A New Tri-Generation System: Thermodynamical Analysis of a Micro Compressed Air Energy Storage

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1 Journal of nergy and Power ngineering 10 (2016) doi: / / D DAVID PUBLISHING A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage Andrea Vallati 1, Stefano Grignaffini 1, Alessandro Quintino 1, Maro Romagna 1 and Lua Mauri 2 1. DIA Department of Astronautial, letrial and nergetial ngineering- Sapienza University of Rome, Via udossiana 18, Rome, Italy 2. Department of Mehanial ngineering, University of Roma TR, Via della Vasa Navale 79, Rome, Italy Reeived: June 22, 2016 / Aepted: July 07, 2016 / Published: November 30, Abstrat: There is a growing interest in the eletrial energy storage system, espeially for mathing intermittent soures of renewable energy with ustomers demand. Furthermore, it is possible, with these system, to level the absorption peak of the eletri network (peak shaving) and the advantage of separating the prodution phase from the exertion phase (time shift). CAS (ompressed air energy storage systems) are one of the most promising tehnologies of this field, beause they are haraterized by a high reliability, low environmental impat and a remarkable energy density. The main disadvantage of big systems is that they depend on geologial formations whih are neessary to the storage. The miro-cas system, with a rigid storage vessel, guarantees a high portability of the system and a higher adaptability even with distributed or stand-alone energy produtions. This artile arries out a thermodynamial and energy analysis of the miro-cas system, as a result of the mathematial model reated in a Matlab/Simulink environment. New ideas will be disussed, as the one onerning the quasi-isothermal ompression/expansion, through the exertion of a biphasi mixture, that will inrease the total system effiieny and enable a ombined prodution of eletri, thermal and refrigeration energies. The exergy analysis of the results provided by the simulation of the model reports that more than one third of the exergy input to the system is lost. This is something promising for the development of an experimental devie. Key words: CAS, thermodynami analysis, exergy analysis, quasi-isothermal ompression/expansion. Nomenlature T 0 T Input temperature at the ompressor ( C) Output temperature at the ompressor ( C) Q h Maximum amount of heat extratable during the ompression phase (kj/kg) Q e Total heat subtrated during the expansion phase (kj/kg) β Pressure ratio C Absorbed energy by the system (ompressor) (kwh) The energy transferred from the system (turbine) (kwh) η h ffiieny of heating side (%) η ffiieny of ooling side (%) el,h el, x yl, Heating energy in the thermal storage Cooling energy in the thermal storage xergy present in the hot liquid absorbed during Corresponding author: Andrea Vallati, assistant professor, researh fields: energy saving, buildings performane, heat exhange. the ompression phase (kj/kg) x yl,e xergy present in the old liquid, due to the ooling expansion phase (kj/kg) x yl,a xergy absorbed by the liquid during the thermal exhange with the air after the ompression (kj/kg) x yl,ae xergy present in the liquid during the thermal exhange with the air after the expansion (kj/kg) η I,CAS Thermodynamial effiieny of the CAS, only mehanial power (%) η pol,cas Polygenerative effiieny of the CAS, mehanial power and heat (%) η ex,cas Total exergy effiieny (%) L e L L ex, L ex,e 1. Introdution Speifi ompression work (kj/kg) Speifi expansion work (kj/kg) xergy destroyed during the ompression phase (kj/kg) xergy destroyed during the expansion phase (kj/kg) The global demand for energy is rising fast and this fat leads to a rapid onsumption of a traditional fossil

