Final Report Summary - RENERGISE (Innovative management of energy recovery for reduction of electrical power consumption on fuel consumption) Executive Summary: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 provides 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 is directly converted to electrical energy with a thermoelectric generator. This thermoelectric generator is 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 drops with the increase of its current; hence an MPPT is 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 is to compensate transient load component, reducing so main generator rating. Regarding Dynamic Recovery (DWHR), the kinetic energy of the exhaust gases is used to rotate a gas turbine connected to an electrical generator. This way, significant amounts of electric energy are regenerated, reducing dramatically fuel consumption. However, the speed and pressure of the gases differ 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 decreases, improving the overall efficiency of the aircraft. The issue of high efficiency has been tackled with appropriate converter design for each subsystem, namely the SWHR and the DWHR; more specifically, the research team has designed and developed the following solutions: • SWHRThe 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).• DWHRAs 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).Project Context and Objectives: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 provides 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.During RENERGISE, in order to increase system’s efficiency, we shall study the introduction of two power generation units, which produce electrical power from waste heat, thanks to an energy recovery system. This power generation will be 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 will be studied and implemented, namely Static and Dynamic Energy Recovery Systems. To implement Static Waste Heat Recovery (SWHR), the heat of the exhaust gases is directly converted to electrical energy with a thermoelectric generator. This thermoelectric generator is 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 drops with the increase of its current; hence an MPPT is required in order to achieve the maximum power output.Another challenge within this project is the introduction of a super-capacitor based energy storage subsystem, either as an intermediate or as an independent energy storage element. The main objective of this subsystem is to compensate transient load component, reducing so main generator rating.Regarding Dynamic Waste Heat Recovery (DWHR), the highly energetic, high temperature gases can be used to rotate an electrical generator thanks to a turbine using steam generated by a boiler placed at the engine nozzle. For reasons of weight, volume, as well as the high rotational speed (above 20000 rpm), the permanent magnet synchronous generator is the more suitable option. The electrical power is then transferred to the DC bus through another power electronics converter connected in between. 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 will decrease, improving the overall efficiency of the aircraft. The issue of high efficiency has to be dealt with appropriate converter design for each subsystem, namely the SWHR and the DWHR; more specifically, the research team will study and develop the following solutions: • SWHR(i) The thermoelectric generator shall feed the super-capacitor subsystem, under MPP conditions, through a unidirectional step up converter. The selection of super-capacitor voltage shall depend on the converter topology, the transient load characteristics and the desired power density.(ii) The super-capacitor bank shall be connected to the 270 VDC bus through a bidirectional converter, the main task of which is to compensate transient load.(iii) An alternative design, proposed by RENERGISE Team, is to connect the thermoelectric generator directly to the 270 VDC bus (injecting so constant active power under MPP conditions), while the super-capacitor subsystem and its DC link-converter are connected as a power active filter, reducing so the necessary capacitance (although higher super-capacitor voltage fluctuation is established).• DWHR(i) As the generator connected to the gas turbine is a three-phase PM synchronous one, an AC/DC (rectifier) or a DC/AC (inverter) conversion will be implemented by the use of single converter or parallel converters. This is a modular design approach, which enables the implementation of different power levels. In the case of parallel converters, the exact power level for each converter module and its dynamic behaviour will be determined during the simulation and evaluation process. Moreover, a control strategy will be determined in order to achieve maximum power factor (PF).(ii) the AC/DC modules shall have unity power factor and low THD (less than 5%) at the generator side (PFC converters); this way, synchronous generator is not charged with reactive power and electromagnetic compatibility issues are eliminated. The selection of the PFC converter topology depends on the relation between output voltage values of the PM generator and the voltage of the DC bus.(iii) the DWHR converter shall inject power to the 270 V DC rig under low dc current divergence, by appropriate filter design, at the converter dc - side, in order to meet MIL STD 704 requirements.For the actual project implementation and in order to achieve the set objectives, different solutions have been examined by the research team, considering the additional requirement to meet the power density demand (3 kW/kg), namely the possibility of introducing a three phase power factor correction technique or a parallel active filter.According to the original proposal, the main results and benefits from the implementation and adaptation of RENERGISE would be:a. Both static and dynamic recovery systems should be designed in a modular and scalable manner, making these solutions applicable to any power level.b. Energy conversion efficiency: RENERGISE should proceed with the appropriate application of the most effective and robust MPPT algorithms to the case of thermoelectric modules, while for dynamic energy recovery an MPPT algorithm based on positive feedback would be developed. The scope was to achieve MPPT detection in both static and dynamic recovery by more than 98% under any operational point.c. Very high management efficiency should be accomplished as the AC/DC conversion in dynamic recovery would be split into parallel operating modules. Two alternative control options (hierarchical and current sharing) should be investigated in terms of dynamic response and efficiency. Furthermore, PFC operation would minimize excessive ohmic losses due to reactive power elimination.d. The converter topologies under study should be characterized by simple and reliable structure, including limited number of semiconductor switches and magnetic components (and consequently minimum cooling volume), as well as small high frequency filters. Thus, RENERGISE should meet the objective of Power density /Unit mass less than 3 kW/kg.e. The recovery system would be sustainable by making a net saving in fuel consumption.f. The use of standard and well established switching converters, the transformer-less design and the use of super-capacitors instead of batteries