The efficiency of reciprocating Hydrogen Compressors varies under different operating conditions and when compared to different types of compressors. The following is a specific analysis:
Compared to diaphragm compressors
Under low flow and high pressure conditions, the efficiency is similar: in scenarios where hydrogen needs to be compressed to high pressure and the flow demand is not high, reciprocating compressors and diaphragm compressors can both function well, with little difference in efficiency. For example, in some small-scale hydrogen high-pressure storage experiments, both can compress hydrogen to a higher pressure, but diaphragm compressors may have smaller flow rates due to membrane limitations, while reciprocating compressors can still maintain good efficiency when processing a certain flow rate.
Under high flow conditions, reciprocating compressors have higher efficiency: when faced with the demand for high flow hydrogen compression, the efficiency advantage of reciprocating compressors is more obvious. Due to the relatively small displacement of diaphragm compressors, multiple devices may need to be operated in parallel under high flow demands, which increases equipment investment and operating costs, and overall efficiency may not be as good as reciprocating compressors. For example, in large hydrogen production plants, a large amount of hydrogen needs to be compressed to medium and high pressure for subsequent processes. Reciprocating compressors can process high flow hydrogen more efficiently through multi-stage compression.
Compared to centrifugal compressors
Under low flow and high pressure conditions, reciprocating compressors have higher efficiency: centrifugal compressors are prone to problems such as surge at low flow rates, leading to a decrease in efficiency, while reciprocating compressors can work stably through multi-stage compression in low flow and high pressure ratio compression tasks, resulting in relatively high efficiency. For example, in some fine chemical production, high hydrogen pressure is required but low flow rate is needed. Reciprocating compressors can flexibly adjust the compression stage and flow rate according to demand to maintain high efficiency.
Centrifugal compressors have higher efficiency under high flow and medium low pressure conditions: Centrifugal compressors are suitable for hydrogen compression scenarios with high flow and medium low pressure. In this operating condition, centrifugal compressors can continuously and efficiently process large amounts of gas, and the rotational motion of their impellers makes the flow of gas smoother, with relatively less energy loss and higher efficiency than reciprocating compressors. For example, in large-scale hydrogen pipeline boosting stations, when a large amount of hydrogen needs to be compressed and transported within a relatively low pressure range, the efficiency advantage of centrifugal compressors is obvious, which can reduce operating costs.
Compared to liquid driven compressors
In terms of compression efficiency, reciprocating compressors are superior: the compression process of reciprocating compressors is relatively direct, and gas compression is achieved through the reciprocating motion of the piston in the cylinder, which can achieve high compression efficiency under appropriate working conditions. Although liquid driven compressors have a simple structure and easy operation, the single-stage pressure ratio is relatively small, and multi-stage compression is often required to achieve higher pressure. In this process, energy loss is relatively large, and the overall compression efficiency may not be as good as reciprocating compressors.
Each has its own advantages in terms of energy utilization efficiency: reciprocating compressors require driving devices such as motors to provide power during operation, and their energy utilization efficiency is related to factors such as the efficiency of the motor and the mechanical transmission efficiency of the compressor. Liquid driven compressors are driven by hydraulic oil, and in some cases, energy recovery technologies such as hydraulic systems can be used to improve energy utilization efficiency. However, if the hydraulic system is not designed or maintained properly, it may also lead to energy waste. For example, in some scenarios where waste heat can be utilized to drive hydraulic systems, the energy utilization efficiency of liquid driven compressors may be improved, but in conventional electric drive situations, reciprocating compressors usually have an advantage in energy utilization efficiency.
Compared to adsorption compressors
Reciprocating compressors have advantages in compression speed and efficiency: Adsorption compressors utilize the adsorption and desorption characteristics of adsorbents for hydrogen gas to achieve compression, and their compression process is relatively slow and inefficient. The reciprocating compressor can quickly suck in and compress hydrogen gas through the rapid reciprocating motion of the piston, and can complete more compression cycles per unit time, thus achieving higher compression speed and overall efficiency than adsorption compressors. For example, in situations where hydrogen needs to be quickly compressed to a certain pressure for filling or supply, reciprocating compressors can meet the demand faster.
Under specific operating conditions, adsorption compressors have certain energy-saving advantages: adsorption compressors have certain energy-saving advantages under low pressure, low flow conditions, and in some situations where compressor operating costs are sensitive. Because it does not require complex mechanical moving parts, the energy consumption during operation mainly lies in the heating and cooling processes of the adsorbent. Under such operating conditions, reciprocating compressors may consume more energy due to mechanical friction and other factors. However, overall, in most common hydrogen compression conditions, the efficiency of reciprocating compressors is higher than that of adsorption compressors.
The key factors affecting the efficiency of reciprocating hydrogen compressors mainly include the following aspects:
The structure and components of the compressor itself
Clearance volume: Clearance volume refers to the volume between the top surface of the piston and the bottom surface of the cylinder head when the piston reaches the top dead center. The larger the clearance volume, the more space the residual gas occupies after expansion, and the less fresh hydrogen gas is sucked in, which reduces the volumetric efficiency of the compressor and affects the overall efficiency. For example, when the clearance volume is too large, the high-pressure gas remaining in the clearance expands during the piston return, occupying some of the suction space and reducing the actual amount of hydrogen gas sucked in, resulting in more cycles and energy consumption for compressing the same amount of hydrogen gas.
