MODELLING OF HVAC SYSTEM COMPONENTS FOR BUILDING DYNAMIC SIMULATION

Transcription

1 13th Coferece of Iteratioal Buildig rformace Simulatio Associatio, Chambéry, Frace, August MODELLING OF HVAC SYSTEM COMPONENTS FOR BUILDING DYNAMIC SIMULATION Luigi Schibuola, Massimiliao Scarpa, ad Chiara Tambai Uiversity IUAV of Veice, Veice, Italy ABSTRACT The reliable assessmet of the aual eergy demad has become ecessary i view of buildig eergy performace certificatio. Accurate models must be used to simulate the behaviour of HVAC compoets i real operatio, usually characterized by a wide variatio of buildig loads. I this cotext, this paper deals with the developmet ad validatio of a algorithm aimed at the assessmet of part load performace of various kids of cotrols for vapour compresio based heat pumps ad chillers, i particular referrig to o-off, iverter-drive ad multi-stage vapour compressio. The reliability of this algorithm i the calculatio of seasoal performaces is checked agaist moitorìg of heat pumps ad chillers operatig uder real coditios. INTRODUCTION Nowadays the importace of part load operatio i seasoal performace assessmet has bee ackowledged, especially with regard to heat pumps ad chillers. As a matter of fact, these machies are usually sized for desig coditios, therefore they ormally work at part load coditios for the most of the seaso. For a correct evaluatio of this behaviour it is ecessary to cosider the real theatig/coolig profiles as well as to ivestigate how the machie works uder such variable coditios. For this purpose, accurate software able to calculate heatig/coolig loads of the buildig i dyamic coditios are today available o the market, also for professioal purposes, ad they will probably become more ad more usual eve i commo desig. At each simulatio time step they may calculate the eergy demad ad the related efficiecy of heat pumps ad chillers as a fuctio of both secodary temperatures ad heatig/coolig loads. For this aim some algorithms able to quatify the ifluece of the load factor o the efficiecy of heat pumps ad chillers have bee developed ad refied. Furthermore, the Committees for Stadardizatio of Europea Coutries are completig ad improvig the techical Stadards aimed at the eergy certificatio of buildigs. I this cotext, it is ow eeded to itroduce a algorithm to calculate the seasoal efficiecy of heat pumps ad chillers, defied as the the ratio of the total amout of heatig/coolig eergy provided by the uit durig the seasoal operatio to the total effective electric eergy cosumed durig the same period. I this paper, chillers will be cosidered i detail, whereas for heat pump aalogous cosideratios may be assumed. I this case the eergy efficiecy is amed (Eergy Efficiecy Ratio) ad S is the Seasoal average of. Oe proposal refers back to the IPLV idex itroduced by ARI i the 1980s with Stadard 550/590 (ARI, 2003), a performace ratig Stadard self-regulated through collaboratio of associatios, such as ARI, ASHRAE, ad participatig maufacturers. More recetly, a ew versio of this idex, ES (Europea S), has bee itroduced by Eurovet, more suitable for Europea climates ad operatig coditios tha IPLV, as a volutary certificatio amog maufactures i Europe (Adot et al. 2003). Both IPLV ad ES characterize the behaviour of the machie i part load coditios, but regardless of the buildig where it is istalled. This way, eve if importat to give more iformatio about the actual behaviour of chillers uder real operatio coditios tha the simple omial at full load, IPLV ad ES ca ot be cosidered a acceptable estimatio of the actual S of a chiller istalled i a particular buildig ad climate. As a cosequece, modificatios are beig itroduced i the procedure to calculate S, as foresee i EN (EN, 2012). Nevertheless, i Italy, two Stadards already exist: UNI (UNI, 2004) ad UNI (UNI, 2001) aimed at the assessmet of the seasoal efficiecy ad at the defiitio of the miimum tests ecessary to trace the behaviour of chillers workig uder part load coditios respectively. Ufortuately, these Stadards are curretly far from beig applied, especially because they eed the executio of specific laboratory tests uder part load coditios, whereas idex ES has met a large favour amog Europea maufactures of chillers because of its simplicity. I this paper, a procedure is preseted to calculate part load efficiecy curves startig from the data used to calculate ES. This way, with o more tests, it is possible to achieve data to be used both i dyamic simulatio ad i buildig eergy certificatios. Obviously, the same procedure ca use IPLV data istead of ES oes

