modelInjectionTwoWayConstant

Information

This model represents a heating system where the configuration Buildings.Fluid.HydronicConfigurations.ActiveNetworks.InjectionTwoWay serves as the interface between a variable flow primary circuit and a constant flow secondary circuit. Two identical terminal units are served by the secondary circuit. Each terminal unit has its own hourly load profile. The main assumptions are enumerated below.

• The design conditions are defined without considering any load diversity.
• Each circuit is balanced at design conditions.
• The pump dissipated heat is not added to the fluid.
• The consumer circuit has either a constant (supply or return) temperature set point if have_resT2=false or a temperature reset if have_resT2=true. The reset logic is based on the terminal valve opening, with the most open valve being kept 90% open.

Without temperature reset (have_resT2=false), the primary flow variation with the load is not optimal (see plot #8): for a load fraction of 30% the normalized primary flow rate is about 60%.

The flow reduction is enhanced when using a reset based on the maximum valve demand: for a load fraction of 30% the normalized primary flow rate is now close to 30%. (Also note the setting of the controller resT2 which ensures a reset at design value when the control loop is enabled).

The flow reduction is further enhanced when using a control based on the return temperature (have_resT2 = false and con(typVar=Types.ControlVariable.ReturnTemperature)): the normalized primary flow rate varies close to linearly with the load fraction. This explains why this control strategy is often adopted as it brings a good flow rate variation with the load at a first cost lower than the previous reset option based on the valve demand. However, it also brings some additional constraints on the sizing of the terminal units. The load diversity must indeed be accounted for. When tracking the return temperature of a constant flow consumer circuit, the supply temperature will vary with the aggregated load. In our example, the actual value of the secondary supply temperature is lower than its design value at partial load, which yields unmet loads (see plot #4). The terminal units should be sized accordingly, based on the lowest possible ΔT when one terminal unit may still be at peak load. Additional caveats speak against the use of return temperature control with this hydronic configuration in the case of variable flow consumer circuits, see Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.InjectionTwoWayVariableReturn.

Parameters

Components

Revisions

• June 30, 2022, by Antoine Gautier:
  First implementation.