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Energy and exergy evaluation of a multiple-PCM thermal storage unit for free cooling applications
Renewable Energy 68 (2014) 452e458
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Energy and exergy evaluation of a multiple-PCM thermal storage unit for free cooling applications A.H. Mosaffa a, *, L. Garousi Farshi b, C.A. Infante Ferreira c, M.A. Rosen d a
Transport Phenomena Research Group, 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 6 July 2013 Accepted 10 February 2014 Available online
Latent heat thermal storage (LHTS) is a technology that can help to reduce energy consumption for cooling applications, where the cold is stored in phase change materials (PCMs). Free cooling is a concept developed for air conditioning applications, in which coolness is collected from ambient air during night and released into the room during the hottest hours of the day. In this work, energy and exergy analyses are performed for a free cooling system using a LHTS unit employing multiple PCMs. The effects of inlet air temperature and air flow rate on the performance of the system are investigated. It is observed that the increase in exergy efficiency due to reducing inlet air temperature is more significant than effect from increasing the air flow rate during the charging process. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage Phase change material Free cooling Exergy efficiency Multiple PCM
1. Introduction Thermal energy storage (TES) systems for heating and cooling capacity are often utilized in applications where the demand for energy and the economically most favorable energy supply are not coincident. Latent heat thermal storage (LHTS) in general and phase change materials (PCMs) in particular have been the focus of much research for the last 20 years. Numerous studies on LHTS applications have been reported [1e4]. PCMs have also been developed to store coolness for air conditioning applications. The heat is released from the PCM during the night and the molten PCM is used to cool the interior of the buildings during the hot hours of day. This concept is known as free cooling [5]. The discharging process is carried out during night when the ambient temperature is low compared to the room temperature. Heat is absorbed by the PCM when the room temperature rises above the comfort limit. Then, hot air which is to be cooled passes through the PCM storage unit and the PCM absorbs heat from the air and melts. The working principle of PCM based free cooling for buildings is shown in Fig. 1. Analyses of energy quality and quantity are important for improving the efficiency of a thermodynamic system. The energy
* Corresponding author. Tel.: þ98 412 4327566. E-mail addresses: [email protected], [email protected] H. Mosaffa). http://dx.doi.org/10.1016/j.renene.2014.02.025 0960-1481/Ó 2014 Elsevier Ltd. All rights reserved.
(A.
efficiency of a LHTS unit conventionally is used to measure its performance. The energy efficiency, however, is inadequate as an index because it does not take into account all relevant aspects of LHTS performance. Exergy analysis is a method that uses the conservation of mass and energy principles together with the second law of thermodynamics for the design of energy systems and efficiency improvement. Exergy analysis evaluates 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. Increasing application of exergy methods and recognition of its usefulness by those in industry, government and academia has been observed recently [7e10]. Through energy, exergy and sustainability analyses for latent, thermochemical and sensible thermal energy storages, Caliskan et al. [11] found that LHTS has the lowest exergy efficiency among them. The maximum exergy efficiency of the LHTS charging process was found to be 4.9%, for a dead-state temperature of 8 C. Rosen [12] described how energy and exergy analyses of TES systems are performed and demonstrated the usefulness of such analyses in providing insights into TES behavior and performance. Ezan et al. [13] investigated the energetic and exergetic performances of a shell-and-tube LHTS unit, concluding that the charging period exergy efficiency increases with increasing inlet temperature. During discharging, however, the irreversibility increases as the temperature difference between the melting temperature of the
A.H. Mosaffa et al. / Renewable Energy 68 (2014) 452e458
Nomenclature cp Ex h L _ m m p Q_ R s T T T0
specific heat at constant pressure (J/kg K) exergy (J) specific enthalpy (J/kg) latent heat (J/kg) mass flow rate (kg/s) mass of PCM pressure (Pa) heat rate (W) ideal gas constant (J/kg K) specific entropy (J/kg K) temperature (K) thermodynamic averaged temperature (K) thermodynamic averaged environment temperature (K)
PCM and the inlet temperature of the heat transfer fluid (HTF) increases, decreasing the exergy efficiency. Oztürk [14] applied energy and exergy analyses to evaluate the efficiency of a large-scale solar LHTS for greenhouse heating, demonstrating that the results of energy and exergy analyses differ significantly and that exergy analysis should be considered in the evaluation and comparison of a LHTS unit. Koca et al. [15] performed energy and exergy analyses for a flat plate solar collector with a PCM filled storage tank. The analyses, performed over three days in October, found the average net energy and exergy efficiencies to be 45% and 22%, respectively. Due to relatively low thermal conductivity of PCMs, many investigations have been performed to improve the heat transfer in LHTS [16]. An interesting technique for enhancing the thermal performance of LHTS units is the use of multiple PCMs [17,18], where the LHTS unit is packed with PCMs of different melting temperatures. Energy and exergy analyses by Gong and Mujumdar [19] of charging/discharging processes in a LHTS system using multiple PCMs demonstrated that the LHTS overall exergy efficiency varies with the number of PCMs used. Domanski and Fellah [20] evaluated the performance of LHTS units from a second law viewpoint, determining the overall exergy efficiency of a three-PCM system to be 74% higher than that of single PCM system. The main purpose of this study is to improve understanding of a free cooling system using a LHTS unit with multiple-PCMs, using energy and exergy analyses. This system consists of several parallel layers of PCM slabs, each containing two PCMs with different melting temperature and equal volumes, and a number of parallel channels through which air flows (Fig. 2). This heat exchanger is manufactured from plastic material plates. Also, an optimization is performed based on the energy and exergy efficiencies to find the optimum performance conditions for the free cooling unit proposed by Mosaffa et al. [21] to provide comfort conditions for the climate of Tabriz, Iran.
