The exergy approach n a legal framewor Prof Danel Favrat Insttut des Scences de l'énerge, PFL Prof Danel Favrat 1 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Preamble Introducton of an energy concept ncludng a exergetc performance ndex, n the energy law of Geneva Necessary vulgarsaton (engneerng offces, archtects, dverse customers etc.) Concerns large development or retroft projects but essentally the followng servces: electrcty, heatng, ar condtonng and refrgeraton 2
Hstory 1: combuston and heatng Smple combuston for heatng and coong (snce around 400000 years) Stll today the majorty of heatng systems (smple fuel or gas bolers, etc.) ffectveness (Frst Law effcency) n today s bolers = around 92% of the Lower Heatng Value (LHV) xergy effcency = around 16% (for an average heatng temperature of 60 C) 3 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Hstory 2 : combuston and wor (or electrcty) ntropy and Second Law ffectveness or Frst Law effcency (smple engne) xergy effcency (smple engne) ds δ T ε = =1 T cold T hot η = = q Carnot 1 T = a T Clausus Perns Gouy (bases) ε 1 T a T Heat pump Artfcal coolng Grove Fuel cell 4 1869 Desel 1919 1969 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xergy The exergy lned to a transfer or a storage of energy s defned as the potental of maxmum wor whch could deally be obtaned from each energy unt beng transferred or stored (usng reversble cycles wth the atmosphere as one of the energy sources ether hot or cold). The exergy approach allows to quantfy n a coherent way both the quantty and the qualty of the dfferent forms of energy consdered. The concept of exergy presents the major advantage of effcency defntons whch are compatble wth all cases of converson of energy resources nto energy servces (heat and electrcty, heat-coldelectrcty, refrgeraton, heat pumps, etc.) and for all domans of use of energy. These effcences are always lower than 100% and gve an ndcaton of the relatve qualty of dfferent techncal concepts n competton.
Frst Law ( ) XRGY BALANC - Second Law T ( a / T ) T a s j Carnot factor θ ( ) h cz j M j j Total coenthalpy cz ( ) d U cz P a V j ( ) 1 T a h cz j T a s j T j ( ) ( ) / dt = Total coenergy J cz a ( M ) j T a ds / dt T a δs / dt = a ( ) / dt T a δs / dt = 0 M j d U cz P a V T a S 6 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Frst Law ( ) - Second Law T a XRGY BALANC h cz j ( / T ) T a s j ( ) 1 T a h cz j T a s j T j j ( M ) j d U cz P a V ( ) j ( ) / dt = a ( M ) j T a ds / dt T a δs / dt = a ( ) / dt T a δs / dt = 0 M j d U cz P a V T a S Worexergy Heat exergy Transformaton exergy (one per networ n) xergy loss q yn L = 0 7 In ths formulaton, each term s ether postve, or negatve n ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Coenergy cartoon representaton of the heatng, cogeneraton and heat pumpng technques 8 Source (partal): Borel L, Favrat D Thermodynamque et énergétque. PPUR 2005
Defnton of terms (1) Terms lned to heat exchange (typcally across a wall) 9 q System x: engne, boler,etc. Y y Terms lned to wor exchange or electrcty (ex: through an engne shaft Terms lned to the transformaton of masses (typcally between nlet and outlet or nsde the system between tme 1 and tme 2) Conventon: exponent means conventon postve enterng exponent - means conventon postve extng xample: = 2W = 2W L xergy losses = ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Defnton of terms (2) Wor (n W) Wor exergy (n W) Frst Law Second Law (exergy) Heat (n W) q Heat exergy (n W) q = 1 T a T Y Transformaton energy (n W) y Transformaton exergy (en W) Y = M ( h cz,n h cz,out ) d(u cz P a V ) dt wth specfc enthalpy h and nternal energy U y = M ( cz,n cz,out ) dj cz dt wth specfc coenthalpy and coenergy J =U P a V T a S L xergy losses (n W) In frst approxmaton: - ndces cz whch represent