Operational Strategy of a Two-Step Thermochemical Process for Solar Hydrogen Production

international journal of hydrogen energy 34 (2009) 4537–4545

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Operational strategy of a two-step thermochemical process for solar hydrogen production

Martin Roeb*, Martina Neises, Jan-Peter Sa¨ck, Peter Rietbrock, Nathalie Monnerie, Ju¨rgen Dersch, Mark Schmitz, Christian Sattler

German Aerospace Center – DLR, Institute of Technical Thermodynamics, Solar Research, Linder Hoehe, 51147 Cologne, Germany article info abstract

Article history: A two-step thermochemical cycle process for solar hydrogen production from water has Received 28 February 2008 been developed using ferrite-based redox systems at moderate temperatures. The cycle Received in revised form offers promising properties concerning thermodynamics and efficiency and produces pure 18 August 2008 hydrogen without need for product separation. Accepted 18 August 2008 The process works by cycling stationary ferrite-coated monoliths through sequential Available online 15 October 2008 oxidation and reduction steps at 1073 and 1473 K, respectively. By using two such monoliths in parallel, it is possible to quasi-continuously produce hydrogen. A prototype solar Keywords: reactor and peripherals suitable to proof the process concept were developed and have Thermochemical cycle been successfully tested in DLR’s solar furnace in Cologne. Repeated cycling operation is Ferrites possible with a reproducible amount of hydrogen produced. In most cases, a distinct decay Solar thermal of the amount of the evolved hydrogen is observed from cycle to cycle due to inhomoge- Water splitting neous heating of the monolithic absorber in the very first cycles and due to disappearance Iron oxide of the porosity and the associated loss of surface area in later cycles. Results from experimental campaigns with the prototype reactor and simulations with a corresponding reactor model support the development and verification of a process strategy for the continuous production of hydrogen in a larger scale. The tasks include the enhancement of long-term stability of the redox system, the development of an

operational strategy and ﬁnally the design and development of a 100 kWth pilot reactor to demonstrate the feasibility of the process on top of a solar tower under real conditions. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1. Introduction and reactor design. Therefore, solar chemistry focuses on thermochemical cycles which proceed in several steps and Thermal splitting of water using concentrated solar energy is enable hydrogen generation at moderate temperatures which an attractive and completely renewable way to produce are manageable by using state-of-the-art engineering mate- hydrogen. However, because of thermodynamic restrictions, rials. The most interesting thermochemical cycle is the two- acceptable yields in the direct thermal splitting of water can step water splitting cycle using redox systems. During the only be achieved at temperatures above 2500 K. Such high ﬁrst step of this cycle (the water splitting) the reduced and temperatures impose extraordinary demands on materials therefore activated material is oxidised by taking oxygen

* Corresponding author. Tel.: þ49 2203 6012673; fax: þ49 2203 4141. E-mail address: [email protected] (M. Roeb). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.08.049 4538 international journal of hydrogen energy 34 (2009) 4537–4545

