Power management control strategy for a stand-alone solar photovoltaic-fuel cell–battery hybrid system

Sustainable Energy Technologies and Assessments 9 (2015) 68–80

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Original Research Article

Power management control strategy for a stand-alone solar photovoltaic-fuel cell–battery hybrid system Vaishalee Dash ⇑,1, Prabodh Bajpai Department of Electrical Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India

a r t i c l e

i n f o

Article history: Received 8 July 2014 Revised 20 October 2014 Accepted 22 October 2014

Keywords: Solar photovoltaic Fuel cell Battery Hybrid renewable energy systems Power management strategy

a b s t r a c t Hybrid renewable energy systems (HRES) help to increase system reliability and improve power quality. However, they face certain challenges in their widespread deployment such as intermittently varying renewable energy (RE) sources like solar radiation and wind speed, high installation costs and limited lifetime of energy storage devices. Thus, there is a need to integrate these sources by means of a suitable power management strategy (PMS) despite the fluctuations in source and load power. This paper aims to formulate a PMS to integrate the power output from solar photovoltaic (PV) array, fuel cell (FC) stack and battery with a provision for onsite hydrogen (H2) generation by means of an electrolyzer and H2 tank. The control strategy handles the source power effectively by considering the limited lifecycle of storage devices. It also eliminates the need for a dump load in the system by matching the source power with load demand when the storage devices are charged to their maximum capacity. Ó 2014 Elsevier Ltd. All rights reserved.

Introduction The shortage of electricity worldwide due to rapid modernization, has intensified research efforts in the direction of RE. Solar PV systems convert solar energy directly into electricity and offer the advantage of long lifetime with minimal maintenance [1]. However, solar resource being intermittent in nature, PV is usually coupled with energy storage devices to ensure reliable supply of electricity to the load. The most commonly used energy storage device along with PV is battery (mainly lead acid type). Recently, energy storage in the form of H2 has received significant attention because of benefits such as longer lifetime of H2 tanks and lower price as compared to battery [2]. Moreover, H2 can be produced by non polluting electrolysis of water; thus reducing dependence on batteries made of toxic materials. In a solar PV based HRES, PV is the primary source of energy to supply the load. Excess energy from PV is primarily stored in the battery and diverted to electrolyzer for generating H2, when the battery state of charge (SOC) reaches its maximum limit. Absence of PV power causes the battery to discharge first. If the battery SOC reaches a minimum limit, the deficit power is met by starting the FC which operates by consuming the stored H2. Thus, the ⇑ Corresponding author. Tel.: +91 7760584240. E-mail address: [email protected] (V. Dash). Present address: Center for Study of Science, Technology and Policy, # 18, 10th Cross, Mayura Street, Papanna Layout, Nagashettyhalli, RMV II Stage, Bangalore 560094, Karnataka, India. 1

http://dx.doi.org/10.1016/j.seta.2014.10.001 2213-1388/Ó 2014 Elsevier Ltd. All rights reserved.

combination of PV, FC and battery serves as an ideal option to power stand-alone loads [3]. A stand-alone PV–FC–Battery hybrid system requires a dedicated control algorithm to manage the frequent interaction and power flow among the source (PV and FC), battery and load (AC, DC or electrolyzer) [4,5]. A study on comparative assessment of three PMSs (PMS1, PMS2 and PMS3) has been carried out taking the specifications of an operational PV–wind–FC hybrid system using an electrolyzer for onsite H2 generation. It was observed that for locations that receive highly intermittent solar and wind resource supply, PMS2 is a viable option. In PMS2, the accumulator or battery is allowed to discharge partially so as to allow electrolyzer operation at a minimum load level for efficient operation thus increasing H2 production [6]. Another study indicates that a single dispatch strategy cannot yield best results under dynamic operating conditions and hence suggests a combined dispatch strategy [7]. In a separate research, the authors analysed a PV–FC–Battery hybrid system using three control strategies, wherein they observed the second control strategy to provide best average hybrid system efficiency [8]. This strategy aimed at maintaining a certain preset level of H2 storage and battery SOC. Load following mode of control for a FC based hybrid power system along with charge sustaining mode of operation for the energy storage system has been shown to help optimize battery capacity and energy efficiency of the FC [9]. In addition, controlled voltage source (CVS) [10] and controlled current source (CCS) [11] non linear controllers have been designed for FC based hybrid system. CVS controller

