a review of hybrid renewable alternative

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Abstract — This paper, prepared by a special task force of the

IEEE PES Renewable Energy Technologies Subcommittee, is a

review of hybrid renewable/alternative energy (RE/AE) power

generation systems focusing on energy sustainability. It

highlights some important issues and challenges in the designand energy management of hybrid RE/AE systems. System

configurations, generation unit sizing, storage needs, and

energy management and control are addressed. Statistics on the

current status and future trend of renewable power generation,

as well as some critical challenges facing the wide-spread

deployment of RE/AE power generation technologies and

vision for future research in this area are also presented. The

comprehensive list of references given at the end of the paper

should be helpful to researchers working in this area. Key Words — Hybrid energy systems, renewable power generation,

energy management, energy storage, generation unit sizing

I. INTRODUCTION

This century is expected to witness unprecedented growth and

challenges in power generation, delivery, and usage.

Environmentally-friendly (renewable and clean alternatives)

power generation technologies will play an important role in

future power supply due to increased global public awareness

of the need for environmental protection and desire for less

dependence on fossil fuels for energy production. These

technologies include power generation from renewable energy

(RE) resources, such as wind, photovoltaic (PV), micro hydro

(MH), biomass, geothermal, ocean wave and tides, and clean

alternative energy (AE) power generation technologies [such as

fuel cells (FC) and microturbines (MT)]. RE/AE generation

sources often come in the form of customized distributed

generation (DG) systems in grid-connected or stand-alone

configuration. FC and MT could also be considered renewable

power generation sources if their input fuel is obtained from

M.H. Nehrir is with Montana State University; C. Wang, Wayne StateUniversity; K. Strunz, Technical University of Berlin, Germany; H. Aki, AIST,Japan; R. Ramakumar, Oklahoma State University; J. Bing, NEO VirtusEngineering, Inc. Littleton, MA; Z. Miao, University of South Florida; and Z.Salameh, University of Massachusetts, Lowell.

renewable sources. For instance, landfill gas has been used t

fuel MT, biomass can be gasified into Syngas and used as fue

for MT and FC, or hydrogen fuel can be generated using wind

or PV-generated electricity (through an electrolyzer) for FC

Though not renewable, diesel generators and reciprocatin

engines are also still commonly used for a wide range of poweapplications, particularly in remote areas, and as backup energ

sources in some stand-alone systems such as a power source fo

a remote telecommunication tower. The diesel engine’s matur

technology, relatively cheaper price, low fuel cost, and hig

fuel efficiency have kept diesel generators in the market. The

are also reasonably fuel tolerant and can be considere

renewable power sources when fueled by renewable fuels suc

as bio-fuel.

In general, the key drivers for the deployment of the abov

energy systems are their perceived benefits, such as reduce

carbon emission, improved power quality and reliability, and i

some cases, combined heat-and-power (CHP) operation (e.g

for MT and FC), which will increase their overall systemefficiency significantly.

In the past half century extensive research has bee

conducted in the RE/AE area world-wide, including feasibilit

studies, computer modeling, control, and experimental work

e.g. [1-15]. As a result, the use of wind and PV powe

generation has become a reality and extensive work i

underway on other RE/AE generation technologies such a

ocean wave and tides, osmotic, geothermal, FC, and MT. Muc

work is also needed on the more mature technologies an

associated energy storage schemes to improve their operationa

performance and reliability.

Because of the intermittent nature of many of RE resource

(e.g., wind, solar, ocean wave), hybrid combination of two omore of their relevant power generation technologies, alon

with storage and/or AE power generation can improve system

performance. For example, wind and solar energy resources i

a given area are somewhat complementary on a daily and/o

seasonal basis. In general, hybrid systems convert all th

resources into one form (typically electrical) and/or store th

energy into some form (chemical, compressed air, therma

mechanical flywheel, etc.), and the aggregated output is used t

supply a variety of loads. Hybridization could result i

increased reliability; however, proper technology selection an

A Review of Hybrid Renewable/Alternative

Energy Systems for Electric Power Generation:

Configurations, Control and Applications

Special Task Force of the IEEE PES Renewable Energy Technologies Subcommittee, Energy Development and

Power Generation Committee:

M.H. Nehrir, C. Wang, K. Strunz, H. Aki, R. Ramakumar, J. Bing, Z. Miao, Z. Salameh





generation unit sizing are essential in the design of such

systems for improved operational performance, and dispatch

and operation control [16-20].

Different generation sources may also help each other to

achieve higher total energy efficiency and/or improved

performance. For instance, a fuel cell/microturbine

combined-cycle system can better utilize the energy available

in the fuel to achieve a significantly higher overall system

efficiency than either source can possibly achieve [21-23], or the response of an energy source with slower dynamic response

(e.g., wind or FC) can be enhanced by the addition of a storage

device with faster dynamics (such as a battery bank,

supercapacitor, or flywheel) to meet different types of load

requirements, e.g. slowly varying loads and fast load transients

[24-26].

