ESPOO V RESEARCH NOES 58 asanori Shukuya & Abdlaziz Hammach Introduction to th Concpt of Exrgy - for a ttr Undrstanding of Low-mpratur-Hating and High-mpratur-Cooling Systms
V IEOEIA RESEARCH NOES 58 Introduction to th Concpt of Exrgy for a ttr Undrstanding of Low-mpratur-Hating and High-mpratur-Cooling Systms Submittd to IEA ANNEX37 Low Exrgy Systms for Hating and Cooling of uildings April 5, asanori Shukuya, Ph.., Profssor, Laboratory of uilding Environmnt, usashi Institut of chnology, 3-, Ushikubo-Nishi 3-chom, suzuki-ku, Yokohama, 4-5, Japan E-mail: shukuya@yc.musashi-tch.ac.jp Abdlaziz Hammach, Ph.., Rsarch Scintist Rssourcs naturlls Canada-Natural Rsourcs Canada Cntr dla thnologi d l'nrgi d CANE -Varnns CANE Enrgy chnology Cntr-Varnns 65 Lionl-oult, C.P. 48, Varnns, Qc, J3X 56, E-mail: habdla@nrcan.gc.ca
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Shukuya, asanori & Hammach, Abdlaziz. Introduction to th Concpt of Exrgy for a ttr Undrstanding of Low-mpratur-Hating and High-mpratur-Cooling Systms. Espoo. V idottita Rsarch Nots 58. 4 p. app. 7 p. Kywords nrgy, hating, buildings, cooling, xrgy, ntropy, calculations Abstract Chaptr dscribs th charactristics of a thrmodynamic concpt, xrgy, in association with building hating and cooling systms. Exrgy is th concpt that xplicitly indicats what is consumd. All systms, not only nginring systms but also biological systms including th human body, work fding on xrgy, consuming its portion and thrby gnrating th corrsponding ntropy and disposing of th gnratd ntropy into thir nvironmnt. h whol procss is calld xrgy-ntropy procss. h faturs of warm xrgy and cool xrgy and also radiant xrgy ar outlind. Gnral charactristics of xrgy-ntropy procss of passiv systms, which would b a prrquisit to raliz low xrgy systms, ar discussd togthr with th xrgy-ntropy procss of th global nvironmntal systm. Chaptr introducs th various forms of xrgy and th mathmatical formulations usd to valuat thm. h xrgy balanc on an opn stady stat systm, which is much mor rlvant to thrmodynamic analysis of nrgy systms, is also dscribd, as wll as th diffrnt xrgtic fficincy factors introducd in th thrmodynamic analysis of nrgy systms. Nxt, an xrgy analysis xampl is outlind through an airconditioning application. Air-conditioning applications ar widly usd in hating and cooling of buildings. Chaptr 3 introducs an xampl of xrgy calculation for spac hating systms. h issus to hav a bttr undrstanding of low-xrgy systms for hating and cooling ar raisd. It is suggstd that a prrquisit for low xrgy systms would b rational passiv dsign of building nvlop systms. 3
Prfac his rport is part of th work don within th projct ralizd for th Intrnational Enrgy Agncy (IEA): Enrgy Consrvation in uildings and Community Systms Programm (ECCS), "Annx 37: Low Exrgy Systms for Hating and Cooling". h gnral objctiv of th Annx 37 is to promot rational us of nrgy by mans of facilitating and acclrating th us of low valud and nvironmntally sustainabl nrgy sourcs for hating and cooling of buildings. V chnical Rsarch Cntr of Finland is th coordinator of th Annx 37. Elvn countris participat: Canada, nmark, Finland, Franc, Grmany, Italy, Japan, h Nthrlands, Norway, Poland and Swdn. his rport has bn writtn by Abdlaziz Hammach, Ph, Rsarch Scintist at th Enrgy ivrsification Rsarch Laboratory (CANE), Canada (chaptr and 3) and asanori Shukuya, Ph, Profssor at th Laboratory of uilding Environmnt, usashi Institut of chnology, Japan (chaptr and 3). A spcial thanks to Cary Simonson Ph, Assistant Profssor at th Univrsity of Saskatchwan, Canada for hlping writing this rport. Espoo July 3, Åsa Nystdt 4
Contnts Abstract...3 Prfac...4 List of symbols...6. Introduction to th concpt...8. Introduction...8.. scription of a systm as an xrgy-ntropy procss...9. Exrgy balanc quation....3 Warm xrgy and cool xrgy...4.4 Radiant xrgy...6.5 Exrgy-ntropy procss of passiv systms...7.6 h global nvironmntal systm...9.7 Conclusion.... athmatical formulations.... Introduction.... Exrgy balanc....3 finitions of xrgtic fficincis...4.3. Convntional xrgtic fficincy...4.3. Rational xrgtic fficincy...5.3.3 Utilizabl xrgy cofficint...7.4 Air-conditioning applications...8.5 Conclusion...36 3. Spac hating xampl...37 3. An xampl of hating xrgy calculation...37 3. Conclusion...39 Rfrncs...4 APPENICES Appndix A: hrmal xrgy containd by a volum of room air Appndix : Radiant thrmal xrgy mittd by a wall surfac Appndix C: Forms of xrgy Appndix : Exampl of xrgtic fficincis Appndix E: Exrgy calculation for spac hating 5
List of symbols C : Vlocity rlativ to th arth surfac (m/s) c p : Spcific isobaric hat capacity (J/(kg K) E : Exrgy (J) : Spcific xrgy (J/kg) g : Acclration du to gravity (m/s ) h : Spcific nthalpy (J/kg) I : Irrvrsibility (J) : olcular wight (g/mol) : ass flow (kg) P : Prssur (Pa) Q : Hat (J) R : Spcific idal gas constant (J/(kg K) S : Entropy (J/(kg K) s : Spcific ntropy (J/(kg K) : mpratur (K) v : Spcific volum (m 3 /kg) W : Work (J) x : ol fraction Z : Altitud abov sa lvl (m) Grk Symbols φ : Rlativ humidity (%) η : Convntional xrgtic fficincy (%) η u : Utilizabl xrgy cofficint (%) τ : Non-dimnsional xrgtic tmpratur ω : Spcific humidity (%) ψ : Rational xrgtic fficincy (%) 6
Subscripts : Rfrnc or ambint stat a : ry air ch : Chmical lc : Elctrical f : Saturatd liquid g : Saturatd vapor gn : Gnration I : Input In : incoming xrgy flow k : Kintic o : Output out : Outgoing xrgy flow p : Potntial ph : physical or thrmochmical r : Hat rsrvoir sh : Shaftwork tr : ransiting v : Watr vapor w : Work Suprscripts Q : Hat 7
. Introduction to th concpt asanori Shukuya. Introduction Chaptr dscribs th gnral charactristics of a thrmodynamic concpt, xrgy, which nabls us to articulat what is consumd by all working systms, whthr thy ar man-mad systms such as thrmo-chmical ngins and lctricity-drivn hat pumps or biological systms such as microbs, plants, and animals including th human body. his articl focuss spcially on its application to dscribing building hating and cooling systms. Popl oftn claim that nrgy is consumd; this is not only in vryday convrsation but also vn in scintific discussion associatd with so-calld nrgy and nvironmntal issus. his claim, howvr, conflicts with th first law of thrmodynamics stating that th total amount of nrgy is consrvd vn though forms of nrgy may chang from on to anothr. All macroscopic natural phnomna happning around us involv th disprsion of nrgy and mattr, which in du cours chang thir forms from on to anothr, but th total amount of nrgy and mattr involvd is nvr consumd but ncssarily consrvd. Whn w us such xprssions as nrgy consumption, nrgy saving, and vn nrgy consrvation, w implicitly rfr to nrgy as intns nrgy availabl from fossil fuls or from condnsd uranium. ut, it is confusing to us on of th most wllstablishd scintific trms, nrgy, to man to b consrvd and to b consumd simultanously. his is why w nd to us th thrmodynamic concpt, xrgy, to articulat what is consumd. Ovr th last two dcads various so-calld nrgy saving masurs hav bn concivd, dvlopd, and implmntd in building nvlop systms and also thir associatd nvironmntal control systms such as lighting, hating, and cooling systms. hos masurs can b catgorizd into two groups: thos for passiv systms and thos for activ systms. Passiv systms ar dfind as building nvlop systms to mak us of various potntials to b found in th immdiat nvironmnt such as th sun, wind, and othrs to illuminat, hat, vntilat, and cool th built nvironmnt. h history of passiv systms is vry long; w may say that it mrgd with th volution of human bing. 8
