At the end of March 2021, Weichai Power Co., Ltd. announced a strategic cooperation with FISCHER Spindle Group Ltd (hereinafter referred to as “Swiss Fast Group”) in Switzerland, investing in its fuel cell air compressor business and establishing a joint venture company in China. China has deployed its fuel cell air compressor business.
Fuel cell air compressor is known as the “lung” of fuel cell power system. Its performance will directly affect the operation index of fuel cell power system. It is also one of the most costly core components. It has always been the short-term development of hydrogen fuel cell engines in China. board. The Swiss Fast Group is an enterprise with a history of nearly 100 years. It is mainly engaged in the business of high-speed and high-precision shafts and fuel cell air compressors. Its products are industry-leading in key indicators such as power density, isentropic efficiency and reliability.
Today, the automotive industry is in a period of transformation in which new and old energy sources are alternated. At present, Weichai Power has fully completed the core technology layout of hydrogen fuel cells and solid oxide fuel cells in the field of new energy powertrains, and the key technical indicators are the world’s leading in the field of commercial vehicles and energy and power. Focusing on the goal of “carbon peak” in 2030 and “carbon neutral” in 2060, we will build a world-leading fuel cell industry chain base and lead the development of the global new energy industry.
Through this strategic cooperation, Weichai Power will further vertically extend the layout of the fuel cell industry chain and improve the core competitiveness of fuel cell powertrains. Prior to this, Weichai Power had strategically invested in Ballard, a hydrogen fuel cell manufacturer, and Syris, a solid oxide fuel cell manufacturer, and both were the largest shareholders. The strategic deployment of fuel cell air compressor technology is also an important step for Weichai Power to achieve the strategic goal of “New Energy Business Leading the Development of the Global Industry”.
Importance of air compressor
In the future, fuel cells will be mainly used in power systems and power sources. Power system applications include scenes such as automobiles, motor cars, ships, unmanned aerial vehicles, forklifts, and motorcycles. In the future electrified transportation scenario, fuel cells will become the main power source for long-distance passenger cars and heavy-duty trucks with heavy loads on long distances, and lithium batteries will become the mainstream in passenger cars with short distances and small loads. In addition, distributed power sources will also be a major application scenario for fuel cells in the future.
Domestically, well-known air compressor companies mainly include: Dongde Industrial, Deran Power, State Power Investment, Snowman, Fuyuan, etc.
In foreign countries, the well-known air compressor companies mainly include: UQM from the United States, Toyota Automatic Loom from Japan, Garrett from the United States, Liebherr from Germany, Bosch from Germany, Aeristech from the United Kingdom, Vairex from the United States, Rotrex from Denmark, Feisu from Switzerland and Hitachi from Japan.
Among the core components of the fuel cell, the stack is the “heart” of the fuel cell, providing electricity; the air compressor is the “lung” of the fuel cell, providing the stack with a proper amount of oxygen at an appropriate pressure. In engineering practice, in addition to the electric stack, the air compressor is often the bottleneck that needs to be solved most. Which type of air compressor is more suitable for fuel cells requires verification of its excellence and reliability in terms of variable operating conditions, cold start, vibration, start and stop, etc. This process requires a large number of samples and time.
The basic requirements of air compressors for proton exchange membrane fuel cells are as follows:
1. No oil. The lubricating oil film covering the proton exchange membrane and the catalyst will isolate the electrochemical reaction of hydrogen and oxygen
2. High efficiency. Air compressor parasitic power seriously affects system efficiency
3. Miniaturization and low cost. Power density and cost limiting factors promote miniaturization and low cost
4. Low noise. Air compressor noise is the main noise source of fuel cell engines
5. Wide range of characteristics. To meet the changing needs of ambient temperature, altitude, etc., air compressors need to have wider MAP characteristics
6. Fast dynamic response. The vehicle power system will move from “hydrogen-electric-lithium-electric hybrid” to hydrogen-electric full-power drive, and air compressors need to provide designated compressed air in time under each working condition.
Mechanical and performance characteristics of different types of air compressors
At present, there are mainly three types of air compressors for fuel cells: centrifugal, roots, and screw.
(1) Centrifugal air compressor
By rotating the impeller to do work on the gas, in the impeller and diffuser flow channel, the use of centrifugal life and decompression expansion effect, the mechanical engineering is converted into gas internal energy. It has the characteristics of compact structure, fast response, long life and high efficiency, but the working area of centrifugal air compressor is narrow, and surge will occur at low flow and high pressure, and surge will seriously affect the life.
