Thermodynamic Analysis of Compressed Air Energy Storage under. Various Ambient Temperature

Transcription

1 7:2 (2015) Journal of Modelng and Optmzaton Thermodynamc Analyss of Compressed Ar Energy Storage under Varous Ambent Temperature Qng He, Hu Lu School of Energy Power and Mechancal Engneerng, North Chna Electrc Power Unversty, 2 Benong Road, Chang Png, Bejng , Chna Emal:hqng@163.com Abstract: In ths paper, effects of ambent temperature on the overall performance and economc benefts of the compressed ar energy storage (CAES) system were nvestgated. Our results revealed that the CAES performance (e.g. effcences of energy, exergy and energy storage) and economc benefts both decrease wth the ncrease of ambent temperature. The economc benefts could be mproved by ncreasng operaton tme untl the desgned maxmum pressure nsde storage cavern s fulflled. To counteract the negatve nfluence of temperature, the performance of CAES system and economc benefts could be mproved by choosng approprate compressor pressure rato durng the desgn process. Keywords: Compressed ar energy storage system; Ambent temperature; Economc benefts; Chargng tme; Compressor pressure rato. 1. Introducton Renewable energy sources (e.g. solar energy and wnd energy) show remarkable uncontrollable characterstcs wth ntermttence and fluctuatons. When the electrcty generated by those erratc renewable energes s nput nto the electrcal grd, t may affect both the qualty and the plannng of power systems. In order to solve ths dlemma, energy storage system, e.g. compressed ar energy storage (CAES) system, s of great mportance for controllng stochastc renewable power plant output [1]. Currently, there are two CAES plants operatng n the world [2] wth one located at McIntosh, Alabama, USA and another located at Huntorf, Germany. In CAES system, ambent ar s compressed and stored n large underground caverns durng off-peak perod of energy consumng. When the peak tme for energy consumng comes, the compressed ar s heated n the combuston chambers and then s fed nto the gas turbne to generate electrcty [3-4]. In order to mprove the CAES performance, ntensve studes have been performed. Recently many novel CAES systems have been proposed. For example, n order to recover the waste heat, Safae et al. [5-6] proposed a dstrbuted compressed ar energy storage (D-CAES) by dstrbutng compressors near heat loads to recover the heat generated durng the compresson stage. A recuperator was used to capture the heat from exhaust of the turbne [7]. Other new system nclude tr-generaton system based on compressed ar and thermal energy storage [8], bomass fuelled CAES [9]. Related thermodynamc and economc analyss of those new systems also have been performed. Najjar and Zaamout [10] analyzed the effects of dry regons on performance of compressed ar energy 14

2 Journal of Modelng and Optmzaton 7:2 (2015) storage plant. Proczka et al. [11] analyzed the effects of pressure and effcent szng of pressure vessels on the cost of compressed ar energy storage. Zhang et al. [12-13] analyzed the thermodynamc effect of thermal energy storage and ar storage chamber model on the compressed ar energy storage system. Jubeh and Najjar [14] and Hartman et al. [15] explored the effects of ambent temperature on the thermal effcency. The ambent temperature of CAES system fluctuates sgnfcantly wth the tmes and locatons. Takng chna as an example, there are fve temperature zones n total wth great changes of geographc and clmate condtons. Fgure 1 shows the monthly averaged temperatures of the representatve cty n the fve zones. Fgure 2 shows the hourly averaged temperature n Bejng [16]. Because, the ambent temperature has great effects on the performance of compressor and turbne [17-20], t s badly needed to nvestgate the ambent temperature on the CAES system. However, to our knowledge, there are stll very lmted researches relatng wth the effects of the ambent temperature on the performance of the CAES. In ths paper, both thermodynamc and economc analyss of the CAES s conducted to nvestgate the effects of ambent temperature on the CAES system. The performance of the analyss model was nvestgated on the bass of thermal, exergy and storage effcency. And the economc analyss s conducted by consderng the operatng costs and benefts. Fnally the effects of chargng tme on the CAES performance and economc benefts were also dscussed n order to counteract the nfluences of the dfferent ambent temperature. Fgure 1 Temperature graph for Chna Fgure 2 Hourly averaged temperature for Bejng n Chna 15

