The micro gas turbine can use natural gas, methane, gasoline, and diesel as fuel. It has the characteristics of small size, light weight, high efficiency, low pollution, and simple operation and maintenance. It is currently a fairly mature and commercially competitive distributed power source. This type of generator set is composed of a micro gas turbine, a built-in high-speed inverter generator directly driven by the gas turbine, and a digital power controller. The high-pressure air from the centrifugal compressor is preheated by the turbine exhaust in the regenerator. Then it enters the combustion chamber to mix and burn with fuel, and the high-temperature gas is sent to the centripetal turbine to perform work, which directly drives the high-speed generator to generate electricity. The generator first sends out high-frequency alternating current, then converts it into high-voltage direct current, and then converts it into alternating current for users to use.
The structure of the micro gas turbine power generation device mainly includes two parts: mechanical power and electric energy conversion. The electric energy conversion part involves rectification and inverter technology in the field of power electronics. The mechanical power part is the core of the entire power generation device, which mainly completes the conversion of chemical internal energy generated by the combustion of fuel (gas or liquid) to mechanical kinetic energy.
Figure 1 shows the internal structure of the C30 micro gas turbine from Capstone, USA. The names of the various components are marked in detail in the figure.
It can be seen in Figure 1 that the rotating shaft of the permanent magnet generator, the impeller shaft of the intake compressor and the gas turbine shaft are coaxially connected. The output AC power of the turbine driven by high temperature and high pressure gas has a high frequency, so the back end must be used by the transformation of power electronic devices. The increase in the output frequency of the generator can effectively reduce its volume and weight, which is conducive to the lightness and miniaturization of the entire device. The casing of the permanent magnet generator is surrounded by radiating fins. Here is the passage through which air enters. The inhaled air passes through the impeller of the intake compressor to make the pressurization ratio 3-5 times. The pressurized high-pressure air will be preheated by the return heat exchanger. The high-temperature and high-pressure air and the fuel injected by the fuel injector will be injected into the combustion chamber in an appropriate ratio, which is strictly controlled by the control system and is fully adjustable. At the same time there is a proportional value with the highest efficiency. Therefore, most of the kinetic energy (about 2/3 of the total kinetic energy) of the gas turbine’s work will be used to drive the intake compressor, which will not reduce the efficiency but make the fuel burn more fully, thereby greatly increasing the heat production and power generation efficiency of the entire device.
The high-temperature and high-pressure gas produced by the violent combustion in the fuel chamber does work on the expansion of the turbine and drives the gas turbine to rotate at a high speed (the rated speed of the Capstone C30 is 96,000 r/min). Because the turbine works in high-temperature gas, the material of the turbine must be an alloy material resistant to high temperature and pressure. The gas after work will enter the return heat exchanger and be discharged from the exhaust port. Compared with the normal temperature air, the gas still contains a lot of heat energy, which can be further used for cold and heat production in combined cooling, heat and power, thereby improving the comprehensive utilization efficiency of the device.
Gas turbines with a power of several hundred kilowatts (kW) have been developed and used in the 1940s and 1960s. They are called small gas turbines for power generation and drive. The characteristic of the unit is tens of thousands of revolutions per minute, and the load is driven after the reduction gear is used to reduce speed. The National Aeronautics and Space Administration (NASA) developed a micro gas turbine power generation device based on a turbocharger in the 1960s. Subsequently, in the 1970s, research on micro gas turbines used as auxiliary power stations on the space shuttle was carried out, and the power generation technology of micro gas turbines has thus been rapidly developed. However, this type of gas turbine was not widely used later, because the efficiency of a simple cycle of a small-power gas turbine was relatively low, and it could not be compared with an internal combustion engine.
The emergence of high-speed permanent magnet generators makes the connection between the generator and the compressor no longer need a reduction mechanism, which greatly reduces the quality of the entire unit, greatly reduces the size, and reduces the cost. At the same time, air bearings are used instead of rolling bearings. Since air bearings do not require a lubrication system, the parts of the unit are greatly reduced and the manufacturing cost is further reduced. In order to improve the thermal efficiency of the unit, high-efficiency compact regenerators are commonly used, with a heat recovery efficiency of up to 90%, making the cycle efficiency of the micro gas turbine generator set up to about 30%.
The advanced micro gas turbine design concept integrates the gas turbine and the generator. The size of the entire gas turbine generator set is significantly reduced, the quality is reduced, and the advantages are obvious. The rotor of the micro gas turbine and the generator rotor are coaxial. The entire gas turbine generator set has only one moving part. The reliability is greatly enhanced, and it is almost maintenance-free. Therefore, the micro gas turbine has strong vitality. After half a century of development, micro gas turbines have been widely used in countries such as the United States, Britain, Japan, Canada, and Russia.
From the perspective of the world’s gas turbine development, the production of advanced gas turbines has been concentrated in several large companies: The American Solar Turbine Company, the American GE Company, the German Siemens Company (Siemens AG), the German MAN Turbo Company, the British Rolls-Royce Company and the Ukrainian Sugon Machinery Consortium have formed a high degree of core technology monopoly and market monopoly, and have become one of the important symbols of a country’s industrial foundation and technological level. With the development of micro gas turbines, the market share is also increasing year by year. According to a related report released by Navigant Research, by 2024, the global micro gas turbine market sales are expected to exceed 10 billion US dollars.