Indirect Evaporative Cooling: Interaction between Thermal Performance and Room Moisture Balance

Transcription

1 Indiect Evapoative Cooling: Inteaction beteen Themal Pefomance and Room Moistue Balance M. Steeman 1, A. Janssens 1, M. De Paepe 2 1 Depatment of Achitectue and Uban Planning, Ghent Univesity - UGent Jozef Plateaustaat, 9Ghent, Belgium 2 Depatment of Flo, Heat and Combustion Mechanics, Ghent Univesity - UGent Sint-Pietesnieustaat 41, 9 Ghent, Belgium 1. Intoduction Diffeent moistue souces exist in buildings e.g. people, plants, cooking and shoeing Apat fom them, the elative humidity in a oom is influenced by ventilation ith outdoo ai, infiltation and the exchange of moistue ith alls and funitue. Hygoscopic mateials such as ood and textile ae able to dampen out elative humidity vaiations and povide theefoe a healthie indoo climate [1]. Svennbeg et al. [2] have shon that funitue plays an impotant ole in the buffe capacity of a oom. 2. Indiect Evapoative Cooling A. Opeation In indiect evapoative cooling (figue 2) the etun ai passes ove the et side of an ai/ai heat exchange. This adiabatic humidification cools the ai steam. At the same time fesh ai flos ove the othe side of the heat exchange and cools don. This method diffes fom diect evapoative cooling in hich the steam is diectly humidified. By humidifying the etun ai the dy bulb tempeatue of the can be loeed ithout inceasing its humidity atio. In this ay a moe comfotable indoo climate can be obtained. Figue 1: Room moistue balance Nevetheless it is difficult to assess the buffe capacity of the indoo climate because of the uncetainty of many paametes: the mateial moistue popeties vay ith the elative humidity and fo many mateials thee is a lack of available data [3] [4]. Futhemoe, it is difficult to pedict the exact suface of buffeing mateial in the indoo climate. Due to this most building simulation pogams, e.g. Tnsys, pedict the elative humidity in a simplified ay [5] [6]. One of the HVAC applications of hich the themal pefomances depend on the indoo humidity is indiect evapoative cooling (IEC), an inteesting passive cooling method in hich a clea inteaction can be obseved beteen the elative humidity of the etun ai and the themal comfot ealized in the building. This pape focuses on the inteaction beteen the themal pefomance of this cooling technique and the indoo elative humidity. The fist pat explains the pinciples of IEC and ho the pefomances ae influenced by the indoo humidity level. The second pat discusses the measued pefomance of an existing installation in a nonesidential building in Oostende and the thid pat pesents some esults fom a paamete study pefomed ith a simple simulation model. 3 Figue 2: Indiect Evapoative Cooling [7] In figue 3 the states of both aisteams duing the pocess ae schematically pesented making use of a Mollie diagam. Duing humidification, the etun ai follos lines of equal et bulb tempeatue until satuation is eached, hile the follos lines of equal humidity atio. et bulb tempeatue > maximum cooling elative humidity % SUPPLY AIR indiect cooling moistue RETURN AIR dy bulb adiabatic tempeatue humidification tempeatue Figue 3: State of the aisteams in IEC 2 1 B. Influence of Buffeing on the Themal Pefomance Because in IEC the etun ai is moistened its elative humidity plays an impotant ole. As moistue buffeing in hygoscopic mateials is able to educe humidity peaks in the indoo climate, a bette pefomance of this technique can be expected. In that case moe moistue could be supplied to the etun ai befoe satuation is eached.