Fig. 1. Free cooling working principle [6].
Tm t V_ W _ W
453
melting temperature (K) time (s) flow rate (m3/h) total energy consumption (W h) power (W)
Greek symbols h efficiency Subscripts exe exergy i initial l liquid s solid
2. Mathematical formulation For the system studied, the charging (melting) and discharging (solidification) processes of the PCM and heat transfer in air are taken to be unsteady two-dimensional problems. To develop a mathematical formulation, several assumptions are made: 1. Temperature variations normal to the flow direction are neglected, in part due to the negligible variation of container wall temperature [22]. 2. Thermophysical properties of PCMs are independent of temperature, but differ between solid and liquid phases [23]. 3. The air velocity profile is fully developed, which is reasonable since the inlet air velocity profile has been shown to have little influence on the outlet air temperature [24]. 4. The effect of natural convection is negligible, which is justified in part because natural convection was shown to be significant for a PCM storage geometry similar to that in the present study only when the PCM slabs are in the vertical position and heated from the side [25]. Fig. 3 shows the computational domain and primary parameters used in the performance analysis. 2.1. Energy analysis The total energy consumption of the system Wfan, is the summation of the energy consumption of the fan during charging and discharging:
Wfan ¼
_ W fan t
charging
_ þ W fan t
discharging
(1)
_ where W fan is the power related to all friction losses (Dp), including pressure losses for flows in ducts, expansion and contraction cross
Fig. 2. Schematic of the LHTS unit.
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Fig. 3. Computational domain during charging: 1) liquid PCM with the mass of ml at T l and 2) solid PCM with the mass of ms at T s .
section changes of ducts, air filter and distribution equipment. That is,
_ W fan ¼
DpV_
.
hfan
(2)
Fig. 4. Comparison of experimental and numerical outlet air temperature with those obtained by present model.
The cooling load of the system, Q_ c , can be determined as follows:
Zt
ExPCM ¼ Q_ ðtÞdt
Q Q_ c ¼ stored ¼ 0 tcharging tcharging
(3)
The coefficient of performance (COP) of the system can be defined as:
Q COP ¼ stored Wfan
(4)
where Ti is the initial temperature of the solid PCM, Ts is the temperature of the solid PCM for t > 0 and Tl is the temperature of the liquid PCM. The total exergy supplied/extracted by the air can be written as follows:
Zt Exair ¼
2.2. Exergy analysis
_ cp;air ðTout Tin Þ T 0 ðsout sin Þ dt m
(8)
0
During charging, air transfers exergy to the PCM, in which part of the exergy is stored and the useful effect is the exergy change of the PCM. During discharging, the exergy output from the PCM becomes available and is gained by the air. Accordingly, the exergy efficiencies can be expressed as [26]:
hexe; charging ¼
h io n þ ml L 1 TTm0 ml cp;s ðTm Ti Þ T 0 ln TTmi h io n þ ml cp;l T l Tm T 0 ln TTml ( " #) (7) Ts þ ms cp;s T s Ti T 0 ln Ti
Total exergy supplied by the HTF Net exergy change in the PCMs þ Power input (5)
Net exergy change in the PCMs hexe; discharging ¼ Total exergy extracted by the HTF þ Power input (6) The net exergy change in each PCM can be expressed as (see Fig. 3)
where T is the thermodynamic averaged temperature and T 0 is defined as follows [27]:
T0 ¼
Tout Tin lnðTout =Tin Þ
(9)
and the specific entropy change can be calculated using
sout sin ¼ cp;air ln
Tout pout R ln Tin pin
(10)
The average exergy efficiency during phase change can be determined by integrating over the process:
hexe ¼
1 t
Zt
hexe dt
(11)
0
The operation of a LHTS unit is a cycle comprised of an energy Table 1 Thermal properties of selected PCMs [29]. Property
CaCl2$6H2O
RT25
Density (kg/m3)
1530 (liquid) 1710 (solid) 1400 (liquid) 2200 (solid) 0.54 (liquid) 1.09 (solid) 190,800 29
749 (liquid) 785 (solid) 1410 (liquid) 1800 (solid) 0.18 (liquid) 0.19 (solid) 232,000 26.6
Heat capacity at constant pressure, cp (J/kg K) Thermal conductivity (W/m K) Latent heat of fusion, L (J/kg) Melting temperature, Tm ( C)
Table 2 System specifications of the storage used in the calculations [21]. Thickness of PCM slabs Thickness of air channels Length of PCM slabs Width of storage Number of PCM slabs Operating time during daytime Efficiency of the fan Pressure drop for air filters and distribution equipment
10 mm 3.2 mm 1.3 m 0.5 m 80 8h 60% 180 Pa
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Table 3 Exergy efficiency (%) during charging process for the system using different PCMs after 2 h (V_ ¼ 1200 m3/h). Tinlet ( C)
32 34 36
Multiple-PCMs (CaCl2$6H2O þ RT25)
56.3 51.9 46.9
Single PCM CaCl2$6H2O
RT25
53.8 49.5 44.7
50.4 46.0 41.9
storage process followed by an energy removal process. The overall exergy efficiency of a PCM free cooling unit can thus be found:
hexe;overall ¼
Exair;supplied Total exergy supplied by the HTF ¼ Total exergy input to the system Wfan (12)
3. Results and discussion Theoretical analyses for the evaluation of the thermal performance of the system were carried out using COMSOL Multiphysics. The numerical approach employed is the effective heat capacity method. The effective heat capacity of the material is directly proportional to both the energy stored and extracted during phase change, and the specific heat capacity [28]. The PCMs considered in this investigation are calcium chloride hexahydrate (CaCl2$6H2O) and RT25; they are packed in the system in decreasing order of their melting points. Thermophysical properties of the PCMs are listed in Table 1. 3.1. Model validation Fig. 4 shows the air outlet temperature profiles predicted by Vakilaltojjar’s experimental model [30], Halawa et al. [31] and the
Fig. 6. Effect of inlet air temperature on the on the outlet air temperature during the day (V_ ¼ 1200 m3/h).
