SIMPLIFIED MODEL-BASED OPTIMAL CONTROL OF VAV AIR- CONDITIONING SYSTEM

Transcription

1 Nnth Internatonal IBPSA Conference Montréal, Canaa August 5-8, 2005 SIMPLIFIED MODEL-BASED OPTIMAL CONTROL OF VAV AIR- CONDITIONING SYSTEM Nabl Nassf, Stanslaw Kajl, an Robert Sabourn École e technologe supéreure, Montreal, Canaa ABSTRACT A smplfe optmzaton process (SOP) for etermnng set ponts s propose an evaluate usng the montorng ata an moel of an exstng VAV system. Controller set ponts, such as supply ar temperature, supply uct statc pressure, an chlle water supply temperature, are etermne by ths propose SOP n orer to mnmze energy use whle respectng thermal comfort. Zone ar temperatures are also consere n orer to obtan further energy savngs. The propose SOP uses the smplfe VAV system moel an certan montore varables avalable n the exstng control system. The evaluaton results on the exstng VAV system obtane by the SOP are consstent wth that reache through the etale optmzaton process (DOP) propose elsewhere. It s conclue that the SOP propose here coul be mplemente to etermne on-lne controller set ponts wthout requrng etale calculatons, nclung the etale VAV moel an optmzaton program. INTRODUCTION Great efforts have been mae n orer to mnmze the energy use assocate wth the operaton of the HVAC system. One of these has nvolve the mprovement of the VAV system performance through the optmzaton of controller set ponts (ASHRAE 2003). Much research has been one wth a vew to such optmzaton on mult-zone VAV systems (Nassf, Kajl, an Sabourn 2004a an Wang an Jn 2000). However, these requre accurate VAV moel an optmzaton algorthms. In aton, certan set ponts etermne by these methos are not verfe n real tme by montore ata for representatve local-loop control; an example s the calculate (or optmze) chlle water temperature, whch s not verfe on-lne through the control valve openng. Ths paper thus proposes a smplfe optmzaton process (SOP) for etermnng set ponts such as supply ar temperature, supply uct statc pressure, an chlle water supply temperature. The avantage of the SOP propose over what has been one n prevous research eneavors s that the SOP oes not requre a etale VAV moel an optmzaton program. Another avantage s that the montore ata for representatve local-loop control s chece on-lne, followng whch the controller set ponts are upate. In ths case, the SOP ensures proper operaton by optng for real stuatons wth mnmum energy use. The smplfe optmzaton process (SOP) s evaluate usng the montorng ata an moel of an exstng VAV system. The smulate energy use s compare to that for the exstng VAV system. The smulaton results obtane by the SOP are also compare wth the results obtane by our optmzaton process evelope elsewhere (Nassf, Kajl, an Sabourn 2005), whch we refer to as the etale optmzaton process (DOP). The comparson ncates that the controller set ponts etermne by the SOP are close or equal to the optmal ones (obtane by the DOP). PROPOSED OPTIMIZATION PROCESS Ths paper proposes a smplfe optmzaton process (SOP) for etermnng the controller set ponts of a mult-zone VAV system. The SOP conssts of: () controller set pont strateges an () a smplfe VAV moel, as shown n Fgure. These two parts are presente an scusse n the next two chapters. The SOP s base on the smulaton of the response of the VAV system performance to the propose changes of controller set ponts. At each smulaton step, three controller set pont values are propose an stue usng the VAV moel; ths s one n orer to select one value for each controller set pont corresponng to the best performance of the VAV system, whch then becomes the best controller set pont. The three propose controller set pont values (propose controller set ponts) are obtane as follows: The propose values of supply ar temperature set ponts nclue the current value an the values obtane by ncreasng or ecreasng the current value by a small fxe amount (see Equaton 3). Verfcatons of the propose values are one by usng the measure ata, such as supply an zone ar temperatures an zone arflow rates (see Equatons, 2 an 4) n orer to respect thermal comfort

