Civil Engineering Reference
In-Depth Information
unintentionally overspecified, and it can be argued that some of the first and
remarkable examples of 'zero-energy houses' in history might have been in this
situations. For example, the 1939 MIT Solar House I, which included a large solar
thermal collection area and water storage (Butti and Perlin
1980
), the 1955 'Bliss
House' (Bliss
1955
), which used large area of solar air collectors and rock mass
storage, or the 1970s Vagn Korsgaard Zero Energy Home in Denmark (Esbensen
and Korsgaard
1977
), all probably had a very large embodied energy associated with
the installed systems. The life cycle performance of those exemplary houses might
have been even better with smaller solar collecting surfaces. Hernandez and Kenny
(
2008a
,
b
) explained how extra care must be taken in current 'zero-energy' building
design to avoid overspecification of certain components, as the use of large areas of
thermal solar collectors for water and space heating together with high levels of
insulation, as is often promoted, might not be the most efficient way of reducing the
life cycle energy for some building typologies. In the context of refurbishment
towards zero-energy buildings, other design strategies might offer more appropriate
solutions, particularly in less extreme climates such as in maritime Europe.
To support practitioners willing to consider building energy refurbishment
projects from a life cycle perspective and use it as an input for the design, the
concept of 'NER' can be introduced. This indicator, frequently used in the
renewable energy field, sometimes also called Energy Return of Investment,
Energy Returned on Energy Invested or Energy Yield Ratio, can be represented for
the refurbishment of an existing building through the following formula:
NER
¼
AEU
1
AEU
2
AEE
2
AEE
1
ð
1
Þ
The NER can be defined for building refurbishment as the ratio of the decrease
in annual energy use (annual energy savings) to the increase in AEE. This ratio can
be used to compare refurbishment options for improving energy performance in
use: the higher the NER of a particular refurbishment strategy, the more effective it
will be in delivering life cycle energy savings.
All options where the NER is greater than one will contribute to an improvement
in life cycle energy performance, an energy saved over the life cycle. The higher the
NER of a refurbishment strategy, the larger the life cycle energy savings.
This introduction of the NER to the built environment allows different refur-
bishment strategies, related to building envelope, building control or energy sys-
tems, lighting, etc., to be compared with NER values of renewable energy systems,
which are extensively published and discussed (Mulder and Hagens
2008
). For
example, the first layer of insulation in a typical existing house would normally
yield very high NER, as would save a large amount of energy with a small amount
of material. Subsequent layers of insulation, while adding to the total embodied
energy, would not deliver an equivalent energy saving, and so a refurbishment of a
building envelope would represent a diminishing NER as we increase the insu-
lation thickness. Technologies such as solar water or space heating systems would