h rcnt dvlopmnt of matrial scinc has brought about various building matrials such as low-missivity coatd glass and othrs; this nabls us to dsign advancd passiv systms. Activ systms ar th systms consisting of various mchanical and lctric componnts such as fans, pumps, hat pumps, and othrs, all of which work by th us of fossil fuls. ost of th activ systms availabl ths days hav bn dvlopd with an assumption of th abundant us of fossil fuls so that thy do not ncssarily work in harmony with passiv systms. Optimal thrmal nvironmntal dsign with thrmally-wll-insulatd glazing matrials with othr thrmally-wll-insulatd building-nvlop matrials having appropriat hat capacity nabls us to raliz passiv solar hating systms. Howvr, it dos not man that activ hating systms ar no longr rquird. W nd nw typs of activ systms that can work in harmony with advancd passiv systms. Low-tmpratur-hating systms ar such kind of activ hating systms which should fit th built nvironmnt to b conditiond primarily by passiv hating systms. A good thrmal-nvironmntal condition within built spacs in th wintr sason can b providd basically with th installation of thrmally-wll-insulatd building matrials with appropriat hat capacity, which mak it possibl to utiliz hat sourcs of lowr tmpratur for hating. In summr sason, a modrat thrmal-nvironmntal condition within built spacs may b providd with a combination of nocturnal vntilation, th installation of appropriat shading dvics for glass windows, and th rduction of intrnal hat gain in addition to th us of thrmally-wll insulating matrials with appropriat hat capacity for building nvlops. his would allow th utilization of cold sourcs with highr tmpratur for cooling. h us of th xrgy concpt in dscribing various hating and cooling systms, whthr thy ar passiv or activ, would nabl us to hav a bttr pictur of what lowtmpratur-hating and high-tmpratur-cooling systms ar... scription of a systm as an xrgy-ntropy procss Lt us assum a building nvironmntal control systm such as lighting, hating, or cooling systms. Enrgy and mattr ar supplid into th systm so that it works. h 9
inputs ar xactly th sam as th outputs undr stady-stat conditions. his is du to th law of nrgy and mass consrvation. If it is so, why do not w rus th nrgy and mattr as output dirctly? his vry fundamntal qustion in trms of lif was onc askd by Schrödingr som fifty yars ago (Schrödingr, 945). If w could hav usd th wastd nrgy and mattr, most of th so-calld nrgy and nvironmntal problms would hav bn alrady solvd. Fig.. Enrgy, xrgy, and ntropy flow in and out a building nvlop systm. h amounts of nrgy flowing in and out ar th sam undr thrmally stady-stat condition according to th law of nrgy consrvation; on th othr hand, th amount of ntropy flowing out is largr than flowing in according to th law of ntropy incras. h amount of xrgy flowing out is smallr than flowing in, sinc xrgy is consumd within th systm to produc ntropy. h most gnral answr to th abov qustion would b that th nrgy and mattr as input ar diffrnt from thos as output; or you may say that th nrgy and mattr as output hav somthing that a systm in qustion must discard. o mak th answr clarr, w us th concpts of xrgy and ntropy, which can xprss th diffrnc in nrgy and mattr btwn input and output xplicitly. Exrgy and ntropy, both of which ar thrmodynamic concpts, can show us what is th rsourc and what is th wast; xrgy is th concpt to articulat what is consumd and ntropy is what is disposd of. Stating in th othr way, xrgy is th concpt, which quantifis th potntial of nrgy and mattr to disprs in th cours of thir diffusion into thir nvironmnt and ntropy is th concpt which quantifis th stat of disprsion, to what xtnt th nrgy and mattr in qustion ar disprsd.
Lt us tak a microscopic viw in ordr to mak th concpts asir to undrstand. Enrgy transfr lik hat transfr is a transfr of th vibration of particls, which compos of, for xampl, a building nvlop systm as shown in Fig.. W assum a stady-stat condition that th right-hand sid of th systm is warmr than th lft-hand sid. h particls in th warmr sid of th building nvlop vibrat rathr strongly; that is, th nrgy flowing into th building nvlop accompanis a crtain amount of xrgy. h vibration disprss in th cours of nrgy transfr; that is, a part of th xrgy is consumd as th xrgy flows. As a rsult, th nrgy flowing out th building nvlop is accompanid with a smallr amount of xrgy. As a rsult of th disprsion of vibration, th stat of disprsion as a whol within th systm incrass. his is th gnration of ntropy, th law of ntropy incras, which is paralll to th law of nrgy and mass consrvation. h amount of incrasd ntropy is proportional to that of consumd xrgy and th proportional constant is th ambint tmpratur in th Klvin scal as dscribd latr. Sinc th stady-stat condition is bing assumd, th distribution of th tmpratur insid th building nvlop is unchangd. his implis that th amount of ntropy containd by th whol of th building nvlop systm is constant. h ntropy of a substanc, which is a function of tmpratur and prssur, rmains unchangd unlss th tmpratur and th prssur of th substanc incrass or dcrass. As dscribd abov, a crtain amount of ntropy is gnratd du to xrgy consumption within th building nvlop systm. his gnratd ntropy must b discardd into th surrounding, namly outdoors, from th building nvlop systm, othrwis it turns out to b contradictory with our assumption of th stady-stat condition and th charactristics of th ntropy as a function of tmpratur and prssur. It is important for us to rcogniz that th nrgy flowing out th building nvlop is accompanid with not only a dcrasd amount of xrgy but also an incrasd amount of ntropy. isposing of th gnratd ntropy from th systm maks room for fding on xrgy and consuming it again. W call th procss dscribd abov as xrgy-ntropy procss (Shukuya and Komuro, 996). abl shows th four fundamntal stps of xrgy-ntropy procss. Any working systms prform ths four stps in sris and cyclically. Hating and cooling systms ar no xcption. hr is a rathr strong blif among scintists and nginrs that ntropy is on of th concpts which is most difficult to undrstand. I think that this is not ncssarily tru. hos who ar intrstd mor in th concpt of ntropy than dscribd hr in this articl should consult, for xampl, a book writtn by Atkins [984], which I think bst dscribs th charactristics of ntropy.