2 The chlle water supply temperature an uct statc pressure set ponts are then etermne accorngly usng strateges presente later n the chapter, Strateges for Controller Set Ponts. Strateges are evelope for etermnng the best uct statc pressure an chlle water supply temperature set ponts at any propose supply ar temperature set pont. In these strateges, the measure ata for representatve local-loop control, such as the postons of the coolng col valve an the VAV box ampers, are use to ncrementally upate the controller set ponts (see Equatons 8 an 0). The smplfe VAV moel evelope an presente n the next chapter etermnes the change n the energy emans of system components for the three controller set pont propose values. The selecte (or the best) controller set ponts correspons to the lowest energy emans. The SOP s evaluate by comparng the smulate energy use wth the results obtane by: () the montorng or moel of an exstng VAV system an () our etale optmzaton process (DOP) evelope elsewhere (Nassf, Kajl, an Sabourn 2005). Ths DOP nclues: () the etale VAV moel, () the two-objectve genetc algorthm optmzaton program, an () an noor thermal loa precton tool. The objectve functons of the DOP are thermal comfort an energy use. To be able to compare the DOP wth the SOP, the former must be mofe to a one-objectve optmzaton problem. The mofe DOP presente here hanles the total energy use as the objectve functon an the thermal comfort as the constrant. In aton, the loa precton tool was also smplfe n the DOP presente n ths paper. A smple loa tool s apple, base on the assumpton that noor sensble loas are equal to the amount of coolng that termnal boxes prove as a prouct of the zone arflow rate an the fference n temperature between the supply an the zone ar. Fgure shows the schematcs of etale an smplfe optmzaton processes (DOP an SOP). In the DOP, the genetc algorthm program sens the controller set ponts to the etale VAV system moel at each optmzaton pero (.e., 5 mnutes), where the energy use (objectve functon) s smulate an returne to the genetc algorthm program. The etale VAV moel s use to etermne the energy use. VAV SYSTEM MODEL The etale VAV system moel evelope an valate (Nassf, Kajl, Sabourn 2004b) s use by the DOP, whle the SOP uses the smplfe one. When the SOP s evaluate, the etale component moels are also use for smulaton purposes. Measure ata Fgure Schematcs of the etale an smplfe optmzaton processes (DOP an SOP) Smplfe VAV component moels The smplfe VAV component moels use by the SOP are evelope to be use n etermnng energy emans at any propose controller set pont. The varatons of the fan, system heat, zone reheats, an chller energy emans are etermne through varatons of the controller set ponts. Durng a small smulaton tme step, the thermal loas are assume to be constant an the relaton between the supply ar temperature (Ts) an the zone arflow rate ( ), at a constant zone ar temperature (Tz) coul be presente as follows: ρ c p ( Tz Ts ) = ρ c p ( Tz Ts ) () DOP Loa precton tool Genetc algorthm Detale VAV moel Controller set ponts The specfc heat c p an ar ensty ρ coul be consere to be constant. The subscrpts, an -, respectvely ncate the current an prevous peros. Fan energy eman ( Wf ) s a functon of the fan arflow rate ( Qf ) an of total statc pressure. The latter s equal to the sum of the uct statc pressure set pont (Ps) an the remanng uct statc pressure rop (VAV component uct), whch s a functon of the fan arflow rate an the flow coeffcent Cf etermne at esgn contons. The relaton between the current an prevous fan energy emans s gven by: 2 Qf Ps + C Qf f Wf = Wf (2) 2 Qf Ps + C Qf f The fan arflow rate ( Qf ) s equal to the sum of the zone arflow rates ( ) that are calculate by: Tz Ts = (3) Tz Ts SOP Detale escrpton n Fgure 2 Set ponts strateges Smplfe VAV moel Controller set ponts

