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Solar Energy 97 (2013) 26–38

Analysis and improvement of a high-efficiency solar cavityreactor design for a two-step thermochemical cycle for solar

hydrogen production from water

Anis Houaijia ⇑, Christian Sattler, Martin Roeb, Matthias Lange, Stefan Breuer,Jan Peter Sack

Institute of Solar Research, German Aerospace Center (DLR), Linder Hoehe, 51147 Cologne, Germany

Received 19 February 2013; received in revised form 24 July 2013; accepted 27 July 2013Available online 30 August 2013

Communicated by: Associate Editor Michael Epstein

Abstract

The present study is related to the European research project HYDROSOL 3D dealing with a two-step thermochemical cycle forwater splitting based on the use of redox materials. The overall process of a plant with a power input of 1 MWth will be developedand analyzed. This includes the definition of core components, e.g. solar reactors, heat exchangers, compressors, hydrogen separationunit and the elaboration of the flow sheet of the process. By process simulation components exergy efficiencies and thus main sourcesfor exergy losses are determined. The results were used to identify and suggest possible improvements. Since the solar receiver-reactorwas identified as the pre-dominant source of exergy losses a new design for such reactor was developed as described in the second part ofthe study based on the experimental experiences with a reactor developed and tested in previous projects. The main objective of theimprovement of the design was to increase the efficiency by minimizing the re-radiation losses. With the support of a raytracing toola combination of a cavity design, a hemispherical absorber shape and a secondary concentrator was derived as the most suitable reactordesign exhibiting at least 25 percentage points less thermal losses than the previous version which was realized and tested as part of a pilotplant.� 2013 Elsevier Ltd. All rights reserved.

Keywords: Solar; Hydrogen; Thermochemical; Two-step cycle; Process simulation; Exergy analysis

1. Introduction

A promising pathway for the solar production of hydro-gen is solar water splitting via a two-step thermochemicalcycle using a metal oxide redox system. The main advanta-ges of this process if compared to solar electricity pluswater electrolysis are the avoiding of electrochemical stepsrequiring expensive electrical power and if compared todirect water thermolysis the avoiding of the H2/O2

0038-092X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.solener.2013.07.032

⇑ Corresponding author. Tel.: +49 2203 601 3999; fax: +49 2203 6014141.

E-mail address: Anis.Houaijia@dlr.de (A. Houaijia).

separation problem since the oxygen is being fixed by themetal oxide. Furthermore, lower temperatures are requiredfor the reaction and the regeneration step compared to thedirect water thermolysis. Thermochemical cycles aimto decompose water into its main parts, hydrogen andoxygen.

The process heat for the thermochemical cycle can beprovided by solar concentrating systems (Eisenstadt andCox, 1975; Bowman, 1980). Temperatures up to 2000 �Ccan be achieved. The usage of solar heat to run a two-stepcycle was proposed by Nakamura (1977). The consideredredox system was Fe3O4/FeO, which allows hydrogen gen-eration through a redox reaction according the followingequations:

A. Houaijia et al. / Solar Energy 97 (2013) 26–38 27

H2OþMOred !MOox þH2 ð1Þ

MOox !MOred þ1

2O2 ð2Þ

The water decomposition occurs at low temperatures inthe range of 500–800 �C by reacting with the reduced formof the oxide MOred and leads to the formation of the oxideMOox and H2. The reduction of the oxidized form of theoxide MOox, which is highly endothermic, is thermody-namically favored at temperatures above 1400 �C. The listof redox pairs, which can be used for this process, is longconsisting of oxide pairs of multivalent metal oxides.Prominent examples are iron oxide Fe3O4/FeO (Nakam-ura, 1977; Charvin et al., 2007), manganese oxideMn3O4/MnO (Sturzenegger and Nuesch, 1999), ceriumoxide CeO2/Ce2O3 (Abanades and Flamant, 2006), ferrites(Tamaura et al., 1998; Agrafiotis et al., 2005; Miller et al.,2006; Kodama and Gokon, 2007) or metal oxide/metalpairs like zinc oxide/zinc ZnO/Zn (Steinfeld, 2002; Schunket al., 2008).

The basic idea of the HYDROSOL projects was to com-bine a thermally stable support structure with a coating ofsuch redox pair systems. The support structure has to standelevated temperatures when heated by solar radiation,while the redox pair systems have to be suitable for the per-formance of water dissociation and regeneration at thesetemperatures (Roeb et al., 2005, 2006). This way, the oper-ation of the whole process can be realized by a single solarenergy converter. Within those projects mostly ferrites wereused as redox pair (Neises et al., 2009, 2010). The redoxactive materials have been fixed on porous ceramic mono-lithic structures (honeycomb structures) which are locatedinside a solar receiver-reactor and flushed with water vaporor inert gas, depending on the operating mode. Receiver-reactors have been developed based on this operation prin-ciple and tested on sun in different sizes (Roeb et al., 2006,2011). Experiments and simulations revealed a draw-backof the current reactor layout (Neises et al., 2009, 2010).Because of the high temperatures and the large exposedabsorber surface area the radiation losses of the receiverare quite high. In the following this effect is addressed inmore detail and a way to overcome this drawback isdepicted.

