Techno-economic analysis of wind turbine–PEM (polymer electrolyte membrane) fuel cell hybrid system in standalone area

Energy 67 (2014) 381e396

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Techno-economic analysis of wind turbineePEM (polymer electrolyte membrane) fuel cell hybrid system in standalone area Sahand Rahimi a, Mousa Meratizaman a, Sina Monadizadeh b, Majid Amidpour a, * a Department of Energy System Engineering, Faculty of Mechanical Engineering, K. N. Toosi University of Technology, No. 15, Pardis St., MolaSadra Ave., Vanak Sq., P.O. Box 11365-4435, Tehran, Iran b Faculty of Industrial Engineering, Islamic Azad University, North Tehran Branch, P.O. Box 19388-13646, Tehran, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 August 2013 Received in revised form 26 December 2013 Accepted 19 January 2014 Available online 22 February 2014

Providing reliable, environmentally friendly, and affordable energy has been a goal for many countries throughout the world. Hydrogen is presented as new energy sources during the last years which can be utilized instead of fossil fuel. One of the most promising clean methods of obtaining hydrogen is using renewable sources like wind and solar energy via electrolysis. In present work, a techno-economic evaluation of wind-hydrogen hybrid system (wind turbine, electrolysis, and PEM (polymer electrolyte membrane) fuel cell) in household size will be considered. In order to save the extra energy of wind turbine, electrolysis is used to convert this energy into hydrogen chemical energy. Generated hydrogen is stored in hydrogen storage tank. PEM fuel cell is applied to convert chemical energy of hydrogen into electrical power with high efficiency when extra power is required. Results show that wind energy in Manjil and Binaloud (two cities which have wind power plant in Iran) has greater wind speed in comparison with other cities. Also result shows that in standalone application, the size of wind turbine is bigger than the on-grid one to supply the full load consumption and it makes the standalone application too expensive. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Windehydrogen hybrid system PEM fuel cell Exergy and economic analysis

1. Introduction Solar and wind are considered as the most preferred renewable energy sources for their availability and inexhaustibility [1]. But because of periodic characteristics of natural resources, it has been a challenge to generate a highly reliable power with PV (photovoltaic) modules or wind turbines. To solve this problem, intermediate energy sources can be used to reduce power production fluctuations. So when extra power is produced, it is converted to intermediate energy sources. In power shortage condition, this intermediate source is used to supply the energy consumers. Hydrogen can be utilized as intermediate energy sources. Several studies have been reported about hydrogen production from renewable energies during the recent years. C.J. Greiner et al. studied the hydrogen production from wind energy in Norwegian case study. Results showed that for isolated system, price of hydrogen is about 8.26 US$ per kilogram [2]. R.J. Mantz et al. worked on a new idea which used idle electrical generation of wind turbine for hydrogen production [3]. D. Honnery et al. worked on estimation of the global hydrogen production from

* Corresponding author. Tel.: þ98 912 1055614. E-mail address: [email protected] (M. Amidpour). 0360-5442/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2014.01.072

wind energy. Generated hydrogen is considered to export to energypoor regions [4]. B. Olateju et al. studied the hydrogen production from wind energy in Western Canada. The generated hydrogen is used for upgrading the bitumen from oil sands [5]. A.U. Chavez et al. worked on a hybrid power plant (solar/wind/hydrogen) based on artificial intelligence in remote area in Mexico [6]. Most of these researches focus on the hydrogen generation process. But one of the important matters is converting the generated hydrogen into electrical energy when the extra power is needed. As mentioned before, hydrogen can be generated as intermediate energy sources when extra power is available. When power shortage takes place, stored hydrogen converts into electrical energy by using PEM (polymer electrolyte membrane) fuel cell with high efficiency. PEM technology was invented at General Electric in the early 1960s. GE (General Electric) announced an initial success in mid-1960 when the company developed a small fuel cell for a program with the U.S. Army Signal Corps. The cell was compact and portable, but its platinum catalysts were expensive. NASA initially researched about PEM fuel cell technology for Project Gemini in the early U.S. space program. Batteries were used for the preceding Project Mercury missions, but Project Apollo required a power source that would last a longer amount of time. Unfortunately, the first developed PEM cells had repeated difficulties with the internal cell contamination and leakage of oxygen

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Nomenclature A CO2 Cp Eact,i 0 ECell Eelec EH2 Eheat; H2 O Eheat,PEM ENernst F g H2,used I i imax J J0,i n O2, used

swept area (m2) and active area of membrane oxygen concentration power coefficient, a function of tip speed ratio and pitch angle activation energy for electrodes open circuit voltage exergy rates of Qelec exergy content of hydrogen exergy rates of Qheat; H2 O exergy rates of Qheat,PEM thermodynamic potential of the cell (V) Faraday constant gravity constant (m/s2) molar hydrogen usage per second current of the cell current density (A/cm2) maximum cell current density (A/cm2) current density exchange current density number of cells in the stack molar oxygen usage per second

