Dr. Nazim Muradov
tests a pilot-scale hydrogen unit at FSEC's Hydrogen
(Photo: Nick Waters)
Hydrogen production from solar-driven thermochemical
water splitting cycles (TCWSCs) provides an approach that
is both energy-efficient and environmentally attractive. Of
particular interest are TCWSCs that utilize both thermal (i.e. high
temperature) and light (i.e. quantum) components of the solar
resource, boosting the overall solar-to-hydrogen conversion
efficiency compared to those with heat-only energy input.
FSEC researchers Ali T-Raissi, Nazim Muradov, Cunping
Huang and Olawale Adebiyi analyzed two solar-driven TCWSCs: a carbon
dioxide (CO2)/carbon monoxide cycle and a sulfur dioxide (SO2)/sulfuric
acid cycle. The first cycle is based on the premise that CO2
becomes susceptible to near-ultraviolet and even visible radiation
at high temperatures (greater than 1300K). The second cycle
is a modification of the well-known Westinghouse hybrid cycle, in
which the electrochemical step is replaced by a photocatalytic step.
Their research has led to the development of a novel
hybrid photo-thermochemical sulfur-ammonia (S-A) cycle. The
main reaction (unique to FSEC's S-A cycle) is the light-induced photocatalytic
production of hydrogen and ammonium sulfate from an aqueous ammonium
sulfite solution. The ammonium sulfate product is processed
to generate oxygen and recover ammonia and SO2 that are then recycled
and reacted with water to regenerate the ammonium sulfite.
The main advantages of the proposed S-A cycle over existing
hybrid and solar high-temperature TCWSCs include the following:
- The S-A cycle includes a step in which the energy of solar photons
is directly converted into the chemical energy of hydrogen (i.e., without
the use of intermediate devices such as photovoltaic cells).
- There is no need for electrical energy input.
- The maximum temperature of the S-A cycle is below 1170K (which
allows the use of less costly materials of construction), and
- The thermochemical step of the process (i.e. decomposition
of sulfuric acid) is a well-developed process since it is common
to all sulfur-family cycles. As a result, S-A cycle has the
potential to attain higher solar-to-hydrogen energy conversion efficiencies
as compared to the state-of-the-art solar concentrator-turbine-electrolyzer
systems. The search for more efficient photocatalysts as
well as thermodynamic and process flowsheet analyses of the proposed
cycle are presently underway.
Click here for a copy of their complete paper
describing their research and this new hydrogen production cycle.
This material was presented in a poster at the National
Hydrogen Association annual conference in Long Beach, CA,
in March. Click
here for an abstract of a second poster presented at the conference: “Zero
Emission Production of Hydrogen via Catalytic Dissociation
of Hydrocarbons” by
N.Z. Muradov, F. Smith and A. T-Raissi.
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