the netc and potental energy are neglected - except when storage s consdered (ex: domestc boler), the terms of the dervatves wth regards to tme of the nternal energy U and of ts exergy counter part coenergy J are neglected (hypothess of steady state) 10 = h T a s ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xamples and smplfcatons (n a frst approach) y,comb = M ˆ A A M F ( ˆ F Δ 0 ) M ˆ G G M F Δ 0 q = 1 T a T q = 0 s T = T a 11 y,water y,water Ar = M water M water Fuel Complete exergy balance: Smplfed exergy balance: Moteur Gc n water out ( n out ) = c T out T n XV M water ( ) 1 T a y,f T y,comb L = XV = exergy value n frst approxmaton: XV = (LHVHHV)/2 where LHV = Lower heatng value HHV = Hgher heatng value Typcally appled to cogeneraton engnes ( out n ) wth T = T T n out 2 L = shaft shaft M F XV L = q,water shaft 0 water y,water 1 T a T water XV (Δ 0 ) (Δh 0 ) (Δh 0 s ) = h T a s ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xergy effcency of a cogeneraton engne (n a frst approach) q = 1 T a T q = 0 s T = T a Ar Fuel n Smplfed exergy balance: y,comb ngne water η = M F XV G shaft out y,comb L = shaft M F XV L = y,comb q,water = XV = exergy value (Δ 0 ) n frst approxmaton: XV = (LHVHHV)/2 where LHV = Lower heatng value HHV = Hgher heatng value shaft shaft q,water water 1 T a water 1 T a M F XV T water T water (Δh 0 ) (Δh 0 s ) 12 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
General formulaton of the balances (usng only terms whch are numercally postve) nergy (Frst Law) Y n a = wor heat Transformaton of masses nergy nput to the system n n Y n nergy output from the system nergy losses xergy balance (Frst and Second Law) q wor heat Transformaton of masses n yn L = q n yn 13 xergy nput to the system xergy losses xergy output from the system ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
ffectveness and exergy effcency ffectveness (Frst Law) (sometmes called thermal effcency) ε = n Y n n Y n Heatng heat pump ε h = cond elec xamples: Coolng heat pump ε f = evap elec! not general! xergy effcency (Frst and Second Laws) η = 14 q n yn q n yn η = General, so can be appled to both examples 1 T a T cond cond η = η = q 1 T evap T a evap ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Famly of engne cycles between two heat sources ε = η = q q q q q ε = η = q η = ε = q q? ε = q q η = q? q ε =? η = q q q q 15 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Famly of heat pump cycles (n a broad sense) between two heat sources ε =? ε =! ε = η = q η =? q q η = ε =? q η = q ε = q η = q q 16 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xergy effcency Is an ndcator of the qualty wth whch men (engneers) convert resources avalable to them Does not: Gve ndcatons concernng the use or not of renewable resources, Account for the relatve dffculty of converson of a gven prmary energy. For example solar energy wth ts low densty of radaton s more dffcult to convert than ol or gas hence usually mples lower exergy effcences, gve ndcatons on the local envronmental mpacts (pollutants affectng health) and only ndrectly gve global envronment ndcatons
From local to global Power plant (away from ctes) 1 cogeneraton and/or Heat pump DH 2 electrcty Buldng utlty plant 3 Convector, 4 Fuel (DH heat exchanger, boler, hp, cogen, etc) Radator,etc 18 η = η 1 η 2 η 3 η 4 xample: Combned cycle power plant wthout cogeneraton (1)Dstrct heatng heat pump (2) buldng DH heat exchanger (3) convector (4) η = el,1 y,1 y,2 el,2 y,3 y,3 q,4 y,4 = q,4 y,1 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xergy effcency of heatng convectors (sub-system 4) (wnter) (hypothess: heat nput to the convector at the average temperature ndquée en column 1 and heat output at room temperature de pèce,.e at 20 C, for an average atmospherc temperature of 0 C) To smplfy the