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174 H. Safaei et al. / Applied Energy 103 (2013) 165–179 3.2. Distribution of electricity generation Figs. 6 and 7 respectively show the share of the generation fleet in supplying electricity at different values of emission tax over the simulation period in the CAES and DCAES configurations. In the absence of any emission tax, all the electric load is sup- plied by the combined and simple cycle gas turbines (98.0% and 2.0%, respectively). As emission tax increase to $30/tCO2e, wind share increases to 12.1% while the share of CCGT and SCGT de- crease to 86.4% and 1.5% in both systems. However, this level of tax is still not high enough to justify building capital intensive compressed air energy storage plants and large wind farms. Never- theless, as more aggressive emission taxes are enforced, the share of gas turbines decreases sharply because of their less efficient operation and consequently higher emission intensity compared to CAES, DCAES and wind. Interestingly, the share of wind energy in the annual electricity generation increases very fast, the share of the expander of the CAES facility remains relatively small, and the share of the CCGT remains intermediate (although declining) at higher emission taxes. These results reveal that even high emission taxes would not significantly increase the market share of the CAES facilities and most of the wind energy would be di- rectly supplied to the grid rather than stored in the CAES facility due to its high capital costs and the requirement for burning fuel Table 4 Share of electricity generation of various components in the CAES and DCAES configurations at a tax level of $60/tCO2e. All values are expressed as percentage of the annual electric load. CAES configuration DCAES configuration CCGT SCGT 39.8 0.2 35.3 0.6 Wind farm Expander 55.5 4.4 56.9 7.2 during its electricity generation cycle (heat rate of 4.19 GJ/MWh in this study). Similar results are observed for the DCAES system as electric load is supplied solely by gas turbines and wind farm at emission tax below $40/tCO2e. Nevertheless, a major difference between the DCAES system and the CAES system is the lower share of CCGT and higher share of wind, expander and SCGT in the annual electricity supply. As tabulated in Table 4, the expander of DCAES supplies 7.2% of the annual electric load while this value is 4.4% in the CAES configuration at an emission tax of $60/tCO2e. In addi- tion, the CCGT share is 35.3% in this configuration while it is 39.8% for the CAES system. This observation is in agreement with the trend of equipment size in Figs. 4 and 5 with wind farm and expan- der replacing a larger market share of the CCGT in the DCAES system due to revenues gained through waste heat recovery from the compressor. Another observation is the small share of the SSGT in the annual electricity generation. Only 0.6% of the annual load is supplied by the simple cycle gas turbine because of its low thermal efficiency and thus high associated fuel costs at high emission taxes. As discussed in Section 3.1, the size of the SCGT plant is larger in the DCAES system at high emission taxes compared to the CAES system in spite of its low thermal efficiency. However, SCGT has a very low capacity factor (annual electricity genera- tion/maximum possible annual electricity generation) of 3.3% in the DCAES system; therefore, its lower specific capital cost would justify building a larger SCGT plant in the DCAES system despite its higher fuel cost per MWh of electricity generated. The capacity factor of each plants with a specific size is driven by its dispatch cost (fuel and emission tax). In other words, the optimization code sizes and dispatches the generation fleet so that the electric load is satisfied at the least cost. 3.3. Distribution of wind energy Although higher emission taxes justify building larger wind farms, the fact that periods with large availability of wind do not necessarily coincide with periods of high electric load necessitates existence of gas turbines and/or compressed air energy storage facilities to fill the gap between the electric load and wind-based electricity generation. Despite the fact that compressed air energy storage plants have a lower heat rate compared to gas turbines, they still burn fuel during the generation phase which translates to high fuel costs for these plants at high levels of emission tax. To further investigate the role of wind energy in meeting the electric demand, the distribution of the available wind energy (sold to the grid for instantaneous use, stored by the compressed air en- ergy storage plant, or curtailed) at various levels of emission taxes in the CAES and DCAES scenarios are illustrated in Figs. 8 and 9, respectively. The availability of wind energy is very limited in both systems below emission taxes of $40/tCO2e; nevertheless, it increases rap- idly beyond this point as compressed air energy storage enters the electricity market. Interestingly, only a small portion of the avail- able wind energy is stored by the compressor of the energy storage facility while a much higher portion is supplied to the grid for instantaneous consumption. This observation is in agreement with 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 CCGT SCGT Wind CAES $0 $10 $20 $30 Emission tax ($/tCO2e ) Fig. 6. Distribution of annual electricity generation in the CAES configuration at various levels of emission tax. $40 $50 $60 $70 $80 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 $0 $10 $20 $30 $40 Emission tax ($/tCO2e ) CCGT SCGT Wind DCAES Fig. 7. Distribution of annual electricity generation in the DCAES configuration at various levels of emission tax. $50 $60 $70 $80 Distribution of electricity generation (TWh/year) Distribution of electricity generation (TWh/year)PDF Image | 20 kW ORC Turbine Off-Design Performance Analysis
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