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Performance investigation of solid desiccant evaporative cooling system configurations in different climatic zones
Energy Conversion and Management 97 (2015) 323–339
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance investigation of solid desiccant evaporative cooling system configurations in different climatic zones Muzaffar Ali a,b,⇑, Vladimir Vukovic c, Nadeem Ahmed Sheikh d, Hafiz M. Ali b a
Energy Department, Austrian Institute of Technology, Giefinggasse 2, 1210 Vienna, Austria Mechanical Engineering Department, University of Engineering and Technology Taxila, Pakistan c Technology Futures Institute, School of Science and Engineering, Teesside University, UK d Mechanical Engineering Department, Muhammad Ali Jinnah University, Pakistan b
a r t i c l e
i n f o
Article history: Received 26 November 2014 Accepted 7 March 2015 Available online 5 April 2015 Keywords: Desiccant evaporative cooling Ventilation cycle Dunkle cycle Climate zones Dymola/Modelica
a b s t r a c t Performance of desiccant evaporative cooling (DEC) system configurations is strongly influenced by the climate conditions and varies widely in different climate zones. Finding the optimal configuration of DEC systems for a specific climatic zone is tedious and time consuming. This investigation conducts performance analysis of five DEC system configurations under climatic conditions of five cities from different zones: Vienna, Karachi, Sao Paulo, Shanghai, and Adelaide. On the basis of operating cycle, three standard and two modified system configurations (ventilation, recirculation, dunkle cycles; ventilatedrecirculation and ventilated-dunkle cycles) are analyzed in these five climate zones. Using an advance equation-based object-oriented (EOO) modeling and simulation approach, optimal configurations of a DEC system are determined for each climate zone. Based on the hourly climate data of each zone for its respective design cooling day, performance of each system configuration is estimated using three performance parameters: cooling capacity, COP, and cooling energy delivered. The results revealed that the continental/micro-thermal climate of Vienna, temperate/mesothermal climate of Sao Paulo, and drysummer subtropical climate of Adelaide favor the use of ventilated-dunkle cycle configuration with average COP of 0.405, 0.89 and 1.01 respectively. While ventilation cycle based DEC configuration suits arid and semiarid climate of Karachi and another category of temperate/mesothermal climate of Shanghai with average COP of 2.43 and 3.03 respectively. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Demand for space cooling and ventilation has increased considerably during the last decade [1–3]. The improved standard of living, changes in life style and climatic conditions have led researchers to find energy efficient alternates of conventional HVAC systems. Cheap and energy efficient alternatives are available in market, however optimal configuration of these alternatives as standalone, hybrid and/or stand-by application is an area of extensive research. For instance, direct and indirect evaporative cooling has gained a lot of success in areas of hot and dry climate [4]. However, research concerning desiccant systems has been mainly concerned with modeling and simulation for performance evaluation of individual system components like desiccant wheels, enthalpy wheels, and evaporative coolers [5–11]. A theoretical ⇑ Corresponding author. Mobile: +92 (0) 3005316356; fax: +92 (51) 9047690. E-mail addresses: [email protected], [email protected] (M. Ali), [email protected] (V. Vukovic), [email protected] (N.A. Sheikh), [email protected] (H.M. Ali). http://dx.doi.org/10.1016/j.enconman.2015.03.025 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
model for the operation of a desiccant air conditioning system was developed on the basis of existing approaches for modeling of the main subsystems of such a system [12]. Various DEC systems can integrate these components to achieve desired comfort conditions. Different DEC systems configurations can be categorized based on operating cycles, for example ventilation, recirculation, and dunkle cycles [13]. Application of an open desiccant cooling process with ventilation and recirculation modes of the system operation was also reported [14]. However, system configuration performance in various climatic zones is relatively less explored in an efficient manner. Few recent studies use ideal models for the performance assessment of a typical DEC system configuration for various climatic conditions. In another study, an evaluation of various solid desiccant cycles for air conditioning in hot and humid climates of 16 cities of India was presented. It was found that the warm and humid climatic conditions result in the highest value of COP [15]. Likewise, a study concluded that that solid desiccant-based hybrid air-conditioning systems can give substantial energy savings as compared to conventional vapor compression
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Nomenclature CC COP Dh Dx En h _ m t T RH
cooling capacity (kW) coefficient of performance (–) change in specific enthalpy (kJ/kg) change in specific humidity (kg of air /kg of water) cooling energy delivered (kW h) specific enthalpy (kJ/kg) mass flow rate (kg/s) time (h) temperature (°C) relative humidity (%)
heater in pro o out supply ret reg
heater heating source inlet process air initial outlet supply air return air regeneration air
Subscripts amb ambient air f final
refrigeration based air-conditioning systems in Indian climates [16]. For climatic conditions in China, a study explored the performance of vapor compressor & desiccant (VC + D) cooling system and hybrid vapor compressor, desiccant and direct evaporative cooler (VC + D + EC) cooling system [17]. Results showed that more energy is saved in hot and dry climatic conditions compared to dry & humid conditions. Similarly a desiccant-evaporative cooling system is investigated to a ventilation and makeup mode operating cycle in Iranian climate. The results showed such systems are more effective than direct and direct–indirect evaporative cooling systems and provide a better thermal comfort even in hot and humid areas [18]. Additionally, an experimental exergo-economic assessment is performed in Turkey [19] and energy saving potential of a hybrid DEC system is determined for Beirut, Lebanon [20]. In another study, performance of two configurations of desiccant systems, conventional and recirculation were investigated through simulation based on outside conditions [21]. Analyses and comparison of a desiccant cooling system using regenerative evaporative cooling and a one-rotor two-stage desiccant cooling system configuration found that the system with regenerative evaporative cooling can handle air to much lower temperature while maintaining good thermal performance [22]. Similarly, various hybrid systems combining DEC with other energy sources are analyzed under various climate conditions [23,24]. Moreover, experimental investigations included a novel desiccant based air conditioning system configuration to improve the indoor air quality and reduce energy consumption [25]. In the system studied, the moisture of the fresh air was reduced passing through a solid desiccant wheel and then its temperature decreased through the ‘‘dry coil’’ of a vapor compression cycle. The study showed that a heat exchanger for pre-heating the regeneration air with exhaust air was feasible to install. Similarly, various other novel configurations of desiccant based evaporative air conditioning system are investigated [26,27]. System modeling and simulation can help in foreseeing the impact of system and sub-system on one another especially for an innovative system design [28]. With the advancements in technology, a number of alternative DEC system configurations are currently available [29]. Selecting an optimal system configuration for each climatic zone is a complicated task. Therefore a comprehensive study on the performance estimation of different DEC configurations under different climatic conditions is vital. Nevertheless, application of DEC systems across the globe is a reality. Although performance of DEC systems under climatic conditions of specific countries were investigated (e.g. India, China, and Iran) no study was found to explore the performance of DEC systems under various climatic conditions around the world.
Additionally, a specific system configuration is considered in various studies. It can be established from published studies that different configurations of DEC system have been investigated in specific climate zones; however no study covered their performances at a global scale. Therefore, the current study presents the performance comparison of five configurations of a DEC system under five different climatic conditions that cover a wide range of climates ranging from arid to continental. An Equation-based Object-Oriented (EOO) modeling and simulation approach [30,31] is applied through Dymola/Modelica tool. The five configurations are analyzed. Three basic configurations include ventilation, recirculation, and dunkle cycles, while the two modified configurations consist of ventilated-recirculation and ventilated-dunkle cycle system configurations. The amount of ventilation air inducted in ventilated recirculation and dunkle cycles is varied from 5% to 40% of outdoor air. Performance comparison of these system configurations is determined in terms of cooling capacity (CC), coefficient of performance (COP), and cooling energy delivered (En). Finally, an optimal DEC system configuration is suggested for each climatic zone based on the analyses of these assessment parameters.
2. Classification of desiccant evaporative cooling systems Classification of DEC systems can be based on the operating cycles. A brief introduction in terms of DEC system schematics and psychrometric representation of different configurations based on the operating cycles is provided in Fig. 1 for the sake of completeness and future referencing in the model development stage.
