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Diving into the hydrogen debate

We have to filter the facts from the hype around hydrogen, says Dr Jacob Klimstra, who offers an optimum use of renewables… and explains where hydrogen fits in.

Many parties consider hydrogen as an important energy carrier for replacing the direct use of coal, oil and natural gas.

Commercial parties such as fuel cell manufacturers see hydrogen as a means to substantially expand their business.

Pipeline operators cherish hydrogen for a future ‘green’ use of their assets. Scientific establishments use the interest for hydrogen as a way to receive grants in research on the matter.

Consequently, many marketeers are strongly lobbying for hydrogen. However, at the same time there are many voices that proclaim a maximum electrification of energy over the use of hydrogen.

Policymakers and citizens hear all these different opinions and it is not easy to distil the facts and ascertain the best way forward.

One might wonder if the moment is already here to use precious renewable electricity for producing hydrogen for a dedicated use or for blending natural gas with it.

Currently, only about 20% of all primary energy supply in Europe is renewable and not even half of that is renewable electricity.

It is true that in Germany renewable electricity production recently exceeded 40% of the total electricity production in that country. However, if the intention is to electrify road and rail transport as much as possible while also moving away from heating of buildings with gas towards using electric heat pumps, much more renewable electric energy is needed.

Renewable electricity is a valuable commodity and it should be used in an optimum way. That is to decrease the greenhouse gas emissions as effectively as possible.

Direct or indirect renewables?

For the heating of buildings, it is possible to use electric energy directly with an electric heat pump or indirectly via the hydrogen route.

For a moderate ambient temperature, an air-to-air electric heat pump needs 1 kWh of electricity to produce 3.5 kWh of heat. One might argue that this does not apply to very low ambient temperatures, but such a situation occurs only a small fraction of the time in the moderate climates of the EU.

Via the hydrogen-based heating route, where electrolysis has to produce the hydrogen and a boiler has to burn the hydrogen, the effectiveness of the renewable electricity is a factor of almost six lower than a direct use.

Hydrogen supporters claim that a coefficient of performance (COP) of 3.5 is only possible in cases of well insulated homes with low temperature heating.

However, equipping newly built homes and well insulated buildings with heat pumps will already substantially increase the demand for electricity. Consequently, in the short and mid term, there is no real ‘excess’ renewable electricity available for producing high amounts of hydrogen for home heating.

A direct use of electricity with heating coils is even more efficient than heating via the hydrogen route. Natural gas and biomethane can be used for heating homes that still require high-temperature heating.

It is nevertheless a fact that the consumption of natural gas for heating will gradually decrease because of the application of heat pumps and improved insulation.

For road transport, the direct use of electricity is also much more effective in combating greenhouse gas emissions than via the hydrogen route.

If a train receives the electricity for its motors from a catenary system, the effectiveness of the electric energy is more than a factor three higher than via the hydrogen route.

In case of the hydrogen route, the electric energy has to be used for producing hydrogen and then a fuel cell has to convert the hydrogen back into electric energy for driving the motors.

The same comparison can be made for road transport, although the use of batteries slightly reduces the total efficiency. This would result in a factor 2.5 better effectiveness when avoiding the hydrogen route.

If natural gas has to be replaced by hydrogen as a major energy carrier in the near future, an option is to produce it by steam reforming of fossil fuels and capturing the resulting CO2.

The big question is if the required CO2 storage can be realised considering the available space, the security of storage and the opposition of citizens. Pyrolysis is another option, where black carbon is a by-product instead of CO2.

The resulting so-called blue hydrogen might be somewhat cheaper than hydrogen produced by electrolysis. Fossil fuel producers as well as major pipeline companies see reforming and pyrolysis as a possibility for a continuation of using their assets.

Hydrogen blending

Manufacturers of gas turbines, reciprocating engines and boilers can certainly modify their products for running on ‘pure’ hydrogen.

However, one should not ignore the issues arising from the fact that hydrogen is a completely different fuel from natural gas. Its high flame speed and the factor 10 lower minimum required ignition energy compared with natural gas and the tendency to produce more NOx require special attention.

Its volumetric calorific value is about a factor three lower than that of natural gas so that much higher flows are required. The flammability limits of hydrogen are about a factor five wider for hydrogen than for natural gas. Nevertheless, the manufacturers are already exploring and testing the techniques.

A number of gas pipeline operators advocate the blending of natural gas with hydrogen. A main argument is greening of an otherwise fossil fuel.

However, because the volumetric calorific value of hydrogen is about a third of that of natural gas, high fractions of hydrogen are needed in order to be effective for decreasing the specific CO2 production.

Currently, many stake-holders consider 20% of hydrogen in natural gas as a limit for avoiding substantial changes in performance, safety and emissions for the end use.

However, the exact allowed fraction also depends on the base gas in which it is injected. Natural gas with a high content of hydrocarbons higher than methane has a higher reactivity than methane and can therefore accept less hydrogen.

The gas sector cannot guarantee that with hydrogen blending the fraction of hydrogen will stay constant. This tends to substantially widen the range in calorific value and Wobbe Index value of the resulting gas while gas applications can in general only accept a limited variation in gas quality.

Figure 1 shows that in order to reduce the specific CO2 emissions from electricity production by adding hydrogen to natural gas, one needs high volume fractions in order to be effective.

If natural gas contains 10% of hydrogen by volume, the energy contribution of the hydrogen in the gas is only slightly higher than 3% and consequently the specific CO2 emission is only reduced by about 3% provided the fuel efficiency remains the same.

The ‘Taxonomy’ process that is under development in Europe requires a maximum CO2 production of 100 g/kWh for new power generators, with a gradual decrease towards the year 2050.

For such a low specific CO2 emission, one needs to have more than 90% of hydrogen in the gas. Such quantities are not available today, so the only possibility to reach such a low level is by using biomethane. However, that is currently also not available in large quantities.

The fraction of renewable electricity in the total energy supply of Europe is currently, and most probably also in the coming decade, not sufficient for producing hydrogen at a scale required to replace a direct use of natural gas.

Yet, in the longer run, there will certainly be a role for hydrogen in the energy supply.

For the time being, by far the best application of renewable electricity with respect to the reduction of greenhouse gases emissions is by replacing electricity produced with fossil fuel and by providing energy for electric heat pumps and electric vehicles.

Any temporary excess of electricity caused by the volatile character of solar and wind based generation can be used for producing hydrogen for dedicated users, such as the fertiliser industry, blast furnaces and refineries.

The ‘learning process’ for electrolyser development can be done at those dedicated locations. As soon as a dedicated hydrogen ‘backbone’ is functioning in Europe, that system can be used for storing and transporting hydrogen resulting from temporary excess in electricity during sunny and windy days with low electricity demand.

Blending natural gas with variable fractions of hydrogen causes too much uncertainty in the gas quality and would be a temporary solution anyway.

Developing inherently complicated gas supply systems for that and adapting the gas applications for blending destroys the intrinsical robustness of the gas supply system.

A reliable and secure energy supply system is of crucial importance for the economy.

About the author

Dr Jacob Klimstra is a senior researcher and engineer with over 50 years of experience in energy, cogeneration and engine technology.

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