Hybrid Energy System

Hybrid energy systems are defined as the integration of several types of energy
generation equipment such as electrical energy generators, electrical energy storage
systems, and renewable energy sources.

From: Classical and Recent Aspects of Power System Optimization, 2018

Related terms:

Energy Engineering, Wind Turbines, Energy Storage, Renewable Energy Source,

Photovoltaics, Diesel Generator, Renewable Energy

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Hybrid Energy Systems

J.F. Manwell, in Encyclopedia of Energy, 2004

8 Trends in Hybrid Energy Systems

Hybrid energy systems are still an emerging technology. It is expected that technol-
ogy will continue to evolve in the future, so that it will have wider applicability and
lower costs. There will be more standardized designs, and it will be easier to select
a system suited to particular applications. There will be increased communication
between components. This will facilitate control, monitoring, and diagnosis. Finally,
there will be increased use of power electronic converters. Power electronic devices
are already used in many hybrid systems, and as costs go down and reliability
improves, they are expected to be used more and more.

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Design and performance optimisation

of stand-alone and hybrid wind energy
systems
E. Kondili, in Stand-Alone and Hybrid Wind Energy Systems, 2010

3.7 Future trends

Hybrid energy systems represent a very promising sustainable solution for power
generation in stand-alone applications. Research and development carried out in
these emerging technologies will certainly result in reducing the cost of the sys-
tems, despite the complex procedure involved in the design and optimisation of
these systems. Optimum resource allocation, based on load demand and renewable
resource forecasting, also promises to significantly reduce the total operating cost
of the system. In addition to mere cost minimisation criteria alone, it is increasingly
important to consider other relevant factors such as minimisation of emissions, or
maximisation of systems reliability.

The optimisation of the configuration, design and operation of hybrid energy

systems is supported by advanced models that describe the systems realistically.
Further research into the development of generic mathematical models will facilitate
the development and application of reliable and easily accessible multi-objective
optimisation tools, such as software programs. The development of a generic, vali-
dated and complete methodology for the synthesis and the design of hybrid energy
systems – incorporating appropriate planning and standardised models to take into
account the characteristics of the location, the suitable hybrid energy system, and
all the potential operational and performance scenarios for its application – would
significantly improve the implementation of these technologies.

The application of modern control techniques (such as a centralised system con-

troller) would further improve the operational performance and energy manage-
ment of these modular hybrid energy systems, allowing the utilisation of the re-
newable resource to be optimised.

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Sustainable Water & Energy Systems

Arun Palatel, in Encyclopedia of Sustainable Technologies, 2017

Conclusions
Isolated HES offers a sustainable option for the electrification of remote locations.
For electrification of a site with a HES, there are several options which may be
selected based on the availability of the resources, techno-economic constraints, and
reliability requirements. The common configurations of HES have been presented.
The determination of suitable ratings of the power generation units and energy
storage is an important step in the design of HES. The appropriate matching of
the electrical demand with the energy supply governs the overall performance of
the system. The methods for sizing of isolated power systems range from simple
correlations to methods based on detailed simulation models. The design space is
the set of all feasible system configurations that are capable of meeting a given
demand ensuring a specified reliability level. Design space for an existing diesel
generator–photovoltaic–battery bank system has been illustrated through a repre-
sentative case study.

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Application of Robust Optimization

Method to Power System Problems
Morteza Nazari-Heris, Behnam Mohammadi-Ivatloo, in Classical and Recent As-
pects of Power System Optimization, 2018

4.5 Economic Dispatch Problem in Hybrid Energy Systems

Hybrid energy systems are defined as the integration of several types of energy
generation equipment such as electrical energy generators, electrical energy storage
systems, and renewable energy sources. Hybrid energy systems may be utilized
in grid-connected mode, isolated from grid, and special aims [40]. Uncertainties
associated with renewable power production and load demands are considered in
recent studies in the area of scheduling of hybrid energy systems.

