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Advanced exergy analysis of an air conditioning system incorporating thermal energy storage
Energy xxx (2014) 1e8
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Advanced exergy analysis of an air conditioning system incorporating thermal energy storage A.H. Mosaffa a, *, L. Garousi Farshi b, C.A. Infante Ferreira c, M.A. Rosen d a
Department of Mechanical Engineering, Azarbaijan Shahid Madani University, Tabriz, Iran Faculty of Mechanical Engineering, University of Tabriz, Iran c Delft University of Technology, Department Process & Energy, Delft, 2628 CB, Netherlands d Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, ON, L1H 7K4, Canada b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2014 Received in revised form 30 August 2014 Accepted 1 October 2014 Available online xxx
Air conditioning incorporating thermal storage, in which the cooling capacity needed for air conditioning is produced during the nighttime when the demand for air conditioning is small and then utilized to satisfy peak loads during the daytime, has received much attention recently. In this work, an air conditioning system consisting of a combination of LHTS (latent heat thermal storage) and VCR (vapor compression refrigeration) is analyzed with advanced exergy analysis. The analysis is performed based on splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts. The results show that all of the exergy destruction of the LHTS unit is endogenous, which indicates that the total exergy destruction of the LHTS unit is due only to its irreversibilities. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Latent heat thermal energy storage Advanced exergy analysis Cooling Exergy Exergy destruction
1. Introduction The demand for AC (air conditioning) normally peaks during the daytime, and air conditioners are used much less during the nighttime. If equipment is designed to match such demands, its availability factor is low. For this reason air conditioning incorporating thermal storage, which produces and stores the cooling capacity needed for air conditioning during nighttime when the demand for air conditioning is small and utilizes the stored cooling capacity during daytime hours, is now receiving much attention. By using thermal storage in this manner, large air conditioning systems that are sized proportional to peak loads during daytime hours become unnecessary, and electricity costs are significantly reduced. Moreover, as the system produces and stores cooling capacity by using inexpensive nighttime electricity (provided the electricity cost differs between peak and off-peak load periods), the overall operating cost is lowered (for those with time-of-use electricity pricing). Thermal energy storages in general and PCMs (phase change materials) in particular have been the subject of much research over the last 20 years [1e6].
* Corresponding author. Tel.: þ98 412 4327566. E-mail address: [email protected] (A.H. Mosaffa).
Analyses of energy quality and quantity are important for improving the efficiency of a thermodynamic system, and the conventional method for performance analysis of LHTS (latent heat thermal storage) systems is based on energy. Many energy analyses of LHTS have been reported. For instance, Mosaffa et al. [7] investigated the energy efficiency for free cooling LHTS systems using multiple PCMs inside flat slabs. Also, they performed a numerical investigation of performance enhancement measures for the same storage to improve energy storage effectiveness and COP (coefficient of performance) [8]. Anisur et al. [9] used a theoretical model to analyze a LHTS system with a cylindrical shell geometry for an air coolingeheating application. They concluded that the PCM container volume also needs to be considered in conjunction with COP for reducing the amount of PCM. However, energy analysis is insufficient for complete thermodynamic evaluation of LHTS systems because it does not take into account all relevant performance aspects. This inadequacy of energy analysis leads to the use of exergy analysis, a technique based on the second law of thermodynamics, in LHTS system assessments. Exergy analysis determines efficiencies which provide a true measure of how nearly actual efficiency approaches the ideal, and identifies more clearly than energy analysis the sources, causes and locations of thermodynamic losses. The thermodynamic performance of cold thermal storage systems was assessed using exergy and energy analyses by Rosen et al.
http://dx.doi.org/10.1016/j.energy.2014.10.006 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
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Nomenclature cp COP _ Ex L m_ m Q_ Q_ RI s T T T0 Tm t _ W
specific heat at constant pressure (J/kg K) coefficient of performance exergy rate (W) latent heat (J/kg) mass flow rate (kg/s) mass (kg) heat rate (W) time averaged heat transfer heat (W) relative irreversibility specific entropy (J/kg K) temperature (K) time averaged temperature (K) thermodynamic averaged environment temperature (K) melting temperature (K) time (s) power (W)
Greek symbols Τ operating time (s) J exergy efficiency Subscripts CM compressor
[10]. Mosaffa et al. [11] performed energy and exergy analyses for a free cooling system using a LHTS unit with multiple PCMs. They showed that the increase in exergy efficiency due to reducing inlet air temperature is more significant than the effect of increasing the air flow rate during the cooling (discharging of LHTS) process. Ezan et al. [12] investigated energetically and exergetically a shell-and-tube LHTS unit during solidification and melting processes. Tyagi et al. [13] reported energy and exergy analyses of LHTS systems for space heating and cooling applications for two LHTS types: rectangular and balls filled with PCM. Li et al. [14] experimentally determined the energy and exergy performance of an adsorption storage system for residential applications. They showed that, as ambient temperature increases, the total energy storage density and energy efficiency increase, while the overall exergy efficiency decreases. Li et al. [15] presented energy and exergy analyses for an adsorption cold thermal energy storage system, for a space cooling application. The results show that the cold energy storage density and recovered exergy increase as the inlet temperature of the heat transfer fluid used in the adsorption system decreases. In the present work, advanced exergy analysis is applied to a cooling system using a LHTS unit which consists of several parallel layers of PCM slabs. A conventional exergy analysis identifies the components of the system with the highest exergy destructions and the processes that cause them. Then, the performance of a component of the system can be improved by reducing its exergy destruction. However, part of the exergy destruction may be unavoidable and part may be due to the exergy destruction occurring in other components (exogenous exergy destruction). This can be identified by performing an advanced exergy analysis. This type of analysis provides enhanced information and can show, for instance, if it is worthwhile to improve other components instead of only the component which has the highest exergy destruction. The LHTS unit cools air during the day and the absorbed heat is extracted during the night by employing an AC system (see Fig. 1).
CD D e el EV EX F i l P s TV
condenser destruction end of the PCM slab in the direction of the air flow electricity evaporator expander fuel initial liquid product solid throttling valve
Superscripts AV avoidable EN endogenous EX exogenous H hybrid R real T theoretical UN unavoidable Abbreviations AC air conditioning LHTS latent heat thermal storage PCM phase change material VCR vapor compression refrigeration
An advanced exergy analysis for the transient processes in the proposed system provides engineers with useful information related to energy system improvement potential. Splitting the exergy destruction within each component into unavoidable/ avoidable and endogenous/exogenous parts via advanced exergy analysis can provide meaningful results not obtained through conventional exergy analysis [16e19]. 2. Description of processes For the system considered, air cooling (PCM melting), releasing heat from LHTS unit (PCM solidification) and air-side heat transfer are taken to be unsteady two-dimensional problems. To develop a mathematical formulation of the melting and solidification processes, it is assumed that the PCM thermophysical properties are independent of temperature, but differ between solid and liquid phases. Further assumptions are that the air velocity profile is fully developed and, due to the negligible variation of container-wall temperature, temperature variations normal to the flow direction are neglected [20]. The effect of natural convection is negligible for this PCM storage geometry [21]. Fig. 2 illustrates the proposed air cooling system using a LHTS unit. Heat is extracted from the PCM by employing a VCR (vapor compression refrigeration) cycle during the night when condensation takes place at lower temperature and, in some regions, electricity is less expensive. The solidified PCM can be used to cool the air in the buildings during the hot hours of the day. 3. Conventional analyses 3.1. Energy analysis An energy analysis of an air cooling system using a LHTS unit has been reported in detail previously by the authors [7,8]. According to
Please cite this article in press as: Mosaffa AH, et al., Advanced exergy analysis of an air conditioning system incorporating thermal energy storage, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.10.006
A.H. Mosaffa et al. / Energy xxx (2014) 1e8
3
Fig. 1. The cooling system employing LHTS. Air is cooled by a PCM in the thermal storage during daytime (left) and the heat is extracted from the thermal storage by employing an air conditioner during nighttime (right).
