Clean power generated in your own basement

23 March 2001

Fewer power plants and transmission lines, more efficient use of resources, and cheaper electricity – this scenario is within reach if a new concept for distributed power generation, the gas fired microturbine, comes into common use. Marco Suter, ABB Corporate Research, Baden-Dättwil, Switzerland

In a liberalised energy market, decentralised power generation (DPG) is an interesting alternative for today's consumers, since it allows control of energy costs and requires only a backup link to the mains grid. As a result of deregulation, many different DPG devices have appeared on the market, for example combined heat and power (CHP) units. One approach to such distributed CHP is to incorporate a microturbine.

The microturbine is well suited as a power source to facilities ranging from hospitals and hotels to shopping malls and factories, especially where a heat load is available. With the help of telecommunication systems, such power plants can be combined with IT functionality into a "virtual utility ", creating network solutions that will provide safe, unsupervised and fully optimised operation, revolutionising the way power is generated and distributed in the future.

Still a relatively new technology, microturbines have the potential to become a 'hot' product as the market grows and commercialisation drives costs downwards. Features likely to speed up this development include:

• Compliance with the latest emissions standards

• High efficiency

• Low noise level

• Minimal maintenance

• Remote control for locally unsupervised operation

• Low upfront investment due to series manufacture and simple design

• Fuel flexibility

• Compact and lightweight, can pass through a standard doorway.

New generation of microturbine

With the aim of developing a new generation of microturbine that would meet such requirements, ABB Distributed Power Generation set up in 1998 a 50/50 joint venture with Volvo Aero Corporation to develop a new generation of microturbine (Figure 1). The partnership draws on Volvo's experience with gas turbine driven hybrid electric vehicles as well as ABB's experience in high-frequency power generation and conversion.

Just recently – with a significant contribution from ABB Corporate Research Ltd in Baden-Dättwil, Switzerland – ABB Industry AG in Turgi, Switzerland and ABB Motors in Västeräs, Sweden have completed the development of the MT100 microturbine CHP unit.

Run on natural gas, the MT100 generates 100 kW of electricity and 152 kW of thermal energy in the form of hot water. The heart of the MT100 is a small gas turbine, mounted together with a compressor on a single shaft and integrated with a new high-speed generator, the HISEM 110/70. Designed for a new era of distributed power generation the MT100 microturbine is a CHP unit. It is mounted in a small cabinet and runs on natural gas. A natural gas compressor may be necessary, depending on the supply pressure. The MT100 is designed for indoor installation with its size of 0.9 m x 1.9 m x 2.9 m and it takes air from an outside intake. Its main components are:

• Gas turbine engine and recuperator

• Electrical generator

• Power conditioning system

• Exhaust-gas heat-exchanger

• Supervision and control system.

The technical data for the CHP unit are given in Table 1.

Design and operation

In the microturbine, (Figures 2, 3) the turbine wheel drives a compressor wheel mounted on the same shaft. The compressor feeds process air into the combustion chamber, where fuel is added and continuous combustion takes place.

The hot gas stream is expanded in the turbine, causing a large part of the thermal energy to be converted into mechanical energy, which drives the compressor and the load. In conventional power plants the load is either a two-pole or four-pole generator, driven via a gearbox. The generator speed is fixed, since it is synchronised with the frequency of an electric network. In the ABB MT100 microturbine the high-speed generator is coupled directly to the turbine shaft and a static frequency converter adjusts the speed electronically. The remaining thermal energy can be dissipated through the stack, but such a gas turbine will suffer from poor efficiency unless several compressor and turbine stages are added. The ABB MT100 overcomes this problem with a recuperator, which recovers the exhaust heat and uses it to pre-heat the compressed air before it enters the combustion chamber. Less fuel is therefore required to reach the desired operating temperature. Another heat-exchanger, after the recuperator, heats the water in the external circuit (Figure 4).

Gas turbine emissions are very low as the continuous combustion can be carefully controlled. The external combustion chamber can also be optimised for low emissions. Gas turbines often feature what is known as 'variable geometry'; adjustable guide vanes control how the gas flows towards the turbine and compressor, allowing the operating point of the gas turbine to be controlled. Here too, the designers of the ABB MT100 took a different approach. The electric power generation system with the frequency converter permits variable speed operation, which allows the turbine speed to be adjusted within a wide range from approximately 50000 to 70000 rev/min, 30 kW to full power, respectively. The partial load operation mode is essential for DPG devices, in particular if connected to island grids. A simpler turbine concept can therefore be chosen, keeping product costs down.

