Exergy Comparison of Hydrogen Production Methods From Renewable Energy Sources

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Cite this: Energy Environ. Sci., 2012, 5, 6640

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A review on exergy comparison of hydrogen production methods fromrenewable energy sources

Koroneos Christopher* and Rovas Dimitrios

Received 26th January 2011, Accepted 17th January 2012

DOI: 10.1039/c2ee01098d

Hydrogen is an important energy carrier which could play a very significant role in the reduction of

emissions of greenhouse gases. The route by which hydrogen is produced is the determining factor for

its environmental performance. Hydrogen can be produced through methane reforming or through the

electrolysis of water with the use of electricity or it can be produced directly by gasification from

biomass. Renewable energy sources (RES) could be the feedstock for the two methods previously

mentioned. The objective of this work is the comparison of hydrogen (H2) production processes using

various renewable energy sources. This comparison is based on the exergy efficiency of each process.

The renewable energy sources that have been used are: wind power, solar energy, hydroelectric power,

and biomass. The solar energy systems that are used are photovoltaic and thermal. The biomass

systems are divided into two categories: (a) electricity production through biomass combustion, (b)

biomass gasification for the direct production of hydrogen. When in any of the processes electricity is

produced, this electricity is used for the electrolysis process of water to produce hydrogen (and oxygen).

Because hydrogen is transported in a liquid form, the liquefaction process is also taken into

consideration in this work. The liquefaction process is very energy intensive and as a consequence it

requires a lot of exergy. It has been found that the hydrogen production process with the highest exergy

efficiency is the electrolysis using electricity from hydro power. This efficiency is 5.6%. The process with

the lowest exergy efficiency is the one with electrolysis driven by electricity from solar energy

photovoltaics. The efficiency of this process is 1.0%.

Laboratory of Heat Transfer and Environmental Engineering, AristotleUniversity of Thessaloniki, P.O. Box 483, GR 54124 Thessaloniki,Greece. E-mail: [email protected]; [email protected];Fax: +30-2310-996012; Tel: +30-2310-995968

Broader context

The performance of exergy analysis of all the hydrogen production r

process. The processes considered included the following: solar elect

electricity production or thermal systems for indirect production

hydrogen by gasification or through combustion for electricity pro

used directly for the production of electricity. The processes involvi

the production of hydrogen. The comparison of the exergy efficienci

hydrogen production process using electricity from the hydropo

photovoltaic process using photovoltaic electricity has the lowest e

extremes. With regards to the issue of environmental impact the em

Since renewable energy is free the cost of the production of hydroge

processes.

6640 | Energy Environ. Sci., 2012, 5, 6640–6651

1. Introduction

The energy used today to satisfy all human needs is mostly based

on fossil fuels. It is estimated that the global distribution of

energy use is 80% fossil fuels, 14% renewable energy and 6%

nuclear power.1 The input of renewable energy sources to the

global energy grid, such as solar, wind and hydropower, is only

3%. The combustion of fossil fuels leads to the emission of great

outes from renewable energy sources has led to the most efficient

ricity production through either photovoltaic systems for direct

of electricity; biomass systems for the direct production of

duction in a power plant; wind and hydroelectric energies were

ng electricity systems were followed by electrolysis of water for

es of all these hydrogen production processes has shown that the

wer input has the highest exergy efficiency at 5.6% and the

fficiency at 1.0%. The rest of the processes fall within these two

issions are very low for even the low exergy efficiency processes.

n is solely based on the construction of the units for the various

This journal is ª The Royal Society of Chemistry 2012

http://dx.doi.org/10.1039/c2ee01098d





amounts of greenhouse gases. Some studies predict that the

global energy consumption will triple in the next thirty years.2

The environmental impact will be significant if fossil fuels

continue to be used at high rates. The use of clean energy

becomes an urgent matter as the energy required keeps on

increasing with the continuous increase of the developing coun-

tries’ gross domestic product. Hydrogen could become a clean

energy carrier if it is produced with the use of RES. This will lead

to the production of hydrogen with a very low carbon footprint.

The hydrogen energy content per weight is 2.5 times higher than

that of any other conventional fuel. Hydrogen has been consid-

ered as the energy carrier of the future.

Most of the hydrogen produced today comes from methane

reforming. The disadvantage of this method is that the co-

product of the reaction is carbon dioxide, which is a greenhouse

gas.

Although there are many methods for the production of

hydrogen from renewable energy sources, the only one currently

practical is the one using electrolysis. The energy efficiency of

electrolysis reaches levels higher than 70%, but it remains

doubtful due to the high cost of the required electricity. It is

estimated that the production cost is about 20.0 $ per GJ of

hydrogen.3 The electricity used can be provided by a single

renewable energy source or by an integrated system of solar

energy, wind power, and hydropower.4,5 These techniques can be

applied to islands or isolated areas where the connection to the

central power grid is difficult or economically impractical. In

such cases, H2 could be utilised as an energy storage medium.6–8

Solar energy can be utilized to produce electricity through

photovoltaic or photothermal methods. Hydrogen can also be

produced by the thermal dissociation of natural gas (methane

thermal splitting) in a high temperature chemical reactor (1400–

1600 K). It features a nozzle-type graphite receiver which absorbs

the solar power and transfers the heat to the flow of methane at

a temperature that allows dissociation.2,9 The products of the

Koroneos Christopher

Christopher Koroneos is

a chemical engineer. He did all

his studies at the Department of

Chemical Engineering of

Columbia University in New

York, USA, where he later was

a professor. Now he is at the

Unit of Environmental Science

and Technology of the Depart-

ment of Chemical Engineering

at the National Technical

University of Athens and the

Laboratory of Heat Transfer

and Environmental Engineering,

Aristotle University of Thessa-

loniki. He is teaching at the

Interdisciplinary Programme of Post Graduate Studies ‘‘Envi-

ronment &Development’’ of the National Technical University of

Athens and at the Hellenic Open University. His research interests

are in Renewable Energy, Exergy Analysis, Life Cycle Assessment

and Sustainable Development.


