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Comparative evaluation of different power management strategies of a stand-alone PV/Wind/PEMFC hybrid power system

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Electrical Power and Energy Systems 34 (2012) 81–89
Contents lists available at SciVerse ScienceDirect
Electrical Power and Energy Systems
journal homepage: www.elsevier.com/locate/ijepes
Comparative evaluation of different power management strategies of a stand-alone
PV/Wind/PEMFC hybrid power system
Erkan Dursun ⇑, Osman Kilic
Marmara University, Technical Education Faculty, 34722 Goztepe, Istanbul, Turkey
a r t i c l e
i n f o
Article history:
Received 7 January 2011
Received in revised form 15 August 2011
Accepted 20 August 2011
Available online 1 November 2011
Keywords:
Stand-alone hybrid power system
Energy management
Battery energy efficiency
a b s t r a c t
This study presents different power management strategies of a stand-alone hybrid power system. The
system consists of three power generation systems, photovoltaic (PV) panels, a wind turbine and a proton
exchange membrane fuel cell (PEMFC). PV and wind turbine is the main supply for the system, and the
fuel cell performs as a backup power source. Therefore, continuous energy supply needs energy storing
devices. In this proposed hybrid system, gel batteries are used. The state of charge (SOC), charge-discharge currents are affecting the battery energy efficiency. In this study, the battery energy efficiency
is evaluated with three different power management strategies. The control algorithm is using MatlabSimulinkÒ.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The demand for new and environmentally friendly energy system is growing worldwide. Wind and solar energy systems are taking the biggest share from, this current trend [1]. To increase the
energy reliability, wind and solar energy are used as dual energy
sources. However, seasonal climatic conditions and geographic conditions affect the wind-solar energy output [2,3]. Therefore, a third
energy system is needed to improve the energy supply reliability.
Thus, the PEM fuel cell ideally fulfills the need for any start up power.
When the wind-solar system energy output is insufficient, the fuel
cell backups the supply system. However, fuel cell lifetime is less
than 2000 h for transportation and 20,000 h for stationary fuel
cells [4]. Frequent start-up and shutdown actions degrade the electrolyzer and the fuel cell performance [5]. In addition, battery
charge-discharge cycle and battery bank energy efficiency gains
importance. Therefore, improved energy management strategies
are proposed, and Matlab/Simulink simulation results are presented. The proposed strategies are implemented as a case study
to a mobile house for two-member family designed by UNIDOICHET. The evaluation of the power management strategy performance is evaluated using real weather data for the region of
installation.
In the literature, there are a few studies related to power management of hybrid power systems. Ipsakis et al. have proposed a
power management strategy for hydrogen production performance
⇑ Corresponding author. Tel.: +90 216 3365770; fax: +90 216 337 89 87.
E-mail addresses: [email protected] (E. Dursun), [email protected]
(O. Kilic).
0142-0615/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijepes.2011.08.025
and system efficiency [6]. Onar et al. proposed a power management
strategy algorithm which dealt with a hybrid (wind turbine/PV/fuel
cell) power system containing a ultra capacitor bank [7]. Ahmed
et al. proposed Power management strategy studied power fluctuations on a hybrid power system [8]. Mohamed and Koivo have
proposed an optimization and simulation algorithm for the microgrid system containing a wind turbine, a micro turbine, a diesel
generator, a photovoltaic array, a fuel cell and a battery storage
[9].
In this study, three new power management strategies are proposed. Their effects on battery bank energy efficiency and PEMFC
membrane life span is truly investigated. The paper is organized
as follows. Section 2, describes of the hybrid power system. Section
3, the structure of the battery bank charge–discharge currents. Finally, presented power management strategies and control
algorithms.
2. Hybrid power system
The hybrid system consists of three power generation systems,
photovoltaic (PV) arrays, a wind turbine and a fuel cell. The PV and
wind turbine are used as the main power generation system for the
system and the fuel cell is assigned as a backup power generator
for the continuous power supply. The hybrid power system consists of an 8 1 array each with a 100 W PV panel, a 1 kW wind
turbine, and a 2 kW fuel cell. The hybrid power system and data
acquisition setup given Figs. 1 and 2. Energy flow route is given
at Fig. 3.
