1. 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

2. 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

3. 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

4. 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

5. 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

6. Project Phases PHASE 1: Cycle Testing and Preliminary Evaluation
PHASE 2: Bench-Scale Testing and Design
PHASE 3: Pilot-Scale Demonstration

7. 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

8. 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

9. 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)

10. S-NH3 Cycle Flowsheet

11. 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

12. 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

13. Solar Hydrogen Production Preliminary Collector/Reactor Configurations

14. 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)

15. 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

16. Solar Hydrogen Production Heliostat/Receiver Configuration
North-facing cavity receiver
Beam-down receiver
Optical Efficiency vs. Heliostat Position in Field

17. 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

18. 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

19. 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