# Peer-to-Peer Energy Systems for Connected Communities: A Review of Recent
Advances and Emerging Challenges

Wayes Tushar${}^{\text{a}}$, Chau Yuen${}^{\text{b,*}}$, Tapan K.
Saha${}^{\text{a}}$, Thomas Morstyn${}^{\text{c}}$, Archie
Chapman${}^{\text{a}}$, M. Jan E. Alam${}^{\text{d}}$, Sarmad
Hanif${}^{\text{d}}$, and H. Vincent Poor${}^{\text{e}}$
${}^{\text{a}}$The University of Queensland, Brisbane, Australia
${}^{\text{b}}$Singapore University of Technology and Design, Singapore
${}^{\text{c}}$University of Edinburgh, United Kingdom
${}^{\text{d}}$Pacific Northwest National Laboratory, WA, USA
${}^{\text{e}}$Princeton University, NJ, USA ∗Corresponding author at:
Engineering Product Development Pillar, Singapore University of Technology and
Design, 8 Somapah Road, Singapore 487372._E-mail addresses:_
<EMAIL_ADDRESS>(W. Tushar<EMAIL_ADDRESS>(C. Yuen),
<EMAIL_ADDRESS>(T. K. Saha<EMAIL_ADDRESS>(T. Morstyn),
<EMAIL_ADDRESS>(A. Chapman<EMAIL_ADDRESS>(M. J. E. Alam),
<EMAIL_ADDRESS>(S. Hanif<EMAIL_ADDRESS>(H. V. Poor).

###### Abstract

After a century of relative stability of the electricity industry, extensive
deployment of distributed energy resources and recent advances in computation
and communication technologies have changed the nature of how we consume,
trade, and apply energy. The power system is facing a transition from its
traditional hierarchical structure to a more deregulated model by introducing
new energy distribution models such as peer-to-peer (P2P) sharing for
connected communities. The proven effectiveness of P2P sharing in benefiting
both prosumers and the grid has been demonstrated in many studies and pilot
projects. However, there is still no extensive implementation of such sharing
models in today’s electricity markets. This paper aims to shed some light on
this gap through a comprehensive overview of recent advances in the P2P energy
system and an insightful discussion of the challenges that need to be
addressed in order to establish P2P sharing as a viable energy management
option in today’s electricity market. To this end, in this article, we provide
some background on different aspects of P2P sharing. Then, we discuss advances
in P2P sharing through a systematic domain-based classification. We also
review different pilot projects on P2P sharing across the globe. Finally, we
identify and discuss a number of challenges that need to be addressed for
scaling up P2P sharing in the electricity market followed by concluding
remarks at the end of the paper.

###### Index Terms:

Peer-to-peer network, energy sharing, connected community, negawatt, review
paper, electric vehicle, solar, storage, renewable.

##  I Introduction

Energy systems are undergoing a rapid transition to accommodate the increasing
penetration of embedded distributed energy resources (DER), such as solar
photovoltaic (PV) arrays and wind turbines. In Australia, for example, 2 GW of
installed rooftop PV capacity has been installed as of December 2019 [1],
which is expected to increase up to 25 GW by 2030 [2]. This extensive
integration of DER to the energy network opens opportunities to provide values
for both the grid and the DER owners. From a grid perspective, on the one
hand, DER can benefit by providing flexibility to improve localized network
performance issues such as voltage fluctuation [3] and network management
capacity [4]. On the other hand, prosumers can reduce their energy cost by
using on-site generation from their DER and make revenues by sharing the
surplus energy [5].

Such capacity of energy sharing in the local energy market makes the DER
owners active prosumers [6] \- energy consumers who also produce energy from
their DER. However, the benefit that a prosumer can reap by trading its energy
in the market could be marginal if it cannot decide on its energy trading
parameters, such as the how much energy to share and the price per unit of
energy, independently [7]. Given this context, peer-to-peer (P2P) energy
sharing has emerged as a platform that can facilitate the independent
decision-making process of prosumers to trade their energy within a connected
community [8]. In P2P sharing, a prosumer can independently decide on its
energy sharing parameters such as how much energy to share and the price and
determine who to share the energy with and when to share. Here, it is
important to note that, in P2P trading, although a centralized controller or a
third party may partially influence the decision-making process of a prosumer,
it cannot directly control what a prosumer chooses to trade with other
community members. For example, a third party or centralized controller may
impose a constraint on the maximum power injection limit for a prosumer in the
P2P market [9], which will influence the decision of the prosumer, but how
much energy the prosumer will trade with other prosumers within the community
given that injection limit is decided by the prosumer independently without
any direct control from the third party (or centralized controller). The
objective of P2P trading also include 1) reducing greenhouse gas emission, 2)
enabling consumers without DER to participate in low-cost energy trading, 3)
providing demand flexibility and energy services to the grid, and 4) ensuring
greater prosumer privacy. Moreover, the advancement in controllable DER
techniques [10] to control the power injection limit of prosumers has further
motivated the evolution of P2P trading with the promise to not violate network
constraints during energy trading.

Consequently, there has been a growing interest in P2P energy sharing research
for the last few years. The focuses of these research can be generally divided
into three categories: 1) research that focuses on the decision-making process
of different participating prosumers with the aim to achieve some targeted
performance improvements either in the individual or in the community level
[11]; 2) research that addresses the impact of P2P sharing on the physical
energy network [12, 13], and 3) research that studies the development of
platforms to enable P2P sharing [14]. Further, a fair number of pilot projects
on P2P sharing and relevant energy management techniques are also being
established in different parts of the world [15]. Interestingly, despite these
extensive efforts, to date, there has been no consideration of pathways to
implementing P2P sharing models in today’s electricity market. One potential
reason could be the lack of a comprehensive understanding of the gap between
what has been developed to date and what else needs to be done for scaling up
P2P sharing.

Given this context, in this paper, we aim to address this gap by shedding some
light on potential barriers to implementing P2P sharing in existing
electricity market frameworks and regulatory regimes. We do so by providing a
comprehensive overview of recent advances in the P2P energy system and an
insightful discussion of the challenges that need to be further addressed to
establish P2P sharing as a viable energy management option in today’s
electricity market through following contributions:

  * •

We provide a detailed background of different aspects of the P2P energy
sharing system.

  * •

We discuss advances in P2P sharing through a systematic domain-based
classification.

  * •

We review different pilot projects on P2P sharing and relevant energy
management technologies across the globe.

  * •

We identify and discuss a number of challenges that need to be addressed for
scaling up P2P sharing in the electricity market.

We note that several recent survey papers have also contributed extensively to
the body of energy sharing knowledge. Most of these studies focused on very
specific topics such as blockchain [16], distributed ledger [14], game theory
[17], computational approaches [18], and markets [19]. However, due to the
lack of a general overview of the topic, their capacities in identifying
further modifications that are needed to prepare P2P sharing as a viable
energy management options in the current electricity market are rather
limited. Two further reviews require additional comments. First, the authors
in [11] provided a comprehensive general overview of various challenges
addressed by existing studies in P2P trading. Nevertheless, the discussion in
[11] revolved around different challenges, rather than different domains of
the P2P energy system. This makes it difficult to identify the developments
and subsequent gaps in various domains of the P2P energy system from the
discussion in [11]. Further, an overview of existing pilot projects is missing
in the study. In this work, we address this issue by choosing a domain-based
classification, while surveying existing studies as well as a providing
comprehensive overview of existing pilots on P2P sharing and relevant energy
trading markets across the globe. The proposed study is also different from
existing reviews in terms of contents, organization, and the focus on
discussion.

Second, [20] consider coordination and optimization methods for facilitating
the integration of small-scale DER into low- and medium-voltage networks. The
authors in [20] focus on three general approaches to coordinating prosumers:
(i) uncoordinated approaches that only consider energy management of an
individual user; (ii) approaches that cast the coordinated energy management
problem as an optimisation problem; and (iii) peer-to-peer energy trading.
Although [20] investigate which integration methods can be implemented with
different levels of network awareness and their capability to address network
or consumer interests, their focus is on computational mechanisms, so they
ignore many of the additional implementation challenges brought to light in
the current work. Similarly, the case studies in [20] focus on energy trading
between simple prosumers, and although they rigorously examine the different
computational approaches to coordinating prosumers, they do not discuss
advances in controllable DER technology that facilitating the participation of
new domains in P2P energy trading, which are considered in this survey.

The rest of the paper is organized as follows. We provide background on
several topics related to P2P energy sharing in Section II. In Section III, a
detailed analysis of recent advancement in P2P energy sharing is given based
on a systematic domain-based classification, followed by a detailed discussion
on various P2P sharing projects around the world in Section IV. We summarize
the overall discussion of the paper along with an explanation of some key
challenges in Section V. Finally, some concluding remarks are drawn in Section
VI.

##  II Background on P2P Sharing

###  II-A Connected community

It is a well-known fact that coordinated DER brings several advantages to the
energy system including reduced cost of transmission and distribution systems,
reduced grid power losses, and a larger share of zero-carbon technologies
[21]. However, to reap these benefits, prosumers have to play active roles in
providing energy services [22] \- a need that ultimately introduces the
concept of a _connected community_.