3 The zone an fan arflow rates are lmte by ther maxmum an mnmum values. When the VAV box ampers are not we open, the zone ar temperatures (Tz) coul be assume to be equal to ther set ponts (.e., 22.5 o C). As shown n Equatons 2 an 3, any change n the controller set ponts (Ps an Ts) generates a change n fan energy eman. It shoul be note that the real-tme measure ata at the prevous tme (-) taes nto account the varatons of thermal loas an outoor ar contons over tme. As mentone above, the thermal loas an outoor contons are assume to be constant urng a small smulaton tme step. Thus, the chller energy eman coul be presente only as a functon of chlle water supply temperature Tw (Wang an Jn 2000): [ + Cc ( Tw Tw ] = Wc Wc ) (4) The parameter Cc can be obtane at esgn system operaton an s approxmately constant for a chller. A comparson of the fan an chller energy emans obtane by Equatons 2 an 4 wth those obtane wth etale fan an chller moels (DOP) shows the acceptable accuracy (wthn 5%), whch mproves when the varable changes (Ps, Tw, Ts) are small. Snce zone ar temperature set ponts are ept constant, the zone ar temperatures, an consequently the temperature obtane from the combnaton of outoor an return ar (Tm), coul be assume to be constant urng a small smulaton tme step. Therefore, the system heat coul then be etermne: Wh = Qf c p ρ ( Ts Tm ) λ (5) The actual zone ar temperature wll only ffer from the set ponts (.e o C) n a lmte number of zones (crtcal zones), as wll be scusse later. Evently, the heat system s not consere when the Tm s greater than Ts. The λ s the converson factor to electrcty energy eman. It s equal to f the electrcty heat system s use. The zone reheat s turne on when the zone arflow rate reaches ts mnmum lmt ( ), an ar mn temperature n the zone s ecrease to a mnmum level (Tzmn). Zone reheats ( Wz ) coul be etermne by Equaton (by ang local reheats): max = Wz + c ρ p Wz max mn α z [( Ts Ts ) + ( Tz Tz )] (6) mn α = (7) z where the max s the maxmum zone arflow rate at esgn uct statc pressure. If the zone reheat s not use at a prevous tme ( Wz ), the effect of the supply ar temperature = 0 ecrease on the local reheat s seen only when the ar zone temperature s close to Tzmn. If the fference (Tz - -Tz mn ) s less than the value of the change n the supply ar temperature set pont (Ts - Ts ), the zone reheat wll tae a negatve value, whch then converts to zero. When the zone reheat s use at a prevous tme ( Wz 0 ), the last term of the equaton above becomes 0 (Tzmn=Tz - ). Equatons 2, 4, 5, an 6 show the varatons n energy eman n response to the varatons of the controller set ponts. It s nown that the supply uct statc pressure set pont shoul be ecrease for fan energy savngs, an that the chlle water supply temperature shoul be ncrease for chller energy savngs. However, the supply ar temperature set pont, whch has conflctng effects on these component energy emans, lays bare the optmzaton problem. Three supply ar temperature values are propose at each smulaton tme, an the assocate total energy eman s smulate n each case. The selecte value then correspons to the least total energy eman. STRATEGIES FOR CONTROLLER SET POINTS As ncate earler, the response of the VAV system performance coul be smulate for any propose controller set ponts. The supply ar temperature set pont ncreases or ecreases by a small fxe value, whle respectng the thermal comfort n the zones. The chlle water supply temperature an uct statc pressure set ponts are then accorngly etermne usng the strateges evelope below. These strateges ensure that the chlle water supply temperature an uct statc pressure set ponts leas to best value an they prove a proper system operaton. The zone ar temperature set ponts n the SOP are not optmze but the zone ar temperatures are nvestgate as presente next. Zone ar temperature Typcally, zone ar temperatures are mantane at constant set ponts n the comfort zone urng occupe peros. However, urng unoccupe tmes, the set ponts are set up for coolng an set bac for heatng, n orer to reuce energy use. A strategy usng the optmzaton of nvual zone temperature set ponts combne wth other controller set ponts urng occupe peros coul further reuce system energy use (Nassf, Kajl, Sabourn 2005). The zones wth the hghest or lowest zone arflow ratos (Ra) are calle crtcal zones. The Ra s the rato of the zone arflow rate to the esgn maxmum arflow rate. The number of crtcal zones selecte wth the hghest Ra ratos s ncate by

