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connection to the long-distance transmission or distribution gridare too high. HPS use few technologies connected with powergeneration such as different power generation devices, differentenergy storage technologies and advanced microprocessor control/supervision systems.

sources and uncontrolled sources. Controlled sources meanprimary energy sources giving rise to the possibility of controllingelectrical power production, for example coal. It is obvious thatpower production from uncontrolled sources is unpredictable andindependent of human action. Solar and wind power plants areuncontrolled sources.

On the other hand, electricity should be produced exactly at thetime it is needed. Sun and wind do not meet this requirement. So,special kind of power plants should be built to avoid shortages of

* Corresponding author. Tel.: 4822 2345864; fax: 4822 2345073.

Contents lists availab

Renewable

els

Renewable Energy 34 (2009) 24142421E-mail address: jozef.paska@ien.pw.edu.pl (J. Paska).electricity or electricity and heat. Mostly, they are connected to thepower grid, but they can also work independently feeding sepa-rated receivers, from one or several homes/farms, small industrialplants to large local communities. Grid-connected HPS provideelectric power reserves and allow surplus power to be fed back tothe grid when HPS generate more power than receivers and localenergy storage systems require. Obviously, the major aim of HPS isto supply remote, off-grid communities where the costs of

2. Why hybrid power plants?

Currently very fast development of new electrical power sourcescalled renewable sources can be observed. These sources areenvironmentally friendly and use primary energy carriers such assolar, wind and water ow, biogas, biomass etc. The sourcesmentioned above can be divided into two groups: controlled1. Introduction

There are many denitions ofdenition of HPS is as follows [1]: sgenerating electricity or electricityprimary energy carriers (renewable aco-ordination of their operation tadvanced power electronics systems.

HPS by denition have been con0960-1481/$ see front matter 2009 Elsevier Ltd.doi:10.1016/j.renene.2009.02.018- the concept of a wind power plant with a battery energy storage,- the utilisation of a DC micro-grid for the integration of different electrical energy sources.

2009 Elsevier Ltd. All rights reserved.

Power Systems. Ouret of co-operating units,heat, with diversiedn-renewable), while thelace by utilisation of

d for the generation of

In our opinion hybrid power systems (plants) are a good way toincrease availability and exibility of power supply systems and tohave available and exible sources of electricity which optimiseutilisation of primary energy carriers. It may be achieved bycombining different primary energy carriers (renewable and non-renewable) utilising, electrical energy storage facilities, andadvanced power electronics and microprocessor systems forcontrol/supervision.Fuel cellsBattery energy storage- the experience of exploitation of a hybrid solar wind power plant,- a solar power plant with a fuel cell,Renewable energy sourcesHybrid power systems An effective w

Jozef Paska*, Piotr Biczel, Mariusz K1osWarsaw University of Technology, Institute of Electrical Power Engineering, Koszykowa

a r t i c l e i n f o

Article history:Received 13 August 2008Accepted 13 February 2009Available online 19 March 2009

Keywords:Hybrid power systemsDistributed generation

a b s t r a c t

Nowadays in many countrframework of so-called disand among them in hybridIn this paper we present oElectrical Power Engineerconsidered:

journal homepage: www.All rights reserved.the increase of generating capacity takes place in small units within theuted power industry (distributed generation DG, embedded generation),wer systems (HPS).xperience of the design, build and exploitation of HPS in the Institute of, Warsaw University of Technology. The following major subjects areof utilising primary energy sources

00-662 Warsaw, Poland

le at ScienceDirect

Energy

evier .com/locate/renene

power and to utilise all available sun or wind power. There are atleast two ways to achieve this aim: electricity energy storage orpower plants using two (or more) primary sources with additionalcontrol systems. One of the sources must be a controlled powersource. Such power plants are hybrid power plants.

