# State-Of-The-Art and Prospects for Peer-To-Peer Transaction-Based Energy System
## 1 Introduction  
The current transition of power infrastructure to sustainable and efficient systems is redefining the roles of stakeholders within the energy value chain. In particular, the proliferation of distributed energy resources (DERs) [1] at the grid edge has accelerated the development of a local grid where a small-scale local production of energy is at the community or household levels [2], which harnesses DERs to form an energy network [3] within the consumer domain. This phenomenon, in effect, balances the power requirement, minimizes energy loss and reduces electricity costs. In a bid to improve the power reliability by reducing dependency on the main grid and to reduce environmental hazard, some countries are encouraging local generation and consumption [4; 5] of energy. Such characteristics empower energy prosumers [6; 7] to use, share, exchange [8; 9] or trade their excess generated energy; thus turning prosumers into proactive prosumers by actively participating in the growing economy [10]. Proactive prosumers are energy producers and consumers that want to be in control of their energy generation and usage. They are always seeking ways to reduce dependence on the main grid and to optimize their energy usage and minimize their energy cost. They are actively engaged in producing more energy than they would utilize in order to trade/share the excess to other prosumers. Depending on the mode of energy generation, photovoltaic cell, wind or hydro, energy can be produced in excess of the required quantity. In seasons of high production, there are available options to utilize the energy surplus generated including selling to a neighbour, or feeding the energy to the main grid by employing net metering. However, exporting the excess energy to the grid is sometimes less beneficial than selling directly to a neighbour because it can limit the possible renewable penetration [11], and the grid can put a cap on maximum power that can be supplied at a time [12]. In addition, the energy producer might not earn the maximum returns on the energy supplied. Thus, a prosumer can optimize the financial returns by directly trading with other prosumers through a peer-to-peer (P2P) energy exchange platform. Energy trading and sharing among prosumers could also improve the balance of energy supply and demand. For instance, prosumers can buy or borrow energy from other prosumers in seasons when energy supply cannot meet demand, or sell or lend energy to other prosumers in seasons of surplus production of energy [6]. Energy trading and sharing among prosumers can only be achieved with the introduction of some enabling technologies such as renewable generation, storage capacity and an information and communication infrastructure (ICT). In addition, operational mechanism (e.g., based on market rules) and optimization techniques (e.g., based on game theory) are required to run a transaction-based energy (TE) system. In the end, the motivation for P2P energy trade could be cost minimization, less reliance on the main grid or improvement in energy management practices. In a well-organized TE system, a trader may employ an algorithm to optimize his/her financial returns during energy trade; either by rescheduling some appliances when energy is inexpensive, interrupting some task execution, or by utilizing DERs and storage units [13]. In addition, the TE platform could also employ an algorithm for matching buyers to sellers in order to reduce communication delay and to ensure prompt transmission of energy between both parties. Therefore, in the over 100 literature works published within the past five years (2012 to today), different energy trading and sharing frameworks, with different objective functions have been proposed. In that regard, this paper provides a detailed discussion of the energy trading and sharing techniques, structure and motivation beyond a basic review such as [10]. This begins with a discussion of the energy trading and sharing concept, followed by classifications of existing sharing/trading methods.