Based on an exergy analysis of the complete plant, astudy to improve the thermal efficiency of the receiver-reac-tor is presented. A basic ray-tracing analysis led to a newreceiver design which is evaluated in the last part of thispaper from a heat loss point of view.

2. Process and plant description

A design study of solar thermochemical water splittingbased on the HYDROSOL technology has been per-formed. The main characteristic of the HYDROSOL pro-cess is that it contains neither moving parts nor movingsolid particles. While HYDROSOL works with honey-combs, other concepts work for example with foam

structures (Furler et al., 2012). The different heat demandsof each step are rather realized by adjusting the flux densityon each reactor module when the status of the cycle isswitched from regeneration to splitting and vice versa byre-alignment of a part of the solar concentrators. By settingup the overall receiver of an array of several receiver-reac-tor modules and by partitioning the heliostat field such aprocess concept was put into practice as a 100 kW pilotplant (Roeb et al., 2011). To investigate measures necessaryto improve the plant and to define components capable totransfer the technology to a larger scale, a design study wasundertaken. The study considers a thermal input of 1 MW.The designed plant includes the solar water splitting reac-tor chambers and all necessary upstream and downstreamunits needed to feed in the reactants and to separate theproducts. Fig. 1 shows the assembly and the piping ofthe reactor chambers on the solar tower platform and theperipheral components. A 4/2-way valve, upstream thereactor inlet piping (5), ensures that the regenerating reac-tor group is fed with nitrogen while overheated steam isbeing injected in the other reactor group (8/9). The outletpiping converges in another 4/2-way valve (6). The productgas from the water splitting step is led through the vapor-izer 2 and the preheater for heat recovery. The gas from theregenerating reactor group is released to the environment,because the recycling by separation into the componentsoxygen and nitrogen is considered too expensive at the1 MW scale examined here.

The product gas consists of mainly hydrogen and watervapor. A condenser liquefies the water vapor. By thatmeans the water is separated from the remaining compo-nents. The condensate is returned to the water tank, whichfeeds the reactors. The cooling water has got an own cir-cuit. Further, the product gas is diluted by nitrogen whenswitching from regeneration to water splitting mode. Thenitrogen is separated by a PSA unit (Pressure SwingAdsorption) from the hydrogen. Two floors below the reac-tor chamber two buffer containers are located, which com-pensate pressure surges in pumping fluids and by thisensure a continuous flow (see Fig. 2).

3. Process analysis

3.1. Process simulation

A steady-state simulation of the processes was carriedout by using the commercial simulation tool Aspen Plus,which is a well-known simulation environment for flowcharts of chemical processes. The following Fig. 3 showsthe flow sheet of the process.

Demineralized water is fed at ambient conditions (25 �Cand 1 bar) to the preheater PREHX, where it is heated to85 �C by the product gas PRODUCT2. After the pre-heat-ing, the water stream WATER3 is divided in the splitterSPLITT1 into the two sub-streams WATER4 andWATER6. The first sub-stream is evaporated in the heatexchanger EVAPORA1 by the product gas PRODUCT1,

Fig. 1. Plant design inside the tower platform.

Fig. 2. Sectional view of the 1 MWth plant.

28 A. Houaijia et al. / Solar Energy 97 (2013) 26–38

while the second one is evaporated in the heat exchangerEVAPORA2/EVAPORA3 by the stream O2N2-1, whichleaves the solar reactor REGNE-R at 1344 �C. After theevaporation, the streams WATER6 and WATER7 aremixed in the mixer MIX1. Then, the stream WATER8 isoverheated in the super-heater SUPERHX1/SUPERHX2to 545 �C by the stream O2N2-2, which contains N2 andO2. The redox material is normally fixed in the reactor,but due to the fact that ASPEN PLUS does not offer areactor model, which treats solids in a way correspondingto the HYDROSOL process, the reduced material has beensimulated as a stream which will be re-introduced to thewater-splitting reactor REAC-R. After regeneration, theFeO stream INT5 is removed in a cyclone and the stream

O2N2-1 is used for the water evaporation of the sub-streamWATER4 and the overheating of the stream WATER8.

The overheated steam WATER9 is introduced to thewater splitting reactor REAC-RE with the redox materialstream FEO, which has been already heated to 992 �C bythe stream INT5. The water stream WATER9 flows intothe reactor REAC-RE at a temperature of 545 �C. Accord-ing to ASPEN calculation, the water splitting reactiontakes place at 934 �C. A H2O to H2 conversion of 35%has been assumed.

Within the project it was decided to select the pressureswing adsorption (PSA) as the separation technology ofthe N2 from the product gas. In order to make the producthydrogen available at elevated pressure, e.g. to make it

Fig. 3. Flow sheet of the process.