2 3. Expeimental Detemination of Themal Pefomance A. Indiect Evapoative Cooling Installation in Oostende Measuements ee caied out in the cafeteia of a holiday cente located at the Belgian Coast (Oostende). Tempeatue and elative humidity ee measued in the installation at all stages of the pocess as ell as in the cafeteia. Also the position of the valves, pumps and fans ee egisteed. The coss flo heat exchange hich is used has a total idth of 9mm, a length of 195mm and is made out of polypopylene. Both the supply and etun fan can ok in to stages ith a maximum ai flo ate of 7m³/h. The installation can opeate at diffeent stages. When the outdoo ai is fesh enough in summe, the ai flo ate is inceased and the ai is used fo fee cooling. As soon as the outdoo ai tempeatue is too high, the fesh ai is adiabatically cooled by moistening the heat exchange. If the desied indoo tempeatue is not yet eached, active cooling may contibute to loe the tempeatue of the. B. Discussion of Measued Pefomance THERMAL SUMMER COMFORT The data pesented hee ae these of July 6 [8]. Measuements ee done evey five minutes. Figue 4 shos the measued tempeatues fom 18 until July. The peiods in hich the IEC is oking ae indicated. Figue 2 shos hee the tempeatue of the aisteams is measued. Duing the fist to days the tempeatue of the donsteam fom the heat exchange section does not diffe much fom that of the outdoo ai. The fesh ai is clealy bypassed alloing fee cooling. The tempeatue measued behind the evapoato is alays loe than befoe, shoing active cooling has been oking duing the hole measued peiod. tempeatue ( C) max 13.9 C ith IEC 1 18/jul/6 19/jul/6 /jul/6 /jul/6 /jul/6 fesh outdoo ai behind heatexch behind evapoato cafetaia IEC Figue 4: Measuements of IEC in Oostende (18/7/6 - /7/6). Duing the measued days the outdoo tempeatue vaied fom C to 35 C. The aveage tempeatue measued in the cafeteia as C. Making use of evapoative cooling, a maximum tempeatue dop of 13.9 C as noticed on /7 hen the outdoo tempeatue eached 34.8 C. On aveage a cooling of 3.3 C could be achieved ith this method ithin the measuing peiod. Duing the nights fee cooling as used fo some hous to cool don the cafeteia. tempeatue ( C) 35 T supply + evapoato T supply adiab 15 heating cooling outside tempeatue ( C) Figue 5: Cooling potential of IEC and Active Cooling fo July 6 (Oostende). Figue 5 shos ho much the is cooled in July. Most of the time the IEC system is able to cool the supply ai to a tempeatue loe then C, subsequently active cooling additionally cools the. It can also be noticed fom the measuements, that in some cases the ai is heated instead of cooled by the heat exchange. This demonstates that the contol algoithm of the ai handling unit may be impoved. The indoo summe comfot is no evaluated by calculating the tempeatue exceedings. In figue 6 only the hous in July hen the cafeteia as occupied (appoximately fom 9-h) ee taken into account. As indoo summe comfot citeia e assume that the indoo tempeatue should not exceed C. Fom the gaph can be seen that a quate of the measued time this citeia is not satisfied. Futhemoe, due to intenal and sola gains, the oveall indoo tempeatue is alays highe than the outdoo tempeatue exceedings (%hous) outside tempeatue aveage tempeatue cafetaia > C > C > C > C > C > C > C > C Figue 6: Indoo summe comfot duing July 6 in Oostende. HYGROTHERMAL INTERACTION Fom figue 7 it is seen that thee is a linea elation beteen the amount of cooling of the and the

3 diffeence beteen the enteing and the et bulb of the etun ai leaving the heat exchange. The smalle the diffeence in dy bulb and et bulb tempeatue, the less the fesh ai can be cooled because satuation is eached faste hen humidifying the etun ai (fig.3). Only the peiod duing the measuements in hich the evapoative cooling as oking is taken into account hee. tempeatue diffeence beteen dy bulb and et bulb etun ai ( C) y x R effectiveness ε tempeatue diffeence of ealized ith IEC ( C) Figue 7: Inteaction beteen humidity level and themal pefomance (Oostende. 