Storage is an integral part of a hybrid RE/AE power

generation system. Capacity-oriented energy storage

technologies, such as pumped hydroelectric systems,

compressed air energy storage (CAES), and hydrogen storage,

generally do not have fast response time and are used for long

term energy storage/release such as managing slow load

variations. On the other hand, access-oriented storage devices

with fast response time, such as batteries, flywheels,

supercapacitors, and superconducting magnetic energy storage

(SMES), are used for responding to short-time disturbances,

such as fast load transients and for power quality issues.

References [27, 28] give a comprehensive explanation of the

performance, purpose, and promise of different storage

technologies.

Table I gives a summary of different RE/AE power

generation technologies and different energy storage schemes

which may be used in hybrid systems. Any combination of the

RE/AE power generation technologies, along with proper

storage and possibly combined with a conventional generationtechnology, e.g. a diesel generator could form a hybrid energy

system. For example, a hybrid system could have any

combination of wind, PV, MH, MT, conventional diesel

generator, storage battery, and FC-electrolyzer hydrogen

storage in grid-connected or stand-alone configuration, often

referred to as a microgrid.TABLE I

DIFFERENT RE/AE POWER GENERATION TECHNOLOGIES AND ENERGY

STORAGE DEVICES

Main RE/AE

technologies

Energy

storage types

Biomass

GeothermalHydro/microhydro

Oceon tidal/wave

Solar PV/thermal

Wind

Fuel cell

Microturbine

Battery

Compressed air Flywheel

Hydrogen

Pumped hydro

SMES

Supercapacitor

Thermal

The outputs from various generation sources of a hybrid

energy system need to be coordinated and controlled to realize

their full benefits. Proper optimization techniques and control

strategies are needed for sizing and for power dispatch from th

energy sources to make the entire system sustainable to th

maximum extent, while facilitating maximum reduction i

environmental emissions, and at the same time minimizing co

of energy production. The optimization problem can therefor

be multi-objective, sometimes with conflicting objectives, an

therefore complex. In such cases, only a global optimal poin

as a trade-off between several local optimal point

corresponding to the different objectives, may be achievedSuch optimization problems are difficult (if not impossible) t

solve using analytic techniques. Heuristic multi-objectiv

optimization techniques [29-31] and goal-oriented multiagen

systems (MAS) [32-34] have shown potential to solve suc

problems.

The remainder of the paper is organized as follows: Hybri

energy system configurations, unit sizing and energy storag

systems are discussed in Section II. Energy management an

control are presented in Section III. Statistics on current statu

and future trend of renewable power generation and sampl

applications of hybrid RE/AE systems and microgrids aroun

the world are given in Section IV. Section V presents som

critical challenges facing the wide-spread deployment o

RE/AE power generation technologies and vision for futur

research in this area. Section V concludes the paper.

II. HYBRID E NERGY SYSTEM CONFIGURATION

A. Integration Schemes

RE/AE sources have different operating characteristics; it i

therefore essential to have a well-defined and standardize

framework/procedure for connecting them to form a hybri

system, or more widely a microgrid, where a local cluster o

DG sources, energy storage, and loads are integrated togethe

and capable of operating autonomously [35]. A robu

microgrid should also have “plug-and-play” operatiocapability. Adapted from the concept widely used in compute

science and technology, plug-and-play operation here means

device (a DG, an energy storage system, or a controllable load

capable of being added into an existing system (microgrid

without requiring system reconfiguration to perform it

designed function, namely, generating power, providing energ

storage capacity, or carrying out load control. A suitabl

system configuration and a proper interfacing circuit [als

called power electronic building block (PEBB)] may b

necessary to achieve the plug-and-play function of a DG system

[36, 37].

There are many ways to integrate different AE powe

generation sources to form a hybrid system. The methods ca

be generally classified into three categories: DC-couple

AC-coupled, and hybrid-coupled [38]–[41]. The AC couple

scheme can further be classified into power frequency AC

(PFAC)-coupled and high frequency AC (HFAC)-couple

systems [42]. These methods are briefly reviewed below.

1) DC-Coupled Systems. In a DC-coupled configuration

shown in Fig. 1, the different AE sources are connected to a D

bus through appropriate power electronic (PE) interfacin

circuits. The DC sources may be connected to the DC bu

directly if appropriate. If there are any DC loads, they can als





be connected to the DC bus directly, or through DC/DC

converters, to achieve appropriate DC voltage for the DC loads.

The system can supply power to the AC loads (50 or 60 Hz), or

be interfaced to a utility grid through an inverter, which can be

designed and controlled to allow bi-directional power flow. The

DC coupling scheme is simple and no synchronization is

needed to integrate the different energy sources, but it also has

its own drawbacks. For instance, if the system inverter is

out-of-service, then the whole system will not be able to supplyAC power. To avoid this situation, it is possible to connect

several inverters with lower power rating in parallel, in which

case synchronization of the output voltage of the different

inverters, or synchronization with the grid, if the system is

grid-connected, is needed. A proper power sharing control

scheme is also required to achieve a desired load distribution

among the different inverters [43].

