Solar Energy 78 (2005) 603–615
www.elsevier.com/locate/solener
Solar thermochemical production of hydrogen––a review
Aldo Steinfeld
a
a,b,*
Department of Mechanical and Process Engineering, ETH––Swiss Federal Institute of Technology, ETH-Zentrum ML-J42.1,
CH-8092 Zurich, Switzerland
b
Paul Scherrer Institute, CH-5232 Villigen, Switzerland
Received 2 June 2003; received in revised form 7 November 2003; accepted 11 December 2003
Available online 15 January 2004
Communicated by: Associate Editor A.T. Raissi
Abstract
This article reviews the underlying science and describes the technological advances in the ﬁeld of solar thermochemical production of hydrogen that uses concentrated solar radiation as the energy source of high-temperature
process heat.
2004 Elsevier Ltd. All rights reserved.
Keywords: Solar; Hydrogen; Thermochemical
1. Thermodynamics of solar thermochemical processes
There are basically three pathways––and their combinations––for producing hydrogen with solar energy:
electrochemical, photochemical, and thermochemical.
The latter is based on the use of concentrated solar
radiation as the energy source of high-temperature
process heat for driving an endothermic chemical
transformation. Large-scale concentration of solar energy is mainly based on three optical conﬁgurations
using parabolic reﬂectors, namely: trough, tower, and
dish systems (Tyner et al., 2001). The capability of these
collection systems to concentrate solar energy is dee
scribed in terms of their mean ﬂux concentration ratio C
over a targeted area A at the focal plane, normalized
with respect to the incident normal beam insolation I,
e ¼ Qsolar
C
I A
*
ð1Þ
Address: Department of Mechanical and Process Engineering, ETH––Swiss Federal Institute of Technology,
ETH-Zentrum ML-J42.1, CH-8092 Zurich, Switzerland. Tel.:
+41-56-310-3124; fax: +41-56-310-3160.
E-mail address: [email protected] (A. Steinfeld).
where Qsolar is the solar power intercepted by the target.
e is often expressed in units of ‘‘suns’’ when normalized
C
to I ¼ 1 kW/m2 . The solar ﬂux concentration ratio
typically obtained is at the level of 100, 1000, and 10,000
suns for trough, tower, and dish systems, respectively.
Higher concentration ratios imply lower heat losses
from smaller areas and, consequently, higher attainable
temperatures at the receiver. To some extent, the ﬂux
concentration can be further augmented with the help of
non-imaging secondary concentrators, e.g., compound
parabolic concentrators (CPC), when positioned in
tandem with the primary parabolic concentrating systems (Welford and Winston, 1989). A recently developed
Cassegrain optical conﬁguration for the tower system
makes use of a hyperboloidal reﬂector at the top of the
tower to re-direct sunlight to a CPC located on the
ground level (Yogev et al., 1998). The aforementioned
solar concentrating systems have been proven to be
technically feasible in large-scale (MW) pilot and commercial plants aimed at the production of electricity in
which a heat transfer ﬂuid (typically air, water, synthetic
oil, helium, sodium, or molten salt) is solar-heated and
further used in traditional Rankine, Brayton, and Stirling cycles (Tyner et al., 2001). Solar thermochemical
applications, although not as far developed as solar
thermal electricity generation, employ the same solar
concentrating technologies.
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.solener.2003.12.012
604
A. Steinfeld / Solar Energy 78 (2005) 603–615
Nomenclature
A
e
C
HHV
I
n_
Qsolar
T
Tstagnation
Toptimum
area of reactor aperture
mean solar ﬂux concentration ratio (–)
high heating value of a fuel
normal beam insolation (kW m2 )
molar ﬂow rate (mol s1 )
solar power input to solar reactor (kW)
temperature (K)
maximum temperature of a blackbody absorber
optimum temperature of the solar cavityreceiver for maximum gexergy
Solar chemical reactors for highly concentrated solar
systems usually feature the use of a cavity-receiver type
conﬁguration, i.e. a well-insulated enclosure with a small
opening, the aperture, to let in concentrated solar radiation. Because of multiple internal reﬂections, the fraction of the incoming energy absorbed by the cavity often
greatly exceeds the surface absorptance of the inner
walls. The larger the ratio of the cavity’s characteristic
length to the aperture diameter, the closer the cavityreceiver approaches a blackbody absorber. Smaller
apertures reduce re-radiation losses, but they intercept a
reduced fraction of the sunlight reﬂected from the concentrators. Consequently, the optimum aperture size is a
compromise between maximizing radiation capture and
minimizing radiation losses.
A comprehensive thermodynamic analysis of solar
thermochemical processes is described by Fletcher
(2001) and by Steinfeld and Palumbo (2001). The principal concepts are summarized herein. The solar energy
absorption eﬃciency of a solar reactor, gabsorption , is deﬁned as the net rate at which energy is being absorbed
divided by the solar power coming from the concentrator. At temperatures above about 1000 K, the net power
absorbed is diminished mostly by radiant losses through
the aperture. For a perfectly insulated blackbody cavityreceiver, it is given by
gabsorption ¼ 1 rT 4
e
IC
ð2Þ
T is the nominal cavity-receiver temperature, and r the
Stefan–Boltzmann constant. The absorbed concentrated
solar radiation drives an endothermic chemical reaction.
