User Manual

White paper | Saving Energy using PICVs | June 2017
© Siemens Switzerland Ltd, 2017 4
Improving Control Accuracy
PICVs Eliminate Overflows with Dynamic-Balancing
As stated in the description of the PICV working principle,
the use of PICVs limits the maximum flow at part-load
conditions and thus avoids the mentioned increase of direct
energy demand (generation, consumption) and indirect
energy demand (transport, distribution).
Hydraulic Cross-Coupling Triggers Variations of
Temperature in the Building
As described above, a section of the heating or cooling
system may temporarily increase (or decrease) its energy
demand, for instance when a meeting room is filled with
people at the beginning of a workshop or empties at the
end. This happens everywhere in the building, at different
moments, in different places.
This increase in energy demand in certain sections of the
system leads to a reduction of the energy supplied to other
areas of the building. The temperature of these areas then
deviates from the setpoint and it takes time until the room
thermostat triggers the appropriate response. The
temperature will then follow a cycle of increases and
decreases of temperature and stabilize again over time
around the desired setpoint (Figure 4). This effect is called
‘‘hydraulic cross-coupling.’’
The first issue with hydraulic cross-coupling is that users of
the building experience periods of discomfort when the
temperature is at its lowest or highest point in the cycle.
Users Shift the Setpoint to Reduce Discomfort
The second issue is that users will typically change the
temperature setpoint when they experience some degree of
discomfort. For example, when the temperature is at its
lowest point in the cycle during the cold months, they may
increase the setpoint by a couple of notches. The whole
curve is shifted up one or two degrees. However, they will
likely not react an hour later, when the room temperature is
a bit higher than usual. The setpoint shift stays for the
whole season.
A similar scenario takes place during the hot months. When
the room is at the hottest temperature in the cycle, the
users may crank up the cooling, without turning it back
down later on when the temperature is at the lowest point.
In both heating and cooling cases, the overall energy
demand is increased because of the variations in
temperature caused by the hydraulic disturbances.
PICVs Nearly Eliminate Temperature Variations
When PICVs are used, their auto-balancing functionality
compensates for the variations in pressure. They allow for
much better control accuracy at the setpoint and hence
virutally eliminate the temperature swings (Figure 5).
Figure 5: PICVs automatically compensate for the
variations in pressure and maintain the room temperature
very close to the setpoint.
Figure 4: Because of cross-coupling, the temperature
deviates from the setpoint. Delayed correction of the
room temperature leads to wide temperature fluctuation,
less comfort and energy loss.