As part of the goals of continuously improving the efficiency of energy conversion whilst simultaneously minimizing emissions of CO2 and other pollutants, as well as increasing operational flexibility, hybrid power plants are a promising solution. They consist of a micro gas turbine (MGT) and a solid oxide fuel cell (SOFC), which, when the two systems are combined, allows a more efficient use of fuel, in turn increasing plant efficiency, cost effectiveness and environmental compatibility. Depending on the size of the plant and the components used, an electrical efficiency of 70% is generally possible (see figure 1).
Particularly on the lower end of the scale of system power size (Pel < 50kW), electrical efficiency factors may be reached of 60% and above. This means that in the long term, hybrid power plants belong to the power plant concepts with the highest electrical efficiency possible. In terms of pollutant emissions, values of under 10 ppm NOx and 25 ppm CO at 15% vol. residual oxygen may be achieved. Additionally, the power plant is able to perform fast load-cycle changes and to ensure an extremely low partial load operation, which simplifies the integration of the volatile renewable energy forms considerably. A further advantage of the system is fuel flexibility. Aside from conventional fuels, hybrid power plants are, for example, usually suited to the use of fuels from the gaseous products of renewable raw materials. Hybrid power plants are also able to offer a wide power spectrum, from just a few kilowatts to several megawatts, thanks to the high scalability of the plant size. The system is suited for use in the decentralized sector (in a similar way to micro gas turbines or SOFC systems) for cogeneration with combined production of electricity and heat. This increases efficiency of the entire plant and with it cost effectiveness, whilst reducing emissions of CO2 and pollutants. Depending on the size of the plant, such decentralized power plant systems can efficiently supply apartment or public buildings, such as hospitals, schools or kindergartens, industrial, business and retail establishments, or even whole neighborhoods.
For the development of an SOFC/MGT hybrid power plant, the DLR Institute of Combustion Technology and the DLR Institute of Technical Thermodynamics have joined forces. The aim of the joint project is to research, develop and realize an SOFC/MGT hybrid power plant system in Germany. A long-term working program has been created to this end, consisting of several different phases.
The potential of the power plant concept lies in the cell interconnection of the high-temperature fuel cells with the gas turbine (fig. 2). Using the compressor in the gas turbine cycle, the process air is compressed for the fuel cell. This compression helps increase the electrical output whilst using the same amount of fuel in the fuel cell. Additionally, the compressed air is pre-heated in the gas turbine’s recuperator to around 600°C. Operating temperature of the pressurized fuel cells is around 650 and 850°C, depending on the point in the operating cycle. Aside from electrical power, hydrogen-rich exhaust gases are produced at a temperature of up to 850°C. These are transferred to a specially-developed combustion chamber which can also be operated using natural gas, thanks to the combination with a conventional combustor stage. The process air’s increased temperature upon entry to the combustion chamber of the gas turbine as well as the hydrogen content mean that in comparison to a conventional gas turbine cycle, far less fuel is consumed. In order to reach maximum efficiency, the individual components must be coordinated so as to make additional supplies of fuels to the combustion system unnecessary. Releasing the exhaust gases to ambient pressure in the following turbine operates a generator which produces electrical power alongside the high-temperature fuel cells.
Current stage of research activities
The first phase consisted of analyzing the operating characteristics of each subsystem separately. A testing laboratory, based on the micro gas turbine Turbec T100, was installed and put into operation for the examination of SOFC/MGT hybrid power plant systems. Furthermore, numerical models were produced for the plant components and the entire system, controling and operating concepts were developed, and cycle analyses were carried out. In the second phase, which is currently underway, the dynamic system behavior of SOFC/MGT hybrid power plants is being examined, and operating and controlling concepts are being tested and optimized. Further emphasis is put on the characterization of operating high-temperature fuel cells under pressure, on the expansion, optimization and validation of the numerical models, as well as on component development. Finally, the knowledge gained will be transferred onto a pilot plant which is connected to an electrical output of 15-30 kW. One advantage of this operation capacity stems from the spatial, financial and above all, the realistic implementation of such a system, particularly with regard to the procurement of an appropriate SOFC. Realizing and building a demonstration plant based on the micro gas turbines by the company MTT is due to happen in phase three, from 2014 onwards. The plant is intended to demonstrate the high level of electrical efficiency of the hybrid power plant as an innovative, decentralized cogeneration plant for use in the industrial, trading and service sectors and for apartment buildings.
Current research topics at the Institute of Combustion Technology
EnBW Energie Baden-Württemberg AG