Unitised Regenerative Fuel Cells for Stand-Alone Photovoltaic

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					Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16-18, 2005 (pp520-525)




                                     Unitised Regenerative Fuel Cells
                             for Stand-Alone Photovoltaic Generation Systems
                                     D. ARDITO, S. CONTI, S. RAITI, U. VAGLIASINDI
                                  Dipartimento di Ingegneria Elettrica, Elettronica e dei Sistemi
                                                Università degli Studi di Catania
                                               Viale A. Doria, 6, 95125 Catania
                                                            ITALY

        Abstract – Photovoltaic (PV) stand-alone applications are able to provide electricity to isolated loads in remote
        areas and they are installed particularly where grid extensions would be uneconomical. However, the limitation
        in power availability of PV systems due to the variability of solar radiation requires the use of storage systems
        in order to supply loads with adequate reliability levels.
        The storage systems have to store a great amount of energy to be maintained for quite long time periods with
        small losses. This is quite difficult to be achieved by using electrochemical batteries and the use of hydrogen in
        regenerative fuel cells as a means for energy storage can represent a solution to reach the aforesaid goals.
        This paper deals with the use of Unitised Regenerative Fuel Cells (URFC) in the realization of stand-alone PV
        generation systems. The study of the generation system with solar hydrogen storage will be carried out using
        analytical models to represent the efficiency of each component in order to assess the capability of the
        generation system to supply its load with an adequate reliability level in terms of Loss Of Load Probability
        (LOLP).
        In this perspective, a comparison between different storage technologies such as Regenerative Fuel Cells (RFC)
        and Unitised Regenerative Fuel Cells will be presented. The performance of a storage system based on
        electrochemical batteries will be taken as reference.

        Key-Words – Renewable Energy, Photovoltaics, Hydrogen Storage System, Unitised Regenerative Fuel Cells.

        1     Introduction                                                    Research in this field proceeds in the development of
        The installation of stand-alone renewable energy                      new technologies to produce hydrogen from water
        generation systems, such as photovoltaic (PV), at                     electrolysis. These technologies are, e.g., the
        those sites were meteorological conditions are                        Unitised Regenerative Fuel Cells (URFC) [2], [3].
        favourable, can bring great benefits in terms of both                    Usually Fuel Cells (FC) are employed for energy
        costs and reliability. In fact, stand-alone applications              generation in Distributed Generation (DG) due to
        are able to provide electricity to isolated loads in                  their high efficiency, reliability and environmental
        remote areas and they are installed particularly                      compatibility. Further, FC can play the role of
        where grid extensions would be uneconomical. Then,                    energy storage systems. To accomplish this FC need
        generally speaking, climatic conditions and grid                      to be coupled with an electrolyser (EZ), which is a
        supply availability have a basic influence on the                     hydrogen generator device, to realize the
        economic evaluation of renewable energy                               Regenerative Fuel Cell (RFC) system. In practice,
        installations with respect to other solutions.                        the RFC system uses two separate cell stacks: an
           However, the limitation in power availability of                   electrolysis cell stack (EZ) to produce hydrogen and
        photovoltaic systems due to the variability of solar                  a separate FC stack to generate electric power from
        radiation calls for the use of storage systems in order               stored hydrogen.
        to supply loads with adequate reliability levels. The                    The URFC refines this concept by using the cell
        storage systems have to store a great amount of                       electrodes to perform both the EZ function and the
        energy to be maintained for quite long time periods                   FC function. Hence, the URFC system uses a single
        with small losses. This is quite difficult to be                      reversible cell stack to alternately produce hydrogen
        achieved by using electrochemical batteries due to                    from electrical energy and regenerate electrical
        low efficiency and self-discharge. At present, the use                energy on demand from stored hydrogen. The URFC
        of hydrogen intended as a means for chemical                          systems have lighter weight and smaller physical
        storage and transfer of solar energy seems to be a                    size than those systems that employ separate cell
        solution to overcome the aforesaid limitations [1].                   stack [4].
Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16-18, 2005 (pp520-525)

