4.2 Microturbines

Microturbines are small electricity generators that burn gaseous and liquid fuels to create high-speed rotation that turns an electrical generator. Today’s microturbine technology is the result of development work in small stationary and automotive gas turbines, auxiliary power equipment and turbochargers, much of which was pursued by the automotive industry beginning in the 1950s. Microturbines entered field testing around 1997 and began initial commercial service in 2000.

The size range for microturbines available and in development is from 30 to 400 kilowatts (kW), while conventional  gas turbine sizes range from 500 kW to 350 megawatts (MW). Microturbines run at high speeds and, like larger gas  turbines, can be used in power-only generation or in combined heat and power (CHP) systems. They are able to  operate on a variety of fuels, including natural gas, sour gases (high sulfur, low Btu content), and liquid fuels such as  gasoline, kerosene, and diesel fuel/distillate heating oil. In resource recovery applications, they burn waste gases  that would otherwise be flared or released directly into the atmosphere.

Designed to combine the reliability of auxiliary power systems used on board commercial aircraft with the design and manufacturing economies of turbochargers, the units are targeted at CHP and prime power applications in commercial buildings and light industrial applications. A schematic of a microturbine-based CHP system is shown in Figure 4-4. In most configurations, a high speed turbine (100,000 rpm) drives a high speed generator producing direct current (DC) power that is electronically inverted to 60 Hz (or 50 Hz) AC. Microturbine systems are capable of producing power at around 25-33 percent efficiency by employing a recuperator that transfers exhaust heat back into the incoming air stream. Efficiencies generally decrease at elevated ambient temperatures. The systems are air cooled and some designs use air bearings, thereby eliminating both water and oil systems used by reciprocating engines. Low emission combustion systems are being demonstrated that provide emissions performance comparable to larger gas turbines. The potential for reduced maintenance and high reliability and durability is currently being demonstrated in actual applications.

chp-microturbine-system

Figure 4-4: Microturbine System

Microturbines are ideally suited for distributed generation applications due to their flexibility in connection methods, ability to be stacked in parallel to serve larger loads, ability to provide stable and reliable power, and low emissions.

Types of applications include:

  • Peak shaving and base load power (grid parallel)
  • Combined heat and power
  • Stand-alone power
  • Backup/standby power
  • Ride-through connection
  • Primary power with grid as backup
  • Microgrid
  • Resource recovery.

Target customers include financial services, data processing, telecommunications, restaurant, lodging, retail, office building, and other commercial sectors. Microturbines are currently operating in resource recovery operations at oil and gas production fields, wellheads, coal mines, and landfill operations, where byproduct gases serve as essentially free fuel. Unattended operation is important since these locations may be remote from the grid, and even when served by the grid, may experience costly downtime when electric service is lost due to weather, fire, or animals.

In CHP applications, the waste heat from the microturbine is used to produce hot water, to heat building space, to drive absorption cooling or desiccant dehumidification equipment, and to supply other thermal energy needs in a building or industrial process.

4.2.1 Technology Description

Microturbines are small gas turbines, most of which feature an internal heat exchanger called a recuperator. In a microturbine, a radial compressor compresses the inlet air that is then preheated in the recuperator using heat from the turbine exhaust. Next, the heated air from the recuperator mixes with fuel in the combustor and hot combustion gas expands through the expansion and power turbines. The expansion turbine turns the compressor and, in single shaft models, turns the generator as well. Two-shaft models use the compressor drive turbine’s exhaust to power a second turbine that drives the generator.

Single-shaft models generally operate at speeds over 60,000 revolutions per minute (rpm) and generate electrical power of high frequency, and of variable frequency (alternating current –AC). This power is rectified to direct current (DC) and then inverted to 60 hertz (Hz) for U.S. commercial use. In the two-shaft version, the power turbine connects via a gearbox to a generator that produces power at 60 Hz. Some manufacturers offer units producing 50 Hz for use in countries where 50 Hz is standard, such as in Europe and parts of Asia.