V. Dash, P. Bajpai / Sustainable Energy Technologies and Assessments 9 (2015) 68–80

helped in reducing the electro-magnetic interference (EMI) of the bi-buck converter and maintaining a low ripple factor for the hybrid power system output voltage. CCS controller helped in mitigating the low frequency ripple in the system current by generating a delayed anti ripple current. Work done in increasing the efficiency of multiport power converter in series or parallel architecture for both charge sustaining and charge depleting mode of operation of the energy storage system in FC vehicles has also been highlighted in [12]. In addition, a wide investigation of PV–FC hybrid systems using battery or super-capacitors has been carried for utility (AC) loads. The authors have described power management of HRES either for a purely AC load [13,14] or a combination of AC and DC loads [15]. However, the motive behind the study is to understand the intricacies involved in designing controllers for PV–FC–Battery hybrid systems catering remote DC loads. A remote DC load could be a telecom load or a critical load such as hospital and defence establishment. The existing literature definitely gives a background for the research. However, an in-depth discussion on design and operation of controllers to regulate the power flow for typical DC load applications is required. The key objective of the work is to design a robust control strategy for a PV–FC–Battery hybrid system that can extract maximum power from the sources (PV and FC) and charge the battery optimally. The excess power in the system should be managed within the storage units’ (battery and electrolyzer-H2 tanks) capacity for typical DC load applications. The present work proposes a PMS which ensures that the load is supplied uninterruptedly and the battery is charged within the maximum specified charging rate by means of a two stage charging technique. The control algorithm suppresses the short term fluctuations in PV and load power by the use of battery. Thus it prevents frequent starting or stopping of the FC and electrolyzer by reducing their switching cycles. In addition, it eliminates the need for a dump load in the system by matching the power drawn from the sources (PV and FC) to the load and electrolyzer demand. This is validated by the simulation results illustrating power sharing among the sources, load and battery under a typical summer scenario and for random weather conditions. The major contribution of this work includes development of detailed models of individual controllers for PV array, FC stack and electrolyzer in MATLAB/SIMULINK for a PV–FC–Battery hybrid system. This work focuses mainly on the electrical aspects of controller design. Thus, modeling of H2 gas actuator and compressor have been ignored. Modeling of HRES components The modeling of HRES components is discussed in this section. This includes the solar PV array, FC stack, electrolyzer stack, battery and H2 tank. Modeling of solar PV array A solar PV system converts solar energy directly into electricity. The simplest PV model is the single diode model as shown in Fig. 1

Fig. 1. Electrical equivalent circuit representing single diode model of a PV cell.

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[16]. PV models with greater accuracy like the two and three diode models are also discussed in [17,18] respectively. In a single diode model, the relationship between load voltage (V) and load current (I) for a fixed temperature and irradiance is expressed as in Eq. (1) [16] which holds good for a PV cell, a module and an array.

I ¼ IL  ID ¼ IL  Io fexp½ðV þ IRs Þ=a  1g  ðV þ IRs Þ=Rsh

ð1Þ

where IL is the PV generated current, Io is the reverse saturation current, ID is the diode current, Ish is the shunt current, Rs is the series resistance, Rsh is the shunt resistance, a is the curve fitting parameter. The value of Rs and Rsh has been neglected by authors in [16,19] respectively to simplify the modeling study. The present study ignores the effect of Rsh only (usually between 250–450 X for mono-crystalline PV) [20]. Eq. (1) is thus modified as

I ¼ IL  Io fexp½ðV þ IRs Þ=a  1g

ð2Þ

Equations described in [21,22] and PV module datasheet (Appendix A, Table A.1) values provided by the manufacturer help in calculating the values of IL, Io, a and Rs at reference conditions or standard test conditions (STC). These reference values can then be used to determine the actual value of IL, Io, a and Rs for a particular value of irradiance (G) and ambient temperature (Ta) based on equations described in [21–22]. Minimum possible temperature is preferred, for best operation of PV. A simple lumped thermal model [16] used in this study, showing the dependence of PV cell temperature (Tc) on ambient temperature (Ta) and irradiance (G) is expressed as

T c ¼ T a þ ½G  ðsa=U L Þ½1  ðgc =saÞ

ð3Þ

where, UL is overall heat loss coefficient, gc is efficiency of PV module and sa is the transmittance-absorption product of PV cells (an estimate of 0.9 holds good for modeling purposes) [16]. The ratio between sa and UL depends on nominal operating cell temperature (NOCT) conditions [16]. Modeling of FC stack Fuel cells are electrochemical energy conversion devices that convert the chemical energy of a fuel directly into electricity. The proton exchange membrane fuel cell (PEMFC) exhibits promising features such as zero emissions, high power density, quick start and have been successfully implemented in distributed generation [23]. PEMFC terminal voltage (Vfc) is less than the internal developed voltage (E) due to the activation, ohmic and concentration voltage drops occurring inside the FC [22,23]. To simplify the FC model, the assumptions made in [22] have been considered. In addition, the mass diffusion equations have also been ignored. The pressure of H2 and O2 at the anode and cathode are directly given as inputs to the FC model and these values are assumed to remain constant at 1.5 atm and 1 atm respectively during the simulation period. The FC output voltage (Vfc) and rate of H2 consumption by FC stack N H2 ; out is given as [24]

V fc ¼ E  V act1  V c  V ohm

ð4Þ

NH2 ;out ¼ Ifc =2F

ð5Þ

where E is the internal voltage developed across the FC, Vact1 is the temperature dependent term of activation voltage drop, VC is the voltage developed across the capacitor, Vohm is the voltage drop due to ohmic losses, Ifc is FC stack current and F is Faraday’s constant. The net generation of heat due to chemical reaction inside the FC, causes the stack temperature to fluctuate during the FC operation. The net heat generation is expressed as [21]

