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This paper aims at carrying out comparative performance analysis of simple and advanced cycles large-scale aero-derivative industrial gas turbines derived from aircraft turbofan engines. The investigation involves technical performances of three large-scale aero-derivative engine cycles based on existing and projected cycles for applications in land based power generation and Combined-Heat-and-Power (CHP). Preliminary design and performance simulation were implemented of a simple cycle (baseline) three-spool 100 MW aero-derivative engine model, intercooled and intercooled/recuperated engine cycles of the same 100 MW nominal power rating. In the analysis, design point and off-design performances of the engine models were established. The results indicate that to a large extent, the advanced engine cycles showed superior performance in terms of thermal efficiency, and fuel flow. In numerical terms, thermal efficiencies of intercooled engine cycle, and intercooled/recuperated engine cycles, over the simple cycle at design point increased by 2.42% and 0.94% respectively, whereas heat rates of these cycles over simple cycle at design point decreased by 2.37% and 0.93% respectively. It is worthy of note that for large-scale aero-derivative gas turbines having power rating of 100 MW and above, intercooled cycle would consume less fuel than intercooled-recuperated and simple cycles. This finding would actually aid good choice of cycle option for large-scale aero-derivative gas turbine designers, manufacturers and users.

Aero-derivative industrial gas turbines (ADIGT) could be grouped into three categories namely: small-scale, medium-scale, and large-scale aero-derivatives, based on different ranges of power ratings. Small-scale ADIGT category can be defined as having the range of power rating from about 0.6 MW up to 5 MW. The range of power rating greater than 5 MW but up to 20 MW could be classified as medium-scale category. Large-scale category would be considered as the class having power rating above 20 MW [

It has been reported that the introduction of modern, high thrust aero-engines for aircraft propulsion has resulted in the development of a commensurate range of new high power, high efficiency ADIGT. This is illustrated for instance by the Rolls-Royce TRENT. The development of these large, efficient ADIGT offers particular chances for use in power generation and combined-heat-and-power plant [

Besides, another instance is the GE LM6000 aero-derivative gas turbine series which has undergone some new innovations in technology especially those in the 35 - 65 MW_{e} range. The GE LM6000 has the latest innovations in the LM6000PG & PH versions. These are denoted as the “PG” for the standard annular combustor (SAC) and “PH” for the dry low emissions (DLE) model. The improved technologies for these new products include new higher temperature alloys and improved cooling pattern to withstand high combustor outlet temperature, LP compressor operating at higher speed and increased mass flow, and higher pressure ratio. The GE LM6000 PG offers a 25% simple cycle power increase compared to the GE LM 2500, its predecessor, owing to advanced technology [

More so, aero-derivative gas turbines can meet stringent NOx control requirements because they are suitable for power augmentation by steam injection. For instance, the GE LM series industrial aero-derivative gas turbines are meeting NOx requirements as low as 25 parts per million (ppm) using steam injection. Other merits of aero-derivative gas turbines include low weight-to-power ratio, compactness, and hence, lesser erection and startup time [

Using heat exchangers (both recuperators and intercoolers) in an engine exhibits tremendous potential to cut fuel consumption and thereby reducing CO_{2} emissions. It was explained that the recuperator utilizes part of heat from the exhaust gas to raise the temperature of the air entering the combustor [

Technically, improvement of thermal efficiency for industrial and aero gas turbines is of paramount importance to the overall performance of the engines. Increase in thermal efficiency depends on certain factors including: Changes in some engine cycle parameters, such as overall pressure ratio (OPR), and turbine entry temperature (TET). Cutting-edge technology of engine components like methods of cooling, efficiencies of components, ducts pressure losses, and introduction of different overall thermodynamic cycle, for example, use of unconventional components like intercoolers and regenerators or recuperators [

The aim of this paper is to compare the technical performances of large-scale aero-derivative industrial gas turbines. The investigation encompasses comparative assessment of simple (SC), intercooled (IC), and intercooled/recuperated (ICR) cycle options. This sort of analysis would surely aid good choice of engine cycle options in the category of large-scale aero-derivative land based gas turbines. GE Power has developed, manufactured and deployed 100 MW large-scale intercooled aero-derivative gas turbine in the LMS100 series [

The Design Point of a gas turbine could be defined as the very condition in the operating range of a gas turbine when the engine is running at the very mass flow, speed, and pressure ratio for which the components were designed [

Besides the DP performance of the gas turbine, it is mandatory to ascertain its general performance over the entire operating range of power output and speed. This is known as Off-Design (OD) performance [

Various performance plots of power output, specific fuel consumption (sfc), thrust, specific thrust or power, etc could be made once the operating conditions of an engine have been determined. It is important to note that off-design performance is very much affected by factors such as ambient conditions of temperature and pressure, altitudes, flight speed (for aero engines), etc. The off-design performance analysis is normally achieved by the use of computer model simulations of engines [

