**5.2 Discussion of repowering performance**

The scheme described above has been numerically studied using a modular simulation code [19], based on fundamental thermodynamic relations. In this simulation code each power plant component (gas and steam turbines, condensers, heat exchangers etc.) is modelled by means of mass and energy balances. A detailed analysis has thus been carried out, simulating the proposed repowering scheme with real data from present day power generating plants [19].

Several gas turbine models, both heavy-duty and aeroderivative units, were tested to assess the feasibility of repowering, the power augmentation achievable and its influence on energy conversion efficiency. The results of the analysis have shown that a power increase of up to 50%, with respect to the existing combined cycle plant, can be achieved. Moreover the additional electricity obtained from repowering is generated at high efficiency (49-52%), though the added section is not so efficient, while the cost of electricity is comparable with that of existing combined cycles. The analysis also showed that the proposed repowering scheme offers a variety of power control strategies and, hence, the possibility of achieving good part load behaviour, especially with the addition of a number of small aeroderivative gas turbines.

A further analysis has been carried out to evaluate the influence of the main added turbine features (*β* and *TTI*), added HRSG operation (steam degree of superheat) and steam-to-air mass ratio (*μ)* on repowering performance.

For this purpose a repowering unit (GT and HRSG) has been added to a given baseline natural gas combined cycle plant, designated GE S109FA. It consists of a single General Electric gas turbine type PG9351FA, with design performance summarized in Table 2. Added gas turbine data have been taken to represent General Electric F-series gas turbines, while added HRSG always produces the maximum amount of steam, 10°C being the minimum temperature difference at pinch point and 90°C the minimum gas temperature within the stack.

Repowering performance has been evaluated in terms of incremental variables, i.e. marginal power output *P* and marginal efficiency *η*. The first is the power of the added gas turbine plus the power increase in the existing combined cycle due to steam injection (including both GT and ST contributions), the second is the ratio between the marginal power output and the marginal primary fuel power related to added GT and to steam injection into existing CC.

The major drawback of this configuration is water consumption related to the flow entering the new HRSG, which is inevitably lost at the stack. This can limit the applicability of the present scheme to sites with large fresh water availability, though the specific water

Moreover, if a low temperature thermal load is available near the power plant, water can be recovered through steam condensation. The implementation of a water recovery technique has to be carefully evaluated, because of the very large size and the very low temperature

The scheme described above has been numerically studied using a modular simulation code [19], based on fundamental thermodynamic relations. In this simulation code each power plant component (gas and steam turbines, condensers, heat exchangers etc.) is modelled by means of mass and energy balances. A detailed analysis has thus been carried out, simulating the proposed repowering scheme with real data from present day power

Several gas turbine models, both heavy-duty and aeroderivative units, were tested to assess the feasibility of repowering, the power augmentation achievable and its influence on energy conversion efficiency. The results of the analysis have shown that a power increase of up to 50%, with respect to the existing combined cycle plant, can be achieved. Moreover the additional electricity obtained from repowering is generated at high efficiency (49-52%), though the added section is not so efficient, while the cost of electricity is comparable with that of existing combined cycles. The analysis also showed that the proposed repowering scheme offers a variety of power control strategies and, hence, the possibility of achieving good part load behaviour, especially with the addition of a number of small aeroderivative

A further analysis has been carried out to evaluate the influence of the main added turbine features (*β* and *TTI*), added HRSG operation (steam degree of superheat) and steam-to-air

For this purpose a repowering unit (GT and HRSG) has been added to a given baseline natural gas combined cycle plant, designated GE S109FA. It consists of a single General Electric gas turbine type PG9351FA, with design performance summarized in Table 2. Added gas turbine data have been taken to represent General Electric F-series gas turbines, while added HRSG always produces the maximum amount of steam, 10°C being the minimum temperature difference at pinch point and 90°C the minimum gas temperature

Repowering performance has been evaluated in terms of incremental variables, i.e. marginal power output *P* and marginal efficiency *η*. The first is the power of the added gas turbine plus the power increase in the existing combined cycle due to steam injection (including both GT and ST contributions), the second is the ratio between the marginal power output and the marginal primary fuel power related to added GT and to steam injection into

requirements are fairly low, as shown in [19].

