One of the main challenges
of developing a hydrogen economy is hydrogen storage. Hydrogen is a reactive gas
with small molecules. It is prone to both leak and react with other gases, liquids,
and solids. Storing it for immediate use requires compressing it, which
requires energy. Its volumetric energy density is much less than natural gas.
Thus, it takes more space to store an equivalent amount of energy.
Ammonia has advantages
over hydrogen for storage. For one, it is a liquid, so it can be contained and
transported much more easily. Ammonia is NH3 and can be converted to hydrogen
when needed. Ammonia “…liquefies at just -40°F and is easier to transport.
Yet, converting ammonia back into usable hydrogen requires temperatures above
572°F, adding a layer of complexity and energy demand.” There are, however,
some challenges in storing and transporting ammonia related to its toxicity
that must be considered.
Below is a table comparing
hydrogen and ammonia as storage media from a 2021 article about recent progress
in ammonia fuel cells published in the Journal of Materials Chemistry A. Below that is a model of the process of ‘green ammonia’ as an energy source.
Ammonia Safety Issues
There are,
however, some challenges in storing and transporting ammonia related to its
toxicity that must be considered. The paper noted above addresses some of
ammonia’s safety issues:
“Ammonia can be dissolved in water to give a solution
with a solubility limit of approximately 35 wt% that does not require any
specialised storage equipment and can simply be stored in a glass bottle.
Ammonia can be compressed into the liquid state at a pressure of 8 bar and
temperature of 20 °C, which is much easier than that of hydrogen. It is deemed
corrosive by nature, with the ability to cause dehydration, severe skin burns,
frostbite and eye damage. On inhalation, ammonia can cause lung damage or
respiratory failure at vapour concentrations of 1700 ppm and can also lead to
fatality if inhaled at excessively high concentrations of 5000 ppm. The use of
ammonia may therefore be monitored under safety regulations and the toxicity
issue can be addressed with appropriate practice. Ammonia can be stored as a
solid in metal–amine complexes such as copper, zinc and their alloys to
alleviate the corrosive properties. It can also be stored in compounds such as
urea, which is a non-toxic solid, to overcome the issue of toxicity associated
with liquid and gaseous ammonia. Storing in the solid form also allows for ease
of transportation and avoids leakage. These solids can then emit ammonia upon
heating or exposure to a vacuum. Further to this, unlike that of hydrogen, the
smell of ammonia is easily detected at concentrations as low as 1 ppm due to
its sharp and irritating odour.”
The New Efficiency Breakthrough that Combines Ammonia
Cracking with the Fuel Cell Reaction
Researchers at Germany’s
Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) have achieved a
breakthrough in converting ammonia to power more efficiently. This involves combining
a high-temperature fuel cell with an ammonia cracker.
“In this setup, the cracker first splits ammonia into
nitrogen and hydrogen at temperatures over 572°F. The hydrogen then flows into
the adjacent fuel cell, where it undergoes a reaction to produce electricity,
while nitrogen is safely released back into the atmosphere.”
The excess heat
from the reaction of burning hydrogen is recovered and used to maintain the
high temperature required to “crack” the ammonia into H2 and N2. The captured
heat can also be used to heat buildings.
“This integrated heat loop raises the system’s overall
efficiency to 60%, putting it on par with conventional natural gas-based power
generation methods.”
“The compact design of this ammonia-powered fuel cell
could be particularly beneficial for industrial facilities, municipalities, and
large vessels like ships. These settings often lack space for bulky hydrogen
storage but require robust energy solutions. With its high energy density and
easier transport, ammonia may serve as an ideal energy source for both
stationary and mobile applications. “Ammonia provides a stable, high-density
form of hydrogen, making it a great candidate for clean power and heat generation,”
Nousch noted, highlighting its potential for various climate-friendly
applications.”
The key to the
breakthrough is attaining 60% efficiency of the entire process, which makes
it more competitive with natural gas. Ammonia is promising as an energy carrier
and fuel cell material. It is thought to be especially promising for marine
propulsion since it requires less space than hydrogen of equivalent energy.
Perhaps I will write a future post on the different types of ammonia fuel
cells and some of their niche applications.
References:
Revolutionizing
Clean Energy: New Fuel Cell System Powers Electricity Directly from Ammonia. Sadie
Watkins. Ever-Growing USA. December 22, 2024. Revolutionizing
Clean Energy: New Fuel Cell System Powers Electricity Directly from Ammonia
Recent
progress in ammonia fuel cells and their potential applications Check for updates. Georgina Jeerh, Mengfei
Zhang, and Shanwen Tao. Journal of
Materials Chemistry A. Issue 2, 2021. Recent
progress in ammonia fuel cells and their potential applications - Journal of
Materials Chemistry A (RSC Publishing)
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