would guarantee low maintenance demand.g. All converters under study would be well-established commercial solutions and should meet applicable standards. Moreover, the expected energy recovery amount would drastically cut down fuel consumption and consequently CO2 emission during flight.Project Results:Static Recovery SolutionAccording to the DoW, in M12 for the Static Waste Heat Recovery (SWHR) subsystem, a SABER behavioural and a functional model for the optimal solution, selected by the TM was submitted. The selected solution (based on the teleconference of 10/10/2012) consists of two converters. The first one supplies power from a thermoelectric generator directly to the 270V dc bus. A unidirectional boost configuration is selected and, for this converter, a MPPT controller has to be implemented. The other converter is a bidirectional buck/boost converter that has to feed energy to the dc bus during fast and slow transients and absorb from the dc bus energy during slow transients. A very large capacitor is connected to the converter input, so as for an exchange of energy between this capacitor and the dc bus to exist. In report ID2.3.1 and its annexes (ID2.3.1.Annex1 ID2.3.1.Annex2 and ID2.3.1.Annex3) a detailed behavioural model has been presented for each of the SWHR converters, taking into account power electronics and passive elements power losses, as well as a functional model which also takes into account those losses. Moreover, the MPPT algorithms which were selected to be tested for the first converter are modelled and simulated. The control for the second converter, in order to be in accordance with the “Demonstration of Energy Management strategies” document was also thoroughly presented. ID2.3.1 concluded with the analysis of the protection scheme of the converter. Dynamic Recovery SolutionAccording to the DoW, a detailed behavioral and functional analysis of the selected dynamic recovery topology has to be submitted with the M12 report. On the 10/10/2012 teleconference, after discussion, the TM decided that the topology for the DWHR converter will be a three-phase square-wave inverter. The control of this converter is done by shifting the AC voltage in order to create a phase delay between the inverter and generator voltage. The power flow from the generator to the converter is a function of the angle of this delay which will be called from now on alpha angle (Figure 2). In report ID2.5.1 and its annexes (ID2.5.1.Annex1 and ID2.5.1.Annex2) a detailed behavioural model is being presented for the DWHR converter, including semiconductor and passive component (inductor, capacitor) losses. Moreover, the model of a controller that complies with the “Energy Management Strategies” document has been presented. After that, the average model of the DWHR was presented, and a comparison with the corresponding behavioural model has been performed. Finally, a protection circuit has been presented, and it has been tested against DC-bus short-circuit and start-up peak currents. Full technical details are given in the attachment. Figure 3: The selected topology for the DWHR systemPotential Impact:The updated (2nd) version of Boost converter demonstrated:• Good operation during output open-short circuit and inrush current• MPPT performance significantly improved• Power efficiency significantly improved following the new inductor• Design of the power devices cooling system should be reviewedAs mentioned above, along with the provided Boost Converter prototype, MILTECH tried to come up with a new and modular design for all SWHR converters. In more details, a common design approach for the CPU cards and the semiconductor switches’ drive was adopted – in order to facilitate any possible future commercialization of these prototypes. Although a Boost Converter with the above concept was designed and constructed, it was not possible to test it. The driving architecture that has been adopted for the power switches was based on the IR2114 bootstrap driver, which is unsuitable for the switching frequency applied to the converter. The Bidirectional Buck-Boost converter was designed using the unified modular architecture as well, which is depicted in Fig. 23 The driving architecture of the power switches is again based on the IR 2114 bootstrap driver (shown in Fig 243), which is incapable of supplying the necessary pulses for high frequencies (100kHz for the Buck-Boost converter). This integrated circuit is not able to provide pulses having a frequency more than 10kHz-15kHz. Moreover, although this converter is a bridge-type one, the fact that the two switches do not operate complementary calls for a more sophisticated design of the bootstrap circuit. The overvoltage and overcurrent protection circuits (fuses, IGBT on the output, varistors) are not implemented, and so it was not possible to test the converter under fault conditions. Furthermore, by optical inspection, there are large current loops on the PCB, which is not suitable for high frequency switching current. What is more, the CPU card has been designed by Miltech in such a way so as to be the same one for any power converter. The heart of the CPU card is the PIC18F67J60 Microcontroller with an integrated 10Mbps Ethernet communications peripheral. This selection was imposed by the RENERGISE prerequisite for Ethernet communication. Testing showed that the selection of PIC18F67J60 Microcontroller is not suitable for the bidirectional buck-boost converter, due to the high sampling rate that is required for the implementation of hysteresis control. So, for the case of this converter a different CPU design approach has to be followed, with more powerful DSP solutions. Finally, the CPU cards include a rather complex “Sample and Hold” circuit, which operation and usefulness is not adequately justified. MILTECH has been provided the supercapacitor modules and has constructed the PCBs for the Supercapacitors balancing action. The DWHR converter prototype demonstrated:• Power circuitry operates without major problems.• Good efficiency (over 99%)• Missing DC-bus filter and synchronisation circuitry. The DWHR prototype tests were limited, due to major deficiencies. The final control board including the synchronisation circuitry, as well as the control software has to be developed so that the DWHR prototype can operate in rectifying mode. Also, the necessary protections, approved in the CDR have to be implemented. In addition, the necessary L-C filter has to be added in order to keep the DC-bus current ripple within the MIL STD704 specifications. Finally, a proper heat sink has to be installed to ensure good heat dissipation during operation at full power 30kW.In general, the following evaluation applies for the whole system:• The Boost converter shows good performance.• The Buck-Boost converter has many design faults and is not capable to be part in the WHR system.• The DWHR needs many modifications (filters, fault protection, voltage synchronization) in order to become fully operational.It should be noted that the converters provided for the integration tests have many modifications to the ones, the schematics of which were presented and accepted during the CDR. In fact the boards were completely redesigned (following the above mentioned modular approach).List of Websites:Not relevant Related documents final1-grc-3-renergise.pdf
