Microturbines as pump drivers have many advantages over other drivers (for example, piston-type engines), including higher power density, smaller footprint, lighter weight, higher availability, better operation, higher reliability, easier maintenance, greater fuel flexibility and fewer moving parts. The old-fashioned emergency diesel engine drivers or battery systems have been used for decades in critical emergency cooling system pumps, but some could be unreliable and unsuitable for modern facilities. An advanced microturbine technology may be a reliable, lightweight and compact alternative for emergency pumps such as emergency cooling pump applications.
Microturbines are small gas turbines with an output power of around 10 to 700 kilowatts (kW). Microturbines mainly consist of a single-stage radial air compressor, one or more radial turbine sections, a combustor and a recuperator. They usually use foil bearings, also called air bearings. In a typical microturbine, the cycle consists of the following four processes:
- The inlet air is compressed in a radial air compressor.
- The air is preheated in the recuperator using the heat from the turbine exhaust.
- The heated air from the recuperator is mixed with the fuel in the combustor and burned.
- The hot gas expands in the turbine stage, and the gas energy converts to mechanical energy to drive the air compressor and the driven equipment (for instance, an emergency cooling pump).
In single-shaft microturbines, a single expansion turbine turns both the air compressor and the driven equipment (the pump). The two-shaft models use one turbine to drive the air compressor and a second turbine to drive the driven equipment. The single-shaft models are designed to operate at high speeds (some models in excess of 10,000 rpm). Often the direct-drive, high-speed cooling pump is proposed. The power turbine on a two-shaft machine can be designed to run at a lower speed matching the driven equipment's speed. A conventional cooling pump can be connected to the power turbine through the single-stage gear unit (the power loss in order of 2 to 4 percent).
There are two main reasons microturbines have not succeeded in ordinary cooling pump applications. First, microturbines have been less fuel-efficient than small piston engines (reciprocating engines). Second, microturbines have historically been more expensive to manufacture than piston engines. The piston engines have been mass-produced in huge quantities for decades. But microturbines are more appropriate for emergency pumps, offshore pumps and similar equipment. The microturbine power-to-weight and power-to-footprint ratios are very high. Microturbine reliability and availability are much better than piston engines.
Microturbines have been used extensively in auxiliary systems and cabin cooling systems on airplanes. Decades of experience in these applications provide a good basis for the engineering and manufacturing technology of emerging microturbine models for emergency cooling pumps, emergency pumps, offshore applications and other services.
The rotational speed of microturbines usually increases as the physical size decreases. For recuperated microturbines, the optimum pressure ratio for the best efficiency is usually about 4-to-1. The turbine section inlet temperatures are generally limited to 1,000 C (within the capabilities of available turbine blade alloys) to avoid the use of expensive materials and special recuperator designs. Modern large gas turbines are currently designed with temperatures in the range of 1,400-1,600 C.
These achievements in large gas turbines have been reached through the development and advancement of the blade and vane internal cooling technologies. The nature of turbine section design (usually the radial type) and its small physical size in microturbines have not yet allowed for the internal cooling.
This is the primary driver for the ceramic technology development for future microturbines. Currently, there are some development projects for microturbine ceramic components with the firing temperature in the range of 1,200-1,400 C (with expected efficiency around 40 percent). Companies will likely produce ceramic components with firing temperatures of more than 1,400 C after the 1,200-1,400 C components have been proven in service.
The most common microturbine design is the single-shaft arrangement, supported by two high-speed bearings. Because this design has only one moving shaft, it offers high reliability and low maintenance.
Recuperators for Microturbines
To be economically viable, most microturbines should have a recuperator that recovers the waste heat. Without the recuperator, the microturbine efficiency is low. The recuperators are gas-to-air heat exchangers that use the hot turbine exhaust gas, typically 650 C, to preheat the compressed air (typically around 150-200 C) before the compressed air goes into the combustor. They reduce the fuel needed to heat the compressed air.
The recuperators are difficult to design and manufacture because they operate under high pressure and high temperature differences. Effectiveness is measured by the ratio of the actual heat transferred to the maximum achievable heat transferred. The recuperator should have a high effectiveness, and usually the target is more than 90 percent. A properly designed recuperator increases the efficiency of a microturbine from less than 25 percent to more than 34 percent.