A connected community enables collaboration between prosumers to take
initiatives that results in collaborative solutions on a local basis to
facilitate the development of sustainable energy technologies [23]. While the
incumbent traditional energy grid suffers a lack of trust from the public,
connected community enhances social acceptance of technology at the local
level - steered by trustworthy individual prosumer and organizations rooted in
the local community [23]. Essentially, a connected community consists of a
group of efficient and interactive prosumers, such as owners of buildings with
diverse and flexible end-user equipment with some electric vehicle (EV)
charging infrastructure111https://www.energy.gov/eere/articles/department-
energy-releases-request-information-potential-funding-grid-interactive., that
can collectively work together to maximize the grid efficiency without
compromising prosumers needs, comfort and convenience. Connected communities
rely on smart technology, DER, flexible loads, and grid integration in order
to reduce energy use and peak demand, improve energy efficiency, while at the
same time, improve user’s experience without compromising any privacy and
security.

The physical electrical connection between different prosumers with a local
community and across different communities is maintained by distribution and
transmission lines of the power network - managed and maintained by
distribution system operators (DSO) and the transmission system operator (TSO)
respectively [24]. The energy sharing services and relevant decision-making
processes, on the other hand, can be provided by different application-
specific energy service providers via transactive energy frameworks [25] using
digital communication, artificial intelligence, and signal processing
techniques over the virtual network. An example of a connected community, in
which energy is managed between different entities using a transactive energy
framework is shown in Fig. 1.

Figure 1: This figure shows an overview of various elements of a connected
community where selected participating prosumers such as households with
renewable generators, community storage, and independent renewable energy
providers generate electricity, which is shared among other energy users
within the network. The energy service provider facilitates the platform for
sharing within the network and provides additional services such as home
energy management and energy donation services if necessary. Utility networks
such as DSO is in charge of delivering electricity within the community. This
figure inspired by [24].

###  II-B P2P energy sharing systems

P2P energy sharing is a branch of transactive energy that considers prosumers’
perspectives while at the same time ensuring that the system is operating
safely and efficiently. In P2P sharing, prosumers can actively participate in
the energy market, negotiate the price with other peers within the connected
community, and then trade their energy and flexibility services as forms of
either watt [26] or negawatt [27]. With the power of setting the terms of
sharing and delivering goods and services, it has been shown in several
studies that the gain that prosumers can reap from participating in P2P
sharing could be substantial [11]. Meanwhile, P2P sharing also benefits the
grid in terms of reducing the peak demand [28], reserve requirement [16],
operational cost [29], and improving reliability [6].

To facilitate this beneficial energy management scheme within connected
communities, the P2P sharing system is divided into two layers - the physical
layer and the virtual layer [29]. In the physical layer, the main elements are
grid connection, smart metering, and communication infrastructure.

  * •

_Grid connection:_ It is important to define the connection points of the main
grid for balancing the demand and generation of energy for both grid-connected
and islanded microgrid based P2P sharing systems. The performance of the P2P
sharing system can be monitored and evaluated by connecting smart meters in
those connection points [17].

  * •

_Smart metering:_ In a P2P energy sharing system, each prosumer is equipped
with a smart meter capable of deciding whether a prosumer should share its
energy with other peers within the community based on the available
information on demand, generation, and market condition. Smart meters also
have the ability to communicate with each other through suitable communication
protocols.

  * •

_Communication infrastructure:_ A communication infrastructure is necessary
within a P2P energy sharing system to discover prosumers and facilitate
information exchange among them. The adopted communication infrastructure
within a connected community needs to fulfill the requirements necessary for
recommended system performance including latency, throughput, reliability, and
security [30].

The virtual layer, on the other hand, comprises information systems, market
operation, pricing, and energy management system.

  * •

_Information system:_ The information system helps prosumers within a P2P
energy sharing system to decide on energy parameters by integrating them to a
suitable market platform with equal access to each participant, monitor the
market operation, and imposing constraints on prosumers’ decisions, if
required, for the network security and reliability purposes.

  * •

_Market operation:_ The purpose of the market operation is to enable prosumers
to experience an efficient energy sharing process by providing services to
match the buy and sell orders in real-time. The different time horizon of the
market operation enables participants to share their resources with different
community members at various time slots of trading at a price that
commensurate with the status of demand and supply of energy within the
community.

  * •

_Pricing mechanism:_ The pricing of P2P sharing balances between the energy
demand and supply within the connected community. Depending on the regulation
within the region, energy prices may or may not include surcharges, taxes, and
subscription fees. Regardless of types, all pricing schemes reflect the state
of the energy within the connected community.

  * •

_Energy management system:_ The energy management system (EMS) of a prosumer
is responsible to bid in the market on behalf of the prosumer to share energy
and simultaneously ensures the security of prosumer’s energy supply. The
decision of an EMS on the participation in energy sharing is triggered by
real-time demand and supply information of the respective prosumer and the
rules set by the prosumer on various market parameters including price, source
of energy, and roles in the market (e.g., watt or negawatt sharer).

In addition, a P2P energy sharing market has other two elements including
prosumers and regulators. Clearly, a sufficient presence of prosumers is vital
for the success of P2P energy sharing within connected communities whereas the
regulation within a region decides whether the P2P energy sharing system
should be facilitated within existing energy market and supply systems. A
demonstration of different layers of the P2P energy sharing system and
relevant elements is shown in Fig. 2.

Figure 2: Demonstration of different layers of P2P energy sharing systems
within a connected community. The virtual layer provides a platform for the
participating prosumers to decide on their energy trading parameters. The
physical layer, on the other hand, facilitates the actual transfer of energy
between the seller and the buyer once the decision is made in the virtual
layer and payment is completed.

###  II-C P2P energy sharing market

Considering how the trading process is performed and how the communication of
information takes place among the participants, the P2P energy market can be
divided into three categories: coordinated market, decentralized market, and
community market, as shown in Fig. 3.

(a) Coordinated market.

(b) Decentralized market.

(c) Community market.

Figure 3: Based on the coordination process of sharing and information
communication, this figure illustrates different types of market that
facilitate P2P sharing within an energy network. In a coordinated market, both
sharing and communication are done in a centralized fashion whereas in a
decentralized market both are performed through decentralized method. In a
community market, the communication process is centralized, but the sharing
process is decentralized.

####  II-C1 Coordinated market

In a coordinated market, both the trading process and the communication of
information are done in a centralized fashion (Fig. 3). That is, a centralized
coordinator communicates with each peer within the network and directly
control the export and import limit of energy that the prosumers share among
themselves through P2P trading. Once the sharing is complete, the revenue of
the entire connected community is distributed among the prosumers by the
coordinator according to pre-set rules. An example of such a pre-set rule is
proportional allocation [31]. While each peer does not communicate and
negotiate energy sharing parameters with other peers in a coordinated market,
they can influence the decision of energy sharing parameters by independently
decide their energy and price before sharing that information with the
coordinator. A key advantage of the coordinated market is the outcome that
maximizes social welfare [32]. However, with increasing penetration of DER,
the computational burden of P2P sharing could be extensive [33]. Further, due
to direct control of peers’ flexible loads and DER, a coordinated market can
potentially compromise the privacy of the prosumers. More discussion on
coordinated markets can be found in [5] and [34].

####  II-C2 Decentralized market

In a decentralized market (Fig. 3), peers can directly communicate with one
another within the connected community and decide on their energy trading
parameters without the involvement of any centralized coordinator [8]. Thus,
in a decentralized market, both the trading process and the communication of
information are done in a decentralized fashion. The main advantage of a
decentralized market is that prosumers are in full control of their decision-
making process, e.g., they can easily decide whether to participate in energy
sharing or not at any given time slot and their privacy is well protected
[11]. Further, the scalability of the decentralized market is also exceptional
[32]. Due to prosumer-centric properties, decentralized markets serve the
prosumers better than the coordinated market.

However, due to a lack of centralized control [35], the efficiency of
decentralized markets is relatively low, and social welfare does not attain
the maximum value. As the overall energy that can be traded within the
community is not very clear to third parties such as network operators,
retailers, and transmission system operators, managing the decentralized
market is more difficult for service providers due to the challenge of
maintaining network constraints and improving the operational efficiency of
the power system. To maintain such a market, sometimes network operators need
to take drastic measures such as load curtailment and blocking peers from the
network [12] in order to maintain the reliability of the grid. Further
examples of P2P sharing in the decentralized market can be found in [36] and
[37].

####  II-C3 Community market

In a community market (Fig. 3), the trading process of decentralized although
the communication between the participating prosumers is done in a centralized
fashion. In this market, a community manager acts as a coordinator of P2P
energy sharing among the prosumers. However, unlike the centralized market,
the community manager cannot directly control the export and import of energy
by different prosumers within the market. Rather, the community manager
influences the prosumers to participate in P2P sharing indirectly via suitable
pricing signals [31]. Thus, in a community market, prosumers need to share
limited information with the community manager while, simultaneously, they can
maintain a higher level of privacy [19]. Further, through indirect control,
the autonomy of prosumers in deciding their energy trading parameters is also
preserved. One core focus of the community market-based energy literature is
to design suitable pricing schemes that can facilitate P2P sharing and at the
same time can provide energy services to different entities within the
network. Pricing schemes also focus on engaging a large number of prosumers in
energy sharing. Different energy sharing mechanisms within community markets
have been discussed in [38], [39], and [40].

