Renewable energy systems require power conditioning to integrate with the electric grid. They may utilize energy storage systems to store excess energy for times when renewable generation is low. Key points: 1) Renewable sources like solar and wind utilize power converters for maximum power point tracking and voltage regulation before connecting to the grid. 2) Energy storage systems store excess renewable energy in batteries, hydrogen, or other means. Batteries provide immediate storage while hydrogen can be transported and stored. 3) Renewable energy networks integrate diverse renewable generators and energy storage to supply power to the electric grid and various loads.

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CHAPTER 10 RENEWABLE ENERGY AND ENERGY STORAGE SYSTEMS
10.1. Introduction
All alternative energy sources have power processing stages for power conditioning. Power
conditioning is to provide voltage, current or impedance variations in order to provide a matching
between the source and load. The source can be in AC or DC that depends on its alternative energy
source. Power processing units are to convert the AC or DC into suitable voltage format and level for its
load or intermediate power stage.
Alternative energy sources such as photovoltaic (PV) systems and fuel cells are DC sources. Their
voltage outputs are not constants and depend on their energy input and the loading. For photovoltaic
systems, their output voltages depend on the illumination level to the cell and loading current. Therefore
a power conditioning unit is usually designed with maximum power point tracing (MPPT) to optimise
the efficiency. Voltage of a fuel cell depends on the input fuel concentration and rate. It also depends
on the loading. Maximum power point tracking is also necessary for fuel cells.
For electromechanical machine types of alternative source, they are usually with induction generator,
synchronous generator or switched-reluctance generator so that their outputs are AC and needed to be
regulated. Some generators also require PWM driver to move the operating point to certain frequency
range for power optimisation and frequency control. Output stages of the generators require AC/DC
voltage conversion for voltage regulation or AC/AC voltage conversion for both voltage and frequency
regulation. The AC/DC converters are bridge types of topologies for high power processing. Power
factor of the system is also needed to be controlled so that the generator is working at almost unity
power factor in order to optimise the efficiency of the machines.
Since power processing stage of renewable energy systems is with high power, switched mode power
high power converter are usually applied such as bridge converters. Today, many power conversion
circuits are available. They provide bidirectional power flow, resonant switching or soft-switching, and
multiple output voltage. Grid connected DC-AC power conversion is also very popular for second stage
power conversion which is further processed the intermediate power stage which is usually a DC to other
load.
10.2. Energy Storage Systems
Renewable energy systems such as photovoltaic systems can be standalone and supplying power to load
or can be connected to the grid. Energy storage systems used in renewable energy system are for storing
the energy when renewable power is generated and releasing power when renewable power is not
sufficient. The followings introduce different topologies of energy storage systems with different energy
storage devices.
10.2.1. Battery Energy Storage Systems
A battery energy storage system (BESS) provides immediate energy storage in a rechargeable battery or
battery bank. High performance batteries, battery chargers and are necessary for high efficiency, fast
response, high power and high energy density. Fig. 10.1 shows an example of a BESS for photovoltaic
(PV) system. In this figure, the DC/DC converter is controlled with maximum power point tracking
(MPPT) to maximise the output power with appropriate output voltage and current of the PV. The
battery charger is responsible for charging the battery when the PV is providing power. When the
renewable power is not sufficient, the battery releases power to the DC/DC converter for providing a DC
link voltage of the DC/AC inverter. The inverter converter injects the AC power to the AC grid.

