Solar Power and Fuels - Forum VERA · 4 D i s h / S t i r l i n g P o w e r T o w e r Reduced...

1

Solar Power and Fuels

Aldo Steinfeld

PAUL SCHERRER INSTITUT


ConcentratedSolar Radiation

Prof. Aldo SteinfeldETH-Zurich

2

PAUL SCHERRER INSTITUTSolar Trough Technology

parabolic trough mirrors

SuperheaterSteam Generator

TurbineElectric Generator

CondenserNatural GasBoiler

Water

SEGS: Solar Electricity Generation Systems

• 354 MWe at Mojave Desert (SEGS I-II), Kramer Junction (SEGS III-VII), and Harper Lake (VIII-IX), since 1985.

• Operated by Kramer Junction Co.

• 2.5 million m2 parabolic troughs.

• Synthetic oil as HTF heated to 391°C.

• Thermal energy fed to turbines: 75% solar / 25% NG fired.

• 80% of all solar world electricity.

• Cost: 0.10 - 0.12 $/kWh (based on 1999 US$).

• Operational strategy: 55% income for selling 16% of solar electricity at peak hours.


3

Heliostat Field

Tower

Receiver

Hot Storage

Cold Storage

Steam Generator

TurbineElectric Generator

PAUL SCHERRER INSTITUTSolar Tower Technology


4

Dish/Stirling

Power Tower

Reduced Costs: 0,04 EURO/kWh

Dish/StirlingPower TowerParabolic Trough

EURO/kWh

0

0,10

0,05

0,20

0,15

0,25

0,30

0,35

Year2000 2005 2010 2015 2020

Parabolic Trough

PAUL SCHERRER INSTITUTSolar Electricity Cost


• Annual World Energy Consumption = 1.2 · 1014 kWh• Annual Irradiation = 2300 kWh/m2

• Conversion Efficiency = 20%

Annual Solar Irradiation (kWh/m2)

<1000 1000 1500 2000 2300 2500 (kWh/m2)

Ref: F. Krieth; J. Krieger,Principles of Solar EngineeringMc Graw Hill, 1978.

Area Requirement = 500 km x 500 km

• intermittent• unequally distributed• very dilute

• concentrated• stored• transported

Conversion into Chemical Energy Carriers


5

Heat

ThermochemistryElectrochemistry

+ -

Photochemistry

E

e-

hν

Solar Hydrogen


Solar FuelsReactants

HeatAbsorption

QH,TH

ChemicalReactor

∆H = 285 kJ/mol

FuelCell

W

QL,TL

∆G = 237 kJ/mol


H2OReactants H2 + ½ O2Solar Fuels



6

Solar FuelsReactants

HeatAbsorption

QH,TH

ChemicalReactor

DH = 285 kJ/mol

FuelCell

W

QL,TL

DG = 237 kJ/mol


H2OReactants H2 + ½ O2Solar Fuels

maximum Carnot 1 L

H

TT

η η= = −

4

absorption 1 HTC Iσ

η⎛ ⎞

= −⎜ ⎟⋅⎝ ⎠

Heat Engine


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Temperature [K]0 500 1000 1500 2000 2500 3000 3500 4000

Carnotabsorption η⋅η

Carnot

10005,000

10,000

20,000

40,000

For:I = 1 kW/m2 (1 sun)σ = 5.67.10-8 W/m2K2

C Tstagnation

1000 2049 K

5000 3064 K

10000 3644 K

C Tstagnation

1000 2049 K

5000 3064 K

10000 3644 K

Toptimal

1106 K

1507 K

1724 KFletcher and Moen, Science 197, 1050-1056, 1977.

Solar Hydrogen

Fossil FuelsH2O

CO2/C Sequestration

ConcentratedSolar Energy

H2OH2O

Thermolysis CrackingThermo-chemical Cycles

GasificationReforming



7

∆H°

∆G°

T∆S°

-50

0

50

100

150

200

250

300

1000 2000 3000 4000 5000

[kJ/

mol

]

Temperature [K]

H2OHOH2OHO2

0

0.10.2

0.30.4

0.50.60.7

0.80.9

1

2000 2500 3000 3500 4000

Temperature [K]

Equilibrium Mole Fractionp = 1 bar

2 2 2H O H + ½ O→PAUL SCHERRER INSTITUT

• Diver et al., Energy 12, 947-955, 1987.