present numerical models and those obtained with CaCl2$6H2O for the melting process. The numerical models are seen to predict slightly higher values in the initial period of melting and lower values in the later stage of melting. Possible reasons for this are the impurity of the test PCM, which degrades its heat transfer performance, and the presence of air in the PCM solid before it melts [31]. The results predicted with the present model and experimental data nonetheless agree well.
3.2. Energy and exergy analyses of system The storage system specifications in Table 2 are used in the analyses. This system is able to meet the requirements for thermal comfort for the climate of Tabriz, Iran. According to summer design conditions, the maximum inlet air temperatures during the day and night are 36 C and 25 C, respectively.
Fig. 5. Air temperature profiles during passage through the LHTS unit (V_ ¼ 1200 m3/h).
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Fig. 7. Effect of air flow rate on the on the outlet air temperature during the day (Tinlet ¼ 36 C).
Fig. 10. Effect of inlet air temperature on the exergy efficiency during daytime (V_ ¼ 1200 m3/h).
Fig. 11. Effect of air flow rate on the exergy efficiency during daytime (Tinlet ¼ 36 C). Fig. 8. Effect of inlet air temperature on the heat transfer rate during the day (V_ ¼ 1200 m3/h).
It is informative to compare the exergy efficiencies of the system when it uses multiple-PCMs or a single PCM during charging (see Table 3). It can be observed that the exergy efficiency when using multiple-PCMs exceeds that for the single PCM system. Furthermore, in a single PCM system, by decreasing melting temperature of the PCM, the temperature difference between Tinlet and Tm increases. Consequently, the exergy destruction increases and exergy efficiency decreases.
Fig. 5 shows the variation of air temperature with both time and distance from the LHTS unit entrance. Air entering the system at a constant temperature of 36 C is cooled as it passes the air passages in the LHTS unit having initial temperature of 25 C. The variation of the air temperature is low near the middle of the storage, where the PCMs have been separated, due to the different heat transfer from the air to PCMs. In the middle of the storage, the temperature of the CaCl2,6H2O is high, preventing the air from cooling significantly. Figs. 6 and 7 illustrate the effect of the inlet air temperature and air flow rate respectively on the outlet air temperature during the day. A greater inlet air temperature or air flow rate results in higher heat transmission, causing the PCMs to melt more quickly and subsequently the heat transfer rate to decrease. Therefore, increasing the inlet air temperature results in a higher outlet air
Table 4 Results for the thermodynamics analyses considering the effects of inlet air temperature and air flow rate.
Fig. 9. Effect of air flow rate on the heat transfer rate during the day (Tinlet ¼ 36 C).
Tinlet ( C)
V_ (m3/h)
32 33 34 35 36 36 36 36 36
1200 1200 1200 1200 1200 800 1000 1400 1600
Q_ c (kW)
textract (h)
Wfan (kW h)
hexe;charging
hexe;discharging
(%)
(%)
2.534 2.966 3.395 3.821 4.234 2.960 3.578 4.803 5.049
8.83 9.88 11.43 12.2 12.51 9.76 11.14 13.13 14.79
3.809 4.118 4.573 4.780 4.890 3.576 4.220 5.371 6.188
57.0 55.4 53.9 52.5 50.8 48.2 49.0 53.0 53.8
59.1 59.6 59.9 60.4 60.9 60.7 60.8 61.0 61.2
A.H. Mosaffa et al. / Renewable Energy 68 (2014) 452e458
457
Fig. 12. Variation of a) the COP and b) overall exergy efficiency with inlet air temperature for various air flow rates.
