Simplified Indices Assessing Building Envelope's Dynamic Thermal Performance: A Survey

Transcription

1 Simplified Indices Assessing Building Envelope's Dynamic Thermal Performance: A Survey Ferrari, S. Building Environmen Science and Technology Deparmen, Poliecnico di Milano ( simone.ferrari@polimi.i) Zanoo, V. Building Environmen Science and Technology Deparmen, Poliecnico di Milano ( valenina.zanoo@mail.polimi.i) Absrac In he las years, grea aenion was reserved o he global energy consumpion and he relaed CO 2 emissions, and in his framework a lo has been done in he consrucion world, inroducing differen kinds of building regulaions and assessmen mehods (i.e. in Europe he EPBD "Energy Performance of Buildings Direcive was adoped) in order o improve he general qualiy of buildings. Thermal behaviour of he building envelope is maybe he elemen which mos srongly influences he energy consumpion during building service life, affecing climaizaion needs. Mos of he simplified procedures used o esimae he building energy consumpion are based on seady-sae indicaors, which canno fully reflec he real building behaviour. This is rue in paricular during he warm season, when i is more difficul o simplify he envelope performance wihou aking ino accoun he hourly variabiliy of he indoor and oudoor climaic condiions. This work surveys he exising indicaors which represen he dynamic hermal envelope performance in a simplified manner, in order o undersand he possible benefis and limis conneced o heir applicaion. Keywords: building performance, building envelope, hermal characerisics, dynamic behaviour 211

2 1. Inroducion In he las decades he worries abou energy resources availabiliy and CO 2 emissions brough o he spread of a lo of regulaions regarding a variey of fields. One of he main source of energy consumpion (equal o 1/3 of he oal amoun) and of greenhouse gases emission in he developed counries is he building sock, mainly because of climaizaion: because of his, new codes relaed o building energy performance were published (i.e. EPBD) aiming a a general increase in he building sock qualiy. Considering he heaing and cooling demand in buildings, he main elemen involved is he envelope, which exchanges hea wih he oudoor environmen. Mos of he currenly available building performance assessmen mehods evaluae he hea exchange hrough he envelope by he means of a seady-sae analysis, bringing o he diffusion of sric regulaions regarding he hea ransmiance of he envelope elemens. This approach is surely simple o use and has been very effecive in improving he qualiy of he building sock. On he oher hand i does no ake ino accoun he unseady sae behaviour of he consrucion maerials, which end o sore hea and release i afer a cerain amoun of ime: his phenomenon is usually referred o as hermal ineria or hermal mass effec. I is more eviden when considering he average heaing or cooling demand, since he usual exercise condiions for buildings are highly unseady, in boh he daily and he yearly inerval, and considering he summer peak load, since i is more srongly influenced by ransien elemens such as he exernal climae and inernal hea loads variabiliy. Now ha he seady-sae analysis of building envelope performance is par of he common pracice and is fully undersood by he consrucion secor and a lo of building codes have se higher sandards regarding he seady-sae parameers, new aenion is given o he ransien behaviour of he consrucion maerials and is effec on he hea exchanges. 2. Hea ransfer Hea ransfer calculaion is based on he law of hea conducion, he Fourier s law, which saes ha hea ransfer hrough a building elemen is direcly conneced o he emperaure disribuion inside he same elemen. This law akes he form of Eq. 1. q T (1) q densiy of hea flow rae [W/m 2 ] λ hermal conduciviy [W/(m K)] T emperaure gradien [K/m] 212

3 2.1 Seady-sae analysis Regarding he calculaion of he heaing peak load, which can ignore he heaing effec of unseady elemens such as he solar radiaion and he inernal hea loads, a seady-sae and one-dimensional analysis is considered accurae enough, and so he hea ransfer by conducion akes a simplified form. Inegraing along a pah of consan hea flow rae and considering is mean lengh L (which is equal o he wall hickness in case of a plane wall), i is possible o define he L/ λ value, which is called hermal resisance R [(m 2 K)/W] and which allowed o creae a model for he hea ransfer wih analogies o he elecrical connecions. From his concep an overall hea ransfer coefficien was developed, o represen boh he hea conducion hrough single- or muli-layer srucures and he hea ransfer due o convecion and radiaion on he wo exernal sides of hem: i is he reciprocal of he overall hermal resisance of he envelope elemen and is called hermal ransmiance [W/(m 2 K)]. Equaion 2, hen, is he mos common way used o calculae he par of he building energy balance which represens he hea exchange by ransmission hrough he envelope. U S T (2) Φ hea flow rae [W] U hermal ransmiance [W/(m 2 K)] S surface area [m 2 ] ΔT emperaure difference beween he indoor and oudoor environmen [K] 2.2 Transien analysis Under ransien condiions, and by combining Eq. 1 wih he energy conservaion principle, he general hea conducion equaion for he one-dimensional hea flows akes he form of Eq. 3, which is a parial differenial equaion (PDE) and canno be simply solved analyically if no for specific and rigid boundary condiion (i.e. periodic analysis). 2 T T c (3) 2 x ρ densiy [kg/m 3 ] c T λ x specific hea [J/(kg K)] emperaure [K] ime [s] hermal conduciviy [W/(m K)] hea flow direcion 213