abl. Four Stps of Exrgy-Entropy Procss.. Fd on xrgy. Consum Exrgy 3. Gnrat Entropy 4. ispos of Entropy isposing of th gnratd ntropy from th systm maks nw room for fding on xrgy and consuming it again. hus th procss cycls.. Exrgy balanc quation Lt us introduc a gnral xprssion of xrgy balanc using th cas of th abovmntiond simpl building nvlop systm. h purpos hr is to outlin th structur of th xrgy balanc quation and w do not discuss th dtaild mathmatical xprssion. hos who ar intrstd in th dtaild mathmatical xprssions should rfr to jan (988), Shukuya (994), and othrs, in addition to Chaptr of this rport. Enrgy is th concpt to b consrvd so that th nrgy flowing in must b qual to th sum of th nrgy stord within th systm and th nrgy flowing out from th systm. his nrgy balanc can b xprssd as follows. (Enrgy input) (Enrgy stord) (Enrgy output) (.) Sinc th stady-stat condition is bing assumd hr, thr is no nrgy storag and hnc th abov quation turns out to b th following simplr form. (Enrgy input) (Enrgy output) (.) Scondly, lt us st up th ntropy quation consistnt with th abov two quations. Enrgy flowing into th systm as hat is mor or lss disprsd nrgy. Hat is a nrgy transfr du to disprsion, thus ntropy ncssarily flows into th systm as hat flows in and som amount of ntropy is gnratd invitably within th systm in th cours of hat transmission. h sum of th ntropy input and th ntropy gnratd must b in part stord or in part flows out of th systm. hrfor th ntropy balanc quation can b xprssd in th following form. (Entropy input) (Entropy gnratd) (Entropy stord) (Entropy output) (.3)
Sinc th stady-stat condition is bing assumd, thr is no ntropy storag as wll as no nrgy storag. hrfor, th abov ntropy balanc quation turns out to b (Entropy input) (Entropy gnratd) (Entropy output) (.4) h fact that th outgoing ntropy from th systm includs th ntropy gnratd within th systm suggsts that th systm disposs of th gnratd ntropy with th ntropy output. Combining th nrgy and ntropy balanc quations brings about th xrgy balanc quation. Entropy (or ntropy rat) has a dimnsion of J/K (or W/K) and nrgy (or nrgy rat) has a dimnsion of J (or W). hrfor w nd a kind of trick to combin th two quations. Gnrally spaking, nrgy containd by a body, which has an ability to disprs, is calld an nrgy rsourc. Such an nrgy rsourc xists within th nvironmntal spac, which is filld with disprsd nrgy. h disprsd nrgy lvl of th rsourc surroundd by th nvironmntal spac can b xprssd as th product of th ntropy containd by th rsourc and its nvironmntal tmpratur in th Klvin scal. h sam xprssion applis to th wast discardd by th systm. hrfor th ntropy balanc quation can b rwrittn as follows. (Entropy input) (Entropy gnratd) (Entropy output) (.5) Whr is th nvironmntal tmpratur. h product of ntropy and nvironmntal tmpratur is calld anrgy, which implis disprsd nrgy. Using th trm anrgy, th abov quation can b xprssd in th following form, anrgy balanc quation. (Anrgy input) (Anrgy gnratd) (Anrgy output) (.6) Providd that anrgy is a portion of nrgy that is alrady disprsd, thn th othr portion is not yt disprsd. Stating in anothr way, nrgy consists of two parts: th disprsd part and th part, which can disprs. h lattr is xrgy. Now lt us tak th diffrnc of th two quations, nrgy balanc quation (.) and anrgy balanc quation (.6). his opration brings about 3
[(Enrgy input) (Anrgy input)] (Anrgy gnratd) [(Enrgy output) (Anrgy output)]. (.7) Anrgy gnratd is such nrgy that originally had an ability to disprs and that has just disprsd. W can stat this in th othr way; that is, xrgy is consumd. Anrgy gnration is quivalnt to xrgy consumption. Using th trm xrgy, th abov quation can b rducd to th following quation. (Exrgy input) (Exrgy consumd) (Exrgy output) (.8) his is th xrgy balanc quation for a systm undr stady-stat condition such as th building nvlop systm shown in Fig.. Exrgy consumd, which is quivalnt to anrgy gnratd, is th product of ntropy gnratd and th nvironmntal tmpratur. (Exrgy consumd) (Environmntal tmpratur) x (Entropy gnratd) (.9) Exrgy consumd is xactly proportional to th ntropy gnratd with th proportional constant of nvironmntal tmpratur..3 Warm xrgy and cool xrgy h amount of xrgy containd by a substanc varis with its tmpratur and also with its nvironmntal tmpratur. Fig. shows an xampl of thrmal xrgy containd by 8 m 3 ( 6m x 5m x.7m) of air as a function of its tmpratur in th cas of an nvironmntal tmpratur of 88 K (5 C). It should b notd that air has a crtain amount of xrgy both whn th air tmpratur is highr than th nvironmnt and whn th air tmpratur is lowr than th nvironmnt. Appndix A shows a mathmatical formula usd to draw Fig.. h xrgy containd by air at a tmpratur highr than its nvironmnt is an ability of thrmal nrgy containd by th air to disprs into th nvironmnt. On th othr hand, th xrgy containd at a tmpratur lowr than its nvironmnt is an ability of th air, in which thr is a lack of thrmal nrgy compard to th nvironmnt, to lt th thrmal nrgy in th nvironmnt flow into it. W call th formr warm xrgy and th lattr cool xrgy (Shukuya, 996). 4
Eithr warm xrgy or cool xrgy dscribd abov is a quantity of stat containd by a substanc. W hav room tmpratur highr than th outdoor nvironmnt whn th spac is hatd. In such a cas room air has warm xrgy as a quantity of stat. On th othr hand, whn w hav a room tmpratur lowr than th outdoor nvironmnt, room air has cool xrgy as a quantity of stat. 8 X r [kj] 6 4 cool o wa r m.8.6.4. x r [kj/m 3 ] 7 8 9 3. 3 r [K] Fig.. hrmal xrgy containd by air as a function of tmpratur, r. Air volum is assumd to b 8m 3 ( 6m x 5m x.7m). Environmntal tmpratur, o, is 88 K(5 C). Air at a tmpratur highr than th nvironmntal tmpratur has cool xrgy and th air at a tmpratur lowr than th nvironmntal tmpratur has warm xrgy (S Appndix A, formula A.). hrmal xrgy, whthr it is warm xrgy or cool xrgy, flows through walls, by a combination of convction, conduction, and radiation. h cas shown in Fig. is whn th nvironmntal tmpratur, namly outdoor tmpratur, is lowr than th indoor tmpratur. In this cas, warm xrgy flows in th intrnal surfac and out th xtrnal surfac of th building nvlop systm. If th nvironmntal tmpratur is highr than th indoor tmpratur, namly th summr condition, th room air has cool xrgy, which flows through th building nvlop systm. h dirction of nrgy flow changs dpnding on th tmpratur profil, whthr th indoor tmpratur is highr or lowr than th outdoor tmpratur, but th dirction of xrgy flow is always th sam from th indoors to th outdoors, xtrnal nvironmnt. What changs is whthr it is warm xrgy or cool xrgy dpnding on, whthr indoor tmpratur is highr or lowr than th outdoor tmpratur. Spac hating systms, whthr thy ar low-xrgy consuming or not, ar th systms that supply and consum xrgy for kping warm xrgy as a quantity of stat 5