AC Energy

Source(s)

DC Energy

Source(s)

AC/DC

Storage

System

Proper

PE Circuit

DC

Loads

DC/AC

60/50 HzUtility

Grid

AC

Loads

DC Bus

AC Bus

DC/DC

(if needed)

Fig. 1. Schematic diagram of a DC-coupled hybrid energy system.

2) AC-Coupled Systems. AC coupling can be divided into

two sub-categories; PFAC-coupled and HFAC–coupled

systems. The schematic of PFAC-coupled systems is shown in

Fig. 2 (a), where the different energy sources are integrated

through their own power electronic interfacing circuits to a

power frequency AC bus. Coupling inductors may also be

needed between the power electronic circuits and the AC bus to

achieve desired power flow management.

The schematic of HFAC-coupled systems is shown in Fig. 2

(b). In this scheme, the different energy sources are coupled to a

HFAC bus, where HFAC loads are connected to. This

configuration has been used mostly in applications with HFAC

(e.g., 400 Hz) loads, such as in airplanes, vessels, submarines,

and in space station applications [4], [44], [45].

In both PFAC and HFAC systems, DC power can be obtained

through AC/DC rectification. The HFAC configuration can

also include a PFAC bus and utility grid (through an AC/AC or a DC/AC converter), to which regular AC loads can be

connected.

3) Hybrid-Coupled Systems. Instead of connecting all the

DG sources to just a single DC or AC bus, as discussed

previously, the different DG sources can be connected to the

DC or AC bus of the hybrid system. Fig. 3 shows a

hybrid-coupled system, where DG resources are connected to

the DC bus and/or AC bus. In this configuration, some energy

sources can be integrated directly without extra interfacing

circuits. As a result, the system can have higher energy

efficiency and reduced cost. On the other hand, control an

energy management might be more complicated than for th

DC- and AC-coupled schemes.

AC Energy

Source(s)

DC EnergySource(s)

AC/AC

(if needed) AC

Loads

60 / 50 Hz

Utility Grid

PFAC Bus

DC/AC

Storage

System

Bi-directional

converter

DC

LoadsAC/DC

(a)

AC Energy

Source (s)

AC/AC

(if needed) HFAC

Loads

60/ 50H

Utility

Grid

HFAC Bus

PFAC

Loads

PFAC Bus

DC Energy

Source (s)DC/AC

Storage

SystemAC/

DC

DC /

AC

DC

Loads

DC Bus

Bi-directional

converter

(b)Fig. 2. Schematic of AC-coupled hybrid energy system: (a) PFAC, (b) HFAC

Non- PFAC

EnergySource (s )

DC Energy

Source(s )

AC /DC

Storage

System

DC/AC

60/ 50 HUtili

Grid

AC

Loads

DC Bus PFAC Bus

DC/DC

(if needed)

PFAC

Energy

Source(s )

Bi- directional

Converter

DCLoads

DC/DC

(if needed)

Fig. 3. Schematic diagram of a hybrid-coupled hybrid energy system.

Different coupling schemes find their own appropriat

applications. If major generation sources of a hybrid system

generate DC power, and there are also substantial amount o

DC loads, then a DC-coupled system may be a good choice. O

the other hand, if the main power sources generate AC (wit

reasonable power quality for the grid and the connected loads

then an AC-coupled system is a good option. If the majo

power sources of a hybrid system generate a mixture of AC an

DC power, then a hybrid-coupled integration scheme may b

considered. It is worth mentioning that the power electroni

interfacing circuits in Figs. 2 and 3 can be made as modula





building blocks, which will give the systems more flexibility

and scalability.

B. Unit Sizing and Technology Selection

Component sizing of hybrid RE/AE systems is important

and has been studied extensively, e.g. [7, 11, 16, 17, 18, 46].

Selection of the most suitable generation technologies (i.e.,

suitable mix of RE/AE/conventional sources) for a particular

application is also equally important. Available applicationsoftware can be used to properly select generation technologies

and their sizes for specific applications. For example, with the

aid of HOMER software, developed at the National Renewable

Energy Laboratory (NREL) [47], a hybrid RE/AE system can

be designed; and with the aid of the Distributed Energy

Resource-Customer Adaption Model (DER-CAM) software,

developed at Lawrence Berkeley National Laboratory (LBL)

[20, 48], optimal technology selection for hybrid systems (to

operate as independent microgrids) can be achieved.

