The Effect of the Isolator Size on the Efficiency of Rotary Piston Compressors

A hybrid renewable-hydrogen green energy system combines renewable energy sources with hydrogen production and storage technologies to create a sustainable and efficient energy system. One of the major components of such systems is compressors, which inﬂuence the system’s overall efficiency. Therefore, this research paper will study design modiﬁcations that improve the efficiency of these components. More precisely, this study examines the correlation between a concentric rotary piston compressor isolator size and efficiency. The objective is to determine the signiﬁcance of size on the compressor’s performance. Two distinct mechanisms and operational designs employed in such compressors are investigated. Irrespective of the compressor design, it is revealed that the isolator’s diameter considerably impacts the pressure ratio of these rotary compressors. Speciﬁcally, the conclusion is that a larger isolator increases efficiency; a 35% larger RSP diameter results in a 145% increase in peak pressure for Mechanism 1. A 100% larger RSP diameter yields a 180% boost in peak pressure for Mechanism 2.

used to categorize energy storage devices, such as capacity and storage period, lifespan, safety, environmental impact, cost and recyclability [3].
Hydrogen energy storage (HES) systems stand apart from other renewable energy storage systems owing to their versatility and ability to deliver multiple services [4]. This attribute is vital for grid operators to sustain system reliability and incorporate RES into infrastructures like electricity, heating, and transportation [4,5]. HES systems facilitate large-scale energy storage, extending from 1 GWh to 1 TWh, whereas battery capacities usually range between 10 kWh and 10 MWh [5].
A compressor is an essential piece of equipment for HES designed to raise the pressure of gases or vapors by decreasing their volume. Various compressor types exist. Two common types are the positive displacement and dynamic compressors. Positive displacement compressors function by trapping the working medium followed by volume reduction, whereas dynamic compressors increase the pressure of the working medium by using its kinetic energy. Positive displacement compressors are distinguished from dynamic turbo-compressors with axial or radial flow configurations by their higher compression ratios and lower mass flow rates [6].
In many systems that depend on compressed fluids, compressors play a crucial role and are widely utilized for energy storage purposes. It is estimated that around 10% of global industrial electrical energy consumption is due to compressed gas [7] and rises to 20% if commercial and residential requirements like air conditioning systems, portable tools, pneumatic heating, ventilation, air pumps, etc. [8] are included. To cut down energy consumption, adopting energy-saving measures is crucial for the gas-compression industry which faces an increasing global demand. mechanical loss, resulting in a greater than 14% improvement in the rolling piston compressor's performance coefficient [11]. Liu et al. employed the gradient-search technique to optimize scroll compressor bearing components and observed a minimum of 14.1% reduction in frictional losses at these components [12]. Etemad and Nieter pinpointed the starting angle and wrap height as critical geometric parameters of scroll compressors but provided no numerical data; their work merely identified parameters with the greatest impact on overall efficiency [13]. According to Hirayama et al., enhancing a rotary vane compressor utilized in refrigeration systems with an extra cylinder and an additional bearing on the main shaft can improve reliability, capacity, performance by as much as 9% [14]. Meng et al. proposed an innovative rolling piston rotary compressor cylinder design that elevated electric efficiency by 4.43% [15]. Savvakis et al. managed to lower the secondary peak pressure of rotary piston compressors by 130% and increasing the efficiency to nearly 14% [16]. Noh et al. examined how cylinder slenderness ratio affects rolling piston compressor performance and discovered that reducing this ratio increased volumetric efficiency by 6.3%, culminating in a total efficiency improvement of 3.7% [17]. Gu et al. scrutinized vane tip gap height sensitivity and deduced that as the gap broadens (0.01-0.05 mm), isentropic efficiency declines by 18.26% [18].
The described piston rotary technology falls under the positive displacement compressor category. This concept is based on a concentric rotary technology that has existed for 40 years and is utilized in internal combustion engines, pumps, expanders, and compressors. The construction of this particular concentric rotary device comprises four primary components: two rotors, a piston, and a housing. One rotor (2) (known as the piston rotor) is connected to the piston (1), while the other rotor (3) (referred to as the isolator rotor) features a cavity (4) that accommodates the piston (1) when it aligns with the isolator rotor's (3) outer edge ( Figure 1). The isolator rotor's (3) function is to confine the working fluid ahead of the piston (1), generating a compression chamber (depicted in orange in Figure 1) between the piston, the isolator rotor's periphery, and the housing.  The four concept designs based on this operating principle are the following. In 1980, Emmanouil Andreas Pelekis originated and patented this approach by developing a compression mechanism utilizing concentric circular motion of vanes within a segmented annular region [19]. His patent dealt with a compression mechanism that achieves compression by a circular concentric motion of a vane (A & B) inside a segmented annular region. In Figure 2  In all these discussed designs, a crucial factor that remains unaddressed by inventors pertains to the size of the isolator rotor. This rotor connects to two distinct shafts, which are concurrently linked through a gear pair. As the piston operates, it compresses the working fluid until reaching its primary peak pressure (denoted in Figure 1 by label 5), which subsequently becomes the output pressure of the Green Energy and Environmental Technology 5/24 compressor following the completion of the compression process. Nevertheless, the isolator's size is critically important for achieving optimal performance in these compressors.
Generally, the compression ratio (CR) is an integral parameter associated with the performance of all types of compressors. For rotary piston compressors, CR heavily relies on the duration of interaction between piston and RSP. A longer interaction time results in a reduced amount of air that can be trapped and compressed by the piston. Consequently, this study aims to minimize this interaction time.
The SARC compressor, illustrated in Figure 1, exhibits several advantages compared to other compressor types. First, it generates enhanced isentropic efficiency (>90%). The absence of contact between moving components or stationary parts enables oil-free operation. Moreover, the avoidance of rotating components to contact stationary machine elements restricts mechanical losses to bearing components only and the frictionless movement of rotating elements along with oil-free assembly contributes to lower construction and maintenance expenses.
Notably, this concentric rotary technology can attain greater speeds than reciprocating and eccentric rotary compressors as it does not encounter any direction-altering inertial forces during operation. Consequently, minimal inertial forces induce no stress-related issues at elevated speeds. Furthermore, these benefits are accompanied by a simple, lightweight design leading to reduced construction costs and increased profitability across various operations.

