Received 30 November 2009; received in revised form 5 March 2010; accepted 19 April 2010

Keywords: Thermochemical cycle; Hydrogen; Ferrites; Receiverreactor; Solar tower

* Corresponding author. Tel.: +49 2203 601 2673.E-mail address: martin.roeb@dlr.de (M. Roeb).

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Available online at www.sciencedirect.com

Solar Energy 85 (2011) 634644Available online 11 May 2010

Communicated by: Associate Editor Tatsuya Kodama

In memoriam: Peter Rietbrock

Abstract

The present work describes the realisation and successful test operation of a 100 kW pilot plant for two-step solar thermo-chemicalwater splitting on a solar tower at the Plataforma Solar de Almera, which aims at the demonstration of the feasibility of the process on asolar tower platform under real conditions. The process applies multi-valent iron based mixed metal oxides as reactive species which arecoated on honeycomb absorbers inside a receiverreactor. By the introduction of a two-chamber reactor it is possible to run both processconcepts in parallel and thus, the hydrogen production process in a quasi-continuous mode. In summer 2008 an exhaustive thermal qual-ication of the pilot plant took place, using uncoated ceramic honeycombs as absorbers. Some main aspects of these tests were the devel-opment and validation of operational and measurement strategy, the gaining of knowledge on the dynamics of the system, in particularduring thermal cycling, the determination of the controllability of the whole system, and the validation of practicability of the controlconcept. The thermal tests enabled to improve, to rene and nally to prove the process strategy and showed the feasibility of the controlconcept implemented. It could be shown that rapid changeover between the modules is a central benet for the performance of theprocess.

In November of 2008 the absorber was replaced and honeycombs coated with redox material were inserted. This allowed carrying outtests of hydrogen production by water splitting. Several hydrogen production cycles and metal oxide reduction cycles could be run with-out problems. Signicant concentrations of hydrogen were produced with a conversion of steam of up to 30%. 2010 Elsevier Ltd. All rights reserved.Test operation of a 100 kW pilot plant for solar hydrogenproduction from water on a solar tower

M. Roeb a,*, J.-P. Sack a, P. Rietbrock a, C. Prahl b, H. Schreiber a, M. Neises a, L. de Oliveira a,D. Graf a, M. Ebert b, W. Reinalter b, M. Meyer-Grunefeldt b, C. Sattler a, A. Lopez c,

A. Vidal c, A. Elsberg d, P. Stobbe d, D. Jones e, A. Steele e, S. Lorentzou f, C. Pagkoura f,A. Zygogianni f, C. Agraotis f, A.G. Konstandopoulos f

aDeutsches Zentrum fur Luft- und Raumfahrt e.V. (DLR), Institute of Technical Thermodynamics, Solar Research, Linder Hohe, 51147 Koln, GermanybDeutsches Zentrum fur Luft- und Raumfahrt e.V. (DLR), Institute of Technical Thermodynamics, Solar Research, Plataforma Solar de Almera,

Carretera de Senes s/n, km 5, E-04200 Tabernas, SpaincCentro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), Plataforma Solar de Almera, Carretera de Senes s/n,

km 5, E-04200 Tabernas, SpaindStobbe Tech Ceramics, Malmmosevej 19c, 2840 Holte, Denmark

e Johnson-Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, UKfAerosol and Particle Technology Lab./Centre for Research and Technology Hellas, 6th km Harillaou-Thermi Road, 57001 Thermi-Thessaloniki, Greece0038-092X/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.solener.2010.04.014

ner1. Introduction

Emission-free hydrogen production from water is possi-ble by solar-powered thermo-chemical cycles. A two-stepcycle can be based on a metal oxide redox pair system,which can split water molecules by abstracting oxygenatoms and reversibly incorporating them into its lattice.During the rst step of this cycle (the water splitting) thereduced and therefore activated material usually thelower-valence oxide of a metal exhibiting multiple oxida-tion states is oxidised by taking oxygen from water andproducing hydrogen according to the reaction:

MOreduced H2O!MOoxidised H2 1In the next step (the regeneration step) the material is

reduced again (regenerated), setting some of its latticeoxygen free according to the reaction:

MOoxidised !MOreduced 1=2O2 2The redox pairs used for this cycle are ferrites and mixed

metal oxides with iron as the main component (Charvinet al., 2007; Ishihara et al., 2008; Kodama et al., 2006;Allendorf et al., 2006; Han et al., 2007; Steinfeld et al.,1999, 2002; Sturzenegger and Nuesch, 1999; Gokonet al., 2008).

