Introduction
The combined cooling, heating, and power (CCHP) system has outstanding energy saving and environmental protection performance as a rapidly developing distributed energy supply system in recent years. The system can realize the cascade utilization of the waste heat from the gas turbine, which significantly increases the overall energy efficiency and reduces the pollutant gas emissions [
1]. From this viewpoint, compared with the internal combustion engine, the micro gas turbine (MGT) power generation has obvious superiority, such as lower cycle life cost, compact structure, simple maintenance, and lower pollutant emission than that of diesel engine, which is beneficial to the environment. It is worth noting that, with the further development of the MGT technology, its advantages will become more obvious [
2,
3]. However, due to the limitation of technology in different countries, the design and manufacture of MGT has always been the bottleneck for its wide application.
At present, Europe and the US are the world leaders in the product maturity of MGTs [
4–
8]. Early in 1995, Allied Signal, Capstone and Elliott demonstrated the 25–75 kW MGT prototypes at the 1995 Annual Power Conference in America. Then MGT was developed rapidly and new prototypes were launched every year. Several products have entered the market, including the 75 kW product from Honeywell (Allied Signal), the 30 kW and 60 kW products from Capstone, the 45 kW and 80 kW products from Elliott, several products with 30–250 kW from Ingersoll-Rand (Northern Research and Engineering Company), a series of products with 75–350 kW from GE, and a series of products with 35–200 kW from Bowman [
4,
5]. Other companies such as Allison Engine Company, Williams International, Teledyne Continental Motors, Volvo and ABB from Europe, and Toyota, IHI and Kawasaki from Japan, have also developed MGT products which have entered the global market [
6]. Obviously, the above MGTs mentioned are widely used in distributed energy supply systems, and power devices for auxiliary power units and so on. It also can be seen from relevant literature that these internationally renowned MGT companies own exclusive technical routes and intellectual property rights, not only from component design to power generation system integration, but also from manufacturing to product promotion [
7–
9], which greatly hinders the improvement of MGT technology in developing countries. In China, the MGT market currently mainly relies on the import, and there is no mature microturbine design system and product development system, which is an important issue worth thinking about by the researchers and relevant manufacturing enterprises to ponder.
Compared with product development, many researchers have conducted a lot of investigation on theoretical innovation and experiment of MGT, which provide indispensable technical guidance for MGT from theoretical level to product promotion [
10–
15]. To solve the efficiency decrease problem caused by losses like seal and tip leakages, an inverted Brayton cycle (IBC) based MGT was introduced in the research by Eleni et al. [
10], which was based on a single shaft, single stage conventional MGT. Reliable start up and stable operation within some operating range were demonstrated. Inoue et al. [
11] from Kawasaki Heavy Industries performed simulations focusing on the development of a 50 kW atmospheric pressure turbine for application in industrial and biomass gasification furnaces. Based on these simulations, they conducted a test run of a single staged atmospheric pressure turbine prototype operating with propane and a power output of 3–5 kW. The prototype using radial turbocharger components reached an electrical efficiency of 8% and a thermal efficiency of 26.2% [
12]. Although the introduction of water in the MGT cycle increases the electrical performance and flexibility of the MGT, only few of these engines have been experimentally tested and up to the present, no cycle is commercially available. Ward et al. [
13] gave a comprehensive review of the literature on research and development of humidified MGTs, pointing out that it is necessary to make the technology commercially available in the future. Alireza et al. [
14] conducted an experimental study to determine the effect of inlet total-pressure distortion on the performance of a MGT. The results showed that the performance was affected significantly with the gas turbine exposed to inlet flow with 60°, 120°, and 180° circumferential distortion patterns at different distortion intensities. Thomas et al. [
15] deve-loped a highly flexible approach on the steady-state analysis of innovative MGT cycles. The results showed that the new solver routine was superior to the standard Simulink algebraic solver in terms of system evaluation and robustness for given applications.