A. Houaijia et al. / Solar Energy 97 (2013) 26–38 29

available for bottling, the pressure of the product streamwas chosen to be 15 bar, which represents the operatingpressure of the PSA. The compression takes place in a 2-stage compressor (COMP1 and COMP2) with inter-cool-ing. The compression ratio of each stage is 3.89 with anisentropic efficiency of 0.69. The storage pressure of thehydrogen has been defined at 30 bar for the demonstrationplant. Therefore, a 2-stage compression with intercooling(COMP3 and COMP4) is required in order to reach thispressure.

3.2. Exergy analysis

Exergy analysis is commonly used to quantify and local-ize the thermodynamic losses in industrial processes. Theprocess components, which exhibit the most importantinfluence on the heat balance, can be identified so thatpotential measures can be taken in order to improve theefficiency of the overall process. Based on the simulationresults carried out with ASPEN Plus, an exergy analysisof the process has been performed. The exergy analysisidentifies the location, the magnitude and the sources ofthermodynamic inefficiencies in a thermal system (Bejanet al., 1996). This information cannot be provided by othermeans (e.g. an energy analysis). The exergy efficiency ofeach component has been calculated according to the fol-lowing equation (Tsatsaronis and Cziesla, 2002):

ek ¼_EP ;k

_EF ;k

ð3Þ

Here, _EP ;k and _EF ;k denote the exergy streams of theproduct and the exergy stream of the fuel. The rate of exer-gy destruction in the kth component is given by:

_ED;k ¼ _EF ;k � _EP ;k � _EL;k ð4Þ

The fuel represents the resources expended to generatethe product, e.g. in the case of the reactor the incident solarradiation and the educt. The value for the chemical exergyin this case is determined mainly by the higher heatingvalue of the produced hydrogen. Here _EL;k represents theexergy loss in the kth component, which is usually zerowhen the component boundaries are at T0. For the overallsystem, the exergy loss includes the exergy flow rates of allstreams leaving the system. In addition to ek and _ED;k; thethermodynamic evaluation of a system component is basedon the exergy destruction ratio yD,k, which compares theexergy destruction in the kth component with the fuel sup-plied to the overall system _EF ;tot:

yD;k ¼_ED;k

_EF ;tot

ð5Þ

The overall exergetic efficiency of the process is given bythe following equation:

etot ¼_EP ;tot

_EF ;tot

ð6Þ

Table 1 shows the results of the exergy analysis for thepresent process flow sheet depicted in Fig. 3.

With the aid of the exergy analysis, the thermodynamicinefficiencies of the process and the amount of the exergydestruction in the system have been determined. The resultshows that the solar reactor is responsible for almost 60%of the exergy destruction within the process. Other compo-nents such as heat exchangers, compressors and hydrogenseparation unit contribute to a very small extent to theoverall thermodynamic inefficiencies of the process. There-fore, the solar reactor is selected as the first option to iden-tify and suggest potential improvements. Measures toimprove the exergy efficiency of this reactor are introducedand described in the following chapter.

30 A. Houaijia et al. / Solar Energy 97 (2013) 26–38

4. Solar reactor

In order to decrease the exergy destruction in the solarreactor, modifications of the reactor design have to be car-ried out. A key issue is to decrease the radiation losses inthe reactor since the exergy destruction is related to thethermodynamic losses, which occur in the solar reactor.The main heat loss mechanism of the reactor was identifiedto be the thermal radiation of the hot absorber front to theambience. Since the necessary temperature levels are giventhe decrease of the reactor aperture and in particular the re-radiation per absorber surface area are regarded the mosteffective measure to decrease those losses – by using a “cav-ity” design. Heat losses in the reactor are also due to theswitching between operation modes. This issue will be ana-lyzed in another paper based on the new designed reactorpresented in this study. The following section presents theconsiderations and calculations carried out to derive a suit-able reactor configuration for the present process.

4.1. Development of absorber shape

A main objective of the new design was to improve itsthermal performance. Since the reactor concept is basedon a gas tight reaction zone, a window at the radiation inletis inevitable. This leads to the limitation that the apertureof a cavity cannot exceed a radius of about 0.5 m, becausequartz windows of larger size would break due to thermalstress under given radiation conditions. As a consequence,a modular reactor design is necessary to reach an overallaperture area large enough for 1 MW thermal input.

When arranging several cavities next to each other, sec-ondary concentrators need to be placed in front of the cav-ities to avoid spillage of radiation between the individualaperture areas, i.e. the openings of the cavities.

A basic study was carried out to conduct a screeningof possible absorber shapes when exposed to radiationfrom a secondary concentrator. To conduct this study,

Fig. 4. Frame of secondary concentrator used for all raytracing runs, cpcshape being foreshadowed.

the raytracing code VeGas (Petrasch, 2010) was used. Inthe following, this study will be described in more detail.

