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As we covered in our previous blog about fuel cell systems, a large contributor to their efficiency is the compressor that is selected for it. But what are the different kinds of compressors, and which one is best for a specific system?

Compressors have a wide variety of designs and types, which differ in pressure and performance, depending on the kind of compressed fluid. Compressors are also classified according to the type of work: dynamic and positive displacement. Figure 1 shows the types and classification of compressors.

Figure 2 shows a comparison of various types of compressors according to several criteria: generated pressures, occupied volume, lubrication requirements, compressor weight, and pressure ripples at the outlet.

As can be seen from the comparison above, we can conclude that centrifugal compressors offer a number of advantages over its positive displacement counterparts:

1. Lightweight;
2. Small volume;
3. Only the bearings require lubrication;
4. Creates a sufficiently high pressure (1.5…6 bar);
5. Has high efficiency (80…82%); and
6. Has a fairly wide performance range.

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Next, we will consider the application of the centrifugal compressor in the fuel cell system.

How, where, and in what systems/areas are centrifugal compressors applied (in a fuel cell system)?

The centrifugal compressor is integrated into various fuel cell systems which are used in energy, transport, and aviation.

The main task of a compressor is to increase the thermodynamic parameters of the oxidizer/fuel and thus obtain higher system efficiency, as well as increase the fuel cell’s power output.

The fuel cell system can be divided into several major sub-systems;

1. Fuel supply system to the anode;
2. Air supply system to the cathode;
3. Fuel cell stack;
4. Heat removal system (radiator);
5. Electricity conversion and storage system.

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All typical fuel cell system designs share these sub-systems. For a more complex system, these sub-systems may change.

Consider an air supply system based on the automotive system shown in Figure 3.  This system includes the following elements:

1. Centrifugal compressor (required to increase air pressure);
2. Electric motor (rotates the impeller);
3. Humidifier (required for the operation of the proton exchange membrane);
4. Separator (required to separate moisture from the reacted air);
5. Tank with water (necessary for humidifying the air);
6. Control valve.

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Below are the schemes for using the compressor in different areas such as: a residential fuel cell system (Figure 4), a gas turbine fuel cell (Figure 5), and an aviation fuel cell system (Figure 6).

What are the constraints and features of the centrifugal compressor in the air supply system to the fuel cell?

There are several constraints of the centrifugal compressor which are associated with the cell itself: the choice of electric drive, design features, and the gas dynamics of the centrifugal compressor or turboexpander. The constraints can be divided into sub-sections:

There are several constraints of the centrifugal compressor which are associated with the cell itself: the choice of electric drive, design features, and the gas dynamics of the centrifugal compressor or turboexpander. The constraints can be divided into sub-sections:

1. Compressor design features and constraints;
2. Constraints of the electric drive;
3. Joint performance mode of network (Compressor + piping/manifold + Fuel Cell stack + Control valve);

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– Consider the design features of a centrifugal compressor for a fuel cell:

For the operation of the fuel cell, relatively low air consumption is required (for auto/aviation 0.05…1 kg/s, for a power generation system 1…2 kg/s). A disadvantage of the centrifugal compressor is the low operating efficiency when the inlet’s axial flow velocity is low. Consequently, the diametrical dimensions of the centrifugal compressor at the inlet will be small.

To obtain higher compressor efficiency, with the required pressure ratio increase in the compressor, there are restrictions on the minimum values of circumferential velocity. These values are directly related to the rotational speed of the compressor impeller. From this, it follows that in order to obtain an optimal compressor design, sufficiently high shaft speeds (above 100k rpm) are required. At low RPM’s, the centrifugal compressor has a suboptimal design. That is, low rotational speeds lead to the need to increase the outlet diameter of the impeller. This is necessary to achieve the optimum level of circumferential velocity. In turn, this entails an increase in the length of the flow path channel, which leads to an increase in hydraulic losses (see Figure 7a). The consequence is a significant decrease in the efficiency of the compressor and the entire system as a whole. Also, an increase in the diameter at the outlet of the impeller leads to a decrease in the height of the blade at the outlet and an increase in the relative clearance (relative clearance = 20%). This also significantly degrades the parameters of the compressor and, makes it impossible to achieve the required pressure ratio. In this shaft speed example with a preliminary estimate completed inside of the AxSTREAM compressor design platform, the efficiency of the compressor at a rotational speed of 100k, , was 56%. 7b also shows a compressor that is designed for a rotational speed of 150k and has an efficiency of 78%.

Note: the difference in scale in the compressor figures. A compressor with a rotational speed of 150k is almost two times less.

-Consider the constraints of the electric drive:

Another constraint of a centrifugal compressor fuel cell system is the constraints of the electric motor used to drive the compressor. The constraints of an electric motor are related to its rotation speed and power output. The peculiarity of the electric drive is that at high rotational speeds its output power decreases (at a speed of more than 100k rpm, the power will not exceed 10kW). Conversely, to obtain power of around 100 kW, the shaft’s rotation speed will not exceed 50,000 rpm in this case.

Figure 8 shows a graph of the dependence of the shaft rotation speed on the power of the electric drives.

There are also experimental electric drives that have high rotational speeds (above 250k rpm and have a power of up to 10 kW).

An alternative for using an electric motor can be a gas turbine (turboexpander), which can have rotational speeds and power characteristics closer to a centrifugal compressor. Using AxSTREAM, an engineer is able to design both an optimized centrifugal compressor as well as an optimized expander to drive the compressor.

– Joint performance mode of network:

As one might expect, the system’s performance relies heavily on ensuring the minimum and maximum required airflow throughout the system. It is also necessary to account for the air pressure in the network and to not exceed maximum permissible

Conclusion: Several approaches are used to increase the efficiency and power of a fuel cell. One popular approach is to increase the thermodynamic parameters of the oxidizer/fuel at the fuel cell inlet.

To increase the air parameters, it is preferable to use a centrifugal compressor, which has a number of advantages  (lightweight, small volume, simplified lubrication requirements, high-pressure ratio – 1.5…6, high efficiency – 80…82%, and a sufficiently wide range of performance with regards to shaft rotation speed, pressure ratio, and airflow rate).

The centrifugal compressor offers versatility and can be used for different fuel cell system applications in energy, transportation, and aviation.

Usually, a compressor designed for a fuel cell is optimized for speeds over 100k rpm. The optimal design for this system has rotational speeds close to 150k rpm and has an efficiency of about 78%. Compressor efficiency decreases at lower shaft rotation speeds due to hydraulic losses (increase in the length of the flow path channel due to the increase in the outlet diameter of the impeller) and increasing the relative clearance.

It is necessary to account for the power and shaft rotation speed of the electric drive. At high rotational speeds in the electric motor and its driveshaft, there are certain difficulties for obtaining high powers (at speeds above 100k rpm, the power is typically below 10 kW).

It is necessary to account for the network’s entire performance to ensure the compressor does not reach off-design modes: stalling or blockage.

To learn more about how AxSTREAM and SoftInWay can help you design an efficient fuel cell system, send us a message at info@softinway.com or request a software trial.

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Source: https://blog.softinway.com/compressors-in-fuel-cell-systems/