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Photovoltaic-assisted alkaline water electrolysis: Basic principles
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 1 7 e4 1 2 4
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Photovoltaic-assisted alkaline water electrolysis: Basic principles A. Djafour a,*, M. Matoug a, H. Bouras a, B. Bouchekima a, M.S. Aida b, B. Azoui c a
Faculty of Sciences and Engineering, Laboratory LENREZA, University Kasdi Merbah of Ouargla, BP511, 30000 Ouargla, Algeria Faculty of Science, University of Constantine, Constantine 25000, Algeria c Faculty of Engineering Sciences, LEB Laboratory, University of Batna, Batna 05000, Algeria b
article info
abstract
Article history:
The purpose of this paper is to provide some general characteristics concerning the coupling
Received 4 July 2010
of a lab scale alkaline water electrolyser powered by a set of photovoltaic panels. A
Received in revised form
description of the different tests made on this system and the development of a program
26 September 2010
under Matlab for the simulation of direct coupling (photovoltaic panels e electrolyser) is
Accepted 27 September 2010
provided. Experimental results provide practical information for the coupling of the panels
Available online 28 October 2010
and the electrolysis cells. The system efficiency is low because of the low efficiency of panels and also the mismatch between the panels and the electrolyser. The simulation results of the
Keywords:
direct coupling with the various combinations of panels and the electrolyser cells according
Photovoltaic system
to the different levels of irradiation, shows the possibility to improve the system perfor-
Electrolysis
mances for a given power of panels.
Hydrogen production
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Direct coupling
reserved.
Performance Adaptation efficiency
1.
Introduction
Hydrogen production using direct solar energy as the primary energy non-fossil source and electrolyser systems can be achieved in various ways. Two main alternatives which use solar electrical energy are: (1) solar thermal electrical power generation and coupling with water electrolysis; (2) photovoltaic electrical power and coupling with water electrolysis. Hydrogen production using these processes has great potentialities in the near future because the different subsystems (thermoelectric solar power, photovoltaic generator and electrolysers) are well developed technologies. Furthermore, the latter process has the additional advantage: the direct current electrical power by a photovoltaic generator can be supplied directly to an electrolyser. Various experimentations [1e6] and theoretical studies
[7,8] have been reported in the literature on photovoltaic-electrolyser systems. In experimental studies, various technologies have been tested, system performances in actual conditions have been studied and safety issues have been addressed. In the theoretical studies, overall system performance for selected conditions has been evaluated, and the cost of hydrogen has been estimated in view of present and future technologies [9]. The photovoltaic hydrogen energy system is one of the methods expected to reduce the global warming, and substitute the fuel of fossil cell. Since that system is expensive to build, it should operate at their maximum output power levels. The PV output fluctuates with solar irradiation level, ambient temperature and load current, then in designing PV-Electrolyser System the effect of these three factors must be considered [10]. Algeria is well endowed with both conventional
* Corresponding author. Tel.: þ21329712627; fax: þ21329711975. E-mail address: [email protected] (A. Djafour). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.099
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 1 7 e4 1 2 4
Nomenclature Rs Iph Vth Is Vop Iop Voc Isc Np Ns E Eref a b PVG aq l g MPP
module series resistance, Ώ photo current, A thermal voltage, v saturation current, A module optimum voltage, v module optimum current, A module open circuit voltage, v module short circuit current, A number of parallel modules number of series modules solar irradiance, W/m2 solar irradiance for the reference, W/m2 the current change temperature coefficient, mA/ C the voltage change temperature coefficient, mV/ C photovoltaic generator water based solution liquid gas maximum power point
(non-renewable) and non conventional (renewable) sources of energy. The largest non-renewable energy source found in Algeria is fossil (i.e. oil and gas), which is being actively exploited. Renewable sources of energy are also abundant in Algeria, the most important one being the solar. Indeed, the mean yearly sunshine duration varies from a low of 2650 h on the coastal line to 3500 h in the south, the potential of daily solar energy is important. It varies from a low average of 4.66 kWh/m2 in the north to a mean value of 7.26 kWh/m2 in the south. This means that the yearly energy potential on 80% of the territory is of the order of 2650 kWh/m2. The total daily available energy is of the order of 16.56 1015 Wh [11]. The availability of solar energy is limited only on shiny days. Therefore, it is important to store the solar energy in other form of energy for the usage at night and gloomy weather. Hydrogen has been identified to be an ideal medium for this purpose with the advantages of being transportable, storable and converting energy with practically no release of environmental pollutant. In view of these considerations, this paper deals with the experimental study of hydrogen production by coupling solar photovoltaic arrays and a water electrolysis unit. A laboratory experimental setup has been designed, constructed and installed in the experimental site in Ouargla University (SouthEast of Algeria). A descriptive study of all the tests made on this system is presented. The performances of the system have been analyzed and modeled to extrapolate its performances for other configurations of direct coupling between the generator and the electrolyser cells, and to choose the most appropriate configuration. A computer simulation program has been used to study the influences of physical and meteorological parameters of Ouargla region on the operation of system, and also to determine the flow rate of hydrogen produced by the system for climate and solar data of this region.
F P Eh Idc Vdc A
module temperature, C reference module temperature, C change in module temperature, C module current, A module voltage, V generator current, A electrolyser current, A hydrogen flow rate, ml/s gas constant, 8.314 J/K mol Faraday efficiency of electrolyser, % number of series cells of electrolyser number of electrons transferred per reaction for hydrogen (Z ¼ 2) Faraday constant, 96485 coulombs/mole ambient pressure, 1.01325 105 Pa calorific value for H2, J/ml directly coupling current, A directly coupling voltage, V PV generator area, m2
2.
The solar-hydrogen system
Tmod Tmod,ref DT I V Ig Ie Q R hF Ncells Z
A solar-hydrogen system usually consists of supplying electric power to a hydrogen generator (electrolyser) by an arrangement of solar panels (photovoltaic system).
2.1.