In order to simplify the raytracing runs, the secondaryconcentrator was modeled as four walls enclosing a nar-rowing cross section towards the cavity aperture area, seeFig. 4.

All edge points of the secondary concentrator are posi-tioned on the shape of a compound parabolic concentratorwhich represents the ideal shape for secondary concentra-tors (Welford and Winston, 1978). The mirrors are mod-eled as specular reflectors, their reflectivity being 90%.The incoming radiation was defined to be homogeneouslydistributed over a half angle of 20�, this value being a typ-ical opening angle of a heliostat field. The total incomingpower was normalized, i.e. set to Pin = 1 W. To allow fora meaningful comparison of the performance of the differ-ent absorber shapes, the secondary concentrator and theincoming radiation are kept constant in the following anal-ysis. Another boundary condition for all absorber geome-tries was that the projection of the absorber shouldalways lie within the projection of the secondary inlet aper-ture (in z-direction). This constraint is necessary to permitthe modular design. The absorber material was alwaysmodeled to have an absorptivity of 90%.

4.1.1. Geometries for absorber shape optimization

In a preliminary screening, two promising absorbertypes were identified: conical and spherical geometries.Since the Hydrosol 2 design is quadratic and rather flat,this shape is also included in the analysis as a reference.

In a parametric study, the following geometric relationswere changed:

1. The distance dz between the absorber and the secondaryconcentrator outlet. To realize a change of this distance,the geometries were changed as depicted in Fig. 5: In thecase of the conical and spherical absorber shapes, analuminum oxide spacer (diffuse reflectivity of 90%) ofvariable height was introduced in the ray-tracing runs.In the case of the flat absorber surface, a top shield con-sisting of four aluminum oxide walls covers theabsorber.

2. For the conical and spherical shapes, another parameterwas changed, this parameter being the opening angle ofthe cone and the curvature of the spherical cap respec-tively. The cone angle is defined as the angle included

Fig. 5. Realization of the change in distance between absorber andsecondary concentrator outlet, shown for the three shapes analyzed.

Fig. 6. Quality values Q (–) for flat absorber shape.

Fig. 7. Quality values Q (–) for conical absorber shape.

Fig. 8. Quality values Q (–) for spherical absorber shape.

A. Houaijia et al. / Solar Energy 97 (2013) 26–38 31

within the cross section of the cone. The curvature of thespherical cap is defined by the fraction of the half widthof the inlet aperture and the sphere radius, i.e. cm ¼ x

R inFig. 5. This way a curvature of cv = 1 describes a hemi-sphere while a curvature of cv! 0 leads to a flat shape.

4.1.2. Quality criterion

In order to compare different absorber shapes, a qualitycriterion has to be defined in the form of a target function.The first criterion to qualify the performance is the inter-cepted power on the absorber surface. The intercept isdefined as the ratio of the power absorbed on the absorbersurface (Pabs) to the power entering the secondary concen-trator (Pin), see Eq. (7). The incoming power is always setto 1 W in this study:

IC ¼ P abs

P in

ð7Þ

Apart from a high intercepted power, a large absorbersurface is of great importance for the present applicationto provide for a large reactive site. This requirement is dif-ferent from other applications with secondary concentra-tors, which usually have the purpose to increase theconcentration ratio. The surface of the absorber isdescribed dimensionless, being divided by the inlet aperturearea of the secondary concentrator:

A0abs ¼Aabs

Asec o;inð8Þ

Another constraint that needs to be accounted for is ahomogeneous flux profile on the absorber surface. This isnecessary, because the reaction kinetics are highly temper-ature dependent. While other receiver concepts of solarthermal applications foresee temperature control by adapt-ing the mass flow in different zones of the receiver, this isnot a reasonable procedure in the present kind of applica-tion: If one changed the mass flow within the reactor, thereaction would be of different speed within the honeycomb.As only the whole honeycomb can be switched from onereaction mode to the other, a constant reaction time withinthe honeycomb should be aimed at.

As a consequence, the flux profile on the absorbershould be as homogeneous as possible to ensure homoge-neous temperatures. In the present study, only areas withan irradiative flux within a limited range are accountedfor as “useable” irradiation on the absorber surface. Themaximum flux on the receiver should be within this rangeto avoid overheating of the honeycombs. The rangebetween the maximum flux and 60% of it are accountedfor to be “useable”. This constraint is integrated in the tar-get function by only taking areas into account which liewithin this useable range (index 60). This is applied as wellfor the area calculation as for the intercept calculation,leading to two new quantities IC60 and A0abs;60.

Although the value of 60% is quite arbitrary, it is stillimportant to include this constraint in the analysis. The

main outcome of the study, which is a recommendationof an absorber shape, does not change if this value of60% is replaced by 70% or 80%, but it does change if thisconstraint is left out completely. As a consequence of theabove, the target function describes the quality Q of anabsorber shape and can be formulated as:

Q ¼ IC60 � A0abs;60 ð9Þ

Fig. 10. Flux profile on spherical absorber (projection in xy-plane)corresponding to cv = 0.8 and dZ/D_abs = 0.25 (poor quality).