        In this paper, the operation of a PV stand-alone
                                                                                              PV                             LOAD
        generation system with solar hydrogen storage will
        be investigated by using analytical models to
        represent the efficiency of each component in order
        to assess the capability of the generation system of                                                 STORAGE

        supplying its load with an adequate reliability level
        in terms of Loss Of Load Probability (LOLP).                            Fig. 1. Simplified block scheme of a PV stand-alone
        Further, a comparison between different storage                                 generation plant with storage systems
        technologies such as Regenerative Fuel Cells (RFC)
        and Unitised Regenerative Fuel Cells (URFC) will                                                                  RFC system
        be carried out by taking as reference the performance                                     H2O
        of a storage system based on electrochemical
        batteries.                                                                                      H2                              in
                                                                                                                                     PRFC
                                                                                                                    EZ
                                                                                      Tank H2                                           out
                                                                                                                    FC               PRFC
                                                                                                        H2
        2     Schemes of a stand-alone PV
                                                                                                 O2 /air
              system with storage                                             a)
        The simplified block diagram of a PV stand-alone                                                                 URFC system
        generation plant with storage system is shown in Fig.                                     H2O
        1.
                                                                                                        H2                              in
            The storage system based on FC technology is                                                                             PURFC
        shown in Figg.2 a) and b), respectively, for RFC and                          Tank H 2                   EZ/FC                  out
                                                                                                                                     PURFC
        URFC. Obviously, the system components of Fig. 1                                                H2
        can not be connected directly to each other for the
                                                                                                 O2 /air
        following reasons:
        • different voltage levels in the system;                                         FC operation
        • control and possible optimisation of global                                     EZ operation
            efficiency would be impossible;                                   b)
                                                                                    Fig. 2. Schemes for RFC and URFC systems
        • necessity to convert cc waveforms into ac ones.
            As a consequence, it is necessary to employ
        DC/DC and DC/AC converters, with different
        control schemes according to the various storage                             PV          MPPT                        DC/AC       LOAD
        technologies, in order to provide power conditioning,
        efficiency optimisation and subsystems coupling [5],
        [6]. This work will deal with the plant typologies
        shown in Fig. 3, where:                                                                              URFC                TANK
           - MPPT = Maximum Power Point Tracker;
           - DC/AC = inverter;                                                a)
           - DC/DC = converter;
           - GC = Gas Compressor.                                                    PV          MPPT                        DC/AC       LOAD


                                                                                                        EZ                FC
        3     Energy flows assessment
            The aim of this Section is to analyse the energy                                            GC               TANK
        flows within the solar hydrogen system. To do this,
        analytical models for the various system components                   b)
        in the considered configurations (Fig. 3) have been
        developed. The basic scheme used for the energy                              PV          MPPT                        DC/AC       LOAD
        flows assessment is shown in Fig. 4. We define the
        following quantities:                                                                       DC/DC                DC/DC

        • λi the irradiance on a surface with a given
                                                                                                             BATTERY
            inclination to the horizontal plane during the i-th
            hour (i=1…24) [kW/m2];                                            c)
        • PLi = load power demand during i-th hour                                 Fig. 3. Configurations with by different storage
            (i=1…24) [kW];                                                         systems: a) URFC, b) RFC, c) Electrochemical
                                                                                                       batteries
        • A is the array surface area [m2].
Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16-18, 2005 (pp520-525)

        3.1        Load energy demand                                                 Obviously, a portion of this surplus energy can be
        Since load power demand PL is considered constant                         stored (ESj). This is due to the storage system
        during the i-th hour, it can be assumed that PLi=ELi                      efficiency, so that:
        (where EL is the load energy demand during the                                                   E         = ηc E                 (6)
        considered hour of the year, i = 1,…, 8760).                                                          Sj      Sj SURj
           The yearly load energy demand, ELy, is given by:                       where η Sj is the storage system efficiency in charge
                                                                                          c