2.1. Ventilation cycle system configuration Ventilation cycle is a rotary desiccant cooling cycle. The process starts with the increase of temperature of ambient air with dehumidification (process 1–2). During the dehumidification, the process air loses moisture and increases temperature. Afterwards, the temperature of this air is reduced sensibly through the heat wheel (process 2–3) with subsequent addition of moisture using a direct humidifier (process 3–4). The regeneration side includes series of processes involving humidification (process 5–6), sensible heating (process 7–8) and desiccant wheel regeneration (process 8–9) as shown in Fig. 1(a) and (a1). In the lack of co-processing, ambient air can be used for regeneration as elaborated in Fig. 1(b) and (b1). The thermal performance of such cycle is lower in terms of specific cooling capacity and coefficient of performance
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Fig. 1. Schematic and psychrometric representation of different DEC system configurations: ventilation (a); modified ventilation (b); recirculation (c); ventilatedrecirculation (d); dunkle (e); and dunkle-ventilated cycles (f).
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(COP) due to the high temperature and humidity ratio of ambient air compared to return air [13]. 2.2. Recirculation and ventilated-recirculation cycle system configuration Recirculation desiccant air cooling cycle is a variant of standard ventilation cycle to increase the cooling capacity of the system as shown in Fig. 1(c) and (c1). In such a cycle, return air is reused as process air and ambient air is used for regeneration. The COP of recirculation cycle is commonly not higher than 0.8 due to relatively low temperature and humidity ratio. The key drawback of such a cycle is lack of fresh air provided by the system because it employs 100% recirculation [13]. However, ventilation air can be added to the return air according to requirements. The system is termed as ventilated-recirculation system as shown in Fig. 1(d) and (d1) [16]. The amount of ventilation air for commercial and institutional buildings is approximately 10–40% of outdoor air [32]. 2.3. Dunkle cycle system configuration Dunkle cycle combines the advantages of ventilation and recirculation cycles. An additional heat exchanger is integrated in the system to take the advantages of relatively low temperature of supply air and large cooling capacity associated with ventilation and recirculation cycles, respectively, as shown in Fig. 1(e) and (e1). Dunkle cycle also has limited use due to lack of fresh air [13]. Ventilated air can be introduced to make it more practical, called ventilated-dunkle cycle, as presented in Fig. 1(f) and (f1) [16]. 3. Climate zones Performance of desiccant evaporative cooling systems is intensively dependent on the ambient conditions. The present investigation analyzes the DEC system performance in different climate zones. Köppen and ASHRAE climate zone classifications [33,34] are being used to divide world climates in different categories. Using Köppen classification world climate can be divided in five groups, namely A, B, C, D, and E. Each major group is subcategorized in different types and subtypes. However, in the current study only three major groups with different subtypes are used to have five climate zones as given in Table 1. Group B represents dry (arid and semiarid) climates. The subtypes are decided based the threshold values with respect to the annual precipitation received. Group C signifies mild temperate/mesothermal climates. Such climates have an average temperature above 10 °C during summer, i.e. April to September in Northern hemisphere and during winter, average temperature is between 3 °C and 18 °C. While group D is representation of continental/MicroTable 1 Climate zones and cooling design days of the selected cities. Sr. no
Location (country)
Latitude and longitude
ASHRAE climate classification
Köppen climate classification
Cooling design day
1
Karachi (Pakistan)
1
BWh
21st June
2
Sao Paulo (Brazil) Shanghai (China) Adelaide (Australia)
24.85°N and 67.02°E 23.5°S and 46.62°N 31.2°N and 121.5°E 34.93°S and 138.58°E 48.208°N and 16.37°E
2
Cwa
3
Cfa
21st February 21st July
4
Csa
21st February
5
Dfb
21st July
3 4
5
Vienna (Austria)
thermal climates. These climates have average temperature above 10 °C in warmest months and below 3 °C in the coldest season. Two remaining climate groups, A and E are not studied in the current analyses as they cover extreme yearly average temperatures i.e. 18 °C or higher, and below 10 °C, respectively. Consequently, the presented analyses cover the most significant climatic conditions occurring in the world. While the climate zones classification according to ASHRAE standard [35], assigns different numbers from 1 to 7 to different climate zones based on cooling degree days (CDD) and heating degree days (HDD) at specific dry bulb temperatures. However, the current study considers only the first five climates for performance analysis of a desiccant cooling system in five different cities. The objective of considering both, Köppen and ASHRAE, classifications is to indicate that the selected five cities have different climate zones in both approaches. Table 1 shows selected cities and their classification in both methods along with their respective cooling design days [36]. Additionally, the selected cities are highlighted in Fig. 2 on the world map according to Köppen climate classification. The climate conditions in terms of dry-bulb temperature and relative humidity of the selected cities according to 12 h (7 am to 7 pm local time) cooling design day data are obtained from IWEC (International Weather for Energy Calculations format from ASHRAE) files of EnergyPlus database [36] are shown in Fig. 3. Though, it is generally thought that the temperature and humidity do not suddenly vary in the design day. However, it is not always true. In current study, the temperature and relative humidity vary suddenly for few climate zones causing certain variations in the performance curves as discussed latter. The approximate variation durations are: Shanghai (temperature vary between time 12.30 to 16.00 h and relative humidity vary between 13.00 to 15.00 h); Karachi (temperature vary between time 12.30 to 16.00 h and relative humidity vary among 12.00 to 15.00 and then 15.00 to 17.00 h); Sao Paulo (temperature vary between time 10.00 to 14.00 h and relative humidity vary between 12.00 to 16.00 h); Adelaide (temperature vary between time 10 to 13.00 h and relative humidity vary among 8.30 to 13.00 and then to 15.00 h). However, such a sudden variation is not observed for Vienna.
4. Component models of DEC system configurations DEC system models are developed through redeclaration of subcomponents in Dymola/Modelica to evaluate different configurations. Modelica supports the redeclaration of the component models. However, such component models are needed to be marked as ‘replaceable’ [30,31]. For the present work, implementation of the method can be described in two phases: component model development phase and implementation phase. In the development phase, a partial base model is developed to provide a common platform to all other component models of a specific component family, e.g. family of heat exchangers. Actually, the base model consists of different connecting ports through which other family component models can be connected. In addition, various conditional component control inputs are included in the base model. Thus, the base model provides a common interface for coupling different component models with the other models. Afterwards, the main component models are developed by extending the base model of each respective component. Thus the base model acts as a ‘base class’ and the component models built on it act as ‘sub class’. For implementation, each component model is needed to be declared as ‘replaceable’ and ‘constrained’ by its base model to change all the possible component models. Furthermore, Modelica annotation ‘ChoicesAllMatching = true’ is also implemented. The
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Fig. 2. Selected cities according to Köppen climate classification.
Fig. 3. Climate conditions of selected cities with respect to cooling design day; dry-bulb temperature (A), relative humidity (B).
annotation ensures to change all possible component models built on a specific base model through ‘change class’ option to vary various possible configurations. However, the variation of component models is beyond the scope of the current work and will be presented in another study.
4.1. Desiccant wheel component models The base model is developed based on the literature [37] with further extensions for two desiccant wheels. The development is carried out in Dymola/Modelica environment. The developed desiccant wheel model is already validated in authors’ previous publication under real-time control strategies of desiccant wheel operation [38]. Fig. 4a presents the block diagram of the model along with the available options for desiccant wheel configurations.
4.3. Humidifier component models The base model of humidifier component models consists of humidifier control signal and conditional water temperature ports in addition to fluid inlet and outlet ports. Two possibilities are included in the change-class list including ideal humidifier [37] and modified validated humidifier component models [39], as shown in Fig. 4c. 4.4. Heat source component models For heat source component model, three model options are used comprising on electric heater, boiler, and constant temperature heater. The base model for all three options consists of fluid inlet and outlet ports with control signal that determines the fluid outlet temperature based on the heat flow rate. The three model possibilities are available in the change-class list as shown in Fig. 4d.