A summary of different types of market for P2P sharing is shown in Table I. We
further note that in a connected community, it is also possible that different
types of markets coexist at the same time. For example, both decentralized and
community market can exist under a composite market paradigm that possesses
the characteristics and advantages/disadvantages of both markets. For details
on P2P energy sharing markets, please see [19] and [32].

TABLE I: Summary of different type of markets for P2P energy sharing. The market structure is based on the characteristic of trading process and the method of communication of information among participants in the market. Type of market | Trading process | Communication of information | References
---|---|---|---
Centralized | Decentralized | Centralized | Decentralized
Coordinated market | ✓ |  | ✓ |  | [31, 32, 33, 5, 34]
Decentralized market |  | ✓ |  | ✓ | [8, 11, 35, 12, 36, 37]
Community market |  | ✓ | ✓ |  | [31, 19, 38, 39, 40]

###  II-D Technologies behind P2P sharing

Successful establishment of P2P sharing of energy has been possible due to
innovations and developments of a number of technologies. What follows is an
overview of technologies that have enable sharing of energy between prosumer
within connected communities.

####  II-D1 Distributed ledger

Due to the nature of the P2P energy sharing mechanism, data security, data
privacy, data integrity, and speed of financial transactions between prosumers
become very critical [14]. As such, distributed ledger technology has been
proven to be very effective in addressing these concerns by providing
prosumers with a platform with transaction security to exchange information
among themselves for both energy and economic transactions without resilience
on trusted third parties. Thus, it is necessary when the communication of
information is decentralized, i.e., decentralized market. The main parts of
any distributed ledger technologies include ledgers, smart contracts, and
consensus protocols [41]. Ledgers record key information and data about the
participants whereas smart contracts define participants’ preferences to
ensure the implementation of agreed terms between two or more parties. The
objective of consensus protocols is to validate transactions. Some examples of
distributed ledger technology include Blockchain [42], Hashgraph [43],
Holochain [44], Directed acyclic graph [45], Hyperledger [46], Ethereum [47],
and Algorand [48]. A detailed survey of different distributed ledger
technology can be found in [41].

####  II-D2 Internet-of-Things

Internet-of-Things (IoT) is a platform that enables devices to communicate
with one another and with humans over the internet to achieve various
objectives, such as energy saving, condition monitoring, predictive
maintenance, and remote monitoring and control [49]. An important part of P2P
energy sharing is that prosumers’ monitor their own energy generation and
demand [50], control and schedule the energy consumption pattern of various
appliances [51], and set the rules for the devices [52] to participate in the
energy sharing over the electricity network based on the information available
in the energy sharing platform. IoT has made all these tasks possible and thus
contributes extensively to prepare P2P sharing as an energy management
technique in the local electricity market. Applications of IoT have been
extensively discussed in [53], and [54].

####  II-D3 Artificial intelligence

Artificial intelligence (AI) is a modern breakthrough in computational
techniques that has the potential to revolutionise automation and decision-
making of agent-based systems. For example, AI could be a vital part of the
‘smart’ of smart grid energy sharing [55]. In general, AI refers to
computational techniques that simulate human intelligence in machines to
enable them to think and act like humans. Machines with AI exhibit
characteristics associated with human mind such as learning and problem-
solving [56]. For P2P sharing, it is important to learn the energy usage
pattern of different flexible loads within a building as well as understand
prosumers’ responses in terms of bidding energy amount and price in various
market conditions. AI has been shown to be very effective to capture these
learning objectives through reinforcement learning [57], deep learning [58],
and artificial neural network [59].

####  II-D4 Responsive buildings

Responsive buildings, also known as grid-efficient buildings [60], refer to
buildings that have capabilities to respond to incentive signals sent from the
grid, other buildings, or third parties by altering their energy generation,
consumption, and sharing behavior. Therefore, responsive buildings have the
capability to monitor and control their real-time energy generation and
dispatch, optimize the energy usage behavior according to occupant needs and
preferences, provide energy services to the grid and other energy entities
within the connected community. To exhibit these capabilities, responsive
buildings are equipped with 1) reliable and low-latency two-way communication
facilities to communicate with devices, appliances, and other responsive
buildings within the community; 2) intelligence management system to monitor,
predict, and learn from occupants’ behavior, weather forecast, and market
condition, and subsequently take complex and intelligent actions that adapt
dynamically over multiple time slots and various conditions; and 3) a secured
and trusted platform that is resilient against cyber attack from unauthorized
sources. An overview of responsive buildings can be found in [61].

####  II-D5 Controllable DER

DERs without coordination and control introduce reverse power flows, voltage
rise, and increased fault currents within the electricity network [62] that
can lead to a widespread catastrophic event like a blackout [63] and therefore
trigger the need for network equipment reinforcement. To avoid this,
significant efforts have been reported recently in advancing the techniques
for controlling DERs like PV inverters, battery storage, electric vehicles,
and demand response asset (flexible loads). Now, PV inverters are smart enough
to self-adjust their active and reactive power injection to the electricity
network in response to the network condition [64], whereas cutting edge
algorithms and techniques are available to opportunistically charge and
discharge the battery storage to provide network services [65] and reduce cost
[66] by the prosumers. Coordinated control of electric vehicle
charging/discharging along with the intelligent management of flexible loads
now enables prosumers to enjoy the benefit of having mobile storage [67] and
participate in P2P trading using the energy stored in the battery of their
vehicles [68]. Further, these DERs represent new points of control on
distribution feeders that can be aggregated meaningfully to impact the bulk
electricity system [10] via P2P trading.

####  II-D6 Design innovation

Design innovation (DI) is a human-centred and interdisciplinary approach that
integrates technology with user experience and positively reflects users’
preferences in energy management schemes such as P2P sharing. As P2P has been
identified as a socio-technical energy management scheme [69], rather than
just a technical scheme, taking participants’ preferences into account while
designing P2P mechanism is of paramount importance [8]. As such, DI, through
its 4D model (discover, define, develop and deliver), provides an excellent
set of tools that can be applied for managing energy generation and
consumption within buildings in a prosumer-centric way [70] with a view to
enable the buildings to participate in the energy sharing market.

####  II-D7 High speed communication

The term _connected community_ clearly emphasizes the need for suitable
communication infrastructures for P2P energy sharing with the capability to
operate remotely and interact with various devices within the connected
community with low-latency. Recent advancement in high-speed communication
such as fifth-generation communication (5G) can fulfill these requirements
with its wide range of network capabilities and abilities to support highly
demanding services. Examples of supports that 5G can provide to advance P2P
energy sharing include, but are not limited to, enabling communication between
massively interconnected devices, enable operations and manipulation of
physical objects over distance with reliability, and ensure low latency
response [71].

Figure 4: A graphical overview of how technologies enable prosumers to share
their energy within a connected community. Distributed ledger facilitates the
information exchange and transaction among the participating entities of
energy sharing. Meanwhile, artificial intelligence, IoT, and controllable DER
critical for producing energy, e.g., by intelligently managing energy within
the households and setting suitable price of sharing, within the community to
be shared by its elements. High speed communication ensure low latency energy
transaction whereas design thinking establishes prosumer-centric outcome of
overall energy sharing within the community.

A graphical overview of how different technologies enable prosumers to share
their energy within a connected community is shown in Fig. 4.

##  III Advancement of P2P in Various Domains

In the last decade, advancements in technological research that are directly
or indirectly contributing to the successful sharing of energy in the P2P
network have been extensive. Considering which network elements have mainly
been utilized to either directly conduct P2P sharing or leverage the sharing
of energy, existing studies in the literature can be divided into three
domains: building domain, storage domain, and renewable domain. Each domain
has its own distinct characteristic and conditions for participating in P2P
sharing. For example, what kind of prosumer in each domain can participate in
P2P sharing and what are the conditions they need to conform to implement P2P
sharing within the domain. Thus, the proposed domain-specific discussion would
help the reader to understand the challenges and conditions that each prosumer
of the domain needs to address for participating in P2P sharing. A graphical
demonstration of these different domains in the energy network is shown in
Fig. 5.

Figure 5: A graphical presentation of different domains in which significant
advancements have been reported in the literature over the past few years. In
the building domain, the main focus has been on building management, co-
generated electricity and heat management, and human-centric management. In
the storage domain, residential storage domain dominates the literature.
However, community storage and mobile storage have also received some
attention in recent times. In the renewable domain, solar, wind, and hydrogen
have been the foci of discussion.

