Solution manual for exercises - MAELabs UCSD

Answer. The table shows the data. The embodied energies are larger, by a factor of 3 to 10, than the
enthalpy of oxidation. This is because of the inherent irreversibility of the reduction process, heat and
other energy losses, and the energy required for mining transporting and concentrating the ores before
reduction.
Material Enthalpy of oxidation
Embodied energy
(MJ/kg)
(MJ/kg) from Chapter 12
Cast iron 5.5 17
Aluminum 20.5 220
E6.7 Estimate the energy to mold PET by assuming it to be equal to the energy required to heat PET
from room temperature to its melting temperature, Tm. Compare this with the actual molding energy.
You will find the molding energy, the specific heat and the melting temperature in the data sheet for
PET in Chapter 12 (use means of the ranges). What conclusions do you draw?
Answer. PET melts at about 238 C, about 200 C above room temperature. Its specific heat is 1445
J/kg.K, so the energy to raise one kg of PET from room temperature (20 o C) to the melting point is
about 0.31 MJ/kg. The extrusion energy is 3.8 MJ/kg, the molding energy 9.6 MJ/kg. Both are more
than ten times greater than the estimate based simply on heat the polymer.
Where does the extra energy go? Part in the relatively low conversion efficiency of fossil-fuel
energy to electric power (about 38%), part in heating the extrusion or molding press, in power
dissipated by the mechanism of the press, and as incidentals – the general energy overhead of the plant.
E6.8. The data sheets of Chapter 12 list eco-indicator values where these are available. As explained
in the text, the eco-indicator value is a normalized, weighted sum involving resource consumption,
emissions and estimates of impact factors. Plot eco-indicator values against embodied energy (a much
simpler measure of impact). Is there a correlation?
Answer. See the chart in the answer to exercise E6.11 (below). It is a CES plot of eco-indicator
values against embodied energy. There is a clear correlation, but with scatter. Is the scatter
significant? Think back to the inherent uncertainty in determining embodied energy (Figure 6.2) and in
the arbitrary nature of the weight factors used to calculate eco-indictors (Figure 3.11 and its
discussion). Given these, you would expect some scatter. We conclude the embodied energy is an
approximate but still useful proxy for the eco-indicator.
Eco text: solution manual 26 MFA, 08/12/2008