Major efforts are underway to develop new process technology for making chemicals using sunlight and the products of combustion
No one can deny that the sun provides more than enough energy to supply the world’s energy and materials needs. After all, Mother Nature has been using sunlight for millennia, making a myriad of chemicals from carbon dioxide and water via photosynthesis. And the fact is, fossil fuels are the remnants of sun-to-chemical production, which humans have been exploiting for the last few centuries as alternatives to the biomass that our ancestors used for cooking and heating needs.
As the energy demands of the human species continue to grow, due to both population growth and usage, it doesn’t take more than a back-of-the-envelope calculation to show that eventually, our reliance on fossil fuels will come to an end — it’s simply a question of when. After all, there is only a finite amount of coal, gas and petroleum. And in the last few decades, most of the conscientious scientific and industrial community have realized that something needs to be done to slow down the release of CO2 into the atmosphere.
Planning for future generations, efforts around the world are growing to ween our current dependence on fossil resources by taking advantage of the ever-present sunshine flooding some part of the planet every day. Although the energy sector tends to receive the most headline-grabbing attention about solar power, there is another less well known, but very active pursuit for using sunlight, CO2 and water for manufacturing chemicals (Figure 1). This article is a modest attempt to highlight some of these activities, which are making progress towards the production of renewable H2 and synthesis gas (syngas; H2 and CO), which is then used for making ammonia, liquid fuels, alcohols and more (see Table 1; a more comprehensive Table 1 can be found later in the article.
Understand the limits
When it comes to making chemicals from CO2, water and sunlight, there are basically three possibilities, explains Christian Sattler, head of solar chemical engineering at the German Aerospace Center (DLR; Cologne, Germany; www.dlr.de). Sunlight can be used as photons for photosynthesis; it can be transformed in photovoltaic (PV) cells into electrons, and then used in electrochemical processes; or it could be used as heat in thermochemical processes. “There is no contradiction, but actually a synergy between the different routes,” says Sattler, adding that “this is the reason why we have a very successful joint solar-fuels topic in the Helmholtz Renewable Energy program [www.helmholtz.de] between the three routes.”
For industrial applications, a basic principle needs to be taken into account seriously, Sattler continues: “Solar radiation is a rather diluted energy source, whereas the produced fuels or chemicals have a very high energy density. Therefore, from a chemical point of view, if sunlight is not concentrated to higher energy densities, the amount of product per irradiated reactor surface is limited to far less than 1 kW/m2. This amount of energy is not available for photochemical processes, but only a fraction of it with the right wavelengths and it has to be multiplied with the efficiency of the chemical process, which gives a rather low number. Therefore, from a large-scale industrial point of view, it makes sense to concentrate solar radiation — either in the form of heat or of electricity — to use it in chemical processes. This makes compact reactors with high throughput possible,” he says.
The electrolysis of water into H2 and O2 has been used industrially for many years, especially in regions of the world where electricity has been inexpensive (such as that from nuclear or hydroelectric plants). Now, as the cost of PV technology has dropped, solar-based renewable H2 production is making its way into the chemical process industries (CPI), including petroleum refineries. In January, for example, Royal Dutch Shell Plc (The Hague, the Netherlands; www.shell.com) and ITM Power (Sheffield, U.K.; www.itm-power.com) announced plans to build the world’s largest water-electrolysis plant to produce H2 for use in Shell’s Rheinland Refinery in Wessling, Germany. When the 10-MW “Refhyne” plant starts up in 2020, it will produce 1,300 metric ton (m.t.) per year of H2 using renewable electricity.