Microturbines operate on the same thermodynamic cycle, known as the Brayton cycle, as larger gas turbines. In this cycle, atmospheric air is compressed, heated, and then expanded, with the excess power produced by the expander (also called the turbine) over that consumed by the compressor used for power generation. The power produced by an expansion turbine and consumed by a compressor is proportional to the absolute temperature of the gas passing through those devices. Consequently, it is advantageous to operate the expansion turbine at the highest practical temperature consistent with economic materials and to operate the compressor with inlet airflow at as low a temperature as possible. As technology advances permit higher turbine inlet temperature, the optimum pressure ratio also increases. Higher temperature and pressure ratios result in higher efficiency and specific power. Thus, the general trend in gas turbine advancement has been towards a combination of higher temperatures and pressures. However, microturbine inlet temperatures are generally limited to 1,800ºF or below to enable the use of relatively inexpensive materials for the turbine wheel, and to maintain pressure ratios at a comparatively low 3.5 to 4.0.

The basic components of a microturbine are the compressor, turbine generator, and recuperator. The heart of the microturbine is the compressor-turbine package, which is commonly mounted on a single shaft along with the electric generator. Two bearings support the single shaft. The single moving part of the one-shaft design has the potential for reducing maintenance needs and enhancing overall reliability. There are also two-shaft versions, in which the turbine on the first shaft directly drives the compressor while a power turbine on the second shaft drives a gearbox and conventional electrical generator producing 60 Hz power. The two-shaft design features more moving parts but does not require complicated power electronics to convert high frequency AC power output to 60 Hz.

Moderate to large-size gas turbines use multi-stage axial flow turbines and compressors, in which the gas flows along the axis of the shaft and is compressed and expanded in multiple stages. However, microturbine turbomachinery is based on single-stage radial flow compressors and turbines. Radial flow turbomachinery handles the small volumetric flows of air and combustion products with reasonably high component efficiency. Large-size axial flow turbines and compressors are typically more efficient than radial flow components. However, in the size range of microturbines — 0.5 to 5 lbs/second of air/gas flow — radial flow components offer minimum surface and end wall losses and provide the highest efficiency.

In microturbines, the turbocompressor shaft generally turns at high rotational speed, about 96,000 rpm in the case of a 30 kW machine and about 80,000 rpm in a 75 kW machine. There is no single rotational speed-power size rule, as the specific turbine and compressor design characteristics strongly influence the physical size of components and consequently rotational speed. For a specific aerodynamic design, as the power rating decreases, the shaft speed increases, hence the high shaft speed of the small microturbines.

The radial flow turbine-driven compressor is quite similar in terms of design and volumetric flow to automobile, truck, and other small reciprocating engine turbochargers. Superchargers and turbochargers have been used for almost 80 years to increase the power of reciprocating engines by compressing the inlet air to the engine. Today’s world market for small automobile and truck turbochargers is around two million units per year. Small gas turbines, of the size and power rating of microturbines, serve as auxiliary power systems on airplanes. Cabin cooling (air conditioning) systems of airplanes use this same size and design family of compressors and turbines. The decades of experience with these applications provide the basis for the engineering and manufacturing technology of microturbine components.

Generator

The microturbine produces electrical power either via a high-speed generator turning on the single turbo compressor shaft or with a separate power turbine driving a gearbox and conventional 3,600 rpm generator. The high-speed generator of the single-shaft design employs a permanent magnet (typically Samarium-Cobalt) alternator, and requires that the high frequency AC output (about 1,600 Hz for a 30 kW machine) be converted to 60 Hz for general use. This power conditioning involves rectifying the high frequency AC to DC, and then inverting the DC to 60 Hz AC. Power conversion comes with an efficiency penalty (approximately five percent). To start-up a single shaft design, the generator acts as a motor turning the turbo-compressor shaft until sufficient rpm is reached to start the combustor. If the system is operating independent of the grid (black starting), a power storage unit (typically a battery UPS) is used to power the generator for start-up.