###  III-A Building domain

To participate in P2P energy sharing in this domain, at least some buildings
within the connected community should have enough provisions to produce energy
to be shared by the community members. For example, a building can be equipped
with DER such as storage and rooftop solar from which it can use the generated
energy to participate in P2P sharing. However, having storage and renewable
generator is not mandatory for buildings to produce energy. A building may use
other DERs such as flexible loads, interruptible appliances such as HVAC and
lights, and electric vehicles to control its energy consumption and thus
reduce demand to share either watt or negawatt with the participants. However,
a key condition that each building needs to fulfill before utilizing the
flexibility for energy sharing is customer preference and comfort. In other
words, energy management within the building should be human-centric to
confirm that human convenience and preferences are prioritized. Further, as
the use of fuel cells and combined heat and power is becoming increasingly
popular in the building, the P2P sharing platform should have enough
adaptability to accommodate this emerging energy system. Given this context,
existing studies in the building domain have focused on P2P sharing from three
different points of view: 1) building management, 2) co-generated electricity
and heat management, and 3) human-centric management.

####  III-A1 Building management

To enable buildings to participate in P2P energy sharing with one another, it
is critical that buildings can produce surplus through proper management and
scheduling of their flexible loads including heating, ventilating, and air-
conditioning (HVAC), lighting, and other adjustable loads. Consequently, in
the building domain, we focus on how the building management system (BMS) [72]
within buildings can create energy surplus or reduce energy deficiency in
order to enable the building to participate in P2P energy sharing [73]. For
example, the authors in [26] and [74] show how smart homes can control HVAC
and other flexible loads in order to participate in P2P energy sharing with
other smart homes within the connected community. Now, based on building
types, BMS finds its application mainly in two kinds of buildings: residential
building [75] and commercial buildings [76]. In both types of buildings,
energy management is done through the management of HVAC control [77],
lighting control [78], and scheduling of flexible loads [79].

##### HVAC control

HVACs are one of the major contributors to energy demand and, at the same
time, have great potential to help save energy and provide energy services
[80] such as P2P sharing. To manage HVAC systems within buildings, a number of
strategies have been reported in the literature including controlling the set-
point temperature and limiting the air distribution and cooling system of the
HVAC system.

_Set-point temperature control:_ The set-point temperature control of the HVAC
system within buildings is motivated by ASHRAE 55-1992 [81] guidelines and is
done by adopting short-term curtailment [82], departure and arrival
preparation [83], pre-cooling [84], modified pre-cooling [85], programmable
thermostat adjustment [86], and temperature reset [77] approaches. Further,
artificial intelligence-based control schemes such as in [87], [88], [89], and
[90] have also been used extensively for the set-point temperature control of
HVAC systems.

_Systematic adjustment:_ The systematic adjustment of HVAC systems is achieved
by placing limits on air distribution and cooling system equipment in the HVAC
system [80]. Although it is difficult to re-balance the system, e.g. due to
the imbalance of cooling (chilled water and supply air) distribution to
individual zones of a building, via systematic adjustment, it provides a fast
power reduction performance.

##### Lighting control

Lighting is the third-largest electricity consumer in both commercial and
residential buildings [91]. Thus, like the HVAC, lighting control also has the
potential to create enough surplus or demand flexibility for buildings to
trade in the local energy market. With the advancement of IoT and sensor
technology, a number of energy savings techniques have been introduced for
building lighting control.

_Occupancy-based strategy:_ With proper occupancy-based lighting control, the
energy usage for lighting can be reduced by between 20% and 60% compared to
the case without control [92]. As such, a large number of studies have
occupancy as the key decision-making parameter to decide the lighting status
of a building. The authors in [93] utilize an integrated room automation
technique to control the lighting of an office building using the occupancy
information in the control scheme. In [94], LightLearn - a reinforcement
learning-based lighting control scheme - is developed using occupant
preferences that successfully balance occupant comfort and energy consumption.
Similar examples of occupancy-based lighting control within buildings can also
be found in [95] and [96].

_Sunlight-based strategy:_ For buildings receiving sunlight, lighting control
schemes based on the availability of sunlight can provide the maximum amount
of savings. In [97], the authors propose a distributed lighting control
mechanism that takes daylight and occupancy status of the building into
account, whereas [98] has developed a fuzzy logic controller for saving energy
in smart LED lighting systems considering sunlight within the space. Further
studies that have focused on lighting control in buildings can be found in
[99].

##### Flexible load schedule and control

The term _flexible load_ refers to the load within buildings that is
controllable and can be scheduled to operate at different times of the day and
night, and therefore is a key factor behind the ability of a building to
participate in P2P sharing. Examples of flexible load include washing
machines, dishwashers, hot water pumps, and electric vehicles (EV). Flexible
loads are required for a building to participate in demand response, for
example, to participate in the P2P market [74] and to provide demand
flexibility to the grid [100]. The scheduling and control of flexible loads
are usually done either via direct [101] or indirect control [102]. Direct
control schemes are mainly incentive-based programs, in which there is an
agreement between the customers and the service provider that provides the
program coordinator with some degree of access to directly schedule, reduce or
disconnect flexible loads within buildings, and building owners receives some
incentives in return for agreeing to provide the coordinator with access to
the building. Examples of direct control load programs include interruptible
load control [103], interruptible tariffs [104], demand-bidding program and
emergency program [105].

Indirect control schemes, on the other hand, rely on different pricing, where
customers are offered time-varying rates that reflect the value and cost of
electricity at different times during the day [106] and are thus encouraged to
individually manage their loads either by reducing their consumption or
shifting their energy activities from peak periods to off-peak hours [107] to
create enough flexibility to participate in P2P sharing [28, 20]. A collection
of research papers that have investigated this particular issue of load
scheduling and control can be found in the review article [108].

A demonstration of how different energy savings can enable buildings to
participate in P2P sharing is demonstrated in Fig. 6.

Figure 6: A demonstration of how buildings can be incentivized by the grid and
energy service providers to participate in P2P sharing by managing their
flexible loads. For example, in response to an incentive signal from the
retailer, a building can control its flexible loads, HVAC, and lighting
systems to produce some surplus energy to share with other buildings within
the connected community. This figure is inspired from the scheme proposed in
[28].

####  III-A2 Co-generated electricity and heat management

Despite relatively high system costs, the use of fuel cells and combined heat
and power (FC-CHP) is becoming popular in residential housing due to high
thermal and electricity efficiency, low emissions, and reduced electricity
bills. As a result, FC-CHP co-generation system is becoming another popular
DER for local generation and consumption. As P2P sharing has been established
as a very promising technique for residents in connected communities to reduce
their cost and improving system performance in terms of balancing local supply
and demand, households with FC-CHP systems are also expected to share their
co-generated heat and electric energy with neighboring households. For
example, in [109], the authors propose a mixed-integer linear programming
approach to determine an operational strategy for a power interchange using
multiple residential solid oxide fuel cell cogeneration systems for saving
energy. An energy management system is developed in [110] that achieves
optimal operation of FC-CHP systems with energy exchange between households
within a connected community. An optimal strategy for managing multiple heat
sources in a connected community is proposed in [111]. Finally, a distributed
energy management system is developed in [112] that optimally schedules
multiple combined heat and power systems in a connected community.

####  III-A3 Human-centric management

Human-centric or prosumer-centric [113] management can be defined as energy
management schemes that consider human convenience and preferences as a
priority and produce outcomes, which ultimately benefit the prosumers. Within
the building domain, human-centric management schemes are closely aligned with
HVAC control, lighting control, and flexible load management and control with
a significant focus on human behavior and convenience. For example, a number
of human-centric building management schemes have been studied in [114, 115,
116] and [117]. In [114], the authors propose a framework of a ubiquitous
thermal comfort assessment for energy-efficient HVAC using RGB video images of
human thermoregulation states. The same authors also study a comparative
assessment of HVAC control strategies using personal thermal comfort and
sensitivity models in [115]. In [116], a set of algorithms is proposed for
controlling HVACs of a group of residential houses that a demand response
aggregator can use to sell regulation service in the wholesale market
consiering comfort requirements of the households.

In addition to human-centric appliance control within buildings for demand
response that subsequently produces resources for trading in the P2P market, a
number of studies are also proposed in recent times with human-centric
outcomes that influence building owners/occupants to participate in the P2P
sharing. For example, in [6], the authors show how P2P trading can be
exploited to incentivize prosumers to coordinate with one another to produce a
federated power plant in the smart grid. The authors in [8], [70], and [118]
utilize motivational psychology to demonstrate the effectiveness of p2p
trading in motivating building occupants to put their renewable resources in
the market for sharing. More examples of similar techniques of human-centric
P2P sharing schemes can be found in [38] and [119].

###  III-B Storage domain

In this domain, prosumers rely on their storage to participate in P2P energy
sharing. A challenging aspect of P2P sharing in this domain is the management
of storages of different size and ownership. For example, residential users
have small-scale storage, whereas, at the community level, medium or large-
scale storage is in use. With the emergence of electric vehicles, charging and
discharging of a large volume of mobile storage devices also have an impact on
the decision-making process of the prosumers. So prosumers should have access
to an intelligent battery storage management algorithm that can enable
prosumers to decide on how to prioritize between different type of storage
within the connected community and condition upon the energy price, demand,
and available energy in the storage and from other sources - rooftop solar,
for example - coordinate the charging and discharging of battery for P2P
sharing to maximize the overall benefit for the community. Another challenge
is the space that is required for the installment of storage, in particular,
for community storage. It might not always feasible to install storage within
connected community for space and cost constraints. Nevertheless, with
storage, prosumers can benefit from its opportunistic utilization in the P2P
market via enjoying better return of investment by selling the energy at peak
pricing period. They can take advantage of their battery to become completely
off-grid, if needed, in case of emergency such as hurricane or bushfire. As
such, to demonstrate how existing studies have captured these conditions and
benefits of using storage in the P2P market, we have classified the research
in the storage domain into three sub-domains: residential energy storage,
community (shared) storage, and mobile storage.