containd by room spac in a crtain dsird rang. Spac cooling systms, on th othr hand, whthr thy ar low-xrgy consuming or not, ar th systms that supply and consum xrgy for kping cool xrgy as a quantity of stat containd by room spac in a crtain dsird rang. As dscribd abov, xrgy consumption is always accompanid with ntropy gnration, thus th gnratd ntropy must b discardd constantly from th room spac to th outdoor nvironmnt to kp warm or cool xrgy within a dsird rang..4 Radiant xrgy Radiant xrgy transfr plays mor important rol in low-tmpratur-hating or hightmpratur-cooling systms than in convntional air hating or cooling systms, bcaus thy rquir hat sourcs with a rathr larg surfac ara whos tmpratur is only slightly highr than room air tmpratur. For this rason, it would b vry important to b abl to valuat radiant xrgy. Fig. 3 shows an xampl of radiant xrgy mittd by a black surfac of m in th cas of nvironmntal tmpratur of C (93 K) givn by akahashi t al. (). Appndix shows a mathmatical formula usd to draw Fig. 3. Supposing that thr is a radiant panl of m with a surfac tmpratur of 4 C, this panl mits 9 W of warm radiant xrgy. If th surfac tmpratur dcrass from 4 C to 3 C, th warm radiant xrgy drops dramatically from 9 W down to W. In th cas of cold sourc of th surfac tmpratur of 6 C, th panl mits 4 W of th cool radiant xrgy. If th surfac tmpratur incrass from 6 C to 4 C, th cool xrgy drops dramatically from 4 W down to. W. his suggsts that low xrgy systms for hating and cooling of buildings ar ralizd providd that hating and cooling xrgy rquirmnts for room spac is dcrasd by th installation of rational building nvlop systms, thus th hating and cooling is providd at a tmpratur clos to room tmpratur. 6
Radiant xrgy [W/m ] 8 6 4 - cool o 3 warm 4 5 Surfac tmpratur [ C] Fig. 3. An xampl of radiant xrgy mittd by a black surfac of m whn th nvironmntal tmpratur is assumd to b 93 K ( C). A surfac with a tmpratur lowr than th nvironmntal tmpratur mits cool radiant xrgy and a surfac with a tmpratur highr than th nvironmntal tmpratur mits warm radiant xrgy (S Appndix, formula.)..5 Exrgy-ntropy procss of passiv systms Hr lt us dscrib th gnral charactristics of six passiv systms from th viwpoint of xrgy-ntropy procss (s (Shukuya, 998) and (Shukuya, )). As suggstd abov, rational passiv (bio-climatic) dsign would b prrquisit to raliz low-xrgy systms for hating and cooling. aylighting: Passiv hating: this is to consum solar xrgy for indoor illumination. Exrgy consumption occurs as solar xrgy is absorbd by th intrior surfacs of building nvlops. Warm xrgy is producd as a rsult of solar xrgy consumption for lighting; this may b consumd for spac hating (Asada and Shukuya, 999). h ntropy gnratd in th cours of solar xrgy consumption for lighting must b discardd into th atmosphr by vntilation cooling or mchanical cooling, hopfully by a low-xrgy systm for cooling. this is to control th rat of solar xrgy consumption during daytim and nighttim by forming th built-nvironmntal spac with th appropriat matrials that hav low thrmal conductivity and high thrmal-xrgy storag capacity. It is also to consum, during nighttim, th thrmal xrgy producd during daytim. ost of th ntropy gnratd is discardd spontanously through th building nvlops into th atmosphr (Shukuya and Komuro, 996). 7
Shading: Vntilation cooling: (Fr cooling) Watr spraying: Composting: this is to lt th xcss solar xrgy, namly th rst of xrgy ncssary for daylighting, b consumd bfor it ntrs th built nvironmnt. It is also to rduc th ntropy gnratd within th built nvironmnt so that mchanical quipmnt for cooling is rquird to consum lss xrgy to rmov th ntropy gnratd within th built nvironmnt. Extrior shading dvics ar vry much attractiv in this rgard, sinc th ntropy gnratd at th dvics is ffctivly discardd into th atmosphr by convction (Asada and Shukuya, 999). this is to consum kintic xrgy of atmosphric air, which is producd by th xrgy-ntropy procss of th global nvironmntal systm dscribd latr (Shukuya and Komuro, 996), for rmoving th ntropy gnratd within th built nvironmnt, such as th ntropy discardd from th body surfac of th occupants and that from th lighting fixturs, lctric appliancs and othrs, into th nar-ground atmosphr. this is to consum th wt xrgy containd by liquid watr, which is vry larg compard to thrmal xrgy, namly warm or cool xrgy, to dcras th warm xrgy producd by solar xrgy consumption and possibly to produc cool xrgy (S (Nishikawa and Shukuya, 999), and (Saito and Shukuya, 998)). Roof spraying and uchimizu, which is to scattr rainwatr on th road surfac, ar also du to this procss. h consumption of wt xrgy to produc cool xrgy or to dcras warm xrgy play a vry important rol in photosynthtic systm of lavs (Saito and Shukuya, 998) and th tmpratur-rgulating systm of human body (Saito and Shukuya, ). this is to lt micro organisms consum activly a larg amount of xrgy containd by garbag and hnc turn it into frtilizr. h warm xrgy producd as a rsult of micro-organisms consuming chmical xrgy can b rationally consumd for maintaining th tmpratur insid th containr at a dsird lvl. his is ralizd by making th walls of a containr thrmally wll insulatd (akahashi and Shukuya, 998). h ntropy gnratd in th procss of composting is discardd into th surrounding of th containr and finally into th nar-ground atmosphr. 8
