Hybrid Energy Storage System Based on Compressed Air and

Extended Summary 本文は pp.911–920

Hybrid Energy Storage System Based on Compressed Air andSupercapacitors with Maximum Efficiency Point Tracking (MEPT)

Sylvain Lemofouet Non-member (Swiss Federal Institute of Technology Lausanne; [email protected])

Alfred Rufer Non-member (Swiss Federal Institute of Technology Lausanne; [email protected])

Keywords: energy storage, compressed air, supercapacitors, maximum efficiency, hydro-pneumatics

This paper presents hybrid energy storage systems mainly basedon Compressed Air and Supercapacitors (CASCES) with high po-tentials in term of life cycle and impacts on environment, and mainlyintended to distributed generation and renewable energy sourcessupport. The idea of associating compressed air and Supercapaci-tors was firstly implemented in the context of an off-line UPS systemwhose principle is illustrated in Fig. 1, with the main goal of devel-oping methods for operating the compressed air energy convertersat their highest efficiency conditions.

All the devices of the system are reversible. There is no pres-sure regulation valve between the air tank and the air machine to

Fig. 1. Principle of CASCES system

Fig. 2. Efficiency curve of the air motor

avoid the additional losses in such a device. The air machine’s inletpressure therefore varies continuously and affects its performances.The typical shape of its efficiency is given in Fig. 2, which showsthe existence of an optimal speed that corresponds to the maximumefficiency. An efficiency optimization algorithm (MEPT) was de-veloped to control the air machine through variable speed, so as toalways keep it at the maximum efficiency operating point. Since themaximum efficiency operation imposes the level of the producedpower, an intermittent time-modulated operation is applied to thePneumatic-to-electric conversion system to modulate the generatedpower in function of the output power demand. A supercapacitiveauxiliary storage device is therefore necessary to obtain a smoothlyvariable, high quality output power, through the regulation of the ca-pacitive intermediary stage voltage. The thermodynamic basics ofcompressed air energy storage are reviewed first, then the principleof the MEPT is presented. Afterwards the strategy of output powermodulation and the design of the supercapacitive auxiliary storageare analyzed.

Despite the implementation of the efficiency optimization strat-egy, the energy efficiency of the air machines remains very low asit can be seen on Fig. 2. This is due to the non isothermal processesand the important leakage and friction in these machines. In ad-dition their limited pressure rating does not allow achieving highenergy density. Oil-hydraulic machines appear as good alternativewith their exceptional performances regarding the energy-efficiency(over 90%). In addition, their relative high operating pressure makesthem interesting for reaching high energy densities. These criteriahave led to the development of hybrid Hydro-pneumatic and Super-capacitors storage systems, where the most critical issue is the air-to-oil interface. Two hydro-pneumatic storage systems called BOP(Battery with Oil-hydraulic and Pneumatics) are also presented. Thefirst one, called BOP-A, uses hydraulic accumulator as storage vol-ume, where an isothermal compression/expression takes place. Thissystem is under experimentation and the first practical results arepresented. The second one, the BOP-B, is more promising in termof energy density because it uses a very small quantity of oil to com-pressed fresh air from outside. In this system, the almost isother-mal compression/expansion takes place in liquid-piston working-chamber with integrated heat exchanger, which leads to high effi-ciency.

Finally, some economical considerations are made, through acomparative cost evaluation of the presented hydro-pneumatic stor-age systems and a lead acid batteries system, in the context of astand alone photovoltaic home application. This evaluation con-firms the cost effectiveness of the presented BOP storage systems.

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Paper

Hybrid Energy Storage System Based on Compressed Air andSuper-Capacitors with Maximum Efficiency Point Tracking (MEPT)

Sylvain Lemofouet∗ Non-member

Alfred Rufer∗∗ Non-member

This paper presents a hybrid energy storage system mainly based on Compressed Air, where the storage and with-drawal of energy are done within maximum efficiency conditions. As these maximum efficiency conditions impose thelevel of converted power, an intermittent time-modulated operation mode is applied to the thermodynamic converterto obtain a variable converted power. A smoothly variable output power is achieved with the help of a supercapacitiveauxiliary storage device used as a filter. The paper describes the concept of the system, the power-electronic interfacesand especially the Maximum Efficiency Point Tracking (MEPT) algorithm and the strategy used to vary the outputpower. In addition, the paper introduces more efficient hybrid storage systems where the volumetric air machine isreplaced by an oil-hydraulics and pneumatics converter, used under isothermal conditions. Practical results are alsopresented, recorded from a low-power air motor coupled to a small DC generator, as well as from a first prototype ofthe hydro-pneumatic system. Some economical considerations are also made, through a comparative cost evaluationof the presented hydro-pneumatic systems and a lead acid batteries system, in the context of a stand alone photovoltaichome application. This evaluation confirms the cost effectiveness of the presented hybrid storage systems.

