The HYDROSOL Projects

 

 
     
 

HYDROSOL Project Final Report

 

Executive Summary

The harnessing of the huge energy potential of solar radiation and its effective conversion to chemical fuels such as hydrogen is a subject of primary technological interest. One of the reactions with tremendous economical impact is the dissociation of water to oxygen and hydrogen. The integration of solar energy concentration systems with systems capable to split water is of immense value and impact on the energetics and economics worldwide. However interesting yields can only be achieved at very high temperatures; thus the current state of the art of solar water splitting chemistry is focused on materials that can act as effective water splitters at lower temperatures in a two-step water splitting process. According to this idea, in the first step the redox material (usually the higher-valence-state of a metal oxide) is reduced delivering some of its lattice oxygen; in the second step the reduced material is oxidized by taking oxygen from water and producing hydrogen, according to the following general scheme:

The disadvantage is that a two-step process is required, consisting of a water splitting and a regeneration (oxygen release) step. The advantage is the production of pure hydrogen and the removal of oxygen in separate steps, avoiding the need for high-temperature separation and the chance of explosive mixtures formation. The concept has been proven experimentally for pairs of oxides of multivalent metals or metal/metal oxide systems (e.g. Mn3O4/MnO, Fe3O4/FeO, ZnO/Zn etc.); however even though water splitting is taking place at temperatures lower than 700°C, material regeneration (i.e. reduction) takes place at much higher temperatures (> 1600°C). In addition, despite basic research with respect to active redox pairs, solar reactor concepts have only recently been reported in the literature and are based on particles fed into rotating cavity reactors, concepts that are complicated and costly to operate.

The uniqueness of the HYDROSOL approach is based on the combination of two novel concepts: nanoparticle materials with very high water-splitting activity and regenerability (synthesised by novel routes such as aerosol processes, combustion techniques and reactions under controlled oxygen pressure) and their incorporation as coatings on special refractory ceramic monolithic reactors with high capacity for solar heat absorption. Prior work of the Consortium members has demonstrated that such refractory ceramics can act as solar heat collectors achieving temperatures in excess of 1100 °C; however the concept of combining them with high-temperature water-splitting materials for the exploitation of solar heat in the field of Solar Chemistry has never before been proposed. With this concept, the full cycle can be performed with the water-splitting material immobilized upon the honeycomb walls avoiding the needs of continuous powder feeding and collection from the reactor.

Through the HYDROSOL Project, the Consortium team has proposed and developed an innovative solar reactor for the production of hydrogen from the splitting of steam using solar energy. The solar reactor contains no moving parts and is constructed from special refractory ceramic thin-wall, multi-channelled (honeycomb) monoliths optimized to absorb solar radiation and develop sufficiently high temperatures. The monolith channels are coated with an active water-splitting material, and the overall reactor looks very similar to the familiar catalytic converter of modern automobiles. When steam passes through the solar reactor, the active coating material splits water vapor by “trapping” its oxygen and leaving in the effluent gas stream as product pure hydrogen, without any need for expensive and complicated gas separation post-processing steps. In a subsequent step, the oxygen “trapping” material is regenerated (i.e. releases the oxygen absorbed), by increasing the amount of solar heat absorbed by the reactor and hence a cyclic operation is established. Highly active oxygen “trapping”/water-splitting materials (based on doped oxides exhibiting redox behavior) have been synthesized employing different techniques the most active been those produced by aerosol synthesis routes in APTL.

The first research period (11/2002-05/2004) involved the parallel development of two aspects: synthesis and development of redox pair powder materials, suitable for water splitting and release of oxygen for regeneration in the temperature range 800-1300 °C with high H2 yield and construction of a dedicated test solar reactor/receiver. For the first task, novel synthesis routes were explored to produce active water-splitting materials. For the second task, a small-scale assembled thermochemical reactor/receiver based on refractory honeycomb structures was designed and built with all necessary peripherals, being capable of achieving uniform temperatures of the order of 1300 °C.

During the second period (05/2004-11/2004), the redox pair compositions meeting the standards defined above were coated on refractory honeycombs and were evaluated with respect to their activity for water splitting and regeneration under solar cyclic operation. The “proof-of-concept” of the HYDROSOL technology has been demonstrated beyond any doubt in a pilot scale solar reactor built and operating at the DLR solar furnace facility in Cologne (Germany): the first “solar Hydrogen” was produced with the proposed concept and multi-cyclic operation was achieved.
During the third research period (11/2004-10/2005) research was focussed on evaluating the durability of the integrated systems under long-term solar cyclic operation and on the evaluation of the concept as a starting point for a plant with significant technical scale. The approach resulted in an integrated solar receiver/reactor concept for the exploitation of solar heat that has never before been realized. The first kind of such a pilot-scale reactor was designed, built and is currently operating in a continuous mode at a solar furnace facility, producing hydrogen by cyclic operation exclusively at the expense of solar energy. The technical and economic feasibility evaluation of a scaled-up plant based on the process parameters completed the Project on October 2005. In fact the HYDROSOL technology is the first demonstration of solar chemistry-based hydrogen production from water, with a future potential - when employed in combination with other solar thermal applications such as power plants - to achieve a hydrogen cost of 24 Eurocent/kWh in the medium-term and of 10 Eurocent/kWh in the long-term. The Project results are summarized below:

  1. Redox pair powder materials synthesized via un-conventional routes (combustion synthesis and aerosol spray pyrolysis) have been proven capable of splitting water at relatively low temperatures (800 °C) with water conversions up to 80 % and Hydrogen being the only reaction product.
  2. A solar reactor based on ceramic refractory honeycomb structures coated with the redox materials above, has been constructed and its capability of achieving uniform temperatures of the order of 1300 °C has been demonstrated.
  3. The evaluation of the redox materials on the solar reactor proved the feasibility of solar hydrogen production by the HYDROSOL process and the stability of the redox/support- assemblies: multi-cyclic solar thermo-chemical splitting of water was successfully demonstrated: the reactor produces hydrogen by cyclic operation exclusively at the expense of solar energy; up to 40 cycles of constant H2 production were operated in a row in a two-day continuous production of hydrogen.
  4. Evaluation of the economic potential of the process and detailed cost analyses indicate that technical improvements provide the potential to reduce the production costs of hydrogen from 24 Eurocent/kWh, to 10 Eurocent/kWh in the long-term.

The developed technology produces hydrogen based on entirely renewable and abundant energy sources and raw materials - solar energy and water respectively – without any CO2 emissions, in an entirely “clean”, natural and environmentally friendly way, addressing thus issues of universal concern and importance. Based on these results, the next efforts involve further scale-up of the technology and its effective coupling with solar concentration systems like solar towers in order to demonstrate large-scale feasibility of a solar plant for the production of solar Hydrogen.


 

 
 
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