Page 91 - Materials Chemistry, Second Edition

3.2 Energy Analysis 75

Table 3.1 The most important forms of energy. 39)

Energy form Example MJ (example)

Kinetic energy Mass of 1 kg moving with 60 km h −1 10 −4 (100 J)
Potential energy Mass of 1 kg 500 m above point of reference 5 × 10 −3 (5 kJ)
◦
Heat 1 kg water at the boiling point, related to 20 C 0.34
Electric current 1 A, 1 h at 230 V 0.83
2
Light Sunlight reaching 1 m on a sunny day at noon 3.4
Chemical energy 1 kg oil burned to CO and H O 45
2 2
6
Nuclear energy 1 kg uranium 235 (nuclear fission) 80 × 10 (80 TJ)
a Note on units:
The SI unit of energy is the Joule (J).
Mechanical definition (J) (N = Newton,SIunitofforce):
2 −2
1J = 1Nm = 1kgms −2 m = 1kgm s .
Combination (J) with power and electrical units:
(W = Watt, SI unit of power; V = volt, SI unit of voltage; A = ampere, SI unit of electric current):
1J = 1Ws; 1 W = 1VA.
In LCAs the megajoule (MJ) is the most common energy unit.
The watt hour (Wh) and the kilowatt hour (kWh) do not belong to ‘the Syst` eme International’, but
according to ISO 1000ISO (1981), they are permitted as supplementary energy units within special
applications. In LCA, (kWh) is often used for electrical energy.
A kilowatt hour converts to exactly:
1kWh ≡ 3.6 MJ.
The conversion factor of 3.61, frequently used in US-American publications, is wrong.

energy conversion (e.g. power plant, heating), and in the case of electricity the grid
losses are considered here. The same applies to feedstock energy, for example, raw
oil, which is a constituent of polyethylene or other synthetic plastics after chemical
transformation. The primary energy can serve as a meaningful measure that,
especially in the case of fossil fuels, directly correlates to the resources demand.
In simple cases, a minimum energy demand can be extracted from material data
if not provided by specific process data. This is demonstrated for aluminium melt
40)
by Boustead and Hancock .
To transfer 1 kg aluminium metal of 290 K (about room temperature) into melt,
the metal first has to be heated up to the melting temperature (932 K). For this,
−1
an amount of heat Q (c = specific heat capacity of aluminium: 913 J kg −1 K )is
p
needed.
−1 −1 –1 –1
Q = c × = 913(Jkg K )×(932–290)(K)= 586146Jkg ≈ 0.586MJkg
p
−1
For melting the metal at the melting point, 0.397 (MJ kg ) × m (kg) is needed,
or 0.397 MJ for 1 kg.
For 1 kg aluminium the total energy for this process thus amounts to at least
0.586MJ + 0.397MJ = 0.983MJ ≈ 1MJ

39) Boustead and Hancock, 1979.
40) Boustead and Hancock, 1979.