through the membrane. GE redesigned their fuel cell and the new model performed adequately for the rest of the Gemini flights. The designers of project Apollo and the Space Shuttle ultimately chose to use alkali fuel cells. Special characteristics of PEM fuel cell make it possible to commercialize PEM fuel cell in vehicle industry, household application and distributed power generation instead of aerospace application [7]. In present work, a techno-economic evaluation of winde hydrogen hybrid system (wind turbine, electrolysis, and PEM fuel cell) in household size will be considered. Windehydrogen hybrid system has been addressed by many researches. Most of them considered the reliable power supply in remote areas. K. Agbossou et al. studied the renewable energy systems based on hydrogen for remote applications. Experimental analysis had been investigated in mentioned study [8]. M. Ni et al. considered the potential of renewable hydrogen production for energy supply in Hong Kong. Solar, wind and biomass were introduced as hydrogen sources [9]. J.G. Carton et al. presented the wind/ hydrogen hybrid system as an opportunity for Ireland sustainable energy supply. Hydrogen engine and PEM fuel cell were installed in experimental set up to evaluate the proposed system [10]. T. Niknam et al. considered a microgrid containing wind/PV and fuel cell from probabilistic energy and operating management points of view [11]. They investigated the self-adaptive charge system in a renewable microgrid with hydrogen production [12]. The presented study has some unique aspects in comparison with available literature. ➢ Most of available literature focused on components modeling or consumption section, but in this article, both of energy producer and consumer are investigated thermodynamically (energy and exergy analysis) and economically simultaneously. ➢ Each component is simulated exactly and commercial codes are not used. ➢ Proposed system is suggested for residential and standalone area.

P0 standard sea level atmospheric pressure (pa) PH2 pressures of the hydrogen PO2 pressures of the oxygen Qelec rate of electrical input energy Qheat,PEM rate of thermal input energy Qheat; H2 O rate of thermal input energy of the heating the input water R specific gas constant for air (J/kmol) and maximum rotor radius (m) Rohm ohmic resistance of a cell T temperature (K) V wind velocity (m/s) and voltage of stack (V) Vcell voltage of the cell Vr local wind velocity z altitude above sea level (m) Greek letter efficiency activation voltage drop (V) of the anode and cathode ohmic voltage drop air density (kg/m3) air density as a function of altitude (kg/m3)

h hact hohm r r(z)

A kilowatt range wind turbine is chosen for obtaining wind energy in five given different points in Iran. In order to save the extra energy of wind turbine, electrolysis is used to convert this energy into hydrogen chemical energy. Generated hydrogen is stored in hydrogen storage tank. When extra power is needed, a PEM fuel cell is applied to convert chemical energy of hydrogen into electrical power with high efficiency. Finally by using simulation results, the energy, exergy and economic analysis are carried out. The proposed system configuration is shown in Fig. 1. 2. Modeling and simulation 2.1. Wind energy in five given points Wind is the second source of renewable energies for power generation in Iran [13]. Based on studies carried out, Iran is a country with 6 m/s average wind speed. In some regions, there are more appropriate and consistent wind for power generation. In this study, real wind data of many cities of Iran have been collected and analyzed. Five cities are chosen which have suitable wind potential. Fig. 2 shows the monthly average wind speed from 2007 to 2012 in these five cities. Average wind speed of selected cities is presented in Table 1. Most of the installed wind turbines of the country are located in Manjil and Binaloud. 2.2. Wind turbine modeling Wind turbines convert the kinetic energy of the wind into electrical energy by rotating the blades. Greenpeace states that about 10% of required electricity can be supplied by the wind until 2020 [14]. The fundamental wind-power equation is used for estimating extractable power from any moving fluid e.g. air [15]:

Pw ¼

1 rAV 3 2

(1)

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Fig. 1. Proposed system configuration.

The density of air depending on the temperature and altitude above sea level can be computed from equation (2) [16].

rðzÞ ¼ P0 =ðRTÞexpð  gz=ðRTÞÞ

(2)

The instantaneous power produced from wind is [17]: =

Pw ¼

1

2

rACp V 3

(3)

The theoretical optimum for utilizing the power of wind by reducing its velocity was first discovered by Betz, in 1926. According to Betz, the theoretically maximum power that can be extracted from the wind is: =

1

2

rAV 3 CP:Betz ¼

=

PBetz ¼

1

2

rAV 3 0:59

(4)

Hence, even if a power extraction without any wastes would be possible, only 59% of the wind power could be utilized by a wind

turbine. Betz did not consider the impact of unavoidable swirl waste [18]. There are lots of complex formulas that are used to find the relation between wind speed and height, as these are too complicated to have any usage for general engineering. The most common of these simpler expressions is power law and is exhibited as [19]:




V2 V1




¼

h2 h1

(5)

where V2 is the known wind speed measured at height h2; h1 is the height at which the wind speed is estimated. a is the friction coefficient of the surface and available from Ref. [20]. From Equation (5), it is clear that wind speed is dependent on the height of turbine hub and varies by the terrain condition. Tip speed ratio is a factor that presents the ratio of tip speed of the wind turbine to speed of the wind as expressed in equation (6). The tip speed is used instead of the rotational speed of rotor [21]. The tip speed ratio, l of a rotor is defined as:

l ¼ UR=Urmax

Fig. 2. Monthly average wind velocity from 2007 to 2012.

a

(6)

To make optimal use of the available wind power, it is necessary to change the rotor speed U in proportion to the wind speed Urmax to hold the maximum value of Cp as the wind speed varies [22]. Tip speed ratio has a major effect on turbine performance because it controls the angle of attack of the blades. For turbines with a low tip speed ratio, e.g. the American farm windmill with l z 1, the swirl wastes reduce the maximum power coefficient, CP,max to approximately (0.42). There are many formulas and definitions to determine the power coefficient based on different simplifications and methods. Here, the more prevalent one is explained. In this study, the power

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Table 1 Annual average wind speed (m/s) in selected cities during the last five years.

Table 2 Parametric coefficients for fuel cell model.