4.2. Heat exchanger component models 5. Model development of system configurations Heat exchanger component models are used for simulating heat wheel of DEC system. The base model comprises of fluid ports for primary and secondary fluid streams. Four different types of heat exchanger component models are used including ideal heat exchanger with constant effectiveness, modified heat exchanger with constant effectiveness, dry-effectiveness-NTU, and constanteffectiveness-mass exchanger [37]. All component models are extensions of the base model as shown in Fig. 4b. In the current study, a constant effectiveness heat wheel is used and its validated model is presented in authors’ another publication [39].
In the current study, all validated component models of DEC are used as described in Section 5. Additionally, the system model is also validated in authors’ previous work [39]. The components and system models are validated with the actual measurement data of ENERGYbase system [40] under transient operating conditions. The desiccant cooling system of ENERGYbase is comprised of two identical units, each of 8240 m3/h design flow rate. However, a single unit is considered in the current study. In the cooling mode, the supply air conditions are maintained with moderately cool
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Fig. 4. Component model options of desiccant evaporative cooling system: desiccant wheel (a); humidifier (b); heat wheel (c); and heat source (d) component model options.
temperature of 24 °C and 60% relative humidity. Thus, in the present work, the validated DEC system model operating in ventilation mode is extended to other configurations for performance analysis in different climate zones.
5% and 40% of design flow [32] is used for the model. A mixing box component model [37] is used to mix fresh air with the return air, keeping the overall flow rate constant. Such configuration models an outside air mixing box with air dampers and a flow path for the minimum outside air flow rate.
5.1. Ventilation cycle model Ventilation cycle employs ambient air as process air and return air for system regeneration. The flow rates of both air streams are kept constant at a design value of 2.5 kg/s (8240 m3/h). The component model ‘Weather_Data’ contains all relevant hourly ambient weather conditions based on respective design day of all five considered climates in terms of dry bulb temperature Tamb, and relative humidity RHamb. The data for each climatic condition is used from the weather data file of EnergyPlus [36]. The return air conditions from room are kept constant at temperature Tret 26.7 °C and RHret 50% relative humidity [40]. Desiccant wheel regeneration temperature, Treg, is kept constant at 70 °C through constant temperature heater component model along with the rotation speed at 20 RPH. Moreover, the validated supply and return humidifier models are controlled at design 50% and 100% of water pump speed, respectively [39]. For the overall system control, temperature and humidity sensors are also used at different state points. Fig. 5 shows the Dymola representation of ventilation cycle model. Weather_Data file contains complete climate data of each zone for simulation. Dx_Pro and Dh_Pro; and Reg_Dx and Reg_Dh are the functions for process and regeneration sides of humidifiers developed in the author’s previous study [39]. Component models with suffix amb, ret, reg, and supply indicate the ambient, return, regeneration, and supply air parameters. 5.2. Recirculation and ventilated-recirculation cycle model In the recirculation cycle model, return air is used as process air, while ambient air is employed for system regeneration. However, standard recirculation cycle lacks the fresh air supply that is undesirable for most practical applications. Therefore, fresh outside air is introduced to develop a ventilated-recirculation cycle model as depicted in Fig. 6. A specified amount of fresh air between
5.3. Dunkle and ventilated-dunkle cycle model development An additional heat exchanger is incorporated in the dunkle cycle model to enhance the system cooling capacity by utilizing ambient conditions. The standard dunkle cycle also faces the same demerit of lacking fresh air similar to the standard recirculation cycle. Therefore, the standard cycle model is modified to induct specified amount of fresh air, termed ventilated-dunkle cycle model, as shown in Fig. 7. 6. Performance analysis of system configurations Performance analysis of ENERGYbase desiccant cooling system is accomplished in five different climate zones using five different system configurations, i.e. ventilation cycle, recirculation and ventilated-recirculation cycles, and dunkle and ventilated-dunkle cycles. The analysis presents the feasibility investigation to decide suitability of the referred desiccant cooling systems in the selected climates. The whole analysis is based on the climate conditions on the respective cooling days of each climate zone. The climate conditions are obtained from the weather data file of EnergyPlus for 12 h (7 am–7 pm) with 1 h time interval. Moreover, three performance parameters are used for the overall system analysis that includes system cooling capacity (kW), COP, and cooling energy delivered (kW h), through Eqs. (1)–(3), respectively.
_ supply ðhamb hsupply Þ CC ¼ m
ð1Þ
_ supply ðhamb hsupply Þ=m _ ret ðhheaterout hheaterin Þ COP ¼ m
ð2Þ
En ¼
Z
tf
to
CC
ð3Þ
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Fig. 5. Dymola representation of ventilation cycle model.
Fig. 6. Dymola representation of ventilated-recirculation cycle model.
_ is air mass flow rate of supply and return air; and h is Here m enthalpy of ambient, supply, and heater; and to and tf are system initial and final operational time (7 am–7 pm). The cooling capacity (CC) as a performance parameter is already used in authors’ previous published work [39], based on the six days transient
measurement data of ENERGYbase system. The parameter En indicates the total cooling capacity delivered by each configuration in each climate during 12 h of system operation. The control strategy uses the conditions of return air with constant temperature of 26.7 °C and relative humidity of 50%
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Fig. 7. Dymola representation of ventilated-dunkle cycle model.
according to ARI rating of room air [41]. Additionally, the control of supply and return humidifiers is also fixed with respect to their design control inputs based on the dehumidification capacity and heat loss. The developed functions used for these two control inputs are fixed for each climate based on the resulting minimum validation error determined in the authors’ previous work [39]. Additionally, a constant temperature heater component model is used to achieve required fixed regeneration temperature at 70 °C. In the presented work, the developed system models of all five configurations are simulated in Dymola software for 12 h of system operation based on the respective design cooling day of each climate zone. All system configuration models are properly constrained through Modelica language code by suitably applying the actual system design and operating parameters of each system component e.g. mass flow rate of process and regeneration air, supply and return air set points, regeneration temperature, desiccant wheel rotation speed, percentage of ventilation air, etc. During the simulation, each configuration automatically retrieves the ambient conditions of each climate zone. The equation of every performance parameter (CC, COP, and En) is also appropriately incorporated in the Modelica code such that these parameters are correctly listed in the result file of Dymola simulations for performance comparison. Finally, the performance of all five DEC system configurations in five selected climate zones is presented after extensive simulations through Dymola/Modelica tool. 7. Results and discussions 7.1. Vienna climate zone The climate conditions of Vienna represents temperate weather that resulted with the fixed return and supply humidifier control strategy, a slightly higher cooling capacity of ventilation cycle during the midday compared to recirculation and dunkle cycles, as shown in Fig. 8(A). Lower values of dunkle cycle cooling capacity
indicate that the additional heat exchanger in such climatic conditions does not enhance the system cooling capacity but results in improved COP due to lesser input energy requirements for regeneration desiccant wheel, as observed in Fig. 8(B). The recirculation cycle shows increased total delivered energy compared to the other cycles as depicted in Fig. 8(C). Overall, the recirculation cycle performance is more suitable in Vienna climate conditions. Figs. 9 and 10 show the effects of variation in fresh air induction in recirculation and dunkle cycles from 5% to 40%, respectively. The cooling capacity and COP of ventilated recirculation and dunkle cycles increase with the increasing percentage of fresh air. The results show that fresh air of Vienna enhances the heat and mass transfer processes of regeneration air through return humidifier, heat wheel, and desiccant wheel that ultimately increases cooling capacity and COP of both cycles. 7.2. Karachi climate zone The ambient conditions of Karachi represents hot-humid climate that favors dunkle cycle in terms of high cooling capacity and cooling energy delivered by the system as shown in Fig. 11. However, the ventilation cycle has high COP that indicates low energy requirements to maintain 70 °C. Cooling capacities and cooling energy delivered for the recirculation cycle match dunkle cycle, however requiring more energy, thus resulting in the lowest COP, as shown in Fig. 11(B). As expected, the ventilation cycle has the highest COP, however it delivers the lowest cooling energy with the least cooling capacity due to humid climatic conditions of Karachi. Additionally, the induction of fresh outside air showed that the actual ambient conditions of Karachi do not enhance the heat and mass transfer processes in the regeneration side of desiccant cooling system as presented in Figs. 12 and 13. However, it makes cycles more realistic in terms of fresh air requirement for thermal comfort. Another possibility is to enhance the ambient
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Fig. 8. Vienna: Performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 9. Vienna: performance analysis of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
conditions by passing the air across the heating coil (using solar energy) before entering the regeneration process. The sudden change in the performance curves during some times of 12 h system operation is due to the variation in temperature and relative humidity as shown in Fig. 3.
