Keywords

1 Introduction

The micro gas turbine (MGT) is a small heat engine that uses gaseous or liquid substances as fuel. It has gradually attracted public attention with the advantages of fuel diversification, low consumption rate, low noise, low emissions, and low maintenance rate [1]. The MGT has modular components with a capacity of more than 1000 kW can be provided to meet larger load requirements through flexible integration and parallel stacking of multiple units [2]; at the same time, when it is used in cogeneration, the heat energy of the micro gas turbine exhaust can be recovered to generate heated water or low-pressure steam which can be used effectively with absorption cooling equipment [3, 4]. At present, there are more than 360 sites in the United States using micro gas turbines for cogeneration with a total capacity of 92 MW, accounting for more than 8% of the total number of cogeneration sites in the United States. It is the best way for multi-purpose small distributed power generation and micro-cogeneration [5].

2 Advanced Micro Turbine Series Plan in the U.S.

The United States is at the dominant level in the development and application of micro gas turbines. As early as 2000, the U.S. Department of Energy Decentralized Energy Office commissioned the United Technology Research Center to carry out the project called “Micro Gas Turbine System Plan 2000”, which was implemented from the perspectives of performance and application to promote the improvement the strategy of effective distributed. The specific targets include that the system cost shall be $500/kW, electrical efficiency shall be 40%, overhaul interval shall be 11,000 h, Ox emission shall be 7 PPM@15%O2, and it is accessible for gas-liquid fuels [6]. At that time, about 50% of the electricity in the United States was provided by coal-fired power plants, the average power efficiency of the grid was about 32%, and it was affected by line losses, especially during peak power periods when the losses were as high as 15% [3]. At the same time, the average value of nitrogen oxides based on grid power output in the United States in 2000 was 3.0 lb/MWh. According to the electrical efficiency of 32%, it is about 70 PPM NOx@15%O2, which is five times higher than the level of gas turbine generator sets with natural gas [7]. Moreover, the high cost of building a new transmission and distribution network and relocation challenges in densely populated urban areas have forced the US Department of Energy to formulate a complete research and development plan for micro gas turbines.

The United Technology Research Center initially adopted an integrated method to realize the R&D plan. The integrated micro-turbine enhanced operational performance, and its electrical efficiency reaches about 33%. The organic Rankine cycle subsystem was able to convert exhaust gas into additional electricity. Among them, the prototype of the micro-turbine is the 400 kW micro turbine ENT400 powered by the ST5 engine of Pratt & Whitney Canada. However, the development of ENT400 was stopped in 2002 due to technical and financial issues. Subsequently, the United Technology Research Center shifted its focus to Capstone’s C200 micro gas turbine (Fig. 1) [8, 9].

Fig. 1.
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Appearance (left) and internal structure (right) of C200 micro gas turbine from Capstone, USA

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In 2016, the Office of Advanced Manufacturing of the U.S. Department of Energy summarized the technical performance characteristics of micro gas turbine cogeneration systems with a scale between 65 kW and 1,000 kW (Table 1). It is believed that an on-board gas compressor is applied in most micro gas turbines to provide all required gas pressure, the inlet fuel pressure of the micro gas turbine usually needs to be close to 75 psig, the overall cogeneration efficiency of the micro gas turbine is 64%–72%, and the power-to-heat ratio is 0.53–0.77 [10]. The basic components of all commercial micro gas turbines include micro gas turbines and power electronic equipment with interconnection and parallel functions. Most micro gas turbine cogeneration systems achieve heat recovery through an integrated heat exchanger with basic components. The maintenance cost varies with scale, fuel type and technology (air bearing or lubricating bearing) (including fixed and random maintenance based on 6000 h/year), and most of them are in the range of 0.8–1.6$/kWh. In addition, the test results of the exhaust gas quantity showed in Table 2 indicate that the maximum NOx, CO, and VOC emissions are 9 PPM, 40 PPM, and 7 PPM, respectively. Moreover, the emissions range of CO2 used for power output and the entire cogeneration system is from 667 lbs/MWh to 804 lbs/MWh. In contrast, the emissions of a typical natural gas combined cycle power plant range from 800 lbs/MWh to 900 lbs/MWh, while the carbon dioxide emissions of coal-fired power plants are close to 2000 lbs/MWh.