4 N max whle the number of crtcal zones selecte wth lowest ratos Ra s ncate by N mn. The strategy, whch s apple by the SOP, nvolves eepng all zone temperature set ponts constant. However, the zone ar temperatures n crtcal zones (N max + N mm ) may be move away from ther set ponts, but shoul always be ept wthn preetermne mnmum or maxmum levels (.e. Tz max = 24.5 o C an Tz mn =2 o C). Ths strategy allows further ecreases or ncreases of the controller set ponts, an thus proves savngs n energy use. In ths case, the crtcal zones (extreme ones) have not prorty to etermne the controller set ponts wth respect to thermal comfort n the zones. Many strateges coul be apple to etermne N max an N mn. A stanar evaton σ, of the normal strbuton calculate by zone arflow ratos Ra coul be use. The number of crtcal zones wll then be hgh when the loa strbutons between zones are sgnfcant. When there s no thermal loa strbuton (zones perform as one zone), the N max an N mn coul be zero. In orer to compare the SOP wth the DOP, the ar temperature set ponts of the crtcal zones are optmze by the DOP, an are ept constant n other (non-crtcal) zones. In aton, N max =3 an N mn =4 are assume to be constant for the SOP an the DOP. Duct statc pressure set pont The uct statc pressure wll ensure the proper operaton of the zone VAV boxes uner varyng loa contons. For a fxe uct statc pressure set pont, all the VAV boxes ten to close as zone loas an flow requrements ecrease. Sgnfcant fan energy savngs are possble f the uct statc pressure set pont s reset such that at least one of the VAV boxes remans open. Several fferent strateges base on ths concept are propose (Englaner an Norfor 992). Our strategy n ths paper taes nto conseraton ths concept as well as the effect of changng other set pont. Assumng that the uct statc pressure ensures proper operaton at esgn contons wth at least one of the VAV boxes fully open, the esgn uct statc pressure (Ps es ) s consere as optmal uner esgn operaton contons. The new (or optmal) statc pressure set pont (Ps) at off-esgn contons coul be smplfe by the frst term of the next equaton (Nassf, Kajl, an Sabourn 2005): 2 ( Ra ) Ps + a ( 0.98 es ps Ps θ ) (8) = hghest The hghest Ra value s consere above, exclung the crtcal zones, whle ther ar temperatures are lower than maxmum values (Tz max ). It s clear that the uct statc set pont etermne above s less than the esgn value at off-esgn contons ( Ra < ). A further ecrease n the uct statc pressure set pont, an consequently n fan energy, coul be also obtane f the hghest Ra values n the crtcal zones N max are not consere whle the ar temperatures are lower than the maxmum values. In orer to tae nto conseraton the real-tme system operaton, the amper poston (θ - ) of the VAV boxes s chece. When the θ - s less than 99% an greater than 97%, the term a ps s equal to zero. Otherwse, t s a fxe value {.e. a ps =0.0 Ps es =2.5 Pa}. Chlle water supply temperature set pont Wth a fxe-spee pumpng chller system, the chlle water temperature set pont must be ajuste to mantan all supply ar temperatures of AHUs wth a mnmal number of coolng col control valves n a saturate (full open) conton (ASHRAE 2003). In Chapter 4 of ASHRAE 2003, the chlle-water temperature (Tw) can be reset n response to suen changes n loa an supply ar temperature set pont: PLR Tw = Ts ( Ts Tw) (9) 0 PLR 0 The equaton above assumes that the chlle water supply temperature assocate wth the last ecson control (ncate by nex 0) was optmal. Assumng that the esgn chlle water supply temperature set pont uner the esgn operaton contons s optmal; the esgn conton coul be use as the last ecson control: Tw Ts PLR Ts Tw) + a (0.98 θ v ) (0) = ( es v For the nvestgate exstng system, the fference (Ts-Tw) es at esgn