Cities

Annual average wind speed (m/s)

x1

x2

x3

x4

Binaloud Hendijan Manjil Zabol Tehran

6.60 4.36 6.40 6.82 4.36

0.948

0.00312

7.6e5

1.93e4

The maximum value of wind speed which is appropriate for wind turbine is determined 20 m/s in this paper. Finally the new power output is presented by equation (11).

coefficient (Cp) is a function of the pitch angle (q) of rotor blades and the tip speed ratio (l). The determination of the power coefficient requires blade element theory and blade geometry science. These complicated issues are empirically involved. In this paper, the numerical approximation is followed, so the power coefficient is defined by equation (7) [23]:

P ¼ Pð1  TCÞ


 121 2:14 Cp ¼ 0:73  0:58q  :002q  13:2 e18:4=li

Producing hydrogen via a water electrolyzer provides a promising and clean way to store and better utilize the renewable energy resources [24]. One advantage of electrolysis is producing high purity hydrogen (especially in fuel cell-vehicle applications) [25]. The modeling of the electrolyzer consists of the relationship between electrical energy consumption and the produced hydrogen. In general, electrochemical cells operate at low voltage. Even when they are connected in series in typical industrial applications, the total voltage across the stack would normally not exceed 50e100 V direct current [26]. The current, however, can range up to several hundreds of amperes depending on the actual size of each cell. The electrical input power is known as:

li

(7)

Here, li is a variable which is defined in order to simplify the equation (7) [23].

li ¼

1   0:003 3

1 ðl0:02qÞ

(8)

q þ1

The output power increases by growing wind speed. Therefore, a control system is required in order to control the tip speed of turbine that is vital factor of safety. Many control systems are used around the world, but a concise model is presented here. One of the methods to control the wind turbine is changing the pitch angel of the blades. The calculation of pitch angel is complicated and difficult and often is obtained by experimental methods. Fig. 3 shows pitch angle variation of blades with the wind speed which is between 13 m/s and 25 m/s in order to avoid over-rated power of the turbine. A creative function equation (9) is used to correct the output power. In this method, TC correction function is used instead of changes in the value of pitch angel.

8 <

TC ¼ 0  2 : TC ¼ tanðqÞ  ðV10:6Þ  0:55 V 0:2

V < 10:6m=s V  10:6m=s

(9)

2.3. Electrolyzer

P ¼ V I

(12)

Pcell ¼ Vcell  I

(13)

Therefore, by determining the value of input voltage, current will be calculated. In order to omit the area effect, using the current density is recommended. It is expressed as:

J ¼ I=A

(14)

where The cell voltage that should be prepared is defined as [27]: 0 Vcell ¼ Ecell þ hact;ða;cÞ þ hohm

where V is the wind speed and q is derived from equation (10).


V q ¼ p 1 Vmax

(11)

(10)

(15)

The model takes various open circuit voltage into account in the calculation of cell polarization. It is calculated by using Nernst equation which considers the effect of temperature and species concentration on the cell. Mostly, the value of E0 is assigned 1.23 V, which is only true at standard temperature and pressure [28]. 0 Ecell

¼

0 Erev

ð3Þ

 0:9  10

p2ðH2 Þ ,pðO2 Þ RT ln ðT  298Þ þ 4F pðH2 OÞ

!

(16) T is the temperature of the electrolyzer. The reversible potential reflects the thermodynamic effect of electrochemical reaction,

Table 3 Different terms of exergy. Physical exergy Chemical exergy

ePh  h0 Þ  T0 ðsi  s0 Þ xi ¼ ðh Pi P eCh xi;j ~εi;j þ RT0 xi;j Lnxi;j xi ¼

Work exergy Heat exergy

eW ¼ W eQ ¼ Q(1  T0/Tb)

j

Fig. 3. Pitch angle variation with wind speed.

j

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Table 5 Input data for cooling and heating load calculation. Parameter

Assumption

Type of windows Height from floor to ceiling Filtration of air-circulation Appliance

Double pane, W strip 8.9 ft 1.5 For bedroom: 400 W For hall: 500 W 2300 Btu/h From walls: 0.094 Btu/h ft3 From partitions: 0.5 Btu/h ft3

Latent and sensible heat gain for kitchen Heat transition coefficient

Fig. 4. Scheme of hydrogen production system.

where PH2 ; PO2 ; and PH2 O are the partial pressures of hydrogen, oxygen and water vapor respectively. In this study the effect of pressure is disregarded and a temperature dependent value of reversible cell voltage is used.

0 Erev

¼ 1:229  0:9  10

3

ðTel  298Þ

2

hact;i

J0;i ¼


Eact;i RT

Jiref exp

(21)

JA cell N_ H2 ¼ 2F

(22)

And the produced hydrogen by the stack:

nJA N_ H2;in  N_ H2;out ¼ 2F

(23)

nJA N_ H2 ¼ 2F

(24)

(17)

Although, the reversible potential of water electrolysis at 25  C is 1.229 V, the water dissociation potential is influenced by the catalyst activity of electrodes [29]. The activation overvoltage is based on electrode kinetics at the reaction site. In other words, the activation overvoltage is a measure of the electrodes activity. This overvoltage can be presented by ButlereVolmer equation [24].

