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Performance investigation of thermal energy storage system with Phase Change Material (PCM) for solar water heating application
International Communications in Heat and Mass Transfer 57 (2014) 132–139
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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Performance investigation of thermal energy storage system with Phase Change Material (PCM) for solar water heating application☆ M.H. Mahfuz, M.R. Anisur, M.A. Kibria, R. Saidur ⁎, I.H.S.C. Metselaar Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Available online 30 July 2014 Keywords: PCM Thermal energy storage Solar water heater Exergy and energy analyses Thermoeconomics
a b s t r a c t In order to harvest solar energy, thermal energy storage (TES) system with Phase Change Material (PCM) has been receiving greater attention because of its large energy storage capacity and isothermal behavior during charging and discharging processes. In the present experimental study, shell and tube TES system using paraffin wax was used in a water heating system to analyze its performance for solar water heating application. Energy and exergy including their cost analyses for the TES system were performed. Accordingly, total life cycle cost was calculated for different flow rates of the Heat Transfer Fluid (HTF). With 0.033 kg/min and 0.167 kg/min flow rates of water as HTF, energy efficiencies experienced were 63.88% and 77.41%, respectively, but in exergy analysis, efficiencies were observed to be about 9.58% and 6.02%, respectively. Besides, the total life cycle cost was predicted to be $ 654.61 for 0.033 kg/min flow rate, which could be reduced to $ 609.22 by increasing the flow rate to 0.167 kg/min. Therefore it can be summarized that total life cycle cost decreases with the increase of flow rate. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction One of the classic devices which can be used to convert the solar energy into thermal energy is solar water heater (SWH). SWH applications largely depend on efficient thermal energy storage (TES). Thermal storage works like a thermal battery where, energy can be stored in the form of latent heat, sensible heat, or both. In this context, TES system with Phase Change Material (PCM) can be more potential for SWH application, as they have high energy density compared with other materials which are used to store only sensible heat [1]. Besides, it also improves the reliability and performance of the SWH system. In literature, there are many researches available on SWH system using TES. Khalifa et al. [2] conducted an experiment to calculate the performance of a flat plate solar collector with a back layer of wax as thermal energy storage. Souliotis et al. [3] studied on solar water heater integrated with collector and storage. They designed and analyzed experimentally the integrated system. Kousksou et al. [4] analyzed the uses of PCMs in solar based Domestic Hot Water (DHW) system. Their model described the heat storage tank with PCM, auxiliary heater, collector, pump, and controller. Al-Hinti et al. [5] also studied experimentally solar water heating system with PCM containers packed in a storage tank. Canbazoglu et al. [6] experimentally investigated passive solar water heating system with sodium thiosulfate pentahydrate as PCM storage. They compared the energy storage performance of PCM ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (R. Saidur).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.07.022 0735-1933/© 2014 Elsevier Ltd. All rights reserved.
storage to the conventional system and found approximately 2.59–3.45 times total accumulated heat. Mehling et al. [7] added PCM module at the top of the solar water tank heating system with stratification. The experiments and simulations showed that energy density could be improved by 20 to 45%. Shabgard et al. [8] developed a network model to predict the performance of thermal storage system including cascaded PCMs and embedded heat pipes. The concept of energy and exergy analyses is important for this performance analysis and also for economic analysis of thermal energy storage systems [9]. Without considering the concept of energy and exergy, it will lead to inappropriate system costing [10]. Exergy concept helps engineers to make logical decisions about the prices, real profits and efficiencies. Jegadheeswaran et al. [11] reviewed PCM storage system performance on the basis of exergy. They presented different procedures to analyze the exergy based performance. Jegadheeswaran and Pohekar [12] studied numerically to investigate the performance enhancement of a shell and tube LHTES unit dispersing high conductive particles in the PCM during charging process (melting). They concluded that in dispersing high conductive material in PCM, the performance could be improved significantly. Koca et al. [13] analyzed the energy and exergy of PCM storage for a flat plate solar collector. They used CaCl2·6H2O as PCM in the latent heat storage tank. The designed collector was a single unit integrated with storage. Energy and exergy analyses were done to measure the performance of the system during charging period. For the selected design, the study showed average energy and exergy efficiencies to be 45% and 2.2%, respectively. Kousksou et al. [14] developed a theoretical model for optimizing the solar air heating system with PCM storage. Energy and exergy analyses were carried out numerically to understand the system
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139
et al. [15] investigated the price of energyand exergy for the solar heating system. The life cycle cost for different PCM types was calculated. They concluded that in choosing the suitable PCM, it is more significant to use the exergy concept. Xiang et al. [16] studied dynamic exergy and exergetic cost of a heating system with ground water source heat pumps. They used structural theory of thermoeconomics. Furthermore, Caliskan et al. [17] conducted exergetic and thermoeconomic analyses of TES for applications in building heating system. They used various reference temperatures for their study. Different reference temperatures affect the thermoeconomic parameters. They concluded that the exergetic cost would become higher at the higher reference condition. Buonomano et al. [18] investigated different control strategies for the thermal storage management. They proposed economic model to assess the operating and capital costs of the systems. Robak et al. [19] proposed the concept of embedded gravity assisted wickless heat pipes within a latent heat storage system. They also analyzed the economic viability by comparing proposed design with sensible heat energy storage system. Results showed that the proposed latent heat storage has the potential to reduce capital costs around 15% compared to sensible heat storage. Tozer et al. [20] and Lozano & Valero [21] investigated thermoeconomic analysis. Both of the researches were led to the development of “Theory of the exergetic cost”. They applied this theory in cost allocation, operation optimization and economic optimization of various thermal energy systems. Most of the researchers proposed TES integrated with solar collector, which needed to change the total collector setup [2–4,22]. In addition, the heat transfer analysis on shell and tube TES system was reported by a few studies [23–27]. Tabassum [28] demonstrated numerical analyses of heat transfer in shell and tube TES system for solar water heating application. To the best of the author's knowledge, no study was conducted on exergy and cost analyses considering the shell and tube LHTS system for solar water heating application. Hence, energy and exergy analyses of the TES system for SWH application were performed in the present study. Moreover, thermoeconomic analysis of the TES system was done with different flow rates of the HTF.
