ALTERNATIVE DESIGN OF HYBRID DESICCANT COOLING SYSTEMS

th Interntionl Conference on Het Trnsfer, Fluid Mechnics nd Thermodynmics ALTERNATIVE DESIGN OF HYBRID DESICCANT COOLING SYSTEMS Nóbreg C.E.L. nd Brum N.C.L. Author for correspondence Deprtment of Computtionl Engineering, CEFET-Rio/Petrópolis, Rio de Jneiro, Brzil, e-mil:nobreg@pobox.com ABSTCT Trditionl vpor-compression refrigertion systems hndle the ir cooling nd dehumidifiction in single process. Depending on the chrcteristics of the therml lod, it is not possible to simultneously meet the cooling nd dehumidifiction requirements, resulting in under dried or overcooled supply ir. Recent studies indicte tht combintion of dsorptive dehumidifiction nd vporcompression might result in n improved system, regrding both energy consumption nd ir-qulity. Accordingly, the present study ims t n lterntive design of hybrid system. It is shon tht such system cn provide ir t the sme supply condition s vpor-compression system, requiring s much s 40% less cooling cpcity. Also, the proposed system orks ith 00% of ir renovtion, being comptible ith ny ir qulity stndrd. INTRODUCTION Modern ir-conditioning design prctice ims t systems ith lo mbient impct nd operting costs, hile providing comfort nd helthy indoor environment. Desiccnt cooling systems emerged s promising lterntive to trditionl vpor compression systems, since it llos for the use of solr energy or lo grde ste het for the rectivtion of the desiccnt rotor, in ddition to the use of ter s refrigernt. Moreover, desiccnt systems often ork ith 00% of fresh ir, thus providing mximum indoor ir qulity. Recent studies, hoever, chnged the perspective from competition to integrtion. In conventionl ir-conditioning systems, ir cooling is simultneously crried out ith dehumidifiction. In humid environments the ir is often overcooled to chieve the dequte humidity level, nd sub sequentilly re-heted so s to bring temperture bck to stisfctory supply condition, in steful process energy ise []. Accordingly, the combintion of the desiccnts nd vporcompression in hybrid cycle hs been incresingly studied. The min dvntge is the therml lod decomposition into sensible nd ltent components, hich re respectively hndled by the cooling coil (vpor-compression cycle) nd the desiccnt rotor. Since the cooling coil hs been unburned of the ltent lod, its refrigertion cpcity is significntly donsized, hen compred to stndrd vpor-compression cycle. Moreover, the cooling coil ill be ble to ork t temperture higher thn the ir de point, hich in turn llos for refrigernt expnsion t higher temperture inside the cooling coil. As consequence, the cycle coefficient of performnce (COP) increses [2]. Also, the condenser ste het cn be hrvested so s to prtilly supply the rectivtion het required by the desiccnt cycle, hich could be supplemented by gs fired or solr source. The hybrid cycle is usully designed ith the desiccnt rotor nd the evportor in n in-line rrngement, in hich the ir is sub sequentilly dried nd cooled, so s to chieve the desired supply condition. The present ork presents lterntive rrngement for the hybrid cycle, ith the desiccnt rotor nd the evportor orking in prllel rther thn in series. It is shon tht the proposed cycle is more verstile thn the current hybrid cycle design, lloing for the ccommodtion of therml lod or outside ir condition fluctutions ith simple ir flo rte control. The combintion of vpor compression nd liquid desiccnt system s reported to increse the COP by s much s 50%, hen compred to the stndrd vpor compression cycle lone []. This very sme increse hs been reported by n experimentl investigtion of the cycle [4]. An experimentl setup of desiccnt heel ssisting 6 kw vpor-compression cycle exhibited energy svings of 7.5%, compred to the stndrd opertion [4]. A similr result (0% of energy svings) s reported by simultion of hybrid cycle hich considered the desiccnt heel nd the evportor orking in series nd in prllel [5]. A close figure (26.