A METHOD TO ASSESS THE SUITABILITY OF A CLIMATE FOR NATURAL VENTILATION OF COMMERCIAL BUILDINGS By James Axley And Steve Emmerch Buldng and Fre Research Laboratory Natonal Insttute of Standards and Technology Gathersburg, MD 20899 USA Reprnted from CD Proceedngs, Indoor Ar 2002, 9 th Internatonal Conference on Indoor Ar Qualty and Clmate n Monterey, Calforna. June 30-July 5, 2002 NOTE: Ths paper s a contrbuton of the Natonal Insttute of Standards and Technology and s not subject to copyrght.
Proceedngs: Indoor Ar 2002 A METHOD TO ASSESS THE SUITABILITY OF A CLIMATE FOR NATURAL VENTILATION OF COMMERCIAL BUILDINGS JW Axley 1 and SJ Emmerch 2* 1 School of Archtecture, Yale Unversty, New Haven, CT, USA 2 Natonal Insttute of Standards and Technology, Gathersburg, MD, USA ABSTRACT Increasngly, European buldng desgners have used natural ventlaton to control ar ualty and cool commercal buldngs to conserve energy compared to mechancal coolng and fan operaton. These advanced natural and hybrd ventlaton systems may be adapted to the North Amercan context, but work s needed ncludng consderaton of the broader dversty of clmates. An approach to the analyss of clmate sutablty s presented and appled to a number of North Amercan clmates. The clmate sutablty method presented offers an approach to predesgn analyss for ventlatve coolng that s clmate specfc and accounts for the expected level of nternal and solar gans for a buldng. The clmate sutablty analyss also yelds estmates of ventlaton rates that are lkely to be needed to acheve ether a drect ventlatve or nght coolng desgn strategy. INDEX TERMS Natural Ventlaton, Hybrd Ventlaton, Desgn Method, Clmate Sutablty INTRODUCTION Coolng resdences by openng wndows and doors durng mld weather s a famlar strategy ndgenous to practcally all clmates. In a few clmates, ndgenous resdental coolng strateges also nclude pre-coolng buldng thermal mass by nghttme ventlaton to mtgate antcpated uncomfortably warm condtons durng the comng day. These ventlatve coolng strateges are more useful n some clmates than others. Unfortunately, lessons learned for resdental ventlatve coolng are not drectly applcable to commercal buldngs because they typcally have larger nternal heat gans and smaller ratos of envelope area to enclosed volumes compared to resdences. Conseuently, coolng tends to be reured for larger portons of the year n commercal buldngs. Ths paper descrbes a method to evaluate clmate sutablty for ventlatve coolng of commercal buldngs. Ths method s appled to specfc clmatc data to characterze the statstcal dstrbuton of the natural drect ventlaton rates needed to offset gven nternal heat gans rates and the potental nternal heat gan that may be offset by nght-tme coolng for those days when drect ventlatve coolng s nsuffcent. THEORY For clmatc sutablty analyss, a commercal buldng may be dealzed as a control volume wth a unform temperature dstrbuton. Applyng conservaton of thermal energy yelds: Dynamc Model dt + = (1) dt KT M E * Contact author emal: Steven.Emmerch@nst.gov 854
Proceedngs: Indoor Ar 2002 wth: K =Σ UA+ mc & P (2) E = KTo + (3) where T o s the outdoor ar temperature, T s the ndoor ar temperature, s the ndoor nternal plus solar gans, M s the ndoor thermal mass, ΣUA s the buldng envelope thermal conductance, and m& s the mass flow rate of ventlaton ar. In ths formulaton, conductve heat transfer s arbtrarly separated nto a rate out eual to the product of the envelope conductance and the ndoor ar temperature (ΣUA)T and a rate n (ΣUA)T o. Thus, the net conductve heat transfer rate s the more famlar product of the envelope conductance and the outsde-to-nsde temperature dfference (ΣUA)(T o -T ). Smlarly, the ventlatve heat transfer rate s separated nto a rate out and a rate n. Together, the combned conductve and ventlatve heat transfer rate out of the control volume s, thus, KT where K s the combned conductve and ventlatve transfer coeffcent defned by Euaton 2. Ths formulaton stresses the fact that the response of the thermal system s excted by the sum of conductve, ventlatve, and nternal gans Kt o + that are defned by Euaton 3 to be the system exctaton E. If ether M s neglgbly small or T s relatvely constant, then the accumulaton term of Euaton 1 may become nsgnfcantly small. Under