fuel cell hybrid power source

Energy Conversion and Management 77 (2014) 763–772

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Load-following mode control of a standalone renewable/fuel cell hybrid power source Nicu Bizon ⇑ University of Pitesti, 110040 Pitesti, 1 Targu din Vale, Arges, Romania

a r t i c l e

i n f o

Article history: Received 22 June 2013 Accepted 9 October 2013

Keywords: Renewable energy Energy storage system Fuel cell Hybrid power source Load-following strategy Distributed generation

a b s t r a c t A hybrid power source (HPS), fed by renewable energy sources (RESs) and fuel cell (FC) sources, with an energy storage device (ESS) to be suitable for distributed generation (DG) applications, is proposed herein. The RESs could be a combination of photovoltaic (PV) panels and wind turbines (WT) based on common DC-bus, which are used as the primary DC source. The FC operates as a backup, feeding only the insufficiency power from the RESs based on the load-following strategy. The battery/ultracapacitor hybrid ESS operates as an auxiliary source for supplying the power deficit based on dynamic power balance strategy (the transient power – mainly via the ultracapacitors stack, and the steady-state power – mainly via the FC and batteries stack). If the FC stack is designed and operates based on average loadfollowing strategy, then the ESS will operate in charge-sustaining mode during a load cycle. This feature permits to optimize the batteries stack capacity and extend its life time as well. The ultracapacitors stack can be designed considering the peaks of RESs power on DC-bus and the imposed window for its state-ofcharge (SOC). This FC/RES/ESS HPS is ideal to be used for standalone plug-in charge station (PCS) or as DG system grid connected. In the last case, which is not analyzed here, the energy management unit (EMU) that communicates with smart grid will establish the moments to match the HPS power demand with grid supply availability, stabilizing the grid. Using load and RES power profiles that have a higher dynamic than in reality, the HPS operation is shown based on an analytical analysis and the appropriate Matlab/ SimulinkÒ simulations. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction A FC/RES/ESS HPS for a PCSs or DG systems grid connected will use the FC and RES systems in relation with a reversible ESS in order to cope with the dynamic load profiles or grid power demands via controlled power converters [1,2]. If the HPS operates into a stand-alone system, then the FC system is used as a backup system that is fueled from a limited hydrogen tank [3] or via an electrolyzer, used as a long-term storage system [4]. The ESS can be implemented using the ultracapacitors [5], batteries [1,6], or both technologies operating in an active hybrid ESS topology [7]. The hydrogen can be produced from water by electrolysis, and until now the water is abundantly available on Earth. The hydrogen can be stored for use with the intermittent and seasonal RES technologies, being one of the most attractive options as energy carrier [8]. In the last decade there have been many proposed technical projects in the development of FC/RES HPSs, mainly based on PV panels and WTs [2,8]. Consequently, the literature in this area has been focused on sizing [2], performance [6], economics [9], and power flow management [5], with high attention paid to ⇑ Tel.: +40 348 453 201/722 895624; fax: +40 348 453 200. E-mail addresses: [email protected], [email protected] 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.10.035

dynamic behavior of energy sources [10] and the appropriate control aspects [11]. The control loops, in this paper, are designed with consideration to the time constants of the energy sources. Thus, the load-following control loop is designed to feed the proton exchange membrane (PEM) FC system based on average power balancing strategy. Because the RESs have substantially different power–current characteristics, each RES will be integrated in the FC/RES HPS via a power converter. Thus, each RES will supply the common DC-bus in an energy harvesting mode if a maximum power point (MPP) tracking controller will be used. Also, the PEMFC is a nonlinear dynamic system and must be integrated in the FC/RES HPS via a power converter, which will operate the PEMFC at MPP. It is known that the energy efficiency is dependent on operating conditions [12], so the MPP depends mainly by the fueling [13], the temperature [14] and the humidity [15]. The FC power can be controlled by the fueling rates, which are considered to be the system maneuvering or energy inputs [16]. Moreover, the FC efficiency depends by the anode and cathode pressures, which can be considered to be the system dynamics’ state variables [17]. Thus, it is challenging to design the control loops to operate the PEMFC system efficiently and sustainably. It is known that the fuel flow cannot follow the current steps because this will essentially