**5.2 Discussion of repowering performance** 

mass ratio (*μ)* on repowering performance.

level of such a heat sink.

generating plants [19].

gas turbines.

within the stack.

existing CC.


Table 2. Design performance of baseline combined cycle

Varying *β* (from 10 to 30), *TTI* (from 1200 to 1600°C) and degree of superheat (saturated and superheated steam), the repowering unit has always been rated such that the amount of steam generated for injection into the existing combined cycle matches the required *μ* value. To avoid compressor and turbine matching problems, considering that General Electric has offered injection for power augmentation for 40 years on all of its production machines [18], a steam-to-air mass ratio *μ*=5% is assumed, corresponding to 30.6 kg/s of steam injected into the GE S109FA combustor.

As shown in Figure 13, injection of superheated steam produces higher values of marginal power and efficiency, for any *β* and *TTI*.

Marginal power output increases with *β*, while it is little influenced by *TTI*; for a pressure ratio of 30, superheated steam injection produces a power increase of 165 MW, of which about 60% (102 MW) produced by the added gas turbine. On the contrary, marginal efficiency is strongly influenced by *TTI*, especially at high pressure ratios*.* In this regard, for the added gas turbine operating at *β*=30, marginal efficiency attains 53.1% at *TTI*=1200°C and 57.4% at *TTI*=1600°C.

More interestingly, the proposed repowering scheme offers the possibility of maintaining high efficiency over a wide range of marginal power outputs. In fact, marginal efficiency is strongly influenced by existing CC and added GT characteristics, but only slightly by the steam-to-air mass ratio *μ*. As shown in Figure 14, by varying *μ* from 3% to 9%, repowering can generate a marginal power output of up to 100 MW and 300 MW, respectively.

Therefore, though the steam mass flow rate for injection is limited by compressor and turbine matching problems or water availability and treatment requirements, the proposed repowering scheme could be beneficially implemented, as it is still characterized by high marginal efficiency and significant marginal power.

The Recovery of Exhaust Heat from Gas Turbines 187

smaller size are considered. For the latter, the effects of repowering are also assessed for

The results obtained show that marginal efficiency is kept above 50%, even with small gas turbine units (around 10MW), while the power increase achieved (marginal power) is directly dependent on the flow rate of injected steam, as shown in Fig.15. Additional gas turbine accounts for about 60% of power increase – 56% for heavy duty GT and 64% for

> 2 x GE LM6000

Added GT power, MW 43.4 86.8 130.2 70.0 85.9 Added GT efficiency, % 41.3 41.3 41.3 36.8 36.5 Injected steam, t/h 45.9 91.7 137.6 99.3 119.8 Steam-to-air mass ratio, % 2.1 4.2 6.3 4.5 5.5 Marginal power, MW 68.2 137.0 205.9 124.0 151.0 Marginal efficiency, % 51.2 51.4 51.4 50.7 50.4

> Siemens V64.3A

> > 2 x GE LM6000

02468 m [%]

Siemens V64.3A

02468 m [%]

3 x GE LM6000

2 x GE LM6000

> Westingh. 401

Siemens V64.3A

3 x GE LM6000

3 x GE LM6000

Westingh. 401

Westing. 401

Table 3. Marginal efficiency and marginal power of repowered gas cycles

GE LM6000

GE LM6000

LM6000

Fig. 15. Marginal efficiency and marginal power as a function of steam-to-air mass ratio

Different techniques for the internal or external recovery of exhaust heat from gas turbines have been investigated. Internal heat recovery techniques can be conventional (regeneration, steam injection) or unconventional (humid air regeneration, steam reforming), while external heat recovery can be performed using a steam bottoming cycle (combined cycle).

integration with two or three gas turbine units.

