The Role of Hydrogen in a Sustainable Energy System
by: Alberto Abánades
One of the most important challenges of humankind is the sustainability of our energy system. Without energy, the metabolism of our society stops, as can happen with our body without nutrients. Our primary energy resources should be kept available in the long term to provide a stable, continuous input to the system to support our activity and social development. A long-lasting energy system (theoretically even almost perpetual in our human history time scale) can only be achieved with the input of renewable energy sources. Nevertheless, renewable sources nowadays have an intermittent production as its original resources depend on fluctuating ambient conditions. This intermittency should be made compatible with the satisfaction of the energy demand that depends on human activity. The decoupling between energy availability and its demand implies the need of a powerful storage capability to manage the system in a satisfying manner. That storage capability is inherent to fossils, as they are, in fact, a chemical storage charged from the accumulation of sun/biomass along the geological lifetime of the earth.
The Achilles heel of renewable energy technologies and approaching economic competitiveness is the baseload energy management. The management of a fluctuating baseload energy source requires a high storage capacity to support at least some daily consumption of the system. The chemical storage shows clear advantages versus other options. Chemical storage in the form of hydrogen, for instance, has a capacity in volume similar to natural gas that is now stored in large quantities providing strategic energy supplies for weeks. In addition, hydrogen utilisation does not produce CO2 emissions, contrary to the case of hydrocarbons that would require a previous decarbonisation or a downstream capture process.
There are different storage options for their application in an energy system based on renewables. Among those, hydro-pumping is the most developed, and it is effectively integrated in those countries with a suitable geomorphology for hydraulic energy stations. Its capacity is large, but depends on the availability of water resources that are shared with other critical human activities such as agriculture and population needs, being sensitive to long drought periods. Other storage concepts might be based on flywheels, compressed air systems, and other electrical storage systems as supercapacitors and batteries. Those concepts have not demonstrated their ability and economic competitiveness for a very high capacity as is required for the management of baseload electricity.
Hydrogen has been postulated as a vector that could be fundamental in a sustainable energy system for its energy density. Its storage at large scales is under development, overcoming a number of technological issues, as safety and volumetric energy density. Moreover, new sustainable energy systems, such as the ‘power to gas’ concept, in which hydrogen constitutes a basic intermediate material for the production of synthetic natural gas (the real storage medium) from CO2, are gaining momentum. In those cases, the generation of hydrogen from renewables, as solar and wind is envisaged via water electrolysis, while thermochemical processes may be used from solar thermal or biomass.
Credit: 51st issue of ESTIEM Magazine (2016)
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