Nomenclature 2. Testing of solar reactor in the solar furnace DLR German Aerospace Center EC European Commission To realise the continuous supply of hydrogen, a quasi- MO metal oxide continuous operating reactor was built at the DLR. It consists of two separate chambers with fixed honeycomb absorbers in from water and producing hydrogen according to the both chambers as shown in Fig. 1a. While in one chamber the reaction: water splitting reaction takes place at temperatures around 1073 K, the other is being regenerated at temperatures of up to / 1473 K. During the water splitting reaction, water vapour (1) MOreduced þ H2O MOoxidised þ H2 diluted with nitrogen is passed through the reaction chamber In the next step (the regeneration step) the material is while during the regeneration the chamber is flushed with reduced again, setting some of its lattice oxygen free accord- pure nitrogen. ing to the reaction: The water splitting process needs significantly less solar power than the regeneration step. Therefore in the first / campaigns with the two-chamber reactor an optical dimin- (2) MOoxidised MOreduced þ O2 isher was used to attenuate the incoming radiation accord- One major advantage is that hydrogen and oxygen are ingly: a metallic mesh was moved in an alternating way in produced in separate steps, i.e. no separation of hydrogen and front of the left or right reaction chamber according to the oxygen is needed. completion of the two half cycles. In the most recent Several redox materials consisting of oxide pairs of multi- campaign, the diminisher was replaced by a newly developed lamellae shutter providing much more flexibility with respect valent metals (e.g. Fe3O4/FeO [1–3],Mn3O4/MnO [4]) or systems of metal oxide/metal (e.g. ZnO/Zn [5]) have been evaluated. to separate adjustment of power and flux density in the two Recently a lot of attention has been paid to ferrites and mixed chambers. The lamellae shutter can regulate separately the metal ferrites [6–9] and different approaches on solar receiver two different temperatures in both chambers needed for concepts for using mixed ferrites [10–12] have been developed. the hydrogen production and the regeneration (Fig. 1b). The In the scope of the project HYDROSOL and the consecutive segment shutter is installed 1000 mm in front of the focus of project HYDROSOL-2, both funded by the EC [14], mixed iron oxides have been used as the reducing material. The basic idea of the HYDROSOL project was to combine a support structure capable of achieving high temperatures when heated by concentrated solar radiation with a redox pair system suitable to carry out water dissociation and for regeneration at these temperatures. That means the complete operation of the whole process (water splitting and regeneration of the metal oxide) can be achieved by a single solar energy converter. The iron oxides have been fixed on ceramic monolithic structures, which are placed inside a solar receiver reactor. Two solar prototype receiver reactors have been designed, built and tested in the solar furnace of DLR in Cologne. With the first reactor, in which a single coated monolith is located in a single reaction chamber, the feasibility of solar chemical hydrogen production by redox systems coated on ceramic- support structures has been shown [13]. With the second reactor, a two-chamber setup, the continuous supply of hydrogen to downstream units was possible by operating both chambers in parallel but in different modes [14]. The goal of the ongoing work is to develop and build an optimized pilot plant (100 kWth) for solar hydrogen production based on this novel reactor concept. The project involves further scale-up of this technology and its effective coupling with solar central receiver systems in order to exploit and demonstrate all potential advantages. Specific challenging problems to be solved include the enhancement and optimi- sation of the metal oxide-ceramic-support system with respect to long-time stability under multi-cycle operation and the development and construction of a complete pilot dual absorber/receiver/reactor unit in a scale of 100 kWth and its Fig. 1 – a) Vertical-horizontal cut of two-chamber reactor coupling to a solar heliostat field. (conti reactor); b) schematic view of the lamellae shutter. international journal of hydrogen energy 34 (2009) 4537–4545 4539