3 7:2 (2015) Journal of Modelng and Optmzaton 2. Thermodynamc analyss In order to analyze the CAES performance, the followng assumptons have been adopted [12]: (a) Each stage of the compressor and turbne s assumed to work at a steady state wth a steady flow. There s no chemcal or nuclear reacton n the storage cavern, and the potental and knematc energy effects are also neglgble. (b) For smplcty, the performance calculated n present paper does not nclude the effect of power consumpton n the cooler. It s also assumed that there s no pressure drop n the heater exchangers. 2.1 Compressor model Assumng that compresson process s polytrophc, the specfc work of the compressor ncludng effcency can be calculated by [15], n1 1 1 n 1 n p 1 w vd p RgT 1 (1) c, c, n1 p, where w s the specfc power consumpton of compressor; n s the polytrophc ndex; R g s the characterstcs gas constant; p and T s the nlet pressure and temperature of the -stage compressor; p +1 s the outlet pressure of the -stage compressor, v s the gas volume; η c, s the polytrophc effcency of the -stage compressor and can be calculated by the emprcal equaton [17], rc, c, 0.91 where P rc, s the pressure rato of the -stage compressor. P 1 (2) 300, 2.2 Turbne model The turbne can be seen as an opposed thermodynamc process of the compresson and the power generated ncludng effcency can be calculated by [15], w p v pv R T n1 1 1 n n d v n p 1 t, d t, t, g 1 n v n 1 p where η t, s the -stage turbne effcency and t can be calculated by the emprcal equaton [17],, (3) t, P 1 (4) 250, rt, 0.9 where P rt, s the pressure rato of the -stage turbne. 2.3 Storage model The temperature and pressure nsde the compressed ar storage can sgnfcantly affect the operatng hours 16

4 Journal of Modelng and Optmzaton 7:2 (2015) and the storage volume [18-19]. Accordng to the underground cavern model developed by Raju and Khatan [18], the pressure and temperature varatons n the cavern can be calculated by, d dt m m n out V, (5) dt m dp c c T T h T T dt V dt n p p a, n eff wall 0, (6) where ρ s the densty of the compressed ar n the cavern storage; the compressor; out m n s the mass flow rate of the nlet ar from m s the mass flow rate of the nlet ar to the turbne; V s the volume of the cavern storage; h eff s the effectve heat transfer coeffcent; T wall s the wall temperature n the cavern; T a,n s the temperature of the ncomng ar from the compressor. 2.4 Impact on the system performance In order to evaluate the effects of ambent temperature on CAES performance, varatons n the thermal effcency, exergy effcency and storage effcency are dscussed. The thermal effcency (η) of CAES can be calculated by, Wt W p 100%, (7) where η s the thermal effcency; W p s the electrcty generated; W p s the energy nput. Exergy effcency (ψ) can be calculated by, E E e p 100%, (8) where E e s the electrc exergy generated; E p s the exergy nput. Storage effcency (η storage ) can be calculated by, storage W out Qft 100% W n, (9) where η t s the gas turbne effcency, whch can be calculated by, 17

5 7:2 (2015) Journal of Modelng and Optmzaton t c 1 cpa Ta ka1 ka T 1 1 Pr 1 k g kg P rt c 1 ka1 k a cpb Tn Ta 1 Pr c c p n t c (10), where w fuel s the electrcal power generaton of the typcal gas turbne under the same heat; η c s the compressor effcency; c pb s the average specfc heat n combuston chamber; T a s the ambent temperature; k g s the combuston gases heat rato; k a s the ar specfc heat rato. The ambent temperature manly affects on the CAES operatng process hence we just calculate the operatng costs and economc benefts. The economc benefts of CAES can be calculated by, E E E E E (11) s p f m& p where E s the CAES operatng economc benefts; Es s the ncomes of sellngelectrcty; Ep s the costs of purchasng electrcty; Ef s the costs of nature gas; Em&p s the costs of CAES mantanng and operatng. 3. Overall performance of CAES system In order to analyze the effects of ambent temperature on the CAES performance, models have been bult based on the CAES located at McIntosh, Amerca. The schematc dagram of CAES system s shown n Fg. 3 and the smulaton data s presented n table 1. The analytcal model manly conssts of compressors, turbne expanders, motor/generator and compressed ar storage cavern. Durng compresson process, the off-peak energy drves the compressors to produce compressed ar. Through the nter-coolng process, the compressed ar s cooled to a defned level and then stored n the compressed ar storage. In the expanson process, the compressed ar s heated n the combuston chambers. Fnally t expands n the turbnes to generate electrcty. The smulaton data of CAES system s set based developed by Bagdanavcus [20]. Durng the analyss, the total power consumpton for compressors s assumed as a constant (P s = MW h). The fuel consumpton rate and nlet ar mass flow rate of the gas turbne are also constants n the expanson process. Hence, the generated power s only decded by the compresson process. Fgure 3 Process of CAES system 18