Unit sizing and technology selection can sometimes be as

straightforward as meeting certain simple requirements such as

using the available generation technology and not exceeding

the equipment power rating, or it can be as complex as

satisfying several constraints and achieving several objectives

to maximum extent at the same time. Normally, based on

available statistical information about generation, load,

financial parameters (e.g., interest rate), geographic factors,

desired system reliability, cost requirements, and other case

specific information, generation technologies and their sizes

can be optimized to satisfy specific objective functions, such as

minimizing environmental impact, installation and operating

costs, payback periods on investment, and/or maximizing

reliability. Power system optimization methods such as linear

programming (LP) [49], interior-point-method (IPM) [50], and

heuristic methods such as genetic algorithms and particleswarm optimization (PSO) can be used for component sizing

and energy management of hybrid RE/AE systems [51-56].

These techniques are especially attractive when multiple

objectives are to be met, some of which may be conflicting, e.g.

minimizing cost, maximizing system availability and

efficiency, and minimizing carbon emission.

C. Storage

1) Storage Diversity. Storage technology is critical for

ensuring high levels of power quality and energy management

of stationary hybrid renewable energy systems. The ideal

storage technology would offer fast access to power whenever

needed, provide high capacity of energy, have a long lifeexpectancy, and is available at a competitive cost. However,

there is no energy storage technology currently available that

can meet all these desirable characteristics simultaneously. In

this section the different types of energy storage devices and

systems are covered without going into the details of operation

of any specific device. The operational performance and

applications of energy storage devices for advanced power

applications (also, equally suited for hybrid RE/AE power

generation system applications) are, for example, discussed in

[27, 28].

2) Storage Types. In analogy to data storage in compute

engineering, a classification in terms of access and capacit

orientation may also be considered for energy storage [57

Among the different types of storage given in Table

supercapacitors, flywheels, and SMES offer fast access to th

stored energy, have a very high cycle life of charge an

discharge operations and very high roundtrip efficiency on th

order of 95 %. However, the cost per unit of stored energy i

also very high. Therefore, all three technologies can bclassified as access-oriented and support power quality. Th

usage of SMES can here only be economically justified fo

applications involving comparatively high levels of powe

Batteries could also be classified as high-power and/o

high-energy types depending on their design. However, i

general, their cycle life of charge/discharge is shorter than th

high-access energy storage devices explained above.

A promising capacity-oriented energy storage technology i

the flow battery. In conventional batteries chemical energy i

stored in reactants, placed near the electrodes inside the batter

cell, but in flow batteries chemical energy is stored in th

electrolyte solutions stored in two tanks outside the battery ce

stacks. As the solution is pumped to circulate from one storag

tank, through a cell stack, to the second tank, ion exchang

takes place through the cell porous membrane, and electron

flow through the load to generate electrical power. Sever

different flow battery chemistries have been developed fo

MW/MWh-level utility applications [58, 59]. The availabl

electrolyte chemistries include zinc-bromine flow batterie

(ZBFB) and vanadium redox batteries (VRB). Othe

chemistries are under development.

An advantage of flow batteries is that their power and energ

capacity can be designed independently. A battery power ratin

can be increased by increasing the cell area where energ

conversion takes place, i.e. by increasing the number of ce

stacks, while its energy capacity can be increased by usin

larger volume of electrolyte solutions in larger tanksFurthermore, flow batteries can be stored and shippe

completely discharged as the reaction only takes place when th

electrolyte circulation pumps are turned on.

Conventional lead-acid batteries are the least expensive fo

hybrid energy system applications, but they suffer from a low

cycle life. Nickel Metal Hydride (Ni-MH) batteries and thos

with Sodium Sulfur (NaS) chemistry offer significan

improvements over lead-acid batteries. Popular commerci

applications for Ni-MH batteries have included usage in hybri

electric vehicles (HEVs) and distributed renewable energ

systems. NaS batteries have been used in Japan in distribute

energy systems and to firm up wind energy in the grid on

large scale, up to 34 MW of power and 245 MWh of energ[60]. The operating temperature of NaS batteries is of the orde

of 300 to 350 ºC, which does not make them attractive fo

mobile applications. This is in contrast with Zinc-Bromin

batteries that operate near ambient temperature.

With increasing interest in electric vehicles, the developmen

of Lithium-Ion batteries has received a significant boost. The

can be well designed as high-power or high-energy batterie

While this is also possible for other battery chemistries, th

Li-Ion type allows reaching particularly high power-to-weigh

or high energy-to-weight densities. Compared with othe

commercially available batteries, conventional Li-Ion batterie





excel in performance with the exception of cost and life

expectancy. Since the cost is relatively high, the main interest

relates to mobile applications. Through the Vehicle-to-Grid

(V2G) concept, Li-Ion batteries are expected to appear as active

resources in distribution networks [61].