With th viw of passiv (bio-climatic) dsign as xrgy-ntropy procss, passiv dsign is to dsign a rout in which th xrgy availabl from our immdiat surroundings is rationally consumd and th gnratd ntropy is rationally discardd into th atmosphr. Again, low-xrgy systms for hating and cooling would b such systms consistnt with passiv dsign dscribd abov..6 h global nvironmntal systm Our nar-ground atmosphr rcivs all th ntropy that is gnratd and discardd by all systms involving lighting, hating, and cooling of th built nvironmnt. his also applis to any living systms such as bactria, plants, and animals, sinc th involvd biological phnomna can b rducd to th combination of chmical and physical phnomna, although such rduction alon cannot giv us an answr to why th biological phnomna ar so complx or how living systms volv. Sinc th ntropy containd by a substanc is, as dscribd in th prvious sction, a function of tmpratur and prssur, th nar-ground atmosphric tmpratur must riss if th nar-ground atmosphr continus to rciv th ntropy discardd from various systms. ut, what is actually occurring in th natur is diffrnt; th avrag atmosphric tmpratur is almost constant from yar to yar. his is du to th fact that th atmosphr has an xrgy-ntropy procss that works fding on and consuming solar xrgy, thrby producing th ntropy, and finally disposing of th producd ntropy into th Univrs. W call this th global nvironmntal systm. Fig. 4 shows schmatically and numrically th xrgy-ntropy procss of th global nvironmntal systm (Shukuya and Komuro, 996). h arth rcivs not only th solar xrgy of.7 W/m but also th cool radiant xrgy of. W/m from th Univrs. hs xrgis ar all consumd soonr or latr within th uppr or lowr atmosphr. h figurs in th squars, 76.6 W/m and 46.3 W/m, show th xrgy consumption in th uppr and lowr atmosphrs rspctivly. h convctiv air currnt nar th ground surfac, a part of which can b usd for vntilation, has.73 W/m of kintic xrgy. h rain drops insid th clouds bfor falling down towards th ground surfac hav.5 W/m of potntial xrgy; a part of this xrgy may b consumd to produc lctric powr. hs kintic and potntial xrgis ar producd by th solar and th cool radiant xrgy consumption. h rsultant gnratd ntropy du to th xrgy consumption is dlivrd first into th uppr atmosphr by convction, vaporation and long-wav radiation and thn into th Univrs by longwav radiation. h total amount of th ntropy gnration is th diffrnc btwn th input and th output ntropy flows across th uppr boundary surfac of th uppr 9
atmosphr. A portion of th. W/m of cool radiant xrgy coming from th Univrs nabls th global nvironmntal systm to hav th outgoing ntropy flow of.39 W/(m K). It should b rcognizd that th cool radiant xrgy of. W/m is vitally important in addition to th solar xrgy, bcaus it is th xrgy that finally swps away all th gnratd ntropy within th uppr and lowr atmosphrs, which includs th ntropy gnratd by bio-climatically dsignd building nvlop systms and also low-xrgy systms for hating and cooling. Fig. 4. Exrgy-ntropy procss of th global nvironmntal systm. h drawing on th top is th xrgy input, output, and consumption in W/m. h othr drawing on th bottom is th ntropy input, output, and gnration in W/(m K). h amounts of th xrgy consumption and th ntropy gnration ar indicatd by th figurs in th squars..7 Conclusion A thrmodynamic concpt of xrgy which xplicitly indicats what is consumd was xplaind from th viwpoint of its application spcially to dscribing building hating and cooling systms, togthr with an xplanation of ntropy, which xplicitly indicats what is disposd of.
All working systms work as xrgy-ntropy procss, in which xrgy is supplid, a portion is consumd and thrby th rsultant ntropy is gnratd, and finally th gnratd ntropy is discardd into th nvironmnt. h structur and function of th xrgy balanc quation wr outlind, and th faturs of warm xrgy and cool xrgy wr prsntd. h gnral charactristics of th xrgy-ntropy procss of passiv dsign wr also dscribd togthr with th global nvironmntal systm. What is suggstd from th discussion hr is that rational passiv dsign is a prrquisit to raliz low xrgy systms for th hating and cooling of buildings.
. athmatical formulations Abdlaziz Hammach. Introduction raditional mthods of thrmal systm analysis ar basd on th first law of thrmodynamics. hs mthods us an nrgy balanc on th systm to dtrmin hat transfr btwn th systm and its nvironmnt. h first law of thrmodynamics introducs th concpt of nrgy consrvation, which stats that nrgy ntring a thrmal systm with ful, lctricity, flowing strams of mattr, and so on is consrvd and cannot b dstroyd. In gnral, nrgy balancs provid no information on th quality or grads of nrgy crossing th thrmal systm boundary and no information about intrnal losss. y contrast, th scond law of thrmodynamics introducs th usful concpt of xrgy in th analysis of thrmal systms. Exrgy is a masur of th quality or grad of nrgy and it can b dstroyd in th thrmal systm. h scond law stats that part of th xrgy ntring a thrmal systm with ful, lctricity, flowing strams of mattr, and so on is dstroyd within th systm du to irrvrsibilitis. h scond law of thrmodynamics uss an xrgy balanc for th analysis and th dsign of thrmal systms. h scond part of this documnt dscribs th various forms of xrgy and th mathmatical formulations usd to carry out th xrgy balanc. h diffrnt xrgtic fficincy factors ar also introducd and xplaind.. Exrgy balanc On of th main uss of th xrgy concpt is in an xrgy balanc in th analysis of thrmal systms. h xrgy balanc is a statmnt of th law of dgradation of nrgy. gradation of nrgy is du to th irrvrsibilitis of all ral procsss. caus opn systm analysis is much mor rlvant to th analysis of thrmal plants or chmical systms than closd systm analysis, th xrgy balanc of an opn stady stat systm, as shown in Fig. 5, is prsntd. h xrgy balanc is statd around a control rgion dlimitd by spcific boundaris.