Meanwhile, there is an ongoing effort to develop alternative methods of making H2 from water, by so-called advanced water-splitting (AWS) technologies. One (of many) such initiatives is the HydroGEN Energy Materials Network (EMN) consortium (www.h2awsm.org), which is led by the U.S. Dept. of Energy’s (DOE; Washington, D.C.; www.energy.gov) National Renewable Energy Laboratory (Golden, Colo.; www.nrel.gov), and includes the Lawrence Berkeley National Laboratory (Calif.; www.lbl.gov), Sandia National Laboratories (Livermore, Calif.; www.sandia.gov), Lawrence Livermore National Laboratory (Calif.; www.llnl.gov) and Savanna River National Laboratory (Jackson, S.C.; www.srnl.doe.gov). Among the AWS technologies under investigation are low-temperature anion-exchange membrane (AEM) and proton-exchange membrane (PEM) electrolysis, high-temperature electrolysis, photoelectrochemical (PEC) pathways and solar thermochemical (STCH) water splitting.
Last November, Proton OnSite (Wallingford, Conn.; www.protononsite.com) received around $1.8 million in a cooperative agreement award to lead the DOE’s AWS Benchmarking Project with the HydroGEN EMN consortium. The goal is to speed the discovery and development of efficient, durable and low-cost AWS materials capable of meeting DOE’s long-term H2 -production goal of less than $2/kg.
Looking at STCH, for example, the Fuel Cell Technologies Office withinDOE (https://energy.gov/eere/fuelcells/fuel-cell-technologies-office) has funded Sandia National Laboratories for many years to verify the potential for solar-thermochemical cycles for H 2 production to be commercially viable, and to advance STCH’s technology readiness level through R&D efforts focused on materials and reactor design, says project lead Anthony McDaniel at Sandia. “Two-step, non-volatile metal-oxide cycles are currently the thermochemistry of choice. Together with DLR (a subcontractor to Sandia on one particular project), the team developed a 3-kWth-scaled prototype and successfully demonstrated Sandia’s moving particle bed, solar-thermochemical reactor technology,” says McDaniel.
“The project with DLR [called HEST-HY in Table 1] was comprised of three main tasks: discovery of redox active materials; reactor prototype design and demonstration; and building a high-fidelity model to predict performance of a large-scale plant (targeting 100 kg/d H2 production),” explains McDaniel. “And while the optimal material still eludes us, the project did complete prototype construction and accomplish the demonstration,” he adds. The high-fidelity plant model is currently being used to conduct detailed technoeconomic studies of the STCH process. Regarding the moving particle-bed reactor, McDaniel says that Sandia’s concepts embodies spatial and temporal separation of pressure, temperature and reaction products, as well as continuous on-sun operation. “These design attributes are a significant departure from the traditional norms of this technology development community and key to high efficiency operation,” he says. Although this specific project was completed last year, the materials work continues through the DOE-funded HydroGEN consortium.
That said, a key metric that drives the development of PEC and STCH is the solar-to-H 2 conversion efficiency — “essentially challenging ourselves to dramatically outdo Mother Nature, and in fact, outdo a conventional electrolyzer (~10% solar-to-H2 efficiency when coupled to a Si-based PV system),” says McDaniel.
“On paper, PEC and STCH promise to dramatically increase the system efficiency (DOE technology development targets >25%),” McDaniel continues. “I favor the STCH approach because it has the theoretical potential to achieve a high solar-to-fuel conversion efficiency. Concentrated solar utilizes the full spectrum (not selectively absorbing photosystems) and converts thermal energy directly into fuel (that is, a reduced oxide) without incurring serial inefficiencies associated with the photon-to-electron intermediate step.”
“In comparing PEC to STCH from a technology development perspective, they each face different challenges,” says McDaniel. The best PEC chip ever devised so far operates at ~16% solar-to-fuel conversion efficiency. But, McDainel points out, it is a small device, producing a few Watts of H 2, that degrades rapidly. The largest STCH process demonstrated is 100 kW (with 750 kW in the Hydrosol Plant endeavor, described below), and operates continuously for weeks — but at a much lower solar-to-fuel conversion efficiency. The highest STCH process efficiency demonstrated thus far is 5.25% at 4 kW. “So, PEC wins on demonstrated efficiency and STCH wins on demonstrated scalability. At the moment, DOE is betting on both,” McDaniel says.