2 13th Coferece of Iteratioal Buildig rformace Simulatio Associatio, Chambéry, Frace, August MODELLING The procedure cosists i a calculatio procedure which is also ormally used together with the dyamic simulatio. I fact, i dyamic buildig eergy simulatios, the calculatio by a dyamic algorithm of the buildig thermal demad is usually accompaied by a quasi steady-state evaluatio of the plat behaviour. The procedure ca be divided ito the followig three calculatios steps: 1- Evaluatio of the declared capacity () ad related uder full load coditios ( ) as fuctios of the mothly mea temperatures of the secodary fluids exchagig heat at the evaporator ad codeser. rformace data from the maufacturer may be used, ad itegrated by meas of liear iterpolatio to adapt to curret boudary coditios. 2- Assessmet of the mothly mea capacity ratio as the ratio of the buildig coolig demad to the maximum eergy which could be supplied i the moth (the last oe is obtaied multiplyig the mea declared capacity by the workig hours of the chiller i the moth). 3- The mothly mea is obtaied multiplyig by a part load factor () aimed to take ito accout the ifluece of part load operatio, beig a fuctio of. This calculatio procedure may be applied at each time step of dyamic buildig eergy simulatios as well as i mothly eergy calculatios, usig mothly meas for temperatures ad eergy ratios, thus achievig mothly meas for ad. As a cosequece, it could be applied also to buildig eergy certificatio calculatios, based o mothly calculatios. Ad S ca be calculated for ay coditio too. As a matter of fact, startig from the i (with i = ) for each time-step i (oe hour log dyamic buildig eergy simulatios or oe moth log i eergy certificatio calculatios) ad idicatig with the umber of time-steps i the seaso, we have: S c, i c, i c, s i1 i1 (1) e, s c, i e, i i1 i1 i where: - c,s ad c,i are the coolig eergies give i the seaso ad i the sigle time-step respectively. - e,s ad e,i are the electric eergies absorbed i the seaso ad i the sigle time-step respectively. CALCULATION I figure 1 some experimetal values of are reported as fuctios of for a air-to-water chiller with scroll compressor ad o-off capacity cotrol (omial coolig capacity: 10 kw). Experimetal values of parameter are also show Air-to-water o-off cotrol Cc=0.9 model model from tests model from tests Figure 1. Curves - ad - ad some values from tests of a chiller with o-off cotrol Parameter is the ratio of the real electric cosumptio of the chiller to the total electric cosumptio uder full load, with the same temperatures of the secodary fluids. Tests i figure 1 suggest a liear tred of agaist, thus resultig i a simple mathematical model for (Bettaii et al., 2003): a b (2) (3) thus achievig: a b (4) The fial coclusio is that oly oe test uder part load coditios i additio to the oe at full loads is sufficiet to defie the correlatio - i case of o-off capacity cotrol. As a matter of fact, figure 1 shows this model fits quite well the moitored values. I particular, i case of o-off chillers, EN proposes the followig correlatio:

3 13th Coferece of Iteratioal Buildig rformace Simulatio Associatio, Chambéry, Frace, August (5) C 1 C ) C ( C Where C C is a degradatio coefficiet. I absece of test values, the default value of C C is 0.9. As show i figure 1, this assumptio leads to a acceptable deviatio from the EN model. I case of variable capacity cotrol chillers, such as multistage or iverter-drive compressors, the - curve is obtaied by liear iterpolatio betwee part load data rated i laboratory tests. For values fallig below the miimum percetage of cotiuous modulatio the o-off behavior takes place, with cosequet adaptio of the above see model Air -to-water modulatig cotrol model from tests from tests from tests Figure 2. Curves - ad - ad values from tests with modulatig cotrol I figure 2 experimetal values of ad are show for a air-codesed chiller equipped with scroll compressor, drive by iverter dow to 30% of the omial capacity (16 kw). I this case, the tred of is ot liear, but a liear correlatio betwee = 0.3 ad = 1.0 gives a acceptable estimatio for i this iterval. Laboratory tests provided aalogous results also for air coditioers, such as split systems (Bettaii et al., 2001). ES CALCULATION Normalized idex ES (Europea S) proposed by Eurovet is itroduced to compare part load performaces of chillers, but this value ca ot represet the seasoal efficiecy of a real chiller istalled i a specific buildig, because it is calculated referrig to predefied ad covetioal coolig load profiles ad secodary fluid temperatures. The coolig load profile as well as the referece secodary fluid temperatures widely vary depedig o buildig iteded use, HVAC termial uits, ad HVAC water heat storages. Ayway, this idex allows a cosistet compariso amog omologous products available o the market, because it highlights the behaviour of chillers uder part load