Merredin Spirulina Algae Farm in Western Australia Credit: ThinkSiD

Susan Kraemer writes in that to use solar thermal energy to convert farmed algae to fuel, the solar fuels research team at Australian National University (ANU), led by John Pye together with Mahesh Venkataraman, proposed a new process, detailed in their paper Modelling of a 50 MWth On-Sun Reactor for SCWG of Algae—Understanding the Design Constraints.

Dr. Pye’s group further described the dynamic process model used in this research in a second paper: System-Level Simulation of a Solar-Driven Liquid Fuel Production Plant via Gasification-Fischer–Tropsch Route and both were presented at the SolarPACES 24th conference in Morocco.

Thermochemical production of solar fuels is an advanced technology being developed in national laboratories around the world. Mirrors concentrate thousands of reflected “suns” onto various kinds of solar reactors to achieve very high temperatures, up to 1500°C.

Solar reactors are used to carry out heat-driven thermochemistry to rearrange molecules to produce hydrogen, or synthesis gas (a mixture of hydrogen and carbon-monoxide), which can be further processed to make hydrocarbon fuels like petrol, diesel and jet fuel.

In the process proposed by the ANU researchers, the solar heat is used to gasify biomass at temperatures up to 605°C. Concentrated solar radiation is used to heat a high pressure algae and water slurry, and the combination of pressure and temperature breaks down the algae bio-molecules into synthesis gas which can then be converted into gasoline.

The process begins with Supercritical Water Gasification (SCWG), a next-generation thermochemical conversion technology for a faster chemical conversion with little waste, in which pressurized water and algae are heated using solar energy to make them go supercritical — a phase which has very different properties compared to room temperature water. SCWG has been proven at lab scale for converting organic feedstock into a gaseous fuel mixture containing CO, CO2, hydrogen and methane. It would also be ideal for a wet feedstock like algae.

While supercritical water gasification is not actually limited to algae, the ANU researchers focused their expertise in solar fuels on algae because it could be farmed in Australia’s sun-drenched deserts with the abundant low-cost space needed for solar fields of heliostats to provide the heat for the reaction. Producing a solar fuel from algae required them to figure out how to design the entire process, including following up the solar SCWG with a modified Fischer-Tropsch (F-T) unit for the conversion of the synthesis gas into a liquid fuel.

As with other solar reactors for solar fuels, heliostats would form a polar arrangement in front of a tower, enabling much more highly concentrated solar radiation to achieve high temperatures with low flux on the receiver tubes. “The way to achieve this is to have a deep cavity with a lot of tubes spread out on the inside cavity and with a cavity you end up naturally with this kind of polar heliostat field layout,” Dr. Pye explained.

In solar reactors, heliostats focus and concentrate sunlight into the receiver at the top of the tower, which heats up the reactor and enables chemical reactions, in this case, the wet algae mix from a nearby algae pond is heated inside the reactor tubes to 605C at a pressure of 24 MPa.

“You need to concentrate the algae solution enough for the reactor to be efficient and then you take this solution up the tower,” said Dr. Venkataraman. “The algae start to gasify in the high temperature and pressure and starts forming syngas which can be converted to liquid fuel.”

Dr. Pye’s group at ANU has shown the techno-economic feasibility of liquid fuel production process via solar driven supercritical water gasification and the solar F-T route. The process has been optimized and the levelized cost of fuel (LCOF) production using this technique comes out to be ~3.2 AUD/L of gasoline equivalent.

That means that their solar algae fuel would cost about three times the 2018 cost of gasoline in Australia. But the currently high cost of algae farming itself accounts for much of this high LCOF. The completely carbon neutral solar process that these researchers have developed would become competitive if the cost of farming algae comes down (or the carbon price goes up).

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