An innovative solar assisted desiccant and evaporative cooling (DEC) system for
building air conditioning has been installed at the Solar Laboratory of the University of
Palermo. In this system, air-to-air packaged wet heat exchangers are used to
maximise the exploitation of the evaporative cooling potential associated with the
exhaust air stream.
Results obtained are very positive, showing that DEC systems can benefit from the
use of wet heat exchangers to efficiently cool the air stream after the adsorption
process in the desiccant wheel, without any change in moisture conten

FINOCCHIARO, Pietro

Archivio istituzionale della ricerca - Università di Palermo

EAI Energia, Ambiente e Innovazione  2/201292Studi&ricerche TECHNICALS PAPERSExperimental results on a solar desiccantand evaporative cooling systemequipped with wet heat exchangersAn innovative solar assisted desiccant and evaporative cooling (DEC) system forbuilding air conditioning has been installed at the Solar Laboratory of the University ofPalermo. In this system, air-to-air packaged wet heat exchangers are used tomaximise the exploitation of the evaporative cooling potential associated with theexhaust air stream.Results obtained are very positive, showing that DEC systems can benefit from theuse of wet heat exchangers to efficiently cool the air stream after the adsorptionprocess in the desiccant wheel, without any change in moisture contentÿ Pietro FinocchiaroRisultati sperimentali di un sistema solare ad adsorbimento conraffreddamento evaporativo realizzato mediante scambiatori di calore bagnatiUn innovativo sistema solare di deumidificazione e raffreddamento evaporativo (DEC) da utilizzareper la climatizzazione degli edifici è stato installato presso il Laboratorio Solare dell’Università diPalermo. Il sistema utilizza scambiatori di calore aria-aria a pacco mantenuti bagnati sul latosecondario al fine di massimizzare il potenziale di raffreddamento evaporativo associato al flusso d’aria di ritorno dall’edificio. I risultati ottenuti sono molto positivi e mostrano che i sistemi DECpossono beneficiare dell’uso di scambiatori di calore bagnati per realizzare un efficaceraffreddamento sensibile del flusso d ’aria di processo dopo la fase di adsorbimento che avvienenella ruota deumidificanteÿ Pietro FinocchiaroUniversity of Palermo, Department of EnergyThe Solar DEC technology is an interesting and fas-cinating solution f or applications in building airconditioning. It is a thermally driven open coolingcycle based on evaporative cooling and adsorptionprocesses. In a solar desiccant cooling cycle, solarenergy is u sed to regenerate a desiccant materialthat dehumidifies moist air by vapour adsorption;the resulting dry and warm air is cooled in a sensi-ble heat exchanger (usually rotating) and then in adirect evaporative cooler. By  a ssociating differentelementary treatments in moist air (dehumidifica-tion, sensible cooling and evaporative cooling)both in the process and exhaust air, t he techniqueuses water as a refrigerant and mostly solar energyas driving heat. In a solar autonomous DEC system,electricity is only used in the auxiliaries, so thetechnique is environmentally friendly. If the pureDEC effect is not sufficient to guarantee indoor airconditions for the s pecific application, a hybridEAI Energia, Ambiente e Innovazione  2/201293Studi & Ricerche|TECHNICALSPAPERSDEC – conventional cooling cycle can be used byadding one or m ore traditional cool ing coils con-nected with an electric w ater refrigerator to thestandard DEC cycle [Henning, 2004].Monthly results are pr esented and elaborated ac-cording to a monitoring procedure developed inthe framework of the In ternational Energy Agency(IEA) Solar Heating and Cooling (SHC) Programme,Task 38 “Solar Air Conditioning and Refrigeration”[Sparber et al., 2008, Sparber et al., 2009].Description of system layoutThe layout of the DEC system is a hybrid configura-tion with two additional auxiliary cooling coils fedby a conventional vapour compression chiller inte-grated in th e DEC air handling cycle (Figure 1).One co il is u sed for pr e-dehumidification o f th eoutside air stream; the o ther coil c ontrols the airtemperature if the desired supply temperature can-not be reached through indirect evaporative cool-ing alone. In the s ystem 22.5 m2 of solar flat platecollectors are us ed with a w ater tank o f 0. 