The RO method is implemented in Ref. [41] to handle the uncertainties associated

with a large-scale intermittent power supply in economic dispatch of a hybrid energy
system, which includes wind turbine, photovoltaic system, hydro, and thermal gen-
eration plants. In this reference, reliability and economy are considered as the main
objectives of the optimization problem, where an adjustable uncertainty budget is
proposed for the RO method to obtain a reliable economic solution. The optimal
solution for an economic dispatch problem for a multienergy system is studied in
Ref. [42] by employment of the RO method. This research considered the realistic
interdependency between the electricity and primary energy infrastructures, which
provided optimal power dispatch by considering the uncertainty in wind power
generation. A multiperiod robust model for economic dispatch problems of a power
system with high penetration of wind generation units is proposed in Ref. [43], which
contains a two-stage decision-making framework. A rolling-horizon operational
structure is proposed in this reference for modeling the real-time ED process, and
dynamic uncertainty sets are introduced to study dynamic relationship between
uncertainties across decision stages.

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Network Planning of Hydrogen Supply

Chain
Hanane Dagdougui, ... Ahmed Ouammi, in Hydrogen Infrastructure for Energy
Applications, 2018

2.2.3.2 The PV Module Subsystem Model

Hybrid energy systems are often taken into account as a viable approach to face
the RES intermittent character. The use of different RES (such as wind and solar)
can enhance the effectiveness to face the load energy demands. The electrical
energy generated from a photovoltaic module can be calculated using the following
formula:

(6.3)

where Spv (m2) is the solar cell array area, PV (–) is the module reference efficiency,
pf (–) is the packing factor, pc (–) is the power conditioning efficiency, and
Git (kWh/m2) is the forecasted hourly irradiation that is predicted by some reliable
meteorological model.

> Read full chapter

Power Electronics for Renewable Ener-

gy Sources
Syed M. Islam, ... Md Mubashwar Hasan, in Power Electronics Handbook (Fourth
Edition), 2018

25.4 Power Electronics for Hybrid Energy Systems

The combination of RES, such as PV arrays or wind turbines, with engine-driven
generators and battery storage, is widely recognized as a viable alternative to con-
ventional remote-area power supplies (RAPS). These systems are generally classified
as hybrid energy systems (HES). They are used increasingly for electrification in
remote areas where the cost of grid extension is prohibitive and the price for fuel
increases drastically with the remoteness of the location. For many applications, the
combination of renewable and conventional energy sources compares favorably with
fossil-fuel-based RAPS systems, in regard to both their cost and technical perfor-
mance. Because these systems employ two or more different sources of energy, they
enjoy a very high degree of reliability as compared with single-source systems such
as a stand-alone diesel generator or a stand-alone PV or wind system. Applications
of hybrid energy systems range from small power supplies for remote households,
providing electricity for lighting and other essential electric appliances, to village
electrification for remote communities have been reported [88].

Hybrid energy systems generate AC electricity by combining RES such as PV array

with an inverter, which can operate alternately or in parallel with a conventional
engine-driven generator. They can be classified according to their configuration as
follows [89]:

• Series hybrid energy systems

• Switched hybrid energy systems

• Parallel hybrid energy systems

The parallel hybrid systems can be further divided to DC or AC coupling. An overview

of the three most common system topologies is presented by Bower [90]. In the
following, a comparison of typical PV-diesel system configurations is described.

25.4.1 Series Configuration

In the conventional series hybrid systems shown in Fig. 25.67, all power generators
feed DC power into a battery. Each component has therefore to be equipped with
an individual charge controller and in the case of a diesel generator with a rectifier.

Fig. 25.67. Series hybrid energy system.

To ensure reliable operation of series hybrid energy systems, both the diesel gen-
erator and the inverter have to be sized to meet peak loads. This results in a typical
system operation where a large fraction of the generated energy is passed through
the battery bank, therefore resulting in increased cycling of the battery bank and
reduced system efficiency. AC power delivered to the load is converted from DC
to regulated AC by an inverter or a motor generator unit. The power generated by
the diesel generator is first rectified and subsequently converted back to AC before
being supplied to the load, which incurs significant conversion losses.