coditios. The ES is calculated through the followig equatio: ES A B C D (6) A B where: - A, B, C, D are the Eergy Efficiecy Ratios at the four partial load workig coditios forward defied; is the ratio of the actual coolig capacity (equal to the coolig load) to the actual power cosumptio. - A, B,C, D are the weightig factors of the correspodig part load s. The test coditios to calculate the part load s are show i Table 1 accordig with the Stadard, depedig o the type of chiller or air-coditioer. I Table 1, Ilet fluid temperatures (evaporator or codeser) are idicated with i, outlet fuid temperatures (evaporator) with o. For example, aircodesed chillers at test coditios A work with eterig air temperature at 35 C ad leavig water temperature at 7 C, i case of fa-coils, or 12 C, i case of cool floors. For every test the Stadard idicates also the ilet outlet temperature differeces i the heat exchagers. Table 1. Ilet (t i ) ad outlet (t o ) temperatures for part load tests for differet chillers ad air-coditioers Type: Test Load ti, Outdoor air ti, Idoor air I Table 2 the weightig factors are summarized o the basis of the type of coolig uit. I particular, two differet series of values are cosidered, depedig o the exteral secodary fluid. I some cases, the Stadard admits the calculatio of the part load efficiecy startig from mere full load tests. For the sake of brevity, these cases are ot preseted here. Table 2. Weightig factors for part load s chose to calculate ES As A show, 100% ES 4% is a performace 4% 3% idex of a 3% chiller or B air-coditioer 75% 26% obtaied 26% assumig 33% covetioal 33% C 50% 40% 40% 41% 41% boudary coditios. Its defiitio has the great C ti, Water tower/sik D ti, Idoor air A 100% /15 27 B 75% /15 27 C 50% /15 27 D 25% /15 27 Type: air-water water-water Test Load ti, Outdoor air air-air to, Water fa-coil/paels ti, Water tower/sik water-air to, Water facoil/paels A 100% 35 7/18 30/15 7/18 B 75% 30 7/18 26/15 7/18 C 50% 25 7/18 22/15 7/18 D 25% 20 7/18 18/15 7/18 Typ e: Test Lo ad air-air water-air air-water water- water D 25% 30% 30% 23% 23%

4 13th Coferece of Iteratioal Buildig rformace Simulatio Associatio, Chambéry, Frace, August relevace to focus the attetio of the HVAC plat desiger ot oly o the full load omial efficiecy but eve o the behaviour uder part load coditios ad allows the HVAC plat desiger to compare more cosistetly differet models available o the market. But ES ca ot represet the real seasoal efficiecy of a particular chiller or aircoditioer i a specific cotext. For example, i case of air-codesed chillers, the calculatio of ES is based o the simplificatio that the coolig load is a liear fuctio of the oly outdoor air temperature, whereas it greatly depeds o iteral heat gais, latet loads, solar radiatio ad occupacy schedules. Obviously this is ot sufficiet for a reliable assessmet of the seasoal efficiecy of machie chiller or air-coditioer istalled i a specific cotext. Therefore it is ecessary to itroduce aother procedure for this purpose. FROM ES DATA As a referece, i this paper the frequet case of aircodesed chillers ad air-coditioers may be cosidered. At fixed temperatures for the iteral secodary fluid, depeds o the outside air temperature ad o the degree of part load. The experimetal tests provide typical profiles as the oes show i figure 3. To reduce the umber of tests eeded to draw such profiles, the - curve (where is the ratio betwee ad ) is assumed as fixed for differet outside air temperatures. I particular, the values required to build this curve ca be obtaied at differet thermal levels, thus leadig to the use of A... D obtaied for the ES calculatio ad adapt them to calculate also four more values of, thus allowig the estimatio of the - curve also for other temperature coditios Ti,co= 20 C Ti,co= 25 C Ti,co= 30 C Ti,co= 35 C Figure 3. - for various codeser ilet air temperatures (Ti,co) for a air-cooled chillert I detail: r r (7) r where: - r is available from the data required by EN (with r = A, B, C, D); - r ca be obtaied from the tables of full capacity performaces provided by maufacturers i usual techical documetatio. Full load (%) To,ev Ti,co ( C) ( C) (kw) (kw) A B C D Part load (%) To,ev Ti,co ( C) ( C) (kw) (kw) A B C D model Figure 4. Full ad part load performace ad cosequet - ad - curves for chiller 1 (water-to-water) The part load coefficiet mostly depeds o the part load effect, whereas secodary fluid temperatures have lower ifluece. The four values of r with r = A, B,C, D correspod to =, 0.75, 0.50, 0.25 respectively. From these four values, the values of the part load coefficiet ca be obtaied by simple iterpolatio for ay capacity ratio. The applicatio of the procedure to some typical commercial chillers is preseted