Microturbine Turbec T100

In this thesis, the micro-CHP unit with Turbec T100 Power and Heat (PH) microturbine, based on a regenerative Brayton cycle, is used. It is a modular unit, generating electricity and heat with high efficiency and low emission levels. This microturbine satisfies the requirements of the European basic health and safety policies and follows Machinery Directive 98/37/EC, Noise Directive 2000/14/EC, Electromagnetic Compatiblity Directive 2004/108/EC and Low Voltage Directive 2008/95/EC. (Turbec 2009)

3.2.1 Main components

The schematic representation of the flows in the power generating unit is illustrated in Figure 3. It provides a visual demonstration of the Turbec T100 operation. The ambient air flows around the generator and enters the compressor in axial direction. After compression, the air leaves the compressor radially. The blue color represents the air with high pressure. It flows through the recuperator where the flue gases preheat it. Pink color represents the preheated air with high pressure. In the combustor, it is mixed with the gaseous fuel and then burned.

The red color represents the flue gases that enter the turbine in radial direction and leave axially. The flue gas preheats the compressed air in the recuperator and leave the unit.

(Turbec 2009)

Figure 3. The schematic representation of Turbec T100 power module (Turbec 2009).

The power module is coupled with an exhaust gas heat exchanger. It is a gas-water counter-current flow heat exchanger, that is located right after the recuperator.The heat from the exhaust gasses is used for water heating in the exhaust gas heat exchanger.The amount of generated heat is directly related to the amount of generated electricity.Sometimes less heat is needed than available. In cases when too much heat is extended to the water, the water can become boiling, which is harmful to the heat exchanger.Thus, the amount of supplied heat must be controlled and this is done using a bypass system, in which exhaust gasses are diverted either totally or partially around the heat exchanger. The outlet water temperature differs according to the input water parameters, temperature, and mass flow. The exhaust gases exit from this gas heat exchanger through an exhaust pipe and chimney. (Turbec 2009)

The power module includes the following subsystems:

• Gas turbine engine.

• Electrical generator. The high-speed generator is water-cooled, and has high efficiency. It generates the electric power by a permanent magnet, supported with two bearings one on each side.

• Electrical system. The high-frequency AC power from the generator is transformed to the needed grid voltage and frequency. A transmission-line filter and a transformer normalizes the AC output.

• The microturbine is guided and governed by an automatic supervision and control system. Due to this, the turbine does not require attendance in person under normal operation. In case of any faulty operation or failure of the grid, the system automatically shuts down. (Turbec 2009)

The major elements of the gas turbine engine are as follows:

• Housing. In the microturbine unit, the generator and the rotating components are installed on the same shaft in the same housing.

• Compressor. A radial centrifugal compressor is used. It has the pressure ratio of 4,5.

• Recuperator. In the recuperator, heat is transferred from the hot exhaust gases to the compressed air entering the combustion chamber.

• Combustion chamber.In the start-up, an electrical igniter in the combustion chamber ignites the mixture of air and fuel. During operation, the combustion process is continuous.

• Turbine.Similarly to the compressor, the turbine is of the radial type. The gases enter the turbine at the temperature of 950 °C and pressure 4,5 bar and leave the turbine at atmospheric pressure and the temperature of 650 °C. (Turbec 2009)

Besides main components, there are several auxiliary systems, which are classified as the following subsystems:

• Lubrication system, is essential for lubricating the squeezed film bearings located on the rotor shaft.

• A separate closed cooling water system, needed for cooling the generator.

• Air intake and ventilation system. A microturbine unit placed indoors draws ambient air. In the unit, the flow of air is separated into 2 different flows. The main air flow is needed for combustion process, and the secondary flow - for ventilation in the power module.

• Fuel gas system includes a fuel control system and fuel booster. In case, the gas pressure is less than 6 bar (g), a fuel booster is used for the gas pressure increasing.

The gaseous fuel enters the fuel booster and then discharged to the fuel control system.

• Buffer air system. The gas turbine compressor supplies this system with air. It is needed for preventing from the ingestion of the lubrication oil in the gas turbine and the electric generator. (Turbec 2009)

3.2.2 Performance

Figure 4 illustrates the influence of an air inlet temperature on the Turbec T100 electrical output and efficiency at full load when using low-pressure gas of 0,02 bar (g). Using high-pressure gas, the electrical output increase is approximately 5 kW and the electrical efficiency increase is 1,5 percent. (Turbec 2009)

Figure 4. The influence of an air inlet temperature on the electrical output and efficiency (Turbec 2009).

Characteristic to gas turbines, the output and efficiency increase as air temperature decreases. Below 10 °C, the performance is maintained at constant level due to limited capacity of the generator and power electronics. (Turbec 2009)

Figure 5 contains 2 charts illustrating how air inlet temperature (left) and water inlet temperature (right) influence on T100 PH microturbine heat output and total efficiency based on low-pressure gas sources of 0,02 bar (g).

Figure 5. The influence of an air inlet temperature (left) and water inlet temperature (right) on heat output and total efficiency (Turbec 2009).

The correction factors in the right chart scale the performance value. In accordance with the factor for heat output for instance, when the inlet water temperature decreases by 10 °C then the thermal output will decrease by nearly 8 kW. (Turbec 2009)

Figure 6 contains 2 charts presenting the influence of inlet pressure drop (left) and outlet pressure drop (right) on the performance characteristics of the microturbine unit.

Figure 6. The influence of inlet pressure drop (left) and outlet pressure drop (right) on T100 performance (Turbec 2009).

Figure 7 contains 2 charts presenting the influence of part load on electrical and total efficiencies (left) and site elevation (right) on the performance characteristics of microturbine unit.

Figure 7. The influence of part load (left) and elevation (right) on performance (Turbec 2009).

In case when less than full load power is needed from a turbine, its output is decreased by decreasing the speed of rotation, which decreases the temperature rise and pressure ratio in the compressor and temperature drop in the turbine, and by decreasing turbine inlet temperature, so the recuperator inlet temperature does not increase. In addition to decreasing power output, this change in working conditions also affects the efficiencies. In the T100

microturbine, the efficiency reduction is minimized by using the speed of rotation as the primary power control method. (Turbec 2009)

The density of air depends on the site elevation above the sea level. The density of air reduces at increasing altitudes, and, as a result, the unit power and heat output reduces. (Turbec 2009)

3.2.3 Maintenance concept

The maintenance concept is based on the several principles:

• Remote monitoring and control system, used for distant control,

• Predeclared functions in case of corrective maintenance,

• A detailed preventive maintenance system,

• Support and maintenance service. (Turbec 2009)

The simple design of the microturbine contributes to the long-term reliability of operation.

Failure diagnostics, monitoring of operation, and device status conditions are enabled by remote monitoring and control system. The module structure of the unit does not require special lifting equipment. The unit is designed with predeclared functions for corrective maintenance, such as filter changes, automatic warnings, and alerts for pertinent conditions.

The design service life is approximately 60 000 hours with a scheduled overhaul after 30 000 hours of operation, and limited inspection and maintenance services in between. The maintenance agreement is based on the service partner coverage. Manufacturer provides the supply of spare parts, technical assistance, and instructions. The on-site assistance is provided by the local service partner. (Turbec 2009)