contons s equal to 5 o C. To conser the real ata, the last term s ae. When the coolng col valve openng θv - s less than 99% an greater than 97%, the term a v s equal to zero. Otherwse, t s a fxe value {.e. a v =0.0(Ts-Tw) es =0.05 o C}. The part loa rato PLR (current coolng col loa to esgn one) coul be calculate from the water or ar se epenng on measure ata. In ths paper, the sensble thermal rato s calculate by usng the fference between the mxng plenum ar temperature an the supply ar temperature. Supply ar temperature set pont As mentone earler, the supply ar temperature set pont has conflctng effects on component energy emans. When ts value s hgh, t may allow a hgher chlle water supply temperature set pont an assocate mprovement n chller effcency. However, when t s low, t wll ecrease fan energy use. As a result, the supply ar temperature set pont shoul not be set too hgh as t may provoe unercoolng n certan zones. When the mnmum arflow rate ntrouce nto nternal zones (zone reheat oes not exst) s lmte n orer to meet ventlaton requrements, the low supply ar temperature set pont may also cause over-coolng n certan zones

5 The supply temperature set pont must thus be properly selecte n orer to mantan the requre comfort n each zone. In the SOP ths s one through a verfcaton of any propose supply ar temperature. The verfcaton (as we wll see n Equaton 4) s meant to ensure that the propose change n the supply ar temperature set pont (.e. 0. o C) s at a level that s lower that the maxmum allowe change, wthout affectng thermal zone comfort. Thus, t s assume that thermal comfort s respecte when the zone ar temperatures n noncrtcal zones are mantane at ther requre set ponts whle those n crtcal zones are wthn maxmum an mnmum lmts [Tz mn - Tz max ]. Ths prevous supply ar temperature set pont coul be ncrease by a maxmum value a,max an ecrease by a maxmum value a,max, an these parameters are etermne by Equaton, where {Ts =Ts - +a,max an = for a max,max an Ts =Ts - a,max an for a = mn,max}. { Tz ( Ra) + Ts ( Ra ) + ( Tz Tz) } a, max = max a mn () Ra Ra mn Tz + Ts + α z α z ( ) (2) Tz Tz, max = mn The parameters above are etermne for each zone, an the mnmum values are selecte to tae nto account the fact that the zone ar temperature Tz must not be hgher than Tz max (or lower than Tz mn ). It shoul be note that all varables n Equatons an 2 are taen from prevous measure ata (-). Three values of supply ar temperature set ponts propose to smulate the performance of the VAV are gven by: ( ). ( ). ( ). Ts Ts Ts = Ts = Ts = Ts + a o + a a The parameters n Equaton 3 are etermne as: (). a = 0. (2). f a (3). f a (5). f a (6). f a (4). f a f a f a,max,max,max a, max, max = 0. a = a p 0 a = a p 0 a a p 0 an a a o = a,max 0, max = a, max o = 0 = a = a p 0 a = a o = a (3) (4) If the zone thermal comfort s ensure, then a o s equal to zero an a an a are equal to a fxe value, 0. o C. When there s a local zone reheat, the a,max s not calculate ue to fact that the supply ar temperature coul be ecrease wthout affectng the zone ar temperatures (zone temperature s mantane by local reheat). When the a,max s negatve, t means that the arflow rate reaches ts maxmum value an the zone ar temperature s hgher than Tz max. Thus, the supply ar temperature set pont shoul only be ecrease (number 4 of Equaton 4). When the a, s negatve, t means that the mnmum arflow rate has reache ts mnmum value an the zone ar temperature s less than Tz mn. Thus, the supply ar temperature set pont shoul be ncrease (number 5 of Equaton 4). To mantan ar temperatures at ther set ponts n non-crtcal zones, the supply ar temperature set ponts shoul be lmte (usng Equaton ): Ts lm = 22.5 D ( Tz n Ts ) (5) For a hgh lmt, the D s Ra an the Tz n s the temperature n the zone havng the hghest Ra, but exclung the crtcal zones (N max ). For a low lmt, the D becomes Ra/α an the Tz n s the temperature n the zone havng the lowest Ra, exclung the crtcal zones (N mn ) SIMPLIFIED OPTIMIZATION PROCESS CALCULATIONS As mentone earler, the SOP conssts of controller set pont strateges an a smplfe VAV moel. The goal of the former s to propose three sets of controller set ponts at each smulaton tme step, whle the latter ams to calculate the assocate energy emans for each set, an thus allow the selecton of the set corresponng to the least energy eman. Fgure 2 shows the smplfe optmzaton process calculatons. Step#: The values of a,max an a,max are etermne by Equatons an 2, usng prevous ata of zone arflow ratos (Ra) an zone an supply ar temperatures (Tz an Ts). The parameters of equaton 3 are etermne usng Equaton 4. The N max an N mn propose here are 3 an 4. Fan energy Chller energy Reheat energy (Calculate or montore values) Smplfe VAV moel (Step#4) Fan, Chller, local reheat, an system heat energy emans Equatons 2, 4, 5, an 6 Sum of Fan, Chller, local reheat, an system heat energy emans Fgure 2 Smplfe optmzaton process calculatons Montore ata Zone an supply ar temperatures Zone arflow rates VAV box an valve postons Chlle water temperature Control strateges Verfcatons of propose values (Step#) (Equaton, 2, an 4) Supply ar temperatures (three values) (Step#2), (Equaton 3) Chlle water temperature an uct statc pressure (Step#3) (Equatons 0 an 8) Controller set ponts selecte corresponng to least energy emans

6 Step#2: The propose supply ar temperature set pont values are etermne by Equaton 3 respectng the lmts presente n Equaton 5. Zone arflow rates (an ratos Ra ) are etermne by Equaton 3. These values are lmte wthn ther upper an lower values. Step#3: The chlle water supply temperature set pont s calculate by Equaton 0. PLR s etermne usng the measure mxng plenum an supply ar temperatures (Tm an Ts) an the calculate fan arflow rate. The uct statc pressure set pont s etermne by Equaton 8. Step#4: The fan, chller, system heat, reheat, an consequently, total energy emans for the propose values of controller set ponts, are etermne by Equatons 2, 4, 5, an 6 of the smplfe VAV moel. The best controller set ponts, corresponng to the least energy eman, are selecte. EVALUATION AND DISCUSSION The SOP s evaluate on the exstng HVAC system nstalle at the École e technologe supéreure (ÉTS) campus. Two ar-hanlng unts of mult-zone VAV systems (AHU-4 an AHU-6) are nvestgate. The AHU-4 meets the loa for 68 west permeter zones, whle the AHU-6 meets the loa for 70 nteror zones. Controller set ponts are etermne by the followng three supervsory control strateges: Strategy S : controller set ponts are exactly the same as n the exstng system Strategy S 2 : controller set ponts are etermne by the SOP Strategy S 3 : controller set ponts are etermne by the DOP. In the exstng system (Strategy S ), the chlle water supply temperature an uct statc pressure set ponts are constant at 7 o C an 250 Pa, respectvely. The supply ar temperature set pont of the exstng system (strategy S ) s set by the operator n the AHU-6 system (.e. 4 o C). However, ths set pont s etermne by applyng the followng strategy (for the AHU-4 system). The set pont changes lnearly wthn the 3 to 8 o C range, wth the outoor temperature between 20 an +20 o C. The set pont calculate above s correcte by ang a value whch vares lnearly from 2 to +2 o C, whch correspons to the varaton of the fan arflow rato from 50% to 90%. The set pont s always lmte between 3 an 8 o C. However, n strateges S 2 an S 3, ths set pont s only lmte at the low value (3 o C). The lowest supply uct statc pressure an hghest chlle water supply temperature set ponts are lmte (50 Pa an o C, respectvely) The evaluatons are one for three wees uner fferent weather contons (summer, mseason, an wnter), but are presente here only for three fferent ays (ay#, ay#2, an ay#3). As mentone above, a hgh supply ar temperature may ecrease chller energy use an a low one may ecrease fan energy use. The best selecte supply temperature epens then on the value of the chller an fan energy use. Thus, two followng cases are assume:. The chller energy use s much hgher than the fan energy use (about fve tmes). The AHU-6 s stue n ths case. There s one chller servng all ar hanlng unts (nclung the AHU-6) that supply contone ar to the ÉTS campus. It s assumng that the chlle water supply temperature s etermne by the AHU-6 system. The montorng ata for two years showe that ths assumpton s qute realstc. As we wll see next (see Fgure 4), to mnmze energy use, the supply ar temperature set pont tens to be hgh n orer to save chller energy, whch becomes hgher than the fan energy. 