reaction are hydrogen and carbon black. The method can be

economically viable due to the fact that carbon black is a high

value nano-material that can easily be absorbed by the existing

market. Thus, there is no CO2 production. This solar route

provides the process energy that otherwise had to be provided by

combustion of fossil fuel. The overall avoided fossil fuel

consumption is equivalent to 277 MJ per kg hydrogen produced

and the avoided CO2 equivalent emissions are 13.9 kg per kg of

produced hydrogen.

Anotherway toharvest solar power is the photoelectrochemical

(PEC) system. In these systems a semiconductor with proper

characteristics is immersed in an aqueous electrolyte and irradi-

ated with sunlight. Depending on the material of the semi-

conductor, the produced electrical current density varies between

10 and 30 mA cm�2 of the semiconductor. At these current

densities, the voltage required for electrolysis is much lower, and

therefore its efficiency reaches high levels.10,11 The effective

voltage to start the water splitting reaction for a photo-

electrochemical system is between 1.35 and 1.4 eV. For practical

applications the band gap is about 1.7 eV. The electrolysis effi-

ciency with this method reaches the levels of 91%, with the overall

hydrogen production reaching levels between 10% and 12.9%.10,11

Additionally, there is a solar reactor technology at laboratory

scale achieving temperatures between 1500 and 2500 K.12 At

these temperatures, hydrogen (and oxygen) can be produced by

thermolysis of water or by the use of specific cycles such as metal

oxide cycles and the S–I cycle. In the case of the thermolysis of

water, the water dissociation efficiency was estimated to be

around 25%. The problems of this technology are related to the

lack of materials that can withstand these temperature levels, to

avoid the backwards reaction, and the need to separate the

mixture of hydrogen and oxygen produced.12 At milder

temperature conditions chemical oxidation/reduction cycles to

dissociate water such as metal oxide cycles and the so-called

Sulfur–Iodine (S–I) cycle can be used.

Rovas Dimitrios

Dimitrios Rovas received his

diploma in Mechanical Engi-

neering in 2003 from the Aris-

totle University of Thessaloniki

and since 2005 he is working as

a research associate at the

Laboratory of Heat Transfer

and Environmental Engineering.

He works in the areas of renew-

able energy sources, environ-

mental management, life cycle

assessment and exergy analysis.

He is co-author of several scien-

tific papers in the field of

Renewable Energy Sources and

Sustainable Development.

Energy Environ. Sci., 2012, 5, 6640–6651 | 6641

Fig. 1 Hydrogen production routes from renewable energy sources.

MxOy / MxOy�1 + 1/2O2

MxOy�1 + H2O / H2 + MxOy

Metal oxide pairs used are Mn3O4/MnO, Co3O4/CoO, ZnO/

Zn and Fe3O4/FeO. The only feasible cycles are ZnO/Zn and

Fe3O4/FeO. Recent studies have shown that the ZnO/Zn cycle

operates at temperatures lower than 1200 K while its exergy

efficiency is between 29% and 44% (ref. 12) with a heat recovery.

The Fe based cycles have theoretical efficiencies of 61% at 1900 K

and 42% at 2500 K.12 The efficiency for the S–I cycle ranges

between 47% and 52%.12,13

The photolytic method uses a photo-catalyst to absorb visible

light and then transmit the energy of appropriate wavelength and

intensity to the water molecule, to liberate H2 and O2. The

reaction of the process is:

H2O + X + light / H2 + 1/2O2 + X

where X is the catalyst. The material used for the photo-catalyst

could be salt compounds, semiconductors, photosynthetic dyes,

green or red algae, etc.1,14

Biomass could be utilized using two different routes: combus-

tion of biomass for the production of electricity and then water

electrolysis, or direct production of hydrogen by gasification or

fermentation/bacteria/moulds. In this way, bio-solids, animal

waste, agricultural residues,municipal solidwastes and sludge can

beused toproduce either synthesis gas or different kinds of biogas.

Both of these gases are high in hydrogen content (30%).15

The disadvantage of the classic gasification process is that it

causes high drying costs of the biomass. The wet biomass

moisture content can reach the levels of up to 95%. Biomass

gasification in supercritical water (SCW) is a promising tech-

nology producing hydrogen from high moisture biomass.

Hydrogen bonds in SCW are lower in number and much weaker.

