18-07-2014, 11:25 AM
MAGNETIC REFRIGERATION
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ABSTRACT
Micro turbines 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 "genets’". 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. Micro turbine 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 fuelled with biogas from landfills and sewage treatment plants. Micro turbine 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. Micro turbines 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, micro turbines 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 abroad. They provide not only electricity, but the thermal energy to provide for all heating and cooling needs.
WHAT IS A MICROTURBINE
Micro turbines 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 a compatible 400-to-480-volts alternating current, 10-to-60 hertz.3phase power.
Micro turbine 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 micro turbine system onsite not only for generating electric power. But also for meeting site heating and cooling needs. Such micro turbine 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, 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. 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 micro turbine 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
COMPARISION WITH TURBINE
Typically micro turbines have power outputs in range of 25 to300KW. In comparison with large power station are entire building and much higher output of around 600MW to 1000MW. One way in which micro turbines can be distinguished from larger turbine is that micro turbine use a single shaft to drive the compressor, turbine and generator. Where is in large power plant, the turbine and generator are on separate shaft and connected by gears that can slow down the high speed rotation of the gas turbines, simultaneously increasing the torque sufficient to run much larger electric generators. Some micro turbines even include the ability to generate electricity from heat of exhaust gases.
CONSTRUCTION
Micro turbines are typically single shaft machines with the compressor and turbine mounted on the same shaft as the electrical generator. It therefore consist of only one rotating part, eliminating the need of gear box and associated numerous moving part. Micro turbines are miniature version of the huge machines used to generate power from natural gas, and evolved from aircraft engines and automotive turbochargers
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 on micro turbine 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 effectiveness in CHP applications.
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
TYPES OF MICRO TURBINES
There are many types of micro turbines. As a example micro turbine development is based on turbine used for aircraft auxiliary power units, which have been used in commercial airlines for decades. Micro turbines like aero derivative turbines, steam micro turbines, and hydro micro turbines.
1.Aeroderivative turbine: An aeroderitive turbine is an engine derived from a jet engine, which operates at very high compression ratios and which tends to be very compact. Aeroderivative technology enables the manufacture of more efficient gas turbine generators for electric utilities. A steam micro turbine is a device for converting energy from high pressure steam produced in a boiler into mechanical power, which can then be used to generate electricity. Micro turbines such as a hydro micro turbine are devices for gathering electric power from moving water sources.
2.Steam micro turbine: It is a kind of micro turbine which takes steam as input.
3.Hydro micro turbine: It’s a kind of micro turbine which takes water as input.
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.
ECONOMICS OF MICROTURBINES
Microturbine capital costs ranges 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 very 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, once-a- year maintenance schedule should suffice. Most manufacturers are targeting maintenance intervals of 5,000-8,000 hours.
Maintenance costs for micro turbine units are still based on forecasts with minimal real-life situation. 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-$1100\Kw
O&M Cost $0.005-0.016\kw
Maintenance Interval 5,000-8,000hrs
CONCLUSTION
Being a new technology many countries have not used micro turbines. With increasing rate of power demand micro turbine will soon find their access to the market. Because of their easy handling system and compact size it is very useful in remote area where it is either expensive or not possible to extend the cable line