Fig. 2, the time averaged heat transfer rate in the LHTS can be expressed as follows:
1 Q_ PCM ¼ t
Zt
_ p T 3 T 20 air Q_ t dt ¼ mc
(1)
0
where t is operating time, T 20 is time averaged inlet air temperature to the LHTS unit, and T 3 is time averaged outlet air temperature from the LHTS unit (see Fig. 2). The time averaged temperature T is defined as follows:
T¼
1 t
Zt Tdt
(2)
0
Q_ PCM _ W
total
3.2. Exergy analysis The total exergy destruction rate can be determined with an exergy balance as follows:
_ P þ Ex _ D _ F ¼ Ex Ex
(4)
The exergy efficiency can be expressed as the ratio of total product exergy output rate to total exergy input (fuel) rate:
j¼
_ P Ex _ F Ex
(5)
The relative irreversibility RI (relative irreversibility) is evaluated as [22],
The COP of the system can be defined as:
COP ¼
_ where W total is the total power consumption during cycle operation (day and night), which consists of power consumptions by the fan and the compressor.
(3)
RI ¼
_ D Ex
_ Ex D;overall
(6)
Fig. 2. Schematic of the cooling system using a LHTS unit.
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During the air cooling process (discharging), air transfers exergy _ _ to the PCM (Ex P;discharging ¼ Exair ), in which part of the exergy is stored. The exergy input causes the exergy change of the PCM. Accordingly, the exergy efficiency can be expressed as [23]:
jdischarging ¼
_ Ex air _ Fan _ ExPCM þ W
(7)
The net exergy change in the PCM for the discharging process can be expressed as [11],
1 _ Ex PCM ¼ t
(
Tm T þ ml L 1 0 Ti Tm " # T þ ml cp;l T l Tm T 0 ln l Tm " #) Ts þ ms cp;s T s Ti T 0 ln Ti ml cp;s ðTm Ti Þ T 0 ln
(8)
where Ti is the initial temperature of the solid PCM, Tm is the PCM melting temperature, and T s and T l are the temperatures of the solid and the liquid PCM respectively for t > 0. Also, T 0 is thermodynamic averaged environment temperature, defined as follows [24]:
T0 ¼
T3 T1 ln T 3 T 1
(9)
where T 1 and T 3 are the inlet and outlet time averaged air temperatures of the system respectively (see Fig. 2). The total exergy supply rate with the air can be written as follows:
1 _ Ex air ¼ t
Zt
m_ cp;air ðT3 T1 Þ T 0 ðs3 s1 Þ dt
(10)
0
During solidification of the PCM (charging), the exergy output _ _ rate from the PCM becomes available (Ex P;charging ¼ ExPCM ) and is gained by the air. The exergy efficiency of the charging process can be expressed as follows:
jcharging ¼
_ Ex PCM _ Fan þ W _ _ Exair þ W CM
Fig. 3. Simplified illustration of various processes for air cooling.
!
(11)
thermodynamic performance of a component through reducing its irreversibility. This splitting of exergy destruction can be expressed _ D ¼ Ex _ UN þ Ex _ AV [16]. as Ex D D Fig. 3 shows a simple diagram of real (consisting only of irreversible processes), theoretical (consisting only of reversible processes) and unavoidable thermodynamic processes during daytime for air cooling. In unavoidable processes, a considered component is taken to operate in an unavoidable condition while other components operate in theoretical conditions in order to calculate the unavoidable part of its exergy destruction. In Fig. 3, DTLHTS is the minimum temperature difference in the LHTS unit (T 3 T e ), where T e is the time averaged temperature at the end of the PCM slab in the direction of the air flow. In this case, the ambient air (point 1) flows through a large number of small parallel channels in the LHTS unit. The cooled air (point 3) is used to cool the interior of the building during the hot hours of the day. The real process is given as 1-2R-a-3. In this case, the fan has a real efficiency (hRFan ), the air exits from the fan with condition 2R, the minimum temperature difference in the LHTS unit has a real R value (DTLHTS ), and air exits the LHTS unit at condition 3. A theoretical process is used for splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts. The theoretical operation conditions for each component correspond to _ D /0. The theoretical process is given as 1-2T-b-3. The unavoidEx able process is based on the theoretical cycle but also considers irreversibilities caused by unavoidable factors. This process is given as 1-2UN-c-3. For calculating the endogenous part of the exergy destruction in each component of a real process, the following processes are analyzed, in each of which there is only one irreversible component:
The exergy destruction rate for the fan can be obtained from an exergy rate balance:
_ F;Fan Ex _ P;Fan ¼ W _ _ D;Fan ¼ Ex _ Ex _ Fan Ex Ex 2 1
(12)
4. Advanced exergy analysis An advanced exergy analysis allows for deeper understanding of the exergy destruction values obtained from an exergy analysis. It identifies the source of exergy destruction in a component. The total exergy destruction rate within each component is split into endogenous part (the exergy destruction rate that is obtained when all other system components are ideal and the component being considered operates with its real efficiency) and exogenous part, _ D ¼ Ex _ EN þ Ex _ EX [17]. It is thus possible to identify the that is, Ex D D impact of a component on the total exergy losses of the system. Moreover, splitting the exergy destruction into unavoidable (the exergy destruction rate that cannot be reduced due to technological limitations and cannot be overcome in the future) and avoidable parts provides an upper limit on the ability to enhance the
Fig. 4. Real, theoretical and hybrid cycles for calculating the endogenous part of the exergy destructions in the components of a VCR cycle.
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5
Fig. 5. Simplified illustration of various processes for extracting heat from a PCM with a VCR cycle.
_ EN ), 1-2R-b-3: The fan has a real efficiency (hR ), the air Fan (Ex D;Fan Fan exits the fan at condition 2R, the minimum temperature differT ence in the LHTS unit has a theoretical value (DTLHTS ) and the air exits the LHTS unit at condition 3. T _ EN LHTS (Ex D;LHTS ), 1-2 -a-3: The fan has a theoretical efficiency T (hFan ), the air exits the fan at condition 2T, the minimum temR perature difference in the LHTS unit has a real value (DTLHTS ) and the air exits the LHTS unit at condition 3. For calculating the unavoidable endogenous part of the exergy destruction in each component of the real process, the approach for calculating the endogenous part of the exergy destruction is used with the condition that each component operates in its unavoidable exergy destruction mode. The real, theoretical and hybrid (consisting of one irreversible process while all other processes are reversible) cycles of the VCR cycle are shown in Fig. 4, where the various conditions which are used to calculate different parts of the exergy destruction are illustrated. For instance, to obtain the endogenous exergy destruction of the condenser, other components operate at theoretical conditions while the condenser operates at the real condition. The saturated refrigerant vapor exits the evaporator after the T T theoretical process with DTmin; EV (point 4 ), enters an isentropic T compressor with hCM and is compressed to the real condenser
Table 1 Various conditions used to calculate endogenous part of exergy destruction rate for each component. _ D Ex
Condition
_ EN Ex D;Fan _ EN Ex D;LHTS _ EN Ex D;EV _ ExEN D;CM _ ExEN D;CD _ ExEN
1-2R-20 T-3T & 4T-5T-6T-7T 1-2T-20 T-3R & 4T-5T-6T-7T 1-2T-20 R-3T & 4R-5H-6T-7H 1-2T-20 T-3T & 4T-5*H-6T-7T 1-2T-20 T-3T & 4T-5**H-6R-7*H 1-2T-20 T-3T & 4T-5T-6T-7**H
D;TV
pressure (point 5**H). The real condenser pressure is obtained by R DTmin; CD and the saturated liquid exits the condenser at condition 6R. The throttling valve is replaced by an ideal expander for the theoretical process (point 7*H). A detailed advanced exergy analysis for a VCR cycle has been performed by Morosuk and Tsatsaronis [19]. Fig. 5 shows a simple diagram of real, theoretical and unavoidable thermodynamic processes during nighttime for extracting heat from a PCM by employing a VCR cycle. That is, a VCR cycle is employed for cooling the ambient air (point 1). Then the cooled air is used to extract heat from the molten PCM. For instance, to obtain the endogenous exergy destruction of the evaporator, other components operate at theoretical conditions while the evaporator operates at the real condition. In this case, the fan efficiency and the minimum temperature difference in the LHTS unit are hTFan and T DTLHTS respectively. The air is cooled by a real process in the R evaporator withDTmin; EV . Therefore the conditions for the air are 12T-20 R-3T. The saturated refrigerant vapor exits the evaporator at condition 4R and enters the isentropic compressor, which has an efficiency hTCM. The superheated vapor (point 5H) is condensed in the condenser, where the minimum temperature difference has a T value of that for a theoretical process (DTmin; ). Then saturated CD liquid enters the ideal expander and its pressure is decreased to the evaporator pressure (point 7H). Table 1 shows the various
Table 2 Thermophysical properties of the PCM [26]. Material Density (kg/m3) Heat capacity at constant pressure, cp (J/kg K) Thermal conductivity (W/m K) Latent heat of fusion, L (J/kg) Melting temperature, Tm (K)
CaCl2.6H2O 1530 (liquid) 1710 (solid) 1400 (liquid) 2200 (solid) 0.54 (liquid) 1.09 (solid) 190, 800 301e302
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Fig. 6. Outdoor temperature, outlet air temperature and PCM temperature at the end of PCM slab profiles for the real process during the day (T 0 ¼ 29.86 C, T 1 ¼ 34.11 C, T 3 ¼ 25.64 C and T e ¼ 24.69 C).