                                              8760
                                     E Ly =    ∑ E Li                       (1)   mode, variable with ESURj .
                                               i =1
                                                                                      The expression of η Sj depends on the technology
                                                                                                          c


        3.2        Photovoltaic energy                                            used to realise the storage system:
                                                                                  • URFC → η Sj = ηURFCj
                                                                                                   c    EZ
        The energy produced (superscript p) by the PV array
        during the i-th hour is:
                                                                                  • RFC → η Sj = η RFCj ⋅ ηcomp
                                                                                            c      EZ
                               p
                             E PVi=A ⋅ ηPVi ⋅ λi          (2)
                                                                                  • Battery → η Sj = η BATTj ⋅ η DC / DC
                                                                                                c         c
        where:
        ηPVi is the efficiency of the PV array, variable with
                                                                                  Discharge mode: definition of “deficit energy” and
        the hour and solar irradiance λi .                                        “provided energy”.
            The hourly PV energy actually available                                   At node N of Fig. 4, during the k-th hour, the
        (superscript a) downstream from the MPPT is given                         following inequality holds:
        by:                                                                                              a
                                                                                                       E PVk < E Lk /ηinv
                              a              p
                                                                                                                                  (7)
                            E PVi =ηMPPT ⋅ E PVi       (3)
                                                                                     This means that the PV energy made available is
        where ηMPPT is the efficiency of the MPPT.                                lower than load energy demand. We have a deficit
                                                                                  energy, EDEFk , given by:
                                          N                                                                                a
                                                                                                  E DEFk = E Lk / ηinv - E PVk =
              PV          MPPT                         DC/AC       LOAD
                                                                                                                                          (8)
                                                                                                  = E Lk / ηinv - Aη MPPT η PVk λk

                                       STORAGE
                                                                                     Obviously, the energy actually provide to the
                                                                                  storage system, EPROVk , will be greater than the
         Fig. 4. Basic scheme used for energy flows assessment
                                                                                  deficit energy.
                                                                                     This is due to the storage system efficiency, so
                                                                                  that:
        3.3        Storage system energy
                                                                                                             E
        Two operation modes for the storage system have                                             EPROVk = DEFCk                (9)
        been identified: the charge mode (superscript c) and
                                                                                                               d
                                                                                                                      ηSk
        the discharge mode (superscript d).                                       where η    d
                                                                                             Sk   is the storage system efficiency in
           Accordingly, the hours of the year will be
                                                                                  discharge mode, variable with EDEFk .
        distinguished in “j” hours (charge hours), when PV
        production exceeds load demand, and “k” hours                                 Similarly to the previous case, the expression of
        (discharge hours), when PV production is lower than                       ηSk depends on the technology used to realise the
                                                                                    d

        load demand. The two operation modes are                                  storage system:
                                                                                  • URFC → ηURFCk
        characterised by different hourly efficiencies,                                     FC

        respectively, η Sj and ηSk .
                        c       d

                                                                                  • RFC → ηRFCk
                                                                                           FC



                                                                                  • Battery → η BATTk ⋅ ηDC / DC
                                                                                                d
        Charge mode: definition of “surplus energy” and
        “stored energy”.
            At node N of Fig. 4, during the j-th hour the
        following inequality holds:
                                                                                  4    Efficiency analytical models of
                           EPVj > ELj / ηinv
                             a
                                                      (4)
                                                                                       system components
        where ηinv is the DC/AC inverter efficiency.                              The efficiency analytical models for each component
           This means that the PV energy made available                           have been derived from experimental data. In the
        exceeds the load energy demand. We have a surplus                         models the efficiency is expressed as a function of
        energy, ESURj , given by:                                                 the component input / output power:
              ESURj = EPVj - E Lj / ηinv = Aη MPPTη PVj λ j - E Lj / ηinv
                       a
                                                                            (5)                           η=η(P)                 (10)
Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16-18, 2005 (pp520-525)