%) s found in n experimentl setup for mobile ir-conditioning system [6]. An even greter energy sving (70%) s reported by hybrid cycle driven by geotherml poer [7]. Figure () shos the schemtic representtion of typicl hybrid desiccnt system. NOMENCLATURE [-] Compressor [-] Condenser DW [-] Desiccnt heel EVAC [-] Evportive Cooler [-] Evportor EXH [-] Exhust f [-] Desiccnt frction h [W/m 2 C] Het trnsfer coefficient h [J/kg m] Air specific enthlpy [-] Het Wheel 468

th Interntionl Conference on Het Trnsfer, Fluid Mechnics nd Thermodynmics k [kg/m 2 S] Mss trnsfer coefficient R [-] Seprtion fctor REH [-] Reheter RH [-] Reltive Humidity RLH [-] Room ltent het RSH [-] Room sensible het RSHR [-] Room sensible Het Rtio T [ C] Temperture Q [kjkg] Adsorption het Y [-] Air bsolute humidity V [m /s] Volumetric ir flo rte W [-] Solid bsolute humidity Specil chrcters λ [-] Non-dimensionl prmeter ρ [kg/m ] Air density Subscripts ir,2, Air Sttes DW Desiccnt Wheel EVAC Evportive Cooler f Fresh Het Wheel Outside ir 2 Air stte REH Rehet supply ir Room ir evportot rec recirculted t totl ll The system consists of desiccnt heel, regenertor, sensible het cooler nd n evportive cooler rrnged in series. The outside ir () is dmitted to the desiccnt heel, undergoing n isenthlpic dehumidifiction process. The regenertor then brings the ir temperture bck to the mbient level. The ir is then cooled under the mbient level t sensible het cooler, in constnt humidity process. The cooling is promoted by vpor-compression cycle. Finlly, the ir is dmitted to n evportive cooler, through hich it suffers n isenthlpic humidifiction process, until it reches the supply ir condition. The min dvntge of this scheme is tht it hndles the ltent lod nd the sensible lod in to seprte processes. The desiccnt heel hndles the outside ir humidity, heres the regenertor nd the het exchnger hndle the sensible het. Since the desiccnt heel cn be driven by lo grde therml energy (or even solr energy), it implies significnt opertion cost reduction hen compring to vpor compression dehumidifiction, hich is typiclly driven by electric poer. This llos for the cooling cpcity of the evportor to be donsized, since it no exclusively hndles the sensible component of the therml lod. In ddition, the evportor ill ork t higher temperture (nd thus pressure) thn tht of the stndrd cycle, hich hs to be kept belo the ir de point. This benefits the COP, implying in further energy svings. Moreover, there is lso the possibility to hrvest the condenser rejected het to pre-het the ir t the therml source inlet, thereby mitigting the fuel consumption t the therml source. EXH DW 2 4 EVAC Figure Typicl Hybrid Desiccnt Cycle METHODOLOGY Figure 2 sho n lterntive design of the hybrid desiccnt system. In the proposed system, the outside ir strem is split in to strems. One strem follos the route to the desiccnt heel, het heel nd evportor, heres the other strem bypsses the desiccnt heel nd is directed to regenertive het heel. At the evportor outlet the strems re mixed, nd the ir mix reches the supply condition. The proposed design hs to dvntges over the typicl hybrid cycle design. First, it lcks the evportive cooler, lloing for diminished initil nd opertion costs. Second, it llos for the ccommodtion of ny fluctution of outside ir conditions. For instnce, should the outside ir humidity be higher thn usul, greter ir flo rte cn be induced through the desiccnt heel. Conversely, for higher outside ir temperture, the bypss rte should be incresed. Het Wheel 2 Desiccnt Wheel Het Wheel 2 Figure 2 Alterntive Hybrid Desiccnt Cycle The objective of the present ork is to compre the energy demnds of the proposed cycle ith the trditionl vpour compression cycle, shon in Fig (4). The comprison ill be crried out for cse study, n ppliction ith chrcteristiclly high ltent component of the therml lod. All clcultions ill be crried from the ir side of the cycle, i.e., the vpor compression cycle simultion is out of the scope of the present ork. Only the evportor cooling cpcity ill be ddressed, from the tempertures T nd T 2 nd the ir flo rte. In ll cycles the exhust strem is regenerted t het heel () so s to pre-cool the fresh ir strem. The systems ill be dimensioned for the cse study depicted in Tble () [8]. 469

