Green hydrogen: the crucial performance of electrolysers fed by variable and intermittent renewable electricity

Abstract

After a short review of past and present EU green energy programmes, the article deals with the great surging interest on green hydrogen worldwide and the consequent programme of the EU. Considering the trend in the increasing percentage of variable wind and solar plants in the EU, a preliminary evidence on their variability is reported and based on their capacity factors during the different months of last year in France, Spain and Italy. The basic characteristics and ‘colours’ of hydrogen are underlined, and emphasis is placed on the current and future global demand in 2030 and 2050. The expected cost of the energy content of green hydrogen just at the exit of electrolysers (power to gas—P2G) fed by renewable electricity as derived by key international well-recognised organisations is presented. The high cost of hydrogen content of energy at the exit of electrolysers is underlined; even with the 2050 prospective low cost around €1.2/kg of hydrogen, it corresponds to €36/MWh to be compared with the EU present gas market price of €13/MWh that with a penalisation of around €130/t CO₂ will reach €36/MWh. Even if the compression, transport and distribution of hydrogen pose important challenges, the article is limited to its production subject to the effects of variability of wind and solar on the performance of large electrolysis plants characterised by a complex balance of plant composed of various subsystems. Based on almost instantaneous data logging from some Italian wind and solar plants, questions are raised about the effects of variability and intermittency on efficiency, flexibility, reduction of the load factor of the electrolyser for its inherent limits. This is relevant due to its minimum load for operation, frequent cold starts, time to ramp up to the operating power. This leads to concerns on the feasibility of operation of an electrolyser plant if it is fed only by a variable and intermittent renewable dedicated plant not connected to the network. Owing to the specific variation versus time of the renewable electricity feeding the electrolyser and of the constraints of hydrogen demand, the cost of hydrogen provided in the literature is only valid for general scenarios. A tailor-made approach case by case for the optimisation and design of the plant appears to be necessary. In contrast the possible smooth behaviour of an electrolyser connected to the grid with power purchasing agreements for electricity from renewable plants spread in the system is analysed; the existing regulations foresee increments in the price of this electricity to remunerate its transport and for ancillary, thus increasing the hydrogen production cost. On the other hand, a better capacity factor could ameliorate the situation but also in this case detailed optimisations are needed. In the conclusions, the still great technological, legal, regulatory and socio-economic challenges for hydrogen production, transport, distribution and final uses are pointed out with the necessity of rigorous analyses and prototypal experiments for a better definition of its market share and relevant timing; specifically it is suggested to concentrate efforts also on a possible new way of operation of electrolysers not as up to now connected to a strong electricity network but fed by RES plants isolated from the grid and characterised by daily, monthly and seasonal variations with time intervals of total lack of electricity at all and sharp up-and-down ramps.