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qnet ¼ qchem  qelec  qsensþlatent  qloss

ð6Þ

where qnet is the net heat energy, qchem is the chemical (or heat) energy, qelec is the electrical energy, qsens+latent is the sensible and latent heat and qloss is the heat loss. During transitions, FC stack temperature (Tfc) will rise or drop according to the FC specific heat capacity and its net heat generation given in Eq. (6) and is now expressed as

M FC C FC

dT fc ¼ qnet dt

ð7Þ

SOC calculates ratio of the amount of charge stored in the battery in time t to its full capacity [28]. 100% SOC indicates the battery is fully charged where as 0% SOC indicates it is completely discharged. Eq. (14) indicates the capacity to which the battery has been charged/discharged from the zero level in time t. Modeling of hydrogen tank The pressure of H2 gas in the tank is calculated using the Beattie–Bridgeman equation [21].

where MFC is the total mass of the FC stack and CFC is the overall specific heat capacity of the stack.

PH2 ¼

Modeling of electrolyzer stack An electrolyzer generates H2 and O2 from water using electricity by the process exactly reversal to that occurring in a FC. Currently alkaline water electrolysis is a more dominant technology in the market as compared to PEM electrolysis [22]. The internal voltage for each electrolyzer cell can be expressed as [22]:

V cell ¼ V rev;cell þ V drop;cell

ð8Þ

where Vrev,cell is the internal electrolyzer cell voltage, Vcell is the input DC voltage to the electrolyzer cell and Vdrop,cell is the voltage drop across the non-linear resistor representing the electrolyzer internal losses. For an electrolyzer stack consisting of ne cells in series, the terminal voltage of the stack is

V el ¼ ne V cell

ð9Þ

The actual rate of H2 production is always lower than theoretical maximum value due to parasitic current losses and is given as

NH2 ;in ¼ nF :ðne Iel =2FÞ

ð10Þ

where nF is the current or faraday efficiency. The operating temperature of an electrolyzer is expressed as [22,25]:

C elec

dT el ¼ Q gen  Q loss  Q cool dt

ð11Þ

where Celec is the overall heat capacity of the electrolyzer stack, Qgen is the heat power generated inside the electrolyzer stack, Qloss is the heat power loss and Qcool is the heat power loss due to cooling. Modeling of battery state of charge Battery is an electrochemical storage device essential for storing electrical energy. The MATLAB/SIMULINK lead acid battery model [26] used in this work, is essentially a controlled voltage source in series with a constant internal resistance. The assumptions considered in the battery model have been mentioned in [27]. The voltage source is described as

E1 ¼ E01  K 01  ½Q =ðQ  Ibat tÞ þ A01 expðB  V bat ¼ E1  Ibat Rbat

Z

Ibat dtÞ

ð12Þ ð13Þ

where E1 is the battery no-load voltage, E01 is battery constant voltage, K01 is polarization constant, Ibat is battery current, Q is maximum battery capacity, A is exponential voltage, B is exponential capacity, Vbat is battery terminal voltage and Rbat is internal resistance of battery. The SOC of the battery at time t is expressed in percentage and calculated as

 Z t   SOC ðtÞ ¼ 1  Ibat  dt =Q  100 0

ð14Þ

N2H2 RT H2 V 2H2

1

cN H2 VT 3H2

!

    A0 1  a1 NH2 N2 H2 VH V H2 bNH2 2  þ B0 1  N H2 V H2 V 2H2 ð15Þ

where V H2 is the volume of the tank (L), T H2 is the temperature of H2 (K), N H2 is the number of moles of H2 in the tank (mol), R is the gas constant in (atm. L/(mol. K)). A0, B0, a1, b, c are constants for H2 gas. The number of moles of H2 in the tank (N H2 ) is given by the difference between the rate of H2 generated by the electrolyzer (N H2 ;in ) and the rate of H2 consumed by the FC N H2 ; out [21]. It is expressed as

