O16 - SOLAR HYBRID PHOTO-THERMOCHEMICAL WATER-SPLITTING CYCLE
WITH IN-SITU THERMAL ENERGY STORAGE
N. Muradov1, A. T-Raissi1, N. Qin1 and K. Kakosimos2
1Florida
2Texas
Solar Energy Center, Cocoa, Florida 32922, USA
A&M University at Qatar, PO Box 23874, Doha, Qatar
email: muradov@fsec.ucf.edu
Abstract
The main limitations of existing solar-powered thermochemical water-splitting cycles (WSC) are that they (i) utilize only
thermal component of the solar irradiation (neglecting quantum component), (ii) do not take into consideration the intermittent
nature of the solar resource, and (iii) involve technically-challenging reagents transport and separation stages. A new family
of hybrid sulfur–ammonia (SA) photo-thermochemical WSC that circumvent the above shortcomings has been developed at
Florida Solar Energy Center. SA cycles through the means of solar beam-splitting utilize the quantum (UV-Vis) and the
thermal (IR) portions of the solar spectrum for hydrogen and oxygen production, respectively, as represented by the following
generic equations:
1.
2.
3.
4.
SO2 + 2NH3 + H2O → (NH4)2SO3
(NH4)2SO3 + H2O → (NH4)2SO4 + H2
(NH4)2SO4 + MO → 2NH3 + MSO4 + H2O
MSO4 → MO + SO3 (SO2 + 1/2O2)
chemical absorption
solar UV-Vis photocatalytic
solar IR thermal
solar IR thermocatalytic
where MO is alkali metal sulfate (e.g., Na2SO4, K2SO4, Rb2SO4). In the photocatalytic step (2) hydrogen production is
accomplished using narrow band gap photocatalysts (e.g., CdS, CdS-ZnS, etc). In the case of, e.g., MO=K2SO4, the reaction
(3) results in the production of K2S2O7 molten salt, which can be pumped through pipes as liquid, thus, simplifying materials
transport and handling. Another advantage of the new cycle is that it provides in situ thermal storage and energy recovery by
means of the molten salt K2S2O7-based system as integral part of the WSC. Thermodynamic process modeling studies
showed the technical feasibility of the hybrid SA WSC.
Keywords: Hydrogen, solar, sulfur-ammonia cycle, thermal storage.
1
Introduction
Solar-powered thermochemical water splitting cycles (WSC)
can potentially reach overall efficiencies of 35-40%, far
exceeding that of other solar-to-H2 conversion systems
(e.g., PV-electrolysis). However, all existing solar-driven
WSC suffer from a number of shortcomings that hinder their
practical applications, such as: (i) utilization of only thermal
component of the solar irradiation (neglecting quantum or
photonic component), (ii) not taking into consideration the
intermittent nature of the solar resource, and (iii) involvment
of technically-challenging reagents transport and separation
stages. In particular, in the existing purely thermochemical
water splitting cycles, the high energy portion of solar
radiation (i.e., UV-Vis) is degraded (or thermalized) to
thermal heat leading to lower overall cycle efficiencies. On
the other hand, in the photoelectrochemical (PEC) water
splitting systems, only a small portion of the solar spectrum
(UV-Vis) is utilized and the thermal component of sunlight is
wasted, resulting in relatively low energy conversion
efficiencies.
The efficiency of solar-driven water splitting systems can be
substantially improved by utilization of both the photonic
(UV-Vis) and thermal (IR) components of the solar
radiation. Thermodynamically, the total energy (AH)
required to split water splitting to H2 and O2 is AH = ∆G +
TAS. At a given temperature T, the overall process will be
more efficient if it can utilize the photonic energy of solar
radiation as Gibbs free energy (∆G), and the thermal
component as TAS.
The objective of this work is to develop a solar-powered
WSC that utilizes both photonic and thermal components of
solar radiation, and provides thermal energy storage
capacity as an integral part of the hybrid WSC.
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HYdrogen POwer Theoretical and Engineering Solutions International Symposium 2015, Toledo, Spain
6 – 9 September 2015
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2
Materials and Methods
Photocatalytic hydrogen production experiments were
conducted using a specially-designed photoreactor made of
glass with a water jacket and a quartz window (Fig. 1). In a
typical experiment, the synthesized photocatalyst combined
with co-catalyst were added to the aqueous solution
containing 1 M (NH4)2SO3 placed inside the photoreactor.
Before each experiment, the photoreactor was purged with
ultra-pure argon gas (Linde, 99.999%) for about 1 hour to
remove dissolved oxygen from the solution. The evolved
hydrogen gas was collected over the water using an
inverted graduated burettes. During the photo-experiments,
the area around the experimental set-up was screened by a
yellow polymer curtain to cut-off the UV radiation.
The relative fraction of solar radiation flux used in the
photochemical and thermochemical steps is determined by
the photon-absorbing characteristics (e.g., the band-gap) of
the semiconductor-based photocatalysts used in the
hydrogen production step. It was determined that the
fraction of the photons that can be utilized by a CdS-based
photocatalysts with photon absorption threshold of 520 nm
amounts to ~20% of the total solar radiation flux.
