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  • International Journal on Architectural Science, Volume 1, Number 4, p.193-213, 2000

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    AIR CONDITIONING SYSTEMS WITH DESICCANT WHEEL FOR ITALIAN CLIMATES L. Bellia, P. Mazzei, F. Minichiello and D. Palma DETEC - University of Naples “Federico II”, P.le Tecchio, 80, 80125 Napoli, Italy (Received 25 October 2000; Accepted 15 January 2001) ABSTRACT Hybrid air conditioning systems based on chemical dehumidification are characterised by high energy efficiency and low environmental impact. They can result profitable if compared to traditional air conditioning systems and allow to obtain better indoor thermal comfort and air quality. In this paper, different hybrid air conditioning system configurations with desiccant wheel are examined; later, a first evaluation of operating costs is carried on, for Italian climates. For this purpose, a commercial computer program, DesiCalcTM, has been employed: from the European file known as TRY, hourly climatic data have been derived and adequately processed. For retail store application, for four Italian sites, maximum saving of about 22% has been obtained, while for theatre obtainable saving is greater and has been evaluated between 23% and 38%. For both the applications, the required comprehensive electric power is reduced (up to about 55%), and also the hours during which the system does not well control indoor relative humidity are strongly reduced. 1. INTRODUCTION It is well known that a considerable part of the primary energy is used for air conditioning purposes. Besides, it is worthwhile observing that: a) the most recent standards regarding environmental comfort and Indoor Air Quality (IAQ) impose both more restrictive limits to indoor R.H. values, and a considerable increase of outdoor airflow rates; b) CFC and HCFC refrigerant fluids are destined to disappearance; c) electric power peaks should be reduced. Therefore it seems necessary to develop new approaches to air conditioning techniques. Hybrid air conditioning systems based on chemical dehumidification are characterised by high energy efficiency and low environmental impact. Besides, they can result profitable if compared to traditional air conditioning systems [1-3]. The moist air chemical dehumidification has been adopted for long time, mostly in the U.S.A., in the industrial and military fields, in ice arenas and, within the commercial fields, in high latent load environments, like supermarkets [4-7]. However, the trend to extend this technique to other applications is evident, such as commercial and residential fields, integrating it with traditional and innovative systems [8-12], in order to obtain energy saving and to get the best thermal and moisture indoor conditions (in particular regarding R.H. values). Such trend is also justified by the required external airflow rates increase for indoor required external airflow rates increase for indoor

    ventilation, in Italy stated by the UNI 10339 [13] and, in the international field, stated by various codes and standards [14-21]. The increase of required external airflow rates implies, for summer Italian climates, a corresponding latent loads increase that must be removed in order to maintain acceptable indoor R.H. values. With the purpose to balance such additional latent loads [22], external air pre-treatment technologies, such as chemical dehumidification, to couple with traditional air conditioning systems, are regarded with increasing interest; the correct equipment component sizing is strictly joined to the external air design conditions definition [21]. The desiccant material may be solid or liquid. Solid desiccants inserted in a rotary heat exchanger [23], called desiccant wheel (DW), are particularly utilised, especially in HVAC applications: they are examined in this paper. In the following, different hybrid air system with desiccant wheel (DWHS) configurations are examined and then a first evaluation of operating costs of such systems, with respect to traditional ones, is provided for Italian climates; for this purpose a commercially available computer program, DesiCalcTM [24], has been employed: hourly climatic data have been derived from the European file known as TRY (Test Reference Year) [25] and adequately processed [26,27]. 2. BASIC CONCEPTS

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    The main concepts about chemical dehumidification, successively employed during the analysis, are herein synthetically presented [28-32]. Air that has to be treated before supplying in indoor ambient is called “process air”. The moisture contained in humid air partially condenses in the chemical desiccant: it is adsorbed because of the vapour partial pressure difference between process air and desiccant surface. So the process air temperature increases because of the conversion in sensible heat of both condensation heat and heat due to the adsorption chemical process. Therefore, process air specific humidity decreases while temperature increases (Fig. 1). For this reason, before supplying to the space, process air must be cooled (Fig. 1b) by means of one or more of the following components: direct expansion or chilled liquid cooling coil (CC); indirect evaporative cooling (IEC); rotary or static heat recuperator (HTX).

    Chemical desiccant – typically chemical compounds such as synthetic polymers, silica gel, titanium silicates, natural or synthetic zeolites, activated alumina, “silica +”, etc… [8,33-38] – must be periodically regenerated using thermal energy obtained by combustion process (generally direct or indirect gas fired heaters) or thermal wastes (hot water from solar panels, condensation heat from refrigeration plant, cogeneration systems, etc.). In Fig. 2 a typical desiccant wheel is shown [30]. The device rotates slowly (6-30 r.p.h.) between process and reactivation air streams: moisture is removed from process air stream by means of the desiccant material; after a partial rotation, the sector of saturated wheel is regenerated by hot and dry air (reactivation air stream) to be utilised again. The desiccant wheel structure is very similar to that of a rotary heat recuperator.

    (a) (b)

    Fig. 1: Air dehumidification by (a) cooling coil and (b) chemical dehumidification

    Fig. 2: Typical desiccant wheel

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    In the field of summer air conditioning for non-industrial application, chemical dehumidification could represent a valid alternative to the traditional cooling and dehumidification coil; hybrid HVAC systems with chemical dehumidification distinguish themselves essentially for the following reasons. • Traditional refrigeration plants are not

    suitable to control separately latent and sensible thermal loads. Often, in order to adequately control ambient relative humidity, it is necessary to cool air up to low temperatures and then to reheat. Consequently COP reduction and high energy consumption take place. Handling sensible loads by means of a traditional refrigeration plant and latent loads by means of chemical desiccant, especially in the presence of a low sensible/latent load ratio, can significantly enhance the system efficiency. With this kind of hybrid system re-heating is never necessary. This advantage is particularly evident in partial-load conditions [39], when ventilation sensible load reduces, while the latent one keeps high. From Table 1 it can be noticed how climate conditions with Te < Tr and ωe > ωr are frequent in Italy.