the solar furnace of DLR in Cologne and is divided into two cycle rather quickly, the rear part of the absorber heats up parts. One part regulates the temperature in the left reactor more slowly and gets slightly hotter from cycle to cycle. chamber and the other part the temperature in the right Therefore the part of the honeycomb reaching a temperature reactor chamber. Each side of the lamellae shutter can be level sufficient for effective regeneration increases moved separately and simultaneously by a stepping motor. throughout the day. The amount of regenerated metal oxide The most recent experimental series in the solar furnace of per cycle increases (see also Section 3.2). DLR was first of all aiming at the evaluation of different Between the second and the third day, the mean hydrogen coating materials, especially concerning their long-term flow rate dropped about 50%. This is partly due to the variation stability and capability to produce constant hydrogen output of experimental parameters on the third day. In particular, over several cycles. Secondly, the experimental series aimed most of the cycles that day were conducted with reduced half- at the development of process parameters and operational cycle times as well for regeneration as for water splitting. But strategies for the pilot plant which has been set up at the this effect alone is not sufficient enough to explain the extent Plataforma Solar de Almerıá(PSA). The strategy for operating of the decrease. Such a massive drop of the hydrogen flow rate the prototype reactor in the solar furnace is taken as a model as seen after the second day was not observed with other strategy to be adapted as the operational strategy for the pilot samples. plant at the solar tower. For this purpose, parametric studies On the third, fourth and fifth day, the hydrogen flow rate concerning regeneration temperature, water splitting stayed constant during the day. But again a slight decrease of temperature and regeneration time were performed. the flow rate can be observed with each day. The fact that the With two different samples tested in parallel in the two hydrogen flow rate only drops from one day to the other but chambers of the solar reactor, it was possible to perform 54 stays constant during the day leads to the conclusion that cycles within five days in a long-term test. Shown in Fig. 2 is some irreversible process is taking place when cooling the the hydrogen flow rate measured for one sample over all five monolith down to room temperature (e.g. recrystallisation days of testing. The conditions of the individual cycles varied processes). Ongoing investigations of surface and layer somewhat due to fluctuating weather conditions and structure suggest that the surface characteristics of the daytime-dependent shape and position of the focal spot. For coating change considerably when the coating is cycled. example, the regeneration temperature of the cycles varied in Especially the disappearance of the porosity and the associ- a range of about 50 K. Moreover, experimental parameters ated loss of surface area probably contribute to the decrease in like regeneration and water splitting temperature as well as hydrogen production over repeated cycles [16]. The drastic the cycle duration were systematically varied from the third drop after the second day might be influenced by the fact that day on. Therefore the analysis of the diagram is possible only on this day the experiment was stopped during a water in a semi-quantitative manner. splitting phase due to clouding. All the other days ended with During the first two days the hydrogen flow rate increased a regeneration phase conducted in both chambers. steadily. This behaviour was also observed in the test of The influence of regeneration temperature is illustrated in several other samples in the solar furnace. The reason for the Fig. 3a. It shows the mean hydrogen flow rate measured for increase is most probably related to the slight increase of one sample for the cycles performed on the third day. In cycles temperature in the rear part of the absorber. Due to the Nos. 18–24 and 32–34 the regeneration was conducted at alternation of temperature levels, it takes a long-time (several temperatures in a range between 1395 and 1425 K whereas in hours) until the reactor and the absorber are heated up cycles Nos. 25–31 the regeneration temperature was only completely. This means that, while the absorber’s front 1350–1365 K. The mean hydrogen flow dropped about 15% temperature achieves the same target temperature in each when reducing the regeneration temperature from cycle No. 24 to No. 25. It recovered to the previous level when the reduction temperature was increased again after cycle 31. An increase of hydrogen production is expected as a result from more effective regeneration of the metal oxide. Due to thermodynamics and kinetics of the redox reaction, higher conversions to the reduced form are expected if the reaction temperature is increased. The experimental results are consistent with this expectation. In further experiments, the influence of splitting temperature and regeneration time was investigated. Water splitting temperature was varied from 1053 K to 1233 K, while regeneration temperature was kept constant at 1423 K. A maximum hydrogen flow rate could be observed at a water splitting temperature of about 1123 K. Furthermore, the influence of the regeneration time on the following water splitting step was investigated. The duration of the regeneration step was varied between 5 and 25 min for constant water splitting and regeneration temperatures (see Fig. 3b). The peak mass flow Fig. 2 – a) Hydrogen flow rate during long-term test of hydrogen increases with an increasing regeneration time of performed with one sample. 5–20 min. The peak hydrogen flow for a regeneration time of 4540 international journal of hydrogen energy 34 (2009) 4537–4545

library Modelica 2.2 in combination with the Modelica_Fluid library and additionally adapted or newly developed models. The main purpose of this ﬁrst model approach is the simulation of the thermal behaviour of the reactor rather than the prediction of the hydrogen production rate.

3.1. Model approach

Fig. 4b shows a screenshot of the overall model. The model is 2-dimensional due to the rotational symmetry of the reactor. The data of ambient temperature and solar power are available from measurements in the solar furnace and are used as input data for the simulations. Because the spatial distribution of solar radiation input is not constant throughout the aperture, but shows a Gaussian-like shape, the total input power is distributed on the reactor front surface following a Gaussian distribution. The ceramic monolith inside the reactor as well as the insulation and the housing are modelled as concentric circular rings built up of several slices in axial direction. Mass and energy balances are set up using the finite volume method. At present, the knowledge about reaction kinetics is only marginal and therefore the reaction rate is calculated by preliminary equations based on the research of Tsuji et al. [3] who investigated the formation of cation-excess magnetite at temperatures between 523 and 623 K. When the model was developed this data was the only kinetic data available for such reactions. The extrapolation to similar redox systems and to a different temperature range introduced some uncertainties. Those will be reduced in future by using Fig. 3 – a) Mean hydrogen flow (o: T > 1373 8C; 6: experimentally determined kinetic data from ongoing exper- T < 1373 8C) of each cycle for one sample at the third day; imental studies. b) peak (-) and mean hydrogen flow (,) versus regeneration time. 3.2. Simulation results

