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9 the return air is pre-heated in counter-flow tothe supply air by means of a high efficientair-to-air heat exchanger, e.g. a heat recoverwheel; 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 regardingtheir 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 C 35.0Ambient air humidity ratio g/kg 25.0Supply air temperature (to room) C 18.0Supply air humidity ratio g/kg 9.0Return air temperature (from room) C 26.0Return air humidity ratio g/kg 11.5Hot water temperature (from COG) C 85.0 trols 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 C.• 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 supply 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 sufficient 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 (air 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 differentsystem configurations can be studied. The computer toolcontains an overall of 21 components which can beswitched on or off 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 inMotta 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 differentcycles the following performance figures have beendefined:• The total cooling, Pcooling,tot is defined as the enthalpydifference between ambient air and supply air multi-plied with the air mass flow:Pcool;tot ¼ _ mair ðhambient hsupplyÞ • The conventional cooling, Pconv denotes 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 difference between the temperature of chilledwater, Tchilledwater and the temperature of ambientair, which defines the condensation condition of thechiller; to calculate the COPchiller the 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:Pcool;sorpt ¼ Pcool;tot Pconv• The sorptive COP, COPsorpt, is defined as the fractionbetween the sorptive cooling and the required heatfor regeneration of the desiccant,