Keywords: energy storage, compressed air, supercapacitors, maximum efficiency, hydro-pneumatics

1. Introduction

World wide, considerations have been made on energy re-sources, about traditional generation and distribution systemsas well as renewable sources. Traditionally, energy storagehas been of high interest, as in the case of non flexible nu-clear or thermal power versus daily cycles. In the context ofenergy storage in relation with power generation, the issuesof fast and strong load variations have been studied also (1) (2).For future concepts of energy production and distribution,like decentralized generation, balance of production and con-sumption is one of the main concerns, because of the loss ofthe effect of a “statistical mean value” for the loads, that nor-mally smoothes their profile. This will reinforce the need forstorage facilities. Considering the integration of renewablemeans, like photovoltaic and wind power that are intermittentpower sources, additional storage facilities are also required.

Recently several new storage technologies have appearedas alternative solutions to classical batteries, and are beingintegrated in grid (3) (4). CAES technologies belong to these in-novative developments, with their high potential of producingless problematic waste materials. In fact, they have been pro-posed from longer date, in the middle and high power range,as well as in electrical power generation and transportationapplications. Ultra-large size facilities have been proposedrecently based on dedicated turbo machinery (5) (6), but theseconventional CAES-systems require additional fuel for com-bustion in order to reach the normal operating conditions of

∗ Swiss Federal Institute of Technology, EPFL-ISE-LEIStation 11, CH-1015 Lausanne, Switzerland

∗∗ Swiss Federal Institute of Technology, EPFL-ISE-LEIStation 11, CH-1015 Lausanne, Switzerland

the turbine. Therefore, they are not classical storage systemas usually considered and their round-trip efficiency is dif-ficult to evaluate. In addition, they are site dependent andmainly restricted to utilities applications. As alternative so-lutions, small size accumulators have been proposed that useintermediary mechanical/hydraulic conversion with the so-called “liquid piston” principle (7). These solutions, which areconsidered in this paper, require no additional fuel and theiroperating principles are compatible with the definition of theround-trip efficiency.

An overview of the today’s main storage technologies isgiven in Table 1, from the most classical pumped hydro-storage for large capacities to the more recently devel-oped Supercapacitors where electrical energy can be directlystored, but in very reduced amounts.

In relation with sustainable development and impact onenvironment, the life cycle of different storage technologiesare given, which show the limited performances of electro-chemical solutions. In addition, these solutions present manydifficulties to be recycled (8). In that context, the develop-ment of alternative solutions to classical batteries is the main

Table 1. Life Cycle and Efficiency of the today’s mainstorage technologies

Storage System Life [Cycles] Efficiency [%]Pumped Hydro 75 Years 70–80Compressed Air 40 YearsFlow Batteries 1500–2500 75–85Metal-Air 100–200 50NAS 2000–3000 89Other advanced batteries 500–1500 90–95Lead-Acid 200–300 75Supercapacitors 10’000–100’000 93–98

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motivation of the current research project, which tries to op-timize old storage techniques based on simple physical pro-cesses like gas compression and expansion, with the help ofmodern power electronics converter and control.

Thus, the studied solution combines Compressed Air asmain storage media with Supercapacitors as auxiliary stor-age device. As the compressed air-based main storage is dif-ficult to operate with low power sources as stand alone pho-tovoltaic supply, the supercapacitive auxiliary storage is usedto adapt the power-levels of the source to that of this mainstorage. In addition, this hybrid topology allows operatingthe main storage conversion chain in intermittent but optimalefficiency conditions. The supercapacitive auxiliary storageis again necessary to filter the intermittent power so as to ob-tain a smoothly variable output power.

The concept of this hybrid storage system is described first,then the Maximum Efficiency Point Tracking algorithm ispresented and a strategy for varying the electric output poweris proposed. The performances of a higher efficiency stor-age device, based on oil-hydraulics machines are also inves-tigated, and then the first experimental results of these inves-tigations are presented. Finally some cost considerations aremade, in the context of a photovoltaic home application.