The electrolyser
The electrolyser is a monocell system, composed of two electrodes, a cathode anode, and an electrolyte, powered by an electric generator. Potentially, the electrolyte can be either an aqueous solution, or a polymeric membrane exchanging of protons or a ceramic membrane conducting of oxygen ions. Electrical energy is needed to produce the electrolytic hydrogen from water according to the following general principles: H2 OðlÞ þ Electric energy/H2 ðgÞ þ O2 ðgÞ
(1)
In an alkaline electrolyser, the anodic and cathodic reactions are [12e14] Anode : 2OH ðaqÞ/1=2OO2 ðgÞ þ H2 OðlÞ þ 2e ;
Erev
an; 25 C
¼ 1:299 V
(2)
Cathode : 2H2 OðlÞ þ 2e /H2 ðgÞ þ 2OH ðaqÞ: Erev
ca; 25 C
¼ 0:00 V
(3)
The net reaction is : H2 OðlÞ/H2 ðgÞ þ 1=2OO2 ðgÞ; ¼ 1:299 V
Erev; 25 C (4)
The steady-state voltageecurrent characteristics of electrolysers can be found in Refs. [12,15,16] and it can be modeled as a temperature-dependent voltage source with an additional non-linear resistor placed in series that also depends on
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 1 7 e4 1 2 4
current and temperature. This resistor models the internal losses in the electrolyser. The hydrogen production rate depends linearly on the current injected into the electrolyser. A simple VeI model for a given electrolyser is given by Eq. (5). Velectrolyser ðTÞ ¼ Vstack ðTÞ þ Ielectrolyser $Relectrolyser ðT; iÞ
The PV generator
2.2.1.
The voltage of the PV generator
Iph I þ 1 Rs I Is
Voc Iph ¼ Is exp 1 Vth
(6)
(7)
The identification of Eq. (6) requires three measurements for a given solar irradiance and the array temperature: the short-circuit current, open circuit voltage and the coordinates of the optimum power point, (Iop, Vop). The voltage Vth, the saturation current (Is) are respectively identified by the following Eqs. (8) and (9): Vth ¼
Is ¼
Vop þ Rs Iop Voc I ln 1 Iop sc
!
I sc Voc Rs Isc exp exp Vth Vth
(8)
DT þ
E 1 Icc Eref
(16)
DV ¼ b DT Rs DI V ¼ DV þ Vref
(18)
I ¼ DI þ Iref
(19)
(17)
3.
Solar PV coupling to the electrolyser
Two types of PV powered electrolyser systems are possible. In the first system, the PV modules are directly connected to the electrolyser to generate hydrogen, and in the second system, the PV output is routed through a DCeDC converter to modify the voltage and current input to the electrolyser [21]. The direct coupled systems of photovoltaic and electrolysis could be of true benefit to system simplicity and at the same time be also very efficient [22].
3.1. Calculation of operating point in the direct coupled systems Graphically, the operating point of the direct coupled solarhydrogen system is defined by the intersection of the generator curve (IeV) with the (IeV) curve of the electrolyser. This coupling must meet the following conditions:
(9)
Isc ¼ Np Isc mod
(10)
Iop ¼ Np Iop mod
(11)
Voc ¼ Ns Voc mod
(12)
Vop ¼ Ns Vop mod
(13)
Ns Rs mod Np
1 e The generator voltage must be equal to the voltage of the electrolyser.
The parameters of the generator with (Ns modules in series and Np modules in parallels) identifying the Eq. (6) are related to those of a single solar module by the following relations and where the index “mod” denotes, module
Rs ¼
E Eref
where V and I are, respectively, the voltage and its corresponding current on the IeV curve.
The voltage (V)ecurrent (I ) characteristics of the modules are described by the non-linear equation Eq. (6). For modules supposed devoid of leaks (Rp infinite) this relationship is [17,18] V ¼ Vth log
DI ¼ a
(15)
(5)
where Vstack(T ) and Relectrolyser(T,i) can be obtained by an experimental setup.
2.2.
DT ¼ Tmod Tmod;ref
4119
(14)
The curve of Eq. (6) is an arbitrary reference IeV curve; it is only applicable at one particular irradiance level and cell temperature. The adaptation of Eq. (6) to define module output as a function of irradiance and module temperature is presented by the SANDSTROM model [19,20]. It can be described by the following equations:
Vth log
Iph Ig þ 1 Rs Ig ¼ Relectrolyser Ie þ Vstack Is
(20)
Relectrolyser and Vstack, are two parameters which are dependent on the electrolyser elements, and the electrolyte temperature. 2 e The generator current must be equal to the current of the electrolyser (Ig ¼ Ie ¼ Idc).
f ðIÞ ¼ Vth log
Iph Idc þ1 Relectrolyser þRs Idc Vstack Is
(21)
This non-linear equation can be resolved through the method of NewtoneRaphson.
3.2.
Power matching of PV-electrolysis systems
Optimum power transfer to the electrolyser requires operation of the solar generator at its maximum power point (MPP), this means that the MPP is the cross section of the actual
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Fig. 3 e Experimental setup. Fig. 1 e Experimental VeI curves of the electrolyser for different temperatures.
characteristic of PV generator and electrolyser. The characteristic of the PV generator is affected by insolation and generator temperature, the characteristic of the electrolyser by electrolyte temperature. For effective hydrogen generation of a PV-electrolysis system the operating point of the total system should equal the MPP of the solar generator. This is usually realized by DC/DC converters that adapt the output of the solar generator to the input of the electrolyser. Another possibility is to vary the configuration of either the solar generator or of the electrolyser [23].
3.2.1.
Power conditioning with DC/DC converters
The most effective, but cost intensive way to maximize the output of the PV field is the decoupling of the PV-electrolyser
system by use of a DC/DC converter. Thereby an MPP tracker guarantees the operation of the PV field at its MPP and a DC/DC converter shifts the power to the characteristic of the load. The maximum efficiency of commercial DC/DC converters is about 90e95% which is a disadvantage compared to the direct coupling [23].