Developers and manufacturers are looking for ways to combinetechnologies to improve performance and efciency of distributedgeneration equipment. Several examples of hybrid systems include:

A solid oxide fuel cell combined with a gas turbine ormicroturbine;

A Stirling engine combined with a solar dish; Wind turbines with battery storage and diesel backupgenerators;

Engines (and other prime movers) combined with energystorage devices such as ywheels.

Hybrid power generation systems contain two or more powergeneration sources in order to balance each others strengths andweaknesses. In Fig. 1 [2], a matrix is used to illustrate those hybridcombinations that are commercially available, in development or,at a minimum, plausible.

The diagram in Fig. 1 covers almost all variety of possible hybridcombinations. However in power systems with large amount of

CHP, such as the Polish (and similar systems in other countries), thediagram seems to miss the options coming from combining theelectricity sector with the heat sector. The diagram already includesCHP units, which makes a restriction on the system. However onecould also add heat pumps and heat storage, which would thenmake a exibility option to the system. Such options have beendescribed among others in [3].

3. Hybrid solarwind and battery power plant

3.1. Idea and realisation of the plant

The team from Warsaw University of Technology, Group ofElectric Power Plants and Power Engineering Economy has builta hybrid solar and wind power plant [4]. That was as a response toa request from a Polish telecommunications company. This hybridpower plant has supplied the telephone exchange. The companywanted to have a clean energy source something what couldreplace diesel generators, particularly in installations placed farfrom the public grid. The power plant had to produce energy all ofthe time without any breaks.

Fig. 2 shows a general view and Fig. 3 shows a block diagram ofthe hybrid power plant.

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J. Paska et al. / Renewable Energy 34 (2009) 24142421 2415Rankine natural gasEngine Cycle diesel/propane

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Not ApplicableStatus:

Commercial or

Legend

Applications:1 Distribution Grid Support2 Green Power3 Power Price Stabilization4 UPS/Power Quality5 Integrated Building Efficiency6 Power Parks7 Remote (Off-Grid) Power8 Village Power9 Water Resource Management10 Energy Securityations overview.

The plant has been tested for several years. Among otherthings, the nature of the produced power has been particularlycarefully observed. Problems, connected with the co-operation

3.2. Some problems caused by uncontrolled power sources

As a result of research we observed a number of problemsconnected to uncontrolled power production and co-operationwith the power grid. We consider the most signicant problems tobe:

Fig. 2. View of solar panels and wind turbine of the hybrid power plant.

Fig. 4. An example of daily production of solarwind power plant.

J. Paska et al. / Renewable Energy 34 (2009) 241424212416between power network and unstable sources have beenstudied. An example of daily power production is shown inFig. 4.

At the heart of the system was a chemical battery. Thebattery was charged by both solar panels and a wind turbine.The main idea was to use only solar panels but there wereinsufcient sunny days in Poland for this approach to besuccessful. Solar panels could however produce enough energyfrom May to September but in winter breaks were veryfrequent. Fig. 5 shows power production through the whole yearfrom the power plant (Pobc means required power demand).Therefore the wind turbine has added. Unfortunately after thatmeaningful improvement wasnt observed. It was caused byvery specic local weather conditions (in the region where thehybrid power plant was installed).

Hot reserve from the power gridDC

DC

AC

DC2

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DC1

IPV

IGen

IGRID

Windgenerator

Solarpanels

PowerGrid

Fig. 3. Block diagram of the hybrid sol- the rapid and unpredictable changes in electricity production,- the sudden disappearances of power generation,- the poor usage of primary energy carriers.

It has to be stressed that time constants of the phenomena aremuch smaller than in classical power plants.