           As previously highlighted, the hourly input or                              η RFC = 0.87289 − 1.38619 ⋅ ( pRFC ) +
                                                                                         EZ                           in

        output power is assumed constant so that, for each                                                       in                     in
                                                                                                                                                   (19)
        given hour energy is numerically equal to power:                                          + 2.55711 ⋅ ( pRFC ) 2 − 1.41009 ⋅ ( pRFC )3
        P=E. Consequently, expression (10) is equivalent to                             η RFC = 0.70721 − 0.74683 ⋅ ( pRFC ) +
                                                                                          FC                           out

        the following:                                                                                           out                    out
                                                                                                                                                   (20)
                             η=η(E)                   (11)                                        + 1.13758 ⋅ ( pRFC ) 2 − 0.77097 ⋅ ( pRFC )3
                                                                                     The coefficients have been derived from
        4.1     PV efficiency model                                               experimental data provided by CNR - ITAE
        The PV hourly efficiency is expressed as a function                       (National Research Council - Institute for advanced
        of global solar irradiance λi :                                           energy technologies), in Messina (Italy).

                                                  7.9 ⋅ 10 −4
                        η PVi = 9.55 ⋅ 10 − 2 −                            (12)   4.4        Battery efficiency model
                                                      λi                          The efficiency model has been developed on the
           The coefficients of the expression have been                           ground of data provided by [9] and [10].
        obtained from experimental measures [7].                                     As said before, it is necessary to take into account
                                                                                  a charge efficiency ( η BATT ) and a discharge
                                                                                                              c

        4.2     URFC efficiency model                                             efficiency         ( η BATT ),
                                                                                                         d
                                                                                                                       respectively,   functions     of
        The efficiencies in EZ operation mode ( η                 EZ
                                                                  URFCj   ) and   p   in
                                                                                             and p   out
                                                                                                            , that are the relative values of input
                                                                                      BATT           BATT
        in FC mode ( ηURFCk ) are, respectively, expressed as
                      FC
                                                                                                    in         out
                                                                                  and output power PBATT and PBATT , referred to peak
                           in           out
        functions of p     URFC   and p URFC   , that are the relative                     EZ         FC
                                                                                  powers PBATTp and PBATTp , i.e.:
        values of input and output power P in and P out ,                                                    in      in       EZ
                                          URFC     URFC                                                     pBATT = PBATT / PBATTp                 (21)
                                        EZ             FC
        referred to peak powers P      URFCp   and P  URFCp     , i.e.:                                      out      out     FC
                                                                                                            pBATT = PBATT / PBATTp                 (22)
                              in         in        EZ
                             pURFC   =P URFC   /P URFCp                    (13)
                                                                                     The expressions for battery efficiency obtained
                              out        out       FC
                             pURFC   =P URFC   /P URFCp                    (14)   are the following:
                                                                                                                   (
                                                                                           η BATT = ⎡1- 0.2 pBATT ⎤ (1- 0.1))
                                                                                             c               in   2          ( N -1)
            The obtained efficiency expressions are the                                                                              (23)
                                                                                                    ⎢
                                                                                                    ⎣               ⎥
                                                                                                                    ⎦
        following:
                                                                                              η BATT = ⎡1- 0.6 ( pBATT ) ⎤ (1- 0.1)
                                                                                                                        2           ( N -1)
         ηURFC = 0.89265 − 1.09878 ⋅ pURFC +
           EZ                              in                                                   d                 out
                                                                                                                                     (24)
                                                                (15)                              ⎢
                                                                                                  ⎣               ⎥
                                                                                                                  ⎦
                                in                      in
                 + 1.81734 ⋅ ( pURFC ) 2 − 1.04471 ⋅ ( pURFC )3                   where N=1…10 is the battery year of life that must
         ηURFC = 0.83465 − 0.69088 ⋅ pURFC +
          FC                          out                                         be taken into account in order to consider the
                               out                    out
                                                              (16)                reduction in efficiency due to self-discharge and
                + 0.89368 ⋅ ( pURFC )2 − 0.64195 ⋅ ( pURFC )3                     electrodes degradation. This is important because the
           The coefficients have been derived from                                battery life-cycle is much shorter than that of FC.
        experimental measures carried out on a test URFC of
        type #9804A (produced by Proton Energy Systems
        Inc., Connecticut, USA, and tested by LLNL -                              5      Loss of Load Probability
        Lawrence Livermore National Laboratory -                                  The efficiency analytical models presented in the
        California, USA) [8].                                                     previous Sections for each component in the various
                                                                                  configurations will be employed to assess the
        4.3  RFC efficiency model                                                 capability of the generation system to supply the load
        The EZ efficiency ( η RFCj ) and the FC efficiency
                              EZ
                                                                                  with an adequate reliability level.
                                                                                      To do this we will define the known reliability
        ( η RFCk ) are, respectively, expressed as functions of
            FC
                                                                                  index called LOLP (Loss of Load Probability) as:
          in       out
         pRFC and pRFC , that are the relative values of input
                           in       out                                                                LOLP =
                                                                                                                 ∑ hLL
        and output power PRFC and PRFC , referred to peak                                                         H                 (25)
                 EZ        FC
        powers PRFCp and PRFCp , i.e.:                                            where the nominator is the sum of the overall “loss
                              in     in      EZ
                             pRFC = PRFC / PRFCp                           (17)   of load hours” (indicated by hLL) - discharge hours -
                                                                                  during which the storage system is not able to meet
                              out     out    FC
                             pRFC = PRFC / PRFCp                           (18)   the load demand; the denominator, H, is the total
            The obtained efficiency expressions are the                           number of hours in the year (8760).
        following:
Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16-18, 2005 (pp520-525)