• Kogan et al., Int. J. Hydrogen Energy 23, 89-98, 1998.



8

H2H2O

½ O2

recycle

HYDROLYSER

xM + yH2O = MxOy + yH2

SOLAR REACTOR

MxOy = xM + y/2 O2

MxOy

M

MxOy



HeatExchanger

Qrerad

ZnO@ 298 K

QuenchQquench

IdealFuelCell

WF.C.

QF.C.

Zn + ½ O2@ 2000 K

C = 5000

I = 1 kW/m2

T = 2000K∆H Zn+½O2 @ 300K → ZnO @ 2000K = 557 kJ/mol

Qsolar = 680 kJ/mol

Qrerad = 123 kJ/mol

Qquench = ∆H Zn + ½ O2 @ 2000K→ Zn + ½ O2 @ 300K

= - 209 kJ/mol

ZnO

QsolarConcentrated

SolarPower

Qsolar

∆Hrxnηabsorption = = 1- = 82%σT4

I×C

Zn½ O2

HydrolyserQhyd

H2 ZnO

IdealFuelCell

WF.C.

QF.C.

H2O

Qhydrolyser = ∆H Zn + H2O→ ZnO + H2 = - 62 kJ/mol

QF.C = (∆H - ∆G) = - 49 kJ/mol

WF.C = -∆G H2 + ½ O2 →H2O @ 300K = 237 kJ/mol

F .C.exergy

solar

WQ

η = 35% no h.r.58% with h.r.

⎧= ⎨⎩


9

600

800

1000

1200

1400

ZnO

-Tem

per

atu

re [

K]

Sh

utter [%

]

1600

1800

2000

2200

2400

10

20

30

40

50

60

Time [seconds]

70

80

90

100

100 200 300 400 5000 7006000

Temperature [K]

Shutter [%]600

800

1000

1200

1400

ZnO

-Tem

per

atu

re [

K]

Sh

utter [%

]

1600

1800

2000

2200

2400

10

20

30

40

50

60

Time [seconds]

70

80

90

100

100 200 300 400 5000 7006000

600

800

1000

1200

1400

ZnO

-Tem

per

atu

re [

K]

Sh

utter [%

]

1600

1800

2000

2200

2400

10

20

30

40

50

60

Time [seconds]

70

80

90

100

100 200 300 400 5000 7006000

Temperature [K]Temperature [K]

Shutter [%]Shutter [%]Shutter [%]Qsolar = 6.3 kWCpeak = 4000 suns

quartz windowCPC

ZnO

Zn + ½O2

rotating cavity

Solar Reactor Technology

ConcentratedSolarPower



PSI’s High-Flux Solar Furnace

kW/m2


10


Thermal Dissociation

ZnO → Zn + ½ O2 ∆G°2300K = 0; ∆H° = 557 kJ/mol

Carbon (coke, biomass, …)

ZnO + C → Zn + CO ∆G°1300K = 0; ∆H° = 370 kJ/mol

H2

• Qsolar = 5 kW• mZnO+C = 10 g/min• molar ratio ZnO:C = 0.9• Tinner cavity = 1700 K• Touter cavity = 1475 K• Zn rate = 0.5 kg/h• Zn purity = 100%• ηthermal = 20%

Experimental @ PSI‘s solar furnace

• Osinga et al., J. Solar Energy Engineering 126, 633-637, 2004.• Osinga et al., Ind. Eng. Chem. Res. 43, 7981-7988, 2004.

CPC

quartz window

inner cavity

outer cavity

ceramic insulation

rotation gear

Zn + CO

ZnO + C

Concentrated Solar Power

PAUL SCHERRER INSTITUTEU-SOLZINC: 300 kW Solar Reactor

• Partners: PSI, ETH, WIS, CNRS, ScanArc, Zoxy

300 kW solar power input

Lower cavity(reaction chamber)

Quartz window

Off-gas

ZnO-C-batch

Upper cavity(absorber)

140 cm

• Qsolar = 300 kW• C = 1500 suns• m0 = 116 kg• Treactor = 1500 K• Zn rate = 45 kg/h• Zn purity = 100%• ηthermal = 30%


11

ZnO/Zn-cycle: Hydrolysis

HydrolysisNanoparticleFormation

T-controlled

Mixing

Zn(g) generator

H2O(g) generator

ProductsHydrolysisNanoparticleFormation

T-controlled

Mixing

Zn(g) generator

H2O(g) generator

Products

Zn

ZnO

H2O


Fossil Fuels (NG, oil)