6 Journal of Modelng and Optmzaton 7:2 (2015) Table 1 Smulaton data of CAES parameter value parameter value Compresson stage Expanson stage Ar mass flow rate 197 kg/s Ar mass flow rate 147kg/s Ambent pressure kpa Turbne nlet temperature 870 Ambent temperature 20 Turbne effcency 0.75 After all heat exchangers 50 Combuston effcency 0.98 Energy effcency of the heat exchanger 0.74 Total heat nput MW Compresson effcency 0.75 Fuel lower heat value (LHV ) kj/kg Electrcal nput MW Electrcty output MW Pressure rato energy effcency of recuperator st compresson 4.6 Storage 2nd compresson 3.7 Ar storage cavern volume 300,000 m3 3rd compresson 2.6 Max pressure n cavern 82 bar 4th compresson 2.1 Mn pressure n cavern 50 bar Total heat output 89.9 MW Wall temperature n the cavern 22 Fg. 4 llustrates the varaton n the power consumpton and generaton of the CAES n relaton to the ambent temperature at the same chargng tme (t c =16 h). The result ndcates that the ambent temperature has strong nfluence on the power consumpton and generaton of CAES system. For example, t can produce a bg fall on the power consumpton and generaton of CAES system from MW h to MW h and from MW h to 2007 MW h respectvely when the ambent temperature ncreases from 253 K to 323 K. The speed of changes of power consumpton and generaton rate versus temperature s 0.91 MW h/k and 3.72 MW h/k respectvely. Hence one can fnd that the power generaton s more senstve to the change of temperature. Fgure 4 Varatons of the CAES power generaton and consumpton versus the ambent temperature Fg.5 shows the varatons of CAES performance versus ambent temperature. One can fnd that the performance of CAES s greatly affected by the ambent temperature. Wth the ncrease of the temperature, all knds of effcency wll reduce. When the temperature ncreases from 253 K to 323 K, the thermal, exergy and 19

7 7:2 (2015) Journal of Modelng and Optmzaton storage effcency have been reduced by 1.63%, 1.56% and 4.25% respectvely. The lop rate of the thermal and exergy effcency are bascally the same, whle the lop rate of storage effcency s much larger than those of the thermal and exergy effcency. Fgure 5 Varatons of CAES performance versus the ambent temperature Economc ncomngs are another crtcal parameter for the development of CAES. Hence, an analyss of the mpact of temperature on the economc performance of CAES s conducted. The ambent temperature manly affects on the CAES operatng process, so we just calculate the operatng costs and economc benefts. The man economc parameters are shown n Table 2. Table 2 Man economc parameters for the CAES Parameter Value (unt) Nature gas cost 0.34 $/m 3 Electrcty purchase prce 0.05 $/kw h Operaton &mantan cost 2.92 $/MW h Electrcty sale prce 0.15 $/kw h Fg.6 shows the varaton of CAES economc benefts versus ambent temperature. From Fg. 6, we can know that the economc benefts decrease wth the ncrease of the ambent temperature. When the ambent temperature ncreases from 253 K to 323 K, the economc benefts drops by $. Fgure 6 Varatons of the economc benefts versus the ambent temperature 20

8 Journal of Modelng and Optmzaton 7:2 (2015) 4. Dscussons Through the above analyss, t s obvous that wth the ncrease of the ambent temperature, both the CAES performance and economc benefts decrease. Ths s not acceptable and some method should be employed to counteract the negatve nfluence of ambent temperature. The operatng parameters (e.g. chargng tme and compressor pressure rato) play an mportant role durng the CAES performance. Hence, n the followng secton, the effects of chargng tme and compressor pressure rato on the CAES performance and economc benefts under dfferent ambent temperature are shown. Fg. 7 shows the effects of the chargng tme on CAES economc benefts. Accordng to Fg. 7, one can conclude that: The varatons of economc performance of the CAES are the same for dfferent ambent temperature. The ncrease of the operaton tme can enhance economc benefts n the compresson process. The varaton of the chargng tme has sgnfcant effects on the CAES economc benefts. For the ambent temperature 313 K and the chargng tme 15 hours, the economc benefts s $, whereas for the ambent temperature 253 K and chargng tme 21 hours, the economc benefts s $. That s, a varaton of $ n the economc benefts was obtaned wth ncreasng chargng tme from 15 hours to 21 hours. Fg. 7 Varatons of the economc benefts versus the chargng tme Although the economc benefts can be mproved by ncreasng operaton tme n the compresson process, the operaton tme s lmted to the parameters of the storage cavern. The storage cavern has a maxmum desgned pressure for the safe operatons. When the pressure nsde the storage cavern reaches the maxmum pressure, the CAES wll stop operaton. We assume that the maxmum pressure n the storage s 80 bar and the mnmum pressure s 30 bar. The varaton of chargng tme versus the ambent temperature s shown n Fg. 8. After hours of chargng, the pressure nsde the storage cavern reaches the maxmum 80 bar when the ambent temperature s 253 K. When the ambent temperature s 323 K, t needs hours to reach the maxmum pressure. Although the economc benefts can be mproved through ncreasng operaton tme, the thermal effcency, exergy effcency and storage effcency wll not be mproved. 21

