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forSystems90139cCentro Ricerche Fiat, 10043 Orbassno, ItalyC211 2005 Published by Elsevier Ltd.systems are used. Thereby the air is cooled far belowthe temperature level needed for comfortable indoorconditions and consequently the chiller works at aCOP lower than if employed for sensible cooling, i.e.temperature control, only.*Corresponding author.E-mail addresses: hans-martin.henningise.fraunhofer.de (H.-M.Henning), tullio.paganoamg.pa.it (T. Pagano), stefano.molacrf.it(S. Mola).Applied Thermal Engineering 271359-4311/$ - see front matter C211 2005 Published by Elsevier Ltd.Keywords: Tri-generation; Desiccant cooling; Humid climates; Dehumidification1. IntroductionAir-conditioning of buildings is a promising applica-tion of co-generation systems during summer. For thispurpose thermally driven equipment to supply coolingdriven with the co-generator waste heat has to be em-ployed. The most widespread technology used for thispurpose is based on heat driven water chillers such asabsorption chillers (e.g. with the material pair lithium-bromidewater) or adsorption chillers. Water chillersare used in combination with dierent techniques topurge cooling loads from the rooms such as e.g. fan-coilsystems, chilled ceilings or centralized air handling units(AHU). However, in order to treat latent loads, air hasto be cooled below the dew-point when chilled waterReceived 24 February 2005; accepted 25 July 2005AbstractMediterranean countries show two specific features regarding air-conditioning of buildings: a highand growingcooling loadand high relative humidity, at least in coastal zones. In this contribution we report on the development of an innovative micro scaletri-generation system (power + heating + cooling), equipped with a rotor based desiccant system adapted to the Mediterranean con-ditions which receives heat for the desiccant regeneration from a combined heat and power (CHP) cycle.The paper presents the design of the advanced desiccant air handling unit which uses a high ecient combination of a vaporcompression chiller working at a high evaporator temperature and a desiccant wheel (silica gel). The electricity of the chiller is sup-plied by the CHP system and the heat to regenerate the desiccant is the waste heat of the CHP. System simulations have been used tooptimize the hydraulic design and the operation strategy in order to minimize operation costs and maximize energy savings. Somenew component models, e.g. for the advanced desiccant cycle were developed for this purpose. The final design of the entire systemconsisting of the CHP system, the vapor compression chiller, the advanced desiccant air handling unit and the load system isdescribed. The load system is composed of an air duct network with induction units and a chilled water network with fan-coilsin the oce rooms.Regarding energy performance results indicate an electricity saving 30% in comparison to state-of-the-art solutions based onconventional technology.Micro tri-generation systemin the MediterraneanHans-Martin Henninga,*, Tullio PaganoaFraunhofer Institute for Solar EnergybAMG ENERGIA s.p.a.,doi:10.1016/j.applthermaleng.2005.07.031indoor air conditioningclimateb, Stefano Molac, Edo WiemkenaISE, 79100 Freiburg, GermanyPalermo, I/locate/apthermeng(2007) 21882194humidity ratios of the ambient air. A pilot system hasbeen installed in fall 2003 at the client building of thescheme are typically employed in temperate climates;the example of Fig. 1 is based on typical design condi-tions in Central Europe (e.g. Germany). The air followsthe following processes during the system:1 ! 2 sorptive dehumidification of supply air; theprocess is almost adiabatic and the air is heatedby the adsorption heat and the hot matrix ofthe wheel coming from the regeneration side;2 ! 3 pre-cooling of the supply air in counter-flow tothe return air from the building;3 ! 4 evaporative cooling of the supply air to thedesired supply air humidity by means of ahumidifier;4 ! 5 the heating coil is used only in the heating sea-son for pre-heating of air;5 ! 6 a small temperature increase is caused by thefan;6 ! 7 supply air temperature and humidity areincreased by means of internal loads;7 ! 8 return air from the building is cooled usingevaporative cooling close to the saturation line;8 ! 9 the return air is pre-heated in counter-flow tothe supply air by means of a high ecientair-to-air heat exchanger, e.g. a heat recoverFig. 