0.102 (99.92%) 0.044 (97.74%) 0.147 (61.76%) 0.0002 (0.29%) 0.0016 (10.91%) 0.0018 (0.79%) 0.074 (99.71%) 0.013 (89.09%) 0.088 (36.98%) 0.0003 (0.17%) 0.0026 (4.42%) 0.0029 (1.26%) 0.177 (99.83%) 0.058 (95.58%) 0.235 (98.74%) 0.102 (57.8%) 0.046 (75.09%) 0.148 (62.23%)
_ EN Ex D _ AV Ex D _ UN Ex D _ T Ex D RI (%)
j (%) _ R Ex D _ R Ex P _ R Ex F
Table 5 _ D in kW). Results of advanced exergy analysis for the cooling system using a LHTS unit during the day (Ex
Numerical analysis of the LHTS unit and evaluation of the thermal performance of the total system were carried out using COMSOL Multiphysics and EES software respectively. The numerical approach employed is the effective heat capacity method [25]. In this work, calcium chloride hexahydrate (CaCl2$6H2O) and R134a are selected as the PCM in the LHTS unit and working fluid in VCR cycle respectively. Thermophysical properties of the PCM are listed in Table 2. The LHTS unit specifications in Table 3 are used in the analyses. This system is able to meet the requirements for thermal comfort for the climate of Tehran, Iran (according to summer design conditions). Table 4 summarizes the assumptions for real and unavoidable operation conditions. The unavoidable values represent technological limitations such as availability, costs of materials, and manufacturing method that cannot be avoided [16,30]. In this case the time averaged heat transfer rate of the air during the day (cooling load) is 8.78 kW, the power consumption rates of the fan are 0.352 kW and 0.345 kW during day and night respectively, and the power consumption of the compressor is 2.161 kW during the night. Consequently, the system has a COP of 3.07 for a real
_ EX Ex D
5. Results and discussion
0.075 (42.2%) 0.015 (24.91%) 0.090 (37.77%)
conditions that are analyzed to calculate endogenous part of the exergy destruction for each component.
0.01 0.007 0.016 (6.88%)
hFan ¼ 75% [27], hel ¼ 98% [29] hCM ¼ 95% [17] DTmin, EV ¼ 0.5 K [17] DTmin, CD ¼ 0.5 K [17] DTLHTS ¼ 0.2 K
74.39 25.61 100
Unavoidable condition
49.66 13.87 2.32
hFan ¼ 65% [27], hel ¼ 80% [28] hCM ¼ 80% [17] DTmin, EV ¼ 10 K [17] DTmin, CD ¼ 10 K [17] DTLHTS ¼ 0.95 K (melting) DTLHTS ¼ 5.42 K (solidification)
_ EXUN Ex D
Fan CM EV CD LHTS
_ ENUN Ex D
Component Real condition
0.177 0.061 0.238
Table 4 Assumptions for real and unavoidable conditions.
0.175 0.01 0.01
_ EXAV Ex D
10 mm 5 mm 1.3 m 1.1 m 34 8h 1 kg/s
_ ENAV Ex D
Thickness of PCM slabs Thickness of air channels Length of PCM slabs Width of storage Number of PCM slabs Operating time during daytime Air mass flow rate
0.352 0.0708 0.423
Table 3 LHTS unit specifications used in the calculations.
0.00008 (0.08%) 0.00103 (2.26%) 0.00111 (0.47%)
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Fan LHTS Overall
6
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(25.08%)
0.001 0 0.330 0.339 0.338 0.301 1.309 (98.98%) (100%) (29.79%) (11.45%) (30.9%)
(6.04%) (13.9%) (10.65%) (18.33%) (0.81%)
(0.24%)
0.00018 0 0.0057 0.0019 0.0048 0.0046 0.017 (99.76%) (100%) (93.96%) (86.1%) (89.35%) (81.67%) (12.5%) (59.43%) (85.95%) (64.1%) (93.7%) (62.42%)
49.21 11.32 25.60 81.63 21.15 66.68 0.61
8.23 6.0 26.6 18.67 25.14 15 100
0.01 0.027 0.081 0 0.033 0 0.152 (7.15%)
0.074 0.029 0.095 0.014 0.046 0.025 0.283
(42.73%) (22.46%) (16.79%) (3.48%) (8.55%) (7.71%) (13.32%)
0.100 0.099 0.470 0.383 0.489 0.301 1.843
(57.27%) (77.54%) (83.21%) (96.52%) (91.45%) (92.29%) (86.68%)
0.174 0.128 0.229 0.055 0.192 0.020 0.799
(99.12%) (100%) (40.57%) (14.05%) (35.9%) (6.3%) (37.58%)
0.0012 0 0.336 0.341 0.342 0.306 1.327
(0.68%)
0.075 0.029 0.089 0.011 0.041 0.021 0.266
_ EXUN Ex D _ ENUN Ex D _ EX Ex D _ EN Ex D _ AV Ex D _ UN Ex D _ T Ex D RI (%)
j (%) _ R Ex D
0.175 0.128 0.565 0.397 0.534 0.326 2.126 0.169 0.016 0.195 1.764 0.143 0.653 0.016 0.345 0.144 0.760 2.161 0.678 0.979 2.649 Fan LHTS CD CM EV TV Overall
Fig. 7. Outdoor temperature, outlet air temperature and PCM temperature at the end of PCM slab profiles for the real process during the night (T 0 ¼ 25.62 C, T 1 ¼ 26.06 C, T 3 ¼ 25.2 C and T e ¼ 30.62 C).