2 698 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage fuel. To address issues related to fossil fuel onsumption, renewable energy resoures, are quikly developing in reent years. However, the intermitteny of renewable power soures, suh as wind and photovoltai, presents a great obstale to their extensive penetration into the power grid. In order to enhane the utilization of the renewable energy soures for power generation, energy storage solutions are indispensable. In fat, the interest in eletrial energy storage systems is due to their ability in deoupling the eletri power generation phase and the onsumption phase [1, 2]. To solve this problem, different energy storage tehnologies suh as pumped hydro, ompressed-air, battery, flywheel and TS (thermal energy storage) systems are explored and developed. Compared to pumped hydro energy storage, CAS (ompressed air energy storage) with artifiial vessel is less dependent on loal topography; ompared to battery, flywheel, apaitor, superapaitor and superonduting magneti, disharge time of CAS is longer, disharge loss is smaller and power rating is bigger; ompared to thermal energy storage, available energy effiieny is higher beause available energy of heat is very small [3]. This kind of tehnology is onsidered as a most promising storage option due to a very high reliability, environmental friendliness, eonomi feasibility, and safe and simple operation [4, 5]. Historially, CAS has been deployed for grid management appliations suh as load shaving, load following, load shifting and regulation. Thus, this tehnology has been applied for large-size power plants. Reently, the attention seems to be also devoted to small-size and/or innovative appliations as onfirmed by the tehnial literatures [6-12]. Baquari and Vahidi [6] proposed a ase study of a S-CAS (small-ompressed air energy storage) system in Iran ities. The S-CAS system is operated in low pressure (not more than 10 bars), so it an be used everywhere. It onsists of a ompressor for hanging the air into the reservoir and a turbo-expander. An indution motor/generator is onneted through luthes to the ompressor and the turbine. During onsumption periods, the CAS system is operated in disharging mode and the ompressed air is released from the reservoir to the expander in order to provide the needed power. They alulated the total effiieny of the system as funtion of the pressure ratio (pressure of the initial state and final state) and showed that, in the range from 3 to 9, the effiieny was not notieable but after 10 it was high ( ). Kim and Favrat [7] performed energy and exergy analyses of different types of miro-cas systems and proposed some innovative ideas for ahieving high effiienies from these systems. They onsidered the possibility of using both the dissipated heat of ompression for heating load and the ompressed air for the power generation and the ooling load. The authors analyzed eight CAS system onfigurations in whih the storage pressure was fixed at 50 bar. These onfigurations are haraterized by: (a) quasi-isothermal or adiabati ompressions and expansions; (b) one or two ompression and expansion stages; () with or without fuel addition for heating the ompressed air before its expansion. The results of their study highlighted that a miro-cas system, espeially with quasi-isothermal ompression and expansion proesses, is a very effetive system for distributed power networks, beause it an be a ombination of energy storage, generation, air-yle heating and ooling system and it has a good effiieny (about 60%). A further innovation proposed by the authors regards harging and disharging proesses of a high-pressure vessel where the pressure ratios are hanging. They onsidered a new storage system that ombines a onstant-pressure air storage and a hydrauli energy storage; as matter of fat, the system produes the required large pressure differene by means of a water olumn. Alami [12] presented an experimental evaluation of two ompat energy storage devies direted towards wind energy storage appliations. The