    • Systems based on chemical dehumidification

    can reduce humidity even when the required dew point temperature is very low, so allowing an easier balance of high latent loads. On the contrary, conventional systems can dehumidify air stream generally only for required dew point temperatures higher than 4°C (in order to avoid frost on evaporator, with consequent performance reduction).

    • In operating conditions, hybrid systems based

    on chemical dehumidification can control separately both temperature and humidity (the DW is connected to a humidity sensor, the CC to a temperature sensor). On the contrary, in traditional cooling systems only temperature is generally directly controlled, while humidity can vary.

    • Since humidity can be accurately controlled,

    hybrid systems assure better thermal comfort and also better air quality. In fact, the absence of condensed water strongly reduces the presence of microorganisms such as bacteria, viruses and fungi [40-43]. So these systems are particularly recommended in applications in which severe hygienic conditions must be maintained (medical facilities and laboratories).

    • Since in hybrid systems the CC task is only sensible cooling of the air stream, the cooling fluid temperature can be higher (for example, the typical 7°C of the chilled water can be changed up to 14°C and over), with a consequent increase of the refrigeration plant COP. Often, hybrid systems can reduce the vapour compression refrigeration plant power because latent load is already balanced by the desiccant system. The smaller size can reduce energy consumption, required electric power and starting investment capital.

    • The technology based on chemical

    dehumidification, reducing electric power and energy requirements and the CFC and HCFC refrigerant fluids consumption, is characterised by a low environmental impact.

    • The desiccant wheel can be applied also to

    existing traditional HVAC systems which are not able to balance latent load, for example when outdoor air percentage is increased in order to conform the plant to the present standards.

    • Installing cost of an hybrid system with

    chemical dehumidification is generally higher than a traditional system, but it can be balanced, in some applications, by lower operating costs [6,7,44-48] due to lack of re-heating and increase of COP.

    • It is possible to use available thermal energy

    [49,50] to reactivate the desiccant. The main disadvantages connected to hybrid HVAC systems with chemical dehumidification are the following: • it is possible that, in the presence of solid

    adsorbent materials, solid particles could be dragged by the air stream, but such inconvenience is decreasing while technology improves;

    • a considerable amount of thermal energy is

    necessary for the reactivation process and it increases with the dehumidification requirements and with the reactivation temperature. Only the last generation of desiccant materials can obtain regeneration temperatures between 40°C and 80°C, so it is possible to satisfy the reactivation needs with low temperature thermal recoveries. However, the payback not always attains acceptable values [51];

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    • the scarce familiarity with such technology and the lack of information about performances and cost/benefit ratio hamper the spreading of hybrid desiccant systems [52],

    even if today chemical dehumidification technology can compete with conventional systems also for residential and commercial applications.

    Table 1: Number of hours during which Te < Tr and ωe > ωr for three Italian localities Tdb,r = 25°C; R.H.r = 50 % ; ωr = 9.9 g/kg

    Site: Crotone Period: 1 June – 30 September Total hours: 2,928 Tdb [°C] ω [g/kg]

    13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5 21.5 22.5 23.5 N. hours

    18.5 0 0 0 0 0 0 0 0 0 0 2 2 17.5 0 0 0 0 0 0 0 0 0 0 5 5 16.5 0 0 0 0 0 0 0 0 1 11 11 23 15.5 0 0 0 0 0 0 0 3 22 24 13 62 14.5 0 0 0 0 0 0 1 9 30 30 21 91 13.5 0 0 0 0 0 4 23 36 34 37 24 158 12.5 0 0 0 0 2 45 46 52 49 41 27 262 11.5 0 0 2 10 41 42 54 34 47 40 37 307 10.5 0 2 10 23 26 35 40 41 41 36 42 296 9.5 4 9 12 14 27 16 22 33 30 29 37 233

    1,439 total percentage on total 49.1 %

    Site: Rome Period: 1 June – 30 September Total hours: 2,928 Tdb [°C] ω [g/kg]

    13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5 21.5 22.5 23.5 N. hours

    18.5 0 0 0 0 0 0 0 0 0 0 0 0 17.5 0 0 0 0 0 0 0 0 0 0 2 2 16.5 0 0 0 0 0 0 0 0 2 9 6 17 15.5 0 0 0 0 0 0 0 1 8 12 9 30 14.5 0 0 0 0 0 0 2 17 26 25 13 83 13.5 0 0 0 0 0 5 32 51 38 21 31 178 12.5 0 0 0 0 16 54 71 75 61 35 33 345 11.5 0 0 1 32 93 94 81 60 43 34 32 470 10.5 0 5 35 75 42 44 47 36 39 28 32 383 9.5 7 32 46 24 31 21 17 16 15 25 26 260

    1,768 total percentage on total 60.4 %

    Site: Milan Period: 1 June – 30 September Total hours: 2,928 Tdb [°C] ω [g/kg]

    13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5 21.5 22.5 23.5 N. hours

    18.5 0 0 0 0 0 0 0 0 0 0 0 0 17.5 0 0 0 0 0 0 0 0 0 0 2 2 16.5 0 0 0 0 0 0 0 0 1 3 5 9 15.5 0 0 0 0 0 0 0 4 10 14 15 43 14.5 0 0 0 0 0 0 12 31 32 31 12 118 13.5 0 0 0 0 0 12 44 59 52 40 25 232 12.5 0 0 0 0 37 68 76 57 33 33 24 328 11.5 0 0 12 89 102 62 43 47 37 36 21 449 10.5 0 48 83 75 32 40 30 26 21 24 23 402 9.5 86 82 34 17 22 18 13 12 11 6 14 315

    1,898 total percentage on total 64.8 %

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    3. AIR CONDITIONING HYBRID SYSTEM WITH DESICCANT WHEEL CONFIGURATIONS

    In the technical literature various hybrid system with desiccant wheel configurations are proposed. Referring to summer conditions, systems can be distinguished [39] between “active” and “passive” ones. In the former, desiccant material is reactivated by heated air, in the latter the desiccant is reactivated by air drier than outdoor air, which is usually the building exhaust air. Passive desiccant systems require no energy different from what is contained in the exhaust air stream, so the operating costs are considerably lower than for active systems. However passive systems only moderate indoor humidity, but are not able to control it. In the following only active systems are considered. In the technical literature many different hybrid system configurations, with one or more [53-57] steps of chemical dehumidification, are presented; in this paper one stage hybrid systems will be analysed. In order to classify the main hybrid system configurations presented in the literature, the typical air-conditioning processes and the devices

    more often utilised for their realisation are herein reported. • Process air cooling: CC; HTX (sensible or

    latent), usually rotating; direct (DEC) or indirect (IEC) evaporative cooling. Sometimes, in the presence of HTX, secondary air is cooled by means of DEC or IEC before entering the recuperator.

