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Abstract—The growing regulatory environment and security concerns about fossil fuels are driving the research and development of new technologies that can contribute significantly to sustainable and resilient urban energy systems. Plug-in electric vehicle (PEV) is just one of those new technologies that meet current and future environmental and economic challenges of transportation. By charging from the electric power grid or renewables, aggregated PEVs can be deployed as dynamically configurable distributed energy storages (DESs) to supply additional power to home appliances, buildings, or the electric power grid when necessary. This growing PEV penetration coupled with the increasing consumers’ interaction with market operations are paving the way for decentralized electric power systems that promise to be efficient, reliable, flexible, economic, and environmentally friendly. This study surveys and summarizes the latest progress and advancement of PEVs, especially as mobile DESs in the areas of technologies, market development, policy, system impact and operations, and relevant pairing infrastructures in smart grid environment.

Index Terms—Demand response, distributed energy storages, distributed generation, plug-in electric vehicles, smart grid, survey, vehicle to grid.

I. INTRODUCTION

he increasing environmental concerns, electricity industry restructuring and deregulated market forces are driving

distributed energy resources (DERs) penetration into distribution systems [1] and [2]. The plug-in electric vehicles (PEVs) are actually one kind of such DERs, more specifically, in the form of distributed energy storages (DESs) that can be utilized for demand-side management, outage management as emergency back-up power supply, and asset management [3]. Since the PEVs directly run on electricity instead of traditional carbon-based fossil fuels, they are considered to be one effective and promising solution to reduce greenhouse gas emissions, promote energy efficiency, and decrease petroleum usage, reliance and price fluctuations [4]. Presently PEVs are treated as a new load for load serving entities that

Xianjun Zhang and Qin Wang are with the Midcontinent ISO, 720 City

Center Drive, Carmel, IN 46032-7574, USA (xzhang@misoenergy.org, qwang@misoenergy.org).

Guangyue Xu is with the Smart Grid Division, Siemens Industry Inc.,Minnetonka, MN 55305, USA. (guangyue.xu@siemens.com)

Ziping Wu is with the Department of Electrical & Computer Engineering, University of Denver, Denver, CO 80208, USA. (ziping.wu@du.edu)

could impact the load profile in distribution systems by causing voltage drop and losses [5]. However, with the on-board batteries it is envisioned that the PEVs could supply to electric power grid as ancillary services when necessary [6].

With the continuing development of battery technologies and price reduction, PEVs can be an economic choice for transportation and gradually displace traditional fossil fuel-based vehicles. The PEV can travel up to 160 km with a battery of 24 kWh recharged using a simple household outlet at 120/240 V and 15/30A or a fast recharge station at 240V [7]. The U.S. DOE estimates that there will be more than 1.2 million of PEVs by 2015 [8]. Vehicle charging standards have also been developed in [9]. Besides the policy initiatives to boost the sales of PEVs from governments [ 10 ], automobile companies have been devoting to the market commercialization of PEVs for the last ten years [11]. In the long term most vehicles might be fully battery-powered.

The advancement of PEV technologies drives the enhancement of system operations and infrastructures to meet this growing demand with economic efficiency and reliability requirement. The PEVs are emerging to be one indispensable component in composing the energy-wise, cost effective, sustainable and resilient urban energy systems, especially in smart grid environment. PEVs, chargers, grids, batteries and storages, renewable based distributed generation (DG), and control facilities, all of these compose microgrid or smart grid system. Consequently, traditional passive distribution systems are now on the way of growing into active systems [12].

Significant studies on PEV performance and impacts have been conducted in existing literatures [13-16]. This paper extends existing literatures of PEV review with the detailed survey and analysis of PEVs as dynamically DESs. This study was divided into several sections to investigate the superior characteristics of PEVs as DESs.

II. SMART PEV INTERFACE INFRASTRUCTURE

To develop the PEV market, a reliable interface infrastructure is required. Enough pairing residential plugs and public charging stations must be developed in large scale to meet the fast growing demand resulting from the penetration of increasing PEVs.