Table 1. The technical performance of cogeneration system for the US micro gas turbine (according to the Office of Advanced Manufacturing of the US Department of Energy, 2016)
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Table 2. Waste emission characteristics of micro gas turbine systems (according to the Office of Advanced Manufacturing of the U.S. Department of Energy, 2016)
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Based on the performance of the commercial products mentioned above, the United Technology Research Center first recognized that the combination of high-performance micro-turbines and organic Rankine cycle systems are useful methods for micro-gas turbine systems to achieve high performance and low emissions. The electrical efficiency is more than 32%, and the NOx and CO emissions under are less than 10 (15% oxygen condition). In addition, the use of ceramic materials and environmentally friendly coatings can extend the durability of the micro-ceramic turbine and reduce the cooling airflow without affecting the turbine performance [7, 11, 12]; On the other hand, the stable micro turbine combustor taking natural gas as fuel can achieve ultra-low emissions in a wide adjustment range [13,14,15], and a high-performance and economical organic Rankine cycle system can be realized through heating, ventilation and air conditioning components [16,17,18,19].

In order to further accelerate the improvement and innovation of the micro gas turbines, the Ministry of Energy jointly released the report in April 2020 with other government agencies, industry and academia called “Gas Turbine Advanced Technology Plan 2030”, which is more inclined to explore new theories, new technologies and new manufacturing processes. The plan advocated complete innovation rather than inheriting the achievements from “Micro Gas Turbine System Plan 2000”. It focused on the power generation capacity of combined cycle gas turbines (large stationary turbines), which supply power to the grid; the simple cycle gas turbine generation capabilities include large stationary turbines (power supply), stationary turbines (oil and gas transmission), and micro gas turbines (power generation); gas turbines for commercial and military aircraft propulsion, including narrow and wide turbines for aircraft propulsion, micro gas turbines for UAV/unmanned vehicle power, medium and small and medium-sized gas turbines for vehicle power/vehicle hybrid power [20].

3 Current Status of Typical Foreign Products

The US Department of Energy has successfully incubated a series of micro gas turbines such as Capstone C series, AE-T100, MT70/MT250, etc., through the “Micro Gas Turbine System Plan 2000”.

Capstone in the United States, as the most famous manufacturer of MGT energy systems, the power of its products is mostly 30 kW–10 MW, which are widely used in energy efficiency management, renewable energy, critical power sources, transportation and various micro-grids. So far, Capstone has more than 9,000 systems and nearly 10,000 equipment in 83 countries. Capstone’s micro gas turbines integrate aviation turbine engines, magnetic generators, advanced power electronic equipment and air bearing technology with low maintenance. A variety of fuels, including natural gas, associated gas, biogas, liquefied petroleum gas/propane and liquid fuels (diesel, kerosene and, including aviation fuel) are used. It can be installed separately or configured in multiple groups. It can run in parallel or independently with the local power grid. The product feature is that there is no problem with cooling system maintenance and coolant treatment. The emission of NOx is less than 5 PPM, and only 65 dB @10 m of noise are generated during operation. It has multi-fuel capability with expandable monitoring functions for remote equipment status performance [21].

The Ingersoll Rand Energy Systems subsidiary, which once belonged to Ingersoll Rand, has released an MT70 micro gas turbine that takes natural gas as fuel with the power generation output of 100 kW, the power factor of 0.82, the heat output of 85.3 btu/h–119.485.3 btu/h when in the open air. The average annual NOx emission is less than 2 t of power plants with the same power [22]. It is compatible with oil-free rotary screw, contact cooling rotary screw and centrifugal air compressor. It can be used in wastewater treatment and agricultural facilities, landfills, oil production facilities, hospitals and factories. However, the Ingersoll Rand Energy Systems subsidiary was sold. Therefore, the further development of the micro gas turbine business was stopped.

Different from the organic Rankine cycle adopted by the U.S. Advanced Micro Turbine System Plan, the British Bowman Company applied the electric turbine compound system. Its products improve the power density and fuel efficiency by recovering the waste energy in the exhaust gas of the generator set, thereby making the gas and diesel generator set work more efficiently. Compared with the organic Rankine cycle, the electric turbine compound system is more compact, cheaper, and easier to install. It is suitable for more than 800 common generators in the range of 150 kW to 2.5 MW, such as CAT 3516 TA /LE, Cummins KTA50-G3, KTA50-G8, QSK60, QSK78, Jenbacher J312 and J320, MAN 2842 and D2676, Mitsubishi 6R41, MTU 4000, Scania DC 12, Volvo TAD1642, rtsilä W20V34SG and W18V50SG, etc. Bowman has also developed customized high-speed motors for large engines equipped with wastegate turbine chargers, such as the 9 MW rtsilä W20V34SG gas turbine. The fuel consumption of the products with an electric turbine compound system is between 4% and 7%, and only 10% of the extra power is generated. More than 800 systems have been sold worldwide, and the cumulative stable operation has exceeded 2 million hours.