Therefore, based on the above research, in order to fill the gaps in the technology and product promotion of MGTs in China, Shanghai Jiao Tong University (SJTU) conducted the research on the theoretical design method to solve the key technology issues of 30 kw MGT with the financial and policy support of Shanghai Science and Technology Commission, and produced the prototype in cooperation with other manufacturers. The research into the combustion flow analysis and burner nozzle manufacture, the manufacture technology and design methods of compressor and turbine impellers, and the reliability test of MGT power generator were conducted, respectively. The theoretical and experimental results obtained are expected to promote the development of MGT technology in China.
Topology and technology of generating set driven by MGT
The MGT power generator mainly comprises a turbine subsystem, a combustion chamber, a generator, a control system, and an oil supply system. The turbine subsystem can be divided into a turbine, a bearing sleeve, and a compressor. The turbine and compressor share the same main shaft and the bearing sleeve locates between the turbine and compressor as the bearing structure in the middle. The main shaft extends outwardly at the end of the compressor and is connected to the generator by means of a coupling to provide work externally. The high temperature gas from the combustion chamber enters the turbine to drive the turbine to produce power. The turbine drives the compressor impeller to induce air from the atmosphere and sends the compressed air into the combustion chamber, where it burns with fuel. The combustion gas then enters the turbine, and forms a complete cycle. Since the turbine provides more power than the compressor impeller consumes, the remaining energy can be supplied to the generator for power generation through the extension shaft. The generating set driven by MGT is shown in Fig. 1.
Turbine
The turbine is composed of four main components such as the volute, the nozzle ring, the turbine rotor, and the heat shield. The volute provides the passage for the combustion gas to enter the turbine, expand and flow out from the turbine at last. The turbine inlet temperature is about 850°C and the working pressure is about 0.28 MPa. To ensure enough mechanical performance and strength at high temperature, the volute is made of high temperature alloy, which can meet the strength requirement under working conditions. By adopting the welding method in the manufacture of the turbine, the cost is reduced and the manufacturing period is shortened. The nozzle ring is located between the turbine inlet and the turbine rotor, which can accelerate the flow expansion and guide the flow direction. The radial flow rotor with a semi-open structure is adopted in the turbine subsystem, which is also made of high-temperature alloy like the turbine volute. It is cast by precision non-allowance casting method and driven by the combustion gas to provide work during operation. It is connected with the compressor impeller and the generator through the main shaft, providing the two components with the energy they need to do work. Because the turbine volute is at a high temperature under the operational condition, an external heat shield is provided to cover the entire turbine in order to reduce the amount of heat escaping into the air and avoid scalding the operator.
Table 1 lists the turbine design and structural parameters. The design mass flow rate is determined by 5% larger than the compressor mass flow rate. The final parameter is obtained after iteration calculations and multiple adjustment of mass flow rate, so that the final net output power is 30 kW.
In this work, VISTACCD, BLADGEN and ANSYS are used for 1D
–3D design of turbine and compressor impellers, and for flow analysis. The structure and aerodynamic parameters of the compressor and turbine are calculated and determined based on the GT design method [
16,
17]. The 3-D turbine is demonstrated in Fig. 2.
In addition, the MSC Nastran software is used to analyze the centrifugal stresses at the highest rotational speed during the operation of aluminum centrifugal compressor impeller. The static structure and vibration mode under operational conditions are analyzed, and the structural strength of centrifugal compressor impeller are investigated. The stress nephogram of the turbine is displayed in Fig. 3.
The analysis of the results indicate that, at 700°C, the tensile strength is 959.6–1088.6 MPa, and the yield limit is from 774–787 MPa. The maximum centrifugal stress is 417 MPa, which is smaller than the yield limit and tensile strength, with a large safety margin, and the temperature stress can be omitted. The first order modal vibration of the impeller and blade are 1819 Hz and 4830 Hz, which are 3.1 and 8.28 times of the working rotation speed, respectively. Therefore, the resonance of the blade and disc does not occur in the whole working process of the turbine.
The strength and modal analysis of the turbine suggest the turbine stress is less than its yield strength, which meets the strength requirement, and the natural frequency of the turbine is far less than its working frequency, which satisfies the vibration requirement. The turbine structural strength can meet the safety requirements under operating conditions, which illustrates the trial-manufacture can be performed in the next step.