Centrifugal air compressors usually have two options. One is the use of a speed increaser + ball bearing. The motor speed is low. The disadvantage is that the structure is complex, it needs oil lubrication (there is the possibility of polluting oxygen), and the life of the speed increaser is short; The other is a dynamic pressure air suspension bearing, which has a small air gap and dry friction during starting. It has high requirements for bearing accuracy and wear resistance, and has certain restrictions on the resistance to vibration and impact and the number of starts and stops. Denmark’s ROTREX air compressor mainly provides pump heads, and its speed increaser is shown in the figure below.
At present, the main bottleneck of dynamic pressure air bearings lies in the coating. The following figure shows a schematic diagram of radial bearings. There are two types of air bearing centrifugal air compressors, one is single-stage compression, with narrow working area, high motor speed and low pressure ratio; the other is two-stage compression, with wide working area, low motor speed and high pressure ratio. .
The Honda FCV Clarity fuel cell vehicle is equipped with a Garrett two-stage centrifugal electric air compressor to achieve an ultra-high pressure ratio. Increasing the inlet air pressure can increase the partial pressure of oxygen in the air into the stack, effectively slowing down the sudden voltage drop of the stack under high current density. Although the increase in inlet air pressure leads to an increase in the energy consumption of the air compressor, the fuel cell will increase the air pressure. The net power of the system will also increase at high pressure ratios. In addition, increasing the inlet pressure optimizes water management capabilities and reduces the volume of the humidifier. According to reports, the domestic Sigatobo has also achieved mass production of two-stage centrifugal air compressors. For the principle of two-stage supercharging, please refer to the Secret of Two-stage Supercharging of Honda Clarity Fuel Cell Air Compressor.
(2) Roots type air compressor
The cavity between the rotors of the Roots-type air compressor does not change. The increase in pressure depends on squeezing the air into the smaller external cavity. The density of the air in the external cavity continues to increase, thereby generating pressure. External compression”.
Roots type adopts volume compression between the rotor and the pump casing. The distance between the rotor and the pump casing is less than 1mm. The high temperature thermal expansion and contraction are easy to jam, and the mechanical strength of the pump casing is high. Roots-type rotors are produced by extrusion, and external compression does not have high precision and wear resistance on the rotor, and the production cost is low. Due to the use of ball bearings, it is necessary to add engine oil for lubrication.
The Roots-type air compressor has a wide working range and is suitable for full-power fuel cell engines. However, the high-frequency noise of the Roots-type air compressor is very high, and the noise spectrum of each operating point is different, so it is necessary to design a muffler specifically.
Toyota used scroll air compressors in the early days, and later changed to six-page Roots air compressors. In response to the noise problem, an intercooler was added to the air compressor outlet to reduce noise. As shown below.
(3) Twin screw air compressor
By forming a compression cavity between the screws, the cavity between the male and female screws gradually shrinks, and the gas house gradually rises, which is called “internal compression”.
Internal compression requires high air quality. Particles in the air will wear the screw and cause sealing problems. Generally, there is a wear-resistant coating on the screw. The internal compression is easy to form a very high compression ratio, and the power consumption is lower than that of the centrifugal Roots type at the high pressure ratio. The shape of the screw and the precision of the bearing are required to be high, and the ball bearing is used, which needs to be lubricated with oil.
The twin-screw air compressor has a wide working range and is suitable for full-power fuel cell engines. However, the high-frequency noise of the twin-screw air compressor is very large, and the noise spectrum of each operating point is different, so it is necessary to design a muffler specifically
At present, only Fujian Snowman has achieved mass production of fuel cell twin-screw air compressors on the market.
Regarding the problem of oil, at present, there are two main ways to reduce the oil penetration to the air side. On the one hand, the oil cavity is an open cavity, and the oil vapor is discharged through the exhaust plug, while keeping the pressure (equal to atmospheric pressure) lower than the air side; on the other hand, On the one hand, in order to prevent the oil from penetrating when the bearing is stationary, the oil level is generally lower than that of the bearing. As long as the oil needs to be added, there is a risk, and the oil content at the outlet of the air compressor is difficult to measure.
Air compressor selection stack is matched with air compressor Map
1. Surge line distance
The centrifugal air compressor has a surge area, and the general design working condition is at least 30% of the flow from the surge line. In practical applications, usually when the stack is working at low power, the air compressor is already in the surge area and needs to be bypassed to release part of the air.
2. Overload condition margin
Outside the rated operating point, the overload operating point is usually designed according to 110% kW, and the maximum operating point is out of the margin.
3. High efficiency area
Generally, the compressor efficiency is marked on the Map. The efficiency is lower at low speeds, and the total efficiency needs to be considered. Frequency converter efficiency is 94~96%, motor efficiency is 87~95%, transmission efficiency is 70~90%, compressor variable efficiency is 80~85%. Therefore, the ratio of the useful power of the general air compressor to the input power of the power supply is about 50%.
