Microturbine

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ABSTRACT

Microturbines are becoming wide spread for distributed power and combined heat and power applications. They range from handheld units producing less than a kilowatt to commercial sized systems that produce tens or hundreds of kilowatts. They are also known as "turbo alternators", or "gensets". Part of their success is due to advances in electronics, which allow unattended operation and interfacing with the commercial power grid. Electronic power switching technology eliminates the need for the generator to be synchronized with the power grid. This allows, for example, the generator to be integrated with the turbine shaft, and to double as the starter motor. Microturbine systems have many advantages over piston engine generators, such as higher power density (with respect to footprint and weight), extremely low emissions and few, or just one, moving part. They accept most commercial fuels, such as natural gas, propane, diesel and kerosene. The are also able to produce renewable energy when fueled with biogas from landfills and sewage treatment plants. Microturbine designs usually consist of a single stage radial compressor, a single stage radial turbine and a recuperator.Typical micro turbine efficiencies are 25 to 35 percent. When in a combined heat and power cogeneration system, efficiencies of greater than 80 percent are commonly achieved.
INTRODUCTION
 
Microturbines are a new type of combustion turbine being used for stationary energy generation applications. They are small combustion turbines, approximately the size of a refrigerator, with outputs of 25kw to 500kw, and can be located on sites with space limitation for power production. Microturbines are composed of a compressor, combustor, turbine, alternator, recuperator, and generator. Waste heat recovery can be used in combined heat and power system to achieve energy efficiency levels greater than 80%. In addition to power generation micro turbines offer an efficient and clean solution to direct mechanical drive markets such as compression and air conditioning. Since making their commercial debut a mere five years ago, microturbines have installed with considerable success in office and apartment building, hotels and motels. Supermarkets, school and college, office and industrial parks, small industries, and numerous other facilities both in the US and abroard.They provide not only electricity, but the thermal energy to provide for all heating and cooling needs. 
WHAT IS A MICROTURBINE

Microturbines are small combustion turbines approximately the size of a refrigerator with outputs of 25kw to 500kw. They evolved from automotive and truck turbochargers, auxiliary power units for airplanes, and small jet engines and are comprised of a compressor, combustor, turbine, alternator, recuperator, and a refrigerator. The engine itself is about the size of a beer keg. The most popular models have just one moving parts”a shaft with a turbine wheel on one end , a permanent magnet generator on other end, and an air compressor wheel in the middle. This assembly rotates at up to 96,000 rpm. At that speed, traditional oil-lubricated bearings are severely challenged. Accordingly the most popular micro turbine engines use air bearing to float the shaft.
Not only is the turbine turning at high rpm, so is the generator. The generator in turn produces a high frequency electrical output, which is then converted by power electronics unit to grid “compatible 400-to-480-volts alternating current, 10-to-60 hertz.3phase power.
Microturbine offer a number of potential advantages compared to other technologies for small-scale power generation. These advantages include a small number of moving parts, compact size, light-weight, greater efficiency, lower emission, lower electricity cost, and opportunities to utilize waste fuels. They have the potential to be located on sites with space limitation for the production of power. Waste heat recovery can be used with these systems to achieve efficiencies greater than 80%.
There is very definitely a trend toward installing microturbine system onsite”not only for generating electric power. But also for meeting site heating and cooling needs. Such microturbine configuration are called combined heat and power, or combined cooling, heat and power (cogeneration) system. The core idea is this: when burning a fuel in a micro turbine unit, donâ„¢t just use the resulting heated gases to spin a turbine and generate electricity. There is still a huge amount of thermal energy in the turbine exhaust. Donâ„¢t waste that valuable energy to the atmosphere”which is what they do in most central power plants (because there is no use for the heat in remote areas). 
Instead, use a heat exchanger to capture much of that thermal energy and use it to meet all the heating and cooling needs of the site. When a microturbine unit is arranged in CHP or CCHP mode, heat from the turbine stack is captured and used to meet some or all the heating and cooling needs of the facility. This makes for much more efficient fuel use. Instead of just using 35% of thermal energy released during fuel combustion (as with a traditional central power plant), with CHP and CCHP one would be using 65% or more of the fuels thermal energy. This realization is a major reason the federal Department of Energy has been strongly encouraging the advance of onsite power generation with CHP and CCHP. 
The 30-kilowatt model of Microturbine is very versatile, being able to burn several gaseous or liquid fuels”natural gas, propane, biogas, diesel, and kerosene.
Microturbine Overview
Commercial Available Yes (limited)
Size Range 25-500 kW
Fuel Natural gas, hydrogen, propane, and 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. 
BASIC COMPONENTS OF MICROTURBINE

TURBO COMPRESSOR:-
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 Microturbines 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, micro turbine turbo machinery is based on single-stage radial flow compressors and turbines. Radial flow turbo machinery handles the small volumetric flows of air and combustion products with reasonably high component efficiency.1 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 micro turbines, the turbo compressor shaft generally turns at high rotational speed, about96, 000 rpm in the case of a 30 kW machine and about 80,000 rpm in a 75 kW machine. One 45kW model on the market turns at 116,000 rpm. 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 micro turbines.

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 permanent magnet (typically Samarium-Cobalt) alternator, and requires that the high frequency 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. Full start-up requires several minutes. If the system is operating independent of the grid (black starting), a power storage unit (typically battery UPS) is used to power the generator for start-up.

RECUPERATOR:-
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. Depending onmicroturbine 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% from that attainable without the recuperator. Recuperators also lower the temperature of the micro turbine exhaust, reducing the micro turbine™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 tomicroturbine 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 there liability 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.

POWER ELECTRONICS:-
As discussed, single-shaft micro turbines 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.