Recuperators

Recuperators are heat exchangers that use the hot turbine exhaust gas (typically around 1,200ºF) to preheat the compressed air (typically around 300ºF) going into the combustor, thereby reducing the fuel needed to heat the compressed air to turbine inlet temperature. Today’s microturbines require a recuperator to achieve the efficiency levels needed to be competitive in continuous duty service. Depending on microturbine operating parameters, recuperators can more than double machine efficiency. However, since there is increased pressure drop in both the compressed air and turbine exhaust sides of the recuperator, power output typically declines 10 to 15%. Recuperators also lower the temperature of the microturbine exhaust, reducing the microturbine’s effectiveness in CHP applications.

Bearings

Microturbines operate on either oil-lubricated or air bearings, which support the shaft(s). Oil-lubricated bearings are mechanical bearings and come in three main forms – high-speed metal roller, floating sleeve, and ceramic surface. The latter typically offer the most attractive benefits in terms of life, operating temperature, and lubricant flow. While they are a well-established technology, they require an oil pump, oil filtering system, and liquid cooling that add to microturbine cost and maintenance. In addition, the exhaust from machines featuring oil-lubricated bearings may not be useable for direct space heating in cogeneration configurations due to the potential for contamination. Since the oil never comes in direct contact with hot combustion products, as is the case in small reciprocating engines, it is believed that the reliability of such a lubrication system is more typical of ship propulsion diesel systems (which have separate bearings and cylinder lubrication systems) and automotive transmissions than cylinder lubrication in automotive engines.

Air bearings have been in service on airplane cabin cooling systems for many years. They allow the turbine to spin on a thin layer of air, so friction is low and rpm is high. No oil or oil pump is needed. Air bearings offer simplicity of operation without the cost, reliability concerns, maintenance requirements, or power drain of an oil supply and filtering system. Concern does exist for the reliability of air bearings under numerous and repeated starts due to metal on metal friction during startup, shutdown, and load changes. Reliability depends largely on individual manufacturers’ quality control methodology more than on design engineering, and will only be proven after significant experience with substantial numbers of units with long numbers of operating hours and on/off cycles. Air bearings significantly lengthen microturbine startup time (one to two minutes).

Power Electronics

As discussed, single-shaft microturbines feature digital power controllers to convert the high frequency AC power produced by the generator into usable electricity. The high frequency AC is rectified to DC, inverted back to 60 or 50 Hz AC, and then filtered to reduce harmonic distortion. This is a critical component in the single-shaft microturbine design and represents significant design challenges, specifically in matching turbine output to the required load. To allow for transients and voltage spikes, power electronics designs are generally able to handle seven times the nominal voltage. Most microturbine power electronics are generating three-phase electricity.

Electronic components also direct all of the operating and startup functions. Microturbines are generally equipped with controls that allow the unit to be operated in parallel or independent of the grid, and internally incorporate many of the grid and system protection features required for interconnect. The controls also allow for remote monitoring and operation.

Microturbines are more complex than conventional simple-cycle gas turbines, as the addition of the recuperator both reduces fuel consumption (thereby substantially increasing efficiency) and introduces additional internal pressure losses that moderately lower efficiency and power. As the recuperator has four connections — to the compressor discharge, the expansion turbine discharge, the combustor inlet, and the system exhaust — it becomes a challenge to the microturbine product designer to make all of the connections in a manner that minimizes pressure loss, keeps manufacturing cost low, and entails the least compromise of system reliability. Each manufacturer’s models have evolved in unique ways.

The addition of a recuperator opens numerous design parameters to performance-cost tradeoffs. In addition to selecting the pressure ratio for high efficiency and best business opportunity (high power for low price), the recuperator has two performance parameters, effectiveness and pressure drop, that also have to be selected for the combination of efficiency and cost that creates the best business conditions. Higher effectiveness recuperation requires greater recuperator surface area, which both increases cost and incurs additional pressure drop. Such increased internal pressure drop reduces net power production and consequently increases microturbine cost per kW.