Hydrogen can serve as an energy carrier for

capacity-oriented energy storage. Hydrogen may be produced

through electrolysis, where water is split into its component

parts of hydrogen and oxygen. The electrolysis can be poweredfrom renewable sources. Hydrogen may also be derived

through steam reforming from methane or natural gas. The

amount of power that can be provided from hydrogen depends

on the size of the fuel cell stacks. Hydrogen is flexible in that it

can be stored in tanks without disturbing self-discharging

effects and can also be used to fill the tanks of fuel cell cars

quickly. In stationary applications that use electrolyzers to

produce hydrogen and fuel cells to generate electric power, the

comparatively low round-trip efficiency is known to be a

disadvantage.

Very popular in large-scale power systems is the usage of

pumped hydro energy storage. It relies on the availability of a

suitable geology and is therefore not considered further here.

Compressed air energy storage is also not considered for the

same reasons. In Table II, the storage types that can be

considered for hybrid energy systems and have been discussed

above are classified. None of the technologies relies on local

geology. Whether a battery is designed for high-power or

high-energy depends on its intended application.

Table II. Classification of storage for hybrid energy systems.

storage class storage technology

access-oriented

Supercapacitor

Flywheel

SMES

High-power battery

capacity-orientedHigh-energy battery

Hydrogen energy storage

3)Multi-level Storage. In computer systems, the

access-oriented storage serves as a cache for the

capacity-oriented storage. This type of integration has allowed

creating a storage system that offers fast and frequent access to

the stored medium through the cache while also offering a high

capacity of storage at a low cost. The concept of cache control

can also be designed for the benefit of power and energy

systems [57]. For the purpose of illustration, a multi-level

energy storage consisting of an access-oriented storage serving

as the cache for the parallel-connected capacity-orientedstorage is shown in Fig. 4 [62-64]. To make sure that the

access-oriented storage deals with fast fluctuations of power,

the cache control can be realized through filtering to separate

the high-frequency power fluctuations from the slowly-varying

and steady-state power spectrum [64]. Alternatively,

knowledge-based control has been proposed for this purpose.

The knowledge-based system uses two neural networks to

capture a set of rules [65].

While the system integration shown in Fig. 4 offers a high

level of performance in the design of hybrid energy systems, it

has been common practice to use just one storage technology

Normally, batteries are used and designed for the intende

usage. For smaller-scale applications, as in single residentia

hybrid energy systems, the Ni-MH battery technology would b

an appropriate candidate. For larger-scale application

involving the compensation of power from wind farms o

multiple residences, flow batteries or NaS batteries have show

to be practical in Japan. V2G concepts are most interesting wit

Li-Ion batteries [61].

Fig. 4. Multi-level energy storage.

III. CONTROLS AND ENERGY MANAGEMENT

Proper control of hybrid energy systems with multipl

RE/AE/conventional-DGs and energy storage (operating a

microgrids) is critical to achieving the highest system reliabilit

and operation efficiency [32]. Typically, a control (or energmanagement) system needs to determine and assign active an

reactive output power dispatch of each energy source whil

keeping its output voltage and frequency at the desired leve

Generally, the control structure of such systems can b

classified into three categories; centralized, distributed, an

hybrid control paradigms. In all three cases, each energy sourc

is assumed to have its own (local) controller which ca

determine optimal operation of the corresponding unit based o

current information. If multiple (and at times conflicting

objectives need to be met, and all energy sources cannot operat

optimally, a compromised (global optimal) operating decisio

may be achieved. A brief description of each control paradigm

follows.

A. Centralized Control Paradigm

In a centralized control paradigm, the measurement signal

of all energy units in a group, i.e. a microgrid, are sent to

centralized controller, as shown in Fig. 5. The centralize

controller acts as an energy supervisor [66, 67] and make

decisions on control actions based on all measured signals and

set of pre-determined constraints and objectives. It wi

prioritize and manage energy utilization among the variou

energy sources of the microgrid [68-70]. The objectiv

functions could be conflicting; for example, minimizing system

operation and maintenance costs and environmental impac

(carbon footprint), and maximizing system efficiency at th

same time may be competing objectives and could mak

solving the problem even more difficult. Often, multi-objectiv

problems do not have a single solution but a comple

non-dominated or Pareto set, which includes the alternative

representing potential compromise solutions among th

objectives. This could make a range of choices available t

decision makers and provide them with the trade-o

information among the multiple objectives effectively [68].

The control signals are then sent to the corresponding energ





sources to output proper power. The advantage of this control

structure is that the multi-objective energy management system

can achieve global optimization based on all available

information. However, the scheme suffers from heavy

computation burden and is subject to single-point failures.

Fig. 5. Illustration of a centralized control paradigm.

B. Distributed Control Paradigm

In a fully-distributed control paradigm, the measurement

signals of the energy sources of the hybrid system are sent to

their corresponding local controller, as shown in Fig. 6 [71-73].

The controllers communicate with one another to makecompromised (Pareto) operating decisions and achieve global

optimization. An advantage of this scheme is the ease of

“plug-and-play operation.” With this control structure, the

computation burden of each controller is greatly reduced, and

there are no single-point failure problems; its disadvantage is

still the potential complexity of its communication system.

