Modeling and Design of Net ZEBs as Integrated Energy Systems - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 19

There are two main steps in creating a mathematical model that describes
the energy transfer processes in a building. First, the thermal exchanges
must be modeled as accurately as is necessary; while an acceptable level of
precision is desired, too much complexity can limit the model usefulness
in analysis and design. Second, an appropriate method of solution must be
chosen to determine the room temperature and auxiliary energy loads. The
type of solution may be numerical or analytical , as long as the variables of
interestcanbedetermined.Asanoptionalthirdstep,a methodofanalyzing
the system without simulation can be developed; this is particularly
important for comparison of design options on a relative basis, for optimal
control studies and peak electricity demand reduction (see Chapter 6 ).
The degree of detail and model resolution required during the energy and
thermal analysis of a building depends on the design stage. For the early
stages of design, when the geometry of the building surfaces is not fully
fixed, a steady-state or an approximate dynamic model is often adequate.
However, more detail is required for a preliminary design, taking into
account all objectives of building thermal design and the specific
characteristics of the HVAC and solar systems considered.
Modeling the longwave radiant heat exchanges of the zone interior is more
important with direct gain systems compared to indirect gain systems and
generally requires more modeling detail, particularly if a floor heating
system is integrated. In designing solar buildings, a key objective is to store
energy in the walls during the daytime for release at night without having
uncomfortable temperature swings. If PCMs are integrated in the room
interior layers the room mean radiant temperature variation is expected to
be reduced.
A basic characteristic of a passive solar building is the strong convective and
conductive coupling between adjacent thermal zones. This coupling is very
important between equatorial-facing rooms receiving a significant amount
of solar radiation transmitted through large windows and adjacent rooms
that receive very little solar radiation. For example, heat transfer by natural
convection through a doorway connecting a warm direct gain room or a
solariumandacoolnorthfacingroom,canbeaneffectivewayofheatingthe
cool room. The transfer of heat can be controlled so as to avoid backflow to
the solarium at night by having motorized inlets that close.
Periodic conditions are usually assumed (explicitly or implicitly) in dynamic
building thermal analysis and load calculations. Heating or cooling load,