3 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage 699 CAS system, that was onsidered as isothermal beause maximum pressure is not exeed 3 bar and has an overall effiieny that is funtion of the ylinder pressure with its maximum value of 84.8%. The maximum theoretial effiieny for the buoyany work system is found to be around 36%. The systems onsidered in the analysis were ompat in size but the results obtained in terms of performane parameters, suh as effiieny and eletrial output, an be generalized as a storage option for real offshore wind farms. A new CAS refrigeration system proposed by Wang et al. [13] onsists of a gas refrigeration yle and a vapor ompression refrigeration yle. Based on the miro-cas and thermal energy storage, the tri-generation system suggested by Li et al. [10] an release energy via three means: eletrial, heating and ooling power. The hybrid wind diesel CAS system for remote areas proposed by Ibrahim et al. [14, 15] an heighten diesel power output, inrease engine lifetime and effiieny and, at the same time, redue fuel onsumption and greenhouse gas emissions. In Italy, Jannelli [16] proposed a small sale adiabati CAS, integrated with a thermal energy storage unit with inter-ooling ompression and inter-heating expansion. This onfiguration, with a ompressor size equal to 3.7 kw provides a storage system effiieny equal to 57%. It should be noted that the disharged air temperature of all CAS systems presented in those referenes is high, therefore a onsiderable amount of thermal energy is lost. In order to solve this problem, an innovative tri-generation CAS system is proposed in this paper. Heat produed in ompression proess is stored and used as heating energy. Compressed air diretly expands in the turbine motor to do work and to produe ooling energy. Unlike other CAS systems, a preheating proess is not required in the proposed CAS and the low-temperature disharged air an be used as ooling energy instead of abandoned. Therefore, the proposed system an simultaneously provide mehanial energy, heating and ooling energy. In Ref. [17] the authors show that, in a tri-generation CAS system, using the air as ooling power, an improve the total energy effiieny of the system by 20-30%. In order to researh the system performane of the proposed tri-generation CAS system, the orresponding thermodynami analysis is arried out and the orresponding theoretial additional effiieny is also alulated in this paper. 2. System Desription Fig.1 shows the shemati onfiguration of the miro CAS proposed in this paper. It is a high-pressure ompressed air storage system (50 bar); it is a small-sale (3 kw) with quasi-isothermal expansion and ompression and a onstant volume storage (1 m 3 ). As an be seen in Fig. 1 the CAS, separate the ompression and expansion yle into two different proesses, allowing the energy storage as ompressed air. The exeution time of the two phases, harge-disharge, is important beause it must onsider the load and supply requirements of energy. During low demand periods, the ompressed air is stored in a vessel. To extrat the energy stored, the ompressed air is taken from the storage tank by means of some valves regulating the output flow and is expanded in the turbine by ativating the eletrial generator whih restores part of the eletri energy previously absorbed. Moreover, the hoie of quasi-isothermal ompression/expansion was due to both the thermodynami advantages and a possible thermal reovery in these two main phases. The first aspet: a quasi-isothermal ompression/expansion (risson yle), allows a higher effiieny than an adiabati proess (Brayton yle). This is due to the fat that the result is a lower work during the ompression phase and a higher work during the expansion phase. The seond aspet: the heat reovered from ompression phase an supply a thermal load, while during the expansion phase, without preheating the gas (on the other hand it is something that happens in onventional

4 700 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage Fig. 1 Layout of proposed miro-cas system. CAS or adiabati systems), the output fluid an be used for the ooling proess of an environment, thus generating a tri-generation system. 3. Methodology We propose a new system that ombines two energy onversion proesses: storing eletriity by generating ompressed air and driving the turbine motor to generate mehanial energy and low temperature disharged air. Therefore it is neessary to establish a thermodynami model and to analyze the energy effiieny of the proposed system. To simplify the theoretial analysis, the following assumptions are used: (a) air an be regarded as ideal gas and meets ideal gas state equation, (b) inompressible, nonvolatile liquid, () no pressure drops, (d) all omponents operate adiabatially (exept the heat exhangers), (e) perfet separation, (f) perfet mixing, (g) ombined liquid and gas flows are in thermal equilibrium. 3.1 Quasi-Isothermal Compression/xpansion The analysis of the ompression/expansion of a gas ontaining the liquid, is based on the model proposed by Hugenroth [18, 19]. This model is used by Bell et al. on the ompression/expansion flooded in a sroll-type volumetri mahine [20, 21]. To obtain this kind of ompression/expansion a big amount of water finely atomized is mixed with the air and inserted into the mahine. The thermal energy reovered during both phases an be sent to a storage, neessary to supply a thermal/ooling load [7]. An analytial ompressor model was developed by examining the behavior of an ideal gas that is in ontat with a liquid and defining for properties of a pseudo-gas that haraterizes the mixture. Using these properties, standard thermodynami relations an be used to determine the ompression power and temperature hange for the ompressed gas. Then the ompression/expansion proess of an ideal gas, with a speifi heat ratio of k

