While the need to develop a low carbon economy is pressing, and electricity production has achieved considerable success with technologies such as wind and solar PV, adapting energy intensive industrial processes and producing green transport fuels from renewable energies has proved challenging. Now, concentrating solar power (CSP) is offering an opportunity to change that.
In a project backed by a consortium including Germany’s DLR, concentrating solar power is being used to produce synthetic hydrocarbon fuels, such as kerosene. While clearly such a development has profound implications for transportation sectors like aviation and shipping that are strongly dependent on drop-in hydrocarbon fuels, it also offers considerable scope for application in other energy intensive sectors. Chemical processes, such as the production of cement, and chemical feedstocks like ammonia for fertilisers, are particular targets for similar emerging technologies.
The SUN-to-LIQUID project has taken the role of solar power a step further by producing renewable transportation fuels from water and carbon dioxide by using concentrating solar power. The research project recently announced that it has now successfully demonstrated the production of kerosene using a solar powered process.
RAMPING UP THE INTENSITY OF CSP
Concentrating solar power is a relatively well-established process by which solar radiation is focused on a receiver where the energy is collected. There are two main types. Parabolic trough is already in commercial use in a number of regions. Here, a mirrored trough focuses sunlight onto a tube containing a heat transfer medium, such as oil. Heating this oil to a temperature of 400°C-500°C, it is then passed through a heat exchanger to generate steam, which is subsequently used in a conventional steam turbine power generation process.
A second type of concentrating solar power involves many digital solar reflectors – heliostats – which angle their position to focus solar rays on a central receiver, typically located atop a tower. Again, such systems are in commercial operation, notably the PS10 plant located near Seville, Spain, which is noted as the world’s first such commercially operating plant. It was commissioned in 2007.
There is a fundamental difference between the two technologies though. Parabolic trough technologies typically concentrate the sun by a factor of around 50 and produce temperatures of between 400°C and 500°C. By contrast, central receiver types can produce far higher levels of solar concentration and consequently far higher temperatures. Given that much higher temperatures are needed for energy intensive chemical processes, such as the production of syngas or cement, it is the favoured approach for those looking to develop a low-carbon approach to sectors previously dominated by fossil fuels.
Dr Martin Roeb, team and project leader at the Institute of Solar Research and Solar Chemical Engineering within the German Aerospace Centre (DLR), explains: “For electricity production, normally lower concentration factors are needed than those needed for fuel production. But, you need much more concentration of sunlight to get to higher temperatures. If you would like to go to 1000°C and beyond, you need higher concentration factors using a technology which is also scalable. That was the reason to choose this solar power tower technology, which in terms of power production, is already commercial. There are plants in California, the Middle East and Australia, in the megawatt and multi-megawatt range.”
He continues: “The idea is to combine such technology, which is already at scale for electricity production, with a method to also use what’s available at scale in terms of synthetic fuel production.”
Within the solar reactor, redox materials, such as nickel ferrite or cerium oxide, are heated to 1400°C, where they are chemically reduced and oxygen is released. In a second step, which takes place at 800°C to 1000°C, water vapour flows through the reactor and the previously reduced material is reoxidised with the oxygen of the water molecules now binding to recreate the metal oxide. The oxygen remains in the reactor, whilst the hydrogen is released until the process is repeated and oxygen is driven off the oxide.
The SUN-to-LIQUID project is a four-year programme supported by the European Union’s Horizon 2020 research and innovation programme and the Swiss State Secretariat for Education, Research and Innovation (SERI).
Started in January 2016, this phase of the project is due to conclude at the end of the year. It follows on from EU SOLAR-JET project, which first developed the technology and demonstrated the proof of concept at lab-scale by achieving the first-ever production of solar jet fuel. A total of 291 stable redox cycles were performed, yielding 700 standard litres of high-quality syngas, which was compressed and further processed via Fischer-Tropsch synthesis to a mixture of naphtha, gasoil, and kerosene. In parallel to that, a solar hydrogen project – HYDROSOL – has been run, which demonstrated solar hydrogen production in 2017.
The SUN-to-LIQUID project has scaled up this technology for testing at a solar tower (main picture). As Roeb observes: “Previous projects like the SOLAR-JET developed a mini plant at lab scale of around five to 10 kW.With SUN-to-LIQUID, we are going up to the >50 kW-scale on the solar tower.”