A. PEV Impacts on Urban Electrical Infrastructure

Either for residential plugs or public charging stations, the increasingly and geographically aggregated PEVs can have

A Review of Plug-in Electric Vehicles as Distributed Energy Storages in Smart Grid Xianjun Zhang, Member, IEEE, Qin Wang, Member, IEEE, Guangyue Xu, Member, IEEE,

Ziping Wu, Student Member, IEEE

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978-1-4799-7720-8/14/$31.00 ©2014 IEEE

2014 5th IEEE PES Innovative Smart Grid Technologies Europe (ISGT Europe), October 12-15, Istanbul

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significant impacts on urban electrical infrastructure or even the electric power grid if no appropriate control or uncoordinated strategies and policies were implemented. Existing local distribution feeders, branches, and transformers could thus experience unprecedented congestion and stresses.

The extent the deployments of PEVs impact urban electrical infrastructure depends on the interface characteristics including charger rating, battery capacity, charging power level, charging/discharging length, charging/discharging time as well as power inverters [17]. As shown in the literatures, these impacts include electrical infrastructure overloading, unbalanced load conditions, increased harmonic distortions and voltage deviations, utility equipment and customer appliances damage, additional investment on distribution reinforcements, and system stability and reliability impairments [5] and [18].

The increased loading current of PEV charging and current harmonics from renewable charging in transformer windings can cause abnormal transformer operation such as operating temperature rise, reduced efficiency, premature insulation, windings or core structure failure, etc. [7] and [16]. One study proposes to quantify the loss of life duration of power transformers [19]. The smart load management with high penetration of PEVs during peak hours has significant benefits in reducing transformer thermal loading and minimizing harmonic losses [20]. B. Bi-directional Communication System

Advanced metering infrastructure (AMI) and smart metering technology can be used to make the PEVs controllable load and facilitate the vehicle to grid (V2G) and renewables’ integration [20]. Advantages could be achieved through the building of a reliable and high speed bi-directional communication system to exchange information among PEVs and charging stations, and provide end users and power generators with the effective information regarding real time system dispatch and demand to facilitate the integration of increasing PEV penetration [21] and [22].

The communication network with broad bandwidth can transport essential information between PEVs and controllers for effective and efficient charging/discharging, selection of public charging points, achieving real time pricing information, and distribution network congestion conditions [23 ]. The communication architecture and protocols were investigated in [24] as the nexus between electric power grid, PEVs, charging stations, generation units, etc. The communication and control of PEVs were investigated with the integration of renewables in [25]. However, the industry-wide code or standards regarding the communication between PEVs and the grid have not been defined yet at this stage [23].

C. PEV Interface Infrastructure with Renewables Integration

Public charging stations could be built like the gas stations with similar functionalities. The present residential and commercial charging facilities are still insufficient because of limited availability and immature PEV market. For a 10 kWh

battery, about 5.5 hours are needed to fully charge in residential garages and commercial buildings or parking lots; 1 to 2 hours are needed in specific charging facilities; less than a half hour are needed in fast charging [21]. The battery chargers in PEV charging stations have many AC-DC power inverters with high ratings, resulting in significant current harmonics that lead to transformer outages and transformer aging [20]. This can significantly impact the reliability, security, efficiency and economy of smart grids. The PEV charging is controllable by altering the timing and level of charging and thus various objectives can be achieved, e.g. valley filling, grid frequency regulation, and grid support [6].

PEVs could be sustainable and resilient automobiles as DESs by charging from renewables and thus reduce the dependence on fossil fuel. A conceptual framework was proposed in [26] for designing the grid-interfaced system by integrating PEV chargers, DG and storage. The intermittent and variable characteristics of wind and solar power from renewable based DG add challenges to system operation and reliability under the large scale of PEV integration [27]. With necessary inverters equipped, PEVs could be utilized as premium medium to facilitate and transport the bidirectional power flow between vehicles, DG and electric power grid through the on-board batteries working as buffers and isolators. This kind of configuration could reduce the reverse power flow from the renewable based DG to the electric power grid and mitigate disruptive impacts to local distribution networks [28].

The on-board PEV battery storage is an essential solution for variable DG output, especially when dealing with reverse power flows, voltage rise, and fluctuation [29]. A hierarchical control algorithm was proposed in [ 30 ] to study the integration of controllable PEV charging and scheduling of wind energy. In [31], the PEV was used as energy storage for a house with PV modules installed, and the PV utilization rate, CO2 reduction, and cost reduction were calculated.