32 A. Houaijia et al. / Solar Energy 97 (2013) 26–38

4.1.3. Results

The quality criterion of Eq. (9) is shown as a function ofthe changed parameters in Figs. 6–8 for the three differentabsorber shapes. Since in the case of the flat absorbershape, only one parameter was changed, a line plot resultsfrom the parameter study as opposed to the surface plots inthe other two cases. While for the flat absorber shape, thequality value stays below 0.3 all the time, it reaches up toabove 0.6 in the case of the conical shape. For the sphericalshape, the best quality value can be reached. It is above 0.8and thus more than three times higher than in the case ofthe flat absorber and about 30% higher than the best valuefrom the conical case.

As not only the qualitative difference between the threeshapes can be observed (which could have been derived bycommon sense), but also the quantitative superiority of thespherical shape is so striking, it was decided to create thenew receiver design based on a spherical shape. The betteroptical performance of the receiver shall make up for thehigher complexity of the mounting structure.

It can be observed that the quality of the sphericalabsorber shape increases strongly when reaching the com-plete hemisphere (curvature towards one). The maximumof the quality function is reached at a spacer length ofabout 0.4 times the inlet diameter of the concentrator.The corresponding flux profile on the spherical absorberis displayed as a projection in the x–y plane in Fig. 9. Asthe entering power was normalized to 1 W, the resultingflux values should not be taken as final values in the plant.As a comparison, the flux profile corresponding to a curva-ture of 0.8 (spherical cap) and a spacer length of 0.25 timesthe inlet diameter is shown in Fig. 10. As this shape is clo-ser to the exit aperture of the secondary concentrator, itreceives more absolute power than the optimum shape.However, the flux distribution is not as homogeneousand furthermore, the area of the spherical cap is smallerdue to the smaller curvature. Therefore, the quality valueof this shape is inferior.

The results of the raytracing analysis are not ofgreat depth, for example the thermal radiation within the

Fig. 9. Flux profile on spherical absorber (projection in xy-plane)corresponding to cv = 1 and dZ/D_abs = 0.48 (optimum).

absorber region was not accounted for. However, thisstudy is rather to be understood as a screening method tocompare different geometries. Thermal radiation will defi-nitely improve the homogeneity of the flux on the absorber,especially in the cases of the conical and spherical absorbershapes. This may be subject to more detailed future work.As the results are quite unambiguous, no further investiga-tion was regarded necessary for the present purpose.

4.2. Reactor design

Based on the findings of the previous section the reactordesign was modified by introducing significant changescompared to the previous reactor designs in particular con-cerning absorber shape and absorber fixation.

The new reactor design is characterized by the followingmain aspects:

� The overall shape of the absorber is close to ahemisphere.� A hemispherical outer hull holds the insulation material.� The reactor body shall be the outer part of the reactor,

hence holding the parts together. In addition, it shallbe the boundary between the feed fluid and theenvironment.� Another intermediate hemispherical shell separates the

feed from the outgoing fluid. In this part, there is anadditional heat exchange between the feed and theoutlet.� The inner part is the holding structure for the coated

honeycombs. It consists of a hemisphere, which holdsthe structures and carries out a good distribution forthe outlet gas stream.� The inlet absorber cup holding structure is welded on

the front plate of the reactor. In this part, the quartzwindow is located, too. Furthermore, this is the bearingpart of the reactor.

A sectional view of the reactor body is shown in Fig. 11.

A. Houaijia et al. / Solar Energy 97 (2013) 26–38 33

The fluid flows through the inlet ring (9), which sur-rounds the outlet pipe. For a most homogeneous currentinside the gas lead, there are baffle plates installed. Thegas stream is led between the outer body (2) and the middlehemisphere (3) towards the inlet absorber (5 and 6). In thisstream, a first heat exchange is taking place between theoutgoing and the incoming fluid. Furthermore, the inletfluid acts as a heat shielding between the hot inner partand the outer part. After this, the fluid is further heatedup in the inlet absorber, because some irradiation hits thesecups. The fluid enters the chamber between the quartz win-dow (7) and the coated absorber cups (13). At this point thereaction takes place. The gases leaving the honeycomb areled to the hot part of the heat exchange to the inlet gas.This is between the inner body (4) and the middle hemi-sphere (3). Subsequently, the gas leaves the reactor at theoutlet (10) to the periphery. The bellow (10) is used to com-pensate thermal expansion of the reactor and the piping.

Almost all individual parts of the reactor are connectedby screw-fixation. This means that they are able to movewhile the thermal expansion takes place, but the parts aregenerally immobile. This is needed to make sure that aneasy disassembling can be carried out. The insulation hullconsists of three different parts. One is the front plate,which is directly fixed on the reactor front plate by screws.Only a quite small space is left for the insulation material.The hull itself is divided into two equal parts, which arefixed by screwing and clip collars to each other and tothe front plate. The reactor main body is assembled tothe front plate by a flange connection. The inlet holdingstructure is welded onto the front plate and has the shapeof a Z-profile, because of its high stability. The inlet absor-ber cups are fixed with a spring ring to stainless steel tubesin the holding structure. On top of the holding structure,the middle hemisphere is radially connected with screws.