Intelligent (model-free) algorithms, such as fuzzy logic, neural

networks, genetic algorithms, and their hybrid combinations

are potential tools for solving such problems, e.g. [71-73].

Fig. 6. Illustration of a distributed control paradigm.

A promising approach for distributed control problems is

multiagent system (MAS) [34], [74]. MAS have been used, for

example, for power system integration, restoration,

reconfiguration, and power management of microgrids [32,33],

[75-77]. A MAS may be distributed as coupled network of

intelligent hardware and software agents that work together to

achieve a global objective.

C. Hybrid Centralized and Distributed Control Paradigm

A more practical scheme, hybrid control paradigm, combines

centralized and distributed control schemes, as shown in Fig. 7

[78], [79]. The distributed energy sources are grouped within a

microgrid; centralized control is used within each group, while

distributed control is applied to a set of groups. With such a

hybrid energy management scheme, local optimization is

achieved via centralized control within each group, while

global coordination among the different groups is achieved

through distributed control. This way, the computationa

burden of each controller is reduced, and single-point failur

problems are mitigated.

Fig. 7. Illustration of a hybrid centralized and distributed control paradigm.

A hybrid control scheme, called multi-level contro

framework, is shown in Fig. 8 [78]. This scheme is similar t

the hybrid control scheme discussed above with an additiona

supervisory (strategic) control level. At the operational leve

basic decisions related to real-time operation are made, an

actual control of each energy unit is performed based on thcontrol objective(s) of the unit very rapidly, e.g., within

millisecond range. The tactical level aims to make operationa

decisions for a group of local control units or the entir

subsystem, with a relatively higher time frame, e.g. in the rang

of seconds to minutes. Strategic decisions concerning th

overall operation of the system, e.g. system “startup” o

“shutdown,” are made at the top level [78]. Two-wa

communication exists among the different levels to execut

decisions.

Fig. 8. Illustration of a multi-level control approach to hybrid power systems

IV. CURRENT STATUS, FUTURE TREND AND SAMPLE

APPLICATIONS OF R ENEWABLE E NERGY GENERATION

In this section a brief overview and growth statistics of pas

current and projection into the future of renewable-base

electricity generation along with some sample applications o

hybrid renewable energy power generation systems around th

world are presented.

A. Current Status and Future Trend of Renewable Powe

Generation

RE/AE has been the fastest growing source of electricit

generation around the world in the past decade. According t

the projection of the US Energy Information Administratio

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Energy

Resource

Energy

Resource

Energy

Resource

Energy

Resource

Energy

Resource

Energy

Resource

EnergyResource

Tactical Level

Supervisor Control

Tactical Level

Supervisor Control

Tactical Level

Supervisor Control

Operational Level

(Individual Control Un

Operational Level

(Individual Control Un

Operational Level

(Individual Control Unit

Operational Level

(Individual Control Unit)

Operational Level


Operational Level


Tactical Level

Supervisor Control

Tactical Level

Supervisor Control

Tactical Level

Supervisor Control

Operational Level


Operational Level


Operational Level


Strategic Level

Strategic Level

Strategic Level





(EIA), non-hydro renewable power generation will continue its

growth well into the future. Fig. 9 shows the history, current

trend, and future projection profiles of non-hydro renewable

generation worldwide compared to total renewable and total

electricity generation [80]. Fig. 10 shows the corresponding

percentages of non-hydro renewable and total renewable

generation over the total electricity generation in the same time

period shown in Fig. 9 [81]. According to these figures,

non-hydro renewable generation has had swift growth in the past decade. This growth is expected to continue in the next

two decades. The non-hydro renewable electricity generation

increased by more than 220% in eight years, from 240 billion

kWh in 2000 to 535 billion kWh in 2008, resulting in an annual

increase of 10.54% [80]. EIA predicts that non-hydro

renewable generation will keep a high growth rate (of about 7%

annually) in the next two decades and will reach 2302 billion

kWh by 2030 [81].

Fig. 11 shows the US non-hydro renewable generation

profiles (by type) between the years 2000 and 2009, and the

EIA’s prediction to year 2035 [81], [82]. The renewable energy

generation has seen its fastest growth: from 85.7 billion kWh in

2000 to 152.2 billion kWh in 2009, at an average annual

increase of 6.6%. According to this prediction, the annual

growth will be higher through 2030, at which time about 11%

of the electricity generated in the US will be from

non-hydroelectric renewable sources.

At present, most non-hydroelectric renewable technologies

(with the exception of bulk wind generation) are still not

economically competitive with fossil fuel based generation

sources, and federal and local governmental incentives are

often a primary motive for installing renewable generation

systems.

A majority of US states have set a Renewable Portfolio

Standard (RPS) that set the goal of generating certain percentage of their electricity from renewable sources. In most

cases, the RPS set by the states is far greater than the 11%

renewable by 2030, predicted by EIA. For instance, California

has set its RPS to 33% of its total electricity generation from

renewables by 2030 [83]. DOE has also been very aggressive in

pushing for electricity generation from renewables and has

envisioned a scenario of 20% electricity from wind by 2030

[84]. Even this figure is lower than many of the RPSs set by the

states.