4 This equaion is usually solved by he means of numerical mehods, which simplify he calculaion bu are usually complicaed o use. The mos common of hese mehods are: conducion ransfer funcion (CTF), which approximae he hea ransfer o an impulse-resul scheme, or finie difference and finie elemen analysis, which simplify he reference domain. 3. Simplified mehods The diffuse pracice for he assessmen of he energy performance of buildings is generally based on he seady-sae calculaion of he ransmission hea ransfer, bu he aenion given o he ransien sae is increasing as he general performance of he building envelopes becomes beer and as he energy analysis of buildings is becoming a sandard procedure. The use of numerical mehods o solve PDEs, on he oher hand, needs a lo of resources in erms of ime and knowledge, and seems o be a big obsacle o he adopion of he ransien analysis. This is one of he reasons ha brough o he developmen of simplified mehods which inroduce simple parameers modificaions o approximae he dynamic effecs of he building envelopes characerisics. According o Barnaby (1982) hese can be divided ino wo groups: hose analysing or simulaing an isolaed building elemen, and hose considering he mass effec in he conex of a ypical whole building or building zone. CEN (European Commiee for Sandardizaion), in collaboraion wih ISO (Inernaional Sandard Organizaion), has already addressed he problem of approximaing he ransien behaviour of he building elemens in he climaizaion demand calculaion procedure and proposes a correcion value, which belongs o Barnaby s second caegory and has o be added o he zone seady-sae hea balance equaion. Because of his approach, he CEN mehod is commonly defined quasi-seadysae. The previously menioned correcion value is he uilizaion facor, which represens he amoun of inernal and solar hea gains (for he heaing season) or of he ransmission and venilaion hea losses (for he cooling season) acually capialised by he effecive mass of he zone. This parameer can be easily calculaed and is specific for he characerisics of he analysed building: i depends on he hea gains on hea losses raio and on he building or zone ime consan. The ime consan is a measure of he oal building hermal ineri and is calculaed essenially by he means of he effecive hea capaciy of boh he envelope and he inernal elemens, derived according o EN ISO (2007). Inernaionally, several alernaive approaches have been developed o approximae he dynamic behaviour of buildings regarding boh he peak load and he energy demand calculaion. In hese work, some of he inernaional simplified mehods analysing isolaed building elemens are surveyed, even aknowledging he limi of hem no considering he coupled effecs of he hermal mass conained in oher room and building elemens. 214

5 3.1 Hea ransmiance correcion values Some simplified mehods apply a correcion o he U-value of he considered elemen, and are usually referred o he heaing season. The firs of hese mehods is he mass facor, developed by Hankins and Anderson (1976), which applies o he heaing season calculaions. The adjusmen facor was defined as he raio of he hea exchange hrough a given wall calculaed by he means of a dynamic simulaion and he one calculaed by he means of he seady-sae mehod applied on hourly inerval. Several reference values were calculaed and proposed by Hankins and Anderson in relaion o he heaing degree-days of he building locaion and he elemen fronal mass, which have been ranslaed in graphs o enlarge he possible use of he mehod. The main problem of his mehod is ha hese values are reliable when dealing wih he winer peak load calculaion, bu heir use for he seasonal energy calculaion does no allow an accurae predicion of he savings due o he envelope mass. Anoher mehod is he effecive U-value, developed by Van der Meer (1978). This parameer is defined as he raio of he seasonal average densiy of hea flow rae and he seasonal average emperaure difference beween he indoor and he oudoor environmens. By he means of an ad hoc developed sofware, Van der Meer performed series of dynamic energy simulaions for a whole se of consrucion echnologies in relaion o he New Mexico climaic condiions, and derived reference values for he mos common wall consrucions. The aim of his parameer is no o deermine a comparison beween differen consrucion, bu o make a correcion of he radiional U-value and o deermine how he elemen behaviour changes as is orienaion or is surface colour are modified. Laer on values of his parameer have been calculaed for a wider range of consrucion and hey have been implemened in he New Mexico Building Code. As for oher simplified mehods, he effecive U-value is no universally applicable, since i is based on a lis of reference values, which were calculaed by he means of deailed dynamic simulaions for specific kinds of envelope echnologies and specific climaic condiions. 3.2 Temperaure difference correcion values Some oher mehods apply a correcion value o he emperaure difference needed o calculae he hea exchange by ransmission, and are usually referred o he cooling season. Since 1940s, ASHRAE sared developing a mehod o calculae he hermal gains for he peak cooling load esimaion hrough he opaque envelope based on an alernaive value of emperaure difference, which could ake ino accoun he combined effecs of solar radiaion and envelope hermal ineria. Saring from experimenal daa and calculaion using he periodic analysis, some defaul daa regarding specific envelope echnologies were derived as he raio of he calculaed hea flow and he nominal U-value and were colleced ino reference ables. In 1972 he oal equivalen emperaure differenial (TETD) was inroduced in ASHRAE Handbook of Fundamenals (1972) o deermine he cooling peak load. The mehod can be applied boh choosing pre-calculaed values from he reference ables, which were derived for specific consrucions and for mild climaes, and 215