65 m3.Rated air flow rate of the air handling unit is 1250m3/h.The main innovation in the DEC process is the useof cross flow plate heat exchangers. These devicesare commonly used in air conditioning systems forheat recovery purposes during the winter.When in-direct evaporative cooling is used, the same heatexchangers can be used to efficiently cool the out-side air during summer, wetting the channels of theexhaust air with water [Rey Martìnez et al., 2004].Therefore, this component can efficiently be inte-grated in a desiccant cooling cycle downstream ofthe adsorption process. Other authors have alreadyconsidered the application of we t surface heat ex-changers for indirect evaporative cooling, but onlyin few ca ses has the coupling with DEC systembeen investigated [Jain e t al. 1995, Rowe et al.2010].The surface of secondary flow (return ai r from thebuilding) air channels is wetted by w ater sprayedby nozzles, su ch that a water film evaporates in tothe cooling air and decreases the temperature ofthe he at exchange surface. Process air flowing inthe primary airflow channels is cooled down due tothe lo wer te mperature su rface of th e separatingwall of the heat exchangers.The sensible heat exchanger normally used in DECsystems is thus r eplaced with two plate heat e x-changers displaced in series for a continuous hu-midification o f th e secondary air flow. Thus returnair is hu midified in tw o steps before leaving theAHU after the h eat exchange with the s upply airstream. Desiccant wheel is regenerated by fresh air,which is heated by the two heating coils. It impliesthe use of an additional fan but, on the other hand,the r egeneration ai rflow can be reduced. No b y-pass is used to limit solar heat consumption for theregeneration and to reduce pressure drops duringthe wintertime. Regeneration flow rate can be mod-ulated independently of the return air flow. Anotherspecific feature o f th e system i s t he use of p art ofthe he at rejected by the refrigerator condenser t opreheat regeneration airflow.The air handling on the psychrometric ch art isLayout of the DEC AHU withwet heat exchangersFIGURE 1EAI Energia, Ambiente e Innovazione  2/201294shown for typical summer operation c onditions inFigure 2. The total cooling power of both wet hea texchangers is the e nthalpy difference betweenpoints 3 and 5. It c an be noted that no additionalcooling coil i s u sed to reach the supply tempera-ture of 22 °C.Experimental resultsMonitoring data were collected during Aug - Sept2010 and June - J uly 2011 and results are sh ownboth in terms of seasonal energy performances.Figure 3 s hows the co ld distribution in  th e AHUwhere DEC indicates the contribution of the desic-cant cool ing process to the whole cooling energydelivered. It c an be noted that the c ontribution ofthe te mperature contr olling coil CC2 i s v ery lowbeing the desired supply temperature o f th e AHUtypically reached without any use of the coolingcoil.The total contribution of the desiccant and evapora-tive cooling effect to the total cooling energy deliv-ered is  ab out 70% . The w ater consumption in  th enew DEC configuration is due to evaporation occur-ring on the secondary side of t he wet heat e x-changers HX1 and HX2. A daily mean value of wa-ter consumption of about 70 lt/day was calculated.Finally, a comparison between the DEC system anda conventional system was performed. The conven-tional system considered is an AHU fed by a waterelectric chiller. The analysis was conducted accord-ing to the mentioned monitoring procedure devel-oped in the framework of the IEA Task 38. Monitor-ing results show a primary energy saving of 53.7%as seasonal mean value. In the calculation it ’s takeninto ac count th at, in a conventional sy