3.2.2. Direct connection with flexible arrangement of PV configuration The operation of the PV field at different voltage levels by changing the number of PV modules connected in series and in parallel can be used to optimize the actual operating point of the system gradually in according to the MPP of the PV field. This configuration has the benefit of direct power transmission between PV field and electrolyser, but requires additional effort in system controlling. The goodness of power
Fig. 2 e Device of characterization and of coupling, PV generatoreelectrolyser.
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Table 1 e Specifications of the PV module TE500 (36 cells connected in series). Parameters of module
Symbol
Area of each module Peak power Peak power voltage Peak power current Short circuit current Open circuit voltage
Amod Pmax Vmax Imax Icc Voc
Value 1.003 0.462 m2 55 W 17.50 V 3.14 A 3.50 A 22.20 V
adaptation depends on the ratio between the voltage steps in which the System voltage level can be increased or decreased and the System voltage itself. This means in practice the actual operating point and the MPP may differ. The voltage can only be changed in steps corresponding to the voltage of a PV module. Due to the fact that commercially available PV modules have an open circuit voltage of over 20 V, the direct connection with flexible arrangement of the PV configuration is not recommended for small power [23].
3.2.3.
Direct connection with flexible cell block configuration
Similarly the cell block configuration can be changed for power adaptation by bypassing of single cells via an externally controlled switch, using the benefit of direct power transmission. Again the goodness of power adaptation depends on the ratio between voltage steps in which the system voltage level can be changed and the system voltage itself. The voltage can only be changed in steps corresponding to the voltage of a single cell of the electrolyser. However, the cell voltage is only 1/10 of the voltage of usual PV modules, so that the direct connection with flexible electrolyser configuration can be recommended for smaller power system [23]. The present paper mainly concerns the analysis of direct coupling between water electrolysis and photovoltaic generator by experimental procedure and simulation.
4.
Experimental procedure and simulation
4.1.
Characteristics of the experimental electrolyser
Fig. 5 e Measured current and voltage as a function of the time.
dimensions are (30 16 15) cm. It has a separator and two soft steel electrodes with a cross section area of 3.14 cm2, each connected to the bottom of a glass-gas compartment. Thus, hydrogen and oxygen are collected by displacing the saturated water in the U-shaped tube. The electrolyte is a 27% solution of KOH (the concentration was chosen to obtain the highest conductivity [24]). The main (IeV) characteristic curves are plotted in Fig. 1 for different temperatures of electrolyte and a picture of the system is provided in Fig. 2. Polarization curves are mostly ohmic, due to a non-optimized design of the electrolysis cell. This was sufficient however to investigate the coupling with the PV module, the main purpose of this work.
4.2. Direct coupling of the electrolyser to the PV generator A combination of photovoltaic and water electrolysis systems is utilized in this research. Use was made at the experimental site in Ouargla University (South-East of Algeria) of an available PV generator composed of two modules TE500 polycrystalline solar panels (55 W each) mounted in parallel to feed the electrolyser. These modules were placed at an optimal angle with the horizontal, while facing south for optimum collection of solar energy; see Fig. 3.
The electrolyser cell used in this work is a simple glass apparatus, locally manufactured and assembled. General
Fig. 4 e Measured electrolyser temperature and irradiance as a function of the time.
Fig. 6 e Calculated power absorbed by the electrolyser.
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Fig. 7 e Calculated hydrogen flow rate.
4.2.1.
Measuring instruments
The overall system parameters are measured accurately and recorded continuously as follows: - The PV module and the electrolyser voltages and currents are measured using digital voltmeters (METEX M-4660M) and ammeters (MULTIMETER GDM 351). - A thermocouple is used to measure the electrolyte and ambient temperatures. - A solarimeter of type MGE mode is used to measure the solar radiation intensity. The solarimeter captor is mounted at the PV module structure and parallel to the module surface. - The hydrogen flow rate is observed by displacing the saturated water in the U-shaped tube.
4.3.
Fig. 9 e Calculated overall system and electrolyser efficiencies.
Simulation
The PV module specifications provided by the manufacturer under standard test conditions (STC), that is, an average solar spectrum at air mass 1.5, irradiance normalized to 1000 W/m2, and a cell temperature of 25 C, are given in Table 1. The (IeV) characteristics are determined experimentally and by simulation for different values of irradiation and temperature. Knowledge of these values and the electrolyser parameters have enabled to determine the system operating points, then the locus of the MPP points, the volume of hydrogen and the
Fig. 8 e Calculated volume of hydrogen.
adaptation efficiency (hae) between the PV generator and the load (electrolyser) can be calculated.
5.
Measurement and simulation results
Experimental runs have been carried out for more than three years (2007e2009) with three months of works for each year. Data obtained during a sunny day’s operation are given in the following figures. In Figs. 4e6 the results of measurements for the day 17/05/2009 have been reported. In Fig. 4 the evolution of irradiance and temperature of the electrolyser are presented. The maximum solar irradiance is 806 W/m2 obtained at 12 h 30 min whereas the maximum temperature is 58 C obtained at 14 h 30 min. Fig. 5 shows the voltage and the current of the load (the electrolyser). The power absorbed by the electrolyser is shown in Fig. 6 practically, it is higher than 50 W for an interval of 3 h. By using the measured values of directly coupling current (Idc) and the electrolyte temperature Te, into the Faraday’s First Law and the ideal gas law Eq. (22), the theoretical (maximum) flow rate of hydrogen (Q) (ml/s) can be calculated by considering the Faraday’s efficiency of the electrolyser, hF ¼ 100%. By integration of this flow following the trapeze method the volume of hydrogen can be obtained during a given interval [14,25]. Figs. 7 and 8 show the evolution of hydrogen production during the day 17/05/2009. A quantity of 20.46 l was obtained
Fig. 10 e Calculated PV generator adaptation efficiency for direct coupling with monocell electrolyser.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 4 1 1 7 e4 1 2 4
Fig. 13 e Simulated performance characteristics of PVG and the electrolyser actual match with direct coupling.
Fig. 11 e Experimental VeI curve of electrolyser at Telc [ 70.05 C.
