Modeling and Design of Net ZEBs as Integrated Energy Systems - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 21

approximations. Examples include combining radiative and convective
heat transfer coefficients (so-called film coefficients commonly
employed in building energy analysis), assuming that many surfaces are
at the same temperature, or considering certain heat exchanges as
negligible. Such approximations need to be carefully selected and
applied by taking into consideration the expected temperature
variations (spatial and temporal) in a zone. For example, a zone with
large windows and floor heating may exhibit large spatial temperature
variations, in which case the use of combined film coefficients would
result in high errors in room operative temperature and floor heating
rate calculations.
A major part of the modeling process considers transient heat conduction
in the building envelope. In most cases relating to heating or cooling load
estimations, energy savings calculations, and thermal comfort analysis, it is
generally accepted that one-dimensional heat conduction may be assumed.
Thermal bridges, such as those present around corners and at the structure,
are generally accounted for in calculating the effective thermal resistance
of building envelope elements by using a more detailed spatial model or
simplified techniques, such as the parallel heat flow path method. However,
the thermal storage process may usually be adequately modeled as
one-dimensional for well-insulated buildings. For steady state calculation
of thermal bridge effects, a 2D or 3D calculation of thermal conductance is
sometimes desirable (e.g., parallel heat flow method (ASHRAE, 2009a)).
Direct gain zone modeling (i.e., a zone with high interior solar gains)
includes certain important requirements in addition to those required for
traditional building modeling. In particular, there is an increased need to
address thermal comfort requirements and to allow the room temperature
to fluctuate so as to enable storage of direct solar gains in building
integrated exposed thermal mass. In addition, for an office environment,
daylighting considerations will dominate, such as the need to uniformly
distribute daylight and to prevent glare.
Peakheating/coolingloadcalculationsareamajoraspectofheating/cooling
equipment sizing and need to take into account building thermal storage
capacity and dynamic variation of both solar radiation and outdoor
temperature, in order to avoid over-sizing of HVAC systems. For most mild
temperate climates, a heat pump will provide an efficient auxiliary heating
and cooling system. Well-insulated buildings with effective shading systems