The issue of high efficiency had to be dealt with with appropriate converter design; more specifically, the research team studied and developed the following solutions:
- AC/DC conversion was implemented by the use of parallel converters. These converters were hierarchically controlled, depending on the recovered energy amount, so each time the number of “active” converters was one that matches electrical power generation. This way the converters always operated close to their nominal power, where they perform maximum efficiency, also maximizing the whole AC/DC conversion efficiency under any point of operation. Moreover, this was a modular design approach, which enabled the implementation of different power levels. The exact power level for each converter module and its dynamic behaviour was determined during simulation and evaluation process.
- AC/DC modules had unity power factor and low THD (less than 5%) at the generator side (PFC converters); this way, synchronous generator was not charged with reactive power and electromagnetic compatibility issues are eliminated. ECRG-UP has significant experience in the development of high-efficiency PFC converter topologies and control techniques, based on PWM DC/DC topologies, as well as by the development of a 2kW PFC on board charger for electric vehicles. The selection of the PFC converter topology initially depended on the output rms voltage value of the PM generator. More specifically, if the peak AC voltage is always lower than DC bus voltage (270V) then boost type topologies can be considered; otherwise step-down configurations have to be adopted. Generally, boost type topologies were considered to be more suitable for PFC applications in the range of 1 – 10kW, due to the continuous input current shape that they produce. Moreover, the production of low AC voltage (100Vrms – 160Vrms) by the PM generator eases human security against electric shock.
The electrical grid of an aircraft is used to supply a variety of different loads. Electrical power can be derived from a variety of sources, categorized as either primary or secondary sources. Batteries and generators are primary sources; inverters and transformer rectifier units are secondary sources of power. Power comes either in the form of direct or alternating current depending on system requirements, even though different load supply voltages can be simultaneously used, depending on load characteristics.
In modern aircraft systems the trend is to reduce emissions and increase efficiency by exploiting residual energy in exhaust gases from the aircraft’s main engine.
In our specific system the main generator provided a 270V DC network, as the intermediate level for absorbing electric energy produced by any possible source (including regenerative schemes) and for feeding aircraft electrical loads.
RENERGISE, in order to increase system’s efficiency, introduced two power generation units, which produce electrical power from waste heat, thanks to an energy recovery system. This power generation is used in replacement of or in addition to usual generation systems, which retrieve their power from the Main Gear Box (i.e. have a direct cost in fuel burning). Two ad hoc methods of converting waste heat into electrical power have been studied and implemented, namely Static and Dynamic Energy Recovery Systems.
To implement Static Recovery (SWHR), the heat of the exhaust gases was directly converted to electrical energy with a thermoelectric generator. This thermoelectric generator was installed on the engine cover where the temperature difference with the ambient corresponds to a typical thermoelectric module’s operating temperature difference. The generated voltage of the module dropped with the increase of its current; hence an MPPT was required in order to achieve the maximum power output. Another challenge within this project has been the introduction of a super-capacitor-based energy storage subsystem, as an intermediate energy bank between the thermoelectric modules’ generation and the main DC bus. The main objective of this subsystem was to compensate transient load component, reducing the main generator rating. Regarding Dynamic Recovery (DWHR), the kinetic energy of the exhaust gases was used to rotate a gas turbine connected to an electrical generator. This way, significant amounts of electric energy were regenerated, reducing dramatically fuel consumption. However, the speed and pressure of the gases differed according to the main engine’s power output, although the waste heat source remains at very high enthalpy levels over all flight conditions.
As a result, the torque/speed characteristic of the turbine may also change; therefore, a feedback control is necessary on the electric generator’s output. Evidently, with the use of the above energy recovery systems, fuel consumption as well as carbon emissions decreased, improving the overall efficiency of the aircraft. The issue of high efficiency had been tackled with appropriate converter design for each subsystem, namely the SWHR and the DWHR; more specifically, the research team had designed and developed the following solutions:
- SWHR - The thermoelectric generator is connected directly to the 270 VDC bus (thus injecting constant active power), while the supercapacitor subsystem and its DC link-converter are connected as a power active filter, reducing the necessary capacitance (although higher supercapacitor voltage fluctuation is established).
- DWHR - As the generator connected to the gas turbine is a three-phase PM synchronous one, an AC/DC conversion has been implemented by the use of a three-phase square wave inverter, operating in the rectifying mode.
Different solutions have been examined by the research team, concerning the cooling design, in order to meet the power density demand (~3 kW/kg).