NH2 ¼

Z

ðNH2 ;in  NH2 ;out Þ  dt

ð16Þ

Solar PV–FC–Battery hybrid system control The block schematic of the solar PV–FC–Battery hybrid system is shown in Fig. 2. The sources (PV and FC) are connected to the load and storage units by means of a DC bus. The battery is directly interfaced with the DC bus, where as the PV array and FC stack are connected to the DC bus by means of DC–DC buck converter 1 and 2 respectively. A DC–DC buck converter 3 is used to interface the electrolyzer with the DC bus. DC–DC converters help to regulate the power flowing from the sources into the DC bus and from the DC bus to the storage devices. Switch SW2 and SW3 connect/ disconnect the FC and electrolyzer to/from the DC bus based on the signal SSW2 and SSW3 respectively. Blocking diodes are used to prevent the accidental reverse flow of current into the sources (PV and FC). The output voltage and current of DC–DC converter 1 are represented as V1o and Ipvdc respectively. The current drawn by the resistive load is Iload. Similarly, the charging/discharging current to/from the battery is denoted as Ibat. The output current from DC–DC converter 2 is denoted as Ifcdc and input current to DC–DC converter 3 is denoted as Idcel. The rate of generation of H2 from electrolyzer and its consumption by the FC are N H2 ;in and N H2 ;out respectively. The parameters used for modeling of the HRES components are shown in Appendix A, Tables A.1–A.6. Individual HRES component ratings are given in Appendix B, Table B.1. The simulation study is carried out for a peak load of 2 kW. Since the battery acts as primary back up source, a 400 Ah rated (nominal voltage = 48 V, maximum current = 40 A (C/10 rate)) battery bank is chosen. The PV array has to supply the battery and load primarily. Thus, a PV array of 4.95 kW (30 modules of 165 W each) is considered. The excess PV power after the losses in converter 1 and meeting the load and battery is fed into the electrolyzer. The peak power generated from PV at STC under a minimum load demand of 5 A, causes the electrolyzer current to increase to a maximum of 110 A. Thus the electrolyzer is rated at 2.75 kW (110 A, 24 V). A FC stack of 2.5 kW is chosen to supply the peak load (2 kW) considering losses in converter 2. The H2 tank size is chosen to have a volume of 100 L, containing an initial 250 mol of H2 at the beginning of simulation.

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Fig. 2. SIMULINK model schematic of the PV–FC–Battery hybrid system.

Power management strategy for hybrid system operation The PMS shown in Fig. 3 makes battery SOC, the key parameter in diverting the power to/from the storage devices. The SOC determines the FC switching ON (FCON) and switching OFF (FCOFF) instants. Battery is prioritized as the primary back up device, thus any excess power from the source is fed into the battery first after meeting the load demand. This helps to prevent frequent starting and stopping of the FC and electrolyzer. Stopping of the FC or electrolyzer in practical systems refers to putting them on standby mode where in only exchange of control signals occur without any actual flow of power. The algorithm initiates with the measurement of PV power (PPV) and load power (Pload). Availability of excess PV power charges the battery either in constant current (CC) or constant voltage (CV) mode depending on its SOC. CC mode of battery charging enables MPPT action on PV output and charges the battery with maximum available PV power in excess. However, MPPT is suspended during CV mode as the battery is charged at a constant voltage and reduced current. This helps in reducing battery wear because of

higher current levels at SOC close to 100%. As the battery SOC becomes greater than or equal to 99.5%, the electrolyzer is switched ON. Excess PV power by MPPT control is fed into the electrolyzer to generate H2 at a rate N H2 ;in The electrolyzer turns OFF when there is unavailability of excess PV power or when the pressure of H2 in the tank (PH2 ) reaches the preset reference value (PH2 ;ref ) or in case the FC is switched ON. When both the storage units reach their maximum storage capacity and excess power is still available, the PMS alters the reference value of the MPPT current from PV to match the load demand. If Pexcess < 0, FC is switched ON only if the battery SOC is less than FCON; else the battery discharges to meet the load demand. In ON state, FC supplies the load and charges the battery in CC mode, consuming H2 at a rate N H2 ;out moles per second. When the battery SOC reaches FCOFF or Ipvdc exceeds Iload by 5 A, the FC is switched OFF. The 5 A margin ensures that the FC is turned OFF only when Ipvdc exceeds Iload by a satisfactory value. This helps to stop FC operation as soon as PV power is restored. Thereafter, based on the availability of excess power and load demand, the battery charging restores and the control logic cycle continues.

Fig. 3. Block schematic of the power management strategy algorithm for the PV–FC–Battery hybrid system.

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PV array MPPT and battery charging controller

The various instances when Iref,pv is limited is as follows:

The control circuit for the extraction of maximum power from PV array and controlled charging of battery is shown in Fig. 4. Out of the several MPPT techniques suggested in literature [29– 31], a current based MPPT (CMPPT) technique is used in the present work owing to its ease of implementation [28]. CMPPT is based on the fact that the maximum PV current (Imp) is linearly related to its short circuit current (Isc). Imp is expressed as [32]

(1) When Iload is less than 33 A and FC is OFF, the upper limit of Iref,pv is set as 0.565 * (40 + Iload). This is because, it is observed that at STC (G = 1000 W/m2 and Ta = 25 °C), the maximum value of Ipvdc is 73 A. Thus if Iload is below 33 A, Ibat shall exceed the 40 A limit. So, in order to limit the output current from DC–DC converter 1 to a value equal to 40 + Iload, the Iref,pv is set as 0.565 * (40 + Iload). The value of 0.565 indicates an approximate value of the duty ratio of the converter and this particular value is chosen by running the simulation for several operating conditions. (2) When electrolyzer is switched OFF and Ipvdc > Iload, the value of Iref,pv is set equal to 0.52 * Iload. Switching OFF the electrolyzer will cause the excess PV current to charge the battery, even though its SOC is 100%. This will hamper the battery lifetime and cause heating problems. Thus, Iref,pv value is limited to a value such that it matches the load demand only. Thus, the output current drawn from PV array is equal to Iload. The value, 0.52 is chosen as it is observed that when the electrolyzer is turned OFF, the DC bus voltage varies between 51–52 V and the PV voltage varies between 95– 100 V for the various operating conditions considered in simulation study. Thus 52/100 = 0.52 is the approximate value of duty ratio at which the converter operates, when the electrolyzer is switched OFF.

