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Due to a multitude of technical reasons such as weight, size and considerable shaking forces, emergency pistontype diesel generators cannot be included in a watertight building or be placed at an elevation high enough to protect them from flood waters. The Fukushima nuclear disaster acts as a recent example of how a tsunami or flood can disable engine-driven generators.
Microturbines are a viable alternative as they offer high availability, easy maintenance, fuel flexibility and fewer moving parts (usually only one moving assembly). These machines are reliable, compact and lightweight compared to diesel engines.
They generate low dynamic forces compared to the shaking produced by diesel engines. They are a superior option for critical emergency power systems. They can be included in watertight buildings or other suitable places to obtain the maximum possible protection and reliability. Further, they can be used to drive emergency cooling water pumps and emergency power generators.
Microturbines are small gas turbines with output power of 30 to 800 kW. A typical microturbine consists of an inlet radial air-compressor, recuperator, combustor and radial turbine section. High-temperature gas is expanded in turbine stages and the energy is converted to mechanical energy to drive the air compressor and the driven equipment (usually a generator).
Single-shaft models are designed for high-speed operation (sometimes more than 70,000 rpm). Generally, a direct-drive high-speed generator is used. The power turbine on a two-shaft machine, however, can be designed to run at a relatively low speed while still maintaining high efficiency. Sometimes, a conventional AC generator is connected to the power turbine through a single-stage gearbox (power loss on the order of 2% to 3%).
Microturbines have been extensively used in auxiliary systems and auxiliary power units on airplanes for years. There are increasingly deployed offshore due to their power-to-weight and power-to-footprint ratios. Their optimum pressure ratio for efficiency is usually around 4:1.
Turbine section inlet temperatures are generally limited to 1,000oC to avoid the use of expensive materials. The nature of their turbine section design (usually radial) and their small dimensions have not yet permitted internal cooling. Therefore, much development work is devoted to ceramics in microturbine applications with firing temperature in the range of 1,200oC to 1,400oC.
Most microturbines use air bearings. Elimination of lubricated bearings and lubrication oil systems increases safety considerably. Air bearing reliability and performance depend to a large extent upon specific design, material selection and individual manufacturer’s quality control methodology.
Because of a higher energy density, a liquid-fuel, microturbine-driven emergency cooling system could offer 20-to-30 times more operating time within the same space limits compared to batteries used in many nuclear facilities. The microturbine can also offer more than 10 days of cooling system circulation compared to conventional eight-hour batteries.
Future microturbines are going to get a lot smaller. Miniature machines could well have speeds in excess of a million revolutions per minute and sizes on the order of a few centimeters. Preliminary designs indicate that such a microturbine needs: a combustion exit temperature above 1,200oC; rotor peripheral speeds of 300 to 600 m/s; rotating components capable of withstanding high stresses; low-friction; high dimensional tolerances; and thermal isolation of the hot and cold sections.
The rotational speed of microturbines increases as the physical size decreases. The shaft turns at around 100,000 rpm in a typical 30 kW microturbine and at about 70,000 rpm in a 100 kW micro-turbine. Miniature versions would have to turn at the same peripheral speed as large gas turbines. High speed implies major centrifugal stresses as stress rises at the square of rotational speed.
Further, advanced materials at microscale would be much more effective than the same materials at macro-scale. The reality is that defect-free, micro-fabricated materials could be quite strong.
Such components most probably will be fabricated from single-crystal ceramics with high strength at an elevated temperature and low density. The high temperature performance of ceramic materials is usually limited by the creep life so more advanced coatings and refractory materials will be needed.
The baseline of a miniature microturbine looks like a small, single-spool turbojet. The design consists of a supersonic radial-flow compressor and radial-flow turbine connected by a hollow shaft to limit heat conduction. It could have a pressure ratio around 4:1.
A thin-film electric starter-generator will mount on a shroud over the compressor wheel and be cooled by the air-compressor discharge air (discharge air will cool the whole structure and casing). A 500-pole planar electric induction machine could be used. Some studies propose one cubic centimeter machines delivering 100 watts of electric power.
Microturbine performance can be enhanced by using an advanced recuperator (to recover the exhaust gas waste heat) with a higher effectiveness and a lower pressure drop; such recuperators would be large and expensive. An effective optimization of recuperator design should balance the performance, weight, size, cost and reliability.
The recuperator durability is also important. Some recuperators have developed leaks due to the differential thermal expansion accompanying the thermal transient. Modern recuperator designs should have an inherently large heat transfer areato- volume ratio that allows for high efficiency in a relatively compact heat exchanger, in addition to low thermal stress and a long service life.
Ceramic material temperature capabilities are known to be higher than those of uncooled metallic components. Microturbine components (hot section parts, particularly the turbine section) fabricated from ceramics have the potential for a significant performance gain in terms of efficiency, reliability and specific power.
Increasing firing temperature by using ceramic materials could increase the turbine exhaust temperature. However, it could also result in a change in recuperator material and design to higher temperature levels.
Many institutions and companies are working on advanced material programs to develop ceramic turbines, combustors and recuperators for microturbines. Progress is required in the following areas, in particular: ceramic component design and fabrication; methods for predicting ceramic component life; non-destructive test methods, for example, to evaluate complex shaped ceramic components; and advanced coatings for ceramics.
Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and UK (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.