5.1.
during this day within 9 h of operation from 08:00 am to 17:00 pm. QðH2 Þ ¼
R Idc Te hF Ncells 106 ZFP
(22)
The electrolyser efficiency (he), and the overall efficiency of the system (hs) are respectively given by the following equations [26]: he ¼
Q Eh Idc Vdc
(24)
The adaptation efficiency (hae) between the PV generator and the load (electrolyser) can be expressed as:
Vdc Idc Vop Iop
Analysis and interpretation of results
The experimental tests which are realized provide the possibility to evaluate the performance and the characterization of system components. From the system and the electrolyser efficiency curves (Fig. 9), it has been noticed that the system efficiency is small due to the generator and the non-optimized electrochemical cell with a thick and poorly conducting separator, and also the mismatch between the generator and the electrolyser, see the efficiency curve of the adaptation of PV generator (Fig. 10).
(23)
Q Eh hs ¼ EA
hae ¼
4123
(25)
Fig. 12 e Simulated PV generator adaptation efficiency for direct coupling with a combination of electrolyser cells.
6. Simulation of coupling (ELePV) with other combinations To reduce losses due to the non adaptation of the system, we simulated the characteristics (IeV) of the coupling with other combinations using the developed program. The electrolyser parameters for simulation have been obtained after linear regression of data measured, see Fig. 11. The equivalent model results in: Relectrolyser ¼ 1.44 U, Vstack ¼ 1.288 V for an electrolyser temperature of 70 C. Figs. 12 and 13 show the simulation results of coupling between two modules in parallel and an electrolyser, which contains fifteen branches in parallel with eight cells in series in each branch. Fig. 12 shows that the efficiency of the generator adaptation is higher than 90% during seven operating hours. The volume of hydrogen produced is 157.99 l of hydrogen per day within 9 h of operation from 08:00 am to 17:00 pm on 17/05/2009. Fig. 13 shows the electrolyser load curve and PV system curves under different conditions of irradiance and temperature (W/m2 and C). Intersection points of these two curves represent the point of work expected when the electrolyser is coupled to the PV system. It is possible to observe in Fig. 13 that the electrolyser generates hydrogen from low irradiances to high irradiances, independently of the temperature, however for low irradiances 300 W/m2 the working point is far from the MPP, in
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which the whole efficiency of the system decreases. Then, under conditions of irradiances between 700 and 770 W/m2, the working point is near the MPP. For irradiance values above 770 W/m2, the working point is very distant from MPP point. These results show a good approximation while coupling curves of electrolyser to solar panel system directly (without auxiliary components), working between 700 and 770 W/m2 of irradiance (this irradiance range is near to average irradiance in Ouargla, Algeria).
7.
Conclusion
From measurements and simulation results, it is concluded that the power transferred from the PV generator toward the electrolyser depends mostly on climatic conditions and the mode of inter-connection between the panels and electrolysis cells. The experimental coupling between the PV generator and the electrolyser is not yet at the maximum power point. The simulation of the coupling with other electrolysers has shown the possibility to improve the system efficiency by using a switching system for a choice of the number of electrolysers to adjust the voltage and current so that the characteristic curve of the load matches the characteristic curve of a PV generator. The hydrogen generation by means of solar PV systems should be of low cost and needs minimum of components and maintenance. In this work it is shown that a direct coupling between a PV system and a stack electrolyser working near its MPP in the range of 700e770 W/m2 of irradiance is possible by a correct design of the electrolyser cells. For the Ouargla region (South-East of Algeria), all the ingredients are available for establishing solar-hydrogen energy system. This paper represents the very initial stage toward a non-fossil era. Although the experimental section is limited, the purpose of this paper is to present the very first work performed in the field of solar hydrogen for Ouargla SouthEast of Algeria. Further experimental work on a larger scale is in progress in Algeria.
Acknowledgement The authors would like to thank Professor Pierre Millet (University Paris Sud 11, France) for fruit full discussions.
references
[1] Brinner A, Bussmann H, Hug W, Seeger W. Test results of the hysolar 10 kW PV-electrolysis facility. Int J Hydrogen Energy 1992;17(3):187e97. [2] Garcı´a-Conde AG, Rosa F. Solar hydrogen production: a Spanish experience. Int J Hydrogen Energy 1993;18(12): 995e1000. [3] Daous MA, Bashir MD, El-Naggar MMA. Experience with the safe operation of a 2 kWh solar hydrogen plant. Int J Hydrogen Energy 1994;19(5):441e5.