6 calculaing direcly he specific value by he means of a general equaion which depends mosly on he decremen facor and on he ime lag of he analysed building envelope, which respecively represen he aenuaion of he hea wave when going hrough i and he conneced delay in he hea ransmission (Eq.4). TETD Tas, e, T i, f Tas, e, Tas, e, (4) T as,e T i f δ oudoor sun-air emperaure [K] indoor air emperaure [K] decremen facor ime lag [h] Because of his las opion, his mehod can be considered simple and a he same ime accurae in calculaing he cooling load, and relaed researches are sill going on, even if adoping more sophisicae calculaions of he ime lag and decremen facor values (Yumruaş e al., 2006, Kaşka e al., 2009). In order o furher simplify he CTF and he TETD mehods, ASHRAE developed also he cooling load emperaure difference (CLTD), which allows o direcly obain he peak load value and is sill adoped as a simplified mehod for residenial buildings (ASHRAE, 2001). The sandard values were derived as he raio of he dynamic hea flow, calculaed by he means of he CTF mehod, and he nominal U-value of some ypical envelope consrucions. As for oher mehods considered, he accuracy of hese adjused values srongly depends on he similariies beween he acual building and he simulaion condiions used by ASHRAE, regarding he envelope characerisics, he inernal hea sources and occupancy schedules, and he climaic condiion, which are ypical for he Unied Saes. To address his limi, ASHRAE (1989) proposed some possible correcions regarding laiude, indoor design emperaure (he sandard value is 25.5 C), oudoor design emperaure (he sandard value is 29.4 C), presence of addiional insulaion and he envelope finishing in erms of colour. Some researches sill propose he use of his mehod, wih a wider range of boundary condiions. In paricular, Bansal e al. (2008) adoped he finie difference mehod o solve he Fourier s equaion and calculaed reference CLTD values for ypical consrucions and climae of India. As a represenaion of he energy consumpion due o he building envelope, always in summer, ASHRAE developed, hen, he overall hermal ransfer value (OTTV), which ries o combine he effec of he envelope elemens exposed o differen orienaions and of boh he opaque and ransparen pars of hese elemens. OTTV A U TD A SC ESM SF op EQ A win (5) A envelope surface [m 2 ] A op surface of he opaque par of he envelope [m 2 ] 216

7 U hermal ransmiance of he opaque par of he envelope [W/(m 2 K)] α absorbance of he opaque par of he envelope TD EQ equivalen emperaure difference for he opaque par of he envelope [K] A win surface of he glazed par of he envelope [m 2 ] SC shading coefficien of he surface ESM exernal shading muliplier (depending on he orienaion) SF solar facor of he glazed surface [W/m 2 ] Considering a single opaque elemen, he characerisics aken ino accoun by his index are he U- value, he hermal absorpance and an adjused value of emperaure difference: he reference values of equivalen emperaure difference were lised according o he surface orienaion and o he elemen weigh, afer being calculaed by he means of dynamic simulaions. Even if since 1989 his index is no par of he ASHRAE Sandard 90.1 anymore, he building codes in some easern counries (i.e. Hong Kong, 1995) sill use i o evaluae he cooling load in case of commercial buildings. More recenly, Nilsson (1994 and 1997), developed a whole mehod for assessing he energy demand of buildings based on he consrucion of duraion diagrams for he variable elemens of he building energy balance equaion. Wihin his mehod, he hea exchange by ransmission hrough he envelope is calculaed by he means of he seady-sae equaion. In order o include he envelope dynamic behaviour in his mehod, Nilsson derived he ficiious ambien emperaure value, which is supposed o be used insead of he oudoor air emperaure in he emperaure difference calculaion. The principle, similarly o wha implied by he TETD bu originally applied o complee envelopes raher han o isolaed elemens, is ha massive consrucions are more influenced by he oudoor emperaure hisory han by he insananeous oudoor emperaure (Eq. 6-7). T * T T T 0 e * (6) or: T * N T N * T a T a τ * Δ,N T N T ficiious ambien emperaure [K] oudoor air emperaure [K] ime coefficien [s] ime inerval [s] curren ime 0 iniial ime N 1 e * (7) Saring from he lumped hea capaciy of he envelope elemens, a characerisic ime coefficien can be calculaed and used o represen he ime lag. Since he lumped hea capaciy value does no 217

