Parasitic power losses

Another good problem for modeling is the micro-DMFC system. Both anode carbon dioxide blockage and cathode flooding are especially acute in microsystems due to the small channel length scale involved, low operating temperature, dominance of surface tension forces, and requirement for low parasitic power losses in these systems. ... 

Figure 11.6. Parasitic power loss for power plants with C02 capture (percent of net power plant power) with state-of-the-art scrubbing technologies.

As shown in Figure 1.21, there is a draining valve at the bottom of the coolant tank for draining the coolant when needed. Typically, the amount of coolant will decrease with use, and fresh coolant needs to be added to the coolant tank when the coolant level drops to a preset value. If possible, a flow meter and a pressure sensor should be avoided in order to lower the cost. Also, their presence will increase the resistance to the flow of the coolant, leading to an increase in parasitic power losses. 

The system efficiency is affecfed by the fuel utilization Tip, the stack efficiency Tis, the system parasitic power losses and the DC-DC converting efficiency line- Th sysfem efficiency is... 

Parasitic power loss is due to the power needs of some fuel cell components, such as sensors, control boards, pumps, fans, blowers (or compressors), solenoid valves, and switches, and due to the power losses when currents pass through certain components such as the diodes and wires. Typically, the sensors, the control boards, the solenoid valves, the switches, the diodes, and the wires consume very little power. The pump for driving the liquid coolant does not consume too much power either. It is the fans (or blowers and compressors) that consume most of the parasitic power. For an air-cooled stack, the total parasitic power loss can be controlled to less than 5% of the stack output power, and for a liquid-cooled stack, the total parasitic power loss can be controlled to less than 10% of the stack output power. 

For a 2530 W load, the stack provided about 2900 W of power, meaning that the sum of system parasitic power loss and the DC-DC converting loss was 370 W, 12.8% of the stack power output. The DC-DC conversion efficiency was around 90%, and therefore, the system parasitic power loss was 2.8% of the stack power, which was 81 W (2.8% x 2900 W). About half of it was consumed by the two fans for the stack, and the other half was consumed by other devices in the system such as the control boards, the small fans (for cooling the control boards and the contactors), sensors, solenoid valves, and wiring. 

Keeping the system pressure drop low is a critical issue for practical systems because compression of gases requires energy. The parasitic power losses originating from compressors are known to reduce the efficiency of fuel-cell systems considerably [10]. Steam reforming of liquid fuels is an exception here because only liquid pumps for both fuel and water and no compressor are required to supply... 

Balance-of-plant components, such as the compressor, pumps and valves, is required to run a fuel cell/fuel processor system. They consume energy. These so-called parasitic power losses are usually in the range of 15-20% of the electrical energy generation of the fuel cell. The major portion of the losses can usually be attributed to the compressor. 

The water management of the systems was also taken into consideration. The water was recovered from the afterburner exhaust. A minimum exit temperature of 48 °C was assumed for the exhaust gas, which is a rather low value, because in practical systems condensers require air cooling. In the summer time it would be difficult to cool the exhaust gases down to 48 °C. To maximise water recovery, the system pressure was set as high as possible. This would be counterbalanced by increased parasitic power losses of the compressor. The parasitic losses for compression were considered by Lattner and Harold [405], consequently a 1.2-bar fuel cell operating pressure was assumed as a compromise. Only the parasitic losses to the compressor (see also Section 8.3) were considered, while the power consumption of valves and pumps was neglected. A 75% efficiency was assumed for the compressor. 

The development of a hybrid compressor/expander module was reported by Selecman and McTaggart [564]. It consisted of a scroll compressor that was fed by a separate compressor/expander module. The latter utilised the remaining compression energy of the fuel cell off-gas. However, the application of expander turbines, though attractive to reduce parasitic power losses of the compressor, is certainly limited to a minimum system power equivalent of 10 kW or higher due to weight, complexity and price. Gee reported on a turbo compressor/expander module developed by Honeywell for a 50-kW fuel cell [565]. The compressor, which worked with up to llOOOOrpm, had 8.0-kW input power with an expander and 14.3 kW... 

A design study for a gasoline fuel processor/fuel cell system was presented by King and O day [616]. It consisted of an autothermal reformer, sulfur removal, water-gas shift and two stage preferential oxidation. The system pressure was close to ambient to reduce parasitic power losses of the compressor (Figure 9.39). 

Maximum power densities range up to 1 W/cm, which are achieved with a pressurized gas supply this mode requires a compressor with an operating pressure of 5 bar and a parasitic power loss of around 12% of the performance. For a low-pressure operation the power densities are below the value given above and lie in the range of around 0.5 W/cm. ... 

The electrical power of the fan in question is just 0.7 W, so the very high stoichiometry for air circulation is supplied using less than 1% of the fuel cell power. This is a very modest parasitic power loss. 

This flow rate is delivered at a pressnre of between 1.10 and 1.15 bar. The pump unit is manufactured by KNF Neuberger and is driven by a 12-VDC motor. According to the published data, this motor/pump combination consnmes between 14 and 19 W. The parasitic power loss is thus about 6%. This represents abont as low a figure as can... 

This would bring the exit gas temperature down to about 55°C. Bearing in mind that the air will have been more or less saturated as it left the fuel cell, we would anticipate a good deal of condensation in the turbine, which would inhibit its performance. We should therefore perhaps round down our estimated power from the turbine to the still by no means negligible 10 kW. The power from the motor driving the screw compressor will therefore be about 47 kW. This is a very substantial parasitic power loss, and largely explains why the traction motor mentioned above is rated at 160 kW, whereas the fuel cell is 260 kW. The other major losses are the cooling system (estimated at 20-kW parasitic losses) and the electrical sub-systems, estimated at 13 kW. 

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