The measure of how well solar energy is converted into
chemical energy stored in H2 for a given process is the
exergy eﬃciency, deﬁned as
gexergy ¼
_
nDGj
H2 þ0:5O2 !H2 O
Qsolar
ð3Þ
Wout
DG
DH
gabsorption
gCarnot
gexergy
r
rate of work output by fuel cell (kW)
Gibbs free energy change (kJ mol1 )
enthalpy change (kJ mol1 )
solar energy absorption eﬃciency
eﬃciency of a Carnot heat engine
exergy eﬃciency of the thermochemical
cycle
Stefan–Boltzmann constant (5.6705 · 108
W m2 K4 )
where n_ is the molar ﬂow rate of H2 produced and DG is
the standard Gibbs free energy change of the reaction at
298 K (237 kJ/mol), i.e., the maximum possible amount
of work that may be extracted from H2 at 298 K, when
both H2 and O2 are available at 1 bar. Since the conversion of solar process heat to chemical energy is limited by both the solar absorption and Carnot eﬃciencies,
the exergy eﬃciency of an ideal solar thermochemical
process is given by (Fletcher and Moen, 1977)
gexergy;ideal ¼ gabsorption gCarnot
rT 4
TL
1
¼ 1
e
T
IC
ð4Þ
with TL as the temperature of the thermal reservoir for
heat rejection, usually ambient temperature. The highest
temperature an ideal solar cavity-receiver is capable of
achieving, deﬁned as the stagnation temperature
Tstagnation , is calculated by setting Eq. (4) equal to zero,
yielding
!0:25
e
IC
ð5Þ
Tstagnation ¼
r
At this temperature, energy is being re-radiated as fast as
it is absorbed. Obviously, an energy-eﬃcient process
must operate at temperatures that are substantially
below Tstagnation . There is an optimum temperature
Toptimum for maximum eﬃciency obtained by setting
ogexergy;ideal
¼0
oT
ð6Þ
Assuming a uniform power-ﬂux distribution, this relation yields the following implicit equation for Toptimum :
!
e
TL I C
5
4
¼0
ð7Þ
Toptimum ð0:75TL ÞToptimum 4r
Toptimum varies between 1100 and 1800 K for uniform
power-ﬂux distributions with concentrations between
A. Steinfeld / Solar Energy 78 (2005) 603–615
605
2.1. H2 from H2 O by solar thermolysis
1000 and 13,000 (Steinfeld and Schubnell, 1993). For
e ¼ 5000, Toptimum ¼ 1507 K––giving a
example, for C
maximum theoretical eﬃciency of 75%, i.e. the portion
of solar energy that could in principle be converted into
the chemical energy stored in H2 . In practice, when
considering convection and conduction losses in addition to re-radiation losses, the eﬃciency will peak at a
somewhat lower temperature.
The exergy eﬃciency is an important criterion for
judging the relative industrial potential of the solar
process. The higher the exergy eﬃciency, the lower is the
required solar collection area for producing a given
amount of solar H2 , and, consequently, the lower are the
costs incurred for the solar concentrating system, which
usually correspond to half of the total investments for
the entire solar chemical plant (Steinfeld, 2002). Thus,
high exergy eﬃciency implies favorable economic competitiveness.
The single-step thermal dissociation of water is
known as water thermolysis,
H2 O ! H2 þ 0:5O2
ð8Þ
Although conceptually simple, reaction (8) has been
impeded by the need of a high-temperature heat source
at above 2500 K for achieving a reasonable degree of
dissociation, and by the need of an eﬀective technique
for separating H2 and O2 to avoid ending up with an
explosive mixture. Among the ideas proposed for separating H2 from the products are eﬀusion separation
(Fletcher and Moen, 1977; Bilgen, 1984; Kogan, 1998)
and electrolytic separation (Ihara, 1980; Fletcher, 1999).
Semi-permeable membranes based on ZrO2 and other
high-temperature materials have been tested at up to
2500 K by Kogan (1998) and by Diver et al. (1983), but
these ceramics usually fail to withstand the severe thermal shocks that often occur when working under highﬂux solar irradiation. Rapid quench by injecting a cold
gas (Lede et al., 1987), by expansion in a nozzle, or by
submerging an irradiated target in liquid water (Olalde
et al., 1988), are simple and workable, but the quench
introduces a signiﬁcant drop in the exergy eﬃciency and
produces an explosive gas mixture. Furthermore, the
very high temperatures demanded by the thermodynamics of the process (e.g. 3000 K for 64% dissociation
at 1 bar) pose severe material problems and can lead to
signiﬁcant re-radiation from the reactor, thereby lowering the absorption eﬃciency, Eq. (2).
