Is Hydrogen Sustainable?
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.
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.
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.
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.
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.
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.
Hydrogen pipe dreams
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.
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.
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.
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.
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.
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.
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.
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.
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