By Rafael Batista, MG Researcher and Juan José Pichardo MG Research Assistant
Challenges of Electrical Mobility Integration
At this moment, we are in the middle of a transportation revolution. Electrical vehicles (EVs) are increasingly replacing petrol cars (as shown in Illustration 1) as a means of transportation, creating an important challenge for electrical utilities. The increase in electrical energy demand due to the integration of EVs could impact the electrical system’s stability and increase electricity costs for customers [1]. Furthermore, there is a necessity for handling the energy demand of EV chargers to avoid increasing the system’s load at the wrong time (during peak hours). One of the strategies for mitigating the effects of EV integration on the electrical grid is to create DC microgrids in parallel with the existing grid in order to be used (almost) exclusively for EV charging purposes.
Microgrids as a solution to the integration of electrical mobility
Microgrids arise as a solution to the integration of electric mobility for many reasons. First of all, To accomplish an effective integration of EVs into the electrical grid, new design parameters need to come into place. This implies that a large scale overhaul of the actual infrastructure of our electrical distribution system would need to be considered, which may result in the replacement or reconfiguration of substation systems, transmission lines, protection systems, and others. It’s important to consider that replacing electrical infrastructure entails the process of dismounting the actual infrastructure and installing newer ones. Being that the case, creating a new infrastructure exclusively for EV loads becomes an easier and economically preferable choice. DC microgrids are one of the best options for creating an EV-exclusive electrical infrastructure.
DC Microgrids and their advantages against AC Microgrids
Microgrids can be defined as decentralized power systems of small scale, which can be operated autonomously or in parallel with the electrical utility. A main component of a microgrid system is distributed generation (DG); enabling loads to be closer to the electrical generation sources, reducing potential losses in the electrical transmission system. When designing a Microgrid, one of the first choices to be made is the preferred wave shape of our system (Yes, we go back to the famous 1890s debate, between Tesla and Edison!). The options are either DC Microgrids or AC Microgrids. For the integration of EV, although at first, it may sound impressive, several aspects benefit the decision towards a DC microgrid.
As stated in [2], “DC microgrids tend to prevail over AC grids because most distributed generation, as well as energy storage devices, operate on DC voltage/current. In addition, DC microgrids exhibit higher efficiency due to the absence of power dissipations caused by AC-DC conversion (and vice versa) and because the control methods are simpler and easier to implement, as there are no frequency issues.”
EVs are an excellent example of loads that would benefit from an integration with a DC voltage bus. As can be seen in Fig. 1, special power electronics must be dedicated to the conversion process between the main electrical utility and the battery management system. This may result in reduced efficiency during the charging process. Additionally, the typical DG sources found on microgrids are inherently DC (photovoltaic panels), for this reason, a direct integration without the use of DC/AC converters could be beneficial.
As it has already been commented, the integration of EVs into our electrical grids is a challenge. But they also offer the possibility of improving the reliability of our grids and even enabling a faster penetration of renewable energy sources (RES). Usually, when considering EVs, only the power demand is taken into consideration. It is important to remember that EVs are in fact moving batteries, which can be used to provide auxiliary services to the electrical utility. Also, the electrical energy storage capacity of a large national-scale vehicle fleet could be greater than the actual capacity of future electrical utilities’ energy storage systems.
Bearing that in mind, utilizing the vehicle fleet as an large-scale energy storage system can have a lot of potential especially in cases of emergency. The integration of the vehicle fleet into the electrical grid is commonly defined as “Vehicle-to-Grid”, or “V2G”.
Vehicle-to-Grid Integration
Vehicle-to-Grid, or simply “V2G”, is a recent power scheme that aims to utilize the growing fleet of electric vehicles to store energy and supply power into the main electrical grid. A more technical definition is “…the control and management of EV loads by the power utility or aggregators via the communication between vehicles and power grid.” [4]. This technology can be very useful when integrated into Microgrids. Microgrids tend to mainly utilize renewable energy generation, which can be a challenge to manage correctly in order to maintain a reliable, consistent supply of power. One of the main components that Microgrids need to have to effectively function are energy storage devices. All electrical vehicles have some sort of energy storing device, plug-in and hybrid cars have large arrangements of Li-ion batteries and fuel-cell vehicles have hydrogen tanks. This storage devices could be aggregated into future renewable Microgrids to improve the stability of the system.
