George Christidis

Postgraduate student
Subject: 
Power Electronics
Research Area: 
DC/DC converters, DC/AC converters, Renewable Energy Sources, Waste heat recovery systems, SiC devices
Tel.: 
(30) 2610.996414
Fax: 
(30) 2610.997362

George Christidis is a PhD candidate in the Electrical and Computer Engineering Department in the University of Patras since 2010. He received his Diploma by the same department in 2010 with a specialization in the area of Electronics. His area of expertise is Power Electronics. His research interests include the analysis, design, simulation and construction of DC/DC and DC/AC converters for use in renewable energy systems, waste heat recovery systems and aeronautics and space applications, as well as the modeling and implementation of SiC power electronics devices. He is a member of IEEE (Power Electronics Society) and a member of the Technical Chamber of Greece.


Research Activity

Voltage step-up DC-DC converters
Two of the various applications of voltage step-up converters are photovoltaic systems and energy recovery systems using thermoelectric generators. In both applications a voltage step-up is required, as the output voltage of these devices is very low in order to connect to the common ac or dc grid. The research is focused on the increase of the converter efficiency, as well as the optimization of its volume, mass and reliability. What is more, various control techniques for maximum power transfer are examined.

Boost-Flyback Converter
This voltage step-up converter is designed for low power (

Voltage multipliers
The main advantage of voltage multipliers when used in voltage step-up of photovoltaic panels is their simplicity, as they only consist of power diodes and capacitors. They are fed by high frequency alternating currents which can be created by either an Inverter or a Boost converter. The operation of the converter is affected by the capacitance value in each stage. A Dickson and a Brugler converter of different stages were developed in order to experimentally study the optimal capacitance to be used in these converters.

Maximum Power Point Tracking
Neither the photovoltaic panels nor the thermoelectric generators are pure voltage or current sources, and therefore the generated electric power is not only a function of the irradiance or the difference of temperature respectively, but also it depends on the operating point. There are different algorithms to find and operate in this maximum power point. Numerous existing methods, as well as new methods, proposed modifications of the existing have been tested in simulations and in a developed step-up converter controlled by a microcontroller.

New power electronics devices
Since 1947 when the transistor was invented, the main material from which all the power electronics devices were built was silicone. However, in recent years, there is a continuous research activity in building and manufacturing devices from silicone carbide. These devices switch faster, can withstand higher voltage and temperature values and have lower power losses. A voltage step-up converter which used a JFET and a Schottky Barrier Diode of Silicon Carbide was examined, and the power losses were determined theoretically as well as with simulations, which was then validated experimentally.

DC-AC converters for photovoltaic systems
In order to connect the photovoltaic panels to the AC grid, the use of an inverter is mandatory. It can be fed either directly by the panel, or by a DC-DC converter, which is usually a voltage step-up converter. In the first case, the inverter has to find and operate in the maximum power point, whereas in the second case, this is done by the DC converter.

Flyback current source inverter
The flyback inverter is widely used for the connection to the AC grid of each single photovoltaic panel and for this reason this system is commonly referred as AC-PV module. Given the large number of units that are used, research is conducted towards the improvement of the weighted efficiency of this converter as well as the power quality and the low cost and volume.

Waste heat recovery systems in rotorcrafts
Due to the cost increase of fossil fuels, as well as for environmental reasons, much is done to reduce the helicopter fuel consumption. A way to achieve this is by cutting down the electrical power provided to the grid by the main generator, which is connected to the main gear box of the helicopter, thus it is powered by the main engine. Instead, waste heat recovery systems are evaluated, which exploit the energy remains of the exhaust gases. This is done either by thermoelectric generators, which are semiconductor devices that generate a voltage as a function of the difference in temperature of the two sides of the device, or conventionally by generating heat and rotating a turbine connected to a synchronous generator. In both of these systems power electronics converters are used in order to supply power to the electric grid, appropriately controlled.

Cubesat electric power supply system
University of Patras takes part in a european mission to send microsatellites at an altitude of about 300km. For the developed cubesat, the electric power supply system is designed and built, which will consist of photovoltaic panels, batteries and voltage step-up and step-down converters driven by a microcontroller.

Average simulation model derivation
For each system control strategy which employs power electronics converters, simulation validation is essential before being tested in the actual system. Power converters operate in as much higher switching frequency as possible, requiring a small simulation time step, whereas operating times to observe the accuracy of the control strategy are quite large and, as a result, the simulation needs a lot of time to complete and demands a lot of space to save the results, without so much detail being required. In order to avoid this, there are average simulation modeling techniques that do not show the operation in each switching cycle, but the whole average operation of the converter. Average models for a unidirectional voltage step-up converter as well as a bidirectional voltage step-up/step-down converter taking account the power losses have been developed, in order to examine the waste heat energy recovery system of the rotorcraft.

Electric vehicles
The Laboratory of Electromechanical Energy Conversion has a variety of electric and hybrid vehicles. One of those is a Fiat Fiorino which is fed by Ni-MH batteries and has a permanent magnet synchronous motor that is driven by a three-phase inverter. At the present stage, all of the vehicle's systems are being checked, in order to develop battery chargers, as well as a data acquisition system in order to record data from all the available sensors.


Research Projects

CLEAN SKY: ITD-GRC-03-004 N°287076 RENERGISE “Innovative management of energy recovery for reduction of electrical power consumption on fuel consumption”
Funded by Cleansky JTI - FP7
Jul 2011 - current

During the project, the consortium will design and manufacture three power electronics converters, suitable for helicopters, which will exploit energy remains in exhaust gases. The first converter will inject energy to the helicopter's electric grid produced by a Thermoelectric Generator, a semiconductor device which converts the heat produced by the main engine into electrical energy. The second converter will will also supply energy to the electric grid and will be powered by a three phase synchronous generator, rotated by a turbine using steam, generated by a boiler placed at the engine nozzle. Finally, the third converter, will connect a supercapacitor bank to the electric grid bidirectionally and will be used to compensate high peak currents demanded by the electric loads.

“Elevators energy models”
Funded by KLEEMAN HELLAS
Apr 2012 - current

In this project the behavior and operation of power electronics converters found in elevators will be analyzed by creating computer simulation models.

“Synergasia” 09ΣΥΝ-32-829 LESS
“Energy Saving in Elevators”
Funded by NSRF 2007-2013
May 2012 - current

The aim of this research project is to find the opimal solution for energy savings in elevators and manufacture prototype which will validate the proposed solution. Energy saving will be accomplished by: minimizing the stand by power, minimizing the power demanded during operation, correcting the power factor and removing higher harmonics, and harvesting the energy during braking.

"UPSAT - University of Patras SATellite"
QB50 - Cubesat
Funded by FP7
May 2012 - current

A research team which consists of laboratories from the Mechanical and Aeronautical Engineering and the Electrical and Computer Engineering Departments is designing a small (10x20) satellite which will carry specific sensors that will explore the lower thermosphere. An essential part of the Cubesat is the Electric Power Supply Unit, which will absorb solar power, store it into a battery pack and feed it to different loads, through power electronics converters.