The control system of the hybrid power system uses the fuel cell
as a backup power. The electrolyzer produces hydrogen for storage.
This hydrogen is kept in a hydrogen tank. This system is installed
82
E. Dursun, O. Kilic / Electrical Power and Energy Systems 34 (2012) 81–89
where In, Vn, and Dn are the current, voltage and duty cycle of
each appliance used during the day, respectively and Ed shows
the total energy demand for the system. The average daily energy
consumption of the system is calculated as 4220 Wh (Table 1). The
total power, 2695 W, declares the maximum instantaneous power,
which the inverter should meet; to maintain the stability of the energy supply, an inverter rated at least 2295 W is required.
Daily average AC loads of the system is shown Fig. 6.
2.2. System components
PV panels are composed of poly-Si PV modules. The commonly
used three types of solar PV system are amorphous, monocrystalline, polycrystalline [12]. Polycrystalline silicon (poly-Si) solar cells
are receiving significant attention in recent years due to the potential of reduced manufacturing cost [13]. The technical specifications of the PV panels are at Table 2.
The PV panels have a tilt angle of 41° with respect to optimum
panel tilt angle according to the latitude of the region’s geographical location [15,16].
The power output of the photovoltaic module is based on current and voltage as follows [17],
a- Wind turbine (Zephyr Air dolphin)
b- PV panels (Solera)
c- Weather station (Davis Vantage Pro2)
Fig. 1. The hybrid power system [10].
PPV ¼ Y PV fPV
2- DC-DC converter for the PV panels
3- DC power supplies
4- Fuses
5- Voltage transducers
6- Current transducer for AC loads
7- DAQ (Data acquisition) NIcFP-180x
½1 þ ap ðT C T C;STC Þ
ð2Þ
1
qAC p ðk; hÞV 3
2
Pw ¼
9- Current transducer for PV panels
q is the air density (kg m3), A the rotor sweep area (m2), Cp the
0
0
in Istanbul, Turkey (41°33 N; 28°59 E). The wind-solar data used in
the study were taken from the weather station for the year 2009
[11]. Solar radiation and wind speed data of the Istanbul are shown
in Figs. 4 and 5.
2.1. Load profile
The load profile of a two-person family is established for annual
power consumption. The load profile of the system is analyzed to
ensure that energy source generates sufficient energy during the
whole year. Eq. (1) shows the estimation of average daily energy
consumption.
In V n Dn
ð3Þ
power coefficient, a function of tip speed ratio (k) and pitch angle
(h), and V is the wind velocity (m s1)Energy output from the turbine can be calculated by
Fig. 2. Data acquisition setup.
i¼1
GT;STC
8- Current transducer for the battery bank
10- DC bus
n
X
!
PPV is the output power of the PV array (kW), YPV the rated capacity
of the PV array, meaning its power output under standard test conditions (kW), fPV the PV derating factor (%), GT the solar radiation
incident on the PV array (kW m2), GT;STC incident radiation at standard test conditions (1 kW m2), ap the temperature coefficient = 0.004 °C1, Tc the PV cell operation temperature (°C), TC,STC
PV cell temperature under standard test conditions (25 °C).
Thermocouples used for PV panel back surface temperatures are
shown in Fig. 7.
Monthly averages PV panel’s output power at Fig. 8.
The wind turbine generates 1 kW of rated power with a permanent magnet synchronous generator. Wind turbine AC/DC converter
is a built in device. The Wind turbine technical specifications are in
Table 3 [18].
Electricity obtained from the wind turbine can be calculated
using the wind speed data.
The instantaneous power produced from wind is [19,20];
1- DC-DC converter for the PEMFC
Ed ¼
GT
ð1Þ
Ew ¼
n
X
P i ðwÞt
ð4Þ
i¼1
where n is the number of hours in the period such as year, season or
month, t is 1 h. Monthly results are given at Fig. 9.