Nomenclature Symbols C Ct E H m˙ T t X
133
specific heat, J/g °C cost at time t, RM energy, kJ latent heat, J/g mass flow rate, kg/s temperature, K time, s exergy, kJ
Greek symbols η energy efficiency ε exergy efficiency
Subscripts charging charging period discharging discharging period HTF Heat Transfer Fluid in inlet melt melting o environment out outlet overall overall store stored
behavior. They concluded that the performance of the system largely depended on the selection of the PCM. The exergy concept is not only essential for efficiency analysis but also a significant tool in economic analysis and cost accounting. Rezaei
SUN Flow separation valve
Cold water inlet
Valve 1
Thermal Energy Storage filled with PCM Water Storage tank
Valve 2
Pump
Backup electric heater Charging loop
Discharging loop
Fig. 1. Proposed water heating system with the thermal storage arrangement.
Hot water outlet
134
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Table 1 Thermophysical properties of paraffin wax.
Output
Node 4
Node 3
Node 2
Node 1
Input
Flow meter
Fig. 2. Latent heat thermal energy storage.
2. Experimental procedure 2.1. System description Fig. 1 presents a proposed solar water heating system with the thermal energy storage arrangement. The proposed system has three main components: a) a solar collector unit, b) a double pipe thermal energy storageand c) a well-insulated water storage tank. In this present study only the thermal energy storage unit was studied experimentally. In this proposed system as illustrated in Fig. 1, charging of the TES is done during the day time when the solar radiation is present. During the charging period or day time, valve 1 is kept open while valve 2 is remained closed. The cold water passes through the solar collector. Water takes heat from the solar radiation and part of the hot water 30
Peak = 61.12°C
Heat flow endo up, mW
Delta H = 200.74 J/g 25
Onset=56.06°C
End = 64.00 °C
20
Onset=58.93°C End=47.51°C
15
Delta H = -195.96 J/g
Peak=55.70°C 10 30
40
50
60
70
Sample temperature, °C Fig. 3. DSC curve of paraffin wax for heating and cooling.
80
Property
Values
Melting point Melting end point Freezing start point Freezing end point Melting peak temperature Freezing peak temperature Specific heat of solid (b30 °C) Specific heat of liquid (N65 °C) Melting latent heat Freezing latent heat
56.06 °C 64.99 °C 58.93 °C 47.51 °C 61.12 °C 55.70 °C 2.565 J/g °C 2.439 J/g °C 200.74 J/g 195.97 J/g
flows through the heat storage unit for charging the PCM. The rest of the water goes directly to the water storage tank. At the time of discharge during night time or in the absence of solar radiation, valve 1 is needed to be closed as it is not necessary to circulate water through the collector and valve 2 is opened to pass the water directly through the TES unit for discharging heat to water. Simultaneously, the PCM starts to be solidified and being fully solidified, this PCM can be used for charging again. This process is continued until the ends of the PCM thermal life cycle. In order to maintain a constant output temperature, an electric heater is also attached to the water storage tank. The electric heater is essential to maintain the preferred temperature for comfortable use. Using a thermostat in the heater, the power consumption and temperature both can be controlled. In the present study, a vertical arrangement of a latent heat TES unit as shown in Fig. 2 was examined. Paraffin wax was poured into the annular space between the copper tube and the outer shell. This TES unit was connected with a thermal bath. The input temperature of 90 °C was selected as the same output from solar collector system [29]. This simulated the charging period of the thermal storage. Charging was continued to reach the PCM heat capacity so that the difference of input and output temperatures became the same. After the charging period, the hot water was drained out and then the thermal bath was filled with ambient temperature water. The discharging cycle began with the flow of water at a temperature around 25 °C. The discharging cycle ended when the output temperature came to be identical to the environment temperature.
2.2. PCM selection The PCMs having a suitable melting temperature should be selected for appropriate application. PCMs with high latent heat, high thermal conductivity and specific heat are desirable for SWH application [30]. The values supplied by the manufacturer might be varied [31,32]. Therefore, pre-characterization of PCM is recommended to get correct thermophysical properties of PCMs. PCMs are generally characterized by calorimetric experiments like Differential Scanning Calorimeter (DSC), and Differential Thermal Analysis (DTA) [33,34]. Usually very small quantity of product is applied to the machine to measure the properties [35]. Paraffin wax is the most suitable option due to its availability, non-corrosiveness, compatible melting temperatures, and low cost [5,36]. Paraffin wax PCM (Sigma-Aldrich product no. 327212 [37]) was selected for this experiment. The melting temperature, enthalpy, and specific heat were measured by a DSC instrument (PerkinElmer Pyris calorimeter DSC4000). DSC was run from 30 °C to 85 °C at 5 °C/min heating rate. The DSC curve as shown in Fig. 3 presentsthe heating and cooling cycles of PCM. In the DSC curve, the melting temperature of the PCM was measured from the corresponding onset temperature of the heating curve [31]. In this experimental study, melting point was found to be 56.06 °C. The thermophysical properties of paraffin wax used in the experiment are presented in Table 1.
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Table 2 The breakdown cost of the prototype. Cost, $
Unit price
Reference
Copper tube Shell/outer tube Manufacturing Total cost
8.95 4.95 10 23.90
1.0 m @8.95 $/m 1.0 m @4.95 $/m – –
[47] [48] – –
ð5Þ
Zt T ðTout −Tin Þ−To ln out dt Tin
ð6Þ
0
At the time of charging and discharging of the TES, the input and output energies were calculated using Eqs. (1) and (2)[38,39]. Zt ½Tin −Tout dt
Zt 0
˙ HTF Xout ¼ mC
2.3. Formulations for energy and exergy analyses
˙ HTF Ein ¼ mC
T ðTin −Tout Þ 1− o dt Tmelt
˙ HTF Xstored ¼ mC
Component
135
ð1Þ
where, Tin and Tout are the temperatures of the water entering and leaving from the thermal energy storage unit, respectively. To and Tmelt are the environment temperature and the melting temperature of PCM, respectively. To calculate the charging, discharging and the overall exergy efficiency of the TES system, Eqs. (7), (8) and (9) were used[11,41]. εcharging ¼
Xstored Xin
ð7Þ
0
˙ HTF Eout ¼ mC
ε discharging ¼
Zt ½Tout −Tin dt
ð8Þ
ð2Þ
0
ε overall ¼ εcharging εdischarging
where Ein is the heat absorbed by the PCM and Eout represents the heat gain by the water from the PCM storage. m˙ and CHTF represent the mass flow rate and the specific heat of the HTF, respectively. In order to determine the thermal storage system efficiency, Eq. (3) was used. η¼
Xout Xstored
Eout Ein
ð3Þ
Exergy was recovered through the water heating process. This is named as output exergy. Input, stored and output exergies were calculated using Eqs. (4), (5) and (6)[8,11,40].
