**2. The recovery of exhaust heat**

The growing popularity of gas turbines in recent years is attributable to the rapid changes in this technology, which have led to improvements in the design of both the individual components and the system as a whole. These technological advances, that concern important developments in materials, construction techniques, blades cooling, control of pollutant emissions, reliability and availability of machines, have enhanced the performance of simple cycle gas turbines, in terms of electrical efficiency and unit size. They have also led

The Recovery of Exhaust Heat from Gas Turbines 167

Recent years have seen a significant increase in the use of combined cycle power plants that, despite the higher fuel cost compared to conventional steam power plants, are currently the

These plants had already become quite popular in the Italian cogeneration sector. Since the mid 1990s, as shown in Figure 2, installed capacity has grown considerably, bringing the

1996 1998 2000 2002 2004 2006 2008

Fig. 2. Combined cycle plants for electric power production and cogeneration in Italy [8]

But since 2000 these plants have found application in the generation of electric power alone, with the installation of 5 sections, increasing to 53 in 2008, with a total installed capacity of around 22000 MW. These systems are characterized by very high efficiency, on average more than 54% [8]. This trend is likely to continue in the future, as shown by the planning permissions granted from 2002 to April 2010, for the construction of new power plants in Italy (Table 1). Indeed, nearly all the plants have efficiencies of above 50%, testifying to the

1996 1998 2000 2002 2004 2006 2008

CC for CHP applications CC for electricity generation



best choice in terms of cost per unit of electricity [7].

number of sections from 57 (1996) to 131 (2008).

0

0

5

10

15

Gross installed power [GW]

20

25

20

40

60

Electricity production [TWh]

80

100

advantages of adopting combined cycle plants.

to a significant increase in temperature and flow rates at the gas turbine exit, thus the need for efficient exhaust heat recovery systems.

The waste heat exhausted from gas turbines can be recovered externally or internally to the cycle itself. External heat recovery can be achieved using a bottoming steam power plant (combined cycle). Internal heat recovery involves reusing the thermal energy exiting the turbine, by means of conventional (thermodynamic regeneration and steam injection) or unconventional techniques (humid air regeneration, steam fuel reforming) [5].

In this paragraph the different recovery techniques will be discussed, highlighting for each of them the main thermodynamic and economic features.

### **2.1 Combined gas-steam cycle power plants**

A combined cycle gas turbine (CCGT) is a fossil fuel power plant that combines the Brayton cycle of the gas turbine with the Rankine cycle of the steam turbine. In a typical layout, shown in Figure 1, exhaust heat from the gas turbine, passing through a heat recovery steam generator (HRSG), produces steam that evolves in the bottoming steam cycle. This type of recovery is said to be "direct", because the heat is transferred directly to the working fluid of another system. In order to improve heat recovery in the HRSG, more than one pressure level is generally required. Combined-cycle configurations, with a triple pressure heat recovery steam generator and steam reheat, attain thermal efficiency of more than 55%.

Fig. 1. Schematic diagram of CCGT cycle

In addition to high efficiencies, combined cycle plants have many other advantages, including [6]:


to a significant increase in temperature and flow rates at the gas turbine exit, thus the need

The waste heat exhausted from gas turbines can be recovered externally or internally to the cycle itself. External heat recovery can be achieved using a bottoming steam power plant (combined cycle). Internal heat recovery involves reusing the thermal energy exiting the turbine, by means of conventional (thermodynamic regeneration and steam injection) or

In this paragraph the different recovery techniques will be discussed, highlighting for each

A combined cycle gas turbine (CCGT) is a fossil fuel power plant that combines the Brayton cycle of the gas turbine with the Rankine cycle of the steam turbine. In a typical layout, shown in Figure 1, exhaust heat from the gas turbine, passing through a heat recovery steam generator (HRSG), produces steam that evolves in the bottoming steam cycle. This type of recovery is said to be "direct", because the heat is transferred directly to the working fluid of another system. In order to improve heat recovery in the HRSG, more than one pressure level is generally required. Combined-cycle configurations, with a triple pressure heat recovery steam generator and steam reheat, attain thermal efficiency of more than 55%.

In addition to high efficiencies, combined cycle plants have many other advantages,


unconventional techniques (humid air regeneration, steam fuel reforming) [5].

for efficient exhaust heat recovery systems.

of them the main thermodynamic and economic features.

**2.1 Combined gas-steam cycle power plants** 

Fig. 1. Schematic diagram of CCGT cycle

volatile hydrocarbons, CO and NOX than oil and coal; - low capital costs and short construction times (often 2–3 years);

including [6]:


Recent years have seen a significant increase in the use of combined cycle power plants that, despite the higher fuel cost compared to conventional steam power plants, are currently the best choice in terms of cost per unit of electricity [7].

These plants had already become quite popular in the Italian cogeneration sector. Since the mid 1990s, as shown in Figure 2, installed capacity has grown considerably, bringing the number of sections from 57 (1996) to 131 (2008).

Fig. 2. Combined cycle plants for electric power production and cogeneration in Italy [8]

But since 2000 these plants have found application in the generation of electric power alone, with the installation of 5 sections, increasing to 53 in 2008, with a total installed capacity of around 22000 MW. These systems are characterized by very high efficiency, on average more than 54% [8]. This trend is likely to continue in the future, as shown by the planning permissions granted from 2002 to April 2010, for the construction of new power plants in Italy (Table 1). Indeed, nearly all the plants have efficiencies of above 50%, testifying to the advantages of adopting combined cycle plants.

