Energy Storage System Design docx.

ENERGY STORAGE SYSTEM DESIGN
Introduction
As the global economy begins to strain under the pressure of rising petroleum prices and
environmental concerns, research have spurred into the development of various types of clean
energy transportation systems such as Hybrid Electric Vehicles (HEVs), Battery Electric
Vehicles (BEVs) and Plug-In Hybrid Electric Vehicles (PHEVs) [1–5]. Especially PHEVs
acquire the most attention due to the combination of electrical source and conventional engine.
This type of vehicle provides the user a considerable pure electrical range and also an extended
range, which can be performed by a conventional Internal Combustion Engine (ICE). The
establishment of a Rechargeable Energy Storage System (RESS) that can support the output
power during acceleration, efficiently use the regenerative energy and perform for a considerable
cycle life are the critical aspects to be met by battery technologies [6–8].
During the last decade, a series of hybridization topologies have been proposed in order to
enhance the power density and cycle life performances of energy storage systems [9–12]. In [13–
19] is documented that the combination of Valve-Regulated Lead-Acid (VRLA) battery and
Electrical Double-Layer Capacitors (EDLCs) can result into an extension of the battery life and
an increase of the energy efficiency and power capabilities. However, in [13] Omar et al.
underlined that the association with EDLCs is still expensive due to the high cost price of the
DC-DC converter.
In [8] Cooper et al. proposed a new technology, called Ultra Battery. This RESS technology
combines in the same battery cell the advantages of the EDLC and lead–acid batteries by using
an asymmetric approach. However, this technology is still in developing process and its real
performances are under investigation.
From the beginning of the nineties, lithium-ion technology has received considerable attention
due to the high energy density, power capabilities compared to VRLA, nickel-metal hydride and
nickel cadmium based technologies [20–34]. Due to the lowest standard reduction potential (E°
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= −3.04 V), equivalent weight (M = 6.94 g/mol) and high exchange current density, lithium as an
electrode can be considered as a most favorable material as it is presented in Table 1.
Design of Energy Storage System
The considered configuration of the VEIL powertrain consists of a battery bank, a supercapacitor
bank, a photovoltaic panels array, a DC/DC converter, an inverter, an AC motor, and a
transmission. The multiple power converter regulates the VFD DC-link voltage and adapts the
different voltage level of the considered Energy Storage Systems (ESSs). The VFD inverter
converts the regulated DC voltage to an AC voltage to drive the AC motor. The transmission is a
gearbox that increases the motor torque using a fixed gear reduction. When the EV demands high
power, the batteries and SCs provides power to the vehicle’s wheels through the DC/DC
converter, the inverter, AC motor, and the transmission. On the other hand, when the EV
demands low power, the batteries provides power to the wheels through the DC/DC converter,
the inverter, the AC motor, and the transmission and charges the SCs through the reversible
DC/DC converter. When the vehicle brakes, the AC motor converts the kinetic energy of the
vehicle into electricity and charges principally the SCs and residually the batteries through the
inverter and the DC/DC converter using the generated energy. The solar panels array works as an
energy back-up, as, whenever solar energy exists, the panels generate energy that is used by the
system. In the tractive mode, this assists the stored energy and when the vehicle is parked, it
provides a charge of the storage systems. The power generated and stored in the energy system
(Pst) at any time should be at least equal to EV power demand (Pdem). st ≥ PP dem (1) Pst is
composed by the batteries power (PBat), the SC power (PSC), the PV array power (Ppv) and the
regenerative break power (Preg): st Bat pvSC +++= PPPPP reg (2) On the other hand, the PV
array has not a response capability for a high power request and should be seen as a backup
system in order to recharge the batteries for a prolonged use, for daily journey and for extended
parking periods, and for covering its self discharge. With this consideration, (2) can be simplified
to (3), and the PV array energy complement is analyzed separately of the other ESSs. st Bat SC +
+= PPPP reg (3) The proposed methodology is based on the combination of ESSs with the
lowest cost and weight, minimizing the number of storage units in series and parallel,
maximizing autonomy and with a relatively better performance. For the considered
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hybridization, the three energy devices can be divided into two types: a classic type (batteries
and SCs), because, traditionally, the EVs use only batteries or more recently batteries and SCs,
and a special type, as it is not common the EVs to use photovoltaic panels. Therefore, the design
methodology needs to divide into a classic energy storage design and a backup energy supply
design.
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