5 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage 701 ( p / v ) and in a thermal balane with a liquid, with a onstant speifi heat ( l ) and a behavior similar to the gas, an be examined through a ratio between speifi heats onerning the mixture, whose value is k*. k * * p, g * v, g mg m g p. g v, g m l m l l l (1) The ompression proess an be onsidered as isentropi where k will be substituted with k*. But first here is the list of the hypothesis adopted: (a) the ompression proess was assumed to our in a stationary ondition, (b) potential and kineti energy are minimum and hemial or nulear reations are absent, () the ompressor and expander isentropi effiieny is 80%, (d) the eletrial generator and motor effiieny is 90%, (e) the proess is divided into two stages with the same pressure ratio for a β TOT = 50. A higher gas temperature, aused by the proess, an be defined through the typial isentropi relation as showed in the following expression: T T o k * 1 * k (2) 3.2 Compressed Air Storage This stage is very important beause it determines the end pressure of the ompression phase one the storage is full. Furthermore, in the disharge step, aording to the load requirements, it is possible to adjust the output flow and onsequently the disharge time. The seleted volume is 1 m 3 with a storage pressure of 50 bar. These data represent a good ompromise between energy density, ost, effiieny of the system and all the problems related to the size [20]. 3.3 Thermal Storage The thermal storage system used for the model is sensible heat type. The thermodynami and energeti relations regulating heating and ooling proesses are basially the same. The only differene an be found in the heat transfer liquid. For the hot-side (ompression) is simply water, whereas for the old-side it will be mixed to an anti-freeze substane that tends to derease the freezing point like an aqueous solution at 56% of 1-, 2-ethanol whih presents a freezing point of 50 C [22]. The thermal energy stored is the result of the algebrai addition of the ontributions of positive energy in input and negative one in output. Assuming that the latter is ignored, there is no onneted load and the tank is onsidered adiabati (dq = 0). The last hypothesis onsidered the energy amount stored in a short time period and then diretly used. Then, for the miro CAS system proposed in this paper, the following parameters are to be onsidered: during the ompression and heat exhange proess, air at pressure p 0 and temperature T 0 from ambient onditions is ompressed by the air ompressor. After this proess eletrial energy is onverted into the potential energy and thermal energy. After the heat exhange proess, hot water at temperature T w is stored in the TS, and the ompressed air at temperature T and pressure p is stored in the vessel for future use. When there is energy demand, ompressed air with potential energy is released from the vessel. During the expansion proess of ompressed air in the turbine, mehanial energy an be generated. Simultaneously, low-temperature disharged air at temperature T e and pressure p e is also generated. The disharged air an be ompletely used as ooling power. 4. nergy and xergy Analysis In the studied system is the energy absorbed by the system during the harging phase. During the disharging phase the energy pressure is transformed into eletrial energy, the energy transferred from the system is e. Then the eletri effiieny of the total storage will be expressed as follows: e I, CAS (3) It is possible to use the heat dissipated during the

6 702 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage ompression phase to supply a thermal load and use the old gas in output from the expander, beause no fuel is used to meet ooling demand. The system performanes onerning this thermal aspets (ompressor) an be stressed as follows: Qh h (4) The performanes onerning the ooling proess (turbine) are: Qe (5) Finally it an be expressed a polygenerative effiieny that takes into onsideration the useful effet determined by the thermal storages added to the one aused by the energy transferred from the turbine. Suh ondition an be obtained by omparing the energy present in thermal storages with the eletri energy required by a heat pump to produe them, as the following relations [23]: (6) pol, CAS e el, H el, C Using the first and seond law of thermodynamis is possible to make a omparison between the different forms of energy and identify the onditions of more effiient use of energy soures. The global exergy effiieny an be estimated by alulating the ratio between the exergy transferred and the one absorbed by the system. The one in input only onerns the energy absorbed during the ompression phase ( + ). The one transferred from the system is the total of the one transferred from the turbine ( - e ) and the one present in the hot liquid absorbed during the ompression phase (x - yl, ) plus the one absorbed by the liquid during the thermal exhange with the air after the ompression (x - yl,a ). Moreover it must be added the exergy present in the old liquid, due to the ooling during the expansion (x - yl,e ) and the one present in the liquid during the thermal exhange with the air after the expansion (x - yl,ae ). The total exergy effiieny of the system, while taking into onsideration what was previously said, an be expressed as follows: e xyl, xyl, a xyl, e xyl, ae (7) ex, CAS 5. Model Validation It is very important to validate the mathematial model to ensure the adequay and validity of the obtained results. The system at issue here is new and therefore the validation proess was onduted by omparing it with other similar works, that are, to be more speifi, the publiations of Kim et al. [7, 23, 24]. Both systems present the same onfiguration and are two tri-generation miro-cas systems where the proesses of ompression and expansion are in quasi-isothermal ondition. Table 1 shows some important energeti parameters, useful to validate our model. In partiular the eletri energy absorbed by the ompressor ( + ) and the one transferred from the turbine ( - e ), whose ratio determines the system effiieny (η I,CAS ). Table 2 shows the ompression ratio (β), the final ompression temperature (T ) and the one of the expansion (T e ), the speifi ompression work (L ) and the one of the turbine (L e ). There is good agreement between the results of our model and the Kim s model results. The different output temperatures whih haraterize the two phases are determined by a different relationship between the liquid and the gas of the mixture. What has been said earlier auses small differenes between the values of the speifi works, thus affeting the effiieny of the system. The differenes that an be notied among the values onerning the thermal energy transferred (ompression) Table 1 Comparison of the energy indexes of the systems. Our model results Kim s results η I,CAS (%) 52% 57% Q h (kj/kg) η h (%) 75% 80% Q e (kj/kg) η (%) 72% 74% - e (kwh) (kwh)