Solar Cracking Reactor

CH2 Sequestration

Fuel cell

Concentrated Solar Energy

Work Output

O2

Feed NG

H2, C, CHx

QuartzEnvelope

SolidC(gr)-absorber

PorousC(gr)-tube

Particle Feed

H2 SweepFeed NG

H2, C, CHx

QuartzEnvelope

SolidC(gr)-absorber

PorousC(gr)-tube

Particle Feed

H2 Sweep

NREL//UC CNRS WIS

2( )2x yyC H xC gr H= +


H2 + C(gr)+ C-particles

CH4+ C-particles

ETH//PSI

• Dahl J. et al., JSEE 127, 76-85, 2005. •Kogan et al., IJHE 30, 35-43, 2005.

Hirsch et al., Chem. Eng. Science 59, 5771-5778, 2004.

Abanades et al., IJHE 30, 843–853, 2005.

Cracking of NG


12

Solar Reforming

CO2


NaturalGas

H2O

COH2

Shift Reactor

Shift Reactor

H2OH2O

CO2H2

CO2H2

Separation

H2H2 Fuel cell WorkOutput

Sequestration (optional)

- 400 kW- Rh catalyst- 1100 K- 10 bar

Catalytic absorber

WindowCPC Reactants inlet

Products outletCatalytic absorber


Products outlet

Reforming of NG

DLR/WIS

EU-Project SOLREFPartners: DLR (D), WIS (IL), Hexion (NE), JM (UK), CERTH (GR), ESCO-Solar (I), ETH/PSI (CH)

2 2·2x yyC H xH O x H xCO⎛ ⎞+ = + +⎜ ⎟

⎝ ⎠



Solar Gasification

CO2


Petcoke

H2O

COH2

Shift Reactor

Shift Reactor

H2OH2O

CO2H2

CO2H2

Separation H2H2 Fuel cell WorkOutput


CC WorkOutput

SYNPETSolar Gasification

cokesolar LHVQOutputWork

+=η

η = 35 %E = 3.5 kWhe/kg

η = 46 %E = 6.1 kWhe/kg

SyngasFC

Rankine

Petcoke

H2

η = 51 %E = 6.6 kWhe/kg

CC

Solar Gasification

0

200

400

600

800

1000 g CO2 / kWhe

Coke-gasification to syngas + 55%-η CCCoke-gasification to H2 + 60%-η fuel cellCoke-combustion + 35%-η Rankine cycle


13


• Trommer et al., Int. J. Hydrogen Energy 30, 605-618, 2005.• v. Zedtwitz et al., Ind. Eng. Chem. Res. 44, 3852-3861, 2005.

Temperature [K]

Pro

duct

Gas

Com

posi

tion

[%]

Solar Gasification

CO2


Petcoke

H2O

COH2

Shift Reactor

Shift Reactor

H2OH2O

CO2H2

CO2H2

Separation H2H2 Fuel cell WorkOutput


CC WorkOutput

SYNPETSolar Gasification

• Qsolar = 5 kW• C = 3000 suns• mC = 4 g/min• Treactor = 1600 K• xC = 87 %• ηthermal = 19 %

ceramic cavity

ConcentratedSolarPower

quartz window

syngas

petcoke

H2O nozzle

Heliostat Field150$/m2

TowerTower Reflector+ CPC

Solar Reactor

Quencher

Hydrolyser

Balance of Plant Heliostat Field150$/m2

TowerTower Reflector+ CPC

Solar Reactor

Quencher

Hydrolyser

Balance of Plant

Cost of Solar H2 = 0.10 – 0.15 US$/kWh

(based on LHV of H2 = 241 kJ/mol)

Quench

Qquench

Zn½O2

Hydrolyser

H2

ZnO

H2O

Qreradiation

Qsolar

Qhydrolyser

SolarReactor

Heliostat Field

ZnO

Zn + ½ O2@ 2300K

Quench

Qquench

Zn½O2

Hydrolyser

H2

ZnO

H2O

Qreradiation

QsolarQsolar

Qhydrolyser

SolarReactor

Heliostat Field

ZnO

Zn + ½ O2@ 2300K

Solar H2 via Zn/ZnO cycle

PAUL SCHERRER INSTITUTCost Analysis for Solar H2


14

Solar Hydrogen

Fossil FuelsH2O


H2OH2O

Thermolysis CrackingThermo-chemical Cycles

GasificationReforming

Long-Term Short/Mid-Term Transition to Solar Hydrogen



• Steinfeld A., Palumbo R., “Solar Thermochemical Process Technology”, Encyclopedia of Physical Science and Technology, Vol. 15, pp. 237-256, 2001.