2. Review of the Basic Thermodynamics of Com-pressed Air Energy Storage

Basic physics theories describe the thermodynamic prop-erties of volumetric compression, under adiabatic as wellas isothermal conditions. For compressed air energy stor-age, adiabatic conditions will lead to an acceptable energy-efficiency only if these conditions can be kept unmodifiedthrough the total storage time (9). However, for storage pur-pose, isothermal compression and expansion represent thebest conditions, because the thermal energy exported to/fromthe external atmosphere yields a higher efficiency and agreater energy density inside of the storage vessel than in theadiabatic case. The phenomena can be easily understood,from the example of a hand pump, with the orifice closed asshown in Fig. 1.

By adding, from a stabilized initial condition of 1 bar pres-sure in a unity volume at 15◦C a supplementary pressure of1.5 bar, the piston will go down reducing the volume to a newvalue of 0.5, while the temperature increases up to 101◦C intotal adiabatic conditions (1st step). In a second step the ther-mal energy ∆Q corresponding to the added mechanical workis exported to the external ambient air by new temperatureequalization at 15◦C. The volume will be reduced again downto 0.4 and the pressure is kept at the previous level of 2.5 bars.Then, by removing the additional pressure (3rd step), the pis-ton comes up again, bringing the volume up to 0.77. Duringthis expansion, adiabatic conditions would decrease the tem-perature down to −51◦C. The new temperature equalization(4th step) will then increase the volume again up to the initialvalue. The match of the initial pressure and temperature con-ditions indicate that the energy stored by isothermal compres-sion can be recovered by isothermal expansion. The energyefficiency is then ideally equal to 1. During isothermal com-pression the internal energy of the reservoir has not changed.This means that in reality, the energy is stored in form of heatin the surrounding during compression and restored during

Fig. 1. Thermodynamic cycle with adiabatic steps andthermal equalization

expansion. The quality of Heat transfer is therefore a keyelement for the performances of this storage process.

3. Principle of the Hybrid Compressed Air andSuperCapacitors Energy Storage (CASCES)System

The idea of associating a high capacity energy storage me-dia (compressed air), and a high power density storage de-vice (Supercapacitors), was firstly implemented at the Indus-trial Electronics Laboratory (LEI) of Swiss Federal Instituteof Technology (EPFL), in the context of an off-line UPS sys-tem whose principle is presented in Fig. 2. This system wasused for the first investigations on CASCES technology, eventhough the energy efficiency of the 300 W power range exper-imental air compressor/motor was known to be very low, as itwill be seen in the next section. The main goal of these inves-tigations was to develop methods for operating that family ofcompressed air energy converters at their optimal efficiencyconditions. A small DC motor/generator was coupled to theair machine in order to get rapidly and easily controllableelectro-mechanical conversion.

All the devices of the system are reversible. During thestorage process, the electric machine works as a motor anddrives the pneumatic machine, which operates as a compres-sor to fill the vessel with pressurized air. During the dischargeprocess, the compressed air is directly expanded in the pneu-matic machine, which operates as a motor to power the DCgenerator. The following technical solutions have been im-plemented to improve the efficiency of the system:• There is no pressure regulation device between the air

tank and the air machine in order to avoid the additionallosses in such a device. The motor inlet pressure there-fore varies continuously.•An efficiency optimization algorithm has been devel-

oped to control the volumetric air machine through vari-able speed, so as to always keep it at the maximum effi-ciency operating point, as illustrated in Fig. 3.

912 IEEJ Trans. IA, Vol.126, No.7, 2006

Hybrid Energy Storage System Based on Compressed Air and Super-Capacitors with MEPT

Fig. 2. Principle of CASCES system

Fig. 3. Efficiency Characteristic of the experimental airmotor

• Since the maximum efficiency operation imposes theproduced power level, an intermittent operation of thecompressed air energy converter is used to modulate theproduced power as a function of the output demand.• The supercapacitive auxiliary storage device is therefore

necessary to obtain a smoothly variable, high qualityoutput power through the regulation of the capacitive in-termediary stage voltage.

4. Strategy of Efficiency Optimization: Princi-ple of Maximum Efficiency Point Tracking(MEPT)

The continuous changes in pressure and load affect thepneumatic machine’s performances, particularly its effi-ciency. The efficiency characteristic for the experimentallow power air motor which has been drawn from measure-ments and data provided by the manufacturer (10) is illustratedin Fig. 3. This shape is typical for volumetric machine. As itcan be noticed on that characteristic, there exists an optimalspeed that corresponds to the maximum efficiency. More-over, this optimal speed is pressure dependent. The purposeof the efficiency optimization strategy is to control the vol-umetric machine so as to always maintain it at the optimaloperating point.