The biomass stock that could be used is wood sawdust, rice

straw, rice shell, wheat stalk, peanut shell and generally residues

of the agricultural industry. Gasification of biomass in SCW has

high gasification efficiency and high molar fraction of hydrogen

(35–45%) and there is no need of a drying process.16

Studies have shown that hydrogen may be produced in the

near term at a cost that is competitive with natural gas reforming

by integrating hydrogen production with existing industrial

utilization of agricultural residues. To be more specific, hydrogen

can be produced by catalytic steam reforming of peanut shell

pyrolysis vapours. In the same framework, residues from the

food industry can be utilized to produce hydrogen via anaerobic

digestion with no emissions of greenhouse gases.15 A case study

reveals that isolated microflora from cow dung exhibited their

capability to generate hydrogen while treating solid waste con-

sisting of jackfruit peel. The biogas generation was found to be

0.72 L g�1 VS (jackfruit peel) converted, with the content of

hydrogen being 55 � 2% and with no methane content.17

Biological treatment of waste organic material with dark and

photo-fermentation can produce hydrogen. Starch and cellulose

as residues of the agricultural and food industry, non-toxic

industrial effluents from the dairy industry, olive mills, bakers

yeast and brewery wastewaters and waste sludge can biodegrade

to produce hydrogen. The highest hydrogen yield obtained from


glucose’s dark fermentation is estimated to be 2.0–2.4 mol of H2

per mol of glucose, while the treatment of starch can theoretically

produce 553 mL of hydrogen per gram of utilized starch.18 Some

photo-heterotrophic bacteria are capable of converting organic

acids (acetic, lactic, and butyric) to hydrogen and carbon

dioxide, while there can be hybrid systems using photosynthetic

and fermentative bacteria. The phototropic treatment of acetate

and butyrate, which are the main products of dark fermentation,

generates 2.5 mol of H2 per mol of acetate and 3.7 mol H2 per

mol butyrate. Another biological process that can be used is bio-

photolysis of water by algae. Algae split water molecules to

NADH and oxygen via photosynthesis while the hydrogen ions

are converted to hydrogen gas by a hydrogenase enzyme.19

Hydrogen could also be produced from post-consumer wastes

such as plastic and grease trap using a two-stage process: fast

pyrolysis to convert polymers to a gas/vapor stream of mono-

mers and other low-molecular weight compounds followed by

catalytic steam reforming of this gas to yield hydrogen and

carbon oxides. ‘‘Trap grease’’, a waste material recovered from

traps on sewer lines, does not require a depolymerization step

and can be directly steam reformed to produce hydrogen. It is

shown that annually, the hydrogen production can reach 6.5 Mt

(6 Mt from plastic and 0.5 Mt from grease trap).20

Hydrogen can also be produced by the treatment of biogas

with hot slag that can act both as a thermal medium and

a catalyst.21 It can be produced efficiently from the electrolysis of

ammonia and black liquor (paper industry effluent) in which

particular 85–90% of the electricity spent in electrolysis is stored

as hydrogen.22,23

The basic idea of the hydrogen production route from

renewable energy sources is illustrated in Fig. 1.

2. Exergy analysis

Exergy is defined as the amount of work available from an energy

source. The maximum amount of work is obtainable when

matter and/or energy such as thermal energy is brought to a state

of thermodynamic equilibrium with the common components of

the environment in which this process takes place with the dead

state by means of a reversible process, involving interaction only

with the above mentioned components of nature.24 Exergy is

a measurement of how far a certain system deviates from a state

of equilibrium with its environment. The quality of energy can be

expressed as the quantity of exergy per unit of energy. The


quality increases with the temperature (provided that the

temperature is higher than that of the environment). It is clear

that the exergy concept incorporates both the quantitative and

qualitative properties of energy.

Every irreversible phenomenon causes exergy losses leading to

exergy destruction of the process or to an increased consumption

of energy from whatever source the energy was derived.

The objective of exergy analysis is to determine the exergy losses

(thermodynamic imperfections) and to evaluate quantitatively the

causes of the thermodynamic imperfection of the process under

consideration. Exergy analysis can lead to all kinds of thermo-

dynamic improvement of the process under consideration.24

The hydrogen production systems studied are all assumed to

operate in a steady state. For a steady state system the mass

balance equation can be expressed by eqn (1)

P_min ¼ P

_mout (1)

where, _m is the mass flow rate, and the subscript in stands for

inlet and out for outlet. The general energy balance can be

expressed by eqn (2)

P_Ein ¼ P

_Eout (2)

E::

kin;inþE:

pot;inþðeventual other forms of energyÞþ: QþX

minhin ¼ E:

kin;inþE:

pot;inþðeventual other forms of energyÞ

þW þX

mouthout: (3)

where _Ein is the rate of net energy transfer in, _Eout is the rate of

net energy transfer out by heat, work and mass; _Q ¼ _Qnet, is the

rate of net heat input, _W ¼ _Wnet is the rate of net work output,

and h is the specific enthalpy.25

In systems studied most often the changes in kinetic and

potential energy are negligible, eqn (3) can then be written as

follows:

_Q +P

_minhin ¼ _W +P

_mouthout (4)

This equation expresses that the total amount of energy input

of the system is equal to the total amount of energy output. The

energy efficiency of the system can be expressed as

hı ¼P

_EoutP_E in

(5)

The exergy of the system can be expressed by the following

equations:

P_Exin �

P_Exout ¼

P_Exdest (6)

or

X��1� T0

Tk

�_Qk �W

::�þX

_minðex tm;in þ exch;inÞ

�X

_mout

�ex tm;out

:

þ ex ch;out

�

¼X

E:xdest (7)

with


extm ¼ (h � h0) � T0(s � s0) (8)

where _Qk is the heat transfer rate through the boundary at

temperature Tk at location k, _W is the work rate, extm is the

thermo-mechanical flow of exergy, s is the specific entropy and

the subscript zero indicates properties at the temperature and

pressure of the environment of the system considered (standard

state).26

Eqn (8) can be expanded to the form of eqn (9)

extm ¼ (h � h0) � (T0s � T0s0) (9)

The term T0s � T0s0 represents the exergy that has been

destroyed and it is presented as Exdest.