The HYDROSOL research group has introduced theconcept of monolithic honeycomb solar reactors for per-forming these redox pair cycles for the production ofhydrogen from the splitting of steam using solar energy(Agraotis et al., 2005) inspired from the well-known auto-mobile catalytic converters. The basic idea proposed, devel-oped and demonstrated within the HYDROSOL andHYDROSOL-II projects was to combine a monolithichoneycomb support structure capable of achieving hightemperatures when heated by concentrated solar radiation,with a redox pair system suitable for the performance ofwater dissociation and for regeneration at these tempera-tures. With this conguration, the complete operation ofthe whole process (water splitting and regeneration of themetal oxide) can be achieved by a single solar energy con-verter. In addition, by using a two-chamber reactor the twosteps of the cycle can be performed in parallel and thereforea hydrogen production process in a quasi-continuous modecan be achieved (Roeb et al., 2006a). After the HYDRO-SOL reactor, several other dierent approaches on solarreceiver concepts for using mixed ferrites have been pro-posed in the scientic literature (Kaneko et al., 2006; Kod-ama and Gokon, 2007; Diver et al., 2006, 2008). Howeverthe essential advantage of the HYDROSOL reactor overthe other reactor concepts above is that, in contrast tothose which in order to meet the technical requirementsof the thermochemical cycle made up of two process stepsperformed at dierent temperature levels with dierent heatdemands either use two dierent reactors or employ reactorparts that are continuously rotating between a higher- and

M. Roeb et al. / Solar Ea lower-temperature zone the HYDROSOL reactor con-tains neither moving parts nor moving solid particles. Thedierent heat demands of each step are realised in theHYDROSOL process not by moving the reactors but byadjusting the ux density on each reactor module whenthe status of the cycle is switched from regeneration tosplitting and vice versa by re-alignment of a part of thesolar concentrators. The proof-of-principle of this conceptwas validated with iron-based oxides coated on ceramicmonolithic structures, which were placed inside a solarreceiverreactor. After successful experimental demonstra-tion of several cycles of alternating hydrogen and oxygenproduction and elaboration of process strategies, presentedin previous contributions (Roeb et al., 2006b, 2008), thepresent work describes the realisation and successful testoperation of a 100 kW pilot plant on a solar tower, whichaimed at the demonstration of the feasibility of the processunder the real operation conditions of a solar towerplatform.

2. Pilot plant

2.1. Layout and operation strategy

The successful testing and the suitable behaviour of thelaboratory-scale reactor in the earlier stages of the projecthave provided a basis for the design of the pilot reactor.Whereas the general reactor concept was kept, the scale-up of the reactor from 10 kWth to 100 kWth was basicallyrealised by increasing the absorber surface. The reactor isset up of two reactor modules to run both steps of the cyclein parallel (photograph in Fig. 1a). Fig. 1b represents aschematic explosion view of the reactor to demonstrateits main components. Three times three (a total of 9) indi-vidual pieces of square-shaped monolithic honeycombabsorbers made of siliconized silicon carbide (siSiC) (Fendet al., 2004; Agraotis et al., 2007), each with a dimensionof 146 146 mm were assembled as one absorber module.The honeycomb absorbers exhibit a length of 60 mm andare mounted to form a square with slightly shaped concavesurface. They are kept in place by springs applying forcefrom the rear side. Into the center channel of each honey-comb monolith a thermocouple has been inserted fromthe back to monitor the absorber temperature near the irra-diated front face. The end of the thermocouple is located5 mm behind the front face and is shielded from directsolar radiation. To additionally monitor the temperaturedistribution on the receiver surface a thermocamera isinstalled in a distance of about 10 m on an arm in frontof the receiver. With dierent lters it is possible to mea-sure the quartz glass window temperature and the temper-ature on the surface of the absorber inside the reactorbehind the quartz glass window.