The actual load demand determines the amount of electric power delivered by the
PV array, the wind generator, the battery bank, or the diesel generator. The solar
and wind charger prevents overcharging of the battery bank from the PV generator
when the PV power exceeds the load demand and the batteries are fully charged. It
may include MPPT to improve the utilization of the available PV energy, although
the energy gain is marginal for a well-sized system. The system can be operated in
manual or automatic mode, with the addition of appropriate battery voltage sensing
and start/stop control of the engine-driven generator.

Advantages are as follows:

• The engine-driven generator can be sized to be optimally loaded while supply-

ing the load and charging the battery bank, until a battery SOC of 70%–80%
is reached.
• No switching of AC power between the different energy sources is required,
which simplifies the electric output interface.
• The power supplied to the load is not interrupted when the diesel generator is
started.
• The inverter can generate a sine wave, modified square wave, or square wave
depending on the application.

Disadvantages are as follows:

• The inverter cannot operate in parallel with the engine-driven generator;

therefore, the inverter must be sized to supply the peak load of the system.
• The battery bank is cycled frequently, which shortens its lifetime.

• The cycling profile requires a large battery bank to limit the depth of discharge
(DOD).
• The overall system efficiency is low, since the diesel cannot supply power
directly to the load.
• Inverter failure results in complete loss of power to the load, unless the load
can be supplied directly from the diesel generator for emergency purposes.

25.4.2 Switched Configuration

Despite its operational limitations, the switched configuration remains one of the
most common installations in some developing countries. It allows operation with
either the engine-driven generator or the inverter as the AC source, yet no parallel
operation of the main generation sources is possible. The diesel generator and the
RES can charge the battery bank. The main advantage compared with the series
system is that the load can be supplied directly by the engine-driven generator, which
results in a higher overall conversion efficiency. Typically, the diesel generator
power will exceed the load demand, with excess energy being used to recharge
the battery bank. During periods of low electricity demand, the diesel generator
is switched off, and the load is supplied from the PV array together with stored
energy. Switched hybrid energy systems can be operated in manual mode, although
the increased complexity of the system makes it highly desirable to include an
automatic controller, which can be implemented with the addition of appropriate
battery voltage sensing and start/stop control of the engine-driven generator (Fig.
25.68).

Fig. 25.68. Switched PV-diesel hybrid energy system.

Advantages are as follows:

• The inverter can generate a sine wave, modified square wave, or square wave,
depending on the particular application.
• The diesel generator can supply the load directly, therefore improving the
system efficiency and reducing the fuel consumption.

Disadvantages are as follows:

• Power to the load is interrupted momentarily when the AC power sources are
transferred.
• The engine-driven alternator and inverter are typically designed to supply the
peak load, which reduces their efficiency at part load operation.

25.4.3 Parallel Configuration

The parallel hybrid system can be further classified as DC and AC couplings as shown
in Fig. 25.69. In both schemes, a bidirectional inverter is used to link between the
battery and an AC source (typically the output of a diesel generator). The bidirectional
inverter can charge the battery bank (rectifier operation) when excess energy is
available from the diesel generator or by the renewable sources and act as a DC-AC
converter (inverter operation). The bidirectional inverter may also provide “peak
shaving” as part of a control strategy when the diesel engine is overloaded. In
Fig. 25.69A, the renewable energy sources (RES) such as photovoltaic and wind are
coupled on the DC side. DC integration of RES results in “custom” system solutions
for individual supply cases requiring high costs for engineering, hardware, repair,
and maintenance. Furthermore, power system expandability for covering the needs
of growing energy and power demand is also difficult. A better approach would
be to integrate the RES on the AC side rather than on the DC side as shown in Fig.
25.69B.

Fig. 25.69. Parallel PV-diesel hybrid energy system: (A) DC decoupling and (B) AC
coupling.
Parallel hybrid energy systems are characterized by two significant improvements
over the series and switched system configuration.