Imp ¼ kcmppt  Isc

ð17Þ

where kcmppt is the current factor for CMPPT control. The values of Imp and Isc are calculated from the PV model under different irradiance conditions. The slope of the line between Imp and Isc indicates kcmppt. The average value of kcmppt is calculated as 0.92. Thus the calculated value of Imp (same as Iref,pv) is used as the reference for MPPT action as shown in Fig. 4. Battery charging is carried out by a two stage process, CC or CV depending on the battery SOC [33]. CC mode basically charges the battery with the maximum available PV power, at a faster rate using CMPPT action. In CV mode, the battery charging current gradually reduces as V1o is maintained constant to avoid heating problems in battery at higher SOC. In both the modes, the charging current is limited to a maximum of 40 A (C/10) for a 400 Ah battery. The switches S1–S4 shown in Fig. 4 are decision making switches, where the middle input is the control signal. Based on whether the control signal is TRUE (T) or FALSE (F), the upper or the lower input is chosen respectively. If the battery SOC is less than 98%, CC mode of battery charging is chosen, wherein Iref,pv is compared with actual PV array current (Ipv) to generate the error signal. A PI controller and limiter tune the error to generate Ibat in CC mode. However, when the battery SOC is greater than 98% AND less than or equal to 99.5%, the battery charges in CV mode. In this case, V1o values just before and after the transition into CV mode are compared and the error that passes through S2 is tuned to generate Ibat. The final error, i.e. Ibat + Iload  (Ipvdc + Ifcdc) is tuned using a PI controller-limiter and compared with a sawtooth waveform to generate the gate pulses for DC–DC converter 1. The position of switches in Fig. 4 depicts CC mode of battery charging. All the DC–DC converters have been modeled using conventional buck converter models described in [34]. Under normal operating conditions, Iref,pv = 0.92  Isc, determined from CMPPT method. However, Iref,pv is limited when the battery charging current exceeds 40 A (maximum allowable battery discharge current).

FC stack MPPT controller The control circuit for FC MPPT operation is shown in Fig. 5. It is observed that each FC module with a capacity of 500 W delivers a maximum of 464 W at 21.38 A and 21.69 V for the specified operating conditions (Appendix A). Thus, MPPT for FC stack is carried out by comparing the actual FC stack current (Ifc) with the maximum FC stack current (Ifc,ref = 21.38 A) under normal operating conditions. Hence, the FC is operated at its peak power under dynamically varying load conditions. Ifc,ref is altered to 0.42 * (40 + Iload  Ipvdc) only when Ibat exceeds 40 A limit during battery charging by the FC. The factor 0.42 is chosen by adjusting the set point such that the combined source current from PV and FC does not cause Ibat to exceed 40 A. Switches S5–S8 operate in similar to switches S1–S4 shown in Fig. 4. The output of switch S5 is labeled as SSW2, which opens or closes the switch SW2 connecting the FC with DC bus as shown in Fig. 2. The signal SSW2 depends on the combined action of a relay with AND operator. The relay gives an output 1 or 0 depending on

Fig. 4. Control circuit layout for PV array MPPT and battery charging.

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Fig. 5. Control circuit layout for FC stack MPPT.

whether the battery SOC is equal to FCON or FCOFF respectively. Switch S6 generates the FC reference current (Ifc,ref) for MPPT. Signal SSW2 drives switch S7 and S8, wherein the error in FC current is tuned by a PI controller and limiter. The limited output is then compared to a saw-tooth waveform to generate the gate pulse for DC–DC converter 2. Electrolyzer controller The control circuit layout showing the DC bus voltage regulation for electrolyzer is illustrated in Fig. 6. The DC bus voltage varies as per the battery voltage (between 47–55 V). The electrolyzer is observed to operate between 20–30 V. Hence, DC–DC converter 3 bucks down the bus voltage to the required voltage level at the electrolyzer input as shown in Fig. 2. Switches S9–S12 shown in Fig. 6 are similar to the switches shown in Figs. 4 and 5. The output of switch S10, denoted as SSW3 determines the switching ON/OFF of the electrolyzer (SSW3 controls SW3 shown in Fig. 2). SSW3 = 1 (electrolyzer is ON) when battery SOC is greater than or equal to 99.5%; SSW3 = 0 (electrolyzer is OFF) if the pressure of H2 in tank exceeds the preset reference pressure (P H2 ;ref ) or if Ifc > 0. The gate pulse generation for DC–DC converter 3 is carried out by a closed loop control on Ibat, which forcibly stops battery charging at 99.5% SOC and diverts the PV current into the electrolyzer. The tuned error in Ibat is compared to actual electrolyzer current (Iel). This difference is again tuned,

