Ammonia Tagged as Storage Medium for Wind Energy
By on March 22, 2018
Henrik Stiesdal is a distinguished figure in the field of wind energy. As such, he has had ample occasion to contemplate the field’s challenges and opportunities. Recently he concluded that ammonia may become an important part of wind energy’s future.
In May 2017 Stiesdal spoke about the hurdles for offshore wind:
The single biggest challenge to the industry is actually not directly related to the technology or business itself, but to the overall character of renewable energy production. In countries and regions with high penetration of wind power the intermittency of the energy production is more and more seen as an obstacle to further development. Consequently, we need to develop storage and interconnectors.
Technical University of Denmark, 3 Questions for Professor Henrik Stiesdal, 05/31/17
From 2004 to 2014 Stiesdal was Chief Technology Officer of Siemens Wind Power. In pursuit of a workable storage method for wind energy, Siemens Wind Power has been working on a system to convert turbine motion into heat and to store the heat in such a way that it can subsequently be converted into electricity.
However, Stiesdal, now an Honorary Professor at Technical University of Denmark, has been doing more thinking. The gist was described earlier this week in a post on the renewable energy Web site Recharge. Stiesdal’s thought process started with the idea of producing hydrogen from wind-generated electricity. It didn’t stop there, though. “Hydrogen is, in many ways, a wonderful fuel,” he said, “but it has some huge drawbacks. You can’t compress it to liquid, so it’s terrible to ship around, it’s aggressive towards steel, limiting the potential use of existing gas pipeline systems, and it’s explosive in a very wide range of mix ratios.”
Ammonia, on the other hand, has a number of important advantages: “The beauty of ammonia is that storing it is no problem. The world is full of ammonia storage tanks. It’s a liquid, doesn’t explode and only burns at high pressure, and it actually works very well in combustion engines. For trucks or for ocean-going shipping . . . ammonia would be a super material.” The one concern, he said, is safety. Ammonia is “toxic, but fortunately you can easily smell it at concentrations that are way below any harmful level.”
In considering the economics of ammonia-based energy storage, Stiesdal started with the observation that “ammonia storage will have lower round-trip efficiency than thermal storage.” The reason for this derives from the essential concept behind wind-to-thermal technology. According to a 2015 paper in the journal Renewable Energy, “there are several methods that convert rotating energy to thermal energy.” The paper says that the best is electromagnetic induction. In this approach, the wind turbine’s rotor causes a conductor to rotate within a static magnetic field. This configuration of a rotating element within a magnetic field is similar to that of a generator, but in the case of an induction system, the electricity that is produced is not given a path to off-site loads. Instead, it takes the form of eddy currents which cause heating of the conductor. A heat transfer fluid conveys this energy to a thermal storage medium, from which it can be extracted in due course by a steam loop that drives a conventional steam turbine.
Siemens Wind Power announced in November 2017 that it had completed three years of research and development on the wind-to-thermal technology and would start construction of a 1.5 MW test plant on the outskirts of Hamburg, Germany. This plant will include a “high-temperature storage unit,” consisting of a 1,000-tonne “basalt stone bed” that can store up to 30 MWh of usable thermal energy at temperatures in excess of 600 degrees C. According to an article in Wind Power Monthly, the storage module will be “capable of producing energy for up to 24 hours.”
The lack of thermodynamically costly energy conversions in the Siemens system accounts for its superior round-trip energy efficiency. However, Stiesdal said, the ammonia “storage system as such is very low cost.” He continued, “The CapEx you need to build storage tanks is actually completely minimal. It disappears in the noise of the cost calculation. And the other thing is that the infrastructure exists to a much larger degree than people imagine. There are pipelines and ships and trucks that are designed for ammonia.”
Hence while thermal energy storage may prove to be the method of choice for storing wind energy on a timescale of hours to days, Stiesdal said that the ammonia “concept is particularly well suited for seasonal storage.”
The key to an ammonia-based system for storing wind energy, in Stiesdal’s view, is electrolyzer economics. “What drives the cost [of green ammonia] is the cost of the electrolyser,” he said. “What I’m pursuing is how does one make a truly low-cost electrolyser that has a decent efficiency. I’m not after the most efficient in the world, because that’s way too expensive, but after the happy compromise between efficiency and CapEx — a cheap electrolyser with a decent efficiency.”
In the meantime, work on electrolyzer affordability continues, not least at Nel Hydrogen in Norway. Last fall, Nel’s Vice President of Market Development and Public Relations Bjørn Simonsen clarified the company’s electrolyzer pricing for Ammonia Energy after a post on an analysis by the International Energy Agency’s Cédric Philibert. The post stated, “Under currently prevailing design parameters, the capital cost of electrolyzers is about $850 per kW of power capacity. Philibert estimates that that cost will fall to $450 per kW for Nel’s 400 MW plant.” In his clarification, Simonsen said that “the 450 $/kW number is our current, not future price of large scale hydrogen production facilities (400 MW).” He also said that Nel is “supplying smaller facilities (100 MW per facility)” at “a price of ~550 $/kW.”
Stiesdal, a native of Denmark, worked with a colleague in the 1980s to develop various aspects of wind turbine technology which were subsequently licensed to the Danish manufacturer Vestas. After a stint as a Vestas employee, he was hired by another Danish company, Bonus Energy, in 1987. Bonus Energy was acquired by Siemens in 2004 and renamed Siemens Wind Power. In 2017 Siemens Wind Power merged with the Spanish wind technology company Gamesa. Stiesdal holds more than 650 patents related to wind power technology.