[4] Lehman PA, Chamberlin CE, Pauletto G, Rocheleau MA. Operating experience with a photovoltaic-hydrogen energy system. Int J Hydrogen Energy 1997;22(5):465e70. [5] Szyszka A. Ten years of solar hydrogen demonstration project at Neunburg Vorm Wald, Germany. Int J Hydrogen Energy 1998;23(10):849e60. [6] Hollmuller P, Joubert J-M, Lachal B, Yvon K. Evaluation of a 5 kWp photovoltaic hydrogen production and storage installation for a residential home in Switzerland. Int J Hydrogen Energy 2000;25(2):97e109. [7] Hammache A, Bilgen E. Hydrogen production for remote communities in northern latitudes. Solar Wind Tech 1987; 4(2):139e44. [8] Friberg R. A photovoltaic solar-hydrogen power plant for rural electrification in India. Part 1: a general survey of technologies applicable within the solar-hydrogen concept. Int J Hydrogen Energy 1993;18(10):853e82. [9] Bilgen E. Solar hydrogen from photovoltaic-electrolyzer systems. Energ Convers Manag 2001;42:1047e57. [10] Soltermann OE, Da Silva EP. Comparative study between the hysolar and a hypothetical international project in Brazil for hydrogen production and exportation (BHP) from photovoltaic energy and secondary hydroelectricity combined supply. Int J Hydrogen Energy 1998;23(9):735e9. [11] Boudries R, Dizene R. Potentialities of hydrogen production in Algeria. Int J Hydrogen Energy 2008;33(17):4476e87. [12] Ulleberg Qystein. Modeling of advanced alkaline electrolyzers a system simulation approach. Int J Hydrogen Energy 2003;28:21e33. [13] Markvart T. Solar electricity. Chirchester, UK: Wiley; 1994. [14] Garcia-valverde R, Miguel C, Martinez-be´jar R, Urbina A. Optimized photovoltaic generator-water electrolyser coupling through a controlled DCeDC converter. Int J Hydrogen Energy 2008;33:5352e62. [15] Ulleberg Qystein, Morner SO. TRNSYS simulation models for solar-hydrogen systems. Sol Energ 1997;59(4e6):271e9. [16] Die´guez PM, Ursu´a A, Sanchis P, Sopena C, Guelbenzu E, Gandı´a LM. Thermal performance of a commercial alkaline water electrolyzer: experimental study and mathematical modeling. Int J Hydrogen Energy 2008;33(24):7338e54. [17] Buresch M. Photovoltaic energy systems. New York: McGraw Hill Book Company; 1983. [18] Lasnier F, Gan Eng T. Photovoltaic engineering handbook. New York, USA: Adam Hilger; 1990. [19] [Chapter 9]Solar cell array design book, vol. 11. Jet Propulsion Laboratory; 1976. [20] Hart GW. Residential photovoltaic system simulation: electrical aspects. In: 16th IEEE photovoltaic specialists conference, San Diego, CA; 1982. [21] ThomasGibson L, NelsonKelly A. Optimization of powered hydrogen production using photovoltaic electrolysis devices. Int J Hydrogen Energy 2008;33:5931e40. [22] Siegel A, Schott T. Optimization of photovoltaic hydrogen production. Int J Hydrogen Energy 1988;13(11):659e75. [23] Steeb H, Abaoud H. GermaneSaudi joint program on solar hydrogen production and utilization, Phase II 1992e1995. Stuttgart: DLR; June 1996. [24] Gilliam RJ, Graydon JW, Kirk DW, Thorpe SJ. A review of specific conductivities of potassium hydroxide solutions for various concentrations and temperatures. Int J Hydrogen Energy 2007;32:359e64. [25] Aj Bard, Faulkner LR. Electrochemical methods, fundamentals and applications. 2nd ed. New York: John Wiley & Sons; 2001. [26] Ahmad GE, El Shenawy ET. Optimized photovoltaic system for hydrogen production. Renew Energ 2006;31:1043e54.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Photovoltaic-assisted alkaline water electrolysis: Basic principles A. Djafour a,*, M. Matoug a, H. Bouras a, B. Bouchekima a, M.S. Aida b, B. Azoui c a
Faculty of Sciences and Engineering, Laboratory LENREZA, University Kasdi Merbah of Ouargla, BP511, 30000 Ouargla, Algeria Faculty of Science, University of Constantine, Constantine 25000, Algeria c Faculty of Engineering Sciences, LEB Laboratory, University of Batna, Batna 05000, Algeria b
article info
abstract
Article history:
The purpose of this paper is to provide some general characteristics concerning the coupling
Received 4 July 2010
of a lab scale alkaline water electrolyser powered by a set of photovoltaic panels. A
Received in revised form
description of the different tests made on this system and the development of a program
26 September 2010
under Matlab for the simulation of direct coupling (photovoltaic panels e electrolyser) is
Accepted 27 September 2010
provided. Experimental results provide practical information for the coupling of the panels
Available online 28 October 2010
and the electrolysis cells. The system efficiency is low because of the low efficiency of panels and also the mismatch between the panels and the electrolyser. The simulation results of the
Keywords:
direct coupling with the various combinations of panels and the electrolyser cells according
Photovoltaic system
to the different levels of irradiation, shows the possibility to improve the system perfor-
Electrolysis
mances for a given power of panels.
Hydrogen production
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Direct coupling
reserved.
Performance Adaptation efficiency
1.
Introduction
Hydrogen production using direct solar energy as the primary energy non-fossil source and electrolyser systems can be achieved in various ways. Two main alternatives which use solar electrical energy are: (1) solar thermal electrical power generation and coupling with water electrolysis; (2) photovoltaic electrical power and coupling with water electrolysis. Hydrogen production using these processes has great potentialities in the near future because the different subsystems (thermoelectric solar power, photovoltaic generator and electrolysers) are well developed technologies. Furthermore, the latter process has the additional advantage: the direct current electrical power by a photovoltaic generator can be supplied directly to an electrolyser. Various experimentations [1e6] and theoretical studies
[7,8] have been reported in the literature on photovoltaic-electrolyser systems. In experimental studies, various technologies have been tested, system performances in actual conditions have been studied and safety issues have been addressed. In the theoretical studies, overall system performance for selected conditions has been evaluated, and the cost of hydrogen has been estimated in view of present and future technologies [9]. The photovoltaic hydrogen energy system is one of the methods expected to reduce the global warming, and substitute the fuel of fossil cell. Since that system is expensive to build, it should operate at their maximum output power levels. The PV output fluctuates with solar irradiation level, ambient temperature and load current, then in designing PV-Electrolyser System the effect of these three factors must be considered [10]. Algeria is well endowed with both conventional
* Corresponding author. Tel.: þ21329712627; fax: þ21329711975. E-mail address: [email protected] (A. Djafour). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.099
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Nomenclature Rs Iph Vth Is Vop Iop Voc Isc Np Ns E Eref a b PVG aq l g MPP
module series resistance, Ώ photo current, A thermal voltage, v saturation current, A module optimum voltage, v module optimum current, A module open circuit voltage, v module short circuit current, A number of parallel modules number of series modules solar irradiance, W/m2 solar irradiance for the reference, W/m2 the current change temperature coefficient, mA/ C the voltage change temperature coefficient, mV/ C photovoltaic generator water based solution liquid gas maximum power point
(non-renewable) and non conventional (renewable) sources of energy. The largest non-renewable energy source found in Algeria is fossil (i.e. oil and gas), which is being actively exploited. Renewable sources of energy are also abundant in Algeria, the most important one being the solar. Indeed, the mean yearly sunshine duration varies from a low of 2650 h on the coastal line to 3500 h in the south, the potential of daily solar energy is important. It varies from a low average of 4.66 kWh/m2 in the north to a mean value of 7.26 kWh/m2 in the south. This means that the yearly energy potential on 80% of the territory is of the order of 2650 kWh/m2. The total daily available energy is of the order of 16.56 1015 Wh [11]. The availability of solar energy is limited only on shiny days. Therefore, it is important to store the solar energy in other form of energy for the usage at night and gloomy weather. Hydrogen has been identified to be an ideal medium for this purpose with the advantages of being transportable, storable and converting energy with practically no release of environmental pollutant. In view of these considerations, this paper deals with the experimental study of hydrogen production by coupling solar photovoltaic arrays and a water electrolysis unit. A laboratory experimental setup has been designed, constructed and installed in the experimental site in Ouargla University (SouthEast of Algeria). A descriptive study of all the tests made on this system is presented. The performances of the system have been analyzed and modeled to extrapolate its performances for other configurations of direct coupling between the generator and the electrolyser cells, and to choose the most appropriate configuration. A computer simulation program has been used to study the influences of physical and meteorological parameters of Ouargla region on the operation of system, and also to determine the flow rate of hydrogen produced by the system for climate and solar data of this region.