2. Thermochemical processes
Five thermochemical routes for solar hydrogen production are depicted in Fig. 1. Indicated is the chemical
source of H2 : water for the solar thermolysis and the
solar thermochemical cycles, fossil fuels for the solar
cracking, and a combination of fossil fuels and H2 O for
the solar reforming and solar gasiﬁcation. All of these
routes involve endothermic reactions that make use of
concentrated solar radiation as the energy source of
high-temperature process heat (Steinfeld and Meier, in
press).
Concentrated
Solar Energy
H22O
Fossil Fuels
(NG, oil, coal)
H
H22O
O
Solar
Thermolysis
Solar
Thermochemical
Cycles
Cycles
Solar
Reforming
H2O
Solar
Cracking
Solar
Gasification
Gasification
CO2/C
Sequestration
Se uestration
Solar Hydrogen
Fig. 1. Five thermochemical routes for the production of solar hydrogen.
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A. Steinfeld / Solar Energy 78 (2005) 603–615
2.2. H2 from H2 O by solar thermochemical cycles
Water-splitting thermochemical cycles bypass the H2 /
O2 separation problem and further allow operating at
relatively moderate upper temperatures. Previous studies performed on H2 O-splitting thermochemical cycles
were mostly characterized by the use of process heat at
temperatures below about 1200 K, available from nuclear and other thermal sources. These cycles required
multiple steps (more than two) and were suﬀering from
inherent ineﬃciencies associated with heat transfer and
product separation at each step. Status reviews on multistep cycles are given by Serpone et al. (1992) and by
Funk (2001) and include the leading candidates GA’s 3step cycle based on the thermal decomposition of H2 SO4
at 1130 K (OKeefe et al., 1982), and the UT3’s 4-step
cycle based on the hydrolysis of CaBr2 and FeBr2 at
1020 and 870 K (Sakurai et al., 1996).
In recent years signiﬁcant progress has been accomplished in the development of optical systems for largescale solar concentration capable of achieving mean
solar concentration ratios exceeding 5000 suns. Such
high radiation ﬂuxes correspond to Tstagnation > 3000 K
and allow the conversion of solar energy to thermal
reservoirs at 2000 K and above which are needed for the
more eﬃcient 2-step thermochemical cycles using metal
oxide redox reactions (Steinfeld et al., 1998a,b):
y
1st step ðsolarÞ : Mx Oy ! xM þ O2
2
ð9Þ
2nd step ðnon-solarÞ : xM þ yH2 O ! Mx Oy þ yH2
ð10Þ
M denotes a metal and Mx Oy the corresponding metal
oxide. The ﬁrst, endothermic step is the solar thermal
dissociation of the metal oxide to the metal or the
lower-valence metal oxide. The second, non-solar,
exothermic step is the hydrolysis of the metal to form
H2 and the corresponding metal oxide. The net reaction is H2 O ¼ H2 + 0.5O2 , but since H2 and O2 are
formed in diﬀerent steps, the need for high-temperature
gas separation is thereby eliminated. This cycle was
originally proposed by Nakamura (1977) using the
redox pair Fe3 O4 /FeO. The solar step, i.e. the thermal
dissociation of magnetite to wustite at above 2300 K,
has been thermodynamically examined by Steinfeld
et al. (1999) and experimentally studied in a solar
furnace by Toﬁghi (1982) and by Sibieude et al. (1982).
It was found necessary to quench the products in order
to avoid re-oxidation, but quenching introduces an
energy penalty of up to 80% of the solar energy input.
The redox pair TiO2 /TiOx (with x < 2) has been considered by Palumbo et al. (1992, 1995). Solar experiments on the thermal reduction of TiO2 , conducted in
an Ar atmosphere up to 2700 K, produced mixtures of
Tin O2n1 with n ranging from 4 to 1, but the chemical
conversion was limited by the rate at which O2 diﬀuses
from the liquid–gas interface. Other redox pairs, such
as Mn3 O4 /MnO and Co3 O4 /CoO have also been considered, but the yield of H2 in reaction (10) has been
too low to be of any practical interest (Sibieude et al.,
1982; Lundberg, 1993). H2 may be produced instead by
reacting MnO with NaOH at above 900 K in a 3-step
cycle (Sturzenegger and N€
uesch, 1999). Partial substitution of iron in Fe3 O4 by other metals (e.g., Mn and
Ni) forms mixed metal oxides of the type
(Fe1x Mx )3 O4 that may be reducible at lower temperatures than those required for the reduction of Fe3 O4 ,
while the reduced phase (Fe1x Mx )1y O remains capable of splitting water (Ehrensberger et al., 1995; Tamaura et al., 1995, 1998).
One of the most favorable candidate metal oxide
redox pair for the 2-step cycle, reactions (9) and (10), is
presumably ZnO/Zn. Several chemical aspects of the
thermal dissociation of ZnO have been investigated
(Palumbo et al., 1998, and literature cited therein). At
2340 K, DG0 ¼ 0 and DH 0 ¼ 395 kJ/mol. The exergy
eﬃciency, Eq. (3), reaches 29% without any heat
recovery (Steinfeld, 2002). The theoretical upper limit in
the exergy eﬃciency, with complete heat recovery during
quenching and hydrolysis, is 82%. Weidenkaﬀ et al.