The incorporation of a recuperator could result in some pressure loss, which reduces the pressure ratio available to the turbine section. The microturbine performance could be enhanced by using a recuperator with higher effectiveness and lower pressure drop. But such recuperators would be large and expensive. The effective optimization of a recuperator design should balance the performance, weight, size, cost and reliability.
Recuperator durability is critical. Some recuperators develop leaks because of differential thermal expansion accompanying the thermal transient. Recent improvements in recuperator reliability and durability have resulted from the use of higher-strength alloys and higher-quality welding, along with an improved engineering design. Modern recuperator designs have an inherently large heat transfer area to volume ratio that allows for high efficiency in a relatively compact heat exchanger. These designs also provide low thermal deformation and stress and a long service life.
Three types of bearings have been used for microturbines:
- Conventional high-speed oil-lubricated bearings, mainly in the form of rolling-element bearings
- Foil bearings (air bearings)
- Ceramic bearings
Air bearings have been used on microturbines for airplane auxiliary systems and cabin cooling systems for many years. When using the air bearing, the high-speed microturbine rotor is aerodynamically supported on a thin layer of air, so friction is low. The oil system and cooling system are not required for this bearing. The elimination of the lubrication oil system improves safety considerably. The air bearing offers operation simplicity, low operating cost, maximum reliability, low maintenance, low power consumption and maximum safety.
There are some concerns for the long-term reliability of air bearings under numerous and repeated starts and stops because of metal-to-metal friction during startup and shutdown. Suitable materials, designs and components for these transient situations should be provided. Reliability depends on the specific design, material selection and the individual manufacturer's quality-control methodology. The air bearing reliability will only be proven after a significant experience with substantial numbers of units working with long operating times and many on-off cycles. Air bearing manufacturer references should be checked carefully.
Case Study: Emergency Cooling System
The Fukushima Daiichi nuclear power plant suffered major damage from an earthquake and subsequent tsunami that hit Japan on 11 March 2011. The earthquake and tsunami disabled reactor cooling systems, leading to radiation leaks and triggering a 30-kilometer evacuation zone surrounding the power plant.
The reactor's emergency diesel engines, crucial components in helping keep the reactor cool in the event of a power loss, were in the basement of the reactor turbine building. For technical reasons, the emergency diesel engines were not included in the watertight building or at a high enough elevation to protect them from flood or tsunami damage. The plant was designed to handle waves up to six meters, but the tsunami produced waves of up to 14 meters. This disabled the emergency engines required to cool the reactor. After the diesel engines failed, the emergency cooling pump system was operated by batteries designed to last about eight hours. All power for the cooling system was lost after eight hours, and the reactor started to overheat.
Microturbines are reliable, compact and light compared with diesel engine drivers. They generate low dynamic forces compared with large shaking forces produced by diesel engines. They are often the superior option for critical emergency pump systems compared with piston engines. They can easily be included in a watertight building or other suitable location to obtain the maximum possible protection.
The energy density of liquid hydrocarbon fuel is about 30 times that of the best available battery technology, so liquid-fueled microturbine-driven emergency cooling pumps could offer around 30 times more operating time within the same space limits. This means a cooling system designed to operate for 10 days could have the same footprint as a battery-powered system designed to operate for eight hours.
The development of small, high-speed microturbines for miniature pump drives could allow for new opportunities in technology. These would be sized by the centimeter. Because of the high energy density of liquid hydrocarbon fuels, the miniature microturbine technology could shrink around 15-20 times the current battery technology sizes, replacing the electric motors and batteries from the miniature equipment market. Preliminary designs indicate that the miniature microturbines (and miniature pumps) require:
- Rotor peripheral speeds of 200 to 500 meters per second
- Rotating components capable of stresses of about 300 to 500 megapascals
- Low-friction, very small bearings
- High-dimensional tolerances
- Thermal isolation of the hot and cold sections in compact designs
The miniature microturbines have to turn at the same peripheral speed as large gas turbines. The result is high shaft speed. The high speed implies high centrifugal stresses because stress rises with the square of rotational speed. On the other hand, materials at micro scale could be much better than at macro scale. Defect-free, micro-fabricated materials are quite strong. The components of miniature microturbines and miniature pumps will probably be fabricated from single-crystal ceramics with high strength and low density.