# State-Of-The-Art and Prospects for Peer-To-Peer Transaction-Based Energy System
## 5 Frameworks for TE
### Classification Based on the Operation Mechanism  
The operation mechanism is highly related to the energy trading structure mentioned in the previous section. Most P2P energy trading and sharing (ETSs) markets deal with energy as the product. Therefore, real-time issues such as balancing are not covered in the P2P ETS operation mechanism so long as the ETS is not implemented for a microgrid. Even if the system is made for a microgrid, energy transaction and real-time operation should be handled separately by the grid operator. In effect, the main operation interest for P2P ETS for both the microgrid and non-microgrid is energy trading, although the trading period and lead time could be differently defined depending on the situation. The operation mechanism in the pilot study presented in [62] is a long-term-based operation: buyers select a provider from a list of possible providers, and the settlement is performed monthly. On the other hand, hourly-based (or shorter period) matching and transaction with short lead times are preferred in other cases (e.g., see [64]). So long as the buyer can respond to the changing price and there is a communication infrastructure support, the transaction period and lead time could be easily reduced.  
Energy transactions could be initiated by either buyers, suppliers or ESP and could occur by using various matching methods. Therefore, the operation mechanisms for P2P ETS are classified into four groups as follows:  
* Buyer selection from amongst supplier products,
* ESP-centered trading,
* Buyer prioritization and ESP-matching and
* Double auction-based energy trading.  
Buyers select the preferred specific provider from a list of energy products posted by suppliers in the case of the buyers' selection approach. The suppliers provide the types of generation and the selling price to the selling list, and the buyers pick from available and preferable providers [62]. In ESP-centered trading, suppliers and buyers make contracts with ESP. ESP forms a big energy pool to trade with utilities or the power market. In the model, participating prosumers do not know how the resources are operated since they are directly controlled by ESP [63]. Therefore, in a strict sense, it is not a P2P ETS, but one of the realistic solutions for prosumers. In the buyers' prioritization and ESP-matching mechanism, a buyer provides the preference of the energy type and/or providers instead of selecting specific providers; then, ESP matches providers and products to buyers considering buyers' preferences and real generation amount. In this way, buyers can increase use choice even though they may not know the exact matching mechanism [64]. Double auction-based P2P energy trading models without the intervention of ESP are proposed in research or pilot projects [29; 68; 69]. It, however, would take a longer time for these models to come to market because of their operation complexity. Furthermore, the basic mechanism of ideal P2P energy trading models would be similar to a conventional power market mechanism except for the existence of a larger external market that is usually an existing power market. The last stage of the transaction is one of the most important roles of ESP: transaction settlement. Once production and consumption occur after the transaction matches, then the settlement should be performed by ESP considering the participants' performance. Mostly the settlement is made up of money in the real-world system [62; 63; 64], but credits or virtual money could also be used for trial projects [29].