OPTIMAL PHASE CHANGE TEMPERATURE FOR BCHP SYSTEM WITH PCM-TES BASED ON ENERGY STORAGE EFFECTIVENESS

Transcription

1 OPIMAL PHASE CHANGE EMPERAURE FOR BCHP SYSEM WIH PCM-ES BASED ON ENERGY SORAGE EFFECIVENESS Yn ZHANG 1*, Xn WANG 2**, Ynpng ZHANG 2 1 College of Archtecture and Envronment, Schuan Unversty, Chengdu , Chna 2 Department of Buldng Scence, snghua Unversty, Bejng , Chna * Correspondng author; E-mal: cdzhangyn@163.com ** Correspondng author; E-mal: wangxnlj@tsnghua.edu.cn Integratng thermal energy storage (ES) equpment wth buldng coolng heatng and power (BCHP) system can mprove system thermal performance. In ths paper, a smplfed model of ES-BCHP system composed of a gas turbne, an absorpton chller and ES equpment wth phase change materals (PCM) s presented. o evaluate the energy savng effect of PCM- ES, a new ndex, energy storage effectveness, s proposed and ts relatonshp wth prmary energy consumpton s establshed. Amed at maxmzng the energy storage effectveness, the optmal phase change temperature of the PCM-ES-BCHP system s obtaned. he results show that the theoretcally optmal phase change temperature s just the geometrcal average value of ambent temperature and exhaust gas temperature from gas turbne for deal PCM-ES equpment wth nfnte NU. It also ndcates that both energy storage effectveness and optmal phase change temperature ncrease wth ncreasng NU. So mprovng the thermal performance of PCM-ES devce s favourable for ncreasng energy effcency and savng prmary energy consumpton accordngly. hs work s of great mportance n gudng the optmzaton desgn of practcal PCM-ES-BCHP systems. Key words: co-generaton, energy storage, phase change materal, thermal optmzaton, energy effcency 1. Introducton Wth the rapd development over the recent two decades, the global total energy consumpton has grown by 49% [1]. heren, buldngs account for about 30% of total energy consumpton and the percentage keeps ncreasng [2]. As a result, the ncreasng demand for coolng, heatng and power supples n buldngs appeals for resurveyng tradtonal energy systems and stmulates the search for more hgh-effcent and low-emsson energy producton, conservaton and utlzaton methods [3]. Buldng coolng heatng and power (BCHP) s a novel knd of buldng energy supply system whch can meet users dfferent load demands smultaneously wth a sngle prmary energy nput [4].

2 Compared to tradtonal separated generaton system, BCHP systems show hgh energy effcency, low pollutons emsson and good economc beneft [5]. However, the energy supply unts n a BCHP system often show poor thermal performance under part load workng condtons, due to the nonsynchronzed and fluctuatng thermal and electrcal demands [5-7]. It s found that ntroducng thermal energy storage (ES) equpment nto BCHP systems proves to be an effectve way to mprove the part load performance of the whole system and savng the prmary energy consumpton [8]. Many researchers nvestgated the thermal and economc performance of the co-generaton or tr-generaton system wth dfferent types of ES equpment [9]. Khan et al. [10] ntegrated heat accumulator wth combned heatng and power system to match the hot water supply and demand through the dynamc charge and dscharge processes of the ES equpment. Bogdan et al. [11] and Campos et al. [12] conducted smlar studes and found that proftablty of co-generaton system wthes equpment was mpacted by varous external factors. he results nferred that the water tanks mght substantally mprove the economc performance when electrcty prce was governed by the dual-tme tarff polcy. Furthermore, Katulc et al. [13] put forward a new approach to determne the optmal daly heat storage tank capacty for a co-generaton system. Fu et al. [14] establshed the dynamc smulaton model for stratfed water storage and conducted experment on the energy savng effect of combned coolng heatng and power system. Baley et al. [15] appled sensble ES equpment to the co-generaton system and optmzed ts nstalled capacty based on completely mxng assumpton for the water tanks. On the other hand, compared to sensble heat storage (e.g., water tank), latent heat storage wth phase change materal (PCM) s of relatvely hgh energy storage densty, whch makes them ncreasngly attractve for applcatons [16]. Pte et al. [17] presented the potental usage of PCM partcles for hgh temperature energy capture and storage n ndustry felds through a fludzed bed. Zhang et al. [18] brefly revewed the ES development, wth specal emphass on the mportant applcatons of PCMs n both solar energy projects and waste heat recovery from ndustral processes. Zeng et al. [19] ntegrated PCMs wth buldng envelopes and optmzed ts thermal physcal propertes, n order to mprove the ndoor thermal comfort and reduce the energy consumpton for passve buldngs. Forentn et al. [20] appled PCM-ES to HVAC systems and found that the PCM tank can effectvely shft the coolng load and ncrease the overall effcency of a heat pump system for space coolng. However, even though PCM-ES applcaton n BCHP system has a great potental for energy savng, relevant researches are not enough. Zhang et al. [21] proposed a new method to preestmate the feasblty of ES-BCHP system before desgn of practcal systems, under deal assumpton that there s no rreversble loss durng the heat transfer processes for the ES equpment. Chen et al. [22] evaluated the energy savng potental of BCHP system wth latent ES equpment based on case study of practcal fluctuatng user loads. Nevertheless, few researchers focused on the key parameter optmzaton, such as the phase change temperature of PCM-ES, even though t had a great nfluence on the performance of the whole PCM-ES-BCHP system. herefore, how to determne the optmal phase change temperature for the PCM-ES-BCHP system s an mportant but unsolved problem. In ths paper, a smplfed model of PCM-ES-BCHP system s establshed and the analytcal optmal phase change temperature s determned based on the proposed energy storage effectveness. Moreover, the mpact of NU of the PCM-ES equpment s analysed to evaluate the energy savng effect of the whole system. hs work can provde gudance for PCM-ES-BCHP system desgn.