Meanwhile, the Hydrosol Plant project started up last November at CIEMAT’s (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas; www.ciemat.es) Plataforma Solar de Almería (PSA) in southern Spain to become the world’s largest solar-chemical installation for producing H2. The plant is the culmination of an international project coordinated by the Greek Aerosol and Particle Technology Laboratory, in collaboration with Germany’s DLR, Spanish CIEMAT, the Dutch company HyGear, and the Greek energy-supply company Hellenic Petroleum. DLR is predominantly responsible for the development of the reactor, which uses solar-thermal energy to make H2 by the reaction of H2O with metal oxides at temperatures of 800–1,400°C. “This is not a laboratory experiment anymore, but it is also no industrial demonstration,” says DLR’s Sattler. “The next step will be to further scale it up into the megawatt range, which will be expensive,” he adds.
While H2 is an important chemical in itself, as well as an energy carrier that can be, and increasingly is being used to power fuel-cell driven vehicles, syngas is the key building block when it comes to producing basic chemicals. And this is an area where the power of the sun is literally heating up interest. In addition to the Hydrosol projects, DLR is active in a number of projects aimed at producing syngas, fuels and other chemicals by solar-thermochemical methods (Table 1). “Some of these are far beyond laboratory scale,” says Sattler.
For example, as part of the Sun-to-Liquid project (www.sun-to-liquid.eu), a small solar tower system was built at IMDEA Energía at Móstoles Technology Park, near Madrid, in 2016. This high-flux solar-concentrating subsystem consists of an ultra-modular solar heliostat central receiver that provides intense solar radiation for high-temperature applications beyond the capabilities of current commercial concentrated solar power (CSP) installations. The heliostat field consists of 169 small size heliostats (1.9 m x 1.6 m). When all heliostats are aligned, it is possible to fulfil the specified flux above 2,500 kW/m2 for at least 50 kW and an aperture of 16 cm, with a peak flux of 3,000 kW/m2. A reliable roadmap for competitive drop-in fuel production from water, carbon dioxide and solar energy will be established in the project.
The E.U.-funded Sun-to-Liquid project is the followup to a predecessor project, Solar-Jet, in which a solar reactor developed at ETH Zurich (Switzerland; www.prec.ethz.ch) was successfully operated for splitting H2O and CO2 to produce syngas, which was subsequently converted via Fischer-Tropsch (F-T) synthesis into kerosene.
“In the framework of the Solar-Jet project, we have experimentally demonstrated at laboratory-scale the entire production chain to renewable jet fuel (kerosene) via solar thermochemical splitting of water and CO2,” says Philipp Furler, a research associate at ETH Zurich and operating agent for solar chemistry research of the International Energy Agency’s (Paris, France; www.iea.org) technology program, SolarPACES. “If coupled to CO2-capture from atmospheric air, such kerosene has zero net CO2 emissions and can be certified for commercial aviation by minor addendum to the existing D7566 specification for synthesized hydrocarbons. Furthermore, we boosted the reactor energy-conversion efficiency by a factor of 13 to a record 5.25%. Within Sun-to-Liquid, we are now scaling up the process to a solar-tower,” says Furler.
In parallel, Sunredox — a spinoff company of ETH Zurich — jointly with Synhelion (Switzerland; www.synhelion.com) and Eni S.p.A (Rome, Italy; www.eni.com) are working in close collaboration to further develop and scale-up the solar chemical technology to the megawatt size over the next years, with the goal of reaching solar-to-fuel energy conversion efficiencies beyond 20% for securing economic competitiveness, says Furler, the co-founder and CEO of Sunredox. ENI and Synhelion signed a cooperation agreement in June 2017 for this purpose. “We are targeting the first 20-MW pilot plant by 2023 and the commercial phase by 2025,” he says.