####  III-B1 Residential energy storage

The majority of research in P2P energy sharing focuses on the participation of
residential prosumers in the market. Consequently, in the storage domain, most
of the existing studies captured how various residential prosumers can utilize
their storage devices in the P2P market. Considering who is the beneficiary of
P2P sharing, the existing research in the residential storage domain can be
divided into two categories. The first category of studies focuses on the
benefit to the prosumers whereas the second kind of studies demonstrate how
the grid can be benefitted as an outcome of the sharing.

##### Prosumers benefit

By participating in P2P sharing, a prosumer can reap economic benefits. For
example, in [120], it is shown that a household can increase its savings by up
to 28% when it involves both its PV and storage in energy sharing. As a
consequence, a large number of studies including recent articles like [121,
122, 123], and [124] have shown how prosumers can engage their residential
storage devices in P2P sharing. In [121], the authors propose a two-stage
control of prosumers PV-battery systems in a community microgrid to
demonstrate an overall savings of the community of 30%. To engage prosumers
P2P sharing, a framework is proposed in [122], which shows that combined P2P
trading and battery storage can lead to savings of almost $60\%$ compared to
the case without P2P trading. Another mechanism is proposed in [123] to enable
prosumers to participate in day-ahead P2P energy trading, which uses a bi-
level optimization-based bidding strategy. The concept of energy loan is
introduced in [124] that can increase the system efficiency and social welfare
of P2P sharing.

While the use of residential storage can benefit the prosumers significantly,
battery storage is also very expensive. The authors of [125] compare the costs
of standalone battery systems versus the potential energy uses that could be
curtailed to avoid those costs when going off grid, and find that in both
cases these costs are significant, especially compared to striking P2P energy
sharing agreements. In addition, extensive charging and discharging could
damage battery life [126]. Therefore, it is important for prosumers to decide
whether to invest in their own storage or share community storage. Further,
they also need to determine the most effective way for their battery storage
devices to participate in P2P sharing. In [127], the authors show that while
owning a battery could be economically beneficial, community storage offers
the prosumers an opportunity to engage in energy sharing and to reduce energy
cost without any investment cost. Meanwhile, [113] proposes a P2P energy
sharing technique that enables prosumers to opportunistically use their
battery energy for trading when the utility of trading is at a maximum.

##### Grid benefit

The grid can also exploit P2P energy sharing within the energy network to
reduce its peak demand and balancing supply and demand within a community
without compromising network security and loss performance [12, 128]. In [28],
a cooperative game theory-based hierarchical energy sharing mechanism is
proposed in which the grid can interact with a community of prosumers that
perform P2P sharing to reduce their peak demand. Similar exploitation of P2P
sharing for peak load shaving is also discussed in [129]. For balancing demand
and supply, prosumers can be incentivized to employ their distributed
resources such as PV and battery storage [130], coordinate their energy usage,
and then buy and sell orders accordingly to balance the demand and supply
within a community [31]. For this purpose, the role of residential storage to
provide flexibility is critical [5]. Examples of P2P mechanisms to balance
demand and supply within communities can be found in [38] and [131].

####  III-B2 Community (shared) storage

Community storage, which is both a technical and social innovation, is
expected to contribute positively to building communities primarily using
renewable energy resources while accommodating the needs and expectations of
the prosumers within local communities [132], while reducing the operating and
capital costs of low and medium networks [133, 134]. After a comparison of
batteries within 4500 households in 200 communities, it is reported in [135]
that community batteries are more effective in promoting PV integration within
communities and promise more benefits compared to household storage systems
[136]. A techno-economic study to improve the feasibility of adopting
community storage in the UK is reported in [137].

Consequently, the use of community storage in P2P sharing to establish both
individual and communal benefits has received much interest in recent times.
For example, in [138], a community-based P2P sharing mechanism is proposed in
which a centralized entity such as a community manager coordinates the sharing
of storage among various prosumers based on their reputations in the
reallocation of available energy in the shared storage. Energy management and
optimal storage sizing technique for a shared community using a multi-stage
stochastic approach is studied in [139]. A trilateral planning model for
community storage using a bi-level stochastic programming approach is designed
in [140] which makes a joint optimal photovoltaic (PV) and energy storage
system plan with a view to maximize prosumers benefit. In [141], a scheme for
multi-resource allocation of shared storage is proposed using a distributed
combinatorial auction approach with the purpose to reduce the electricity
bills of the consumers. Finally, a detailed discussion on equilibrium prices
for shared storage in a spot market, technology platforms necessary for the
physical exchange of power, and market platforms necessary to trade
electricity storage can be found in [142].

####  III-B3 Mobile storage (electric vehicle)

Electrification of road transportation is an essential part of alleviating the
impact of greenhouse gas on climate change. With much innovations in the
design and technicalities of electric vehicles (EV) [143, 144], also known as
mobile storage [145], large penetration of EVs on the road have demonstrated
the potential to support the grid in terms of providing demand flexibility
[146] and frequency regulation [147] through their charging and discharging
capacities. Additionally, due to the capability of storage mobility, EVs are
considered as promising elements for P2P energy sharing [148] where power can
be transferred from the battery of one EV to the battery of another EV [149].
The possibility of EVs to be connected together for sharing energy is further
enhanced by the recent advancement of artificial intelligence in the EV domain
[150].

As a result, several studies have reported different mechanisms to execute P2P
energy sharing among EVs. For instance, a distributed EV power trading model
based on blockchain and smart contract is proposed in [151] to realize the
information equivalence and transparent openness of power trading. A similar
technique of smart contracts based proof-of-benefit consensus protocol is also
used in [152] to design a P2P energy sharing mechanism for EVs to balance
local electricity demand within the community. A multi-objective techno-
economic environmental optimization is proposed in [153] to enable EVs for
energy services such as P2P trading where the EVs much need to cooperate
together to achieve the overall social benefit. In [68], a consortium
blockchain-based energy trading mechanism is implemented to enable plug-in EVs
to share energy in a localized P2P market. Finally, an operational framework
is developed in [154] for peer-to-peer (P2P) energy trading between an EV
charging station and a business entity equipped with a solar generation unit
that outputs significant benefits for both parties compared to having sole
agreements with the utility.

TABLE II: Summary of advancement of P2P energy sharing schemes in different domains of the energy system. Each domain is further divided into sub-domains and an overview of existing studies in those sub-domains are outlined. Domain |  Sub-domain |  Overview of the sub-domain |  References
---|---|---|---
Building |  Building management |  To develop an efficient building management system, which can be utilized to control HVAC, lights, and flexible loads to create energy surplus or reduce energy deficiency in order to contribute to P2P energy sharing. |  [72, 73, 26, 74, 75, 76, 77, 78, 79, 80, 77, 87, 90, 91, 95, 96, 98, 100, 101, 102, 105, 106, 107, 108, 155, 156, 157]
Co-generated electricity and heat management |  To develop P2P sharing scheme for households with FC-CHP systems in connected communities to reduce their cost and improving system performance in terms of balancing local supply and demand. |  [109, 110, 112]
Human-centric management |  To humanizing the design of HVAC, lighting, and flexible load management based on human behavior and convenience to encourage building owners to participate in P2P sharing. |  [113, 114, 115, 116, 117, 6, 8, 70, 118, 38, 119]
Storage |  Residential storage |  To investigate the use of P2P sharing of residential storage to benefit prosumers and the grid. |  [120, 121, 122, 123, 124, 127, 113, 28, 129, 130, 31, 5, 38, 131]
Community storage |  To understand the use of community storage in P2P sharing to establish both individual and communal benefits in the smart gird. |  [132, 135, 136, 137, 138, 139, 140, 141, 142]
Mobile storage |  To demonstrate the application of mobile storage, that is EV, to support the grid in terms of providing demand flexibility and frequency regulation and enable prosumers to reduce energy and transport-related costs. |  [143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 68, 154]
Renewable |  Solar |  To design P2P energy sharing schemes focusing on the participation of residential households in reducing energy cost, balancing supply and demand, reducing peak load, and managing network loss. |  [158, 113, 159, 38, 160, 121, 161, 16, 162, 163, 39, 7, 28, 6, 5, 164, 11, 165, 166, 167, 168, 169, 170]
Wind |  To develop mechanisms that can be used for wind energy to be shared among participants within connected communities. |  [171, 172, 173, 174, 122, 175]
Hydrogen |  To explore the opportunities for P2P hydrogen sharing in the energy network that includes fuel cell vehicles, energy storage, combined heat and power system, and renewable energy. |  [176, 177, 178, 179, 180]

###  III-C Renewable domain

In general, solar, wind, and hydrogen are becoming popular to produce
renewable energy for usage within the community. The benefits of using
renewable energy for P2P sharing such as cost reduction, demand-supply
balance, and peak reduction are well established. However, to enjoy these
benefits, at least some prosumers within the connected community should be
able produce surplus energy to be shared by other participants. Further,
sharing renewable energy in P2P decentralized market without any coordination
can compromise the operation of the network within its technical limit. Hence,
prosumers’ transactive meter should have the capacity to decide how much
energy it can push to the P2P network at any given time without compromising
the network security. Nonetheless, sharing renewable energy is the most
popular and well investigated P2P sharing mechanism in the literature. It
offers benefit to prosumers to enjoy low cost clean energy without investing
in energy storage. Meanwhile, the advancement in inverter technology has
further opportunity to benefit prosumers economically by enabling them to
provide regulation services to the grid. In this section, we discuss existing
P2P sharing techniques for three different renewable sources: solar, wind, and
hydrogen.

