2018

The role of membranes in the use of natural salinity gradients for reverse electrodialysis

In this thesis reverse electrodialysis (RED) is central in which energy is harvested from a salinity gradient. For RED, two streams with a difference in salinity are used in combination with ion exchange membranes. These charge-selective membranes allow either the transport of cations (for cation exchange membranes, CEMs) or anions (for anion exchange membranes, AEMs).

‘Feed streams can be natural waters, for example a river and seawater,’ says Timon Rijnaarts. ‘However, divalent ions, such as Mg2+  and Ca2+ have several negative effects. We studied these using monovalent-selective CEMs to block divalent cations. Alternatively, we used multivalent-permeable CEMs to allow divalent cation transport.’

Other strategies to counteract ‘uphill transport’ – as this phenomenon is called – were studied as well: to use pretreatment and tunable monovalent-selective membranes.

‘We fabricated defect-free multilayer coatings, showing improved cation selectivity in large-scale electrodialysis stack experiments,’ says Timon. ‘Our collaboration with Fujifilm and Wetsus, operating the RED stack at the Afsluitdijk, proved very fruitful. We were able to test our jointly-developed, novel membranes in full swing, under practical operating conditions. The new selective membranes we fabricated, are now standard operable at the Afsluitdijk pilot project.’

Timon enjoyed collaborating with FujiFilm and Wetsus partners.

‘They closely followed my work, and they were very involved,’ he says. ‘Above that, they have a good feeling for the scientific approach we advocate, here at Mesa+. The pilot-phase the RED Stack pilot project is in, contributes to that. Working in the development stages of this technology, fundamental questions are welcomed, and suggested improvements can be implemented rapidly. It makes you feel excited, to participate at the front edge of research and development.’

Profiled membranes

Another aspect of this thesis work involved profiled membranes. Different types were compared with conventional membranes using spacers.

‘It was found that profiled membranes have a decreased stack resistance, due to absence of the spacer shadow effect,’ Timon says. ‘These profiled membranes do not block parts of the membrane surface, nor the channels for ion transport. In my opinion profiled membranes have a great future, especially when rapid prototyping using 3D printing techniques are used designing them.’

Timon enjoyed working on these profiled membranes. ‘We benefited from our collaboration with colleagues from the Universidade NOVA de Lisboa,’ he says. ‘Their computational, modelling expertise was the inspiration for us to fabricate and test various advanced, profiled membranes.’

In one of the last chapters, the effect of water fouling was discussed. Here, Timon tested six different AEMs, together with Wetsus colleague Jordi Moreno, using the natural waters at the Afsluitdijk. ‘We identified the losses of power density to be caused mainly by spacer fouling,’ Timon says. ‘This adds to the argument to design profiled membranes in the future.’

EMI

After his PhD project, Timon started working at the European Membrane Institute (EMI). ‘Here we perform contract research projects,’ Timon says. ‘I like working in an academic setting on practical issues coming from industry partners. Also, we look to commercialize some ideas resulting from my PhD work, for other applications.’

‘This mix of research and consulting activities appeals to me very much so. In my contact with industry partners, I learnt that their commercial interests always prevail. At Mesa+ we apply science with a good fundamental basis. Because of that we can think along with customers on practical issues better, while understanding this fundamental background. This approach in Mesa+ is valued greatly now in my work at the EMI.’