temperature (Fig. 6). Also, when the air flow rate increases, the outlet air temperature curve is seen in Fig. 7 to shift up and lead to a higher outlet air temperature. Figs. 8 and 9 show the effect of the inlet air temperature and air flow rate respectively on the heat transfer rate in the LHTS unit during the day. The heat transfer rate increases because of the increase in inlet air temperature and air flow rate, permitting more heat to be absorbed by the PCMs. Moreover, the PCMs melt quickly when the heat transfer rate is high, resulting in a shorter melting time due to an increased heat transfer rate at higher inlet air temperatures or air flow rates. The effect of inlet air temperature and air flow rate on the exergy efficiency is shown in Figs. 10 and 11, respectively. Decreasing the temperature difference between the air and the PCMs decreases the irreversibility and hence increases the exergy efficiency, with time. Also, the exergy efficiency is seen to increase with decreasing inlet air temperature and increasing air flow rate. It can thus be concluded that decreasing the inlet air temperature or increasing the air flow rate leads to improved exergy efficiency, and that the air flow rate has a larger impact on the exergy efficiency compared to the inlet air temperature, due to the variation of Wfan. To find the optimum performance (in terms of maximum energy and exergy efficiencies) for the free cooling system described in Table 2, energy and exergy analyses were performed and the results are shown in Table 4. For the analysis, it is assumed that the difference between inlet air temperatures during day and night is constant for all cases. To solidify PCMs completely within a reasonable period, the air flow rate during the solidification process
has been chosen to be 1980 m3/h. As expected, the highest cooling load, Q_ c , is obtained at higher inlet air temperature and air flow rate. In addition, due to the increased cooling load and stored heat in the LHTS unit, the required time to extract the stored heat from the PCMs, textract, during night increases and hence the power consumption of the fan increases. Furthermore, the effect of decreasing the inlet air temperature on exergy efficiency is apparently more significant than increasing the air flow rate during charging. The results show that by decreasing the inlet air temperature from 36 to 32 C (11%), the exergy efficiency of the charging process increases 6.2%. In contrast, increasing the air flow rate from 800 to 1600 m3/h, the exergy efficiency of the charging process increases 5.6%. Fig. 12 illustrates the variation of the COP and overall exergy efficiency with inlet air temperature for different air flows. These calculations have been performed on the basis of equivalent air flow rate (1980 m3/h) during discharging to extract the stored heat in the LHTS unit. It is evident that the cooling load increases when inlet air temperature or air flow rate increase. Although the increase of the air flow rate leads to an increase of the power consumption, the COP increases with the air flow rate (Fig. 12a). Also, due to an increase in irreversibility when inlet air temperature increases, the overall exergy efficiency decreases (Fig. 12b). Nevertheless overall exergy efficiency reduces smoothly for higher inlet air temperatures. Fig. 13 shows the variation of outlet air temperature of the LHTS unit and the exergy efficiency for different times during the day of 14 August 2012. The outlet air temperature is seen to remain lower than 27 C during the entire operating time, indicating that this system can provide comfort at all times. Also, due to the increased temperature difference between inlet and outlet air temperatures, the heat transfer rate and hence irreversibilities increase. The variation of exergy efficiency during the charging process can be explained considering the temperature difference between inlet and outlet air temperatures. For this case, the free cooling LHTS unit has a COP of 7.00 and overall exergy efficiency of 15.15%. 4. Conclusions
Fig. 13. Outlet air temperature and exergy efficiency subject to the variable inlet air temperature during the peak summer day of 2012 (V_ ¼ 1200 m3/h, COP ¼ 7.00 and hexe,overall ¼ 15.15%).
Energy and exergy analyses are performed for a free cooling system using multiple PCM LHTS unit. The performance of a LHTS unit employing multiple PCMs is studied numerically using COMSOL Multiphysics based on the effective heat capacity method. The effects of the inlet air temperature and the air flow rate on the performance of the system are ascertained from the energy and exergy associated with the melting period of the system during daytime. From the thermodynamic analyses the following conclusions are drawn:
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1. The present numerical model, after validation using existing experimental data, permits predictions of adequate accuracy for engineering applications. 2. Higher inlet air temperature and air flow rate increase the heat transfer rate and shorten the charging time but increase the outlet air temperature and the amount of heat absorbed by the PCMs. 3. The exergy efficiency decreases with increasing inlet air temperature during charging, and during charging the effect of decreasing inlet air temperature on increasing exergy efficiency is more significant than increasing air flow rate. 4. Higher COPs are obtained for higher inlet air temperatures, while higher overall exergy efficiencies are obtained for lower inlet air temperatures.
References [1] Farid MM, Khudhair A, Razack S, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Convers Manag 2004;45:1597e 615. [2] Oró E, de Gracia A, Castell A, Farid MM, Cabeza LF. Review on phase change materials (PCMs) for cold thermal energy storage applications. Appl Energy 2012;99:513e33. [3] Waqas A, Din Z. Phase change material (PCM) storage for free cooling of buildings: a review. Renew Sustain Energy Rev 2013;18:607e25. [4] Gil A, Oró E, Peiró G, Álvarez S, Cabeza LF. Material selection and testing for thermal energy storage in solar cooling. Renew Energy 2013;57:366e71. [5] Butala V, Stritih U. Experimental investigation of PCM cold storage. Energy Build 2009;41:354e9. [6] 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. [7] Rosen MA, Pedinelli N, Dincer I. Energy and exergy analyses of cold thermal storage systems. Int J Energy Res 1999;23:1029e38. [8] Dincer I, Rosen MA. Exergy, energy, environment and sustainable development. 2nd ed. Amsterdam: Elsevier; 2013. [9] Rosen MA, Tang R, Dincer I. Effect of stratification on energy and exergy capacities in thermal storage systems. Int J Energy Res 2004;28:177e93. [10] Qi Li Y, Ling He Y, Feng 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. [11] Caliskan H, Dincer I, Hepbasli A. Thermodynamic analyses and assessments of various thermal energy storage systems for buildings. Energy Convers Manag 2012;62:109e22. [12] Rosen MA. The exergy of stratified thermal energy storages. Sol Energy 2001;71:173e85.