3 Method 2.1. BCHP system he typcal BCHP system under summer workng condton s shown n Fg. 1. he gas turbne (G) s drven by natural gas and the mechancal energy s further changed nto electrcty power, whch s then delvered to the users drectly. At the same tme, the absorpton chller (AC), actvated by the hgh temperature exhaust gas, produces low temperature water to fulfl the coolng requrement. For the operaton strategy, eng et al. [23] found that followng thermal load (FL) was more energysavng than followng electrcal load (FEL) for BCHP systems. hus the system gves prorty to meet coolng demand, and nsuffcent electrcty can be bought from the power grd. Power grd Electrcty Natural gas Exhaust gas Coolng Gas turbne Absorpton chller Buldng users Fgure 1. ypcal BCHP system n summer workng condton So for the whole BCHP system, the total prmary energy consumpton (PEC) comprses two parts: the consumed natural gas by the gas turbne and the mported electrcty from the power grd. Q Q Q Q PEC QNG η [1 η ] COP η E, grd C E E, G grd G AC grd In Eq. (1), the bought electrcty from the power grd s converted to the equvalent heat value of correspondng prmary fuel (e.g., coal and natural gas) through the converson parameter η grd, the electrcty generaton effcency of the power plant [5] Waste heat utlzaton subsystem (WHUS) As Fg. 2 shows, n order to mprove the energy effcency under part load workng condtons, a ES devce wth PCM s nstalled between the gas turbne and the absorpton chller. Durng the charge process (.e., off-peak hours), hgh temperature exhaust gases (Q exhaust ) from the gas turbne flow nto the PCM-ES equpment for heat storage. Whereas durng the dscharge process (.e., peak hours), stored heat (Q ES ) s released and flows nto the absorpton chller to produce coolng water. (1) Q NG Gas turbne Q E, G PEC grd Electrcty Q E 110 Q exhaust PCM- ES Q ES Absorpton chller Waste heat utlzaton subsystem Coolng Q C

4 Fgure 2. Schematc dagram of a PCM-ES-BCHP system For the absorpton chller, from the perspectve of thermodynamcs, t can be regarded as a heat engne combned wth a heat pump, so that ts thermal performance hghly depends on generaton, evaporaton and condensaton temperatures [24]. o smplfed analyss, the nlet exhaust gas temperature ( AC, ), coolng water temperature ( w ) and ambent temperature ( a ) are substtuted for the generaton, evaporaton and condensaton temperatures of the absorpton chller, respectvely [5]. Hence, the coeffcent of performance (COP) of the absorpton chller can be obtaned by usng the smplfed thermodynamc model: COP X AC, a w AC, a w where X means the thermodynamc perfectness of the absorpton chller, whch represents the thermal performance dfference between practcal devce and deal one. As shown n Fg. 3, the nlet temperature of the absorpton chller just equals the outlet temperature of the PCM-ES equpment durng dscharge process ( AC, = ds,o ). herefore, the COP s hghly nfluenced by the heat transfer performance of the PCM-ES equpment. c, c, o (2) PCM ds, o ds, ch, m ds, o ch, o ds, 129 Absorpton chller Q Fgure 3. Heat transfer process n the PCM-ES equpment It can be seen that the nlet temperature of the PCM-ES equals the exhaust gas temperature ( ch, = exhaust ). he outlet temperatures depend on not only the phase change temperature ( m ) of the energy storage materal but also the heat transfer performance (NU) of the PCM-ES equpment. Accordng to the heat transfer model shown n Fg. 3, the outlet temperatures of PCM-ES durng charge and dscharge processes can be expressed by ( )[1 exp( NU )] (3) ch, o ch, ch, m ch ( )[1 exp( NU )] (4) ds, o ds, m ds, ds After ntroducng PCM-ES devce, the gas turbne can work steadly under rated condton n theory [21]. herefore, the total prmary energy consumpton s manly mpacted by the waste heat utlzaton subsystem (.e., PCM-ES and absorpton chller). o evaluate the overall energy