_ R Ex P
Fig. 7 shows the outdoor air temperature entering the system, the actual outlet air temperature from the system and the PCM temperature at the end of the PCM slab in the direction of the air flow during the night. These results are obtained for a real process. Due to the high temperature of the outdoor air, a VCR cycle is employed to decrease the outdoor air temperature before it enters the LHTS unit to solidify the molten PCM. The VCR cycle decreases
_ R Ex F
5.2. Charging process
Table 6 _ D in kW). Results of advanced exergy analysis for a cooling system using a LHTS unit during the night (Ex
Fig. 6 shows the outdoor air temperature entering the system, the actual outlet air temperature and the PCM temperature at the end of the PCM slab in the direction of the air flow during the day. These results are obtained for the real process. In this case, the initial temperature of the solid PCM is assumed to be fixed at 10 C and the time averaged heat transfer rate to the PCM is 8.78 kW, _ P ) in all calculations which is held constant as the total product (Ex for real, theoretical and unavoidable processes. To keep total product constant, the mass of the PCM changes. The results obtained from the advanced exergy analysis of the system during the day are presented in Table 5. A conventional exergy analysis shows that the fan and the LHTS unit should be improved, because they exhibit the highest values of RI. Considering the information obtained from splitting the exergy destructions, the results show that, for the real process, 0.09 kW of the overall exergy destruction rate (33.77% of _ _ Ex D;overall ) is unavoidable and 0.235 kW (98.74% of ExD;overall ) is due only to irreversibilities within components themselves. Also, _ EN the value of Ex shows that, after all possible improvements, D;overall the system has an exergy destruction rate of 0.088 kW (36.98% of _ EN > > Ex _ EX for each component, which sug_ EN ). Moreover, Ex Ex D D D gests that the fan and the LHTS unit have little influence on each _ other's exergy destruction rates (i.e., 0.17% of Ex D;LHTS is due to the _ fan and 4.42% of ExD;Fan is due to the LHTS unit). In other words, improving each component itself is more important for decreasing the value of its exergy destruction rate rather than modifying other components. In order to improve the exergy efficiency of the LHTS unit, several methods have been proposed [31e33].
_ ENAV Ex D
5.1. Air cooling process
0.099 0.099 0.140 0.044 0.151 0 0.533
_ EXAV Ex D
process. This value is comparable to that for a conventional AC system working in the same conditions with a COP of 3.6, where, in case day/night electricity rates apply, the device consumes high-cost electricity during the day.
(70.21%) (88.55%) (69.1%) (100%) (61.6%)
7
(1.02%)
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A.H. Mosaffa et al. / Energy xxx (2014) 1e8 R (T 2
R
the air temperature by 10 K for the real process T 20 ¼ 10 K). Also, the secondary working fluid of the condenser is ambient air which is assumed to increase by 10 K as it passes through the condenser. In this case, we assume the initial temperature of the liquid PCM to be 36 C and the time averaged heat release rate from the PCM to be 8.78 kW, which is held constant as the total product _ P ) in all calculations for real, theoretical and unavoidable rate (Ex processes. To maintain the total product as a constant, the temperature of point 20 is adjusted. The results of the advanced exergy analysis of the system during the night are presented in Table 6. According to conventional exergy analysis, the following components should be improved in the given order (excluding the throttling valve) based on their high values of RI: condenser (26.6%), evaporator (25.14%), compressor (18.67%), fan (8.23%) and LHTS unit (6.0%). The results of splitting the exergy destruction rate show that for the real process, 0.283 kW _ of the exergy destruction rate (13.23% of Ex D;overall ) is unavoidable _ and 0.799 kW (37.58% of Ex ) is due only to irreversibilities D;overall within the components themselves. It can be observed that, after all possible improvements, the system exhibits an exergy destruction _ ENUN of 0.266 kW (12.5% of Ex _ EN ). rate Ex D D ENAV _ The value of ExD indicates that the exergy destruction rates for the evaporator and the condenser are important, and focusing improvement efforts on them is merited. Furthermore, the result _ EX thatEx D;LHTS ¼ 0 indicates that the total exergy destruction rate of the LHTS unit is due only to its irreversibilities. 6. Conclusions A combination of an LHTS unit and a VCR cycle is proposed as an air conditioning system in which the VCR cycle operates only during nighttime to solidify the molten PCM. An advanced exergy analysis is applied based on splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts for the system. The following conclusions are drawn from the analyses: In the air cooling process during the day, the relative irreversibility of the fan exceeds that of the LHTS unit. Also, 37.77% of the overall exergy destruction rate of the system is unavoidable and after all possible improvements the system has an exergy destruction rate of 0.088 kW (36.98% of the overall exergy destruction rate). The exergy destruction rates for each component are due to irreversibilities within the components _ EN =Ex _ D z1). themselves (Ex D For the charging process during the night, due to their exhibiting the highest values of RI, several components should be improved in the following priority order: condenser, evaporator, compressor, fan and LHTS unit. Also, 13.32% of the overall exergy destruction rate of the system is unavoidable and, after all possible improvements, the system has an exergy destruction rate of 0.266 kW (12.5% of the overall exergy destruction rate). _ EN _ Since Ex D;LHTS =ExD;LHTS ¼ 1, the total exergy destruction rate of the LHTS unit is due only to its irreversibilities. References [1] Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Convers Manage 2004;45: 1597e615. [2] Osterman E, Tyagi VV, Butala V, Rahim NA, Stritih U. Review of PCM based cooling technologies for buildings. Energy Build 2012;49:37e49. [3] Al-Abidi AA, Mat SB, Sopian K, Sulaiman MY, Lim CH, Th A. Review of thermal energy storage for air conditioning systems. Renew Sustain Energy Rev 2012;16:5802e19.
[4] Li G, Hwang Y, Radermacher R, Chun HH. Review of cold storage materials for subzero applications. Energy 2013;51:1e17. [5] Zhao D, Tan G. Experimental evaluation of a prototype thermoelectric system integrated with PCM (phase change material) for space cooling. Energy 2014;68:658e66. [6] Li G, Hwang Y, Radermacher R. Review of cold storage materials for air conditioning application. Int J Refrig 2012;35:2053e77. [7] Mosaffa AH, Infante Ferreira CA, Talati F, Rosen MA. Thermal performance of a multiple PCM thermal storage unit for free cooling. Energy Convers Manage 2013;67:1e7. [8] Mosaffa AH, Infante Ferreira CA, Rosen MA, Talati F. Thermal performance optimization of free cooling systems using enhanced latent heat thermal storage unit. Appl Therm Eng 2013;59:473e9. [9] Anisur MR, Kibria MA, Mahfuz MH, Saidur R, Metselaar IHSC. Analysis of a thermal energy storage system for air coolingeheating application through cylindrical tube. Energy Convers Manage 2013;76:732e7. [10] Rosen MA, Pedinelli N, Dincer I. Energy and exergy analyses of cold thermal storage systems. Int J Energy Res 1999;23:1029e38. [11] Mosaffa AH, Garousi Farshi L, Infante Ferreira CA, Rosen MA. Energy and exergy evaluation of a multiple-PCM thermal storage unit for free cooling applications. Renew Energy 2014;68:452e8. [12] Ezan MA, Ozdogan M, Gunerhan H, Erek A, Hepbasli A. Energetic and exergetic analysis and assessment of a thermal energy storage (TES) unit for building applications. Energy Build 2010;42:1896e901. [13] Tyagi VV, Pandey AK, Buddhi D, Tyagi SK. Exergy and energy analyses of two different types of PCM based thermal management systems for space air conditioning applications. Energy Convers Manage 2013;69:1e8. [14] Li G, Qian S, Lee H, Hwang Y, Radermacher R. Experimental investigation of energy and exergy performance of short term adsorption heat storage for residential application. Energy 2014;65:675e91. [15] Li G, Hwang Y, Radermacher R. Experimental investigation on energy and exergy performance of adsorption cold storage for space cooling application. Int J Refrig 2014;44:23e35. [16] Tsatsaronis G, Park MH. On avoidable and unavoidable exergy destructions and investment costs in thermal systems. Energy Convers Manage 2002;43: 1259e70. [17] Kelly S, Tsatsaronis G, Morosuk T. Advanced exergetic analysis: approaches for splitting the exergy destruction into endogenous and exogenous parts. Energy 2009;34:384e91. [18] Morosuk T, Tsatsaronis G. New approach to the exergy analysis of absorption refrigeration machines. Energy 2008;33:890e907. [19] Morosuk T, Tsatsaronis G. Advanced exergetic evaluation of refrigeration machines using different working fluids. Energy 2009;34:2248e58. [20] Zhang Y, Faghri A. Semi-analytical solution of thermal energy storage