The feed gas is fed in through channels in the outer partof the reactor surrounding the hot core. Product andsweep gases are collected in one central exhaust pipeattached to the rear part of the housing. Each module of

gy 85 (2011) 634644 635the reactor is equipped with a quartz window xed by awater-cooled window frame. The distance between the cen-

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ner(a)

636 M. Roeb et al. / Solar Eters of the two modules is 1.3 m so that the dierent andhomogeneous ux distributions required for each processstep can be realised separately on each module withoutaecting the other one. The pilot reactor has been installedon the tower of the so-called Small-Solar-Power-System(SSPS) solar tower plant (Fig. 2) of the Plataforma Solarde Almera (PSA), Spain. The heliostat eld of this solarfacility is able to provide about 1.5 MWth (Geyer andSchiel, 1987). Thus, for the pilot plant only a part of theheliostat eld is needed.

For powering the two modules of the reactor with dier-ent solar ux it is necessary to divide the heliostat eld intodierent parts and to actuate those separately (Roeb et al.,2008, Fig. 3). The heliostats (of the so-called Martin Mar-

(b)

housing

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se

Fig. 1. (a) 100 kWth pilot reactor and (gy 85 (2011) 634644ietta type), consist of 12 facets each with a total surface of39.3 m2. The power supply of each heliostat is realised

aling

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EExxppeerriimmeennttaall PPllaattffoorrmm

Fig. 2. SSPS heliostat eld and tower.

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M. Roeb et al. / Solar Eindependently by an own photovoltaic cell. All in all 93individual-tracking heliostats are available with an averageclean reectivity of 87% concentrating the power at theaperture. During the splitting process, steam is fed in froma steam generator into the so-called east reactor chamberoperating at 800 C. At the same time, nitrogen as ushinggas is fed into the west reactor chamber operating at1200 C, in order to release the oxygen from the metaloxide redox system. After a half-cycle of 2030 min, a partof the heliostats focus is moved from the west to the eastchamber to realise the necessary temperature increase upto 1200 C to perform the regeneration of the redox systemin the east chamber, whereas the west chamber is cooleddown to 800 C to proceed with the splitting of water atthat temperature.

2.2. Hardware

Besides the solar part and all peripheral units for gasand steam supply and feeding, pre-heating, and productgas piping and treatment, the system additionally includesseveral devices for analytics. Those are besides thermocou-ples at various positions of the absorber/reactor, a moving

Focus 1 (east): Groups 1, 2, 3, 4

Focus 2 (west): Groups 5, 6, 7, 8

Switch-over groups (east and west): 0, 9, 10, 11

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gy 85 (2011) 634644 637bar ux measurement system, an infrared camera to mon-itor alternatively the surface temperature of the absorber orof the quartz window, a mass spectrometer and a gas chro-matograph including pumping and gas cooler to enable themonitoring of the hydrogen produced. Fig. 4 gives a sche-matic overview on the peripheral hardware of the pilotplant.

The feed streams of steam and nitrogen are adjusted bymass ow controllers. Up- and downstream the controllersa temperature and pressure measurement probe is installed.The steam generator is equipped with a reservoir includingcharging level sensors. Three gas pre-heaters are installed,two for heating the nitrogen feed stream and one for hold-ing the temperature of the nitrogen/water steam mixtureabove 110 C in order to avoid condensation. For theswitch between hydrogen production and metal oxideregeneration and vice versa magnetic valves are used toalternate the ow between the two modules.

Temperature measurements are performed at the inletand at the outlet of the reactor. In the exhaust line probesfor the gas chromatograph (GC) and for the mass spec-trometer (MS) measurements are integrated. There, a partof the product gas is sucked from the reactor o-gas stream

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ner638 M. Roeb et al. / Solar Einto those systems for analysis. All these components arehoused beside the reactor in a cabinet on the tower. Out-side the tower, a thermo camera is located in a distanceof about 10 m on an arm in front of the receiver, whichmeasures the temperature distribution at the absorber.Using several lters, measurements of the temperature onthe quartz glass window and on the surface of the absorberbehind the window can be carried out.