The inverter and the diesel generator capacity rather than their individual compo-
nent ratings limit the maximum load that can be supplied. Typically, this will lead to
a doubling of the system capacity. The capability to synchronize the inverter with the
diesel generator allows greater flexibility to optimize the operation of the system.
Future systems should be sized with a reduced peak capacity of the diesel generator,
which results in a higher fraction of directly used energy and hence higher system
efficiencies.

By using the same power electronic devices for both inverter and rectifier operation,
the number of system components is minimized. Additionally, wiring and system
installation costs are reduced through the integration of all power-conditioning
devices in one central power unit. This highly integrated system concept has advan-
tages over a more modular approach to system design, but it may prevent convenient
system upgrades when the load demand increases.

The parallel configuration offers a number of potential advantages over other system
configurations. These objectives can only be met if the interactive operation of the
individual components is controlled by an “intelligent” hybrid energy management
system. Although today's generation of parallel systems includes system controllers
of varying complexity and sophistication, they do not optimize the performance of
the complete system. Typically, both the diesel generator and the inverter are sized
to supply anticipated peak loads. As a result, most parallel hybrid energy systems
do not utilize their capability of parallel, synchronized operation of multiple power
sources.

Advantages are as follows:

• The system load can be met in an optimal way.

• Diesel generator efficiency can be maximized.

• Diesel generator maintenance can be minimized.

• A reduction in the rated capacities of the diesel generator, battery bank,

inverter, and renewable resources is feasible while also meeting the peak loads.

Disadvantages are as follows:

• Automatic control is essential for the reliable operation of the system.

• The inverter has to be a true sine-wave inverter with the ability to synchronize
with a secondary AC source.
• System operation is less transparent to the untrained user of the system.
25.4.4 Control of Hybrid Energy Systems
The design process of hybrid energy systems requires the selection of the most
suitable combination of energy sources, power-conditioning devices, and energy
storage system together with the implementation of an efficient energy dispatch
strategy. System simulation software is an essential tool to analyze and compare pos-
sible system combinations. The objective of the control strategy is to achieve optimal
operational performance at the system level. Inefficient operation of the diesel
generator and “dumping” of excess energy is common for many RAPS, operating
in the field. Component maintenance and replacement contributes significantly to
the life-cycle cost of systems. These aspects of system operation are clearly related to
the selected control strategy and have to be considered in the system design phase.

Advanced system control strategies seek to reduce the number of cycles and the
DOD for the battery bank, run the diesel generator in its most efficient operating
range, maximize the utilization of the renewable resource, and ensure high reliability
of the system. Due to the varying nature of the load demand, the fluctuating power
supplied by the photovoltaic generator, and the resulting variation of battery SOC,
the hybrid energy system controller has to respond to continuously changing oper-
ating conditions. Fig. 25.70 shows different operating modes for a PV single-diesel
system using a typical diesel dispatch strategy:

Fig. 25.70. Operating modes for a PV single-diesel hybrid energy system.

Mode (I). The base load, which is typically experienced at nighttime and
during the early morning hours, is supplied by energy stored in the batteries.
Photovoltaic power is not available, and the diesel generator is not started.
Mode (II). PV power is supplemented by stored energy to meet the medium
load demand.
Mode (III). Excess energy is available from the PV generator, which is stored in
the battery. The medium load demand is supplied from the PV generator.
Mode (IV). The diesel generator is started and operated at its nominal power to
meet the high evening load. Excess energy available from the diesel generator
is used to recharge the batteries.
Mode (V). The diesel generator power is insufficient to meet the peak load
demand. Additional power is supplied from the batteries by synchronizing the
inverter AC output voltage with the alternator waveform.
Mode (VI). The diesel generator power exceeds the load demand, but it is kept
operational until the batteries are recharged to a high SOC level.