    • Dehumidification: CC; DW. • Heating: HC; HTX. • Humidification: with vapour or liquid water

    (DEC). • Desiccant reactivation: - return air or outside air; - thermal energy from heaters using fuels, from

    solar panels and heat wastes. The following cases, referring to process air, can be distinguished: 1) All recirculation air systems (“recirculation

    mode” or “recirculation cycle”)

    It is a very rare case, only for applications without occupancy, for example stores with high latent load. A possible configuration is shown in Fig. 3, while in Table 2 some features of the main configurations presented in literature are reported1. The configuration proposed by Collier [58] can work also in “ventilation mode” and “make-up mode”, successively defined.

    Fig. 3: All recirculation air hybrid system

    Table 2: Mean characteristics of some all recirculation air hybrid systems reported in literature

    Source

    HTX

    CC HC-

    proc. IEC –

    processDEC – process

    DEC - secondary

    side of HTXRegeneration

    air

    Regeneration thermal energy

    Particular character-

    istics Http…[66],

    case a Rotary Yes No Optional No No Outdoor air Gas -

    Collier [58] Rotary No No alternative to HTX

    Yes Yes Outdoor air Gas -

    Jurinak [50], case 2

    Rotary No No No Yes Yes Outdoor air Solar energy – gas

    -

    _______________________________ 1 Even if not reported in Tables 2, 3 and 4, DW is always present in described systems.

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    2) All outdoor air systems

    Sometimes all outdoor air is supplied in ambient for the following reasons: a) to obtain better indoor air quality, b) due to the particular application, c) because recirculation air is treated by traditional systems (for example, fan-coil), d) because the potential saving due to thermal and moisture conditions of the return air is estimated to be greater with an HTX on exhaust air rather than with recirculation. In Fig. 4 two possible system configurations are shown, while in Table 3 some features of the main configurations presented in literature are reported. The following can be noticed2: a) for the reactivation, some systems use return

    air stream (“ventilation mode” or “ventilation cycle” or “Pennington cycle”, Fig. 4a), while others use outdoor air (“make-up mode”, Fig. 4b);

    b) some configurations employ energy recovery

    systems (rotating or static heat recuperator, heat wastes);

    c) some configurations [24-Fig.63,30,59] present

    a pre-cooling coil upstream the desiccant wheel, so it is possible to obtain a first dehumidification of the outdoor air, even for coil surface temperature values not too low (for example, with chilled water at 10-15°C), to improve the wheel performance and to reduce the outlet process air temperature;

    d) sometimes a humidifier on process side is

    downstream the desiccant wheel [51,60]: this apparent contradiction is explained by the fact that in some conditions the wheel could dehumidify too much with respect to the actual needs, despite the presence of a regulation system; such observation is valid

    also for some all-recirculation air [59] and partial recirculation air [61] systems;

    e) the system proposed in [62] combines radiant

    cooling system (chilled ceiling) and desiccant dehumidification, in order to achieve independent humidity and temperature control, as suggested in [63]. The plant is composed by two systems: a primary air system, with a DW, which guarantees the required outdoor air flow, balances the latent load and part of the sensible load, and a chilled ceiling system which balances the remaining sensible load;

    f) the configurations proposed by Schibuola

    [51,59,64,65] refer only to the primary air treatment in air-and-water systems; the regulation system is also indicated: regulation with by-pass on process air and on reactivation air [59,64], regulation of reactivation air temperature [51,65].

    3) Partial recirculation air systems

    In Fig. 5 a possible system configuration is shown, while in Table 4 some features of the main configurations reported in literature are presented. It can be noticed that it is possible to further classify partial recirculation air systems based on different criteria: according to whether outdoor air or return air is used for regeneration; according to the energy recovery system chosen (rotating or static heat recuperator, recovery of thermal wastes, no thermal recovery); according to whether DW dehumidifies the whole mixture of outdoor air and recirculation air (“closed cycle”) or only outdoor air fraction (“open cycle”) [57], so it is possible to use a smaller DW. We speak of “mixed mode” [66] when both reactivation air and process air are composed by a mixing of outdoor and indoor air.

    Fig. 4: All outside hybrid system: (a) ventilation mode, (b) make-up mode

    ____________________________ 2 Considerations reported in b), c), d) are substantially valid also for all and partial recirculation air systems (points 1

    and 3).

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    4. THERMODYNAMIC ANALYSIS

    FOR A PARTIAL RECIRCULATION AIR HYBRID SYSTEM

    In the technical literature energy comparisons between some of the above-mentioned system configurations are reported [7], but none of them has resulted strongly better than the others, depending the comparison on many parameters, such as the heat exchangers (IEC, DEC, HTX) efficiency. However, it can be observed a significant energy saving with respect to traditional air systems. Therefore, a comparison, only referring to operating costs, between traditional air system with CC dehumidification and hybrid air system with DW is presented. The comparison has been carried out by means of DesiCalcTM software tool [24]. DesiCalc™ provides the capability to compare operating energy costs of a traditional HVAC system with respect to an hybrid system with DW. A total of 11 building templates are provided in the software (supermarket, movie theatre, retail store, hospital,…) and a set of 236 U.S. weather data sites are available to represent a wide range of climatic locations; the version of DesiCalc™ utilised for this study was integrated with TRY climatic data [25] of 10 Italian sites. DesiCalc™ is able to evaluate solutions including reheat systems, enthalpy wheels, heat recovery and, obviously, desiccant wheels. It is possible to insert energy price data, together with a wide variety of parameters relative to occupancy, lights and equipment thermal loads, with full user access to customize rates for any location. The tool employs a simple Windows™ interface over the DOE-2.1E; default input values are available to help the user. It is important to observe that DesiCalc™ represents a desiccant system screening tool rather than a design tool. It is a tool flexible enough to evaluate a variety of commercially available desiccant systems, so complex design parameters are not present in DesiCalc™ ’s user input. As a result, most of the application and equipment

