6. November 2017

Energy saving potential with oil-free turbo compressors in fuel cell systems

Due to their compact design and high efficiency, oil-free electrical turbo compressors are ideally suited for mobile fuel cell systems, e.g. in vehicles and thus have great energy saving potential compared to standard compressors. This article presents a comparison with other compressor technologies and the potential for improvement in converter topology.

Most efforts to improve fuel cell systems are aimed at reducing production costs and increasing the efficiency of the fuel cell stacks. As a result, the pressure to increase efficiency is continuously growing as is the cost pressure on ancillary components, the so-called balance of plant (BoP). This applies in particular to the compressor for supplying air to the fuel cell. The compressor requires between 10% to 20% of the fuel cell’s gross output and simultaneously accounts for most of the volume, weight and costs of the BoP. A fuel cell system including the subsystem (compressor BoP) is shown in Figure 1.

Conventional fuel cell system including subsystem
Figure 1: Conventional fuel cell system including subsystem (compressor BoP).

Most fuel cell systems are operated with scroll compressors, side channel compressors or displacement compressors. Displacement compressors can be further divided into various subtypes (rotary vane, piston, diaphragm, and wobbling disc compressors, etc.). These compressors are already used in the automotive sector for air conditioners. Although the technology is widely available and already industrialized, it is limited in terms of miniaturisation and efficiency optimization. Displacement compressors are available for practically the entire range of pressure ratios and mass flows.

It is true that side channel compressors can be miniaturised by increasing their rotational speed; however, this principle decreases efficiency, which further decreases for greater pressure ratios. Side channel compressors can only be used at low pressure ratios, typically in the range of 1 to 1.3.

Scroll compressors also belong to the displacement compressor group; however, they are listed separately. Scroll compressors have medium to high efficiency, depending greatly on the manufacturing tolerances. Miniaturisation by increasing rotational speed is only possible to a limited extent, since the rotational speed will be limited by the design of the rotor.

Turbo compressors have the highest efficiency. They can also be miniaturized by increasing rotational speed. However, their operating range is limited since speed, mass flow and pressure ratio cannot be selected arbitrarily. Pressure ratios up to approx. 2.5 are possible for single-stage fuel cells with typical mass flows.

In terms of weight (and size), the oil-free turbo compressor from Celeroton performs better than well-established fuel cell compressors by a factor of 3 to 17. Side channel compressors perform second best with a factor of 3 and are therefore very often used in mobile fuel cell systems; however, they are suitable for fuel cell applications only to a limited degree, since they have about half the efficiency of turbo compressors. With constant tank filling and fuel cells, the power consumption of compressors equipped with Celeroton technology can be halved from approximately 20% of the output power of the fuel cell to about 10%, thus increasing the range of the vehicle by about 10%. As an example, the following table shows a 5 kW fuel cell and its operating point. For a given constant driving distance, a compressor with Celeroton technology consumes 1,260 kWh less energy than a side channel compressor with a running time of 3,000 hrs. If the hydrogen cycle is included from production (electrolysis, approx. 70% efficiency) via the conversion in the fuel cell (fuel cell, approx. 50% efficiency), the energy saving increases “from the socket” to about 3,600 kWh.

Despite the high rotational speeds of oil-free turbo compressors (CT-17-1000.GB) of Celeroton at a rated speed of 280,000 rpm), they do guarantee high dynamics.

TypeSide channel compressorScroll compressorDisplacement compressorOil-free Turbo Compressor (Celeroton CT-17-700.GB)
Operating point pressure ratio / mass flow1.4 / 5 g/s1.4 / 5 g/s1.4 / 5 g/s1.4 / 5 g/s
Rotational speed (rpm)12,0001,0001,725220,000
Weight (kg) 425251.5
Efficiency (%)21292652
Power consumption (W)700500560280
Estimated energy saving – network energy (kWh)1)0 (Reference)1,7001,2003,600

Figure 2: Comparison of compressor technology. 1) Estimated energy saving calculated with 3,000 h running time at the specified operating point, a fuel cell efficiency rating of 50% and an electrolysis efficiency rating of 70% (energy saving due to weight reduction not calculated).

In addition to the compressor, the compressor’s control electronics must be taken into account as part of the overall view of the BoP in fuel cell systems. Currently, several electronic components are being used, as shown in the Figure 1 above. These include the actual drive converter with constant input voltage for the compressor. The components also include a DC/DC converter for providing constant input voltage to the drive converter starting with a variable fuel cell voltage (high voltage) as well as another DC/DC converter for feeding constant output voltage to the drive converter from the battery for start-up. These three components can be efficiently covered with a single application-specific compressor converter, as shown in the following figure.

Application-specific compressor converter system
Figure 3: Application-specific compressor converter system.

The potential for energy saving and for reducing the complexity of various concepts and solutions in terms of the electronics has not been investigated in research and development projects, or if so, only to a marginal extent. A reduction in the use of DC/DC converters signifies a reduction in the complexity of the BoP. However, the use of fewer electronic components also leads to an increase in efficiency of the BoP and thus an increase in the efficiency of the entire fuel cell system which ultimately yields an increase in the range of fuel cell vehicles. Costs are further reduced, since fewer electronic components, cables and plugs must be installed.

For a simplified example of energy saving, assume that the compressor requires 10% of the fuel cell output power. Thus, this value would be 10 kW for a fuel cell system with fuel cell output power of 100 kW. In a typical fuel cell system, the compressor output power flows through two DC/DC converters between the fuel cell output and the constant input voltage of the drive converter, and from there to the compressor via the drive converter. A medium efficiency rating of 90% (subject to load operation) can be assumed for both converters. Therefore, if the electronics system could be replaced, this would save 10% of the compressor output, which corresponds to 1 kW, and thus 1% efficiency of the entire fuel cell system. In addition, both the volume and the weight of the fuel cell system can also be reduced. For a given constant driving distance, a running time of 3,000 hrs would thus save 3,000 kWh of energy. If we calculate in the production (electrolysis, approx. 70% efficiency) and conversion of hydrogen (fuel cell, approx. 50%), the energy saving from the socket increases to approx. 9,000 kWh per converted BoP as shown in Figure 3.