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 !2 u u RT 6 J J 7 t ln4 ¼ þ þ 15 F 2J0;i 2J0;i

JA cell cell N_ H2;in  N_ H2;out ¼ 2F

i ¼ a; c

Similarly, at the anode side, oxygen is the only gas which is transported in the porous electrode layer. At the anodeeelectrolyte interface, the rate of O2 diffused away from the interface equals to O2 generation rate in a steady state. The flow rate of H2O and O2 of each cell at the electrolyzer outlet can be calculated as:

JA cell N_ O2;out ¼ 4F

(25)

JA cell cell N_ H2 Oout ¼ N_ H2 Oin  2F

(26)

(18)

Then for the stack will be:

i ¼ a; c

(19)

The activation overvoltage is extremely affected for these values which depend on the used electrocatalyst, electrode morphology, age, pressure, temperature and other factors. Value of Jiref at reference temperature must be chosen from literature, from manufacture’s information or obtained empirically by polarization curve fitting from experimental data [30]. Ohmic overvoltage is the overvoltage because of ohmic resistance. These include the electronic wastes due to the resistance of bipolar plates, electrode, current collector, etc. [31].

nJA N_ O2;out ¼ 4F

(27)

nJA N_ H2 Oout ¼ N_ H2 Oin  2F

(28)

Inlet water flow rate is assumed to be known parameter. In order to calculate the energy efficiency, it is necessary to specify the input and output energies. It is obvious that the output energy is the chemical energy of the produced hydrogen, but the other side is more complicated. :

hohm ¼ J  Rohm

(20)

According to Faraday’s law, the amount of produced hydrogen (in mol/s) in electrolyzer for each cell in series is [32]:

Table 4 Climate and geographic specifications of 5 selected areas.

(29)

where HHVH2 is the higher heating value of H2.

2.4. PEM fuel cell modeling

City

Climate

Latitude

Longitude

Tehran Manjil Binaloud Zabol Hendijan

Semi-arid Mediterranean Cold Semi-arid Hot Semi-arid Humid subtropical

35 36 35 31 30

51 49 59 61 49

N N N N N

hen

HHVH2  NH2 ¼ Qelec þ Qheat;PEM þ Qheat;H2 O

E E E E E

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of H2 and O2 directly into electrical energy, with water and heat as the only by-products. The low operating temperature allows for quick start-up and the high power density and mechanically robust construction make PEM fuel cells an attractive Replacement for the I.C. (internal combustion) engine [33].

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Fig. 5. The plan of considered building.





In order to model a fuel cell stack some parameters are required to fit the model. Although, most of the parameters are obtained from the manufacturer’s data sheet, a few are still required from experimentation and from the available literature. The output voltage of a single cell can be defined as the result of the following expression [34]:

hohm ¼ 0:01605  3:5  105  T þ 8  105  i  i

Vfc ¼ ENernst  hact  hohm  Vcon

hcon ¼ B ln 1 

(30)

ENernst is presented as following equation.

(31) PH2 and PO2 are calculated via StefaneMaxwell equation. hact can be calculated as follow [35].









The concentration polarization loss caused by the inability of the oxygen and hydrogen gases to diffuse at a sufficient speed through the porous components of the cell is given by Andujar [39].





i imax

; B ¼

RT 2F

(35)

For a stack containing N cells, the output voltage and power are given respectively by:

  ENernst ¼ 1:229  0:85  103  ðT  298:15Þ      

þ 4:308  105  T  ln PH2 þ 1=2 ln PO2

hact ¼  x1 þ x2  T þ x3  T  ln CO2 þ x4  T  lnðiÞ

(34)

(32)

Vstack ¼ N  Vfc

(36)

Pstack ¼ Vstack  I

(37)

To find a relation between current density and hydrogen consumption, the following equations are used.

H2used ¼ 2O2used ¼

NAfc I 2F

(38)

x is parametric coefficients and it has specific quantity for each fuel cell. In this study, the following coefficients are used [36] (see Table 2). The oxygen concentration can be calculated by using the following equation [37].

CO2 ¼

PO2 5:08 

106

  exp 498 T

(33)

hohm is a measure of the ohmic voltage drop associated with the conduction of the protons through the solid electrolyte and electrons through the internal electronic resistances [38]:

Table 6 Economic data of considered system. Component

Cost

Lifetime (years)

Wind turbine Wind tower (more than 2 kW) Wind tower (less than 2 kW) Electrolyzer Compressor Hydrogen tank PEM fuel cell

1100 $/kW [44] 250 $/m [46] 100 $/m 1500 $/kW [47] 400 $/kW [48] 500 $/kg [47] 2000 $/kW [47]

20 25 25 10 15 25 10

[45] [46] [47] [47] [48] [47] [47]

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3.1. Exergy analysis of wind turbine The exergy efficiency of a wind turbine is usually characterized by its power coefficient as given below. The maximum possible value of CP is 0.5926 according to the Betz criterion.

CP ¼

I,v

hmechanic ,halternative ,0:5rpR2 Vr3

¼

Wu Wa

(40)

The exergy content of the blowing air is equal to the kinetic energy as equation below [41].

Exergy of kinetic energy ¼ availability ¼ ke1 ¼

Vr2 2

(41)

In general, the energy balance equation for a wind turbine can be represented by:

ke1 ¼ wu þ ke2

(42)

To determine the available power, amount of air passing through the rotor is required. Assuming standard atmospheric conditions (25  c, 101 kPa) in this study, the density of air is 1.18 kg/ m3 and its mass flow rate is:

_ ¼ rAVr ¼ rpR2 Vr m

(43)

The portion of incoming kinetic energy is not converted to electric power and leaves the wind turbine as outgoing kinetic energy. Noting that the mass flow rate remains constant, the exit velocity can be determined by using equation (44).

    _ 1 1  Cp /0:5mV _ 2 ¼ mke _ 12 1  Cp ¼ 0:5mV _ 22 /V2 mke qffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ V1 1  Cp

Fig. 6. Validation of wind turbine developed model.