Gas turbine engine

The gas turbine is a single-shaft engine with the following main components:

• Housing

• Compressor

• Recuperator

• Combustion chamber

• Turbine

Housing: The electrical generator and the rotating components of the gas turbine are mounted on the same shaft. The engine parts and the shaft are located in the same housing.

Compressor: In the MT100, a radial centrifugal compressor is used to compress the ambient air. The pressure ratio is about 4.5:1. The compressor is mounted on the same shaft as the turbine and the electrical generator.

Recuperator: The recuperator is a gas-to-air heat-exchanger attached to the microturbine. It increases the efficiency of the turbine by transferring the heat from the hot exhaust gases to the compressed air fed to the combustion chamber.

Combustion chamber: The reheated compressed air is mixed with the natural gas, and an electrical igniter in the combustion chamber ignites the mixture. The combustion chamber is of the lean re-mix emission type, guaranteeing low emissions of NOx, CO and low unburned hydrocarbon content in the exhaust gases.

Turbine: The radial turbine drives the compressor and the generator at a nominal speed of 70 000 rev/min. On leaving the combustion chamber the combustion gases have a temperature of approximately 950°C and are at a pressure of about 4.5 bar. As the gases expand through the turbine the pressure decreases to close to atmospheric and the temperature drops to approximately 650°C.

The exhaust-gas heat exchanger is of the gas/water counter-current flow type. It transfers the thermal energy contained in the exhaust gases, which enter the heat exchanger at a temperature of approximately 270°C, to the hot-water system. The outlet water temperature depends on the incoming water conditions, ie its temperature and mass flow. A typical example would be a fixed water temperature increase from 50°C to 70°C and a volume flow of water corresponding to the current status of the microturbine.The exhaust gases leaving the heat exchanger pass through an exhaust pipe to the stack.

Generation and power conditioning

Small gas turbines benefit in particular when the gearbox that reduces the turbine shaft speed to the speed of conventional electrical machines is eliminated. The result is a more efficient, compact and reliable machine. With such a system, the shaft speed is normally above 30 000 rev/min and may exceed 100 000 rev/min.

In the MT100 electric power is generated by a HISEM 110/70 high-speed permanent magnet synchronous generator, which is integrated with the microturbine. An advantage of the high-speed generator is that the size of the machine decreases almost in direct proportion to the increase in speed, leading to a very small unit that can be integrated with the gas turbine.

Before the generated power can be sent to the grid, it has to be converted to the grid voltage and frequency. The AC of the generator is first rectified to DC and then converted to three-phase AC. An inductor stabilises the AC output, while an EMC filter protects the grid against generated interference.

The high speed generator uses state-of-the-art high-energy permanent magnets and high yield-strength materials. For example, the use of neodymium-iron-boron (NdBFe) magnets reduces the generator rotor losses. The rotor is suspended by one bearing on each side of the permanent magnet rotor; there are no additional bearings on the turbine shaft. The output frequency of the generator is 2.3 kHz. The generator can be used in reverse as a motor, in which mode it acts as an electric starter for the gas turbine. The generator design is derived from ABB’s HSG100 generator, developed for use in hybrid vehicles (Figure 5).

It is important to make sure that the rotor never reaches a temperature that would de-magnetise the magnet.This is ensured both by reducing the rotor losses and providing efficient cooling in the air-gap.

Two factors, air friction and asynchronous mmf (magneto motive force) waves in the air-gap, are the main cause of temperature rise in the rotor. The latter is mainly owing to harmonics in the stator current. Because the high speed causes high friction losses and the carbon fibre retaining band acts as a thermal insulator, the rotor is more sensitive to current harmonics than the rotor in a conventional machine. A current harmonics limit was therefore defined which is based on the acceptable rotor losses. During the development of the microturbine, additional technical requirements came up that led to a further improvement of the generator unit in the areas of reliability and production costs. Also of benefit was ABB Motors’ experience with high volume production, which could also be incorporated in the design.

Frequency converter

A prerequisite for direct mechanical coupling is highly efficient frequency conversion. Insulated gate bipolar transistors (IGBTs) not only provide this high efficiency, they can also be switched at an appropriately high frequency. Machines based on these devices are tailor- made for certain high-speed applications.

The factors that provided the biggest challenge to the designers of the frequency converter were the high input frequency of 2300 Hz and the product cost target.The microturbine converter is derived from ABB’s ACS 600 converter platform. However, extensive application programming was needed to implement the interface and control functions required for the microturbine application.