[13] 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. [14] Oztürk HH. Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating. Energy Convers Manag 2005;46:1523e42. [15] Koca A, Oztop HF, Koyun T, Varol Y. Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector. Renew Energy 2008;33:567e74. [16] Jegadheeswaran S, Pohekar SD. Performance enhancement in latent heat thermal storage system: a review. Renew Sustain Energy Rev 2009;13: 2225e44. [17] Cui H, Yuan X, Hou X. Thermal performance analysis for a heat receiver using multiple phase change materials. Appl Therm Eng 2003;23:2353e61. [18] Seeniraj RV, Narasimhan NL. Performance enhancement of a solar dynamic LHTS module having both fins and multiple PCMs. Sol Energy 2008;82: 535e42. [19] Gong ZX, Mujumdar AS. Thermodynamic optimization of the thermal process in energy storage using multiple phase change materials. Appl Therm Eng 1997;17:1067e83. [20] 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. [21] Mosaffa AH, Infante Ferreira CA, Talati F, Rosen MA. Thermal performance of a multiple PCM thermal storage unit for free cooling. Energy Convers Manag 2013;67:1e7. [22] 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. [23] Halawa E, Bruno F, Saman W. Numerical analysis of a PCM thermal storage system with varying wall temperature. Energy Convers Manag 2005;46: 2592e604. [24] Vakilaltojjar SM, Saman W. Analysis and modelling of a phase change storage system for air conditioning applications. Appl Therm Eng 2001;21:249e63. [25] 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. [26] Rosen MA, Dincer I. Exergy methods for assessing and comparing thermal storage systems. Int J Energy Res 2003;27:415e30. [27] 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. pp. 57e79. [28] 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. [29] 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. [30] Vakilaltojjar SM. Phase change thermal storage system for space heating and cooling. Ph.D. thesis. Adelaide: University of South Australia; 2000. [31] Halawa E, Saman W, Bruno F. A phase change processor method for solving a one-dimensional phase change problem with convection boundary. Renew Energy 2010;35:1688e95.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Energy and exergy evaluation of a multiple-PCM thermal storage unit for free cooling applications A.H. Mosaffa a, *, L. Garousi Farshi b, C.A. Infante Ferreira c, M.A. Rosen d a
Transport Phenomena Research Group, 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 6 July 2013 Accepted 10 February 2014 Available online
Latent heat thermal storage (LHTS) is a technology that can help to reduce energy consumption for cooling applications, where the cold is stored in phase change materials (PCMs). Free cooling is a concept developed for air conditioning applications, in which coolness is collected from ambient air during night and released into the room during the hottest hours of the day. In this work, energy and exergy analyses are performed for a free cooling system using a LHTS unit employing multiple PCMs. The effects of inlet air temperature and air flow rate on the performance of the system are investigated. It is observed that the increase in exergy efficiency due to reducing inlet air temperature is more significant than effect from increasing the air flow rate during the charging process. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Thermal energy storage Phase change material Free cooling Exergy efficiency Multiple PCM
1. Introduction Thermal energy storage (TES) systems for heating and cooling capacity are often utilized in applications where the demand for energy and the economically most favorable energy supply are not coincident. Latent heat thermal storage (LHTS) in general and phase change materials (PCMs) in particular have been the focus of much research for the last 20 years. Numerous studies on LHTS applications have been reported [1e4]. PCMs have also been developed to store coolness for air conditioning applications. The heat is released from the PCM during the night and the molten PCM is used to cool the interior of the buildings during the hot hours of day. This concept is known as free cooling [5]. The discharging process is carried out during night when the ambient temperature is low compared to the room temperature. Heat is absorbed by the PCM when the room temperature rises above the comfort limit. Then, hot air which is to be cooled passes through the PCM storage unit and the PCM absorbs heat from the air and melts. The working principle of PCM based free cooling for buildings is shown in Fig. 1. Analyses of energy quality and quantity are important for improving the efficiency of a thermodynamic system. The energy
* Corresponding author. Tel.: þ98 412 4327566. E-mail addresses: [email protected], [email protected] H. Mosaffa). http://dx.doi.org/10.1016/j.renene.2014.02.025 0960-1481/Ó 2014 Elsevier Ltd. All rights reserved.
(A.