Consequently, about 80% of the total solar irradiance
comprising mostly of thermal energy with wavelengths
above 520 nm (i.e., visible and IR photons) can be utilized
in the oxygen generation step.
In principle, photonic and thermal means of the utilization
of the solar energy source can be accomplished via splitting
of solar spectrum into two beams: UV-Vis for the
photochemical reactor and Vis-IR for the thermochemical
reactors. This approach would result in a relatively compact
system and allow more efficient use of solar resource, but it
would be more complex and require special beam-splitting
dichroic mirrors.
3.1 Photocatalytic Hydrogen Production
Earlier, we reported CdS-photocatalyzed H2 production
from aqueous ammonium sulfite (NH4)2SO3 solutions [5-7].
In the present work, we examined the catalytic effect of
transition metals, including, Pt, Pd, Ru, Ni and Co on the
kinetics of H2 photogeneration. These metals were added to
3 Results and Discussion
the photocatalytic system in the form of fine polymerA novel solar-powered hybrid “Sulfur-Ammonia” (SA) WSC stabilized colloidal particles in the quantity about 1% to the
has been developed at Florida Solar Energy Center (FSEC) amount of a photocatalyst. CdS was used as a baseline
by introducing ammonia (NH3) as a working reagent to hybrid- photocatalyst. The results are summarized in Fig. 2.
sulfur cycle to allow more efficient solar interface, facile
100
product separation steps and all-fluid operation [1-4] (Note:
Pt
the initial development of the SA cycle was conducted under
80
US DOE funding). In this hybrid photo/thermo-chemical water
Pd
splitting cycle the quantum portion of the solar spectrum (UVVis) is used for the production of hydrogen and the thermal
60
energy (i.e. IR) portion of solar radiation ̶ for generating
oxygen, as represented by the following generic reactions:
Ru
(1) SO2 + 2NH3 + H2O → (NH4)2SO3
(2) (NH4)2SO3 + H2O → (NH4)2SO4 + H2
(3) (NH4)2SO4 + MO → 2NH3 + MSO4 + H2O
(4) MSO4 → MO + SO3 (SO2 + 1/2O2)
where MO is alkali metal sulfate (e.g., Na2SO4, K2SO4,
Rb2SO4)
H2 evolved, mL
Fig. 1. Experimental setup for conducting photocatalytic
experiments
40
20
Ni
Co
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Time, hours
Fig. 2. Effect of different co-catalysts on the rate of CdSThe reaction (1) is a chemical absorption reaction occurring at
photocatalyzed H2 production.
near ambient conditions; reaction (2) is a solar photocatalytic
stage (25-80oC); reaction (3) is solar thermal stage (400- It is evident from the Fig. 2 that Pt cocatalyst shows the
500oC); and reaction (4) is solar high-temperature highest activity among transition metals tested.
thermocatalytic stage (up to 850oC, depending on particular
WSC and presence of catalytic additives).
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HYdrogen POwer Theoretical and Engineering Solutions International Symposium 2015, Toledo, Spain
6 – 9 September 2015
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The overall efficiency of a thermochemical cycle for
hydrogen production via water-splitting is a function of the
efficiencies of all the individual steps/processes that
comprise the cycle, including the photon conversion
efficiency (PE) of the hydrogen production step, which is
defined as the ratio of the chemical energy of hydrogen
product to the photon (or irradiation) energy absorbed by
the reacting system:
The spectrum of Xe-lamp light contains a wide range of
wavelengths, but only the wavelengths ranging from 300 to
500 nm are capable of activating the CdS photocatalyst.
The photon conversion efficiency of the photocatalytic
process is estimated at 20-25% range depending on the
particular photosystem.
and recovery in the operational temperature range as part
of the cycle operation. NH3 and SO3 can be stored as
liquids at modest pressures (less than 10 atm) and ambient
temperature, and on-demand recombine in presence of
water to produce ammonium sulfate and release stored
thermal energy (Q).
(8) NH3 + SO3 + H2O → NH4HSO4 + Q
Fig.3 shows AHS in the form of molten salt. AHS possesses
different physical and chemical properties than ammonium
sulfate, which substantially expands the range of operating
temperature for thermal energy storage.