limited and compared to a saw-tooth waveform to generate the duty ratio for converter 3. Simulation results and discussion The PV–FC–Battery hybrid system is simulated under two different operating scenarios. The results are shown for a typical summer scenario and an arbitrarily varying weather profile taken as inputs to the simulation model. Peak values of solar radiation and ambient temperature for the summer scenario are based on data collected from HOMER [35] and meteorological information available at [36] respectively. The simulation is considered for 48 h and 24 h duration in summer and arbitrary varying weather cases respectively. SOCmin (minimum SOC limit till which the battery is allowed to discharge), FCON and FCOFF are considered as 40%, 40% and 80% respectively for both the cases. For the summer scenario, system operation is analyzed for two different initial settings of battery SOC (SOCin) i.e. at 85% and 45%. For the arbitrary variation in weather data, simulation results are discussed only for SOCin at 85%. Summer scenario A typical summer data profile is taken as input to simulate the case for an arbitrary step varying load profile. FCON and FCOFF settings are such that the FC charges the battery from 40% to 80%

Fig. 6. Control circuit layout for input voltage regulation to the electrolyzer.

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SOC or till PV power is restored. The input data is used to simulate two different cases with SOCin settings at 45% and 85%. The solar irradiance (G), ambient temperature (Ta) and cell temperature (Tc) of the PV array for 48 h duration are illustrated in Fig 7. The first peak insolation is 1033 W/m2, occurring at 12th hour and the second peak of 1003 W/m2 occurs at 34th hour. Ta increases with increase in G and decreases with fall in G. Tc rises faster as compared to Ta with increase in G and becomes equal to Ta when G reduces to zero. Battery SOCin at 85% The battery SOC is set at 85% before starting the simulation and the variation in key parameters are shown in Figs. 8–11. The output voltage of DC–DC converter 1 (V1o), battery voltage and SOC of the battery are shown in Fig 8. V1o is initially zero (absence of PV output) and gradually increases with increase in G. It is maintained at a value higher than the battery voltage which helps in transferring PV output power to the battery and load. The battery voltage and SOC increases during charging and decreases during the discharging period. Battery charging is suspended during electrolyzer operation, i.e. between 9.5th–16.5th hour. SOC rises to a maximum of 99.9% and does not fall below 40% as the FC is switched ON (23.5th hour) at 40% SOC of battery and helps in charging the battery. The sharing of current among PV array (Ipvdc), resistive load (Iload), battery (Ibat), electrolyzer (Idcel) and the FC cell stack (Ifcdc) at the DC bus are shown in Fig 9. Iload varies in steps from a minimum of 8.8 A to a maximum of 39.6 A. The battery discharges initially (shown as positive Ibat) in the absence of Ipvdc (G = 0) till 5th hour. Increase in PV output, charges the battery (shown as negative Ibat) by MPPT action. When the battery SOC reaches 99.5% at 9.5th

hour, electrolyzer gets switched ON (Idcel > 0). Gradual decrease in G, switches OFF the electrolyzer at 16.5th hour followed by battery discharging till 23.5th hour (SOC = 40%). FC is switched ON at this instant to supply the load and charge the battery again. FC switches OFF as the PV power is restored at 32.5th hour. Battery charging continues till the electrolyzer is switched ON again at 35.5th hour. The PV array operates at its MPP except when Ibat exceeds its C/10 charging limit between 7.5th–10th hour and between 33rd–36th hour. The PV array output, FC stack output and electrolyzer input powers are shown in Fig. 10. In the first 24 h duration, peak PV power is 4708 W, extracted at the 12th hour in correspondence with peak G. However, the second peak PV power of 4287 W is extracted at the 37th hour, incoherent with the second peak G, available at 34th hour (in Fig. 7). This is evident as the Iref,pv (actual MPPT reference signal) value is less than Imp (theoretical MPPT reference signal defined in Eq. (14)) at the 34th hour, as shown in first subplot of Fig. 11. The PV array operates at MPP except in the interval between 7.5th–10th hour and between 33rd–36th hour (Fig 11). Such a control action restricts Ipvdc and indirectly limits Ibat below 40 A. The FC when switched ON at 23.5th hour operates at its MPP and delivers 2321.6 W as shown in the second subplot of Fig. 10. The third subplot indicates that during electrolyzer operation starting at 9.5th–35.5th hour, a peak power of 2115 W–2257 W is fed respectively into the electrolyzer. P H2 is indicated in the second subplot of Fig. 11. During electrolyzer operation between 9.5th–16.5th hour and between 35.5th– 40.5th hour, H2 is generated (shown as increment in P H2 ). However, PH2 decreases during FC operation between 23.5th–32.5th hour, as FC consumes H2 from the tank. A minor change in PH2 is observed

Fig. 7. Irradiance, ambient and cell temperature of the PV array in a typical summer scenario.

Fig. 8. PV converter voltage, battery voltage and battery SOC in summer at 85% Bin.