In the first simulation runs, the model was verified with 25 min is less than that for 20 min, which is probably due to respect to the pure thermal behaviour of the reactor by leaving the fact that the cycle with 25 min regeneration time was out the chemical reaction. A good matching of simulated and conducted several cycles later. No dependency of the mean measured temperatures was observed. This is true not only for hydrogen flow rate on regeneration time could be found. The steady state conditions but also for periods with distinct intense peak at the beginning of each hydrogen production temperature transients. After that, typical process situations cycle is short in comparison to the whole duration of the half- including chemical reactions were simulated, in particular the cycle. Therefore its influence on the mean hydrogen produc- start-up and sequential oxidation and reduction steps, during tion rate is almost negligible. The peak hydrogen mass flow which temperatures are cycled and feed gas composition is might be caused by a specific surface activation which is alternated. Fig. 5a shows a simplified power input and two intensified by longer regeneration times. calculated temperatures for such a situation. T1 is the Even when regenerating the sample for 25 min, the temperature in the centre of the monolith near the front side hydrogen flow rate was constant over the whole time. A and T6 is the temperature at the outer rim of the monolith near decrease of hydrogen production was not observed during the the backside (Fig. 4a). After a heat-up phase the feeding of water splitting which means that the limit for the cycle steam starts at t ¼ 3600 s. At the same time, the hydrogen duration was not reached yet. This could mean that the water production starts with a peak mass flow rate, which decreases splitting is a rather slow reaction compared with the oxygen during the next 1200 s of the experiment (Fig. 5b). At t ¼ 4800 s release. the steam input is stopped, the solar power input is increased immediately to heat up the reactor and the regeneration period starts. At t ¼ 6000 s the hydrogen production starts again, but 3. Simulation with a lower initial rate than the first time. This periodic operation is repeated two more times. The temperature In a first simulation approach, a model of the first laboratory difference between T1andT6 is about 100–170 K, which reactor built at the DLR (Fig. 4a) has been set up. The model represents the largest temperature difference inside the was built in Dymola using partial models from the standard monolith. This temperature difference is the result of different international journal of hydrogen energy 34 (2009) 4537–4545 4541

a inlet of process gas

housing (stainless steel)

to mass spectrometer

honey comb product gas probe structure (SiC) quartz window SiSiC funnel

T6 to offgas treatment T1

concentrated solar radiation tube (stainless steel)

SiSiC cylinder ceramic blender

diluting gas inlet

insulation

Fig. 4 – a) Cross section of the laboratory scale reactor modelled in Dymola; b) screenshot of the overall model in Dymola used for the simulation of the laboratory scale reactor.

contributions. One reason for this difference is the inhomo- with the centre of the monolith’s front face and with a signifi- geneous flux distribution of the concentrated solar radiation cant decrease of intensity towards the outer edges of the focal hitting the front face of the monolithic absorber. The solar spot. This causes a radial temperature gradient at the front furnace provides a typical Gaussian-shaped flux distribution in which is transmitted with somewhat reduced extent towards its focal plane with a maximum in the centre which coincides the backside of the monolith. Together with thermal losses 4542 international journal of hydrogen energy 34 (2009) 4537–4545

Fig. 5 – a) Solar power input and calculated temperatures of a reactor start-up and 3 cycles of hydrogen production and regeneration (see Fig. 4a for the position of T1 and T6); b) comparison of calculated and measured hydrogen production rates; c) calculated values of available ferrite for the centre part, the outer part and the whole monolith during the periodic operation.

