HYDROSOL Project Final Report
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
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
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
- 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.
- 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.
- 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.
- 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.