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lead to the degradation of the stack lifetime, brought by the apparition of the fuel starvation phenomenon [18]. Therefore, if utilizing fuel cell in dynamic applications, then it is mandatory that the FC current slopes will be limited as response to the sharp load profiles [18–20]. Besides the use of constant rate limiters [18], some adaptive techniques are proposed in the literature such as the use of the wavelet transform [19] or use of the Extremum Seeking Control (ESC) [20] for analyzing the dynamic load profile to evaluate the appropriate profile of the FC current. As it is known, the air flow control is one of the most important control methods for maintaining the stability and reliability of a fuel cell system, which can avoid starvation or saturation of the oxygen [18,21]. The air flow is controlled by the fuel cell processor based on different control strategies in order to match the optimal oxygen excess ratio [22]. Nonetheless, to shown the advantages of the fuel flow control in the load following loop, this paper considers a constant air flow corresponding to nominal value of 50 lpm. In this case, the PEMFC will always have enough oxygen, operating in the region between oxygen starvation and oxygen saturation. However, if the air flow is too large, the net power will decrease due to the excessive power demanded by the air compressor. This aspect of maximize the net power based on both fuel rates will be considered in the following work, where a one input – two outputs ESC is proposed. Here a one input – one output ESC scheme is used [23]. Beside the RESs above mentioned, the PEMFCs are considered to be the most promising alternative among next DG systems due to their high energy density and clean energy [24]. On the other side, the RES technologies have recently attracted specialists’ attention, being intensively investigated to solve current energy crisis [2]. Note that the PV power flow is influenced by several factors, including irradiance, temperature, shading, degradation, mismatch losses, soiling, etc [25]. Also, the uncertainty in the WT power flow is very large due to the inherent variability in wind speed [26]. So, the RES power flow fluctuates depending on weather conditions, and this issue must be solved based on power balancing strategy considering the difference power and battery voltage value [27]. Here the FC power reference is also computed based on power balancing strategy, but in a different control way. Thus, a fuzzy logic controller is used to implement the intelligent energy management strategy proposed. Note that a excellent review of main energy management strategies of PEMFC hybrid systems is shown here, too. All strategies are based on an optimal reference power signal that is calculated by minimizing a cost function. In this paper the reference power (which is the difference power that must be supplied by the PEMFC) is processed to obtain the reference fuel flow rate for the hydrogen. As it was mentioned before, there are many proposed techniques to compute the fuel flow rates. Because fuel cell is supplied with gas through pumps, valves and compressors, large time constants are involved in such signal processing blocks. Therefore, in the hybrid FC system, the PEMFC will operate in nearly steady state conditions and ultracapacitors will function during transient load demand [18] or variation of the RES power [27]. Also, in this paper it is proposed a control to regulate the DC bus voltage through the power delivered by the fast energy storage device, the ultracapacitors [18]. To better stabilize the DC bus, a small battery is used. This battery will operate in the charge sustaining mode based on the load following strategy. It is know that fluctuating power causes frequency deviations and reduction in reliability of the smart grid when a large power flow, from several RES HPSs, is used [28]. Due to the high variability of available RES power flow, the batteries used in RES/ESS HPS can operate in the irregular cycles, under partial charge/discharge mode [25,26]. In turn, this can also have a detrimental effect on

battery lifetime [29,30], so an average charge sustained mode is proposed for the batteries stack used in this FC/RES/ESS HPS architecture. However, limited by their inherent time response, the PEMFC stack has a long start-up time and limited slope to instantaneous power demands [13]. Thus, combining FC with ESS will obtain a FC/ESS HPS architecture that makes the best use of the merged technologies [31]. In the FC/battery HPS, the FC system is controlled to satisfy load average power requirements, but the battery, on the other hand, is used to serve high pulse power requirements in short intervals. Consequently, the battery/ultracapacitor hybrid ESS topology is required to make face to a pulsed load [1]. Furthermore, the ultracapacitor stack is used to regulate the DC-bus based on a semi-active hybrid ESS topology [1]. The overall energy efficiency of the FC/RES/ESS HPS could be maximized by identifying the best degree of hybridization [32] and the appropriate energy management strategy of multiple sources and loads [33]. The challenge for the EMU design based on the load-following strategy is to enhance the performance of all technologies working together and to minimize fuel consumption while reducing system degradation. What remains of this paper is organized as follows. The second section presents the RES/FC HPS architecture and defines the control loops used for (I) the FC power control, (II) DC-bus voltage, and (III) RES energy harvesting. Also, some design considerations are briefly shown. The third section details the used models for all blocks from the RES/FC HPS architecture. The forth section introduces the power balancing strategies and compares two of them that are based on grid-following and load-following concept. The features of the load-following strategy applied to the RES/FC HPS architecture are highlighted through the simulation, some of the obtained results being shown in next section. Finally, some conclusions are given related to the RES/FC HPS architecture under loadfollowing strategy. 2. RES/FC HPS architecture The purpose of this paper is to demonstrate that load-following EMU strategy is feasible and flexible, offering robust design options to control the power flow and improve the operation of the whole RES/FC HPS (see Fig. 1). 2.1. The FC control The FC system powers the DC bus via a DC–DC power converter that boosts the FC voltage. The FC system usually operates between the maximum efficiency point and maximum power point (MPP) for both high efficiency and reliability [12]. If energy harvesting mode is selected to operate the FC system, then the boost converter is necessary to be appropriately controlled based on MPP tracking control loop. The MPP tracking controller based on advanced ESC proposed in [23] offers high performances in both search speed and tracking accuracy indicators. The MPP ESC generates the reference current, Iref, by processing the FC power signal to extract the gradient signal to the MPP point [23]. The hysteretic controller is chosen as current-mode controller because it is robust and quite simple to design [23]. The PEMFC systems can operate efficiently and durable if the fueling rates are appropriately controlled [34,35]. A dynamic PEMFC model that sufficiently represents its dynamics is necessary to validate the proposed control loops [36]. The PEMFC air flow can be adjusted by robust control of a compressor [37], but it is also known that the FC power can be directly regulated by controlling the hydrogen feed [34]. The hydrogen and oxygen (or air) feeds will be adjusted to stoichiometrically match

