Evolution of the security of supply concept

Leonel Carvalho

  (1)Senior Researcher, INESC TEC

The major evolution in the concept of security of supply is the result of demand-side developments, due to the profusion of dispersed photovoltaic generation on medium and low voltage grids and also the active participation of consumers in the electricity and services markets - especially those who have the capacity to defer consumption and/or store energy through electrochemical and/or thermal systems.

The security of supply, in this case of electricity, is undeniably one of the major challenges that both Portugal and Europe must face in the upcoming years. In fact, vulnerabilities have been reported in this sector, particularly due to the lack of primary energy resources for electricity generation, leading to inevitable price spikes in wholesale markets. The need to mitigate the clear effects of climate change requires the fast decarbonisation of the economy and, consequently, of the electricity sector - which implies, on the one hand, the progressive declassification of fossil-fuelled power plants and, on the other hand, the increase in the demand via the electrification of consumption (e.g. electric mobility, air conditioning, water heating, etc.). Against this background, a new challenge arises: designing an electric power system (EPS) based mainly on renewable endogenous resources which can keep or even improve the security of supply standards to which modern societies have become accustomed to.

The solution to this challenge is not simple, and perhaps a recollection of the historical evolution of the national electricity system (SEN) - particularly in terms of generation and transmission subsystems -, could help. The SEN, as we know it today, results from the integration of various independent grids that supplied cities and/or regions and that were based on small hydro or thermal power plants (e.g., the Tejo Plant, in Lisbon). With the construction of large hydroelectric plants in the 1950s, 60s and 70, and the creation of the high and extra high voltage transmission grid, the system achieved a national span allowing not only to diversify the energy mix, but also to ensure the necessary redundancies towards the security of supply. In this accelerated expansion phase, which was characterised by the strong electrification of consumption and the relative abundance of oil, natural gas and coal, the security of supply was assumed as guaranteed if the installed capacity was greater than the maximum demand forecasted for the planning horizon, plus a gap or static reserve. Said reserve, which is nothing more than an extra generation capacity, is essential to cope not only with momentary power shortages caused by sudden outages and scheduled maintenance actions, but also to mitigate short-term deviations between production and consumption associated with forecasting errors. Basically, this gap quantifies the redundancy required for the generation system to guarantee the security of supply based on the logic that if the system can meet the maximum demand, then it should also be able to supply lower load levels. The minimum reserve required to avoid excessive investment costs in idle capacity was defined through deterministic criteria based on the installed capacity of the largest unit and/or percentages of the maximum demand (e.g., 10% of the maximum demand). Currently, the SEN interconnects with the Spanish system and, through the latter, with the rest of Europe, allowing exchanges of energy and reserves between countries, promoting a more efficient use of generation resources of diverse nature available in distant areas. The collective effort in the progressive interconnection of European EPS has granted the access to greater redundancies and increasingly tighter requirements for continuity of supply. Currently, the security of supply monitoring report (RMSA – Relatório de Monitorização da Segurança de Abastecimento, in Portuguese) elaborated by the Directorate General for Energy and Geology (DGEG) defines as planning criterion an average of no more than five hours per year with load disconnection, which represents a remarkable availability of about 99.95%! Despite its simplicity, this new criterion represents an improved approach to define the static reserve levels, since it is based on a probabilistic modelling for the SEN operation. The tools for quantifying this indicator include extensive historical information about the variability of the primary renewable energy resources (hydro, wind, solar, biomass, etc.), of forced outage rates of generation units and interconnection circuits, of scheduled maintenance actions, of planned exchanges with Spain, and of the stochastic behaviour of the demand along the year. These tools are extremely useful in a context where large quantities of power flow from the production centres to the bulk consumption. However, new sources of uncertainty are anticipated in the upcoming future, which will certainly influence the definition of the reserves required to ensure an adequate continuity of supply. On the supply side, the resilience of the generation system against the progressive scarcity of water must be properly addressed, since it not only affects the capacity of hydro units, but also of conventional thermal units if the water used as coolant becomes suddenly unavailable or increases in temperature. Dust clouds from the Sahara can also cause issues, as photovoltaic (PV) production is affected not only during such events, but also over subsequent days (because of dust deposition on the panels). The role of hydrogen in storing surplus wind and solar production is also an important issue to ensure firm capacity in the months when demand is at its peak.

In addition to these central issues, the security of supply concept will be most affected by the current developments on the demand side, fuelled by the profusion of dispersed PV production in medium and low voltage networks and the active participation of consumers in electricity and ancillary services markets, especially the consumers who can defer consumption and/or store energy through electrochemical and/or thermal systems. A first tentative approach to model such behaviour in security of supply studies is very much in line with the concept of the renewable energy community (REC), which aims to promote the use of local renewable energy sources to supply the local loads. Consequently, this approach bundles all these effects into net load diagrams according to the projects that are expected to be developed and connected to the distribution grids. Please keep in mind that this type of simplification does not account for the power and energy that consumers might have at their facilities over time, nor does it allow to understand when and if this capacity can be used for security of supply purposes. To understand these issues, let us look at the case of electric vehicles (EV), when a considerably high number is connected to the grid. If the EPS needs generating capacity, then momentary decrease of the total charging power can be an important lever, especially in cases where EV users do not need to use the EV over the following hours. Moreover, the anticipation of EV charging when there is an excess of renewable generation can also be beneficial, especially if it allows to avoid charging at peak hours. This reasoning can easily be extended to other equipment available in households, retail, or industry, whose utilisation can be adjusted over time and/or enables energy storage. Given this situation, it is expected that a significant part of the load can adapt itself according to the power available in the EPS, effectively contributing to the security of supply, which is clearly different from assessing its contribution based on immutable net load profiles. In an extreme scenario, the intelligent management of consumption, distributed generation and storage will allow the creation of real energy islands that will probably use the large power generation centres only in the event of a failure of local resources, which configures a situation resembling the initial development stages of the EPS. Hence, it is crucial to develop methodologies and tools to simulate the flexibility available in distribution networks in an adequate manner, both in temporal and spatial terms, to quantify the contribution of this important mechanism to the security of supply indicators and truly develop a reliable and sustainable EPS.