In principle, most efficient operation is achieved if the generated power is supplied

directly to the load from all energy sources, which also reduces cycling of the
battery bank. However, since diesel generator operation at light loads is inherently
inefficient, it is common practice to operate the engine-driven generator at its
nominal power rating and to recharge the batteries from the excess energy. The
selection of the most efficient control strategy depends on fuel, maintenance and
component replacement cost, the system configuration, environmental conditions,
and constraints imposed on the operation of the hybrid energy system.

25.4.5 Grid-Connected Hybrid Energy Systems

Grid-connected hybrid renewable energy systems (RES), such as PV arrays or wind
turbines, engine-driven generators, and battery storage, increase the reliability of
the renewable energy supply to the grid [91]. Different RES are connected to a
common DC or AC bus, as illustrated in Section 25.4.1. Usually, a power transformer
is required to step up the inverter output voltage to a medium-/high-voltage grid.
Such power transformer is subjected to a high initial cost, regular maintenance
cost, and large physical size [40]. The application of a suitable dielectric material
enables the power conditioning of RES to be cheaper, more efficient, and lighter
in weight. In [40], a high-frequency, 10–20 kHz magnetic linked cascaded multilevel
inverter (CMLI) is proposed, which eliminates the necessity of using a step-up power
transformer to interface the CMLI to the grid. The block diagram of such grid-con-
nected hybrid energy systems is shown in Fig. 25.71. Various RES are connected in
a common DC bus through suitable converter and high-frequency magnetic link
(HFML) that is energized by the common DC bus. The HFML facilitates isolated
DC supplies for the three-phase CMLI as shown in Fig. 25.72. The HFML converts
the DC bus voltage into 10–20 kHz square-wave voltage using a full bridge. This
high-frequency square-wave voltage is fed to the primary of a multiwinding trans-
former (MWT), which involves full-bridge diode rectifier connected to the MWT
secondary windings as shown in Fig. 25.72A. The output terminals of the rectifiers
are connected to the cascaded units of the CMLI as shown in Fig. 25.72B. The CMLI
inverter output terminals are connected to the grid. The main reasons of utilizing
high-frequency voltage for the MWT are to reduce the transformer core size and to
create galvanic isolation between the PV panels and the grid [92]. It is possible to
reduce the size of harmonic filter at the inverter output as CMLI generates output
voltage with inherently low harmonic contents. Moreover, the cost of the overall
conversion system can be reduced by the elimination of the step-up transformer
[93,94].

Fig. 25.71. Block diagram of a typical Grid-connected RES hybrid energy system.

Fig. 25.72. Circuital diagram of the Grid-connected hybrid energy systems. (A) High
frequency magnetic link, (B) Grid connected CMLI..

> Read full chapter

Waste management
Majid Saidi, Aliakbar Ghaffari, in Design and Operation of Solid Oxide Fuel Cells,
2020

14.10 Hybrid solid oxide fuel cell energy systems

Hybrid energy systems are capable of exploiting the advantages of two or more
energy technologies simultaneously. The SOFC as a high-temperature technology
can be combined with bottoming cycles such as gas turbines and steam turbines
in cascade to meet higher efficiency suggesting great potential as a green
decentralized CHP system. SOFC offers high WtE or fuel-to-electricity ratios and,
on the other hand, GT improves the utility of thermal energy of exhaust/flue gases
derived from high-temperature SOFCs. Accordingly, SOFC–GT hybrid systems have
attracted much attention in recent literature [178–183]. A 5 kW planar SOFC module
integrated with a microgas turbine unit including two radial turbines and one radial
compressor was thermodynamically optimized by Facchinetti et al. [178]. Sensitivity
analyses on the more influential parameters were also carried out. The optimization
results indicated the existence of optimal designs achieving exergy efficiency higher
than 65%. Wongchanapai et al. [179] evaluated the combination of direct-biogas
SOFC with an MGT–CHP system. The effect of key parameters such as SOFC
reforming agent, SOFC fuel utilization factor (Uf), turbine inlet temperature, and
compression ratio on system performance was examined. The results showed the
feasibility of 55% electrical and more than 80% overall CHP efficiency. An integrat-
ed MSW gasification combined with SOFC and stirling engine for decentralized CHP
plant of 120 kW electricity power was presented and thermodynamically studied by
Rokni [180]. A block schematic of the utilized plant is depicted in Fig. 14.13. Plant
electrical efficiency up to 48% and CPH efficiency up to 95% was possible,
depending on the plant design and MSW composition.

