Is Hydrogen Sustainable?

By Oliver Sylvester-Bradley

A critical review of the sustainability of a hydrogen economy

Introduction
Hydrogen is often cited as the solution to global energy problems and the
perfect energy carrier for a sustainable society due to its' non polluting
nature and suitability to store and transport energy generated from
renewable sources. This essay examines the practicalities of a
hydrogen-based energy system and its' ability to provide a sustainable
future.
Figure 1
Many people including Romano Prodi, president of the European Commission,
President Bush and numerous "deep greens" including Amory Lovins, author of
Natural Capitalism advocate the "hydrogen economy" as the future of world
energy supply. The justification seems well founded; a renewable energy
plant generates electricity sporadically, hydrogen can be produced by the
electrolysis of water, storing energy in the form of hydrogen negates the
need for large and inefficient batteries (and associated pollution) and fuel
cells (fig. 2) can turn hydrogen back into useful electrical energy (using
oxygen from the atmosphere) as and when required, the only by-product being
steam or water.
Figure 2
The associated benefits include the elimination of exhaust pollution
(sulphur dioxide, carbon monoxide, carbon dioxide and other particulates
associated with the combustion of fossil fuels) particularly in cities, the
associated reduction in global warming and the ability to generate power
locally promoting the development of a decentralised energy system. It is
even suggested that ultimately a hydrogen economy would help avoid global
economic downturn due to unmanageable increase in peak demand and the
limited supply and increasing costs of fossil fuels.
Don't believe the hype
It's easy to be convinced by stories we want to believe. Ergo magazines
claims hydrogen 'is the one'. Jeremy Rifkin, head of the Foundation on
Economic Trends claims the hydrogen economy will be 'the next great economic
revolution' James Burges thinks it 'may be our only option for survival';
hydrogen is the answer. However, critical analysis of the properties of
hydrogen and the necessary steps in a hydrogen society reveal a far less
optimistic picture.
Net Energy
It takes energy to make energy. We use energy to find and pump oil but,
luckily for industrial society, oil has huge net energy and we can usually
obtain more than 200 times the amount of energy from oil than is required
for its' extraction. Oil has the highest net energy return of any fuel; gas,
coal, wind and solar all have dramatically less. Hydrogen has negative net
energy meaning it takes more energy to produce it than it contains, this
highlights the first problem of a hydrogen society.
Counting Calories
Although it is the most abundant element in the universe hydrogen is not
very easy to obtain. Ironically, petrol is the most concentrated form of
hydrogen available for human consumption containing more hydrogen by volume
than pure hydrogen itself, since the structure of the atoms in hydrocarbons
use less space. Hydrogen also has a very low calorific value (a gallon of
petrol has 115,000 btus, a gallon of liquid hydrogen has only 30,000 btus)
so it takes about 4 times the volume of hydrogen (compared with petrol) to
travel the same distance, requiring larger and heavier fuel tanks
(compressing or liquifying hydrogen uses more energy) which require
additional energy to transport. This low calorific value dictates increased
volumes (or high pressures) throughout the entire hydrogen system.
Hydrogen manufacture
There are several ways to obtain hydrogen; by electrolysis of water, by
splitting water using light, by collecting and reforming gas from biomass,
by reforming natural gas or any other fossil fuel. Each of these processes
is extremely energy intensive and always results in hydrogen with a negative
net energy, electrolysis (the cleanest and most appropriate process for
obtaining hydrogen from wind and solar power) is the most energy intensive
of them all - roughly 75% efficient and costs roughly four times as much as
reformation. The simplest, cheapest and most efficient process is the
reformation of natural gas, an established industry, yet this is still only
85% efficient. It is therefore less polluting and resource intensive to
simply burn natural gas.