Valve performance: The opening and closing time, sealing performance, and flow capacity of the valve have a significant impact on the efficiency of the compressor. If the gas valve cannot be opened and closed in a timely and accurate manner, it will cause gas leakage, reflux, and increase energy loss. For example, if the air valve is not tightly sealed, high-pressure gas will leak through the air valve to the suction side during the compression process, resulting in a decrease in compression efficiency; If the flow capacity of the air valve is insufficient, it will limit the suction and discharge speed of the gas, affecting the displacement and efficiency of the compressor.
Piston ring sealing: The piston ring is used to seal the gap between the piston and the cylinder wall, preventing hydrogen gas leakage. If the piston ring is severely worn or improperly installed, resulting in a decrease in sealing performance, hydrogen gas will leak from the gap between the piston ring and the cylinder wall, causing a portion of the energy during the compression process to be used to compress the leaked gas, reducing the efficiency of the compressor. Meanwhile, hydrogen leakage may also lead to abnormal pressure between compression stages, further affecting the overall performance of the compressor.
The fit between cylinder and piston: The fit clearance between cylinder and piston should be appropriate. Excessive gap will increase gas leakage; A small gap will increase frictional resistance, both of which will reduce the efficiency of the compressor. For example, when the clearance is too small, the friction force of the piston moving inside the cylinder increases, requiring more energy to overcome the friction force. At the same time, it may also lead to increased wear of the cylinder and piston, affecting the service life and operating efficiency of the equipment.
In terms of operating conditions
Inhalation temperature: If the inhalation temperature is too high, the specific volume of gas increases, the mass of hydrogen gas inhaled under the same volume decreases, the displacement of the compressor decreases, and the efficiency also decreases accordingly. Moreover, high-temperature gases require more energy to reach the set pressure during compression, which increases energy consumption. For example, in the high temperature environment of summer, if the suction cooling system of the compressor is not effective and the suction temperature increases, the operating load of the compressor will be significantly increased, and the efficiency will decrease.
Exhaust pressure: The higher the exhaust pressure, the greater the compression ratio, and the more energy the compressor needs to consume. When the exhaust pressure exceeds the design value, the operating efficiency of the compressor will significantly decrease, and may even lead to equipment failure. On the contrary, if the exhaust pressure is too low, although the energy consumption is reduced, it may not be able to meet the requirements of subsequent processes for hydrogen pressure. In addition, the allocation of compression ratios at all levels is also important. Unreasonable allocation of compression ratios can lead to excessive load on a certain level, affecting overall efficiency.
Gas flow rate: Within a certain range, the efficiency of the compressor will increase with the increase of gas flow rate. However, when the flow rate exceeds the rated flow rate of the compressor, the efficiency of the compressor will decrease due to the increased resistance of components such as valves and pipelines, as well as the rapid movement of the piston. Moreover, excessive flow fluctuations can also make the operation of the compressor unstable, affecting efficiency.
Cooling effect: The cooling effect of the intercooler and cylinder cooling system is crucial to the efficiency of the compressor. Good cooling effect can reduce the temperature of gas during compression, making the compression process closer to isothermal compression and reducing energy consumption. If the cooling system malfunctions or the cooling water volume is insufficient or the water temperature is too high, it will cause the gas temperature to rise, which not only affects the efficiency of the compressor, but may also cause a decrease in the performance of the lubricating oil and exacerbate component wear.
Lubrication and sealing aspects
Lubricating oil quality and supply: The viscosity, cleanliness, oxidation resistance and other quality indicators of lubricating oil directly affect the lubrication effect of the compressor. Lubricating oil with inappropriate viscosity cannot form a good oil film on the surface of moving parts, increasing friction loss; Lubricating oil containing impurities or oxidized deterioration can block the oil circuit, exacerbate component wear, and reduce the mechanical efficiency of the compressor. Meanwhile, insufficient supply or unstable pressure of lubricating oil can also lead to poor lubrication, affecting the normal operation and efficiency of the compressor.
Sealing performance: In addition to piston ring sealing, the sealing performance of compressor shaft seals, pipeline connection parts, etc. is also important. Any leakage in any part will result in hydrogen loss, requiring additional energy for the compressor to compensate for the pressure loss caused by the leakage and reduce efficiency. For example, a damaged seal at the shaft seal can cause hydrogen gas to leak into the atmosphere, which not only wastes energy but may also pose safety hazards.
In terms of drive and control systems
Drive motor efficiency: The efficiency of the drive motor directly affects the overall energy consumption of the reciprocating hydrogen compressor. Efficient motors can more effectively convert electrical energy into mechanical energy, providing power for compressors. If the motor efficiency is low, a large amount of electrical energy will be converted into heat energy and wasted, increasing operating costs. For example, using old and inefficient motors to drive compressors will significantly increase power consumption compared to using new high-efficiency energy-saving motors.
Control system accuracy: Advanced and precise control systems can adjust parameters in real-time based on the operating conditions of the compressor, such as speed, load, etc., to ensure that the compressor always operates in the high-efficiency range. If the precision of the control system is insufficient and cannot be accurately adjusted, it may cause the operating parameters of the compressor to deviate from the optimal values, affecting efficiency. For example, when the gas flow rate changes, the control system cannot adjust the compressor speed in a timely manner, resulting in a situation of "big horse pulling small car" or "small horse pulling big car", causing energy waste or compressor overload.