Microturbine performance, in terms of both efficiency and specific power, is highly sensitive to small variations in component performance and internal losses. This is because the high efficiency recuperated cycle processes a much larger amount of air and combustion products flow per kW of net powered delivered than is the case for high pressure ratio simple-cycle machines. When the net output is the small difference between two large numbers (the compressor and expansion turbine work per unit of mass flow), small losses in component efficiency, internal pressure losses and recuperator effectiveness have large impacts on net efficiency and net power per unit of mass flow. For these reasons, it is advisable to focus on trends and comparisons in considering performance, while relying on manufacturers’ guarantees for precise values.

4.2.2 Performance

Table 4-2 summarizes the performance characteristics of typically commercially available microturbines. Heat rates and efficiencies shown were taken from manufacturers’ specifications and industry publications. Electrical efficiencies are net of parasitic and conversion losses. It should be noted that performance is also affected by ambient air temperatures. High ambient air temperatures do result in a significant drop in efficiency.

Available thermal energy is calculated based on manufacturer specifications on turbine exhaust flows and temperatures. CHP thermal recovery estimates are based on producing hot water for process or space heating applications. Total CHP efficiency is the sum of the net electricity generated plus hot water produced for building thermal needs divided by total fuel input to the system. The data in the table show that electrical efficiency increases as the microturbine becomes larger. As electrical efficiency increases, the absolute quantity of thermal energy available decreases per unit of power output, and the ratio of power to heat for the CHP system increases. A changing ratio of power to heat impacts project economics and may affect the decisions that customers make in terms of CHP acceptance, sizing, and other characteristics.

Each microturbine manufacturer represented uses a different recuperator, and each has made individual tradeoffs between cost and performance. Performance involves the extent to which the recuperator effectiveness increases cycle efficiency, the extent to which the recuperator pressure drop decreases cycle power, and the choice of what cycle pressure ratio to use. Consequently, microturbines of different makes will have different CHP efficiencies and different net heat rates chargeable to power.

As shown, microturbines typically require 50 to 80 psig fuel supply pressure. Because microturbines are built with pressure ratios between 3 and 4 to maximize efficiency with a recuperator at modest turbine inlet temperature, the required supply pressure for microturbines is much less than for industrial-size gas turbines with pressure ratios of 7 to 35. Local distribution gas pressures usually range from 30 to 130 psig in feeder lines and from 1 to 50 psig in final distribution lines. Most U.S. businesses that would use a 30, 70, or 100 kW microturbine receive gas at about 0.5 to 1.0 psig. Additionally, most building codes prohibit piping higher-pressure natural gas within the structure. Thus, microturbines in most commercial locations require a fuel gas booster compressor to ensure that fuel pressure is adequate for the gas turbine flow control and combustion systems.

Table 4-2: Microturbine CHP – Typical Performance Parameters

Cost and Performance Characteristics System System System System
Nominal Electricity Capacity (kW)30kW70kW80kW100kW
Net Electrical Capacity (kW)286776100
Package Cost (2003 $/kW) $1,280$1,070$1,100$1,000
Total Installed Cost for Power-only (YR 2003 $/kW)$2,263$1,658$1,663$1,526
Total Installed Cost for CHP (YR 2003 $/kW)$2,636$1,926$1,932$1,749
Electric Heat Rate (Btu/kWh), HHV15,07113,54414,10313,127
Net Electrical Efficiency (%), HHV22,6%25,2%24,2%26,0%
Fuel Input (MMBtu/hr)0.4230.911.091.31
Required Fuel Gas Pressure (psig)55708590
Required Fuel Gas Pressure w/GBC (psig)0.2-150.2-150.2-150.3-15
CHP Characteristics
Exhaust Flow (lbs/sec)0.681.601.671.76
GT Exhaust Temp (degrees F)530°450°500°520°
Heat Exchanger Exhaust Temp (degrees F) 220°220°220°220°
Heat Output (MMBtu/hr)0.1860.3250.4120.466
Heat Output (kW equivalent)5495121136
Total CHP Efficiency (%), HHV67%61%63%62%
Thermal Output/Fuel Input0.440.360.380.35
Power/Heat Radio0.520.700.630.73
Net Heat Rate (Btu/kWh)6,7957,4857,3207,300

Most microturbine manufacturers offer the equipment package with the fuel gas booster included. This packaging facilitates the purchase and installation of a microturbine, as the burden of obtaining and installing the booster compressor is no longer placed on the customer. Also, it might result in higher reliability of the booster through standardized design and volume manufacture.