2. Standard desiccant cooling cycle in the Tx-diagram of humidair.gas utility of the municipality of Palermo (AMG) in Sic-ily/Italy and the system is commissioned and operatedwith accompanying monitoring during 2004.2. Desiccant air handling unit configurationAs a first step of the project dierent configurationsof desiccant air handling units were compared in orderto identify the configuration which is able to provide de-sired supply air conditions with a minimum of energyconsumption. However, before the dierent new designsare presented and compared, the standard desiccantcycle as used in temperate climates is described in detailin order to show the general operation principle. Basedon this cycle dierent modifications were made in orderto adjust it to the specific needs in a warm-humidclimate.2.1. Standard desiccant cooling cycleThe standard cycle which is mostly applied today usesrotating desiccant wheels, equipped either with silica gelor lithium-chloride as sorption material. All requiredcomponents are standard components and have beenin use for air-conditioning of buildings or factories sincemany years.The standard cycle using a desiccant wheel is shownin Fig. 1 and the corresponding states of the air in theAn alternative to treat latent loads by cooling air be-low the dew-point is the direct treatment of ventilationair in an open sorptive cooling cycle, also referred toas desiccant cooling system. In such a cycle air dehumid-ification is realized using a sorptive component such as asorptive wheel. Additionally, a temperature decrease canbe achieved by combination of the sorptive dehumidifi-cation with either direct, indirect or combined (direct +indirect) evaporative cooling.However, the standard desiccant cooling cycle, whichis for instance installed in temperate climates like Cen-tral Europe, is not able to cope with the conditions ofwarm and humid climates such as for instance in thecoastal zones of the Mediterranean countries. Thereforeapplication of desiccant technology in such climatesusing sorptive rotors requires specific configurations.In the framework of the project MITES (Micro Tri-generation System for Indoor air conditioning in theMediterranean Climate), a project supported by theEuropean Union, a novel configuration of an open cool-ing cycle based on sorptive rotor technology has beendeveloped. This heat driven air handling unit receivesits driving heat from a motor co-generation unit and isspecially designed for weather conditions with highH.-M. Henning et al. / Applied Thermalcycle are shown in Fig. 2. Systems according to thisFig. 1. Standard desiccant cooling system.Engineering 27 (2007) 21882194 2189wheel;9 ! 10 regeneration heat is provided for instance bymeans of a co-generation system;10 ! 11 the water bound in the pores of the desiccantmaterial of the dehumidifer wheel is desorbedby the hot air;11 ! 12 exhaust air is blown to the environment bymeans of the return air fan.Application of the cycle described above is limited totemperate climates. Reason is, that the achievable sup-ply air dehumidification is not high enough to enable di-rect evaporative cooling at conditions with far highervalues of the humidity of ambient air.3. Cycles adjusted to humid climatesFor all studied cycles the same boundary conditions,i.e., temperature and humidity values of ambient air,supply air to the building, return air from the buildingand regeneration air to regenerate the sorption materialwere assumed. These values are shown in Table 1. Thefollowing modified cycles which all use cooling coils inaddition to the sorptive wheel were studied regardingtrols the air to achieve the final desired humidity(air states 3 ! 4). A re-heater (air states 4 ! 5) isneeded, if the supply temperature shall enter theroom with a comfortable temperature, i.e., a temper-ature not below 18 C176C. Cycle using two sorptive wheels which are operated inseries with an intermediate cooling coil (air states2 ! 3); a scheme is shown in Fig. 5 and the corre-sponding air states in Fig. 6. Using this system thecomplete dehumidification of ambient air is realizedby sorption. A second cooling coil (air states 5 ! 6)is necessary in order to achieve the desired supplyReturn air humidity ratio g/kg 11.5Hot water temperature (from COG) C176C 85.0Fig. 4. Cycle of Fig. 3 in the Tx-diagram of humid air.2190 H.