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HIGH EFFICIENCY SOLAR PRODUCTION OF HYDROGEN FROM WATER
National Hydrogen Association Annual Hydrogen Conference Sacramento, California April 2, 2008 Rob Taylor Science Applications International, Corporation San Diego, California Roger Davenport Science Applications International, Corporation San Diego, California Ali T-Raissi Florida Solar Energy Center University of Central Florida Cocoa, Florida
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Hybrid Photo/Thermo-Chemical Water Splitting Cycle
No carbon emissions/no fossil fuels Solar or nuclear energy source Uses high value photonic energy plus remaining “low-energy” (thermal) energy of solar spectrum Doesn’t require extreme high temperatures (<1,000oC) or exotic materials Potential to reach 57% overall efficiency with 32% solar photocatalytic step
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Solar High Temperature Water Splitting Cycle with Quantum Boost
Award Notification - October 19, 2004 Contract Start – September 1, 2007 Three Phase, Four Year, $5M Program
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Project Participants Science Applications International Corp.
Contract Management Solar Concentrator System Development and System Integration Florida Solar Energy Center of the University of Central Florida Photo/Thermo-Chemical Cycle Evaluation/Selection Chemical Reactor Design
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Project Objectives Evaluate hybrid photo/thermo-chemical water splitting cycles that employ the UV-visible portion of the solar spectrum for hydrogen production Select a cycle/solar system that has the best potential for cost-effective production of hydrogen from water Demonstrate technical feasibility of the selected cycle using solar input in a bench-scale reactor Demonstrate pre-commercial feasibility through demonstration of a fully-integrated pilot-scale solar hydrogen production system and economic analysis of the selected cycle
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Project Phases PHASE 1: Cycle Testing and Preliminary Evaluation
PHASE 2: Bench-Scale Testing and Design PHASE 3: Pilot-Scale Demonstration
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Phase 1 Tasks Cycle Testing and Evaluation
Photo/Thermo-chemical Cycle Analysis Lab Testing of Cycles Report: Preferred Cycle Selection Reactor/Process Configuration Solar Concentrator Design Concentrator Specifications Preliminary Concentrator Design Subsystem Tests Report: Preliminary Solar Concentrator Design Economic Evaluation
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Comparison of Solar Hydrogen Production Processes
Attributes Natural photosynthesis < 1% solar energy conversion efficiency Direct decomposition Temp. > 2,500oC & high temp. materials, gas recombination issue, degradation of photonic energy to heat Photoelectrochemical <15% solar to H2 efficiency, bias voltage is needed, photo-electrode corrosion issues, not efficient & stable photocatalyst has been found PV-electrolysis High potential, E > 1.23 V requirement, high cost of PV cells Photocatalysis Need for high band gap photocatalysts Eg> 3.0 eV, <5% UV in terrestrial insolation, recombination of H2 & O2 Thermochemical High efficiency (> 50%), H2 & O2 are produced in separate processes
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FSEC’s Sulfur-Ammonia Cycle
FSEC’s hybrid photo/thermo-chemical water splitting cycle employs the high energy photon portion of the solar spectrum for the production of H2 & the thermal energy (i.e. IR) portion of sunlight for generating O2: SO2(g) + 2NH3(g) + H2O(l) (NH4)2SO3(aq) (Chemical Absorption, 25oC) (NH4)2SO3(aq) + H2O (NH4)2 SO4(aq) + H2(g) (Solar Photocatalytic, 80oC) (NH4)2SO4(s) + ZnO(s)→ 2NH3(g) + ZnSO4(s) + H2O(g) (Thermocatalytic, 500oC) ZnSO4(s) → SO2(g) + ZnO(s) + 1/2O2(g) (Thermolytic, 900oC)
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S-NH3 Cycle Flowsheet
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Solar Hydrogen Production System Characteristics
Simple separations; hydrogen and oxygen produced in different reactors Energy requirement at high temperature is reduced because some work is done in photocatalytic reaction 32% efficiency has been measured for H2 production step Photocatalysis involves aqueous materials at atmospheric pressure and moderate temperatures Thermochemical processes involve solids and gases at high temperatures
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Solar Hydrogen Production Solar Field Characteristics
SAIC has started evaluation of collector field and reactor configurations suitable for this process For highest efficiency, we would like to separate high-energy and low-energy photons: High-energy photons photoreactor for photocatalysis reaction at approx. one sun and low temperature (60ºC-90ºC) Low-energy photons thermal reactor for high temperature reactions at 300ºC and 900ºC Spectral separation can be done with “hot” mirrors and “cold” mirrors Hot mirror reflects low-energy photons and passes through high-energy photons Cold mirror reflects high-energy photons
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Solar Hydrogen Production Preliminary Collector/Reactor Configurations
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Solar Hydrogen Production Preliminary Solar Field Evaluation
Several considerations lead to preference of a heliostat/central receiver system with a separate photocatalytic reactor Temperatures are too high for flat plate, evacuated tube, or trough collectors Hot/cold mirrors are expensive. Their elimination also reduces the size of the high-temperature system % Equipment for handling of solids and high-temp gases not suitable for installation on dish (motion, flex hoses, etc.) Scale of equipment for solids handling more suitable to large systems (1+MW) and dishes are limited to kW Photoreactors can be very low cost (e.g., SAIC has built plastic film reactors for solar water detox)
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Solar Hydrogen Production Heliostat Field Characteristics
A North field arrangement provides more energy to the receiver per heliostat mirror Smaller heliostats give smaller images and a smaller minimum system size For the high temperature reactor, prefer a cavity receiver with a secondary concentrator to minimize thermal losses The heliostat field is a major system cost item so reducing heliostat costs directly reduces hydrogen cost
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Solar Hydrogen Production Heliostat/Receiver Configuration
North-facing cavity receiver Beam-down receiver Optical Efficiency vs. Heliostat Position in Field
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Solar Hydrogen Production Heliostat Cost Reduction
SAIC has identified a potential for cost savings using a GRC (Glass-Reinforced Concrete) heliostat structure : Very low cost material ($0.07/lb) Easy to process (spray on mold) Excellent weathering Design flexibility (molded in reinforcing ribs, pretensioning) Small (10-15 m2) heliostat, factory- produced, self-powered, with wireless communication to minimize field wiring costs Factory-built concrete track foundation to simplify installation
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Solar Hydrogen Production Heliostat Cost Reduction
GRC Heliostat design is proceeding, with a potential prototype this summer Preliminary cost estimate is $105/m2 for prototypes, compared to conventional designs at $126/m2 in large-scale production Economic analysis is ongoing. Goal is to demonstrate hydrogen production at less than $3/kg
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Conclusions Important not to degrade solar energy for optimum efficiency S-NH3 cycle demonstrated 57% conversion efficiency with moderate temperatures Heliostat field preferred solar concentrator GRC shows promise to lower heliostat field cost S-NH3 cycle is a leading cost effective H2 conversion option
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