these condtons, the thermal response of the buldng system wll be governed by the steady-state lmtng case: Steady State Model KT = E (4) Ths steady-state approxmaton s the essental bass of the heatng and coolng degree day methods used for prelmnary determnaton of annual heatng or coolng energy needs and as metrcs of a gven clmate's heatng and coolng season. It wll also provde an approxmate means to characterze the ventlatve coolng potental of a gven clmate. The heatng balance pont temperature T o-hbp establshes the outdoor ar temperature below whch heatng must be provded to mantan ndoor ar temperatures at a desred nternal heatng set pont temperature T -hsp. Hence, when outdoor temperatures exceed T o-hbp, drect ventlatve coolng can usefully offset nternal heat gans to mantan thermal comfort. At or below T o-hbp, ventlatve coolng s no longer useful although ventlaton would stll be mantaned at the mnmum level reured for ar ualty control. At T o-hbp, the combned conductve and ventlatve heat loss from the buldng just offsets nternal gans or, usng the steady state approxmaton: ( hsp o hbp ) = Heatng Balance Pont K T T (5) Solvng ths euaton for T o-hbp and expandng we obtan: To hbp= T hsp (6) m& mncp +ΣUA where the ventlaton flow rate has been set to the mnmum reured for ar ualty control. 855
Thermal Comfort and Humdty Control The heatng balance pont temperature, based on a prescrbed T -hsp set eual to the lowest T that s acceptable for thermal comfort, establshes a lower bound of acceptable outdoor temperatures for ventlatve coolng. The T o eual to the hghest acceptable temperature for thermal comfort establshes an upper bound above whch ventlatve coolng wll not be useful. Here, ths lmtng temperature wll be assumed to eual the ndoor coolng set pont temperature T -csp above whch mechancal coolng would normally be actvated to mantan thermal comfort. In addton, ndoor ar humdty must be lmted to acheve comfortable condtons and to avod mosture-related problems. Dstnct thermal comfort zones may be dentfed for summer and wnter condtons. However, due to nternal gans, natural ventlaton may be expected to be useful to lmt overheatng n commercal buldngs durng both summer and cooler perods of the year. Conseuently, for ventlatve coolng of commercal buldngs, t s useful to use a comfort zone that covers all seasons of the year. A reasonable comfort zone for ventlatve coolng, based on combnng ASHRAE's wnter and summer comfort zones [ASHRAE 1997b], would be delmted by lower and upper dry bulb temperatures of 20 C and 26 C and a lmtng dew pont temperature of 17 C. Thus, the Drect Ventlatve Coolng Crtera may be defned as: T (, T = 20 C) T T = 26 C and T 17 C (7) o hbp hsp o csp o dp For nght ventlatve coolng, no lower lmt need be placed on outdoor ar temperatures but the humdty lmt wll be mantaned to avod mosture-related problems n buldng materals and furnshngs. Thus, the Nght Ventlatve Coolng Crtera are: T T = 26 C and T 17 C (8) o csp o dp Proceedngs: Indoor Ar 2002 METHOD Wth the theory and comfort crtera establshed above, a method to evaluate the sutablty of a gven clmate for ventlatve coolng may be formulated. Ths method ncludes both a drect ventlaton procedure and a nghttme coolng procedure. Drect Ventlaton Relatve to enclosed volume, commercal buldngs typcally have small envelope surface areas yet reure relatvely large mnmum ventlaton rates for ar ualty control. Conseuently, the conductve conductance of commercal buldngs may be expected to be small relatve to the mnmum ventlatve conductance: m& c >ΣUA mn p (9) Thus, the heatng balance pont temperature of commercal buldngs may be roughly estmated by ntroducng the condton of Euaton 9 nto Euaton 6 to obtan: T = T T (10) m c UA m& c o hbp hsp hsp & mn p +Σ ASHRAE Standard 62 [ASHRAE 1999] prescrbes mnmum ventlaton rates for commercal buldngs. For offces, due to relatvely low occupancy levels (e.g., 7 person/100 m 2 ) and moderate rate reurements (.e., 10 L/s-person), the specfc ventlaton rate reured s 0.7 L/s-m 2. Thus, T o-hbp s 8 C, -4 C, -28 C, and 75 C for nternal gans of 10 W/m 2, 20 mn p 856