III. SYSTEM OPERATIONAL IMPACTS AND DEMAND RESPONSE

MECHANISMS

With the rapid development and significant penetration of PEVs, they are emerging to be a good solution to utilize the spare generation resources, and facilitate the system operation through demand response (DR) mechanisms.

A. PEV Impacts on Grid System Operation

In most of the time, generation resources are underutilized based on the fact that even though the peak demand lasts only for a few hours, the system must strike to meet this demand considering the system reliability [ 32 ]. Therefore, it is meaningful and challenge to sufficiently utilize generation resources and transmission constraints.

The charging/discharging of PEVs has to be controllable and adjustable rates and time scheduling have to be deployed to prevent the additional demand burden during the peak hours. Significant number of studies has been conducted on

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the impact of PEV charging on system operation [5] and [33]. Charging is generally dependent on the driving profile of individual customers, and the driving profiles can be generated from empirical data [34]. In an ideal model where partial loads are replaced with PEVs and these PEVs are fully controllable in charging, monthly electric bill should keep unchanged. However, research in [ 35 ] indicates that uncontrolled charging can increase the monthly electric bill by 22%, even with a 10% low PEVs penetration. Centralized coordination methods of EV charging were introduced in [36]. The simple dual time-of-use pricing schemes were investigated in [37] as a form of tariffs for PEV charging coordination. With the optimized charging and discharging, the aggregated PEVs can effectively shift part of the grid’s load peak to off-peak hours and enhance the overall effective load carrying capacity of the grid [38].

Nowadays the electricity in distribution systems is supplied by load serving entities to the end users with fixed retail price by periods, not like the real time wholesale electricity market, where the pricing information is updated every five minutes. If similar market was developed in distribution systems, they might be able to obtain relevant pricing information based on the provided historical pricing information and load forecast and thus make decisions on charging/discharging strategies [ 39 ]. Despite the charging stations are most probably operated by private enterprises for profit purposes, charging/discharging strategies and interface prices should be developed for PEV users to minimize the electricity cost in charging stations. The distribution substations are used to provide reliable electricity supply to the end users. A market mechanism needs to be developed for the PEVs to fill in the valley and chop the load peak with the coordination of distribution substation [39] and [40].

B. DR Mechanisms

Most DR mechanisms, e.g. time of use, pricing, and real time pricing bring economic incentives for PEV owners besides the benefits of fuel cost savings and emissions reduction [41]. DR mechanisms can also enhance robustness since the demand side management (DSM) enables the users to employ the energy more efficiently by interacting between participants such as wholesale markets, retail utilities and customers [21]. Here DSM differs from DR. DR encourages end users to make short term reductions in energy demand; while DSM encourages end users to use energy more efficiently by improving or upgrading their devices.

Based on tariffs, compel or incent PEV charging towards off-peak hours, DR (Type II), the real time pricing program should be designed for PEV charging. A good overview of existing DR mechanisms is described in detail in [42]. The DR and wind power integration were investigated in [27] with the penetration of PEVs. The joint use of DR schemes and traffic patterns or consumption data has been applied to the problem of charging batteries for PEVs [43].

The electric power system normally has the peak period

starting from 6:00 am to 10:00 pm while the rest hours are in the off-peak period. Most PEVs are charged between 18:00 pm and 6:00 am at home; while between 8:00 am and 17:00 pm this charging will occur in commercial areas and public charging stations, which coincides with the peak hours. During the off-peak hours, the PEV charging behavior should be incentivized with lower off-peak rates to minimize the bill cost and maximize the utilization of the spare generation resources that otherwise would not have been committed and dispatched. While during the peak hours, since the PEV charging coincides with the system peak load, this additional demand burden should be prevented for the system. Lower rates should be offered to compel PEV users to switch into charging in off-peak hours. Contrarily, the PEV discharging to the grid should be encouraged and incentivized with higher rates to provide ancillary service and relieve system tight conditions. A study in [38] was performed to investigate the effective load carrying capacity of PEVs to the grid.

