Solar Fuels Technology — One Step Closer to Commercialization


The emirate of Qatar is an oil and gas rich nation – having the third largest gas reserves in the world. For many years, Qatar had enjoyed double-digit economic growth. Qatar’s GDP dropped to 2.2% in 2016, however, weighed down by the spillover effects of unpredictable and downward moving oil prices. In contrast, emirate holds modest oil reserves – with a daily output of 800,000 barrels. Projections show that current oil reserves should ensure output for only the next 23 years.

The electricity demand in Qatar has grown rapidly in recent years, having increased by 17 percent in the past two years due to increased population and energy-intensive water desalination. The emirate has been expanding Qatar’s renewable energy portfolio and in particular, has focused on rapid deployment of solar energy technologies. Qatar has extensive solar resource and according to the Climate Technology Centre and Network, the annual solar energy potential of each square kilometer of Qatari soil is equivalent to 1.5 million barrels of oil. The emirate has targeted solar energy and water as research hotspots to pursue.

Researchers at the Texas A&M-Qatar University (TAMU-Q) became interested in hydrogen production using solar energy. Hydrogen is the largest chemical commodity produced and used in the world. It has a broad range of applications that include engine and fuel cell fuel, ammonia production, and feedstock for petroleum processing at refinery operations, among others. Researchers at the UCF have been conducting research on solar thermochemical production of hydrogen since 2005. With funding from the Qatar National Research Fund (QNRF), a team of researchers from UCF-FSEC, TAMU-Q and TAMU-College Station began further development of a UCF-invented technology for the solar hybrid sulfur-ammonia (HySA) thermochemical water splitting cycle.

The new HySA cycle opted to solve two main limitations of previous solar thermochemical water-splitting cycles, namely, absorption of solar energy in the form of thermal energy only, and not able to function continuously due to the intermittent nature of the solar resource. To address these challenges, FSEC researchers introduced ammonia as a reagent into the solar HySA cycle. The new HySA cycle incorporates a more efficient solar interface, less problematic chemical separation steps and, most importantly thermal storage as an integral part of the cycle. In the HySA cycle, hydrogen generation occurs in a photo-reactor illuminated by the ultraviolet and visible portion of the solar spectrum. Oxygen production occurs in a dark reactor with thermal (IR) heat portion of solar flux. The utilization of the full solar spectrum allows the cycle to reach a higher overall solat-to-H2 energy conversion efficiency – in the order of 40-45%.

UCF-TAMU team focused on three parallel tasks: (1) a comprehensive thermodynamic analysis and modeling of the complete/closed cycle, (2) on-sun demonstration of photochemical hydrogen and thermochemical oxygen production steps, and (3) validation of the integrated thermal energy storage system. Notably, two energy storage approaches were tested: use of a molten salt and alkali sulfate/pyrosulfate system. Advantageously, alkali-metal sulfates form binary mixtures with corresponding pyrosulfates (molten salts) having melting point temperatures in the range of 300-600oC. This allowed an all-fluid operation (via molten salt transport), but also efficient thermal energy storage and recovery at the temperature range of interest and as an integral part of the cycle operation.

[Block diagram of O2 production sub-cycle with molten salt and thermochemical energy storage systems]

Another advantage of the new HySA cycle is that both intermediate products of the cycle: NH3 and SO3 can be stored as liquids at ambient temperature and modest pressures (less than 10 atm), and on-demand recombine in presence of water vapor to produce ammonium sulfate (or bisulfate) – releasing heat in the process.

[photo of Ali, Nazim, Nan]

The UCF-TAMU project had nine major tasks (aims) divided among all partners. The UCF team also included several ChE students and post-docs (not shown in the photo). Most of the laboratory work was conducted at UCF’s FSEC, including the on-sun testing of both hydrogen and oxygen production segments of the water-splitting cycle. In addition, using the feasibility of operating integrated thermal storage was validated. The results of the experimental and analytical (modeling) work have been published (four papers), and presented in seven international meetings, including Intern. Conf. Hydrogen Production, Hangzhou, China (2016), where the research team received the Best Paper Award.