hrmal nrgy rsrvoirs r r r Q, Q, Q, r r Q E Q Surfac Control Input of mattr Control rgion E i E o I Output of mattr Shaft work W sh Q Q E,, x,,,,... x Environmnt Fig. 5. Stady stat procss in an opn control rgion. Q i E Eo Wsh I E (.) whr: i m i i E i (.) o m o o E o (.3) E Q Qr r r (.4) h xprssion for spcific xrgy is writtn as: C ( h h ) o ( s s ) ch gz (.5) h xrgy flow to th control rgion is always gratr that that from th control rgion. h diffrnc btwn th two, th rat of loss of xrgy, is calld th irrvrsibility 3
rat. h irrvrsibility rat is calculatd from th Gouy-Stodola rlation, which stats that th irrvrsibility rat of a procss is th product of th ntropy gnration rat for all systms participating in th procss and th tmpratur of th nvironmnt. Qr I S gn mos misi o i r r (.6) h various xrgy trms, which go into th xrgy balanc, ar prsntd in Appndix C..3 finitions of xrgtic fficincis hr dfinitions of xrgtic fficincis for stady stat procsss ar found in th litratur. hs ar th convntional or simpl xrgtic fficincy, th rational xrgtic fficincy and th utilizabl xrgy cofficint..3. Convntional xrgtic fficincy h simplst form of xrgtic fficincy is th convntional xrgtic fficincy. For th formulation of this fficincy th xrgy balanc for th incoming and outgoing flows is st up, whr I is th irrvrsibility. in Eout I E (.7) Rfrring to Fig. : in E i E E Q (.8) E out E W o sh (.9) h traditional xrgtic fficincy is th ratio of th total outgoing xrgy flow to th total incoming xrgy flow: 4
η E out E in (.) his is an unambiguous dfinition and can b usd for all procss plants and units. Unfortunatly, it givs a good imprssion of th thrmodynamic prfction of a systm only whn all th componnts of th incoming xrgy flow ar transformd to othr componnts,.g., in th cas for powr stations or for building hating and cooling systms. h traditional xrgtic fficincy for powr stations is xprssd as: W η E lc ful (.) Whn all th componnts of th incoming xrgy flows ar not transformd to othr componnt, th untransformd componnts giv a fals imprssion of th prformanc of th procss plant or unit. For xampl, if w considr a chmical ractor with a zro ractiv convrsion factor, th input xrgy rat will qual th output xrgy rat and th traditional xrgtic fficincy will qual. hr ar no irrvrsibilitis in th ractor but it dos not produc anything! In this cas, th traditional xrgtic fficincy givs a fals imprssion of th thrmodynamic prformanc of th ractor. o solv this problm othr xrgtic fficincis hav bn proposd..3. Rational xrgtic fficincy h rational xrgtic fficincy is dfind by Kotas (985) as a ratio of th dsird xrgy output to th xrgy usd or consumd. E dsird output I ψ E usd E usd (.) E dsird output is th sum of all xrgy transfrs from th systm, which must b rgardd as constituting th dsird output, plus any by-product, which is producd by th systm. h dsird output is dtrmind by xamining th function of th systm. E usd is th rquird xrgy consumd for th procss to b prformd. h rational fficincy can b applid to any systm, xcpt to purly dissipativ systms, bcaus no dsird product can b dfind in this cas. 5
As an xampl of formulation of rational fficincy considr th rfrigration plant vaporator shown in Fig. 6 (Kotas, 985). E E L Q L Cold Chambr Fig. 6. Rfrigration plant vaporator. h cold chambr may b considrd as a thrmal nrgy rsrvoir at a tmpratur L <. Rfrring to Fig. 6, th xrgy balanc for th control rgion indicatd by th dashd contour may b writtn: E E I Q L L (.3) h dsird output is th incras in th xrgy of th cold chambr, which sinc L <, is associatd with hat transfr from th cold chambr. hus: E dsird output Q L L (.4) y incorporating Eqs. (3) and (4) into Eq. (), th rational fficincy for this systm is xprssd as: Q L L ψ E E I E E (.5) and th irrvrsibility rat is: 6
Q L I S S L (.6).3.3 Utilizabl xrgy cofficint rodyansky, Sorin and LGoff (994) introducd this form of xrgtic fficincy, calld utilizabl xrgy cofficint. his form of fficincy is an improvmnt on th traditional xrgtic fficincy, bcaus it subtracts th untransformd componnts from th incoming and outgoing strams. h following sction introducs this concpt. o any matrial, hat and work stram can b associatd an xrgy contnt, which is compltly dfind by tmpratur, prssur and composition of th stram itslf and of a rfrnc stat, which is normally th nvironmnt in which th systm oprats. It is, thrfor, possibl to comput th xrgy contnt of all incoming and outgoing strams to and from a systm and to stablish an ovrall xrgy balanc ovr any systm, as shown in Fig. 7. h total xrgy input, E in, of a ral systm is always highr than its xrgy output, E", bcaus a crtain amount of xrgy is irrvrsibly dstroyd within th systm. his xrgy, gnrally rfrrd to as th intrnal xrgy losss or xrgy dstruction, I int is dirctly linkd to th thrmodynamic irrvrsibilitis in th systm. Intrnal Exrgy Losss (I int ) Extrnal Exrgy Losss (I xt ) Producd Utilizabl Exrgy, E pu Exrgy Input, E in Consumd Exrgy, E c Exrgy Output, E" Producd Exrgy, E p Utilizabl Exrgy, E out ransiting Exrgy, E tr Fig. 7. Graphical prsntation of ovrall xrgy balanc. 7
As illustratd in Fig. 7, part of th xrgy output from th systm may dissipat into th nvironmnt as hat losss, swag wast or smokstack fflunts, for xampl. his wastd xrgy, no longr usabl by subsqunt procsss, constituts th xtrnal losss, I xt. It is mor appropriat, from th standpoint of downstram oprations, to considr th xrgy that rmains utilizabl, E u, rathr than th total output, E". Only part of th utilizabl xrgy is producd by th systm through th physicochmical phnomna that tak plac within its boundaris. h rst of th xrgy that lavs th systm with th utilizabl xrgy stram is a part of th xrgy input, which has simply gon through th systm without undrgoing any transformation. his fundamntal fact was first rcognizd by Kostnko (983), who gav th nam transiting xrgy, E tr, to this fraction of th xrgy supplid to a systm. ypically in a chmical ractor, part (but not all, bcaus of tmpratur and prssur changs) of th xrgy associatd with unractd fd or inrts would constitut transiting xrgy. ransiting xrgy was furthr charactrizd and algorithms hav bn