    variables are fixed rather than user definable. Therefore, while DesiCalc™ helps the user to evaluate the competitive position of desiccant system options versus alternative HVAC systems, it does not provide results for a specific project, for which the user has to perform a more detailed study, using more complex software codes, such as DOE [59]. In order to carry on an energetic comparison between the traditional and the hybrid system with DW, the following assumptions are considered in DesiCalcTM: • single-duct and constant air flow system (both

    traditional and hybrid); • outdoor air temperature and relative humidity

    change each hour, assuming, for Italian sites, the TRY climatic file values;

    • variable total thermal load during systems operation;

    • sometimes the latent load is such to require re-heating for traditional system;

    • desiccant wheel main features: - desiccant material: silica gel; - temperature of reactivation air entering

    desiccant wheel: 250°F ≈ 121°C; - regulation by variation of regeneration air

    flow; - variable ratio between process and

    regeneration air flows; - process air velocity in the wheel: 3.6 ms-1. In Fig. 6 the traditional air system configuration is reported: the system has been equipped with enthalpy heat recuperator on exhaust air (“Base system – enthalpy heat recovery”). Among the hybrid air system configurations implemented in DesiCalcTM, the following are herein considered: 1) “Pre-cool enthalpy relief air heat exchanger”

    (Fig. 7);

    Fig. 5: Post-cool sensible relief air heat exchanger (with or without evaporative cooler)

    Desiccant wheelOutdoor air

    (economizer)

    Outdoor air (ventilation air)

    Sensible heat recovery

    To outdoors

    Regeneration heater

    Relief air to outdoors

    C C-

    H C

    +

    Humidifier

    +-

    Conditioned space

    Return from space

    Evaporative cooler

    e

    ro rg xl

    dw

    r

    hx m s r Supply fan

    Return fan

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    2a) “Post-cool sensible relief air heat exchanger” (Fig. 5 with disabled evaporative cooler); in Fig. 8 qualitative air treatments on the psychrometric chart are reported for a typical Italian site in summer design conditions;

    2b) “Post-cool sensible relief air heat exchanger with evaporative cooler” (Fig. 5 with evaporative cooler, on regeneration side, enabled);

    3a) “Post-cool sensible outdoor air heat exchanger” (Fig. 9 with disabled evaporative cooling); this configuration differs from that reported in Fig. 5 because outdoor air is used instead of return air on reactivation side;

    3b) “Post-cool sensible outside air heat exchanger with evaporative cooler” (Fig. 9 with evaporative cooler, on reactivation side, enabled).

    For the above reported equipment configurations an enthalpic economizer (linked to three coupled dampers) is enabled, that varies the outdoor air

    stream percentage: when outdoor air conditions allow it, partial recirculation mode gives the place to the temporary more convenient all-external air mode. The DesiCalcTM code assumes, both for the traditional system and for the hybrid one, that CC can be a chilled water or direct expansion coil: in the first case the handling air unit is separated from the refrigeration system (“Central plant”), in the second there is a packaged direct expansion unit (“Roof-top”)3. In order to evaluate the possibility to use in Italy a hybrid air conditioning system with DW, hourly weather data derived from the European file [25] TRY (“Test Reference Year”) and opportunely processed4 [27] have been inserted in DesiCalcTM. Italy is subdivided into 10 climatic zones (considered approximately homogeneous) identified by the name of a single city, for each of them the hourly data of the main outdoor air thermodynamic properties are provided.

    _______________________________ 3 The term “Roof-top” refers to the position of the unit, generally on the building roof. 4 TRY file properties (TDB, R.H.) have been integrated with specific humidity, enthalpy, wet bulb temperature and

    density. Furthermore the file format has been changed to fit DesiCalcTM requirements.

    Return from space

    Outdoor air

    Fig. 6: Base system – Enthalpy heat recovery

    Enthalpy wheel C C

    Relief air to

    outdoors

    HumidifierH C

    Conditioned space Supply fan

    Return fan

    Relief air to outdoors

    Outdoor air (ventilation)

    Enthalpy wheel

    Return from space

    Outdoor air (economizer)

    C C H C Humidifier

    Regeneration heater

    Outdoor air

    Desiccant wheel

    To outdoors

    -

    -

    +

    +

    Fig. 7: Pre-cool enthaply relief air heat exchanger

    Conditioned space

    Return fan Supply fan

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    Fig. 8: Psychrometric chart relating to the system in Fig. 5 (without evaporative cooler)

    Fig. 9: Post-cool sensible outside air heat exchanger (with or without evaporative cooler) 5. OBTAINABLE ENERGETIC AND

    ECONOMIC SAVINGS WITH RESPECT TO A TRADITIONAL AIR CONDITIONING SYSTEM

    In order to evaluate system operating costs and savings, besides the 5 hypotheses made in section 4, the following assumptions have been considered: • air-cooled condenser of the refrigeration plant,

    without condensation heat recovery;

    • mean seasonal COP of the refrigeration plant: 3 for the traditional system, 4 for the hybrid one;

    • summer post-heating (for the traditional system), regeneration air heating (for the hybrid system) and winter heating (for both the systems) by means of gas-fired heater with mean seasonal effectiveness equal to 80%;

    • gas-powered steam humidifier; • presence of an enthalpic economizer, formerly

    described; • heat-exchanger efficiency: 70%;

    Outdoor air (ventilation air)

    outdoor air (economizer)

    Desiccant wheel

    Sensible heat recovery

    To outdoors

    Regeneration heater

    Outdoor air

    C C H C Humidifier

    Return from space

    Conditioned space

    Evaporative cooler

    Supply fan

    Return fan

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    • electric energy unitary mean cost refers to the present time-dependent electric energy rates in force in Italian cities;