Booster compressors can add from $50 to $100 per kW to a microturbine CHP system’s total cost. As well as adding to capital cost, booster compressors lower net power and efficiency so operating cost is slightly higher. Typically, the fuel gas booster requires about 5% of the microturbine output. For example, a single 60 kW unit requires 2.6 kW for the booster, while a booster serving a system of three 30 kW units would require 4.4 kW. Such power loss results in a penalty on efficiency of about 1.5 percentage points. For installations where the unit is located outdoors, the customer can save on cost and perating expense by having the gas utility deliver gas at an adequate pressure and obtaining a system without a fuel gas booster compressor.

4.2.3 Emissions

Microturbines have the potential for extremely low emissions. All microturbines operating on gaseous fuels feature lean premixed combustor technology, which was developed relatively recently in the history of gas turbines and is not universally featured on larger gas turbines. Because microturbines are able to meet emissions requirements with this built-in technology, post-combustion emission control techniques are not needed.

The primary pollutants from microturbines are oxides of nitrogen (NO x ), carbon monoxide (CO), and unburned hydrocarbons. They also produce a negligible amount of sulfur dioxide (SO 2 ). Microturbines are designed to achieve the objective of low emissions at full load; emissions are often higher when operating at part load.

NO x is a mixture of mostly NO and NO 2 in variable composition. In emissions measurement it is reported as parts per million by volume in which both species count equally. NO x forms by three mechanisms: thermal NO x , prompt NO x , and fuel-bound NO x . The predominant NO x formation mechanism associated with gas turbines is thermal NO x . Thermal NO x is the fixation of atmospheric oxygen and nitrogen, which occurs at high combustion temperatures. Flame temperature and residence time are the primary variables that affect thermal NO x levels. The rate of thermal NO x formation increases rapidly with flame temperature. Prompt NO x forms from early reactions of nitrogen modules in the combustion air and hydrocarbon radicals from the fuel. It forms within the flame and typically is about 1 ppm at 15% O 2 , and is usually much smaller than the thermal NO x formation. Fuel-bound NO x forms when the fuel contains nitrogen as part of the hydrocarbon structure. Natural gas has negligible chemically bound fuel nitrogen.

Thermal NO x formation is a function of both the local temperatures within the flame and residence time. In older technology combustors used in industrial gas turbines, fuel and air were separately injected into the flame zone. Such separate injection resulted in high local temperatures where the fuel and air zones intersected. The focus of combustion improvements of the past decade was to lower flame local hot spot temperature using lean fuel/air mixtures whereby zones of high local temperatures were not created. Lean combustion decreases the fuel/air ratio in the zones where NO x production occurs so that peak flame temperature is less than the stoichiometric adiabatic flame temperature, therefore suppressing thermal NO x formation.

All microturbines feature lean pre-mixed combustion systems, also referred to as dry low NO x or dry low emissions (DLE). Lean premixed combustion pre-mixes the gaseous fuel and compressed air so that there are no local zones of high temperatures, or “hot spots,” where high levels of NO x would form. DLN requires specially designed mixing chambers and mixture inlet zones to avoid flashback of the flame. Optimized application of DLN combustion requires an integrated approach to combustor and turbine design. The DLN combustor is an intrinsic part of the turbine design, and specific combustor designs are developed for each turbine application. Full power NO x emissions below 9 ppmv @ 15% O 2 have been achieved with lean premixed combustion in microturbines.