ð9Þ
2.4. Formulations for cost analysis This is the most important part of the present study. To design the best system, total life cycle cost should be analyzed. Cost analysis was done with respect to different flow rates of the working fluid of the thermal energy storage system. The loss of energy can be considered as cost directly [15]. To get a more accurate picture of the system cost, price of lost exergy was also added to the lost energy cost. To find the total life cycle cost along with energy and exergy cost, material cost should be included. It can be written as: Total life cycle cost ¼ lost energy cost þ lost exergy cost þ material cost:
Zt Tin ˙ HTF dt ðTin −Tout Þ−To ln Xin ¼ mC Tout
ð10Þ
ð4Þ The net present value of all expenditures should be computed to determine the total cost. Present value can be calculated with Eq. (11).
0
Ct ¼ present valueðPVÞ=discount factorðDÞ
Table 3 Estimated solar energy and exergy unit price for each year. Year
Solar energy ($/kWh) at time t
Solar exergy ($/kWh) at time t
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0.264 0.273 0.283 0.293 0.303 0.314 0.325 0.336 0.348 0.360 0.372 0.385 0.399 0.413 0.427 0.442 0.458 0.474 0.490 0.508
0.293 0.304 0.314 0.325 0.337 0.348 0.361 0.373 0.386 0.400 0.414 0.428 0.443 0.459 0.475 0.491 0.509 0.526 0.545 0.564
input
ð11Þ
output
100 90 80 70 60 50 40 30 0
20
40
60
80
100
Fig. 4. Input and output temperatures during charging.
120
140
136
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139
(a) 0.167 kg/min. Input
Output
Environment
Temperature, °C
Temperature, °C
Environment
(b) 0.133 kg/min.
60 50 40 30 20 0
50
100
Input
60 50 40 30 20
150
0
50
Time, min
150
(d) 0.067 kg/min Input
Output
Environment
Temperature, °C
Environment
Temperature, °C
100 Time, min
(c) 0.100 kg/min 60 50 40 30 20 0
Output
50
100
Input
Output
60 50 40 30 20
150
0
50
Time, min
100 Time, min
150
200
(e) 0.033 kg/min Temperature, °C
Environment
Input
Output
60 50 40 30 20 0
100
200
300
Time, min Fig. 5. Input, output and environmental temperatures for fluid flow rate a) 0.167 kg/min, b) 0.133 kg/min, c) 0.1 kg/min, d) 0.067 kg/min and e) 0.033 kg/min.
where, Ct is the cost at time t. I is the interest rate. Ct is the predicted price at future time period. Discount factor is calculated from Eq. (12). ðt−1Þ
ð12Þ
D ¼ 1=ð1 þ IÞ
In this study the period of economic analysis was taken as 20 years (average life of a solar collector system) [42]. Maintenance cost per month was predicted as RM 3.00. For the calculation, the installation cost was calculated as $ 23.90 (RM 76.49). The breakdown of this prototype setup cost is presented in Table 2. The annual energy from the system could be 47.07 kWh which was calculated from the experimental data in this prototype water heating system and this amount was assumed to be constant throughout the 20 year period. The interest rate was assumed to be 3.5% for Malaysia [43]. Calculating with these data, the total cost found was $ 351.26. The price of energy will not be the same for the 20 year period. The price of energy will vary with the effect of the discount factor value. To predict the energy price ($/kWh) for each year, trial and error method was used. The first year price was estimated and other year's price Table 4 Input, output, lost energy and efficiency of the TES.
was calculated using Eqs. (11) and (12) according to the estimated first year's price. Then, the cost of producing energy was computed using these values. The first year price was estimated in such a way that the total cost of producing energy became equal to the total cost of thermal storage ($ 351.26). This equality was attained by changing the first year's estimated unit cost of energy. For solar energy Dincer [44] has referred the value of 0.9 for energy price ratio to the exergy price. The prices of energy and exergy for each year are tabulated in Table 3.
3. Results and discussion 3.1. Charging of the TES unit The paraffin wax was charged in the experimental setup with 0.167 kg/min flow rate of HTF. Initially, thermal bath took about 8 min to reach the desired temperature of input fluid. Input temperature was varied between 81 °C to 87 °C. Fig. 4 shows the input and output temperatures of the thermal storage unit during charging period. It Table 5 Exergy input, stored, output, and lost exergy of the thermal storage.
Flow rate (kg/min)
Ein, (kJ) at 0.133 kg/min
Eout (kJ)
Energy lost = Ein − Eout (kJ)
Energy efficiency (%)
Flow rate (kg/min)
Xin (kJ) at 0.133 kg/min
Xstore (kJ) at 0.133 kg/min
Xout (kJ)
Exergy lost = Xin − Xout (kJ)
0.033 0.067 0.100 0.133 0.167
464.26 464.26 464.26 464.26 464.26
296.56 318.22 340.92 350.62 359.40
167.70 146.04 123.34 113.64 104.86
63.88 68.54 73.43 75.52 77.41
0.033 0.067 0.100 0.133 0.167
72.59 72.59 72.59 72.59 72.59
49.73 49.73 49.73 49.73 49.73
6.95 6.39 4.66 4.48 4.37
65.64 66.21 67.94 68.12 68.23
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Charging
Discharging
Overall
Table 6 Total price of lost energy for 20 years. 16
70
Charging efficiency, %
15 60
13 12
50
10
40
9 30 7 20
6
10 0.034
0.068
0.102
0.136
0.17
Discharging and overall exergy efficiency, %
80
0
4 0.204
Flow rate, kg/min
Fig. 6. Efficiency values in charging, discharging and overall exergy efficiency.