efficiency of a LHTS unit conventionally is used to measure its performance. The energy efficiency, however, is inadequate as an index because it does not take into account all relevant aspects of LHTS performance. Exergy analysis is a method that uses the conservation of mass and energy principles together with the second law of thermodynamics for the design of energy systems and efficiency improvement. Exergy analysis evaluates 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. Increasing application of exergy methods and recognition of its usefulness by those in industry, government and academia has been observed recently [7e10]. Through energy, exergy and sustainability analyses for latent, thermochemical and sensible thermal energy storages, Caliskan et al. [11] found that LHTS has the lowest exergy efficiency among them. The maximum exergy efficiency of the LHTS charging process was found to be 4.9%, for a dead-state temperature of 8 C. Rosen [12] described how energy and exergy analyses of TES systems are performed and demonstrated the usefulness of such analyses in providing insights into TES behavior and performance. Ezan et al. [13] investigated the energetic and exergetic performances of a shell-and-tube LHTS unit, concluding that the charging period exergy efficiency increases with increasing inlet temperature. During discharging, however, the irreversibility increases as the temperature difference between the melting temperature of the
A.H. Mosaffa et al. / Renewable Energy 68 (2014) 452e458
Nomenclature cp Ex h L _ m m p Q_ R s T T T0
specific heat at constant pressure (J/kg K) exergy (J) specific enthalpy (J/kg) latent heat (J/kg) mass flow rate (kg/s) mass of PCM pressure (Pa) heat rate (W) ideal gas constant (J/kg K) specific entropy (J/kg K) temperature (K) thermodynamic averaged temperature (K) thermodynamic averaged environment temperature (K)
PCM and the inlet temperature of the heat transfer fluid (HTF) increases, decreasing the exergy efficiency. Oztürk [14] applied energy and exergy analyses to evaluate the efficiency of a large-scale solar LHTS for greenhouse heating, demonstrating that the results of energy and exergy analyses differ significantly and that exergy analysis should be considered in the evaluation and comparison of a LHTS unit. Koca et al. [15] performed energy and exergy analyses for a flat plate solar collector with a PCM filled storage tank. The analyses, performed over three days in October, found the average net energy and exergy efficiencies to be 45% and 22%, respectively. Due to relatively low thermal conductivity of PCMs, many investigations have been performed to improve the heat transfer in LHTS [16]. An interesting technique for enhancing the thermal performance of LHTS units is the use of multiple PCMs [17,18], where the LHTS unit is packed with PCMs of different melting temperatures. Energy and exergy analyses by Gong and Mujumdar [19] of charging/discharging processes in a LHTS system using multiple PCMs demonstrated that the LHTS overall exergy efficiency varies with the number of PCMs used. Domanski and Fellah [20] evaluated the performance of LHTS units from a second law viewpoint, determining the overall exergy efficiency of a three-PCM system to be 74% higher than that of single PCM system. The main purpose of this study is to improve understanding of a free cooling system using a LHTS unit with multiple-PCMs, using energy and exergy analyses. This system consists of several parallel layers of PCM slabs, each containing two PCMs with different melting temperature and equal volumes, and a number of parallel channels through which air flows (Fig. 2). This heat exchanger is manufactured from plastic material plates. Also, an optimization is performed based on the energy and exergy efficiencies to find the optimum performance conditions for the free cooling unit proposed by Mosaffa et al. [21] to provide comfort conditions for the climate of Tabriz, Iran.
Fig. 1. Free cooling working principle [6].
Tm t V_ W _ W
453
melting temperature (K) time (s) flow rate (m3/h) total energy consumption (W h) power (W)
Greek symbols h efficiency Subscripts exe exergy i initial l liquid s solid
2. Mathematical formulation For the system studied, the charging (melting) and discharging (solidification) processes of the PCM and heat transfer in air are taken to be unsteady two-dimensional problems. To develop a mathematical formulation, several assumptions are made: 1. Temperature variations normal to the flow direction are neglected, in part due to the negligible variation of container wall temperature [22]. 2. Thermophysical properties of PCMs are independent of temperature, but differ between solid and liquid phases [23]. 3. The air velocity profile is fully developed, which is reasonable since the inlet air velocity profile has been shown to have little influence on the outlet air temperature [24]. 4. The effect of natural convection is negligible, which is justified in part because natural convection was shown to be significant for a PCM storage geometry similar to that in the present study only when the PCM slabs are in the vertical position and heated from the side [25]. Fig. 3 shows the computational domain and primary parameters used in the performance analysis. 2.1. Energy analysis The total energy consumption of the system Wfan, is the summation of the energy consumption of the fan during charging and discharging:
Wfan ¼
_ W fan t
charging
_ þ W fan t
discharging
(1)
_ where W fan is the power related to all friction losses (Dp), including pressure losses for flows in ducts, expansion and contraction cross
Fig. 2. Schematic of the LHTS unit.
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Fig. 3. Computational domain during charging: 1) liquid PCM with the mass of ml at T l and 2) solid PCM with the mass of ms at T s .
section changes of ducts, air filter and distribution equipment. That is,
_ W fan ¼
DpV_
.
hfan
(2)
Fig. 4. Comparison of experimental and numerical outlet air temperature with those obtained by present model.