The principle of this strategy is shown in Fig. 4. On the ba-sis of various measurements (pressure, flow rate, speed, etc),the maximum efficiency-tracking module determines the op-timal speed. This optimal speed serves as reference to thespeed control module and is achieved through the control ofthe electromagnetic torque of the DC generator by the means

Fig. 4. Structural diagram of the MEPT control strategy

Fig. 5. Flow chart of the MEPT algorithm

of the current regulator of the motor drive.Fig. 5 presents the flow chart of the MEPT algorithm which

is mainly based on two techniques:• The Quadratic Interpolation (Q.I), which exploits the

quadratic shape of the efficiency at start up. During thisphase, the quadratic shape is approximated by a secondorder polynomial function and the interpolated optimalspeed reference Ni is determined by:

Ni =ηk−1N2

k − ηkN2k−1

ηk−1Nk − ηkNk−1· · · · · · · · · · · · · · · · · · · · · (1)

where ηk and Nk are the kth measurements of efficiencyand speed respectively. This interpolation allows reach-ing the first optimum after a few steps from the start.• The well-known Perturbation-Observation (P.O) tech-

nique for the tracking of optimal speed. The determina-tion of the speed increment ∆N is crucial for the stabilityand accuracy of the algorithm. It should be chosen smallenough to avoid mechanical oscillation. In our case, ∆Nhas been determined experimentally to 5 rpm.

The experimental results recorded from a small vane typeair motor are shown in Fig. 6. As it can be seen, there is agood superposition of the theoretical optimal speed (blackcurve) and the experimental one (gray curve), which con-firms the effectiveness and accuracy of the proposed MEPT

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Fig. 6. Performances of the MEPT algorithm

algorithm. The P.O technique used for optimum tracking pro-duces very low speed oscillations that do not generate any no-ticeable torque ripple. It should be notice that this algorithmuses no parameter of the controlled machine; therefore, it canbe generalized for the control of any kind of volumetric ma-chine, as it will be shown in the next section. Furthermore,the proposed control strategy can be easily adapted to controlthe motor in other operation modes like the Maximum PowerPoint Tracking (MPPT) operation mode (14).

Despite the implementation of the efficiency optimiza-tion strategy, the energy efficiency of the air machine re-mains very low (less than 20% for the experimental machine,around 30% for high power machine) due to non isothermalconversion as explained in Section 2 and important leakageand friction. In addition their low pressure rating does notallow the achievement of high energy density. Consequently,air machines are definitely not suitable for energy storage ap-plications. Therefore another conversion solution needed tobe found and oil-hydraulic machines appear as ideal devices;however they require an oil-to-air interface which is not al-ways easy to implement.

5. Oil-Hydraulics and Pneumatics: Key Compo-nents for High Efficiency Conversion

Hydraulic accumulators are well known components in in-dustrial applications. They are based on high-pressure steelvessel, where a trapped volume of gas (generally nitrogen)is compressed, up to 350 bars and more, by injecting oil inthe shell or in the body, using a gas/liquid separation devicethat can be a diaphragm, a bladder or a piston. Fig. 7 showsclassical accumulators as can be found from many hydraulicsmanufacturers. These equipments are generally designed todeliver high instantaneous power and are therefore oversizeand costly for battery-type energy storage as considered inthis paper.

On the other hand, hydraulic motors are highly interestingdevices, because they exhibit exceptional performances re-garding the energy-efficiency. In addition, their high pressurerating, in the range of several hundreds of bars, makes thema key component for achieving with CAES energy-densitiescomparable to that of lead acid battery. Typical housings andenergy efficiency of these devices are given by Figs. 8 and 9respectively.

Combining these two hydraulic components, a hydro

Fig. 7. Hydraulic accumulators: Bladder, Diaphragmand Piston technologies

Fig. 8. Hydraulic motor/pump: Typical housings

Fig. 9. Hydraulic motor/pump: Typical efficiencycharacteristics

pneumatic storage system can be realized, where interestingperformance can be reached. The principle of such a system,which is also called Battery with Oil-hydraulics and Pneu-matics, type A: BOP-A, is illustrated in Fig. 10. As it can beseen, the hybrid topology with an auxiliary supercapacitivestorage is again used for the same reasons as explain in Sec-tion 3: the possibility to boost the source power and to varythe load power from the optimal tracked level. From the effi-ciency curves of Fig. 9, it is obvious that the MEPT algorithmpresented earlier is again necessary to achieve optimal energyconversion efficiency through variable speed operation.