shows that input and output temperatures become identical within 135 min. Therefore, the present study considered 135 min as the charging time of the TES unit. 3.2. Discharging of TES unit During discharging period, the HTF gained heat from PCM and simultaneously PCM started to be solidified. This discharging process varied between 110 min to 256 min for different flow rates of HTF. Discharging period ended when the output temperature became in equilibrium with the environment temperature. Fig. 5(a)–(e) shows the input and output water temperatures for different flow rates. The water input temperature was around 25 °C and output temperature was observed between 40 °C to 60 °C at the initial state. After 60 min, output temperature comes around 30 °C. At the end, the output temperature became identical with the environment temperature. 3.3. Energy and exergy efficiency of the TES unit The inlet energy from the solar collector to the thermal storage and outlet energy from the thermal storage unit to the water were calculated using Eqs. (1) to (3). The energy efficiency of the storage unit and the lost energy were also calculated and tabulated in Table 4. Charging was done on the TES with 0.133 kg/min flow rate. Hence, energy input was the same in all the cases. From Table 4, it can be perceived that the loss
Exergy
100
10
90
9
80
8
70
7
60
6
50 0
0.034
0.068
0.102
0.136
Flow rate, kg/min Fig. 7. Energy and exergy efficiency.
0.17
5 0.204
Exergy efficiency, %
Energy efficiency, %
Energy
137
Flow rate (kg/min)
Energy lost (kWh/year)
Total price of lost energy for 20 years ($)
Exergy lost (kWh/year)
Total price of lost exergy for 20 years ($)
0.033 0.067 0.100 0.133 0.167
17.00 14.81 12.51 11.52 10.63
126.94 110.54 93.36 86.02 79.38
6.66 6.71 6.89 6.91 6.92
55.21 55.68 57.14 57.29 57.38
of energy increases by reducing the flow rate. With the increase of flow rate the energy efficiency increases. Exergy input, storedand output, and loss of exergy were calculated using Eqs. (4) to (6). Table 5 presents input, stored, output, and lost exergies of the thermal storage. This table shows that exergy loss is increasing with the increase of flow rate of HTF. Exergy efficiency was calculated using Eq. (7) to (9). Charging efficiency was the same in all the cases because charging of the TES was done with the same flow rate of HTF. The data from exergy efficiency are graphed in Fig. 6 and clearly shows that the amount of exergy efficiency is maximized by lowering the flow rate of HTF. Fig. 7 depicts the energy and exergy efficiencies of the thermal energy storage unit. This is interesting to see that energy efficiency is increasing with the flow rate of the HTF while exergy efficiency is decreasing with the rise of flow rate of the HTF. Similarly, Kousksou et al. [45] also reported that higher flow rate of HTF resulted in higher entropy generation of the system so that exergy efficiency also decreased with the increase of flow rate. Since from Fig. 7, it is still hard to determine the cost effective flow rate, thermoeconomic analysis can be done to find the optimum design condition. 3.4. Cost analyses of the thermal energy storage unit All the previous calculations were done to be used in this section for cost analysis. This cost analysis consists of loss of energy, loss of exergy and PCM material price. The energy and exergy associated with economy throughout the total life cycle of the system is presented in Table 6. This table shows the total costs of lost energy and exergy for different flow rates in 20 years. The service life of the material should be known to determine the material cost. Materials can sustain its properties more than 2500 cycle or around 7 years for composite paraffin wax [46]. Therefore, for 20 years, setup needs to change the material approximately 3 times. Experimental setup used 0.703 kg of paraffin. Table 7 presents the material cost calculation for the prototype setup. Fig. 8 illustrates the cost of lost energy, lost exergy and PCM material cost. Material cost remains the same for all the flow rate of HTF. The lost energy costs are decreasing with the increase of flow rate. On the other hand, lost exergy cost is increasing with the rise of flow rate. Therefore, total life cycle cost graph is necessary to make a decision. To find the total life cycle cost, it is necessary to add the price of lost energy, price of lost exergy and PCM material price. Using the values from Tables 6 and 7 total life cycle cost of the system was attained. Fig. 9 displays the total life cycle cost for different flow rates. From Fig. 9 it can be clearly visualized that, with the increase of flow rate the life cycle cost decreases. Table 7 Material cost for the TES system. Component
Cost ($)
Unit price
Reference
Material cost
148
3 × 0.703 kg @ 70.01 $/kg
[37]
138
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Lost energy
Material
Lost exergy
5. Recommendations for future work 58
In order to obtain the optimum result or test feasibility, the following research areas are indicated. 120
57
80
56
40
55
0 0
0.034
0.068
0.102
0.136
Lost exergy cost, $
Lost energy and material cost, $
160
• Optimization with different pipe diameter, annulus width, and pipe length can be studied. • Test existing setup with different types of PCM. • Different Heat Transfer Fluid (HTF) can be used as an alternative of water to obtain better heat exchange. • Nanoparticle enhanced PCM can be used to investigate the system performance. • Different types of fins can be used to increase heat transfer rate on the inside copper tube.
54 0.204
0.17
Acknowledgments
Flow rate, kg/min. Fig. 8. The cost of lost energy, lost exergy and PCM material cost.
The present study was supported by the HIRG project UM.C/HIR/ MOHE/ENG/21 of the University of Malaya, Malaysia. References
4. Conclusion In this study, a shell and tube thermal energy storage for solar water heater system has been examined experimentally. As a first objective, energy and exergy efficiencies of the TES system were determined for different flow rates of water. Subsequently, the energy and exergy cost for the TES system was calculated. Finally, total life cycle cost was also determined for different flow rates of the HTF. Results obtained for the TES unit with different flow rates of HTF are summarized here. ▪ With the increase of flow rate of HTF i.e. water from 0.033 kg/min to 0.167 kg/min, energy efficiency increases from 63.88% to 77.41%. ▪ With the increase of flow rate of HTF from 0.033 kg/min to 0.167 kg/min, exergy efficiency decreases from 9.58% to 6.02%. ▪ Highest exergy efficiency at the time of discharging is found to be about 13.98%. ▪ With flow rate of 0.167 kg/min, total price of lost energy and exergy was found to be around $ 79.38 and $ 57.38, respectively. ▪ With 0.033 kg/min flow rate of HTF, the total life cycle cost has been estimated $ 654.61. However, the total life cycle cost can be reduced to $ 609.22 by increasing the flow rate to 0.167 kg/min.