The standard chemical exergy of gaseous reference species can

therefore be expressed as,27

_Ech;i ¼ _miRT0lnP0

Pk

(10)

where, R is the specific gas constant.

Different ways of formulating exergetic efficiency proposed in

the literature have been given in detail elsewhere.28 The exergetic

efficiency expresses all exergy input as used exergy, and all exergy

output as useful exergy. Therefore, the exergetic efficiency

becomes:

hii ¼ hex ¼E:xout

E:xin

(11)

The efficiency will be improved to the maximum when the

exergy loss or irreversibility _Exin � _Exout is minimized.29

When heat at a temperature Tt > T0 is available and the

temperature of a thermodynamic system is Tt and the tempera-

ture of the environment is T0 its exergy quality can be expressed

by the quality factor.

Quality factor ¼ 1� T0

Tt

(12a)

When the temperature Tt, at which the heat is available,

increases, the quality factor increases too.

Exergy analysis is employed to detect and to evaluate quan-

titatively the causes of thermodynamic imperfection of the

process under consideration. Exergy analysis indicates the

possibilities of thermodynamic improvement of the process

under consideration.30–35

The exergy analysis of the hydrogen production system under

consideration can be split into two stages. At the first stage the

exergy analysis of the electrolysis is performed so that the exergy

efficiency for this process can be determined, this value is utilized

for renewable energy sources based processes to produce elec-

tricity. At the second stage, exergy analysis is performed for each

renewable energy system. In the third stage the hydrogen

produced in each of the processes is liquefied. Also, the exergy

efficiency for the liquefaction process is common for all processes

investigated.

2.1 Exergy analysis of the electrolysis process

The exergy analysis of the water electrolysis is based on the

electrolysis process shown in Fig. 2.36 The electricity production


shown in the figure comes from renewable energy sources. In the

case under study, the O2, H2 purification and the H2 compres-

sion processes will be excluded. Thus, the only steps that will be

taken into account are the electricity transformation and recti-

fication and the water splitting. The numbers inside parentheses

in Fig. 2 represent the inlet and outlet exergy of an individual

process.

The efficiency of the electrolysis process can be calculated by

analyzing each individual unit of the problem. Taking into

consideration the data of the electrolysis units of Fig. 2, its exergy

efficiency is calculated by eqn (12b)

hex ¼Exout

Exin¼ 25

37¼ 67:5% (12b)

The ‘‘electrolysis unit’’ investigated in this analysis will be the

same for all renewable energy sources based electricity produc-

tion processes investigated.36

2.2 Exergy analysis of the hydrogen liquefaction process

The hydrogen liquefaction process used in this application is

shown in Fig. 3, where a closed nitrogen loop is used for

additional cooling. Hydrogen gas enters the compressor at 300

K and pressure 1 bar and exits at 300 K and pressure 100 bar.

The compressed hydrogen passes through a heat exchanger

network and the outlet temperature is at 35 K.34 The network

consists of two heat exchangers. The working fluids in the first

exchanger are the hydrogen that needs to be liquefied, liquid

nitrogen and the non-liquefied hydrogen of the process, which

will return to the compressor. The working media of the second

exchanger are the hydrogen at the liquid and vapor phases. The

data of all the streams of this figure are shown in Table 1.34

Assume reversible isothermal compression: the total ideal work

requirement for compression of both hydrogen and nitrogen is

given by eqn (13).

(Wtot)rev ¼ mH(w12)rev + mN(wab)rev (13)

Fig. 2 The electrolysis process.


where, (w12)rev is the work of the hydrogen compressor expressed

by eqn (14)

(w12)rev ¼ h2 � h1 � (q12)rev ¼ h2 � h1 � T1(s2 � s1)¼ ex2 � ex1 ¼exH (14)

(wab)rev is the work of the nitrogen compressor expressed by eqn

(15)

(wab)rev¼ hb� ha� (qab)rev¼ hb� ha� T1(sb� sa)¼ exb� exa¼exN (15)

Using the data from Table 1 (wtot)rev and taking r ¼ mN/mH ¼1, it is then calculated:

(Wtot)rev ¼ mH(w12)rev + mN(wab)rev ¼ mH[(w12)rev + (wab)rev] ¼6167.69mH (kJ)

Assuming that both compressors have an efficiency of 0.85, the

work needed for both compressors is

Wtot ¼ (Wtot)rev/ncomp ¼ 7256.1mH (kJ) (16)

The work can be expressed as a function of the liquefaction

ratio of hydrogen (eqn (17))

W0tot ¼

Wtot

y(17)

where y is the liquefaction ratio of hydrogen (eqn (18)).

y ¼ ðh1 � h2Þðh1 � h8Þ þ r

ðha � hbÞðh1 � h8Þ (18)

As mentioned before, the fraction of nitrogen to hydrogen

mass (r) can be taken to be equal to 1.