9 7:2 (2015) Journal of Modelng and Optmzaton Fgure 8 Varatons of the chargng tme versus the ambent temperature 5. Conclusons The ambent temperature has sgnfcant effects on the CAES performance and economc benefts. The followng conclusons can be drawn: 1. The CAES performance and economc benefts all decrease wth the ncrease of ambent temperature. When the temperature ncreases from 253 K to 323 K, the thermal, exergy and storage effcency has been reduced by 1.63%, 1.56% and 4.25% respectvely and the economc benefts drop by $. 2. The economc benefts ncrease wth the ncrease of the operaton tme n the compresson stage. The varaton of the chargng tme could lead to a sgnfcant varaton of the economc benefts. References [1] Francsco Daz-Gonzalez, Andreas Sumper, Orol Goms-Bellmunt, et al. A revew of energy storage technologes for wnd power applcatons [J]. Renewable and sustanable energy revews, 2012, 16: [2] CAES-Located n Mclntosh, Ala, the110-megawatt Compressed Ar Energy Storage (CAES) faclty s PowerSouth s most unque generatng source. [3] Grazzn G, Mlazzo A. Thermodynamc analyss of CAES/TES systems for renewable energy plants [J]. Renewable Energy, 2008, 33(9): [4] Foley A, Lobera I D. Impacts of compressed ar energy storage plant on an electrcty market wth a large renewable energy portfolo [J]. Energy, 2013, 57: [5] Hugo R, Keth D, Safae H. Dstrbuted compressed ar energy storage system and method. USA: Unted States Patent and Trademark Offce; 2012, PCT/US12/ [6] Safae H, Keth D W. Compressed ar energy storage wth waste heat export: An Alberta case study [J]. Energy Converson and Management, 2014, 78: [7] L Y, Wang X, L D, et al. A trgeneraton system based on compressed ar and thermal energy storage [J]. Appled Energy, 2012, 99:

10 Journal of Modelng and Optmzaton 7:2 (2015) [8] Karellas S, Tzouganatos N. Comparson of the performance of compressed-ar and hydrogen energy storage systems: Karpathos sland case study [J]. Renewable and Sustanable Energy Revews, 2014, 29: [9] Denholm P. Improvng the techncal, envronmental and socal performance of wnd energy systems usng bomass-based energy storage [J]. Renewable Energy, 2006, 31(9): [10] Najjar Y S H, Zaamout M S. Performance analyss of compressed ar energy storage (CAES) plant for dry regons [J]. Energy converson and management, 1998, 39(15): [11] Proczka J J, Muraldharan K, Vllela D, et al. Gudelnes for the pressure and effcent szng of pressure vessels for compressed ar energy storage[j]. Energy Converson and Management, 2013, 65: [12] Zhang Y, Yang K, L X, et al. The thermodynamc effect of thermal energy storage on compressed ar energy storage system [J]. Renewable Energy, 2013, 50: [13] Zhang Y, Yang K, L X, et al. The thermodynamc effect of ar storage chamber model on Advanced Adabatc Compressed Ar Energy Storage System [J]. Renewable Energy, 2013, 57: [14] Jubeh N M, Najjar Y S H. Power augmentaton wth CAES (compressed ar energy storage) by ar njecton or superchargng makes envronment greener [J]. Energy, 2012, 38(1): [15] Hartmann N, Vöhrnger O, Kruck C, et al. Smulaton and analyss of dfferent adabatc compressed ar energy storage plant confguratons [J]. Appled Energy, 2012, 93: [16] PSD Grdded Clmate Datasets: Surface Temperature [17] Yousef S.H. Najjar, Mahmoud S. Zaamout. Performance analyss of compressed ar energy storage plant for dry regons [J]. Energy Converson and Management, 1998, 39(1): [18] Raju M, Kumar Khatan S. Modelng and smulaton of compressed ar storage n caverns: a case study of the Huntorf plant [J]. Appled Energy, 2012, 89(1): [19] Hoffens H. Huntorf ar storage gas turbne power plant. Energy supply, Brown Bover Publcaton DGK E: [20] Audrus Bagdanavcus, Nck Jenkns. Exergy and exergoecomc analyss of a compressed ar energy storage combned wth a dstrct energy system [J]. Energy converson and management, 2014(77):