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Thermodynamic Analysis on an Integrated Liquefied Air Energy

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Thermodynamic Analysis on an Integrated Liquefied Air Energy energies Article Thermodynamic Analysis on an Integrated Liquefied Air Energy Storage and Electricity Generation System Yingbai Xie and Xiaodong Xue * Department of Power Engineering, North China Electric Power University, Baoding 071003, China; [email protected] * Correspondence: [email protected] Received: 22 August 2018; Accepted: 20 September 2018; Published: 23 September 2018 Abstract: For an integrated liquefied air energy storage and electricity generation system, mathematical models of the liquefied air energy storage and electricity generation process are established using a thermodynamic theory. The effects of the outlet pressure of the compressor unit, the outlet pressure of the cryogenic pump, the heat exchanger effectiveness, the initial air temperature and pressure before throttling on the performances of the integrated liquefied air energy storage, and the electricity generation system are investigated, using the cycle efficiency and liquid air yield ratio as the evaluation indexes. The results show that if the compressor outlet pressure is raised, both the compression work and the expansion work increase, but because the expansion work increases more slowly, the cycle efficiency of the system gradually decreases. Increasing the cryogenic pump outlet pressure and heat exchanger effectiveness can significantly increase the cycle efficiency of the system; the higher the air pressure and the lower the air temperature before throttling, the greater the liquid air yield after expansion, and the higher the cycle efficiency. The theoretical analysis models and research results can provide a reference for the development of an integrated system of liquefied air energy storage and electricity production, as well as for the development of medium-capacity energy storage technology. Keywords: liquefied air energy storage; cycle efficiency; liquid air yield ratio; electricity generation 1. Introduction Solar, wind, and other renewable energies are widely used to generate electricity in the world [1–5]. For these energy forms, because of their characteristics of instability and intermittence [6,7], efficient energy storage technologies are required in order for a sustained and stable output [8–12]. Energy storage technologies, such as bulk power management, compressed air energy storage (CAES), and pumped hydroelectricity storage (PHS) [13–16], are presently relatively mature and reliable. However, these two technical schemes [17] are limited by geographical or hydrogeological conditions. PHS technology needs abundant water resources for support, while CAES requires high-performance natural underground reservoirs. According to the Electric Power Research Institute (EPRI), the total cost for CAES is around 1000 $/kW. It may be double this for PHS. As a result, there is a demand to develop a general, cost-effective energy storage technology, regardless of local conditions. Liquid air energy storage (LAES) is an innovative and leading universal industrial energy storage technology [18–21]. The idea of LAES began in 1977 at the University of Newcastle, and was tested by Mitsubishi Industries Ltd. (Tokyo, Japan) in 1998 [1]. Researchers at the University of Leeds together with the Highview Power Storage Company developed the first 350 kW/2.5 MWh pilot demonstration plant at the University of Birmingham in 2010. The data gathered from this pilot plant showed that the efficiency of the total cycle is in the range of 50–60%. Energies 2018, 11, 2540; doi:10.3390/en11102540 www.mdpi.com/journal/energies Energies 2018, 11, 2540 2 of 12 Since 2014, Viridor has selected Highview to design MW level pre-commercial, multi-MW, and conceptual Giga-Plant LAES. However the round-trip efficiency is still approximately 60%. Some literature has contributed to this topic. The process can be divided into two parts, namely air liquefied and electric generation. As the air liquefied process needs a lower temperature, while the electric generation process needs a higher temperature, internal heat exchangers, such as regenerators, are needed to connect these two processes in order to improve the cycle efficiency. An integrated system was put forward so as to investigate the operational parameters of the major devices effecting the cycle efficiency. Chino and Araki [22] also proposed an air liquefaction plant integrated with a conventional combined cycle power plant. Li et al. studied a LAES system integrated with a nuclear power plant [23]. The overall system efficiency is improved, owing to the reheating arrangement. In this paper, more detailed effects will be discussed regarding the outlet pressure of the compressor and cryogenic pump, the efficiency of the heat exchanger, the air temperature and pressure before the throttle on the cycle efficiency, and the liquid air yield. 2. System Description Figure1 shows the layout of an integrated liquefied air energy storage and electricity generation system, referring to the literature [8]. The system adopts a two-stage compression and two-stage expansion. Compressor #1 Expander #2 5S 6S 1L 7E 12S Hot Storage 2L Tank 6E 11S Heat Cooler #1 Exchanger #2 Compressor #23L 5E Expander #1 4L 4E 7S Heat Cooler #2 Cold Storage Exchanger #1 Tank #1 10S 9S 8S 5L 3E 10L 1S 3S Cold Cold Storage Gasification Heat Box Tank #2 Exchanger 6L 2S 4S 9L J-T Valve 2E Cryogenic 7L Pump Separator Liquid 1E 8L Air Tank Figure 1.1. SchematicSchematic diagram diagram of of an a integratedn integrated liquid liquid air energy air energy storage storage and electricity and electricity generation generation system. system. The left side is the air liquefying process. It contains two air compressors, a J–T valve (It is 3.a throttleMathematical valve), M aodels separator, for Processes a liquid air tank, and some heat exchangers. When there is surplus electricity from the renewable energy sources or from the grid, the outer air is compressed by the two 3.1.air compressors,Air Liquefying andProcess itstemperature and pressure are raised. Then, the gaseous air is cooled down and throttled in the J–T valve. The liquefied air is then collected in the separator and stored in the The outlet pressure and inlet pressure of the air compressors are as follows: liquid air tank. ppac,, out= ac in ac (1) where pac,out is the outlet pressure of the air compressor, Pa; pac,in is the inlet pressure of the air compressor, Pa; and πac is the compression ratio. The relationship between the inlet and outlet temperature of the air compressor is as follows: nc −1 nc (2) TTac,, out= ac in ac where Tac,out is the outlet temperature of the air compressor, K; Tac,in is the inlet temperature of the air compressor, K; and nc is the polytropic index of the compression process. The efficiency of the compressor ƞac can be expressed as follows: −1 nc ac = (3) nc −1 where κ is the adiabatic index. The specific work, wac, done to the compress air is as follows: 2 wac= c air()()() T ac, out , i − T ac , in , i = h 2 L − h 1 L + h 4 L − h 3 L (4) i=1 where cair is the specific heat capacity of air, J/(kg·K). After the two air compressors, two internal heat exchangers are used. The high-temperature and high-pressure air is cooled down to heat the low temperature cold fluid within the internal heat Energies 2018, 11, 2540 3 of 12 When there is an insufficient supply of electricity available to meet the consumers’ demands, the cryogenic pump is activated. The liquid air in the liquid air tank increases its temperature through the heat exchangers, and recovers to a gaseous state. When it reaches the set-points for the temperature and pressure, the air enters the two-stage expander in order to generate electricity. This is the electricity generation process. There are some regenerators between the two processes. To decrease the system fluctuation, the large capacity storage method is used. The above integrated system is modeled with the following assumptions: • Ignoring other components, it is assumed that the air is a mixture of 21% oxygen and 79% nitrogen. The thermodynamic properties of nitrogen and oxygen are evaluated in REFPROP (Reference Fluid Thermodynamic and Transport Properties Database), according to the authors of [24,25], respectively. • According to thermodynamics, it is assumed that the compression and expansion processes are polytropic processes. • The pressure losses along the cycle have been ignored, in order to have a solution that compares different cycles under the same conditions. In the analysis, the system is assumed to be in a steady state condition, and the thermal losses in the heat exchangers are ignored [26]. 3. Mathematical Models for Processes 3.1. Air Liquefying Process The outlet pressure and inlet pressure of the air compressors are as follows: pac,out = pac,inpac (1) where pac,out is the outlet pressure of the air compressor, Pa; pac,in is the inlet pressure of the air compressor, Pa; and pac is the compression ratio. The relationship between the inlet and outlet temperature of the air compressor is as follows: nc−1 Tac,out = Tac,inpac nc (2) where Tac,out is the outlet temperature of the air compressor, K; Tac,in is the inlet temperature of the air compressor, K; and nc is the polytropic index of the compression process. The efficiency of the compressor hac can be expressed as follows: k − 1 nc hac = × (3) k nc − 1 where k is the adiabatic index. The specific work, wac, done to the compress air is as follows: 2 wac = ∑ cair(Tac,out,i − Tac,in,i) = (h2L − h1L) + (h4L − h3L) (4) i=1 where cair is the specific heat capacity of air, J/(kg·K). After the two air compressors, two internal heat exchangers are used. The high-temperature and high-pressure air is cooled down to heat the low temperature cold fluid within the internal heat exchanger. Ignoring the heat dissipated to the surroundings, the outlet air temperature of the cold side, Thex,cold,out, is as follows: Thex,cold,out = (1 − #)Tac,out + #Thex,cold,in (5) Energies 2018, 11, 2540 4 of 12 where # is the efficiency of the heat exchanger, and Thex,cold,in is the inlet air temperature of the internal heat exchanger on the cold side, K.
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