The uncontrolled sources power production depends mainly onsun irradiation and wind speed. The power versus time curve,called the production prole, follows the primary energy carrieravailability versus time curve. In fact, the changes are extremelyrapid (Fig. 4). In the case when many similar power plants areinstalled, source power changes result in the need to increase thehot capacity reserve in the power grid. The reserve has to becapable to cover load demand in case electricity production fromDC

AC

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ACLOAD

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Hybrid solar-wind power plant

arwind and battery power plant.

power from sources to the load. The unit allows the maximisation

Hansen et al. [8], and Hogue andWahid [9], a simple PEMFCmodel,

Energy 34 (2009) 24142421 2417wind or sunshine fall. The additional reserve is necessary in gridswith relatively high capacity in power sources like wind turbines.

The sudden disappearance of power productionwas observed inthe power plant. In the aftermath of that, it could be a large powershortage in the power grid. The shortage has to be immediatelyreplaced by other sources. The problem is that turbine sets or dieselsets cannot be speeded up enough quickly and the problem couldnot be solved by increasing the hot reserve. The reason is the poordynamics of turbine sets. Thus, other methods have to be applied.One such method could be to apply an energy storage system ora new, fast enough, controlled power source.

It is almost impossible to produce power simultaneously fromboth renewable sources in plant shown in Fig. 3. This is due to thenature of the DC link. The power converters (DC/DC, AC/DC1 andAC/DC2) had diodes in the output circuit. Those diodes werenecessary to protect the sources (especially solar panels) fromopposite polarization (mostly the diodes are structural part ofconverters). So there were two parallel connected diodes inrenewable sources connection. In consequence only one from twosources could supply the load at the same time. If the sun has givene.g. 50% of load needs and wind 40%, it was necessary to supply theload from chemical battery although the sources together couldproduce enough power to meet needs. But neither of them couldsupply the load alone.

The best way, in our opinion, to solve the problems described inthis chapter is to build the power plant as a controlled sourcewhich

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Fig. 5. Power produced by the solarwind power plant during a year.

J. Paska et al. / Renewablewill be possible if the additional controlled source is used. There areseveral possible additional sources and different possible schemesof connectionwith wind power turbines. But it is sure that the newhybrid power plant has to be renewable or at least a green source.The additional source could be green fuel supply diesel generatorfor example. We have proposed fuel cells [4] for this purpose,especially fuelled by hydrogen [1,5,6].

4. Hybrid solar-fuel cell power plant

4.1. Used fuel cell and plant concept

Hydrogen fuelled fuel cells are new, efcient and clean DCpower sources. They have also very good dynamic properties. Wehave tested the PEM fuel cell Nexa [7], produced by Ballard(Fig. 6).

As was mentioned above the additional source has to be able tocome on line very fast. PEM fuel cell meets this criterion. Fig. 7shows an example of time response for sudden load increase (from0 to 30% of Imax).of the usage of renewable uncontrolled source. Then sources will beable to supply the load together. So, even if photovoltaic panels arenot able to meet the power demand, they could be used. The lack ofpower is met with fuel cell power.

The most important advantage of the proposed system is that itmaximises solar panel working time and minimises fuel demand.The control system can keep the output power xed independentlyof solar irradiation.

4.2. Results of experiments

We have prepared some simulations and physical models toconrm the advantages of proposed hybrid power plant.

First, a mathematical model using Simulink was prepared. Itcontained a PV array model, using some concepts presented byWe have developed a photovoltaic and fuel cell hybrid system.The systems block diagram is shown in Fig. 8a, and the equivalentelectrical circuit in Fig. 8b. There are photovoltaic panels and PEMfuel cell as power sources. The fuel cell is fuelled by hydrogen. Atthe heart of the control system is a special microprocessor controlunit. The unit controls power electronic converters transferring

Fig. 6. General view of NEXA 1.2 kW PEM fuel cell.power electronics converters and control unit models. The model

Fig. 7. Nexas time response for sudden load increase.

allowed us to recognise problems with source co-operation and tomatch parameters with power converters regulators. Solar irradi-ation and ambient temperature were given as input parameters.The load power and current and sources currents were the outputs.An example of the simulation results is shown in Fig. 9. It could beobserved that the output current (Itotal) was xed and independentof solar irradiation. The solar current has changed due to irradia-tion. Hence, fuel cell current has fallenwhen solar current has risenand the converse is also true it has risen when the solar currenthas fallen.