(2000a,b) reported activation energies determined by
thermogravimetry in the range 310–350 kJ/mol. Moeller
and Palumbo (2001a) derived the reaction rate law and
Arrhenius parameters for directly irradiated ZnO pellets. Weidenkaﬀ et al. (1999) studied the condensation of
zinc vapor in the presence of O2 by fractional crystallization in a temperature-gradient tube furnace. The oxidation of Zn is a heterogeneous process and, in the
absence of nucleation sites, Zn(g) and O2 can coexist in a
meta-stable state. Otherwise, they need to be quenched
to avoid their recombination. In particular, the quench
eﬃciency is sensitive to the dilution ratio of Zn(g) in an
inert gas ﬂow and to the temperature of the surface on
which the products are quenched. Alternatively, electrothermal methods for in situ separation of Zn(g) and
O2 at high temperatures have been pioneered by
Fletcher and his group, and experimentally demonstrated to work in small-scale reactors (Fletcher, 1999;
Fletcher et al., 1985; Parks et al., 1988; Palumbo and
Fletcher, 1988). High-temperature separation further
enables recovery of the sensible and latent heat of the
products (e.g., 116 kJ/mol during Zn condensation).
Various exploratory tests on the dissociation of ZnO
were carried out in solar furnaces (Bilgen et al., 1977;
Elorza-Ricart et al., 1999; Weidenkaﬀ et al., 2000a,b;
Lede et al., 2001; Moeller and Palumbo, 2001b). Fig. 2
shows the schematic conﬁguration of a solar chemical
reactor concept designed by Haueter et al. (1999) that
features a windowed rotating cavity-receiver lined with
ZnO particles that are held by centrifugal force. With
A. Steinfeld / Solar Energy 78 (2005) 603–615
Fig. 2. Schematic of the ‘‘rotating-cavity’’ solar reactor concept
for the thermal dissociation of ZnO to Zn and O2 at 2300 K. It
consists of a rotating conical cavity-receiver (#1) that contains
an aperture (#2) for access of concentrated solar radiation
through a quartz window (#3). The solar ﬂux concentration is
further augmented by incorporating a CPC (#4) in front of the
aperture. Both the window mount and the CPC are watercooled and integrated into a concentric (non-rotating) conical
shell (#5). ZnO particles are continuously fed by means of a
screw powder feeder located at the rear of the reactor (#6). The
centripetal acceleration forces the ZnO powder to the wall
where it forms a thick layer of ZnO (#7) that insulates and
reduces the thermal load on the inner cavity walls. A purge gas
ﬂow enters the cavity-receiver tangentially at the front (#8) and
keeps the window cool and clear of particles or condensable
gases. The gaseous products Zn and O2 continuously exit via an
outlet port (#9) to a quench device (#10) (Source: Paul Scherrer
Institute, Switzerland).
this arrangement, ZnO is directly exposed to high-ﬂux
solar irradiation and serves simultaneously the functions
of radiant absorber, thermal insulator, and chemical
reactant. Solar tests carried out with a 10 kW prototype
subjected to a peak solar concentration of 4000 suns
proved the low thermal inertia of the reactor system––
ZnO surface temperature reached 2000 K in 2 s––and its
resistance to thermal shocks.
The carbothermal reduction of metal oxides using
coke, natural gas (NG), and other carbonaceous materials as reducing agents brings about reduction of the
oxides at much more moderate temperatures. The corresponding overall chemical reactions may be represented as: 1
Mx Oy þ yCðgrÞ ! xM þ yCO
ð11Þ
Mx Oy þ yCH4 ! xM þ yð2H2 þ COÞ
ð12Þ
1
CH4 is taken as representative of NG, and carbon
(graphite) as representative of coal/coke.
607
Using NG as a reducing agent, Eq. (12), combines in a
single process the reduction of metal oxides with the
reforming of NG for the co-production of metals and
syngas (Steinfeld et al., 1998a,b). Thus, CH4 is reformed in the absence of catalysts and, with proper
optimizations, may be made to produce high-quality
syngas with an H2 :CO molar ratio of two, which is
especially suitable for synthesizing methanol––a potential substitute for petrol. Carbothermal reductions of
Fe3 O4 , MgO, and ZnO with C(gr) and CH4 to produce
Fe, Mg, Zn, and syngas have been demonstrated in
solar furnaces using packed/ﬂuidized beds and vortextype reactors (Steinfeld and Fletcher, 1991; Steinfeld
et al., 1993, 1995, 1998a,b; Kr€
aupl and Steinfeld, 2003;
Osinga et al., in press; Schneider, 2003). These reactions
are highly endothermic and proceed to completion at
reasonable rates above about 1500 K for Zn and Fe,
and 1800 K for Mg. Two examples of solar chemical
reactor concepts for producing Zn via reactions (11)
and (12) are shown in Figs. 3 and 4, respectively: the
‘‘two-cavity’’ solar reactor based on the indirect irradiation of ZnO + C (Osinga et al., in press), and the
‘‘vortex’’ solar reactor based on the direct irradiation of
Fig. 3. Schematic of the ‘‘two-cavity’’ solar reactor concept for
the carbothermal reduction of ZnO. It features two cavities in
series, with the inner one functioning as the solar absorber and
the outer one as the reaction chamber. The inner cavity (#1) is
made of graphite and contains a windowed aperture (#2) to let
in concentrated solar radiation. A CPC (#3) is implemented at
the reactor’s aperture. The outer cavity (#4) is well insulated
and contains the ZnO/carbon mixture that is subjected to
irradiation by the graphite absorber separating the two cavities.
