EnergyPlus is an integrated simulation. This means that all
three of the major parts, building, system, and plant, must be
solved simultaneously. In programs with sequential simulation,
such as BLAST or DOE-2, the building zones, air handling
systems, and central plant equipment are simulated
sequentially with no feedback from one to the other. The
sequential solution begins with a zone heat balance that
updates the zone conditions and determines the heating/cooling
loads at all time steps. This information is fed to the air
handling simulation to determine the system response; but that
response does not affect zone conditions. Similarly, the
system information is passed to the plant simulation without
feedback. This simulation technique works well when the system
response is a well-defined function of the air temperature of
the conditioned space. For a cooling situation, a typical
supply and demand situation is shown schematically in the
Figure 3. Here, the operating point is at the intersection of
the supply and demand curves.
Sequential Simulation
Supply/Demand Relationship.
However, in most situations the system capacity is
dependent on outside conditions and/or other parameters of the
conditioned space. The simple supply and demand situation
above becomes a more complex relationship and the system curve
is not fixed. The solution should move up and down the demand
curve. This doesn’t happen in sequential simulation methods
and the lack of feedback from the system to the building can
lead to nonphysical results. For example, if the system
provides too much cooling to a conditioned space the excess is
reported by the program as “overcooling”. Other categories of
unmatched loads exist and are similarly reported by the
program. While this kind of reporting enables the affected
system or plant components to be properly sized, the system
designer would, in most cases, prefer to see the actual change
in zone temperature. The same mismatches can occur between the
system and plant simulations when they are simulated
sequentially.
To obtain a simulation that is physically realistic, the
elements have to be linked in a simultaneous solution scheme.
The entire integrated program can be represented as a series
of functional elements connected by fluid loops as shown in
Figure “Schematic of Simultaneous Solution Scheme”. In
EnergyPlus all the elements are integrated and controlled by
the Integrated Solution Manager. The loops are divided into
supply and demand sides, and the solution scheme generally
relies on successive substitution iteration to reconcile
supply and demand using the Gauss-Seidell philosophy of
continuous updating.
Schematic of Simultaneous
Solution Scheme
In the sections which follow, the various individual
functions of the integrated solution will be described.
Integrated Solution Manager[LINK]
EnergyPlus is an integrated simulation. This means that all three of the major parts, building, system, and plant, must be solved simultaneously. In programs with sequential simulation, such as BLAST or DOE-2, the building zones, air handling systems, and central plant equipment are simulated sequentially with no feedback from one to the other. The sequential solution begins with a zone heat balance that updates the zone conditions and determines the heating/cooling loads at all time steps. This information is fed to the air handling simulation to determine the system response; but that response does not affect zone conditions. Similarly, the system information is passed to the plant simulation without feedback. This simulation technique works well when the system response is a well-defined function of the air temperature of the conditioned space. For a cooling situation, a typical supply and demand situation is shown schematically in the Figure 3. Here, the operating point is at the intersection of the supply and demand curves.
However, in most situations the system capacity is dependent on outside conditions and/or other parameters of the conditioned space. The simple supply and demand situation above becomes a more complex relationship and the system curve is not fixed. The solution should move up and down the demand curve. This doesn’t happen in sequential simulation methods and the lack of feedback from the system to the building can lead to nonphysical results. For example, if the system provides too much cooling to a conditioned space the excess is reported by the program as “overcooling”. Other categories of unmatched loads exist and are similarly reported by the program. While this kind of reporting enables the affected system or plant components to be properly sized, the system designer would, in most cases, prefer to see the actual change in zone temperature. The same mismatches can occur between the system and plant simulations when they are simulated sequentially.
To obtain a simulation that is physically realistic, the elements have to be linked in a simultaneous solution scheme. The entire integrated program can be represented as a series of functional elements connected by fluid loops as shown in Figure “Schematic of Simultaneous Solution Scheme”. In EnergyPlus all the elements are integrated and controlled by the Integrated Solution Manager. The loops are divided into supply and demand sides, and the solution scheme generally relies on successive substitution iteration to reconcile supply and demand using the Gauss-Seidell philosophy of continuous updating.
In the sections which follow, the various individual functions of the integrated solution will be described.
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This documentation is made available under the EnergyPlus Open Source License v1.0.