-M. Henning et al. / Applied Thermal Engineering 27 (2007) 21882194Fig. 3. Standard cycle with additional cooling-coil behind heattheir energy performance: Standard cycle with a cooling coil added behind theheat recovery wheel on the supply air side; a schemeis shown in Fig. 3 and the corresponding air statesin Fig. 4. The sorptive wheel realizes a pre-dehumid-ification (air states 1 ! 2) and the cooling coil con-Table 1Boundary conditions for cycle designParameter Unit ValueAmbient air temperature C176C 35.0Ambient air humidity ratio g/kg 25.0Supply air temperature (to room) C176C 18.0Supply air humidity ratio g/kg 9.0Return air temperature (from room) C176C 26.0recovery wheel.Fig. 5. Cycle with two sorption wheels.Fig. 6. Cycle of Fig. 5 in the Tx-diagram of humid air.air temperature. Two heating coils are necessary inorder to provide regeneration heat for the first sorp-tive wheel (air states 9 ! 10) and the second sorptivewheel (air states 11 ! 12). Since no dehumidificationis realized by cooling air below the dew-point, therequired cold water temperature is relatively high. Cycle employing two cooling coils, a first one in frontof the sorptive wheel for pre-dehumidification (airstates 1 ! 2) and a second one for control of supplyair temperature (air states 4 ! 5); a scheme is shownin Fig. 7 and the corresponding air states in Fig. 8.Although pre-dehumidification is realized by coolingthe air below the dew-point, a high value of chilledwater temperature is sucient since the dehumidifica-tion takes place at a high value of the humidity ratioand thus at a high saturation temperature of watervapor. At last a conventional system has been modeled inorder to compare the sorptive cycles with a reference;a scheme is shown in Fig. 9 and the corresponding airstates in Fig. 10. This system consists of a conven-tional air handling unit in which evaporative coolingof the return air is used to pre-cool the ambient airwith a heat recovery system (air states 1 ! 2). A cool-ing coil guarantees that the desired level of dehumid-ification is achieved (air states 2 ! 3). A re-heater (airstates 3 ! 4) is needed, if the supply temperatureshall enter the room with a comfortable temperature,i.e., a temperature not below 18 C176C.The calculation of the air states has been carried outusing a design tool developed at the Fraunhofer Insti-tute for Solar Energy Systems in which many dierentsystem configurations can be studied. The computer toolcontains an overall of 21 components which can beswitched on or o in order to derive a new configurationout of the complete system. Standard performance fig-ures for all components were used; the used desiccantwheel model has been developed by Motta 1 and is de-scribed in Motta et al. 2. A by-pass fraction of 20% wasused for sorptive wheel regeneration, i.e., only 80% ofthe return air have to be heated up to the regenerationtemperature and pass the sorptive wheel.In order to compare the performance of the dierentcycles the following performance figures have beendefined: The total cooling, Pcooling,totis defined as the enthalpydierence between ambient air and supply air multi-plied with the air mass flow:Fig. 10. Cycle of Fig. 9 in the Tx-diagram of humid air.H.-M. Henning et al. / Applied Thermal Engineering 27 (2007) 21882194 2191Fig. 7. Cycle with a cooling coil before the sorption wheel.Fig. 8. Cycle of Fig. 7 in the Tx-diagram of humid air.Fig. 9. Conventional reference cycle without sorption wheel.Pcool;tot _mairC1hambientC0hsupply The conventional cooling, Pconvdenotes the the cool-ing supplied by the cooling coils, for instance usingchilled water from a compression chiller. The chiller COP, COPchiller, denotes the COP of aconventional vapor compression chiller and dependson the dierence between the temperature of chilledwater, Tchilledwaterand the temperature of ambientair, which defines the condensation condition of thechiller; to calculate the COPchillerthe performanceof a typical market available compression chilleremploying FKW 134a as refrigerant has been used. The sorptive cooling, Pcool,sorpt,defines the amount ofthe total cooling which is not covered by the coolingcoils:Pnominal air flow of 1000 m3/h at return air conditions(26 C176C). The following conclusions can be drawnfrom this comparison: The standard cycle with additional cooling coilbehind the heat recovery wheel (scheme of Fig. 3)requires the lowest amount of conventional cooling.However, this cooling is needed at a low temperaturelevel, since the final humidity control of the supply airis realized with this cooling coil. Both, the 2-wheels cycle (Fig. 5) as well as the 2-cool-ing-coils cycle (Fig. 7) need chilled water at a farhigher temperature. This means that eventually otherenvironmental heat sinks such as well water might beused if available. The 2-wheels cycle requires a far higher amount ofthe gas utility of the municipality of Palermo (AMG)the 2-cooling-coils configuration was selected, although2192 H.