O2used ¼

Pstack 4F  Vstack

(44)

(39)

Thus, the available power is calculated by using equation (45).

_ 1Þ available power ¼ Wa ¼ ðmke 3. Exergy analysis Exergy can be defined as the quality of energy which combines the first and second law of thermodynamics. Therefore, it is more appropriate measure for analyzing energy process. Exergy is always evaluated with respect to a reference environment (dead state), a restricted form of equilibrium where only the conditions of mechanical and thermal equilibrium must be satisfied. This state of the system is called the restricted dead state. In this article, the temperature T0 and pressure P0 of the environment are taken as standard-state values such as 25  C and 101,325 pa [40]. The exergy of a stream can be divided into different terms. In the absence of nuclear, magnetism, electricity and surface tension effects, the specific total exergy is the sum of four terms. Table 3 shows these terms. Only hydrogen production system is considered in exergy analysis. Fig. 4 shows the scheme of hydrogen production system.

(45)

This is the maximum power available to the wind turbine. Today, most of the wind turbines use about 20e40% of the kinetic energy of the wind. Exergy efficiency, useful work (Wu) and exergy destruction (losses) can be calculated by using the equations (46)e(48) respectively. The exergetic efficiency of a turbine is defined as a measure of how well the stream exergy of the fluid is converted into inverter work output. Applying this to the wind turbines, exergy efficiency is obtained as:

ε ¼

We We ¼ _ _  Ex Wu Ex 1 2

(46)

Table 7 Specifications of the GenHyÒ1000. Number of cells Temperature Membrane type Membrane thickness Membrane active area

12 50  C NAFIONÒ N117 177.8 mm 250 cm2

Fig. 7. Voltage versus current density of GenHyÒ1000 and the developed model.

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Table 8 Parametric coefficients for BCS 500 W PEM fuel cell.

Table 9 Characteristics of hydrogen production system.

Parameter

Coefficient

Number of cells Temperature Area (cm2) H2 pressure (atm) O2 pressure (atm)

32 333 64 1 0.2095

where the useful work is:

Wu ¼ ðP1  P2 Þ,

_ m

19 m 104 kW 40 m 15  C 65  C 0.8 178 mm 0.25 square meters 20

(47)

r

The exergy losses are defined as:

_ Ex dest ¼ Lex ¼ Wu  We ¼

Wind turbine Rotor diameter Nominal power Height of tower Electrolyzer Inlet water temperature Outlet water temperature Hydrogen exchange factor Membrane thickness Area of each cell Number of cell





_ _  Ex Ex 1 2  We

_ 1 ðex2  ex1 Þ m _ W

(52)

_ m _ 1 ðex2  ex1 Þ E_ x;dest;C ¼ W C

(53)

hexe;C ¼ (48)

C

3.2. Exergy analysis of PEM electrolyzer The exergy balance of the system is shown in equation (49). Ein and Eout represent the input and output rate of exergy while, Edis and Edes express the exergy dissipation and exergy destruction rates respectively. The rate of exergy loss is expressed in equation as well:

Ein ¼ Eout þ Eloss

(49)

Eloss ¼ Edis þ Edes

(50)

Edes is associated with the destruction of exergy due to irreversibilities and exergy loss Edis represents the exergy of streams which are thrown to the ambient. The exergy efficiency of a PEM electrolyzer can be expressed as: :

hex ¼

EH2  N H2 Eout ¼ Ein Eelec þ Eheat;PEM þ Eheat;H2 O

(51)

4. Sizing and economic 4.1. System sizing The size of the system is considered in this stage. The products of system are electrical energy (kWh) and hydrogen (kg). This part of the study deals with sizing of considered system and its economic analysis. For this purpose, five different climate areas are chosen. The variety of the climate and high potential for wind energy are the main reasons for choosing these five points. The climatic and geographic specifications of selected points are shown in Table 4. The energy consumption in a one-hundred square meters residential unit is calculated in this section. Cooling and heating load of the building and required hot water are calculated for four people consumption. The standard hot water consumption per capita could be found in literature [43]. Cooling and heating load of the

3.3. Compressor To calculate the exergy efficiency and destruction of compressor the following equations are used [42].

Fig. 8. Comparison between BCS 500 W and the developed model.

Fig. 9. Exergy analysis of wind turbine in various wind speeds.

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Table 10 Size of wind turbine in selected cities. City

Tehran

Manjil

Zabol

Binaloud

Hendijan

Turbine diameter (m)

31

24

19

16

27

the fifteenth day of each month. Now by using this wind speed, size of wind turbine is calculated for each month to supply the maximum power requirement. Size of wind turbine is determined by the length of rotor blades. Now, average of blade length plus 30% (as the safety factor) is chosen as final size of blade in selected cities. In rated wind speed of 13 m, the nominal power production of wind turbine is calculated.

4.1.2. Size of PEM electrolyzer Maximum of extra energy consumption in each month is chosen as an electrolyzer input energy. Working time for electrolyzer is assumed to be 8 h a day. Fig. 10. Exergy analysis of electrolyzer in various wind speeds.

building are computed by Carrier Commercial software. Input data and other assumptions are shown in Table 5. To calculate the energy consumption for lighting according to geographic characteristics of each selected area, time of darkness during 24 h is considered. Overlapping index (0.75) is used to achieve the real time of lighting per day. To determine other required energy consumptions such as refrigerator, television, etc., the commercial catalogs are used. It is supposed that this building is in a standalone area so it is not possible to receive electrical energy from the grid power. It is assume that extra electrical energy is converted into hydrogen and can be sold to local consumer. According to these assumptions, the power system should be designed for maximum load. Fig. 5 shows the plan of the considered building. The size of proposed system is chosen by attention to maximum required energy of each selected city. The effect of climate characteristics on size of system can be explained as follow:  Average monthly temperature is used in energy requirement calculation  Cooling and heating loads are different in each climate

4.1.3. Size of compressor Maximum hydrogen production on the fifteenth day of each month is chosen as the mass flow of compressor. Working time of compressor is assumed to be 8 h a day. It should be noted that the hydrogen compressor needs electrical power for compression so it is considered as an electrical consumer. Try and error method is applied to consider the electrical power consumption of compressor in size of wind turbine.