####  III-C1 Solar

As discussed before, most existing literature on P2P energy sharing focuses on
the participation of residential households, which are usually equipped with
rooftop solar panels. As a result, in the renewable domain, most P2P sharing
mechanisms are developed considering solar as the main source of energy. In
particular, the use of solar in P2P trading has been extensively used for
reducing energy cost, balancing supply and demand, reducing peak load, and
managing network losses.

##### Cost reduction

Reducing energy cost is possibly the most important characteristic of P2P
sharing that encourages prosumers to install rooftop solar and participate in
the local energy market. However, how much cost prosumers can save relies on
the energy sharing price and strategy of each participating prosumer [158].
For example, in [113] the authors proposed an opportunistic energy sharing
mechanism using solar panel and batteries that helps prosumers to reduce their
energy costs. A multi-leaders and multi-followers based P2P sharing model is
proposed in [159] that reduces the cost of buyers by 4.36% while improves the
benefit of sellers by 12.61% compared to the feed-in-tariff scheme. It is
reported in [38] and [160] that extensive engagement of prosumers in P2P
sharing is the key to reduce energy cost, which can further be improved by
additional inclusion of energy storage devices [121, 161].

##### Supply-demand balance

Within a connected community, it is critical that some prosumers have energy
generation capacity to enable prosumers to participate in energy sharing with
one another. Now, to reap the maximum economic benefit, it is important that
the local generation balances the local demand, which is generally monitored
through distributed ledger techniques for P2P sharing [16]. For example, in
P2P sharing, prosumers can monitor their own energy generation and demand
through smart meter [162], access the energy offered by other prosumers for
sharing within the market through distributed ledger [163], and then
subsequently create the buy and sell order to share with one another within
the community [39] to balance the supply and demand. However, if there is a
deficiency of energy, it can be supplied by the grid [7], community storage
[135], or diesel generators [181] with relatively higher cost.

##### Coordinated peak demand reduction

An important service that can be provided by prosumers to the grid by
participating in P2P sharing is peak demand reduction. Several studies have
reported techniques that have proven effective to reduce peak demand. For
example, in [28], the authors propose a cooperative game theory-based P2P
sharing scheme that helps a centralized power system to reduce the total
electricity demand of its customers at peak hours. A federated power plant
using P2P energy sharing platforms is introduced in [6], which incentivizes
prosumers to shift their loads aware of predictable peak demand periods. In
[5], the authors show how adjusting the structure of demand tariffs can reduce
their contribution to peak demand. Further applications of P2P energy sharing
in reducing peak demand can be found in [164].

##### Network loss management

P2P energy sharing can incur additional power loss within the system due to
the transacted electrons by different prosumers to the network [11]. For
example, the impact of P2P sharing on the change in network power loss is
discussed in [165]. The cost of this network loss needs to be distributed. As
such, in [166], the authors propose a graph-based scheme for allocating cost
among the participating prosumers. Fees are used by the system operator in
[167] to allocate market-related grid costs to the participants. Further, an
optimal power routing strategy is used in [168] to optimize the power dispatch
by different prosumers with the objective of minimizing the power loss ratio
between the buyers and sellers within the network, while[169] introduces
energy classes to treat energy as a heterogeneous product and coordinate P2P
sharing to minimize the cost of network losses. As an alternative, [128]
investigate how a P2P trading mechanism can be designed to bias the matching
of electrically close peers, in order to reduce losses.

####  III-C2 Wind

Compared to solar, the number of studies covering P2P sharing of wind energy
is relatively small. This is mainly due to the fact that residential houses
usually do not install wind turbines to produce onsite energy. Wind turbines
are generally used as small or medium scale wind farms within microgrids
[171]. However, some mechanisms have been reported that can be used for wind
energy to be shared among participants within connected communities. For
example, in [172], a stochastic decision-making framework is designing in
which a wind power generator can provide some required reserve capacity from
demand response aggregators in a P2P structure. The proposed framework is
formulated as a bilevel stochastic model incorporating the capability to
assess risk associated with wind power generator’s decision and to determine
the effect of scheduling reserves on the profit variability.

In [173], a distributed control methodology is proposed to control the power
outputs of deloaded wind double-fed induction generators under dynamical
conditions and through peer-to-peer information exchange. P2P energy trading
for enhancing the resilience of networked microgrids with wind turbines is
studied in [174]. After defining a community of houses containing a mix of
prosumers (with a PV and/or wind power installation) and consumers, a two-
stage stochastic model of P2P sharing model is proposed in [122]. The authors
have verified the proposed approach and assessed its economic potentials.
Lastly, a critical P2P vision of renewable energy including wind energy to
describe the relationship between energy transitions and social change, and to
offer one plausible socio-cultural vision of the era of renewable energy is
discussed in [175].

####  III-C3 Hydrogen

With the establishment of the Paris agreement on November 4, 2016, it is
expected that the utilization of hydrogen will not only enhance the
sustainability and reliability of the energy system but also improve the
system’s flexibility [176]. While today’s energy sector is heavily dependent
on fossil fuel, hydrogen can play a pivotal role in the future to establish a
low carbon economy by linking various layers of energy transmission and
distribution infrastructure [176]. For example, to improve the absorption of
renewable energy, hydrogen is already used for small scale residential
storages (fuel cells) and transportations. In [177], the results from a
demonstration project on a zero-energy residential building are discussed,
which consist of both solar panels and hydrogen fuel cell EV for combined
transport and power generation. According to [177], the project has
demonstrated a reduction of energy import from the grid by 71% over one year,
which is an excellent outcome towards achieving a net-zero carbon energy
system.

As hydrogen has begun to penetrate to the energy system, researchers have also
begun to explore the opportunities for P2P hydrogen sharing in the energy
network. In [178], a multi-agent management framework including fuel cell
vehicles, energy storage, combined heat and power system, and renewable energy
is proposed for energy sharing among multiple microgrids. The main purpose is
to schedule the arrangements of fuel cell vehicles to improve the local
absorption of renewable energy and enhance the economic benefits of
microgrids. A P2P home energy sharing technique is studied in [179] to find
the optimal size and operational schedule for hydrogen storage and solar
systems incorporated within homes. In addition, [180] develops a local energy
market framework, in which electricity and hydrogen are shared. Participants
in the market consist of renewable distributed generators, loads, hydrogen
vehicles, and a hydrogen storage system operated by an agent and an iterative
market-clearing method is designed where participants submit offers/bids with
consideration of their own preferences and profiles according to the utility
functions. It is shown that the proposed clearing process avoids complex
calculations and preserves players’ privacy.

A summary of the advancement of P2P energy sharing in different domains of the
energy network is given in Table II.

##  IV Pilot projects around the world

To demonstrate the effectiveness of P2P energy sharing in the smart grid, a
large number of pilot projects are being trialed in different parts of the
world. In particular, countries in North America, Europe, Australia, and Asia
are heavily engaged in studies under various testbed settings. To that end,
what follows is an overview of different projects on P2P energy sharing in
four continents of the world. For each continent, we provide some detail of
one particular project and followed by a summary of other projects on P2P
sharing.

####  IV-1 North America

In North America, almost all P2P projects are based in the USA. For example, a
P2P project can be found in Brooklyn in the Brooklyn microgrid testbed, in
which prosumers within a community can trade their on-site produced energy
with the neighbors by using the typical power network [29]. The Brooklyn
Microgrid project consists of a microgrid energy market in Brooklyn, New York,
where participants are located across three distribution grid networks. Severe
weather events (e.g. hurricanes, heat waves, etc.) increase, which raises
operation issues of the (already) outdated electrical grids in Brooklyn. In
this project, the consumption and generation data is transferred from the
prosumers Transactive Grid smart meters to their blockchain accounts to create
buy and sell orders. Orders are sent to the market mechanism which is
sustained by a smart contract. When buy and sell orders match, payment is
carried out and a new block with all current market information is added to
the blockchain. This microgrid is helping the grid to reduce the impact of
grid issues through complete decoupling and control the energy supply within
the community in the event of natural calamity. Further, it is also helping
the grid to accommodate the growing number of electric vehicles on the US road
[29].

Among other projects, a cloud-based software platform is used in the project
TeMiX [182] for decentralized network management purposes, which also enabled
automated energy transactions between peers within the community. Similar
solar implementation and software platform for trading surplus solar
generation between houses was trialed in Boston [183]. Another new project on
P2P sharing is going to begin in PowerNet’s headquarter in Florida, in which
Power Ledger’s xGrid platform will be used to trade solar power with neighbors
connected in its office park [184].