PEMFC is a common type of fuel cell, and traditionally uses
hydrogen [21]. PEMFC has been regarded as the most adequate
system as a power source for many portable electric devices. PEMFC could also be a sustainable alternative for the power generation
in zero-emission automotive applications as well as for stationary
power stations [22,23]. PEMFC technical specifications are at Table
4 [24]. The PEM air-cooled fuel cell is from the FutureE model
Jupiter the rated power 2 kW.Peak values show the load demand
current rate with respect to time.
A daily PEMFC power output at Fig. 10. The fuel cell was not active in February and July from Fig. 11. Hence, the wind turbine and
E. Dursun, O. Kilic / Electrical Power and Energy Systems 34 (2012) 81–89
83
Fig. 3. Energy flow route of the system.
Fig. 4. Monthly average global solar radiation of Istanbul.
Fig. 5. Monthly average wind speed of Istanbul.
PV panels supplied the total load demand. The battery bank consists of eight batteries each of 200 Ah 12 V. The battery bank capacity is 19.2 kW h. The usable capacity is 9.6kWh, which is sufficient
for 1.8 days of autonomy. The hydrogen tank has 10 kg of capacity,
which is enough to absorb all produced hydrogen, and are connected to the fuel cell. The system is composed of a 0.4 kW electrolyzer and a 3.5 kW inverter. Electrolyzer specifications are shown
in Table 5.
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E. Dursun, O. Kilic / Electrical Power and Energy Systems 34 (2012) 81–89
Table 1
Calculated energy consumption of the system.
Appliance
Power (W)
Air conditioner
Refrigerator
LCD TV
Electrolyzer
Lighting
Water heater
Total
a
560
70
65
400
100
1500
2695
Used/day (h)
a
5
8a
2a
0.5
2a
0.22
Table 2
PV panel parameters [14].
Energy used/day (Wh)
Maximum power (W)
Maximum current (A)
Short circuit current (A)
Open circuit voltage (V)
Number of cells and type
Weight (kg)
Length (mm)
Depth (mm)
Efficiency (%)
2800
560
130
200
400
330
4220
100
5.86
6.44
21.4
36 poly-Si
12
1490
35
16
Discrete work hours.
3. The battery energy efficiency
Usually, two indexes, the state-of-charge (SOC) and the terminal
voltage mainly characterize a lead–acid battery. Besides, the
charge or discharge time and the current value are required [26].
SOC ¼ SOC 0 þ
Z t
t0
Capbat
Ibat
Ibat
dt
Capbat
ð5Þ
battery capacity ðAhÞ
the battery current ðAÞ
where SOC0 the battery SOC of the starting point; t0 and t, are
the time of the starting point and the time of interest, respectively.
Kattakayam and Srinivasan [27] recommended through trial and
error and pro-longed experimentations that 50% < SOC < 80%
would be ideal working range for the lead–acid batteries. The lower limit is in tune with the findings of Lancashire [28] and Calloway
[29], which observed the life of the battery bank, expands to 4000
charges/discharges cycles at 60% SOC or to 2000 cycles at 40% SOC.
The upper limit arises out of heating effects and gassing.
Fig. 12 shows the annually SOC variation of the battery bank.
Most storage systems are not ideal, losses occur in charging and
discharging cycles during storing periods [30,31].
4. Discharge and charge currents
The value, at any time, of the battery charge and discharge currents will vary according to the excess or shortage of local power
available. However, it is sensible, through a charge controller, to
limit the discharge and charge rates to maximum values to protect
Fig. 7. Thermocouples used for PV panel operation temperature.
the battery and ensure an efficiently operation [32–34]. When the
fuel cell does not work, the battery current can be calculated as
follows.
Ibat0 ¼
pPV greg1 þ Pwind greg2 P dem =ginv erter
V bat
ð6Þ
PPV is the PV output power (kW), Pwind the wind turbine output
power (kW), greg1 the PV panel’s regulator efficiency (%) = 90%, greg2
wind turbine’s regulator efficiency (%) = 90%, pdem the demand
power (kW), ginverter the inverter efficiency (%) = 92%, Vbat battery
bank voltage (V), Pfc fuel cell output power (kW), and gcon is the fuel
cell’s converter efficiency (%) = 90%.
