## 2 State of the Art
### Consensus Protocols for Ethereum and Energy Efficiency
On the other hand, PoA keeps being one of the most used consensus algorithms for permissioned and private scenarios in the literature [45]. For example, the suitability of PoA for the supply chain is explored in [46], where they use Quorum as a private ledger for a dairy production factory. Quorum is an Ethereum fork that provides permissioned or private access to blockchain data and smart contracts and maintains interoperability and compatibility with the Ethereum ecosystem. Finally, it is worth indicating that Ethereum's clients like Geth offered the possibility of alternating between PoW and a specific implementation of PoA called Clique in previous versions. Thanks to such a feature, in this article it was possible to compare PoA and PoW in a fair way through multiple tests, which are later detailed in Section 5. It is important to note that the scope of this article is centered in a resource consumption analysis between PoA and PoW, aiming at quantifying the resource savings of using an energy-efficient consensus protocol. Consequently, other essential aspects of blockchain performance such as scalability or security are beyond the scope of this work and are therefore not examined in detail. Nevertheless, recognizing the relevance of these topics, we have included selected references to guide interested readers toward further exploration. For instance, in [47], the authors conduct a more extensive performance analysis and comparison of the three main algorithm implementations of PoA: Clique, Istanbul Byzantine Fault Tolerance (IBFT) 2.0 and Quorum Byzantine Fault Tolerance (QBFT). This comparison was performed in terms of transaction throughput, latency, resource utilization and scalability within a Hyperledger Besu private setting. The authors find that the block size is one of the most significant variables when it comes to performance, which is a fact that was also validated in Section 5. Although ref. [47] provides valuable insights into the impact of PoA algorithms on various parameters, the work has certain limitations when it comes to assessing their practical applicability in real-world environments, specifically, networks comprising tens of validators and thousands of geographically distributed nodes. In [48], the authors explore scalability, arguing that as the number of authorities grows, transaction throughput in Aura and Clique drops significantly. They find that Clique achieves the greatest availability, Aura ensures the most robust security, and their solution delivers the quickest transaction processing along with the shortest consensus time. In addition, future researchers and developers should be aware of the security of vanilla PoA algorithms. For example, in order to prevent key compromise, redundancy can be introduced by replicating keys across multiple nodes and conducting routing failure tests to identify compromised nodes [49]. In [50], a comparison of PoA and Practical Byzantine Fault Tolerance (PBFT) is made in terms of consistency, availability and partition tolerance (CAP), finding out that a PBFT algorithm is more suitable than PoA for permissioned blockchains deployed over the internet. However, PBFT is not designed to scale outside a small network, as it relies on a leader node to coordinate consensus, and as the number of nodes increases, communication and coordination become much more complex and slower [48]. According to [51], PoA has two main security vulnerabilities: network synchronization and potential centralization of authorities. For example, clock synchronization issues may split the network into two groups. The group with the majority of authorities needed to make decisions would have the ability to outvote the other group. On the other hand, the fact of having a limited number of identified authorized nodes to validate blocks generates a degree of centralization that cannot be dismissed.