In Spawn there can be both connected and unconnected zones defined in the EnergyPlus input file. Connected zones have a corresponding zone model Buildings.ThermalZones.EnergyPlus_9_6_0.ThermalZone in Modelica that communicates with the EnergyPlus zone heat balance model. Unconnected zones are thermal zones which are defined entirely within the EnergyPlus input file, and for these zones the conventional EnergyPlus algorithms are used to simulate the zone conditions, including the air temperature and humidity, which are free floating. In contrast, for connected zones, Modelica models the temperature and humidity. During the initialization of a new simulation it is necessary to compute initial values for the zone air conditions as well as the conditions of any thermal mass, such as for walls, floors and ceilings. Conventionally, EnergyPlus handles this requirement using a warmup period, and in Spawn the traditional EnergyPlus warmup algorithm is employed to initialize unconnected zones. The EnergyPlus warmup algorithm is described in the EnergyPlus Engineering Reference, and summarized in the following steps.
Zone and wall surface temperatures are initialized to 23°C.
Zone humidity ratios are initialized to the outdoor conditions.
During warmup, the outdoor conditions are determined by the EnergyPlus weather file.
The first day of the simulation is repeated until warmup convergence, which occurs when the minimum and maximum air temperatures during the warmup day remain nearly the same between two successive iterations.
Spawn initializes unconnected zones using the warmup algorithm that was just described. However, connected zones are treated differently than in a conventional EnergyPlus simulation because initial zone air properties are specified in the Modelica zone model. During Spawn warmup, the following steps occur:
All wall surface temperatures are initialized to 23°C just as they are in a conventional EnergyPlus warmup period. However, as in EnergyPlus, during the warmup iterations, the exterior walls will be subject to the ambient conditions defined by the weather file. Therefore, exterior surface temperatures will not remain fixed at their 23°C initial condition during the warmup process. Similarly, room-facing wall surfaces will be exposed to the zone temperature, and therefore approach a quasi-steady state at the conclusion of warmup.
The air temperatures of unconnected zones are initialized to 23°C.
The humidity ratios of unconnected zones are initialized to the outdoor conditions.
The air temperatures and humidity ratios of connected zones are initialized to the initial values defined in Modelica, and held fixed during the warmup period.
During warmup, the outdoor conditions are determined by the EnergyPlus weather file in the same way as a conventional EnergyPlus simulation.
The first day of the simulation is repeated, but Spawn uses a different criteria for stopping the iteration compared to a conventional EnergyPlus simulation. In EnergyPlus, the first day is repeated until the zone air temperature reaches a periodic steady state as indicated by the minimum and maximum temperatures for the warmup day stablizing. In Spawn, the exit criteria is similarly based on reaching a periodic steady state, however Spawn exits warmup when the surface temperatures stabilize instead of the air temperature.