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Fig. 9. Sharing of current at the DC bus among the sources, load and battery in summer at 85% Bin.

Fig. 10. PV, FC and electrolyzer power in summer at 85% Bin.

Fig. 11. MPPT reference current and P H2 in summer at 85% Bin.

during FC operation as the rate of H2 consumption is less compared to its generation. Hence the final value of P H2 at 48th hour is more than the initial level. Battery SOCin at 45% The battery SOC is initially set at 45% and the variation in important component parameters are shown in Figs. 12–15. V1o, battery voltage and battery SOC are shown in Fig. 12. V1o increases with available G and is maintained at a value higher than the battery voltage. The battery voltage and SOC increases during charging and decreases during the discharging period. Battery SOCin set at 45% decreases due to discharging till 2nd h. FC starts charging the battery and stops at 7th hour (SOC = 40%). Then PV continues charging the battery till 99.9% SOC. Battery

charging ceases during 9.5th–16.5th hour(electrolyzer operation). Thereafter, the discharging-charging cycle continues as per the control logic. The distribution of component currents at the DC bus is illustrated in Fig. 13. The load profile remains unchanged from the previous case. Absence of irradiance till 5th hour causes the battery to discharge and its SOC decreases to 40%. This switches ON the FC at the 2nd hour and battery starts charging. With increase in PV output, FC turns OFF at the 7th hour. Battery charging by PV continues till 99.9% SOC. Thereafter, the electrolyzer operates between 9.5th– 16.5th hour. After the 16.5th hour, battery discharges in absence of source current and causes the FC to start again (23.5th hour). Thereafter the charging cycle of battery continues till the electrolyzer is switched ON again at 35.5th hour.

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Fig. 12. PV converter voltage, battery voltage and battery SOC in summer at 45% Bin.

Fig. 13. Sharing of current at the DC bus among the sources, load and battery in summer at 45% Bin.

Fig. 14. PV, FC and electrolyzer power in summer at 45% Bin.

The PV array, FC stack and electrolyzer power characteristics are shown in Fig. 14. The peak power extracted from the PV array at the 12th and 37th hour is 4680 W and 4287 W respectively. The second peak power does not correspond to peak G occurring at 34th hour. This is because Iref,pv during the second peak irradiance (33rd–36th hour) is limited to restrict Ibat within 40 A as shown in the first subplot of Fig. 15. A peak power of 2317 W and 2338 W is extracted from FC stack under MPPT action, as shown in the second subplot. Similarly, the third subplot shows a maximum power of 2115 W and 2257 W fed into the electrolyzer at the 12th and 37th hour respectively.

It is observed that P H2 shown in Fig. 15 increases during electrolyzer operation (9.5th hour–6.5th hour and 35.5th hour–41st hour) and decreases during FC operation (23.5th hour–32.5th hour). This is because electrolyzer generates H2 and FC consumes H2 stored in from the tank. The simulation study depicted in the above two cases with Bin at 85% and 45%, ensures reliable operation of the control scheme even with lower value of Bin. However, it is observed that with Bin set at 45%, the FC operates for a longer duration compared to the case Bin set at 85%. This is evident as the PV power alone is insufficient to

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Fig. 15. MPPT reference current and P H2 in summer at 45% Bin.

Fig. 16. Arbitrary variation in irradiance, ambient and cell temperature of the PV array.

Fig. 17. PV converter voltage, battery voltage and battery SOC plots under arbitrary variation in irradiance and ambient temperature.

maintain higher SOC levels. As the SOC decreases to 40% value, the FC starts to charge the battery. Arbitrary variation in irradiance and ambient temperature The profile of arbitrary weather as input for 24 h duration is shown in Fig. 16. This profile incorporates all possible variations in irradiance such as sudden changes, gradual rise and fall, hence reflects a practical scenario of intermittently varying solar irradiance. In addition, the electrolyzer switching OFF condition due to maximum H2 storage (preset P H2 ;ref ¼ 70 atm) is also highlighted in this case study.

V1o, battery voltage and SOC variations are shown in Fig. 17. The higher value of V1o compared to battery voltage enables power flow from PV to battery and load. Battery voltage and SOC plots indicate an increase/decrease during battery charging/discharging. The battery discharges initially till the 5th hour until Ipvdc > Iload. With increasing PV output, battery charges till 99.5% SOC, beyond which the electrolyzer switches ON at 9.5th hour. The electrolyzer shuts down at 12.5th hour as PH2 ¼ P H2 ;ref ¼ 70 atm (shown in Fig. 19). However, battery SOC does not increase in spite of available excess PV power, as the battery is already at its maximum SOC. This is achieved by disabling the MPPT action such that Ipvdc reduces to match Iload. This is evident from the first subplot of

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Fig. 18. Sharing of current at the DC bus among the sources, load and battery under arbitrary variation in irradiance and ambient temperature.

Fig. 19. MPPT reference current and P H2 under arbitrary variation in irradiance and ambient temperature.