2. The chller energy use s close to the fan energy use. The AHU-4 system s stue n ths case, assumng that t s serve by one small chller. As wll see next (see Day# n Fgure 8), to mnmze energy use, the supply ar temperature set pont tens to be low n orer to save fan energy, whch becomes hgher than the chller energy. AHU-6 System Fgures 3 through 5 show energy emans, supply ar an chlle water temperatures, an uct statc pressure for three nvestgate strateges (S, S 2, an S 3 ). It s note that there are no local reheats nstalle n the nteror zones serve by the AHU-6 system. The controller set ponts an resultng energy eman etermne by the SOP s very close to the value etermne by the DOP. Snce these two strateges start at 9:00 usng real set pont values (.e., 4 o C for supply ar temperature, 7 o C for chlle water supply temperature, an 250 Pa for supply uct statc pressure), the controller set ponts etermne by the SOP nees a certan amount of tme to reach the optmal values etermne by the DOP ue to a small ncremental change n supply ar temperature. Ths coul also happen when the thermal loas change sgnfcantly. The ar temperatures n the zones must be mantane at the requre set ponts. However, a certan amount of floatng s allowe between maxmum an mnmum levels n crtcal zones. Fgure 6 shows the zone ar temperatures etermne by the propose SOP. It s able to mantan all requre zone temperatures (except the crtcal zone temperatures) at the set pont (22.5 o C). As has been state, the values of Ra n the crtcal zones (N max =3) are exclue n the calculaton of the uct statc pressure set pont, whch becomes less than 250 Pa, an further fan energy savngs are obtane. Snce the ar temperatures n the crtcal

7 zones are allowe to float wthn maxmum an mnmum values, there s consequently lttle restrcton wth respect to selectng the supply ar temperature set pont. Ths leas to further energy savngs an a convenent system operaton. For example, n Fgure 6, the ar temperature of one of the crtcal zones s at approxmately 2 o C at 2:00. To mantan the temperature n ths zone at 22.5 o C, the supply ar temperature set pont shoul be ncrease, whch coul lea to poor thermal comfort n other crtcal zones havng hgher arflow ratos. Fgure 6 Zone ar temperatures for strategy S 2 Fgure 3 Energy emans n AHU-6 for summer ay Fgure 4 Supply ar an chlle water temperature set ponts n AHU-6 for summer ay Fgure 5 Duct statc pressure set pont n AHU-6 for summer ay The set ponts are also etermne for mseason an wnter ays. Snce the AHU-6 system servng the nteror zones only yels coolng thermal loas, the results are not much fferent from what s scusse above, an are thus not presente here. Comparng the results obtane by the three nvestgate strateges, we foun that the SOP s able to successfully etermne the set ponts that are close to those etermne by the DOP consere to be optmal. The energy savngs obtane for three wees s 6.2% when the SOP s apple an 6.6% when the DOP s apple, versus the energy use by exstng system. AHU-4 System The three nvestgate strateges are also evaluate on the AHU-4 system. Fgures 7 an 8 show the energy emans an supply ar an chlle water temperature set ponts. All zones serve by the AHU- 4 have the same orentaton (south-west). Consequently, the values of Ra are qute the same, an thermal comfort restrctons are less sgnfcant. On Day#, the supply ar temperature an resultng uct statc pressure set ponts for two strateges, S 2 an S 3, are the same. Gven that the thermal loas, an consequently the zone arflow rates, are relatvely low on Day#2 an Day#3, the uct statc pressure set ponts are at ther lowest value (.e. 50 