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Fig. 1. RES/FC HPS architecture based on the load following control.

the required FC current, Iref, in the load-following control loop. So, the FC current is the input variable of hysteretic controller to manage the FC power [33]. Note that the regulation of the hydrogen feed could be profitable for relative small PEMFC systems, where recycling of the unreacted hydrogen may be impractical [38]. In this paper this variant is considered based on a load-following control scheme of singleinput single-output type.

2.3. The DC voltage control

where PESS, PRES, PFC, and PL are the average power of the ESS, RES, FC system, and load, and g1 is the energy efficiency of the boost converter. The charge-sustaining mode of the ESS (PESS = 0) is obtained based on load-following control. Thus, the average FC current must be:

Note that charge-sustaining mode of the ESS is possible if PL > PRES during a load cycle. In this case the ESS topology could be of passive type [1,23]. The necessity of a hybrid batteries/ultracapacitors ESS appears related to peak power demands, high slopes of load power profile and statistical distribution of the RES power. Also, note that the slopes of FC power must be limited up to the admissible limits using a rate limiter block in order to avoid the apparition of the fuel starvation phenomenon [13]. Consequently, only the low frequency variation of the PL  PRES will be considered in the control of the FC system [39]. Furthermore, the time constants of the battery, PEMFC and ultracapitors are different, and this aspect must be considered for the control designed to regulate the voltage on DC bus [5,10]. In this paper a batteries/ultracapacitors ESS semi-active topology was considered to regulate the DC voltage based on control of the buck-boost converter. The ultracapitors will exchange power on the DC bus via this bidirectional power converter. So, besides the voltage regulation, the power flow balance on the DC bus will be assured on all frequency scales [32].

PFCðAVÞ ¼ ðPL  PRES Þ=g1 ) IFCðAVÞ ¼ ðPL  PRES Þ=ðV FCg1 Þ

2.4. The RES technologies

2.2. The load following control The power flows’ balance on the DC bus:

PRES þ PESS þ g1 P FC ¼ PL

ð1Þ

ð2Þ

The fuel flow regulator set the fuel flow rate (FuelFr) for the hydrogen based on (3) [16]:

FuelFr ¼

60000  R  ð273 þ hÞ  NC  IFCðAVÞ 2F  ð101325  Pf Þ  ðU fðH2Þ =100Þ  ðxH2 =100Þ

ð3Þ

where R = 8.3145 J/(mol K); F = 96485 A s/mol; NC is the number of cells in series (65); Uf(H2) – nominal hydrogen utilization (99.56%); h – operating temperature (65 °C), Pf – fuel pressure (1.5 bar); xH2 – H2 composition (99.95%). The load-following control loop is shown in Fig. 1, where the Average Value (AV) value of signals is estimated using the AV blocks, having the AV time of 10 times the sampling period (Ts = 20 ls).

