6 May 2016

Nuon’s power plant in Eemshaven is being repurposed: in the future, it will run on ammonia. This no-carbon fuel is produced when there’s an excess of solar and wind energy.

Now that the transition to renewable energy is picking up speed, we need to find ways to accommodate the peaks and valleys in our future power supply. Several time scales are at play here. A solution that skims off short-term excesses is not necessarily the best way to handle a multiple-day surplus. And storing excess solar energy in summer for use in winter is another challenge entirely.

The power company Nuon is working with a handful of chemical companies, Delft University of Technology and the University of Twente to solve that last puzzle. The consortium wants to investigate whether Nuon’s gas-fired power plant in the northern city of Eemshaven can be converted into a ‘seasonal battery’, storing excess energy in ammonia that can be burned to generate electricity at times of shortage.

Nuon energiecentrale
Photo: Nuon

‘At this time scale and size, the obvious solution is to store energy in molecules,’ explains Tjeerd Jongsma, the director of the Institute for Sustainable Process Technology (ISPT). The Nuon project is part of the Power to Ammonia project being run by ISPT, which is investigating ways to use ammonia as an energy carrier and fuel.

Ammonia is an excellent choice for an energy carrier, says Jongsma: ‘It’s a liquid at -35 degrees Celsius or at less than ten bars at room temperature. It has a high energy density, contains no carbon and thus releases no CO2 when it’s burned, and under the right conditions no NOx either. It’s poisonous, but because we already have a century’s worth of industrial experience with it, we know how to safely process, transport and store it.’

What’s more, ammonia is an economically smart choice for a seasonal battery. ‘Ammonia is a high-value substance with a large market, because it’s an important raw material used in making chemical fertilizer and other products. Roughly 7 per cent of the Netherlands’ natural gas is currently used to make ammonia. It shores up the business case when you can also sell the ammonia you produce.’

Dumped

The consortium can use the proven Haber-Bosch process to create ammonia. In this process, nitrogen (from the air) reacts with hydrogen using a metal catalyst under high pressure and temperature. For this purpose, the hydrogen is extracted not from natural gas but through the electrolysis of water. The product is stored in giant tanks as a liquid. To give you an idea: a tank holding sixty thousand tons of ammonia represents 375 gigawatt-hours of energy. For comparison: in a year Amsterdam uses roughly double that in electricity.

The burning of ammonia has been less thoroughly studied than its synthesis, because interest in the compound as a fuel is a recent phenomenon. That’s not surprising; ammonia doesn’t burn as well as most fuels. Nonetheless, Japanese researchers recently succeeded in running a gas turbine on 100 per cent ammonia. Indeed, Jongsma expects that the conversion of Nuon’s power plant will not be the biggest problem.

‘The greatest obstacle is the electrolysis of water. The technology is known, but too expensive. That’s why we’re working with TU Delft, where they have a number of new ideas for making better electrolysis devices.’ What exactly those ideas are Jongsma cannot say, because the patents have not yet been granted.

Even if the production of hydrogen is perfected, the ammonia cycle will still contain significant losses: because each step in the process is less than 100 per cent efficient, in winter you recover only a fraction of the energy contained in last summer’s sunlight. But that’s not a problem, says Jongsma, because we’re talking about excess energy that otherwise would have been ‘dumped’ in the first place. Which is not to say that the project’s partners won’t try to maximize the efficiency of each individual step in the process, from the compound’s synthesis through the optimal use of the residual heat from its combustion.

The goal is to demonstrate the technology at a significant scale in five years. It may be ten years, however, before the ammonia-fired power plant in Eemshaven comes online.