The market for distributed power is huge, both in industrialised and developing countries, but particularly in the USA where diminishing operating margins in the existing generation, transmission and distribution infrastructure are now causing regular disruptions in a computer based industrial economy less tolerant of them than ever before. This has prompted many users to consider generating power locally. In the developing world the lack of a power transmission structure capable of sustaining industrial growth is likewise creating a market for distributed power.


To take advantage of this market Turbo Genset Inc and DTE Energy Technologies (aka Dtech) a subsidiary of DTE Energy Co, have signed a collaborative agreement to jointly develop a 400 kW mini-turbo-generator system to be designated the ENT 4000 (Figure 1). The generator will be powered by a mini-turbine, the ST5, being developed by Pratt & Whitney Canada Corp. Under the agreed terms Dtech will provide overall system controls for integration into microgrids as well as interfaces to traditional utility grids and will be responsible for packaging and marketing the system. Woodward Governor Company will develop the fuel system, to include a patented metering valve and control system for accurate flow control “at a very low cost compared to conventional turbine control systems”. The target audience is small to medium commercial and industrial customers and micro-grids serving residential and commercial developments.

To this partnership PWC brings its proven expertise in small to medium gas turbines, Turbo Genset the compactness and scalability, of its alternator technology; DTE brings enormous resources and immediate access to the distributed generation market. DTE’s confidence in the market and in this development is such that it has ordered 100 of the yet to be developed 400 kW TGI generators, with the associated power electronics, at a cost of $8.5 million. The first pre-commercial units are scheduled for delivery in July 2001 and delivery of commercial units should start in early 2002.

High speed generator

Turbo Genset’s part is to develop the existing high speed generator from the current standard size of 50 kW to a 400 kW unit. This is a matter of scaling up, not radically changing the design, according to the company. The new units are based on the existing design, and will exhibit the same power density.

Patented disc rotor

The Turbo Genset alternator adopts a patented disc configuration (Figure 2) in which the magnetic field is axial, rather than the radial configuration normally used. High-speed alternators have only become feasible with the advent of high-field strength rare-earth magnetic materials such as neodymium-iron-boron and samarium-cobalt. Because these materials tend to be very brittle and have poor tensile strength, they need to be supported by a high-strength retaining shell under the extreme centrifugal loads experienced at high speeds. Nonetheless the disc configuration overcomes a number of difficulties associated with high rotational speeds, including heat build up and heat dissipation.

The rotor consists of two or more discs which share the same shaft and contain the permanent magnets, while the stator consists of one or more discs which contain the windings, each rotor/stator pair presently producing about 10 kW of electrical output (Figure 3). The 50 kW alternator in the company’s first commercial turbo gensets contain 5 stator discs connected in series and 6 rotor discs sharing the same shaft. The device is roughly the same size as a car alternator, but produces over 50 times the electrical power.


Development in this case consist mainly of scaling up the existing design by adding more rotor/stator pairs on the same shaft. It is also believed that the modules, ie the rotor stator pairs, can be scaled up in size. Development work will largely consist of stress testing of a larger diameter alternator at the speed of the turbine, around 30 000 rpm. Development will be carried out with PWC on a joint test bed over a 12 to 18 month period.

TGC have already produced a small scale electrical power generator which uses a gas turbine engine as the prime mover directly coupled to a high speed permanent magnet alternator. The technology is currently being further developed at Imperial College, London. TGC’s innovation is a high speed, highly compact alternator directly powered by a small gas turbine using various oil and gas fuels, including natural gas. A Turbo Genset machine that can produce 50 kW weighs only 150 kg compared to 1-2 tonne for a conventional reciprocating engine genset. The company is also developing a 100 kW unit, but expects the existing design ultimately to support output powers up to 1-2 MW.

A key feature of the alternator is its high operating speed. Because it is capable of operating at the same speed as the gas turbine engine, the engine shaft can be coupled directly to the alternator without the need for a reduction gearbox. This further improves the power-to-weight ratio and reliability of the overall system.

The design has several interesting points. The rotor discs are optimised for the operating speed and power, the stator discs for the power and voltage; therefore the machines can be easily adapted to suit different operating speed, power and voltage requirements. The axial magnetic field means that the rotor stengthening does not interfere with the field.

Importantly, the motors feature integral air cooling, and therefore do not degrade engine performance by relying on the engine air intake for cooling. Claimed advantages over conventional units are compactness, reliability, low noxious emissions, low manufacturing cost, low installation cost (no special base or vibration mountings), low maintenance, multi-fuel capability, and high grade exhaust heat.

ST5 gas turbine

Turbo Genset have formed a public company, Turbo Genset Inc, registered in Canada and quoted on both the Toronto and London stock exchanges, and in combination with Pratt & Whitney Canada are carrying out joint product development of mini-turbines. Initially these will be sized up to 450 kW (Figure 4) although the company believes that in principle 2MW machines are possible with the same technology.

For this application, development of rotor size and speed is necessary. Standard disc pairs can run at 65 000 rpm. But tests currently running at the PWC/TGI test site for the PWC combination set are at 30 000 rpm, the speed of the turbine, using a single rotor/stator pair of larger than standard size. For the first engine test run in March 2001 an eight stage alternator is envisaged; though the precise combination of stages/size/speed will depend on refinements during development. This first generation will be used for two types of tests – alternator testing, and package and controls development. The second generation engine is scheduled for December 2001, in time for precommercial trials.

PWC’s power unit (Figure 5), a new high efficiency mini gas turbine being developed from the standard version ST5, is based on the aerodynamics of a PW207 aero engine (actually a helicopter engine) developed into a full industrial engine. Although most of the parts have the same function not a single component of the original has survived intact; PWC literally took the flow path of the PW207 and built a new turbine around it. The result was a free turbine engine with a gas path design based on other well proven turboshaft engines. Efficiency is further enhanced by the addition of an optional recuperator (Figures 6,7,8). To date modifications to the ST5 include the replacement of the original annular combustor with an external silo type combustor, so that there would be no performance compromises. Addition of the recuperator has increased peak rating efficiency from 24.6 to 34.4 per cent.

Air enters the engine (Figure 6) through a radial intake and flows through a single stage centrifugal compressor. The compressed air is then directed into an exhaust recovery heat exchanger (in the case of the recuperated version) before entering the single can low emissions combustor, where fuel is introduced. The combustion products are expanded through a compressor turbine and then a power turbine, each being a single stage axial type, mechanically unconnected. The flow then passes through the recuperator (excluded in the simple cycle vesion) before finally exiting through a single port exhaust.