3. Thermal qualication

First, a series of thermal tests with uncoated monoliths,i.e. without a redox system, were carried out. The objectivewas to gain knowledge about the thermal behaviour of thepilot plant in particular of the receiverreactor and thepossibility and practicability of controlling it by dierentmeans. All tests were carried out with air instead of nitro-gen and without water vapour. A specic intention was todetermine a practicable operational strategy concerning theoptimum of temperatures and mass ows. These tests werealso aimed at the denition of standard operational param-eters as reference for parametric tests on thermal perfor-mance and hydrogen production. The experimental focusof attention was on the one hand the temperature at thecentral honeycomb of both modules which in most casesexhibited also the maximum temperature and on theother hand the average temperature of all monoliths. Tocontrol the temperature, the volume ow and pre-heater

Fig. 4. Flow sheet of thgy 85 (2011) 634644temperature for the working medium air as well asthe number of heliostats focused on the two modules ofthe receiver were systematically varied or used to compen-sate for the uctuations of the receiver temperature causedby other (outer) parameters like Direct Normal Irradiance(DNI), ambient temperature, wind speed and direction.

During the thermal test campaign, the two monolithswere operated individually with dierent solar power, dif-ferent operating temperatures and dierent mass ow ofthe feed gas to most authentically simulate the operationwhen producing hydrogen. Thus, dierences and inuencesof both modules can be observed by parallel operation(Fig. 5). The operational conditions were switched after atypical time of about 30 min for a half-cycle by movingthe focus of some heliostats from one module to the otherand by switching the valves for the gas supply.

The inuence of defocusing and focusing of heliostatswas examined for both temperature levels (Fig. 6). It wasobserved that the absorbers heat up faster than they cooldown. Thus, after the switch, the level of 1200 C targetedfor the regeneration is reached earlier than the level of800 C targeted for water splitting. The time for switchingthe temperature levels is dened as the period until bothreach 95% of their desired level. The speed by which helio-stats are shifted determines the time needed to mutuallytransfer the status of the modules. So far, more than tenminutes are needed to cool down the hotter modules to800 C. This is in particular true for the rear part of the

e process hardware.

nerM. Roeb et al. / Solar Emonoliths and the outlet temperature of the gas: a sharpreaction after changing solar ux is observed at the absor-ber front, where the thermocouples are located, whereasthe outlet temperature needs more time to react on achange of solar power like the one described. In fact, theeective time for a switch is much shorter (

3era

tim

nerof the redox material should not signicantly exceed1250 C to avoid the degradation of the material.

To achieve a fast switching of the temperature levels, thenumber of heliostats moved should be as high as possibleuntil the target temperatures are reached. Fig. 6 gives animpression on the strategy of realising such a fast change-over and on the number of heliostats needed to keep thetwo temperature levels. The number of heliostats focusedon the modules was adjusted according to the require-

0 10 200.00

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Fig. 7. Specic amount of additional hydrogen versus regeneration

640 M. Roeb et al. / Solar Ements; during the switchover of temperature levels betweenthe modules, the heliostats were not simply refocused fromone module to the other. Additionally to the refocusedheliostats, some heliostats were moved to standby toaccelerate the cool down, and on the other hand on themodule supposed to heat up, additional heliostats werefocused to facilitate the switchover. The number of helio-stats was further adjusted in order to achieve the exact levelof required temperatures. Fig. 8 shows the solar power pro-vided by the heliostats to each of the two chambers, asmeasured by a moving bar system in front of the receiverapertures. To keep a temperature level of 800 C roughly17 kW were necessary, whereas about 45 kW were neededto keep a temperature level of 1200 C.

Parametric studies were carried out to nd out parame-ters suitable to control the process, especially to adjust andmaintain the temperature levels within the half-cycles.Those parameters were rstly the mass ow of air (usedas the working medium), secondly the pre-heating temper-ature of the feed (here air), and thirdly the number of helio-stats focussed on the two apertures.

