Introduction
    Chemical hydrogen
    Electrolytic hydrogen
Hydrogen distribution
Cost of producing hydrogen
Safety
Environmental effects
Conclusion

Introduction

Much has been published in the popular press, in recent years, about hydrogen being the fuel of the future. Nothing is less certain.

In reality, hydrogen can never be considered as a fuel, in the same way as oil or natural gas. It is a means of storing energy because it has to be manufactured using an energy-intensive process. It is physically impossible to obtain more energy out of the manufactured hydrogen than was put in to manufacture it. In reality, the holistic EROEI (Energy Returned Over Energy Invested) is typically less than 0.8. It is therefore evident that hydrogen should not be used when the original energy invested could be used to better advantage. For example, there are two principal ways of driving a car from an electricity supply: either one can generate hydrogen and use a fuel cell to regenerate electric power to drive a motor or one can use a rechargeable battery to drive an identical motor. The former will have an electrical efficiency (kWh in to kWh out) of, typically 45%: the latter will have an efficiency of better than 80%. So, a priori, there is a better case for using rechargeable batteries. This simplistic statement does, however, have many provisos, such as autonomy, weight, lifetime and volume issues which could modify the equation.

The notion of using hydrogen is based on the premiss that, when burnt or used in a fuel cell, the exhaust gases consist only of water vapour. Even this is true only for fuel cells. If used as a combustible gas, then polluting nitrogen oxides are also produced in small quantities. However, one has to have a more holistic approach.

Proposals have been made to use hydrogen for:

This idea presupposes an important new infrastructure for hydrogen production and distribution. Can such an infrastructure be implemented in Cyprus?

Hydrogen production

There are two ways of producing hydrogen. 

Chemical hydrogen

The cheapest, easiest and most common way is chemically from hydrocarbons. Let us assume, for simplicity, that our starting point is methane, CH₄. This is done in two phases. The first is reacting it with water vapour over a catalyst:

            CH₄ + H₂O > CO + 3H₂

The second phase is to react the highly toxic carbon monoxide with more water:

            CO + H₂O > CO₂ + H₂

Thus, we get four molecules of hydrogen out of each molecule of methane. The problem lies in that we also get just as much carbon dioxide, the main "greenhouse gas", as if we simply burnt the methane, in the first place. Analogous reactions can take place with nearly every hydrocarbon, even coal, but most other hydrocarbons have a much higher carbon:hydrogen ratio, so would produce disproportionally more carbon dioxide than hydrogen. 

Electrolytic hydrogen

This is produced by passing a low voltage DC electric current through slightly acidified fresh water. Molecular hydrogen bubbles off the cathode, while half the volume of molecular oxygen bubbles off the anode.

            2H₂O > 2H₂ + O₂

This is a very exact physical phenomenon and a current of 1 ampere flowing through the water for 1 second will produce precisely 0.00001045 grams of hydrogen. The voltage required to do this depends on the conductivity of the water and the configuration of the electrodes and would normally be between 2 and 4 volts. To take a hypothetical example of hydrogen required to fuel a car, a tankful would typically be, say, 30 kg of highly compressed hydrogen to give the car an autonomy of 500 km. This would require 2.87 billion A.s to produce. At about 3 V, this would require 2.4 MWh. Let's imagine that, one day, there will be 200,000 hydrogen-burning (e.g., fuel cell) cars in Cyprus, each averaging 40 km/day, it would require an extra power station of 1.6 GW to provide the required amount of hydrogen; this is almost twice the current peak electrical capacity for all industrial and domestic requirements. By coincidence, 1.6 GW is the output power of a standard European pressurised water reactor nuclear power station.

The overall efficiency of high-pressure electrolysers is about 75%, on condition they work 24/7, but it drops very considerably if they are not used continuously at full output. This makes them unsuitable to be run uniquely from variable renewable sources. Combined with the efficiency of the generating plant and that of the fuel cell (currently about 50%) cars, their overall energetic efficiency from the primary energy source to the cars' wheels is not likely to be better than with ordinary internal combustion engines.

Furthermore, to produce this quantity of hydrogen, it would require an annual consumption of over 1.5 million tonnes of fresh water - in a country already strapped for sufficient water in periods of low rainfall. By extra desalination in a small plant, say, with a capacity of about 1/8 that of the existing Dhekelia plant, this quantity would be feasible. However, the power required to desalinate this volume of water would be considerable and would add to the energetic and economic costs of the produced hydrogen.

Where would the energy for all this come from? We can see in the essay on Renewables that such a requirement would be impossible to supply from wind or solar panels in the context of this island. Yet many persons are banking on the advent of the fuel-cell car (see the essay on Cars) within a 1 - 2 decade time frame. As we can see in the essay on Electricity, this leaves us with the choice of using fossil fuels, with the consequent inability to meet Cyprus' commitments to the Kyoto Protocol (see the essay on Climate change) and the EU or nuclear fission, which would probably be unacceptable to many people on the island.