stem, addi-tional cooling energy is needed for the dehumidifi-cation pr ocess to reach dew po int temperatures,with consequent re-heating of the air up to the de-sired inlet temperature conditions [Beccali M. et al.2012].ConclusionsIn the present work, an innovative desiccant coolingcycle using wet heat exchangers is presented. TheSequence of air handling onreturn and supply air stream fortypical summer conditions onthe psychrometric chartFIGURE 2Distribution of the seasonal cooling energy in kWhproduced by the DEC AHUFIGURE 3EAI Energia, Ambiente e Innovazione  2/201295system presented is a Solar DEC air handling unit,which was recently modified and updated accord-ing to a new concept m ainly based on the use ofwet plate heat exchangers. Monitoring results relat-ed to the mo dified configuration showed th at, dueto the optimisation of the indirect evaporative cool-ing process, a supply temperature in  th e range of21-22 °C can be achieved without the use of an aux-iliary cooling coil. Therefore, the electricity con-sumption o f th e auxiliary cooling machine i n t henew system configuration can be significantly re-duced in comparison to the previous one.Monitoring data show very good operation a ndhigh efficiency of wet pl ate heat e xchangers. F ur-thermore, the use of the plate heat exchanger elimi-nates the possible moisture ca rryover th at can oc-cur in  the rotative heat exchangers normally usedin DEC systems and air leakages between two air-flows.The use of a d edicated fan for the regeneration o fthe de siccant wheel seems to be a g ood solutionpermitting an independent control of the regenera-tion process.The aut hors are wo rking on the efficiency assess-ment of wet heat exchangers through targeted ex-perimental tests carried out in a specific f acility.Numerical s imulations of t he whole DEC systemwill be utilized to e valuate performances in otherclimatic conditions.AcknowledgmentsThe authors acknowledge the support provided byENEA Research Agency for this research project inthe framework of the “Accordo di Programma MSE-ENEA sulla Ricerca di Sistema Elettrico”.[1] Beccali M., Finocchiaro P., Nocke B., “Energy performance evalua-tion of a demo solar desiccant cooling system with heat recovery forthe regeneration of the adsorption material”, Renewable Energy,Available online 1 February 2012, ISSN 0960-1481,10.1016/j.renene.2011.12.021.[2] Jain S., Dhar P.L., “Evaluation of solid-desiccant-based evaporativecooling c ycles for typical hot and humid climates”, InternationalJournal of Refrigeration, Elsevier, Vol 18. No. 5 pp 287-296 (1995).[3] Henning H.M., Solar-Assisted Air-Conditioning in Buildings – AHandbook for Planners, Springer, Wien - New York (2004).[4] Rey Martìnez F.J., Velasco Gómez E., Herrero Martìn R., MartìnezGutiérrez J., Varela Diez F. , “Comparative st udy of two d ifferentevaporative systems: an indirect evaporative cooler and a semi-indi-rect ceramic evaporative cooler”, Energy and Buildings, Elsevier 36pp. 696-708 (2004).[5] Rowe D., White S., Goldsworthy M., Spillmann T., Reece R., Ross-ington D., Dolly B., Larre M., Thomson R., “Preliminary Findings onthe Performance of a New Residential Solar Desiccant Air-Condi-tioner”, Proceedings of EUROSUN 2010, Graz, Austria (2010).[6] Sparber W., 240 - Unified Monitoring Procedure and PerformanceAssessment for Solar Assisted Heating and Cooling Systems, Euro-sun 2008, Lisbon (2008).[7] Sparber W., Na politano A., Be sana F., Thür A., N ocke B., Finoc-chiaro P., Nieto L .A. Bujedo, Rodriguez J., Nù ez T., “Comparativeresults of monitored solar assisted heating and cooling installations”,Proc. of the 3rd Intern. Conference Solar Air-Conditioning, Palermo(I), 30 Sept-2 Oct (2009).ReferencesStudi & Ricerche|TECHNICALSPAPERS

Experimental results on a solar desiccant
and evaporative cooling system
equipped with wet heat exchangers

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Experimental results on a solar desiccant and evaporative cooling system equipped with wet heat exchangers

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