**1. Introduction**

The aim of this work is to visualize how variations in a future northern European electricity system can be cost-efficiently managed. The work first examines climate neutral energy systems with extensive electrification of the industry, transport and heating sectors. Outlooks for Nordic [1], northern European [2,3] and European [4] electricity systems have been presented in recent years. The common features of these outlooks are a substantial increase in electricity demand which, to a large extent, is supplied by wind and solar power. Given the varying generation of wind and solar power, the ability of the electricity system to manage these variations have become a question of high priority [5].

There exists a wide range of technologies and strategies to provide flexibility, ranging from shifting the electricity demand for heat pumps in time in single-family dwellings by exploiting the thermal inertia of buildings, to using large-scale underground storage of hydrogen produced for industrial applications. Lund and Lindgren [6] have compiled a comprehensive overview of the different approaches for increasing energy system flexibility. Flexibility measures serve two main purposes: to assure the reliability or increase the cost efficiency of a given electricity system. This work concerns the latter subset, i.e., flexibility measures with the purpose to manage variability such that the social cost of electricity is reduced. This subset is referred to as variation management strategies (VMSs) [7,8]. The potential of flexibility measures to reduce the social cost of meeting the demand for electricity is well documented. Previous work investigating the impact of flexibility measures on the operation of a given electricity system reported reduced costs because

**Citation:** Göransson, L. Balancing Electricity Supply and Demand in a Carbon-Neutral Northern Europe. *Energies* **2023**, *16*, 3548. https:// doi.org/10.3390/en16083548

Academic Editors: Luis Hernández-Callejo, Jesús Armando Aguilar Jiménez and Carlos Meza Benavides

Received: 13 March 2023 Revised: 16 April 2023 Accepted: 18 April 2023 Published: 19 April 2023

**Copyright:** © 2023 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of reduced curtailment [7,9] and because of reduced operation of peak generation units in favor of varying renewable electricity (VRE) and base load generation [10]. Studies on the impact of flexibility measures on the cost-optimal system composition reported reduced costs as investments in varying renewable electricity generation capacity replace investments in more expensive electricity generation options [8,11]. Johansson et al. [8] investigated the impact of a range of variation management strategies on the cost-optimal system composition of electricity systems with different preconditions for VRE. It was found that different strategies influenced the operation of the electricity system and investment in electricity generation capacity in different ways. In addition, it was found that the costoptimal investments in variation management strategies depends on the preconditions for VRE generation. These findings indicate that there are different types of variability which need to be matched with the most cost-efficient strategies for each type. It is reasonable to assume that all types of variations are present in all electricity systems, albeit to different degrees, and what we look for is a balanced combination of strategies adequate for the given electricity system.

Few outlooks for the northern European electricity system address in detail how variations in the electricity system can be met cost efficiently. Pursiheimo and Kiviluoma [3] analyzed electrification scenarios for northern Europe with a focus on Finland. They applied the model of Backbone [12], an investment model with a high time resolution and consequentially with the ability to balance options to cost-efficiently manage variations. Variation management was addressed by illustrating the dispatch of one winter week and one summer week for the Finnish electricity system. The illustrations of the dispatch of electricity systems provided a detailed understanding of how the demand for electricity was met every hour, including how variations were managed when plotted for a limited time span such as a couple of weeks. However, to give an overview of how variations are managed more generally across one or several years, other options for visualization are needed. Several efforts have been made to measure and graphically represent flexibility provision in the electricity system. IEA [13] proposed a flexibility assessment tool (FAST) to estimate if there is sufficient flexibility in a given electricity system to accommodate additional wind and solar power, accounting for flexibility on the demand and supply side as well as the storage and interconnection capacity. Yasuda and Carlini [14] introduced flexibility charts which map the capacity for flexible generation and interconnections relative to the peak capacity of a given electricity system. The tools by IEA and Yasuda et al. rely on statistics which are easy to access to give a first idea of the ability of a given electricity system to manage variations. Using time series from actual or modelled electricity system operations, the contribution of each flexibility supplier can be assessed in more detail. Heggarty and Bourmaud [15] introduce two graphical tools, the flexibility solution modulation stacks and the flexibility solution contribution distribution, to visualize the contribution of flexibility suppliers on annual, weekly and daily timescales.

This paper adds to the previous outlooks for future northern European electricity systems by focusing on how variations in the electricity system are managed using a novel approach of visualizing the net load variability and the contribution of different strategies to reduce this net load. A cost-efficient combination of strategies to manage the net load was derived using a cost-minimizing electricity system model with high temporal resolution representing 11 strategies to manage variations on the demand side and the generation side, as well as pure storage technologies. The contribution of each strategy was visualized using a novel graphical representation in the space spanned by the amplitude, duration and the number of occurrences of the net load variations. Based on the results, the functionality of different strategies to manage variations can be generalized and connected to a functionality-based framework for strategies to manage variations which facilitate the choice of a strategy for a given electricity system context.

#### **2. Method**

To find cost-efficient strategies to meet the demand for electricity, heat and hydrogen in a carbon-neutral northern Europe, the cost-minimizing electricity system model ENODE was applied. Given a time-resolved demand for electricity, heat and hydrogen, together with costs and technical limitations of generation and storage technologies, ENODE generates investments in and operations of technologies for generating and storing electricity, heat and hydrogen. From these results, net loads (i.e., electricity demand which cannot be moved in time, reduced by wind and solar power generation) for each region with and without accounting for strategies to manage variations were produced to visualize the role of each strategy in a carbon-neutral northern Europe.