18/7/6 - /7/6). The lineaity can be expessed by the effectiveness ε of indiect evapoative cooling: ε s,1 s,2 (1) s,1 ',1 In (1) s,1 and s,2 ae espectively the tempeatue of the enteing and leaving the heat exchange, and ',1 the et bulb tempeatue of the etun ai enteing the heat exchange. The effectiveness thus compaes the actual tempeatue diffeence ealized by IEC ith the maximum possible diffeence. Calculated fom the measuements in July (figue 7) the effectiveness as aveagely 88%. 4. Simulations A. Paamete Study The definition of effectiveness ε (1) can be used as a fist step in evaluating the pefomance of an IEC system having a constant effectiveness. As the et bulb tempeatue of the etun ai depends on the moistue balance indoo, the influence of moistue, buffeing and ventilation ate can be easily studied. Theefoe in Tnsys [6] a simple model as built that evaluates the indoo tempeatue at evey time step and calculates the tempeatue of the (at the same time step by iteation). Fo the indoo envionment, the BESTEST building fom Common Execise1 [9] as used. The indoo climate thus consisted of a volume of 1.6m³, all alls ae built fom 15cm concete. Assuming that all boundaies ae adiabatic, it is possible to study only the effect of the installation on the indoo conditions. The outdoo climate is stationay C and 5% elative humidity. Thee is no infiltation, no adiation and no cooling o heating device. The ai is assumed to be ell mixed. Moistue and heat as poduced evey day beteen 9 and 17h due to pesons and household activities. Thee is no othe ventilation pesent then the IEC. To model the moistue buffeing the Simple Capacitance Ratio model fom Tnsys, assuming that the capacity of the ai is fictively inceased, as used. Runs ee caied out ith houly values until stationay conditions ee eached. Fist the effectiveness of the IEC system itself as vaied (figue 8). In the simulations thee is a moistue of.35kg/h and heat of 6W due to five pesons occupying the oom (seated, light ok) [6][]. It can be undestood easily that a bette effectiveness of the system ill esult in a loe supply tempeatue and theefoe a moe comfotable indoo climate. aveage tempeatue ( C) 31 indoo ai effectiveness (%) Figue 8: Relation effectiveness indoo and supply tempeatue (n 2h -1 /.35kg/h moistue & 6W / Ratio 5) MOISTURE PRODUCTION Seveal simulations ee un in hich the indoo moistue as vaied. Fist e inceased only the moistue (due to exta activities: case A, B&C), late e inceased the moistue and heat (due to lage occupancy: case A, D&E) (Table1). As the moistue indoo inceases, the IEC cools the less, esulting in a less comfotable indoo climate. Futhemoe ith highe moistue s the vaiations in indoo tempeatue ae shon to be lage due to lage vaiations in the supply tempeatue (figue9). Fom figue the inteaction beteen heat and moistue is demonstated: a lage occupancy of the oom causes a highe cooling demand due to intenal gains, but at the same time the supply tempeatue inceases due to highe moistue. Table 1: Moistue Occupation Moistue & activities (kg/h) Exta activities (kg/h) Heat (W) A 5pesons 5*.7 / 5*1 B 5p+kitchen 5* *1 C 5p+shoes 5* *1 D p *.7 / *1 E p *.7 / *1

4 tempeatue ( C) elative humidity (%) time (hs) aveage tempeatue ( C) A A D D poductio n Tin A Tin B Tin C A B C RHin A RHin B RHin C E B B E moistue (kg/h) indoo ai indoo ai Figue 9&: Influence of indoo moistue on themal pefomance IEC (ε 85% / n 2h -1 / Ratio5) MOISTURE BUFFERING Simulations ee done ith and ithout taking into account moistue buffeing in the indoo climate. A Capacitance Ratio of 5 equals ith thee alls (+/- 52.5m²) finished ith gypsum plaste, hile a atio of equals ith all alls and ceiling (+/- 1.6m²) finished ith plaste. Fom figue 11 can be seen that the aveage indoo tempeatue does not change much fo diffeent buffe capacities. This can be explained by the fact that moistue buffeing does not have a lage effect on the aveage elative humidity, but athe dampens out the vaiations. This has also been seen by [2]. C C Figue 11: Influence of moistue buffeing on themal pefomance IEC (ε 85% / n 1h -1 /.35kg/h moistue & 6W) VENTILATION RATE As a last paamete the ventilation ai change ate as vaied fom.5 to 5h -1. High ai flo ates cause moe moistue to be emoved fom the oom poviding a loe supply