Intermittent renewable energies and electricity supply system

Renewable wind and solar energies offer so much scope for development that their worldwide growth is currently unrivalled by anything other than coal-fired power plants. And yet, renewable energies appear to be far more harmless to health and the environment, in terms of the use of natural resources, climate impact and air quality. As a result, their development is strongly encouraged in many countries and enjoys a generally positive public image. As the electricity they produce is an energy carrier with numerous and expanding uses, the development of wind and solar power looks set to continue at a steady pace over the coming decades.

What share can wind and solar power take in the future electricity mix? Does their growth depend solely on investment capacity and project completion times, or are there other reasons or physical causes limiting it? One such physical cause is security of electricity supply. This security is an imperative for any developed society, whose activities and infrastructures cannot be subjected to power supply hazards without damage. The security of the electricity network is based on a balance in which all production equals the load, i.e. the power required for consumption. This balance is made necessary by the fact that electricity is difficult to store, and in small quantities compared to the quantities passing through the network.

Wind and solar power generation are subject to instantaneous variations in the natural flows imposed by the Earth's rotation and the dynamics of the atmosphere. As a result, these renewable energies supply electricity intermittently. The power grid must be balanced to take account of these intermittent flows when they have priority access, as they do today. In such cases, the grid must be balanced by adjusting the output of other controllable power plants (thermal or hydroelectric power plants), which provide the necessary back-up or complement to match the load. Will this balancing still be possible when the share of intermittent renewable energies increases? This is the central question addressed in this article, based on existing data for France and Germany.

Germany is at the forefront of wind and solar power development, and may therefore be the first country to see the effects of their expansion. F. Wagner was a forerunner, carrying out a detailed study of the consequences of a growing share of these energies in the electricity mix, working on the basis of values recorded on the German grid