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4. Energy Management 279

15000

Load(W) 10000

5000

0
0 0.5 min 2 min 4 min 6 min
Time
FIGURE 8.17
Time required for the diesel generator to supply stable power to the load.

does not consider the time between turning the DG ON and reaching its steady-state
operation. The DGs requires certain time to reach its steady-state output power
depending on starting conditions of the prime movers (cold start or warm start).
The adopted DG model showed that it needs about 30 s to deliver stable output po-
wer as shown in Fig. 8.17. Typically, in the field 2 min are budgeted for the DG to
reach steady output. This time must be taken into account when designing power
management scheme for a renewable system to avoid power blackouts. Thus, the
DG must be turned ON before the batteries dod reaches a maximum set value
(e.g., 80%) to avoid blackout. Optimal values of dod are found in this section for
different load demands at which the DG is turned ON before the batteries state of
charge reaching a maximum dod value. The optimal values are found for the system
that was sized in Section 3 [9].

4.2 HYBRID RENEWABLE ENERGY SYSTEM ENERGY MANAGEMENT:
CASE STUDY
Fig. 8.18 illustrates a standalone hybrid PV/batteries bank/diesel generator renew-
able energy system. The hybrid system of this case study is designed such that PV
panels and batteries bank only supply the load during the daytime while the DG
operates only at night. The batteries are used to minimize the DG operational
time to minimize cost and emitted pollution. A novel energy management strategy
is used during the night to control when to turn ON the DG to avoid blackout. The
dod maximum value used in the literature is 80% and the DG is turned ON when
the batteries bank dod reaches 80%. However, the DG has slow dynamic behavior
while starting as shown in Fig. 8.17. Thus, the DG must not wait until the dod value
reaches 80% because that will cause blackout. On the other hand, a dod value of
70%, for example, cannot be used because there will be huge waste of power avail-
able in the batteries bank which will result in an extra consumption of fuel. Conse-
quently, an optimal dod value must be found for each load required power at which
theDGmustbeturnedONbeforebatteries dod reaches 80% value.