varies with variation of the OA supply temperature (Zhu et al.,
2014b). The VRF unit which is basically designed for maintaining
the indoor air temperature is also greatly affected by
the OA supply temperature. On the other hand, the outdoor air
flow rate, determined by the DCV strategy, does not need to be
further optimized. Therefore, only the OA supply temperature
set-point is adopted as the optimizing (control) variable. The
OA supply temperature set-point should not be higher in
cooling mode (and lower in heating mode) than OA dry bulb
temperature. It is also found that there is a reciprocal relationship
between the cooling capacity of the VRF unit and the
OAP unit as the OA supply temperature changes. Therefore, to
decide the optimal OA supply temperature is to find the best
load ratio (LR) of the OAP unit to the whole load. In other words,
if the OAP unit provides LR of the total needed cooling/heating,
the rest part, i.e., 1-LR, should be provided by the VRF unit
to maintain indoor temperature at the set-points. Therefore,
the problem of searching the best OA supply temperature
turns into optimization of LR.
The logic of the optimal control strategy is shown in Fig. 3.
During the time of the system startup, the optimal control is
not activated and the OA supply temperature is set at a fixed
set-point until the room temperature equals to or closes to its
set-point. Once the optimal control is activated, predicting
models of the energy consumption of VRF unit and OAP unit
are implemented first, from which energy consumption of the
system in the next time-step can be predicted based on the
operating data of the present and previous steps. These
operating data are updated and stored every time-step. The
premise is that the operating conditions are constant or their
changes are negligible during a short prediction time step
(Wang and Jin, 2000). The following sub-sections discuss the
energy consumption predicting models both in cooling and
heating modes, and the optimization algorithm of LR.
3.2. Energy consumption predicting model of VRF unit
The VRF unit includes an outdoor unit and several indoor
units as shown in Fig. 1. Each indoor unit consists of a DX coil
and a supply fan. The OAP unit can also be simply treated as
the combination of a DX coil and a supply fan. Without loss of
generality, all the DX coils are assumed to be of the same type
and have similar characteristics. Similar to the modeling
methodology illustrated in literature (Zhou et al., 2008), the
overall energy consumption of a VRF unit consisting of several
DX coils is estimated using the following equations: