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176 H. Safaei et al. / Applied Energy 103 (2013) 165–179 in the annual electricity supply increases. The annual cost of elec- tricity in DCAES and CAES scenarios respectively reaches $81.72 and $82.60/MWh at emission tax of $80/tCO2e. The primary drivers in the trade-off analysis between a DCAES and a CAES plant are the capital cost of the pipeline and the reve- nues gained through waste heat recovery in the DCAES scenario. Furthermore, the HRU of the DCAES system would replace some capacity of the boilers of the district heating plant which could be considered as an extra financial profit at a system level analysis. In order to obtain better insight to this trade-off analysis, the size and associated capital fuel costs of various components of the DCAES system are shown at the cross-over carbon price of $40/ tCO2e in Table 5. It should be noted that the capital and fuel cost of the heat recovery unit are negative since they represent the sav- ings in the capital cost (shaving of peak load of the boilers due to the existence of the HRU) as well as the fuel cost of the boilers of the heating system. As shown, the levelized capital cost of the air pipeline is approximately 8.5% of the total capital cost of the DCAES plant (expander, compressor, cavern, and pipeline). However, the annual fuel savings from the heat recovery unit is $2.86 million per year, more than 50% of the annual fuel consumption of the expander. One should note that the exact location of the cross-over point would obviously depend on various system parameters such as the nature of the heat and electricity markets (composition of the gen- eration fleet, value of ancillary services, etc.). The main contribu- tion of this paper is to introduce and investigate the opportunity of the DCAES system to economically compensate for fluctuations of wind-based electricity in niche markets. Recovery of the low- quality waste heat of compression form a compressed air energy storage facility for heating applications instead of using it to lower fuel consumption of the plant itself (i.e. adiabatic design) is a new approach that could be valuable in a carbon-constrained economy. The authors acknowledge that this system does require further evaluation prior to industrial development; however, believe that this model illustrates the potential benefits of DCAES over conven- tional CAES technology. 3.6. Sensitivity analysis on pipeline length As discussed, the cross-over point is sensitive to the inputs and assumptions made in the simulation. Since the major difference between the conventional CAES and DCAES systems is the air pipe- line, the possible effects of the length of the pipeline on the cross- over point and thus the minimum effective fuel price required for economic competitiveness of DCAES with CAES is evaluated in this section. In order to investigate the effect of the capital cost of the pipeline in this tradeoff analysis, the simulations were run again with two different pipeline lengths to evaluate the change in the cross-over point. Two new systems were simulated, one with an increased pipeline length of 100 km and another with a decreased length of 25 km. DCAES system with a pipeline length of 100 km: this increased length would not have any impact on the optimal configuration of the CAES system. However, it is expected to increase the COE for the DCAES system since it would require a longer and more Table 5 expensive pipeline. The cross-over emission tax in this scenario is $50/tCO2e as compared to the base case with a threshold tax of $40/tCO2e. The optimal values of the system components for the two DCAES systems with 50 and 100 km pipelines are tabu- lated in Table 6. As shown, an increase in the length of the pipeline makes wind energy and compressed air storage less favorable and results in lower sizes for the wind farm, expander, compressor, cavern and air pipeline. Since a smaller compressor and wind farm are used, there are fewer opportunities for waste heat recovery and a smaller HRU and larger HOB are required in the DCAES system with a 100 km long pipeline. A longer pipeline, smaller wind farm, smaller HRU, and thus higher demand for fuel to meet the electric and heat load would cause an increase of $0.33/MWh in the aver- age annual cost of electricity in the DCAES scenario with a 100 km long pipeline (a value of $76.57/MWh). Although this change in COE may not seem very high, it indicates that DCAES systems could compete with conventional CAES systems effectively only under certain circumstances (niche markets) and highlights the impor- tance of custom designing these facilities in accordance to the local parameters and local energy market conditions. Simulation with 25 km pipeline: the cross-over emission tax for the system with a shorter pipeline length of 25 km is $30/tCO2e corresponding to an effective fuel price of $7.0/GJ (at a base natural gas price of $5.0/GJ). These values are $40/tCO2e and $7.6/GJ in the base-case system with a pipeline length of 50 km. The optimal size of various components of the two DCAES systems at the carbon price of $30/tCO2e are tabulated in Table 7. As shown, high capital costs associated with a 50 km long pipeline leads to the superiority of conventional gas turbines so that the optimal system configura- tion only includes gas turbines and wind farms in this scenario. In other words, revenues from waste heat recovery from the compressor would not be high enough to justify building a 50 km pipeline between the compression unit (heat load) and the cavern of the energy storage plant at low emission taxes. However, these revenues are high enough to justify building a 25km pipeline. The optimal design of the system with a 25 km distance between the heat load and the cavern includes a DCAES facility with a 193 MW expander and a 23 MW compressor. Size of the energy storage plant and wind farms sharply increase past this point and the cost of electricity remains lower in this system compared to the configuration with the pipeline length of 50 km. 3.7. Emission tax and emission reduction This study optimizes both the size and the dispatch of the gas turbines, wind farm, and CAES and DCAES facilities to satisfy a var- iable electric load at variable emission taxes. This approach pro- vides insight into the opportunity of reducing the carbon intensity of the electric sector, a major contributor to GHG emis- sions but relatively easier to be managed, through the introduction of emission taxes. The interesting question is how much of an emission tax would be required to justify investing in more capital intensive but cleaner technologies (wind and energy storage in this case) and drive the overall emissions associated with electricity generation below a certain level. The carbon intensity of satisfying the electric load considered in this study at the base-case natural Optimal size, fuel consumption and associated costs of the DCAES plant at a tax level of $40/tCO2e (at the cross-over point for the scenario with a 50 km long pipeline). Expander Compressor Cavern Pipeline HRU Size 284 (MW) 41 (MW) 4106 (MWh) 437 (mm) 29 (MW) Fuel consumption (TJ/year) Capital cost ($ million/year) 734.7 14.62 NA 1.94 NA 1.26 NA 1.65 374.1 0.04 Fuel cost ($ million/year) 5.61 NA NA NA 2.86PDF Image | 20 kW ORC Turbine Off-Design Performance Analysis
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