The RESs that are usually combined in such of active RES/FC HPS architecture are the PV panels [4] and WTs [11]. Both RES technologies can cover our energy needs [26], and an increasing use helps to reduce our impact to the environment by decreasing CO2 emissions into the atmosphere [24]. In addition, both technologies are ideal for distributed power generation as DG systems connected into smart grids [8], but also being highly suitable for remote autonomous applications, such as in a remote farmhouse [9]. The PV panels are often considered as the most reliable elements in RES systems [9]. Furthermore, even if the PV panel performance decreases steadily over time if these are deployed outdoors, losses up to 20% of efficiency are achieved after 20 years of continuous operation [24,31]. Unlike WTs systems, the PV systems operate autonomous usually without any noise generation if the sun tracking systems are not incorporated. In addition, unlike other

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RES systems, the PV systems require minimal maintenance costs related only to cleaning the panel surface, keeping them at the highest efficiency in operation. So, the PV and WT systems represent a reliable and matured green technology for the exploitation of solar and wind energy [24]. It should be noted that successful penetration of the RES technologies into the DG systems is clearly dependent by the HPS architecture and EMU design, which can resolve the RES reliability issues such as unpredictability of environmental conditions and intermittent supply of power generation [3,26]. The ESS is the key device for resolving these issues through energy balancing techniques [39,40]. The technological progress in this field now offers efficient and sustainable solutions [1,6].

2.5. The energy harvesting from the RES Different energy harvesting techniques are proposed in the literature [23]. It may be noted that regardless of the technique used, the distribution of power flow on the DC bus is statistical distributed, being dependent by environmental conditions [26]. So, because this issue is outside the goal of this paper, the RES power

variation during a short time period will be simply modeled based on a random power generator. 2.6. Design considerations The maximum value of the load profile used in simulation is of 6 kW. If the battery SOC must be almost constant during a load cycle, when the RES power could be zero, then the maximum FC power must be higher than 6 kW. If the battery SOC can vary in an admissible window, then the FC current will be given by (3). The hybrid ESS can be designed based on frequency domain analysis of the power flow split by the stacks of batteries and ultracapacitors [1]. The DC-bus voltage (VDC) and maximum FC voltage (VFC(MPP)) define the voltage boost ratio, which sets the duty cycle (D) of the boost converter:

D ¼ 1  V FCðMPPÞ =V DC

ð4Þ

If the D value is chosen on the higher admissible range, 0.5 < D < 1, then the control sensitivity, @IFC/@D, will result larger [16]. For example, if D = 0.8 and VDC = 210 V, then VFC(MPP) = 42 V.

Fig. 2. The batteries/ultracapacitors ESS semi-active topology.

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3. Modeling of the RES/PF HPS architecture Since the problem of modeling is not the main purpose of this paper, default models from the Matlab/SimulinkÒ toolboxes will be used in simulations. The used parameters are briefly shown below. 3.1. The FC model A preset model of 6 kW – 45 V PEMFC stack from the SimPowerSystemÒ toolbox has been used to model the FC system, which is fueled at the nominal flow rates for hydrogen and air of 50 lpm and 300 lpm, respectively (see Fig. 3). The energy harvested from the FC system is of 6160 W from the maximum power of 6174 (see the zooms from Fig. 3). Thus, the tracking accuracy is of 99.77% without power ripple during the stationary phase [23]. The 6 kW PEMFC stack has the cell membrane aria of about 65 cm2 and 65 cells in series, thus the voltage at light load is about 60 V. The time constant and temperature were set to 2 s and 338 K. 3.2. The ESS model A lithium-ion batteries stack of 500 Ah/210 V from the SimPowerSystemÒ toolbox was used in all simulations. The battery voltage, EBatt, is given by charge (fc) or discharge (fd) function, as it is shown in Fig. 2, where:

Fig. 4. The RES model.

superposition effect of switching in power converters and sharp changes in the RES power profile (see Fig. 4). The ultracapacitors stack was modeled by its nominal capacitance (1 F) and series resistance (0.01 X). The initial state-ofcharge and time response of lithium-ion battery were set at 80% and 10 s. The simulation results will show that the 500 Ah capacity is enough to sustain the power balance (1) during low-frequency (LF) changes in the RES and load power profile (see Fig. 5), until the FC power will track (with limited slope for the controlled fuel flow rates) the average FC power (2). 3.3. The RES model

Ew is a constant – the working battery voltage (V). Kc – polarization constant V/(Ah). Kr – polarization resistance (X). iLPFbatt – low frequency battery current dynamics (A). Q – maximum battery capacity (Ah). Eexp(q) = A  exp (B  q). q = ibattt is current battery capacity (Ah). ibatt – battery current (A). A – exponential voltage (V). B – exponential capacity (Ah)1. The preset values for the parameters of the battery model (shown in Fig. 2) were used. The batteries’ stack was hybridized by an ultracapacitors stack to regulate the voltage on DC bus at 210 V (see Fig. 2) and better mitigate the high-frequency (HF) ripple. The HF ripple appears as