Then the experimental plant was created (Fig. 10) and someexperiments were done (Figs. 11 and 12).

pendent fuel cells. Such installations can give energy all the time

and do not produce any pollutants. On the other hand, the problemwith the fuel cell is limited hydrogen tank capacity. So, becausesolar panels do not need fuel, using both sources permits tomaximise refueling period.

Fig. 8. Hybrid solar-fuel cell power plant block diagram (a) and its equivalent electricalcircuit (b).

Fig. 9. Simulation results currents ow in the hybrid solar-fuel cell power plant.

J. Paska et al. / Renewable Energy 34 (2009) 2414242124184.3. Advantages and disadvantages of hybrid solar-fuel cellpower plant

The main reason to build the system described was to supplya stand-alone telecom system using renewable energy sources. So,the power plant has to produce energy independently from anyexternal (weather) uctuations. This could be obtained by usingtwo sources: weather dependent solar panels and weather inde-Fig. 10. GPV 110ME solar panel (a) and NP 50 PEM fueIn comparison to solar power plant energy production, thedescribed hybrid installation will give power as shown in Fig. 13.

In addition, the cost of produced energy has decreased despitethe equipment cost increase. This was possible because the totalnumber of PV working hours in the year was almost doubled usinga hybrid system.

Now the question is how much we can earn having such hybridsystem. At rst sight hybrid system seems much more expensive.We have discussed this problem previously [5,10,6].

The most important thing is to determine total PV working timein a year with load power. This could be done using meteorologicall cell with power converters used in experiments.

supply the load together in variant 2 (Figs. 11 and 12).

quality improvement.On the other hand, another EU goal is sustainable development.

In the case of power it means increasing efciency, RES utilisation

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J. Paska et al. / Renewable Energy 34 (2009) 24142421 2419In variant 1 a working time of about 1200 h per year wasobtained with a load power out of PVSIM simulation software forthe location in Warsaw (Poland). The working time was only takeninto consideration when the PV array has produced more powerthan the load needs. The plant construction did not allow the use ofsolar power when the produced power level is lower in variant 1.Then, the total power cost per 1 kWh was calculated using theUNIPEDE method. The cost at level 5.5 V/kWh was calculated.

In variant 2 it was possible to supply the load by solar powereven if production was smaller than needs. Then it reacheda working time level of 2200 h per year with the load power. A costof approximately 4.8 V/kWh was achieved. So, a cost reduction of10% was achieved.

5. Wind power plant with battery energy storage

The improvement of wind power plant availability is a majoraim of all international research centres, which are concerned withthis subject. One of the ways to achieve this target is to builda hybrid power plant using wind turbine and energy storage. Thetask of energy storage is to compensate for the surpluses anddata and statistical information about the weather in the placewhere the plant is installed. We have performed such calculationusing the PVSIM simulation tool [11]. Two variants were analysed.

Variant 1 is a power plant without a special control unit andvariant 2 is a power plant with such a system. The sources cannotwork together in variant 1. So the load can be powered only by solarpower or fuel cell power. The control unit allows the sources toshortages of wind power generation. The simplest version to realise

solar current

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Fig. 12. Currents in the solar-fuel cell power plant sunrise.is a connection of the wind plant with battery energy storage (BES)[12]. A block diagram of the proposed system is shown in Fig. 14.

Besides the major lines of energy ow (wind generatorconverterinvertertransformer-grid),wecanmarkout the followingadditionalBESblocks:RL blockofbatterycharging current regulator,RZ block of battery discharging current regulator (supply of inverterfrom battery), B block of battery.We can increase the availability byconnecting another source of energy, for example a diesel generatoror fuel cells. Fig. 15 shows a block diagram of a hybrid wind powerplant with a DC transmission line [12].