“Most personal transportation vehicles sit parked for about 22 hours each day, during which time they represent an idle asset. EVs could potentially help in micro-grid energy management” [3].
Classification of Vehicle-to-Grid concepts
There are three emerging concepts of grid-connected EV technologies, which are the Vehicle to Home (V2H), Vehicle to Vehicle (V2V) and Vehicle to Grid (V2G). Also, a global concept is Vehicle to Everything (V2X), which encompass all the previous three technologies.
-Vehicle to Home:
V2H refers to the power exchange between the EV battery and home power network. In this case, EV battery can work as energy storage, which provides the backup energy to the home electrical appliances and to the home renewable energy sources.
-Vehicle to Vehicle:
V2V is a local EV community that can charge or discharge their battery’s energy among them.
-Vehicle to Grid:
V2G utilizes the energy from the local EV community and trades them to the power grid through the control and management of local aggregators.
A key enabler hardware for the integration of the V2G concept is the use of manageable, bidirectional, coordinated, and monitored EV chargers. This implies that the development of grid-level control algorithms is needed to harvest the advantages of this new technology. Also, DC Microgrids would ease this kind of implementation and promote a faster integration of the V2G concept.
Advantages and Disadvantages of V2G
One-way V2G provides “load-only” ancillary services to the grid by controlling electric vehicle charging rates at the request of grid operators. The aggregator manages and controls a large fleet of electric vehicles to achieve the ancillary services. This services could be:
-Active power compensation:
Use the excess of power from electric vehicles to provide active support to the power grid. Active power support requires vehicles to discharge battery power and therefore could only be achieved using bidirectional V2G, but not unidirectional V2G. The goal of this service is to flatten the load profile of the grid through peak load reduction and load leveling.
-Reactive power compensation:
Reactive power compensation is a technique to regulate the voltage in the power grid. Reactive power support also provides power factor correction, which reduces generation current flows and power losses on the power line. In addition, this service can reduce the load on power equipment, leading to an increase in the operating efficiency of the power system.
-Support for renewable energy sources:
The integration of electric vehicles into the power system can be a solution to these problems. The problem of the intermittency of renewable energy sources can be solved by using a fleet of electric vehicles as energy backup or energy storage. The vehicle fleets act as an energy backup to supply the necessary energy when renewable energy generation is insufficient. At the same time, they act as energy stores to absorb the excess of energy generated by renewable energy sources, which would otherwise be “wasted”.
Nevertheless, there are important disadvantages that need to be considered related to the use of V2G technology. Higher rates of battery deterioration, higher investments for intelligent bidirectional EV chargers, and social issues related to the compensation of sharing vehicle battery power.
A correct policy for rewarding the users in allowing the use of their EV systems in the V2G concept may be a possible solution for these disadvantages.
The PUCMM’s Microgrid Laboratory is currently working in several concepts related to the integration of V2G technologies and the exploration of DC Microgrids. This post shows the relevance to the consideration of the use of DC Microgrids for the integration of V2G technology.
References
[1] S. Khan and M. Kushler, “Plug-In Electric Vehicles: Challenges and Opportunities,” 2013. [Online]. https://www.aceee.org/research-report/t133
[2] I. Skouros and A. Karlis, “A study on the V2G technology incorporation in a DC nanogrid and on the provision of voltage regulation to the power grid,” Energies (Basel), vol. 13, no. 10, May 2020. https://doi.org/10.3390/en13102655
[3] W. Kempton and J. Tomić, “Vehicle-to-grid power implementation: From stabilizing the grid to supporting large-scale renewable energy,” J Power Sources, vol. 144, no. 1, pp. 280–294, Jun. 2005. https://doi.org/10.1016/j.jpowsour.2004.12.022
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This article is derived from the Subject Data funded in whole or part by NAS and USAID under the USAID Prime Award Number AID-OAA-A-11-00012. Any opinions, findings, conclusions, or recommendations expressed in this article are those of the authors alone and do not necessarily reflect the views of USAID or NAS.