Indeed, to demonstrate the technology, a unique solar concentrating plant was built at the IMDEA Energy Institute in Móstoles, Spain, with a concentration factor of some 2,500. This is around three times greater than existing central receiver solar installations used for electricity generation.
The high intense solar flux, verified by a DLR measurement system, allows reaction temperatures of more than 1500°C to be achieved within the solar reactor positioned at the top of the tower. Starting with water and CO2, the solar reactor produces syngas, a mixture of hydrogen and carbon monoxide, via the thermochemical redox cycle. For the demonstration plant, the solar reactor was developed by ETH Zurich. An on-site gas-to-liquid plant that was developed by industrial gases company HyGear processes this gas to Fischer-Tropsch hydrocarbon fuels, like kerosene. Solar-to-syngas energy conversion efficiencies exceeding 30% can potentially be realised thanks to favourable thermodynamics at high temperature and utilisation of the full solar spectrum, the project’s backers claim.
However, although this latest phase of the development is due to conclude at the end of the year, further advances are expected. As Roeb says: “More complex fuels and better concentrating factors are also part of adapting existing technology to this fuel production process. Early next year, following the end of the project, we are looking at two options on how to continue. We’ll seek further funding to scale it up but also industry contributions. The idea is to have the follow-up project begin sometime next year.”
Roeb adds: “That next phase is expected to be a pre-commercial validation, following on from the proof of concept and proving that the system can work at a reasonable scale. The next phase is proving the cost, the economics of the still pre-commercial system but at multi-megawatt scale.
“The scale up is one central point, but we also plan to look a little bit more into heat recovery. At the moment, we still lose too much of the heat and we need to recover that because it’s high temperature heat, which is very valuable, and we still have to improve our system to do so. There is an option to do so from other technologies like recuperators.
“Potentially this could lead to the development of a combined plant that uses a very high temperature chemical reaction process to produce the liquid hydrocarbons and then a conventional steam generation power plant or process steam.”
Alongside project partners DLR, ETH Zurich, HyGear Technology & Services B.V and IMDEA Energy, other partners are Abengoa Energía and project coordinator Bauhaus Luftfahrt e.V. ARTTIC supports the Research Consortium with project management and communication.
SOLAR POWERED INDUSTRIAL FUTURE
In addition to transport, other energy intensive industries are also likely to benefit from this technology. For example, the EU-backed SOLPART project is investigating the use of high temperature solar reactors for the production of calcium oxide or quicklime (CaO), a major component in cement.
Under the coordination of French research centre, CNRS, a 30 kWth pilot reactor is being constructed within the solar furnace. Again, this project will end in December 2019. DLR is contributing to this project by investigating the use of a rotary kiln reactor to split limestone into CaO and CO2.
Despite the breakthroughs, there are still some technical challenges to demonstrate this technology is able to operate at scale in an industrial arena. For example, one of the challenges is to determine how to run such processes continuously. Roeb explains: “Many of those kinds of processes need to be operated 24 hours for several reasons, such as they are integrated with other downstream processes which don’t like too much variation in plant load. Therefore, we are always looking into options to combine these technologies with thermal storage.”
Today’s solar thermal power plants are typically equipped with thermal storage that is capable of storing the energy needed to keep the plant at full-load operations for between 12 to 24 hours, but at significantly lower temperatures than those required for solar reactors of this type.
“We are looking at a combination with this thermal storage option, but we are also looking into thermo-chemical storage options,” says Roeb. “It’s another field of our research to explore how we can increase the storage density. We are looking into, for example, redox storage to create the temperatures. There are different materials available in the range of between 700°C and 1500°C.”
According to the project’s backers, compared to conventional fossil-derived jet fuel, the net CO2 emissions can be reduced by more than 90% using the solar reactor process. The solar energy-driven process also uses abundant feedstocks like water and CO2, and unlike many biofuels, does not compete with food production.
With its numerous advantages expected to see additional demonstration projects and further breakthroughs for the technology, fully commercial solar reactor systems are anticipated by the 2030s. As Roeb concludes: “It will take some years to further scale up the technology and tackle the remaining challenges, but I would say, in a couple of years, we would have the core technology in place.”