The minimum work of hydrogen liquefaction is equal to the

liquefied hydrogen’s exergy:

(W0)min ¼ mH[h8 � h1 � T0(s8 � s1)] (19)

The exergy efficiency of the liquefaction process is expressed

by eqn (20)

z ¼ ðW 0Þmin

ðW 0totÞ

¼ mH½h8 � h1 � T0ðs8 � s1Þ�mH � ðW 0

tot=yÞ

¼ ½h8 � h1 � T0ðs8 � s1Þ�W

0tot=y

(20)

Using the data of Table 1 the exergy losses of the process are

calculated to be equal to 86.7%. This leads to liquefaction

exergy efficiency of 13.3% (Fig. 4). Depending on how the

liquid hydrogen is used, part of its exergy value can be

recovered.

2.3 Exergy analysis of the renewable energy system

2.3.1 Exergy analysis of the photovoltaic panel. The exergy

efficiency of a photovoltaic panel can be expressed as the ratio

of the exergy output, the electricity produced, to the exergy

input, in this case the exergy of the solar radiation


Fig. 3 The liquefaction process.34

Fig. 4 Grassmann diagram for the liquefaction process.

hpv ¼Expv;out

Ex0pv;in

(21)

Expv;in ¼ ESUN ¼ e�

u�RsA (22)

where, Rs is the solar radiation, A is the collector surface and �e/�u

is the maximum energy available from radiation (black body

exergy factor),31,37,38

e�

u�¼ 1þ 1

3

�T0

T1

�4

� 4

3

�T0

T1

�(23)

In this study, the I-165 MonoCrystalline Si solar panel by

Isofoton has been used. Its technical characteristics and the

solar radiation are shown in Table 2. The solar radiation is

taken to be 1000 W m�2, the sun surface temperature is

considered to be 6000 K (T1) and the ambient temperature

equals to 300 K (T0).

The exergy output of the solar panel considered is given by

eqn (24).39

Expv,out ¼ UIsT (24)

where, sT is a temperature correction coefficient expressing the

exergy losses due to the temperature differences between the panel

surface temperatureTp and the ambient temperatureT0, equal to34

sT ¼ 1 � 0.005(Tp � T0) (25)

Table 1 Pressure, temperature, enthalpy and entropy values for theliquefaction processa (ref. 34)

Point Pressure/bar Temperature/KEnthalpy (h)/kJ kg�1

Entropy (S)/kJ kg�1 K�1

(1) 1 300 4227 70.63(2) 100 300 4273 51.55(3) 100 77 1146.4 32.5(4) 100 35 521.3 20.91(5) 1 20 521.3 30.19(6) 1 20 714.8 39.299(7) 1 77 1322.35 53.474(8) 1 20 268.3 16.996(a) 1 300 460 4.39(b) 100 300 445 3.035(c) 1 77 100 1.4654

a Streams indicated with numerals are hydrogen streams and streamsindicated with letters are nitrogen streams.


The surface temperature Tp has the value of 330 K.

Using the data from Table 2,40 and eqn (21)–(25), the exergy

efficiency is calculated to be equal to: hpv ¼ 12.3%.

The exergy losses of the photovoltaic panel are 87.8% of the

solar input, of the remaining 12.2% of the exergy 0.325 � 11.8%

¼ 4.0% is lost in the electrolysis process, and of the remaining

8.2% of the exergy 0.867� 8.2%¼ 7.2% is lost in the liquefaction

process. This leads to an overall exergy efficiency of the whole

process of 1.0% (Fig. 5).

2.3.2 Exergy analysis of the photothermal process. A photo-

thermal process consists of two subsystems, a solar collector

system and an electricity production system based on the

Rankine cycle. The produced electricity is used in the production

of H2 through the electrolysis process. A typical photothermal

process is shown in Fig. 6.

The exergy efficiency of the collector–receiver system is given

by

Table 2 Technical characteristics of the photovoltaic panel and the solarradiation40

Rs: Solar radiation/W m�2 1000

Monocrystalline Si photovoltaic panelLength/m 1310 mmWidth/m 969 mm�Nominal voltage[ 12 VMaximum current Imax 9.48 AMaximum voltage Vmax 17.4 V


Fig. 5 Grassmann diagram for the hydrogen production process using

a photovoltaic panel.

hC/R ¼ ExC/R/Exs,in (26)

where, Exs,in is the exergy input of the solar radiation and ExC/Ris the exergy increase of the medium in the collector

ExC/R ¼ N[mf(hf0 � hfi) � T0(sf0 � sfi)] (27)

N is the number of collectors, mf is the mass flow rate of the

working fluid in each of the collectors, hf0 is the specific enthalpy

of the working fluid exiting the heat exchanger, and sf0 is the

specific entropy of that stream at the same point, hfi and sfi are the

specific enthalpy and entropy respectively of the stream entering

the heat exchanger, T0 is the ambient temperature.25 The exergy

of the incoming solar energy (received by the collector) is

Exs;in ¼ Qs

�1� T0

Ts

�(28)

where Qs is the total amount of solar energy input for N collec-

tors and Ts is the surface temperature of the sun. In this analysis

an ammonia Organic Rankine Cycle (ORC) will be used (Fig. 6).

The electricity production efficiency is given by

hel ¼Wnet

Ex1f(29)

where Wnet is the net output of electricity and Ex1f the increase in

exergy due to the heat transfer from the collectors, of the working

fluid (ammonia), of the ORC cycle.

