My last post (12 May 2011) was concerned with the Expansion-Cycle Evaporation Turbine (ECET), in particular how the ECET can be used to provide a 20% boost to the output of an Open-Cycle Gas Turbine (OCGT). Today I’ll examine the cost of electricity under the OCGT+ECET scenario. To be specific, I’ll compare the Levelised Electricity Cost (LEC) for the following four generation options:
2. Combined-Cycle Gas Turbine (CCGT, in which the hot OCGT exhaust is used to drive a conventional Rankine-cycle steam turbine)
3. coal-fired Rankine-cycle steam turbine
4. OCGT+ECET (see Figure 1 below)
The material in today’s post is taken from www.sunoba.com.au/ECET
Figure 1 shows the OCGT+ECET flow-sheet (click to see original jpg figure).
Figure 1: Flow-sheet for the OCGT+ECET option. The Expansion-Cycle Evaporation Turbine (ECET) exploits the hot exhaust of the Open-Cycle Gas Turbine (OCGT). The thermodynamic cycle of the ECET is based on evaporative cooling of hot air at reduced pressure.
A case study has been made (see www.sunoba.com.au/references, article 8) in which the inlet ECET 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. The estimated OCGT dry air flow-rate is 195 kg/s and, assuming the OCGT fuel is natural gas, the estimated ECET 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 was 59.0 kJ/kg dry air, or 20.5% that of the upstream OCGT.
In the LEC comparison, the OCGT and ECET properties are based on the case study referred to above, except that the adiabatic efficiency of the ECET turbine was increased slightly. The comparison 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 CCGT supplementary system downstream of OCGT, otherwise 0.39 for coal-fired system
· ECET output/OCGT output: 0.23 [Note: now 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 Figure 2 (click to see original jpg figure), the Capacity Factor (CF) is the fraction of time that the generator is active.
Figure 2: LEC comparisons for the four generation options.
The conventional wisdom (in the absence of the ECET boost) 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.
For more details, see www.sunoba.com.au