system with conjugate laminar forced convection. Int J Heat Mass Transf 1996;39: 717e24. [21] Laouadi A, Lacroix M. Thermal performance of a latent heat energy storage ventilated panel for electric load management. Int J Heat Mass Transf 1999;42:275e86. [22] Szargut J, Ziebik A, Stanek W. Depletion of the non-renewable natural resources as a measure of the ecological cost. Energy Convers Manage 2002;43: 1149e63. [23] Rosen MA, Dincer I. Exergy methods for assessing and comparing thermal storage systems. Int J Energy Res 2003;27:415e30. [24] Infante Ferreira CA. Refrigeration and heat pumping in food processing plants. In: Stougie L, editor. Energy efficiency and the quality of energy in the food processing industry. Delft: Interduct: Delft University of Technology; 2002. p. 57e79. [25] Chen C, Guo H, Liu Y, Yue H, Wang C. A new kind of phase change material (PCM) for energy-storing wallboard. Energy Build 2008;40:882e90. [26] Agyenim F, Hewitt N, Eames P, Smyth M. A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS). Renew Sustain Energy Rev 2010;14:615e28. [27] Cziesla F, Tsatsaronis G, Gao Z. Avoidable thermodynamic inefficiencies and costs in an externally fired combined cycle power plant. Energy 2006;31: 1472e89. [28] Dincer I, Rosen MA. Exergy: energy, environment and sustainable development. Amsterdam: Elsevier; 2007. [29] Petrakopoulou F, Tsatsaronis G, Morosuk T, Carassai A. Conventional and advanced exergetic analyses applied to a combined cycle power plant. Energy 2012;41:146e52. [30] Bejan A, Tsatsaronis G, Moran M. Thermal design and optimization. New York: Wiley; 1995. [31] Domanski R, Fellah G. Exergy analysis for the evaluation of a thermal storage system employing PCMs with different melting temperatures. Appl Therm Eng 1996;16:907e19. [32] Li Y, He Y, Wang Z, Xu C, Wang W. Exergy analysis of two phase change materials storage system for solar thermal power with finite-time thermodynamics. Renew Energy 2012;39:447e54. [33] Gong Z, Mujumdar AS. 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Advanced exergy analysis of an air conditioning system incorporating thermal energy storage A.H. Mosaffa a, *, L. Garousi Farshi b, C.A. Infante Ferreira c, M.A. Rosen d a
Department of Mechanical Engineering, Azarbaijan Shahid Madani University, Tabriz, Iran Faculty of Mechanical Engineering, University of Tabriz, Iran c Delft University of Technology, Department Process & Energy, Delft, 2628 CB, Netherlands d Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, ON, L1H 7K4, Canada b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 May 2014 Received in revised form 30 August 2014 Accepted 1 October 2014 Available online xxx
Air conditioning incorporating thermal storage, in which the cooling capacity needed for air conditioning is produced during the nighttime when the demand for air conditioning is small and then utilized to satisfy peak loads during the daytime, has received much attention recently. In this work, an air conditioning system consisting of a combination of LHTS (latent heat thermal storage) and VCR (vapor compression refrigeration) is analyzed with advanced exergy analysis. The analysis is performed based on splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts. The results show that all of the exergy destruction of the LHTS unit is endogenous, which indicates that the total exergy destruction of the LHTS unit is due only to its irreversibilities. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Latent heat thermal energy storage Advanced exergy analysis Cooling Exergy Exergy destruction
1. Introduction The demand for AC (air conditioning) normally peaks during the daytime, and air conditioners are used much less during the nighttime. If equipment is designed to match such demands, its availability factor is low. For this reason air conditioning incorporating thermal storage, which produces and stores the cooling capacity needed for air conditioning during nighttime when the demand for air conditioning is small and utilizes the stored cooling capacity during daytime hours, is now receiving much attention. By using thermal storage in this manner, large air conditioning systems that are sized proportional to peak loads during daytime hours become unnecessary, and electricity costs are significantly reduced. Moreover, as the system produces and stores cooling capacity by using inexpensive nighttime electricity (provided the electricity cost differs between peak and off-peak load periods), the overall operating cost is lowered (for those with time-of-use electricity pricing). Thermal energy storages in general and PCMs (phase change materials) in particular have been the subject of much research over the last 20 years [1e6].
* Corresponding author. Tel.: þ98 412 4327566. E-mail address: [email protected] (A.H. Mosaffa).
Analyses of energy quality and quantity are important for improving the efficiency of a thermodynamic system, and the conventional method for performance analysis of LHTS (latent heat thermal storage) systems is based on energy. Many energy analyses of LHTS have been reported. For instance, Mosaffa et al. [7] investigated the energy efficiency for free cooling LHTS systems using multiple PCMs inside flat slabs. Also, they performed a numerical investigation of performance enhancement measures for the same storage to improve energy storage effectiveness and COP (coefficient of performance) [8]. Anisur et al. [9] used a theoretical model to analyze a LHTS system with a cylindrical shell geometry for an air coolingeheating application. They concluded that the PCM container volume also needs to be considered in conjunction with COP for reducing the amount of PCM. However, energy analysis is insufficient for complete thermodynamic evaluation of LHTS systems because it does not take into account all relevant performance aspects. This inadequacy of energy analysis leads to the use of exergy analysis, a technique based on the second law of thermodynamics, in LHTS system assessments. Exergy analysis determines efficiencies which provide a true measure of how nearly actual efficiency approaches the ideal, and identifies more clearly than energy analysis the sources, causes and locations of thermodynamic losses. The thermodynamic performance of cold thermal storage systems was assessed using exergy and energy analyses by Rosen et al.
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Nomenclature cp COP _ Ex L m_ m Q_ Q_ RI s T T T0 Tm t _ W
specific heat at constant pressure (J/kg K) coefficient of performance exergy rate (W) latent heat (J/kg) mass flow rate (kg/s) mass (kg) heat rate (W) time averaged heat transfer heat (W) relative irreversibility specific entropy (J/kg K) temperature (K) time averaged temperature (K) thermodynamic averaged environment temperature (K) melting temperature (K) time (s) power (W)
Greek symbols Τ operating time (s) J exergy efficiency Subscripts CM compressor
[10]. Mosaffa et al. [11] performed energy and exergy analyses for a free cooling system using a LHTS unit with multiple PCMs. They showed that the increase in exergy efficiency due to reducing inlet air temperature is more significant than the effect of increasing the air flow rate during the cooling (discharging of LHTS) process. Ezan et al. [12] investigated energetically and exergetically a shell-and-tube LHTS unit during solidification and melting processes. Tyagi et al. [13] reported energy and exergy analyses of LHTS systems for space heating and cooling applications for two LHTS types: rectangular and balls filled with PCM. Li et al. [14] experimentally determined the energy and exergy performance of an adsorption storage system for residential applications. They showed that, as ambient temperature increases, the total energy storage density and energy efficiency increase, while the overall exergy efficiency decreases. Li et al. [15] presented energy and exergy analyses for an adsorption cold thermal energy storage system, for a space cooling application. The results show that the cold energy storage density and recovered exergy increase as the inlet temperature of the heat transfer fluid used in the adsorption system decreases. In the present work, advanced exergy analysis is applied to a cooling system using a LHTS unit which consists of several parallel layers of PCM slabs. A conventional exergy analysis identifies the components of the system with the highest exergy destructions and the processes that cause them. Then, the performance of a component of the system can be improved by reducing its exergy destruction. However, part of the exergy destruction may be unavoidable and part may be due to the exergy destruction occurring in other components (exogenous exergy destruction). This can be identified by performing an advanced exergy analysis. This type of analysis provides enhanced information and can show, for instance, if it is worthwhile to improve other components instead of only the component which has the highest exergy destruction. The LHTS unit cools air during the day and the absorbed heat is extracted during the night by employing an AC system (see Fig. 1).