tempeatue and theefoe a bette indoo climate. On the othe hand the vaiations in elative humidity and thus in supply tempeatue ae dampened out. tempeatue ( C) elative humidity (%) Tin n.5/h Tin n1/h Tin n2/h Tin n5/h n.5/h n1/h n2/h n5/h RHin n.5/h RHin n1/h RHin n2/h RHsupply n5/h Figue 12: Influence of ai flo ates on themal pefomance IEC (ε 85% / Ratio5 /.35kg/h & 6W). B. Wet Suface Heat Exchange - Model The measuements have only been the fist step in studying indiect evapoative cooling. A model fo a coss flo et suface heat exchange is developed, hich ill be validated against the measuements. The aim is develop a model that can be easily used in the Tnsys envionment to be able to simulate buildings hee IEC is applied. i -1 i i +1 etun ai,i x,i,i s,i ate film tempeatue ( C) elative humidity (%) Tin no buffe Tin buff5 Tin buff _no buffe buff5 buff RHin no buffe RHin buff5 RHin buff Figue 12: Woking scheme of the simulation model fo a et suface heat exchange. Heat balances G c G c + xα + xα (2) ( ) ( xα + xα s ) + xα s s ( p p ) i 1, xα a β xh ev a ( G c + xα ) + G c x f f f f f f f i+ 1, f (3) α (4) Mass balance ξ + β x p β xp + G ξp (5) ( G ) i 1,

5 Fou balances descibe the system: a heat balance fo the etun ai (2) and the (4), and a heat and mass balance at the ate film suface (3) (5). To model et suface heat exchanges, the equations ae solved simultaneously using a contol volume method. The model ill allo defining of the inlet conditions of both ai steams and the dimensions and chaacteistics of the IEC system. 5. Conclusions Indiect evapoative cooling is an inteesting passive cooling technique in hich the pefomances depend mainly on the indoo humidity. Measuements caied out in July 6 sho that by using this technique the supply tempeatue can be cooled up to 14 C. In ode to establish a comfotable indoo climate the ai as futhe cooled by active cooling. Some fist simulations ith a simple model based on the systems effectiveness aleady sho that the knoledge of the inne moistue balance is essential to pedict the pefomance of IEC ell. The indoo moistue plays the most impotant ole in loeing the indoo tempeatue, hile the amount of moistue buffeing mateial athe influences the stability of the climate (less vaiation in elative humidity and tempeatue occus). Highe ventilation ates also help to loe the indoo tempeatue. Futhe eseach ill be caied out using a et suface heat exchange model alloing a bette undestanding of the paametes influencing the cooling technique. [4] Nodtest. Moistue Buffe Capacity Initial evie of definitions and methods. Meeting may 14, 4, ETH Züich. 4. [5] Janssens, A., De Paepe, M. Effect of moistue inetia models on the pedicted indoo humidity in a oom. Poceedings of the th AIVC Confeence, Bussels. 5. [6] SEL, TRANSSOLAR, CSTB, TESS. Tnsys 16: A Tansient System Simulation Pogamme. Univesity of Wisconsin, Madison, USA. 4. [7] Menega Klimatechnologie..menega.de [8] Koninklijk Meteoologisch Instituut van België..kmi.be [9] Rode, C., Woloszyn, M. 4. Annex41 Subtask1: Common Excecise1 (Whole building heat and moistue analysis) [] Haiman, L., Bundett, G., Kittle, R. 1. Humidity Contol Design Guide fo Commecial and Institutional Buildings. ASHRAE. ISBN Nomenclatue G mass flo ate [kg/s] c specific heat [J/kg K] tempeatue [K] x contol volume suface [m²] α heat tansfe coefficient [W/m²K] p vapo pessue [Pa] β vapo tansfe coefficient [kg/s/pa/m²] ξ vapo capacity 6.1 e(-6) [kg/kg/pa] Subscipts etun ai s ate film 1 ai enteing heat exchange 2 ai leaving heat exchange et bulb i cuent contol volume Acknoledgements This PhD eseach is established ith financial suppot of the Flemish Institute fo the Pomotion of Innovation by Science and Technology in Flandes (IWT). Refeences [1] Simonson, C.J., Salonvaaa, M., Ojanen, T. The effects of stuctues on indoo humidity - possibility to impove comfot and peceived ai quality. Indoo Ai 12, p [2] Svennbeg, K.,Hedegaad, L., Rode, C. Moistue Buffe Pefomance of a Fully Funished Room, ASHRAE Special Publications, Poceedings of Buildings IX-Confeence. 4. [3] IEA Annex 14. Condensation and enegy. Volume 3. Catalogue of mateial popeties. Belgium, K.U.Leuven