around the edge and with the axial temperature gradient due additionally by conductive heat losses at the contact face to to the fact that heat has to be transferred by convection and surrounding layers. The percentage of regenerated metal conduction from the front to the backside this sums up to the oxide can be significantly increased by homogenising the mentioned difference between T1andT6. temperature level of the honeycomb structure. The easiest The resulting temperature gradients in the monolith in way to do this is to provide a homogeneous solar flux to the general, and in particular the temperature difference between absorber. In the solar furnace, this could be done by beam T1 and T6inFig. 4a explains the decreasing initial hydrogen manipulation. In the focal area of central receiver systems production rate. Due to these temperature gradients, a part of (solar towers), the solar flux is much more homogeneously the monolith does not reach a sufficient high temperature distributed. Therefore much more effective regeneration is level which is needed to enable the regeneration reaction. expected for a scaled-up version of the reactor operated on This part with no or incomplete regeneration is predomi- a solar tower platform. nantly the back and the edge of the monolith. The model is able to predict the thermal dynamics of the Fig. 5c shows the absolute mass of ferrite, which contains reactor. Because heat production and heat consumption are iron in a lower valence state and therefore is capable of combined with the chemical reactions the proper calculation water splitting. This amount determines the potential of of the reaction rates is important for the heat balances. hydrogen production since the oxygen is captured only by Experimental investigations are continuing in order to obtain ferrite in the reduced state. Three curves are shown: the total a deeper insight into the reaction. Next steps are model amount of ferrite, the amount in the centre and at the outer refinements as well as the set up of a structured library and rim of the monolith. The amount of reduced and therefore simulations of an up-scaled plant. ‘‘active’’ ferrite of the fresh monolith starts at a high initial level, decreases during the first hydrogen production period (3600–4800 s) and increases again during the regeneration 4. Design of the solar 100 kW pilot plant period (4800–6000 s). The regeneration is incomplete and the next cycle starts at a lower initial ferrite concentration 4.1. Design of reactor layout compared to the first cycle. After the second regeneration cycle, it reaches an almost constant peak level. From Fig. 5c The general design of the two-chamber prototype reactor was it is apparent that, in addition to the rear of the monolith, chosen as a starting point and a sound base for the design of the the edges are also not regenerated fully, as they do not attain pilot reactor. Scale-up to 100 kWth has been achieved mainly by sufficient high temperatures. increasing the absorber surface by using nine individual pieces As mentioned above the lower temperatures coincide with of square shaped monoliths with dimensions of 146 146 mm a flux minimum of the solar furnace’s focal spot at its rim and each. Each reactor assembly (one for the water splitting and international journal of hydrogen energy 34 (2009) 4537–4545 4543

one for the regeneration) consists of a configuration of 3 3 Because the regeneration step is endothermic and per- (total 9) SiSiC blocks. Partial heat recovery is realised by formed at 1423–1473 K and the water splitting step is slightly transferring heat from the product gas to the feed stream exothermic and performed at 1073–1123 K, the regenerating through a metal sheet separating the ducts for the two streams. part of the modules needs a higher solar flux density than the Fig. 6a shows the general setup of the reactor. second part for the water splitting. The latter only needs a low amount of solar power to compensate heat losses. This 4.2. Refinement of concept for a solar concentrating means, the flux density has to be changed when the status of system providing alternating solar flux the cycle in the related reaction chamber is switched from regeneration to water splitting and vice versa. This can be The temperature level of up to 1473 K in combination with realised by partitioning the heliostat field. Fig. 6b shows a mass production of hydrogen can only be realised by using a layout example of partitioning the existing heliostat field, a solar tower system, a so-called central receiver system, which corresponds to the field of the SSPS central receiver consisting of a set of tracked mirrors, tower and receiver. system at the Plataforma Solar de Almerıá [15]. The heliostat

Fig. 6 – a) General setup and explosive view of pilot reactor; b) possible grouping of heliostats. 4544 international journal of hydrogen energy 34 (2009) 4537–4545

field is made up of 93 heliostats in total with a total reflective idea is the partitioning of the heliostat field and the use of area of 3655 m2. The pilot receiver reactor is positioned on an different groups of heliostats. The pilot reactor has already intermediate tower platform at 26 m height above ground been installed on site. First thermal tests have been conducted level. recently and the first operation with coated monoliths and One group of heliostats (see Fig. 6b, grey encircled area) is hydrogen production is scheduled for late 2008. responsible for the base load for both chambers needed to keep the temperature level of the water splitting. A second group of heliostats (light grey encircled area) is flexible and is switched from one chamber to the other after each half-cycle. Acknowledgements This second group provides the power needed for the regeneration at higher temperatures. A third group of heliostats The authors would like to thank the European Commission for (black encircled area) is responsible for the heating phase, in partial funding of this work within the Project HYDROSOL-2 order to change the temperatures from 1123 K to 1473 K in ‘‘Solar Hydrogen via Water Splitting in Advanced Monolithic a sufficient short period of switching time. This latter group Reactors for Future Solar Power Plants’’ (SES6-CT-2005- provides an extra amount of power (about 100 kW/m2) only 020030), under the Sixth Framework Programme of the Euro- during those switching periods and is used in addition to the pean Community (2002–2006). other two groups. A fourth group consists of some heliostats which are partially covered to allow for a ‘‘microcontrol’’ of references the flux and the solar power on the aperture. This group is not yet included in Fig. 6b. 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