dvlopd for computing it dirctly (Sorin and rodyansky, 985; rodyansky t al. 994). On th basis of ths obsrvations a nw cofficint of thrmodynamic fficincy, th utilizabl xrgy cofficint, η u, has bn dfind (Sorin t al., 998). h xrgtic fficincy with transiting xrgy is dfind as follows: η u out E tr E in E tr E in I int I xt E tr E in E tr E E pu E c (.7) whr E tr is th transiting xrgy rat, E pu is th producd utilizabl xrgy rat and E c is th consumd xrgy rat. As has bn dmonstratd by Sorin and co-workrs (998), th dcras in th transiting xrgy, E tr, improvs th convrsion prformanc of th systm. h utilizabl xrgy cofficint dcrass as I int, I xt and E tr dcras. An xampl on th us of th various xrgtic dfinitions is shown in Appndix..4 Air-conditioning applications Air-conditioning applications ar important and widly usd in hating and cooling of buildings. his sction prsnts th us of th concpt of xrgy in th assssmnt of air-conditioning applications. h concpts of physical xrgy and chmical xrgy 8
play an important rol in assssing th tru thrmodynamic mrit of air-conditioning applications. h objctiv of most air-conditioning applications is to bring a humid air mixtur to a stat (tmpratur and composition) that diffrs from th conditions found in th atmosphric air. h classical way of dscribing th thrmodynamic proprtis of humid air is to viw it as a prfct gas mixtur of dry air (a) and watr vapor (v) (abl ). h idal gas constants of ths two componnts ar th valus corrsponding to 3 K and th low-prssur limit (jan, 988). abl. Idal gas constants of dry air (a) and watr vapor (v). ry Air (a) Watr Vapor (v) R a. 87 kj ( kg. K) R v. 465 kj ( kg. K) c a p. 3 kj ( kg. K) c. 87 kj ( kg. K), p, v a 8. 97 kg kmol v 8. 5 kg kmol R 8. 34 kj ( kmol. K) R 8. 34 kj ( kmol. K) c a p, 9. 57 kj ( kmol. K) c p, v 33. 74 kj ( kmol. K) h stat of any humid air is spcifid by its tmpratur, its prssur P, and on of th mol fraction x a or x v sinc x x. a v h composition of humid air is dscribd in th fild of air conditioning by diffrnt ways: h mass ratio calld spcific humidity or humidity ratio, ω, which rprsnts th numbr of kilograms of watr to kg of dry air in th givn mixtur : ω m m v a (.8) 9
3 h mol fraction ratio, ϖ, which rprsnts th numbr of mols of watr corrsponding to mol of dry air in th givn mixtur : a v x x ω (.9) h rlativ humidity, φ, which rprsnts th numbr mols of watr in th actual mixtur ovr th numbr of mols of watr in th saturatd mixtur at tmpratur : [ ] [ ] ) (,,, P P P x P x sat v sat sat v v φ (.) h spcific total flow xrgy of humid air is dducd from th dfinition of th physical flow xrgy applid to a mixtur of idal gass. It can b xprssd diffrntly dpnding on how th composition of humid air is dscribd (jan, 988): h spcific total flow xrgy pr mol of a humid air mixtur is: ( ) v v v a a a v p v a p a t x x x x x x R P P R c x c x,,,, ln ln ln ln (.) whr, subscript indicats ambint proprtis. wo altrnativ vrsions of this quation ar usd for nginring calculations. h first altrnativ uss th mol ratios ϖ and ϖ to dscrib th composition of th actual and th ambint air mixturs: ( ) v p a p t R P P R c c ω ω ω ω ω ω ω ω ω ln ln ln ln,, (.) h scond altrnativ rports th spcific total flow xrgy pr kilogram of dry air: ( ) v p a p t R P P R c c ω ω ω ω ω ω ω ω ω ln ln ln ln,, (.3)
3 (.4) ( ) ( ) ( ),, ln ln ln ln ω ω ω ω ω ω ω ω R P P R c c a a v p a p t h spcific total flow xrgy of dry air is dducd by stting ω and ϖ to zro. ( ) a a a p a t R P P R c ω ln ln ln,, (.5) h spcific total flow xrgy of liquid watr is also rquird for th cas of xrgy analysis of air-conditioning applications. h spcific total flow xrgy pr kilogram of liquid watr, w, yilds: ( ) ( ) ( ) ( ) ( ) w w w w w t P s P s P h P h,,,,,,, (.6) whr th partial prssur of watr vapor in atmosphric air is givn by: v w P x P,, (.7) h total flow xrgy of liquid watr can b approximatd by using th proprtis of rspctiv nighboring stats on th two-phas dom of th ollir chart (jan, 988): ( ) ( ) ( ) ( ) ( ) [ ] ( ) v f sat g f g f w t R v P P s s h h φ ln ) (, (.8) An Evaporativ Cooling Exampl (jan, 988). h vaporativ cooling systm is shown in Fig. 8.
Liquid Watr m w (, P ) ry air (, P ) Coold Humid air m ma mv a Scrn (, P ) mv mw Ambint (, P, φ ) Fig. 8. Adiabatic vaporativ cooling procss. Qustion: How much watr is ndd in ordr to lowr th tmpratur of th outgoing mixtur to a prscribd lvl? First law for th control volum: ( ) m w h (, P ) m a h ( ) m v h ( P ) m a h, a w a v w v ( ) h ( P ) h ( ) h ( P ) v m m h,, a w a v m a m a a ( ) ω h (, P ) h ( ) ωh ( P ) h, w a v v v (.9) (.3) (.3) y using approximat rlations, w obtain an xplicit rlationship for calculating th ndd humidity ratio whn th xit tmpratur is spcifid. h a ( ) ω h ( ) h ( ) ωh ( ) f a g (.3) ω h c g ( ) p, a ( ) h ( ) f (.33) Qustion: How much xrgy is bing dstroyd during th vaporativ cooling procss? 3
h xrgy balanc for th control volum is: a t, a m m w t, w m a t S gn (.34) whr t is th spcific total flow xrgy of th humid air xprssd pr kg of dry air. h rat of xrgy dstruction pr kg of dry air is, thrfor, S gn a m ω t, a t, w t (.35) h xrgy fficincy of th vaporativ coolr can b dfind as th ratio: ε t ω t, a t, w (.36) Numrical Application: 5ºC 98.5 K P atm φ.6 P sat ( ).366 bar.33 atm ω. 6. 6 9 P φ P. 6. 33. v sat a ω ω ( ) 8. 97 8. 5 (. 9) 9. (.37) (.38) Assum that P P P and 33
Assum that th function of th vaporativ cooling procss is to lowr th tmpratur to 5ºC 88.5 K From stam tabls: h g ( 88. 5K) 59. kj kg h f ( 98. 5K) 4. 77 kj kg ( 98. 5 88. 5). 3 ω 59. 5 4. 77 8. 97 ω 8. 5 (. 44). 665. 44 ln P ln ln c R R t, a p, a a a P ( ω ) (.39) Sinc P P and R ln ω ( ). 87 98. 5ln(. 9) 6 kj kg. t, a a ( ) h ( ) s ( ) s ( ) ( ) [ P P ( )] v ( ) R lnφ f g sat f v h t, w f g (.4) (.4) Sinc P P and ( ) h ( ) s ( ) s ( ) ( ) h ) s ( h ) f g f g fg fg [ P )] v ( ) P sat f ( (.4) ( (.43) thn (. 6) 7 kj kg R lnφ. 46 98. 5 ln. t, w v (.44) 34