The RES model is quite simple, but has all features implemented to obtain a RES power profile that is close to reality. The control signal of the controlled current source is sum of the LF and HF sequences (see Fig. 4). As it was mentioned above, the HF sequence is based on random generator, having the maximum power and sampling time of 1 kW and 1 ms. 3.4. The load model The load model is also based on a controlled current source, having the control signal sum of load sequence and load ripple (see Fig. 5). The load ripple approximates the inverter ripple that is back propagated to the FC system. The load sequence could include the loading of DC bus during the operating of the electrolyzer that is usually used into RES/FC HPS architecture in order to use the surplus energy, PRES  PL during a load cycle. The electrolyzer is fed with this energy to produce hydrogen that is stored for later use when the demanded energy exceeds the energy supplied by the FC system and RES. Hydrogen technologies, combining FC systems and electrolyzes with hydrogen tanks, are interesting for long-term energy storage because of the inherent high mass-energy density [41]. This should allow the number of PV panels or WTs to be reduced, as well as requiring less battery capacity [42]. 3.5. The DC bus modeling

Fig. 3. The FC parameters, P–I characteristic, and MPP tracking (zooms).

In many grid connected applications, e.g. RES DG systems, the DC-bus voltage cannot be considered constant since the variation in the grid voltage (e.g. due to voltage sags [28]) will result in a change in the DC-bus voltage. Consequently, the DC-bus voltage should be regulated to assure a correct operation of the inverter system and to avoid damage of the ESS and DC-bus capacitor. The HPS operates under a dynamic load, so the DC-link current is not constant. Because the variations on the DC-bus are slower than the response time of the control loop, which is based on an

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4. Power balancing strategies In order to transform the RES/FC/ESS HPS into an active generator, it is important to define the energy management strategy for the power flows of different RESs, FC system and ESS. The voltage on the DC bus is the direct consequence of the power flow exchange among these sources, and it should be well regulated for the stable and robust operation of the HPS, optionally connected to the grid via an inverter system. The exchanged power with the grid should be precisely controlled to provide ancillary services to the power system of the smart grid. If the HPS grid connected is part of a plug-in station for FC electrical vehicles, then the power flow could be bidirectional [7]. This is an advanced control function for the plug-in stations. In the RES HPS without ESS (nondispatchable DG) only the grid connection system can be used to regulate the DC-bus voltage. The fluctuant RES power is totally delivered to the smart grid [26]. Consequently, the RES HPS operate a passive generator, whose power supply does not depend on the grid’s requirement but on the environmental conditions. On the other hand, in RES HPS with ESS (disptachable DG plus ESS) it can be noted that the DC-bus voltage and the grid power can be both controlled based on an appropriate power balancing strategy [43]. The required control and operational strategies of a smart grid are conceptually different than those of conventional power systems [44]. Two power balancing strategies are usually used:

Fig. 5. The load model.

ultracapacitors stack and the controlled buck-boost converter (see Fig. 2), the DC-bus can be modeled as a capacitor, CDC, with controlled current sources in parallel (see Fig. 1). 3.6. The power converters modeling Two types of power converters are used in the RES/FC HPS architecture to interface the FC (and other RESs) and the ultracapacitors stack to the DC bus: (1) a unidirectional boost and (2) a bidirectional buck-boost converter (see Fig. 2), respectively. The basic topologies [3] were implemented using power devices from the SimPowerSystemÒ toolbox. For example, the operating relationships of the boost converter are

diFC þ V DSðonÞ dt diFC ¼ ðr L þ RDfonÞ Þ  iFC þ L þ V DðonÞ þ v DC dt

v FC ¼ ðrL þ RDSðonÞ Þ  iFC þ L v FC

ð5Þ

for the IGBT on-state and FW-Diode on-state. The preset values are used for the on-state parameters: (RDS(on), VDS(on)) and (RD(on) and VD(on)), respectively. The magnitude of the HF components of FC current, DI, is set by the hysteretic band of the MPP controller. Thus, the value of boost inductance is given by the well known relationship:

L ¼ DV FCðMPPÞ =ð2f sw DIÞ

ð6Þ

where D is the duty cycle given by (4), and fsw = 1/Tsw is the switching frequency. The switching frequency must be of 100-times higher than dither frequency (100 Hz) to assure an appropriate tracking of the reference current by the FC current based on MPP hysteretic control:

irefðLFÞ ffi iFCðLFÞ

ð7Þ

In the same manner is modeled and designed the buck-boost converter.