With this arrangement, the inner cavity protects the window
against particles and condensable gases coming from the reaction chamber. Uniform distribution of continuously fed reactants is achieved by rotating the outer cavity (#5). The reactor is
speciﬁcally designed for beam-down incident radiation, as obtained through a Cassegrain optical conﬁguration that makes
use of a hyperbolical reﬂector at the top of the tower to re-direct
sunlight to a receiver located on the ground level (Source: Paul
Scherrer Institute, Switzerland).
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A. Steinfeld / Solar Energy 78 (2005) 603–615
Fig. 4. Schematic of a ‘‘vortex’’ solar reactor concept for the
combined ZnO reduction and CH4 reforming. It consists of a
cylindrical cavity (#1) that contains a windowed aperture (#2)
to let in concentrated solar energy. Particles of ZnO, conveyed
in a ﬂow of NG, are continuously injected into the reactor’s
cavity via a tangential inlet port (#3). Inside the reactor’s cavity, the gas-particle stream forms a vortex ﬂow that progresses
towards the front following a helical path. The chemical
products, Zn vapor and syngas, continuously exit the cavity via
a tangential outlet port (#4) located at the front of the cavity,
behind the aperture. The window (#5) is actively cooled and
kept clear of particles by means of an auxiliary ﬂow of gas (#6)
that is injected tangentially and radially at the window and
aperture planes, respectively. Energy absorbed by the reactants
is used to raise their temperature to above about 1300 K and to
drive reaction (12). (Source: Paul Scherrer Institute, Switzerland.)
ZnO + CH4 (Steinfeld et al., 1998a,b). Indirect-irradiated reactors such as the one depicted in Fig. 3 have the
advantage of eliminating the need for a transparent
window. The disadvantages are linked to the limitations
imposed by the materials of construction of the reactor
walls: limitations in the maximum operating temperature, thermal conductivity, radiant absorptance, inertness, resistance to thermal shocks, and suitability for
transient operation. Direct-irradiation reactors such as
the one depicted in Figs. 2 and 4 provide eﬃcient
radiation heat transfer to the reaction site where the
energy is needed, by-passing the limitations imposed by
indirect heat transport via heat exchangers. The major
drawback when working with reducing or inert atmospheres is the requirement for a transparent window,
which is a critical component in high-pressure and severe gas environments.
Calculation of the chemical equilibrium composition
for various metal oxides of interest shows that only the
carbothermic reduction of Fe2 O3 , MgO, and ZnO will
result in signiﬁcant free metal formation (Murray et al.,
1995). The carbides Al3 C4 , CaC2 , SiC, and TiC are
thermodynamically stable in an inert atmosphere; the
nitrides AlN, Si3 N4 , and TiN are stable in N2 atmosphere. These valuable high-temperature materials were
produced in solar furnaces (Duncan and Dirksen, 1980;
Murray et al., 1995; Smeets, 2003). CaC2 is well known
as the feedstock for the production of acetylene. The
nitrides and carbides AlN, Fe3 C, and Mn3 C may also
be used in cyclic processes as feedstock to produce
hydrogen and hydrocarbons, or may serve as intermediaries in the production of the metal. The hydrolysis of
AlN yields NH3 , the hydrolysis or acidolysis of Fe3 C
yields liquid hydrocarbons, and the hydrolysis of the
various carbides of manganese yields H2 and hydrocarbons in diﬀerent proportions. Thus, thermal and
carbothermal reduction processes may be incorporated
in 2-step thermochemical cycles of the type shown in
Fig. 5, in which the metal oxides that result from the
hydrolysis are recycled to the solar reactor. Metals and
lower-valence metal oxides can also serve as reducing
agents, as for the case of Fe and Ti reduction of ZnO
(Epstein et al., 2002; Palumbo et al., 1992). Solar
electrothermal reduction of metal oxides is another
alternative route for lowering the reduction temperature
and simultaneously accomplishing product separation.
It has been demonstrated experimentally for ZnO, using
an electrolytic cell housed in a solar cavity-receiver
(Palumbo and Fletcher, 1988). At 1000 K, up to 30% of
the total amount of energy required to produce Zn
could be supplied by solar process heat. Other interesting candidates for solar high-temperature electrolysis
are MgO and Al2 O3 .