GT Number and model GE

50.0

50.4

50.8

h [%]

P [MW]

51.2

51.6

aeroderivative GT.

**6. Conclusions** 

Fig. 13. Performance of CC repowering scheme in terms of marginal efficiency and power

Fig. 14. Influence of steam-to-air mass ratio on performance of CC repowering scheme

Referring again to the combined cycle GE S109FA, Table 3 summarizes the improved performance that can be achieved adopting the repowering scheme for specific commercial gas turbines. In particular, two heavy-duty gas turbines (Siemens and Westinghouse V64.3A 401), with mechanical power in the 70-85 MW range, and aeroderivative (GE LM6000) of

8 12 16 20 24 28 32 b

> saturated steam injection superheated steam injection

> > =1200°C =1600°C TTI TTI

Fig. 13. Performance of CC repowering scheme in terms of marginal efficiency and power

b =30

30

8 12 16 20 24 28 32 b

T =1200°C TI T =1400°C TI T =1600°C TI

m=3% m=6% m=9%

30

<sup>30</sup> <sup>30</sup>

30

30 30 30

48% 50% 52% 54% 56% 58%

51% 52% 53% 54% 55% 56% 57% 58%

h

h

P [MW]

Fig. 14. Influence of steam-to-air mass ratio on performance of CC repowering scheme

<sup>10</sup> <sup>10</sup> <sup>10</sup>

Referring again to the combined cycle GE S109FA, Table 3 summarizes the improved performance that can be achieved adopting the repowering scheme for specific commercial gas turbines. In particular, two heavy-duty gas turbines (Siemens and Westinghouse V64.3A 401), with mechanical power in the 70-85 MW range, and aeroderivative (GE LM6000) of

50 100 150 200 250 300 P [MW]

smaller size are considered. For the latter, the effects of repowering are also assessed for integration with two or three gas turbine units.

The results obtained show that marginal efficiency is kept above 50%, even with small gas turbine units (around 10MW), while the power increase achieved (marginal power) is directly dependent on the flow rate of injected steam, as shown in Fig.15. Additional gas turbine accounts for about 60% of power increase – 56% for heavy duty GT and 64% for aeroderivative GT.


Table 3. Marginal efficiency and marginal power of repowered gas cycles

Fig. 15. Marginal efficiency and marginal power as a function of steam-to-air mass ratio

### **6. Conclusions**

Different techniques for the internal or external recovery of exhaust heat from gas turbines have been investigated. Internal heat recovery techniques can be conventional (regeneration, steam injection) or unconventional (humid air regeneration, steam reforming), while external heat recovery can be performed using a steam bottoming cycle (combined cycle).

The Recovery of Exhaust Heat from Gas Turbines 189

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In order to compare the capabilities of the different solutions, a characteristic plane of exhaust heat recovery, based on a unified analysis approach, has been introduced. The performance plane of exhaust heat recovery is an effective tool for comparing various design solutions that are conceptually different and not directly comparable. On this plane each recovery technique is identified by a region, whose position and extent depends on typical parameters and characteristics of the baseline gas turbine, as well as on limitations related to minimum stack gas temperature and maximum mass flow rate increase in the turbine.

The characteristic plane indicates directly the performance obtainable with various heat recovery techniques. The analysis carried out has shown that performances close to combined cycle plants can only be achieved with combined recovery techniques (humid air regeneration or steam reforming of fuel), where the efficiency penalty is small at high maximum gas temperatures and low compression ratios.

Conventional combined recovery techniques (regeneration and steam injection) can compete with combined cycle plants at low turbine inlet temperature (1200°C), as they offer greater design simplicity, in spite of an efficiency penalty of a few percentage points.

Lastly, from the unified thermodynamic approach an innovative repowering scheme has been proposed. This allows to repower existing combined gas-steam power plants through the addition of a gas turbine and a one-pressure level HRSG, that feeds the output steam to the combustor of an existing gas turbine. This scheme significantly increases power output (50%) with fairly high marginal efficiency, in spite of the relative simplicity of the added components.
