# State-Of-The-Art and Prospects for Peer-To-Peer Transaction-Based Energy System
## 5 Frameworks for TE
### Classification Based on Energy Trading Structure
#### 5.1.1 Classification Based on Distributed Structure  
Until recently, energy trading has been a wholesale business, mostly among big corporations. However, recent advancements in the use of DER have inspired the trials of small and medium-scale P2P trading systems in different parts of the globe such as The Netherlands [62], sonnenCommunity [63] in Germany, Pico [64] in the U.K. and Energy Internet [65] in China. Additionally, some energy traders are inspired by blockchains as in EnerChain [66] in Europe and Brooklyn P2P energy trade [67]. The authors in [68] proposed a distributed energy trading structure in a competitive market, while [69] reported a framework for energy sharing and coordination in a time-slotted P2P fashion. The authors developed a model of bilateral energy sharing and coordination by assuming that prosumers that are directly linked with each other are neighbouring partners. If the directly connected neighbour cannot satisfy an energy demand, the demand and/or supply will be requested from the utility company. The work in [26] presented a P2P energy trading system among EVs. In this case, the energy trading system is based on an activity model to predict the daily activity of drivers in Belgium in particular. This reduces the impact of the charging process on the power system during business hours.  
Various distributed trading mechanisms for energy networks were also proposed in [8; 70; 71; 72]; in particular, [PERSON]. [8; 73; 74; 72] considered energy market design and studied the behaviour of the actors and their impacts on the market operation. Interestingly, Ref. [24] provides an analytical solution for distributed energy trading without a central coordinator to allow several MGs to interact during energy exchange in order to minimize the network operation cost. Furthermore, [PERSON]. [36] addressed P2P energy trading among smart grid households with the aim of optimizing cost by considering components with a significant impact on cost such as storage and renewable resources. The smart homes in their model are connected through a bi-directional distribution network. Energy generation and consumption data are processed through a cloud-based control system. In addition, Ref. [7] reported distributed P2P energy trading as a way to reduce the total cost emanating from energy generation and transportation. In [25], energy trading in grid-connected MG was studied, and a customer-to-customer business model was introduced based on the generalized architectural model for P2P energy trading. Their test results however showed that P2P energy trading can balance local energy demand with generation and reduce the chances of overloading the distribution lines [75].  
Of course, interconnecting MG with the marketplace comes with additional monitoring and control complexities. In this regard, a recent survey on control techniques for MG in [76; 77] reported that as MGs become sophisticated, each of them will deliver new energy services that could be of mutual interest among MGs or clusters of MGs. In [30], a description of a marketplace for P2P electricity sharing is presented. This resource-sharing network enables electricity access in off-grid areas. With the use of power management units, the overall system is less expensive and more scalable than conventional MG, thus providing more affordable and scalable electricity access.  
It follows that distributed energy exchange including P2P energy trading and sharing provides some optimization benefits in terms of cost reduction from transportation and direct transaction between neighbours. In addition, it can balance local energy demand and generation. However, given the peculiarity of electricity, this approach lacks control. For instance, there is no assurance that a buyer will get the right amount of energy purchased, just as there is no known medium for energy transaction management. This and some other challenges posed by a P2P structure to TE foster further discussion of centralized structure as discussed in the next subsection.