The cooling load of the system, Q_ c , can be determined as follows:
Zt
ExPCM ¼ Q_ ðtÞdt
Q Q_ c ¼ stored ¼ 0 tcharging tcharging
(3)
The coefficient of performance (COP) of the system can be defined as:
Q COP ¼ stored Wfan
(4)
where Ti is the initial temperature of the solid PCM, Ts is the temperature of the solid PCM for t > 0 and Tl is the temperature of the liquid PCM. The total exergy supplied/extracted by the air can be written as follows:
Zt Exair ¼
2.2. Exergy analysis
_ cp;air ðTout Tin Þ T 0 ðsout sin Þ dt m
(8)
0
During charging, air transfers exergy to the PCM, in which part of the exergy is stored and the useful effect is the exergy change of the PCM. During discharging, the exergy output from the PCM becomes available and is gained by the air. Accordingly, the exergy efficiencies can be expressed as [26]:
hexe; charging ¼
h io n þ ml L 1 TTm0 ml cp;s ðTm Ti Þ T 0 ln TTmi h io n þ ml cp;l T l Tm T 0 ln TTml ( " #) (7) Ts þ ms cp;s T s Ti T 0 ln Ti
Total exergy supplied by the HTF Net exergy change in the PCMs þ Power input (5)
Net exergy change in the PCMs hexe; discharging ¼ Total exergy extracted by the HTF þ Power input (6) The net exergy change in each PCM can be expressed as (see Fig. 3)
where T is the thermodynamic averaged temperature and T 0 is defined as follows [27]:
T0 ¼
Tout Tin lnðTout =Tin Þ
(9)
and the specific entropy change can be calculated using
sout sin ¼ cp;air ln
Tout pout R ln Tin pin
(10)
The average exergy efficiency during phase change can be determined by integrating over the process:
hexe ¼
1 t
Zt
hexe dt
(11)
0
The operation of a LHTS unit is a cycle comprised of an energy Table 1 Thermal properties of selected PCMs [29]. Property
CaCl2$6H2O
RT25
Density (kg/m3)
1530 (liquid) 1710 (solid) 1400 (liquid) 2200 (solid) 0.54 (liquid) 1.09 (solid) 190,800 29
749 (liquid) 785 (solid) 1410 (liquid) 1800 (solid) 0.18 (liquid) 0.19 (solid) 232,000 26.6
Heat capacity at constant pressure, cp (J/kg K) Thermal conductivity (W/m K) Latent heat of fusion, L (J/kg) Melting temperature, Tm ( C)
Table 2 System specifications of the storage used in the calculations [21]. Thickness of PCM slabs Thickness of air channels Length of PCM slabs Width of storage Number of PCM slabs Operating time during daytime Efficiency of the fan Pressure drop for air filters and distribution equipment
10 mm 3.2 mm 1.3 m 0.5 m 80 8h 60% 180 Pa
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Table 3 Exergy efficiency (%) during charging process for the system using different PCMs after 2 h (V_ ¼ 1200 m3/h). Tinlet ( C)
32 34 36
Multiple-PCMs (CaCl2$6H2O þ RT25)
56.3 51.9 46.9
Single PCM CaCl2$6H2O
RT25
53.8 49.5 44.7
50.4 46.0 41.9
storage process followed by an energy removal process. The overall exergy efficiency of a PCM free cooling unit can thus be found:
hexe;overall ¼
Exair;supplied Total exergy supplied by the HTF ¼ Total exergy input to the system Wfan (12)
3. Results and discussion Theoretical analyses for the evaluation of the thermal performance of the system were carried out using COMSOL Multiphysics. The numerical approach employed is the effective heat capacity method. The effective heat capacity of the material is directly proportional to both the energy stored and extracted during phase change, and the specific heat capacity [28]. The PCMs considered in this investigation are calcium chloride hexahydrate (CaCl2$6H2O) and RT25; they are packed in the system in decreasing order of their melting points. Thermophysical properties of the PCMs are listed in Table 1. 3.1. Model validation Fig. 4 shows the air outlet temperature profiles predicted by Vakilaltojjar’s experimental model [30], Halawa et al. [31] and the
Fig. 6. Effect of inlet air temperature on the on the outlet air temperature during the day (V_ ¼ 1200 m3/h).
present numerical models and those obtained with CaCl2$6H2O for the melting process. The numerical models are seen to predict slightly higher values in the initial period of melting and lower values in the later stage of melting. Possible reasons for this are the impurity of the test PCM, which degrades its heat transfer performance, and the presence of air in the PCM solid before it melts [31]. The results predicted with the present model and experimental data nonetheless agree well.
3.2. Energy and exergy analyses of system The storage system specifications in Table 2 are used in the analyses. This system is able to meet the requirements for thermal comfort for the climate of Tabriz, Iran. According to summer design conditions, the maximum inlet air temperatures during the day and night are 36 C and 25 C, respectively.
Fig. 5. Air temperature profiles during passage through the LHTS unit (V_ ¼ 1200 m3/h).
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Fig. 7. Effect of air flow rate on the on the outlet air temperature during the day (Tinlet ¼ 36 C).
Fig. 10. Effect of inlet air temperature on the exergy efficiency during daytime (V_ ¼ 1200 m3/h).
Fig. 11. Effect of air flow rate on the exergy efficiency during daytime (Tinlet ¼ 36 C). Fig. 8. Effect of inlet air temperature on the heat transfer rate during the day (V_ ¼ 1200 m3/h).
It is informative to compare the exergy efficiencies of the system when it uses multiple-PCMs or a single PCM during charging (see Table 3). It can be observed that the exergy efficiency when using multiple-PCMs exceeds that for the single PCM system. Furthermore, in a single PCM system, by decreasing melting temperature of the PCM, the temperature difference between Tinlet and Tm increases. Consequently, the exergy destruction increases and exergy efficiency decreases.
Fig. 5 shows the variation of air temperature with both time and distance from the LHTS unit entrance. Air entering the system at a constant temperature of 36 C is cooled as it passes the air passages in the LHTS unit having initial temperature of 25 C. The variation of the air temperature is low near the middle of the storage, where the PCMs have been separated, due to the different heat transfer from the air to PCMs. In the middle of the storage, the temperature of the CaCl2,6H2O is high, preventing the air from cooling significantly. Figs. 6 and 7 illustrate the effect of the inlet air temperature and air flow rate respectively on the outlet air temperature during the day. A greater inlet air temperature or air flow rate results in higher heat transmission, causing the PCMs to melt more quickly and subsequently the heat transfer rate to decrease. Therefore, increasing the inlet air temperature results in a higher outlet air
Table 4 Results for the thermodynamics analyses considering the effects of inlet air temperature and air flow rate.
Fig. 9. Effect of air flow rate on the heat transfer rate during the day (Tinlet ¼ 36 C).
