Semi-rotary and Linear Actuators for Compressed Air Energy Storage and Energy Efficient Pneumatic Applications

The motivation for the use of compressed air as an energy carrier and as a storage means for many industrial applications resides in the simplicity and low-cost conditions of its implementation. However, conventional pneumatic technology suffers from a very low energetic efficiency even if the production, use and recycling of the components can be said to be environmentally friendly, and it does not use problematic materials. This introductory chapter positions pneumatic technology and discusses a possible extension of the applications to the sector of energy storage in a general manner. The chapter gives the historical background of the presented developments of the book and gives an overview of the content of the document. 

The elementary principles related to compressed air are presented, describing the basic compression and expansion characteristics. The adiabatic, polytropic and isothermal phenomena are described together with the definitions of the energy content of a given volume. Different loss factors related to compressed air are enumerated together with the advantages and drawbacks of pneumatic technology. Then, the possibility of storing energy under low pressure conditions as the so-called Underwater CAES system is discussed. Such systems have the interesting property of being realized with a very low amount of grey energy.

The chapter presents the main principle on which the proposals of this book are based. In this principle, the energetic efficiency of pneumatic actuators is strongly increased by adding an amount of expansion work to the classical work produced by constant pressure displacement. Such a principle has already been applied in steam machines at the beginning of the 20th century or in existing pneumatic converters used as motors for automotive vehicles.

This chapter presents the first example of the combination of two actuators of different volumes with which the principle of adding an expansion work can be realized. The two semi-rotary actuators are mechanically coupled and describe an oscillatory motion. Then the oscillatory motion is transmitted to an electric generator through a so-called motion rectifier. The structure of the new system is presented with the control valves and control circuitry. The different variables of the system as pressure, torques, and mechanical work are calculated by simulation. The efficiency of the new system is calculated and compared with the efficiency of a single actuator without expansion. The principle of adding an expansion work with semi-rotary actuators is then presented but with one actuator only where the expansion occurs in the same and unique chamber. Efficiency, torque waveform and produced mechanical work are presented, as well the control circuits. The power reversibility of a system using semi-rotary actuators is addressed, and a solution with a crankshaft is studied. 

 A pneumatic motor is studied where the pneumatic actuators consist of linear cylinders. This mechanical principle based on the use of a crankshaft and piston rods has the inherent property of being reversible. For this system, the same principle of adding expansion work with an additional volume is applied as it was in the previous chapter for semi-rotary actuators. The mechanical behavior of the crankshaft and piston rod is described, and the following pneumatic displacement and expansion work is simulated. Two different architectures are simulated, namely, first, a system with pistons operating in phase and second, with alternating pistons. The energy efficiency of the new motor is calculated and compared with the efficiency of a system using a single linear cylinder. Further, the principle of realizing the expansion work in the same cylinder as for the displacement work is applied to the motor with linear cylinders. The torque, power and converted work are presented with the simulation results. The study is completed with the presentation of a physical demonstrator system.

The method of adding expansion work to pneumatic actuators is studied for classical linear cylinders. The operating principle of new cylinder assemblies is presented. A first simulation set illustrates the performance of the new assembly and tries to define the parameters of a single cylinder which produces the same mechanical performance. Acceleration, speed and reached position within a given time are the conditions for the comparison. Then, the air consumption of both compared systems is calculated. With an experimental set-up, a parasitic effect is observed, which consists of a pre-expansion transient due to parasitic dead volumes related to the tubing and internal volumes of the valves. A second assembly is realized with larger volumetry in order to observe the dependency of the parasitic effect from the size of the cylinders. For the control, a system with simpler control valves is also studied. 

In Chapter 6, the parasitic effect of pre-expansion due to the presence of dead volumes has been observed. This effect will now be analyzed in more detail in this chapter. Especially the influence of this pre-expansion on the total produced mechanical work is calculated. Different pre-expansion factors are considered and the mechanical work of an ideal system without pre-expansion will be compared with the reduced mechanical work of a cylinder assembly affected by pre-expansion. 

In this chapter, an application example is studied where the energetic efficiency of a pneumatically driven device is of importance. The chosen example consists of an air-driven gas booster used as a Hydrogen compressor in a refuel station for H2 driven cars. The needed force for the driving of the compression cylinders is calculated, and a new pneumatic motor based on the principle of adding expansion work is proposed. The new motor is designed for sufficient effort for moving the mobile equipment under the maximum compression force. The air consumption of the new system is calculated, and finally, the air savings in comparison to a classical air-driven booster. The simulation is completed with a dynamic part showing the dynamic performance in terms of velocity and time to reach the final position of the pistons.

The chapter serves as a conclusion to the different principles, systems and application examples described in this book. From the original air-powered diving lamp driven by a semi-rotary pneumatic actuator to the final example of the gas booster, different systems have been proposed with a large benefit in terms of energetic efficiency or in other terms, in the reduced amount of air consumed.