Since the compression and expansion processes take placein the storage vessel, very high thermodynamic efficiency canbe easily obtained by operating the system in isothermal cy-cles as explained in Section 2. This is done when the charg-ing time of the accumulator is much higher than the time-constant needed for the thermal exchange between the accu-mulator’s gas chamber and the external environment. Typi-cally, photovoltaic generators would charge the accumulator



Fig. 10. Principle of hydro-pneumatic energy storagesystem

within up to 8 hours, while the thermal exchange will needa tenth of minutes. This leads to isothermal conditions andyields high-value efficiency.

However, the closed nitrogen cycle of this storage sys-tem, together with the needed important volume of the com-pressing hydraulic-oil would lead to a very low energy den-sity, around 3 Wh/l and 2.5 Wh/kg. Because of this low en-ergy density, the BOP-A system is more suitable for lowenergy applications or stationary applications where volumeand weight are not critical criteria.

A system with a much higher energy density can be real-ized using an open gas-cycle, with air from the atmosphere,and operating with a low oil-volume reciprocating compres-sion/expansion interface. This system must be designed withan integrated heat exchanger that can achieve nearly isother-mal air compression and expansion. Such a system is de-scribed in the next section, as well as in Refs. (11) and (12).

6. BOP-B Storage System: A High PerformanceBattery with Oil-Hydraulics and Pneumatics

In order to exploit all the promising features of pneumaticenergy storage, a converter is needed which would compressand expand the air with high efficiency, acting as interface tostandard forms of energy like electricity or rotating shafts.The key to high conversion efficiencies is to maintain al-most constant temperatures during compression and expan-sion (a swerve of 30◦C induces an efficiency drop of 5%).So far, only positive displacement machines could be fittedwith intermediary heat exchangers which would allow nearlyisothermal cycles. If the reciprocator is a liquid piston, theprovisions for good and simple heat exchange are quite easyand reliable and the efficiency grows as no seal friction is in-volved. The working principle of such a converter is shownin Fig. 11, in a simplified manner in order to explain thesequences of one cycle. A detailed description is given inRefs. (11) and (12).

During discharge (expansion), the compressed air enters,through the opened valve (D), the work-chamber (1R) ofthe right cylinder comprising the liquid piston (2R), the saidvalve (D) being controlled so as to admit exactly the por-tion of compressed air which, once expanded, will reach theatmospheric pressure. The pressure established in the rightcylinder is transmitted through the exchanger coil (3R) tothe hydrostatics (4), passing the four-way-valve (5). Thisvalve remains in the position (b) and thus activates the mo-

Fig. 11. Principle of the BOP-B storage system

tor port. This leads to the expulsion of the air in the work-chamber (1L) by the return flow of (4), which joins the muf-fler (6) through the opened valve (B). The air in the right-handed work-chamber (1R) is squeezed between metal plateswhen expanding; these metal plates just emerge from thethermally stabilized liquid, so any cooling down of the airis seriously hampered (the same would happen during com-pression, where a temperature rise would be limited, as theexternal exchanger (3R) always tends towards the surround-ing temperature of the fan air flow. As the stroke ends, the4-Way-Valve (5) inverts the interface flow by switching tothe position (a) without changing the rotational direction ofthe hydraulic motor, the inertia of the liquid pistons beingnegligible.

The conversion chain between air and wire consists ofthree stages: Air-Oil (called the interface: A), Oil-Shaft (hy-drostatics: B), and Shaft-Wire (motor-generator: C). Smallloss-sources like the flywheel and the fan of the heat ex-changer have been neglected. In the kW range, efficienciesof 90% for A, 95% for B and 90% for C are attainable with-out excessive R&D efforts, which would yield a round tripefficiency of about 60% for the overall system. The open-gascycle together with the reduced amount of oil yield to spe-cific energy of about 30 Wh/l and 20 Wh/kg for steel vesselsto 50 Wh/kg Carbon fibber wounded vessels, which is com-petitive with lead acid batteries. In addition, the BOP systemsare mainly based on mature, reparable and reliable technolo-gies, with easy-to-recycle material that produces no or verylittle problematic waste. This leads to other important advan-tages over electrochemical technologies: high life cycles andlow impacts on the environment. Because of these long term

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advantages, BOP technologies are highly compatible with re-newable energy sources.