7.3. Sao Paulo climate zone In the Sao Paulo climate zone, it is evident that the performance of standard recirculation cycle configuration in terms of cooling capacity is much better than the other two cycles, as
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Fig. 10. Vienna: performance analysis of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 11. Karachi: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
shown in Fig. 14. While the low COP of recirculation cycle represents high input energy demands compared to other cycles, the dunkle cycle has higher COP due to the presence of additional heat recovery. However, the dunkle cycle lacks the cooling capacity due to fixed return air conditions. On the
other hand, ventilation cycle has the lowest cooling capacity and cooling energy delivered. Moreover, the induction of fresh air in the ventilated-recirculation cycle decreases the system performance with low cooling capacity, as shown in Fig. 15. The rapid changes in the ambient
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Fig. 12. Karachi: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 13. Karachi: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
conditions in terms of temperature and relative humidity are causing sudden variation in the performances curves during certain time intervals. However, the overall impact of fresh air is almost negligible for ventilated-dunkle cycle, as presented in Fig. 16.
7.4. Shanghai climate zone Shanghai climate conditions represent humid subtropical weather. In such climate, the performance of recirculation and dunkle cycles is similar and much better in terms of cooling
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Fig. 14. Sao Paulo: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 15. Sao Paulo: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
capacity compared to the ventilation cycle, as shown in Fig. 17. However, the dunkle cycle is built with an additional heat exchanger that increases the initial system cost. In addition, the two cycles require more energy input compared to the ventilation cycle that has higher COP. Fig. 18 shows that the cooling capacity of ventilated recirculation cycle with 10% fresh air is higher than for the other
amounts of fresh air. The COP of the cycle with different amounts of fresh air is almost the same, except in the case of 5% fresh air. The performance of ventilated dunkle cycle decreases with increased amount of fresh air, as shown in Fig. 19. Moreover, the rapid change in performance curves during time interval of 14–16 h is due to variation in ambient conditions in this time duration.
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Fig. 16. Sao Paulo: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 17. Shanghai: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
7.5. Adelaide climate zone The city of Adelaide is representation of dry-summer subtropical climates. The performance of the ventilation cycle is slightly better than the other two cycles, as shown in Fig. 20. Additional heat exchanger enhances the COP of the dunkle cycle. Moreover,
the effects of different amounts of fresh air are less significant in both ventilated recirculation and ventilated dunkle cycles, as shown in Figs. 21 and 22, respectively. The sudden variations between time intervals of 8.00 to 10.00 h and 14.00 to 16.00 h are because of the changes in the climates conditions of Adelaide.
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Fig. 18. Shanghai: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 19. Shanghai: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
8. Summary of results The performance analysis of three configurations of desiccant cooling systems in terms of the operational cycles is performed in five climate zones. Additionally, the standard recirculation and dunkle cycles are modified by the induction of fresh air to analyze
more practical options. Tables 2 and 3 summarize the results of all climates for all configurations. In these tables, average simulated values of cooling capacity (CC) and COP are given, while the maximum value of cooling energy delivered (En) is mentioned. The arrow directions show the increasing ("), decreasing (;), or constant effects (?) for ventilated recirculation and dunkle cycles. It
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Fig. 20. Adelaide: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 21. Adelaide: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
can be observed from Table 2 that ventilated-dunkle cycle is the most suitable DEC system configuration for continental/microthermal climate of Vienna, temperate/mesothermal climate of Sao Paulo, and dry-summer subtropical climate of Adelaide climate
zones (the highlighted values). The arid and semiarid climate conditions of Karachi and another category of temperate/mesothermal climate of Shanghai favor standard ventilation cycle configuration as presented in Table 3 (the highlighted values).
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Fig. 22. Adelaide: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Table 2 Performance summary of three climates.
Table 3 Performance summary of two climates.
9. Conclusions In the current investigation, performance analysis of a commercial desiccant cooling system is presented to select an appropriate configuration through an EOO modeling and simulation approach. Three standard and two modified system configurations are
analyzed based on the operating cycle in five climate zones. In practice, supply air temperature and humidity requirements may substantially differ depending on the occupant preferences, e.g. due to acclimatization, personal circumstances etc. Such personalized ventilation requirements however fall outside of comparative analyses presented in this paper.
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In general, the ventilated-dunkle cycle is the most appropriate configuration for Vienna, Sao Paulo, and Adelaide climate zones that represent continental, temperate and dry-summer subtropical climate, respectively. While the DEC system performance is much better with ventilation cycle configuration in Karachi and Shanghai climates representing arid and semiarid and a different category of temperate climates, respectively. Acknowledgements The authors express profound gratitude to the Energy Department of Austrian Institute of Technology (AIT), Vienna, Austria for its support to perform such study on their ENERGYbase desiccant evaporative cooling system. References [1] Ellis MW, Mathews EH. Needs and trends in building and HVAC system design tools. Build Environ 2002;37(5):461–70. [2] Perez-Lombard L, Ortiz J, Pout C. A review on buildings energy consumption information. Energy Buildings 2008;40(3):394–8. [3] U.S. Department of Energy, Buildings Energy Data Book 2011, Washington; 2012. [4] Daou K, Wang RZ, Xia ZZ. Desiccant cooling air conditioning: a review. Renew Sustain Energy Rev 2006;10:55–77. [5] Niu JL, Zhang LZ. Effects of wall thickness on heat and moisture transfer in desiccant wheels for air dehumidification and enthalpy recovery. Int Commun Heat Mass Transfer 2002;29(2):255–68. [6] Sphaier CM, Worek WM. The effect of axial diffusion on enthalpy wheels. Int J Heat Mass Transf 2006;49:1412–9. [7] Zhang XJ, Dai YJ, Wang RZ. A simulation study of heat and mass transfer in a honeycomb structure rotary desiccant dehumidifier. Appl Therm Eng 2003;23:989–1003. [8] Shang W, Besant RW. Theoretical and experimental methods for the sensible effectiveness of air-to-air energy recovery wheels. HVAC&R Res 2008;14(3):373–96. [9] Shang W, Besant RW. Effectiveness of desiccant coated regenerative wheels from transient response characteristics and flow channels-Part I. HVAC&R Res 2008;15(2):329–45. [10] Shang W, Besant RW. Effectiveness of desiccant coated regenerative wheels from transient response characteristics and flow channels-Part II. HVAC&R Res 2008;15(2):347–66. [11] Nobrega CEL, Brum NCL. Modeling and simulation of heat and enthalpy recovery wheels. Energy 2009;34:2063–8. http://dx.doi.org/10.1016/ j.energy.2008.08.016. [12] Panaras G, Mathioulakis E, Belessiotis V, Kyriakis N. Theoretical and experimental investigation of the performance of a desiccant airconditioning system. Renewable Energy 2010;35:1368–75. [13] La D, Dai YJ, Li Y, Wang RZ, Ge TS. Technical development of rotary desiccant dehumidification and air conditioning: a review. Renew Sustain Energy Rev 2010;14:130–47. [14] Kanoglu M, Bolattürk A, Altuntop N. Effect of ambient conditions on the first and second law performance of an open desiccant cooling process. Renewable Energy 2007;32:931–46. [15] Join S, Dhar PL, Kaushik SC. Evaluation of solid-desiccant-based evaporative cooling cycles for typical hot and humid climates. Int J Refrig 1995;18(5):287–96. [16] Dhar PL, Singh SK. Studies on solid desiccant based hybrid air-conditioning systems. Appl Therm Eng 2001;21:119–34.