F P Eh Idc Vdc A
module temperature, C reference module temperature, C change in module temperature, C module current, A module voltage, V generator current, A electrolyser current, A hydrogen flow rate, ml/s gas constant, 8.314 J/K mol Faraday efficiency of electrolyser, % number of series cells of electrolyser number of electrons transferred per reaction for hydrogen (Z ¼ 2) Faraday constant, 96485 coulombs/mole ambient pressure, 1.01325 105 Pa calorific value for H2, J/ml directly coupling current, A directly coupling voltage, V PV generator area, m2
2.
The solar-hydrogen system
Tmod Tmod,ref DT I V Ig Ie Q R hF Ncells Z
A solar-hydrogen system usually consists of supplying electric power to a hydrogen generator (electrolyser) by an arrangement of solar panels (photovoltaic system).
2.1.
The electrolyser
The electrolyser is a monocell system, composed of two electrodes, a cathode anode, and an electrolyte, powered by an electric generator. Potentially, the electrolyte can be either an aqueous solution, or a polymeric membrane exchanging of protons or a ceramic membrane conducting of oxygen ions. Electrical energy is needed to produce the electrolytic hydrogen from water according to the following general principles: H2 OðlÞ þ Electric energy/H2 ðgÞ þ O2 ðgÞ
(1)
In an alkaline electrolyser, the anodic and cathodic reactions are [12e14] Anode : 2OH ðaqÞ/1=2OO2 ðgÞ þ H2 OðlÞ þ 2e ;
Erev
an; 25 C
¼ 1:299 V
(2)
Cathode : 2H2 OðlÞ þ 2e /H2 ðgÞ þ 2OH ðaqÞ: Erev
ca; 25 C
¼ 0:00 V
(3)
The net reaction is : H2 OðlÞ/H2 ðgÞ þ 1=2OO2 ðgÞ; ¼ 1:299 V
Erev; 25 C (4)
The steady-state voltageecurrent characteristics of electrolysers can be found in Refs. [12,15,16] and it can be modeled as a temperature-dependent voltage source with an additional non-linear resistor placed in series that also depends on
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current and temperature. This resistor models the internal losses in the electrolyser. The hydrogen production rate depends linearly on the current injected into the electrolyser. A simple VeI model for a given electrolyser is given by Eq. (5). Velectrolyser ðTÞ ¼ Vstack ðTÞ þ Ielectrolyser $Relectrolyser ðT; iÞ
The PV generator
2.2.1.
The voltage of the PV generator
Iph I þ 1 Rs I Is
Voc Iph ¼ Is exp 1 Vth
(6)
(7)
The identification of Eq. (6) requires three measurements for a given solar irradiance and the array temperature: the short-circuit current, open circuit voltage and the coordinates of the optimum power point, (Iop, Vop). The voltage Vth, the saturation current (Is) are respectively identified by the following Eqs. (8) and (9): Vth ¼
Is ¼
Vop þ Rs Iop Voc I ln 1 Iop sc
!
I sc Voc Rs Isc exp exp Vth Vth
(8)
DT þ
E 1 Icc Eref
(16)
DV ¼ b DT Rs DI V ¼ DV þ Vref
(18)
I ¼ DI þ Iref
(19)
(17)
3.
Solar PV coupling to the electrolyser
Two types of PV powered electrolyser systems are possible. In the first system, the PV modules are directly connected to the electrolyser to generate hydrogen, and in the second system, the PV output is routed through a DCeDC converter to modify the voltage and current input to the electrolyser [21]. The direct coupled systems of photovoltaic and electrolysis could be of true benefit to system simplicity and at the same time be also very efficient [22].
3.1. Calculation of operating point in the direct coupled systems Graphically, the operating point of the direct coupled solarhydrogen system is defined by the intersection of the generator curve (IeV) with the (IeV) curve of the electrolyser. This coupling must meet the following conditions:
(9)
Isc ¼ Np Isc mod
(10)
Iop ¼ Np Iop mod
(11)
Voc ¼ Ns Voc mod
(12)
Vop ¼ Ns Vop mod
(13)
Ns Rs mod Np
1 e The generator voltage must be equal to the voltage of the electrolyser.
The parameters of the generator with (Ns modules in series and Np modules in parallels) identifying the Eq. (6) are related to those of a single solar module by the following relations and where the index “mod” denotes, module
Rs ¼
E Eref
where V and I are, respectively, the voltage and its corresponding current on the IeV curve.
The voltage (V)ecurrent (I ) characteristics of the modules are described by the non-linear equation Eq. (6). For modules supposed devoid of leaks (Rp infinite) this relationship is [17,18] V ¼ Vth log
DI ¼ a
(15)
(5)
where Vstack(T ) and Relectrolyser(T,i) can be obtained by an experimental setup.