As far as the hydrolysis step is concerned, reaction
(10), laboratory studies on the kinetics and preliminary
tests with a novel concept of a hydrolyser indicate that
the water-splitting reaction proceeds exothermally at
reasonable rates when steam is bubbled through molten
zinc at above about 700 K (Berman and Epstein, 2000;
Cortina, 2001). In principle, the heat liberated could be
used in an auto-thermal type of hydrolyser to melt zinc
and produce steam. Alternatively, if the H2 production
plant is realized next to the solar plant, molten zinc
could be withdrawn from the quencher at 700 K (or
higher) and fed directly to the hydrolyser. On the other
hand, transportation of solid zinc to the site where H2 is
ﬁnally utilized eliminates the need for troublesome
storage and transportation of H2 .
2.3. H2 by decarbonization of fossil fuels
Three solar thermochemical processes for H2 production using fossil fuels as the chemical source are
considered: cracking, reforming, and gasiﬁcation. These
routes are shown schematically in Fig. 6. The solar
cracking route refers to the thermal decomposition of
A. Steinfeld / Solar Energy 78 (2005) 603–615
609
Fig. 5. Scheme of 2-step thermochemical cyclic processes for the production of hydrogen and other synthetic ﬂuid fuels using water
and a reducing agent (coke or natural gas, except for the pure thermal decomposition) as feedstock, solar energy as the source of
process heat, and a metal as energy carrier and storage. In the ﬁrst, endothermic step, the metal oxide is thermally reduced to the metal,
a lower-valence metal oxide, a metal carbide, or a metal nitride. In the second, exothermic step, the reaction with water produces H2 ,
syngas, hydrocarbons, or ammonia. The metal oxide obtained in the second step is recycled to the ﬁrst step. M denotes metal, Mx Oy
metal oxide, Mx0 Oy0 lower-valence metal oxide, Mx Cy metal carbide, and Mx Ny metal nitride.
NG, oil, and other hydrocarbons, and can be represented by the simpliﬁed net reaction:
y
ð13Þ
Cx Hy ¼ xCðgrÞ þ H2
2
The steam-reforming of NG, oil, and other hydrocarbons, and the steam-gasiﬁcation of coal and other solid
carbonaceous materials can be represented by the simpliﬁed net reaction:
y
þ x H2 þ xCO
Cx Hy þ xH2 O ¼
ð14Þ
2
Other compounds may also be formed, depending on
the reaction kinetics and on the presence of impurities
in the raw materials. Reaction (13) yields a carbon-rich
condensed phase and a hydrogen-rich gas phase. The
carbonaceous solid product can either be sequestered
without CO2 release or used as material commodity or
reducing agent under less severe CO2 restraints. Reaction (14) yields syngas, the building block for a wide
variety of synthetic fuels including Fischer–Tropsch
type chemicals, hydrogen, ammonia, and methanol. Its
quality is determined mainly by the H2 :CO and
CO2 :CO molar ratios. For example, the solar steam-
gasiﬁcation of anthracite coal at above 1500 K yields
syngas with a H2 :CO molar ratio of 1.2 and a CO2 :CO
molar ratio of 0.01 (von Zedtwitz and Steinfeld, 2003).
The CO content in the syngas can be shifted to H2 via
the catalytic water-gas shift reaction (CO + H2 O ¼
H2 + CO2 ), and the product CO2 can be separated from
H2 using, for example, the pressure swing adsorption
technique. Some of these processes are practiced at an
industrial scale, with the process heat supplied by
burning a signiﬁcant portion of the feedstock. Internal
combustion results in the contamination of the gaseous products while external combustion results in a
lower thermal eﬃciency because of the irreversibility
associated with indirect heat transfer. Alternatively,
using solar energy for process heat oﬀers a threefold
advantage: (1) the discharge of pollutants is avoided;
(2) the gaseous products are not contaminated; and (3)
the caloriﬁc value of the fuel is upgraded by adding
solar energy in an amount equal to the DH of the
reaction.
Steinberg (1999) compared the diﬀerent decarbonization routes. From the point of view of carbon
sequestration, it is easier to separate, handle, transport,
and store solid carbon than gaseous CO2 . Further, while
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A. Steinfeld / Solar Energy 78 (2005) 603–615
The exergy eﬃciency is deﬁned as the ratio of the work
output (Wout ) by a 65%-eﬃcient H2 /O2 fuel cell 2 to the
total thermal energy input by solar and by the heating
value of the reactants:
Concentrated
Solar Energy
Solar
Cracking
Fossil Fuels
(NG , oil)
H2
gexergy ¼
C
Sequestration
Concentrated
Solar Energy
Fossil Fuels
(coal, NG, oil)
SolarGasification/
Reforming
H2O
H2
CO
H2 O
Shif t
Shift
Reactor
Reactor
H2
CO 2
Separation
CO 2
Sequestration
H2
Fig. 6. Schematic of solar thermochemical routes for H2 production using fossil fuels and H2 O as the chemical source: solar
cracking (upper box), and solar reforming and gasiﬁcation
(lower box).
the steam-reforming/gasiﬁcation method requires additional steps for shifting CO and for separating CO2 , the
thermal cracking accomplishes the removal and separation of carbon in a single step. In contrast, the major
drawback of the thermal decomposition method is the
energy loss associated with the sequestration of carbon.
