Electric vehicle (EV) technology attracts public and government interests due to increasing environmental concerns and rising fossil fuel prices. The convergence of the transport and power system would lead to numerous problems for the smart grid. The wide penetration of EVs would, for example, increase the grid load while they are charging. However, the expected adoption of EVs has also opened the way for vehicle to grid (V2G) technology deployment.
V2G involves the management and control of electric vehicle loads via the local aggregators or electrical utility. They do so by communicating with both the smart grid and vehicles. V2G uses power from the regional EV population and transmits it to the smart grid. In general, V2G includes systems such as vehicle to grid chargers, power grid aggregators, electric vehicles, power loads, power transmission systems and communication systems.
V2G technology offers various supports and benefits. For example, the successful deployment of V2G can supply harmonic filtering and also failure recovery in case of a power shutdown. EV owners also benefit from this technology, it’s not just the electrical utility. EV users can get back-up energy storage if they are using renewable energy resources at home. This in turn provides an uninterrupted power supply.
These are just a few benefits. A deeper look involves discussing the power flow between the smart grid and electric vehicle.
Unidirectional and bidirectional power flow
To attain the intended benefits, the smart grid uses the communication systems to manage and control the flow of power with the EV battery. In the majority of cases, V2G management's goals are to reduce emissions, maximize benefit and improve smart grid power efficiency. Moreover, power can flow in both unidirectional and bidirectional ways.
Unidirectional V2G technology regulates the EV battery charging rate between the smart grid and EV when power is flowing in a single direction. Realizing unidirectional V2G is inexpensive, as it incorporates a basic device for controlling the charging rate. It can provide power grid support facilities, such as spinning reserve and power grid control. This allows more flexible smart grid operations. The deployment of the unidirectional V2G requires an alluring power exchange policy between electrical utility and EV users. To increase the EV user’s participation, this power exchange strategy must promise them income if they don’t charge their EVs during peak time. The electrical company can simultaneously prevent peak hours overloading as EVs will be charged during off-peak hours.
However, the potential to supply support services to the grid is limited for unidirectional V2G services. The premium supports such as reactive power support, peak load shaving, frequency regulation and voltage regulation can only be provided by bidirectional power flow. Bidirectional V2G technology allows two-way power flow between EV and the smart grid.
In general, a two-way EV battery charger is made up of DC-to-DC converter and AC-to-DC converter. During EV charging, the AC-to-DC converter transforms the AC power produced in the smart grid into DC power, and then during discharging, it changes that DC power back into AC before sending back the power to the grid. On the contrary, the DC/DC converter uses current control methods to regulate the bidirectional power flow. It acts as a boost converter during discharging and as a buck converter during the charging of EV.
The bidirectional V2G offers more possibilities and flexibility to boost power system operations. The major advantages are reactive and active power support, deployment of renewable energy support, preventing power grid overloading, failure recovery, reduced power grid losses and power factor regulation. The active power support further provides load-leveling services and peak load shaving. Furthermore, the grid voltage can be controlled with proper control switching and suitable sizing of the DC link charger capacitor.
The power generation from solar photovoltaic and wind turbines and other renewable energy resources are inconsistent and intermittent as these resources vary according to climatic conditions. However, in bidirectional V2G, EV acts as energy supplier and storage to resolve the unpredictable nature of these renewable energy resources.
Integration challenges of V2G technology
Currently, the deployment of bidirectional V2G faces several challenges. One major issue is the degradation of battery because of consistent discharging and charging cycles. In a battery, there are irreversible chemical reactions, which reduce the battery's usable power and increase its internal resistance. Extra investment is also needed because additional hardware is required for a complex two-way EV battery charger.
Moreover, social isolation is another crucial issue for the deployment of bidirectional V2G technology. The EV owner will also attempt to keep EV’s battery in a high charge state to get security in case of sudden traveling usage. As a result, he or she will be reluctant to regularly contribute in V2G bidirectional services.
Many countries are deploying unidirectional V2G technology to mitigate the social barrier problem and introduce EV into the marketplace. Bidirectional V2G is likely to be accepted in the future when the technology and market are organized. The major drawback of unidirectional V2G technology is its limited services. That is why it is cheaper than its bidirectional counterpart.
Overall, V2G technology demands the up-gradation of the current power system, it therefore has a high investment cost. The current software and hardware infrastructure of smart grids must be restructured to fulfill all the V2G requirements. Every EV participant will need a complex and expensive bidirectional battery charger. The frequent charging and discharging in bidirectional V2G also has the potential to increase energy losses of smart grids. Also, the EV participant can become anxious while sharing the EV’s battery power with the grid. This worsens with a smaller number of charging stations.
In conclusion, further technological enhancements are required for the successful deployment of V2G technology. There is no doubt that it will bring numerous flexibilities and advantages to the smart grid but it requires time. The technology is not yet matured. For its widespread adoption, many technical, economic and social issues must be dealt with.