here. For each chiller a table is preseted with workig data uder full ad part load coditios. I deeper detail, the coolig capacity P c, absorbed electric power P e ad cosequet are give for the secodary fluid temperatures provided i the S procedure. The ratio of to gives the correspodig value. I figure 4, a water-codesed chiller (machie 1) is cosidered with just oe refrigeratio circuit equipped with a scroll compressor. The set of the full ad part load performaces ad the correspodig - ad - profiles are show i figure 4. For this o-off chiller the liear tred of is cofirmed. The performace data at full load have bee obtaied from the available techical documetatio. The part load performace is available from the maufacturer as well. I particular, it is obtaied by certified tests doe i authorized laboratories as required by Eurovet/CEN ad used by the maufacturer to calculate the ES

5 13th Coferece of Iteratioal Buildig rformace Simulatio Associatio, Chambéry, Frace, August reported i the commercial catalogue. For this chiller ES is Full load (%) To,ev Ti,co (kw) ( C) ( C) (kw) A B C D Part load (%) To,ev Ti,co (kw) ( C) ( C) (kw) A B C D Figure 5. Full ad part load performace ad cosequet - ad - curves for chiller 2 (air-cooled) I figure 5 the data ad the cosequet profiles of parameters ad Y are preseted for a aircodesed chiller with oly oe circuit ad oe scroll compressor (machie 2). As ofte happes, i this case the full load capacity at 20 C is ot preset i the techical documetatio provided by the maufacturer. P (kw) (kw) (kw) tred tred y = x R² = y = x R² = Ti,co ( C) Figure 6. Coolig capacities ad absorbed electric power as fuctios of outdoor temperature (Ti,co) for chiller 2 (air-cooled). It is although possible to use a mathematical extrapolatio, because the coolig capacities ad absorbed electric powers have usually a liear profile, as show i figure 6. The validity of the liear regressio is quatified by the high value of the correlatio coefficiet R 2. R 2 2 ( calculated P c ) (8) 2 ( ) real P c is the mea value of all the real measured where (or ). For chillers 1 ad 2, characterized by o-off capacity cotrol, the liear profile of parameter cofirms the possibility to use the model of equatio (4) as a alterative of the iterpolatio amog the four poits from tests. But this simple model is o more usable i presece of multistage or ivert-drive compressors. A chiller provided with multistage capacity cotrol is ow preseted i figure 7, cosistig i a air-codesed chiller with four scroll compressors ad two refrigeratig circuits (machie 3), with ES equal to 4.40,. Also i this case the correlatio of is ot liear but the four values of permit a acceptable evaluatio of this factor i the whole rage. Startig from the certified values of, available because eeded to calculate the S, the maufacturer ca immediately calculate four values of the Y coefficiet for four load percetages (25%, 50%, 75%, ad 100%). Therefore it is sufficiet that the maufacturer itroduces these four values i the techical documetatio besides the ES value. This way, with o more tests, the HVAC plat desiger will be able to apply the proposed procedure to calculate the seasoal efficiecy, thus achievig the electric eergy demad for coolig both i buildig eergy dyamic simulatios ad i steadystate procedures for buildig eergy certificatio Figure 7. Full ad part load performace ad cosequet - ad - curves for chiller 3 (air-cooled) SIMULATION VS MONITORING A compariso betwee simulatio results ad data from moitorig of a chiller istalled i a residetial buildig is preseted. The uit is a air-cooled chiller with oe scroll compressor ad o-off cotrol itegrated ito a HVAC plat based o fa-coil termial uits. The omial capacity is 5 kw ad the cold water is ormally produced at 7 C. The ES of the uit is By usig the procedure previously

6 13th Coferece of Iteratioal Buildig rformace Simulatio Associatio, Chambéry, Frace, August preseted, the four values of s are calculated from ES data provided by the maufacturer. The moitorig system has collected the chiller ilet/outlet water temperatures, the outdoor air temperature ad the electric eergy cosumptio of the uit with a oe-miute iterval for a summer i Italy (from Jue to September). The water flow rate to the evaporator was costat ad it was measured. Startig from these measures, the hourly average of the coolig capacity ad of the electric cosumptio have bee calculated ad the cosequet the values. By iterpolatio betwee the data from the maufacturer, the are evaluated as a fuctio of the measured water ad outdoor air temperatures. This way experimetal s (ratio / ) have bee obtaied. I figure 9 a compariso betwee the calculated values ad the correspodig measured oes is proposed. You ca ote the high