CD D e el EV EX F i l P s TV
condenser destruction end of the PCM slab in the direction of the air flow electricity evaporator expander fuel initial liquid product solid throttling valve
Superscripts AV avoidable EN endogenous EX exogenous H hybrid R real T theoretical UN unavoidable Abbreviations AC air conditioning LHTS latent heat thermal storage PCM phase change material VCR vapor compression refrigeration
An advanced exergy analysis for the transient processes in the proposed system provides engineers with useful information related to energy system improvement potential. Splitting the exergy destruction within each component into unavoidable/ avoidable and endogenous/exogenous parts via advanced exergy analysis can provide meaningful results not obtained through conventional exergy analysis [16e19]. 2. Description of processes For the system considered, air cooling (PCM melting), releasing heat from LHTS unit (PCM solidification) and air-side heat transfer are taken to be unsteady two-dimensional problems. To develop a mathematical formulation of the melting and solidification processes, it is assumed that the PCM thermophysical properties are independent of temperature, but differ between solid and liquid phases. Further assumptions are that the air velocity profile is fully developed and, due to the negligible variation of container-wall temperature, temperature variations normal to the flow direction are neglected [20]. The effect of natural convection is negligible for this PCM storage geometry [21]. Fig. 2 illustrates the proposed air cooling system using a LHTS unit. Heat is extracted from the PCM by employing a VCR (vapor compression refrigeration) cycle during the night when condensation takes place at lower temperature and, in some regions, electricity is less expensive. The solidified PCM can be used to cool the air in the buildings during the hot hours of the day. 3. Conventional analyses 3.1. Energy analysis An energy analysis of an air cooling system using a LHTS unit has been reported in detail previously by the authors [7,8]. According to
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A.H. Mosaffa et al. / Energy xxx (2014) 1e8
3
Fig. 1. The cooling system employing LHTS. Air is cooled by a PCM in the thermal storage during daytime (left) and the heat is extracted from the thermal storage by employing an air conditioner during nighttime (right).
Fig. 2, the time averaged heat transfer rate in the LHTS can be expressed as follows:
1 Q_ PCM ¼ t
Zt
_ p T 3 T 20 air Q_ t dt ¼ mc
(1)
0
where t is operating time, T 20 is time averaged inlet air temperature to the LHTS unit, and T 3 is time averaged outlet air temperature from the LHTS unit (see Fig. 2). The time averaged temperature T is defined as follows:
T¼
1 t
Zt Tdt
(2)
0
Q_ PCM _ W
total
3.2. Exergy analysis The total exergy destruction rate can be determined with an exergy balance as follows:
_ P þ Ex _ D _ F ¼ Ex Ex
(4)
The exergy efficiency can be expressed as the ratio of total product exergy output rate to total exergy input (fuel) rate:
j¼
_ P Ex _ F Ex
(5)
The relative irreversibility RI (relative irreversibility) is evaluated as [22],
The COP of the system can be defined as:
COP ¼
_ where W total is the total power consumption during cycle operation (day and night), which consists of power consumptions by the fan and the compressor.
(3)
RI ¼
_ D Ex
_ Ex D;overall
(6)
Fig. 2. Schematic of the cooling system using a LHTS unit.
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A.H. Mosaffa et al. / Energy xxx (2014) 1e8
During the air cooling process (discharging), air transfers exergy _ _ to the PCM (Ex P;discharging ¼ Exair ), in which part of the exergy is stored. The exergy input causes the exergy change of the PCM. Accordingly, the exergy efficiency can be expressed as [23]:
jdischarging ¼
_ Ex air _ Fan _ ExPCM þ W
(7)
The net exergy change in the PCM for the discharging process can be expressed as [11],
1 _ Ex PCM ¼ t
(
Tm T þ ml L 1 0 Ti Tm " # T þ ml cp;l T l Tm T 0 ln l Tm " #) Ts þ ms cp;s T s Ti T 0 ln Ti ml cp;s ðTm Ti Þ T 0 ln
(8)
where Ti is the initial temperature of the solid PCM, Tm is the PCM melting temperature, and T s and T l are the temperatures of the solid and the liquid PCM respectively for t > 0. Also, T 0 is thermodynamic averaged environment temperature, defined as follows [24]:
T0 ¼
T3 T1 ln T 3 T 1
(9)
where T 1 and T 3 are the inlet and outlet time averaged air temperatures of the system respectively (see Fig. 2). The total exergy supply rate with the air can be written as follows:
1 _ Ex air ¼ t
Zt
m_ cp;air ðT3 T1 Þ T 0 ðs3 s1 Þ dt
(10)
0
During solidification of the PCM (charging), the exergy output _ _ rate from the PCM becomes available (Ex P;charging ¼ ExPCM ) and is gained by the air. The exergy efficiency of the charging process can be expressed as follows:
jcharging ¼
_ Ex PCM _ Fan þ W _ _ Exair þ W CM
Fig. 3. Simplified illustration of various processes for air cooling.
!
(11)
thermodynamic performance of a component through reducing its irreversibility. This splitting of exergy destruction can be expressed _ D ¼ Ex _ UN þ Ex _ AV [16]. as Ex D D Fig. 3 shows a simple diagram of real (consisting only of irreversible processes), theoretical (consisting only of reversible processes) and unavoidable thermodynamic processes during daytime for air cooling. In unavoidable processes, a considered component is taken to operate in an unavoidable condition while other components operate in theoretical conditions in order to calculate the unavoidable part of its exergy destruction. In Fig. 3, DTLHTS is the minimum temperature difference in the LHTS unit (T 3 T e ), where T e is the time averaged temperature at the end of the PCM slab in the direction of the air flow. In this case, the ambient air (point 1) flows through a large number of small parallel channels in the LHTS unit. The cooled air (point 3) is used to cool the interior of the building during the hot hours of the day. The real process is given as 1-2R-a-3. In this case, the fan has a real efficiency (hRFan ), the air exits from the fan with condition 2R, the minimum temperature difference in the LHTS unit has a real R value (DTLHTS ), and air exits the LHTS unit at condition 3. A theoretical process is used for splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts. The theoretical operation conditions for each component correspond to _ D /0. The theoretical process is given as 1-2T-b-3. The unavoidEx able process is based on the theoretical cycle but also considers irreversibilities caused by unavoidable factors. This process is given as 1-2UN-c-3. For calculating the endogenous part of the exergy destruction in each component of a real process, the following processes are analyzed, in each of which there is only one irreversible component:
The exergy destruction rate for the fan can be obtained from an exergy rate balance:
_ F;Fan Ex _ P;Fan ¼ W _ _ D;Fan ¼ Ex _ Ex _ Fan Ex Ex 2 1
(12)
4. Advanced exergy analysis An advanced exergy analysis allows for deeper understanding of the exergy destruction values obtained from an exergy analysis. It identifies the source of exergy destruction in a component. The total exergy destruction rate within each component is split into endogenous part (the exergy destruction rate that is obtained when all other system components are ideal and the component being considered operates with its real efficiency) and exogenous part, _ D ¼ Ex _ EN þ Ex _ EX [17]. It is thus possible to identify the that is, Ex D D impact of a component on the total exergy losses of the system. Moreover, splitting the exergy destruction into unavoidable (the exergy destruction rate that cannot be reduced due to technological limitations and cannot be overcome in the future) and avoidable parts provides an upper limit on the ability to enhance the
Fig. 4. Real, theoretical and hybrid cycles for calculating the endogenous part of the exergy destructions in the components of a VCR cycle.
