Page 96 - Materials Chemistry, Second Edition

80 3 Life Cycle Inventory Analysis

In CED guideline VDI 4600 it is pointed out that the evaluation of nuclear energy
and renewable energies is ‘not clearly definable’, and the following proposals are
made:

1. Hydropower: The system boundary is the intake structure of the power plant. The
efficiency of the energy supply is, according to this definition, the relationship
of the net energy production (electric current) to the processable energy of the
water, thus the potential energy that results from the usable gross height of
fall.
2. Wind power: Analogous to hydropower, the system boundary is equal to the
rotor disc of the power plant. The supply level, according to this definition, is
the relationship of the net energy production (electric current) to the kinetic
energy of the wind that passes the rotor blades.
3. Photovoltaic energy: The system boundary is the gross module surface. The
supply level is, according to this definition, the relationship of the net energy
production (electric current) to the solar energy irradiated on the gross surface.
4. Nuclear energy: Evaluation of the primary energy is done with the thermal
efficiency of the nuclear power stations and the efficiency of utilisation for
nuclear fuels. For Germany this results in an average value of 0.33.
5. Fuels and biomass: For fuels used to generate energy (including also garbage,
etc.) the low heat value is inserted, in the case of biomass related to the
harvested plants.
The use of these definitions and specifications is recommended until an Inter-
national Standard is provided. Such definitions can never be completely satisfying.
Thus Frischknecht has correctly pointed out that water craft is also based on solar
56)
energy, which induces evaporation. Because photovoltaic energy can only attain
20% efficiency (compared to 80% by water craft) with respect to primary energy
and electricity production, the determination of the system boundary seems to be
inequitable at first sight. A closer look reveals that 100 − 20 = 80% (solar) energy
is not lost and can be applied for thermal use as in the case of the conversion of
fossil into electrical energy. A photovoltaic system that uses waste heat (e.g. for
the supply of industrial water) will fare better in the analysis than a system that
only provides electricity! In addition, it is to be considered that these specifications
produce higher overall efficiencies for solar cells, and this will yield a lower CED
which will fare better in comparative assessments.
Wood as biomass according to (5) is introduced with low heat value, that is, the
efficiency of wood production as related to solar energy is avoided. This efficiency
is low and, as it appears in the denominator in Equation 3.6, would lead to a
dominance ‘of CED wood’ in all wooden products. This approach would only be
justified if solar energy were a scarce source (as in case of the fossil energy sources).
For renewable energies, Frischknecht and co-workers propose to use the energy
57)
extractable with today’s technology as weighting factors consistently. This is an

56) Frischknecht, 1997.
57) Frischknecht et al., 2007b; SIA, 2010.