The main components of MGTs are usually: a compressor, a heat exchanger (recuperator), a combustion system, a turbine, a generator and its associated power electronics, and a system to utilise the heat from the exhaust gases. All of the components have complex interactions with each other, so making a change to one component influences the characteristics of the entire cycle in different ways.
For example, if the pressure loss in the combustion chamber is reduced by the implementation of a new combustion system, the turbine operation point shifts to higher pressure levels, enabling an overall higher level of efficiency. Alternatively, a more compact, and therefore cost-effective, recuperator with a higher pressure loss can be used without compromising plant performance.
To examine these characteristics and dependencies and demonstrate the potential of new cycles like the Inverted Brayton Cycle and the coupling of an MGT with a wood gasifier, or with an advanced high-temperature fuel cell, the department has developed numerical simulation tools. These allow both the calculation of steady-state variables and the analysis of dynamic changes in which the interaction of thermodynamics, rotor dynamics and machine control can be examined. Unlike detailed, three-dimensional flow calculations using CFD, the goal of these simulations is to determine important characteristics of the cycle even without precise geometric data and with little computational effort.
The modular and flexible structure of the simulators allows for continuous development and permits adaptation to different applications. For example, special consideration was given to a freely expandable species interface, which allows easy implementation of new fuels. This allowed, for example, an assessment to be made in advance of the impact of using wood gas in the MGT test facility, which was designed for natural gas combustion. The changes in the operating points of the turbine and compressor due to differences in exhaust gas volume and composition and the expected increase in efficiency through the reduction of combustor pressure loss could both be explored.
Due to the low computing time demands, sensitivity analyses can be carried out efficiently, such as the study shown in the accompanying image (2), to achieve increases in efficiency by improving individual components of an MGT-based range extender as an example for use in cars.
By simulating entire system networks, consisting of MGTs, thermal storage systems, consumers, etc., operating strategies can be subsequently simulated and optimised. One possible application for this is the optimisation of the operation of district or local heating power plants, depending on load profiles. The goal of such optimisation is to increase the hours of operation and reduce the number of start-stop cycles, resulting in a shorter payback period and an increase in the service life of the plant.
Accuracy of the simulation results can be significantly improved by better determination of model parameters based on experimental data. In return, simulation data show potential for optimisation of the instrumentation in test equipment and test procedures, and can speed up troubleshooting. Because software development and operation of various MGT test rigs is combined within the unit, the benefits of close collaboration between software developers, users and testers are able to fully evolve.
[1] Study on Range Extender Concepts for Use in a Battery Electric Vehicle - REXEL