5 converson effect, the effcency of the waste heat utlzaton subsystem (η WHUS ) can be desgnated by the rato of output coolng to nput heat: η Q C WHUS (5) Qexhaust Integratng Eq. (5) wth Eq. (1), t can be obtaned that Q η 1 Q PEC PECG PECgrd (1 ) 1 η η η η C G E G grd WHUS grd In practcal engneerng felds, the coolng and electrcal loads (Q C, Q E ) are determned by users. he power generaton effcences (η grd, η G ) are determned by the power grd and chosen gas turbne respectvely. Moreover, n most stuatons, there s η G <η grd [23]. So from Eq. (6), t can be seen that PEC always decreases monotoncally wth ncreasng η WHUS Energy storage effectveness he waste heat utlzaton subsystem s ndeed an energy storage unt, where the stored heat s converted to the coolng water through the absorpton chller. he effectveness of such an energy storage unt can be defned as QC ηwhus Qexhaust QC QES COP ε η Q Q Q COP Q WHUS,max C,max C,max ES,max max exhaust It s the rato of the practcal provded coolng power to the theoretcally maxmal one for the energy storage unt. Furthermore, Eq. (7) ndcates that the defned energy storage effectveness s ndeed the product of two effcences: η Q ES 1, η2 QES,max COP (8) COP max η 1 means the rato of stored heatng power to ts maxmal one (.e., the heat storage capacty of deal ES equpment) and η 2 means the rato of practcal COP of the absorpton chller to ts maxmal one (.e., often the rated COP). Accordng to the system process (Fg. 2), by combnng Eqs. (2) to (4) wth Eq. (8), there are ( ch, m ) [1 exp( NUch)] η1 ch, AC, o (6) (7) (9)

6 η 2 ( ) [(1 exp( NU )] ds, m ds, ds a ds, ( m ds, ) [(1 exp( NUds )] ch, ch, a So the defned energy storage effectveness of energy storage unt can be changed nto ( ) [1 exp( NU )] ( ch, m ) [1 exp( NUch] ds, ( m ds, ) [1 exp( NUds )] ε η1 η2 ch, a ( ch, AC, o) ds, m ds, ds a ch, (10) (11) It s clear that the phase change temperature ( m ) have mpacts on both η 1 and η 2. For gven energy supply devces (gas turbne and absorpton chller), the maxmal heat storage capacty and the rated COP are all known and η WHUS,max s a constant value. Accordng to Eq. (7), energy storage effectveness (ε) ncreases monotoncally wth ncreasng η WHUS. As a consequence, there s mn PEC max η max ε (12) WHUS In other words, for the PCM-ES-BCHP system optmzaton, mnmzng the prmary energy consumpton s just equvalent to maxmzng the overall energy converson effcency of the waste heat utlzaton subsystem, also equvalent to maxmzng the defned the energy storage effectveness. 3. Results Based on the prevous analyss and establshed model, the energy storage effectveness (ε) s the functon of phase change temperature ( m ), ambent temperature ( a ), nlet temperatures ( ch,, ds, ) and numbers of heat transfer unt (NU ch, NU ds ) of PCM-ES. It s assumed that the numbers of transfer unt equal each other durng charge and dscharge processes (NU ch =NU ds =NU). Amed at maxmzng ε and mnmzng PEC accordngly, the optmal phase change temperature ( m,opt ) can be deduced out from Eq. (11): ε exp( NU ) (exp( NU ) [1 exp( NU )] 0 m, opt 1 exp( NU ) m ds, ds, a a ch, Eq. (13) gves the analytcal optmal phase change temperature based on the smplfed PCM- ES-BCHP system model. Accordng to Eqs. (3) and (4), f the heat exchange area of the PCM-ES equpment s nfnte (NU + ), the outlet temperatures just equal the phase change temperature ( ch,o = ds,o = m ). In that deal stuaton, m,opt can be expressed by (14) m, opt a ch, a exhaust (13)