7 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage 703 Table 2 Comparison between speifi works and maximum heat exhanged during both phases. Our model results Kim s results β 50 (7.1 & 7.1) 50 (7.1 & 7.1) T ( C) L (kj/kg) T e ( C) -8-6 L e (kj/kg) Table 3 Comparison of the speifi exergy results. Our model results Kim s results x + (kj/kg) L ex, (kj/kg) x e - (kj/kg) L ex,e (kj/kg) x - yl, (kj/kg) x - yl,a (kj/kg) x - yl,e (kj/kg) x - yl,ae (kj/kg) η ex,cas (%) 62% 68% and subtrated (expansion) from the liquid phase of the mixture are determined by the model omparison whih imposes the effiieny values of thermal exhange (80% ompression 74% expansion) [21]; instead, in this study, they are provided by the mathematial model of the system. The final steps of the validation proess are the omparison onerning the exergy analysis that determines the quality of the energy, performed by omparing the energy onservation and its hange over time thus ausing the degradation of the energy, due to the irreversibility of the proess. Table 3 shows the exergy balane of the system with an effiieny exergeti evaluation (η ex,cas ). 6. Results and Disussion The mathematial model implemented was used to assess the energy features of a 3 kw CAS, with storage apaity of 1 m 3 and a pressure of 50 bar. Figs. 2a-2 show the main harateristis of the system whih have been already reported in the previous tables. As it an be seen the performane inreases by 20% when onsidering not only the mehanial power effiieny but the ombined heat and mehanial power effiieny. This result is in line with those found by Liu and Wang [17] and they strongly suggest how quasi-isothermal ompression and expansion proesses are very helpful to minimize exergy loss in heat transfer proess and how they result effetive for a miro CAS system installed to meet a residential heating and ooling demand. To understand how this system ould be inserted into an integrated plant of a residential onfiguration, the harge and disharge times of the system were evaluated too. The system was examined in an operating period of a whole day during whih a harge/disharge yle was performed in 1.88 h for the storage phase (Fig. 3) and 1.08 h to perform the full disharge of the system (Fig. 4). Moreover the system absorbed 5.34 kwh to exeute the harge phase; it is important to underline that the energy required by this proess was thought it would be provided almost entirely by some kind of renewable energy soure, thus atually reating a tri-generation system. The trend of temperature in the thermal storage is shown in Fig. 5. During the disharge phase there was a release of 2.81 kwh for a round-trip effiieny of 52%. The thermal storage determined by the heat subtrated during the ompression phase was of 1.89 kwh while the refrigeration energy stored during the expansion phase was of 1.39 kwh. While examining these two aspets representing the peuliarity of the system, if ompared to other types of storage suh as batteries, it an be notied how the power effiieny reported a net value of 70%. However ompression air systems an offer some advantages respet to onventional batteries whih require preious and expensive materials (haraterized by a short servie life), an effiieny affeted by the operating onditions and high dissipation osts.

8 704 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage (a) nergy results (b) xergy results () Polygenerative system results Fig. 2 Summary of the model simulation results.

9 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage 705 Fig. 3 Trend of eletrial power during ompression phase. Fig. 4 Trend of eletrial power during expansion phase. Fig. 5 Trend of temperature in the thermal storage.