660
650
Total cost, $
640
630
620
610
600 0
0.034
0.068
0.102
0.136
Flow rate, kg/min. Fig. 9. Total life cycle cost for different flow rates.
0.17
0.204
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Contents lists available at ScienceDirect
International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt
Performance investigation of thermal energy storage system with Phase Change Material (PCM) for solar water heating application☆ M.H. Mahfuz, M.R. Anisur, M.A. Kibria, R. Saidur ⁎, I.H.S.C. Metselaar Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Available online 30 July 2014 Keywords: PCM Thermal energy storage Solar water heater Exergy and energy analyses Thermoeconomics
a b s t r a c t In order to harvest solar energy, thermal energy storage (TES) system with Phase Change Material (PCM) has been receiving greater attention because of its large energy storage capacity and isothermal behavior during charging and discharging processes. In the present experimental study, shell and tube TES system using paraffin wax was used in a water heating system to analyze its performance for solar water heating application. Energy and exergy including their cost analyses for the TES system were performed. Accordingly, total life cycle cost was calculated for different flow rates of the Heat Transfer Fluid (HTF). With 0.033 kg/min and 0.167 kg/min flow rates of water as HTF, energy efficiencies experienced were 63.88% and 77.41%, respectively, but in exergy analysis, efficiencies were observed to be about 9.58% and 6.02%, respectively. Besides, the total life cycle cost was predicted to be $ 654.61 for 0.033 kg/min flow rate, which could be reduced to $ 609.22 by increasing the flow rate to 0.167 kg/min. Therefore it can be summarized that total life cycle cost decreases with the increase of flow rate. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction One of the classic devices which can be used to convert the solar energy into thermal energy is solar water heater (SWH). SWH applications largely depend on efficient thermal energy storage (TES). Thermal storage works like a thermal battery where, energy can be stored in the form of latent heat, sensible heat, or both. In this context, TES system with Phase Change Material (PCM) can be more potential for SWH application, as they have high energy density compared with other materials which are used to store only sensible heat [1]. Besides, it also improves the reliability and performance of the SWH system. In literature, there are many researches available on SWH system using TES. Khalifa et al. [2] conducted an experiment to calculate the performance of a flat plate solar collector with a back layer of wax as thermal energy storage. Souliotis et al. [3] studied on solar water heater integrated with collector and storage. They designed and analyzed experimentally the integrated system. Kousksou et al. [4] analyzed the uses of PCMs in solar based Domestic Hot Water (DHW) system. Their model described the heat storage tank with PCM, auxiliary heater, collector, pump, and controller. Al-Hinti et al. [5] also studied experimentally solar water heating system with PCM containers packed in a storage tank. Canbazoglu et al. [6] experimentally investigated passive solar water heating system with sodium thiosulfate pentahydrate as PCM storage. They compared the energy storage performance of PCM ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (R. Saidur).
http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.07.022 0735-1933/© 2014 Elsevier Ltd. All rights reserved.
storage to the conventional system and found approximately 2.59–3.45 times total accumulated heat. Mehling et al. [7] added PCM module at the top of the solar water tank heating system with stratification. The experiments and simulations showed that energy density could be improved by 20 to 45%. Shabgard et al. [8] developed a network model to predict the performance of thermal storage system including cascaded PCMs and embedded heat pipes. The concept of energy and exergy analyses is important for this performance analysis and also for economic analysis of thermal energy storage systems [9]. Without considering the concept of energy and exergy, it will lead to inappropriate system costing [10]. Exergy concept helps engineers to make logical decisions about the prices, real profits and efficiencies. Jegadheeswaran et al. [11] reviewed PCM storage system performance on the basis of exergy. They presented different procedures to analyze the exergy based performance. Jegadheeswaran and Pohekar [12] studied numerically to investigate the performance enhancement of a shell and tube LHTES unit dispersing high conductive particles in the PCM during charging process (melting). They concluded that in dispersing high conductive material in PCM, the performance could be improved significantly. Koca et al. [13] analyzed the energy and exergy of PCM storage for a flat plate solar collector. They used CaCl2·6H2O as PCM in the latent heat storage tank. The designed collector was a single unit integrated with storage. Energy and exergy analyses were done to measure the performance of the system during charging period. For the selected design, the study showed average energy and exergy efficiencies to be 45% and 2.2%, respectively. Kousksou et al. [14] developed a theoretical model for optimizing the solar air heating system with PCM storage. Energy and exergy analyses were carried out numerically to understand the system
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139
et al. [15] investigated the price of energyand exergy for the solar heating system. The life cycle cost for different PCM types was calculated. They concluded that in choosing the suitable PCM, it is more significant to use the exergy concept. Xiang et al. [16] studied dynamic exergy and exergetic cost of a heating system with ground water source heat pumps. They used structural theory of thermoeconomics. Furthermore, Caliskan et al. [17] conducted exergetic and thermoeconomic analyses of TES for applications in building heating system. They used various reference temperatures for their study. Different reference temperatures affect the thermoeconomic parameters. They concluded that the exergetic cost would become higher at the higher reference condition. Buonomano et al. [18] investigated different control strategies for the thermal storage management. They proposed economic model to assess the operating and capital costs of the systems. Robak et al. [19] proposed the concept of embedded gravity assisted wickless heat pipes within a latent heat storage system. They also analyzed the economic viability by comparing proposed design with sensible heat energy storage system. Results showed that the proposed latent heat storage has the potential to reduce capital costs around 15% compared to sensible heat storage. Tozer et al. [20] and Lozano & Valero [21] investigated thermoeconomic analysis. Both of the researches were led to the development of “Theory of the exergetic cost”. They applied this theory in cost allocation, operation optimization and economic optimization of various thermal energy systems. Most of the researchers proposed TES integrated with solar collector, which needed to change the total collector setup [2–4,22]. In addition, the heat transfer analysis on shell and tube TES system was reported by a few studies [23–27]. Tabassum [28] demonstrated numerical analyses of heat transfer in shell and tube TES system for solar water heating application. To the best of the author's knowledge, no study was conducted on exergy and cost analyses considering the shell and tube LHTS system for solar water heating application. Hence, energy and exergy analyses of the TES system for SWH application were performed in the present study. Moreover, thermoeconomic analysis of the TES system was done with different flow rates of the HTF.