Thus, the solar cracking may be the preferred option for
NG and other hydrocarbons with high H2 /C ratio. For
coal and other solid carbonaceous materials, the solar
gasiﬁcation via reaction (14) has the additional beneﬁt of
converting a solid fuel traditionally used to generate
electricity in Rankine cycles into a cleaner ﬂuid fuel––
cleaner only when using solar process heat––that can be
used in highly eﬃcient fuel cells.
Hirsch et al. (2001) and von Zedtwitz and Steinfeld
(2003) performed 2nd law analyses of the solar NG
cracking and the solar coal gasiﬁcation, respectively.
Wout
Qsolar þ HHVreactant
ð15Þ
where Qsolar is the speciﬁc solar energy input and
HHVreactant is the high heating value of the fossil fuel
being processed, e.g. about 890 kJ mol1 for NG, and
35,700 kJ kg1 for anthracite coal. The solar reactor is
assumed a blackbody cavity-receiver operated in the
temperature range 1350–1500 K and subjected to a mean
solar ﬂux concentration ratio in the range of 1000–2000.
For the solar thermal cracking of NG, the exergy eﬃciency amounts to 30%. This route oﬀers zero CO2
emissions as a result of carbon sequestration. However,
the energy penalty for completely avoiding CO2 reaches
30% of the electrical output, vis-
a-vis the direct
conventional use of NG for fueling a 55%-eﬃcient
combined Brayton–Rankine cycle. Higher exergy eﬃciencies––exceeding 65%––can be obtained when the
carbon is either steam-gasiﬁed to syngas in a solar gasiﬁcation process and the syngas further processed to H2 ,
or used as a reducing agent of ZnO in a solar carbothermal process for producing Zn and CO that are
further converted via water-splitting and water-shifting
to H2 . Any of these two alternative solar processes yield
2 additional moles of H2 per mole C(gr) and oﬀer a net
gain of 40% in the electrical output (and, consequently,
an equal percent reduction in the corresponding speciﬁc
CO2 emissions), as compared to the conventional combined cycle power generation. Thus, CO2 emissions are
reduced and NG is conserved. For the solar coal gasiﬁcation, the exergy eﬃciency amounts to 46%. This
route oﬀers a net gain in the electrical output by a factor
varying in the range 1.7–1.8 (depending on the coal
type), vis-
a-vis the direct use of coal for fueling a 35%eﬃcient Rankine cycle. Speciﬁc CO2 emissions amount
to 0.53–0.56 kg CO2 /kW he , about half as much as the
speciﬁc emissions discharged by conventional coal-ﬁred
power plants.
Reaction (13) has been eﬀected using solar process
heat with CH4 and C4 H10 at 823 K for the catalytic
production of ﬁlamentous carbon (Steinfeld et al., 1997;
2
State-of-the-art stationary SOFC fuel cells feature energy
conversion eﬃciencies in the range 55–60% when fed with
natural gas, and in the range 65–70% when fed directly with
hydrogen since the relative loss in the reformer is in the order of
10%. The H2 /CO2 separation unit is assumed to be based on the
PSA technique at 90% recovery rate (Ruthven et al., 1993). Its
minimum energy expenditure is equal to the DG of unmixing,
about 1% of the electric output of the fuel cell.
A. Steinfeld / Solar Energy 78 (2005) 603–615
Meier et al., 1999). The decomposition of several
hydrocarbons (methane, propane, gasoline) over carbon
catalysts in a bench-scale ﬂuidized bed was eﬀected at
1123 K and a corresponding kinetic model for CH4 decomposition was developed by Muradov (2000). It
was found that the crystallographic structure and the
speciﬁc surface area of the carbon species mostly
determine their catalytic activity. Lewandowski, Weimer, and co-workers designed and tested a solar tubular
quartz reactor containing ﬁne carbon black particles
suspended in a CH4 feed gas stream, and obtained up to
90% dissociation (Dahl et al., 2000, 2002). Such an
‘‘aerosol’’ solar reactor concept, shown in Fig. 7, features two concentric graphite tubular reactors, the outer
solid tube serving as the solar absorber and the inner
porous tube containing the reacting ﬂow. The vortex
solar reactor conﬁguration of Fig. 4 was also tested for a
CH4 ﬂow laden with carbon particles that serve simultaneously as radiant absorbers and nucleation sites for
the heterogeneous decomposition reaction (Hirsch and
Steinfeld, 2004).
The steam-gasiﬁcation of carbonaceous materials
and related reactions has been performed using concentrated solar energy in exploratory early studies with
coal (Gregg et al., 1979; Beattie et al., 1983; Kubiak and
Lohner, 1992), and with oil shales (Gregg et al., 1980;
Fig. 7. Scheme of the ‘‘aerosol’’ solar reactor concept for the
thermal cracking of NG (Source: National Renewable Energy
Laboratory, USA).
