Microturbines

by Barney L. Capehart, PhD, CEM
College of Engineering, University of Florida

Last updated: 02-20-2007

Introduction

Microturbines are a relatively new distributed generation technology being used for stationary energy generation applications. They are a type of combustion turbine that produces both heat and electricity on a relatively small scale. Microturbines offer several potential advantages compared to other technologies for small-scale power generation, including: a small number of moving parts, compact size, lightweight, greater efficiency, lower emissions, lower electricity costs, and opportunities to utilize waste fuels. Waste heat recovery can also be used with these systems to achieve efficiencies greater than 80%.

Because of their small size, relatively low capital costs, expected low operations and maintenance costs, and automatic electronic control, microturbines are expected to capture a significant share of the distributed generation market. In addition, microturbines offer an efficient and clean solution to direct mechanical drive markets such as compression and air-conditioning.

Description

A. What is a Microturbine?

Microturbine Overview
Commercially Available	Yes (Limited)
Size Range	25-500 kW
Fuel	Natural gas, hydrogen, propane, diesel
Efficiency	20-30% (Recuperated)
Environmental	Low (<9-50 ppm) NOx
Other Features	Cogeneration (50-80°C water)
Commercial Status	Small volume production, commercial prototypes now.

(Courtesy of California Distributed Energy Resources Guide on microturbines)

Microturbines are small combustion turbines approximately the size of a refrigerator with outputs of 25 kW to 500 kW. They evolved from automotive and truck turbochargers, auxiliary power units (APUs) for airplanes, and small jet engines. Most microturbines are comprised of a compressor, combustor, turbine, alternator, recuperator (a device that captures waste heat to improve the efficiency of the compressor stage), and generator. The figure below illustrates how a microturbine works.

B. Types of Microturbines

Microturbines are classified by the physical arrangement of the component parts: single shaft or two-shaft, simple cycle, or recuperated, inter-cooled, and reheat. The machines generally rotate over 40,000 revolutions per minute. The bearing selection—oil or air—is dependent on usage. A single shaft microturbine with high rotating speeds of 90,000 to 120,000 revolutions per minute is the more common design, as it is simpler and less expensive to build. Conversely, the split shaft is necessary for machine drive applications, which does not require an inverter to change the frequency of the AC power.

Microturbine generators can also be divided into two general classes:

• Unrecuperated (or simple cycle) microturbines—In a simple cycle, or unrecuperated, turbine, compressed air is mixed with fuel and burned under constant pressure conditions. The resulting hot gas is allowed to expand through a turbine to perform work. Simple cycle microturbines have lower efficiencies at around 15%, but also lower capital costs, higher reliability, and more heat available for cogeneration applications than recuperated units.
• Recuperated microturbines—Recuperated units use a sheet-metal heat exchanger that recovers some of the heat from an exhaust stream and transfers it to the incoming air stream, boosting the temperature of the air stream supplied to the combustor. Further exhaust heat recovery can be used in a cogeneration configuration. The figures below illustrate a recuperated microturbine system. The fuel-energy-to-electrical-conversion efficiencies are in the range of 20 to 30%. In addition, recuperated units can produce 30 to 40% fuel savings from preheating.

Cogeneration is an option in many cases as a microturbine is located at the point-of-power utilization. The combined thermal electrical efficiency of microturbines in such cogeneration applications can reach as high as 85% depending on the heat process requirements.

Microturbine Efficiency
Configuration	Efficiency
Unrecuperated	15%
Recuperated	20-30%
With Heat Recovery	Up to 85%

(Courtesy of California Distributed Energy Resources Guide on Microturbines)

Advanced materials, such as ceramics and thermal barrier coatings, are some of the key enabling technologies to further improve microturbines. Efficiency gains can be achieved with materials like ceramics, which allow a significant increase in engine operating temperature.

C. Characteristics of Microturbines

Some of the primary applications for microturbines include:

• Distributed generation—stand-alone, on-site applications remote from power grids
• Quality power and reliability—reduced frequency variations, voltage transients, surges, dips, or other disruptions
• Stand-by power—used in the event of an outage, as a back-up to the electric grid
• Peak shaving—the use of microturbines during times when electric use and demand charges are high
• Boost power—boost localized generation capacity and on more remote grids
• Low-cost energy—the use of microturbines as base load or primary power that is less expensive to produce locally than it is to purchase from the electric utility
• Combined heat and power (cogeneration)—increases the efficiency of on-site power generation by using the waste heat for existing thermal process.

