A.A. 2011-2012

Sustainable Energy

Mod.1: Fuel Cells & Distributed

Generation Systems

Thermochemical Power Group (TPG) - DiMSET – University of Genoa, Italy

Dr. Ing. Mario L. Ferrari

A.A. 2011-2012

Lesson XVII

Lesson XVII: Gas Turbines

A.A. 2011-2012

Simple cycle (1/4)

Lesson XVII

A.A. 2011-2012

Simple cycle (2/4)

Lesson XVII

�Ideal Brayton cycle: efficiency

�Ideal Brayton cycle: maximum specific work

It is possible to demonstrate (first order derivative = 0) that the expression

has a maximum for:

β

A.A. 2011-2012

Simple cycle (3/4)

Lesson XVII

�Real Brayton cycle: efficiency (1/2)

l: limit (real fluid)

i: internal (real cycle)

r: real

A.A. 2011-2012

Simple cycle (4/4)

Lesson XVII

�Real Brayton cycle: efficiency (2/2)

β

η

•In the case of real cycle efficiency curve

shows a maximum value

•The compression ratio for maximum

efficiency is higher than the compression

ratio for maximum specific work

•The designer has to decide if optimising

the turbine for maximum efficiency or

maximum specific work

•Usually aeroengines (and aeroderivative

turbines) are optimized for ηmax typically

with high β (e.g. 30) and heavy-duty

turbines for Wmax with lower β values

(e.g. 17).

A.A. 2011-2012

Recuperated Cycle (1/2)

Lesson XVII

Effectiveness or regeneration rate

•Efficiency increase

A.A. 2011-2012

Recuperated cycle (2/2)

Lesson XVII

•Efficiency:

A.A. 2011-2012

Intercooled Cycle

Lesson XVII

•Specific work increase

•Efficiency decrease (if ideal cycle)

•Efficiency increase possible (if real cycle)

•More efficiency benefit if recuperator is includes

•Example: LMS100 GE machine (P=100 MW, η=46%)

A.A. 2011-2012

Reheated Cycle

Lesson XVII

•Specific work increase

•Efficiency decrease

•Increase of heat content at turbine outlet (useful aspect for combined cycles or

recuperated cycles

•Essential for recuperator application in high β cycles

A.A. 2011-2012

Ericsson Cycle

Lesson XVII

•Not real cycle because too many exchangers and machines

•Efficiency equal to Carnot cycle if cycle with R=1 (recuperated)

H

←Q

HL

L

3 4

52

A.A. 2011-2012

Mixed Cycles: STIG (or Cheng) Cycle

Lesson XVII

•Efficiency increase (up to 46% in simple cycle), power increase

•Low cost in comparison with combined cycles

•Disadvantages: large water consumption, power limitation and surge risks

Examples: Allison/General Motors, 4/6 MWe; Kawasaki, 2/4 Mwe, GE LM5000)

A.A. 2011-2012

Mixed Cycles: RWI Cycle

Lesson XVII

•Efficiency increase (up to 52%), power increase (lower than STIG)

•Disadvantages: large water consumption, power limitation and surge risks, large

dimension heat exchanger

•No commercial application because it is no too cheap in comparison with CC

Water inlet

A.A. 2011-2012

Mixed Cycles: HAT Cycle

Lesson XVII

•Efficiency increase (up to 55%), power increase

•Disadvantages: large water consumption, power limitation and surge risks,

saturator is critical

•No commercial application, but prototype developments

A.A. 2011-2012

Gas Turbine Emissions

Lesson XVII

�Chemical pollution:

�CO emission are not significant (high air excess): <15-20 ppm

�HC emission are not significant (high air excess and natural gas – no

benzene based composites)

�NOx (to be reduced with apt devices)

�SOx (not significant if natural gas, low if kerosene): < 2-4 ppm (n.g.)

�Carbon particulate (not present if natural gas)

�Thermal pollution:

�CO2 emission (low if natural gas)

Emission content depends on turbine type, combustor geometry, operating

conditions (temperature), maintenance type, fuel composition.

Zeldovich reactions

(significant if T > 1600 K)

A.A. 2011-2012

Gas Turbine Emissions Abatement

Lesson XVII

Different approaches are considered:

�Low temperature operation (not efficient)

�Steam injection at combustor level

�Selective catalytic reduction devices

�Dry low NOx combustors (premixed flames)

Steam injection:

�Steam injection in STIG cycles is useful to have more uniform

temperatures in combustors (50% reduction in Nox)

Premixed flames (emissions < 15-20 ppm):

�With premixed flows (upstream of combustor inlet) it is possible to have

uniform combustion and no peak temperature (<1600 K) avoiding

significant thermal NOx formation.