Hybrid power plants – consisting of a micro gas turbine (MGT) and a high-temperature fuel cell system (SOFC) – are considered as the most promising future power plant concept for improving the efficiency of power generation and the operating flexibility while reducing the emission of carbon dioxide and other pollutants. By coupling a MGT with a SOFC, the fuel is converted more efficiently, thereby increasing the efficiency of the plant, which in turn makes it more economical and reduces its environmental impact. Depending on the size of the plant and the components used, electrical efficiencies up to 70 percent are feasible (see Image 1).
Achieving electrical efficiencies of more than 60 percent is possible even in small facilities that operate in the lower kilowatt range (Pel < 50 kilowatts). Hybrid power plants are considered one of the power plant concepts with the highest attainable electrical efficiency levels. With respect to pollutants, emission levels lower than 10 ppm of nitrogen oxides (NOx) and 25 ppm carbon monoxide (CO) can be achieved (at 15 Vol% oxygen (O2)). In addition, the power plant is capable of performing fast load changes and of ensuring very low part load operation – which simplifies the integration of renewable energy technologies. Image 2 illustrates the calculated operating range of a hybrid power plant consisting of a Turbec T100 MGT and a planar SOFC. The power output can be varied in a wide range within the temperature limits of the SOFC and the MGT rotational speed.
Another advantage of this system is its fuel flexibility. Besides conventional fuels, hybrid power plants in principle are suitable for using, different types of biogas (-> Bio-HyPP EU). In addition, thanks to their scalable plant capacity, hybrid power plants have a broad power output spectrum ranging from just a few kilowatts to several megawatts. Like MGTs and SOFCs, hybrid power plants are suitable for decentralized combined heat and power systems (CHP) increasing the overall system efficiency, economy and reducing carbon dioxide and other pollutant emissions. Depending on the plant size, these decentralized power plant systems can efficiently supply power to multi-story dwellings, public facilities – such as hospitals, schools and kindergartens – industrial, commercial and trading establishments, as well as city districts.
The Institute has been working on a long-term project on the research, development and implementation of a SOFC/MGT hybrid power plant system in collaboration with the DLR Institute of Engineering Thermodynamics since 2006. A work plan consisting of several stages was established.
Functionality of the hybrid power plant
The potential of the hybrid power plant concept is based on the interconnection of the high-temperature SOFC with a MGT (Figure 3). First, the process air is compressed using the gas turbine compressor. With the compressed air, the SOFC pressure vessel is purged, equalizing any pressure differences between the interior of the SOFC and the outside. This also avoids the formation of explosive mixtures in case of leakages. The compressed air is pre-heated up to 720 degrees Celsius in the recuperator and supplied to the cathode side of the SOFC. Pressurizing the fuel cell increases its electric power output at the same fuel usage.
The operating temperature of the pressurized fuel cell is between 680 and 850 degrees Celsius, depending on its operating point. Desulfurized and reformed natural gas is fed to the anode side of the fuel cell. The anodic gas is partially recirculated to maintain the reformation process, to preheat the fuel and to increase the fuel utilization. The fuel utilization in a SOFC is below 1. Therefore the SOFC exhaust gas at 850 degree Celsius still contains hydrogen. These gases are then fed into a specially designed combustion chamber, capable of operating with both the fuel cell exhaust gas and natural gas. Due to the increased inlet temperature and hydrogen content of the anodic exhaust gas being fed into the turbine combustion chamber, considerably less fuel than required by conventional gas turbines – or no fuel at the optimum operating point – is needed to attain the necessary turbine inlet temperature. The exhaust gases are expanded to atmospheric pressure in the turbine, which powers a generator that produces electricity alongside the high-temperature fuel cell. The hot exhaust gas is then fed to the recuperator to preheat the SOFC inlet air.
Status of the research work
In the first phase, the operational characteristics of the SOFC and MGT subsystems were analyzed separately and a test rig to investigate SOFC/MGT hybrid power plant systems based on the micro gas turbine Turbec T100 was installed and put into operation. Simulation models were developed generated for the individual plant components and the entire system, controlling and operational concepts were developed, and analyses of cycle designs were conducted.
In the second phase, the dynamic behavior of SOFC/MGT hybrid system concepts was studied by using a virtually coupled test facility. The test rig is composed of a Turbec T100 MGT, a piping system and a SOFC emulator (see image 4). By analyzing the transient behavior of this test plant, it was possible to refine the operational concepts gained in Phase 1 and develop control concepts. Additional research topics were the characterization of a pressurized high-temperature fuel cell – for which a high pressure SOFC test rig was built – as well as the expansion, optimization and validation of the numerical models and the development of the components.
Current research projects at the Institute of Combustion Technology
During the ongoing third phase, the results are being transferred to a smaller test facility, which is based on a MTT EnerTwin MGT with an electric power output of three kilowatts that will be coupled to a high-temperature fuel cell with an electric power output of approximately 30 kilowatts. One of the advantages of this power range is the spatial and financial applicability of a coupled system, in particular with regard to the procurement of a suitable SOFC fuel cell. Another advantage of this size concerns usage: a plant of this scale can be used in larger residential units, in small and medium-sized enterprises, and in companies in the CTS sector (Commerce/Trade/Services).
The EnerTwin micro gas turbine was experimentally characterized and an analysis of the influence of increased pressure loss between the compressor and the turbine was conducted. An SOFC exhaust gas burner for the hybrid power plant was designed, manufactured and is currently tested.
The construction of a demonstration plant is under way, being carried out step by step. As in the demonstration plant based on the Turbec T100, two separate hybrid power plant test rigs of the MGT and SOFC subsystems are set up. In the MGT test plant, the SOFC properties (pressure loss, residence time and temperature increase) are being emulated. The influence of the MGT on the SOFC will be studied at a SOFC test plant by the Institute of Engineering Thermodynamics. The system characteristics and the control concepts will be tested and optimized for both test plants before the two systems are coupled.
In addition, a MGT and SOFC hybrid power plant using biogas is currently being developed and built in collaboration with industrial and scientific partners under the EU project 'Bio-HyPP'. Within the framework of this project various system components (turbo components, recuperator, etc.) for the hybrid power plant are being optimized (-> Bio-HyPP EU).
Institute of Engineering Thermodynamics
EnBW AG (Energie Baden-Württemberg AG)
Micro Turbine Technology BV (MTT)
University of Genoa (UNIGE)
Technische Universiteit Eindhoven (TU/e)
EnBW Energie Baden-Württemberg AG
Support code: 03ET6032
Project Bio-HyPP funded by the European Union's Horizon 2020 research and innovation programme under grant agreement No 641073