Comparing a Flywheel vs Supercapacitor Power Buffer System to Enable Deployment of Fast EV Chargers in Weak Grids
Area: Electrical, Power conversion, Energy storage
Subarea: Power Electronics
Nature: Modelling - Simulation
Distinctive skills to be gained: Understand the principle of power electronics, energy conversion and energy storage how to model the losses and determine conversion efficiencies
Background: Electrical energy can be transported and converted with high efficiency into other type of energy (such as mechanical in case of vehicles) but its storage is still a problem. The most known electrochemical storage device is the battery, but this has disadvantages such as a limited lifetime/number of charging/discharging cycles and a long charging and discharging time (hours) at a very limited current which is obviously not suited for EVs on road.
Fast charging is a technique developed for such cases but handling short time (minutes) / large currents/powers (100+A/100kW) is very demanding both for the existing power grid which may not be designed to handle significantly higher powers. Using an intermediary energy/power store embedded with the EV charger may offer the solution to reduce the grid power peak and provide fastest recharge of EV battery but supercapacitors (electrochemical storage) or flywheels (mechanical storage) need to be used.
Developing an efficient power converter able to perform the AC/DC conversion (in the grid side converter that needs to control active power whilst maintaining grid side current sinusoidal and balanced, may only have unidirectional power flow unless it may be required to also support local grid during grid faults) and DC/DC conversion (convert constant DC to variable DC voltage as seen across the battery or supercap stack or to interface with a variety of vehicles having different battery voltage and power ratings, that needs to control the power flow into the supercap stack) power conversion and its associated control to implement the required power and energy management (limit the recharge power of supercap between EV charges from grid and implement power sharing grid/storage sys during fast charging) . The other option is to design a flywheel and a high performance AC machine (permanent magnet of induction type) electric drive to interface the main DC bus of the EV charger to the fast mechanical flywheel storage system. The following directions of research are relevant to above topic and may be explored in more detail:
a. Investigate the particularities of implementing a large supercapacitor stack vs implementing the flywheel storage in terms of physical size and the required power electronic/electrical drive interface
b. Investigate and evaluate the potential advantages offered by wide-band gap switching devices over conventional silicon-based semiconductor devices
c. Investigate the advantages offered by a modular converter design
d. Investigate the possible advantages offered by smart prioritization of multiple vehicle charging depending on power available, urgency/need of certain vehicles/price premium
e. Investigate a large installation such as a parking lot that is equipped with renewable energy, and the possibility of having energy stored on parked vehicles for longer durations to support other needs (airport, grid etc.)
Project Aims: This project will investigate the implementation of the simulation models for power loss evaluation and control for a multi -stage AC/DC and DC/DC power converter with embedded supercapacitor or battery energy storage.
Project Objectives (a shorter selection will be agreed with supervisor after the literature review stage is completed and the exact direction of research decided) :
1) Understand requirements for EV fast charging (derive V/I/P/energy ratings and times)
2) Review latest developments in the field of energy storage devices (batteries and supercaps) relative to the specification of fast charging
3) Review power converter topologies able to implement the fast charging functionality (DC/DC and A/DC DC/AC)
4) Implement simulation models for AC/DC rectifiers and DC/DC converter using the example models provided in PSIM or PLECS and build an EV charger model for the system (AC/DC + DC/DC) and also of the electric drive (AC motor + AC/DC inverter) . This study can be augmented by implementing design of magnetic components (interface inductors) and estimation of semiconductor losses (thermal model) which can then be used if exploring (a) or (b)
5) Investigate smart management techniques relevant to multicar simultaneous charging (c) or large car park with renewable generation (d)