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Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance investigation of solid desiccant evaporative cooling system configurations in different climatic zones Muzaffar Ali a,b,⇑, Vladimir Vukovic c, Nadeem Ahmed Sheikh d, Hafiz M. Ali b a
Energy Department, Austrian Institute of Technology, Giefinggasse 2, 1210 Vienna, Austria Mechanical Engineering Department, University of Engineering and Technology Taxila, Pakistan c Technology Futures Institute, School of Science and Engineering, Teesside University, UK d Mechanical Engineering Department, Muhammad Ali Jinnah University, Pakistan b
a r t i c l e
i n f o
Article history: Received 26 November 2014 Accepted 7 March 2015 Available online 5 April 2015 Keywords: Desiccant evaporative cooling Ventilation cycle Dunkle cycle Climate zones Dymola/Modelica
a b s t r a c t Performance of desiccant evaporative cooling (DEC) system configurations is strongly influenced by the climate conditions and varies widely in different climate zones. Finding the optimal configuration of DEC systems for a specific climatic zone is tedious and time consuming. This investigation conducts performance analysis of five DEC system configurations under climatic conditions of five cities from different zones: Vienna, Karachi, Sao Paulo, Shanghai, and Adelaide. On the basis of operating cycle, three standard and two modified system configurations (ventilation, recirculation, dunkle cycles; ventilatedrecirculation and ventilated-dunkle cycles) are analyzed in these five climate zones. Using an advance equation-based object-oriented (EOO) modeling and simulation approach, optimal configurations of a DEC system are determined for each climate zone. Based on the hourly climate data of each zone for its respective design cooling day, performance of each system configuration is estimated using three performance parameters: cooling capacity, COP, and cooling energy delivered. The results revealed that the continental/micro-thermal climate of Vienna, temperate/mesothermal climate of Sao Paulo, and drysummer subtropical climate of Adelaide favor the use of ventilated-dunkle cycle configuration with average COP of 0.405, 0.89 and 1.01 respectively. While ventilation cycle based DEC configuration suits arid and semiarid climate of Karachi and another category of temperate/mesothermal climate of Shanghai with average COP of 2.43 and 3.03 respectively. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Demand for space cooling and ventilation has increased considerably during the last decade [1–3]. The improved standard of living, changes in life style and climatic conditions have led researchers to find energy efficient alternates of conventional HVAC systems. Cheap and energy efficient alternatives are available in market, however optimal configuration of these alternatives as standalone, hybrid and/or stand-by application is an area of extensive research. For instance, direct and indirect evaporative cooling has gained a lot of success in areas of hot and dry climate [4]. However, research concerning desiccant systems has been mainly concerned with modeling and simulation for performance evaluation of individual system components like desiccant wheels, enthalpy wheels, and evaporative coolers [5–11]. A theoretical ⇑ Corresponding author. Mobile: +92 (0) 3005316356; fax: +92 (51) 9047690. E-mail addresses: [email protected], [email protected] (M. Ali), [email protected] (V. Vukovic), [email protected] (N.A. Sheikh), [email protected] (H.M. Ali). http://dx.doi.org/10.1016/j.enconman.2015.03.025 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.
model for the operation of a desiccant air conditioning system was developed on the basis of existing approaches for modeling of the main subsystems of such a system [12]. Various DEC systems can integrate these components to achieve desired comfort conditions. Different DEC systems configurations can be categorized based on operating cycles, for example ventilation, recirculation, and dunkle cycles [13]. Application of an open desiccant cooling process with ventilation and recirculation modes of the system operation was also reported [14]. However, system configuration performance in various climatic zones is relatively less explored in an efficient manner. Few recent studies use ideal models for the performance assessment of a typical DEC system configuration for various climatic conditions. In another study, an evaluation of various solid desiccant cycles for air conditioning in hot and humid climates of 16 cities of India was presented. It was found that the warm and humid climatic conditions result in the highest value of COP [15]. Likewise, a study concluded that that solid desiccant-based hybrid air-conditioning systems can give substantial energy savings as compared to conventional vapor compression
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Nomenclature CC COP Dh Dx En h _ m t T RH
cooling capacity (kW) coefficient of performance (–) change in specific enthalpy (kJ/kg) change in specific humidity (kg of air /kg of water) cooling energy delivered (kW h) specific enthalpy (kJ/kg) mass flow rate (kg/s) time (h) temperature (°C) relative humidity (%)
heater in pro o out supply ret reg
heater heating source inlet process air initial outlet supply air return air regeneration air
Subscripts amb ambient air f final
refrigeration based air-conditioning systems in Indian climates [16]. For climatic conditions in China, a study explored the performance of vapor compressor & desiccant (VC + D) cooling system and hybrid vapor compressor, desiccant and direct evaporative cooler (VC + D + EC) cooling system [17]. Results showed that more energy is saved in hot and dry climatic conditions compared to dry & humid conditions. Similarly a desiccant-evaporative cooling system is investigated to a ventilation and makeup mode operating cycle in Iranian climate. The results showed such systems are more effective than direct and direct–indirect evaporative cooling systems and provide a better thermal comfort even in hot and humid areas [18]. Additionally, an experimental exergo-economic assessment is performed in Turkey [19] and energy saving potential of a hybrid DEC system is determined for Beirut, Lebanon [20]. In another study, performance of two configurations of desiccant systems, conventional and recirculation were investigated through simulation based on outside conditions [21]. Analyses and comparison of a desiccant cooling system using regenerative evaporative cooling and a one-rotor two-stage desiccant cooling system configuration found that the system with regenerative evaporative cooling can handle air to much lower temperature while maintaining good thermal performance [22]. Similarly, various hybrid systems combining DEC with other energy sources are analyzed under various climate conditions [23,24]. Moreover, experimental investigations included a novel desiccant based air conditioning system configuration to improve the indoor air quality and reduce energy consumption [25]. In the system studied, the moisture of the fresh air was reduced passing through a solid desiccant wheel and then its temperature decreased through the ‘‘dry coil’’ of a vapor compression cycle. The study showed that a heat exchanger for pre-heating the regeneration air with exhaust air was feasible to install. Similarly, various other novel configurations of desiccant based evaporative air conditioning system are investigated [26,27]. System modeling and simulation can help in foreseeing the impact of system and sub-system on one another especially for an innovative system design [28]. With the advancements in technology, a number of alternative DEC system configurations are currently available [29]. Selecting an optimal system configuration for each climatic zone is a complicated task. Therefore a comprehensive study on the performance estimation of different DEC configurations under different climatic conditions is vital. Nevertheless, application of DEC systems across the globe is a reality. Although performance of DEC systems under climatic conditions of specific countries were investigated (e.g. India, China, and Iran) no study was found to explore the performance of DEC systems under various climatic conditions around the world.
Additionally, a specific system configuration is considered in various studies. It can be established from published studies that different configurations of DEC system have been investigated in specific climate zones; however no study covered their performances at a global scale. Therefore, the current study presents the performance comparison of five configurations of a DEC system under five different climatic conditions that cover a wide range of climates ranging from arid to continental. An Equation-based Object-Oriented (EOO) modeling and simulation approach [30,31] is applied through Dymola/Modelica tool. The five configurations are analyzed. Three basic configurations include ventilation, recirculation, and dunkle cycles, while the two modified configurations consist of ventilated-recirculation and ventilated-dunkle cycle system configurations. The amount of ventilation air inducted in ventilated recirculation and dunkle cycles is varied from 5% to 40% of outdoor air. Performance comparison of these system configurations is determined in terms of cooling capacity (CC), coefficient of performance (COP), and cooling energy delivered (En). Finally, an optimal DEC system configuration is suggested for each climatic zone based on the analyses of these assessment parameters.
2. Classification of desiccant evaporative cooling systems Classification of DEC systems can be based on the operating cycles. A brief introduction in terms of DEC system schematics and psychrometric representation of different configurations based on the operating cycles is provided in Fig. 1 for the sake of completeness and future referencing in the model development stage.