Engine components operating point matching to establish OD performance is normally a tedious and time consuming task since it is an iterative process. Computer based simulation is normally employed to accomplish the task. TURBOMATCH is an in-house gas turbine engine performance software developed and established at Cranfield University (CU). It is employed to simulate the DP and OD performances of a broad range of aero and industrial gas turbines. Simple single shaft engines, complex multi-spool engines, as well as novel cycle engine configurations can be modelled adequately using the scheme [

In the scheme, different engine components (intake, compressor, combustor, turbine, nozzle, etc) are repre- sented by bricks (building blocks of the programme). These bricks are pre-programmed routines deployed to simulate, on a modular basis, the performance of the various engine components they represent. The cycle thermal efficiency, specific fuel consumption, power, or thrust of the engine, etc. are essential performance output parameters that are obtained as desired results of the simulation. Besides these overall cycle results, individual component performance characteristics, and the working-fluid properties at various stations within the engine are also outputted [

In this paper, three-spool turboshaft engine with a free power turbine (FPT) was considered in which a high pressure compressor (HPC) is driven by the high pressure turbine (HPT) and a low pressure compressor (LPC) is driven by an intermediary low pressure turbine (IPT). The schematic representation of such engine is shown in

The T-S diagram of the simple cycle is shown in

where c_{pi} is the specific heat capacity at constant pressure, which varies with temperature at the given engine component station i.i represents engine station numbers 1, 2, 3, 4, 5, 6 etc.

Heat rejected at constant pressure (process 8 - 2) in the exhaust is given by Equation (2).

Equation (3) or Equation (4) gives the total compressor work (CW) (process 2-3-4) per unit air mass flow, where process 2 - 3 occur in the LPC and process 3 - 4 occur in the HPC.

High pressure turbine work (HPTW) (process 5 - 6) per unit air mass flow is defined by Equation (5).

Intermediary pressure turbine work (IPTW) (process 6 - 7) given by Equation (6)

Free power turbine work (FPTW) (process 7 - 8) given by Equation (7)

This implies that total expansion work (EW) is obtained as stated in Equation (8)

The thermal efficiency is calculated using Equation (9) below.

To increase the efficiencies of the simple-cycle, unconventional components are added to the cycle. These components include intercoolers, regenerators (recuperators), or reheaters. However, the initial and maintenance costs of the cycle may increase due to these additional components. The improvements in cycle performance brought about by these components can only be justified if the decrease in fuel costs offsets the increase in other costs. There is the general urge to reduce fuel consumption in gas turbine operation [

Incorporating an intercooler between the LPC and HPC of the simple cycle engine in section 2.4 such that air leaving the LPC is cooled before entering the HPC, results in an intercooled cycle. Intercooling reduces the total compressor work, thereby, increasing useful work output, turbine work remaining the same [

Incorporating a recuperator between the outlet of the HPC and outlet of the LPT of the intercooled engine in section 2.5.1 such that air leaving the HPC is heated before entering the burner, results in an intercooled/recup- erated cycle. The recuperator or regenerator is a heat exchanger connected between the turbine exhaust and the compressor exit. The thermal efficiency of the cycle increases due to recuperation because the portion of heat in the exhaust gases that is supposedly wasted by flaring is now utilised to preheat the air at the exit to the compressor. This, in effect, reduces the heat gain from burning fuel, and hence, decreases fuel consumption for same power output. However, if the compressor outlet temperature is equal or higher than the turbine exhaust temperature, the use of a regenerator is not recommended. Else, there will be a reversal of heat flow to the exhaust gases, causing the efficiency to decrease. Very high pressure ratios in gas-turbine engines could cause this adverse

situation [

Referring to the cycle in _{9} is the maximum temperature that can occur within the recuperator which is the temperature of the exhaust gases entering the recuperator and leaving the turbine. Air in the regenerator (recuperator) can only be preheated to a temperature below T_{9}, and air will normally exit the regenerator at T_{10}, a lower temperature [

Using the station numbering and notations in the T-S diagram of

The heat input per unit air mass flow here is given by Equation (10).

Here, compressor work is given by Equation (11), and Equation (12) gives the expansion work.

Equation (13) calculates the thermal efficiency in this case with reference to Equation (11) and Equation (12).

