Z E M C H 2 0 1 2 I n t e r n a t i o n a l C o n f e r e n c e
38
nearby. Since the Directive does not specify minimum or maximum harmonized
requirements as well as details of energy performance calculation framework, it is up to
the Member States to define the exact meaning of “high energy performance” and
“amount of energy from renewable sources” according to their own local conditions and
strategic interests. Nearly zero-energy performance derives from net zero-energy
concept which in case of buildings is usually defined as a high energy performance
building that over a year is energy neutral (i.e. net balance of primary energy is 0
kWh/(m² y)). Therefore, a possible way to assess the nearly zero-energy performance is
by analysing the annual energy balance in Net Zero-Energy Buildings. Net Zero-Energy
Buildings have been the object of various studies in recent years as various countries
have set this performance as long
‐
term goal of their energy policies (Ayoub 2009,
Aelenei et al. 2011, Sartori et al. 2012). The International collaborative research initiative
between the Solar Heating and Cooling (SHC) and the Energy Conservation in Buildings
and Community Systems (ECBCS) through Task 40/Annex 52 - “Towards Net
‐
Zero
Energy Solar Buildings”, summarises most of the recent developments in the field (IEA
2008).The approximate 55 National Experts, among which can be found the authors of
this work, together with other 25 regular participants and contributors, are currently
researching net
‐
zero, plus energy and near net
‐
zero energy buildings and in order to
develop a common understanding, a harmonized international definitions framework,
tools, innovative solutions and industry guidelines for NZEBs.
NZEB performance
In the international context, there are four main types of NZEBs: Net Zero Site Energy,
Net Zero Source Energy, Net Zero Energy Cost and Net Zero Energy Emissions. Net
Zero Site Energy means that the annual balance is based on the grid interaction at the
boundary of the building site, i.e. the overall energy delivered to the building from the
utility grid has to be offset by the overall energy feed in to the grid. In the Net Zero
Source Energy definition, which is the one that matches the currently used by EPBD
recast in a nearly zero-energy context (EPBD 2010), the energy (delivered from and feed
into the grid) has to take into account primary energy conversion factors. Net Zero
Energy Cost buildings definition is based on a economic balance (the energy bills of a
building are equivalent the amount of money the utility pays the owner for renewable
energy the building feeds to the grid) whereas in the Net Zero Energy Emissions case,
buildings produce and export at least as much emissions-free renewable energy as they
import and use from emission-producing sources on an annual basis (Torcellini 2006).
Although there is no standard approach for designing and realizing a Net Zero Energy
Building (there are many different possible combinations of building envelope, utility
equipment and on-site energy production equipment able to achieve net-zero energy
performance and also the balance boundary, which defines which consumers are
included in the balance differs in known approaches) there is some consensus that zero
energy buildings design should start from passive sustainable design as this level of
performance is achieved as a result of executing two fundamental steps: (a) reduce
building energy demand and (b) generate electricity or other energy carriers to get
enough credits to achieve the desired energy balance (Fig. 1) from RES. As one can
easily imagine, passive approaches play a crucial role in addressing NZEB design as
they directly affect the heating, cooling, ventilation and lighting loads put on the buildings
mechanical and electrical systems, and indirectly, the strive for renewable energy
generation.
Case Studies
Table 1 presents a summary of the main technical features of the 9 projects selected
from the IEA Task 40 project data base for analysis (Musall 2012). As it can be seen
from Table 1, buildings are characterised according to location, conditioned floor area,
