The conceptual design and simulation of a micro gas turbine generator.
Micro gas turbine generator
-system
8.1 THE MICRO GAS TURBINE GENERATOR SYSTEMThe aim of this study is to do a conceptual thermo-fluid design of a low cost micro-turbine power generator with commercially available turbo machinery and to simulate this system using the thermal fluid network solver ~ l o w n e x ~ .
Operation of elementary turbo machinery was explored, evaluation of different variations of the real Brayton cycle was done and the influence of performance parameters on the systems was explained during the initial parts of this study. It was followed by a detail discussion of the turbo machinery that includes information on types of turbine machinery and the turbine components (compressor and turbine). The selection process that was followed in this study to appoint the relative machines to be used in the micro gas turbine generator was shown.
Part of this included an investigation into the custom design of turbine machinery, especially for this application. Once the selection process was concluded, it was possible to calculate the selected combination of turbo machinery's ideal operating conditions by doing system analysis.
Limitations of the selected turbine machinery were used to calculate the operating conditions of the other subsystem that are part of the micro gas turbine system. The results of the system analyses set the initial boundary conditions for the rest of the subsystems like heat exchangers and the combustion chamber.
The system's combustion chamber can operate with various fuel sources with little or no changes to the fuel control system. This ability to operate on a variety of fuels, including biogas, highlights the wide spectrum of application of this micro gas turbine system. It can be used in rural environments without a constant supply of fuel.
Detail design of the heat exchanger and combustion chamber subsystems were done in the Chapter 5 of this study, followed by a steady state simulation done in the code ~ l o w n e x @ . This simulation proved that the configuration of the combination of turbine machinery, together with the designed heat exchangers and combustion chamber can produce an operating micro gas turbine system, which proved that this system is operational under steady state conditions.
Therefore, transient simulations that prove the operation of the system at different operating conditions had to be done. Firstly, start-up was proven by controlling the system from low speeds onwards to high speeds (close to that of working conditions) without any load on the generator.
Gentle load following simulation followed to prove the system's ability to generate power under normal operating conditions, followed by instantaneous load following. Here, the micro gas turbine generator system was subjected to harsher load following conditions as associated with load rejection. Thereafter, the system's performance to off-design conditions like overload and surging were investigated, but showed that it cannot be accommodated and control system
accessibility to remote locations around the world that can only be reached by land makes it an attractive commodity for users in remote places that do not have the transportation infrastructure to accommodate big systems. Another factor that offers interest is the flexibility of the system, i.e: Temporary sites can make use of this system wherever the location, as long as the necessary auxiliary systems is available also.
Figure 8.1: Illustration of the physical layout of the micro gas turbine system 8.3 RECOMMENDATIONS
The conceptual design and simulation of a micro gas turbine generator system was done as part of Master's study in order to prove the concept, therefore with limited funds. With the concepts proven more effort should be put in to demonstrate a functional operating unit. For this more attention should be paid to:
Turbo machinery:
Although touched during this study, more money and time has to be invested into the feasibility study of using custom designed turbo machinery. In this case, funding play an essential role, for turbine machinery is expensive, while developing turbine machinery are extremely expensive.
Henceforth the need for a good simulation package like Concepts NREC - COMPAL is emphasised to limit expenses of developing machinery.
During this study, only centrifugal turbine machinery was evaluated. More research is needed to assess the feasibility to use axial turbines for gas turbine systems with higher efficiencies, but also a higher cost.
Thermal barrier technology is constantly growing allowing higher TIT'S by using barrier technology on turbine blades. If this technology is acquired to allow higher TIT'S significantly higher thermal efficiencies as well as specific work output can be reached even with the current conceptual micro turbine design.
Heat exchangers
The conceptual design and simulation 8 1
of a micro gas turbine generator.
Micro gas turbine generator system
More research on heat exchanger designs that have an improved efficiency. An increase in efficiency means that a smaller heat exchanger can be used and therefore lower costs to produce the heat exchanger. These include spherical heat exchangers that are incorporated around the turbo machine, allowing the gas turbine to occupy a lot less space, while producing high efficiencies.
Lower the coolant inlet temperature. This will cause the drop in temperature between the LP and HP compressors bigger, therefore enhancing the system's thermal efficiency.
Combustion chamber:
The design done for the combustion chamber in this study for this micro gas turbine generator system is a theoretical design. Therefore, more time and research are needed to be able to design a more efficient combustion chamber.
Simulation packages of combustion chambers are limited and even ~ l o w n e x ~ was unable to simulate this rather common combustion chamber satisfactorily. Using CFD with more complex models will enable the designer to produce a combustion chamber with satisfactory performances and efficiencies. This can be done by optimisation of the combustion chamber combustion zones, dilution and wall cooling need to be researched in more depth.
The fuel supply and control system need to be designed. The combustion chamber controls the TIT, therefore, it is essential that the fuel control system is efficient to be able to react swiftly in order to supply high volumes of fuel, followed immediately by low fuel requirements moments later.
Auxiliaries
Load following control system: in order to allow for the reliable operation of the micro gas turbine system, a system is needed that is able to follow the electrical power load, while being able to protect the system against overloading and sudden load changes.
Lubrication system: As stated in this study, a reliable lubrication system is needed for both the lubrication as well as cooling of the turbo machinery's bearings. This system needs a back-up system as well.
Cooling system: A constant supply of coolant is needed; therefore the logistics of the cooling system has to be evaluated.
Generatorlmotor configuration: this unit serves as both the generator systems, as well as the start- up motor, that need to operate at different shaft speeds. An inverter is needed to control the motor's speed, as well as inverting the developed power to a constant frequency of 50 Hz.
8.4 CONCLUSION
The conceptual design and simulation of a micro gas turbine system that generates 72kW of electrical power continuously has been proved during in this study. Turbo machinery that has been used in this study is standard centrifugal turbo machines commonly used in big internal combustion, diesel powered machinery. As stated in the study, development of a custom turbo