Comparison of microturbine technologies

Microturbines are small combustion turbines that produce power in the range of 25 to 500 kilowatts (kW). They are well suited for small commercial buildings such as hotels, small retail stores, and restaurants, as well as for farms and light industry. Units that burn a wide variety of fuels-natural gas, hydrogen, propane, methanol, and diesel- while causing minimal emissions are being developed. The main components of a microturbine include the turbine, recuperator (heat exchanger), compressor, and electrical generator.

Processes and computer systems have grown increasingly dependent on reliable power. From water and sewer districts to small manufacturing facilities to poultry farms, many organizations now realize they need standby generation to avoid disruptions due to power failure or outages. In some cases, even a short-term loss of power can result in downtime, restart expenses, and product losses.

Standby capabilities can be provided by microturbines, together with an energy storage system, such as an uninterruptible power system (UPS) or fuel cells. Even though microturbines are not suited for emergency standby applications that require dual-fuel sources and 10-second start up times, microturbines can provide outage protection for many non-emergency loads.

When microturbines are used to provide standby capabilities, they usually employ a load management controller that senses power failure on the grid and automatically transfers from the grid to stand-alone mode, restoring power to the facility. When grid power returns, the controller transfers back to the grid supply.

Many utilities and some energy services companies (ESCOs) provide standby generation services to improve electric reliability. Some provide incentives when the facility-owned generator is used to help the utility provide peak demand power. They may provide equipment and maintenance services. Some make incentive payments for use of the facility generator in non-emergency situations, which can lower energy costs. Where there are curtailment rates, further savings can be realized, especially during peak summer months.

Efficiency

Early simple cycle designs had low efficiencies-on the order of 15%. However, most commercially available products today are recuperated microturbines that recover heat from the exhaust stream, which boosts the temperature of combustion air, increasing efficiency. The heat-exchange device used to recover heat is called a recuperator. Air heated using the high temperature exhaust requires less fuel to reach the turbine inlet temperature. Recuperated designs currently have fuel energy-to-electrical conversion efficiencies in the range of 20-30%. In cogeneration designs, additional heat is recovered from the exhaust to heat other processes, which can raise efficiency up to 85%. These other thermal processes may include heating water, drying systems, and cooling using absorption chillers.

Tests of a number of installed products (30-70 kW) show that electrical efficiency is around 25%, and thermal efficiency 47%, bringing total efficiency to about 75%.

Cost

The capital cost of new installations runs from $700 to $1100/kW, which includes all hardware, software, and initial training. Heat recovery adds $75 to $350/kW to the initial cost. Manufacturers expect future capital costs to fall below $650/kW, when volume production increases.

Operation and maintenance costs are estimated based on comparable small reciprocating engines, since few real-life data are available. These costs are estimated from $0.005 to $0.016/kW. Maintenance intervals are estimated at 5000-8000 hours with annual maintenance on new products.

Newly developed models have reduced parts counts, which improves reliability and lowers maintenance costs. Early models employed a gearbox to reduce the high turbine speed to operate a slower generator, which added many parts that negated the low maintenance advantage of microturbines, which by themselves have few parts.

Newer models use high-speed alternators and inverter technology to increase electrical efficiency and reduce maintenance costs. In one model, the stator of the high-speed alternator generates an alternating current voltage of approximately 600 volts (v) at a frequency of 2267 hertz (Hz). This output is converted from alternating current to direct current then back to alternating at 480 V, 60 Hz.

Current Products

More than 20 companies worldwide are involved in the development and commercialization of microturbines. One major manufacturer now provides an 80-kW microturbine with 28% efficiency and a heat rate of 11,350 British thermal units per kilowatt-hour

Tests show NOx emissions increase when smaller units (30 kW) are operated at part load (below 75% of full load). This is not true of larger units (70 kW).

Development is continuing in a number of areas:

• Heat recovery/cogeneration packaged with microturbines

• Hybrid systems: fuel-cell/microturbine, flywheel/microturbine

• Fuel flexibility

• Fuel gas compression

• Advanced materials and manufacturing

• Ultra-low NOx combustion

• Advanced sensors and controls

The California Energy Commission's Public Interest Energy Research (PIER) program promotes the research, development, and demonstration of microturbines through its Environmentally Preferred Advanced Generation research program. The objective of this program is to advance the technical and market status of environmentally preferred technologies such as fuel cells, micro and small turbines, and fuel cell/turbine hybrid systems.

Some of the desired performance targets and stretch goals for microturbines are shown in the table at left.

Other advantages include:

• Compact size and lightweight

• Good efficiencies, especially w/cogeneration

• Low emissions

• Low maintenance costs, compared with reciprocating technology

• High-speed operation with no low-frequency vibration

• Simple design with a minimum of moving parts

Some disadvantages are:

• Low fuel-to-electricity efficiencies on noncogen units

• Loss of power output and efficiency with higher ambient temperatures and elevation

• Increase in NOx emissions at part-load of smaller units

Test Your Understanding Answers

1. Recuperated microturbines are more efficient than simple cycle designs.

True

2. Cogeneration applications can achieve higher efficiency than applications without additional heat recovery because they:

Use additional waste stack gas heat to heat other processes

3. Microturbines have high capital and O&M costs.

False

4. With volume production, the capital cost of microturbines is expected to be:

Approximately $650/kW

5. One disadvantage of microturbines is:

The increase in NOx emissions at part-load of smaller units