7. Strategy of Output Power Variation

7.1 Introduction The strategy of output power vari-ation is described on the basis of the pneumatic system pre-sented in Section 3, but it is also valid and can be used in asimilar form for the BOP systems. Only the discharge pro-cess is considered, but the approach is identical for the stor-age process.

As mentioned earlier, the variation of electric output powerPL is based on the intermittent, time-modulated operation ofthe pneumatic-to-electric conversion chain. The excess of thegenerated power PE is stored in the supercapacitors and usedto supply the load during the stop-time of the air machine, oras power-assistance during peak power demand. The control-scheme of this strategy is presented in Fig. 12.

By maintaining the intermediary dc bus voltage Uinter con-stant, the voltage regulator automatically compensates thelack or excess of the generated power, allowing thus the out-put power to vary freely. The time-modulated on-off opera-tion of the pneumatic energy conversion is made through theelectro-valve control module. There are two possible oper-ation modes: The Fixed-frequency operation mode and theFree-oscillating operation mode.

7.2 Fixed-Frequency Operation Mode In thismode, the cycle period T is constant. The converted energyWe is then proportional to the duty cycle D as presented inFig. 13. As the reservoir pressure drops during operation, theproduced power PE decreases. When the system is suitablyadjusted, the energy Ws stored during the running time ton isequal to the energy Wr required by the load during the stoptime to f f .

Fig. 12. Principle of output power variation

Fig. 13. Power modulation

Ws = Wr ↔∫ D.T

0PEdt =

∫ T

0PLdt · · · · · · · · · · · · · (2)

If we consider the simple case where the produced powerPE and the load power PL are constant, we can get fromEq. (2) the Duty cycle as:

D =PL

PE· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (3)

Since it’s not possible to predict the behavior of the loadpower, D is determined once at the beginning of each cy-cle using Eq. (3). This approach is effective only if the cycleperiod T is small enough compared to the time constant ofthe power variations. It may happen that the load power in-creases strongly within a cycle in such a way that the storedenergy becomes insufficient to supply the load. In that case,the pneumatic (or hydraulic) machine is turned on as soonas the minimum allowable value of the supercapacitive bankvoltage is reached. It’s also turned off at its maximum voltageand at the minimum operating pressure as shown on Fig. 12.The simulation results of this mode are presented in Fig. 14.As it can be seen, the cycle period is not always constant,particularly when the load power is important and the super-capacitive bank discharged.

7.3 Free-Oscillating Operation Mode In this mode,the operation cycle period is never constant. The only con-trol parameter is the supercapacitive bank voltage. When itreaches the minimum value, the air (or hydraulic) machineis turned on and when it reaches the maximum value, it isturned off. As the intermediary voltage is held constant, thefrequency and duty cycle of the operation cycle are automat-ically fitted to the ratio of PE and PL. This is the simplestway to modulate the produced power. As it can be seen fromthe simulation results presented in Fig. 15, for the same load

Fig. 14. Fixed Frequency Operation mode



Fig. 15. Free Oscillating Operation mode

power profile, the number of switchings of the pneumatic ma-chine is lower than in the previous mode.

7.4 Sizing the Supercapacitive Bank The superca-pacitive bank must be able to provide the energy Wr requiredby the load during the stop time to f f . This energy evaluatedas:

Wr =

∫ T

DTPLdt · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (4)

The most critical case is that where we have D = 0 andPL = PLmax = Cste. In that case, the energy to provide to theload is:

Wr =

∫ T

0PLmaxdt = PLmax.T · · · · · · · · · · · · · · · · · · · · (5)

The maximum amount of energy WS Cmax that can be storedin a supercapacitive bank of total capacitance CT and maxi-mum voltage UTmax is given by:

WS Cmax =12

CT U2Tmax · · · · · · · · · · · · · · · · · · · · · · · · · · · · (6)

Because of the losses in the Supercapacitors and the min-imum voltage UTmin required by the interface converter, thisenergy cannot be restored totally. If we consider a depth ofdischarge (DOD) d = UTmin

UTmax∗100% and a discharge efficiency

ηd, the maximum available energy is:

WUmax =12ηdCT U2

Tmax

⎡⎢⎢⎢⎢⎢⎣1 −(

d100

)2⎤⎥⎥⎥⎥⎥⎦ · · · · · · · · · · · · · · (7)

From Eqs. (5) and (7) we can get the value of CT :

CT =2T PLmax

ηdU2Tmax

[1 −

(d

100

)2] · · · · · · · · · · · · · · · · · · · · · · (8)

This design criterion takes into account the internal lossesproduced inside the supercapacitive bank, due to the inter-nal series resistance (13). A more accurate design should alsoinclude the losses inside the interface converter.