The generator input stage, including the start-up converter, is based on hardware from the ACS 600, but a new control hardware had to be developed to regulate the speed of the turbine and to control the start-up process. In the start mode, the rectifier is used as an inverter to accelerate the turbine to 30 000 rev/min, the speed from which the turbine starts to produce net power for further acceleration to the operating range of 50 000 to 70 000 rev/min. The start up converter then acts as a rectifier and supplies electric power to the DC bus, from where the grid converter transfers it to the utility network. Today's MT100 microturbine has no internal energy storage and therefore needs power from the mains for start-up. Current development is aimed at making black starts be possible with the next generation of turbines.

The ACS 600 series converters are designed to operate in an industrial environment,while in microturbine application they have to meet the requirements for residential area installations. Although distributed power generation applications are still a 'grey zone’ with respect to grid regulations, it was decided to adapt the converter to comply with the EN50081 standards..While the source of the functional hardware is the ACS 600 series,the mechanical installation has been tailored for the microturbine application. An important feature here is the use of water-cooling.

Control system

The ABB MT100 is controlled and operated automatically by the power module controller (PMC), so that in normal use the CHP unit can be left without an operator. If a critical fault should occur, the PMC initiates either a normal stop or an emergency shutdown, whichever is necessary. A fault code is recorded by the PMC and shown on the display on the control panel or sent to the owner of the microturbine via SMS or email.

The gas turbine and the electric power generation system are operated and controlled automatically by the PMC, which inputs values (internal parameters) from several sensors:

• Gas pressure

• Oil temperature

• Vibrations

• Speed.

The user defines the value of the following external parameters (demand side input):

• Heat demand

• Electric power demand.

Integration and grid connection

In a typical microturbine application it is likely that the grid topology will not be known, so that controlling the system stability can be a challenging task. In cases where several distributed power generation units have to be co-ordinated, the risk of grid instability may be very pronounced. Since there is more than one controller inside each microturbine, even internal oscillations may appear rapidly, as tests on the pre-prototype CHP unit at ABB Corporate Research showed (Figure 6).

To avoid this kind of instability, different simulation models have been established, which allowed a consolidated analysis. As a result, changes in the controls and filter do guarantee a stable operation of the microturbine system. The simulation models were derived from previous work by ABB on the hybrid bus, from the topology of the CHP system and from measurements. Some of the models are time-domain based and are used to understand and study the entire system as well as the different interfaces between the sub-systems. It goes without saying that the MT100 must be able to handle all critical situations without damage being caused to itself or to equipment outside the microturbine system.

Other simulation models used are frequency-domain-based, allowing a more straightforward analysis and identifying possible locations of instabilities in the system, mainly towards the grid. This work is based on experience with railway systems, where a shutdown due to such instabilities occurred some years ago. Such models are defined by generating a frequency spectrum of every component involved in the system, eg the inverter or a special type of grid topology. By combining these spectra and analysing them, instabilities in the system can be rapidly located.This analysis results in the components – mainly the controllers – being designed so that stable interoperability of the entire system and its neighbours is ensured.

Maintenance concept

A key property of the ABB MT100 CHP unit is that it requires very little maintenance. Preventive, ie scheduled, maintenance is divided into inspections and overhauls. Owing to the small number of moving parts, maintenance can be limited to the work shown in the following lists. An inspection takes place after 6000 hours of operation, causes an outage of at most 24 hours and includes:

• General checks

• Inspection of combustion chamber and replacement of fuel nozzle

• Replacement of consumables, oil and water refill

• Fuel gas compressor inspection, oil refill

• New oil and air filters

• Cleaning of cooling water strainer.

An overhaul takes place after 30 000 hours of operation, causes an outage of 48 hours maximum and includes the same procedure as on an inspection, plus:

• Replacement of the entire combustion chamber.

• Engine refurbishment

• New bearings for the lubrication oil pump, ventilation fan and buffer air pump.

Clean energy

The emissions of the microturbine are very low with e.g. NOx less than 15 ppmv and a target value of 5 to 9 ppmv. The emissions for gas and diesel engines are higher than 300 ppmv and therewith factors higher than for the microturbine. On the efficiency side, the total efficiency of the microturbine is approximately 80 per cent. Typical applications for the CHP unit are small industries, hospitals, theme parks, hotels, shopping centres, multi-storey offices, sheltered housing, convention centres, etc. A very specialised application would be in greenhouses, where even the exhausted CO2 can be used.

The first installation of an MT100 microturbine is in the central heating plant room of ABB in Baden-Dättwil.

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