Microturbine-generators (MTGs) are small, high-speed power plants that usually include the turbine, compressor, generator, and power electronics to deliver power to the grid. They typically operate on natural gas. Future units may have the potential to use diesel and lower energy fuels such as gas produced from landfill or digester gas.

Most MTG designs have a high-speed gas turbine engine driving an integral electrical generator that produces 20-100 kWe power while operating at high speed, generally in the range 50 000-120 000 rpm. In the single shaft designs of MTG electric power is produced in the 10 000s of Hz, converted to high voltage DC, and then inverted back to 60 Hz, 480 VAC.

Most MTG engine designs typically have one or several power producing sections, which include the turbine, compressor and generator on a single shaft. Other designs offer two shafts.

During operation, engine air is drawn into the unit and passes through the recuperator where the temperature is increased by hot exhaust gas. The air flows into the combustor where it is mixed with fuel, ignited and burned. The ignitor is used only during startup, and then the flame is self-sustaining.

The combusted gas passes through the turbine nozzle and turbine wheel, converting the thermal energy of the hot expanding gases to rotating mechanical energy. The turbine drives the compressor and generator. The gas exhausting from the turbine is directed back through the recuperator, and then out the stack.

Economy of manufacturing vs economy of scale

To be competitive with existing technology, most MTG manufacturers rely on enhanced reliability and lower maintenance costs. MTG manufacturers expect to achieve greater reliability and lower costs by using fewer moving parts and lower manufacturing costs. Manufacturers thus expect economy of manufacturing of microturbines to replace the economies of scale of central plants.

For MTGs to be competitive in the marketplace, minimum customer expectations are:

  • 40 000 h “wheel life”;
  • Heat rate of 12 000 to 16 000 Btu/kWh;
  • Good part load performance;
  • Emissions less than 9ppm;
  • Noise less than 70 dB;
  • Cheap and easy installation and maintenance.

    There is a tremendous potential market for microturbine-generators if the manufacturers can make their products competitive with the alternatives. Using turbo-charger technology, the cost of producing an MTG can be progressively lowered – depending on the manufacturer’s expertise in achieving economies of manufacturing and particularly if casting can be used instead of machining.

    The manufacturers realize that with an adequate volume of sales, relying on low cost economics of manufacturing, MTGs have stronger potential to compete at the meter with large central power plants. Additionally, on site power maybe able to pick off other markets within niches to provide for future product development.

    MTGs are intended to provide the energy industry with dispersed power generation assets that may be located close to the loads they serve. For utilities, interest in MTGs is based on deferred central power plant construction, deferred distribution line upgrades, and improved reliability. End use customers may view MTGs as an alternative to other small generators, an environmentally acceptable power generation device, and a reliability improvement mechanism.

    There is speculation that MTGs may be an integral part of the future utility infrastructure. This envisages numerous, small generators scattered throughout a utility’s traditional distribution network working in parallel with central power plants. Some believe this will emulate what personal computers and local area networks did by working in parallel with mainframes.

    Measuring reliability

    MTG manufacturers and others are reporting the performance capabilities of the turbines; however, no consistent, independent, third party independent testing has been done to confirm or discredit such performance claims.

    However, MTGs will only be considered if they perform acceptably and meet customers’ requirements for power quality, reliability, availability, environmental considerations, cost effectiveness, usability and system efficiency.

    To address these issues a US government/industry programme is now underway. Under this programme, MTGs are purchased, installed, operated and tested to assess their performance.

    The programme includes the following performance measures: starts/stops; overall unit efficiency; net power output; operability; emissions; power quality; endurance; O&M requirements.

    The test programme is expected to provide valuable insights, both qualitative and quantitative, into the installation, performance and maintenance requirements of units presently available to the market. Test results are based on actual operating conditions at the test site in Irvine, California. In addition to the results and experiences derived from installing and operating these units, performance data are collected to trend and profile operating characteristics via a data acquisition system and manually.

    The data acquisition system installed at the test site provides interval sampling of MTGs in operation. Raw data are collected at 5-minute intervals from various measurement sensors that feed a datalogger with either pulse or analog signals. The raw data is collected nightly, and processed once a month.

    Each MTG is fitted with sensors at various locations. Additionally, environmental parameters are collected for the entire site. Measurements include: kWh produced; amount of fuel consumed; fuel temperature; gas pressure; power quality snapshots (total harmonic distortion); ambient temperature, relative humidity and barometric pressure. In the case of MTGs with boilers, water flows, boiler air temperature and water temperature are measured.

    Tests are categorized into three phases: installation and start-up; operation and maintenance; and performance.

    Once installed, the start and stop capabilities of the microturbine-generators are tested. Units are expected to withstand the wear of daily starts and stops.

    Machine performance test criteria include:

  • Endurance. For the test programme, MTGs are operated for as long as practicable at nominal load. Daily operating parameters such as fuel flow, ambient air pressure, temperature and humidity, energy (kWh), operating temperatures and pressures are recorded. Critical MTG parameters are recorded with the intent of correlating degradation with factors other than wear and tear.
  • Transient response. MTGs should be able to respond adequately to load changes. Units that are not capable of isolated bus operation run in parallel with the system grid. Changes in system load are picked up by the grid and not by MTG units. Load changes on these MTG units are accomplished by manually setting load using the control system.
  • Harmonic distortion. The power output is measured with a BMI or equivalent recorder, which measures total harmonic distortion (THD). The BMI is also used to determine the power factor of the fully loaded unit during the endurance test. The measured power factor is used to verify that the package achieves rated output when connected to the utility grid.
  • Noise measurement. Ambient noise levels are measured using a handheld noise meter. Each unit is operated independently to take noise measurements during operation.
  • Emissions measurement. For each MTG type tested, one certified test is conducted to determine compliance with South Coast Air Quality Management District Rule 2005 for NOx emissions. In addition, periodic measurements are done with handheld equipment to determine trends and degradation that might occur with operation.
  • Peak load. Peak load gross and net measurements are taken with a BMI meter or equivalent power measurement recorder. For units without compressors, or compressors that are externally powered, the net output must be determined by subtracting the external power requirements to sustain MTG operation. The results of this test yield performance characteristics such as efficiency, heat rate, fuel consumption and operating hours, which can be compared with manufacturer specifications.

    Future hurdles

    If current technology proves itself, the next hurdles relate to specific applications such as power quality, standby power, and peak shaving. The next planned step in the test programme is to field test the promised benefits of MTGs in actual applications.

    In the longer term, there is the possibility of hybrid combination of MTGs with other technologies such as fuel cells. The fuel cell supplants the combustor on the MTG while the MTG can be used to pressurize the fuel cell. In such a hybrid combination, efficiency is expected to be as much as 60 per cent and emissions less than 1.0 ppm NOx, with less-than-detectable SOx and other target pollutants (sulphur compounds are removed from the fuel prior to use).

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