ω t w. 44 7.. 9, kj kg (.45) t, a p, v ln ( c ωc ) p (.46) P ( ) ( ) ω ω ω R a ln Ra ω ln ω ln P ω ω Sinc P P t ( ) ( ) ω ω c ωc ln R ω ln ω ln p, a p, v a ω ω (.47) t (. 3. 44. 87). 87 98. 5 88. 5 88. 5 98. 5 ln 98. 5 98. 5. 9. 665. 665. 9 (. 665) ln. 666 ln. 63 kj kg (.48) Exrgy fficincy: ε t, a ω t, w. 63 33. 6. 9 t. (.49) Which mans that th vaporating cooling procss du to thrmodynamic irrvrsibility dstroys two thirds of th xrgy brought to th control volum. On important obsrvation that must b dalt with is that th choic of th ambint conditions affcts quit strongly th numrical rsults of th xrgy analysis. h choic of P atm is quit obvious, howvr thr is a lack of a convntion for th slction of and φ. Fig. 9 shows how th numrical rsults of th xrgy fficincy ar affctd by th slction of and φ. 35
Scond Law Efficincy, Φ.6 Scond Law Efficincy, 5 C Exrgy fficincy.4.4.38.36.34.3.3 3 4 5 6 7 8 9 3 Ambiant mpratur, ( C) Exrgy fficincy.5.4.3....4.6.8 Rlativ humidity, Φ Fig. 9. Exrgy fficincy of th adiabatic vaporativ cooling procss as a function of φ and..5 Conclusion h xrgy analysis of a thrmal nrgy systm has bn prsntd through th xrgy balanc of an opn systm. h mathmatical formulations of th various forms of xrgy and th xrgy fficincy factors wr prsntd. Fw xampls wr also prsntd to illustrat th us of th mathmatical formulation. In th contxt of spac hating and cooling in buildings, it is obvious that th us of ithr a convntional or a rational xrgtic fficincy is sufficint to compar btwn diffrnt hating and cooling systms sinc no chmical ractions ar involvd in hating and cooling applications. 36
3. Spac hating xampl asanori Shukuya and Abdlaziz Hammach 3. An xampl of hating xrgy calculation Lt us compar thr numrical xampls of xrgy consumption during th whol procss of spac hating from th powr plant, through th boilr to th building nvlop in th stady stat as shown in Fig. (Shukuya, 994). Cas assums that th thrmal insulation of th building nvlop systm is poor; that is, singl window glazing and an xtrior wall with only a thin insulation board, and a boilr with a modrat thrmal fficincy. Cas manwhil assums that th thrmal insulation of th building nvlop is improvd by a combination of doubl window glazing and an xtrior wall with improvd insulation, whil th boilr fficincy rmains unchangd. Cas 3 assums in addition that th boilr fficincy is improvd to nar its limit. abl 3 summarizs th assumptions for calculation in thr Cass. Fig.. A spac hating systm assumd for xampl calculation of xrgy consumption. Fig. shows rspctiv thr sris of xrgy input, xrgy consumption, and xrgy output from th boilr, to th watr-to-air hat xchangr, to th room air, and finally to th building nvlop in thr Cass. Exrgy consumption within th boilr systm is th largst among th sub-systms. Consuming a lot of xrgy is unavoidabl whn xtracting thrmal xrgy by a combustion procss from th chmical xrgy containd in LNG. caus of this, on may considr that th improvmnt of boilr fficincy is ssntial. h dashd lin indicatd blow Cas shows th rsult of th improvmnt of boilr fficincy from.8 to.95 in Cas. h dcras of xrgy consumption is marginal. On may, thn, considr that incrasing th outlt watr tmpratur of th boilr maks xrgy output 37
from th boilr largr and hnc th boilr mor fficint. his, howvr, rsults in th consumption of mor xrgy within th watr-to-air hat xchangr and also within th room air, in which th rquird tmpratur is 93 K ( C). hs facts imply that an xtrmly high boilr fficincy alon cannot ncssarily mak a significant contribution to rducing xrgy consumption in a whol procss of spac hating. abl 3. Assumptions for xampl calculation of xrgy consumption. Cas Hat loss cofficint of building nvlop hrmal fficincy of boilr 8.7 W/K (3. W/m K) 8 % 57. (.59) 8 3 57. (.59) 95 Hat-loss-cofficint valus in th brackts ar thos pr unit floor ara. A 6.m x 6.m x 3.m room with on xtrior wall having a.5m x 6m glazd window is assumd. h xtrior-window and wall U valus ar 6. and.67 W/m K for Cas ; 3.6 and.4 for Cass and 3. h numbr of air changs du to infiltration is.8 h - for Cas ; and.4 h - for Cass and 3. h room air tmpratur is idally controlld and kpt constant at 93 K ( C) in all cass whil th outdoor air tmpratur is assumd to b constant at 73 K ( C). Outlt air tmpratur, inlt and outlt watr tmpraturs of th hat xchangr ar assumd to b 33 K (3 C), 343 K (7 C), and 333 K (6 C), rspctivly, for all Cass. h rats of lctric powr supplid to a fan and a pump ar 3 W and 3 W in Cas ; 6 W and W in Cass and 3. h ratio of th chmical xrgy to th highr hating valu of liquidifid natural gas (LNG) is.94. h thrmal fficincy of th powr plant, that is, th ratio of producd lctricity to th highr hating valu of LNG supplid is.35. 3 5 Exrgy [W] 5 Cas Cas Spac Hating Exrgy Load 5 Cas 3 oilr Hat xchangr Room air 3 uilding nvlop 4 Fig.. A comparison of xrgy consumption for four stags of th spac hating systms. Exrgy consumption is th diffrnc in xrgy btwn input and output; for xampl, in Cas, 554 W of xrgy is supplid to th boilr and 4 W of warm xrgy is producd and dlivrd to th hat xchangr by hot watr circulation so that thir diffrnc, namly 34 W (554-4), is consumd insid th boilr. 38
h hating xrgy load, which is th xrgy output from th room air and th xrgy input to th building nvlop is 48 W in Cas and 78 W in Cas and 3. It is only 6 to 7 % of th chmical xrgy input to th boilr so that on may rgard a masur rducing th hating xrgy load as marginal. ut, as can b sn from th diffrnc in th whol xrgy consumption profil btwn Cas and Cas, it is mor bnficial to rduc th hating xrgy load by installing thrmally wll-insulatd glazing and xtrior walls than to dvlop a boilr with an xtrmly-high thrmal fficincy, in ordr to dcras th rat of total xrgy consumption. h rduction in xrgy consumption of th boilr sub-systm indicatd by th diffrnc btwn Cas and Cas 3 du to th improvmnt in boilr fficincy turns ssntially maningful togthr with th improvmnt of building-nvlop thrmal insulation. hos intrstd in numrical calculation of th xampl xplaind abov ar ncouragd to consult Appndix E, which dscribs th dtaild calculation procdur to obtain Fig.. 3. Conclusion A fw xampls of xrgy calculations wr prsntd. What is suggstd from th discussion hr is that rational passiv dsign is a prrquisit to raliz low xrgy systms for th hating and cooling of buildings. 39