- Advantages of hybrid energy storage system
In recent years, energy has become one of the major issues facing the whole mankind. With the continuous development of new energy, how to make more effective use of existing energy has been paid more and more attention. The storage and form conversion of electric energy has become a hot research issue in the field of energy. At present, there are many ways to store electric energy on new energy vehicles, but in terms of energy storage mechanism, energy storage systems mainly include battery energy storage system, flywheel energy storage system and super capacitor energy storage system.
At this stage, the requirements of safety, high specific power, high specific energy, long service life, mature technology and wide working temperature range can not be fully taken into account, regardless of ultra-high speed flywheel, superconducting inductor, various fuel cells, electrochemical batteries or supercapacitors. Generally speaking, the specific power of energy storage devices with high specific energy will not be too high; Similarly, if the specific power of an energy storage device is relatively high, its specific energy is not necessarily very high. These energy sources can not provide high specific energy and high specific power at the same time, so it is difficult to be used as the energy of electric steam year alone. Using hybrid energy system, that is, hybrid power system, has become a practical scheme in the design of electric vehicles.
Broadly speaking, hybrid electric vehicle refers to a vehicle equipped with two driving devices with different characteristics. One of the two driving devices is the main power source of the vehicle, which can provide stable power output and meet the power demand of stable driving of the vehicle; In addition, there is an auxiliary driving device, which has good off design characteristics and can balance power, regenerate and store energy.
According to the recommendations of the technical committee of electric vehicles under the international Electromechanical Commission, hybrid electric vehicle refers to a vehicle driven by two or more energy accumulators, energy sources or converters, in which at least one kind of vehicle can provide electric energy, which is called hybrid electric vehicle. According to this definition, there are many forms of hybrid electric vehicles, which can be divided into oil electric hybrid, flywheel battery hybrid, fuel cell battery hybrid and electric electric hybrid. In order to avoid confusion, the hybrid electric vehicle is usually represented by the vehicle powered by internal combustion engine and battery in the industry. The hybrid vehicle of fuel cell and battery is called fuel cell electric vehicle, and the vehicle powered by battery and capacitor is called super capacitor auxiliary electric vehicle.
As the number of energy used increases, the layout and control of the whole power system becomes more difficult. Generally, the hybrid power system with two kinds of energy (one has relatively high specific energy and the other should have relatively high specific power) is adopted. The hybrid power system used in electric vehicles has the following advantages.
(1) The separation of energy and power requirements of electric steam is realized, so that electric vehicles can use high-energy energy energy such as battery and fuel cell and high-power energy such as super capacitor and ultra-high-speed flywheel.
(2) Since there is no need to pursue high specific energy and specific power at the same time, the cycle life of energy can be prolonged and the production cost can be reduced.
(3) It makes full use of the advantages of various energy sources of electric vehicles, such as mature technology and low cost of battery, high specific energy and high fuel efficiency of fuel cell, huge specific power and instantaneous charge / discharge capacity of super capacitor, huge specific power of ultra-high speed flywheel and unlimited cycle life.
The hybrid electric vehicle using high specific energy and high specific power energy supplies energy to the motor through the power converter under normal driving conditions. In order to make the energy with high specific power have high power output capacity at all times, the energy with high specific energy charges the energy with high specific power from the energy with high specific energy under the condition of light load driving. When the vehicle is accelerating or climbing, Energy with high specific power and energy with high specific energy supply the motor at the same time. When the vehicle is braked or driving downhill, the motor works in the generator mode, and the regenerative energy charges the energy with high specific power through the power converter. If the energy with high specific power cannot accept all the renewable energy, the rest will be absorbed by the energy with high specific energy.
- Several typical hybrid energy storage systems
Based on various existing energy technologies, the hybrid power system available for electric vehicles includes battery battery hybrid power, battery super capacitor hybrid power, fuel cell battery hybrid power and flywheel hybrid battery.
2.1 mixing of super capacitor and battery
Because the specific energy of super capacitor is low, it is difficult to meet the practical application of electric vehicles, so it must be combined with other energy sources. As a hybrid energy storage system of electric and electric super capacitor and battery, the battery provides high energy density and the super capacitor provides high power density, which can simultaneously prolong the driving range of electric vehicles, improve the starting / accelerating performance of electric vehicles and recover braking energy. The research on this hybrid energy storage structure will help to accelerate the realization of the development goal of replacing internal combustion engine vehicles with pure electric vehicles, Reduce environmental pollution and energy crisis. The hybrid system needs to install a power converter between the battery and the super capacitor, which is considered based on the relatively low working voltage of the super capacitor (even if multiple capacitors are used in combination, the working voltage is usually less than 100V). At present, the hybrid system using valve regulated lead-acid battery and super capacitor (the average energy consumption of vehicles output by valve regulated lead-acid battery and the peak power output by super capacitor) has attracted extensive attention. When the vehicle is running at low power, the super capacitor is charged by the valve regulated lead-acid battery, and the regenerative energy is absorbed by the super capacitor during the regenerative braking of the vehicle. Other possible hybrid systems include MH / Ni battery and supercapacitor hybrid, lithium-ion battery and supercapacitor hybrid.
