Overall efficiency PEMFC

The utilisation and the on/off cycles of a CHP system affect its potential benefit. Furthermore, higher overall efficiency leads to lower fuel consumption, while longer lifetimes lead to lower annualised capital costs. Concerning the relative influence of the demand profile, the nominal electric capacity, the efficiency and the lifetime of AFC, PAFC, PEMFC and SOFC systems, target costs were found to lie in a rather wide range. 

Toshiba Fuel Cell Power Systems (TFCPS) (Japan) a subsidiary of Toshiba, has the sole purpose of commercialisation of its 1 kW residential PEMFCs by 2008. This is termed the Dash to 2008 when it is planning on having a unit priced on the open market at less than 1.2 miUion Yen (approximately US 9500). The technical targets for the system are a cold start time of less than 10 minutes, overall efficiency of >77% higher heating value (HHV) and 80°C waste heat. In the new company, TFCPS will produce the stacks. 

Phosphoric acid fuel cell - PAFC is the first fuel cell to have reached commercialization. Phosphoric acid is used as the electrolyte. More than 75 MW PAFC systems are in operation in 85 cities in 19 countries so far. Similar to the high temperature PEMFC, the operating temperature of a PAFC is higher than 200°C. This helps increase the tolerance of the Pt catalyst against carbon monoxide. Hydrocarbon reforming gas is a promising fuel source for PAFC. The overall efficiency of PAFC can reach 80% if the cogenerated heat is harnessed. Due to these characteristics of the PAFC, its main apphcation focuses on stationary power source. 

A fuel cell system for automobile application is shown in Figure 1.5 [41]. At the rated power, the PEMFC stack operates at 2.5 atm. and 80°C to yield an overall system efficiency of 50% (based on lower heating value of hydrogen). Compressed hydrogen and air are humidified to 90% relative humidity at the stack temperature using process water and heat from the stack coolant. A lower system pressure is at part load and is determined by the operating map of the compressor-expander module. Process water is recovered from spent air in an inertial separator just downstream of the stack in a condenser and a demister at the turbine exhaust. 

At this point we would like to leave the field of low temperature PEMFCs for an excursion to solid oxide fuel cells (SOFCs) and consider an example of an LCA for SOFC modules (with each module combining many stacks within a steel pressrrre vessel) that are rrrass-produced for central power production. It can be assumed that the length of the stack flow field can be related to not orrly the mass of stack materials (e.g. the amount of stainless steel or cerarrrics used) but also to the stack efficiency, which subsequently dictates in part the nurrrber of modules needed to achieve the desired power. Including corrsideration of the balance of plant, the length of the flow field also dictates the amonrrt of fuel available for combustion off the stacks (also related to the cherrristry of balance-of-plant emissions) and therefore the energy that might also be obtained by the overall... 

In most stationary fuel cell plants, natural gas is the fuel of choice. It is widely available, clean, and the processing of natural gas is generally straightforward. For the PEM fuel cell and the PAFC, steam reforming is the preferred option, ensuring high overall fuel conversion efficiencies. In both PEMFC and PAFC systems the sulphur removal can be achieved by HDS. 

BASF Fuel Cells (formerly PEMEAS or Celanese Ventures) produces polybenzimidazole (PBI)-based high-temperature membrane and electrode assemblies sold under the brand name Celtec . These MEAs operate at temperatures between 120 °C and 180 °C. One of the distinct advantages of high-temperature PEMFCs is exhibited in their high tolerance toward fuel gas impurities, such as CO (up to 3%), H2S (up to 10 ppm), NH3, or methanol (up to several percent), compared to low-temperature PEMFCs. Additionally, waste heat can be effectively used and, therefore, the overall system efficiency is increased. 

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