8. Practical Results

In order to verify the performance of the proposed systemsand especially the use of a hybrid configuration with a hydro-pneumatic main storage device combined to a Supercapaci-tors auxiliary storage, a dedicated setup has been realized.The diagram of which is presented in Fig. 16 that shows thehydro-pneumatic chain composed of the inverter, the PMSYmachine, the oil pump/motor and the hydraulic accumulator.Under the electric drive, the auxiliary storage device is rep-resented, with the interface converter, facing the active loadrealized with a transistor and a dissipating resistor. For theevaluation of the storage system in the laboratory conditions,the PV source is replaced by a line supply made of a smalltransformer and a three-phase diode rectifier.

This installation has been realized with the goal of experi-menting the alternating operation of the two storage subsys-tems, and to further implement the MEPT algorithm. Anadditional extension of this equipment for the experimenta-tion of a BOP-B system is expected, and will be presentedin further contributions. A view of this experimental setup isgiven by the photograph of Fig. 17. The basic operation ofthe system for a storage process is illustrated by the curvesof Fig. 18. The pressure of the gas in the accumulator is rep-resented, as well as the speed of the hydraulic motor/pump.While the main storage subsystem is charging the hydraulicaccumulator through oil injection, the relatively high powerof the pump is provided by the supercapacitive auxiliary stor-age device. When the oil-pump is stopped, this auxiliarystorage is recharged by the low power primary source thatcould be the PV-system. The intermittent time-modulated op-eration allows varying the mean value of the needed power,to meet the variations of the compression power due to thechanges in the gas pressure. The curves of a cycle operationin MEPT conditions as well as the results of efficiency mea-surements will be presented in further publication when thefinal and optimized installation will be realized.

9. Economical Considerations: ComparativeCost Evaluation

9.1 Specifications of the Studied Case A compar-ative storage-cost evaluation has been made for a Day-to-Night power shift in a Photovoltaic supplied home applica-tion. The daily scenario of the application is shown in Fig. 19for a normal sunny day. The mean power of the system is4 kW, the storage peak-power 10 kW and the discharge peak-power 5 kW. 3-days autonomy is required, which leads to astorage capacity of 60 kWh. Three storage technologies areconsidered with DC voltage output: A tubular plate lead acidbatteries system as shown in Fig. 20, a BOP-A system asshown in Fig. 10 and a BOP-B system as shown in Fig. 11with Supercapacitors as auxiliary storage. The minimum lifecycle requirement is 3’500 cycles.

9.2 Cost Evaluation and Comparison Table 2shows the cost evaluation of the considered storage technolo-gies for the specified application. The cost of the PV panel

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Fig. 16. Schematic Diagram of experimental setup for BOP-A system

Fig. 17. View of the experimental setup for BOP-A system

and its interface converter is not taken into account. The firstremark is that for BOP systems, the storage part is completelyseparated from the conversion part, thus offering more flex-ibility for the design and sizing of the storage. Secondly,BOP-B is for far the best storage solution for this applica-tion. BOP-A is much more expensive than the other systems,because of the large storage unit due to its low energy den-sity. However, in comparison with lead acid, this high cost isbalanced by a high life cycle so that the two systems presentalmost the same specific energy cost.

Fig. 21 shows the evolution of the System-cost and the

Fig. 18. Basic operation curves of BOP-A

Energy-cost for the three considered technologies with re-gard to the storage capacity. As it can be seen, there is anoffset-cost for BOP systems that corresponds to the conver-sion cost. In addition, BOP-A is more sensitive to the storagecapacity than the other systems. These graphs confirm thecost effectiveness of BOP-B which makes this system very



Fig. 19. Daily scenario

Fig. 20. Principle of the PV supply with the Batteries storage

Table 2. Cost evaluation for the three (60 kWh-10 kWpeak) storage technologies

Characteristic Lead Acid BOP-A BOP-BStorage ( e ) 18’000 60’000 4’500Conversion ( e ) / 15’000 18’000Technology Cost ( e ) 18’000 75’000 22’500Operation & 30% 20% 20%Maintenance Cost ( e ) 5’500 15’000 6’000Total Cost( e ) 23’500 90’000 28’000Life Cycle (cycles) 3’500 15’000 15’000Total Energy ( kWh ) 210’000 900’000 900’000Specific Cost ( e/kWh ) 0.11 0.1 0.03