When charging stations are based on the renewable generation resources, lower rates should be provided for the PEV charging in both peak and off-peak periods. Also, the PEV discharging in this case should be greatly promoted to increase the renewable energy utilization. However, it should be noted that no matter the charging stations are fossil-fuel based or renewables based, the PEV users should be informed of the real time variable charging/discharging prices by smart meters to maintain the equilibrium of market mechanism [44].

IV. MOBILE DISTRIBUTED ENERGY STORAGES

The advantages of DR mechanisms for PEVs have been proven in helping electric power grid shift the peak load. The increasingly aggregated PEVs could even more efficiently and economically supply electricity to the electric power grid.

A. On-board Batteries

Presently, costly batteries and limited driving range are constraining the market development of PEVs. However, the cost will reduce gradually with the fast development in battery technologies. The charging levels defined in Society of Automotive Engineers (SAE) J1772 [9] and [ 45 ] are shown in Table I. The distribution and fuel efficiency of vehicles are shown in Table II [46] and [47]. The Nissan Leaf PEV has a 24kWh battery with a Depth of Discharge (DOD) of 80%, which will be fully charged via Level 1 (1.44kW) in 13.3 hours, via Level 2 in 5.8 hours, and via DC fast charge (50 kW) in 23 minutes; the Chevy Volt PEV has a 16kWh battery with a 65% DOD, which will be fully charged via Level 1 for 7 hours, and Level 2 for 3 hours [35].

Currently, batteries, e.g. Pbacid, Ni-Cd, Ni-MH, Li-ion, etc., have been developed focusing on high power density, long cycles, fast recharging capabilities and low cost, with different charging methods depending on the type, capacity and other characteristics, namely, constant voltage, constant current charge, taper current charge, pulsed charge, etc. [48]. The load profiles of lithium-ion batteries have been studied

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based on optimization techniques by vehicle companies [49]. Despite PEV batteries can be utilized for voltage regulation and frequency regulation, battery life will be degraded due to frequent charging and discharging [50].

TABLE I CHARGING CONFIGURATIONS AND RATINGS

Charging Method Supply Voltage (V)

Maximum Current (A)

Maximum Power (kW)

AC Level 1 120 12/16 1.4/1.9 DC Level 1 200-450 80 36 AC Level 2 208-240 80 19.2 DC Level 2 200-450 200 90 AC Level 3 (TBD) 208-240 TBD >20 DC Level 3 (TBD) 200-600 400 240

TABLE II PEV ATTRIBUTES

Car Type % of PEV Vehicles

Battery Size (kWh)

Fuel Efficiency (Wh/km)

Nissan Leaf 50 24 173 Mitsubishi i-Miev 25 16 135 Chevy Volt 20 16 224 Tesla Roadster 5 53 110

Either at the public charging stations or residential garages, bi-directional charger and controller need to be equipped to allow battery charge/discharge when necessary. In [51], a bi-directional power converter (BPC) is introduced that can perform the demand-response service according to the received commands from system operators. Besides, the PEV charging/discharging, metering and control equipment need to be designed and operated at different kW levels as well.

B. Vehicle to Grid Operation

Interactions between vehicles and grids are described as grid to vehicle (G2V) and vehicle to grid (V2G), the benefits of using PEVs as distributed energy storage could be fully realized based on the on-board batteries. Various potential benefits and implementation issues of V2G have been investigated in [50], [52] and [53]. The PEVs can provide reactive power support to the electric grid by adjusting the power factor of vehicles through power inverters [20]. The PEVs can also be estimated and utilized to supply ancillary services such as regulation and stabilization for the electric power grid [53] and [54]. In [55], the V2G was investigated for its capability of dealing with the wind power fluctuation.

The increasingly aggregated PEVs can potentially promote the V2G transactions during peak hours and offer services of emergency DR and operational capacity planning. In addition, the complementing nature of travel behavior and system demand makes them suitable to provide ancillary services through V2G transactions during peak hours [3]. However, since the high speed bi-directional communication infrastructure has not been developed, the charging/discharging of such batteries poses new challenges to system dispatch due to the difficulties of remote control and coordination of such batteries [31].