        6     Monte Carlo Simulation                                             In physical terms, the quantity he represents the
        LOLP calculation for the various system                               number of hours during which the storage system is
        configurations has been carried out by means of a                     able to meet a load demand equal to PLav.
        software tool developed by the Authors on                                The results obtained are referred to an ideal load
        MATLAB® platform, based on Monte Carlo (MC)                           diagram (shown in Fig. 5) which brings about the
        method. This method allowed to obtain a realistic                     maximum energy storage.
        assessment of system reliability by using the                         PLmax
        statistical variations of load demand and PV
        production. This has been done by means of
                                                                               PLav
        appropriate statistical models for power demanded
        by the load and produced by the PV generator [11].




                                                                                 PL (kW)
        Once the statistical models are defined in terms of
                                                                                                                                  0.4PLmax
        probability density functions (pdfs) the procedure
        involves repeating the simulation using each time
                                                                                            1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
        (hour by hour) a particular value of the random
                                                                                                                              hours (h)
        variables (load demand and PV production),
        generated according to the corresponding pdfs.                                                   Fig. 5. Ideal daily load diagram
            For each hourly simulation it is possible to assess
        whether the considered hour is a “loss of load hour”                      This is because the concern is on the operating
        or not. The simulations are then extended to the                      condition in which the storage system assumes the
        overall year, thus obtaining the sum of the loss of                   most critical role from the reliability viewpoint. This
        load hours and then the value of the LOLP for that                    condition is when the generation diagram has its
        year (LOLPy).                                                         maximum during minimum load hours. Hence the
            To ensure a reasonable accuracy of the                            surplus energy is maximum with respect to load
        calculation performed by the Monte Carlo method,                      demand.
        an appropriate number of years (Y) is to be                               Graphs of Figg. 6, 7, 8 and 9 show that the PV
        considered. The final result will be the average                      generation system with hydrogen-based storage
        LOLP value:                                                           technology is more reliable than the system with
                                       Y                                      electrochemical batteries. Further, the configuration
                                      ∑ LOLP
                                       y =1
                                                   y                          with URFC has a lower LOLP (reduced by a 10%)
                            LOLP =                                 (26)       than the configuration with RFC. Consequently, the
                                              Y                               URFC, besides being advantageous in terms of light
                                                                              weight and small physical size, ensures higher
        7     Numerical results                                               reliability levels than those systems that employ
        This section presents the results of the analysis                     separate cell stacks (RFC).
        carried out to assess the reliability in terms of LOLP
        index for the three configurations characterized by                                 1                                               LOLP
                                                                                                                                           LOLPurfcURFC
        different storage systems. In particular the values of                                                                              LOLP
                                                                                                                                           LOLPrfc RFC
                                                                                           0,9
        LOLPURFC, LOLPRFC LOLPBATT will be reported in                                                                                     LOLPbattBATT
                                                                                                                                            LOLP
                                                                               LOLP