The Recovery of Exhaust Heat from Gas Turbines 169

Compared to simple cycle gas turbines, the higher costs of constructing a combined cycle plant are not always offset by higher efficiency, especially for small and medium size plants. The need to combine the high efficiency of combined cycles with the low cost of simple cycles has raised the interest in new technologies that enable internal waste heat recovery

Internal heat can be recovered through the working fluid (fuel, air) or an auxiliary fluid (usually water). In the first case internal heat recovery is defined as "direct", in the second as

Thermodynamic regeneration is a direct internal recovery technique, since thermal energy is transferred directly from exhaust gas to air at the compressor exit. This produces an efficiency gain due to the reduction in primary thermal energy requirements without

Steam injection on the other hand is an indirect internal recovery technique. In this case the recovered thermal energy is transferred to an auxiliary fluid (water), which is then injected into the combustor. This increases the primary thermal energy required to keep the temperature at the turbine inlet constant, but results in a power increase and, consequently,

Direct and indirect recovery can also be combined, as for instance in HAT and CRGT cycles. In humid air plants (HAT) , the saturation of air at compressor exit extends the regeneration margins, thanks to the greater temperature difference between the exhaust gas at turbine exit and the compressed air at regenerator inlet. In chemically recuperated plants (CRGT), exhaust heat is recovered through an endothermic steam-reforming process of the primary fuel. More specifically, a portion of the recovered heat is transferred directly to the fuel,

In thermodynamic regeneration, the exhaust heat at the turbine exit is used to preheat the air entering the combustion chamber. The heat exchange between the two gas streams is achieved by means of a countercurrent heat exchanger, known as a regenerator or

The thermal efficiency of the Brayton cycle is enhanced since regeneration decreases the heat input required to produce the same net work output. Heat recovery through a gas-to-gas heat exchanger is limited by a characteristic value of the compression ratio, beyond which the temperature of the exhaust gas falls below that of the air at the compressor outlet,

The efficiency gain achieved through regeneration strongly depends on the heat exchanger effectiveness, defined as the ratio of the actual heat transfer rate to the air and the maximum possible heat transfer rate, that would exist were the heat exchanger to have infinite heat transfer surface area. More specifically, gas turbine efficiency increases with heat exchanger effectiveness, as the air at the combustion chamber inlet is preheated at higher temperatures,

recuperator. Figure 3 shows a schematic diagram of the regenerative cycle.

**2.2 Gas turbine configurations with internal heat recovery** 

changing, as a first approximation, the mechanical power output.

while the remainder is used to produce the required steam.

**2.2.1 Heat recovery without auxiliary fluid** 

thereby deteriorating efficiency.

resulting in greater fuel savings.

from the gas turbine.

"indirect"[5].

an efficiency gain.


Table 1. Planning permissions for power plants in Italy, granted from 2002 to April 2010 [9]

**COMPANY PLANT LOCATION MWe MWt MWe/MWt**  EDISON MARGHERA environmental remediation

 *Total 4340 7610 57.0%*  ENIPOWER FERRERA ERBOGNONE 1040 1850 56.2%

 *Total 4575 8190 55.9%*  ENEL PRODUZIONE CASTEL SAN GIOVANNI 80 120 66.7%

PIACENZA 58 MWe for summer peak

 ABRUZZOENERGIA GISSI 760 1400 54.3% ACEAELECTRABEL PRODUZIONE LEINI' 380 700 54.3% AEM MI -ASM BS CASSANO D'ADDA 390 700 55.7% AEM TORINO MONCALIERI 770 1350 57.0% ASM BS e AMGS VR PONTI SUL MINCIO 250 450 55.6% CALENIA ENERGIA SPARANISE 800 1400 57.1% E.ON ITALIA PRODUZIONE LIVORNO FERRARIS 800 1400 57.1% ELECTRABEL ITALIA ROSIGNANO SOLVAY 400 750 53.3% EN PLUS SAN SEVERO 390 700 55.7%

 ENERGIA MODUGNO MODUGNO 750 1350 55.6% ENERGIA MOLISE TERMOLI 750 1300 57.7% ENERGY PLUS SALERNO 780 1370 56.9% EUROSVILUPPO ELETTRICA SCANDALE 800 1390 57.6% IRIDE ENERGIA TORINO NORD 400 710 56.3% ITALGEN VILLA DI SERIO 190 365 52.1%

PORTOGRUARO PORTOGRUARO planning permission expired RIZZICONI ENERGIA RIZZICONI 800 1400 57.1% SARMATO ENERGIA SARMATO 47 70 67.1% SET TEVEROLA 400 750 53.3% SORGENIA APRILIA 750 1350 55.6% TERMICA CELANO CELANO 70 100 70.0% TIRRENO POWER VADO LIGURE conversion to combined cycle TIRRENO POWER NAPOLI LEVANTE 400 700 57.1% VOGHERA ENERGIA VOGHERA 400 750 53.3% *Total 21800 38695 56.3%* 

ENDESA ITALIA TAVAZZANO conversion to combined cycle

 CIVITAVECCHIA fuel change EDIPOWER TURBIGO repowering

ENERGIA BERTONICO/TURANO

MIRANT GENERATION

TORVISCOSA 800 1500 53.3% ORTA DI ATELLA 780 1340 58.2% ALTOMONTE 800 1400 57.1% SIMERI CRICHI 800 1360 58.8% PIANOPOLI 800 1360 58.8% CANDELA 360 650 55.4%

MANTOVA 780 1370 56.9% RAVENNA 785 1370 57.3% BRINDISI 1170 2200 53.2% FERRARA 800 1400 57.1%

LIVORNO environmental adaptation CAVRIGLIA/SANTA BARBARA 390 700 55.7%

FIUME SANTO 80 220 36.4%

LODIGIANO 800 1400 57.1%

load

Table 1. Planning permissions for power plants in Italy, granted from 2002 to April 2010 [9]