2.1. Ventilation cycle system configuration Ventilation cycle is a rotary desiccant cooling cycle. The process starts with the increase of temperature of ambient air with dehumidification (process 1–2). During the dehumidification, the process air loses moisture and increases temperature. Afterwards, the temperature of this air is reduced sensibly through the heat wheel (process 2–3) with subsequent addition of moisture using a direct humidifier (process 3–4). The regeneration side includes series of processes involving humidification (process 5–6), sensible heating (process 7–8) and desiccant wheel regeneration (process 8–9) as shown in Fig. 1(a) and (a1). In the lack of co-processing, ambient air can be used for regeneration as elaborated in Fig. 1(b) and (b1). The thermal performance of such cycle is lower in terms of specific cooling capacity and coefficient of performance
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Fig. 1. Schematic and psychrometric representation of different DEC system configurations: ventilation (a); modified ventilation (b); recirculation (c); ventilatedrecirculation (d); dunkle (e); and dunkle-ventilated cycles (f).
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(COP) due to the high temperature and humidity ratio of ambient air compared to return air [13]. 2.2. Recirculation and ventilated-recirculation cycle system configuration Recirculation desiccant air cooling cycle is a variant of standard ventilation cycle to increase the cooling capacity of the system as shown in Fig. 1(c) and (c1). In such a cycle, return air is reused as process air and ambient air is used for regeneration. The COP of recirculation cycle is commonly not higher than 0.8 due to relatively low temperature and humidity ratio. The key drawback of such a cycle is lack of fresh air provided by the system because it employs 100% recirculation [13]. However, ventilation air can be added to the return air according to requirements. The system is termed as ventilated-recirculation system as shown in Fig. 1(d) and (d1) [16]. The amount of ventilation air for commercial and institutional buildings is approximately 10–40% of outdoor air [32]. 2.3. Dunkle cycle system configuration Dunkle cycle combines the advantages of ventilation and recirculation cycles. An additional heat exchanger is integrated in the system to take the advantages of relatively low temperature of supply air and large cooling capacity associated with ventilation and recirculation cycles, respectively, as shown in Fig. 1(e) and (e1). Dunkle cycle also has limited use due to lack of fresh air [13]. Ventilated air can be introduced to make it more practical, called ventilated-dunkle cycle, as presented in Fig. 1(f) and (f1) [16]. 3. Climate zones Performance of desiccant evaporative cooling systems is intensively dependent on the ambient conditions. The present investigation analyzes the DEC system performance in different climate zones. Köppen and ASHRAE climate zone classifications [33,34] are being used to divide world climates in different categories. Using Köppen classification world climate can be divided in five groups, namely A, B, C, D, and E. Each major group is subcategorized in different types and subtypes. However, in the current study only three major groups with different subtypes are used to have five climate zones as given in Table 1. Group B represents dry (arid and semiarid) climates. The subtypes are decided based the threshold values with respect to the annual precipitation received. Group C signifies mild temperate/mesothermal climates. Such climates have an average temperature above 10 °C during summer, i.e. April to September in Northern hemisphere and during winter, average temperature is between 3 °C and 18 °C. While group D is representation of continental/MicroTable 1 Climate zones and cooling design days of the selected cities. Sr. no
Location (country)
Latitude and longitude
ASHRAE climate classification
Köppen climate classification
Cooling design day
1
Karachi (Pakistan)
1
BWh
21st June
2
Sao Paulo (Brazil) Shanghai (China) Adelaide (Australia)
24.85°N and 67.02°E 23.5°S and 46.62°N 31.2°N and 121.5°E 34.93°S and 138.58°E 48.208°N and 16.37°E
2
Cwa
3
Cfa
21st February 21st July
4
Csa
21st February
5
Dfb
21st July
3 4
5
Vienna (Austria)
thermal climates. These climates have average temperature above 10 °C in warmest months and below 3 °C in the coldest season. Two remaining climate groups, A and E are not studied in the current analyses as they cover extreme yearly average temperatures i.e. 18 °C or higher, and below 10 °C, respectively. Consequently, the presented analyses cover the most significant climatic conditions occurring in the world. While the climate zones classification according to ASHRAE standard [35], assigns different numbers from 1 to 7 to different climate zones based on cooling degree days (CDD) and heating degree days (HDD) at specific dry bulb temperatures. However, the current study considers only the first five climates for performance analysis of a desiccant cooling system in five different cities. The objective of considering both, Köppen and ASHRAE, classifications is to indicate that the selected five cities have different climate zones in both approaches. Table 1 shows selected cities and their classification in both methods along with their respective cooling design days [36]. Additionally, the selected cities are highlighted in Fig. 2 on the world map according to Köppen climate classification. The climate conditions in terms of dry-bulb temperature and relative humidity of the selected cities according to 12 h (7 am to 7 pm local time) cooling design day data are obtained from IWEC (International Weather for Energy Calculations format from ASHRAE) files of EnergyPlus database [36] are shown in Fig. 3. Though, it is generally thought that the temperature and humidity do not suddenly vary in the design day. However, it is not always true. In current study, the temperature and relative humidity vary suddenly for few climate zones causing certain variations in the performance curves as discussed latter. The approximate variation durations are: Shanghai (temperature vary between time 12.30 to 16.00 h and relative humidity vary between 13.00 to 15.00 h); Karachi (temperature vary between time 12.30 to 16.00 h and relative humidity vary among 12.00 to 15.00 and then 15.00 to 17.00 h); Sao Paulo (temperature vary between time 10.00 to 14.00 h and relative humidity vary between 12.00 to 16.00 h); Adelaide (temperature vary between time 10 to 13.00 h and relative humidity vary among 8.30 to 13.00 and then to 15.00 h). However, such a sudden variation is not observed for Vienna.
4. Component models of DEC system configurations DEC system models are developed through redeclaration of subcomponents in Dymola/Modelica to evaluate different configurations. Modelica supports the redeclaration of the component models. However, such component models are needed to be marked as ‘replaceable’ [30,31]. For the present work, implementation of the method can be described in two phases: component model development phase and implementation phase. In the development phase, a partial base model is developed to provide a common platform to all other component models of a specific component family, e.g. family of heat exchangers. Actually, the base model consists of different connecting ports through which other family component models can be connected. In addition, various conditional component control inputs are included in the base model. Thus, the base model provides a common interface for coupling different component models with the other models. Afterwards, the main component models are developed by extending the base model of each respective component. Thus the base model acts as a ‘base class’ and the component models built on it act as ‘sub class’. For implementation, each component model is needed to be declared as ‘replaceable’ and ‘constrained’ by its base model to change all the possible component models. Furthermore, Modelica annotation ‘ChoicesAllMatching = true’ is also implemented. The
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Fig. 2. Selected cities according to Köppen climate classification.
Fig. 3. Climate conditions of selected cities with respect to cooling design day; dry-bulb temperature (A), relative humidity (B).
annotation ensures to change all possible component models built on a specific base model through ‘change class’ option to vary various possible configurations. However, the variation of component models is beyond the scope of the current work and will be presented in another study.
4.1. Desiccant wheel component models The base model is developed based on the literature [37] with further extensions for two desiccant wheels. The development is carried out in Dymola/Modelica environment. The developed desiccant wheel model is already validated in authors’ previous publication under real-time control strategies of desiccant wheel operation [38]. Fig. 4a presents the block diagram of the model along with the available options for desiccant wheel configurations.
4.3. Humidifier component models The base model of humidifier component models consists of humidifier control signal and conditional water temperature ports in addition to fluid inlet and outlet ports. Two possibilities are included in the change-class list including ideal humidifier [37] and modified validated humidifier component models [39], as shown in Fig. 4c. 4.4. Heat source component models For heat source component model, three model options are used comprising on electric heater, boiler, and constant temperature heater. The base model for all three options consists of fluid inlet and outlet ports with control signal that determines the fluid outlet temperature based on the heat flow rate. The three model possibilities are available in the change-class list as shown in Fig. 4d.
