.Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.DecouplingTemperature

Information

This example is similar to Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.Decoupling except that an alternative control logic is implemented, based on the measurement of the return temperature in the consumer circuit and in the primary branch.

This control logic intends to keep constant the difference between those two measurements. Considering that we have: T_{1, ret} - T_{2, ret} = (ṁ₁ - ṁ₂) / ṁ₁ * (T_{1, sup} - T_{2, ret}), the control objective can be expressed based on the set point ΔT_set and the consumer circuit temperature differential ΔT₂ (ΔT₂ = T_{2, sup} - T_{2, ret} = T_{1, sup} - T_{2, ret}) as: ΔT_set = (ṁ₁ - ṁ₂) / ṁ₁ * ΔT₂. For consumer circuits that have a temperature differential relatively constant (see for instance Buildings.Fluid.HydronicConfigurations.ActiveNetworks.Examples.ThrottleOpenLoop), the control logic will thus maintain a nearly constant fraction of primary flow recirculation. However, at very low load if ΔT₂ value drops (due for instance to a secondary flow recirculation ensuring a minimum flow for the secondary pump) the control valve will be fully open, and the primary pump speed potentially maxed out, trying to compensate for the vanishing ΔT₂. In other words, taking the example of a heating circuit, at low load the control logic cannot infer that the low value of T_{1, ret} - T_{2, ret} is due to a consumer circuit return temperature that is too high (as it tends towards the supply temperature). It will work under the assumption that T_{1, ret} is too low, and open the control valve to try and increase the primary flow recirculation. This flawed control is showcased in this example when the parameter is_cor is set to false (the default), see plots #1 and #2 between 14 and 16 h.

Now setting is_cor to true, a correction is used to counteract this low load effect by limiting the set point to the consumer circuit temperature differential ΔT₂. Note that the implementation of that correction is specific to a change-over operation and needs to be adapted for heating-only or cooling-only applications.

Revisions

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