All the coated absorber cups are fixed the same wayonto the inner body by spring rings. This fixation has

Fig. 11. Sectional view of t

been proofed within the HitRec concept (Hoffschmidtet al., 2001). The outlet pipe is flanged at the middle hemi-sphere and has a duct through the outer body, where the inletpipe composes the outer ring. The bellow is flanged alike.

The inlet absorber cups have a square shape to assemblea good ring structure. The coated absorber cups are ofthree different shapes for realizing an almost sphericalstructure, the so-called “football shape”. The three shapesare pentagonal, hexagonal and half hexagonal. The foot-ball shape is a truncated icosahedron, belonging to thefamily of Archimedean bodies.

4.3. Thermal balancing of the reactor

The reactor design was accompanied by repeatedly car-rying out a thermal balancing to be sure to provide a designexhibiting the intended reduction of radiation losses. Thecalculation is shown exemplarily for the final design inthe following to validate its suitability. The main task ofthermal balancing is to calculate the thermal efficiency.This efficiency results from the needed chemical energyand the losses in the following way:

gthermal ¼_Qnet

_Qnet þ _Qloss

ð10Þ

The net power _Qnet, which is the power transferred to thereactant either in the form of sensible heat or as chemicalenergy, will be used as a set value. Therefore the efficiencywill be determined by the amount of the thermal losses_Qloss, the calculation of which will be depicted in the follow-ing. The sum of _Qloss and _Qnet equals the thermal energyentering the reactor, the solar input. gthermal is the thermalefficiency of the reactor. Different parts and loss mecha-nisms contribute to the overall thermal losses. These partsare the radiation heat transfer, the transfer via conductionthrough the reactor hull and the convective heat transfer atthe outside.

he solar reactor model.

34 A. Houaijia et al. / Solar Energy 97 (2013) 26–38

_Qlos ¼ _Qcond þ _Qconv þ _Qrad ð11Þ

4.3.1. Radiation heat transfer

To calculate the radiative heat transfer between theabsorber, the window and ambience, Eq. (12) for grey radi-ators is applied (Kabelac and Vortmeyer, 2006):

_Q12 ¼r � e1 � e2 � u12

1� ð1� e1Þ � ð1� e2Þ � u12 � u21

� A1 � ðT 41 � T 4

2Þ ð12Þ

Here, _Q12 is the heat transfer from surface 1, the win-dows, to 2, the ambience, u the view factor, r the Ste-fan–Boltzmann-constant, T the surface temperature and ethe emissivity of the surface. The view factors and surfaceswere calculated according the geometric configuration. Forthe view factors related to the honeycomb surface, theshape is assumed as a half sphere, although in reality it isa football shape made of hexagonal and pentagonal shapedabsorber cups.

At high temperature a main impact to heat losses is thewindow reradiation. The transmitting term is influenced bywavelength and can be read in manufacturers diagrams, seeFig. 12 for the used quartz-glass window (Heraeus, 2004).

It should be noted that the window temperature in thismodel is assumed uniform for each operating state. Thismeans, the term in Eq. (12) only shows the temperatureinfluence onto the emissivity for each state, not its distribu-tion along the window.

The radiation heat transfer also occurs at the front plateand the outside of the reactor body. This case is much eas-ier to calculate, because only parts of the outer body areradiating into the environment. Due to the modular assem-bly, only the outer surfaces are radiating. Furthermore, thefront plate radiates onto the secondary concentrator, and

Fig. 12. Transmission of a 10

not into the environment. The same calculation methodswere applied for all remaining surfaces.

4.3.2. Heat transfer by conduction

In the case for conductive heat transfer, the transfer hasto be divided into three different parts: Heat transferthrough a plane, multilayer structure, heat transfer througha multilayer half sphere and heat transfer through a cylin-drical, multilayer body. The calculation methods are simi-lar, the governing equations can be found in Elgeti et al.(2006). Exemplarily, conduction through a plane multi-layer structure is shown in Eq. (13) for conductive trans-port from the window to the outside.

_Qcond ¼A

dlay1

klay1þ . . .þ dlayn

klayn

� ðT out � T inÞ ð13Þ

Here, dlay1 is the first layer’s thickness, dlayn the nthlayer’s thickness, klay1 the first layer’s thermal conductivityand klayn the nth layer’s conductivity. Fig. 13 shows theheat transfer and all considered layers. Conduction fromthe reactor to the window is calculated in an analogousway.