Hydrogen distribution

If hydrogen were to be a popular source of energy, whether for fixed or mobile uses, it would have to be distributed. It would seem likely that a pipeline may be considered. Experience with natural gas pipelines has revealed that there are always considerable losses, often in the range of 1 - 2 per cent per 100 km, averaged with pumping stations. Natural gas has a molecule which is 8 times more massive than that of hydrogen, which would filter through the smallest leak much more readily than would methane, especially as it would be necessary to pump hydrogen through at a higher pressure (or to have an expensive and potentially leaky pump at each filling station).

An alternative method would be by pressurised insulated tankers, transporting liquid hydrogen at -253°C. A spherical tank could be dropped off at a filling station. As the contents boiled, the pressure would rise. The problem with this solution is that the outtake of hydrogen would have to match the amount of heat being inputted, to keep the pressure safe. This would involve a quite sophisticated installation, to prevent leakage to the atmosphere. The advantage is that liquid hydrogen would be less likely to produce an explosion than an equivalent weight of compressed gaseous hydrogen. The density of liquid hydrogen is just under 71 g/l, so the tanks would have to be very large.

Cost of producing hydrogen

Hydrogen, no matter how produced, is a costly fuel. The following estimates, just for the production, have been published by the International Energy Agency, a pro-hydrogen lobby.

The cost of producing electricity from fossil fuels, for comparison, is typically in the USD/GJ 6-12 region and for nuclear generation USD/GJ 10-12 (including externalised costs such as insurance and decommissioning).

No figures have been published on the cost of hydrogen distribution, but it is estimated that this may approach the doubling of the cost to the consumer.

Safety

If hydrogen is mixed with air in any proportion between 4 and 74 per cent in a confined space, ignition will produce an explosion. In the open, the risk is lower but not inexistent, because the low density of the gas will cause it to dissipate upwards very rapidly. A sudden leak in the open, such as a bursting hydrogen tank, where there is a source of ignition, would usually produce a massive fireball which would consume everything within a certain radius, but travel upwards, away from the earth, within seconds. An indoor leak could be very violent.

As a matter of comparison, most violent accidental gas explosions causing severe damage to buildings are caused by natural gas or bottled LPG (butane or propane). The explosive limits for methane are 5 to 15 per cent, and for propane and butane 2 to 9 per cent, in round figures. It can therefore be seen that the danger of ignition with hydrogen covers a much wider range of concentrations.

However, large quantities of hydrogen are produced and used by the chemical industries, for example in the production of ammonia, in almost complete safety, so the technology is available, at least within the confines of a chemical plant. I am personally very concerned about its use in general public applications, such as in cars. A leak, especially in a confined space such as the body of a passenger car or in a closed building, could possibly have disastrous effects, even if the tanks were somehow made safe from rupture in the event of a road accident. Similarly, I don't think I would wish to live within a few hundred metres of a filling station dispensing hydrogen! (See the page Hydrogen myths for further discussion).

Environmental effects

As far as is known, widespread use of hydrogen as a combustible, is not likely to have any important negative environmental effects. The quantity of water vapour produced would be small compared with the natural loading and it would be quickly dissipated. In large cities, the relative humidity may rise slightly during periods of heavy use (e.g., rush hour traffic). Natural atmospheric mixing would soon dissipate this. It should not be forgotten that petrol and diesel traffic today also produce large quantities of water vapour, so the difference would not be great.

If all cars throughout the world "burnt" hydrogen, there could be a slight rise in global humidities, which would result in slightly more cloud formation. This may affect precipitation patterns in some places and even increase the earth's albedo, causing a minute global cooling. However, major climate changes are not foreseen.

There is one cause for concern that has been raised. Because hydrogen is light, any leakage will travel to tropopausal levels quite rapidly, before any natural oxidation can occur. Transtropopausal mixing mechanisms between the troposphere and the stratosphere are quite violent and hydrogen entering the stratosphere will cause ozone depletion (4H₂ + 2O₃ > 4H₂O + O₂). This is not a catalytic chain reaction as is caused by, for example, the halogens in CFCs and halons, but it is expected that the sheer volume of hydrogen potentially reaching the stratosphere would cause significant depletion and the resultant increase of stratospheric water vapour could also contribute to global climate change. These phenomena have not yet been fully modelled by atmospheric scientists, so there is no scientific assessment of the potential effects of a global change to a hydrogen-based economy, available at this time. Many scientists have expressed concern, though.

Conclusion

Hydrogen may seem to offer a good solution in the long term, as a combustible fuel or for fuel cells, but there remain many technical and economic problems to be resolved before it can be considered as a viable substitute for fossil fuels. In the Cyprus context (and this also applies to many other countries), the most environmentally-friendly way of generating hydrogen would be via electrolysis with power coming from a nuclear fission plant, which would also provide the energy for desalinating sea water for the electrolysis.

Last modified 05/03/2005

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