correspodece quatified by a high correlatio coefficiet R 2. O the other had, the validity of the mathematical model i presece of o-off capacity cotrol is cofirmed also by the good predictio of show i figure 10. Furthermore, i each hourly time step, the mea electric cosumptio is calculated as the ratio of the mea measured coolig capacity provided to the correspodig calculated i the same iterval. Figure 11 shows the calculated mea electric cosumptio values versus the correspodig measured oes. Fially, i table 3, the seasoal results are resumed, startig from the same measured coolig eergy provided to the buildig R 2 = s from ES data model measure Figure 8. measures for the moitored chiller ad model obtaied by meas of s from ES data. I figure 8 a compariso betwee measured s ad calculated oes (by liear iterpolatio betwee the fourth values calculated from ES data) is show. For low values better correlatio with the mathematical model of equatio (4), also reported i figure 8, is idetified, as the uit presets a o-off capacity cotrol. A validatio of the procedure to simulate the seasoal performace has bee carried o, coisiderig, for the whole summer, the actual coolig eergy measured i every hour. For each hour, o the basis of the water ad outdoor air temperatures, the chiller performace at full load are calculated as well as the ad cosequet. Calculated R 2 = Measured Figure 9. Compariso betwee measures ad calculated s for the moitored chiller. Calculated Calculated (kw) Measured Figure 10. Compariso betwee measures ad calculated values for the moitored chiller R 2 = Measured (kw) Figure 11. Compariso betwee electric absorptio (kw), measured ad calculated, for the moitored chiller The good predictio of the real S ca be appreciated i table 3. Obviously, this aalysis is ot a validatio of the global model of the buildig-plat system, but oly a test for a procedure to assess S to be implemeted i simulatio models. Ideed the validatio of buildig models is ot the scope of this paper

7 13th Coferece of Iteratioal Buildig rformace Simulatio Associatio, Chambéry, Frace, August Table 3. Compariso betwee measured ad simulated seasoal performace of the moitored chiller Measured Simulated Cold demad (kwh) 1071 Electric cosumptio (kwh) S CONCLUSION A simple algorithm to simulate the part load efficiecy of chillers ad air-coditioers has bee preseted ad validated i a case study. Its fudametal prerogative is the capacity to evaluate the behaviour of the uit i part load coditios. The data required to characterize each particular uit ca be derived from iformatio usually available i maufacturer s catalogues ad specificatios sheets. The goal of the model is to allow a correct assessmet of the seasoal performace of chillers ad air-coditioers. NOMENCLATURE = capacity ratio Cc = degradatio factor = eergy efficiecy ratio = eergy efficiecy ratio at full load (=Declared Capacity) S = seasoal eergy efficiecy ratio = absorbed electric power, kw = absorbed electric power at full load, kw = coolig capacity, kw = coolig capacity at full load, kw = part load factor c = coolig eergy produced, kwh c,s = seasoal coolig eergy produced, kwh e = electric cosumptio, kwh e,s = seasoal electric cosumptio, kwh Ti,co = secodary fluid ilet temperature at the codeser, C To,ev = secodary fluid outlet temperature at the evaporator, C z REFERENCES = ratio of electric power absorptio to the electric absorptio at full load with the same thermal levels of the secodary fluids Adot J., Waide P Eergy Efficiecy ad Cetral Air Coditioers, Fial Report Eurovet. ARI Stadard 550/590, Stadard for performace ratig of water chillig packages usig the vapor compressio cycle, Air Coditioig ad Refrigeratig Istitute (ARI), Arligto Virgiia. Bettaii E., Romagoi P., Schibuola L Seasoal performace calculatio for air coditioig equipmets, Clima 2000, Word Cogress proceedigs, Naples. Bettaii E., A. Gastaldello G., L. Schibuola L Simplified models to simulate part load performaces of air coditioig equipmets, proceedigs of the 8th Coferece of the Iteratioal Buildig rformace Simulatio Associatio (IBPSA), Eidhove. EN EN 14825:2012 Air coditioers, liquid chillig packages ad heat pumps, with electrically drive compressors, for space heatig ad coolig Testig ad ratig at part load coditios ad calculatio of seasoal performace. Schibuola L.,Tambai C., Baldassa P., ecchi R., Part load curves from test data to evaluate the seasoal performaces of refrigeratio machies proceedigs 10 th REHVA World Cogress 2010 Atalya, Turkey. UNI 2004, Italia stadard UNI 11135, Air coditioers, water chillig packages ad heat pumps- Calculatio of the seasoal efficiecy. UNI Italia Stadard UNI 10963,Air coditioers, liquid chillig packages ad heat pumps-rformace tests at part load