Energy droughts from variable renewable energy sources in European climates

Highlights

• Energy droughts are defined as periods of low production or high production/demand imbalance.

• Energy droughts present various characteristics within Europe and between energy sources.

• The fluctuations of the energy demand impact energy droughts especially for solar and hydro power.

• Using several energy sources in an energy mix reduces the frequency and the duration of energy droughts.

• Small storage systems limit wind power droughts and Southern Europe solar power droughts.

Abstract

The increasing share of variable renewable energy sources in the power supply system raises questions about the reliability and the steadiness of the production. In this study, we assess the main statistical characteristics of “energy droughts” for wind, solar and run-of-the-river hydro power in Europe. We propose two concepts of energy droughts, considering either: Energy Production Droughts (EPD) as sequences of days with low power production or Energy Supply Droughts (ESD) as sequences of days with a high production/demand mismatch.

Using a set of adhoc weather-to-energy conversion models, we characterize energy droughts in 12 European regions from 30-yr series of daily wind, solar, hydro power and energy demand. The characteristics of EPD are very different between sources with short but frequent wind power droughts and rare but long hydro power ones. Solar power droughts are very region-dependent with much longer droughts in Northern Europe. ESD are next characterized in a 100% renewable energy scenario. The features of EPD and ESD differ significantly, highlighting the interplay with the energy demand. Moreover, both duration and frequency of energy droughts decrease when mixing energy sources or when some storage capacity balances the temporal production/demand mismatch.

Introduction

During the 21st Conference of Parties (COP21), the 2015 United Nations Climate Change Conference in Paris, 175 countries agreed on limiting the temperature increase due to global warming to 2 °C above preindustrial levels. Such an ambitious goal necessitates a deep transformation of our societies and first and foremost a reduction of the anthropogenic greenhouse gas emissions. Many countries have already started their energy transition. In Europe, increasing the share of renewables in the electricity production mix is one of the main targets for the next decade. The European Climate Foundation even set a 100% renewable mix objective to be met by 2050 [1]. In Europe, just like in most regions worldwide, this goal is at least physically realistic, since the resource in renewable energy balances the energy demand several times [2].

Variable renewable energies (VRE) and especially those driven by weather, namely wind, solar and hydro-power from river flow, are expected to play a key role in increasing the share of renewables. Indeed, the installed capacity of VRE is quickly growing worldwide [3]. Exploitable roughly everywhere, wind and solar resources are, for instance, already important contributors of the energy mix in Europe. For some countries, they largely contributed to the early achievement of the 27% share of renewables targeted for 2030 by the European Council [4]. Hydropower from river flow, classically obtained from run-of-the river power plants (further referred to as RoR power plants), is often given less consideration, especially when compared to the key role of hydropower from large water reservoirs [5]. Even if individual RoR plants have fairly no balancing capacity and only small to very small power capacity (e.g. often smaller than a few MW), the overall RoR production is far from negligible in some regions [6,7]. It is also expected to increase significantly in the next years worldwide with the construction of new plants, the upgrade of old ones and the deployment of new technologies such as river hydrokinetic turbines [8,9].

Wind power, solar power and hydro-power from river flow are by definition very sensitive to weather conditions, and thus present high space/time variability [10]. Moreover, wind and solar power are highly intermittent. Consequently, the integration of VRE in the power system is often difficult. They also make power systems rather vulnerable to the hydro-climatic variability and hydro-meteorological extreme events. These issues become even more critical for high shares of VRE [11,12].

In the last decade, many studies have focused on the intermittency of VRE production at small time scales [[13], [14], [15]]. These fluctuations result from the strong variability of weather variables and, to a lower extend, from the cut-in and cut-off thresholds of power generators that only function for a specific range of meteorological conditions (e.g. wind-speed based cut-in and cut-off thresholds for wind turbines). Covering timescales ranging from seconds to several hours, these high-frequency variations in power production lead to a number of critical operational issues (e.g. maintaining the system's stability in order to avoid the system collapse). This especially calls for the support of sufficient mechanical inertia in the system, fast responding back-up power and flexible storage facilities as well as demand-side management [16].

Low VRE production conditions can be also problematic. They result from weather or hydrometeorological configurations with low VRE resources or from situations for which a production curtailment is necessary to avoid damage on power production units (e.g. when wind speed exceeds the cut-off threshold of wind turbines). They can induce critical situations in term of reliability of the power supply potentially and require the use of large back-up sources or energy storage facilities [17,18]. Low VRE production conditions have been the centre of attention of numerous studies. They either aim to assess the risks related to low production sequences, in terms of probability of occurrence or duration for both current and future climate conditions, or to improve their predictability, with the objective of supporting the operational management of transmission system operators and guarantee system reliability.

Low river flows affect the hydropower production and a number of other water-related systems. Consequently, a large number of studies focused on the characterization of these conditions in terms of occurrence, durations and severity from local to continental scales (e.g. Ref. [19] for the European domain). A lot of effort is also made to improve their predictability from daily to seasonal scale [20,21]. The persistence of wind calms and/or the characteristics of low-wind speed periods (for various thresholds) have been similarly analysed for a number of sites worldwide [[22], [23], [24]]. Recent studies also consider the occurrence of simultaneous low wind conditions across large areas in the context of modern transmission grids [[25], [26], [27]]. Similarly, low solar power periods due to overcast conditions, persistent low level clouds or dust outbreaks have been recently analysed [[28], [29], [30]].

The impact of low VRE production periods is also likely to be increased if they co-occur with high energy demand [[31], [32], [33]], a situation which can result from the fluctuation of some large scale climate phenomenon such as the North Atlantic Oscillation [34].

In this study, we propose to complement and enlarge these analyses by characterizing and comparing the periods of low VRE production in Europe for the three main renewable power sources, namely wind, solar and hydro power from river flows. We consider the three sources separately and also combined in an energy mix. For the three sources, we use a same analysis framework and focus on what we call “energy droughts” whose concept rests on the analogy with the classical hydro-meteorological droughts, defined as long periods of very low river flows [22] or periods with no or fairly no precipitation [35,36]. We here propose two definitions of “energy droughts”, considering either 1) Energy Production Droughts as uninterrupted sequences of days with low power production or 2) Energy Supply Droughts as uninterrupted sequences of days with a high production/demand mismatch. We here characterize Energy Supply Droughts within a hypothetical 100% VRE system where electricity generation comes from wind, solar and/or run-of-the-river hydro-power only. We do not base our analysis on site measurements but on high resolution gridded hydrometeorological datasets obtained for a 30-year historical period from satellite observations and from outputs of weather and hydrological models. This allows us to conduct our study on different European regions (12) and thus explore how the characteristics of energy droughts also depend on the climatic spatial variability.

The meteorological datasets and the different weather-to-energy conversion models are presented in section 2 together with the two definitions of energy droughts. Section 3 gathers the results of this study and a comparison of the two droughts definitions. The results are discussed in Section 5 and section 6 concludes our study.