• Steinfeld A., “Solar Thermochemical Production of Hydrogen - A Review”, Solar Energy, Vol. 78/5, pp. 603-615, 2005.

• www.pre.ethz.ch


Solar Energy 78 (2005) 603–615

www.elsevier.com/locate/solener

Solar thermochemical production of hydrogen––a review

Aldo Steinfeld a,b,*

a Department of Mechanical and Process Engineering, ETH––Swiss Federal Institute of Technology, ETH-Zentrum ML-J42.1,

CH-8092 Zurich, Switzerlandb 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 thermo-

chemical 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 com-

binations––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 en-

ergy 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 de-

scribed in terms of their mean flux concentration ratio eCover a targeted area A at the focal plane, normalized

with respect to the incident normal beam insolation I ,

eC ¼ Qsolar

I � A ð1Þ

*Address: Department of Mechanical and Process Engi-

neering, 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).

0038-092X/$ - see front matter � 2004 Elsevier Ltd. All rights reserv

doi:10.1016/j.solener.2003.12.012

where Qsolar is the solar power intercepted by the target.eC is often expressed in units of ‘‘suns’’ when normalized

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 sys-

tems (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 com-

mercial 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 Stir-

ling cycles (Tyner et al., 2001). Solar thermochemical

applications, although not as far developed as solar

thermal electricity generation, employ the same solar

concentrating technologies.

ed.

mailto:[email protected]

Nomenclature

A area of reactor apertureeC mean solar flux concentration ratio (–)

HHV high heating value of a fuel

I normal beam insolation (kWm�2)

_n molar flow rate (mol s�1)

Qsolar solar power input to solar reactor (kW)

T temperature (K)

Tstagnation maximum temperature of a blackbody ab-

sorber

Toptimum optimum temperature of the solar cavity-

receiver for maximum gexergy

Wout rate of work output by fuel cell (kW)

DG Gibbs free energy change (kJmol�1)

DH enthalpy change (kJmol�1)

gabsorption solar energy absorption efficiency

gCarnot efficiency of a Carnot heat engine

gexergy exergy efficiency of the thermochemical

cycle

r Stefan–Boltzmann constant (5.6705 · 10�8

Wm�2 K�4)

604 A. Steinfeld / Solar Energy 78 (2005) 603–615

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 radi-

ation. Because of multiple internal reflections, the frac-

tion 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 cavity-

receiver approaches a blackbody absorber. Smaller

apertures reduce re-radiation losses, but they intercept a

reduced fraction of the sunlight reflected from the con-

centrators. 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 prin-

cipal concepts are summarized herein. The solar energy

absorption efficiency of a solar reactor, gabsorption, is de-

fined as the net rate at which energy is being absorbed

divided by the solar power coming from the concentra-

tor. At temperatures above about 1000 K, the net power

absorbed is diminished mostly by radiant losses through

the aperture. For a perfectly insulated blackbody cavity-

receiver, it is given by

gabsorption ¼ 1� rT 4

I eC� �

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� _nDGjH2þ0:5O2!H2O

Qsolar

ð3Þ

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 con-

version of solar process heat to chemical energy is lim-

ited 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

¼ 1

�� rT 4

I eC� 1

�� TL

T

�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

Tstagnation ¼ I eCr

!0:25

ð5Þ

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

oT¼ 0 ð6Þ

Assuming a uniform power-flux distribution, this rela-

tion yields the following implicit equation for Toptimum:

T 5optimum � ð0:75TLÞT 4

optimum � TLI eC4r

!¼ 0 ð7Þ

Toptimum varies between 1100 and 1800 K for uniform

power-flux distributions with concentrations between

A. Steinfeld / Solar Energy 78 (2005) 603–615 605

1000 and 13,000 (Steinfeld and Schubnell, 1993). For

example, for eC ¼ 5000, Toptimum ¼ 1507 K––giving a

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 addi-

tion 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 com-

petitiveness.