    • gas unitary mean cost refers to the present rates in force in Italian cities;

    • primary energy equivalences: for electric energy, 0.23⋅10-3 tep correspond to 1 kWh; for gaseous fuel, 0.0232 tep correspond to 1 MBtu (≅ 1055 MJ);

    • design outdoor thermal and moisture conditions for each site:

    - DB 1% - MCWB (Dry Bulb temperature 1% - Mean Coincident Wet Bulb temperature) for the traditional system, DP 1% - MCDB (Dew Point temperature 1% - Mean Coincident Dry Bulb temperature) for the hybrid system;

    • design indoor thermal and moisture conditions: - in winter: Tdb ≈ 20°C, R.H. = minimum 40%;

    in summer: Tdb ≈ 26°C, R.H. = maximum 60%; • main features of the retail store application: - lighting and other electric loads: 13 Wm-2 and

    9 Wm-2 (optimised values for high energy efficiency buildings);

    - occupancy level (from Italian standard UNI 10339): 0.25 persons/m2;

    - outdoor air flow (from Italian standard UNI 10339): 6.5 l/(s⋅person);

    - infiltrations: 0.3 exchanges/hour; • main features of the theatre application: - lighting and other electric loads: 11 Wm-2 and

    22 Wm-2; - occupancy level (from Italian standard UNI

    10339): 1.5 persons/m2; - outdoor air flow (from Italian standard UNI

    10339): 5.5 l/(s⋅person); - infiltrations: 0.1 exchanges/hour. With these assumptions, for retail store and theatre applications, yearly energy consumption and operating total costs for traditional (TS) and hybrid systems have been evaluated (Tables 5-11). Also displayed in these tables are: i) the attainable savings (RC), also in terms of primary energy (RE), expressed as a percentage with respect to the traditional system; ii) the comprehensive required electric power (P); iii) the number of yearly hours during which the system is not able to keep an indoor R.H. less or equal to 60%, also expressed in percentage with respect to the yearly total. In particular, for retail store application (Tables 5-8) 4 Italian sites are considered and, for each case, two different air-conditioned building sizes are examined: the tables refer to a retail store of 4,600 m2 (case of time-independent electric energy rates) and to 14,000 m2 (time-dependent electric energy

    rates). The reported values refer to the retail store of 4,600 m2 while, in the rows showing the percentage savings in terms of operating costs or primary energy, the values relative to 14,000 m2 appear in brackets. The greatest annual savings attainable using the hybrid system are obtained with the “Pre-cool enthalpy relief air heat exchanger” configuration (Fig. 7), in the central plant case, reaching values around 22%. For the site of Foggia, less humid, the savings are lower (maximum about 16%), while for Trapani, particularly wet, the number of yearly hours during which the traditional system does not well control indoor R.H. is greater. It can be also noted that the hybrid system, unlike the traditional one, perfectly controls in every conditions indoor R.H.. Besides, the required comprehensive electric power is reduced for the hybrid system, up to 30%. Table 9 refers to a theatre of 1,200 m2 with climatic conditions of Rome (time-independent electric energy rates, gas rates for great tertiary users). The obtainable savings are greater with respect to retail store, between 23% and 38%. The savings attain about 38% both for “Post-cool sensible relief air heat exchanger with evaporative cooler” configuration (Fig. 5 with evaporative cooler, on regeneration side, enabled), and for “Post-cool sensible outdoor air heat exchanger with evaporative cooler” configuration (Fig. 9 with evaporative cooler, on regeneration side, enabled), both in roof-top case. Anyway, for all the system configurations, with roof-top option, savings greater than 35% are attained. Primary energy savings are close to those relative to operating costs. Also the reduction of the required comprehensive electric power is greater, particularly in the case of central plant (reductions of about 44-50%). With regard to discomfort hours, conclusions similar to the previous case are valid. Finally, for the same theatre application, Tables 10 and 11 show how savings obtainable using hybrid system increase (up to a maximum of about 45%) when occupancy level (Table 10) and required outdoor air flow rates per person (Table 11) increase. Referring to the “Roof-top post-cool sensible relief air exchanger with evaporative cooler” system, more detailed results are displayed in Fig. 10. The economic savings RC versus the external air flow rate are plotted in Fig. 10 using the occupancy level as a parameter. It is noteworthy to observe the sharp increase of RC with the external air flow rate, particularly for the more common occupancy level. Such trend confirms the suitability of the hybrid system in handling high latent loads due to the increase of required external flow rate and occupancy level.

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    The results of Tables 5-11 also show a considerable spread of RC with the hybrid system configuration. Since such RC variations do not exhibit a definite trend, it is recommended to analyse the different

    possible system configurations in order to get maximum savings.

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    6. CONCLUSIONS Referring to operating costs and by means of DesiCalcTM software tool, in this paper traditional air system and various hybrid air system with desiccant wheel configurations have been compared, in order to evaluate attainable annual savings, for Italian climates and for some typical applications. With this aim a file containing European hourly climatic data, TRY, has been utilised and opportunely processed. For retail store application, for four Italian cities, the configuration with process air pre-cooling upstream the desiccant wheel (“Pre-cool enthalpy relief air heat exchanger”) is the most profitable and maximum savings of about 22% have been obtained with respect to the corresponding traditional system. For theatre application, for Rome, obtainable savings are greater and have been evaluated between 23% and 43%. For both the applications, the required comprehensive electric power is reduced (up to about 55%) with respect to the traditional air system; besides, the number of yearly hours during which the traditional system does not well control indoor relative humidity is much greater with respect to the hybrid system. The following conclusions can be drawn: • energy savings obtainable using hybrid

    systems with desiccant wheel depend on: - climatic conditions of the considered site; - system configuration; - latent loads due to external air flow rate and

    occupancy level; - utilised desiccant wheel performance; • economic savings significantly depend on the

    local electric energy and gas fares, strongly variable from nation to nation and, with regard to gas, also from city to city.