4. Curve slope: flow fluctuation
It can be seen on the MAP chart that the air compressor working line and the speed line intersect. The smaller the absolute value of the slope of the speed line at the intersection, the worse the stability of the working condition.
5. Influence of ambient temperature
Under high temperature conditions, the Map moves to the left, and the adaptability of the maximum operating conditions needs to be considered; under low temperature conditions, the surge line moves to the right, and the distance between the operating point and the surge line needs to be considered. For centrifugal air compressors, the inlet air temperature of the air compressor has a certain influence on the pressure ratio of the air compressor.
6. The influence of plateau altitude on Map
On the plateau, the environmental pressure decreases and the air compressor Map moves to the left. It is also necessary to consider the adaptability of the maximum operating point. Usually, when it is unable to adapt, it will adjust the control strategy, such as correcting the fuel cell demand power and pull current.
7. Supply voltage, pre-charge
Air compressor voltage currently has two main specifications, 250-450 VDC and 450-750 VDC. The voltage is mainly adapted to the voltage of the lithium battery on the hybrid power of hydrogen and lithium battery. Although the Chinese standard GBT 31466 stipulates the high voltage specifications of electric vehicles, the actual situation is somewhat different. When the air compressor is started, the pre-charging circuit is generally used for soft start, so that the voltage rises slowly and the influence of electromagnetic interference is reduced. At present, the air compressor controller has very few pre-charge functions, and the workaround is to do it in DC-DC.
It generally complies with CAN 2.0B, but it has not been carried out in accordance with SAE-J1939 or ISO11898. Electric vehicles have more high-voltage electrical appliances, which have a great impact on communications. Under normal circumstances, the controller has done drive circuit isolation and CAN transceiver isolation.
9. Power consumption
The air compressor is the largest electrical energy consumer among the auxiliary parts of the fuel cell engine. Generally, the power consumption of a 50kW stack air compressor does not exceed 8 kW, and the power consumption of an 80 kW stack air compressor does not exceed 12 kW. The difference in energy consumption of different air compressors can reach 10-20%.
The noise of centrifugal air compressors is generally in the range of 80-90 dBA, the original noise of Roots and screw air compressors is in the range of 110-120 dBA, and the noise of Roots type and screw type is high-frequency noise, and the noise of different operating points is very different. Big.
11. Air compressor cooling
The coolant inlet temperature requirements of different air compressors are generally between 50 and 75°C, which is mainly limited by the temperature resistance of the controller chip. Usually, the air compressor cooling passage is separated from the electric stack, because of the large difference in the temperature of the cooling liquid. If the same kind of coolant is used, it is necessary to consider whether the ion extraction rate of the air compressor meets the requirements. Different types of air compressors have different cooling power and cooling flow resistance, and it is necessary to integrate the overall cooling flow resistance of the fuel cell engine for matching design.
Surge line confirmation method
At the same speed, the back pressure valve gradually closes, and when it is close to the surge line, the gradient of the back pressure valve opening change is small. When the back pressure is almost constant and there is no decreasing trend, the abnormal sound of the centrifuge can be heard. At this time, it has already surged, and it is found that it needs to leave the surge area 1-2s after the surge.
Flow accuracy, pressure ratio accuracy
In the air compressor Map test, in order to ensure the accuracy of the flow, temperature and pressure sensors and flow meters are generally installed at the inlet and outlet of the air compressor, and the inlet and outlet flow meters generally adopt different forms.
Shutdown energy release
When the air compressor is working at the highest speed, it is generally not suitable to stop it directly. Since the electrical energy generated during the shutdown process needs to be released, if the power used is not a bidirectional load, the returned electrical energy cannot be absorbed, which may cause some harm.
Air bearings and twin-screw air compressors all have high requirements for air quality. Confirm that the air filter has been reliably connected before testing.
In the study of fuel cell test conditions, the steady-state conditions are the main ones, including idling, starting, accelerating, decelerating and stopping. There are two main factors. On the one hand, because fuel cells are more inclined to hybrid systems, the working conditions of fuel cells are relatively stable; on the other hand, fuel cells only discharge water without considering pollutant emissions. The durability assessment of an air compressor generally includes full load working conditions and variable working conditions. Variable working conditions are generally set with reference to NEDC working conditions of the vehicle.
At present, the cold start of fuel cells can generally be started in a low temperature environment of -20℃, and some manufacturers can achieve -30℃. The air compressor needs to perform a cold start test in a high humidity and low temperature environment.
At present, the random vibration 5 g assessment and the mechanical shock 30 g assessment are generally performed according to the battery pack vibration standard. For air bearing compressors, endurance tests are required for the number of starts and stops.
The air compressor controller needs to complete the EMC assessment.