When the proposed integrated process is operated solely in standalone electricity production mode, the resulting SWP cycle can generate electricity with “unprecedented” efficiencies of 40-46%. Similarly, in standalone hydrogen mode, pressurized hydrogen is produced at efficiencies approaching ~50%.

In the co-production mode, the co-produced hydrogen is stored for use in a turbine-based hydrogen water power (H2WP) cycle with calculated hydrogen-to-electricity efficiency of 65-70%—comparable to fuel cell efficiencies. The H2WP cycle utilizes much of the same equipment as the SWP cycle, thereby reducing capital outlays.

The overall sun-to-electricity efficiency (OSTE) of the hydricity process, averaged over a twenty-four hour cycle, approaches ~35%—nearly the efficiency attained by using the best multi-junction photovoltaic cells along with batteries.

The team noted that, in comparison, to the PV/battery system, the proposed process:

1. stores energy thermochemically with a two- to three-fold higher density;

2. does not discharge the stored energy over time and the storage medium does not degrade with repeated uses, as is the case with batteries; and.

3. coproduced hydrogen has alternate usages in transportation/chemical/petrochemical industries.

In the solar water hydrogen power (SWH₂P) co-production cycle, the hydrogen and power production units are integrated, allowing mass exchange and heat exchange—the entire water superheating step is a part of the power production unit and a portion of the pressurized superheated water after high pressure turbine expansion is directed to hydrogen production unit.

The unconverted water stream can also be sent to the power production unit and/or can be cooled down against a process stream in the electricity production unit.

The process integration increases the overall production efficiency by minimizing the exergy losses associated with the transfer of high temperature heat across large temperature differences and supplying high efficiency electricity for compression of hydrogen and oxygen, the team noted in their paper.

In all but thermal and chemical energy storage systems, solar energy is converted to electricity prior to conversion to the ultimate energy storage form. In the absence of solar energy, the stored energy form is then converted back to the electricity and supplied to the grid. Thermal and chemical energy storage eliminate the redundant sun-to-electricity conversion step while providing the stored solar energy as high temperature heat when necessary. An advantage of chemical/thermal storage is that the power production part of the solar thermal power plant can be operated round-the-clock by substituting the direct solar heat with the heat supplied either through thermal storage or via combustion of the stored chemical.

—Gençer et al.

For the study, the team modeled two solar hydrogen production systems: (i) direct thermal hydrogen production, and (ii) two-step hydrogen production.

Hydrogen can also be combined with carbon from agricultural biomass to produce fuel, fertilizer and other products.

If you can borrow carbon from sustainably available biomass you can produce anything: electricity, chemicals, heating, food and fuel.

—Rakesh Agrawal, Purdue University’s Winthrop E. Stone Distinguished Professor in the School of Chemical Engineering and corresponding author of the PNAS paper

The system has been simulated using models, but there has been no experimental component to the research.

The research has been funded by the US Department of Energy through the DOE’s Center for Direct Catalytic Conversion of Biomass to Biofuels at Purdue’s Discovery Park and through a “Solar Economy” project led by Agrawal under the National Science Foundation’s Integrative Education and Research Traineeship Program.

Resources

• Emre Gençer, Dharik S. Mallapragada, François Maréchal, Mohit Tawarmalani, and Rakesh Agrawal (2015) “Round-the-clock power supply and a sustainable economy via synergistic integration of solar thermal power and hydrogen processes” PNAS doi: 10.1073/pnas.1513488112