4.1.4. Size of hydrogen tank Maximum produced hydrogen in a day is calculated for computing the size of hydrogen storage tank capacities. Double of computed capacities is chosen as final capacities of hydrogen storage tank in each city.

4.1.5. Size of PEM fuel cell Maximum energy shortage on fifteenth day of each month is calculated. PEM fuel cell should supply this energy shortage in about 8 h.

4.2. Economic analysis Size of each component is calculated according to below. 4.1.1. Size of wind turbine From wind speed data during five last years, monthly average wind speed is calculated and considered as average wind speed on

Economic analysis for proposed system is examined in this section. The economical approach according to the concept of ACS (annualized cost of system), is developed in this study. ACS is composed of annualized capital cost Cacap, annualized replacement cost Carep and annualized maintenance cost Camain. Then ACS of the system can be expressed accordingly by:

ACS ¼ Cacap ðWind Turbine þ Wind Tower þ Electrolyser þ Compressor þ Hydrogen Tank þ PEM fuel cellÞ þ Carep ðWind Turbine þ Wind Tower þ Electrolyser þ Compressor þ Hydrogen Tank þ PEM fuel cellÞ þ Camain ðWind Turbine þ Wind Tower þ Electrolyser þ Compressor þ Hydrogen Tank þ PEM fuel cellÞ (54) Fig. 11. Exergy destruction and exergy efficiency in hydrogen production system.

Table 6 shows the economic data which is used in this section [44e48].

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Table 11 Average energy production and consumption on the fifteenth day of each month (kWh). Cities

Tehran (31 m)

Month

Consumption

Production

Consumption

Manjil (24 m) Production

Consumption

Zabol (19 m) Production

Consumption

Production

Consumption

Production

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total

140 104.47 57.41 28.44 349.71 365.45 362.56 345.38 30.34 65.11 110.56 142.74 2102.17

250.4368 354.0148 418.237 490.9605 419.0261 335.0744 241.3969 186.1158 185.5036 234.8958 241.944 195.145 3552.751

105.6 69.67 36.81 321.54 344.31 355.55 349.86 332.08 30.64 57.21 96.46 115.74 2215.47

61.24003 105.3673 399.0595 489.9202 973.4682 1311.513 1311.513 1311.513 1276.82 351.3676 55.12079 43.7098 7690.611

127.5 96.27 52.31 27.44 364.21 379.35 377.26 361.58 26.74 53.41 97.96 131.74 2095.77

80.25528 139.4617 141.821 257.1018 509.5364 821.9723 821.9723 821.9723 815.0725 247.8099 103.2594 71.40439 4831.639

139.6 103.77 57.41 28.34 339.21 355.15 352.16 334.58 31.24 66.41 111.96 142.64 2062.47

230.1232 269.1134 267.0159 270.0987 206.8164 483.5548 582.8945 471.0465 440.2887 166.9964 150.5681 132.335 3670.852

92.4 61.87 34.41 344.74 368.61 378.65 373.66 357.68 27.14 42.81 81.46 102.94 2266.37

147.0381 217.4284 198.8465 349.2633 436.1889 470.8948 377.7102 280.0441 197.0986 167.587 156.6609 170.9611 3169.722

(55)

i0  f 1þf

i

(58)

ð1 þ iÞYrep  1

where Crep is replacement cost of the component in US$, Yrep is component lifetime in year and SFF is sinking fund factor which is a ratio to calculate the future value of a series of equal annual cash flows. 4.2.3. Annualized maintenance cost System maintenance cost is deemed to be constant every year and it is related to the lifetime of components. In this study, it is assumed as 2.5%e6.66% of each component capital cost.

where Ccap is initial capital cost of each components, Yproj is the project lifetime, CRF is capital recovery factor which is the ratio to calculate the present value of an annuity (series of equal annual cash flows). The annual real interest rate i is related to the nominal 0 interest rate i and the annual inflation rate f. Annual interest rate is calculated according to:

i ¼

Hendijan (27 m)

  Carep ¼ Crep ,SFF i; Yrep ¼ Crep ,

4.2.1. Annualized capital cost Annualized capital cost of each component (wind turbine, wind tower, electrolyzer, compressor, hydrogen tank and PEM fuel cell unit) is [48]:

  i,ð1 þ iÞYproj Cacap ¼ Ccap ,CRF i; Yproj ¼ Ccap , ð1 þ iÞYproj  1

Binaloud (16 m)

4.2.4. NPV (net present value) Net present value is the present value of installing and operating the system over its lifetime in the project and it is referred to lifecycle cost. NPV is calculated according to equation (59) [50]:

(56)

In Iran, the nominal interest rate and the annual inflation rate as referred in April 2012 are 20% and 17% respectively. Therefore, the annual real interest rate of 2.56% is used in this simulation [49].

