Net ZEB Design Processes and Tools - Modeling, Design, and Optimization of Net-Zero Energy Buildings - page 203

and Galal, 2010a; Chen, Galal, and Athienitis, 2010b). The requirement for
a coupled model in this case was undeniable; the usefulness of the thermal
energyoutputofthecollectorishighlydependentonthedemandforheating
within about 12 h (the approximate storage capacity of the ventilated slab).
Furthermore, the ability of the collector to offset purchased heating energy
is highly dependent on the collector outlet air temperature and the slab
temperature. The collector outlet temperature must be at least as warm as
the ventilated slab; ideally at least 5 °C warmer to justify the fan energy use
and compensate for losses in the duct that runs to the slab. In general, it is
essential for renewable energy systems that are integrated into the HVAC
system to be modeled as a coupled system because usability of the collected
energy must be quantified. Arguably the most powerful and flexible tool for
modeling custom mechanical systems is TRNSYS.
Most modern detailed BPS engines have evolved from using separate
building and HVAC models to combining them such that their performance
is solved simultaneously at each time step. Figure 4.17 depicts the flow of
data between buildings and HVAC equipment in conventional and modern
tools. For each timestep, decoupled models (i.e., the “loads-systems-plant
sequence”(McQuiston,Parker,andSpitler,2005))proceedfromlefttoright
in this figure with no feedback loops. This means, for example, that the
heat load data required to maintain a space at the setpoint temperature is
sent to the secondary or terminal HVAC equipment (e.g., a hydronic radiant
heating panel). This, in-turn is used to calculate how much thermal energy
is required from the plant. Finally, the plant model determines the amount
of fuel or electricity required to supply that amount of thermal energy. Each
step is essentially a transfer function that quantifies the energy conversion
efficiency. However, the underlying assumption is that each system to the
right of the previous one is capable of supplying exactly the requested
amount of energy and at that exact moment or timestep. Thus, complex
interactionsbetweenaplant,thedistributionsystem,andspacesmaynotbe
accurately represented. This may be a major limitation for Net ZEBs, which
generallyseekpassivesolutionsinordertoreducebothenergyuseandplant
capacity. Furthermore, decoupled HVAC models also have limited accuracy
if the HVAC system has substantial thermal mass (e.g., a concrete radiant
heating floor) (McQuiston, Parker, and Spitler, 2005).
Program such as BLAST and DOE-2 use decoupled models, while
EnergyPlus and ESP-r use coupled models (McQuiston, Parker, and Spitler,
2005). Since the burden of coupling the building and HVAC models is