4.2. Heat exchanger component models 5. Model development of system configurations Heat exchanger component models are used for simulating heat wheel of DEC system. The base model comprises of fluid ports for primary and secondary fluid streams. Four different types of heat exchanger component models are used including ideal heat exchanger with constant effectiveness, modified heat exchanger with constant effectiveness, dry-effectiveness-NTU, and constanteffectiveness-mass exchanger [37]. All component models are extensions of the base model as shown in Fig. 4b. In the current study, a constant effectiveness heat wheel is used and its validated model is presented in authors’ another publication [39].
In the current study, all validated component models of DEC are used as described in Section 5. Additionally, the system model is also validated in authors’ previous work [39]. The components and system models are validated with the actual measurement data of ENERGYbase system [40] under transient operating conditions. The desiccant cooling system of ENERGYbase is comprised of two identical units, each of 8240 m3/h design flow rate. However, a single unit is considered in the current study. In the cooling mode, the supply air conditions are maintained with moderately cool
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Fig. 4. Component model options of desiccant evaporative cooling system: desiccant wheel (a); humidifier (b); heat wheel (c); and heat source (d) component model options.
temperature of 24 °C and 60% relative humidity. Thus, in the present work, the validated DEC system model operating in ventilation mode is extended to other configurations for performance analysis in different climate zones.
5% and 40% of design flow [32] is used for the model. A mixing box component model [37] is used to mix fresh air with the return air, keeping the overall flow rate constant. Such configuration models an outside air mixing box with air dampers and a flow path for the minimum outside air flow rate.
5.1. Ventilation cycle model Ventilation cycle employs ambient air as process air and return air for system regeneration. The flow rates of both air streams are kept constant at a design value of 2.5 kg/s (8240 m3/h). The component model ‘Weather_Data’ contains all relevant hourly ambient weather conditions based on respective design day of all five considered climates in terms of dry bulb temperature Tamb, and relative humidity RHamb. The data for each climatic condition is used from the weather data file of EnergyPlus [36]. The return air conditions from room are kept constant at temperature Tret 26.7 °C and RHret 50% relative humidity [40]. Desiccant wheel regeneration temperature, Treg, is kept constant at 70 °C through constant temperature heater component model along with the rotation speed at 20 RPH. Moreover, the validated supply and return humidifier models are controlled at design 50% and 100% of water pump speed, respectively [39]. For the overall system control, temperature and humidity sensors are also used at different state points. Fig. 5 shows the Dymola representation of ventilation cycle model. Weather_Data file contains complete climate data of each zone for simulation. Dx_Pro and Dh_Pro; and Reg_Dx and Reg_Dh are the functions for process and regeneration sides of humidifiers developed in the author’s previous study [39]. Component models with suffix amb, ret, reg, and supply indicate the ambient, return, regeneration, and supply air parameters. 5.2. Recirculation and ventilated-recirculation cycle model In the recirculation cycle model, return air is used as process air, while ambient air is employed for system regeneration. However, standard recirculation cycle lacks the fresh air supply that is undesirable for most practical applications. Therefore, fresh outside air is introduced to develop a ventilated-recirculation cycle model as depicted in Fig. 6. A specified amount of fresh air between
5.3. Dunkle and ventilated-dunkle cycle model development An additional heat exchanger is incorporated in the dunkle cycle model to enhance the system cooling capacity by utilizing ambient conditions. The standard dunkle cycle also faces the same demerit of lacking fresh air similar to the standard recirculation cycle. Therefore, the standard cycle model is modified to induct specified amount of fresh air, termed ventilated-dunkle cycle model, as shown in Fig. 7. 6. Performance analysis of system configurations Performance analysis of ENERGYbase desiccant cooling system is accomplished in five different climate zones using five different system configurations, i.e. ventilation cycle, recirculation and ventilated-recirculation cycles, and dunkle and ventilated-dunkle cycles. The analysis presents the feasibility investigation to decide suitability of the referred desiccant cooling systems in the selected climates. The whole analysis is based on the climate conditions on the respective cooling days of each climate zone. The climate conditions are obtained from the weather data file of EnergyPlus for 12 h (7 am–7 pm) with 1 h time interval. Moreover, three performance parameters are used for the overall system analysis that includes system cooling capacity (kW), COP, and cooling energy delivered (kW h), through Eqs. (1)–(3), respectively.
_ supply ðhamb hsupply Þ CC ¼ m
ð1Þ
_ supply ðhamb hsupply Þ=m _ ret ðhheaterout hheaterin Þ COP ¼ m
ð2Þ
En ¼
Z
tf
to
CC
ð3Þ
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Fig. 5. Dymola representation of ventilation cycle model.
Fig. 6. Dymola representation of ventilated-recirculation cycle model.
_ is air mass flow rate of supply and return air; and h is Here m enthalpy of ambient, supply, and heater; and to and tf are system initial and final operational time (7 am–7 pm). The cooling capacity (CC) as a performance parameter is already used in authors’ previous published work [39], based on the six days transient
measurement data of ENERGYbase system. The parameter En indicates the total cooling capacity delivered by each configuration in each climate during 12 h of system operation. The control strategy uses the conditions of return air with constant temperature of 26.7 °C and relative humidity of 50%
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Fig. 7. Dymola representation of ventilated-dunkle cycle model.
according to ARI rating of room air [41]. Additionally, the control of supply and return humidifiers is also fixed with respect to their design control inputs based on the dehumidification capacity and heat loss. The developed functions used for these two control inputs are fixed for each climate based on the resulting minimum validation error determined in the authors’ previous work [39]. Additionally, a constant temperature heater component model is used to achieve required fixed regeneration temperature at 70 °C. In the presented work, the developed system models of all five configurations are simulated in Dymola software for 12 h of system operation based on the respective design cooling day of each climate zone. All system configuration models are properly constrained through Modelica language code by suitably applying the actual system design and operating parameters of each system component e.g. mass flow rate of process and regeneration air, supply and return air set points, regeneration temperature, desiccant wheel rotation speed, percentage of ventilation air, etc. During the simulation, each configuration automatically retrieves the ambient conditions of each climate zone. The equation of every performance parameter (CC, COP, and En) is also appropriately incorporated in the Modelica code such that these parameters are correctly listed in the result file of Dymola simulations for performance comparison. Finally, the performance of all five DEC system configurations in five selected climate zones is presented after extensive simulations through Dymola/Modelica tool. 7. Results and discussions 7.1. Vienna climate zone The climate conditions of Vienna represents temperate weather that resulted with the fixed return and supply humidifier control strategy, a slightly higher cooling capacity of ventilation cycle during the midday compared to recirculation and dunkle cycles, as shown in Fig. 8(A). Lower values of dunkle cycle cooling capacity
indicate that the additional heat exchanger in such climatic conditions does not enhance the system cooling capacity but results in improved COP due to lesser input energy requirements for regeneration desiccant wheel, as observed in Fig. 8(B). The recirculation cycle shows increased total delivered energy compared to the other cycles as depicted in Fig. 8(C). Overall, the recirculation cycle performance is more suitable in Vienna climate conditions. Figs. 9 and 10 show the effects of variation in fresh air induction in recirculation and dunkle cycles from 5% to 40%, respectively. The cooling capacity and COP of ventilated recirculation and dunkle cycles increase with the increasing percentage of fresh air. The results show that fresh air of Vienna enhances the heat and mass transfer processes of regeneration air through return humidifier, heat wheel, and desiccant wheel that ultimately increases cooling capacity and COP of both cycles. 7.2. Karachi climate zone The ambient conditions of Karachi represents hot-humid climate that favors dunkle cycle in terms of high cooling capacity and cooling energy delivered by the system as shown in Fig. 11. However, the ventilation cycle has high COP that indicates low energy requirements to maintain 70 °C. Cooling capacities and cooling energy delivered for the recirculation cycle match dunkle cycle, however requiring more energy, thus resulting in the lowest COP, as shown in Fig. 11(B). As expected, the ventilation cycle has the highest COP, however it delivers the lowest cooling energy with the least cooling capacity due to humid climatic conditions of Karachi. Additionally, the induction of fresh outside air showed that the actual ambient conditions of Karachi do not enhance the heat and mass transfer processes in the regeneration side of desiccant cooling system as presented in Figs. 12 and 13. However, it makes cycles more realistic in terms of fresh air requirement for thermal comfort. Another possibility is to enhance the ambient
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Fig. 8. Vienna: Performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 9. Vienna: performance analysis of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
conditions by passing the air across the heating coil (using solar energy) before entering the regeneration process. The sudden change in the performance curves during some times of 12 h system operation is due to the variation in temperature and relative humidity as shown in Fig. 3.