Pa), an so they are not llustrate. It shoul be note that the coolng col valve s close before :00 on Day#2. On wnter ays (Day #3), great energy savngs are obtane by the DOP an the SOP. The fan energy eman oes not vary sgnfcantly wth the varaton of supply ar temperature, ue to the saturaton of the fan arflow at ts low lmt (40% of esgn value). The man energy eman s from zone reheats. The supply ar temperature set ponts etermne by the SOP an the DOP are hgher than that for the exstng system. As a result, the requre local reheats are lower than for the exstng one. The supply ar temperature set ponts etermne by the SOP an the DOP coul be equal to the return an requre outoor ar mxng temperature

8 Fgure 7 Energy emans of AHU-4 for three nvestgate ays the exstng system. Energy savngs obtane for three wees coul be 6.2% when the SOP s apple to the exstng AHU-6 system. Comparng SOP wth the etale optmzaton propose DOP, t s foun that SOP s capable of successfully etermnng the set ponts that are close to those obtane by the DOP, an consere as optmal. The savantage of usng the SOP nstea of the DOP s that the controller set ponts etermne by the SOP nee a certan amount of tme to reach the optmal values etermne by the DOP when the outoor contons or thermal loas are sgnfcantly change. Ths coul be overcome by usng a ynamc ncremental value, whch taes nto conseraton the varatons of the supply ar temperatures urng a sample prevous pero. Thus, the propose SOP coul be mplemente n orer to etermne the on-lne controller set ponts wthout requrng etale calculatons, nclung the VAV moel an optmzaton program. REFERENCES Fgure 8 Supply ar an chlle water temperature set ponts of AHU-4 for three ays We conclue that the performance of the SOP propose n paper s close to that of the DOP. To mprove the performance of the SOP, the ynamc ncremental value coul be use nstea of a fxe value (0. o C) n orer to ncrease ts responses wth any sgnfcant conton changes (Day#3 n Fgure 8). For example, the fxe ncremental value coul be ajuste to tae nto conseraton varatons of the supply ar temperatures urng a sample prevous pero. The propose strategy S 2 (SOP) proves great energy savngs compare to strategy S (exstng one). As shown n Fgure 7, the energy savngs are hghest on wnter ays. The etale VAV moel use by the DOP s consere to be accurate. Thus, the controller set ponts, such as uct statc pressure an chlle water temperature, ensure that at least one of the VAV box an coolng col valves are we open. However, snce n realty, the VAV moel s not completely accurate, these set ponts coul not qute so optmal, n vew of whch, the SOP, by usng the montore ata (VAV box an coolng col valve openngs) coul perform better than the DOP unless the DOP were to use a very accurate an aaptve VAV moel. CONCLUSION Evaluatons of smulatons one on the exstng HVAC system show that the smplfe optmzaton process SOP evelope n ths paper proves great energy savngs compare to the strategy apple n HVAC Applcatons ASHRAE hanboo. Amercan Socety of Heatng, Refrgeratng an Ar-contonng Engneers, Atlanta, USA. Branemuel, M. J., Gabel, S., an Anersen, I A Toolt for Seconary HVAC System Energy Calculaton. Unversty of Colorao at Bouler. Englaner, S. L. an. Norfor, L.K Savng fan energy n VAV systems parts 2: Supply fan control for statc pressure mnmzaton usng DDC zone feebac. ASHRAE Transactons, 98 (), Nassf, N., S. Kajl, an R. Sabourn Optmzaton of HVAC Control System Strategy Usng Two-Objectve Genetc Algorthm. HVAC&R (3): In press. Nassf, N., S. Kajl, an R. Sabourn. 2004a. Two- Objectve On-Lne Optmzaton of Supervsory Control Strategy. Bulng Serv. Eng. Res. Technol 25(3): Nassf, N., S. Kajl, an R. Sabourn. 2004b. Moelng an Valaton of Exstng VAV System Components. Proceeng of Esm, Canaan Conference on Bulng Smulaton, Vancouver, Canaa. Wang, S. an X. Jn Moel-Base Optmal Control of VAV Ar-Contonng System Usng Genetc Algorthm. Bulng an Envronment 35: Wang, S Dynamc Smulaton of Bulng VAV Ar-Contonng System an Evaluaton of EMCS On-Lne Control Strateges. Bulng an Envronment,