Small gas turbine technology

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Abstract

Small Gas Turbine Technology: Small gas turbine, in the power range up to 500 kW, requires a recuperated thermodynamic cycle to achieve an electrical efficiency of about 30%. This efficiency is the optimum, which is possible for a cycle pressure ratio of about 4–1. The cycle airflow is function of the power requirement. To increase the efficiency, in view to reduce the CO2 emission, it is mandatory to develop a more efficient thermodynamic cycle.

Different thermodynamic cycles were examined and the final choice was made for an Intercooled, Recuperated cycle.

The advantage of this cycle, for the same final electrical efficiency of about 35%, is the smaller cycle airflow, which is the most dimensional parameter for the important components as the heat exchanger recuperator and the combustion chamber.

In parallel with the thermodynamic cycle it is necessary to develop the High Speed Alternator technology, integrated on the same shaft that the gas turbine rotating components, to achieve the constant efficiency at part loads, from 50% up to 100%, by the capacity to adjust the engine speed at the required load.

To satisfy the stringent requirement in pollutant emissions of NOx and CO, the catalytic combustion system is the most efficient and this advance technology has to be proven.

The major constraints for the small gas turbine technology development are the production cost and the maintenance cost of the unit. In the power range of 0–500 kW the gas turbine technology is in competition with small reciprocating engines, which are produced in large quantity for automotive industry, at a very low production cost.

Introduction

Small gas turbines, in the power range from 0 to 500 kW, have an electrical efficiency of about 18%.

This fact is mainly due to the scale effect on the aerodynamic components, the assembly clearances and the relatively low turbine inlet temperature compared with large gas turbine.

To increase this efficiency, up to a value, which can permit to the gas turbine to compete with the reciprocating engine and decrease the CO2 emission, the simplest way is to add a recuperator or regenerator on the gas exhaust.

A small gas turbine unit fitted with a recuperator, with an efficiency of 90%, can achieve an electrical efficiency of about 30%, slightly lower than reciprocating engines, which offer in this range of power an efficiency of about 35%.

To pass a new step in efficiency, two different ways are opened:

  • Dramatically increase the Turbine Inlet Temperature (TIT), solution, which requires on small gas turbine the development on ceramic components in hot parts.

  • Develop new thermodynamic cycles, which are well adapted to a small co-generation gas turbine system.


For the first way, the technology of the ceramic turbine wheel is unfortunately not yet available. As the demand for a more efficient unit is still existing, this presentation describes the choice made of a more efficient cycle and the feasibility of the major components of this cycle to satisfy the stringent requirements in terms of pollutant emissions, production and maintenance costs, and, reliability.

This R and D study was supported by the European Commission as an EC funded contract: ENK5 CT2000 00070.

Acronym: CHEP.

Title: «Research and Development of high efficiency components for an intercooled, recuperated CHP gas turbine for Combined Heat and Efficient Power».

Project co-ordinator: Microturbo S.A.
Partners:

Section snippets

Thermodynamic cycle choice

A parametric study was conducted to compare the simple cycle with the recuperative cycle, the others possible cycles known as recuperative-intercooled cycle recuperative-under-pressurised cycle and recuperative inverse cycle. The aerodynamic pathflow of each cycle is given in Fig. 1, Fig. 2, Fig. 3, Fig. 4. To compare the efficiency of different thermodynamic cycles, the max continuous Turbine Inlet Temperature (TIT) was selected at the conservative level of 950 °C (1223 K).

The efficiency of

Thermodynamic rotating components

The thermodynamic rotating components of the 350 kW recuperative intercooled gas turbine are:

  • Compressor 1

  • Compressor 2, after the intercooling heat exchanger

  • Turbine wheel, radial or two stages of axial turbines


The main dimensional characteristics of each component are given in Table 3.

The parametric study of the influence of the pressure ratio repartition between the compressor 1 and 2, for a constant cycle pressure ratio of 6/1, shows that this parameter is not critical.

The cycle efficiency is

High speed generation

The High Speed Generator (HSG) with associated starting and power converters is the classical solution for Microturbine units. Mostly of the existing systems are in the power range from 30 to 100 kW. For the 350 kW Recuperated-Intercooled unit it is necessary to design and prove the high power HSG.

The design of the HSG was made for the turbine max nominal speed of 42 000 rpm (model G185).

The main characteristics are given below:

Recuperator

The thermal exchange efficiency of the recuperator on the cycle performance is of the first importance.

As shown in Fig. 12, a loose of 5% in recuperator efficiency conducts to a drop of 2% on the cycle performance and, by the way, on fuel consumption.

Recuperator efficiency is one of the major criteria to reduce the cycle fuel consumption, but, the size and the cost. Of this component is directly linked to its efficiency and increase of about 40% for an efficiency change from 85% to 90%.

A state

Catalytic combustion chamber

The catalytic combustion configuration is based on a two steps combustion system:

  • Step one is a catalytic partial oxidation (CPO) in rich condition, it can be considered as a fuel processing system, without NOx emission.

  • Step two is a homogenous combustion operating in very weak conditions. Due to high dilution, combustion, with H2 high percentage, is done at low temperature, thus reducing NOx emissions.


This stepwise combustion is expected to produce very low NOx emission because its both steps

Turbine mechanical arrangement

A preliminary mechanical arrangement of the recuperative intercooled turbine unit is given on the following lay out.

Note that the first compressor and the High Speed Alternator are on the same shaft.

The second stage compressor with the radial turbine are on a second shaft.

The air and gas inlets into the spiral recuperator need to be optimised by a Fluent simulation to achieve the lowest pressure drops.

Conclusion

This study demonstrates the feasibility of a competitive recuperative-intercooled co-generation unit of a nominal power of 350 kW, with the advance technical solutions to achieve the following targets:

  • lowest CO2 emission by higher cycle efficiency;

  • reduction of NOx and CO emissions by Catalytic Partial Oxidation combustion system;

  • low cost, high efficiency spiral, laser welded heat exchanger recuperator;

  • High Speed Generator integrated on the turbine shaft to minimise the number of components.

Acknowledgements

We thank the European Commission for the financial support given to the R and D project subject of this paper.

We also acknowledge our partners for their kind co-operation to achieve this project.

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