Furler points out three main advantages of the thermochemical route to renewable fuels compared to alternative approaches. Firstly, the thermochemical route uses the entire solar spectrum and bypasses critical energy conversion stages, such as electricity generation, electrolysis, or reverse-water-gas-shift and thus can exceed the energy conversion efficiencies of alternative processes and become cheaper. Secondly, it is applied in desert regions with high direct normal irradiance (DNI), whereas fuels derived from biomass use valuable agricultural land and suffer from low energy-conversion efficiencies (typically below 1%) resulting in critical land footprint. Thirdly, it can be combined with inexpensive thermal-energy storage solutions, such as a packed bed of rocks ($15/kWh), to enable continuous and uninterrupted 24/7 operation of the plant, regardless of the sunlight intermittency. In contrast, approaches based on electricity lack inexpensive storage solutions, says Furler.
In January, Siemens AG (Munich; www.siemens.com) and Evonik Industries AG (Essen, both Germany; www.evonik.com) started the Rheticus project — a two-year project to use electricity from renewable sources and bacteria to convert CO2 and water into specialty chemicals, such as butanol and hexanol. With €2.8 million funding from the Federal Ministry of Education and Research (BMBF; Bonn and Berlin, Germany; www.bmbf.de), the companies plan to bring a test plant onstream by 2021 at Evonik’s site in Marl, Germany. After that, the next step could be a plant with a production capacity of 20,000 m.t./yr, says Evonik.
In the Rheticus project, syngas will be produced electrolytically (using electricity generated by renewable sources, such as photovoltaic (PV) cells or wind generators) by the reduction of CO2 into syngas at the cathode, while water is split into O2 and H+ ions at the anode. The syngas will then be fermented into alcohols using aerobic microorganisms. The complete process has been demonstrated at small scale, as described in a Nature Catalysis article published in January. In that study, a PV module supplied the electricity (480 mA per 10 cm2 at 3.7 V) operating at 11% conversion efficiency. The electrolyzer used a commercial Ag-based gas-diffusion electrode (GDE) from Covestro AG (Leverkusen, Germany; www.coversto.com) for the cathode, and an indium/mixed-metal-oxide-coated titanium plate from ElectroCell A/S (Tarm, Denmark; www.electrocell.com) for the anode. When coupled to a fermentation module, the syngas from the electrolyzer is metabolized into butanol and hexanol with high carbon selectivity. The study demonstrated that the conversion of PV electricity, CO2 and water to the desired alcohols achieved close to 100% Faradic efficiency. Evonik says that artificial photosynthesis is closer than expected with this scalable hybrid system.
Meanwhile, researchers around the world are regularly reporting new “breakthroughs” in catalyst development targeting the steps of artificial photosynthesis — either H2 production or CO2 reduction, or both simultaneously. These efforts are often reported in the Chementator pages of this magazine. For example, researchers from the National University of Singapore (www.nus.edu.sg) have developed a prototype device that uses artificial photosynthesis to produce ethylene using only sunlight, water and CO2, at room temperature and pressure (Chem. Eng., January 2018, p. 7; www.chemengonline.com/making-ethylene-artificial-photosynthesis). Another example can be found in this month’s Chementator Briefs on p. 8, which mentions the findings of a recent study for making solar H2 in an international colaboration led by the University of Twente’s MESA+ Institute.
Meanwhile, progress continues at the Joint Center for Artificial Photosysthesis (JCAP; www.solarfuelshub.org), which was established in 2010 by the DOE, and is the U.S.’s largest research program dedicated to advance artificial-fuels generation science and technology. The second phase of this program started in 2015, with the focus on solar CO2-reduction to fuels. JCAP is led by the California Institute of Technology (Pasadena; www.caltech.edu), in partnership with the Lawrence Berkeley National Laboratory. Other partners include the University of California at Irvine and San Diego, and the SLAC National Accelerator Laboratory, operated by Stanford University.
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