Fig. 14 shows the side wall of the reactor, here conduc-tive heat transfer through cylindrical layers applies. Thedivision of the heat loss into inner and outer heat transfer( _Qside;in and _Qside;out) is needed, because the fluid works as aninsulation part. It is heated, due to convective heat transferto a certain temperature. This is the starting point for cal-culations for conduction. This means, the very high tem-perature inside the reactor does not have a high impacton the heat losses.

Fig. 15 shows the reactor back side. Here, conductiveheat transfer through spherical layers applies. The hull isapproached by a half sphere as the other layers. In reality

mm quartzglass window.

Fig. 13. Scheme for heat transfer (window).

Fig. 14. Scheme for heat transfer (side).

Fig. 15. Scheme for heat transfer (back).

A. Houaijia et al. / Solar Energy 97 (2013) 26–38 35

the shape is hexagonal cylindroid. But it is easier to calcu-late when the whole system is a half sphere and the resultsare more conservative, meaning the heat losses are biggerwith this kind of calculation. This leads to an overestima-tion of the heat losses. This is in favor of the calculation,

because the real heat losses will be even lower than the cal-culated one.

4.3.3. Convective heat transfer

For convective heat transfer, the governing equation is:

_Qconv ¼ a � AðT fluid � T wallÞ ð14Þ

The heat transfer coefficient a was calculated based onempirical equations for the Nusselt number, which can befound in the literatures Gnielinski (2006) and Klan(2006). Depending on the flow regime at the outside andinside of the window, equations for forced and natural con-vection or a combination of both were applied.

4.3.4. Results of thermal balancing

As the thermal balancing is connected through all differ-ent parts it is only possible to make an overall statement incase of the results. This means, all calculations were exe-cuted at once and iteratively. Nevertheless, some resultsare presented separately.

Due to the two different reactor-operating temperaturesthe window temperature alters, too. This means, the radia-tive loss.

_Qwin ¼ _Qsol;in � _Qrerad � _Qrad;rec�win � _Qwin;cond

� _Qwin;conv ð15Þ

While _Qwin has to be zero in steady state, _Qsol;in is theabsorbed incoming solar radiation, _Qrerad is the reradiatedheat of the window, _Qrad;rec�win is the radiation transferbetween receiver and window, _Qwin;cond is the conductionaland _Qwin;conv the convective heat loss. With this informationthe window temperature can be calculated. Fig. 16 showsthe results of the calculation for window temperature in agraph, for the splitting and the reduction state.

This part is essentially important for the calculations ofradiation losses. As mentioned before, the window dimen-sion is the limiting factor for radiation. The smaller it is,the less losses occur. But as seen in Fig. 15 the smallerthe window is built, the higher its temperature will rise.A window diameter of 0.29 m was chosen. Resulting fromthis a window temperature of 685 �C was calculated tooccur for the water splitting and 1020 �C for the regenera-tion step. Important for the determination of the windowdiameter was the conditions that the temperature needsto stay below the maximal allowable long-term usage tem-perature of quartz glass while having at the same time stilla small diameter to ensure low reradiation losses. A safetymargin of about 150 �C for the window temperature wasconsidered. The actual long-term usage temperature isabout 1150 �C (Schmidt, 2012). At last, the heat lossesare shown in Table 2.

As seen in the above table, the heat losses at the rela-tively small window are around one third of the totallosses. The other main part for heat losses is the back-radi-ation of the receiver to the environment, which is alsodirectly connected to the window size. Meaning, the

Table 1Results of the exergy analysis.

Process unit _EF in kW _EP in kW _ED in kW eK in % yk in %

Solar reactor 896 283.05 612.95 31.59 60.51Preheater 3.26 1.26 2 38.65 0.19Evaporator 1 35.89 14.02 21.87 39.06 2.15Evaporator 2 41.03 20.59 20.44 50.18 2.01Super-heater 4.69 3.97 0.7 84.64 0.001PSA 16 3.84 12 23.92 1.18Hydrogen compression 16.04 7.06 8.98 44.01 0.88Overall system 1012.95 333.79 679.16 32.95

Fig. 16. Window temperature (Schmidt, 2012).

Table 2Heat losses.

Name Splitting Regeneration(W) (W)

Heat conduction and carriage at

Front 515 760Side 135 510Back 230 1060

Total conduction heat losses 880 2330

Convection at

window 580 940Radiation at

Window-Environment 2075 4835Receiver-Environment 1820 9680Hull-Environment 60 190

Total radiation losses 3955 14,705

Total heat losses 5415 17,975

36 A. Houaijia et al. / Solar Energy 97 (2013) 26–38

window size describes 70–80% of the total heat losses. Theheat losses are at a level of 5.5 kW and 18 kW respectivelyfor the new reactor type. With this result the thermal effi-ciency can be calculated. This is done in the following forthe regeneration mode. The efficiency is defined as seen in