7.3. Sao Paulo climate zone In the Sao Paulo climate zone, it is evident that the performance of standard recirculation cycle configuration in terms of cooling capacity is much better than the other two cycles, as
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Fig. 10. Vienna: performance analysis of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 11. Karachi: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
shown in Fig. 14. While the low COP of recirculation cycle represents high input energy demands compared to other cycles, the dunkle cycle has higher COP due to the presence of additional heat recovery. However, the dunkle cycle lacks the cooling capacity due to fixed return air conditions. On the
other hand, ventilation cycle has the lowest cooling capacity and cooling energy delivered. Moreover, the induction of fresh air in the ventilated-recirculation cycle decreases the system performance with low cooling capacity, as shown in Fig. 15. The rapid changes in the ambient
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Fig. 12. Karachi: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 13. Karachi: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
conditions in terms of temperature and relative humidity are causing sudden variation in the performances curves during certain time intervals. However, the overall impact of fresh air is almost negligible for ventilated-dunkle cycle, as presented in Fig. 16.
7.4. Shanghai climate zone Shanghai climate conditions represent humid subtropical weather. In such climate, the performance of recirculation and dunkle cycles is similar and much better in terms of cooling
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Fig. 14. Sao Paulo: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 15. Sao Paulo: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
capacity compared to the ventilation cycle, as shown in Fig. 17. However, the dunkle cycle is built with an additional heat exchanger that increases the initial system cost. In addition, the two cycles require more energy input compared to the ventilation cycle that has higher COP. Fig. 18 shows that the cooling capacity of ventilated recirculation cycle with 10% fresh air is higher than for the other
amounts of fresh air. The COP of the cycle with different amounts of fresh air is almost the same, except in the case of 5% fresh air. The performance of ventilated dunkle cycle decreases with increased amount of fresh air, as shown in Fig. 19. Moreover, the rapid change in performance curves during time interval of 14–16 h is due to variation in ambient conditions in this time duration.
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Fig. 16. Sao Paulo: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 17. Shanghai: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
7.5. Adelaide climate zone The city of Adelaide is representation of dry-summer subtropical climates. The performance of the ventilation cycle is slightly better than the other two cycles, as shown in Fig. 20. Additional heat exchanger enhances the COP of the dunkle cycle. Moreover,
the effects of different amounts of fresh air are less significant in both ventilated recirculation and ventilated dunkle cycles, as shown in Figs. 21 and 22, respectively. The sudden variations between time intervals of 8.00 to 10.00 h and 14.00 to 16.00 h are because of the changes in the climates conditions of Adelaide.
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Fig. 18. Shanghai: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 19. Shanghai: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
8. Summary of results The performance analysis of three configurations of desiccant cooling systems in terms of the operational cycles is performed in five climate zones. Additionally, the standard recirculation and dunkle cycles are modified by the induction of fresh air to analyze
more practical options. Tables 2 and 3 summarize the results of all climates for all configurations. In these tables, average simulated values of cooling capacity (CC) and COP are given, while the maximum value of cooling energy delivered (En) is mentioned. The arrow directions show the increasing ("), decreasing (;), or constant effects (?) for ventilated recirculation and dunkle cycles. It
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Fig. 20. Adelaide: performance comparison of three cycles with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Fig. 21. Adelaide: performance comparison of ventilated-recirculation cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
can be observed from Table 2 that ventilated-dunkle cycle is the most suitable DEC system configuration for continental/microthermal climate of Vienna, temperate/mesothermal climate of Sao Paulo, and dry-summer subtropical climate of Adelaide climate
zones (the highlighted values). The arid and semiarid climate conditions of Karachi and another category of temperate/mesothermal climate of Shanghai favor standard ventilation cycle configuration as presented in Table 3 (the highlighted values).
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Fig. 22. Adelaide: performance comparison of ventilated-dunkle cycle with respect to: cooling capacity (A), COP (B), and cooling energy delivered (C).
Table 2 Performance summary of three climates.
Table 3 Performance summary of two climates.
9. Conclusions In the current investigation, performance analysis of a commercial desiccant cooling system is presented to select an appropriate configuration through an EOO modeling and simulation approach. Three standard and two modified system configurations are
analyzed based on the operating cycle in five climate zones. In practice, supply air temperature and humidity requirements may substantially differ depending on the occupant preferences, e.g. due to acclimatization, personal circumstances etc. Such personalized ventilation requirements however fall outside of comparative analyses presented in this paper.
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In general, the ventilated-dunkle cycle is the most appropriate configuration for Vienna, Sao Paulo, and Adelaide climate zones that represent continental, temperate and dry-summer subtropical climate, respectively. While the DEC system performance is much better with ventilation cycle configuration in Karachi and Shanghai climates representing arid and semiarid and a different category of temperate climates, respectively. Acknowledgements The authors express profound gratitude to the Energy Department of Austrian Institute of Technology (AIT), Vienna, Austria for its support to perform such study on their ENERGYbase desiccant evaporative cooling system. References [1] Ellis MW, Mathews EH. Needs and trends in building and HVAC system design tools. Build Environ 2002;37(5):461–70. [2] Perez-Lombard L, Ortiz J, Pout C. A review on buildings energy consumption information. Energy Buildings 2008;40(3):394–8. [3] U.S. Department of Energy, Buildings Energy Data Book 2011, Washington; 2012. [4] Daou K, Wang RZ, Xia ZZ. Desiccant cooling air conditioning: a review. Renew Sustain Energy Rev 2006;10:55–77. [5] Niu JL, Zhang LZ. Effects of wall thickness on heat and moisture transfer in desiccant wheels for air dehumidification and enthalpy recovery. Int Commun Heat Mass Transfer 2002;29(2):255–68. [6] Sphaier CM, Worek WM. The effect of axial diffusion on enthalpy wheels. Int J Heat Mass Transf 2006;49:1412–9. [7] Zhang XJ, Dai YJ, Wang RZ. A simulation study of heat and mass transfer in a honeycomb structure rotary desiccant dehumidifier. Appl Therm Eng 2003;23:989–1003. [8] Shang W, Besant RW. Theoretical and experimental methods for the sensible effectiveness of air-to-air energy recovery wheels. HVAC&R Res 2008;14(3):373–96. [9] Shang W, Besant RW. Effectiveness of desiccant coated regenerative wheels from transient response characteristics and flow channels-Part I. HVAC&R Res 2008;15(2):329–45. [10] Shang W, Besant RW. Effectiveness of desiccant coated regenerative wheels from transient response characteristics and flow channels-Part II. HVAC&R Res 2008;15(2):347–66. [11] Nobrega CEL, Brum NCL. Modeling and simulation of heat and enthalpy recovery wheels. Energy 2009;34:2063–8. http://dx.doi.org/10.1016/ j.energy.2008.08.016. [12] Panaras G, Mathioulakis E, Belessiotis V, Kyriakis N. Theoretical and experimental investigation of the performance of a desiccant airconditioning system. Renewable Energy 2010;35:1368–75. [13] La D, Dai YJ, Li Y, Wang RZ, Ge TS. Technical development of rotary desiccant dehumidification and air conditioning: a review. Renew Sustain Energy Rev 2010;14:130–47. [14] Kanoglu M, Bolattürk A, Altuntop N. Effect of ambient conditions on the first and second law performance of an open desiccant cooling process. Renewable Energy 2007;32:931–46. [15] Join S, Dhar PL, Kaushik SC. Evaluation of solid-desiccant-based evaporative cooling cycles for typical hot and humid climates. Int J Refrig 1995;18(5):287–96. [16] Dhar PL, Singh SK. Studies on solid desiccant based hybrid air-conditioning systems. Appl Therm Eng 2001;21:119–34.
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