W/m 2, 40 W/m 2, and 80 W/m 2 respectvely. It s clearly evdent that nternal gans expected n commercal buldngs can ute easly extend the ventlatve coolng season well nto wnter months n North Amerca. When outdoor ar temperatures exceed T o-hbp, yet fall below T -csp, ventlaton can drectly offset nternal gans. Recognzng conductve losses durng warm perods are typcally small relatve to nternal gans for commercal buldngs (.e., (ΣUA)(T o -T )< ), the ventlaton rate reured to offset nternal gans whle mantanng ndoor ar temperatures wthn the comfort zone may be estmated usng the steady state model, Euaton 4 as: ΣUA( T T ) 0 & cool = (11) cp( T To) cp( T To) m Proceedngs: Indoor Ar 2002 If T o <T o-hbp, no ventlatve coolng wll be reured. When outdoor ar temperatures fall wthn an ncrement of (T -csp -T -hsp ) above T o-hbp, the mnmum ventlaton rate wll suffce: m& cool = m& mn when To hbp To To hbp + ( T csp T hsp ) Above ths range, the ventlaton rate wll have to ncrease as T o ncreases: m& = when T + ( T T ) < T T c ( T T ) cool o hbp csp hsp o csp p csp o (12) (13) Euatons 11, 12, and 13 may be used to determne perods when drect ventlatve coolng may be appled and to estmate the ventlaton rates needed to mantan thermal comfort durng these perods. If T o >T -csp or T o >17 C then ventlatve coolng s not useful and evaluaton of coolng usng nghttme ventlaton s pursued as below. Nghttme Coolng When daytme outdoor temperatures exceed T -csp, drect ventlaton s no longer useful. Coolng the buldng's thermal mass wth outdoor ar durng the prevous nght may be able to offset daytme nternal gans, however, f the outdoor ar temperature drops below T -csp durng the nght. When ths s possble, the heat transfer rate at whch energy may be removed from the buldngs thermal mass nght approaches, n the lmt for a very massve buldng: ( ) mc & T T when T < T nght p csp o o csp The total energy removed from the buldng's thermal mass durng the evenng may then be used to offset nternal gans on the subseuent workday. The average nternal gan that may be offset s eual to the ntegral of the nght removal rate dvded by the workday tme perod: nghttme (14) cool = (15) nght t Euaton 15 wll be used to estmate the nternal gan that may be offset for a nomnal unt nghttme ar change rate to mantan thermal comfort. RESULTS Ths method was appled to usng WYEC2 hourly annual clmatc data publshed by ASHRAE [ASHRAE 1997a]. The WYEC data sets were devsed to be typcal year data sets 857
Proceedngs: Indoor Ar 2002 to evaluate typcal year energy consumpton. When evaluatng the performance of a specfc (proposed) natural ventlaton system, t may be reasonable to consder extreme year rather than typcal year condtons [Levermore and Wrght 2000]. Calculatons were made for specfc nternal gans from 10 W/m 2 to 80 W/m 2. The low end of ths range corresponds to the combnaton of low-energy lghtng systems wth mnmal plug-loads and relatvely low occupant denstes. The upper end corresponds to very ntensve lghtng, plug loads, and occupancy levels. A story heght of 2.5 m s assumed for these calculatons. Table 1 presents the results for four U.S. locatons (results for addtonal locatons may be found n Axley 2001 and Emmerch, Dols and Axley 2001). The table contans a set of four columns that report the drect ventlatve coolng results: the average ar change rate reured to effect drect ventlatve coolng for each of four specfc nternal gan rates when drect coolng s effectve, the standard devaton of these ventlaton rates, and the fracton of the year drect coolng s effectve for each case.e., the number of hours drect ventlaton s effectve out of the total number of hours n a year's record. A fnal column reports the results for complmentary nght coolng: the average specfc nternal gan that can be offset by a nomnal unt ar change rate of (prevous) nghttme coolng for overheated days, the fracton of overheated days that mght be cooled usng nghttme ventlaton, and the total number of days per year that nghttme coolng mght be effectve. Table 1. Clmate sutablty statstcs for four U.S. locatons Drect Coolng Nght 10 W/m 2 20 W/m 2 40 W/m 2 80 W/m 2 Coolng 1 Mam, FL FLMIAMIT.WY2 data {FLMIAMIT.WY2 data} Vent. Rate or 3.1 ±2.6 6.0 ±5.3 11.9 ±10.6 Coolng Potental 23.9 ±21.2 Effectve 2 26.5 27.3 27.3 27.3 Hot-Humd-Coastal 2.9 ±1.9 W/m 2-26 (79 days) Los Angeles, CA CALOSANW.WY2 data Hot-Ard-Coastal Vent. Rate or Coolng Potental 1.5 ±1.0 3.0 ±2.1 5.9 ±4.2 11.8 ±8.4 5.9 ±2.3 W/m 2-93 Effectve 2 94.9 97.8 97.8 97.8 (27 days) Kansas Cty, MO MOKANCTW.WY2 data Temperate-Contnental Vent. Rate or Coolng Potental 1.9 ±1.8 2.6 ±3.1 4.8 ±6.1 9.7 ±12.1 4.5 ±3.2 W/m 2-57 Effectve 2 37.8 67.4 73.9 73.9 (81 days) Madson, WI WIMADSNT.WY2 data Cold-Contnental Vent. Rate or Coolng Potental 1.8 ±1.7 2.4 ±2.8 4.1 ±5.2 8.2 ±10.4 6.0 ±3.0 W/m 2-82 Effectve 2 39.3 72.4 88.7 88.7 (68 days) 1 Nght coolng for subseuent days when drect coolng s not effectve. 