Bearing sleeve
The bearing assembly is composed of the bearing shell, the bearing, the bearing housing, and the main shaft, etc. The bearing shell is the support of the whole turbine, which is installed on the oil tank as the base. The turbine and compressor are located at either side of the bearing shell and are screwed to the bearing shell respectively. The oil lubricated bearing is adopted. The bearing shell has an oil cavity inside which is connected to the oil supply system through an oil inlet port at the bottom to supply the oil to the bearing through an oil circuit inside the shell. The oil finally falls freely and is collected by the bearing shell and returns to the oil tank through the oil return port at the bottom. The turbine engine has two radial bearings and an axial thrust bearing. The radial bearing belongs to floating sleeve, and there is an oil wedge surface inside the floating sleeve, which forms an oil film to provide the radial support for the main shaft to maintain stability with the bearing when the main shaft rotates at a high speed.
Compressor
The compressor mainly consists of a muffler, an impeller covering, a compressor impeller, a diffuser and a volute. The muffler is located at the compressor inlet, which can effectively reduce the noise caused by inlet high-speed airflow. Besides, the external mesh screen can prevent large foreign matter from entering the compressor and colliding with the high-speed rotating impeller, which may, otherwise, cause damage to the impeller. The inlet impeller casing collects air and lets it into the impeller runner. The compressed air enters the combustion chamber through a pipeline and is combusted with natural gas in the combustion chamber, and the high-temperature gas after combustion is used to drive the turbine and output the power. BLADGEN is used for the two-dimensional design of compressor, mainly for the study of internal and external meridians of the meridional plane, axial length, inlet and outlet height, wheel diameter, thickness distribution, blade root and tip angle distribution, etc. After repeated checking, the ideal state can be obtained.
The compressor design and structural parameters are summarized in Table 2.
The 3-D compressor and its stress nephogram are exhibited in Figs. 4 and 5, respectively.
The analysis of the results show that the maximum equivalent stress of the centrifugal compressor is 127 MPa at the maximum speed, which is far less than the maximum allowable stress (385 MPa) of the material, and has a sufficient safety margin. The 1st order natural vibration frequency of the blade is 4697 Hz in the assembled condition, which is 8.1 times of the maximum speed and avoids the possibility of the blade resonating at this speed and meets the use of the compressor impeller. Since the centrifugal stress is small, it is not necessary to consider the influence of the aerodynamic force on the structure. The structural strength of the compressor impeller can meet the strength requirement of the fourth strength theory, thus, the next trial production can be performed.
After the completion of the aerodynamic design, structural design and strength check of each component, the 3-D compressor and turbine assembly is presented in Fig. 6. The gas turbine is manufactured by a turbo-machinery factory in Chongqing, China, as shown in Fig. 7.
Combustor
The combustor is composed of a flame tube, an outer casing, an intake manifold, and a fuel nozzle. The performance index of the combustor mainly involves combustion efficiency, pressure loss, stability, ignition range, outlet temperature distribution, and heat capacity.
According to the relevant technical index of commercial and military micro gas turbines, and considering the technical requirements of this project and the achievability of specific index, the technical index of the micro gas turbine combustion chamber is determined as follows:
(1) Combustion efficiency ηb: 0.98;
(2) Total pressure recovery coefficient σb: 0.95;
(3) Lean fuel flameout boundary (burning natural gas): residual air coefficient≤12;
(4) Pollutant emission: in line with the statistical level of domestic gas turbines;
(5) Weight: less than 30 kg;
(6) Unit body structure: a two-stage combustion chamber casing which can be separated for easy inspection and maintenance.
To ensure that the combustion chamber has a good ignition characteristic and stable combustion environment, the design method of combustor in air vortex axis engine is borrowed for this work. The combustor prototype is shown in Fig. 8.
High speed permanent magnet generator
High-speed rotor is one of the research hotspots in this field. It has two main features: the high rotational speed and the high frequency of the winding current and the stator core flux, which determine the unique technology of the high-speed generator.