(1) The grid-following strategy is based on line-current loop to regulate the DC-bus voltage, while the ESS power flow compensates the RES power flow to balance the power requirements of the smart grid. (2) The load-following strategy (or the ‘‘power dispatching’’ strategy) is based on line current loop to control the HPS power, while the DC-bus voltage is regulated based on ESS power flow. The load-following strategy has better performances on the grid power control than the grid following’’ strategy [40,43]. The energy management of multi-power sources has been studied intensively in the last decade based on multi-port power converter topologies [7]. For example, the control based on efficiency map of a FC/ultracapacitor HPS is proposed in [33]. The control based on a differential flatness system is studied in [45] and applied to regulate the DC-bus voltage of a FC/RES/ESS HPS [5]. The control strategy based on the flatness technique does not use a commutation algorithm when the operating mode changes with the load power variation. The second energy source (ultracapacitors stack, batteries stack, or hybrid ESS) is used to compensate the dynamic of the main source dynamics. Thus, the chattering effects are avoided, but it can be noted the relative large number of sensors used. If a separate control algorithms are proposed to control the HPS when the operating mode changes with the load power variation, then high instantaneous power demand could appear for the main energy source [4]. Furthermore, the controls and energy management strategies envisioned for a smart grid are mainly determined based on the used RES technologies, load requirements, and the expected operational scenarios [24]. The load-following strategy can be used to optimize the ratings of ESSs and dispatchable HPSs by reducing the peak load in a wide range of load demand. Thus, the advanced EMU controls the power flow of the dispatchable HPSs and controllable loads, and the consumption level of the utility grid based on the present and forecasted values of load, generation, and market information received [3,24].

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Fast response of each HPS grid-connected is necessary in presence of multiple HPSs on the DC bus that has no dominant source of energy generation during an autonomous mode, which means lack of infinite bus. Consequently, a dynamic power compensator device based on ultracapacitors stack and bidirectional power converter is necessary [46]. Uncontrolled, fast response of HPSs grid-connected can adversely affect the voltage/angle stability if appropriate provisions are not in place [24]. Furthermore, note that HPSs have significantly different power capacities and characteristics that depend by the RESs used. Consequently, appropriate local control loops must to be used for each RES. In general, these are of MPP tracking type to harvest all the available energy from each RES [23]. In this paper a load-following strategy have been proposed for a FC/RES/ESS HPS, having the following features: – assures the grid active power based on power balancing strategy for the backup power source, which is the FC system; – optimizes the energy efficiency of the FC system and each RES based on energy harvesting technique of MPP tracking type; – compensates the energy gap between the output power from the RESs and FC based on batteries stack; – mitigates the impact of HF perturbation on the load power due to the fluctuations of the electric utility grid and RES power based on controlled power flow exchanged with the ultracapacitors stack. Excepting the second feature, which was investigated in [23], the above features will be shown in the next section by simulation performed. The power controllers are used to implement the power balancing strategy in order to coordinate the FC, ESS, and RES power flow inside the HPS (shown in Fig. 1). The power exchanges on DC-bus (pdc) lead directly to the stability of the HPS and have impact on the DC-bus voltage (udc). As it was mentioned above, the DC-bus can be modeled by (8)

pdc ¼ dEdc =dt ¼ C dc udc dudc =dt ¼ pRES þ pESS þ g1 pFC  pL

ð8Þ

where pESS, pRES, pFC, and pL are the instantaneous power of the ESS, RES, FC system, and load. The FC system has relatively slow power dynamics, and fastvarying fuel flow rate references are not welcome for its operating lifetime. Therefore, a low-pass filter (the AV block) with a slope limiter should be added in the load-following control loop. In order to focus on the power-balancing strategies of the HPS, the control schemes of the power conversion systems through different power converters will not be detailed in this paper more than mentioned above.

Fig. 6. The testing diagram to evaluate the FC energy harvested.

5. Simulation Results 5.1. FC energy harvesting The testing diagram is shown in Fig. 6, where two FC systems are fueled at the same FuelFr value. One is used as reference (without MPP tracking control) and the second is controlled in a MPP tracking loop. The load sequence is set to draw the P–I characteristic and show the MPP tracking process (see Fig. 3), or to simulate a specific load cycle (see Fig. 7). Due to including of a rate limiter block in the flow rate regulator, it can be observed in Fig. 7 that the FuelFr has limited slopes. Thus, the FC power slopes are limited under admissible limits [13]. Also, for the same FuelFr used for both FC systems, the FC energy harvested increases with about (120/6000)  100 = 2%, if the MPP tracking control is used.

Fig. 7. The load power profile to evaluate the FC energy harvested.