Microturbines offer many potential advantages for distributed power generation. Selected strengths and weaknesses of microturbine technology are listed in the following table from the California Distributed Energy Resources Guide on Microturbines.

Microturbines
Strengths	Weaknesses
Small number of moving parts	Low fuel to electricity efficiencies
Compact size	Loss of power output and efficiency with higher ambient temperatures and elevation
Lightweight
Good efficiencies in cogeneration
Low emissions
Can utilize waste fuels
Long maintenance intervals
No vibrations
Less noise than reciprocating engines
Strengthens energy security

Accessible: N/A

Aesthetics:

• Improves sightlines and views with off-the-grid systems, which eliminate the need for overhead power lines

Cost-Effective: (See Section F: Economics of Microturbines)

• Enables cost savings by reducing the peak demand at a facility, therefore lowering demand charges

Functional:

• Provides better power reliability and quality, especially for those in areas where brownouts, surges, etc. are common or utility power is less dependable
• Provides power to remote applications where traditional transmission and distribution lines are not an option such as construction sites and offshore facilities
• Can be an alternative to diesel generators for on-site power for mission critical functions (e.g., communications centers)
• Possesses combined heat and power capabilities
• Reduces upstream overload of transmission lines
• Optimizes utilization of existing grid assets—including potential to free up transmission assets for increased wheeling capacity
• Improves grid reliability
• Facilitates faster permitting than transmission line upgrades
• Can be located on sites with space limitations for the production of power

Productive:

• Provides high-quality power for sensitive applications
• Responds faster to new power demands—as capacity additions can be made more quickly
• Facilitates less capital tied up in unproductive assets—as the modular nature of microturbines means capacity additions and reductions can be made in small increments, closely matched with demand, instead of constructing central power plants sized to meet estimated future (rather than current) demand
• Stand-by power decreases downtime, enabling employees to resume working
• Produces less noise than reciprocating engines

Secure/Safe:

• Strengthens energy security
• Stand-by power provides quick recovery after an event

Sustainable:

• Produces the lowest emission of any noncatalyzed fossil fuel combustion system
• Has a small footprint, minimizing site disturbance
• Reduces or defers infrastructure (line and substation) upgrades
• For recuperated microturbine, possesses higher energy conversion efficiencies than central generation
• Enables more effective energy and load management

D. Economics of Microturbines

Microturbine capital costs range from $700-$1,100/kW. These costs include all hardware, associated manuals, software, and initial training. Adding heat recovery increases the cost by $75-$350/kW. Installation costs vary significantly by location but generally add 30-50% to the total installed cost.

Microturbine manufacturers are targeting a future cost below $650/kW. This appears to be feasible if the market expands and sales volumes increase.

With fewer moving parts, microturbine vendors hope the units can provide higher reliability than conventional reciprocating generating technologies. Manufacturers expect that initial units will require more unexpected visits, but as the products mature, a once-a-year maintenance schedule should suffice. Most manufacturers are targeting maintenance intervals of 5,000-8,000 hours.

Maintenance costs for microturbine units are still based on forecasts with minimal real-life situations. Estimates range from $0.005-$0.016 per kWh, which would be comparable to that for small reciprocating engine systems.

Microturbine Cost
Capital Cost	$700-$1,100/kW
O&M Cost	$0.005-0.016/kW
Maintenance Interval	5,000-8,000 hrs

(Courtesy of California Distributed Energy Resources Guide on Microturbines)

Application

Microturbines can be used for stand-by power, power quality and reliability, peak shaving, and cogeneration applications. In addition, because microturbines are being developed to utilize a variety of fuels, they are being used for resource recovery and landfill gas applications. Microturbines are well suited for small commercial building establishments such as: restaurants, hotels/motels, small offices, retail stores, and many others.

The variety of energy consumers that are already using microturbines is large and growing fast. For example:

• A McDonald's restaurant in Chicago, Illinois, gets most of its electricity from a natural-gas-powered microturbine, cutting $1,500 off its total monthly power bill. More

• Landfill gas-fired microturbines installed at the Jamacha Landfill in Spring Valley, California supply power on-site and back to the grid. More
• A textile mill in Lawrence, Massachusetts, ensures continuous operation by getting its power from microturbines.
• The Chesapeake Building on the University of Maryland campus, College Park, Maryland has a cooling, heating, and power (CHP) system consisting of microturbines, chiller, and stack that uses waste heat to cool and heat the building, significantly increasing system efficiency. More

The development of microturbine technology for transportation applications is also in progress. Automotive companies are interested in microturbines as a lightweight and efficient fossil-fuel-based energy source for hybrid electric vehicles, especially buses.