th Interntionl Conference on Het Trnsfer, Fluid Mechnics nd Thermodynmics Figure Psichrometric representtion of the proposed hybrid cycle The typicl vpour-compression system ill be dimensioned ccording to the usul ir-conditioning design methodology [9,0]. Tble (2) shos the ir sttes, the totl ir flo rte, required rehet nd cooling cpcity for the evportor. Tble (): Cse Study Outside Air Conditions: T : 2.2 C RH : 44.5% Room Air Conditions: T : 25.9 C Y : 50.0% Fresh ir flo rte: V :.8 m /S Room Sensible Het: RSH: 5.6 kw Room Ltent Het: RLH: 9.05 kw Tble (2): Typicl Vpour-compression cycle T( C) Y(Kg/Kg) h(kj/kg) Stte 2.2 0.06 67.50 Stte 26.9 0.005 5.70 Stte 2 9.8 0.007 28.4 Stte 5. 0.007.89 Stte 24.9 0.009 49. V t (m /s).2 Q reh (kw) 2.00 Q cool (kw) 0.00 Figure 5 Psichrometric representtion of the vpourcompression cooling cycle While the methodology for sizing the stndrd compression ir-conditioning system is strightforrd nd cn be found in ny HVAC hndbook, the dimensioning of the hybrid cycle requires grphicl procedure, long ith effectiveness vlues for the het exchngers. The desiccnt heel outlet stte cn be obtined by running desiccnt heel simultion:. Set the stte nd the het heel effectiveness, nd use Eq.() to determine T 2. Stte 2 is then defined, since Y 2 Y. 2. Stte is obtined s the output of the desiccnt heel simultion progrm, fter setting the regenertion temperture.. Set stte nd the het heel effectiveness nd use Eq.() to determine T 2. Stte 2 is then defined, since Y 2 Y. 4. Set T evp nd the evportor effectiveness nd use Eq.() obtin T, ith Y Y 2. 5. Determine the supply ir stte () t the intersection of line -2 ith the RSHR line. 6. With sttes nd defined, clculte the totl volume flo rte V t ccording to Eq.(4). Figure () shos the psichrometric representtion of the cycle. The resulting ir sttes, flo rtes, required het nd cooling cpcity re shon in Tble (). The folloing equtions re used in the methodology: EXH 2 REH Figure 4 Typicl vpour-compression cooling cycle T T 2 T T T T2 T T 2 T2 T T T ( h h ) RSH + RLH V t ρ () (2) () (4) 470

th Interntionl Conference on Het Trnsfer, Fluid Mechnics nd Thermodynmics Detils of the mthemticl model development nd vlidtion of the desiccnt heel simultion re idely vilble in the literture. [-4]. In short, s shon in Fig. (6), energy nd mss blnces re pplied to elementry control volumes enclosing the sorbent lyer nd the flo chnnel, resulting in equtions (5) to (8), nd n lgebric eqution (9) hich reltes the dsorptive cpcity s function of the vpor pressure in the pore vicinity, knon s isotherm. The prmeter R is mesure of the strength of dsorption of prticulr desiccnt mteril. Y λ ( Y Y ) (5) W ( Y Y ) λ 2 W W mx λ λ λ 2 T T ( T T) λ ( Y Y) + R ( R + ) φ Q h Cr h f k h h (6) (7) (8) (9) (0) () (2) Tbles (2) nd () sho the sme sttes nd sme totl ir flo rtes for both cycles, hich imply the sme condition of therml comfort. Hoever, the proposed cycle orks ith 00% of fresh ir, heres tble () shos tht the vpor compression cycle runs ith nerly 60% of recirculted ir, resulting in poorer indoor environment qulity ise. Also, it cn be seen tht the required cooling cpcity (chiller size) on the Hybrid cycle is 40% less hen compred to tht of the vpour compression cycle. Even though it hndles greter mount of outside ir. ( Y Y ) Y λ T T W Figure 6 Computtionl domin (desiccnt heel) Tble (): Proposed Hybrid cycle ( Y Y ) λ 2 ( T T) λ ( Y Y) T( C) Y(Kg/Kg) h(kj/kg) Stte 2.2 0.06 67.50 Stte 2 27.0 0.06 6.80 Stte 55.0 0.0047 67.50 Stte 2 4.0 0.0047 44.42 Stte 9.0 0.0047 20.90 Stte 5. 