HOW MICROTURBINE WORKS
Microturbine engine has only one moving part, basically a shift. At one end of that shaft is a turbine wheel; at the opposite end of the shaft is a permanent magnet electric generator; and positioned at the mid point of that shaft is an air impeller wheel (ie; an air compressor) for drawing ambient air , compressing it , then pumping it into combustor. Fuel is then injected into the combustor, where it then mixes with compressed air. Combustion occurs and the resulting gasses expand and rush out through the turbine, spinning it to a very high rpm.
This whole microturbine system is packaged in an enclosure not much bigger than a refrigerator”about 7 feet tall, 2.5 feet wide and 6.5 feet deep. Ambient air is first drawn into the microturbine system enclosure, filtered, then passed over the electric generator, which is kept cool by this passing air. Next, the air is drawn into the impeller (or compressor), which compresses the air before pumping it into the combustor
Now, a part of that compressed “air stream exiting the impeller (compressor) is diverted to the air bearing. The microturbine shaft in effect now rides on a thin film of compressed air”this being in the thin annular space between the rotating shaft and the stationary bearing housing 

TYPES OF MICROTURBINE
Microturbine are classified by the physical arrangement of the component parts; single Shaft, simple cycle, or recuperated, inter-cooled, and reheat. The machines generally rotate over 40000 revolutions per minute. The bearing selection “oil or air- is dependent on usage .A single shaft microturbine with high rotating speeds of 90000 to 120,000 revolutions per minute is the more common design ,as it is simpler and less expensive to built. Conversely, the spilt shaft is necessary for machine drive applications, which does not require an inverter to change the frequency of the AC power.
Microturbine generator can also be divided into two general classes:
Unrecuperated (simple cycle) micro turbine”in a simple cycle, or unrecuperated, turbine. Compressed air is mixed with fuel and burned under constant pressure condition. 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 application than recuperated unit.
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 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.
Recuperated Microturbine

CHARACTERITICS OF MICROTURBINES
Some of primary applications for microturbine include:
Distributed generation”stand “alone, on site applications remote from power grids.
Quality power and reliability”reduced frequency variation, voltage transients, surges, dips, or other disruptions.
Stand by power”used in the event of an outage, as back up to 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 primary power that is less expensive to produce locally than it is to produce from the electric utility
Combined heat and power (cogeneration )”increase the efficiency of on-site power generation by using the waste heat for existing thermal process.

DISTRIBUTED ENERGY GENERATION
Energy is produced on a large scale in large thermal and hydro electric power plants and is then distributed to the users through network of lines called the power grid. These plants meet the need of consumers over a large area. In distributed energy generation on the other hand involves the on site generation of small scale power. On-site power generation means power is generated right where it is needed.
Advantages of Distributed Generation
As energy need not be transmitted there is no need of any large transmission infrastructure. Thus the losses during power transmission are greatly reduced. The combined heat and power (CHP) technology can be applied to micro turbines to increase its efficiency. This lowers emission and operating cost by reducing losses and increasing efficiency. From a companyâ„¢s point of view, it gives greater control, choice and flexibility in meeting needs for power and heat energy.
Selected strength and weaknesses of microturbine technology are:
Strengths
Small number of moving parts
Compact size
Lightweight
Good efficiencies in cogeneration
Low emission
Can utilize waste fuels
Long maintenance interval
No vibration
Less noise than reciprocating engines
Weaknesses
Low fuel to electricity efficiencies
Loss of power output and efficiency with higher ambient temperature and elevation.

APPLICATIONS OF MICROTURBINES
Microturbines are being increasingly preferred over reciprocating engines in many applications. These include:
Combined heat and power (co-generation)
Waste heat from the micro turbine can be transferred via a heat exchanger to produce steam or provide hot water for local area. The hot water can be used in a green house to grow plants; water can duct to provide central heating in building in winter. Thermal hosts can found easier because the the produced by each microturbine unit is so much that by a large power station.
Distributed power generation
Electricity is generated locally to meet demand in the local area, for example a small townâ„¢s electricity supply. This can relieve congestion of the distribution network or power grid. Hospitals, hotels, factories and holiday resorts can install distributed power at remote sites without grid access.
Distributed generation provides a wide range of services to consumers and utilities, including standby generation, peak shaving capability, base load generation and co-generation.
Hospitals
The waste heat from the generator can be used to create for the sterilization of medical equipment as well as for laundry purposes, like the daily changing of bed linen. It can also act as backup power supply, which is critical for the smooth functioning of various life-supporting equipments.
Backup generators
Microturbines can also be used in remote areas where there is no access to electricity. It could provide electricity for research station in the middle of a jungle or desert, where there is no ready access to diesel supplies but is located near gas wells. 
Vehicle applications
Hybrid vehicle ( microturbine to high speed alternator)
In hybrid vehicle applications, the power produced by a microturbine is converted into electricity by a high “speed alternator. The power is used to drive electric motors connected to the wheels. Any excess energy is directed to an energy storage system such as batteries or flywheels. The operating mode of the hybrid approaches ranges from battery-primary systems where the microturbine can be a ˜battery charger™, to engine-primary system where the batteries help the micro turbine meet peak power needs, e.g. during acceleration.
Hybrid vehicle (microturbine and fuel cell together)
A hybrid combination of micro turbines with fuel cells can increase overall system efficiencies. Hybrid systems take advantage of an increase in fuel cell efficiency with an increase in operating pressure. The microturbine compressor stage is used to provide this pressure. The fuel cell produces heat along with power, and this heat energy is used to drive the microturbines turbine stage. If the fuel cell produces enough heat the micro turbine can generate additional power. For the hybrid combination, efficiency is expected to be as much as 60% and emission less than 1.0 ppm NOx, with negligible SOx and other application.