Fig. 6. Daily average AC loads curve in February, 2009.
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E. Dursun, O. Kilic / Electrical Power and Energy Systems 34 (2012) 81–89
Fig. 8. Monthly averages PV panels output power.
Table 4
PEM fuel cell parameters.
Table 3
Wind turbine parameters.
Wind turbine type
Rotor diameter (mm)
Weight (kg)
Blade number
Generator type
Cut-in (m s1)
Cut-off (m s1)
Nominal power (W)
Maximum power (W)
Output voltage
Vertical axis
1800
17.5
3
Permanent magnet synchronous
2.5
50
1000
3200 (20 m s1)
25 VDC
Fuel cell type
PEM
Nominal power (kW)
Number of cells
Output voltage
Efficiency (%)
Consumption
2
70
48 VDC
50
22 slpm
1400
1200
Ibat00
pPV greg1 þ Pwind greg2 þ Pfc gcon Pdem =ginv erter
¼
V bat
ð7Þ
1000
Power [W]
If battery current is positive, the battery is discharging. If battery current is negative, the battery is charging.
When fuel cell is working, battery current;
800
600
400
200
5. Power management strategies
0
The main decision factors for the power management strategies
are the level of the power provided by the renewable energy system (wind-solar) and the state of charge (SOC) of the battery bank.
The battery bank or the fuel cell should be capable of providing the
needed power. Power energy generated by wind turbine (Pwind) and
Time [min]
Fig. 10. PEMFC output power in a day.
PV panels (Psolar) is summed up at renewable energy system power
(Pres). Load (Pload) is subtracted from the Pres and the excess power
Fig. 9. Monthly average wind turbine output power.
86
E. Dursun, O. Kilic / Electrical Power and Energy Systems 34 (2012) 81–89
Fig. 11. Monthly average PEMFC output power.
Fig. 12. Annual SOC [%] of the battery bank.
Table 5
Electrolyzer specifications [25].
Maximum power (W)
Hydrogen flow rate (slh1)
Max outlet pressure
Purity
Operating temperature
Storage temperature
Efficiency
400
60
10.5 bar
99.9999%
15–40 °C
2–60 °C
35%
(Pexcess). The stand-alone system is composed from renewable
power sources. Therefore, the power management strategies became even more complex. The algorithms for strategy1, strategy2
and strategy3 are given at Figs. 13–15 respectively. At strategy1,
SOCmax > SOC > SOCmin and Pexcess > 0, the electrolyzer will run and
the battery bank will discharge. When, SOC 6 SOCmin and hydrogen
tank pressure P 4, the fuel cell will run and the battery bank will
be charged (Fig. 13). At strategy2, SOCmax > SOC > SOCmin and
Pexcess 6 0, the fuel cell will not run, and the battery bank will be
discharged. When, SOC 6 SOCmin, the battery bank will charged
(Fig. 14). At strategy3, SOCmax > SOC > SOCmin and Pexcess > 0, the
electrolyzer will run, and the battery bank will be charged. If,
SOC 6 SOCmin or Pexcess 6 0, the fuel cell will run, and battery bank
discharges (Fig. 15). The best result for the battery bank energy
Fig. 13. Power management strategy1 for the hybrid power system.
efficiency was obtained from the strategy3 algorithm. In strategy3,
renewable energy system (RES) power is then directed to the electrolyzer for hydrogen production. This strategy aims to protect the
battery bank from overcharging. The developed algorithms have
E. Dursun, O. Kilic / Electrical Power and Energy Systems 34 (2012) 81–89
87
degrade their performance and possibly reduce their lifespan with
strategy3.
The battery bank energy efficiency is obtained with Eq. (8). Hybrid power system algorithm for power management is shown
Fig. 16. The control subsystem driven from Eq. (8) is shown at
Fig. 17.