The stator, mainly composed of the base, the main magnetic pole, the commutating pole, and and the brush device, is used to generate a magnetic field. The rotor, consisting of an armature core, an armature winding, a commutator, a shaft, and a fan, is used to generate electromagnetic torque and induce electromotive force. In the generator system, the permanent magnetic pole of the rotor is made of high-performance NdFeB permanent magnet material. The residual magnetic induction strength Br at room temperature is up to 1.5 T, the coerced force Hc is up to 25 kOe, and the maximum magnetic energy product is up to 40 MGOe. The material has a temperature resistance of 180°C. High frequency cold rolled silicon steel dw250-25 is used as the armature core. The armature winding wire is a high frequency electromagnetic wire with a temperature index of 180. The conversion of electromagnetic energy and mechanical energy in the generator is completed in a magnetic field. In this design, a permanent magnet is used to establish a magnetic field to complete the energy conversion. Then the rotor strength of high-speed permanent magnet generator is analyzed. Based on the theory of elastic mechanics and the finite element contact theory, the stress calculation model of high-speed permanent magnet rotor is established, and the interference magnitude between the sheath and the permanent magnet is determined. Finally, the strength of the permanent magnet and the sheath is analyzed. An interference fit is applied between the permanent magnet and the sheath, and a static pre-pressure is applied to the permanent magnet by the sheath to offset the tensile stress generated by the high-speed rotation, so that the permanent magnet can withstand a certain compressive stress when it rotates at a high speed, the safe operation of the permanent magnet rotor is ensured.
The generator has a rated power of 30 kW, a rated output voltage of 380 V AC/50 Hz, and a rated speed of 34000 r/min.
Control system
The control of the MGT generator set is mainly composed of the start-stop control, the speed control, the temperature control, the load control, as well as the over-speed, over-temperature, flameout, vibration and other safety protection of micro gas turbine. The control system communicates by network, with the main control cabinet and generator control cabinet. The control system consists of a PLC controller, a frequency converter, a rectifier, a speed governor, a fuel regulating valve, a speed sensor, a temperature sensor, and a pressure sensor.
The motor control is the core of the control system. The speed of permanent magnet synchronous motor has to be 35000 r/min according to the generator parameters provided by the user. Besides, the motor is required to switch the working state to power generating after a 35000 r/min is reached and ensure the power generation can be adjusted at the same time. Therefore, the selected driver has to have a high-speed multi-state switching control function for the permanent magnet synchronous generator. The control scheme is depicted in Fig. 9.
Oil supply and cooling system
The lubricating oil for turbine and generator are provided by a set of oil supply system devices, and the fuel tank is also used as the base of the whole unit. Because the oil supply pressure between the turbine and the generator is different, the lubricating oil is divided into two branches by a pressure reducing valve after being filtered and cooled by the oil pump. Meanwhile, the pressure reducing valve ensures that the pressure in each branch is not affected and maintains pressure stability.
Results and analysis
Combustion characteristics
Based on the design requirements of the micro gas turbine, an emission test is conducted. To evaluate the exhaust emissions of the combustor, a measurement experiment of the CO, NOx and smoke in the combustor exhaust gas is performed on the combustion simulator. The inlet flow rates of air and natural gas are within 0.150–0.252 kg/s and 0.0039–0.0086 kg/s, respectively. Besides, the outlet temperature of the combustor is controlled to within 800°C–900°C. A German Testo350 gas analyzer is equipped with the combustion system for the gas test. A Testo350 flue gas analyzer is suitable for heavy pollution sources monitoring, industrial boiler, burner, and the construction of power plants. An ultra-high temperature pressure sensor (PTG701 with a pressure range of 0– 10 MPa, a temperature range of 0°C–1000°C, a comprehensive precision of 0.5%fs, an output signal of 4–20 ma, and a response time of less than or equal to 20 ms) and a Pt100 temperature sensor (with a measurement range of –200°C–850°C, an allowable deviation of grade A+ 0.15+ 0.002 (t), a thermal response time of less than 30 s, a thermal resistance placement depth greater than or equal to 200 mm, and an output current signal of less than or equal to 5 mA.) are selected for the measurement of the turbine inlet pressure and temperature. The amount of pollutant emissions in combustor exhaust gas is given in Table 3 at the design condition.
