Centrifugal Compressors for Fuel Cells

The development of fuel cell technologies and improvements in fuel cells power densities combine to make the use of fuel cells possible in different power sectors as primary or secondary power sources for commercial purpose, residential power requirements, and automobiles, etc. The fuel cell harnesses the chemical energy of a fuel along with an oxidizing agent by converting it into electrical energy through a pair of reactions. For example, in a hydrogen fuel cell, as shown in Figure 1, the hydrogen combines with oxygen from the air to produce electricity and releases water.

Fuel Cell System
Figure 1 Fuel Cell System [1]
The design of a fuel cell system is quite complex and depends on fuel cell types and their applications. With so many possible combinations of fuel cells, this article will not focus on different type of fuel cells, but on Air Management Systems which may significantly affect the overall performance of a fuel cell system.

Air Management Systems

Key sub-systems of any fuel cell system are the fuel processor, fuel cell stack, air management and power management systems. The air management system strongly affects the fuel cell stack efficiency and the power loss of the fuel cells. Therefore, it is necessary to develop a clean, reliable, cost-effective oil-free air system [2].

Major tasks in air management system are Air Supply, Air Cleaning, Pressurization and Humidification.

Fuel cell performance depends on the reactant gases’ pressures, where relatively high-pressure ratios close to the fuel cell working pressure ensure maximum efficiency. A compressor in an air system is used to increase the air pressure. Generally, the compressor needs to have a pressure ratio of 2 to 4. The development of a compressor with high efficiency and wide operating range is very critical to the fuel cell system, in which, the reactants need to be delivered with the right pressures and flows.

Centrifugal Compressor Design

Generally, twin-screw compressors or centrifugal compressors are used in the fuel cell air systems. Screw compressors’ noisy characteristics and need for lubrication during operation make the centrifugal compressors more preferred in this application. ,. The centrifugal compressors have better efficiency than twin-screw compressors. Thus, the centrifugal compressors provide a promising future for fuel cell air systems. Normally, low specific speed compressors are required which can handle low flow rates with higher pressure ratios at lower speeds due to the maximum speed restriction of an electric motor. Such compressors are highly specific to the operating conditions of the process for which they are designed. This makes them highly expensive. So, compressors should be designed and operate with a high level of care and accuracy to avoid any failure and to extract the best performance possible from the machine.

Centrifugal Compressor in AxSTREAM
Figure 2 Centrifugal Compressor in AxSTREAM®

These centrifugal compressors can be designed quickly with the AxSTREAM® platform with performance predictions which  matches reasonably on the field.  This software platform  can utilize Preliminary Design based on Inverse Solver and can generate thousands of designs within a minute as shown in Figure 3.

3D Design Generation in Preliminary Design
Figure 3 Design Generation and Review in Preliminary Design

All of the generated designs can be reviewed easily based on non-dimensional stage parameters, geometrical parameters, performance parameters and loss parameters, which ensures the optimum design goes to the Detailed Analysis Module based on Through Flow Analysis, where fine-tuning can be performed along with the generation of compressor characteristic curves as shown in Figure 4(A). 3D blade profiles can also be controlled in the blade design module as shown in Figure 4(B).

Figure 4 (A) Compressor Characteristic Curves, (B) 3D Blade Design
Figure 4 (A) Compressor Characteristic Curves, (B) 3D Blade Design

A 3D CFD and structural analysis can be performed on the same software platform with an automatic mesh generation and simplified settings.

Air Management System Design and Analysis

Apart from compressor performance, other components such as the condenser, humidifier etc. of an air management system also affects the performance and effectiveness of the air management system and thus the fuel cell. So, it is also recommended to model and simulate the entire air flow system.

Modelling such a system in CFD needs expertise in CAD modelling, meshing along with the computational time and resources. One of the simplest ways to model an air management system is as a one dimensional thermal fluid network in AxSTREAM NET™ which is a part of the AxSTREAM® Platform. The entire system can be split into different 1-D elements to represent heat exchangers, condensers, humidifiers etc. (shown in Figure 5) and then analyzed using low computational resources and less time than other methods.

A condenser and a cooling system humidifier is shown modeled as sub-systems in Figure 5. For simplification, these are represented as two elements.  Figure 6 shows the detailed modelling of a heat exchanger which can be part of Air Flow Management system or any other flow system.

Figure 5 Simple Layout of Air Management System in AxSTREAM NET
Figure 5 Simple Layout of Air Management System in AxSTREAM NET™
Figure 6 Detailed Modelling of Heat Exchanger
Figure 6 Detailed Modelling of Heat Exchanger

Centrifugal compressors have a promising future in fuel cell applications. That being said, compressor performance affects the air system significantly. So, to have an efficient air system which can meet fuel cell stack requirements, designers need a well-established and validated tool like AxSTREAM® to perform time efficient and performance rich design activities.

Interested in trying AxSTREAM® for yourself? Get in touch at Sales@Softinway.com to schedule a demo and trial today!

References:

[1] Benjamin BLUNIER, Student Member, IEEE, Air Management in PEM Fuel Cells: State-of-the-Art and Prospectives, Abdellatif MIRAOUI Universit ́e de Technologie de Belfort-Montb ́eliard (UTBM), Belfort CEDEX 90010, France

[2] S. Pischinger, C. Schönfelder, W. Bornscheuer, H. Kindl, A. Wiartalla, Integrated Air Supply and Humidification Concepts for Fuel Cell Systems, SAE Paper 2001-01-0233, SAE International, Warrendale, PA, (2001).

[3] M. G. Turner, A. Merchant, D. Bruna, A Turbomachinery Design Tool for Teaching Concepts for Axial-Flow fans, compressor, and Turbines, GT2006-90105, May 8-11, 2006, Barcelona, Spain.