Proposed solution
This research demonstrates that the CR increases as the RSP diameter increases It is important to note that while temperature does increase, it does so to a lesser extent compared to pressure. Nevertheless, consideration of thermal expansion due to elevated temperatures is crucial. Furthermore, heat management should address higher temperatures arising from an increased CR, and accommodating larger diameters must take into account greater dynamic loads and spatial constraints. To summarize, identifying an optimal diameter constitutes an optimization problem involving all previously mentioned parameters.
Green Energy and Environmental Technology 6/24  As depicted in Figure 4, a higher CR also results from a larger RSP reducing the size of compression chamber (6). A portion of this chamber is occupied by the RSP

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(depicted by region 7 in the same figure). However, the shorter interaction time between piston and RSP is the main reason for the higher CR rather than the smaller compression chamber (6) caused by the region 7.

Methodology
Initially, a watertight 3D fluid model was developed using the pre-processor  the concentric configuration of this compressor permits high-speed operation.
Notwithstanding, rotary vane compressors [14] surpass 4000 rpm, while screw compressors reach beyond 6000 rpm [26]. The maximum achievable speed for the current compressor is not limited to 4000 rpm; its weight-balanced design enables even higher speeds (pending patent application prior to publication). In theory, it operates at an equivalent rotational speed to a gas turbine (e.g., 30,000 rpm).
However, based on CFD simulations, the peak pressure ratio (22.8) is attained at 4000 rpm and optimal efficiency at 6400 rpm for the present design. High speed is crucial for maintaining sealing efficacy and promoting heavy-duty industrial usage with elevated flow rates. Consequently, SARC is a smaller and lighter machine in comparison to other commercial compressors due to high flow rates and high speeds.
Conversely, flow rates are directly proportional to speed-requiring machinery adjustments such as smaller piston orbit or diameter for low flow rate applications while maintaining high-speed performance. Regarding boundary conditions, smooth walls were assumed and a symmetry plane was employed to expedite convergence. Neither of these factors adversely affect result accuracy because the focus lies on a qualitative comparison of RSP sizes rather than achieving a more realistic simulation outcome. Furthermore, a sealing feature was implemented with the symmetry plane to ensure compressed air remains within the domain. The fluid flow is governed by the Redlich-Kwong equation for compressible gas and air which offers greater accuracy than Van der Waals or ideal gas equations. The wall properties are considered to be smooth, having an absolute roughness set to 0.
As for turbulence modelling, the RNG k-ε RANS model was chosen due to its ability to enhance accuracy in rotating flow scenarios [29]. The scheme for the solution method is transient, with the convergence criterion based on the PISO algorithm alongside a tolerance value of 10 −3 .
Upon defining a grid-independent mesh, the subsequent convergence criterion stipulated that maximum pressure and temperature variations between subsequent cycle calculations must not exceed 3%. To achieve a constant primary peak pressure value, each case required simulation over five or six cycles.
The compression chamber lacks a discharge port; instead utilizing a blind configuration because discharge port location, size and timing significantly According to a previous optimization study (pending paper approval), which determined the intake port's position and size, the optimal intake port (7) is tangential to the intake chamber's outer periphery and proximate to the isolator periphery. The design should be circular and of minimal dimensions, considering the specific compressor size. In this instance, the diameter measures 12.5 mm (see   The laboratory is also QMS-certified and adheres to ISO 9001:2008 requirements.