0.007.89 Stte 24.9 0.009 49. V t (m /s).2 Q DW (kw) 40.00 Q cool (kw) 60.50 Cution should be tken hen compring the het requirements. Tble (2) indictes rehet demnd of 2kW, hile Tble () indictes het demnd of 40kW on the desiccnt heel rectivtion. Although the het requirement of the vpor compression is much loer, one hs to consider tht the rehet is ccomplished using electricl heters, heres the desiccnt heel my be rectivted using solr poer or ste het. Even compressor ste het cn be use to pre-het the rectivtion ir, hich cn be supplementry heted ith gs so s to rech the required rectivtion temperture. A meningful comprison on the energy consumption nd operting costs of ech cycle ould require simultion of the vpor compression cycle, so s to ccount for the compression ork in ech cse. This mtter is hoever beyond the scope of the present study nd shll be ddressed in future effort. + 47

th Interntionl Conference on Het Trnsfer, Fluid Mechnics nd Thermodynmics CONCLUSION Modern HVAC design ims t lo-energy systems hich re ble to provide indoor mbient ith improved ir qulity. An lterntive design for Hybrid vpor-compression system hs been presented, the performnce of hich hs been compred to trditionl vpor-compression cycle. It hs been shon tht the proposed design cn provide the sme therml comfort s the vpor-compression cycle, using less electricl poer ith totl ir renovtion, in contrst ith stndrd systems hich recircultes s much s 2/ of the totl ir flo. Also, the proposed system lcks n evportive cooler, in opposition to the current Hybrid system design. ACKNOWLEDGEMENTS The uthors ould like to express his grtitude to the finncil support of the Brzilin Government gency CAPES nd CNpQ throughout this reserch project. REFERENCES []McQuiston F.C., Prker J.D., Spliter J.D., Heting, Ventilting nd Air Conditioning, John Wiley & Sons, 6 th ed., Hoboken, NJ, 2004. [2] Nellis G., Klein S., Thermodynmics, Cmbridge University Press, Cmbridge, 2009. [] Di Y., Wng R., Zhng W., Yu J., Use of liquid desiccnt cooling to improve the performnce of vpor compression ir-conditioning, Applied Therml Engineering (2), 200, pp. 85-202. [4] Ji C., Di J., Wu J., Wng R., Anlysis on hybrid irconditioning, Applied Therml Engineering (26), 2006, pp. 292-2400 [5] Ling J., Hng Y., Rdermcher R., Theoreticl study on seprte sensible n ltent cooling ir-conditioning system, Interntionl Journl of Refrigertion, (), 200, pp, 50-520. [6] Lee H., Ling X., Hng Y., Rdermcher R., Performnce investigtion on desiccnt ssisted mobile ir-conditioning system, Interntionl Journl of Refrigertion, (0), 206, pp, 870-880. [7] Css,W., Schmitz, G., Experiences ith gs driven, desiccnt ssisted ir conditioning system ith geotherml energy for n Office building, Energy nd Buildings (7), 2005, pp.49-50. [8]Crrier, W. Hndbook of ir-conditioning system design, McGr- Hill, Ne York,Ny, 965. [9]Pitt E.G. Air Conditioning Principles nd Systems, 4 rd ed., Prentice-Hll, Upper Sddle River, NJ, 2002. [0] Thelkheld J.L, Rmsey, J.W. nd Kuehn, T.H., Therml Environmentl Engineering, rd ed., Prentice-Hll, Upper Sddle River, NJ, 998. [] Nóbreg C.E., Brum N.C.L, Modelling nd Simultion of Het nd Enthlpy Recovery Wheels, Energy (4), 2009,pp. 206-2068. [2] Nóbreg C.E.L., Brum, N.C.L., Influence of Isotherm Shpe over Desiccnt Cooling Cycle Performnce, Het Trnsfer Engineering, vol.0 (4), 2009, pp.22-28. [] Nóbreg C.E., Brum N.C.L., A Grphicl Procedure for Desiccnt Cooling Cycle Design, Energy (6), 20p p: 206-2068. [4] Chung JD, Lee, DY., Effect of desiccnt isotherm on the performnce of desiccnt heel, Interntionl Journl of Refrigertion (2), 2009, pp.720-726.. 472