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168 H. Safaei et al. / Applied Energy 103 (2013) 165–179 electricity. To the knowledge of the authors this idea has not been discussed in literature before and this paper is the first to investi- gate the opportunities in enhancing the economics of CAES through recovery of the otherwise wasted heat of compression from CAES plants to satisfy heating loads (instead of wasting the heat to the ambient in conventional CAES plants or storing it to la- ter heat the compressed air during the power generation cycle as proposed in the Adiabatic CAES design). The primary contribution of this paper is providing insight into the potential financial gains through waste heat recovery from the compression trains of CAES for municipal heating applications in order to facilitate R&D work on detailed engineering design of such plants. It bears mentioning that a different the concept of distributed (decentralized) compressed air energy storage has been introduced and discussed in the literature which is based on replacing the gen- erator of individual wind turbines with compressors to directly store mechanical energy of wind in the form of potential energy of compressed air [20,21]. The main advantage of this concept is eliminating the energy losses during the process of converting mechanical energy of wind to electricity and then to potential en- ergy of compressed air. Nevertheless, distributing the compressors near heat loads to enable the use of compression heat for space and water heating applications, as proposed in the DCAES configura- tion, has not been studied before. 2. Methodology Although the economic performance of CAES plants depends on the nature of the electricity market that they participate in, some recent studies have shown that conventional CAES plants may not be able to compete economically with other alternatives, espe- cially conventional gas turbines, in support of wind-based electric- ity [12,22,23]. This fact is due to the relatively high capital cost of the CAES facility in comparison to gas turbines. However, at ele- vated fuel prices or in the case of high emission taxes, CAES tech- nology would become more attractive because of its better fuel economy compared to conventional gas turbines. In a carbon-con- strained world, technologies with high carbon footprints would be phased out and the electricity generation mix would move towards cleaner technologies which would make the already complicated market dynamics more difficult and uncertain to model. Moreover, compressed air energy storage plants should ideally be designed and operated as independent entities in the electricity market to maximize their economic performance [24]. These plants would buy and store off-peak electricity from a variety of sources and sell electricity back to the grid during periods of peak demand and also provide ancillary services to the grid to maximize their competi- tiveness and profit in the electricity market. However, because of the complexities associated with forecast- ing and modeling this real world scenario with higher fuel costs, we study a simplified system to investigate the economics of DCAES and CAES plants in supporting wind-based electricity. In or- der to avoid the complex dynamics of the electricity market but at the same time provide insight into the performance of the DCAES configuration in a close-to-real world scenario, a hypothetical sce- nario which represents a high-level policy making approach was used in this study. This hypothetical scenario represents a planning firm designing the electricity and heating infrastructure of a por- tion of a city in a carbon-constrained world. It is assumed that good wind resources are available; therefore the planning firm is consid- ering harnessing this clean energy resource to satisfy a portion of the electric load. Natural gas-based power plants (simple cycle gas turbine, SCGT, and combined cycle gas turbine, CCGT) and nat- ural gas-based compressed air energy storage plants (either con- ventional CAES or DCAES) are also considered as candidates for electricity generation because of their relatively low GHG emis- sions. In addition, the firm plans to utilize district heating technol- ogy to satisfy the heat load of a portion of the city with a high load intensity (e.g. downtown core) because of the potential of the dis- trict heating technology in supplying cleaner and more efficient heating energy compared to individual heating facilities. The heat- ing energy of this district heating facility could be supplied through either direct combustion of natural gas in heat-only-boilers (HOB) or heat recovery from the DCAES plant. The overall objective of the planner is to satisfy both the electric and heat load at the lowest levelized cost (or maximizing the net social welfare). It is also as- sumed that the electric load of the city has to be satisfied by its own generation fleet and import and export of electricity are ig- nored. One should note that in the real world, cities are not nor- mally isolated from the electric grid and can buy/sell electricity from/to other grids. The authors acknowledge this simplification in the model presented but decide to use this assumption for the sake of simplicity and to evaluate the conditions under which the DCAES design might be economically preferable to CAES at a high level policy making approach. The performance of the DCAES configuration depends most strongly on the size and diurnal and seasonal fluctuations of the heat load profile. In other words, if the heat load is too small then the economic gains from waste heat recovery for heating purposes would not be sufficient to justify the DCAES system (mainly due to the requirement for capital intensive compressed-air pipelines and the effect of economy of scale on this cost). However, in the real world the magnitude of heat load supplied by district heating net- works is limited by increased heat losses in geographically-dis- persed networks. In order to build a close to real world scenario, the size of the heat load connected to the district heating network was chosen so that it represented a concentrated municipal heat demand center. It should be noted that the DCAES system might only be chosen over the conventional CAES configuration when it is possible to pair with a district heating network. Therefore, in- stead of modeling the heat demand of an entire city, the authors only focused on satisfying a concentrated heat load (a portion of the city) supplied by a district heating network. 2.1. System of study As mentioned, two different configurations can be considered to evaluate the performance of conventional CAES and DCAES sys- tems in meeting the electric and heat loads. 2.1.1. System with a conventional CAES plant As shown in Fig. 1, a wind farm, a conventional CAES plant lo- cated outside a city at a favorable geological storage site, and con- ventional gas turbine plants (SCGT and CCGT) could be utilized to meet the electric demand. The heat load would be satisfied by large scale boilers connected to district heating networks. Since this study focuses on providing reliable and dispatchable wind-based electricity, it is assumed that the CAES facility can only be charged by the wind farm while SCGT and CCGT can only provide electricity for instantaneous use and not for compression in the CAES plant. 2.1.2. System with a DCAES plant The major difference between this configuration (Fig. 2) and the previous system is that the compressor of the compressed air stor- age facility is located within the city and close to a concentrated heat load. A heat recovery unit (HRU) is utilized to recover the heat of compression, thereby negating the fuel consumption of the heat-only-boiler (HOB) of the district heating system. The gener- ated compressed air is transported via a pipeline network to a suit- able geological storage site outside of the city for storage. The expander of the compressed air storage plant is located at this sitePDF Image | 20 kW ORC Turbine Off-Design Performance Analysis
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