In addition to the typical day-night cycle of photovoltaics, wind power plants are also subject to strong fluctuations – a plant designed for 100 MW can produce an average of 40-50 MW of power per year, but this average power is distributed between power peaks and valleys.
On the other hand, there is a hunger for energy with different demand cycles: Apart from the additional feed-in into city gas grids, almost every application requires a certain amount in a certain time – so much for the succinct reality.
The big task of our time can be described like this: Wind and sun are not constantly and uniformly available. If we want to use their energy to fuel cars and buses every day, to produce steel and to heat houses, then there must be a balancing of fluctuations between generation and consumption.
So somewhere between generation and energy delivery, the volatile generation profiles of renewables must be “balanced”.
A wide variety of models and technologies are competing:
From the classic pumped-storage power plants to air liquefaction, mechanical processes such as flywheels, heat storage, e.g. with liquid salt, to various battery technologies – and hydrogen. Green hydrogen is seen as having good chances due to its storability, application and the calculated price per KW and KWh for power output and capacity.
But when it comes to the concrete planning of hydrogen projects, the price tag becomes clear. It must be borne in mind, however, that hydrogen not only serves as a storage medium, but by transporting energy it also allows the various sectors of industry, mobility and heat to be coupled with each other.
Whatever technology is used to ” soften the peaks” – it will only be possible at the price of “occasional use” – precisely when production peaks of renewable electricity occur. But this implies two things:
- the plants are coupled in terms of their utilisation to the ratio of peaks and dips of the renewable generation. For a wind farm with 100 MW peak and 40 MW average output per year, the utilisation of a directly connected electrolysis plant is also only 40% – which significantly increases the capital costs.
2 In addition, the overall system with buffer storage must be sized in such a way that the demands of the consumers can be met. If a municipality wants to convert its entire bus fleet to fuel cell buses, then refuelling must be possible every day of the year – a delivery reliability of 99.5% for gas, which is common in the industry, is not sufficient for 365 days, because then the buses would be standing around unfuelled for 1.8 days.
It follows from both points that the sizing of electrolysis capacity and intermediate storage size is an essential task in the modelling of the new hydrogen plants. The sour apple of the lack of utilisation due to fluctuating renewables could, if necessary, also be temporarily replaced by “conventional” grid electricity in order to recoup the high investments, at least initially, with a high utilisation.
In the second case, fallback scenarios with the use of grey hydrogen or alternative processes may have to be considered – a close look at the supply requirements of the customers and a coordination of the overall system to meet these is inevitable in any case – otherwise the buses will stand still even though the wind is blowing.