2.2.
DT ¼ Tmod Tmod;ref
4119
(14)
The curve of Eq. (6) is an arbitrary reference IeV curve; it is only applicable at one particular irradiance level and cell temperature. The adaptation of Eq. (6) to define module output as a function of irradiance and module temperature is presented by the SANDSTROM model [19,20]. It can be described by the following equations:
Vth log
Iph Ig þ 1 Rs Ig ¼ Relectrolyser Ie þ Vstack Is
(20)
Relectrolyser and Vstack, are two parameters which are dependent on the electrolyser elements, and the electrolyte temperature. 2 e The generator current must be equal to the current of the electrolyser (Ig ¼ Ie ¼ Idc).
f ðIÞ ¼ Vth log
Iph Idc þ1 Relectrolyser þRs Idc Vstack Is
(21)
This non-linear equation can be resolved through the method of NewtoneRaphson.
3.2.
Power matching of PV-electrolysis systems
Optimum power transfer to the electrolyser requires operation of the solar generator at its maximum power point (MPP), this means that the MPP is the cross section of the actual
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Fig. 3 e Experimental setup. Fig. 1 e Experimental VeI curves of the electrolyser for different temperatures.
characteristic of PV generator and electrolyser. The characteristic of the PV generator is affected by insolation and generator temperature, the characteristic of the electrolyser by electrolyte temperature. For effective hydrogen generation of a PV-electrolysis system the operating point of the total system should equal the MPP of the solar generator. This is usually realized by DC/DC converters that adapt the output of the solar generator to the input of the electrolyser. Another possibility is to vary the configuration of either the solar generator or of the electrolyser [23].
3.2.1.
Power conditioning with DC/DC converters
The most effective, but cost intensive way to maximize the output of the PV field is the decoupling of the PV-electrolyser
system by use of a DC/DC converter. Thereby an MPP tracker guarantees the operation of the PV field at its MPP and a DC/DC converter shifts the power to the characteristic of the load. The maximum efficiency of commercial DC/DC converters is about 90e95% which is a disadvantage compared to the direct coupling [23].
3.2.2. Direct connection with flexible arrangement of PV configuration The operation of the PV field at different voltage levels by changing the number of PV modules connected in series and in parallel can be used to optimize the actual operating point of the system gradually in according to the MPP of the PV field. This configuration has the benefit of direct power transmission between PV field and electrolyser, but requires additional effort in system controlling. The goodness of power
Fig. 2 e Device of characterization and of coupling, PV generatoreelectrolyser.
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Table 1 e Specifications of the PV module TE500 (36 cells connected in series). Parameters of module
Symbol
Area of each module Peak power Peak power voltage Peak power current Short circuit current Open circuit voltage
Amod Pmax Vmax Imax Icc Voc
Value 1.003 0.462 m2 55 W 17.50 V 3.14 A 3.50 A 22.20 V
adaptation depends on the ratio between the voltage steps in which the System voltage level can be increased or decreased and the System voltage itself. This means in practice the actual operating point and the MPP may differ. The voltage can only be changed in steps corresponding to the voltage of a PV module. Due to the fact that commercially available PV modules have an open circuit voltage of over 20 V, the direct connection with flexible arrangement of the PV configuration is not recommended for small power [23].
3.2.3.
Direct connection with flexible cell block configuration
Similarly the cell block configuration can be changed for power adaptation by bypassing of single cells via an externally controlled switch, using the benefit of direct power transmission. Again the goodness of power adaptation depends on the ratio between voltage steps in which the system voltage level can be changed and the system voltage itself. The voltage can only be changed in steps corresponding to the voltage of a single cell of the electrolyser. However, the cell voltage is only 1/10 of the voltage of usual PV modules, so that the direct connection with flexible electrolyser configuration can be recommended for smaller power system [23]. The present paper mainly concerns the analysis of direct coupling between water electrolysis and photovoltaic generator by experimental procedure and simulation.
4.
Experimental procedure and simulation
4.1.
Characteristics of the experimental electrolyser
Fig. 5 e Measured current and voltage as a function of the time.
dimensions are (30 16 15) cm. It has a separator and two soft steel electrodes with a cross section area of 3.14 cm2, each connected to the bottom of a glass-gas compartment. Thus, hydrogen and oxygen are collected by displacing the saturated water in the U-shaped tube. The electrolyte is a 27% solution of KOH (the concentration was chosen to obtain the highest conductivity [24]). The main (IeV) characteristic curves are plotted in Fig. 1 for different temperatures of electrolyte and a picture of the system is provided in Fig. 2. Polarization curves are mostly ohmic, due to a non-optimized design of the electrolysis cell. This was sufficient however to investigate the coupling with the PV module, the main purpose of this work.
4.2. Direct coupling of the electrolyser to the PV generator A combination of photovoltaic and water electrolysis systems is utilized in this research. Use was made at the experimental site in Ouargla University (South-East of Algeria) of an available PV generator composed of two modules TE500 polycrystalline solar panels (55 W each) mounted in parallel to feed the electrolyser. These modules were placed at an optimal angle with the horizontal, while facing south for optimum collection of solar energy; see Fig. 3.
The electrolyser cell used in this work is a simple glass apparatus, locally manufactured and assembled. General
Fig. 4 e Measured electrolyser temperature and irradiance as a function of the time.
Fig. 6 e Calculated power absorbed by the electrolyser.
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Fig. 7 e Calculated hydrogen flow rate.
4.2.1.
Measuring instruments
The overall system parameters are measured accurately and recorded continuously as follows: - The PV module and the electrolyser voltages and currents are measured using digital voltmeters (METEX M-4660M) and ammeters (MULTIMETER GDM 351). - A thermocouple is used to measure the electrolyte and ambient temperatures. - A solarimeter of type MGE mode is used to measure the solar radiation intensity. The solarimeter captor is mounted at the PV module structure and parallel to the module surface. - The hydrogen flow rate is observed by displacing the saturated water in the U-shaped tube.
4.3.