Tinlet ( C)
V_ (m3/h)
32 33 34 35 36 36 36 36 36
1200 1200 1200 1200 1200 800 1000 1400 1600
Q_ c (kW)
textract (h)
Wfan (kW h)
hexe;charging
hexe;discharging
(%)
(%)
2.534 2.966 3.395 3.821 4.234 2.960 3.578 4.803 5.049
8.83 9.88 11.43 12.2 12.51 9.76 11.14 13.13 14.79
3.809 4.118 4.573 4.780 4.890 3.576 4.220 5.371 6.188
57.0 55.4 53.9 52.5 50.8 48.2 49.0 53.0 53.8
59.1 59.6 59.9 60.4 60.9 60.7 60.8 61.0 61.2
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Fig. 12. Variation of a) the COP and b) overall exergy efficiency with inlet air temperature for various air flow rates.
temperature (Fig. 6). Also, when the air flow rate increases, the outlet air temperature curve is seen in Fig. 7 to shift up and lead to a higher outlet air temperature. Figs. 8 and 9 show the effect of the inlet air temperature and air flow rate respectively on the heat transfer rate in the LHTS unit during the day. The heat transfer rate increases because of the increase in inlet air temperature and air flow rate, permitting more heat to be absorbed by the PCMs. Moreover, the PCMs melt quickly when the heat transfer rate is high, resulting in a shorter melting time due to an increased heat transfer rate at higher inlet air temperatures or air flow rates. The effect of inlet air temperature and air flow rate on the exergy efficiency is shown in Figs. 10 and 11, respectively. Decreasing the temperature difference between the air and the PCMs decreases the irreversibility and hence increases the exergy efficiency, with time. Also, the exergy efficiency is seen to increase with decreasing inlet air temperature and increasing air flow rate. It can thus be concluded that decreasing the inlet air temperature or increasing the air flow rate leads to improved exergy efficiency, and that the air flow rate has a larger impact on the exergy efficiency compared to the inlet air temperature, due to the variation of Wfan. To find the optimum performance (in terms of maximum energy and exergy efficiencies) for the free cooling system described in Table 2, energy and exergy analyses were performed and the results are shown in Table 4. For the analysis, it is assumed that the difference between inlet air temperatures during day and night is constant for all cases. To solidify PCMs completely within a reasonable period, the air flow rate during the solidification process
has been chosen to be 1980 m3/h. As expected, the highest cooling load, Q_ c , is obtained at higher inlet air temperature and air flow rate. In addition, due to the increased cooling load and stored heat in the LHTS unit, the required time to extract the stored heat from the PCMs, textract, during night increases and hence the power consumption of the fan increases. Furthermore, the effect of decreasing the inlet air temperature on exergy efficiency is apparently more significant than increasing the air flow rate during charging. The results show that by decreasing the inlet air temperature from 36 to 32 C (11%), the exergy efficiency of the charging process increases 6.2%. In contrast, increasing the air flow rate from 800 to 1600 m3/h, the exergy efficiency of the charging process increases 5.6%. Fig. 12 illustrates the variation of the COP and overall exergy efficiency with inlet air temperature for different air flows. These calculations have been performed on the basis of equivalent air flow rate (1980 m3/h) during discharging to extract the stored heat in the LHTS unit. It is evident that the cooling load increases when inlet air temperature or air flow rate increase. Although the increase of the air flow rate leads to an increase of the power consumption, the COP increases with the air flow rate (Fig. 12a). Also, due to an increase in irreversibility when inlet air temperature increases, the overall exergy efficiency decreases (Fig. 12b). Nevertheless overall exergy efficiency reduces smoothly for higher inlet air temperatures. Fig. 13 shows the variation of outlet air temperature of the LHTS unit and the exergy efficiency for different times during the day of 14 August 2012. The outlet air temperature is seen to remain lower than 27 C during the entire operating time, indicating that this system can provide comfort at all times. Also, due to the increased temperature difference between inlet and outlet air temperatures, the heat transfer rate and hence irreversibilities increase. The variation of exergy efficiency during the charging process can be explained considering the temperature difference between inlet and outlet air temperatures. For this case, the free cooling LHTS unit has a COP of 7.00 and overall exergy efficiency of 15.15%. 4. Conclusions
Fig. 13. Outlet air temperature and exergy efficiency subject to the variable inlet air temperature during the peak summer day of 2012 (V_ ¼ 1200 m3/h, COP ¼ 7.00 and hexe,overall ¼ 15.15%).
Energy and exergy analyses are performed for a free cooling system using multiple PCM LHTS unit. The performance of a LHTS unit employing multiple PCMs is studied numerically using COMSOL Multiphysics based on the effective heat capacity method. The effects of the inlet air temperature and the air flow rate on the performance of the system are ascertained from the energy and exergy associated with the melting period of the system during daytime. From the thermodynamic analyses the following conclusions are drawn:
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1. The present numerical model, after validation using existing experimental data, permits predictions of adequate accuracy for engineering applications. 2. Higher inlet air temperature and air flow rate increase the heat transfer rate and shorten the charging time but increase the outlet air temperature and the amount of heat absorbed by the PCMs. 3. The exergy efficiency decreases with increasing inlet air temperature during charging, and during charging the effect of decreasing inlet air temperature on increasing exergy efficiency is more significant than increasing air flow rate. 4. Higher COPs are obtained for higher inlet air temperatures, while higher overall exergy efficiencies are obtained for lower inlet air temperatures.
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