5.2. The load-following strategy The simulation results for the FC/RES/ESS HPS, which is shown in Fig. 1, are presented in Figs. 8–10. The first plot of Fig. 8 shows the load power, which was set constant at 4 kW, and the RES power profile. The RES power sequence is (2 kW, 4 kW, 2 kW,

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Fig. 8. The simulation results for the FC/RES/ESS HPS.

Fig. 9. The simulation results for the FC/RES/Battery HPS.

0 kW) and the fluctuating RES power is shown in the zoom. The second plot shows the FC power based on the FuelFr estimated in the control loops (see the 7-th plot). The FuelFr value is computed based on the load-following loop and adjusted in the MPP tracking loop. The last loop compares the FC current, Ifc, with the FC reference current, Iref (see the 8-th plot and the zoom). The slopes of the power delivered by the boost converter, Pboost = g1PFC, is compared with the average FC power, PFC(AV), computed in the load-following loop based on (2) (see the third plot). Note that the slopes of the average FC power are determined by the load and RES power profile. The limited slopes for the FC power

Fig. 10. The simulation results for the FC/RES/ESS HPS under variable RES power profile.

delivered are obtained based on the FuelFr regulator, which include a rate limiter block, besides the control block given by (3).

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Thus, the fuel starvation phenomenon is avoided for any power profile of the load or RES. The forth plot shows the voltage of the battery stack, which is the DC-bus voltage, too. Besides of (4/ 200)  100 = 2% overvoltage to 2 kW step-up (-down), the DC-bus voltage is regulated to 210 V. The HF ripple is lower than 1% based on voltage regulation via the bidirectional converter supplied by a small ultracapacitors stack (1 F). The ultracapacitors voltage varies around initial value set to 100 V (see the 5-th plot), offering a voltage window of 100 V, which means 5 kJ of energy that can be used for dynamic compensation. Part of this energy is used to regulate the DC-bus voltage under buck and boost commands (see the 6th plot). The switching frequency of the hysteretic voltage-mode controller is in range of 1 kHz to 10 kHz (being about 8 kHz in the zoom on the 6-th plot). So, the FC current tracks the FC reference current, Iref, with a small HF ripple (see the zoom on the 8-th plot). If the ultracapacitors stack is not used to regulate the DC-bus voltage, then the battery stack and the DC capacitor try to compensate the power balance based on the load-following control loop. The simulation was made using the same load power and RES power profile (see the first plot of Fig. 9), but without fluctuating RES power to better highlight the charge sustaining mode for the batteries stack (see the ESS power on the third plot of Fig. 9). The FC power has almost the same profile (see the second plot of Fig. 9), demonstrating the operation of the load following control with or without fluctuating RES power. It is easy to compute the ESS energy exchanged with the DC-bus during the about 1.6 s of the transitory response to 2 kW step in the FC power. This ESS energy (about 1600 J) is sustained by the battery stack and the DC capacitor. If only the DC capacitor is used, then this must to have about 100 F to obtain 8 V ripple on the DC-bus during the transitory response (see the 4-th plot of Fig. 9). The CDC capacity was considered 1 F in all simulations, but this capacity could be lower that this value if a higher HF power ripple is accepted for this capacitors. Different load and RES power profiles were used in simulation to validate the operation of the load following control loop in case when PL > PRES during the load cycle. The FC power will have the profile that will results from (2) with about 1.5 kW/s limited slopes (see the second plot of Fig. 9). For example, simulation for a variable RES power profile is shown in Fig. 10. Note that PL > PRES during a time sequences of the load cycle. The FC stack will supply the load during the time when PL > PRES (see the second plot of Fig. 10), demonstrating once again the operation of the load following control loop. The RES power profile is shown in the third plot of Fig. 10. If PL > PRES then the battery operates in charge sustaining mode based on load-following control. The voltage regulation on the DC-bus is started at 11 s (see the 4-th plot of Fig. 10).

6. Conclusion In this paper a systemic analysis of power flow control applied to a standalone FC/RES/ESS HPS is shown. The analysis has been focused on the load following control of the FC power based on a power balancing strategy. Besides this control loop, which assures a charge sustaining mode for the batteries stack, a FC MPP tracking control is also used to improve the energy efficiency of the whole HPS. The performance of the FC/RES/ESS HPS under different load/ RES power scenarios has been verified by carrying out simulation studies using a practical load demand profile (see Fig. 7) and RES power profile sharper than in reality. When the RESs power becomes insufficient with respect to the load power requirement, the stored hydrogen is used to feed the FC system to produce