        the graphs of Figg. 6, 7, 8 and 9 as a function of p                               0,8

        (adimensional) which is defined as the relative value                              0,7

        of PPvpeak (kW) referred to the daily average value of                             0,6

        load demand, PLav (kW):                                                            0,5
                                   PPV peak                                                      0   2       4       6   8   10       12   14     16    18        20   22
                               p=                          (27)                                                                   p
                                    PL av                                                                Fig. 6. LOLP graphics with he=1
        where PPvpeak is the value of power generated by the
        PV system in standard conditions (Solar                                             1                                                    LOLPURFC
                                                                                                                                                LOLPurfc
        Irradiation=1kW/m2 and Cell Temperature=25ºC).                                     0,9                                                   LOLPRFC
                                                                                                                                                LOLPrfc
            The aforesaid graphs are characterised by                                      0,8                                                   LOLPBATT
                                                                                                                                                LOLPbatt

        different capacities, Es (kWh), of the storage system                              0,7
                                                                               LOLP




                                                                                           0,6
        expressed in terms of “equivalent hours” (he),                                     0,5
        defined as:                                                                        0,4

                                              ES                                           0,3
                                    he =                           (28)                          0       2       4       6        8        10      12        14        16
                                           PL av                                                                                      p

                                                                                                     Fig. 7. LOLP graphics with he=12
Proc. of the 5th WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, December 16-18, 2005 (pp520-525)


                   1                                              LOLP
                                                                 LOLPurfcURFC
                                                                                              References:
                 0,9                                              LOLP
                                                                 LOLPrfc RFC                  [1] Mitlitsky F., Myers B. and Weisberg A.H.
                 0,8
                                                                  LOLP
                                                                 LOLPbattBATT                      (LLNL, California), “Regenerative Fuel Cell
                 0,7
          LOLP




                 0,6                                                                               System R&D”, Proceedings of the 1998 U.S.
                 0,5                                                                               DOE Hydrogen Program Review (NREL/CP-
                 0,4
                                                                                                   570-25315).
                 0,3
                 0,2                                                                          [2] F. Mitlitsky, B. Myers and N.J Colella.
                       0       2   4     6   8    10
                                                       p
                                                           12     14   16      18   20   22        “Unitized regenerative fuel cell for solar
                                                                                                   rechargeable aircraft and zero emission
                               Fig. 8. LOLP graphics with he=24                                    vehicles”, Fuel Cell Seminar, San Diego
                                                                                                   Nov/Dic 1994.
                   1                                             LOLP
                                                                LOLPurfcURFC                  [3] J.F. McElroy, “Unitized Regenerative Fuel Cell
                 0,9
                 0,8                                             LOLP
                                                                LOLPrfc RFC                        Energy Storage Systems For Aircraft And
                 0,7                                             LOLP
                                                                LOLPbattBATT                       Orbital Applications” UTC Hamilton Standard
                 0,6
                                                                                                   Div., Rept BD94-02, Mar 1994.K.
          LOLP




