Long-term planning and the sustainable power system: a focus on flexibility needs and network reliability

Long-term planning models are useful to describe future energy and technology options and to analyze environmental issues. They propose solutions for meeting future energy consumption. Focusing on the electricity sector, we argue that in order to provide a more relevant assessment of the power supply system ahead we need to tackle both flexibility needs and network reliability. On the one hand, flexibility is integrated in long-term planning models as an additional criterion for new investment decisions: it allows electricity generation capacity with short start-up periods to be part of the mix to satisfy peak demand, despite their higher marginal costs in comparison with the cost of base load technologies. On the other hand, and in order to assess network reliability, a suitable representation of dynamic dissipative processes over the electrical network is necessary. We introduce the notions of conveyance and reliability losses and their qualitative impacts on power transmission. We propose a methodology to exhibit the level of losses associated with a given level of reliability, whether generation capacities are centralized or decentralized. Our methodology is based on a thermodynamic description of the electric system and lumps it into a "one-loop grid". It provides a figure for the amount of reactive power and kinetic reserve needed to ensure network reliability and face admissible load fluctuations.