TURBOMATCH code is capable of computing engine performance parameters using thermodynamic relations and models, considering variations of properties of working fluid at different conditions of pressure and temperature [

For the engine model, a simple cycle three-spool engine inspired by the aero-derivative GE LMS100 core is chosen as the baseline engine. The LMS100 turbine has a free power turbine (FPT), an intermediary turbine (IPT) that drives the LPC, and an intercooler. With an overall compression ratio of 42:1, it has an annular combustor equipped with dry low emission (DLE) technology, and an air-cooled HPT which drives the HPC. The LMS100 was derived from the LM6000 by the addition of the intercooler and free power turbine delivering an

output power of about 100 MW [

It should be understood that the design point of the inspiring engine core is proprietary information of the original equipment manufacturer (OEM), and as such, the design point is reasonably chosen by engineering judgment. This is because some key defining parameters of the DP like the turbine entry temperature (TET), etc. are not usually disclosed by the OEM [

The engine components were modelled in TURBOMATCH bricks shown in

The simple cycle LS-ADIGT engine described in section 3.1 is modified by introducing an intercooler between the LP and HP compressors. While retaining component efficiencies, TET, and inlet mass flow, the air leaving the LP compressor is cooled to a temperature of 320˚C, with intercooler effectiveness of 30%, and intercooler pressure loss of 3% of LPC delivery pressure. However, a mass bleed totaling 27% of intake air mass flow is channeled for both HPT and IPT inlet blades cooling. The intercooled engine arrangement is shown in

The intercooled/recuperated three-spool large-scale aero-derivative engine components in TURBOMATCH bricks is shown in

Performance parameter | Value at DP of simulated three-spool LS2-ADIGT engines | ||
---|---|---|---|

Simple cycle | Intercooled | ICR | |

Power turbine rating (kW) | 100,000 | 100,000 | 100,000 |

Inlet mass flow (kg/s) | 215.5 | 215.5 | 215.5 |

Exhaust mass flow (kg/s) | 220.59 | 220.47 | 220.54 |

Fuel flow (kg/s) | 5.09 | 4.97 | 5.04 |

Exhaust gas temperature (K) | 783 | 692 | 690 |

Overall compression pressure ratio | 42.15:1 | 42.15:1 | 42.15:1 |

Thermal efficiency | 0.457 | 0.467 | 0.460 |

are retained except for the inclusion of a recuperator of 75% effectiveness, with cold side and hot side pressure losses of 1% and 2% respectively, of the inlet pressure, and 2% mass leakage. The inlet mass flow remains 215.5 kg/s with a total of 27.4% bleed for both HPT and IPT inlet blades cooling. Also, keeping ISA SLS as design point, the engine was simulated and the summary of its DP performance shown in

To verify the results of performance parameters of the simulated baseline engine, comparison is made with public domain source reference data of LMS100. This is shown in

By the use of component maps in TURBOMATCH codes, the off-design performances of the SS-ADIGT engines were simulated and the variation of some key engine output parameters were plotted and presented in

With reference to the simulation results shown in

In

The negative sign on the heat rates in

However, it is important to note that though the thermal efficiency is improved by using the advanced cycles, the incorporation of intercoolers and recuperators would make the engine more complex. This would increase the capital and maintenance cost actually, but cost of fuel would reduce due to reduction in heat rate and fuel consumption.

These results compare favorably with values obtained in the literature. For instance, it was reported that the 1.4 MW intercooled/recuperated Heron-1 turbo-shaft gas turbine manufactured by EECT of the Netherland exhibits thermal efficiency of 42.9% while a simple cycle gas turbine of same power range has thermal efficiency

Performance parameter | Value at ISA SLS | |||
---|---|---|---|---|

Inspiring core LMS100 | Simulated baseline engine | Variation | % variation | |

Power turbine rating | 100,000 kW | 100,000 kW | 0.00 | 0.00 |

Inlet mass flow | N/A | 215.5 kg/s | - | - |

Exhaust mass flow | 220 kg/s | 220.47 kg/s | −0.47 | −0.21 |

Fuel flow | N/A | 4.97 kg/s | - | - |

Exhaust gas temperature | 686 K | 692 K | −6 | −0.87 |

Overall pressure ratio | 42.00:1 | 42.15:1 | −0.15 | −0.36 |

Thermal efficiency | 0.440 | 0.467 | −0.027 | −6.1 |

of about 26% - 34%. This represents a thermal efficiency increase of about 26.2% at the minimum [

Performance simulation of a simple cycle (baseline), IC and ICR three-spool large-scale aero-derivative industrial gas turbine derived from turbofan engine has been implemented. In doing so, design and off-design point performances of the engine models were established. It is found that the IC and ICR cycles exhibit better thermal efficiency than the simple engine. Similarly, heat rate in combustor is reduced in the advanced cycles than the simple engine. It is, however, worthy of note that for large-scale aero-derivative gas turbines having power rating of 100 MW and above, intercooled cycle would consume less fuel than intercooled-recuperated and simple cycles. This finding would actually aid good choice of cycle option for large-scale aero-derivative gas turbine designers, manufacturers and users.

The authors would like to thank Prof. Pericles Pilidis and Dr. Theoklis Nikolaidis of the Department of Power and Propulsion of Cranfield University United Kingdom for their valuable contributions.

Barinyima Nkoi,Thank God Ebi Isaiah, (2016) Advanced Cycles Large-Scale Aero-Derivative Gas Turbines: Performance Comparison. Journal of Power and Energy Engineering,04,7-19. doi: 10.4236/jpee.2016.45002