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Expansion-Cycle
Evaporation Turbine |
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Expansion-Cycle Evaporation
Turbine* * Patents pending |
Sunoba Renewable Energy Systems |
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The
Expansion-Cycle Evaporation Turbine (ECET) is the continuous-flow version of
the evaporation engine. The principal
target application is to boost the power output of Open-Cycle Gas Turbines
(OCGTs) by exploiting the heat energy in the OCGT’s hot exhaust. This web
page highlights a case study that shows OCGT power output can be boosted by
more than 20% without any further fuel consumption. Also presented is an economic analysis
comparing the Levelised Electricity Cost for the following four generation
options: ·
OCGT ·
Combined-Cycle Gas Turbine (CCGT, in which the hot OCGT exhaust
is exploited in a conventional Rankine-cycle steam turbine) ·
coal-fired Rankine-cycle steam turbine ·
OCGT+ECET This analysis shows that the OCGT+ECET
combination has a lower electricity cost than the OCGT for all Capacity
Factors. Moreover, the OCGT+ECET
combination has a lower electricity cost than the CCGT and coal-fired power
options provided the Capacity Factor is not too high. The figure below shows the flowsheet for a
4-stage ECET with one expansion turbine and four evaporation/re-compression
stages mounted on one shaft, on which is also mounted a generator to convert
mechanical energy into electrical energy:
The power output of such installations is
analysed in Article [8]. The basis for the analysis is that air and
water vapour are ideal gases with constant specific heat capacities and the
jump in enthalpy on evaporation is given as a function of temperature by
interpolation from tabulated values.
Realistic values are used for the adiabatic efficiencies of turbines
and compressors. In the case study, the inlet air stream is
taken as the exhaust of a 56 MW OCGT that was commercially available in
2007. The OCGT mass flow-rate is 197
kg/s with exhaust temperature 508°C.
At the ECET inlet, the estimated dry air flow-rate is 195 kg/s and,
assuming the OCGT fuel is natural gas, the estimated inlet partial pressures
are 92.5 kPa dry air and 8.8 kPa water vapour. The specific output of the OCGT is
estimated as 287.2 kJ/kg dry air. The ECET case study used the following
parameters: ·
ratio of inlet pressure to pressure in the first-stage
evaporator: 6.5 ·
number of evaporation/re-compression stages: 4 ·
adiabatic efficiency of turbine and compressors: 0.90 ·
energy cost for reverse osmosis water purification: 15 kJ/litre ·
energy cost for water injection: 10 kJ/litre The ECET
output is 59.0 kJ/kg dry air, or 20.5% that of the upstream OCGT. The sensitivity of this result to changes
in the parameters is explored in Article [8]. The
economic analysis is expressed in a spreadsheet that predicts the Levelised Electricity
Cost (LEC) as a function of various parameters for the four generation
options listed earlier (OCGT, CCGT, coal-fired, OCGT+ECET). The OCGT and ECET properties are based on
the case study referred to above, except that the adiabatic efficiency of the
ECET turbine has been increased slightly.
The example presented here uses the following parameters: ·
specific capital cost, OCGT:
AUD 750,000/MW ·
specific capital cost, ECET: AUD 750,000/MW ·
specific capital cost, Rankine steam turbine: AUD 1,500,000/MW ·
thermodynamic efficiency, OCGT:
0.34 ·
thermodynamic efficiency, Rankine: 0.34 for supplementary system
downstream of OCGT, otherwise 0.39 for coal-fired system ·
ECET output/OCGT output: 0.23 [Note: assumes adiabatic
efficiency for ECET expansion turbine is 0.92] ·
cost of capital: 0.07 × amount owing ·
loan repayment period: 25
years ·
non-fuel O&M costs: 0.015 × capital cost ·
cost of natural gas: AUD 6.00/GJ ·
energy content of coal: 28 GJ/t ·
cost of coal: AUD 80/t In the
figure below, the Capacity Factor (CF) is the fraction of time that the
generator is active.
The conventional wisdom is that coal-fired
power is cheapest for base load (CF large), OCGT power is cheapest for peak load
(CF small) and CCGT power is cheapest in some middle range (‘shoulder’). That holds for the parameters used in the
economic analysis. When the ECET boost is applied to the OCGT,
the LEC order of merit (from cheapest to most expensive) depends on the
Capacity Factor as follows: 0 < CF < 0.14 OCGT+ECET < OCGT < CCGT
< coal 0.14 < CF < 0.23 OCGT+ECET
< CCGT < OCGT
< coal 0.23 < CF < 0.25 OCGT+ECET
< CCGT < coal < OCGT 0.25 < CF < 0.34 CCGT < OCGT+ECET < coal < OCGT 0.34 < CF < 0.44 CCGT < coal < OCGT+ECET < OCGT 0.44 < CF < 1 coal < CCGT
<
OCGT+ECET < OCGT This example indicates there is economic
advantage in using the Expansion-Cycle Evaporation Turbine to boost the power
of installed OCGTs for peak duty and some shoulder duty in the electricity
grid. The broad features of the
interpretation above are robust, although the CF values at which the order of
merit changes depend on assigned parameters. Note that rapid response time is also
important for peaking plants. In that
respect, the ECET boost should be as quickly obtainable as OCGT power. Rankine-cycle steam plants do not have a
quick response time. Calculations
based on your data Upon request, the thermodynamic output of
the ECET, as predicted by Article [8], can be
calculated for you. The following
parameters must be supplied (preferred units are shown, but other common
units are acceptable): ·
temperature of OCGT exhaust [°C] ·
partial pressure of water vapour in the OCGT exhaust [Pa] ·
ambient pressure [Pa] ·
ratio of ambient pressure to pressure in first-stage evaporation
chamber [-] ·
number of evaporation/re-compression stages [-] ·
adiabatic efficiency, turbine [-] ·
adiabatic efficiency, compressors [-] ·
temperature of injected water [°C] The output file gives the
specific work [J/kg dry air throughput] and specific water consumption
[litres/kg dry air throughput], as well as state variables at all stages. Upon request, an economic
analysis can be prepared based on the thermodynamic calculation for your
data. The data before the figure above
are typical of the economic parameters that need to be supplied. Further technical information would also be
required about the upstream OCGT, namely dry air flow-rate [kg/s], power output
[MW] and fuel consumption [J/s]. |
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© Sunoba Pty Ltd 2 June 2011 |
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