## 6 Category II: Consensus Mechanism
### Proof-of-Work  
Since the proof-of-work mechanism was initially used in Bitcoin, the most common early-stage consensus mechanism in blockchain-based energy trading was POW. The consensus proof is achieved by solving a difficult problem, that is, finding a nonce to form a hash value satisfying the condition. Double-spending can be avoided to some extent because finding an answer requires a lot of computing resources, but it is easy for other nodes to verify the answer. However, its weaknesses are obvious, such as the long delay to confirm a trade [111]. Additionally, POW in Bitcoin cannot be directly used for energy transactions, because it can only provide integrity check for things, which is far from enough for energy transactions [84]. Later, many improved methods have been proposed on this basis.The first emerging solution is to reduce the burden by outsourcing the computations. For example, the auction-based market model is designed precisely because of the characteristics of POW mechanism in [60]. Limitations on computing resources have led miners to outsource computing to cloud computing servers. The overhead of proof-of-work is too high for mobile devices, so edge computing [104; 116] and cloud/fog computing [105] are used as a network support device. In [58], the interests of edge computing service providers are also taken into account. To maximize their interests, the marginal resource allocation mechanism that ensures incentive compatibility and individual rationality is still open. Two pricing models for uniform and discriminatory pricing schemes are further proposed to assist [105]. That is, regarding the rewards given to the miners, there are fixed rewards and variable rewards, so that the competitive factors of the miners are not only related to the calculation force, but also related to the size of the block for certification. The second is the proof-of-work based on energy contribution. For example, in [88], the traditional proof-of-work and proof of energy contribution are combined to achieve consensus. The former is for local aggregators (LAGs) about data audit; it is similar to the traditional pow in Bitcoin, which generate a unique hash value of difficulty for each block. The latter is for Plug-in Hybrid Electric Vehicles(PHEVs) about energy contributions, the discharge PHEV that contributes the most to the power supply is awarded with energy currency as an additional incentive. The total energy emitted is measured and recorded by smart meters, which is PHEV's specific working proof of energy contribution. Similarly, in [117], the authors first defined two concepts, the data coin and the energy coin, as cryptocurrencies for Electric Vehicles (EV) applications. During the interaction, vehicle records are stored in the consortium blockchain, and the proof of work is done with the Proof of Data Contribution Frequency and the Proof of Energy Contribution Amount. In the process of information interaction, the data coin is defined according to the proof of frequency contribution of electric vehicle data, and in the process of energy interaction, the energy coin is defined according to the proof of its energy contribution. On the basis of these two proofs, the representativeness of majority decision is determined. That is, if the EV contributes to the collaborative intelligence more frequently, it will receive more data coins, will be given higher priority to access the resource pool, and its data will be given greater credibility. If it gives more energy back to the grid or other entities, it will have more energy coins and get higher priority or a lower energy price. A final improvement is to authorize only a portion of the nodes for the consensus process. This method is used in [103], and an additional leader node is added to issue the audit content to the authorized node for verification. Then, the authentication results are compared. If the authentication results are consistent, the transaction will be made, and the leader node will receive additional rewards.