With the aim of preventing global warming, the Kyoto Protocol demands that Japan reduces its emissions of carbon dioxide (CO₂) to a level that is 6 % lower than 1990 levels. To this end, it is necessary to further promote energy conservation. One means of doing so is by introducing cogeneration systems. In addition, it is important to develop a technology that can make use of recyclable energy such as biomass fuel instead of fossil fuel. For Japan, moreover, it is good policy to have an optimum mix of energy sources, given that the country has meager energy resources of its own. In other words, there is a huge demand for energy-efficient and environmentally friendly technologies. The development of small-size independent distributed electricity generating systems is therefore important both technically and socially in order to promote the spread of cogeneration systems.

A gas turbine is a promising power source in that it can cleanly burn a range of fuel types and be used as the basis of a high-efficiency system. Therefore, using the technology accumulated from the development of gas turbines for automobiles, we developed a micro gas turbine for a cogeneration system that realizes an overall energy efficiency (electricity and heat) of 70 %. In this article, we will consider the past and present state of micro gas turbine technology, and consider its future prospects.

Simple-cycle and recuperated-cycle micro gas turbines (MGT) were developed for use in cogeneration systems. A simple-cycle MGT is better suited to applications that require steady heat energy (steam) more than electric power. A recuperated-cycle MGT, however, can be used for those applications that need both electricity and heat energy.

Firstly, lean premixed combustion with a multistage fuel supply was investigated for a 300-kWe class simple-cycle MGT. An NOx emission level of less than 15 ppm (O₂ = 16 %) with town gas as the fuel was demonstrated when the equivalence ratio of the primary lean pre-mixture was held at a constant value of less than 0.6 and the pilot fuel constituted about 10 % of the total fuel flow rate. Secondly, to investigate low-NOx combustion for a 50-kWe class recuperated-cycle MGT, we examined lean premixed combustion that produces a NOx level of less than 10 ppm (O₂ = 16 %) with town gas and also lean premixed, pre-vaporized combustion with kerosene that produces a NOx level of less than 20 ppm (O₂ = 16 %).

Masayoshi Otsuka

One of the advanced technologies incorporated into a micro gas turbine (MGT) is the dynamic air-lubricated (no oil lubrication) bearing. We investigated the performance characteristics of a self-acting air-lubricated bearing in a 50-kWe MGT. From the results of our experiments, we were able to clarify the following.

(1) The thrust load of 300 N incurred by a 50-kWe MGT cannot be supported by the dynamic air pressure generated by a pocket-type bearing with a micro-meter size groove.

(2) The target performance (a film thickness of at least 20 μm at a load of 300 N) can be achieved by introducing compressed air (air pressure of equal to or more than 120 kPa) into the rotor disc surface through a pocket-groove.

(3) The performance characteristics of the self-acting air-lubricated bearing were improved when a new structure featuring a rear-mounted thrust bearing with a sheet-spring was adopted.