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energy. On the other side, the excess of power produced could be used by an electrolyzer for hydrogen production. The simulation results clearly indicate that a standalone FC/RES/ESS HPS is operational and reliable. The EMU proposed has features that (I) assures the load power requirement based on controlled FC power, (II) optimizes the energy efficiency of whole HPS based on energy harvesting technique of MPP tracking type applied to FC system and each RES, (III) compensates the LF and HF profile of the pdc power based on batteries and ultracapacitors stacks, and (IV) regulates the DC-bus voltage based on controlled power flow exchanged with the ultracapacitors stack. As it was mentioned in the introduction, on the basis of the measurement obtained from the HPS sensors, the EMU will determine the conditions for (I) charging the batteries stack by the RESs, (II) producing of hydrogen by the electrolyzer, (III) generating electrical energy by the FC system, and so on. The EMU will generate command signals to the power converters controllers based on this logical flow chart. A pulsed power profile could appear for the FC system and batteries stack due to switching of the power flows. Compared to this switching controlled mode of the power flows, all the control loops included in the EMU are of adaptive type. The main control loop is of load-following type based on the average power balancing strategy defined by (2) and (3), including signal conditioning blocks from the control loop (the rate limiter and saturation blocks). The FC system will power the DC bus with the required difference of the load power, which is the part that cannot be supported by the RESs. Note that all the power flows between the FC system, RESs, ESS, and DC load (which includes’ the hydrogen generation via the electrolyzer) take place through the DC bus based on power balancing equation defined by (8). From this equation, different dynamic control loops are implemented. In this implementation, the LF power profile is mainly supplied by the batteries stack that operates in the charge sustained mode. It is known that deep discharges or large LF pulses of the power exchanged can be very dangerous for the batteries. This remarkable feature of the load-following control to keep the batteries SOC almost constant during the entire cycle of operation will avoid these operation modes, increasing the batteries lifetime. The batteries really act as a short-term energy buffer, thus its capacity is low in comparison with other HPS implementations proposed. The HF power profile exchanged on DC bus is compensated by the ultracapacitors stack via a bidirectional power converter. This feature of the DC-bus voltage regulation avoids many charge–discharge HF cycles to the batteries, thus again contributing to extend their lifetime. The last feature of the energy harvesting was shown here for the FC system, contributing with 2% increasing at the energy efficiency of the whole HPS if a MPP tracking control technique will be applied to all RESs. Fig. 10 clearly shows that when the net power flow PRES  PL is positive the FC system could be stopped or will operate at low power to avoid the complex starting procedure. On the other hand, in this case the electrolyzer can be started to store hydrogen. Thus, besides an appropriate power design for the HPS topology and a protection procedure of its energy sources, the EMU must contain a monitoring procedure of the power flows limits in relation with batteries SOC window. It was shown that extending the battery operation modes to charge-depletion (CD) and charge-increasing (CI), besides charge-sustaining (CS), the efficiency of the whole HPS can be increased based on advanced EMU that considers all operation modes [7]. The EMU will control the power flows in different battery operating modes (CS, CD, and CI on average), monitoring its SOC based on current limits of the power flows, the load power profile to obtain higher efficiency and low charge/discharge ratio for the battery power profile. Thus, besides

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efficiency, functionality and reliability, other performance parameters, such as specific energy, specific power, maintenance requirement, cost, and safety, must be taken in consideration in designing the EMU. The impact of the FC/RES/ESS HPSs in the DG sector depends on (I) magnitude of the market penetration (cost of the HPS), (II) timing of the battery maintenance, (III) energy management (advanced features for the EMU), (IV) structure of the power sector, and, finally but not least, (V) availability of reliable FC systems for such applications. FC/RES/ESS HPSs grid connected can contribute by an efficient use of the renewable intermittent potential and can potentially solve imbalances between electricity supply and demand. From these conclusions, a number of questions arise for further exploration: (I) What type of energy management strategy and safe control is suitable for the advanced EMU of the HPSs in relation with upcoming smart grids? (II) What are the optimum ESS requirements for various HPS configurations? (III) Can the impact of the load-following control feature on battery life be quantified? (IV) Can the impact of the HPS use on renewable intermittent potential be quantified? (V) Can the hydrogen consumption characteristics under a given EMU strategy be evaluated? This paper proposes a simple alternative to operate the FC/RES/ ESS HPS based on load-following control and energy harvesting concepts. Overall, the potential to increase the HPS efficiency is shown in this paper for the CS mode, but could be extended to both CD and CI operation modes. The use of multi-operating modes makes the HPS attractive, but also makes it difficult to estimate its efficiency due to complications surrounding of using three operating modes in designing of the best EMU strategy.

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