Environmental Engineering Reference
In-Depth Information
10.3 Structures and Products: From Buildings to
Computers
Process analysis of the energy embodied in buildings
and manufactured products is considerably more difficult
than the quantification of energy used in producing basic
materials. Input-output analysis is of a limited use be-
cause many of the examined sectors produce an enor-
mous variety of items. Process analysis must consider not
only the components used in the final assembly but also
the cost of long-distance transportation, which has be-
come common in the global economy. For example, pas-
senger cars are now assembled from components made in
more than a dozen countries, and the raw materials used
to make auto parts may have come from a different set of
more than a dozen countries.
These realities, as well as different assumptions regard-
ing the recycling and waste rates, analytical boundaries,
and typical conversion efficiencies, result in often sub-
stantial differences in published energy costs. And the
results of specific process analyses may not be applicable
to other items even within the same category. This is
well illustrated by focusing on three ubiquitous posses-
sions in modern societies: houses, passenger cars, and
computers. The energy cost of houses eludes easy gener-
alizations because of very different shares of principal
construction materials and the quality of interiors. Highly
automated car assembly may have uniform energy costs
no matter where it takes place, but at the century's turn
there were more than 700 car models on the market,
ranging in weight from barely 0.5 t to nearly 5 t. And
even when the range is restricted to personal and office
computers, these machines range from small all-purpose
laptops to powerful large desktops dedicated to
computer-assisted design.
The mass of modern structures is dominated by one
of three main structural materials—wood, steel, or con-
crete—but houses include a large assortment of other
components whose embodied energies range from less
than 10 GJ/t (tiles) to more than 200 GJ/t (machined
aluminum alloys). As a result, the energy cost of residen-
tial space in rich countries varies from 3 GJ/m
2
to 9
GJ/m
2
, with floors and roofs usually the largest items
(Baird 1984; Buchanan and Honey 1994; CWC 2004).
These values translate to a wide range of grand totals for
single-family houses, from as little as 200-300 GJ for
small wooden houses and 500-700 GJ for small steel-
based houses to more than 2 TJ for large houses using
a mixture of materials. About 500 GJ of energy is
embodied in an average three-bedroom, wood-framed
North American bungalow.
Several studies have compared the embodied en-
ergy costs of identically sized family houses using the
three principal structural materials. Predictably, concrete
houses have the largest mass, and depending on the
design, either they or the steel houses require the most
energy (Glover, White, and Langrish 2002). A study of
220-m
2
family houses in the Toronto area found final
embodied energies of 1.12 TJ for wood-based, 1.42 TJ
for steel-based, and 1.76 TJ for concrete-based designs
(CWC 2004), implying energy intensities of approxi-
mately 5, 6.5, and 8 GJ/m
2
, respectively. Similarly, a
process analysis of a multistory Swedish building showed
the concrete-frame structure embodying 60% more en-
ergy than an identically sized wood-framed one (B
¨
rjes-
son and Gustavsson 2000).
Nationwide U.S. data show the expected progres-
sion for other structures: warehouses are relatively cheap
(5-7 GJ/m
2
), high-rise apartments take 8-9 GJ/m
2
,