The Italian Ansaldo Energy Company mainly produces AE-T100 micro gas turbines for thermal power generation and three-heat power generation using natural gas as fuel. The pressure of the combustor is 4.5 bar, the turbine inlet temperature is 950 ℃, the fuel pressure is (0.02–0.1) bar, fuel temperature is 0 ℃–60 ℃, electrical efficiency is about 30%. It is low in maintenance requirements with an interval of 6000 working hours. It can be installed indoors or outdoors and can be remotely monitored. It is mostly used in the food processing industry, chemical preparation manufacturing, laundry, paper machine, logistics center, hotels, hospitals, office/commercial centers and other places [23, 24].

4 Current Status of Domestic R&D

In contrast, the level of research and development and manufacturing for domestic MGT power generation technology is relatively low. The commercial models used on a large scale are all from the U.S. Capstone Company. Some scholars and research institutions have achieved certain results in basic research theories and certain models of prototypes. However, the overall manufacturing process, materials and equipment maturity are far behind those of developed countries.

With the distributed power generation and cogeneration systems are more and more widely used in industry, China’s demand for micro gas turbines has also grown rapidly where products with a power of less than 100 kW account for 56%, and products with a power of 100 kW–300 kW accounts for 29%. The research and development and manufacturing for micro gas turbines with independent intellectual property rights are still in the initial stage. The products are either OME of foreign companies or co-production with foreign companies. For example, Aero Engine Corporation Hunan Power Machinery Research Institute had self-developed 75 kW micro gas turbine by referencing the micro gas turbines experience in Germany, France, Britain and other countries in the 1990s, which has a market share of 100% in the domestic vehicle power market; Shanghai Jiao Tong University had co-developed 30 kW and 50 kW low-calorific value micro gas turbine generation devices with the Commonwealth Research Institute of Australia, and produced a small batch of micro gas turbines for water pumps with 37 hp; Shanghai Astronautics Energy Co., Ltd. and U.S. Capstone jointly manufactured parts and components; Harbin Dongan Technology Development Company, successfully developed the first domestic 18 kW micro gas turbine generator set with independent intellectual property rights in 2015, which can be used for auxiliary power devices of vehicles and emergency backup power supply for satellite communication base stations; Shanghai Fanzhi Energy Equipment belonging to ENN Group Co., Ltd. completed the 100 kW micro gas turbine development project in March 2017 and handed it over to ENN Power for small-scale production; in August 2020, the major key scientific and technological project of the Municipal Science and Technology Commission called “Research on the Key Technologies of High-Efficiency Transmission and Combustion of Micro Gas Turbines” was accepted by the Shanghai Municipal Science and Technology Commission which was led by ENN Power and participated by Tongji University and Shanghai GENE Automation Technology Co., Ltd. in Shanghai. Other research institutions and manufacturing companies are still in the laboratory prototype stage without industrialized complete machine products with independent intellectual property rights. Among the domestic micro gas turbines mentioned above, small-batch industrialized products with a high degree of independent intellectual property rights were represented by ENN100 micro gas turbines developed by ENN Energy Power. Since the operation of the cogeneration system of Lanxi Best Aluminum Products Co., Ltd. in March 2018, the system has been operating stably, and the cost is about 8,000 RMB/kW, which is greatly reduced compared to imported micro-gas turbines. On the other hand, the thermoelectric ratio can be adjusted flexibility, and the exhaust temperature can be continuously adjusted between 270 ℃ and 650 ℃. Compared with a single regenerative cycle unit, it is more widely applicable. However, there are still problems such as high maintenance rate, frequent shutdowns, and multiple failures of equipment components.

Given the fact that the R&D and manufacturing capabilities of China’s gas turbine (including micro gas turbine) are far behind those of developed countries in Europe and the United States, in September 2019, the National Energy Administration issued Response of The National Energy Administration on the Inclusion of 24 Projects Including the Gas Turbine Power Generation Project of Huaneng Nantong Power Plant in the First Batch of Gas Turbine Innovation and Development Demonstration Projects which clearly demonstrates 22 gas turbine models and two operation and maintenance service projects covering heavy gas turbines and small and medium-sized gas turbines developed by major domestic gas turbines development companies such as Harbin Power Plant Equipment Corporation, Dongfang Electric Corporation, Shanghai Electric Group, Aero Engine Corporation, and China State Shipbuilding Corporation.

5 Conclusion

Micro gas turbine generation technology has significant advantages. It can guarantee the safety and efficiency of electric energy in various environments compared with the existing diesel generator sets with inherent disadvantages such as large volume, high noise, high vibration, obvious target characteristics, single fuel, and poor power adjustment ability. Therefore, it has strong application potential in the civilian and military fields. Compared with developed countries, China’s manufacturing process, materials and equipment maturity are backward. Therefore, it is necessary to carry out more research to solve the problems such as high maintenance rate, frequent shutdown and multiple faults of equipment components.