*1.1. Context*

The operation and planning of modern electric power systems face major transformations nowadays, due to the increasing share of renewable generation (e.g., wind or solar) in the electricity mix, which is uncertain by nature and tends to be deployed in a decentralized way, and to the liberalization and unbundling of the electricity supply chain which occurred in the 1990s in Europe.

The main challenge with electricity systems consists in the fact that generation and consumption must be physically equal at every instant in order to maintain system stability, since electrical energy is as for now difficultly storable at a large scale. Extreme problems of coordination must thereby be solved by modern Transmission System Operators (TSOs), which are furthermore complicated by the fact that they do not own the generation (and consumption) assets, since the liberalization of the electricity sector. The coordination is in that context performed through market platforms on which the market actors can interact. The wholesale market level allows in that way interactions between large producers, large consumers and entities known as Access Responsible Parties (ARPs) (or Balance Repsonible Parties—BRPs—depending on the country), which are responsible for maintaining the balance in their portfolio (containing injection, offtakes and possibly exchanges with other ARPs/BRPs). Bilateral contracts, and power exchange platforms such as EPEX SPOT [1] in Western Europe, provide such opportunities at the wholesale level, with exchange horizons starting from years ahead to close to real time. The retail market enables on the other hand interactions between small end-users (consumers and prosumers) and electricity suppliers (through e.g., fix and varying tariff contracts), which are often themselves ARPs/BRPs.

Currently, new modes of exchange of electricity tend to emerge at the local level, which question the market structure depicted above. This is motivated firstly by the proliferation of decentralized renewable energy resources (owned by small end-users or prosumers), following the ambitious environmental targets promoted at the European and worldwide scale, for which a more efficient coordination could be achieved locally. The increasing willingness of the citizen to play an active role in the electricity supply chain is another important driving factor. The literature speaks of 'consumer-centric electric systems', for which the end-user is placed at the centre of the electrical energy value chain.

Some studies propose in that way to keep a centralized market structure, while adapting the wholesale markets to extend their conditions of access to small end-users [2,3]. On the other hand, fully decentralized structures relying on peer-to-peer exchanges, in which all prosumers and consumers are directly interconnected between each other for buying and selling energy services, are discussed in [4,5]. An intermediate solution promotes the grouping of local consumers/prosumers into organized communities, in which energy resources are pooled and allocated to reach a common objective. The modes of exchanges of energy inside the community may however vary depending on authors: local competitive markets are for instance established in [6,7], whereas collaboration prevails over competition in [8,9]. Peer-to-peer exchanges inside communities are also studied in [10,11].

In its directive 2018/2001 on the promotion of the use of energy from renewable sources [12], the European (EU) Commission has formalized the concept of Renewable Energy Communities (or RECs), in which end-users would be allowed to exchange renewable energy produced locally. The directive has since been transposed into decrees and legal frameworks in many countries of the European Union, e.g., in Wallonia in Belgium [13], in France [14] or in Italy [15]. The science and technology communities have in parallel launched many initiatives to study and implement pilot projects of RECs: the cVPP project [16], lead by the Technische Universiteit of Eindhoven, and the E-Cloud project [17,18], lead by ORES (one of the main Walloon Distribution System Operator or DSO) and which will further be described in Section 3, are two striking examples. The present paper focuses more particularly on the case of such RECs.
