## 2 State of the Art
### Consensus Protocols for Ethereum and Energy Efficiency  
As it was indicated in the previous subsection, consensus protocols are used to reach an agreement among the blockchain nodes. The first consensus protocol made use of a PoW mechanism that was utilized by Bitcoin. Although PoW provides trust and security, its massive energy consumption brought in recent years discussions about its long-term sustainability [33; 34; 35]. For instance, The Cambridge Bitcoin Electricity Consumption Index(CBECI) [36] estimated that PoW consumption involved around 147 terawatt-hours and 74 metric tons of CO\({}_{2}\) a year [37], which is the same consumption as a small country like Denmark. As a consequence, it has been successfully deprecated in recent years by new consensus protocols that are able to maximize energy efficiency without losing the trustworthiness provided by PoW [38; 39; 40]. In general, the trade-off for increasing energy efficiency is to decrease decentralization. In practice, PoW cannot always provide complete decentralization either, since, when the network is not big enough, a few nodes with high mining capacity can monopolize the validation process. In a permissioned scenario, the network is populated with nodes that have a certain degree of trust in each other. Therefore, there is no need to provide computational competition among the participating parties to ensure they are following the rules of the network. Hence, in permissioned blockchains, the use of an alternative protocol can accelerate block creation, since there is no need for keeping a high block time. Moreover, in this type of networks, blocks can be larger than in a mainnet, resulting in a higher transaction throughput. To tackle the previous issues, different consensus protocols have been proposed in recent years to decrease energy consumption. Proof-of-Stake (PoS) and PoA are two protocols that provide a good balance between efficiency and performance without losing excessive decentralization. In addition, they are suitable for environments where the involved parties have similar interests (like the validation of ARs proposed in this article):  
* Proof-of-Stake (PoS). It is the protocol currently supported by the mainnet of the Ethereum network and one of the most popular. There is a lot of research with respect to its energy consumption and, as demonstrated by the Crypto Carbon Ratings Institute [41], its energy consumption is orders of magnitude below Bitcoin's PoW mechanism. In PoS, validator nodes (i.e., the ones responsible for validating transactions and for creating new blocks) do not come to an agreement through the resolution of a computational problem as in PoW, but by making use of the amount of stake they possess. This eventually can lead to an imbalance on the validation process where the nodes with most stake are always selected, so parameters like randomness or coin age can be adjusted to keep the validation decentralized. Thus, PoS may involve new security risks, but it prevents other type of attacks, since an attacker needs to get stake to perform fraudulent transactions. * Proof-of-Authority (PoA). PoA establishes a pool of known and trustworthy validator nodes. The guarantor element is not the amount of stake the validator nodes contribute with, but their public identity. This acts as an incentive to improve their standing on the network to keep them as validators [42]. Moreover, validators need to comply with pre-requisites and regulations present in a smart contract. These rules and the lack of competition among the stakeholders provide a good level of trust. Furthermore, PoA is more resistant than PoW to 51% attacks [38], since it is far more complicated to control half of the nodes than half of the computational power. Current analyses on consensus algorithms' carbon footprint indicate that PoA stands out as the most carbon-efficient algorithm, as it operates in trusted environments with limited nodes and validations [43; 44].