2 For drect coolng = hours effectve 8760 hours; for nght coolng = days effectve days needed. These statstcs have been devsed to provde desgn gudance for prelmnary consderatons. The decson to employ ventlatve coolng strateges depends n part on the ventlaton rates that wll be reured to effect the coolng and n part on the relatve effectveness of the strategy. For example, the Mam, FL data shows that drect coolng wll not only reure 858
Proceedngs: Indoor Ar 2002 relatvely hgh ventlaton rates but even then t wll be useful for only a small fracton of the year. For ths locaton, nghttme coolng also proves to be relatvely margnal demandng relatvely hgh nghttme ventlaton rates to offset moderate specfc nternal gans and then only effectve for 26 of the overheated days of the year. Examnng these results reveals that drect ventlatve coolng may be expected to be most feasble and effectve n the cooler locatons for moderate to hgh specfc nternal gans a reasonable result that may at frst seem unntutve. Drect coolng wll not be partcularly feasble or effectve n the hot-humd locatons for moderate to hgh specfc nternal gans. DISCUSSION The proposed method has a ratonal physcal bass and, therefore, should be consdered relatvely general. The frst results of ts applcaton ndcate that t s able to reveal sgnfcant dfferences between clmates. Furthermore, the method has been devsed to provde buldng desgners wth useful prelmnary desgn gudance relatng to the levels of ventlaton reured to mplement the drect and nghttme coolng strateges. These observatons suggest that the proposed method may prove to be a practcal desgn tool. The method s not wthout ts shortcomngs, however. Estmates of the nternal gans that may be offset by nghttme coolng are based on the assumpton that the buldng has, essentally, nfnte thermal mass thus these results may sgnfcantly overestmate the beneft of nghttme coolng. Also, the statstcal representaton of the reured ventlaton rate to offset a gven specfc nternal gan may reure reconsderaton. Bourgeos has publshed a smlar method [Bourgeos et al. 2000] that doesn t use the concept of the balance pont temperature but lmts drect ventlatve coolng to outdoor ar temperatures exceedng 12 C (.e., to avod cold drafts). Conseuently, Bourgeos method more narrowly lmts the ventlatve coolng season. CONCLUSION A method to evaluate the clmate sutablty of a gven locaton for drect ventlatve coolng and complmentary nghttme ventlatve coolng of a buldng's thermal mass has been presented. Importantly, the method may be appled, n prncple, to ventlatve coolng acheved by natural, mechancal, or mechancally asssted natural means. Ths method allows the buldng desgner to evaluate the feasblty and potental effectveness of ventlatve coolng strateges, gven knowledge of the lkely nternal gans n the buldng, and make frst estmates of the reured ventlaton rates. REFERENCES ASHRAE (1997a). WYEC2: Weather Year for Energy Calculatons 2. ASHRAE. ASHRAE (1997b) 1997 ASHRAE Handbook Fundamentals. SI Edton ed., ASHRAE. ASHRAE (1999), ASHRAE Standard 62-99: Ventlaton for Acceptable Indoor Ar Qualty, ASHRAE. Axley, J.W. (2001). Applcaton of Natural Ventlaton for U.S. Commercal Buldngs Clmate Sutablty, Desgn Strateges & Methods, and Modelng Studes. GCR-01-820, Natonal Insttute of Standards and Technology. Bourgeos, D., A. Potvn, and F. Haghghat. (2000) Hybrd Ventlaton Of Canadan Non-Domestc Buldngs : A Procedure For Assessng IAQ, Comfort And Energy Conservaton. n RoomVent 2000. Unversty of Readng: Elsever. Emmerch, S.J., W.S. Dols, and J.W. Axley. (2001) Natural Ventlaton Revew and Plan for Desgn and Analyss Tools. NISTIR 6781, Natonal Insttute of Standards and Technology. Levermore, G.J., A.M. Jones, and A.J. Wrght (2000). Smulaton of a Naturally Ventlated Buldng at Dfferent Locatons. ASHRAE Transactons, Vol. 106 (Part 2). 859