Concerning the mass ow of air it can be said in generalthat a variation of the mass ow of the feed could be a use-ful instrument to simultaneously inuence the temperatureprole of the whole receiver. This is at least true for thene-tuning of the operational temperature, which meansthat temperature uctuations caused by solar ux uctua-tions of up to about 80 K could be compensated by massow adjustments. For such compensation the mass owhas to be changed signicantly (5080% mass ow reduc-tion of the initial value for temperature increase, or by afactor of 45 for a comparable temperature decrease).Since this inuences the conditions for the chemical processdrastically in particular the reaction kinetics the mass

0 40 50 60

T = 1080 C T = 1135 C T = 1150 C T = 1180 C

tion time in min

e for dierent regeneration temperatures (ironzinc mixed oxide).

gy 85 (2011) 634644ow is not regarded as the rst choice of controlparameters.

Feed pre-heating had only a very small eect on absor-ber temperatures if the temperature of the working mediumair is raised from ambient temperature to 250 C at theinlet of the reactor. Therefore they are not suitable as ameans to inuence the operational temperature signi-cantly and to control the process.

The preferred way of controlling the process tempera-ture is by the heliostats themselves. Fig. 9 represents theresults of an experiment involving control by heliostatsonly. The individual heliostats of the SSPS eld providedierent foci which dier in size and solar ux distribution.Some provide very sharp-edged foci, others provide verybig images with low solar ux. This fact was used to setup a control strategy by using so-called high-ux mirrorsfor the coarse adjustment, i.e. temperature steps of about100 K, and low-ux heliostats for the ne-tuning of thetemperature control, i.e. temperature steps of about 1020 K. An accurate temperature control is in particular nec-essary for the high temperature reaction, the regeneration,to avoid on the one hand overheating and on the otherhand to ensure sucient reaction rates (see above). Fig. 9demonstrates the eect of varying the number of those dif-ferent heliostats focused on the two apertures and the fea-

nerM. Roeb et al. / Solar Esibility of the described control concepts by just using thenumber of heliostats for temperature control. For bothtemperatures, 800 C and 1200 C, the control by heliostatscan be applied to suciently ensure steady states.

4. Hydrogen production experiments

When preparing the hydrogen production experimentspreliminary tests with water vapour in the reactor were car-ried out. A mass ow of 3.5 kg/h steam and a mass ow of15 Nm3/h nitrogen turned out as the most suitable opera-tion conditions to start with. After that the blank siSiCmonolithic absorber structures were replaced by a set of

Fig. 8. Power on the aper

Fig. 9. Eect of varying numbergy 85 (2011) 634644 64118 monoliths coated with ironzinc mixed oxide. In thebeginning of the hydrogen production tests, both chamberswere initially heated up to 800 C to achieve steady-state asfar as possible. Afterwards, two full cycles (twice heatingup to 1200 C and twice cooling down to 800 C) were car-ried out in both reactor modules (Fig. 10).

The composition of the o-gas stream was detected bythe GC for all cycles for both modules and by the MS justfor the western module. The hydrogen concentrationsdetected in one of the rst experiments are displayed inFig. 11. There, the dots symbolize the signal given by theGC and the line in between was calculated as linear inter-polation between two sampling points. This curve only

ture of each module.

of heliostats on temperature.

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ner642 M. Roeb et al. / Solar Egives a rough idea about the progress of hydrogen concen-tration during the test, since only every 100 s a measure-ment was performed and recorded. Nevertheless, somequalitative information can be drawn out. The rst broadpeak at t = 5000 s was obviously caused by splitting resid-ual water in the apparatus. But apparently the highest out-put of hydrogen was produced in the eastern moduleduring the rst cycle at about t = 7000 s. The measuredconcentration corresponds to a conversion of 30% of thesteam fed in. After that a reduction of hydrogen concentra-tion and therefore of also the yield by a factor of about twooccurred. This eect is similar to what has been observedearlier in smaller reactors in the lab and in the solar furnaceand is attributed mainly to deactivation of the particularredox system and to a minor extent to inhomogeneous tem-perature distribution of the absorber. There is evidencethat some of the zinc material in the mixed oxide, volatiliz-es during cyclic operation resulting in a reduction of theactivity of the redox material from its initial value. The

Fig. 10. Temperature and steam mass

Fig. 11. Concentration of hydgy 85 (2011) 634644strongly diminished hydrogen production indicated bythe last peak was caused by the occurrence of a leakageand therefore by inltrated air in the reactor. Nevertheless,about 35 g of hydrogen were produced in the very rstthree half-cycles, which would mean that about 500 g ofhydrogen could have been produced, if the experimentwas extended all over the day. The target is to increasethe daily production to more than 3 kg per day by mainlyimproving the hydrogen production rate of the redox mate-rial, by decreasing the heat rejection of the receiver and byimproving the recovery of sensible heat in the process.