3.2 Integration of thermal energy storage with SA watersplitting cycle.
Due to the intermittent nature of solar resource, there is a
need to couple solar-powered thermochemical cycles with
thermal energy storage systems. A new type of O2generation sub-cycle having all-fluid reaction media has
been developed at FSEC. This sub-cycle includes the alkali
metal sulfates and pyrosulfates. In this sub-cycle,
ammonium sulfate produced in the photocatalytic step 2
releases one molecule of NH3 and forms ammonium
hydrogen sulfate (AHS):
(5) (NH4)2SO4 → NH3 + NH4HSO4
AHS reacts with alkali metal sulfate (e.g., Na2SO4, K2SO4,
or Rb2SO4) liberating NH3 and water, and generating the
corresponding pyrosulfate salt, as shown below for the case
of K2SO4:
(6) NH4HSO4 + K2SO4 → NH3 + H2O + K2S2O7
This is followed by thermal decomposition of pyrosulfate
K2S2O7 with production of the original metal sulfate and
evolution of SO3 or SO3-SO2-O2 mixed gas, depending on
the operational parameters and presence of catalytic
additives:
(7) K2S2O7 → K2SO4 + SO3 (SO2, O2)
The reactions (5), (6) and (7) are carried out at the range of
temperatures of 150-200oC, 200-500oC and 500-850oC,
respectively. SO3 could be further catalytically decomposed
to SO2 and O2.
The main advantages of the developed alkali metal
sulfate/pyrosulfate sub-cycle is that ammonium sulfate,
ammonium bisulfate and alkali metal pyrosulfate exist in a
molten salt form in the wide range of temperatures. It allows
for all-fluid operation and efficient thermal energy storage
Fig. 3. Ammonium hydrogen sulfate in molten form.
Experimental studies indicated that that the NH4HSO4K2SO4 mixture with the ratio of 1:1.25 mol showed the best
performance in terms of better solubility of reagents in the
molten salt system. In general, the use of molten salts is
well known and it has been utilized industrially for many
decades, thus, the thermal energy recovery from the molten
salts is doable and can be readily practiced in the SA cycle.
In the present study, we conducted a thermodynamic
analysis of the O2-producing sub-cycle, using the advanced
thermodynamic equilibrium calculator FactSageTM, and
available literature data on the thermodynamic properties of
compounds involved in the sub-cycle. The results were
compared to our preliminary thermal analysis (TGA)
experiments to verify the validity of above configuration.
The available thermodynamic data were transformed into
the appropriate formats following Lindberg experimental
work [8] and FactSageTM database. In particular, we
extracted ∆Ho298.15 [J/mol], So298.15 [J/mol·K] Cp [J/mol·K]
(temperature dependent functions) and Gibbs free energy
of formation for the quadruplets. Then, an exploratory and
non-constrained thermodynamic analysis (i.e. inclusion of
all possible compounds) was performed.
Fig. 4 illustrates the temperature-dependent thermodynamic
equilibrium of an equimolar mixture of (NH4)2SO4 and
K2SO4.
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HYdrogen POwer Theoretical and Engineering Solutions International Symposium 2015, Toledo, Spain
6 – 9 September 2015
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- simplified thermal storage and energy recovery as an
integral part of the cycle
- facile product separation steps, and
- potentially high solar-to-H2 energy conversion efficiency.
Acknowledgements
Fig. 4. Temperature-dependent thermodynamic equilibrium
of the oxygen sub-cycle with initial stoichiometric ratios
(some compounds have been omitted for clarity).
The evolution of the compounds as well as the phase
changes in the system can be observed in Fig. 4. This
representation is considered to be more representative than
previous ones (especially at temperatures lower than
400oC) because of the inclusion of ammonium bisulfate as
an intermediate product. The same figure presents the
existing proposed steps of the oxygen evolution sub-cycle
at corresponding temperatures in order to achieve adequate
reactions’ extent and maintain all compounds in liquid state.
The inclusion of ammonium bisulfate reveals that there is
one more temperature range where all compounds are
liquid, around 375oC. In other words the oxygen sub-cycle
could include one more step at lower range of
temperatures, which could potentially reduce the energy
requirements of the process. Also, the interim product could
serve as thermochemical energy storage material. Further
experimental work is in progress to investigate the impact
on the alkali-metal based sulfates solution. The calculations
will be conducted with different compositions in order to
increase the efficiency of the whole cycle while maintaining
a liquid salt mixture in a molten state.
4
Conclusions
A novel “Sulfur-Ammonia” solar hybrid photothermochemical water splitting cycle has been developed
by introducing ammonia (NH3) as a working reagent to
hybrid-sulfur cycle to allow more efficient solar interface,
facile product separation steps and all-fluid operation. The
unique feature of the developed SA water-splitting cycle is
that it is capable of utilizing both photonic (UV-Vis) and
thermal (IR) components of solar radiation, and provides
thermal energy storage capacity as an integral part of the
cycle. The main advantages and features of the SA cycle
over existing solar-driven cycles are summarized below; in
particular, it involves:
This work was supported by the Qatar National Research
Fund (a member of The Qatar Foundation) under NPRP
award: NPRP 6 - 116 - 2 – 044. The authors thank prof.
Arun Srinivasa (Texas A&M) for fruitful discussions.
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- a photocatalytic step that generates hydrogen using
visible fraction of the solar spectrum,
- a novel O2 evolution sub-cycle incorporating sulfatepyrosulfate regenerable system, which allows all fluidic
operation (no solids are involved),
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HYdrogen POwer Theoretical and Engineering Solutions International Symposium 2015, Toledo, Spain
6 – 9 September 2015
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