Research and digital simulations of power electronics devices,which determine the efciency of this HPS, have conrmed that it ispossible to build such a construction.

6. Integration of DG sources using DC micro-grid

The strategic aims of the EU clearly describe the role ofdistributed generation (DG) and renewable energy resources (RES)in electricity generation. Experts are unanimous that DG partici-pation in generation will increase further due to economic reasons.Unfortunately it will result in a need to changes in the powersystem. The current centralised solutions will have to be trans-formed. The transformation is forced by another EU aim energy

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Fig. 13. Hybrid solar and fuel cell power plant production.and local energy balancing.Local balancing means that almost all energy consumed in

separated area needs to be produced within this area using localprimary carriers. Such a solution allows the minimisation of powerlosses due to power transmission and power distribution andencourages investment in local small production units.

AC

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Fig. 14. Block diagram of grid-connected wind plant with battery energy storage.

The goals and development directions described above requirethe transformation of the current distribution networks of mediumand low voltage. It is necessary to introduce the possibility of localcontrol of power ows. Protection systems need to work with bi-directional energy ows. There is no data communicationsubsystem necessary to control all components in current distri-bution network. As a consequence the idea of micro-grids wasintroduced. A micro-grid is a small, balanced power subsystemwhich connects distributed power stations and consumers locatedin a small area.

connected to a power system it has to be constructed in such a waythat the power interchange is planned as in case of big powerplants.

Unfortunately, a disadvantage of micro-grids is a problem ofvoltage and frequency control (reactive and active power ow). It isclear that there will also be problems with voltage distortion due tothe wide utilisation of power electronic converters.

We propose to use DC micro-grids as a solution which avoidsmost of problems described above. The solution allows costs to bekept low in many cases and makes easier some issues connectedwith the control of quality parameters. The problem of quality in DCsystems is reduced to keep voltage or current in the required range.

An idea, which is fundamental to the DC micro-grid concept isthe realisation of the postulate of locally balancing production andconsumption as in case of the AC micro-grid. In addition highquality parameters will be kept. Thanks to the introduction of DCthere will not be problems associated with frequency, shape andreactive power control. A block diagram of the system proposed isshown in Fig. 16. DC micro-grid principle of operation is based onthe conversion of all kind of produced power into DC. In the case ofgenerators with variable frequency the conversion is alwaysapplied. A rectier and inverter also realise the connection withpower grid. All power balancing and control functions are per-formed in DC circuit.

Consumers are connected by inverters. If the connection needsto be bi-directional it has to be done by a bi-directional converter orthe parallel connection of a rectier and inverter.

The problem of power quality in the micro-grid is reduced tovoltage level control. It is obvious that consumers need to be joinedto the AC 50 Hz sources. So there are inverters which produce

AC

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Fig. 15. Block diagram of grid-connected wind plant with DC transmission line.

J. Paska et al. / Renewable Energy 34 (2009) 241424212420The principle of micro-grid operation is balancing productionwith consumption. In the case of stand-alone micro-grid systemthis has to be balanced all the time. In the case of a subsystemFig. 16. DC micro-a 50 Hz sinusoidal voltage. Due to the power control is executed inDC circuit, if the DC voltage is kept higher than the minimumthreshold value, the output AC voltagewill meet all requirements. Itgrid concept.

is particularly interesting that the voltage in DC circuit does nothave to be kept with very high precision. Modern inverters cankeep a stable sinusoidal waveform with relatively wide changes ofDC input voltage.

The described solution can have a few variants depending on thenumber of inverters and their power and localization. The optimalsolutions are, the one inverter supplying all consumers on one side,and individual inverters for each consumer on the second side. Thesecond possibility allows for instance for the measurement ofenergy in a DC circuit.