7 η ε m,opt ( o C) a ( o C) m_exhaust=300 m_exhaust=400 m_exhaust=500 a me (h) Fgure 4. optmal phase change temperature for BCHP system wth deal PCM-ES equpment (NU + ) For deal PCM-ES equpment wth nfnte NU, the optmal phase change temperature ( m,opt ) s just the geometrcal average value of the ambent temperature ( a ) and the exhaust gas temperature ( exhaust ) from the gas turbne. For nstance, the exhaust gas temperature ( exhaust ) often ranges from about 300 to 500 o C for dfferent gas turbnes [6]. Whereas the ambent temperature vares wdely for dfferent clmate zones as well as fluctuates tmely n one day. Fg. 4 gves the hourly outdoor ar temperature n a typcal summer day n Bejng, Chna (from Chnese Archtecture-specfc Meteorologcal Data Sets for hermal Envronment Analyss). hen accordng to Eq. (14), the optmal phase change temperature for BCHP system wth deal PCM-ES (NU + ) can be obtaned (Fg. 4). It can be seen that optmal phase change temperature vares slghtly wth changng ambent temperature n one day, but vares consderably wth changng exhaust gas temperature. On the other hand, for practcal PCM-ES equpment wth fnte NU, optmal phase change temperature ( m,opt ) s mpacted by varous factors (Eq. (12)). It s assumed that exhaust =300 o C, ch, =140 o C, a =20 o C, NU=1, the energy storage effectveness s shown n Fg η η ε m / o C Fgure 5. Energy storage effectveness varatons wth changng phase change temperature

8 ε m,opt / o C It can be seen that wth the ncreasng m, η 1 decreases whle η 2 ncreases. On the one hand, hgher phase change temperature leads to hgher outlet temperature of PCM-ES durng dscharge process, whch s favourable for ncreasng the COP of the absorpton chller. On the other hand, hgher phase change temperature also leads to relatvely lower temperature dfference ( exhaust - m ) durng charge process, resultng n low energy storage capacty for the PCM-ES equpment. hus due to such two counteractve nfluences, the energy storage effectveness (ε) ncreases frst and then decreases after the peak value, wth ncreasng phase change temperature ( m ). And ε reaches the maxmal value (0.28) only f m =183 o C for ths case ε m,opt NU Fgure 6. Optmal phase change temperature under dfferent NU of PCM-ES As Fg. 6 shows, wth the ncreasng NU of the PCM-ES equpment, both the energy storage effectveness (ε) and the optmal phase change temperature ( m,opt ) ncreases. In other words, mprovng the thermal performance of the ES devce can reduce the heat transfer rreversble losses durng the charge and dscharge processes, so that the overall energy converson and usage effcency of the PCM-ES-BCHP system ncreases, whch s favourable for prmary energy consumpton savng. 4. Conclusons Integratng PCM-ES equpment wth BCHP system can mprove the thermal performance and reduce the prmary energy consumpton. In ths paper, a new ndex, energy storage effectveness, s proposed to evaluate the energy savng effect of the PCM-ES. Based on the smplfed system model, the relatonshp s establshed between the energy storage effectveness and the prmary energy consumpton for the whole PCM-ES-BCHP system. Amed at maxmzng the energy storage effectveness, the optmal phase change temperature of the PCM-ES equpment s obtaned. he results of an llustratve example show that the theoretcally optmal phase change temperature s just the geometrcal average value of the ambent temperature and the exhaust gas temperature from the gas turbne for deal PCM-ES equpment wth nfnte NU. It also ndcates that both energy storage effectveness and optmal phase change temperature ncrease wth ncreasng NU. So mprovng the thermal performance of PCM-ES devce s favourable for ncreasng overall energy converson and usage effcency of the PCM-ES-BCHP system and savng the prmary energy consumpton

9 accordngly. hs work s of great mportance n gudng the optmzaton desgn of PCM-ES-BCHP systems. Acknowledgement hs research s fnanced by Natonal Key Research and Development Program of Chna (2016YFB ), Natonal Natural Scence Foundaton of Chna ( and ) and Schuan Scence and echnology Program of Chna (17YYJC0994). Nomenclature Q heat energy capacty [kw] temperature [ o C] Greek symbols ε η energy storage effectveness effcency Abbrevatons AC AHP BCHP COP FEL FL G NG NU PCM PEC ES WHUS absorpton chller absorpton heat pump buldng coolng heatng and power coeffcent of performance followng electrcal load followng thermal load gas turbne natural gas number of transfer unts phase change materal prmary energy consumpton thermal energy storage waste heat utlzaton subsystem Subscrpts a C ambent coolng

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