As the penetration level of renewable generation (in

particular wind and solar PV) will increase in an unprecedented

pace, given the intermittent nature of the energy resources, the

issues associated with grid integration of these technologies become more challenging. In general, control and power

management of these generation sources will be easier when

they operate independently or in a microgrid setting, together

with the use of energy storage (discussed in Section II). When

used in such manner, energy source hybridization is an

effective approach that can mitigate many of the problems

associated with the intermittent nature of the energy resources

to certain extent and will increase system reliability. This issue

has been extensively reported in the literature, e.g. [16] – [18],

[20], [47], [56], [67].

The rest of this section gives an overview of some sampl

applications of hybrid RE/AE systems around the world

Experiences in developing hybrid RE/AE systems ar

indicative of improved system performance and reliability, an

reduced system cost.

Fig. 9. History and future projection profiles of world renewable generatioand the total electricity generation [80], [81].

Fig. 10. The percentage of world renewable generation over the total electrici

generation [80], [81].

Fig. 11. US Renewable generation profiles [81], [82].

B. Sample Applications around the World

The area of hybrid RE/AE systems implementation is ver

dynamic and various demonstration projects on such system

0

5000

10000

15000

20000

25000

30000

35000

40000

2000 01 02 03 04 05 06 07 08 15 20 25 30 35

Non-Hydroelectric Renewable

Total Renewable

Total Electricity

History Projection(Billion kWh)

0.00

5.00

10.00

15.00

20.00

25.00

2000 01 02 03 04 05 06 07 08 15 20 25 30 35

Non-Hydroelectric

Renewable

Total Renewable

History

% over the total electricity generation

Projection

0

2

4

6

8

10

12

0

100

200

300

400

500

600

00 01 02 03 04 05 06 07 08 09 20 30 35

% o v e r t h e T o t o a l E

l e c t r i c i t y

B i l l i o n k W h

Geothermal

Solar,Tide and Wave

Wind

Biomass and Waste

% over theTotoal Electri city

History Projection





have been (and are currently being) conducted around the world

for different purposes, such as electrification of remote villages,

remote telecommunication units, or in the form of microgrids

for clean and secure electricity production purposes in urban

centers. Inclusion of all reported projects is beyond the scope

of this paper. In this section, seven sample applications of

hybrid RE/AE systems and microgrid test facilities (deploying

hybrid RE/AE systems) from around the world, which vary in

purpose as well as in system configuration, are brieflysummarized. For further information about these projects and

their actual configuration, the reader is referred to the

reference(s) given for each system.

1. Utsira Island, Norway: An autonomous hybrid

wind-hydrogen-fuel-cell system with flywheel and battery

storage and a hydrogen engine supplies power to the island

residents [85], [86].

2. Kahua Ranch, Hawaii Hydrogen Power Park, USA: This

hybrid wind-PV-electrolyzer-FC energy system was

initially developed as a part of Hawaii Hydrogen Power

Park program for technology validation and research. The

system provides emission-free electricity to Kahua Ranch

located on the Hawaii Island [87].

3. Starkenburger Lodge, Austria: A completely off-grid

hybrid PV-storage system with CHP-based engine supplies

the power demand of the remote Alpine Lodge in Austrian

Alps [88].

4. Fuel cells and Energy Networks of Electricity, heat, and

hydrogen, Japan (Project NEXT 21): More than 5,000

PEMFCs (fueled by natural gas) were installed in

residential households on an experimental basis by the end

of 2009. The PEMFC units provide electricity and hot

water to the households [89]-[91].

5. The Kythnos Island microgrid project, Greece: The system

is a single-phase PV-storage-diesel-engine microgridinstalled on in the island to provide uninterruptible power

to the island residents. The system is used to test

centralized and decentralized control and power

management of the microgrid in islanded mode [92].

6. The Hachinohe microgrid project, Japan: The purpose of

this renewable-energy-based microgrid, built in the urban

city of Hachinohe, is to investigate its grid integration and

stabilization, as well as control and reliable operation in

island mode [93].

7. Wind2H2 Project, National Wind Technology Center

(NWTC) at the National Renewable Energy Laboratory

(NREL), USA: The Wind2H2 system was initially

approved for demonstration operation in March 2007. Thesystem produces hydrogen directly from RE sources

through electrolyzers. The generated hydrogen is stored in

high pressure storage tanks and can be used both as a

transportation fuel and as an energy storage medium [94].

Table III gives a list of components (generation sources, etc.)

and their capacity, storage type, integration scheme (bus types,

AC/DC/hybrid), and control paradigm (central, distributed, or

hybrid) used in each project. Central control paradigm is

applied to all projects except the Kythnos Island Project in

Greece, where droop control is applied for load sharing amon

the PV inverters of the system. The fast response function o

the hybrid systems is realized either by access-oriented energ

storage devices or by fast-acting controllable generators.