Nomenclature Symbols C Ct E H m˙ T t X
133
specific heat, J/g °C cost at time t, RM energy, kJ latent heat, J/g mass flow rate, kg/s temperature, K time, s exergy, kJ
Greek symbols η energy efficiency ε exergy efficiency
Subscripts charging charging period discharging discharging period HTF Heat Transfer Fluid in inlet melt melting o environment out outlet overall overall store stored
behavior. They concluded that the performance of the system largely depended on the selection of the PCM. The exergy concept is not only essential for efficiency analysis but also a significant tool in economic analysis and cost accounting. Rezaei
SUN Flow separation valve
Cold water inlet
Valve 1
Thermal Energy Storage filled with PCM Water Storage tank
Valve 2
Pump
Backup electric heater Charging loop
Discharging loop
Fig. 1. Proposed water heating system with the thermal storage arrangement.
Hot water outlet
134
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Table 1 Thermophysical properties of paraffin wax.
Output
Node 4
Node 3
Node 2
Node 1
Input
Flow meter
Fig. 2. Latent heat thermal energy storage.
2. Experimental procedure 2.1. System description Fig. 1 presents a proposed solar water heating system with the thermal energy storage arrangement. The proposed system has three main components: a) a solar collector unit, b) a double pipe thermal energy storageand c) a well-insulated water storage tank. In this present study only the thermal energy storage unit was studied experimentally. In this proposed system as illustrated in Fig. 1, charging of the TES is done during the day time when the solar radiation is present. During the charging period or day time, valve 1 is kept open while valve 2 is remained closed. The cold water passes through the solar collector. Water takes heat from the solar radiation and part of the hot water 30
Peak = 61.12°C
Heat flow endo up, mW
Delta H = 200.74 J/g 25
Onset=56.06°C
End = 64.00 °C
20
Onset=58.93°C End=47.51°C
15
Delta H = -195.96 J/g
Peak=55.70°C 10 30
40
50
60
70
Sample temperature, °C Fig. 3. DSC curve of paraffin wax for heating and cooling.
80
Property
Values
Melting point Melting end point Freezing start point Freezing end point Melting peak temperature Freezing peak temperature Specific heat of solid (b30 °C) Specific heat of liquid (N65 °C) Melting latent heat Freezing latent heat
56.06 °C 64.99 °C 58.93 °C 47.51 °C 61.12 °C 55.70 °C 2.565 J/g °C 2.439 J/g °C 200.74 J/g 195.97 J/g
flows through the heat storage unit for charging the PCM. The rest of the water goes directly to the water storage tank. At the time of discharge during night time or in the absence of solar radiation, valve 1 is needed to be closed as it is not necessary to circulate water through the collector and valve 2 is opened to pass the water directly through the TES unit for discharging heat to water. Simultaneously, the PCM starts to be solidified and being fully solidified, this PCM can be used for charging again. This process is continued until the ends of the PCM thermal life cycle. In order to maintain a constant output temperature, an electric heater is also attached to the water storage tank. The electric heater is essential to maintain the preferred temperature for comfortable use. Using a thermostat in the heater, the power consumption and temperature both can be controlled. In the present study, a vertical arrangement of a latent heat TES unit as shown in Fig. 2 was examined. Paraffin wax was poured into the annular space between the copper tube and the outer shell. This TES unit was connected with a thermal bath. The input temperature of 90 °C was selected as the same output from solar collector system [29]. This simulated the charging period of the thermal storage. Charging was continued to reach the PCM heat capacity so that the difference of input and output temperatures became the same. After the charging period, the hot water was drained out and then the thermal bath was filled with ambient temperature water. The discharging cycle began with the flow of water at a temperature around 25 °C. The discharging cycle ended when the output temperature came to be identical to the environment temperature.
2.2. PCM selection The PCMs having a suitable melting temperature should be selected for appropriate application. PCMs with high latent heat, high thermal conductivity and specific heat are desirable for SWH application [30]. The values supplied by the manufacturer might be varied [31,32]. Therefore, pre-characterization of PCM is recommended to get correct thermophysical properties of PCMs. PCMs are generally characterized by calorimetric experiments like Differential Scanning Calorimeter (DSC), and Differential Thermal Analysis (DTA) [33,34]. Usually very small quantity of product is applied to the machine to measure the properties [35]. Paraffin wax is the most suitable option due to its availability, non-corrosiveness, compatible melting temperatures, and low cost [5,36]. Paraffin wax PCM (Sigma-Aldrich product no. 327212 [37]) was selected for this experiment. The melting temperature, enthalpy, and specific heat were measured by a DSC instrument (PerkinElmer Pyris calorimeter DSC4000). DSC was run from 30 °C to 85 °C at 5 °C/min heating rate. The DSC curve as shown in Fig. 3 presentsthe heating and cooling cycles of PCM. In the DSC curve, the melting temperature of the PCM was measured from the corresponding onset temperature of the heating curve [31]. In this experimental study, melting point was found to be 56.06 °C. The thermophysical properties of paraffin wax used in the experiment are presented in Table 1.