Experimental procedure and verification
Pressure measurements were obtained using the AVL X-Ion high-speed, modular data acquisition system and power analyzer. Capable of capturing signals at exceptionally high-frequency sampling rates (>100 kHz), the AVL X-Ion connects to Kistler 5011 signal amplifier tied to an in-cylinder pressure sensor. During experimentation, a variable frequency motor maintained a consistent speed of 2576 rpm while driving the compressor. Outlet pressure readings were obtained from an outlet pressure sensor situated where peak pressure development was theorized. Notably, measured peak pressure at 2576 rpm closely aligned with CFD outcomes. Attributed to the first five rotations, CFD simulations indicated a peak pressure of 20.477 bar, while measurements during testing exceeded 1000 rotations.
As illustrated in Figure 9, peak pressure values oscillated between 15.53 and 21.29 bar.
The horizontal axis in the graph represents time in milliseconds (msec), while the vertical axis indicates pressure in bar.

Mechanism 1
In order to expedite the simulation process, the RSP size for the first mechanism is not made twice as large as the nominal size.   development across both RSP sizes can be observed in Figure 12. A 27.35% increase in RSP diameter causes the peak pressure to occur 10°later, resulting in an elevated CR and peak pressure value. Peak values for each RSP size are presented in Table 2.     (RSP), necessitating alternative materials or a modified cooling management strategy due to the higher CR.
Similarly, Figure 14 reveals that density follows a pattern comparable to pressure.
The maximum value is considerably higher with a larger RSP diameter, and the two curves closely resemble each other, except for the area surrounding the compression process' completion where the pressure/density rises substantially more rapidly.  Consequently, it requires a smaller arc to encompass and facilitate the piston. This shorter interaction arc primarily accounts for the elevated CR and peak pressure observed with a larger RSP ( Figure 16).
Interestingly, despite having a higher peak temperature, lower temperatures develop along the side areas of an enlarged RSP.
Green Energy and Environmental Technology 16/24  Convergence diagrams for this mechanism are depicted in Figures 19 and 20, which display analogous behavior to Mechanism 1 by converging after four cycles. The temperature exhibits a quicker convergence than pressure; however, an additional cycle is required to achieve a divergence goal below 3%.
The progression of pressure, temperature, and density is illustrated in Figures 21,   22 and 23, respectively.
Green Energy and Environmental Technology 17/24        at that moment. The values are obviously higher for the bigger RSP, since the CR in the case of a bigger RSP is higher.
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Conclusion
Amplifying the rotary sliding port (isolator) diameter enhances the compression ratio and pressure output of rotary piston compressors without altering any other critical components, irrespective of the compressor's mechanism. However, Mechanism 1 attains a superior compression ratio compared to Mechanism 2 due to its involvement with the RSP periphery as part of the compression chamber.
Consequently, increasing the RSP diameter reduces the end volume of the compression chamber (as depicted in Figure 7), leading to a further escalation in peak-pressure value relative to Mechanism 2.