611
Fletcher and Berber, 1988; Ingel et al., 1992; Flechsenhar and Sasse, 1995). The CO2 -gasiﬁcation of coal was
eﬀected using a ﬂuidized bed reactor under direct irradiation (Kodama et al., 2002), and the heat transfer
characteristics have been analysed when using an
external radiative source for process heat (Belghit and
Daguenet, 1989). More recently, the reaction kinetics of
steam-gasiﬁcation of coal were investigated for a quartz
tubular reactor containing a ﬂuidized bed and directly
exposed to an external source of concentrated thermal
radiation (M€
uller et al., 2003). Several solar reactor
concepts have been proposed and tested with small-scale
prototypes (Malburg and Treiber, 1980; Gregg, 1980;
Frosch and Qader, 1981).
The solar reforming of NG, using either steam and
CO2 as partial oxidant, has been extensively studied in
solar concentrating facilities with small-scale solar
reactor prototypes using Rh-based catalyst (Levy et al.,
1989; Hogan et al., 1990; Richardson and Paripatyadar,
1990; Buck et al., 1991; Levy et al., 1992; Buck et al.,
1994; Muir et al., 1994; W€
orner and Tamme, 1998); and
in molten salt using other metallic catalysts (Kodama et
al., 2001; Gokon et al., 2002). The solar reforming
process has been scaled-up to power levels of 300–500
kW and tested at 1100 K and 8–10 bars in a solar tower
using two solar reforming reactor concepts: an indirectirradiation tubular reactor (Epstein and Spiewak, 1996)
and a direct-irradiation volumetric reactor (Tamme
et al., 2001; Moeller et al., 2002). The indirect-irradiated
solar reactor consists of a ceramic-insulated pentagonal
cavity-receiver containing a set of vertical Inconel tubes
ﬁlled with a packed bed of catalyst, usually 2% Rh on
Al2 O3 support. A matching CPC is implemented at the
windowless aperture for capturing radiation spillage,
augmenting the average solar ﬂux concentration, and
providing uniform irradiation of the tubes. The directirradiated solar reactor, also referred to as the ‘‘volumetric’’ reactor, is shown in Fig. 8. The main component
is the porous ceramic absorber, coated with Rh catalyst,
which is directly exposed to the concentrated solar
radiation. A concave quartz window, mounted at the
aperture, minimizes reﬂection losses and permits operation at elevated pressures. Similar to the indirect-irradiated reactor, a CPC is implemented at the aperture.
Dry-reforming of CH4 was also preformed in the solar
aerosol reactor of Fig. 7. Operating with residence times
on the order of 10 ms and temperatures of approximately 2000 K, CH4 and CO2 conversions of 70% and
65%, respectively, were achieved in the absence of any
added catalysts (Dahl et al., in press).
An optional source of H2 is H2 S, a toxic industrial
by-product derived from the natural gas, petroleum, and
coal processing. Current industrial practice uses the
Claus process to recover sulfur from H2 S, but H2 is
wasted by oxidizing it to H2 O. Alternatively, H2 S can be
thermally decomposed at 1800 K to co-produce H2 and
612
A. Steinfeld / Solar Energy 78 (2005) 603–615
CPC
Window
Catalytic absorber
Reactants inlet
Products outlet
Fig. 8. Scheme of a ‘‘volumetric’’ solar reactor concept for the reforming of NG (Source: Deutsches Zentrum f€
ur Luft- und Raumfahrt
e.V., Germany).
sulfur, which after quenching have a natural phase
separation,
H2 S ¼ H2 þ 0:5S2
ð16Þ
In contrast to H2 O thermolysis, solar experimental
studies on H2 S themolysis indicate that high degree
of chemical conversion is attainable and that the
reverse reaction during quench is negligible (Noring
and Fletcher, 1982; Kappauf et al., 1985; Kappauf
and Fletcher, 1989). A study delineating the chemical kinetics gives a quantitative rate expression for
H2 S decomposing in an Al2 O3 reactor (Harvey et al.,
1998).
2.4. Economical assessments
The economics of solar hydrogen production have
been assessed for H2 produced via reaction (12) (Steinfeld
and Spiewak, 1998), via reaction (13) (Spath and Amos,
2003), via reaction (14) (Spiewak et al., 1992), via reaction (16) (Diver and Fletcher, 1985), and via reactions (9)
and (10) (Steinfeld, 2002). These assessments indicate
that the solar thermochemical production of hydrogen
can be competitive with the electrolysis of water using
solar-generated electricity, and, under certain conditions,
might become competitive with conventional fossil-fuelbased processes at current fuel prices, even before the
application of credit for CO2 mitigation and pollution
avoidance. The weaknesses of these economic evaluations are related primarily to the uncertainties in the
viable eﬃciencies and investment costs of the various
components due to their early stage of development and
their economy of scale. Further development and largescale demonstration are warranted.
3. Summary
This paper is a review of research on the solar thermochemical production of hydrogen. Comprehensive
literature reviews on solar thermochemical possessing
have been carried out by Fletcher (2001) and by Steinfeld and Palumbo (2001).
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