Headlines declaring hydrogen
as the clean fuel for the future are becoming all too frequent. Governments and
private companies in Australia, India, China, Germany, Saudi Arabia, and many
other countries have announced large projects for producing, storing, and
transporting hydrogen. Globally, over 300 projects are being undertaken with
investments amounting to $500 billion. India has unveiled plans for producing 5
million tonnes of hydrogen in a bid to become a global export hub.
The promise of hydrogen is
that it produces only water when it burns. Thus, using it instead of coal, oil,
or natural gas would eliminate greenhouse gas (GHG) emissions. Hydrogen
certainly has a role in the net-zero emissions scenarios of tomorrow, for
example those proposed by the Intergovernmental Panel for Climate Change (IPCC),
the International Energy Agency (IEA) and many other organizations. Most
scenarios for decarbonization rely on electrifying as much as possible. While
the bulk of decarbonization will come from the electrification of home
appliances, vehicles, and industries there are many sectors such as metal refining,
long-distance trucking, and shipping that are hard to decarbonize with
electricity alone and here hydrogen could play a crucial role. But calling it
the fuel of the future is a stretch too far. Hydrogen currently represents less
than 1% of total global energy and even in the net-zero emissions scenarios it
barely increases to 5% by 2050; nowhere near enough to justify the appellation.
Some exaggeration by
promoters of any technology is understandable, but when these pronouncements
begin to change government policy, it behooves everyone to take a closer look
at the contributions hydrogen can make in the future. If the primary reason for
using hydrogen is to reduce greenhouse gas emissions, we must consider the
extent to which hydrogen will need to replace fossil fuels and ensure that the
hydrogen is produced in ways that do not emit greenhouse gases. This essay will
review the ways hydrogen is produced as well as point out the areas where use
of hydrogen will be critically important in reducing GHG emissions.
While hydrogen is the most
abundant element in the universe, it is not present as such on Earth. On Earth
hydrogen is mostly present in combination with oxygen as water. It is also
present in combination with varying amounts of carbon in fossil fuels such as
natural gas, oil, and coal, as well as in biomass. Hydrogen can be produced
from any of these sources, but the processing will entail energy consumption
and/or emission of carbon dioxide. Hydrogen is not a source of energy;
it is an energy carrier. In that respect, it resembles electricity–we
must expend energy from another source to produce it.
Hydrogen Production. Hydrogen
is a widely used industrial gas. About 90 million metric tons are produced each
year, equivalent to about 6% of global oil consumption. Most of the hydrogen is
used in petroleum refining, and for producing ammonia and methanol. Currently,
half of the hydrogen is produced by the reaction of natural gas (methane) with
steam in a process known as steam reforming. Analogous reactions with
petroleum, coal, or biomass provide most of the remainder. Steam reforming is
the cheapest source of hydrogen and is used in petroleum refining operations
and for producing ammonia. However, each tonne of hydrogen produced by this
process entails producing about 6 tonnes of carbon dioxide, and hence hydrogen
produced using current technologies would not be helpful for a transition to a
clean future unless the carbon dioxide is captured and sequestered. Technology
for carbon capture and sequestration (CCS) is still very expensive and not
practiced at anywhere near the required scale.
Hydrogen can also be
produced by the electrolysis of water as well–passing electricity through water.
The electrolysis process does not entail emission of carbon dioxide, but there could
be emissions in producing the electricity. Commercial electrolyzers have an
efficiency of 75% and require over 50 MWh of electricity to produce a tonne of
hydrogen. Producing one tonne of hydrogen by electrolysis would result in
emitting 20 tonnes of carbon dioxide if the electricity was generated by a
natural gas power plant and over 50 tonnes if coal was burned to generate the
electricity. Either way, it is a situation far worse than with steam reforming!
Colors of Hydrogen. Hydrogen
itself is a colorless gas. However, depending on the process used to produce it
different colors have been assigned to it to reflect the varying amounts carbon
emissions. Indeed, there is full rainbow of hydrogen designations (Figure). Hydrogen
produced by fossil fuels has the highest emissions and is labeled black
or grey. If carbon-capture is employed in conjunction with such
production, the resulting hydrogen is labeled blue. If we use a clean
source of electricity to produce hydrogen, it could be a desirable fuel. Indeed,
promoters of hydrogen are talking about using wind and solar power to produce
what is called green hydrogen. Nuclear power could also be used to
produce emissions-free hydrogen; it is referred to as pink hydrogen. Other
sources of clean electricity include hydro and geothermal power.
Figure: Colors of hydrogen depending on production
technologies.
Source: Global
Energy Infrastructure
To produce the 500 million
tonnes of hydrogen projected in the net-zero scenarios would require 2,600 TWh
of clean electricity, an amount that could be generated from 1,200 GW of wind
or solar farms. To put in perspective, current global installed capacity of wind
and solar power is only 1,400 GW. The recently announced Adani–Total venture
seeks to dedicate 2.3 GW of solar to produce green hydrogen, capable of
producing only 10,000 tonnes of hydrogen a year—a tiny fraction of what is
needed.
Hydrogen Consumption.
One large application of hydrogen is in metals refining. Use of hydrogen instead
of coal/coke for reducing iron ore and producing steel has been developed but
it is currently being practiced at only a very small scale because the process
is more expensive. Expanding hydrogen’s role in metallurgical operations could reduce
up to 20% of greenhouse gases, but that would require producing over 200
million tonnes of emissions-free hydrogen.
Transportation contributes
to about one third of greenhouse gas emissions and use of hydrogen in this
sector would be very impactful. Hydrogen packs far more energy per unit of weight
or volume than batteries, but hydrogen has to be contained in a vessel. Because
storage vessels must withstand high pressures, they must be constructed from
heavy steel or other materials bolstered by reinforced fiber, resulting in increased
weight for the overall system. For cars and light duty vehicles, battery EVs
outperform hydrogen FC-EV.
There is another reason why
fuel-cell EVs have not gained traction whereas battery EVs are rapidly
penetrating this sector; it has to do with efficiency. Batteries return around
95% of the electrical energy saved in them. In the case of a fuel-cell vehicle
we lose 30% of the energy in first producing hydrogen from electricity, and
then another 35% in the regenerating electricity using the fuel cell, for a
combined efficiency of 45%. Increasing the efficiencies of electrolyzers and fuel
cells could allow FC-EVs to gain market share in this sector. Until then they
will remain a minor player.
The chief drawback of
battery EVs is their relatively lower capacity and slow recharging. For
long-distance trucking and other heavy-duty applications where large amounts of
on-board energy needs to be stored, hydrogen fuel cells technology becomes
attractive. Storing compressed hydrogen becomes more practical in large
vehicles and ships.
Hydrogen, like batteries, is
a way of storing electricity. If electric power is generated at times when
there is low demand, it makes sense to store it—put it in a bank if you like.
However, there are substantial energy losses both during conversion of
electricity to hydrogen (30%) and regeneration of electricity from hydrogen
(40%). The situation is akin to a bank that charges you a 30% fee to deposit
money and again charges you a 40% fee during withdrawal! You must be desperate
to save money in such a bank. For this reason, schemes to produce hydrogen at wind
and solar facilities to ameliorate the problem of intermittency makes limited
sense. It would be far better to use the excess electricity directly for water
treatment, desalination, or whatever else the local region may need.