2.2 improvement of electrical performance
Compared with the voltage waveform during the starting process when the super capacitor is connected in parallel with the battery, the starting instantaneous voltage increases from 3.2V when the battery is only used to 7.2V; The starting current is increased from 560A to 1200A; The power output power at the instant of start-up is increased from 2kW to 8.7kw; The steady voltage in the starting process is increased from 7V to 9.4v; The steady current during start-up is increased from 280A to 440A; The power supply in the starting process is stable, and the output power is increased from 2.44kw to 4.12kw.
2.3 improvement of startup performance
The application of super capacitor in parallel with battery can improve the starting performance of locomotive. Start the vehicle with 1.9l diesel engine in parallel with super capacitor (450F / 16.2v) and 12V and 45A · h battery, and start it smoothly at 10 ℃. Although in this case, the battery can also be started without super capacitor, the speed and performance of starting motor when super capacitor is connected in parallel with battery are very good. Due to the increase of the output power of the power supply, the starting speed increases from 300r / min when only using the battery to 450R / min; Especially in terms of improving the starting performance (higher starting torque) of the vehicle in cold weather, the super capacitor is very meaningful. At minus 20 ℃, due to the great decline of the performance of the battery, it is likely that it will not start normally or need multiple starts to succeed. When the super capacitor is connected in parallel with the battery, it only needs one ignition. Its advantages are very obvious.
2.4 improvement of battery application status
When the super capacitor is connected in parallel with the battery, since the equivalent series resistance (ESR) of the super capacitor is much lower than the internal resistance of the battery, at the moment of starting, 800A of 1200A starting current is provided by the super capacitor, and the battery only provides 400A. It is significantly lower than the 560A of the battery only, which effectively reduces the polarization of the battery plate, prevents the rise of the internal resistance of the battery, and improves the stable voltage in the starting process.
The most important thing is that the reduction of battery plate polarization is not only conducive to prolong the service life of the battery, but also eliminate the impact of frequent startup on the service life of the battery.
1) Flywheel hybrid battery
Ultra high speed flywheel is installed on the vehicle powered by chemical battery or internal combustion engine to act as a buffer and work together with the system, which is called flywheel hybrid battery. As an auxiliary power source, the role of ultra-high speed flywheel in electric vehicles mainly has two aspects.
(1) Stabilize the power output of the active power source, that is, when the electric vehicle starts, climbs and accelerates, the flywheel accumulator can quickly and energetically provide power (discharge), provide auxiliary power for the active power source, and reduce the power output of the active power source.
(2) Improve the efficiency of energy recovery during braking, that is, when the electric vehicle goes downhill, taxis and braking, the flywheel energy accumulator can store kinetic energy (charging) quickly and with large energy. The charging speed is not affected by the chemical reaction speed of active substances, which improves the efficiency of energy recovery during regenerative braking.
From the vehicle power load diagram, it can be seen that during stable operation, the vehicle power demand is only 1 / 4 of the peak power, so it is urgent to introduce a high-power output energy storage system, and the flywheel system has special requirements to meet this output power and energy density.
The existing research shows that the reasonable design of hybrid flywheel battery can save energy by 30% and reduce exhaust emissions by 75%. Theoretically, the ultra-high speed flywheel has high kinetic energy, but because the general metal materials can not bear the stress caused by its high-speed rotation and the gyro torque suppression problem that is difficult to solve at present, there are still many difficulties in practical application, but this can not bury the advantages of the ultra-high speed flywheel as an auxiliary power source.
With the deepening of research and the invention of high-strength carbon fiber composites, ultra-high speed flywheel will supply power to the power system as an independent energy system like battery and fuel cell. As a long-term goal, it is expected to become an energy system with the highest specific energy (higher than any other battery, even higher than internal combustion engine) and specific power.
2) Battery and battery hybrid
In battery and battery hybrid system: one battery has high specific energy; Another battery has high specific power. Considering the technical maturity and cost, Zn air battery and valve regulated lead-acid battery become the best choice. The specific energy of Zn air battery is up to 230W · H / kg, which is used to improve the driving range of the vehicle. The specific power of valve regulated lead-acid battery is up to 300W / kg, which is used for vehicle acceleration and climbing. This hybrid scheme overcomes the disadvantage that mechanically rechargeable Zn air battery can not recover the regenerative braking energy of the vehicle.
Other possible battery and battery hybrid systems include Zn air and MH / Ni hybrid, Zn air and lithium ion hybrid.
3) Hybrid of fuel cell and battery
Although the fuel cell has a very high specific energy, the specific power is low and it is difficult to recover regenerative energy. Therefore, the hybrid system of fuel cell and battery is also a scheme, in which the battery is selected to make up for the shortcomings of fuel cell. SPFC fuel cell and VRLA, SPFC and MH / Ni, as well as SPFC and Li lon hybrid are commonly used hybrid systems of several fuel cells and batteries.