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5
Fig. 5. Simplified illustration of various processes for extracting heat from a PCM with a VCR cycle.
_ EN ), 1-2R-b-3: The fan has a real efficiency (hR ), the air Fan (Ex D;Fan Fan exits the fan at condition 2R, the minimum temperature differT ence in the LHTS unit has a theoretical value (DTLHTS ) and the air exits the LHTS unit at condition 3. T _ EN LHTS (Ex D;LHTS ), 1-2 -a-3: The fan has a theoretical efficiency T (hFan ), the air exits the fan at condition 2T, the minimum temR perature difference in the LHTS unit has a real value (DTLHTS ) and the air exits the LHTS unit at condition 3. For calculating the unavoidable endogenous part of the exergy destruction in each component of the real process, the approach for calculating the endogenous part of the exergy destruction is used with the condition that each component operates in its unavoidable exergy destruction mode. The real, theoretical and hybrid (consisting of one irreversible process while all other processes are reversible) cycles of the VCR cycle are shown in Fig. 4, where the various conditions which are used to calculate different parts of the exergy destruction are illustrated. For instance, to obtain the endogenous exergy destruction of the condenser, other components operate at theoretical conditions while the condenser operates at the real condition. The saturated refrigerant vapor exits the evaporator after the T T theoretical process with DTmin; EV (point 4 ), enters an isentropic T compressor with hCM and is compressed to the real condenser
Table 1 Various conditions used to calculate endogenous part of exergy destruction rate for each component. _ D Ex
Condition
_ EN Ex D;Fan _ EN Ex D;LHTS _ EN Ex D;EV _ ExEN D;CM _ ExEN D;CD _ ExEN
1-2R-20 T-3T & 4T-5T-6T-7T 1-2T-20 T-3R & 4T-5T-6T-7T 1-2T-20 R-3T & 4R-5H-6T-7H 1-2T-20 T-3T & 4T-5*H-6T-7T 1-2T-20 T-3T & 4T-5**H-6R-7*H 1-2T-20 T-3T & 4T-5T-6T-7**H
D;TV
pressure (point 5**H). The real condenser pressure is obtained by R DTmin; CD and the saturated liquid exits the condenser at condition 6R. The throttling valve is replaced by an ideal expander for the theoretical process (point 7*H). A detailed advanced exergy analysis for a VCR cycle has been performed by Morosuk and Tsatsaronis [19]. Fig. 5 shows a simple diagram of real, theoretical and unavoidable thermodynamic processes during nighttime for extracting heat from a PCM by employing a VCR cycle. That is, a VCR cycle is employed for cooling the ambient air (point 1). Then the cooled air is used to extract heat from the molten PCM. For instance, to obtain the endogenous exergy destruction of the evaporator, other components operate at theoretical conditions while the evaporator operates at the real condition. In this case, the fan efficiency and the minimum temperature difference in the LHTS unit are hTFan and T DTLHTS respectively. The air is cooled by a real process in the R evaporator withDTmin; EV . Therefore the conditions for the air are 12T-20 R-3T. The saturated refrigerant vapor exits the evaporator at condition 4R and enters the isentropic compressor, which has an efficiency hTCM. The superheated vapor (point 5H) is condensed in the condenser, where the minimum temperature difference has a T value of that for a theoretical process (DTmin; ). Then saturated CD liquid enters the ideal expander and its pressure is decreased to the evaporator pressure (point 7H). Table 1 shows the various
Table 2 Thermophysical properties of the PCM [26]. Material Density (kg/m3) Heat capacity at constant pressure, cp (J/kg K) Thermal conductivity (W/m K) Latent heat of fusion, L (J/kg) Melting temperature, Tm (K)
CaCl2.6H2O 1530 (liquid) 1710 (solid) 1400 (liquid) 2200 (solid) 0.54 (liquid) 1.09 (solid) 190, 800 301e302
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Fig. 6. Outdoor temperature, outlet air temperature and PCM temperature at the end of PCM slab profiles for the real process during the day (T 0 ¼ 29.86 C, T 1 ¼ 34.11 C, T 3 ¼ 25.64 C and T e ¼ 24.69 C).
0.102 (99.92%) 0.044 (97.74%) 0.147 (61.76%) 0.0002 (0.29%) 0.0016 (10.91%) 0.0018 (0.79%) 0.074 (99.71%) 0.013 (89.09%) 0.088 (36.98%) 0.0003 (0.17%) 0.0026 (4.42%) 0.0029 (1.26%) 0.177 (99.83%) 0.058 (95.58%) 0.235 (98.74%) 0.102 (57.8%) 0.046 (75.09%) 0.148 (62.23%)
_ EN Ex D _ AV Ex D _ UN Ex D _ T Ex D RI (%)
j (%) _ R Ex D _ R Ex P _ R Ex F
Table 5 _ D in kW). Results of advanced exergy analysis for the cooling system using a LHTS unit during the day (Ex
Numerical analysis of the LHTS unit and evaluation of the thermal performance of the total system were carried out using COMSOL Multiphysics and EES software respectively. The numerical approach employed is the effective heat capacity method [25]. In this work, calcium chloride hexahydrate (CaCl2$6H2O) and R134a are selected as the PCM in the LHTS unit and working fluid in VCR cycle respectively. Thermophysical properties of the PCM are listed in Table 2. The LHTS unit specifications in Table 3 are used in the analyses. This system is able to meet the requirements for thermal comfort for the climate of Tehran, Iran (according to summer design conditions). Table 4 summarizes the assumptions for real and unavoidable operation conditions. The unavoidable values represent technological limitations such as availability, costs of materials, and manufacturing method that cannot be avoided [16,30]. In this case the time averaged heat transfer rate of the air during the day (cooling load) is 8.78 kW, the power consumption rates of the fan are 0.352 kW and 0.345 kW during day and night respectively, and the power consumption of the compressor is 2.161 kW during the night. Consequently, the system has a COP of 3.07 for a real
_ EX Ex D
5. Results and discussion
0.075 (42.2%) 0.015 (24.91%) 0.090 (37.77%)
conditions that are analyzed to calculate endogenous part of the exergy destruction for each component.
0.01 0.007 0.016 (6.88%)
hFan ¼ 75% [27], hel ¼ 98% [29] hCM ¼ 95% [17] DTmin, EV ¼ 0.5 K [17] DTmin, CD ¼ 0.5 K [17] DTLHTS ¼ 0.2 K
74.39 25.61 100
Unavoidable condition
49.66 13.87 2.32
hFan ¼ 65% [27], hel ¼ 80% [28] hCM ¼ 80% [17] DTmin, EV ¼ 10 K [17] DTmin, CD ¼ 10 K [17] DTLHTS ¼ 0.95 K (melting) DTLHTS ¼ 5.42 K (solidification)
_ EXUN Ex D
Fan CM EV CD LHTS
_ ENUN Ex D
Component Real condition
0.177 0.061 0.238
Table 4 Assumptions for real and unavoidable conditions.
0.175 0.01 0.01
_ EXAV Ex D
10 mm 5 mm 1.3 m 1.1 m 34 8h 1 kg/s
_ ENAV Ex D
Thickness of PCM slabs Thickness of air channels Length of PCM slabs Width of storage Number of PCM slabs Operating time during daytime Air mass flow rate
0.352 0.0708 0.423
Table 3 LHTS unit specifications used in the calculations.
