To address the challenge posed by potentially large amount of PEVs and PHEVs, we must develop advanced communication and management tools to allow utility to actively manage and control these distributed assets. In addition, significant improvements in various subsystems such as battery, power electronics and motors are needed to continue the revolution towards gasoline free transportation. ATEC will establish research, development and demonstration of these advanced technologies.
Vehicle to grid (V2G) concept envisions the plug-in hybrid or plug-in electric vehicle as a resource for the electric grid, where power can be absorbed or sourced by the vehicle energy storage system. However, there are many intermediary steps that have to be achieved, before this vision comes to fruition.
ATEC will research and implement various technologies that will allow for V2G to become a reality. We will look at the benefits and the issues with each stage of V2G implementation. The three stages we have identified are explained below. Each provides additional flexibility to the electrical distribution system.
Smart Charging (V1G): The vehicle charging rate is controlled remotely based on grid conditions and user preferences. The benefits include:
- Using electricity when it would otherwise be wasted
- Minimize additional load at peak times (load as spinning reserve)
- Allow easier integration of intermittent renewable resources (such as wind and solar) to the grid
Vehicle to Building (V2B): This system would allow for the charger to be controlled remotely; in addition, the charger would be able to feed power back to the home to which it is plugged in. This means that the charger will have to be bi-directional. Such a system boasts additional benefits compared to V1G:
- Provide back-up power for buildings
- Ensure high power quality for buildings
- Help supply power to building when grid power is costly
Vehicle to Grid (V2G): allows for the vehicle to feed power back directly to the grid. In addition, the power converters used to charge the vehicle can also be used to support the stability of the grid by providing ancillary services. Such a system would require constant bi-directional communication between the charger and the grid. V2G offers:
- Grid-stabilizing ancillary services (reactive power and voltage control, loss compensation, energy imbalance)
- Supply power to grid when economically viable
- Allow easier integration of renewable resources by ensuring high power quality from the resource
Modern vehicles design is becoming more of a multidisciplinary field. We plan to incorporate automotive research of the many faculty at NCSU. The developed technologies will be showcased on a vehicle testbed. In particular, ATEC research will focus on improving the electronic components of the vehicle electric drivetrain.
Electric motor: currently permanent magnet motors and induction motors are typically used on vehicles with electric propulsion. We will look at how we can improve the efficiency, and reduce the cost of these electric motors. ATEC will also work on developing other motor types such as reluctance motors for electric propulsion, as well as on developing novel concepts such as integration of the electric motor with the power electronics and placing the motors in the vehicle’s wheels (hub motors).
Power electronics: there are a number of power conversion and conditioning devices on the electric vehicle (eg. motor drivers, voltage boosters, on-board battery charger). We will research into methodologies to simplify these electronics systems, make them more efficient, more robust and smaller. In addition, we are developing the next generation post-silicon devices that operate more efficiently and at higher temperatures. The use of SiC devices will enable us to make the power electronics systems more compact, more efficient, and to require a smaller cooling unit.
Energy storage devices: in addition to extensive research on batteries with the goal of increasing life and fast charging capability, ATEC will also investigate other power sources such as super capacitors, fuel cells and flywheels. Super capacitors in particular are of interest due to their long life and very fast charging characteristics.
System studies: PHEVs and PEVs are complex systems that are made of a number of sub-components. To maximize vehicle efficiency, a control strategy has to be developed where each subsystem operates at its most efficient point. In addition, the overall vehicle performance can be tailored to the driving conditions to improve the system efficiency.. By considering the vehicle as a system the vehicle overall efficiency can improve dramatically without cost increments or technological breakthroughs.
The stumbling block to introducing all-electric vehicles on the road today is the fact that the batteries are not up to the task to mimic all of the conveniences that we have come to expect from vehicles powered by internal combustion engines. The issues with the commercially available batteries in the vehicle application are:
- Battery energy and power densities need to improve. To match gasoline, energy and power densities of batteries have to increase significantly.
- Batteries need to be charged in a time that is comparable to the refueling time of the gasoline tank. Therefore batteries must accept charge at high rates safely.
- Battery life is an issue. Vehicle owners expect their batteries to last as long as the vehicle. Even though some batteries are able to last for thousands of cycles, when hundreds of batteries are put in a vehicle pack, in some cases, one failure makes the pack unusable.
- Battery safety must be assured, especially during fast charging and in hot weather.
- Battery cost needs to be reduced.
ATEC research will attempt to address all of these issues. At the fundamental level, Dr. Zhang and his research team are developing lithium-ion batteries based on nanofiber electrodes. Dr. Zhang synthesizes these electrodes by using an inexpensive electrospinning process. When these nanofiber electrodes are used in lithium-ion batteries, the stable nanofiber structure eliminates the existence of inactive materials such as polymer binders and carbon black conductors, so they can hold more energy for a longer time; this implies higher energy density and more stable capacity. In addition, the large surface area of the nanofibers implies better charge acceptance and therefore faster charging.
To improve the charging time of the pack, each battery will be managed at the individual level to insure safety during fast charging. Algorithms will be developed to charge the battery at the fastest rate at which the chemistry is able to absorb the charge safely. To improve the life of the pack to match that of a single battery, weak batteries will be taken out of the circuit electronically. Finally, as the vehicles retire, used PHEV batteries can be deployed to electric substations to support the grid.
ATEC research will also consider alternative energy sources other than batteries. For instance, supercapacitors exhibit very long life, great charge and discharge characteristics, and very high power density. On the other hand they typically have low energy density. Therefore, combining batteries and supercapacitors in one storage device will result in a superior system.