Fig. 21. Total-Cost (ke) and Specific energy-Cost (ctse/kWh)versus Storage capacity

promising as future storage solutions associated with renew-able energies sources

10. Conclusion

Compressed air in vessel storage systems has been con-sidered with the target to develop a high life-cycle and cost

effective system. The Efficiency optimization strategy pro-posed in this study can significantly improve the efficiency ofthe thermodynamic conversion. The presented hybrid topol-ogy, which combines compressed air and Supercapacitors,improves flexibility and dynamic performances of the stor-age system and makes it suitable for a wide range of lowpower autonomous and grid-connected applications, like re-newable support and power quality enhancement for sensitiveusers. The typical advantages over classical CAES plants in-clude site independence, fuel free operation and environmen-tal harmlessness. However, low-pressure and low-efficiencyare weaknesses of the purely pneumatic studied system andare mainly related to the characteristics of the air motor. Toimprove the efficiency of the conversion and the energy den-sity of the storage, the dedicated system must be able to op-erate at high and variable pressure in an isothermal process.For this purpose, oil-hydraulic machines offer the best per-formances, but they need a particular air-oil interface. Fur-ther developments are still going on at EPFL’s LEI, in orderto verify the quality of the integrated heat exchanger for theisothermal process. Global system optimization and dedi-cated valve control and coordination belongs also to furtherexpected developments. Experimental results have been pre-sented for the pneumatic system that uses air motor as well asfor a BOP or hydro-pneumatic system which is based on highefficiency hydraulic motor/pump. Finally, the cost evaluationin the context of a practical application has proved the costeffectiveness of the studied hydro-pneumatic systems com-pared to a classical electrochemical storage technology.

(Manuscript received May 12, 2005,revised Dec. 26, 2005)

This paper was presented at IPEC-Niigata 2005, and approved forpublication in the IEEJ Transactions on Industry Applications Soci-ety.

References

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( 5 ) J. Lehmann: “Air storage gas turbine power plants, a major distribution forenergy storage”, International Conference on Energy Storage, pp.327–336,Brighton UK (1981-4)

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( 7 ) A. Reller and I. Cyphelly: “Speicherung gasofmiger Energientrager: EineBestandsaufnahme”, VDE-Berichte, Energiespeicher 1734, pp.37–45 (2002)

( 8 ) D.C.R Espinosa, A.M. Bernardes, and J.A.S. Tenoio: “An overview onthe current processes for recycling of Batteries”, J. of Power Sources 135,pp.311–319 (2004)

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Sylvain Lemofouet (Non-member) was born in Bertoua, Cameroon,in 1970. He received the B.S. degree and the M.S.degree in Electrical Engineering in 1991 and 1999respectively, from the Advanced Teacher’s TrainingCollege for Technical Education (ENSET), Univer-sity of Douala, Cameroon. He worked as Electron-ics Teacher at the technical high school of Garoua,Cameroon from 1991 to 1997, and as Industrial Con-sultant and Assistant-Lecturer in the Electrical de-partment of ENSET from 1999 to 2001, before join-

ing the Industrial Electronics Laboratory (LEI) of Swiss Federal Institute ofTechnology (EPFL) in Lausanne, Switzerland, where he is pursuing a PhDdegree in the frame of the Doctoral school in Energy. His theses researchproject deals with Hybrid Compressed Air and Supercapacitors energy stor-age for distributed generation and renewable energy sources support.

Alfred Rufer (Non-member) was born in Diessbach, Switzerland, in1951. He received the M.S. degree from the SwissFederal Institute of Technology Lausanne (EPFL),Lausanne, Switzerland, in 1976. In 1978 he joinedABB, Turgi, Switzerland, where he was involved inthe fields of power electronics and control, such ashigh-power variable frequency converters for drives.In 1985, he was a group leader involved with powerelectronic development in ABB. In 1993, he becamean Assistant Professor at EPFL. Since 1996, he has

been a full professor and Head of the Industrial Electronics Laboratory,EPFL. He has authored and coauthored several publications on power elec-tronics and applications and he holds several patents. Prof. Rufer was core-cipient of two IEEE Prize Paper Awards. He was newly elected as an IEEEFellow.


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Hybrid Energy Storage System Based on Compressed Air and