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Table 2 The breakdown cost of the prototype. Cost, $
Unit price
Reference
Copper tube Shell/outer tube Manufacturing Total cost
8.95 4.95 10 23.90
1.0 m @8.95 $/m 1.0 m @4.95 $/m – –
[47] [48] – –
ð5Þ
Zt T ðTout −Tin Þ−To ln out dt Tin
ð6Þ
0
At the time of charging and discharging of the TES, the input and output energies were calculated using Eqs. (1) and (2)[38,39]. Zt ½Tin −Tout dt
Zt 0
˙ HTF Xout ¼ mC
2.3. Formulations for energy and exergy analyses
˙ HTF Ein ¼ mC
T ðTin −Tout Þ 1− o dt Tmelt
˙ HTF Xstored ¼ mC
Component
135
ð1Þ
where, Tin and Tout are the temperatures of the water entering and leaving from the thermal energy storage unit, respectively. To and Tmelt are the environment temperature and the melting temperature of PCM, respectively. To calculate the charging, discharging and the overall exergy efficiency of the TES system, Eqs. (7), (8) and (9) were used[11,41]. εcharging ¼
Xstored Xin
ð7Þ
0
˙ HTF Eout ¼ mC
ε discharging ¼
Zt ½Tout −Tin dt
ð8Þ
ð2Þ
0
ε overall ¼ εcharging εdischarging
where Ein is the heat absorbed by the PCM and Eout represents the heat gain by the water from the PCM storage. m˙ and CHTF represent the mass flow rate and the specific heat of the HTF, respectively. In order to determine the thermal storage system efficiency, Eq. (3) was used. η¼
Xout Xstored
Eout Ein
ð3Þ
Exergy was recovered through the water heating process. This is named as output exergy. Input, stored and output exergies were calculated using Eqs. (4), (5) and (6)[8,11,40].
ð9Þ
2.4. Formulations for cost analysis This is the most important part of the present study. To design the best system, total life cycle cost should be analyzed. Cost analysis was done with respect to different flow rates of the working fluid of the thermal energy storage system. The loss of energy can be considered as cost directly [15]. To get a more accurate picture of the system cost, price of lost exergy was also added to the lost energy cost. To find the total life cycle cost along with energy and exergy cost, material cost should be included. It can be written as: Total life cycle cost ¼ lost energy cost þ lost exergy cost þ material cost:
Zt Tin ˙ HTF dt ðTin −Tout Þ−To ln Xin ¼ mC Tout
ð10Þ
ð4Þ The net present value of all expenditures should be computed to determine the total cost. Present value can be calculated with Eq. (11).
0
Ct ¼ present valueðPVÞ=discount factorðDÞ
Table 3 Estimated solar energy and exergy unit price for each year. Year
Solar energy ($/kWh) at time t
Solar exergy ($/kWh) at time t
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
0.264 0.273 0.283 0.293 0.303 0.314 0.325 0.336 0.348 0.360 0.372 0.385 0.399 0.413 0.427 0.442 0.458 0.474 0.490 0.508
0.293 0.304 0.314 0.325 0.337 0.348 0.361 0.373 0.386 0.400 0.414 0.428 0.443 0.459 0.475 0.491 0.509 0.526 0.545 0.564
input
ð11Þ
output
100 90 80 70 60 50 40 30 0
20
40
60
80
100
Fig. 4. Input and output temperatures during charging.
120
140
136
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139
(a) 0.167 kg/min. Input
Output
Environment
Temperature, °C
Temperature, °C
Environment
(b) 0.133 kg/min.
60 50 40 30 20 0
50
100
Input
60 50 40 30 20
150
0
50
Time, min
150
(d) 0.067 kg/min Input
Output
Environment
Temperature, °C
Environment
Temperature, °C
100 Time, min
(c) 0.100 kg/min 60 50 40 30 20 0
Output
50
100
Input
Output
60 50 40 30 20
150
0
50
Time, min
100 Time, min
150
200
(e) 0.033 kg/min Temperature, °C
Environment
Input
Output
60 50 40 30 20 0
100
200
300
Time, min Fig. 5. Input, output and environmental temperatures for fluid flow rate a) 0.167 kg/min, b) 0.133 kg/min, c) 0.1 kg/min, d) 0.067 kg/min and e) 0.033 kg/min.
where, Ct is the cost at time t. I is the interest rate. Ct is the predicted price at future time period. Discount factor is calculated from Eq. (12). ðt−1Þ
ð12Þ
D ¼ 1=ð1 þ IÞ
In this study the period of economic analysis was taken as 20 years (average life of a solar collector system) [42]. Maintenance cost per month was predicted as RM 3.00. For the calculation, the installation cost was calculated as $ 23.90 (RM 76.49). The breakdown of this prototype setup cost is presented in Table 2. The annual energy from the system could be 47.07 kWh which was calculated from the experimental data in this prototype water heating system and this amount was assumed to be constant throughout the 20 year period. The interest rate was assumed to be 3.5% for Malaysia [43]. Calculating with these data, the total cost found was $ 351.26. The price of energy will not be the same for the 20 year period. The price of energy will vary with the effect of the discount factor value. To predict the energy price ($/kWh) for each year, trial and error method was used. The first year price was estimated and other year's price Table 4 Input, output, lost energy and efficiency of the TES.
was calculated using Eqs. (11) and (12) according to the estimated first year's price. Then, the cost of producing energy was computed using these values. The first year price was estimated in such a way that the total cost of producing energy became equal to the total cost of thermal storage ($ 351.26). This equality was attained by changing the first year's estimated unit cost of energy. For solar energy Dincer [44] has referred the value of 0.9 for energy price ratio to the exergy price. The prices of energy and exergy for each year are tabulated in Table 3.
3. Results and discussion 3.1. Charging of the TES unit The paraffin wax was charged in the experimental setup with 0.167 kg/min flow rate of HTF. Initially, thermal bath took about 8 min to reach the desired temperature of input fluid. Input temperature was varied between 81 °C to 87 °C. Fig. 4 shows the input and output temperatures of the thermal storage unit during charging period. It Table 5 Exergy input, stored, output, and lost exergy of the thermal storage.
