Civil Engineering Reference
In-Depth Information
between changes in the impulse air temperature
T
a
imp
and a global control signal
u
are of the form
P
085
◦
C
(
)
=
k
sg
/(ʸ
ol
s
+
)
where
k
sg
=
.
/
ʸ
ol
=
s
1
0
% and
40 s for
0819
◦
C
winter mode, and
k
sg
=−
0
.
/
% and
ʸ
ol
=
36 s for summer. Therefore, as a
function of the previous transfer functions,
P
(
s
)
, the PI controllers can be defined as
C
PI
(
s
)
=
K
(
1
+
sT
i
)/
sT
i
where a closed loop time constant,
ʸ
cl
, two times bigger
than the open loop one,
ʸ
ol
, is desired. Although the controller can be considered
too conservative with these parameters tuning, this election has been made with the
main idea of avoiding abrupt changes in the indoor air temperature which can affect
users. Thus, the PI parameters for both operation modes must be
K
k
sg
and
T
i
=
ʸ
ol
. In addition, to avoid actuator saturation in this layer, anti-reset windup
functions (Ogunnaike and Harmon Ray
1994
) have been added to the PI controllers,
=
0
.
5
/
which are defined by a tracking time constant of
T
t
=
√
T
i
s
.
Finally, from that global control signal,
u
, estimated by each one of the PI con-
trollers, the associated control signals for the aperture of the valve,
Ap
Va l ve
, and the
fan velocity,
V
Fan
are obtained in an efficient way through their associated split-range
control strategy.
5.4.2.2 Split-Range Control
The split-range control technique is commonly used when there are several
manipulated variables to reach a certain reference. Furthermore, in general, the split-
range control technique is used for temperature control of small plants, and for year-
round heating and cooling of office buildings (Ogunnaike and Harmon Ray
1994
). In
this case, split-range controllers are used to define a fixed relationship between the
global control signal provided by the PI controllers,
u
, and the available manipulated
variables of the fancoil unit: the aperture of the water flow valve,
Ap
Va l ve
, and the
fan velocity,
V
Fan
.
These relationships are estimated based on the combined influence of each manip-
ulated variable in the impulse air temperature, and also, taking the energy consump-
tion derived from the use of each one of the manipulated variables into account.
Hence, after a detailed study of the energy consumption derived from each of them,
it has been deduced that the use of the fan velocity is more efficient, in energy terms,
than the use of the water flow valve, since in the first one only the motor from the
fan has to be considered, while in the other, the use consumption derived from the
use of the solar cooling installation (mainly pumps and the boiler) should also be
taken into account since the use of the water flow valve entails the intervention of
the regulation system of the water impulsion pump and, therefore, a rise in energy
costs with respect to the fancoil blower. Therefore, to develop ideal energy efficient
controllers, they should, in a first attempt, try to reach impulse air temperature set-
point through the use of the fan velocity, and only when it is not possible to reach this
setpoint with the fan velocity, combine the use of the fan velocity and the water flow
valve. Thus, split-range controllers have been defined as a function whose behaviour
can be observed in Fig.
5.23
for winter operation mode, and Fig.
5.24
for summer. In
addition, in Figs.
5.23
and
5.24
, it is shown that, the point in which the water flow