More recently another experimental campaign using thesame coated monoliths has been carried out. Despite sev-eral weeks of interruption the redox material exhibitedthe same activity with that during the cycles shown inFig. 11. After several cycles the hydrogen yield settled ata more or less constant level, which is about a factor of veless than the rst cycle. Currently the redox material hasbeen renewed by replacing the coated absorber monoliths

ow rate of rst water splitting tests.

rogen detected by the GC.

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Solar Energy Engineering 128, 37.

nerto enable to start another experimental campaign withoptimised process conditions.

5. Conclusions

A 100 kW pilot plant for two-step solar thermo-chemi-cal water splitting via monolithic honeycomb solar reactorshas been developed, installed and test operated at the SSPSsolar tower plant at PSA in Spain. The reactor concept isstrongly based on the HYDROSOL two-chamber solarreceiverreactor developed and tested for quasi-continuoushydrogen production in the solar furnace of DLR inCologne. The feasibility of the process has been demon-strated under real conditions at PSA. An exhaustive ther-mal qualication of the pilot plant has been carried outusing uncoated ceramic honeycombs as absorbers. Thosetests helped to develop and validate operational and mea-surement strategy as well as to create essential knowledgeon the dynamics of the system, in particular during thermalcycling. One main aspect was the necessity of a rapidchangeover between the modules as a central benet forthe performance of the process. Such quick changeoverafter completion of a half- cycle could be realised by mov-ing as many as possible heliostats until the target tempera-tures are reached.

Potential control parameters have been analysed whichare capable of ensuring sucient constant temperature lev-els. It was concluded that there is only little usefulness ofemploying feed gas pre-heating and mass ow of the feedgas as control parameters, whereas the preferred way ofcontrolling the process temperature is by the heliostatsthemselves. Solar ux uctuations are compensated byadding or removing individual heliostats to the two foci.A control strategy was set up by using high-ux mirrorsfor the coarse adjustment and low-ux heliostats for thene tuning of the temperature control. For both testedcycle temperatures, 800 C and 1200 C, the control byheliostats can be applied to ensure suciently steady states.In the end the practicability of the implemented controlconcept could be shown.

Some test series applying honeycombs coated with redoxmaterial have been also carried out. This allowed practicaltesting of solar hydrogen production by water splitting in a100 kW-scale. Several hydrogen production andmetal oxidereduction cycles could be run without problems. Signicantconcentrations of hydrogen were produced with a conver-sion of steamof up to 30%.Like in lab-scale experiments deg-radation of the redox material is an issue in the pilot plant aswell. This problem is being addressed in on-going materialdevelopment studies. The pilot plant tests are being contin-ued by using absorber monoliths with dierent coating andby stepwise optimising the process conditions.

Acknowledgements

M. Roeb et al. / Solar EThe authors would like to thank the European Commis-sion for co-funding of this work within the ProjectRoeb, M., Monnerie, N., Schmitz, M., Sattler, C., Konstandopoulos, A.,Agraotis, C., Zaspalis, V.T., Nalbandian, L., Steele, A., Stobbe, P.,2006a. Thermo-chemical production of hydrogen from water by metaloxides xed on ceramic substrates. In: Proceedings of the 16th WorldHydrogen Energy Conference, Lyon, France, June 1316.

Roeb, M., Sattler, C., Kluser, R., Monnerie, N., de Oliveira, L.,HYDROSOL-II Solar Hydrogen via Water Splitting inAdvanced Monolithic Reactors for Future Solar PowerPlants (SES6-CT-2005-020030), under the Sixth Frame-work Programme of the European Union (20022006).

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Test operation of a 100kW pilot plant for solar hydrogen production from water on a solar towerIntroductionPilot plantLayout and operation strategyHardware

Thermal qualificationHydrogen production experimentsConclusionsAcknowledgementsReferences