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DC/DC
Converter
Inverter
Battery
Charger
Battery
Photo Voltaic AC
DC
Fig. 10.1. A BESS for a PV system
Inverter
AC
Battery
AC/DC Conversion
Charging Battery
Fig. 10.2. A BESS connecting to AC grid
BESS can be also applied in power utility. There are many topologies of BESS. Most of them are based
on bidirectional inverter for the connection between the AC grid and the battery as shown in Fig. 10.2.
Many types of rechargeable batteries can be used as the energy storage devices in BESS. Table 10.1
lists the comparison of different types of batteries. It shows that the efficiency and the energy density of
lithium-ion (Li-ion) batteries are the highest. There are many types of Li-ion batteries developed such as
lithium cobalt oxide (LiCoO2) batteries which are commonly used and high energy density, and lithium
iron phosphate (LiFePO4 or LFE) batteries which are lower safety risk but lower energy density.
Table 10.1. Comparison of different types of batteries
Type Cell voltage (V) Density (Whr/kg) Efficiency (%)
Lead-acid 2.1, 2.2 30-40 70-92
Ni-Cad 1.2 40-60 70-90
Ni-iron 1.2 50 65
NiMH 1.2 30-80 66-95
Li-ion Based 3.6 160 99
Nano Titanate 13.8 90 87-95
10.2.2. Hydrogen Energy Storage Systems
Besides BESS, hydrogen energy storage
system (HESS) has been using recently. Fig.
10.3 shows a block diagram of a HESS. This
type of systems usually consist of the fuel
cells for energy conversion. In a HESS,
power from alternative energy source is not
storage in battery but through electrolyser to
chemically decompose water into oxygen and
Electrolyser Fuel Cell
Water Oxygen
Alternative
Energy Source
Load or DC Bus
H Hydrogen
Storage Tank
Hydrogen
Storage Tank
Oxygen
or Air
Water
or Steam
Fig. 10.3. Block diagram of a HESS with Fuel Cell
hydrogen. The hydrogen is stored in the compressed high pressure storage tank. The storage tank can
be transported to other location for energy source. The stored hydrogen can also be transported through
gas pipe to other location. When generating power from the HESS, the fuel cell in the HESS converts
hydrogen and oxygen to water, and generates DC power. MPPT is necessary for the fuel cell.
The best efficiency of electrolyser is around 85%. The general efficiency of a fuel cell is around 35%
and the best efficiency is around 40-50%. Efficiency of a HESS is expected to be 30%. Comparing with
the information listed in Table 10.1, it is obvious that efficiency of BESS is higher than HESS, however,
energy density of hydrogen tank is higher. For a low pressure tank, 170 Whr/kg of energy density can be
achieved. For some commercial tanks, such as the hydrogen energy storage manufactured by
Millennium Cell, energy density can be up to 425 Whr/kg.
10.2.3. Other Energy Storage Systems
Except BESS and HESS, other energy storage systems have been developed. Ultracapacitor (or called
supercapacitor) is the most popular alternative energy storage device for energy storage systems because

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of its high power density, i.e., high charging and discharging current ratings and fast dynamic response.
Comparing to batteries, energy density of ultra-capacitors is low which is around 3-6 Whr/kg. Voltage
of each ultracapacitor is around 2.7-16.2V. Their capacitance is very high and can be over thousands of
Farads. The energy stored in an ultracapacitor is 0.5CV2
. Because of the low energy density and high
price, ultracapacitors are rarely used for high energy storage. With faster dynamic response than
batteries, ultracapacitors can be used as assistive energy storage devices such as connected in parallel
with batteries to improve the total dynamic performance. A typical example of application of
ultracapacitor is as the assistive energy storage device in electric vehicles.
Another one is superconducting magnetic energy storage system. In this system, a coil constructed with
superconductor acts as the energy storage device. Energy is stored in DC current manner. Since
resistance of superconductor is very low, the stored DC current flows and freewheels internally in the
superconducting coil. The energy stored in the superconducting coil is 0.5LI2
. As the high cost for cold
superconductor cold support, this type of energy storage system is not popular.
10.3. Renewable Energy Systems for Utility
Energy stored in energy storage systems can be converted directly to AC grid by bidirectional inverters.
Some renewable energy systems, such as photovoltaic systems and wind power systems, use very little
or no energy storage system, and are connected to the grid directly.
10.3.1. Photovoltaic Systems
Ele
Fig. 10.4 shows a typical connection of photovoltaic (PV) systems generating power to an AC grid via a
DC bus. In this figure, the DC bus is connected to a numbers of PV panels. The PV panels are
connected in series or parallel. For each parallel connection, each set of PV panels is connected to the
DC bus through a DC/DC converter. Parallel sharing current control is applied in order to ensure there
is no circulating current in the system. Maximum power-point tracking (MPPT) is applied to control the
DC/DC converters to prevent from overloading of the PV panels. The power of the DC bus is converted
to AC to the grid by a voltage source inverter. Modulation index (M) of the inverter can be varied from
0.4 to 1.15. Too small and too high M gives poor harmonic spectrum so that it has to be avoided.
Typical M of the inverter is selected to be around 0.9.
PV PV PV
DC/DC
Converter
PV PV PV
DC/DC
Converter
PV PV PV
DC/DC
Converter
Parallel
Current
Control
DC
Bus
Inverter
AC
Grid
Fig. 10.4. Typical connection of PV systems to an AC grid
10.3.2. Renewable Energy Systems with Electromechanical Machine
Many renewable energy sources rely electromechanics to convert the natural power to electrical power
such as wind power, tidal power and hydro power. Some combustion generators from green or organic
gas such as hydrogen and organic methane are also based on electrical machines. There are many