2. Thermochemical processes

Five thermochemical routes for solar hydrogen pro-

duction 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 H2O 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).

Solar Hydr

SolarThermolysis

SolarThermo-chemicalCycles

SolarReformin

H2O

Solar Ene

H2O

SolarThermo-chemicalCycles

SolarReformin

H2O

H2O

Concentra

Fig. 1. Five thermochemical routes for

2.1. H2 from H2O by solar thermolysis

The single-step thermal dissociation of water is

known as water thermolysis,

H2O ! 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 sepa-

rating 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 ther-

mal shocks that often occur when working under high-

flux solar irradiation. Rapid quench by injecting a cold

gas (L�ed�e 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 thermody-

namics 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 low-

ering the absorption efficiency, Eq. (2).

ogen

SolarCracking

SolarGasificationg

Fossil Fuels(NG, oil, coal)

CO2/CSe uestration

rgy

H2O

SolarCracking

SolarGasificationg

Fossil Fuels(NG, oil, coal)

CO2/CSequestration

H2O

ted

the production of solar hydrogen.


2.2. H2 from H2O by solar thermochemical cycles

Water-splitting thermochemical cycles bypass the H2/

O2 separation problem and further allow operating at

relatively moderate upper temperatures. Previous stud-

ies performed on H2O-splitting thermochemical cycles

were mostly characterized by the use of process heat at

temperatures below about 1200 K, available from nu-

clear 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 multi-

step cycles are given by Serpone et al. (1992) and by

Funk (2001) and include the leading candidates GA’s 3-

step cycle based on the thermal decomposition of H2SO4

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 accom-

plished in the development of optical systems for large-

scale 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):

1st step ðsolarÞ : MxOy ! xMþ y2O2 ð9Þ

2nd step ðnon-solarÞ : xM þ yH2O ! MxOy þ yH2

ð10Þ

M denotes a metal and MxOy 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 reac-

tion is H2O¼H2 + 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 Fe3O4/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 con-

sidered by Palumbo et al. (1992, 1995). Solar experi-

ments on the thermal reduction of TiO2, conducted in

an Ar atmosphere up to 2700 K, produced mixtures of

TinO2n�1 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 Mn3O4/MnO and Co3O4/CoO have also been con-

sidered, 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 substi-

tution of iron in Fe3O4 by other metals (e.g., Mn and

Ni) forms mixed metal oxides of the type

(Fe1�xMx)3O4 that may be reducible at lower temper-

atures than those required for the reduction of Fe3O4,

while the reduced phase (Fe1�xMx)1�yO remains capa-

ble of splitting water (Ehrensberger et al., 1995; Tam-

aura 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 pel-

lets. Weidenkaff et al. (1999) studied the condensation of

zinc vapor in the presence of O2 by fractional crystalli-

zation in a temperature-gradient tube furnace. The oxi-

dation 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, elec-

trothermal methods for in situ separation of Zn(g) and

O2 at high temperatures have been pioneered by

Fletcher and his group, and experimentally demon-

strated 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;

L�ed�e 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

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 water-

cooled 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).

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 reac-

tion chamber. Uniform distribution of continuously fed reac-


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 mate-

rials as reducing agents brings about reduction of the

oxides at much more moderate temperatures. The cor-

responding overall chemical reactions may be repre-

sented as: 1

MxOy þ yCðgrÞ ! xMþ yCO ð11Þ

MxOy þ yCH4 ! xM þ yð2H2 þ COÞ ð12Þ

1 CH4 is taken as representative of NG, and carbon

(graphite) as representative of coal/coke.

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 re-

formed 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 poten-

tial substitute for petrol. Carbothermal reductions of

Fe3O4, 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 vortex-

type 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 irra-

diation of ZnO+C (Osinga et al., in press), and the

‘‘vortex’’ solar reactor based on the direct irradiation of

tants is achieved by rotating the outer cavity (#5). The reactor is

specifically designed for beam-down incident radiation, as ob-

tained 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).

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 cav-

ity, 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, Switzer-

land.)


ZnO+CH4 (Steinfeld et al., 1998a,b). Indirect-irradi-

ated 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 tempera-

ture, thermal conductivity, radiant absorptance, inert-

ness, 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 atmo-

spheres is the requirement for a transparent window,

which is a critical component in high-pressure and se-

vere gas environments.