CO and unburned hydrocarbons both result from incomplete combustion. CO emissions result when there is insufficient residence time at high temperature. In gas turbines, the failure to achieve CO burnout may result from combustor wall cooling air. CO emissions are also heavily dependent on operating load. For example, a unit operating under low loads will tend to have incomplete combustion, which will increase the formation of CO. CO is usually regulated to levels below 50 ppm for both health and safety reasons. Achieving such low levels of CO had not been a problem until manufacturers achieved low levels of NOx, because the techniques used to engineer DLN combustors had a secondary effect of increasing CO emissions.

While not considered a regulated pollutant in the ordinary sense of directly affecting public health, emissions of carbon dioxide (CO 2 ) are of concern due to its contribution to global warming. Atmospheric warming occurs because solar radiation readily penetrates to the surface of the planet but infrared (thermal) radiation from the surface is absorbed by the CO 2 (and other polyatomic gases such as methane, unburned hydrocarbons, refrigerants, water vapor, and volatile chemicals) in the atmosphere, with resultant increase in temperature of the atmosphere. The amount of CO 2 emitted is a function of both fuel carbon content and system efficiency. The fuel carbon content of natural gas is 34 lbs carbon/MMBtu; oil is 48 lbs carbon/MMBtu; and (ash-free) coal is 66 lbs carbon/MMBtu.

4.2.4 CHP Applications

In CHP operation, a second heat exchanger, the exhaust gas heat exchanger, transfers the remaining energy from the microturbine exhaust to a hot water system. Exhaust heat can be used for a number of different applications, including potable water heating, driving absorption cooling and desiccant dehumidification equipment, space heating, process heating, and other building or site uses. Some microturbine-based CHP applications do not use recuperators. With these microturbines, the temperature of the exhaust is higher and thus more heat is available for recovery.

Thermal loads most amenable to CHP systems in commercial/institutional buildings are space heating and hot water requirements. The simplest thermal load to supply is hot water. Retrofits to the existing hot water supply are relatively straightforward, and the hot water load tends to be less seasonally dependent than space heating, and therefore, more coincident to the electric load in the building. Meeting space heating needs with CHP can be more complicated. Space heating is seasonal by nature, and is supplied by various methods in the commercial/institutional sector, centralized hot water or steam being only one example.

4.2.5 Thermal Energy Generation

Effective use of the thermal energy contained in the exhaust gas can improve microturbine system economics. Exhaust heat can be recovered and used in a variety of ways, including water heating, space heating, direct or indirect drying, and driving thermally activated equipment such as an absorption chiller or a desiccant dehumidifier.

While, electrical efficiency is a function of the temperature drop across the turbine expansion stage, microturbine CHP total system efficiency is a function of exhaust temperature. Recuperator effectiveness strongly influences the microturbine exhaust temperature. Consequently, the various microturbine CHP systems have substantially different CHP efficiency and net heat rate chargeable to power. These variations in CHP efficiency and net heat rate are mostly due to the mechanical design and manufacturing cost of the recuperators and their resulting impact on system cost, rather than being due to differences in system size.

4.2.6 Current Market Applications

Microturbines are currently being tested in a number of different market segments. Applications include CHP, power-only applications (sometimes referred to as Prime Power), peak generation, premium power (High Reliability/Power Quality) applications, and resource recovery. Relatively new to commercial use, the outlook for microturbine-based CHP systems in the restructured electric industry is still uncertain. Primary targets for microturbine CHP in the commercial/institutional sectors are those building types with electric to hot water demand ratios consistent with microturbine capability: Education, Health Care, Lodging, and certain Public Order and Public Assembly applications. Office Buildings, and certain Warehousing and Mercantile/Service applications can be target applications for CHP if space conditioning needs can be incorporated.

4.2.7 CHP Potential

The simplest integration of microturbine-based CHP into the commercial, institutional and industrial sectors is in applications that meet the following criteria:

  • relatively coincident electric and thermal loads
  • thermal energy loads in the form of hot water
  • electric demand to thermal demand ratios in the 0.5 to 2.5 range
  • moderate to high operating hours (>3000 hours per year)

Microturbine exhaust can also be used to produce heat at high temperatures that can be used directly or by means of a heat exchanger to drying or pre-heating processes. Exhaust can also be used for preheated combustion air.

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