Fig. 6 The photothermal process.39


Ex1f ¼ Qf

�1� T0

TR0

�(30)

where Qf is the increase in energy in the evaporator of the ORC

and TR0 is the boiling temperature of the medium in the

evaporator.

The exergy efficiency of the system is calculated by multi-

plying the individual efficiencies. The efficiencies of the parts of

the photothermal process are shown in Table 3. Using the given

data for the efficiency of the different parts of the photothermal

process, from Table 3, and carrying out the calculation with

eqn (26)–(30), the exergy losses are calculated to be equal to

87.3%. Of the remaining 12.7% of the exergy 0.325 � 12.7% ¼4.1% is lost in the electrolysis process and of the remaining

8.6% of the exergy 0.867 � 8.6% ¼ 7.4% is lost in the lique-

faction process. This leads to an overall exergy efficiency of

1.2% (Fig. 7).

2.3.3 Exergy analysis of the wind turbine. The wind turbine

exergy41 is expressed by eqn (31)

exWE ¼ 1

2V 2

1 (31)

the following data are used:

� air pressure is at 1 atm

� air temperature is at 300 K

� air density is 1.225 kg m�3.

The received power of the engine is given by

Ein ¼ mexWE (32)

where m is the air mass flow through the wind turbine.

m ¼ rAV1 ¼ rpD2

4V1 (33)

The exergy efficiency of the wind turbine is

hex,WE ¼ Exout/Exin (34)

The nominal power outputs of the wind turbines for several

wind speeds are presented in Table 4.42 Taking into consideration

the nominal power of the Neg-Micon wind engines,42 and the

exergy input calculated from eqn (31)–(34), the exergy efficiencies

for various wind turbines at different wind speeds are calculated.

The results are shown in Table 5. The exergy efficiency of this

engine (Table 5) is equal to 45.6%. The exergy lost in the elec-

trolysis process is 0.325 � 45.6% ¼ 14.8% while of the remaining

30.8% of the exergy 0.867 � 30.8% ¼ 26.7% is lost in the lique-

faction process, leading to an overall exergy efficiency of 4.1%

(Fig. 8).

2.3.4 Exergy analysis of the hydroelectric process. The

hydroelectric process produces power from the water flow

through a turbine. In order to perform the exergy analysis of

a hydroelectric power plant, the data of the Meshohora hydro-

electric dam in Greece are used.43

� The water flow is _V ¼ 45 m3 s�1

� The length of the pipe flow is: 7.469 km

� The height of the fall is: 230 m, and

� the tube diameter is: 3.0 m.


Table 3 The efficiencies of individual parts of the photothermal process30

System Received exergy (1) Delivered exergy (2) Exergy losses/kW (3)Exergy losses(%) (4)

Exergy efficiencies(%) (5)

Collector Exi ¼ 270.82 Exc ¼ 78.82 I1 ¼ 192.19 70.96 29.03Receiver Exc ¼ 78.82 Exu ¼ 53.768 I2 ¼ 24.862 31.618 68.38Collector–receiver Exi ¼ 270.82 Exu ¼ 53.768 I1,2 ¼ 217.05 80.146 19.858Boiler heat exchanger Exu ¼ 53.77 Exu ¼ 51.845 Ihx ¼ 1.92 3.577 96.424Heat engine Exu

1 ¼ 51.845 W ¼ 34.511 I3 ¼ 17.334 33.434 66.566Total system Exi ¼ 270.82 W ¼ 34.511 I ¼ 236.30 87.254 12.743

Fig. 7 Grassmann diagram for the hydrogen production process from

a photothermal process.

The exergetic efficiency of the turbine is taken as 92.5%, and

the exergetic efficiency of the electric motors as 95%.

Due to friction along the pipe, pressure loss occurs which can

be expressed through the head loss (hf) according to eqn (35)

DPloss ¼ rghf, (35)

hf ¼ fLu2mean

2Dg(36)

where f is a friction coefficient related to the Reynolds number

and the pipe’s roughness, L the pipe length, umean the mean

velocity, D the hydraulic diameter of the pipe and g the gravity

acceleration.

umean is calculated by

umean ¼_V

A¼

_V

pðD2=4Þ ¼45

7:065¼ 6:4 m s�1 (37)

while the Reynolds number is given by eqn (38)

Re ¼ rumean

D

m(38)

Table 4 Technical characteristics of the three wind engines42

Wind velocity/m s�1

Wind turbine type

600 kW/48 m

750 kW/48 m

1 MW/60 m

6 93 95 1507 153 168 2488 235 259 3859 329 362 535


where r is the density of water and m is its dynamic viscosity

(1.002 � 10�3 N m�2, water at 25 �C). Using the Moody diagram

(3 ¼ 0.025) the friction coefficient is calculated to be 0.005.

Using eqn (36) hf is calculated at 25.8 m. Assuming that the

velocity of the flow is constant at 6.4 m s�1 the energy losses due

to friction and the pressure loss are

Eloss,fri ¼ m � DPloss ¼ A � r � L � g � hf ¼ 7.065 � 7.469 �9.81 � 25.8 ¼ 13 � 109 Joules ¼ 3.6 � 106 W h (39)

The potential energy of the water is calculated from eqn (40)

Epot ¼ mgH ¼ A � r � L � g � H ¼ 7.065 � 1000 � 7469 �9.81 � 230 ¼ 11.9 � 1010 Joules ¼ 33 � 106 W h (40)

where m is the mass of water in kg (52.8 � 106 kg), g the gravity

acceleration in m s�2, and H is the height of the fall in m.

