The current traditional power grid is designed to transport energy over long distances. This characteristic of the traditional system implies that certain limitations exist, such as the maximum voltage capacity supported by the distribution lines [1]. When this capacity is exceeded, the heat generated by a line can cause it to sag or break, resulting in power supply instabilities such as phase and voltage fluctuations. Because the capacity of a line depends on its length and the transmission voltage, one solution is to create shorter lines and distribute the functionalities of the current power grid in smaller smart networks. These networks are called smart micro-grids, which are a type of discrete energy system that includes appropriated energy sources as well as power loads that provide power to residential, commercial, industrial, and governmental consumers. The main purpose of smart micro-grids is to provide affordable energy to areas independently of the main power supply network while optimizing the transmission of the energy.

In the current context of energy generation, thanks to renewable sources such as solar or wind, and together with the emergence of a new type of actor that consumes and produces energy within the system—the so-called prosumers—micro-grids have the potential to replace the traditional energy transmission system in the near future [2]. However, the rise of smart micro-grids comes with some challenges that must be faced. These challenges range from the vulnerability of platforms to DDOS attacks, to the emergence of intermediaries that do not contribute to energy distribution but end up making it more expensive [3].

In the past, some authors have proposed strategies for energy management on micro-grid platforms. For example, in [4], the excess or shortage of energy could be compensated by exchanging it with the utility grid or other external sources. However, that papentryr did not allow for direct energy exchange between individuals, nor did it allow for the automation and distribution of the platform. Without the use of blockchain technology, democratized energy markets could not be created. A blockchain network acts as a reliable distributed ledger that is governed by the platform and where information of value is stored. The network can be utilized to distribute the control and governance of the smart grid, along with the communication that is carried out within it, thus avoiding the single point of failure and eliminating those intermediaries that do not give any value to the platform. Moreover, blockchain technology (BT) provides a mechanism for protecting the actors against identity theft by signing direct communications between peers [5].

2. Conceptual Foundations of Micro-Grid Platforms and Blockchain Technology

Traditional power grids deliver energy from a few central generators to a large number of consumers. This creates a closed market in which energy prices are dictated in a monopolistic way. Sometimes, to avoid abusive pricing by companies towards consumers, states need to implement regulatory measures, with the European Union ^[6][7][6,7] being an example in this case. In the face of this monopolistic behavior, the literature has proposed the distribution of the traditional main grid into smaller micro-grids [2]. These micro-grids are comprised of a set of loads and generators. The set of generators can be composed of individual houses with solar panels on the roof. The entry of more entities into the energy market reduces the risk of oligopolies and avoids the intervention of states by imposing the use of regularization measures. This way, the energy market is converted into a more democratic market in which the offer and demand of energy will be the only factors that can regulate the energy price. Micro-grid platforms make use of a great number of Internet of Things (IoT) devices that exchange crucial information between them. The continuous communication between the devices allows for the distribution of the management and control of any IoT platform. This helps with the optimization of the workflow of the system, but not without some drawbacks [8].

• Heavy reliance on exchanged messages. Since each part of the system is controlled by an independent entity, the other entities have to trust the messages received to understand the system’s global state. If a malicious entity could somehow modify the content of those messages, the proper functioning of the entire platform would be compromised.
• Reliance on the truthfulness of the transmitted data. Entities of the platform have to rely on the fact that the data transmitted have not been tampered with by the sender entity to make an unfair profit. In addition, it is a possibility that databases will be attacked in order to steal, modify, or delete sensitive information about the entities that are taking part in the system’s workflow.

In the literature, the use of BT has been proposed to overcome the listed flaws of this kind of platform. BT consists of a peer-to-peer (P2P) network of nodes, governed by a consensus algorithm that dictates how the information is stored within the network. This technology allows for the creation of a distributed ledger where anything of value can be stored. The use of a blockchain network within any IoT system makes it possible to distribute the process workflow while eliminating other centralized entities [8]. In addition, by eliminating the single point of failure factor of centralized platforms, protection against some traditional forms of cyberattacks is gained. In this way, the blockchain is used as a bulletin where important information about the system is stored. Furthermore, the data stored within the blockchain network are kept in the same state after their storage, which means that the information is tamper-proof [9]. Within a blockchain-based system, a cryptographic mechanism of pairs of asymmetric keys is used, which signs and encrypts the data transmitted. Hence, as long as the blockchain network is big enough, the consensus algorithm keeps the information in a consistent state [10], the keys are not compromised, and the information transmitted and stored is secure from any attack, thus maintaining its integrity and authorship [11]. If this mechanism is also used in the exchange of messages between individuals of the system, then the messages are protected from being read and modified by unauthorized third parties [12]. A user needs to generate a random private key to make use of a blockchain protocol. This key is usually part of a cryptography mechanism that uses a key pair mechanism: the random private key mentioned and a public one derived from that. This public-key cryptography mechanism is used, not only because they allow for an efficient management of the keys, but also because it is impossible for an attacker to obtain the private key even when knowing the public one. To interact with a blockchain protocol, an individual needs to generate at least one wallet address as an identifier. It is a three-step process, which starts with the generation of a random private key that only the owner should know. Then, through a one-way algorithmic transformation, the public key is obtained, which is shared with the network and is used to verify the signatures made by the user with their private key. Finally, the public key is hashed in order to obtain the wallet address to be used in the exchange of virtual assets between individuals within the blockchain protocol. The process of exchanging assets is quite straightforward and shares the same steps as in every blockchain. Figure 1 illustrates how a user, Alice, wants to initiate a transaction with Bob with 2 coins. To do that, Alice signs the transaction (Tx) with her private key and broadcasts it in the network. Then, each node of the network verifies Alice’s signature with her public key, and if the check is correct and the transaction is proven to have come from Alice, the network validates that she has the coins she wants to spend. If everything goes well, the transaction will be added to the blockchain.

