Microturbines for emergency cooling systems

Due to a multitude of technical reasons such as weight, size and considerable shaking forces, emergency piston type 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.

(You can read the full article in the 2014 Turbomachinery International Handbook)