Instead of quantifying energy use numerically the annualized
life cycle energy equation (Eq. (2)) can be represented by an XY
graph where the horizontal axis is the annualized embodied
energy (AEE) and the vertical axis the annual energy use (AEU). LCZEB
is represented by a line at 458, and typically occupies the fourth
quadrant representing buildings where AEU is negative and equal
to the AEE. Buildings along the horizontal axis represent zero
energy buildings (with zero annual energy use), but it must be
noted that they could have different embodied energy, therefore
appearing closer or further from being LC-ZEBs. Fig. 2 illustrates
the representations of some generic building cases and showing
how they relate to the concept of a LC-ZEB.
As the annualized embodied energy (AEE) is likely to be always
above zero, representing the energy used in the delivery process of
the building and its components and systems, a LC-ZEB would
generally have an annual energy use (AEU) below zero. To achieve
an AEU less than zero requires that a building would need to
produce more energy than is used for running it and so requiring
the installation of some form of renewable energy systems.
However the renewable energy systems must be considered as any
other building component and so their additional embodied
energy is also annualized and enters the equation as a part of the
AEE. Therefore renewable energy installations with a high ‘net
energy’ input, that is a large ratio of energy produced to their
embodied energy, would be part of an optimum solution towards
LC-ZEB. Over-sizing of building components or renewable energy
systems with the sole intention of bringing the AEU to zero could
result in a high increase of AEE, meaning that the total annualized
life cycle energy (ALCE) might not be significantly reduced or could
even increase. The ALCE indicator would provide a true value of the
efforts to minimize energy use in the built environment and, as an
analogy to Odum’s ‘net-energy’ definition, what ‘the true value of a
design decision that decreases the annual energy use of a building is
after the embodied energy of additional building components and
systems are subtracted’