or energy yield ratio, can also be introduced into the building
energy analysis to aid in decision-making and to optimize building
design towards LC-ZEBs. The NER corresponding to a change
between two building options 1 and 2 can be presented as:
NER ¼
AEU1 AEU2
AEE2 AEE1
(3)
The NER, defined for the built environment as the ratio of the
decrease on annual energy use to the increase in annualized
embodied energy, can be used to compare design options that are
intent on to improving AEU but increase the AEE. The higher the
NER of a particular technology or the NER associated to a design
decision that affects building energy performance, the more
effective it will be in reducing the life cycle energy use and moving
towards LC-ZEB.
Options where the NER is greater than one will contribute to a
reduction of the life cycle energy. These need not only include
technologies but might also include decisions made at the early
design stage such as those around building form, orientation,
layout, etc. These options can decrease AEU without increasing AEE
would have an NER value of infinity, meaning that they represent
ideal options from a life cycle perspective and, in many cases, could
mean prioritising architectural solutions over those of high AEE
system based technologies.
This introduction of the NER to the built environment allows
building design and construction options to be compared with NER
values of renewable energy systems, which are extensively
published and discussed [21]. For example, the first layer of
insulation in a house would normally have a very high NER and
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 would represent a diminishing NER. Technologies such as
solar water or space heating systems would also represent a
diminishing NER as the annual solar input rate per square meter of
installation decreases as can occur with large installations
oversized for the summer, as in those cases, increasing collector
size and embodied energy does not proportionally increases the
solar energy input. Technologies such as PV, however, will have a
practically constant NER independent of their size as the
production of electricity will be proportional to their quantity of
materials used in their production and installation.