The kinetic energy of the water is calculated by eqn (41)

Ekin ¼ 1

2mu2mean ¼ ð0:5Þ�52:8� 106

�ð6:4Þ2¼ 1:08� 109Joules

¼ 0:3� 106W h

(41)

where, umean is the water speed in m s�1.

The energy of the water just before the turbines is equal to the

sum of the kinetic and potential energy:

Etot ¼ Epot + Ekin � Eloss,fri ¼ 33 + 0.3 � 3.6 ¼ 29.7 GW h(42)

The exergy losses in the two serial placed turbines are:

Exloss,turb ¼ (2)(1 � 0.925) ¼ (2)(0.075) ¼ 0.15 or 15% (43)

Thus, the exergy output in the turbines is:

Eturb ¼ 29.7 � (0.15)(29.7) ¼ 25.3 GW h (44)

Table 5 Exergy efficiency of three wind engines30

Wind velocity/m s�1 Exergy efficiency

Wind turbine type

600 kW/48 m

750 kW/48 m

1 MW/60 m

6 hex,WE 38.8% 39.7% 40%7 40.24% 44.19% 41.75%8 41.41% 45.64% 43.42%9 40.71% 44.8% 42.37%


Table 6 Contents (mass% in dry sample) of C, H, N, S, Cl, O, and ash;and HHV in kJ kg�1 ref. 44

Biomass HHV/kJ kg�1 C H N S Cl O Ash

Wood waste 18 467 49.7 6.0 1.7 0.0 0.1 41.0 1.5


wind power.

Taking into consideration the exergy efficiency of the electric

motor, the exergy losses are:

Exloss,motor ¼ (2)(1 � 0.9) ¼ (2)(0.1) ¼ 0.2 or 20% (45)

Therefore, the exergy output is:

Eout ¼ 25.3 � (0.2)(25.3) ¼ 20.3 GW h (46)

The overall exergy efficiency of the electricity production in the

Meshohora hydroelectric power plant is:

hex ¼Exout

Exin¼ Exout

Etot þ Eloss;fri

¼ 20:3

33¼ 0:615 or 61:5%

The Grassmann diagram of the process is presented in Fig. 9.

The exergy loss of the power plant is 61.5%. Taking into

consideration the exergy efficiency of the electrolysis process, the

efficiency after this process is at 0.325 � 61.5% ¼ 41.6%. The

exergy lost in the liquefaction process is 0.867 � 41.6% ¼ 36%

leading to a remaining exergy efficiency of 5.6%, that is, the

overall efficiency of the process.

2.3.5 Exergy analysis of the biomass combustion process. The

biomass combustion produces the essential heat to operate

a power plant. The inlet exergy of the biomass is

Exbio,in ¼ _mDH (47)

where m, the biomass flow rate, is assumed to be 3000 kg h�1 and

DH, the average caloric potential of the inlet, is taken to be


hydro power.


18 467 kJ kg�1 (ref. 44). The biomass used is wood waste and its

weight composition is given in Table 6.

The exergy efficiency using the Carnot cycle is given by

hex ¼DH

Exbio;inhboiler

�1� T0

Tm

�(48)

where Tm ¼ ((h2 � h1)/(s2 � s1)) is the average thermodynamic

temperature of the heat received by water, hboiler ¼ the boiler’s

efficiency, DH ¼ the caloric potential of biomass,

Exbio,in ¼ biomass exergy.

The assumptions made for the analysis are:

� the combustion temperature is 793 K and the ambient

temperature is 300 K

� the boiler efficiency is 90%

� the generator’s exergy losses represent the 3% of the exergy

output of the steam cycle.

� Tm is taken to be 546 K.

Taking into consideration these factors, the exergy effi-

ciency of biomass combustion is calculated to be 41.4% and

the exergy losses of the electric power production are equal to

1.2%. The electricity production efficiency is 0.987 � 41.4%

¼ 40.9%, of which 0.325 � 40.9% ¼ 13.3% is lost in the

electrolysis process. Of the remaining 40.9% � 13.3% ¼ 27.6%

exergy 0.867 � 27.5% ¼ 24.2% is lost in the water electrolysis.

This results in a hydrogen production process overall effi-

ciency of 3.9%. The results are illustrated in Fig. 10, where the

exergy losses throughout the process are shown.

2.4 Exergy analysis of the biomass gasification process

A typical biomass gasification process is shown in Fig. 11. The

reaction that describes the gasification of the biomass is:

C6H10O5 + 4.5H2O / 0.5CO + 7.5H2 + 4.5CO2 + CH4


biomass combustion.


Fig. 11 The biomass gasification process.42

Table 7 Heat and energy flows in the SCWG processa (ref. 43)

Heat flows (106 kJ kmol�1

biomass) Energy flows (106 kJ kmol�1 biomass)

DHF DHP DHR Biomassb Biomassc Biomassd Hydrogen

Input 2.597 3.078 1.044 0.256Output 2.111 0.250 1.777hE 40.6%

a DHF: heat for heating feedstock; DHP: heat released from productstream; DHR: heat of reaction; hE: the exergy efficiency. b Cellulose asfeedstock. c Providing process heat by wood combustion with anefficiency of 75%. d Generating electricity by wood conversion with anefficiency of 40%.