    Starting cost of the hybrid system with desiccant wheel is generally greater with respect to the traditional system, so a cost-benefit analysis is required in order to evaluate the profitability of the investment. Acceptable pay-back values could be obtained using wheels more efficient than those used in this paper. Wheels with innovative desiccant materials [38,51] allow a regeneration temperature (up to 40-60°C) significantly lower with respect to traditional wheels (regeneration temperature higher than 80°C), as that utilised in DesiCalcTM. Consequently such wheels allow a reduction of the required thermal energy for

    regeneration and of the total required energy, and permit heat waste exploitation. This subject needs further investigation, which will be developed also utilising both more complex commercially available software tools, such as DOE, and self-developed software tools. ACKNOWLEDGEMENT The authors would like to thank Michael Witte for his information about DesiCalcTM. NOMENCLATURE TRY test reference year IEC indirect evaporative cooling DEC direct evaporative cooling DW desiccant wheel DWHS desiccant wheel hybrid system HTX heat recuperator CC cooling coil HC heating coil TS traditional system TDB dry bulb temperature of the humid air, °C R.H. relative humidity of the humid air, % ω specific humidity of the humid air, gv/kga e external air point on psychrometric chart r recirculation air point on psychrometric

    chart s supply air point on psychrometric chart Ct operating cost of the traditional system, $ Ch operating cost of the hybrid system with

    DW, $ RC = (Ct – Ch)/Ct ⋅100 = economic saving in

    utilising hybrid system with respect to traditional one, %

    RE primary energy saving in utilising hybrid system with respect to traditional one, %

    P required electric power, kW Subscripts r recirculation air e external air a dry air v vapour contained in humid air REFERENCES 1. R.M. Lazzarin, G.A. Longo and A. Gasparella,

    “Theoretical analysis of an open-cycle absorption heating and cooling system”, Int. J. Refrig., Vol. 19, No. 3, pp. 160-167 (1996).

    2. M. Meckler, “Desiccant-assisted air conditioner improves IAQ and comfort”, Heating/Piping/AirConditioning, October, pp. 75-84 (1994).

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    3. K. McGahey, “New commercial applications for desiccant-based cooling”, ASHRAE Journal, July, pp. 41-45 (1998).

    4. N.J. Banks, “Desiccant dehumidifiers in ice arenas”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 69-71 (1992).

    5. D.S. Calton, “Application of a desiccant cooling system to supermarkets”, ASHRAE Transactions, Vol. 91, Part 1B, pp. 441-446 (1985).

    6. D.L. Manley, K.L. Bowlen and B.M. Cohen, “Evaluation of gas-fired desiccant-based space conditioning for supermarkets”, ASHRAE Transactions, Vol. 91, Part 1B, pp. 447-456 (1985).

    7. P.R. Burns, J.W. Mitchell and W.A. Beckman, “Hybrid desiccant cooling systems in supermarket applications”, ASHRAE Transactions, Vol. 91, Part 1B, pp. 457-468 (1985).

    8. S.J. Slayzak, A.A. Pesaran and C.E. Hancock, “Experimental evaluation of commercial desiccant dehumidifier wheels”, National Renewable Energy Laboratory (NREL), prepared under Task No. BE513001, May (1996).

    9. D.R. Kosar et al., “Dehumidification issues of standard 62-1989”, ASHRAE Journal, March, pp. 71-75 (1998).

    10. G. Meckler, “Efficient integration of desiccant cooling in commercial HVAC systems”, ASHRAE Transactions, Vol. 94, Part 2, pp. 2033-42 (1988).

    11. W.C. Griffiths, “Use of liquid sorption dehumidification to improve energy utilization of air systems”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 33-39 (1992).

    12. S. Jain, P.L. Dhar and S.C. Kaushik, “Experimental studies on the dehumidifier and regenerator of a liquid desiccant cooling system”, Applied Thermal Engineering, Vol. 20, pp. 253-267 (2000).

    13. UNI 10339, “Impianti aeraulici a fini di benessere - Generalità, classificazione e requisiti – Regole per la richiesta d’offerta, l’offerta, l’ordine e la fornitura” (1995).

    14. ASHRAE Standard 62-1989, Ventilation for acceptable indoor air quality (1989).

    15. ASHRAE Public Review Draft 62-1989R, Ventilation for acceptable indoor air quality (1989).

    16. CEN prENV 1752, Ventilation for buildings: design criteria for the indoor environment (1996).

    17. CIBSE Guide A, revision section 2, Environmental criteria for design, Chartered Institute of Building Service Engineers, U (1993).

    18. DIN 1946 Part 2, Ventilation and air conditioning: technical health requirements (1994).

    19. NKB Report no. 61, Indoor climate – air quality, Nordic Committee on Building Regulations NKB (1991).

    20. B.W. Olesen, “International development of standards for ventilation of buildings”, ASHRAE Journal, April, pp. 31-39 (1997).

    21. L. Bellia, P. Mazzei, F. Minichiello and A. Palombo, “Outdoor-air design conditions relating to the capacity of air-conditioning systems”, International Journal of Energy Research, Vol. 24, pp. 121-135 (2000).

    22. L.G. Harriman III et al., “Dehumidification and cooling loads from ventilation air”, ASHRAE Journal, November, pp. 37-45 (1997).

    23. E. Van den Bulck, “The design of dehumidifiers for use in desiccant cooling and dehumidification systems”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 118-126 (1992).

    24. InterEnergyTM Software for the Energy Industry, DesiCalcTM User’s Manual version 1.1g, December (1998).

    25. CEE, Test Reference Years TRY – Weather data sets for computer simulations of solar energy systems and energy consumption in buildings, Commission of the European Communities, Directorate General XII for Science, Research and Development (1985).

    26. L. Bellia, P. Mazzei and A. Palombo, “Simplified energy and cost toolkit for hybrid evaporative system”, Papers and abstracts from the 3rd International Symposium on Humidity and Moisture, London, Vol. 2, pp. 2-10 (1998).

    27. L. Bellia, P. Mazzei, F. Minichiello and A. Palombo, “Cooling energy consumption and operating costs: evaporative all-air and air-and-water systems in the Italian climate”, International Journal of Energy Research, Vol. 24, pp. 163-175 (2000).