                 0,5
                 0,4
                 0,3
                                                                                              [4] A. Burke, “High Energy Density Regenerative
                 0,2                                                                               Fuel Cell Systems for Terrestrial Applications”,
                                                                                                   IEEE AES Systems Magazine, December 1999.
                 0,1
                   0
                       0   2   4   6   8 10 12 14 16 18 20 22 24 26 28 30 32                  [5] H. Solmecke, O. Just, D. Hackstein,
                                                   p                                               “Comparison of Solar Hydrogen Storage
                               Fig. 9. LOLP graphics with he=48                                    Systems with and without Power-Electronic
                                                                                                   DC-DC Converters”, Renewable Energy
                                                                                                   (Pergamon), 19 (2000), pp. 333-338.
                                                                                              [6] J. Appelbaum, “The Operation of Loads
        4 Conclusion
                                                                                                   Powered by Separate Sources or by Common
        In this paper, the operation of a PV stand-alone
                                                                                                   Source of Solar Cell”, IEEE Transactions on
        generation system with solar hydrogen storage has
                                                                                                   Energy Conversion, Vol. 4, No. 3, September
        been investigated by using analytical models to
                                                                                                   1989.
        represent the efficiency of each component in order
                                                                                              [7] M. Guerra, A. Sarno, S. Raiti, G. Tina,
        to assess the capability of the generation system to
                                                                                                   “ENEA's facilities to test small-size inverters
        supply the load with an adequate reliability level.
                                                                                                   for low voltage grid-connected PV systems”,
            This allowed a comparison between different
                                                                                                   16th European Photovoltaic Solar Energy
        storage technologies such as Regenerative Fuel Cells
                                                                                                   Conference and Exhibition, EPSE'2000, 1-5
        (RFC) and Unitised Regenerative Fuel Cells
                                                                                                   May, 2000, Glasgow, Scotland
        (URFC), carried out by taking as reference the
                                                                                              [8] F. Mitlitsky, A.H. Weisberg, B. Myers,
        performance of a storage system based on
                                                                                                   “Vehicular Hydrogen storage using Lightweight
        electrochemical batteries.
                                                                                                   Tanks (Regenerative Fuel Cell systems)”, Proc.
            The aforesaid comparison resulted in the higher
                                                                                                   of the 1999 U.S. DOE Hydrogen Program
        reliability level of the PV stand-alone generation
                                                                                                   Review (NREL/CP-570-26938).
        system with hydrogen storage as referred to the use
                                                                                              [9] H. Senoh, Y. Hara, H. Inoue, C. Iwakura,
        of electrochemical batteries. Further, as for the
                                                                                                   “Charge Efficiency of Misch Metal-Based
        hydrogen storage system, the URFC guarantees
                                                                                                   Hydrogen Storage Alloy Electrodes at
        higher reliability level than those systems that
                                                                                                   Relatively Low Temperatures”, Electrochimica
        employ separate cell stacks (RFC).
                                                                                                   Acta (Pergamon), 46 (2001), pp. 967-971.
            This makes the URFC technology very attractive
                                                                                              [10] M. Pedram, Q. Wu, “Design Considerations for
        to store energy in the form of hydrogen and to
                                                                                                   Battery-Powered Electronics”, Proceedings of
        produce electrical energy from the stored hydrogen
                                                                                                   the 36th ACM/IEEE Conference on Design
        as well.
                                                                                                   Automation (ACM Press, June 1999).
            However, URFC technology still involves
                                                                                              [11] S. Conti, T. Crimi, S. Raiti, G. Tina, U.
        uncompetitive costs as compared to RFC technology
                                                                                                   Vagliasindi, “Probabilistic Approach to Assess
        (URFCs are only employed in space applications
                                                                                                   the Performance of Grid-Connected PV
        where the high costs are determined also by the use
                                                                                                   Systems”, Proceedings of 7th International
        of precious materials).
                                                                                                   Conference on Probabilistic Methods applied to
                                                                                                   Power Systems, September 22-26., Naples,
                                                                                                   Italy.