Calculation of the chemical equilibrium composition

for various metal oxides of interest shows that only the

carbothermic reduction of Fe2O3, MgO, and ZnO will

result in significant free metal formation (Murray et al.,

1995). The carbides Al3C4, CaC2, SiC, and TiC are

thermodynamically stable in an inert atmosphere; the

nitrides AlN, Si3N4, and TiN are stable in N2 atmo-

sphere. 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, Fe3C, and Mn3C may also

be used in cyclic processes as feedstock to produce

hydrogen and hydrocarbons, or may serve as interme-

diaries in the production of the metal. The hydrolysis of

AlN yields NH3, the hydrolysis or acidolysis of Fe3C

yields liquid hydrocarbons, and the hydrolysis of the

various carbides of manganese yields H2 and hydro-

carbons 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 inter-

esting candidates for solar high-temperature electrolysis

are MgO and Al2O3.

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 pro-

duction 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

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, MxOy

metal oxide, Mx0Oy0 lower-valence metal oxide, MxCy metal carbide, and MxNy metal nitride.


NG, oil, and other hydrocarbons, and can be repre-

sented by the simplified net reaction:

CxHy ¼ xCðgrÞ þ y2H2 ð13Þ

The steam-reforming of NG, oil, and other hydrocar-

bons, and the steam-gasification of coal and other solid

carbonaceous materials can be represented by the sim-

plified net reaction:

CxHy þ xH2O ¼ y2

�þ xH2 þ xCO ð14Þ

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. Reac-

tion (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+H2O¼H2 +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 gas-

eous 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 decarboni-

zation 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

Fossil Fuels(NG, oil)

SolarCracking

CH2 Sequestration



SolarGasification/Reforming

CO2

Fossil Fuels(coal, NG, oil)

H2OH2CO

Shif tReactor

ShiftReactor

H2OH2O

Separation

H2

Sequestration

H2CO2

Fig. 6. Schematic of solar thermochemical routes for H2 pro-

duction using fossil fuels and H2O as the chemical source: solar

cracking (upper box), and solar reforming and gasification

(lower box).

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.


the steam-reforming/gasification method requires addi-

tional steps for shifting CO and for separating CO2, the

thermal cracking accomplishes the removal and sepa-

ration 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.

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:

gexergy ¼Wout

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 kJmol�1 for NG, and

35,700 kJ kg�1 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 effi-

ciency 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 effi-

ciencies––exceeding 65%––can be obtained when the

carbon is either steam-gasified to syngas in a solar gas-

ification process and the syngas further processed to H2,

or used as a reducing agent of ZnO in a solar carbo-

thermal 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 com-

bined cycle power generation. Thus, CO2 emissions are

reduced and NG is conserved. For the solar coal gasi-

fication, 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 C4H10 at 823 K for the catalytic

production of filamentous carbon (Steinfeld et al., 1997;


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, Wei-

mer, 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, fea-

tures 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 simul-

taneously 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 con-

centrated 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).

Fletcher and Berber, 1988; Ingel et al., 1992; Flechsen-

har and Sasse, 1995). The CO2-gasification of coal was

effected using a fluidized bed reactor under direct irra-

diation (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 indirect-

irradiation 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

Al2O3 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 direct-

irradiated solar reactor, also referred to as the ‘‘volu-

metric’’ 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 oper-

ation at elevated pressures. Similar to the indirect-irra-

diated 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 approxi-

mately 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 H2S, 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 H2S, but H2 is

wasted by oxidizing it to H2O. Alternatively, H2S can be

thermally decomposed at 1800 K to co-produce H2 and

Catalytic absorber


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,

H2S ¼ H2 þ 0:5S2 ð16Þ

In contrast to H2O thermolysis, solar experimental

studies on H2S 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 chemi-

cal kinetics gives a quantitative rate expression for

H2S decomposing in an Al2O3 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 reac-

tion (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-fuel-

based processes at current fuel prices, even before the

application of credit for CO2 mitigation and pollution

avoidance. The weaknesses of these economic evalua-

tions 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 large-

scale demonstration are warranted.

3. Summary

This paper is a review of research on the solar ther-

mochemical production of hydrogen. Comprehensive

literature reviews on solar thermochemical possessing

have been carried out by Fletcher (2001) and by Stein-

feld and Palumbo (2001).

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