TABLE III: Summary of different projects on P2P energy sharing in North America and Europe. Continent |  Country |  Overview of the project |  References
---|---|---|---
North America |  USA |  A P2P energy sharing demonstration project in Brooklyn using the blockchain where prosumers share their energy using typical distribution network |  [29]
A cloud-based software platform TeMiX for energy trading that provide automated energy transaction and decentralized network management services |  [182]
Kealoha project has implemented P2P markets using solar generation where a software platform enable exchange of excess solar generation between houses |  [183]
Power Ledger’s xGrid platform will be used in American PowerNet’s headquarters to trade solar power with neighbors connected in its office park |  [184]
Europe |  Germany |  Share&Charge is a decentralized blockchain based market for EV charging, transactions, and data sharing |  [185]
Peer energy cloud is a cloud based local energy platform for local energy trading and smart homes |  [186]
Sonnen community considers storage and storage systems to provide a platform for virtual energy pool |  [187]
Netherlands |  Powerpeers is a blockchain based market that has enabled residential buildings to share energy with one another |  [188]
Vandebron helps electricity consumers to select local sustainability produces of their choices for energy purchase |  [189]
Norway |  EmPower is a local energy trading platform in which prosumers can share their energy with one another in a local energy market |  [190]
UK |  Piclo is a software platform for selling and buying smart grid flexibility services and trading energy among peers |  [191]
Finland |  SmartTest is a smart energy system with the consideration of information and communication technology and P2P sharing of resources |  [192]

####  IV-2 Europe

Based on the number of projects on P2P sharing, undoubtedly Europe is leading
the world with a number of demonstration trials in Germany, Netherlands,
Norway, Finland, and the UK. For instance, three projects on P2P sharing are
available in Germany. _Share &Charge_ is a blockchain energy market for EV
charging transactions, and data sharing [185]. _Peer energy cloud_ [186] is a
cloud-based platform that enable local energy sharing by considering
Information and communication technology and P2P approaches. Sonnen Community
in Germany considers solar and storage systems to create a virtual energy pool
[187]. Meanwhile, in the Netherlands, two projects on P2P sharing are running
at this moment including 1) a project - known as _Powerpeers_ \- for
residential buildings to share their energy with one another using a
blockchain-based energy market [188] and 2) _Vandebron_ \- a platform for
electricity consumers to select desirable local sustainability producers
[189]. In Norway, _EMPower_ provides a trading platform for local energy
exchange between prosumers in a local market [190], whereas _Piclo_ is a UK
based software platform for selling and buying of smart grid flexibility
services and P2P energy trading [191].

Lastly, _P2P-SmartTest_ in Finland demonstrates a smart grid based transactive
energy concept to perform P2P energy sharing [192]. The objective of the
P2P-SmartTest project is to investigate and demonstrate a smarter electricity
distribution system integrated with advanced ICT, regional markets, and
innovative business models. The developed P2P approaches ensure the
integration of demand-side flexibility and the optimum operation of DER and
other resources within the network. Meanwhile, they are also capable of
maintaining a second-to-second power balance and the quality and security of
the supply. A part of the project also develops and demonstrates the
capability of the distributed wireless ICT solutions in offloading the
required traffic of different applications of energy trading, network
optimization, and real-time network control. For proper operation of the
distributed network, the project integrates the necessary network operation
functions for resilient distribution system operation. Table III summarized
the projects on P2P sharing in North America and Europe.

####  IV-3 Australia

With extensive government subsidy from both Federal and State governments, for
example, see [193], P2P energy sharing has also gotten significant momentum in
Australia. With the establishment of Power Ledger, in particular, P2P energy
sharing pilots are being demonstrated in several areas in Australia. For
example, in RENew Nexus project, Power Ledger has conducted trials with
different households, where the households trade excess energy generated from
rooftop solar panels with their with neighbors using the existing electricity
network and retailers [194]. It is an on-going project in Western Australia
with the purpose to understand the potential of localized energy markets and
how technology platforms can facilitate more efficient outcomes to the energy
system. The project ran in three parts. In part 1 - Freo 48 - a solar P2P
trial was run in two phases. Phase 1 ran for seven months in 2018-19 with 18
participants whereas Phase 2 initially had 30 participants (later reduced to
29) and ran from October 2019 to January 2020. In part 2 - Loco 1 - the
modeling of a residential Virtual Power Plant (VPP) was completed to better
understand the financial benefits that prosumers could realize from having a
battery installed and participating in a VPP. Finally, part 3 - Loco 2 - a
microgrid with a 670kWh shared battery system which will facilitate 36
households is currently under construction. This will trade excess energy with
each other via the battery. The purpose of these trials was (1) to demonstrate
proof of concept test for P2P electricity trading; (2) to understand the value
of P2P for customers and project partners; and (3) to trial the technological
interoperability between the Power Ledger platform, Synergy (energy retailer),
Western Power (network operator) and supporting technologies, and test the
Power Ledger platform capability as a client P2P solution. Other examples of
P2P projects in Australia are explained in Table IV.

TABLE IV: Summary of different projects on P2P energy sharing in Australia and Asia. Continent |  Country |  Overview of the project |  References
---|---|---|---
Australia |  Australia |  Power Ledger partnered with Nicheliving to deploy its energy trading platform to deliver 100% renewable energy at 62 apartments |  [195]
An Australian retail investment company uses Power Ledger’s content to trial trading solar energy within its shopping centers |  [196]
Energy retailer Powerclub will use Power Ledger’s VPP enabled platform to enable Powerclub households with batteries to sell their stored solar energy during peak demand period and price spikes |  [197]
RENeW Nexus involved two trials whereby 48 households used Power Ledger’s platform to trade excess energy generated from rooftop solar panels with their neighbours via the existing electricity network and retailer |  [194]
36 homes with rooftop solar in East Village, WA will use Power Ledger’s platform to trade solar energy with each other via the battery |  [198]
The Gen Y Demonstration Housing Project in WA uses Power Ledger’s blockchain platform to sell excess solar energy to neighbors at peak demand, rather than back to the grid |  [199]
In Wongan-Ballidu, Australia nine commercial sites will use Power Ledger’s P2P energy trading platform to monetize their excess solar energy |  [200]
The first P2P commercial project in the National Energy Market in the Australian Capital Territory, which will enable the customer to save on their energy costs and decrease consumption from fossil fuel sources |  [201]
In DeHavilland Apartments & Element47, Australia, Power Ledger will have its peer-to-peer (P2P) energy trading technology will be used to enable households to trade solar energy between one another |  [202]
Asia |  Japan |  Kansai Electric Power Co (Kepco) is leading a project that will enable solar power suppliers to sell extra electricity to consumers via a blockchain-enabled system |  [203]
KEPCO used Power Ledger’s platform to create, track, trade and provide a marketplace for the settlement of renewable energy credits or non-fossil certificates, generated by rooftop solar systems |  [204]
Blockchain-enabled peer-to-peer technology demonstration with households trading excess solar energy between each other in the Kanto region |  [205]
India |  A peer-to-peer pilot energy trading project in India’s Lucknow will demonstrate the feasibility of Power Ledger’s platform to trade energy from rooftops with solar power to neighboring households/buildings |  [206]
A large-scale desktop P2P energy trading trial across existing 300 kW solar infrastructure servicing a group of gated communities in Delhi’s Dwarka region |  [207]
Thailand |  Power Ledger, in partnership with TDED, will create a blockchain-based digital energy business developing peer-to-peer energy trading solutions in Thailand |  [208]
Thai renewable energy business BCPG and Thai utility Metropolitan Electricity Authority are using Power Ledger’s software for tracking and settling the energy generated from renewable sources and transactions between the participants |  [209]
South Korea |  Electron, a blockchain startup in UK, will test their own energy flexibility trading platform in South Korea to prove the benefits of flexibility trading in the South Korean market as it starts to decarbonize and decentralize. |  [210]
KEPCO will trial a blockchain-based peer-to-peer energy trading system in two apartments in Seoul and nine buildings within KEPCOs facilities to lower energy bills by enabling businesses and households to buy surplus electricity of their peers |  [211]
Singapore |  Electrify’s Marketplace 2.0 uses smart contracts where consumers can buy electricity from retailers or even from their own peers with the purpose to eliminate or reduce many of the fees and transaction costs with the automatic nature of smart contracts |  [212]
Malaysia |  A pilot trial aims to demonstrate the feasibility of solar energy trading in the Malaysian energy market by enabling consumers to choose whether they wish to purchase clean, renewable energy or power from fossil fuels |  [213]

####  IV-4 Asia

Asia is also not behind when it comes to demonstrate the effectiveness of P2P
energy sharing for energy producers and consumers. For example, in Japan, the
Australia-based Power Ledger made a partnership with Japanese solar provider
Sharing Energy and electricity retailer eRex to trial its P2P trading platform
in Kanto, Japan [204]. The trial aimed to show how a group of households can
trade excess solar energy between each other and follows a trial in Osaka with
Japan’s utility KEPCO which achieved consumer acceptance. In the trial, Power
Ledger’s platform was integrated with existing smart meter systems in homes to
enable participants to set prices and track energy trading in real-time. The
trial was scheduled to run until December 2019.