    28. ASHRAE, ASHRAE Handbook – Systems and equipment, Atlanta GA, U.S.A, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (1996).

    29. Munters Cargocaire, The dehumidification handbook, 2nd edition, edited by Lewis G. Harriman III, Amesbury, MA, U.S.A. (1990).

    30. L.G. Harriman III, “The basics of commercial desiccant systems”, Heating/Piping/Air-Conditioning, July, pp. 77-85 (1994).

    31. S. Neti and E.I. Wolfe, “Measurements of effectiveness in a silica gel rotary exchanger”, Applied Thermal Engineering, Vol. 20, pp. 309-322 (2000).

    32. C.J. Simonson and R.W. Besant, “Heat and moisture transfer in desiccant coated rotary energy exchangers: Part I. Numerical model”, HVAC&R Research, Vol. 3, No. 4, pp. 325-350 (1997).

    33. R.K. Collier Jr., D. Novosel and W.M. Worek, “Performance analysis of open-cycle desiccant cooling systems”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 82-87 (1992).

  • International Journal on Architectural Science

    212

    34. A.W. Czanderna, “Polymers as advanced materials for desiccant applications: 1 – Commercially available polymers”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 88-97 (1992).

    35. A.A. Pesaran and A.F. Mills, “Moisture transport in silica gel packed beds – I. Theoretical study”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 98-109 (1992).

    36. A.A. Pesaran and A.F. Mills, “Moisture transport in silica gel packed beds – II. Experimental study”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 110-117 (1992).

    37. J. Schraemli, S. Malaterra, “Il deumidificatore dinamico: tecnologia e applicazioni (The dynamic dehumidifier: technology and applications)”, Condizionamento dell’Aria Riscaldamento Refrigerazione, Vol. 20, No. 12, pp. 1197-1204 (1997).

    38. H.G. Spanninga, L.L. Van Dierendonk and G. Nesterov, “Desiccant cooling with silica +”, 19th International Congress of Refrigeration, Proceedings III b, The Hague, The Netherlands, August 20-25, pp. 873-880 (1995).

    39. L.G. Harriman III et al., “Evaluating active desiccant systems for ventilating commercial buildings”, ASHRAE Journal, October, pp. 28-37 (1999).

    40. B. Kovak et al., “The sanitizing effects of desiccant-based cooling”, ASHRAE Journal, April, pp. 60-64 (1997).

    41. A.V. Arundel et al., “Indirect health effects of relative humidity in indoor environments”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 3-12 (1992).

    42. M.K. West and E.C. Hansen, “Effect of hygroscopic materials on indoor relative humidity and air quality”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 178-182 (1992).

    43. M.K. West and E.C. Hansen, “Determination of material hygroscopic properties that affect indoor air quality”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 183-186 (1992).

    44. B.M. Cohen and R.B. Slosberg, “Application of gas-fired desiccant cooling system”, ASHRAE Transactions, Vol. 94, Part 1, pp. 525-536 (1988).

    45. T.J. Marciniak et al., “Gas-fired desiccant dehumidification system in a quick-service restaurant”, ASHRAE Transactions, Vol. 97, Part 1, pp. 657-666 (1991).

    46. D. Novosel, “Advances in desiccant technologies”, Energy Engineering, Vol. 93, No. 1, pp. 7-19 (1996).

    47. J.W. Spears and J. Judge, “Gas-fired desiccant system for retail super center”, ASHRAE Journal, October, pp. 65-69 (1997).

    48. G. Scalabrin and G. Scaltriti, “A new energy saving process for air dehumidification: analysis and applications”, ASHRAE Transactions, Vol. 91, Part 1A, pp. 426-441 (1985).

    49. W. Kessling, E. Laevemann and M. Peltzer, “Energy storage in open cycle liquid desiccant cooling systems”, Int. J. Refrig., Vol. 21, No. 2, pp. 150-156 (1998).

    50. J.J. Jurinak, J.W. Mitchell and W.A. Beckman, “Open-cycle desiccant air conditioning as an alternative to vapor compression cooling in residential applications”, Journal of Solar Energy Engineering, August, Vol. 106, pp. 252-260 (1984).

    51. L. Schibuola, “Possibilità applicative di sistemi essiccanti ad alta efficienza nell’ambito della climatizzazione”, Atti Convegno annuale AICARR, Milano, pp. 301-319 (2000).

    52. V.C. Mei et al., “Desiccant cooling and dehumidification technology”, Oak Ridge National Laboratory, ORNL/Con-309, Oak Ridge, Tennessee, U.S.A. (1992).

    53. B.K. Hodge et al., “Thermodynamic assessment of desiccant systems with targeted and relaxed humidity control schemes”, ASHRAE Transactions, Vol. 104, Part 2, pp. 313-319 (1998).

    54. M. Meckler, “Integrated desiccant cold air distribution system”, ASHRAE Transactions, Vol.95, Part 2, pp. 1085-1097 (1989).

    55. G. Meckler, “Two stage desiccant dehumidification in commercial building HVAC systems”, ASHRAE Transactions, Vol. 95, Part 2, pp. 1116-1123 (1989).

    56. G. Meckler, “Use of desiccant to produce cold air in gas-energized cold air HVAC system”, ASHRAE Transactions, Vol. 96, Part 1, pp. 1257-1261 (1990).

    57. S.K. Wang, Handbook of air conditioning and refrigeration, McGraw-Hill Inc., 1221 Avenue of the Americas, New York, NY 10020, U.S.A. (1994).

    58. R.K. Collier, B.M. Cohen and R.B. Slosberg, “Desiccant properties and their effects on the performance of desiccant cooling systems”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 75-81 (1992).

    59. DOE, release 2.1E, Department of Energy program, Simulation Research Group, Lawrence Berkeley Laboratory, Berkeley, California (1994).

    60. M. Mariotti and L. Schibuola, “Possibilità d’impiego di sistemi di deumidificazione mediante assorbimento chimico e ciclo rigenerativo in impianti di condizionamento dell’aria”, Atti Convegno nazionale AICARR, Roma, pp. 581-598 (1991).