Eq. (17). The estimation of the heat demand results ofthe Aspen flow sheet, shown in Fig. 3. In the relevant casethe total heat demand is 650 kW for the regeneration step.Basis for this assumption is the availability of sufficientlyfast reaction rates, which was assumed. The above consid-erations only relate to the design point and to steady-stateconditions. Effects due to transient operational behaviorare not considered here. The reaction is carried out in sevenreactor modules per focal point. The estimated use of ther-mal power is 93 kW. With this information the thermal effi-ciency of the solar receiver-reactor is calculated to be:

gthermal;reg ¼_Qnet;reg

_Qnet;reg þ _Qlos;reg

¼ 93 kW

93 kWþ 18 kW

¼ 0:84 ð16Þ

To point out the improvement in relation to the Hydro-sol 2 pilot reactor, the thermal efficiencies are compared.The heat losses of that reactor were approximated on thebasis of the Stefan–Boltzmann law. In this case, only theradiation heat losses are of concern. This is justified sincethe re-radiation losses from the almost plane absorber sur-face at high temperature contribute the by far greatestshare to the overall losses (Roeb et al., 2008) The pilot

Fig. 17. HYDROSOL 2 reactor.

A. Houaijia et al. / Solar Energy 97 (2013) 26–38 37

plant of HYDROSOL 2 is built of two chambers, each withnine monolithic honeycombs. Each honeycomb is dimen-sioned with 146 � 146 � 50 mm (Gohring, 2008).

_Qlos;rad;reg ¼ �r � erec � Arec � T 4reg ¼ 48:5 kW ð17Þ

This value was confirmed by later experimental cam-paigns (Roeb et al., 2011). As seen in Eq. (18), the heatlosses of radiation are about 50% of the whole thermalenergy input to the pilot plant, which is close to 100 kW.Considering 65 kWth thermal power needed for the regen-eration step, the reactor’s thermal efficiency is:

gthermal;reg;Hydrosol2 ¼_Qnet;reg

_Qnet;reg þ _Qlos;reg

¼ 65 kW

65 kWþ 48:5 kW¼ 0:57 ð18Þ

Even with this estimation, which neglects a part of theoverall losses, the pilot reactor efficiency is much lower –more than 25 percentage points – than that of the newdesign. A more detailed simulation of the pilot reactorshowed an even higher amount of waste heat. The thermalefficiency of the pilot reactor was only about 35% (Goh-ring, 2008). Thus, it can be said, that the measures andcharacteristics constituting the new reactor designexplained in the present paper are expected to lead to a sig-nificantly higher thermal efficiency in relation to the previ-ous Hydrosol 2 pilot reactor. Fig. 17 shows theHYDROSOL 2 reactor.

5. Conclusion

A plant and process concept for solar thermochemicalhydrogen production from water based on a redox cyclewas introduced and analyzed. A plant suitable to workwith 1 MW thermal power was pre-designed. All necessarycomponents have been integrated in a flow sheet. Thepower and/or heat for preheating, evaporation and over-heating were calculated. The piping, storage tanks, the heatexchanger, the reactor modules, the separation and storageof the product gas were laid out.

An exergy analysis has been carried out. The analysisshows that the solar reactor has the highest exergy destruc-tion value (�60%) due to high operational temperature anddue to the nature of two-step process (water splitting and

regeneration). The evaporators are ranked in the secondplace according to their contribution to the exergy destruc-tion of the overall system due to the heat transfer on a lowtemperature level. Due to this finding measures have beentaken to significantly improve the energy balance of thesolar reactor. Based on the original concept of theHYDROSOL project series, a new reactor has beendesigned in order to decrease the exergy destruction. Anoptimization of the reactor shape has been carried out suchthat the heat losses and in particular re-radiation losses aremuch lower than in earlier versions of the reactor. Animportant tool to derive the most suitable geometry ofthe reactor was based on ray tracing. With a given shapeof a secondary optics and by defining a quality criterionthe most suitable geometry of the absorber could bederived. It was shown that a spherical absorber is superiorto conical and flat absorber shapes and that the quality ofthe spherical absorber shape increases strongly when reach-ing the complete hemisphere. The introduction of a spher-ical shape of the absorber and a suitable secondaryreflector ensures a more homogenously distributed solarflux and therefore a more homogeneous temperature distri-bution than in the previous version which exhibited a flatdesign of the absorber. The reduction of losses was mainlybecause of the smaller ratio between the quartz windowsurface and the absorber surface. Another reason for thereduced radiation losses is the cavity design ensuring thatthe thermal radiation is rather absorbed inside the reactorthan emitted through the window. Furthermore, the wholereactor set-up and all components were designed in a wayallowing easy maintenance and replacement of parts, inparticular of the individual absorber monoliths. To ensurethe reliability of the design it was supported by calculatingthe thermal balance of the reactor and heat losses by con-duction, irradiation and convection. The analysis and com-parison to the previous reactor design revealed that asignificant increase of the thermal reactor efficiency of morethan 25 percentage points can be expected.

Acknowledgement

The authors acknowledge the co-funding of the JTI-FCH for the project HYDROSOL 3D (Contract No.245224).

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