Fig. 9 e Calculated overall system and electrolyser efficiencies.
Simulation
The PV module specifications provided by the manufacturer under standard test conditions (STC), that is, an average solar spectrum at air mass 1.5, irradiance normalized to 1000 W/m2, and a cell temperature of 25 C, are given in Table 1. The (IeV) characteristics are determined experimentally and by simulation for different values of irradiation and temperature. Knowledge of these values and the electrolyser parameters have enabled to determine the system operating points, then the locus of the MPP points, the volume of hydrogen and the
Fig. 8 e Calculated volume of hydrogen.
adaptation efficiency (hae) between the PV generator and the load (electrolyser) can be calculated.
5.
Measurement and simulation results
Experimental runs have been carried out for more than three years (2007e2009) with three months of works for each year. Data obtained during a sunny day’s operation are given in the following figures. In Figs. 4e6 the results of measurements for the day 17/05/2009 have been reported. In Fig. 4 the evolution of irradiance and temperature of the electrolyser are presented. The maximum solar irradiance is 806 W/m2 obtained at 12 h 30 min whereas the maximum temperature is 58 C obtained at 14 h 30 min. Fig. 5 shows the voltage and the current of the load (the electrolyser). The power absorbed by the electrolyser is shown in Fig. 6 practically, it is higher than 50 W for an interval of 3 h. By using the measured values of directly coupling current (Idc) and the electrolyte temperature Te, into the Faraday’s First Law and the ideal gas law Eq. (22), the theoretical (maximum) flow rate of hydrogen (Q) (ml/s) can be calculated by considering the Faraday’s efficiency of the electrolyser, hF ¼ 100%. By integration of this flow following the trapeze method the volume of hydrogen can be obtained during a given interval [14,25]. Figs. 7 and 8 show the evolution of hydrogen production during the day 17/05/2009. A quantity of 20.46 l was obtained
Fig. 10 e Calculated PV generator adaptation efficiency for direct coupling with monocell electrolyser.
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Fig. 13 e Simulated performance characteristics of PVG and the electrolyser actual match with direct coupling.
Fig. 11 e Experimental VeI curve of electrolyser at Telc [ 70.05 C.
5.1.
during this day within 9 h of operation from 08:00 am to 17:00 pm. QðH2 Þ ¼
R Idc Te hF Ncells 106 ZFP
(22)
The electrolyser efficiency (he), and the overall efficiency of the system (hs) are respectively given by the following equations [26]: he ¼
Q Eh Idc Vdc
(24)
The adaptation efficiency (hae) between the PV generator and the load (electrolyser) can be expressed as:
Vdc Idc Vop Iop
Analysis and interpretation of results
The experimental tests which are realized provide the possibility to evaluate the performance and the characterization of system components. From the system and the electrolyser efficiency curves (Fig. 9), it has been noticed that the system efficiency is small due to the generator and the non-optimized electrochemical cell with a thick and poorly conducting separator, and also the mismatch between the generator and the electrolyser, see the efficiency curve of the adaptation of PV generator (Fig. 10).
(23)
Q Eh hs ¼ EA
hae ¼
4123
(25)
Fig. 12 e Simulated PV generator adaptation efficiency for direct coupling with a combination of electrolyser cells.
6. Simulation of coupling (ELePV) with other combinations To reduce losses due to the non adaptation of the system, we simulated the characteristics (IeV) of the coupling with other combinations using the developed program. The electrolyser parameters for simulation have been obtained after linear regression of data measured, see Fig. 11. The equivalent model results in: Relectrolyser ¼ 1.44 U, Vstack ¼ 1.288 V for an electrolyser temperature of 70 C. Figs. 12 and 13 show the simulation results of coupling between two modules in parallel and an electrolyser, which contains fifteen branches in parallel with eight cells in series in each branch. Fig. 12 shows that the efficiency of the generator adaptation is higher than 90% during seven operating hours. The volume of hydrogen produced is 157.99 l of hydrogen per day within 9 h of operation from 08:00 am to 17:00 pm on 17/05/2009. Fig. 13 shows the electrolyser load curve and PV system curves under different conditions of irradiance and temperature (W/m2 and C). Intersection points of these two curves represent the point of work expected when the electrolyser is coupled to the PV system. It is possible to observe in Fig. 13 that the electrolyser generates hydrogen from low irradiances to high irradiances, independently of the temperature, however for low irradiances 300 W/m2 the working point is far from the MPP, in
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which the whole efficiency of the system decreases. Then, under conditions of irradiances between 700 and 770 W/m2, the working point is near the MPP. For irradiance values above 770 W/m2, the working point is very distant from MPP point. These results show a good approximation while coupling curves of electrolyser to solar panel system directly (without auxiliary components), working between 700 and 770 W/m2 of irradiance (this irradiance range is near to average irradiance in Ouargla, Algeria).
7.
Conclusion
From measurements and simulation results, it is concluded that the power transferred from the PV generator toward the electrolyser depends mostly on climatic conditions and the mode of inter-connection between the panels and electrolysis cells. The experimental coupling between the PV generator and the electrolyser is not yet at the maximum power point. The simulation of the coupling with other electrolysers has shown the possibility to improve the system efficiency by using a switching system for a choice of the number of electrolysers to adjust the voltage and current so that the characteristic curve of the load matches the characteristic curve of a PV generator. The hydrogen generation by means of solar PV systems should be of low cost and needs minimum of components and maintenance. In this work it is shown that a direct coupling between a PV system and a stack electrolyser working near its MPP in the range of 700e770 W/m2 of irradiance is possible by a correct design of the electrolyser cells. For the Ouargla region (South-East of Algeria), all the ingredients are available for establishing solar-hydrogen energy system. This paper represents the very initial stage toward a non-fossil era. Although the experimental section is limited, the purpose of this paper is to present the very first work performed in the field of solar hydrogen for Ouargla SouthEast of Algeria. Further experimental work on a larger scale is in progress in Algeria.
Acknowledgement The authors would like to thank Professor Pierre Millet (University Paris Sud 11, France) for fruit full discussions.
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