    61. K. Matsuki and Y. Saito, “Desiccant cooling R&D in Japan”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 134-143 (1992).

    62. J.L. Niu, “Developing a decoupled cooling and dehumidification air-conditioning system”,

  • International Journal on Architectural Science

    213

    ISHVAC’99 The International Symposium on Heating, Ventilation and Air Conditioning, November 17-19, Shenzen, China, Vol. 2, pp.735-745 (1999).

    63. W. Coad, “Conditioning ventilation air for improved performance and air quality”, Heating/Piping/AirConditioning, September, pp. 49-56 (1999).

    64. L. Schibuola, “Performance comparison of desiccant systems in air conditioning applications”, 19th International Congress of Refrigeration, Proceedings III b, The Hague, The Netherlands, August 20-25, pp. 850-856 (1995).

    65. L. Schibuola, “High efficiency desiccant systems for air conditioning applications”, Proceedings of the 8th International Conference on Indoor Air Quality and Climate, Indoor Air 99, Edinburgh, Vol. 5, pp. 37-42 (1999).

    66. “Two-wheel desiccant dehumidification system”, pp. 1-22 (1998). http://www.pnl.gov/fta/8_tdd.htm.

    67. N.J. Banks, “Utilization of condenser heat for desiccant dehumidifiers in supermarket applications”, Seminar at the 1982 ASHRAE Annual Meeting, Energy conservation through alternate energy sources for regeneration of sorption equipment (1982).

    68. W.A. Belding, “Impact of evaporative cooling technology on desiccant based ventilation systems”, NovelAire Technologies, www. Novelaire.com (2000).

    69. R.K. Collier, T.S. Cale and Z. Lavan, “Advanced desiccant materials assessment – phase 1”, GRI Report no. 8610182 (1986).

    70. L.G. Harriman III, “Moisture control in photolithography areas”, Desiccant cooling and dehumidification, ASHRAE, Atlanta GA U.S.A., pp. 29-32 (1992).

    71. P. Lamp and F. Ziegler, “European research on solar-assisted air conditioning”, Int. J. Refrig., Vol. 21, No. 2, pp. 89-99 (1998).

    72. F.C. Winkelmann et al., DOE-2 Supplement, Version 2.1 E, Lawrence Berkeley Laboratory, University of California and Hirsh & Associates, Berkeley, California, USA (1993).

    73. W. Yuan, Y. Jiang and L. Fu, “Feasibility study of desiccant cooling in combined heating, cooling and power system”, ISHVAC’99 The International Symposium on Heating, Ventilation and Air Conditioning, November 17-19, Shenzen, China, Vol. 2, pp. 778-784 (1999).

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International Journal on Architectural Science, Volume 1, Number 4, p.193-213, 2000 193 AIR CONDITIONING SYSTEMS WITH DESICCANT WHEEL FOR ITALIAN CLIMATES L. Bellia, P. Mazzei, F. Minichiello and D. Palma DETEC - University of Naples “Federico II”, P.le Tecchio, 80, 80125 Napoli, Italy (Received 25 October 2000; Accepted 15 January 2001) ABSTRACT Hybrid air conditioning systems based on chemical dehumidification are characterised by high energy efficiency and low environmental impact. They can result profitable if compared to traditional air conditioning systems and allow to obtain better indoor thermal comfort and air quality. In this paper, different hybrid air conditioning system configurations with desiccant wheel are examined; later, a first evaluation of operating costs is carried on, for Italian climates. For this purpose, a commercial computer program, DesiCalc TM , has been employed: from the European file known as TRY, hourly climatic data have been derived and adequately processed. For retail store application, for four Italian sites, maximum saving of about 22% has been obtained, while for theatre obtainable saving is greater and has been evaluated between 23% and 38%. For both the applications, the required comprehensive electric power is reduced (up to about 55%), and also the hours during which the system does not well control indoor relative humidity are strongly reduced. 1. INTRODUCTION It is well known that a considerable part of the primary energy is used for air conditioning purposes. Besides, it is worthwhile observing that: a) the most recent standards regarding environmental comfort and Indoor Air Quality (IAQ) impose both more restrictive limits to indoor R.H. values, and a considerable increase of outdoor airflow rates; b) CFC and HCFC refrigerant fluids are destined to disappearance; c) electric power peaks should be reduced. Therefore it seems necessary to develop new approaches to air conditioning techniques. Hybrid air conditioning systems based on chemical dehumidification are characterised by high energy efficiency and low environmental impact. Besides, they can result profitable if compared to traditional air conditioning systems [1-3]. The moist air chemical dehumidification has been adopted for long time, mostly in the U.S.A., in the industrial and military fields, in ice arenas and, within the commercial fields, in high latent load environments, like supermarkets [4-7]. However, the trend to extend this technique to other applications is evident, such as commercial and residential fields, integrating it with traditional and innovative systems [8-12], in order to obtain energy saving and to get the best thermal and moisture indoor conditions (in particular regarding R.H. values). Such trend is also justified by the required external airflow rates increase for indoor required external airflow rates increase for indoor ventilation, in Italy stated by the UNI 10339 [13] and, in the international field, stated by various codes and standards [14-21]. The increase of required external airflow rates implies, for summer Italian climates, a corresponding latent loads increase that must be removed in order to maintain acceptable indoor R.H. values. With the purpose to balance such additional latent loads [22], external air pre-treatment technologies, such as chemical dehumidification, to couple with traditional air conditioning systems, are regarded with increasing interest; the correct equipment component sizing is strictly joined to the external air design conditions definition [21]. The desiccant material may be solid or liquid. Solid desiccants inserted in a rotary heat exchanger [23], called desiccant wheel (DW), are particularly utilised, especially in HVAC applications: they are examined in this paper. In the following, different hybrid air system with desiccant wheel (DWHS) configurations are examined and then a first evaluation of operating costs of such systems, with respect to traditional ones, is provided for Italian climates; for this purpose a commercially available computer program, DesiCalc TM [24], has been employed: hourly climatic data have been derived from the European file known as TRY (Test Reference Year) [25] and adequately processed [26,27]. 2. BASIC CONCEPTS
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