Due to various practical concerns and reasons, wide deployment of V2G is not mature and still in the early stage of the market, which might be envisioned in the next 10 to 20

years. However, vehicle to building (V2B) operation could be feasible to serve the load inside a building in the next coming few years based on the relatively fewer technology requirements compared to directly connect to the electric power grid [56]. Despite the PEVs normally have a limited short driving range, typically less than 200 km after a full charge [ 57 ], the capacity of the on-board batteries are considered to be large enough to partially support the household electric consumption management [56].

C. V2G Development

The superiority of V2G regulation has been proved from the economic perspective in [6] and [55]. Based on the location and duration of parking at public charging stations or residential garages, a vision was presumed that the V2G technology could provide a high revenue stream for PEVs by supplying peak power, spinning reserves, and/or regulation [58]. The requirements to realize this vision include pairing charging/discharging equipment with grid control and kW levels, expected PEV discharging frequency and period capabilities for providing the required kW and/or kWh of electricity, and higher revenues to cover the costs of the provided battery services [55] and [58].

If the energy market was developed that allows the participation of PEVs, then the V2G capabilities of multiple PEVs will be aggregated and bid into the market [59] and [60] but every PEV will be treated as a single market participant. The applications of V2G for load shifting and frequency regulations have been studied from different aspects with promising results achieved [55], [58], and [61]. One further study also indicates that the AC/DC power converters used for charging PEVs can provide extra reactive power compensation and voltage regulation [62]. Unlike frequency regulation by PEVs, this voltage regulation does not affect the battery degradation and could be a potential way of gaining profit in a foreseen reactive power market [45] and [63].

V. APPLIED RESEARCH METHODOLOGIES

A variety of targeted methodologies were developed to analyze and investigate the impacts and development of PEVs as DESs from various aspects and disciplines. Monte Carlo methodology was used to investigate the PEVs, including the forecast of thermal loading on transformers and identification of the associated risk due to its stochastic characteristics [7], and evaluation of PEV impacts in distribution system [64]. Stochastic approaches could be a good way to fairly understand the impacts of PEV charging on distribution systems [65-67]. Optimization based approach was used to find the optimal scheduling of the PEVs’ charging, and maximize the vehicle owners’ benefit [ 68 ]. A linear optimization approach was developed in [26] to design grid interfaced PEV chargers, DG and storage by maximizing grid power usage and minimizing system lifecycle cost. The pricing strategies on the return of interest of charging stations were analyzed based on coordinated charging and discharging

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optimization [ 69 ]. Moreover, a deterministic linear optimization approach was employed in [34] to minimize charging cost, maximize average use of wind power or minimize load factor for the charging time of each EV customer. In [70], an economic optimization approach was proposed to minimize cost and emissions.

The control and agent based methodologies were also found in several articles. New management system based on multi agent transport simulation (MATS) agents was developed by using the energy hub concept in [ 71 ]. A hierarchical control algorithm was proposed in [30] to investigate the integration of controllable PEV charging and scheduling of intermittent wind energy. The simulation based methodologies were also developed to conduct the PEV research. The impact of high penetration of charging stations on the reliability of distribution systems was analyzed based on simulation models in MATLAB Simulink [41]. A control strategy based on DC bus voltage sensing was proposed in [72] to ensure the optimal usage of available power, charging time and grid stability using MATLAB Simulink.

VI. CONCLUSIONS

The future urban energy infrastructures should become resilient and sustainable that can no longer stand alone and be operated independently [73]. The demand reduction through improved energy efficiency is one of the essential metrics in engineering this kind of urban energy infrastructures [74-76]. PEVs could work as an effective means of promoting energy efficiency by coupling different energy infrastructures, including the urban electricity, water, and natural gas, which are becoming more interconnected with the increased coupling of distributed energy resources and storages.

This paper has surveyed and reviewed the potential benefits, development and impacts of the PEVs as DESs in smart grid environment. Presently PEVs as DESs are still under investigation as a long-term solution for composing the sustainable and resilient urban energy infrastructure. The industry sector of PEV market is still in the infancy stage and it might take 10 to 20 years before existing distribution systems are ready to support the large scale of PEV penetration. Besides, the availability of PEVs due to customers’ choices and the comfort loss for customers are considered as significant obstacles on the way to deploy PEVs as mobile storages.

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