It can be seen from Table 3 that with the increase in fuel/air ratio, the emission of NOx increases as well. When the inlet air flowing rate is set at 0.250 kg/s and natural gas at 0.0079 kg/s, NO emission reaches its maximum of 33.7 ppm. Meanwhile, the emission of NO2 reaches its maximum of 8.3 ppm when the air flowing rate and gas flowing rate are set at 0.248 kg/s and 0.0080 kg/s, respectively. It also can be seen from these results that emission of NOx is within acceptable range (≤35 ppm) under the design condition. The ignition success rate of combustion chamber reaches 98% at a low rotational speed, which guarantees the normal operation of the combustor.
High-speed permanent magnet generator test
The high-speed permanent magnet generator can start, drive, and generate output power. The electric motor operations mainly include electric operation and power generating operation. In the starting stage, the electric motor is in the electric operation status. Then it is in the power generating operation status most of the time. This system also can realize flexible switch from start motor mode to generator mode, and from grid-connected mode to off-grid mode, because of the multi-state switching control system adopted. In this work, the motor driver model of SP4403 (380–480 V, heavy load 45 kW, 96 A continuous current, closed-loop peak current 168 A) and speed feedback sensor (TAMA-KAWA TS2605N1E64 rotating transformer, maximum speed 40000 r/min) are selected.
The operation state of the generator at each stage is as follows:
The start-up stage: The electric motor drives the gas turbine forward to a speed of 20000 r/min. At this time, the electric motor works in the electric speed control mode, and the target speed (which can be adjusted) is 20000 r/min. At this stage, the motor acceleration time can be adjusted according to the actual situation.
The first acceleration stage: After the starting speed of the motor reaches 20000 r/min, the speed is kept constant, and the engine ignition command is issued to the upward system. When the gas turbine is ignited, the electric motor speed is set to enter the next rising speed stage.
The second acceleration stage: After the gas turbine engine is ignited, the motor needs to be adjusted to a target speed of 35000 r/min or slightly below that speed (The second rise speed target value of speed can be modified according to the technical requirement set.). In this condition, the motor and turbine are to drive the load at the same time to achieve the target speed, and the output of the electric motor is gradually reduced to zero. This phase should be adjusted during the operation of electric motor acceleration time and gas turbine ACC time matching, to prevent the electrical and the gas turbine from influencing each other.
The second velocity keeping stage: After the second target speed is reached, the motor control mode is adjusted to make the motor stop electric operation until the gas turbine speed rises to 35000 r/min (adjustable). When the speed reaches 35000 r/min, the motor control mode is changed to power generation state, and the motor power output power is adjustable within the full power range (0–30 kW). There is no vibration when the motor is switched to the generating state.
The stopping stage: Since all power sources are provided for the gas turbine in the second velocity keeping stage, the motor works in the state of reverse power generation. After the shutdown, only the fuel quantity of the gas turbine is set to 0. The gas turbine will not output power. The electric motor running signal is cut off.
A prototype of the generator is completed and the controller is debugged with several adjustments of motor parameters. The generator no-load test and the generator load test were conducted.
In the generator no-load test, the controller terminal voltage is set to 380 V and the no-load current is 3.44 A. The no-load rotating speed reaches 32960 r/min and the generator could operate well. However, due to the restriction of controller frequency, the generator does not reach its maximum rotating speed.
In the generator load test, the two generators are paired by a flexible coupling, one operating as the motor and the other as the generator.
First the controller terminal voltage is set to 480 V and the current is 68 A. The load rotating speed is 25000 r/min. The output voltage of the generator is 280 V and the output current is 91.5 A. The generator operates with an output power of 25.62 kW.
Next, the controller terminal voltage is set to 480 V and the current is 43.8 A. The load rotating speed is 33000 r/min. The output voltage of the generator is 376 V and the output current is 55 A. The generator operates with an output power of 20.68 kW.
Because the inductance value of the reactor equipped for generator controller is larger than the expected value, the generator does not operate at rated power and rotating speed. Nevertheless, it could be concluded from the data above that, the generator could meet the design requirements as long as the controller and reactor are properly matched.
Test and experiment of generator set
Based on the machine assembly and component test of the 30 kW-class MGT power generation system, a joint debugging is conducted. The calibrating of the controlling system and a reliability demonstration test are completed. The prototype of the power generation system is shown in Fig. 10.