The figures may not seem too alarming and advocates of the hydrogen economy
will correctly point out that if we can obtain abundant renewable energy
conversion efficiencies are negligible however, this is only the first step
of the process. In a sustainable hydrogen economy we must also store and
transport hydrogen for other uses and this process is also inefficient.
Relative energy consumption for road based transportation is uneconomical at
almost any distance (Fig 3), 40% of the transported energy being consumed
every 200Km.
Piping hydrogen is also problematic due to the energy required for pumping
and the low volumetric energy density of hydrogen, demanding higher flow
rates which in turn lead to greater flow resistance. Consequently about 4.6
times more energy is required to move hydrogen through a pipeline than for
natural gas (Fig 4) and 10% of the energy is lost every 1000Km, the
additional problem that hydrogen is not compatible with the current piping
infrastructure due to brittleness of material, seals and the incompatibility
of pump lubrication poses further problems.
Figure 3
Figure 4
The major hurdle in realising hydrogen fuelled systems has been the creation
of successful storage devices. Hydrogen can be stored as a gas, a liquid or
a solid (below 4.2_K), as a hydride (a hydrogen compound) or by absorbing
the gas in a microporous medium. Gas and liquid storage tanks are already in
use (Fig 5) and low weight carbon tanks lined with polymer 'bladders'
promise up to 12wt% hydrogen storage. Microporous carbon nanotubes are under
development which can currently store 7wt% of hydrogen. The advantages of
such 'solid state' systems being the increase in storage capacity for a
given volume, increased safety and lower energy costs of storage.
Figure 5
Current practice however, is far from ideal. Hydrogen pressure tanks filled
to 200bar can only ever be emptied to 42bar to accommodate the pressure in
the receiving system. This means that any tanked storage device only ever
returns 80% of its' potential, the remaining 20% returns in the tank or
leaks through the walls of the tank at up to 2% per day.
Ways around this problem, like using extra compressors, to remove the final
20%, complicate the transfer process and, of course, require additional
energy.
Cumulative efficiencies
As we have seen there are several conversion efficiencies involved in a
hydrogen society all of which decrease delivered energy. The cumulative
effect of these illustrates the impracticality of the system. For example,
using photo voltaic collectors (Fig 6) to generate the energy (with an
ambitious efficiency of 20%) we see that converting solar energy to
hydrogen, storing it and then using it to power a fuel cell vehicle results
in only 6% of the primary (solar) energy arriving at the wheels of the car.
Sending hydrogen down a 1000km pipe or a 200km road (an essential part of a
hydrogen economy) results in just 5% and 3% (respectively) of the primary
energy reaching the wheels. Without radical improvement in renewable energy
generation efficiency it seems highly unlikely these small percentages could
power vehicular travel at all.
Figure 6
Flights of fancy
The figures above illustrate the problems of efficiency regardless of cost.
The actual price of delivered energy would also be much higher than
conventional energy due to the higher cost of renewable energy, hampering
the introduction of a hydrogen economy further still. And this is just one
scenario based around fuel for cars. What about boats and planes, can these
energy hungry, seemingly essential (to industrial society) modes of
transport ever really be fuelled in this way?

For an aeroplane to fly on hydrogen it's fuel tank would need to be so large
it would be inefficient to make a plane any smaller than three times the
size of a current Boeing 747.
Is this a realistic proposal for a sustainable future?
Is generating the 10,000 Mtoe of energy currently consumed by the world from
entirely renewable resources actually possible, without turning the entire
surface of the planet into solar panels and windmills?
Possibly, but considering the example above it seems we need to generate 33
times (100%/3%) more energy than we actually need to account for conversion
inefficiencies. This is not a plausible proposal.
Total sustainability
But that's not the end of the story. In order to build this implausible
economy we need energy. Energy to make solar panels, energy to make
electrolysers, energy to make storage tanks and fuel cells and cars.Take the
photovoltaic (PV) panels as an example. Energy payback for current PV is
estimated at anywhere between 3 and 12 years,,,. If it was 5 years,
considering PV panels only last for 30 years this means we would need to
re-invest a sixth of the energy produced by the panels into making new
panels for the system to be sustainable. Which means the optimistic 20% of
the primary energy leaving the panels should actually only be 16.6%, apply
this logic to every stage of the process (especially to products with large
embodied energy such as fuel cells) and the proposition of a sustainable
hydrogen economy is no longer simply implausible but ridiculous.
Conclusion

The only way a hydrogen economy could possibly function sustainably would be
if renewable energy plant produced enough energy to cover ALL the conversion
inefficiencies of the hydrogen system, plus ALL the energy required to
manufacture and dispose of (or recycle) the system, plus the delivered
energy we needed in the first place.
Hydrogens main use is undoubtedly as a sophisticated battery; to store
energy but, considering hydrogens low volumetric energy density it seems
there may be other energy carriers better suited to this task. Other fuels
(eg. ethanol, methanol or alcohol) based around shorter carbon cycles (than
oil) involving photosynthesis seem equally, if not better, suited to
sustainable energy systems with decreased conversion efficiencies.
US commitment to develop a hydrogen based economy (and the worlds first zero
emission power station which burns coal and sequesters CO2 at sea) has been
compared with 'putting the cart before the horse' because until plentiful
renewable energy can be obtained using hydrogen as an energy carrier is
simply a waste of energy.
Increasing efficiencies of individual components in the hydrogen system will
undoubtedly improve its potential however, it will always take more energy
to produce the hydrogen than it contains.


The possibility of a completely sustainable hydrogen economy therefore seems
so distant that the billion dollar investment in hydrogen and fuel cell
research from the automotive industry must have more to do with vested
interests than any concern for a sustainable future.

Further Research
Extensive research into every aspect of the hydrogen economy is well
underway. The EU funded Clean Urban Transport in Europe (CUTE) project plans
to provide three hydrogen powered buses to each of ten european cities,
including London, by late 2003. The USHER project aims to demonstrate the
feasibility of PV produced hydrogen with the introduction of the largest
single PV array in the UK on the Cambridge science park in England, which
will generate hydrogen to power a fuel cell bus. The 3500m2, 300kWp array is
expected to power a 144kw electrolyser which will produce 30m3 of hydrogen
per hour. A sister project in Gotland aims to make the island completely
self sufficient, by introducing a renewable energy powered hydrogen economy
within one generation. Results from these schemes will be key to
understanding the sustainability of a hydrogen economy.

However, given the obvious demand for copious renewable energy, I believe
that any funding or time invested in the hydrogen economy would be far
better spent on increasing efficiencies of renewable energy plant,
particularly PV without which, there will never be a sustainable hydrogen
economy.