NPV ¼

(59)

where ACS is the annualized cost of system ($/year) which includes capital, replacement, annual operating and maintenance. CRF is capital recovery factor which is a ratio to calculate the present value of series of equal annual cash flows, i is the real interest rate (%) and Yproj is the project lifetime (in year).

4.2.2. Annualized replacement cost Annualized replacement cost is the annualized value of all replacement costs occurring throughout the lifetime of the project. To do this, first future cost of each component should be calculated by using following equation:

Crep ¼ Ccap ðIn Base YearÞ,ð1 þ iÞYrep

ACS ð1 þ iÞYproj  1   ¼ ACS, CRF i; Yproj i,ð1 þ iÞYproj

4.2.5. LCOP (levelized cost of product) Levelized cost of product is the average cost per unit ($/unit of product) of useful total products. It is calculated as follows:

(57)

Summation of these costs is equal to Crep. Then, by using the following equation, Carep is calculated.

LCOP ¼

ACS Annual output product of the system

(60)

Table 12 Characteristics of calculated electrolyzer in selected cities. Cities

Inlet water temperature ( C)

Outlet water temperature ( C)

Heat exchanger factor

Membrane thickness (micro meters)

Area (square meters)

Number of cells

Max power (W)

Tehran Manjil Zabol Binaloud Hendijan

15 15 15 15 15

65 65 65 65 65

0.8 0.8 0.8 0.8 0.8

178 178 178 178 178

0.05 0.05 0.05 0.05 0.05

12 30 20 10 5

58,000 156,000 99,000 52,000 22,000

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Table 13 Characteristics of PEM fuel cell in selected cities.

Tehran Manjil Zabol Binaloud Hendijan

Area (square meters)

Temp ( C)

Number of cells

H2 pressure (atm)

O2 pressure (atm)

Max current (A/cm2)

Max H2 consumption (mol/s)

Max power (W)

0.05 0.05 0.05 0.05 0.05

65 65 65 65 65

80 35 30 20 37

10 10 10 10 10

0.2 0.2 0.2 0.2 0.2

1 1 1 1 1

0.2073 0.0907 0.0777 0.0518 0.0959

21,700 9500 8100 5500 10,050

Fig. 12. Hydrogen production and consumption on the fifteenth day of each month in selected cities.

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Fig. 13. Portion of wind turbine and PEM fuel cell in power supply of the considered building in selected cities.

In this equation, ACS is annualized cost of system ($/year) and the denominator is the total annual products (unit of product/year). For wind/hydrogen hybrid system, product is defined as an electrical energy (kWh). It should be noted that extra hydrogen can be sold to local market and its value minus from ACS.

5. Results and discussion 5.1. Results of system modeling 5.1.1. Wind turbine modeling validation To validate the developed wind turbine model, the compar-

ison with two commercial ones is examined. Rotor diameter is equal to 33.4 m for Enercon E-33 and 101 m for Enercon E-101 (Fig. 6).

5.1.2. PEM electrolyzer modeling validation In order to check the reliability of the developed model which is presented, a GenHy PEM electrolyzer is considered. GenHy PEMÒ is a research program on PEM water electrolysis supported by the European commission [51]. Specifications of the GenHyÒ1000 are presented in Table 7. Fig. 7 shows voltage versus current density of GenHyÒ1000 and the developed model. This figure is generated by increasing power up to 2600 W approximately.

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5.3.1. Size of wind turbine According to wind turbine sizing procedure which is explained before, Table 10 shows the size of wind turbine (rotor diameter) in selected cities. Now, by using the calculated size of wind turbine in selected cities, Table 11 shows the average energy production and consumption on the fifteenth day of each month during a year. It should be noted that the average time of wind turbine activation is assumed to be 6e10 h a day according to wind flow availability in selected cities. 5.3.2. Size of PEM electrolyzer According to PEM electrolyzer selection which is mentioned before, Table 12 shows the characteristics of electrolyzers in selected cities. Fig. 14. Extra annual hydrogen and its value (5 US$ per kilogram) in selected cities.

5.1.3. PEM fuel cell modeling validation Presented model for PEM fuel cell is validated by using a 500 W PEM fuel cell (BCS). Technical details of BCS product are shown in Table 8 [52]. Fig. 8 shows the comparison results of presented model and the commercial one.

5.2. Results of exergy analysis Table 9 shows the characteristics of hydrogen production system which is used in exergy evaluation. According to exergy analysis relations which are explained in this section, Figs. 9 and 10 show the exergy inlet, exergy outlet, exergy destruction and exergy efficiency in various wind speeds and in selected hydrogen production system. Results show that by increasing the wind speed, an exergy efficiency in electrolyzer decreases. After rated speed, exergy efficiency remains constant. Rated speed in this study is assumed to be 12 m/s. Fig. 11 shows the exergy efficiency and exergy destruction in hydrogen production system. Results illustrate that the maximum exergy destruction occurs in wind turbine and by improving the blade design and alternator (electrical generator) system, exergy destruction can decrease.

5.3. Results of system sizing The household energy consumption for selected cities is shown in Table (Appendix 1e5). It should be mentioned that all calculations are done for one day (the fifteenth day of each month) then is generalized to whole days of month.

Table 14 Calculated LCOP, ACS and NPV for all selected cities.