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different types of machines suitable for electrical generation. The most typical machines are doubly-fed
induction generators, synchronous generators, induction generators and switched reluctance generators.
Fig. 10.5 shows schematic diagrams of the popular connections of generators for wind power systems.
M
Mechanical
Subsystem
Doubly-fed
Induction Generator
AC/AC
Converter
AC
Rotor
Stator
(a) Doubly-fed inductor generator
M AC/DC
Converter
+
Inverter
Induction Generator
AC
DC
Mechanical
Subsystem
(b) Induction generator
M AC/DC
Converter
+
Inverter
Synchronous Generator
AC
DC
Inverter
Mechanical
Subsystem
(c) Synchronous generator
M SRG
Converter
+
Inverter
Switched Reluctance Generator
AC
DC
Mechanical
Subsystem
(d) Switched reluctance generator
Fig. 10.5. Topologies of wind turbine
Some generators produce variable frequency and variable voltage output. In the past, mechanical
systems were used for regulating the output stage to produce constant frequency and constant voltage.
Nowadays, variable frequency to constant frequency (VCVF) converter can be used to instead of using
mechanical method. There are different types of topologies suitable for this application. They are
AC/DC/AC converters, matrix converters and cycloconverters. Indeed, cycloconverters have less
flexibility on voltage and frequency control and is less popular.
10.3.3. AC/DC/AC Converters for Renewable Energy System
Basically an AC/DC/AC converter is based on a cascade connection of an AC/DC conversion circuit and
a DC/AC inverter. The intermediate stage is a DC link stage which has some capacitors or even
batteries for intermediate energy storage and voltage smoothing. The capacitor or the battery can also
provide a prolonged uninterrupted time in case the input stage fails to supply power. The AC/DC
conversion circuit is usually a rectifier or a power factor correction converter. The DC/AC inverter is a
voltage source inverter.

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10.3.4. Matrix Converters for Renewable Energy
System
Fig. 10.6 shows a typical matrix converter for AC/AC
power conversion. A matrix converter is constructed
with many bidirectional switches. In practice, two
back-to-back IGBTs with their anti-parallel diodes
form a bidirectional switch. The output voltage level,
waveforms and frequency can be varied with different
switching patterns and different duty ratio of the
switches so that it is a good candidate of renewable
energy generator. PWM sinusoidal output voltage with
minimal high order harmonics can be obtained by the
converter.
Vin
Vout
Fig. 10.6. Matrix converter for AC/AC power
conversion
In a matrix converter, no intermediate energy storage device, such as intermediate DC link capacitors or
batteries, is needed. Reliability of the converter also gets rid of the lifetime of capacitors. The power
circuit is small size because there is no magnetic component and capacitor. Its input current is sinusoidal
with high order harmonics only. The power factor is unity with any load. It is also a kind of
bidirectional converter allowing bidirectional power flow and power conversion.
10.4. Grid and Renewable Energy Network
Fig. 10.7 shows a grid network with renewable energy systems. It consists of numbers of renewable
energy systems supplying power to the grid and to the load. Battery energy storage system (BESS) and
hydrogen energy storage systems (HESS) with fuel cells are also connected to the grid for energy
storage.
Wind
Power
Solar
Photovoltaic
Bio-power
Generation
Fuel Cell
BESS
Electric
Vehicle
Building
Power
Distribution
Street
& Traffic
Lights
Rail
Transportation
EV
Charging
HESS
Hydrogen
LED
Lighting
Motor
Drives
Computers
AV
Entertainment
Elevator
Escalator
Air-con
Waste
Food
Fig. 10.7. Renewable energy network and grid