To effectively perform the whole process from start to stop, the following execution schemes are adopted:
The automatic control mode: The motor works in the speed control mode, and its rotational speed is increased to the ignition speed based on the set ramp rate.
The manual control mode: The motor works in the speed control mode, and its rotating speed is increased to the manually given value based on the set rate. Then, the given speed is manually increased until the motor reaches the ignition speed. The upper limit of the manually given speed value is the set point (ignition speed as initial value). After the ignition of the gas turbine, the manual control mode is locked.
The first acceleration stage: The motor keeps operating at the first target speed, and the ignition command is automatically or manually sent to the upper system. The diagram of the experimental data for the speed, temperature, and fuel flow rate in this stage is demonstrated in Fig. 11.
The second acceleration stage: After the output power of the motor reaches zero, the motor stops working until the rotating speed of the gas turbine rises to the rated value. In this case, the motor controlling mode is switched to the power generation state. It is assumed that output power of the generator under the controlling system is close to zero. When the motor is switched to the power generation state, the oscillation and shock of system should be prevented. Therefore, when the output power of the generator system is close to zero (with rotating speed possibly lower than the rated value), the motor shall not be switch from the power generation state to the electro motion state.
The grid loading operation stage: Under the command of the controlling system, the output power of the generator returns to the power grid in the form of energy feedback. The output power is adjustable within the full power range (0–30 kW).
The off-grid loading operation stage: After receiving the off-grid command from the upper system, the operation mode of the MGT system is switched from the grid-connected mode to the off-grid mode. The electricity generated by the system is consumed by the local load resistance. Then, the output power is adjustable within the full power range (0–30 kW).
The normal shutdown experiment: The motor still works in the power generation state. When the engine throttle is closed by the main controlling system, the gas turbine no longer operates as the power source. In this case, the motor speed decreases with the turbine to zero.
Test data and analysis
MGT system vibration test
Two large vibrations are detected as the turbine rotating speed increases from 0 to 22220 r/min. The possible reason for this might be that the gas turbine reaches its critical speed. The experimental data are basically consistent with theoretical calculation. The first vibration occurs at the speed of 9994.9 r/min, and the amplitude is 1.81 mm/s, while the second occurs at the speed of 19922.6 r/min, and the vibration amplitude is 2.9 mm/s. The vibration amplitudes of other rotating speeds are 1 mm/s, indicating that the vibration of the system is within an acceptable range.
MGT system power generation test
Three tests are carried out for the power generation of the micro gas turbine system, which indicate that the temperature of the combustor could basically reach the design temperature of 870°C, the inlet temperature of the turbine is 857°C, and the output power reaches 24.4 kW. The rotating speed of the micro gas turbine is 34050 r/min, and the air flowing rate is 0.48 kg/s; the compressor outlet pressure is 0.135 MPa (gauge pressure, actual value is 2.35 bar), the compressor outlet temperature is 138°C; The turbine inlet pressure is 0.131 MPa, and the inlet temperature is 857°C, the turbine outlet temperature is 669°C; and the fuel valve opening is 59% and the output electric power of system is 24.7 kW.
Conclusions
This research has achieved a series of important results, including several breakthroughs in the technical bottlenecks of 30 kW micro gas turbine. The continuous operation of the micro gas turbine power generation system is achieved.
The MGT developed in this project is based on the simple Brayton cycle with a low efficiency. The gas turbine is suitable for distributed combined heat and power system. To improve the efficiency, it is necessary to adopt a recuperative cycle by increasing the recuperation. This system also can realize flexible switching from the start motor mode to the generator mode, and from the grid-connected mode to the off-grid mode, because of the multi-state switching control system adopted.
This 30 kW MGT can achieve steady operation within a low rotational speed from 10000 r/min to 34000 r/min. The lubricating oil bearings instead of air bearings, and the design method of combustion chamber using the way applied in axial flow aero-engine are used in to realize economy and environmental friendliness of MGT, which can provide theoretical and experimental support for the industrialization of MGT in China.
From the current manufacturing cost point of view, the price of the micro gas turbine power generation system is still very high, compared with the same-scaled internal combustion engine system. Therefore, there remains lots of work to be done on overall design and material selection in the future.
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