Farmed microalgae have the potential to capture carbon dioxide at scale and to make valuable chemicals with a low carbon footprint – but how can we maximise the performance of the photobioreactors used to grow them? As part of a collaboration with Swiss researchers, TTP has developed a mathematical model to speed up the process of finding the best design for a given application and scale.
The promise – and challenge – of scaling up microalgal transformations
To understand the sensitivity of photobioreactor performance metrics to particular parameters, we needed a sophisticated design tool, and that’s exactly what the talented team at TTP created for us. We can now quickly explore the complex interplay of parameters involved, and confidently move towards optimum designs on larger scales.
Professor Dr Mirko Kleingries
HSLU, Switzerland
Microalgae may be tiny, but they could be future industrial powerhouses, thanks to highly efficient photosynthetic machinery, rapid growth rates, and ability to thrive in saltwater.
The spotlight is currently on two application areas – carbon dioxide removal (CDR) to mitigate climate change, and the manufacture of valuable chemicals, including food supplements, dyes, cosmetics, pharmaceuticals, fertilisers and biological signalling chemicals, but also bioplastics, platform chemicals and biofuels.
But the performance of algae in photobioreactors is critically dependent upon the details of the reactor design and the operating conditions. And although algae are the best photosynthesisers in the natural world, their photosynthetic efficiency still only reaches low double-digits.
That means reactors need to operate close to the theoretical maximum to make them economically viable, which leads many in the field to take a “best guess” approach to selecting reactor parameters. This makes it nearly impossible to be sure that a peak in performance has been found, rather than just a local maximum.
Modelling the workings of an algal photobioreactor
But things worked out quite differently for Mirko Kleingries, a researcher at HSLU in Lucerne, Switzerland, who leads a team that investigates the use of microalgae for CO2 sequestration.
Kleingries first made contact with TTP at Climeworks’ Direct Air Capture Summit in Zurich in June 2022. As a result of that conversation, our modelling experts at TTP developed a mathematical model of the physics and biochemistry inside a photobioreactor that enabled Kleingries’ team to push forward rapidly in the development of scaled-up algal photobioreactors.
TTP’s model accounts for all the basic parameters needed to correctly mirror the function of any photobioreactor – such as cell concentration, algal metabolism, CO2 concentration, nutrient levels, light flux, temperature, pH, and more. Also included are equations on mass and heat transfer, and feedback loops such as the effect of algal growth on light penetration.
To validate this model, we plugged this “core module” into the equations defining some well-known bioreactor designs, to give what was in essence a preliminary “digital twin” for each setup. Encouragingly, this gave results in line with expectations – for example, in terms of cost per unit of biomass, or productivity per unit area of the reactor footprint.
Photobioreactors for chemical synthesis
Kleingries and his team then applied the core module to a reactor format typically used in the production of fine chemicals. With the fundamental parameters already targeted by the core module, the process of maximising yields was made much easier, said Kleingries.
“To understand the sensitivity of photobioreactor performance metrics to particular parameters, we needed a sophisticated design tool, and that’s exactly what the talented team at TTP created for us”, he explained.
To understand the sensitivity of photobioreactor performance metrics to particular parameters, we needed a sophisticated design tool, and that’s exactly what the talented team at TTP created for us.
Mirko Kleingries
Researcher at HSLU in Lucerne, Switzerland
Photobioreactors for carbon capture
Kleingries’ team is now focusing on a photobioreactor design for outdoor direct air capture (DAC), with plans to bury the resulting dried biomass securely below ground. To maximise the photosynthetic efficiency, this photobioreactor has a thin-layer structure filled with a high concentration of a microalga known to have high carbon content and rapid generational turnover.
TTP’s core module has adapted very well to this different reactor setup, and we’ve learnt a lot about the optimum conditions already.
Mirko KleingriesResearcher at HSLU in Lucerne, Switzerland
As a result, his team has been able to shift their focus from modelling to technical implementation. This has boosted progress towards their next goal, which is to scale up and commercialise the system in a location with more sunshine and consequently higher algal growth rates.
And whether the focus is on using microalgae for carbon capture or for chemical synthesis, Kleingries’ view is that modelling has a vital role to play:
One thing is certain – fast-growing algae, grown on a large scale, will be increasingly important for mitigating climate change and in sustainably producing the chemicals that society needs. With these modelling tools now at our disposal, we can quickly explore the complex interplay of parameters involved, and confidently move towards optimum designs on larger scales.
Mirko Kleingries
Researcher at HSLU in Lucerne, Switzerland