-M. Henning et al. / Applied Thermal Engineering 27 (2007) 21882194Table 2Comparative results of the studied system configurationsParameter Unit StandardScheme shown in Fig. 3Total cooling, Pcool,totkW 19.1Conventional cooling, PconvkW 11.2TchilledwaterC176C 8Chiller COP, COPchiller 3.99Sorptive cooling, Pcool,sorptkW 7.9Regeneration heat, PregkW 5.9Sorptive COP, COPsorpt 1.34Electricity demand ventilators, Pel,ventkW 0.6Electricity demand chiller, Pel,chillerkW 2.81Total electricity demand, Pel,totkW 3.41Electricitymarized in Table 2. All given values refer to aPel;tot Pel;chillerPel;ventResults of the comparison of all the performance fig-ures defined above for the studied systems are sum-The total electricity demand, Pel,tot, is the sum of theelectric consumption of the chiller and the ventilators:dierent for the dierent designs due to the imple-mented components. Pel,chilleris the electricity demand of the chiller whichis defined by the fraction between the conventionalcooling and the chiller COP:Pel;chillerPconvCOPchillerPel,ventdefines the electricity demand of the ventila-tors. This electricity demand depends on the overallpressure drop of the air handling units which is quiteCOPsorptcool;sorptPregfor regeneration of the desiccant, Preg:Pcool;sorpt Pcool;totC0Pconv The sorptive COP, COPsorpt, is defined as the fractionbetween the sorptive cooling and the required heatsaving % 31.8this configuration shows a higher regeneration heatdemand compared to the standard configuration.However, since the amount of waste heat from the co-generation system is far higher than the heat neededby the desiccant system, this is no limiting condition.4. Design of the complete systemAfter the design of the desiccant air handling unit thedesign of the complete system consisting of the co-gener-ation unit, the air handling unit, the compression chillerand the building related components has been made,Based on a detailed cooling load calculation it becameobvious that not all cooling loads can be covered by2 Wheels 2 Cooling coils ReferenceFig. 5 Fig. 7 Fig. 919.1 19.1 19.112.7 11.7 18.814.9 15.1 8.34.35 4.36 4.006.4 7.4 15.5 7.2 0.41 1.03 0.8 0.6 0.32.92 2.68 4.703.72 3.28 5.00regeneration heat than both other sorptive cyclesand the highest electricity demand for the ventilators. The lowest overall electricity demand is shown by the2-cooling coils cycle and a reduction of electric con-sumption of about 34% in comparison to a conven-tional system (Fig. 9) is achieved.Finally, for the installation at the client building of25.6 34.3 utilitytoTable 3Energy balance of the complete systemdesign caseParameter Unit ValueVolume flow fresh air m3/h 1100Cooling power for AHU kW 12.8Cooling for fan-coils kW 12.0EHP electric demand kW 5.70Ventilators electric demand kW 0.66Appliances electric demand kW 3.00Total electric demand kW 9.36COG waste heat kW 21.85Regeneration heat kW 7.92Excess heat (+DHS) kW 13.93H.-M. Henning et al. / Applied Thermalthe air handing unit. Therefore a fan coil system is oper-ated in addition which is supplied with chilled waterfrom the compression chiller as well. A scheme of theoverall system is show in Fig. 11. Fig. 12 presents a pho-tograph of the system showing the air handling unitmounted outdoors and the hydraulic pipe network.During summer the co-generation system provideselectricity for the compression heat pump (indicatedEHP in Fig. 11) and for the other appliances (e.g. com-puters,artificiallighting,printersetc.)oftheocerooms.The waste heat of the co-generation unit is used to heatthe regeneration air of the desiccant system. Excess heatcan be used for domestic hot water preparation (DHS)or is rejected to the environment by the excess water-to-Fig. 11. Scheme of the complete system at the client building of the gastesting and supplies the second floor of the building. Meaning of abbreviationchiller, air cooled); AHU = air handling unit with sorptive wheel accordingexchanger for rejection of excess heat.air heat exchanger (EHX). An energy balance of thewhole system for the design case is shown in Table 3.It become