The battery energy efficiency (gb) of the battery is expressed as
the ratio between the output energy from the battery, Eout
(kW h year1), and the total inputs, Ein (kW h year1).
gb ¼
Eout
100%
Ein
ð8Þ
Eout ¼ P out t
Fig. 14. Power management strategy2 for the hybrid power system.
Fig. 15. Power management strategy3 for the hybrid power system.
improved overall efficiency closed to 80–85% range (Fig. 18). Thus,
this efficiency range is well above than previously proposed results
[35]. The energy efficiency of the battery bank was between 73.1%
and 84% [36,37] from previous studies. Frequent start-up and shut
down actions for the electrolyzer, and the fuel cell will eventually
ð9Þ
Ein ¼ Pin t
ð10Þ
Pout V bat Ibatdischarge
ð11Þ
Pin ¼ V bat Ibatcharge
ð12Þ
Ibatcharge the charge current of the battery bank, Ibatdischarge the discharge current of the battery bank, Pout instantaneous output power
of the battery bank, Vbat instantaneous voltage of the battery bank.
The higher is the total energy efficiency of the battery. The lower is the cost of charging the battery. Therefore, the control algorithm is needed to keep the battery efficiency is as high as
possible. The energy efficiency of the battery is affected by the
charging and discharging currents.
All three strategies have fully supplied the connected loads.
Since, the battery bank is a bit oversized, the supply capacity for
the system is high. The PEMFC start-up number is less in strategy3
(Fig. 19). This improves the fuel cell membrane lifetime. The PEMFC hydrogen consumption reached it is lowest level with strategy3.
Thus, the PEMFC energy production is also at its minimum. Even
though the hydrogen consumption is at its minimum, the energy
need is supplied continuously. PMS3 resulted in the highest hydrogen production because the entire RES power was directed to the
electrolyzer whenever the maximum SOC limit was reached. However, the largest variability in the SOC for the accumulator caused
the fuel cell to operate more intensively. Therefore, hydrogen
inventory was depleted during the four-month time period leading
to significant deficit in hydrogen. The PEMFC running time at PM
strategy3 is shown in Fig. 20.
Ò
Fig. 16. Hybrid power system algorithm in Matlab/Simulink .
88
E. Dursun, O. Kilic / Electrical Power and Energy Systems 34 (2012) 81–89
Fig. 17. Control subsystem.
6. Conclusions
Fig. 18. Comparison of the annual battery bank energy efficiency.
100
PEMFC electrical production
(kWh)
PEMFC start-up number
90
80
Using stand-alone wind-solar energy system has become popular in recent years. Stand-alone power system depends on the geographical and meteorological conditions of the installed region.
Therefore, the wind turbine and solar cells may not meet the energy demand. So, a third power supply source might be needed.
This source should not be affected from any geographical or meteorological conditions. PEMFC is an ideal power generation system
for such implementations. However, the price of PEMFC is high
and its membrane lifetime is short. Thus to increase the operation
time of the membrane and to enable the continuous energy flow,
three power management strategies are proposed. The proposed
power management strategies for the hybrid power system satisfy
the load and battery bank SOC. Battery bank’s maximum and minimum SOC levels are determining the operation of the fuel cell. All
these strategies have enhanced the energy efficiency of the battery
bank, and the results are compared at Fig. 18. The best result for
energy efficiency is obtained with strategy3. The battery bank’s energy efficiency has reached up to 85% with this proposed power
management algorithm.
70
60
50
Acknowledgments
40
30
20
10
0
PM strategy1
PM strategy2
PM strategy3
Fig. 19. Annual electrical production and start-up number of the PEMFC.
The project is supported by UNIDO-ICHET (United Nations
Industrial Development Organization – International Centre for
Hydrogen Energy Technologies) with the mission of raising public
awareness for renewable energy and demonstrating viable implementations of hydrogen energy technologies in developing countries. Partial support from NANOCOFC project under EC-FP6
program is also acknowledged. Special thanks to Dr. M. Suha Yazıcı
and Mehmed Eroglu from UNIDO-ICHET.
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Fig. 20. The PEMFC running time at PM strategy3.
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