ACS (US$) NPV (US$) LCOP (US$ per kWh)

Tehran

Manjil

Binaloud

Zabol

Hendijan

72,406 1,121,957 0.67

78,033 1,209,148 0.33

31,641 490,288 0.28

52,415 812,195 0.36

43,016 666,552 0.45

5.3.3. Size of PEM fuel cell It is mentioned before that first, the Maximum energy shortage on fifteenth day of each month is calculated. Then the selected PEM fuel cell should be able to supply this energy shortage in about 8 h. According to this method, size of PEM fuel cell is calculated. Table 13 shows the characteristics of PEM fuel cell in selected cities. Now by using the size of system in each city, hydrogen production and consumption on the fifteenth day of each month are calculated and shown in Fig. 12. Also, Fig. 13 shows the portion of wind turbine and PEM fuel cell in power supply of considered building in selected cities. 5.4. Results if economic analysis Fig. 14 shows the extra annual hydrogen and its value (5 US$ per kilogram) in selected cities. According to economic analysis, the LCOP, NPV and ACS for all selected cities are shown in Table 14. It should be noted that LCOP is not a criterion for comparison of product cost in market because LCOP is calculated based on all costs in the project lifetime. 6. Conclusion Techno-economic analysis of wind turbineePEM fuel cell hybrid system is considered in this article. Complete modeling of hybrid system components and energy consumption modeling of 100 square meters building are carried out. Energy, exergy and economic evaluation are done by using simulation data. Results show that wind energy in Manjil and Binaloud (two cities which have wind power plant in Iran) has greater wind speed in comparison with other cities. Also result shows that in standalone application, the size of wind turbine is greater than the on-grid one to supply the full load consumption and it makes the standalone application more expensive. So, hybrid system is suggested to save the extra energy in form of hydrogen. In this method, PEM fuel cell can cover the power production fluctuations by using the generated hydrogen. So the smaller wind turbine is required. Result of exergy analysis in hydrogen production system shows that the majority of exergy destruction occurs in wind turbine and by improving the Blade aerodynamic and using new types of alternator (electrical generator), this destruction can decrease.

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Appendix 1 Table 1 Monthly (the fifteenth day) energy consumption in Tehran (kWh). Tehran Month

Lighting

TV

Refrigerator

Heating

Cooling

Warm water

Other

Total (kWh)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1.042 0.918 0.756 0.586 0.45 0.397 0.408 0.525 0.686 0.854 1.002 1.08

0.1479 0.1479 0.1479 0.1479 0.1479 0.1479 0.1479 0.1479 0.1479 0.1479 0.1479 0.1479

1.0575 1.0575 1.0575 1.0575 1.0575 1.0575 1.0575 1.0575 1.0575 1.0575 1.0575 1.0575

113.8 78.4 31.5 2.7 0 0 0 0 4.5 39.1 84.4 116.5

0 0 0 0 324.1 339.9 337 319.7 0 0 0 0

19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95

4 4 4 4 4 4 4 4 4 4 4 4

139.998 104.473 57.411 28.441 349.705 365.452 362.563 345.38 30.342 65.11 110.558 142.736

Table 2 Monthly (the fifteenth day) energy consumption in Manjil (kWh). Manjil Month

Lighting

TV

Refrigerator

Heating

Cooling

Warm water

Other

Total (kWh)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1.042 0.918 0.756 0.586 0.450 0.397 0.408 0.525 0.686 0.854 1.002 1.080

0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148

1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058

79.4 43.6 10.9 0 0 0 0 0 4.8 31.2 70.3 89.5

0 0 0 295.8 318.7 330.0 324.3 306.4 0 0 0 0

19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95

4 4 4 4 4 4 4 4 4 4 4 4

105.598 69.673 36.811 321.541 344.306 355.552 349.863 332.080 30.642 57.210 96.458 115.736

Table 3 Monthly (the fifteenth day) energy consumption in Zabol (kWh). Zabol Month

Lighting

TV

Refrigerator

Heating

Cooling

Warm water

Other

Total (kWh)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1.042 0.918 0.756 0.586 0.450 0.397 0.408 0.525 0.686 0.854 1.002 1.080

0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148

1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058

101.3 70.2 26.4 1.7 0 0 0 0 0.9 27.4 71.8 105.5

0 0 0 0 338.6 353.8 351.7 335.9 0 0 0 0

19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95

4 4 4 4 4 4 4 4 4 4 4 4

127.498 96.273 52.311 27.441 364.206 379.352 377.263 361.580 26.742 53.410 97.958 131.736

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Table 4 Monthly (the fifteenth day) energy consumption in Rasht (kWh). Binaloud Month

Lighting

TV

Refrigerator

Heating

Cooling

Warm water

Other

Total (kWh)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1.042 0.918 0.756 0.586 0.450 0.397 0.408 0.525 0.686 0.854 1.002 1.080

0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148

1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058

113.4 77.7 31.5 2.6 0 0 0 0 5.4 40.4 85.8 116.4

0 0 0 0 313.6 329.6 326.6 308.9 0 0 0 0

19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95

4 4 4 4 4 4 4 4 4 4 4 4

139.598 103.773 57.411 28.341 339.206 355.152 352.163 334.580 31.242 66.410 111.958 142.636

Table 5 Monthly (the fifteenth day) energy consumption in Hendijan (kWh). Hendijan Month

Lighting

TV

Refrigerator

Heating

Cooling

Warm water

Other

Total (kWh)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1.042 0.918 0.756 0.586 0.450 0.397 0.408 0.525 0.686 0.854 1.002 1.080

0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148 0.148

1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058 1.058

66.2 35.8 8.5 0 0 0 0 0 1.3 16.8 55.3 76.7

0 0 0 319 343 353.1 348.1 332 0 0 0 0

19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95 19.95

4 4 4 4 4 4 4 4 4 4 4 4

92.398 61.873 34.411 344.741 368.606 378.652 373.663 357.680 27.142 42.810 81.458 102.936

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