Other ongoing developments to improve microturbine design, lower costs, and increase performance in order to produce a competitive distributed generation product include heat recovery/cogeneration, fuel flexibility, and hybrid systems (e.g., fuel cell/microturbine, flywheel/microturbine).

Relevant Codes and Standards

• Executive Order 13423, "Strengthening Federal Environmental, Energy, and Transportation Management"

National Energy Policy. Report of the National Energy Policy Development Group, May 2001.

• Encourages "increasing energy conservation and efficiency" and "increasing America's use of renewable and alternative energy."

In March 1999, the Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA) Board voted to undertake the development of uniform standards for interconnecting distributed resources with electric power systems. The IEEE Standards Coordinating Committee 21 (IEEE SCC21), the committee responsible for developing technical standards for distributed technologies, is now working to develop IEEE P1547, the Standard for Distributed Resources Interconnected with Electric Power Systems. The consensus standard will contain specific requirements related to performance, operation, testing, safety, and maintenance of interconnections between distributed resources and other electric power systems.

Additional Resources

WBDG

Building / Space Types

Applicable to all building types and space types

Design Objectives

Aesthetics, Cost-Effective, Functional / Operational, Productive, Secure / Safe, Sustainable

Products and Sytems

Federal Green Construction Guide for Specifiers

Project Management

Building Commissioning

Tools

LEED® Version 2.1 Credit / WBDG Resource Page Matrix, LEED®-DoD Antiterrorism Standards Tool, Electricity Rates (ERATES), EMISS, Energy Escalation Rate Calculator (EERC), Energy Plus, ENERGY-10, Facility Energy Decision System (FEDS), Federal Renewable Energy Screening Assistant (FRESA)

State and Federal Agencies

• California Energy Commission (CEC)
• CEC California Distributed Energy Resource Guide on microturbines
• Department of Energy (DOE)
• DOE Office of Energy Efficiency and Renewable Energy (EERE)
• DOE Distributed Energy Program (DE), Microturbines
• DOE Advanced Microturbine Program

Associations and Organizations

• California Alliance for Distributed Energy Resources (CADER)—A voluntary collaborative committed to facilitating the successful deployment of highly efficient and environmentally responsible distributed energy resources into competitive energy markets.
• Distributed Power Coalition of America (DPCA)—An advocacy organization committed to advancing the use of distributed power. Members include: electric and gas utilities, consumers, distributed power equipment manufacturers and suppliers, natural gas transmission pipeline companies, gas and electric marketers, and research organizations. The site provides information on DCPA and its activities.
• U.S. Combined Heat and Power Association (USCHPA)—An association that brings together diverse market interests to promote the growth of clean, efficient CHP in the United States. Its mission is to create a regulatory, institutional, and market environment that fosters the use of clean, efficient CHP as a major source of electric power and thermal energy in the U.S.
• World Alliance for Decentralized Energy (WADE)—Launched by a group of major companies and national industry associations to accelerate the development of high efficiency cogeneration and decentralized energy (DE) systems which will deliver substantial economic and environmental benefits worldwide.

Manufacturers

There are more than twenty companies worldwide that are involved in the development and commercialization of microturbines for distributed generation applications. Below are links for five of the leading microturbine manufacturers.

• Bowman Power Systems—A U.K. company that develops 80-kW microturbine power generation systems for DER and mobile power applications.
• Capstone Turbine Corporation—Based in Chatsworth, CA, Capstone is a leader in the commercialization of low-emission, high-reliability microturbine power generators. The company offers 30-kW and 60-kW systems for DER applications.
• Elliott Energy Systems—Located in Stuart, FL, Elliott Energy Systems develops and manufactures 80-kW microturbines now with plans for larger units later.
• Ingersoll-Rand Company (IR)—Based in Portsmouth, NH, IR develops the PowerWorks™ line of microturbine generators with output of 70-kW now with plans for larger units later.
• Turbec AB—A Swedish company jointly owned by ABB and Volvo Aero. The company offers a 100-kW microturbine power generator for commercial DER applications.