The critical problem is a data communication subsystem andproper control strategy. The strategy will have a direct impact onpower quality, power sources utilisation, and the balancing leveletc. As a consequence it will have a crucial inuence on energy costsin the micro-grid.

7. Conclusions

Very good simulation and experimental results allow us to saythat the proposed hybrid power plants have worked as was plan-ned. Hybrid power systems (plants) are a good way to have avail-able sources of electricity which optimise utilisation of primaryenergy sources.

The important problem is the integration of distributed gener-ation and renewable energy sources into existing power networks,especially local power networks.

We propose a DC micro-grid which integrates all the abovedescribed circuits. The DC micro-grid allows:

- the development of a new method of energy measurement,- the introduction of load active control methods.

Technical realisation of the DC micro-grid is possible and quitesimple at the present time at relatively lowcost. It is the result of therapid development of power electronic converters and datacommunicationequipment anda signicant reduction in their costs.

References

[1] Paska J. Technologies of distributed generation, vol. 4. Elektroenergetyka Technika, Ekonomia, Organizacja; 2002 [in Polish].

[2] Burch GD. Hybrid renewable energy systems. In: U.S. DOE natural gas/renewable energy Workshops. Golden, Colorado; August 21, 2001.

[3] Blarke MB, Lund H. The effectiveness of storage and relocation options inrenewable energy systems. Renewable Energy 2008;33.

[4] Dmowski A, Biczel P, Kras B. Stand-alone telecom power system supplied byPEM fuel cell and renewable sources. In: International fuel cell workshop2001. Kofu, Japan; 1213 November, 2001.

[5] Biczel P. Optimal usage of primary energy carriers on example of hybrid solarand fuel cell power plant. Ph.D. dissertation, electrical engineering faculty,Warsaw University of Technology, Warsaw, Poland; 2003 [in Polish].

[6] Paska J, Biczel P, K1os M. Experience with hybrid power generating systems. In:Eighth international conference, electrical power quality and utilisation EPQU05. Cracow-Poland; September 2123, 2005.

[7] Bonhoff K. The NEXA 1200 watt compact power supply. The fuel cell world.Switzerland: Lucerne; July 2002. p. 15.

[8] Hansen AD, Srensen P, Hansen LH, Binder H. Models for a stand-alone PVsystem. Roskilde: Ris National Laboratory; December 2000.

[9] Hoque A, Wahid KA. New mathematical model of a photovoltaic generator(PVG). The Institute of Engineers, Bangladesh. Journal of electrical engineering2000;1.

[10] Paska J, Biczel P. Hybrid photovoltaic power plant with fuel cell as an exampleof optimal utilisation of primary energy sources in distributed power industry,vol. 4. Elektroenergetyka Technika, Ekonomia, Organizacja; 2003 [in Polish].

J. Paska et al. / Renewable Energy 34 (2009) 24142421 2421- the avoidance of many difculties associated with the controlof energy parameters,

- a simplication of control strategy and control units,- cost reduction,- a reduction in transmission losses,[11] King DL, Dudley JK, Boyson WE. PVSIM: a simulation program for photovoltaiccells, modules, and arrays. In: Twenty-fth IEEE PVSC, Washington, DC; May1317, 1996.

[12] K1os M. Technical and economic aspects of electrical energy storage on theexample of wind power plant. Ph.D. dissertation, electrical engineeringfaculty, Warsaw University of Technology, Warsaw, Poland; 2006 [in Polish].

Hybrid power systems - An effective way of utilising primary energy sourcesIntroductionWhy hybrid power plants?Hybrid solar-wind and battery power plantIdea and realisation of the plantSome problems caused by uncontrolled power sources

Hybrid solar-fuel cell power plantUsed fuel cell and plant conceptResults of experimentsAdvantages and disadvantages of hybrid solar-fuel cell power plant

Wind power plant with battery energy storageIntegration of DG sources using DC micro-gridConclusionsReferences