Fig. 19 where Iref,pv is less than Imp between 12.5th–14.5th hour. MPPT action resumes only when Iload exceeds Ipvdc at 14.5th hour. Thereafter, the battery discharges followed by FC operation at 22nd hour. The distribution of source, battery and load currents are shown in Fig. 18. Initially the battery discharges/charges based on the available excess PV power. At the 7.5th hour, due to a sharp drop in irradiance, Ipvdc decreases significantly and causes Ibat to reduce. With switching ON of electrolyzer at 9.5th hour, P H2 increases and reaches 70 atm (Fig. 19). This causes the electrolyzer to turn OFF (Idcel = 0 A) at the 12.5th hour. Between 12.5th–14.5th hour, MPPT action is temporarily disabled and Ibat is maintained at zero. This clearly shows that due to integrated action of electrolyzer controller and MPPT controller, the PV power is limited to match the load when both battery and H2 reach their maximum SOC. Hence the need for a dump load in the system is also eliminated. Beyond this, the decreasing G causes the battery to discharge till 40% SOC, followed by FC operation at 22nd hour. It is clearly observed that P H2 shown as the second subplot in Fig. 19 increases during electrolyzer operation and does not exceed the preset reference value of 70 atm. PH2 starts decreasing after 22nd hour due to FC operation.

in the present work. The results are illustrated to validate the proposed control strategy for the distribution of current at the DC bus indicating the power sharing among the sources (PV and FC), battery and load (resistive load and electrolyzer). The control algorithm diverts the excess PV power into the electrolyzer when the battery SOC reaches 99.5% and hence protects the battery from overcharging. The control strategy also eliminates the need for a dump load by limiting the power drawn from the sources so as to match the load demand. The output power from solar PV is given priority over the FC in supplying the load due to the high cost of electricity generation from H2. The load demand is uninterruptedly supplied by a combination of PV, FC and battery. The proposed control logic has been verified for different profiles of irradiance and ambient temperature in MATLAB/SIMULINK. A typical summer and intermittent solar radiation profile for a step varying load is considered. The simulation results highlight the efficacy of the control scheme with different battery SOC initial settings to integrate solar PV, FC as power sources and battery as back up source to supply the load reliably.

Acknowledgements Conclusions The operation of PV–FC–Battery hybrid system along with an electrolyzer and H2 tank for on-site H2 generation is investigated

The authors would like to express their gratitude towards Electrical Engineering Department, IIT Kharagpur for their encouragement and support. We would also like to thank VICET, IIT Kharagpur for funding the work.

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Appendix A. Specifications of the model of PV–FC–Battery hybrid system

Table A.1. PV module specifications at STC (1000 W/m2 irradiance, 25 °C module temperature and 1.5 air mass ratio). Nominal power Voltage at MPP Current at MPP Open-circuit voltage Short-circuit current Module efficiency No. of solar cells in series in a module Short-circuit current cell temperature coefficient Nominal operating cell temperature

165.0 W 35.0 V 4.7 A 42.0 V 5.2 A 13.0% 72.0 0.007% 40.0 °C

DC–DC converter 1

DC–DC converter 2

DC–DC converter 3

3500 lF, 0.001 X 5000 lF, 0.001 X 30 mH, 0.001 X 3000 lF, 0.001 X 2000 lF, 0.001 X 15 mH, 0.001 X 0.7 lF, 0.001 X 5 mH, 0.001 X

Appendix B. Component ratings of PV–FC–Battery hybrid system

500 W 48 5–35 °C P H2  1:5 atm 44 kg 500 J/(kg K) 0.1 F (4.8 F for each cell)

Device

Specification

Values

PV array

Number of modules in each string Number of strings in parallel Rated power at STC Maximum voltage at STC Maximum current at STC

3

FC stack

Number of modules in series Rated power Maximum voltage Maximum current

5 2.5 kW 108 V 21.3 A

Battery

Ampere hour rating Nominal voltage

400 Ah 48 V

Electrolyzer

Maximum power Maximum current

2.75 kW 110 A

Hydrogen tank

Volume Initial number of moles of H2 in tank

100 L 250 mol

Table A.3. Battery specifications.

Type Ampere hour rating Nominal voltage Fully charged voltage (No load) Charging rate

Input capacitor Output capacitor Inductor Input capacitor Output capacitor Inductor Output capacitor Inductor

Table B.1. Component ratings of the PV–FC–Battery hybrid system.

Table A.2. Fuel cell module specifications.

Rated capacity Number of cells (nfc) Operating environmental temperature Operating pressures Weight Specific heat capacity of stack CFC

Table A.6. DC–DC converters specifications.

Lead acid 400 Ah 48 V 55.2 V C/10

10 4.95 kW 105 V 47 A

Table A.4. Electrolyzer stack specifications. References Number of cells (ne) Celec

10 6.25  105 J/°C

Table A.5. Hydrogen storage tank specifications.

A0 B0 a1 b c

0.1975 atm L2/mol2 0.02096 L/mol 0.00506 L/mol 0.04359 L/mol 0.05  104 L K3/mol

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