year s dvded nto two seasons: a heatng season at an average atmospherc temperature of 0 C and an ar condtonnng season wth an average atmospherc temperature of 30 C. It s however mportant to pont out that t s only a assumpton to smplfy and these values cannot be used for desgn. Networ Low temperature (45/35 C) Medum temperature (65/55 C) Hgh temperature (75/65 C) Technologes Floor heatng or large convector areas or pulsed ar xergy effcency [%] 53 Convectors 38 Convectors 33 Drect electrcal lectrcal radators 7 19 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Power plants (wthout cogeneraton hence usually out of town) (sub-system 1) Technologes xergy effcency [%] Nuclear 32 Combned cycle 54 Coal 42 hydro 88 Waste ncneraton 23 Wnd 48 Solar photovoltac 11 20 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xample of smple system: drect electrc heatng (sub-systems 1 et 4) Power plant (away from ctes) 1 cogeneraton and/or Heat pump DH 2 electrcty Buldng utlty plant 3 Convector, 4 Fuel η = η 1 η 4 xample: combned cycle power plant wthout cogeneraton (1) electrcal radator (4) (DH heat exchanger, boler, hp, cogen, etc) Radator,etc 21 η = el,1 y,1 q,4 el,4 = q,4 η = 0.54 y,1 ( ) 1 273 273 20 1 = 0.037 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Buldng plant (sub-system 3) Technologes xergy effcency [%] Nomnal temperatures (supply/return) 45/ 35 C 65/ 55 C 75/ 65 C Dstrct heatng heat exchanger (supply 85 C) 54 76 86 Non condensng boler 11 16 18 Condensng boler 12 16 18 lectrcal heat pump 45 45 45 Cogeneraton gas engne 42 45 46 Cogeneraton fuel cell 49 51 52 Cogeneraton engne & heat pump 22 25 26 Cogeneraton fuel cell & heat pump 25 27 28 22 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Towards multstage heat pumps 23
Dstrct heatng heat pump (3 stages) Condenser Goteborg: 45 MW th vaporator 24 xergy effcency > 60%
25 NH3 heat pump of PFL dstrct heatng (twn-screw wth ntermedate vapor njecton, 3.5 MW th )
ffectveness and exergy effcency of heatng Frst Law η I and Second Law η II Heatng ffcences 3.5 3.0 2.5 2.0 1.5 1.0 0.5 η I η II HP HP HP HP HP HP HP 0 1 2 3 4 5 6 7 8 9 10 11 26
Smple example: buldng boler electrcty Buldng utlty plant 3 Convector, 4 Fuel boler Radator,etc 27 η = ch M LHV η = η 3 η 4 LHV XV 1 T a T ch 1 T a T pèce 1 T a T ch η = 0.16 η = ( )( 0.38) = 0.07 y,3 y,3 = 0.92(0.966) 1 q,4 y,4 273 1 273 60 1 = q,4 y,3 273 273 20 273 273 60 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Power plant wth fossl or bomass fuels (sub-system 2) (wth DH networ at an average temperature of 85 C) Technologes xergy effcency [%] Cogeneraton combned cycle 55 Cogeneraton waste ncneraton 33 Boler 20 Centralzed electrcal heat pump (sub-system 2) Technology xergy effcency [%] Dstrct heatng electrcal heat pump 61 28 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xample Power plant (away from ctes) 1 DH Heat pump 2 electrcty Buldng utlty plant 3 Convector, 4 ε LHV =0.58 (gven) η=0.56-4% grd loss η 1 =0.54 η = Fuel η = η 1 η 2 η 3 η 4 el,1 M comb XV η = ( 0.54) ( 0.61) ( 0.76) ( 0.38) = 0.094 y,2 el,2 y,3 y,3 q,4 y,4 DH heat exchanger, Radator,etc T 1 a = (273 20) M comb XV 29 gven for DH HP 273 1 273 60 273 1 273 85 273 1 273 20 273 1 273 60 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Same example but T h dff. Power plant (away from ctes) 1 DH Heat pump 2 electrcty Buldng utlty plant 3 Convector, 4 Fuel DH heat exchanger, Radator,etc η = ( 0.54) ( 0.61) ( 0.76) ( 0.38) = 0.094 30 Gven for HP gven fo HP 1 1 η = ( 0.54) 0.61 273 273 60 273 273 85 1 273 273 40 273 1 273 85 273 1 273 20 273 1 273 60 ( )( 0.54) ( 0.53) = 0.094 273 1 273 20 273 1 273 40 9.4%!! ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xample: electrcty Buldng utlty plant 3 Convector, 4 31 η = η 3 η 4 LHV η 3 = [ε el Hypotheses: XV ( ε th,hp ) ε th,hp ( ε cogen ε el ) LHV XV ] 1 T a T η 3 = [ el LHV M comb LHV XV η T hp cogen LHV ΔT hp LHV XV ] 1 T a T 273 40 273 η 3 = [0.37(0.966)0.45 ( 0.9 0.37)0.966] 1 = 0.22 40 273 40 xample:cogeneraton gas engne and buldng heat pump (3) convector (4) η = y,3 y,3 Fuel q,4 y,4 ε hp = q,4 η = 0.22 y,3 cogeneraton engne HP Radator,etc ( )( 0.53) = 0.118 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xample: Hydro power plant 1 electrcty Buldng utlty plant 3 Convector,4 40 C Fuel domestc heat pump Radator,etc 32 η = η = η 1 η 3 η 4 el,1 y,1 y,3 el,3 q,4 y,4 = q,4 y,1 η = ( 0.88) ( 0.45) ( 0.53) = 0.212 after account of 4% of grd losses ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
xamples of technologes Power plant Dst. plant Supply/return temperatures 45 / 35 Buldng plant Room convector Overall exergy effcency [%] Drect electrc heatng (nuclear power) 0.32 0.07 0.07 0.07 2.2 2.2 2.2 Drect electrc heatng (combned cycle cogeneraton) 0.55 65 / 55 75 / 65 45 / 35 65 / 55 75 / 65 45 / 35 65 / 55 75 / 65 0.07 0.07 0.07 3.7 3.7 3.7 Drect electrc heatng (hydro power) 0.88 0.07 0.07 0.07 6.0 6.0 6.0 Dstrct boler 0.2 0.54 0.76 0.86 0.53 0.38 0.33 5.8 5.8 5.8 Buldng non-condensng boler 0.11 0.16 0.18 0.53 0.38 0.33 6.1 6.1 6.1 Buldng condensng boler 0.12 0.53 6.6 Dstrct heat pump (nuclear power) 0.32 0.61 0.54 0.76 0.86 0.53 0.38 0.33 5.6 5.6 5.6 Domestc heat pump (nuclear power) 0.32 0.45 0.45 0.45 0.53 0.38 0.33 7.6 5.4 4.8 Domestc cogeneraton engne and heat pump Dstrct heat pump (combned cycle power) Domestc heat pump (combned cycle power) Domestc heat pump (cogeneraton combned cycle power) Cogeneraton fuel cell and domestc heat pump 0.22 0.25 0.26 0.53 0.38 0.33 11.8 9.4 8.7 0.54 0.61 0.54 0.76 0.86 0.53 0.38 0.33 9.4 9.4 9.4 0.54 0.45 0.45 0.45 0.53 0.38 0.33 12.9 9.2 8.1 0.55 0.45 0.45 0.45 0.53 0.38 0.33 13.2 9.4 8.3 0.25 0.27 0.28 0.53 0.38 0.33 13.4 10.4 9.5 Dstrct heat pump (hydropower) 0.88 0.61 0.54 0.76 0.86 0.53 0.38 0.33 15.4 15.4 15.4 Domestc 33 heat pump (hydropower) 0.88 0.45 0.45 0.45 0.53 0.38 0.33 21.2 15.1 13.3 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Ar condtonng or refrgeraton convector (Summer hypothess: cold nput to the convector at an average temperature gven n column 1 and cold output to the room at 20 C, for an average atmospherc temperature of 30 C) Networ temperatures Technologes 10/15 C Water or ar 56 5/10 C Water or glycol water 43 0/5 C glycol water 34-5/0 C glycol water /ce slurry 28-10/ 5 C glycol water 24-15/ 10 C glycol water 21-20/ 15 C glycol water 18 xergy effcency [%] 34 xample of calculaton η = T a T 1 = T a T 1 273 30 273 20 1 273 30 27312.5 1 = 0.56 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Ar condtonng / refrgeraton (hypothess : electrc compresson chller wth an exergy effcency of 40%) Power plant technologes Power plant Supply/return temperatures Dst. plant Buldng plant 10 / 15 5 / 10 0 / 5 10 / 15 Room convector 5 / 10 0 / 5 Overall exergy effcency [%] Nuclear power 0.32 0.4 0.4 0.4 0.56 0.43 0.34 7.1 5.4 4.3 10 / 15 5 / 10 0 / 5 Gas motors 0.36 0.4 0.4 0.4 0.56 0.43 0.34 8.1 6.2 4.9 Combned cycle power plant wthout cogeneraton 0.54 0.4 0.4 0.4 0.07 0.07 0.07 12.1 9.3 7.3 hydropower 0.88 0.4 0.4 0.4 0.53 0.38 0.33 19.8 15.2 12.0 35 ÉCOL POLYTCHNIU FÉDÉRAL D LAUSANN
Conclusons Introducng an exergy ndcator exerge n a law on energy hghlghts: The mportance of a low temperature heatng, respectvely of a hgh temperature coolng The wea effcences of today s domnant heatng technologes and the prospect for mprovement A coherent ranng of the alternatves facltatng a better plannng of future energy systems After ths frst step a major effort s stll requred to educate staeholders and secure ts mplementaton at all levels It s vtal to ensure a rgourous teachng of exergy at all professonal and unversty levels 36