Due to environmental concerns and rising fuel prices, there has been a striving impetus towards transportation electrification. Electric vehicles (EVs) such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and fuel cell electric vehicles (FCEVs) are more fuel efficient and environmental friendly compared to conventional vehicles propelled by an internal combustion engine (ICT) [1]. PHEVs stand out from the electric vehicle market due to several advantages. These advantages are that they are mainly powered by an on-board energy storage system and can be recharged from a power utility grid as well as renewable energy sources [2]. PHEV batteries act as energy storage to counter the intermittency of renewable energy sources as well as to supply power back to the grid via the vehicle-to- grid (V2G) technology [2]. PHEVs provide a way of using clean energy in transportation sectors and could provide further energy savings if equipped with regenerative braking.
Among all the renewable energy sources, solar is clean, inexhaustible and widely accessible. The problems for solar energy are that solar power varies rapidly with respect to ever-changing environmental conditions. The maximum power generated by photovoltaic (PV) system does not happen at the time when there is a peak load demand. Thus, the value to the system may be enhanced if the energy harvested by PV panels can be stored in energy storage systems and released to cover the peak load at different times. Fig. 1.1 demonstrates the total PV installations in Australia from April 2001 to June 2017. Up to July 2017, the number of PV installations in Australia exceeds 1.7 million, with an aggregated capacity of over 6.2 gigawatts [3].
Fig. 1.1: Australian PV installations from April 2001 to June 2016 [3].
Two major configurations of PV generation systems are stand-alone systems or off-grid systems and grid-connected systems. Stand-alone PV systems are designed to operate without a connection to the utility grid and are often installed in remote areas where extreme weather often happens [4]. Energy storage integration is necessary when an uninterruptible electricity supply is required [5]. Grid-connected PV systems provide a connection between the PV system and the grid via inverters. Grid-connected PV systems are often equipped with maximum power point tracking (MPPT) techniques to extract as much power as possible from the sun and deliver electricity to the grid for a reasonable tariff. The purpose of implementing MPPT techniques is to ensure that the PV array always operates at its maximum power point (MPP). This is accomplished by controlling the duty cycle to determine the on/off state of the IGBT switch within the converters interfaced with the PV array.
PV material recycling is another problem associated with PV generation [6]. The normal lifetime of a PV module is 25 years. A PV module needs to be decommissioned and recycled at the end of its lifetime [7] for environmental and economic purposes. It is estimated that about 80 metric tons of waste will be generated from 1 MW of end-of-life PV modules [6].
PHEVs can be a good choice to implement energy storage as they provide an eco-friendly medium of transportation. The V2G technology enables the onboard energy storage system of the vehicle to supply power to the grid when the load demand is high. V2G technology can be defined as a system that provides a controllable, bi-directional power
flow between a vehicle and the grid. A PHEV can be recharged from the grid when its battery’s state of charge (SOC) is low, whereas a PHEV can supply power to the grid when there is an imbalance of generation and load. The power management unit (PMU) is used to implement V2G technology. Although there is no such vehicle designed to incorporate PMU in its energy storage system, the potential for V2G technology is significant. Research shows that vehicles are not in use for transportation up to 95% of the time, when the energy storage system can be utilized to stabilize the electricity network without compromising its primary transportation function [8].
The distribution networks were initially constructed for a centralized generation paradigm [9]. Power generated from the central power plant is transmitted to the end users through the transmission line [10]. With an increasing number of distributed generation resources (DGRs) such as solar, wind, biomass and fuel cell integrated into the network, some instability issues and operational challenges may arise in the distribution network [11], [12]. These challenges include but are not limited to: bidirectional power flow [13], poor power quality [12], dysfunction of protection equipment, reactive power shortage, and steady-state voltage rise [12]. These challenges can be mitigated by the integration of energy storage systems. Energy storage systems are able to store excessive energy for further use and supply power to local loads and the grid at night time. They can also improve the distribution network reliability by mitigation of the aforementioned challenges. PHEV energy storage systems are one of the cost-effective energy storage technologies, and provide an effective approach to solar energy utilization for PHEV batteries charging. With an increasing penetration of PHEVs in the transportation sector, the operational challenges attributed to DGRs can be significantly reduced and power system reliability and security can be maintained or improved.
There is increasing attention being paid to the electric vehicle market, as PHEVs and HEVs have significantly lower carbon emissions, less operational costs and consume less fuels compared to the vehicles propelled by internal combustion engines [14]. DERs can be used to charge PHEVs with low fuel costs. During the time when DERs are abundant, such as strong wind and high solar irradiance, the energy generated from these DERs can be stored into the energy storage system of PHEVs. The utilization of PHEVs could improve the coordination of the DERs connected to the distribution grid and increase the penetration level of DERs [15]. However, the massive usage of PHEVs may lead to
several potential issues for power systems. One problem could be that charging large numbers of PHEVs simultaneously cause some technical problems in maintaining the reliability and security of power systems. These technical problems include huge power losses [16], voltage deviations at some local buses and peak loads of distribution transformers at a particular time of a day [17].
PHEVs and PV systems both have a positive influence on the environment with decreasing carbon emissions and low fuel consumption. Increasing PV penetrations without integration with PHEVs (or similar energy storage facilities) may lead to voltage and frequency instability of the power system, whereas connecting PHEVs into the network without any PV system integration could cause considerable increased power losses and voltage deviations. Grid-connected PV systems integrated with PHEVs utilizing appropriate energy management strategies can neutralize their negative effects on the power system.