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penetration of wind energy into the electric grid. Although PHS has approximately 40 and 20 GW of installed capacity in Europe and US respectively [3], development of PHS pro- jects is geographically limited due to the necessity for two large natural or artificial water reservoirs with sufficient ele- vation difference. Environmental licensing and long con- struction time (in 10 years scale) are among other factors limiting large expansion of PHS capacities [10,11]. On the other hand, CAES technology can use a variety of both underground and aboveground storage facilities and CAES plants can be constructed in a relatively short time (around 3 years). CAES systems use inexpensive off-peak electricity to compress air into underground or aboveground storage reservoirs, which are later used to power modified gas turbines and generate electricity. CAES technology was extensively investigated in the 1970s to provide load following services and to gain a high capacity factor for base load power plants (especially nuclear) by storing off-peak electric- ity. The first CAES plant was commissioned in Huntorf, Germany in 1978 to provide black-start services to nuclear plants as well as provide relatively inexpensive peak power [12]. The Huntorf plant, which is still in operation, stores up to 310,000 m3 of compressed air at a pressure range of 44–70 bar in two salt caverns and can produce up to 290 MW of electricity at full capacity for 4 h at an air discharge flow rate of 417 kg/s. The second utility scale CAES plant was commissioned in 1991 in McIntosh, Alabama and is still in operation as well. The McIn- tosh plant can generate 110 MW of electricity at full capacity for 26h at an air discharge rate of 154kg/s. It stores up to 540,000 m3 of compressed air at a pressure range of 45–74 bar in a salt cavern [13]. The Alabama plant consumes up to 25% less nat- ural gas than the Huntorf plant as waste heat from the exhaust of the low pressure expander is recuperated to preheat discharge air from the cavern prior to entering the high pressure combustor [13]. Despite the successful operation of these two CAES plants, a ser- ies of events caused the development of CAES technology to slow down during the late 1980s and the 1990s. These events included the loss of momentum in the nuclear industry, the development of efficient and low capital intensive single and combined cycle gas turbines, a drop in natural gas prices, and an overbuilt generation capacity [3,11]. However, the desire for higher penetration of clean but intermittent wind and solar energy sources into the electric grid has renewed interest in CAES as a method to overcome their intermittency and thus lower the GHG emissions from electricity generation [11]. CAES plants are not pure energy storage facilities since their operation requires the combustion of fuel during the generation cycle (the only exception is the Adiabatic CAES configuration which is still in the research and development phase). Compressed air is heated during the generation cycle (to prevent water vapor from freezing) and then electricity is generated through expansion of heated compressed air in expanders. As a case in point, the Hun- torf plant uses 5800 kJ of natural gas per kWh of peak electricity that it generates1 [13]. A variety of newer CAES designs have been proposed in the past few decades to improve the storage efficiency of conventional CAES plants. One design introduced by Energy Stor- age and Power Corporation in the 1990s is based on pairing CAES plants with conventional gas turbines. The main idea of this ap- proach is eliminating the combustor in the CAES facility and utilizing 1 Each kWh of energy equals to 3600 kJ. Therefore, the thermal efficiency of the Huntorf plant would be 62% solely based on its ‘‘inside-the-fence’’ fuel consumption. This CAES plant also uses 0.82 kWh of electricity during the compression cycle for each kWh of electricity that it delivers during the generation cycle (energy ratio of 0.82). the exhaust stream from the gas turbine instead of the combustor to heat the compressed air and thus improving the overall efficiency [10,14]. In contrast to this approach which focuses on waste heat recov- ery during the discharging process, the Adiabatic CAES design is based on storing the heat of compression in a thermal energy stor- age facility. This stored heat then would be utilized to heat the compressed air during the generation process and thus lower (or even eliminate) the fuel consumption of the CAES plant [15]. This concept was introduced in the 1980s and the interest in this con- cept is recently renewed both in Europe and the United States [10,16]. However, this concept is still in the research and develop- ment phase and its development is challenged by few major tech- nical issues including design of high pressure, high temperature, large scale and economically attractive thermal energy storage sys- tems, high pressure and high temperature compressors, and high pressure expanders [17,18]. This paper introduces and evaluates a different approach to im- prove the efficiency of conventional CAES plants. This approach fo- cuses on utilizing the otherwise wasted heat of compression for heating needs and thus improving the overall thermal efficiency of the CAES plant. This new configuration, which is called Distrib- uted CAES (DCAES), is realized by distributing air-compression sta- tions near heat loads such as district heating facilities instead of siting the compression train at the storage facility (as the case for a conventional CAES plant). An integrated compressed air pipe- line network, supplied by these distributed compressors located near high heat-load facilities (e.g. hospitals and office towers), would use off-peak electricity to compress air. The heat produced by air-compression would then be utilized or stored for heating needs, and thus lower the overall cost of the DCAES system by negating the demand for heating fuel usage within these high heat-load regions. The compressed air from this system would be pipelined to favorable geological sites for underground storage. The expander of the DCAES system located at the storage site would generate electricity via combustion and expansion of stored compressed air, similar to the conventional CAES designs. Further details on different variations of the DCAES concept may be found in Hugo et al. [19]. The DCAES concept is based on improving the economics of the conventional CAES facilities through the use of the low quality heat of compression for space and water heating demands. Waste heat recovery from industrial compressors is a mature technology and therefore introduces less technical complexity compared to the Adiabatic CAES design which requires heat recovery and storage at high pressures and temperatures. On the other hand, the DCAES design requires a pipeline between the compression site (region with high density of heat load) and the storage site (e.g. cavern). In the admittedly unrealistic case where the heat load and the CASE storage facility are co-located, then D-CASE would always be preferred to CASE since it provides heat ‘‘for free’’. The essence of our analysis is to explore how the relative competitiveness of CAES and D-CASE depends on pipeline length. The tradeoff be- tween the increased capital cost of the DCAES system compared to conventional CAES (mainly due to the air pipeline) and savings on fuel (used for heating purposes) can make the DCAES system cheaper compared to the conventional CAES in certain situations. The intensity and fluctuations of the heat load, size and fluctu- ations of the electric load, distance between the heat load and stor- age facility, and the fuel and construction costs are the major players in this tradeoff. As mentioned earlier, ambitious plans for higher penetration of wind energy into the electric grid is one of the main drivers for the renewed interest in the CAES systems in the twenty first century. Therefore, this paper focuses on economic evaluation of DCAES concept, as an alternative to conventional CAES systems, to provide economic and dispatchable wind-based H. Safaei et al. / Applied Energy 103 (2013) 165–179 167PDF Image | 20 kW ORC Turbine Off-Design Performance Analysis
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