplied to the load or inverter. The dynamic
characteristics of PEMFC generators show obvious instantaneous voltage drops
when a sudden increase in load occurs, causing the subsequent DC/DC and DC/AC
(direct/AC conversion) to be protected and unable to work normally. Using
supercapacitors to compensate the dynamic characteristics of the PEMFC generator
can remove the voltage drop peak when the load suddenly increases, thereby
improving the dynamic output performance of the generator and providing stable
DC voltage for subsequent DC loads and DC/AC.
4.5 Application in low-power electrical equipment
Traditional battery-powered flashlights have a limited life. Even modern
LED flashlights take hours to fully charge, and the battery has a short cycle
life. A flashlight that uses supercapacitors as energy storage components only
takes 90 seconds to charge and has a cycle life of up to 500,000 times. If
charged and discharged once a day, it can be used for about 135 years. This
tactical flashlight was developed by the police and military. The use of
supercapacitors as energy storage components ensures that emergency lighting has
the characteristics of power saving, high brightness, uninterruptibility and
long life.
5. Knot terminology
As an energy storage system with large energy storage, fast charging and
discharging speed, wide operating temperature range, safe and reliable
operation, and no need for maintenance, supercapacitor will gradually replace
batteries with the development of supercapacitor technology, and its application
fields will continue to expand. It will surely promote technological progress
and achieve greater economic and social benefits.
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