ETH Zurich produces hydrocarbons with solar mini-refinery

Materials World magazine
27 Sep 2019

ETH Zurich has developed a solar mini-refinery capable of producing carbon neutral synthetic fuels from sunlight and air. But can it be scaled up?

Over the last few decades, the renewable energy sector has boomed, as countries, companies and individuals look for ways to reduce their carbon footprints. This has led to a push to electrify a number of industries that has historically been powered by a diverse range of fuels.

Electric vehicles (EVs) are a familiar proponent of this movement and global sales appear to be steadily growing. According to the International Energy Agency report, Global EV outlook 2019, there were 5.1 million EVs in the world in 2018, an increase of two million from the previous year.

But while electrifying industries in an effort to reduce emissions has been successful for some, others have found it more challenging. In particular, the aviation industry has seen slow progress. Despite companies like Rolls-Royce, Airbus and Siemens working to develop electric engines capable of powering aeroplanes, these are unlikely to come into commercial operation until the mid-2020s, and even then they will only serve short-to-mid-range distances.

Electrifying aviation is especially problematic, as according to ETH Zurich, liquid fuels contain 20-60 times more power per unit mass than batteries. As such, the weight of a battery large enough to power an aeroplane, even for short journeys, is difficult to achieve.

This situation is exacerbated for long-distance flights. ‘Unless there is some radical, yet-to-be invented paradigm shift in energy storage, we are going to rely on hydrocarbon fuels for the foreseeable future,’ United Technologies Chief Technology Officer, Paul Eremenko, told BBC News.

This is concerning given the amount of emissions produced by flying. For example, a one-way flight from London Heathrow, UK, to JFK, New York, USA, produces 986kg of CO₂. Air travel is also rising – 4.3 billion passengers flew in 2018, an increase of 38 million from the previous year. During the same time, the demand for diesel and jet fuel rose by 3%, putting more pressure on the aviation industry to find ways to reduce its greenhouse emissions.

There is hope in the form of cleaner hydrocarbons. A team from the Swiss Federal Institute of Technology (ETH) Zurich, led by Professor Aldo Steinfeld, has developed a process to create carbon neutral fuel using just sunlight and air.

Solar mini-refinery

For more than a decade, the Zurich team has been developing a solar mini-refinery that can produce liquid hydrocarbons such as methanol or kerosene. ‘These are “drop-in” fuels – synthetic and completely interchangeable substitutes for conventional petroleum-derived hydrocarbons – for example gasoline, jet fuel, diesel and methanol – compatible with the worldwide existing infrastructures for fuel distribution, storage and utilisation,’ Steinfeld said.

‘The amount of CO2 released during their combustion is equal to the amount removed from the air for their production.’ As such, the solar-made hydrocarbon fuels are carbon neutral, in contrast with the conventional fossil-derived hydrocarbons, as petrol for example that discharges 2.31kg of CO2 for every litre used, and diesel achieved 2.68kg/L.

The system uses three integrated thermochemical conversion units. The first is the direct air capture unit, which can extract CO2 and H2O directly from ambient air. It is based on the adsorption-desorption cyclic process applied to an amine-functionalised sorbent. This process uses amine - a group of substances formed from ammonia by replacing hydrogen atoms with a group of atoms containing carbon - to absorb the gas and water available in the air. Secondly, the mini-refinery uses a solar redox unit, which can convert the CO2 and H2O into a desired mixture of CO and H2, or syngas.

‘The solar redox unit is based on the thermochemical splitting of CO2 and H2O via a reduction-oxidation cyclic process using non-stoichiometric ceria,’ Steinfeld explained. ‘The redox cycle comprises two steps – in the first endothermic step, ceria is thermally reduced to generate O2 using concentrated solar energy as the source of high-temperature process heat. In the second exothermic step, reduced ceria is re-oxidised with CO2 and H2O to generate a desired mixture of CO and H2.’

The solar reactor consists of a cylindrical cavity-receiver, containing a reticulated porous ceria structure. This structure is directly exposed to the high-flux solar irradiation from a solar parabolic concentrator, which concentrates the radiation by a factor of 3,000, enabling it to reach temperatures of 1,500°C.

The ceria used is an oxide of cerium, a rare earth metal found in cerium-rich monazite and bastnasite. It is mined in the USA, China, Russia, Australia and India, and currently supply exceeds demand. Although classified as rare, it is as abundant as copper. As such, supply for materials should not be impacted if the project was scaled up, as ceria is not consumed by the process.

The thermochemical process in the solar redox unit uses the entire solar spectrum. Steinfeld said this is an important feature to enable high energy conversion efficiency, which leads to economic competitiveness for the technology.

A further advantage is that only the endothermic reduction, whereby the energy is absorbed, requires a solar input so it is possible to use two identical solar reactors to perform redox reactions simultaneously by alternating the concentrated solar input between them. This further increases the solar redox unit’s efficiency. Finally, a gas-to-liquid unit converts the created syngas into liquid hydrocarbons, such as jet fuel and others.

Limited fuel

Steinfeld’s team has proved it is possible to create synthetic hydrocarbons using solar energy and air under real field conditions, which gives hope for an easy and clean solution for a number of industries. But the solar mini-refinery of Zurich can currently only produce a small amount of fuel, and there are challenges to scale it up to a commercially viable level, one of which is the efficiency of the solar reactor in relation to the amount of fuel it can actually create.

At present, the demonstration project produces less than a decilitre of fuel a day, therefore production costs are high. Without increases in efficiency, the area required to make any meaningful quantity of fuel would be huge. While carbon-neutral fuels are hugely appealing, if thousands of square kilometres of solar heliostats are required to make it, it will never take off. The goal is to reach an overall efficiency exceeding 15%, so that a solar commercial plant of 1km2 area can produce 20,000L of kerosene per day.

Steinfeld’s team is already making a larger demonstration project and, jointly with European partners Imdea, Bauhaus, DLR, and Hygear, and has built a larger solar refinery in Madrid, Spain, as part of the EU SUN-to-Liquid project. The team is testing its solar reactor on top of a solar tower, and the project demonstrates the scalability of the solar technology from a 4kW setup to a 50kW pre-commercial plant.

The technology has achieved a degree of success, as there are already two spin-off companies. Synhelion was launched in 2016, building on Steinfeld’s research. The company recently partnered with Italian energy giant Eni, as it continues to grow. ‘The production of methanol using solar energy is a milestone towards Synhelion’s goal of replacing conventional fuels by CO2-neutral drop-in fuels,’ the company said. ‘Preliminary data show that the project being developed with Eni will cut more than 50% of emissions released in conventional methanol production.’

Climeworks, the second spinoff company from Steinfeld’s research, was founded in 2010 and commercialises the technology for the capture of CO2 directly from the air. ‘Climeworks takes this CO2 back out of the air, and produces a pure gas which can be combined with H2O to create fuel, as well as other carbon-based products such as plastics,’ the company said. ‘Instead of releasing more CO2 from fossil fuels, we recycle CO2 that has already been released into the air. We close the carbon cycle.’

The success of ETH Zurich’s demonstration projects going forwards, and the ability to scale-up the technology will be essential if the technology is to give the hydrocarbons industry cause for concern. The team is planning on launching a full-size commercial system by 2025, which will have a production volume of 10 million litres of methanol fuel a year.

Moving towards more environmentally friendly fuel sources is gathering pace, particularly encouraged by the steady reduction in the costs involved. However, upfront infrastructure costs can be the downfall of many environmental endeavours. ETH’s drop-in fuels could be the sustainable alternative industries including aviation need to reduce emmissions.