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2. Hybrid Renewable Energy System Modeling 259

customers at lower prices compared with what they pay for any consumption
from the grid. Renewable energy systems that are connected off-grid require sup-
plement energy sources to enable continuous supply of power to the premises.
The supplement sources could be either another renewable energy source com-
bined with storage energy subsystem and/or a diesel generator (DG). Available
storage energy systems in the market differ depending on the used renewable en-
ergy source [1]. Batteries in all kinds are mainly used to store energy for PV and
wind energy systems [2]. Different kinds of fluids are also used to store energy for
concentrated solar power (CSP) systems that are also known solar thermal systems
[3]. Other energy storage subsystems such as hydrosystems [4], supercapacitors
[5], and fuel cells [6] are also used to store energy produced by renewable energy
sources.
In this chapter, hybrid PV solar energy systems are investigated for off-grid
applications. These systems are constructed from PV solar panels, batteries
bank, and a DG. The sizing of these renewable energy systems is done for
different loads in Tunisia using particle swarm optimization (PSO) algorithm.
Optimal sizing of each part of this hybrid renewable energy systems is achieved.
The optimization process criterion for these systems is either lower overall sys-
temcostorlowerCO 2 emission of the system. The main source of pollution in
the hybrid energy system is the DG. The optimal depth of discharge (dod) value
of the batteries bank is also investigated for both conditions of lower cost of the
renewable energy system and for lower pollution emission. Energy management
of the hybrid renewable energy system is also studied and different proposals
are discussed. These proposals are introduced to avoid blackouts that may result
when switching the DG ON when the load power requirement is not satisfied by
available energy of the PV subsystem and batteries bank. Blackouts typically occur
because of an amount of time needed by the DG to reach steady-state power level
after it is turned ON. Thus, the DG must be turned ON before the batteries bank
dod reaches the set value, which is typically 80% [7]. This condition will allow
the DG to stabilize before the batteries stored charge reaches the depleted level
of 80%.

2. HYBRID RENEWABLE ENERGY SYSTEM MODELING
Fig. 8.1 shows a general block diagram of a hybrid renewable energy system. The
main parts used in the system include PV panels, batteries bank, and a DG. Each
part of thehybridenergysystemis modeled andanalyzedinthissection. Atwo-
diode nonlinear model that depends on solar irradiation and ambient temperature
is used to characterize the PV cells. The battery model includes the dod param-
eter, which is used to characterize the status of charge of the battery. The dod
parameter is used to control turning ON/OFF the DG. The DG model comprises
a diesel engine (DE), a synchronous generator, and an excitation subsystem
model.