Figure 14.13. Block scheme of the municipal solid wastes gasification with solid
oxide fuel cell and Stirling engine plant hybrid systems. SOFC, solid oxide fuel cell.

On the other hand, the high-temperature exhaust gas from SOFC have high exergy
that can be exploited in other cycles for additional power generation or for heating
and cooling purposes (cogeneration/trigeneration) with different configurations
[184]. These systems are referred to as SOFC hybrid systems. The basic idea of an
SOFC/GT hybrid cycle is quite simple. Overall, SOFC/GT layouts are classified to
two sections due to the applied pressure: SOFC/GT pressurized and atmospheric
cycles. Furthermore, the SOFC/GT pressurized cycles differ due to internal reforming
operation which includes anode gas recirculation or heat recovery steam generator
and external reforming operation. Different bottoming cycles have also been consid-
ered, such as Rankine cycles which this configuration means hybrid SOFC/ORC or
SOFC/GT–ORC [182,185–187] and also novel hybrid layout of a trigeneration system
based on SOFC, ORC, solar collectors, and absorption chillers [188]. In addition, in-
novative hybrid SOFC based system in research and development projects with high
potentials for commercialization involve: SOFC-GT hybrid desalination plants for
freshwater power generation [183,189], SOFC–combined cooling and powers hybrid
systems (SOFC-CCP) [190], photovoltaics (PV)-coupled SOFC, and GT-(electrolyzer)
hybrid power generation systems [191,192], and others [181].

However, highly efficient electrochemical reactions of SOFCs when running on

biogas possess the desired characteristic that the exhausts of the plant are pure CO2
and water which makes biogas one of the best candidates for hybrid SOFC-based
applications. A comprehensive study on the utility of gasified biomass in a hybrid
SOFC/GT power plant was presented by Toonssen et al. [193]. The authors inves-
tigated the effect of gasification technology, gas-cleanup technology, and system
scale on the overall performance of the system considering four different systems.
All these systems were based on a recuperated internally reformed SOFC/GT hybrid
system equipped with anode and cathode recirculation arrangements in which the
former was used to provide the steam required for the internal SR reaction, and the
latter was utilized for air preheating. The lignocellulosic biomass conversion process
into electricity using integrated gasification and SOFC/GT systems was studied in
small and medium-scale applications (100 kW and 8 MW of dry-biomass input) by
Caliandro et al. [194]. They conducted multiobjective optimization for maximizing
the efficiency and minimizing the capital investment costs. The overall energy
efficiency for converting woody biomass into electricity was up to 70%. The
economic analysis of the optimal configurations showed the great impact of the
fuel cell cost on the total cost. The analyses showed the promising potential of the
economic and thermodynamic feasibility of such plants.

In the framework of the European project SOFCOM, a 2 kW SOFC based demon-
stration plant was installed in Torino, Italy, in 2013 within one of the largest WWTP in
Europe [195,196]. The design of the main plant sections included the gas cleaning
unit, fuel processor, SOFC “hot-box,” oxycombustor, CO2/H2O condensation unit
and photobioreactors further utilized for algal growth. Later in a similar work about
the context of carbon recovery and utilization, the results of EU-funded SOFCOM
project on the performance of the proof-of-concept plant units were investigated
[172]. The main outcomes from the general environmental perspective were: mi-
croalgae are a fast-growing biomass and thus an effective solution; the SOFCOM
 system was successful in complete abolishing of CO2 emission from an SOFC; and
carbon capture from biogas power plants contributed to an overall emissions cut-off.
Nevertheless, photobioreactors faced technological problems which require further
considerations and solutions for operation enhancement.

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