Fig. 13 The exergy efficiencies of the hydrogen production methods.

The weight fraction of the feedstock is 20% of cellulose

and 80% of water. The reactor operates at 600 �C and 350

bar producing SynGas that is made up of 55.6% hydrogen,

33.3% carbon dioxide, 7.4% methane and 3.7% carbon

monoxide.45

Using the data of Table 7, the gasification exergy efficiency is

assumed to be equal to 40.6%.46 The liquefaction process effi-

ciency is calculated to be 13.3%, and the overall efficiency of the

hydrogen production process is 5.4%. The Grassmann diagram

of the process is presented in Fig. 12.


biomass gasification.


3. Comparison and conclusions

An exergy analysis of all the hydrogen production routes from

renewable energy sources was performed. The renewable energy

sources that were used included wind power, solar energy,

hydroelectric power, and biomass. Because these sources could

be used in different ways, the most reasonable ones are all

considered. The solar energy systems were either photovoltaic

for direct electricity production or thermal for indirect

production of electricity. Biomass fed systems either produced

hydrogen directly by gasification or combusted to produce

electricity in a power plant. Wind and hydroelectric energies

were used directly for the production of electricity. Since

hydrogen had to be produced, all processes in which electricity

was produced were followed by electrolysis of water. Its exer-

getic efficiency was calculated to be 67.5%. A great impact on

the final exergy efficiency of all processes was that of hydrogen

liquefaction. Because hydrogen is transported in a liquid form,

the liquefaction process is also taken into consideration in this

work. The liquefaction process is very energy intensive and as

a consequence it requires a lot of exergy. Hydrogen liquefaction

has a very low exergetic efficiency value of 13.3%. The exergy

efficiency for electrolysis is taken to be the same for all processes

using renewable energy sources.

The comparison of the exergy efficiencies of all the hydrogen

production processes using renewable energy sources is given in

Fig. 13. This figure shows that the process with the hydropower

Fig. 14 The exergy efficiencies of the hydrogen production methods

before and after liquefaction.


input has the highest exergy efficiency at 5.6% and the photo-

voltaic process has the lowest at 1.0%. The biomass gasification

energy system has the second highest exergetic efficiency

reaching the level of 5.4%. Biomass gasification is a direct route

to hydrogen production, so there is no need for water elec-

trolysis. In the event that no liquefaction takes place (Fig. 14),

the hydro process efficiency reaches the level of 41.6% and that

of the photothermal process is equal to 8.6%. The exergy effi-

ciencies before and after the liquefaction process have great

fluctuations. That part of the process needs to be redesigned. If

there was development of an efficient hydrogen gas collection

system, the use of renewable energy sources could be highly

efficient, especially in the case of the use of hydropower, wind

power and biomass gasification. Even though there is a low

exergy efficiency, the emissions of the processes are very low

and there are no greenhouse gas emissions.47 It must also be

noted that renewable energy sources are free so the exergy

efficiency has an impact on the cost of the construction of the

processes.

Nomenclature

Symbols

A

6650 | Energy Env

surface (m2)

D
the turbine diameter (m)
E
energy (kW) _E rate of net energy (kW s�1)
Ex
exergy (kW)
ex
specific exergy
�e/�u
the black body exergy factor
f
friction factor
g
gravity acceleration (m s�2)
H
height of the fall (m).
h
specific enthalpy (kJ kg�1)
hf
head loss (m)
I
current (Amp)
_m
mass flow (kg s�1)
m
mass (kg)
N
number of collectors
Rs
solar radiation (W m�2)
R
specific gas constant (J kg�1 K�1)
P
pressure (Atm)
q
specific heat (kJ kg�1)
Q
heat (kW) _Q heat rate (kW s�1)
r
(kg of N2)/(kg of H2)
s
entropy (kJ kg�1 K�1)
T
temperature (K)
Tm
thermodynamic temperature of water (K)
T0
temperature of the environment, 300 K
V
velocity (m s�1). _V volume rate (m3 s�1)
U
voltage (Volts)
umean
mean velocity (m s�1)
w
specific work (kJ kg�1)
W
work (kJ) _W work rate (kJ s�1)
iron. Sci., 2012, 5, 6640–6651

ExWE

This

the wind turbine exergy (kW)

y
liquefaction ratio of hydrogen
Greeks

z
exergy efficiency of the liquefaction process
DH
heating value of biomass (kJ kmol�1 biomass)
h
efficiency (%)
DS
difference in entropy values (kJ kg�1 K�1)
m
dynamic viscosity of water (kg m�1 s�1)
r
density (kg m�3)
sT
temperature correction coefficient
Subscripts

0
environment, initial state
1,2,.
process steam indicator
ab
nitrogen compression
bio
biomass
C/R
Collector/Receiver
comp
compressor
ch
chemical
dest
destroyed
ex
exergy
f
working medium
in
inlet
k
location k
kin
kinetic
loss
losses
min
minimum
out
outlet
p
panel
pot
potential
pv
photovoltaic
R
Rankine
rev
reversible
s
solar
t
the system
tm
thermomechanical
tot
total
turb
turbine
u
collector
w
water
WE
wind energy
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Documents

Exergy Comparison of Hydrogen Production Methods From Renewable Energy Sources