In 2008 US DOE awarded nine renewable and distribute

system integration (RDSI) projects aimed at modernizing th

US utility grid [95]. In addition, there are many activ

microgrid demonstration projects around the world integratin

RE/AE and conventional energy sources. The detailedescriptions of several such projects appear in [96], [97].

V. CHALLENGES AND VISION FOR THE FUTURE OF

RE/AE POWER GENERATION TECHNOLOGIES

In this section a partial list of challenges facing th

wide-spread deployment of RE/AE power generatio

technologies and future visionary research areas are presented

A. Challenges

Despite their significant benefits to the environment an

great long-term potential for sustainable energy developmen

hybrid RE/AE systems are currently in an economidisadvantage position because of their high installation cost

compared with traditional electricity generation technologies

In the majority of cases, the incentives from federal and stat

governments and local utilities are necessary to make a hybri

system economically viable, which, in turn, makes th

incentive policies so critical to the wide-spread deployment o

such systems [98].

Energy storage is necessary for stand-alone hybrid RE/A

systems to have continuous, reliable power supply with desire

power quality. Energy storage is also one of the enablin

technologies to accommodate grid-scale renewable generatio

sources into power systems at high penetration. Among th

different energy storage techniques discussed in Section II.C

only pumped hydroelectric storage and underground CAES ar

the two technologies which can provide a competitive system

cost [99]. However, they are heavily geographicall

constrained and only suitable for large grid-scale energ

storage applications. On the other hand, batteries are the mo

common energy storage technologies for distributed hybri

RE/AE systems. Though the requirement of energy density an

specific energy are not so critical to stationary energy storag

applications, system cost and durability are still the key barrier

for battery storage systems. Moreover, it is a very challengin

task to accurately gauge and estimate the state of charge (SOC

and state of health (SOH) of batteries [100-105], in particulaas electric vehicles are being put on the road around the world

Therefore, new battery technologies deserve more researc

attention and efforts to improve their durability an

performance, and lower their cost.

B. Vision for the Future

A partial list of the future research topic areas, which ca

impact RE/AE power generation and management, are give

below:





TABLE IIIEXAMPLE APPLICATIONS OF HYBRID SYSTEMS AROUND THE WORLD [85] – [97]

x Energy Management and Standardization: As the

deployment of hybrid RE/AE systems in the form of

independent microgrid increases, the need for real-time

energy management of such systems, and robust

communication between the individual energy sources of

the microgrid become an important task and therefore

deserve further attention. Furthermore, systematic

approaches and standardization, e.g. IEEE Standard 1547

[106] and IEC 61850 [107], are needed for efficient and

safe deployment of such systems.x DC Distribution: With the development of modern

equipment and household appliances that use dc voltage,

several researchers have explored the virtue of dc

Microgrid for localized loads and the idea of completely

rewiring homes to run on dc, e.g. [108,109]. This venue

deserves further attention to explore its technical and

economic feasibility.

x New Semiconductor Devices: The rugged electronic power

switching technology using Silicon Carbide and Gallium

Nitride semiconductors is rapidly advancing [110].

Devices made out of these materials can be operated at

much higher frequencies, and such operations can lead to

compact inverters, choppers and other interface systems,which can enhance the overall performance of hybrid

RE/AE systems. As these devices become available,

research efforts are needed to integrate them into the

evolving hybrid systems.

x Excitonic Solar Cells: This class of solar cells uses Titania

nanotube arrays [111, 112] shows considerable promise to

harness a larger fraction of the solar spectrum. The

availability of this class of devices should be closely

monitored for potential use in RE/AE systems.

x Nanotechnology: In general, the application of

nanotechnology to improve various components of hybrid

systems should be a constant topic of research an

investigation.

x Hydrogen: Last but not least, the production of hydroge

and hydrogen economy should be a constant futur

research topic. A breakthrough in this area coul

revolutionize the way we live.

V. CONCLUSIONS

This paper provides a summary of available approaches an

those currently under research for optimal design of hybri

RE/AE energy systems. Different approaches for system

configuration, unit sizing, and control and energy managemen

of hybrid systems are presented. Current Status and Futur

Trend of RE Power Generation, the challenges facing th

wide-spread deployment of RE/AE systems, and researc

vision for the future of RE/AE power generation technologie

have been discussed. The comprehensive list of references a

the end of the paper is aimed to help interested researchers i

the design and power management of hybrid RE/AE energ

systems with focus on energy sustainability.

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Access oriented X X --- --- --- --- ---

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ACKNOWLEDGMENT

The authors wish to acknowledge the valuable contribution

of Dr. Zhehua Jiang to the Control and Energy Managemen

part of the paper and his input to the general structure of th

paper.