0.00008 (0.08%) 0.00103 (2.26%) 0.00111 (0.47%)
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Fan LHTS Overall
6
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(25.08%)
0.001 0 0.330 0.339 0.338 0.301 1.309 (98.98%) (100%) (29.79%) (11.45%) (30.9%)
(6.04%) (13.9%) (10.65%) (18.33%) (0.81%)
(0.24%)
0.00018 0 0.0057 0.0019 0.0048 0.0046 0.017 (99.76%) (100%) (93.96%) (86.1%) (89.35%) (81.67%) (12.5%) (59.43%) (85.95%) (64.1%) (93.7%) (62.42%)
49.21 11.32 25.60 81.63 21.15 66.68 0.61
8.23 6.0 26.6 18.67 25.14 15 100
0.01 0.027 0.081 0 0.033 0 0.152 (7.15%)
0.074 0.029 0.095 0.014 0.046 0.025 0.283
(42.73%) (22.46%) (16.79%) (3.48%) (8.55%) (7.71%) (13.32%)
0.100 0.099 0.470 0.383 0.489 0.301 1.843
(57.27%) (77.54%) (83.21%) (96.52%) (91.45%) (92.29%) (86.68%)
0.174 0.128 0.229 0.055 0.192 0.020 0.799
(99.12%) (100%) (40.57%) (14.05%) (35.9%) (6.3%) (37.58%)
0.0012 0 0.336 0.341 0.342 0.306 1.327
(0.68%)
0.075 0.029 0.089 0.011 0.041 0.021 0.266
_ EXUN Ex D _ ENUN Ex D _ EX Ex D _ EN Ex D _ AV Ex D _ UN Ex D _ T Ex D RI (%)
j (%) _ R Ex D
0.175 0.128 0.565 0.397 0.534 0.326 2.126 0.169 0.016 0.195 1.764 0.143 0.653 0.016 0.345 0.144 0.760 2.161 0.678 0.979 2.649 Fan LHTS CD CM EV TV Overall
Fig. 7. Outdoor temperature, outlet air temperature and PCM temperature at the end of PCM slab profiles for the real process during the night (T 0 ¼ 25.62 C, T 1 ¼ 26.06 C, T 3 ¼ 25.2 C and T e ¼ 30.62 C).
_ R Ex P
Fig. 7 shows the outdoor air temperature entering the system, the actual outlet air temperature from the system and the PCM temperature at the end of the PCM slab in the direction of the air flow during the night. These results are obtained for a real process. Due to the high temperature of the outdoor air, a VCR cycle is employed to decrease the outdoor air temperature before it enters the LHTS unit to solidify the molten PCM. The VCR cycle decreases
_ R Ex F
5.2. Charging process
Table 6 _ D in kW). Results of advanced exergy analysis for a cooling system using a LHTS unit during the night (Ex
Fig. 6 shows the outdoor air temperature entering the system, the actual outlet air temperature and the PCM temperature at the end of the PCM slab in the direction of the air flow during the day. These results are obtained for the real process. In this case, the initial temperature of the solid PCM is assumed to be fixed at 10 C and the time averaged heat transfer rate to the PCM is 8.78 kW, _ P ) in all calculations which is held constant as the total product (Ex for real, theoretical and unavoidable processes. To keep total product constant, the mass of the PCM changes. The results obtained from the advanced exergy analysis of the system during the day are presented in Table 5. A conventional exergy analysis shows that the fan and the LHTS unit should be improved, because they exhibit the highest values of RI. Considering the information obtained from splitting the exergy destructions, the results show that, for the real process, 0.09 kW of the overall exergy destruction rate (33.77% of _ _ Ex D;overall ) is unavoidable and 0.235 kW (98.74% of ExD;overall ) is due only to irreversibilities within components themselves. Also, _ EN the value of Ex shows that, after all possible improvements, D;overall the system has an exergy destruction rate of 0.088 kW (36.98% of _ EN > > Ex _ EX for each component, which sug_ EN ). Moreover, Ex Ex D D D gests that the fan and the LHTS unit have little influence on each _ other's exergy destruction rates (i.e., 0.17% of Ex D;LHTS is due to the _ fan and 4.42% of ExD;Fan is due to the LHTS unit). In other words, improving each component itself is more important for decreasing the value of its exergy destruction rate rather than modifying other components. In order to improve the exergy efficiency of the LHTS unit, several methods have been proposed [31e33].
_ ENAV Ex D
5.1. Air cooling process
0.099 0.099 0.140 0.044 0.151 0 0.533
_ EXAV Ex D
process. This value is comparable to that for a conventional AC system working in the same conditions with a COP of 3.6, where, in case day/night electricity rates apply, the device consumes high-cost electricity during the day.
(70.21%) (88.55%) (69.1%) (100%) (61.6%)
7
(1.02%)
A.H. Mosaffa et al. / Energy xxx (2014) 1e8
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8
A.H. Mosaffa et al. / Energy xxx (2014) 1e8 R (T 2
R
the air temperature by 10 K for the real process T 20 ¼ 10 K). Also, the secondary working fluid of the condenser is ambient air which is assumed to increase by 10 K as it passes through the condenser. In this case, we assume the initial temperature of the liquid PCM to be 36 C and the time averaged heat release rate from the PCM to be 8.78 kW, which is held constant as the total product _ P ) in all calculations for real, theoretical and unavoidable rate (Ex processes. To maintain the total product as a constant, the temperature of point 20 is adjusted. The results of the advanced exergy analysis of the system during the night are presented in Table 6. According to conventional exergy analysis, the following components should be improved in the given order (excluding the throttling valve) based on their high values of RI: condenser (26.6%), evaporator (25.14%), compressor (18.67%), fan (8.23%) and LHTS unit (6.0%). The results of splitting the exergy destruction rate show that for the real process, 0.283 kW _ of the exergy destruction rate (13.23% of Ex D;overall ) is unavoidable _ and 0.799 kW (37.58% of Ex ) is due only to irreversibilities D;overall within the components themselves. It can be observed that, after all possible improvements, the system exhibits an exergy destruction _ ENUN of 0.266 kW (12.5% of Ex _ EN ). rate Ex D D ENAV _ The value of ExD indicates that the exergy destruction rates for the evaporator and the condenser are important, and focusing improvement efforts on them is merited. Furthermore, the result _ EX thatEx D;LHTS ¼ 0 indicates that the total exergy destruction rate of the LHTS unit is due only to its irreversibilities. 6. Conclusions A combination of an LHTS unit and a VCR cycle is proposed as an air conditioning system in which the VCR cycle operates only during nighttime to solidify the molten PCM. An advanced exergy analysis is applied based on splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts for the system. The following conclusions are drawn from the analyses: In the air cooling process during the day, the relative irreversibility of the fan exceeds that of the LHTS unit. Also, 37.77% of the overall exergy destruction rate of the system is unavoidable and after all possible improvements the system has an exergy destruction rate of 0.088 kW (36.98% of the overall exergy destruction rate). The exergy destruction rates for each component are due to irreversibilities within the components _ EN =Ex _ D z1). themselves (Ex D For the charging process during the night, due to their exhibiting the highest values of RI, several components should be improved in the following priority order: condenser, evaporator, compressor, fan and LHTS unit. Also, 13.32% of the overall exergy destruction rate of the system is unavoidable and, after all possible improvements, the system has an exergy destruction rate of 0.266 kW (12.5% of the overall exergy destruction rate). _ EN _ Since Ex D;LHTS =ExD;LHTS ¼ 1, the total exergy destruction rate of the LHTS unit is due only to its irreversibilities. References [1] Farid MM, Khudhair AM, Razack SAK, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Convers Manage 2004;45: 1597e615. [2] Osterman E, Tyagi VV, Butala V, Rahim NA, Stritih U. Review of PCM based cooling technologies for buildings. Energy Build 2012;49:37e49. [3] Al-Abidi AA, Mat SB, Sopian K, Sulaiman MY, Lim CH, Th A. Review of thermal energy storage for air conditioning systems. Renew Sustain Energy Rev 2012;16:5802e19.
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Please cite this article in press as: Mosaffa AH, et al., Advanced exergy analysis of an air conditioning system incorporating thermal energy storage, Energy (2014), http://dx.doi.org/10.1016/j.energy.2014.10.006