Although Japan, Thailand, South Korea, and India are leading the efforts with
more than one demonstration project, Singapore and Malaysia are also in the
race to contribute towards P2P demonstration through different pilots on
distributed energy sharing. A summary of different projects of P2P trading
that are currently being trialed in Asia is given in Table IV. Note that,
except P2P sharing, other relevant energy trading techniques such as
transactive control [214] and negawatt trading [27] are also trialed in many
parts of the world. For details of these demonstration projects, readers are
referred to [24], [16], and [215].

##  V Summary & Challenges

Based on the discussion in this paper, clearly, the benefit of P2P sharing to
both prosumers and the grid is well demonstrated. Prosumers can reduce the
cost of electricity while enjoying clean energy by participating in P2P
sharing. By encouraging community members to interact with one another to
share their energy and introducing provisions for energy donation, P2P sharing
improves social values within the members and help them to establish an
environment-friendly energy neighborhood. Previously, for example, in Feed-in-
Tariff only prosumers with renewable sources could enjoy the economic and
environmental benefits of energy trading. However, with P2P, prosumers with
any renewable energy status can have that luxury. Now, in some P2P markets,
prosumers can make their decision on not only the energy sharing parameters
but also whether or not to participate in energy sharing without any influence
from a third party. Thus, P2P empowers the prosumers and give them the true
independence of the energy they produce and manage.

Meanwhile, the grid can also benefit significantly from P2P sharing. For
instance, by reducing the demand, P2P sharing can help the grid to reduce its
investment for infrastructure upgrades to cope with the increasing energy
demand. This also opens the opportunity for the grid to accommodate new
emerging demand for energy from the electric vehicle with its current
infrastructure with a minimal upgrade. Now, building within a connected
community can provide demand flexibility to the grid through which the grid is
able to reduce its peak load demand and improve the security of the power
system by addressing the voltage and frequency disturbances through the
ancillary service market.

However, despite demonstrated benefit and ample opportunities, the
establishment of P2P sharing in today’s energy market relies on the decision
of the regulatory board. That is, the regulatory board determines how P2P
energy markets fit into the current energy policy. Thus, legislative rules
establish which market design is allowed, how taxes and fees are designed, and
in which way the market can be integrated into today’s energy market and
energy supply system. Thus, the government of country/region can easily
support P2P markets to enable the efficient utilization of local renewable
energy resources and decrease environmental degeneration by regulatory
changes, e.g. the introduction of subsidies. The regulatory board can also
discourage the implementation of P2P markets if they determine that the
subsequent result could have negative impacts on the current energy system.

Meanwhile, to present P2P sharing to the regulatory board as a viable energy
management option and convince them to allow it to practice in the current
energy market, a number of potential challenges that are yet to be addressed,
These challenges can be summarized as follows.

###  V-A Co-existence of different stakeholders

Of course, the research in P2P energy sharing has been extensive for settings
where prosumers are engaged in sharing energy among themselves with very
little (or, not at all) with little (or, no) interaction with the grid or an
aggregator. However, while this assumption is valid for the establishment of
sharing mechanisms in a small-scale setting, to successfully achieve
objectives of higher monetary benefit, supply-demand balance, incentivizing
prosumers, and ensuring secure transactions, large-scale development of energy
sharing requires to consider other existing participating stakeholders of the
network. Examples of such stakeholders may include generators, retailers, and
distribution network service providers (DNSP).

Now, the involvement of stakeholders with conflicting interests in using
prosumers’ energy makes the decision-making process of energy sharing a
complex task. For example, a conflicting scenario may arise between a retailer
and a DNSP serving the same set of prosumers when the retailer requests
prosumers to discharge their batteries to meet the excess demand of its
additional customers, whereas, at the same time, the DNSP may send a signal to
prosumers not to push any electron to avoid a potential network voltage
violation expected by the DNSP. Hence, there is need to develop techniques
that will prepare P2P energy sharing mechanisms for implementation in systems
where stakeholders like retailers, generators, and DNSPs co-exist with other
network elements without affecting each other’s interests and roles in the
network.

###  V-B Network constraints, losses, and management

As the number of prosumers participating in P2P energy sharing will increase,
the risk of voltage rise at various nodes of the power system network will
increase as well. Of course, one potential way to mitigate this risk could be
to regulate how much each prosumer can export to the network at any given time
slot. However, imposing such rigid restrictions on prosumers’ choices may
detrimentally affect the potential revenue that a prosumer would expect from
participation in the trading and therefore could cause prosumers to lose
interest in future participation. On the other hand, unchecked injection of
power from all prosumers will compromise the security of the network.

Another important issue is that P2P transactions may raise power losses across
the network. Therefore, the P2P price, which has been assumed to be
significantly more lucrative than other existing trading schemes in most
existing studies will need to consider this loss factor. This will increase
the price of energy buyers. Furthermore, a large number of renewable energy
plants are being connected to the network, which will allow the retailers to
sell energy at a much cheaper rate than before, which may also affect the P2P
price as well. Hence, how to allocate the loss factor within P2P price while
simultaneously being competitive need to address.

Furthermore, it is important to note that the flow of electricity cannot be
controlled. Therefore, it is not likely that for a very large network the
intended receiver will receive the actual power that has been sent to it by
the seller over the distribution network. As a consequence, the power loss due
to P2P sharing trading would be different compared to the case when the buyer
actually receives the power sent by the seller (e.g. if they are located side-
by-side). Hence, how to calculate the actual loss in this kind of scenario and
then decide the P2P energy sharing price needs further investigation.

###  V-C Post-settlement uncertainty

Although P2P sharing can offer significant benefits to its customer in terms
of different energy services and lower energy cost, the outcome of the sharing
could jeopardize the overall market structure and trust between the buyers and
sellers if the interaction and negotiation between prosumers are poorly
designed due to communication delay, insufficient forecasting, less visibility
and understanding of network condition, and lack of customer information. For
instance, assume a scenario where a prosumer commits a certain amount of
energy for a given time slot and gets paid for the committed energy from the
seller. However, due to a lack of accurate forecasting of its own demand and
generation, it is possible that the actual energy that is transferred to the
buyer is less than what was committed. Such a market outcome will surely be
suboptimal and not suitable for long-term sustainability.

Hence, before deploying in the energy market, the computation and
communication complexity issues must be resolved for the robust operation of
the system. Accurate forecasting techniques need to be employed and network
conditions for each P2P transaction periods need to be identified through
sophisticated algorithm. Importantly, prosumers need to be educated about the
importance of P2P sharing and the discloser of honest information about their
demand, generation, and energy commitment to the market for the sustainability
of the market.

###  V-D Low cost privacy and security

Finally, one significant importance of P2P energy sharing is the availability
of accurate and statistically useful energy transaction and usage data across
communities for better prosumer decision-making. However, such data sharing
could potentially compromise the privacy of individual participants.
Therefore, accessible data also needs to ensure that everyone’s private
information is safe. For example, a provably-private transformation of
prosumers energy data is needed to facilitate data accessibility while
simultaneously granting data enough statistical accuracy for interrogation of
data.

Furthermore, although P2P energy sharing provides a platform to engage
individual prosumers to share their energy with one another and with the grid,
without cryptography, this could also increase the vulnerability of the
network if one or more prosumers act as adversaries and plan to breach the
security of the system, for example, by injecting false information. At
present, the security of transactions in P2P energy sharing has been confirmed
through the use of distributed ledger techniques such as blockchain. However,
it is important to note that providing security via blockchain could be
computationally expensive and therefore very costly [216]. As such, there is a
need for methodologies and techniques that will confirm the security of the
network, not by integrating expensive measures, but enabling the trading
decision in such a way that the security of the transaction is preserved.

##  VI Conclusion

This review article has provided an overview of existing peer-to-peer energy
sharing literature in order to identify recent progress in this area of
research as well as to discover the challenges that are preventing peer-to-
peer sharing to become a viable energy management option in today’s
electricity market. As such, first, we have added background on the connected
community, peer-to-peer energy sharing systems, peer-to-peer energy markets,
and distributed ledger technology for readers to easily follow the rest of the
discussion in the paper. Then, we have discussed recent advancements in peer-
to-peer energy sharing research in different domains including building
domain, storage domain, and renewable domain. Following the discussion on
advancement, a detailed list of existing trial projects on peer-to-peer energy
sharing in North America, Europe, Australia, and Asia has been provided.
Finally, we have identified a number of challenges that are need to be
addressed before deploying peer-to-peer sharing as an energy management
technique in today’s electricity market.

## Acknowledgement

This work was supported in part by the Queensland State Government under the
Advance Queensland Research Fellowship AQRF11016-17RD2, in part by the
University of Queensland Solar (UQ Solar; solar-energy.uq.edu.au), and in part
by the SUTD-MIT International Design Centre (idc; idc.sutd.edu.sg), in part by
the U.S. National Science Foundation under Grants DMS-1736417 and
ECCS-1824710, and in part by the Engineering and Physical Sciences Research
Council (award references EP/S000887/1 and EP/S031901/1).

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