10 706 A New Tri-Generation System: Thermodynamial Analysis of a Miro Compressed Air nergy Storage 7. Conlusions In this paper a new tri-generation system based on quasi isothermal ompression in a ompressed air energy storage is presented. Compared to other traditional CAS systems, the proposed CAS system has several advantages. First, the proposed system an simultaneously provide mehanial energy, heating and ooling energy. Seond, disharged air of the turbine in the proposed system is not abandoned, instead it is reused ompletely as ooling power. Thermodynami and energeti analysis of the proposed system have been arried out and the thermodynami model was validated by omparing it with the Kim s model. Some important onlusions an be drawn from the analysis of the system with the validated thermodynami model. The results indiate that using disharged air as ooling power an improve the total energy effiieny of the CAS system by 20% in theory. Therefore, this system is very important and worthy of being onsidered for residential and where the environmental aspet is important. Referenes [1] Tan, W. S., Hassan, M. Y., Majid, M. S., and Rahman, H. A Optimal Distributed Renewable Generation Planning: A Review of Different Approahes. Renewable and Sustainable nergy Reviews 18 (February): [2] Mahlia, T. M. I., Saktisahdan, T., Jannifar, A., Hasan, M., and Matseelar, H A Review of Available Methods and Development on nergy Storage; Tehnology Update. Renewable and Sustainable nergy Reviews 33 (May): [3] Akinyele, D. O., and Rayudu, R. K Review of nergy Storage Tehnologies for Sustainable Power Networks. Sustainable nergy Tehnologies and Assessments 8 (Deember): [4] Foley, A., and Lobera, I. D Impats of Compressed Air nergy Storage Plant on an letriity Market with a Large Renewable nergy Portfolio. nergy 57 (August): [5] Lund, H., Salgi, G., lmegaard, B., and Andersen A. N Optimal Operation Strategies of Compressed Air nergy Storage (CAS) on letriity Spot Markets with Flutuating Pries. Applied Thermal ngineering 29 (5-6): [6] Baqari, F. F, and Vahidi, B Small-Compressed Air nergy Storage System Integrated with Indution Generator for Metropolises: A Case Study. Renewable and Sustainable nergy Reviews 21 (May): [7] Kim, Y. M., and Favrat, D nergy and xergy Analysis of a Miro-Compressed Air nergy Storage and Air Cyle Heating and Cooling System. nergy 35 (1): [8] Zhao, P., Dai, Y., and Wang, J Design and Thermodynami Analysis of a Hybrid nergy Storage System Based on A-CAS and FSS for Wind Power Appliation. nergy 70 (June): [9] Bagdanaviius, A., and Jenkins, N xergy and xergoeonomi Analysis of a Compressed Air nergy Storage Combined with a Distrit nergy System. nergy Conversion and Management 77 (January): [10] Li Y., Wangb X., Li D., and Ding Y A Trigeneration System Based on Compressed Air and Thermal nergy Storage. Applied nergy 99 (November): [11] Grazzini, G., and Milazzo, A Thermodynami Analysis of CAS/TS Systems for Renewable nergy Plants. Renewable nergy 33 (9): [12] Alami, A. H xperimental Assessment of Compressed Air nergy Storage (CAS) System and Buoyany Work nergy Storage (BWS) as Cellular Wind nergy Storage Options. Journal of nergy Storage 1 (June): [13] Wang, S., Chen, G., Fang, M., and Wang, Q A New Compressed Air nergy Storage Refrigeration System. nergy Convers Manage 47 (18-19): [14] Ibrahim, H., Younes, R., Ilina, A., Dimitrova, M., and Perron, J Study and Design of a Hybrid Wind Diesel Compressed Air nergy Storage System for Remote Areas. Applied nergy 87 (5): [15] Ibrahim, H., Younes, R., Basbous, T., Ilina, A., and Dimitrova, M Optimization of Diesel ngine Performanes for a Hybrid Wind Diesel System with Compressed Air nergy Storage. nergy 36 (5): [16] Jannelli,., Minutillo, M., Lubrano Lavadera, A., and Falui, G A Small Sale CAS (Compressed Air nergy Storage) System for Stand-Alone Renewable nergy Power Plant for a Radio Base Station: A Sizigi-Design Methodology. nergy 78 (Deember): [17] Liu, J. L., and Wang, J. H Thermodynami Analysis of a Novel Tri-Generation System Based on Compressed Air nergy Storage and Pneumati Motor.

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