2.1. Blockchain Consensus Algorithms

A consensus algorithm describes the mechanism that allows all agents in the system to coordinate in a distributed environment. It constitutes the only source of truth. Thanks to the consensus algorithm employed by the network of nodes, it is possible to keep the information stored and replicated in a consistent state. Among the functions of any consensus algorithm is ensuring that there is only one blockchain in the system, which can be an issue when a part of the network accepts a blockchain while the remaining nodes accept a different one (Fork). The consensus algorithm should enable the convergence of the chains into one as soon as possible. Moreover, it should offer resilience against attempts by malicious actors to take over the network and guarantee that there will not be any consensus failure when nodes try to add new blocks of data to the blockchain. Keeping the data stored in a blockchain makes it more difficult for attackers to take down the services of a system, and the attacker is forced to take down the majority of them to successfully hack the data [8]. There is a great variety of consensus algorithms, including the Proof of Work (PoW), Proof of Stake (PoS), and Practical Byzantine Fault Tolerance (PBFT) (see Table 1), or any of their variants that are the most widespread and have proven their effectiveness in practice [13]. PoW requires work to be performed by the miner and then verified by the network. The work required usually consists of the performance of a series of operations, algorithms, and mathematical calculations to be solved by the miners. These calculations vary and are different depending on the blockchain network they want to participate in. Each mathematical problem posed can only be solved by a very high computational calculation, which then encourages the nodes to behave in a certain way on the platform as compared with the simplicity of verifying the block mined. The greater problem of this algorithm is that a network using a consensus algorithm based on PoW wastes a massive amount of energy and is very slow. Therefore, it is not environmentally friendly and also not suitable for platforms that need to store information quickly [14]. PoS algorithms are based on the assumption that those who own more units of a PoS-based coin are especially interested in the survival and good functioning of the network that gives value to those coins. Therefore, they are the most suited to bearing the responsibility of protecting the system from possible attacks. That is why the protocol rewards them with lower difficulty in finding blocks (it is inversely proportional to the number of coins they prove to possess). The PoS algorithm has a theoretical vulnerability called the Nothing at Stake Theory, which has not been proven in practice. That theory states that forks in the blockchain network will occur more frequently [15]. In a PBFT consensus algorithm, all nodes communicate with each other, with the objective that honest nodes reach an agreement on the state of the system following the majority rule. Nodes not only have to verify that the message comes from a specific node, but they also have to verify that the message has not been tampered with. For the model to work, it is assumed that the number of simultaneous malicious nodes can never be equal to or greater than one-third of the total number of nodes. Therefore, the more nodes there are in the system, the more difficult it will be to reach that third. It is called practical in the sense that this proposal can work in asynchronous environments. This algorithm is used only in permissioned platforms and cannot be used in a public one, where nodes can access it freely [16]. In [17], the authors discussed the “blockchain trilemma”, a term coined by Ethereum’s founder Vitalik Butering to explain the problem of developing blockchain technology. According Nto this study, no blockchain satisfies the following three characteristics: scalability, consistency, and decentralization (see Table 1). For example, PoW solves the consistency and decentralization problems, but it lacks scalability. On the other hand, PoS can offer scalability and decentralization, but at the cost of consistency. Finally, PBFT-based algorithms can solve consistency problems while being scalable, but they centralize the process.

2.2. Blockchain Accessibility

The implementation of blockchain technology in the real world depends on the accessibility of the network underlying this kind of platform. If a player needs permission to be part of the blockchain network, it is said that it is a permissioned one. These kinds of networks are used in platforms where the actors are known, although they each have different interests. On the other hand, if anyone can be part of the network without requirements, the network is called a public blockchain. A public blockchain, based on PoW, is less efficient in terms of reaching consensus and therefore managing transactions per second because it offers a truly decentralized ecosystem with proven security against attacks, with Bitcoin and Ethereum being their main representatives [10] (see Table 1). Public blockchains that make use of another consensus algorithm, such as PoS or any of its variants, are far more efficient, although they lose some consistency. Blockchain networks that use PBFT-based consensus algorithms could only be used in permissioned environments because they lose decentralization in favor of scalability; to have consistency, it is required that the actors are known.

2.3. Smart Contracts

Another relevant aspect of some blockchain technologies is that they allow for the deployment and execution of coded scripts called smart contracts. Those scripts, due to the immutability feature of the blockchain technology, are considered self-enforcing and are used to automatize some processes, such as payments between entities within a platform that would otherwise need human intervention and/or that of third parties [18]. The code of smart contracts is transparent to the players that can make use of it, which means that they know the programmed clauses that rule it. Then, when those parties agree to use a smart contract, the workflow of the interactions between them is governed by the rules coded in the smart contract, all without the need for human hands to verify the process [19]. A smart contract ensures that the agreement will be carried out automatically when the conditions agreed upon are met [20].