8 change depending on he insulaion posiion in he srucure cross-secion, an adjusmen value can be used o disinguish beween specific layers layous wih he same lumped mass value (Eq. 8). * cm UA ξ correcion coefficien due o he layers layou c specific hea [J/(kg K)] m mass [kg] U hea ransmiance [W/(m 2 K)] A surface [m 2 ] (8) Sandard reference values of he correcion coefficien were calculaed for differen sandard layous (such as massive layers boh on he inside and on he ouside, only on he inside, only on he ouside, and insulaion layers boh on he inside and on he ouside) comparing he ficiious ambien emperaure mehod resuls and he ones from dynamic building simulaions: he resuls were laer lised in reference ables. 4. Conclusion The currenly available simplified mehods o adjus he seady-sae hea ransmission calculaion are only few and usually developed on he basis of a limied number of case sudies (considering consrucions and climaic condiions), which is a srong limi o heir possible use. Mos of hem are also old, and were herefore developed using very differen consrucions compared o he ones currenly adoped, especially considering he saring U-value. I would be ineresing o develop new reference ables of he same correcion values using a wider range of wall samples and boundary condiions, in order o fully undersand heir reliabiliy as he simulaion condiions change. Mos of hese mehods are hen relaed o he peak load calculaion more han o he general demand assessmen: his is easy o undersand, since he hermal mass effec on he peak loads (aenuaion) is considerably more eviden and is commonly represened by he decremen facor. Moreover, limiing he analysis o he hea exchange hrough a single envelope elemen causes a lack of consideraion of oher unseady parameers of he room hea balance equaion, such as he hermal mass effec due o he oher building elemens (walls, slab, ceiling and inernal mass), he inernal hea sources and he venilaion hea losses. These parameers srongly influence he boundary condiions of a single wall behaviour, bu heir effec can be surely beer calculaed when considering a whole room analysis. This is he reason why some of he previously described simplified mehods, in paricular he ones no specifically addressing he peak load calculaion (i.e. effecive U-value, OTTV, ficiious ambien emperaure), refer o values derived by several simulaions of case-sudy buildings. Also he CEN 218

9 mehod, which regards he whole zone hea balance calculaion, akes ino accoun all he above menioned unseady parameers. References ASHRAE (1972) Handbook of Fundamenals, Chaper 26 Air-condiioning and cooling load. ASHRAE (1989) Handbook of Fundamenals, Chaper 26 Air-condiioning and cooling load. ASHRAE (2001) Handbook of Fundamenals, Chaper 3 Hea Transfer and Chaper 28 Residenial Cooling and Heaing Load Calculaions. Bansal K., Chowdhury S. and Gopal M.R. (2008) Developmen of CLTD values for buildings locaed in Kolka India, Applied Thermal Engineering, 28(10): Barnaby C.S. (1982) A survey of simplified echniques for calculaing energy effecs of building mass, Proceedings of he Building Thermal Mass Seminar, 2-3 June 1982, Knoxville TN, USA. EN ISO (2007), Thermal performance of building componens Dynamic hermal characerisics Calculaion mehods. EN ISO (2008), Energy performance of buildings Calculaion of energy use for space heaing and cooling. Hankins and Anderson, Inc. (1976) Repor on he effec of wall mass on he sorage of hermal energy, Boson, Repor o he Masonry Indusry Coomiee. Hong Kong Building Auhoriy (1995) Code of pracice for Overall Thermal Transfer Values in buildings (available online hp:// [accessed on 21/12/2009]) Kaşka Ö. and Yumruaş R. (2009) Experimenal invesigaion for oal equivalen emperaure difference (TETD) values of building walls and roofs, Energy Conversion and Managemen, 50 (11): Nilsson P.E. (1994) Heaing and cooling requiremens in commercial buildings a duraion curve model including building dynamics, Ph.D. Thesis, Dep. of Building Service Engineering, Chalmers Universiy of Technology, Gohenburg, Sweden. Nilsson P.E. (1997) Shor-erm hea ransmission calculaions by inroducing a ficiious ambien emperaure, Energy and Buildings, 25: Tubi N. (1981) La realizzazione di muraure in laerizio, ANDIL, Ed. Laerconsuls. 219

10 Van der Meer W.J. (1978) Effecive U values: a mehod for calculaing he average hermal performance of building componens, Albuquerque, New Mexico Energy Insiue Universiy of New Mexico. Yumruaş R., Kaşka Ö. and Yıldırım E. (2007) Esimaion of oal equivalen emperaure values for mulilayer walls and fla roofs by using periodic soluion, Building and Environmen, 42(5):