9 L IFE CYCLE SCREENING OF SOLID OXIDE FUEL CELLS
9.3 L IFE CYCLE SCREENING OF THE CONSTRUCTION PHASE
The primary energy consumption related to the production of the SOFC is used as an indicator of the environmental impacts and resource consumptions in the production phase. A tubular design of the SOFC, which was the design initially developed, is now aban‐
doned in favour of a planar alternative with a more compact scalable design and lower production cost. The first generation planar cells are electrolyte‐supported. The second generation cells, which are primarily in focus at the moment, are anode‐supported. Third generation, metal‐supported cells are currently being developed and tested [1]. These will potentially lower the material costs, as the use of rare earth is minimised in favour of more stainless steel. Such a development is also associated with a reduction of operation tem‐
perature, increasing the lifetime and the efficiency of the SOFC by lowering internal resis‐
tance; hence, this development is required for the SOFC to be competitive.
In Fig. 42, the distribution of primary energy consumption for the production of materials and manufacturing of a first generation cell and system is illustrated. In the perspective of the development towards third generation cells, the first generation cell represents a worst case scenario. The data set used in Fig. 42 is based on a planar 1 kW SOFC from Karakoussis (2001) [70] and can be considered as the first estimate of the potential primary energy con‐
sumption related to the construction of SOFCs. For this type of fuel cell, the main part of
chromium alloy used in the interconnector and the production of steel used for heat ex‐
changers, air and fuel supply, etc., are the two most important factors at the production stage of the life cycle of this fuel cell.
0% 20% 40% 60% 80% 100%
Primary energy consumption
Anode/elec./cathode &
other materials Intercoonect material
Cell manufacturing
Fig. 42, Distribution of primary energy consumption for materials and manufacture of cell and system forming a fuel cell.
The data is based on a first generation 1 kW planer SOFC.
When the cells develop towards third generation cells, the relative contribution from the anode/electrolyte/cathode will diminish, as these parts will become thinner and will be supported by the interconnector. The interconnector may also become thinner as the cells develop; thus, the system surrounding the cells will become more and more important. The energy consumption related to the manufacture of the anode, cathode and electrolyte has been assessed using the energy consumption for aluminium production per mass in the cell analysed here. Due to commercial confidentiality, no exact data have been acquired on the production of these ceramic materials in the cell itself. Doubling the energy use for manu‐
facturing the anode, cathode and electrolyte will only increase the total energy require‐
ment for materials and manufacturing by 1.6 per cent. This is due to the fact that the inter‐
connector is made from chromium alloy and the fact that the steel for the system by far has the largest energy consumption in the cells themselves [70].
The production of the anode, cathode and electrolyte is not likely to generate higher en‐
ergy consumption in the future. It is only expected to contribute marginally to the total energy consumption in the production of the fuel cells. In addition to the cells, the sur‐
rounding system also generates energy consumption. The system counts for approximately 40 per cent of the energy consumption of the cell and, also here, the material production has a significant contribution.
The processes used in Karakoussis (2001) [70] are not optimised for mass production. As an example, the anode and cathodes are not co‐sintered, thus increasing the energy demand in the data used here. Furthermore, no recycling of the materials in the system has been assumed, which can prove important in terms of lowering the energy consumption for the production of materials for the fuel cell. Re‐cycling may also prove important in terms of reducing the use of rare earth for the ceramics of the SOFCs.
The power density of the fuel cell, i.e. the capacity of the individual cell per cm2, is 0.2 W/cm2, and the cell has an operating temperature of 900°C.. The power density is expected to exceed to 0.5 W/cm2 [55], which means that the energy consumption for producing a 1 kW fuel cell would decrease by 40 per cent. Electrolyte‐supported cells have reached a power density of 0.48 W/cm2, and experimental second generation cells have performed 0.8 W/cm2 [55]. Third generation interconnector metal‐supported cells are still at the ex‐
perimental stage. However, these are expected to increase the power densities even more.
The running temperature is lowered to 550‐650°C, compared to 900‐1.000°C in the first generation cells. This will lower the internal resistance. The power density will increase from the first generation cell analysed here and, subsequently, the overall energy con‐
sumption related to the production of 1 kW SOFC will decrease.
In Fig. 43, the energy consumption per kW of capacity related to the production of SOFCs and traditional power‐producing units is illustrated. Two SOFCs are shown; one with a power density of 0.2 W/cm2 and another SOFC using the same data, but scaled for an im‐
proved power density of 0.5 W/cm2. These figures are compared to the primary energy consumption related to the production of a large coal‐fired power plant and three sizes of gas turbine power plants, all of which represent current technologies. For these power plants, existing data from theEcoInvent database has been used. The EcoInvent database is one of the most comprehensive and up‐to‐date life cycle inventory databases available. The 2.500 processes, products, and services in the database are applicable to a European con‐
text [14]. This database contains data gathered in 2004 for processes, products, and ser‐
vices in the year 2000 and was constructed from several Swiss databases covering data for both Switzerland and Europe.
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000
1 kW SOFC (0,2 W/cm2)
1 kW SOFC (0,5 W/cm2)
460 MW Coal PP 100 MW GT PP 1 MW GT PP 50 kW GT PP Primary energy consumption (MJ pr. kWe)
In terms of primary energy consumption, the production of SOFC is already at this point more efficient than the production of large coal‐fired power plants, as a power density higher than 0.5 W/cm2 has been achieved. The lifespan, however, is still a problem which requires further development. Coal‐fired power plants generate a large energy consump‐
tion per kW, because of the use of large amounts of steel for the production of the plants.
The production of gas turbines is still less energy consuming than the production of SOFCs.
The SOFC would have to reach a power density of 1 W/cm2 and reuse at least one third of the interconnector and system material in order to be comparable to gas turbines at the production stage.