Flow rate (kg/min)
Ein, (kJ) at 0.133 kg/min
Eout (kJ)
Energy lost = Ein − Eout (kJ)
Energy efficiency (%)
Flow rate (kg/min)
Xin (kJ) at 0.133 kg/min
Xstore (kJ) at 0.133 kg/min
Xout (kJ)
Exergy lost = Xin − Xout (kJ)
0.033 0.067 0.100 0.133 0.167
464.26 464.26 464.26 464.26 464.26
296.56 318.22 340.92 350.62 359.40
167.70 146.04 123.34 113.64 104.86
63.88 68.54 73.43 75.52 77.41
0.033 0.067 0.100 0.133 0.167
72.59 72.59 72.59 72.59 72.59
49.73 49.73 49.73 49.73 49.73
6.95 6.39 4.66 4.48 4.37
65.64 66.21 67.94 68.12 68.23
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Charging
Discharging
Overall
Table 6 Total price of lost energy for 20 years. 16
70
Charging efficiency, %
15 60
13 12
50
10
40
9 30 7 20
6
10 0.034
0.068
0.102
0.136
0.17
Discharging and overall exergy efficiency, %
80
0
4 0.204
Flow rate, kg/min
Fig. 6. Efficiency values in charging, discharging and overall exergy efficiency.
shows that input and output temperatures become identical within 135 min. Therefore, the present study considered 135 min as the charging time of the TES unit. 3.2. Discharging of TES unit During discharging period, the HTF gained heat from PCM and simultaneously PCM started to be solidified. This discharging process varied between 110 min to 256 min for different flow rates of HTF. Discharging period ended when the output temperature became in equilibrium with the environment temperature. Fig. 5(a)–(e) shows the input and output water temperatures for different flow rates. The water input temperature was around 25 °C and output temperature was observed between 40 °C to 60 °C at the initial state. After 60 min, output temperature comes around 30 °C. At the end, the output temperature became identical with the environment temperature. 3.3. Energy and exergy efficiency of the TES unit The inlet energy from the solar collector to the thermal storage and outlet energy from the thermal storage unit to the water were calculated using Eqs. (1) to (3). The energy efficiency of the storage unit and the lost energy were also calculated and tabulated in Table 4. Charging was done on the TES with 0.133 kg/min flow rate. Hence, energy input was the same in all the cases. From Table 4, it can be perceived that the loss
Exergy
100
10
90
9
80
8
70
7
60
6
50 0
0.034
0.068
0.102
0.136
Flow rate, kg/min Fig. 7. Energy and exergy efficiency.
0.17
5 0.204
Exergy efficiency, %
Energy efficiency, %
Energy
137
Flow rate (kg/min)
Energy lost (kWh/year)
Total price of lost energy for 20 years ($)
Exergy lost (kWh/year)
Total price of lost exergy for 20 years ($)
0.033 0.067 0.100 0.133 0.167
17.00 14.81 12.51 11.52 10.63
126.94 110.54 93.36 86.02 79.38
6.66 6.71 6.89 6.91 6.92
55.21 55.68 57.14 57.29 57.38
of energy increases by reducing the flow rate. With the increase of flow rate the energy efficiency increases. Exergy input, storedand output, and loss of exergy were calculated using Eqs. (4) to (6). Table 5 presents input, stored, output, and lost exergies of the thermal storage. This table shows that exergy loss is increasing with the increase of flow rate of HTF. Exergy efficiency was calculated using Eq. (7) to (9). Charging efficiency was the same in all the cases because charging of the TES was done with the same flow rate of HTF. The data from exergy efficiency are graphed in Fig. 6 and clearly shows that the amount of exergy efficiency is maximized by lowering the flow rate of HTF. Fig. 7 depicts the energy and exergy efficiencies of the thermal energy storage unit. This is interesting to see that energy efficiency is increasing with the flow rate of the HTF while exergy efficiency is decreasing with the rise of flow rate of the HTF. Similarly, Kousksou et al. [45] also reported that higher flow rate of HTF resulted in higher entropy generation of the system so that exergy efficiency also decreased with the increase of flow rate. Since from Fig. 7, it is still hard to determine the cost effective flow rate, thermoeconomic analysis can be done to find the optimum design condition. 3.4. Cost analyses of the thermal energy storage unit All the previous calculations were done to be used in this section for cost analysis. This cost analysis consists of loss of energy, loss of exergy and PCM material price. The energy and exergy associated with economy throughout the total life cycle of the system is presented in Table 6. This table shows the total costs of lost energy and exergy for different flow rates in 20 years. The service life of the material should be known to determine the material cost. Materials can sustain its properties more than 2500 cycle or around 7 years for composite paraffin wax [46]. Therefore, for 20 years, setup needs to change the material approximately 3 times. Experimental setup used 0.703 kg of paraffin. Table 7 presents the material cost calculation for the prototype setup. Fig. 8 illustrates the cost of lost energy, lost exergy and PCM material cost. Material cost remains the same for all the flow rate of HTF. The lost energy costs are decreasing with the increase of flow rate. On the other hand, lost exergy cost is increasing with the rise of flow rate. Therefore, total life cycle cost graph is necessary to make a decision. To find the total life cycle cost, it is necessary to add the price of lost energy, price of lost exergy and PCM material price. Using the values from Tables 6 and 7 total life cycle cost of the system was attained. Fig. 9 displays the total life cycle cost for different flow rates. From Fig. 9 it can be clearly visualized that, with the increase of flow rate the life cycle cost decreases. Table 7 Material cost for the TES system. Component
Cost ($)
Unit price
Reference
Material cost
148
3 × 0.703 kg @ 70.01 $/kg
[37]
138
M.H. Mahfuz et al. / International Communications in Heat and Mass Transfer 57 (2014) 132–139 Lost energy
Material
Lost exergy
5. Recommendations for future work 58
In order to obtain the optimum result or test feasibility, the following research areas are indicated. 120
57
80
56
40
55
0 0
0.034
0.068
0.102
0.136
Lost exergy cost, $
Lost energy and material cost, $
160
• Optimization with different pipe diameter, annulus width, and pipe length can be studied. • Test existing setup with different types of PCM. • Different Heat Transfer Fluid (HTF) can be used as an alternative of water to obtain better heat exchange. • Nanoparticle enhanced PCM can be used to investigate the system performance. • Different types of fins can be used to increase heat transfer rate on the inside copper tube.
54 0.204
0.17
Acknowledgments
Flow rate, kg/min. Fig. 8. The cost of lost energy, lost exergy and PCM material cost.
The present study was supported by the HIRG project UM.C/HIR/ MOHE/ENG/21 of the University of Malaya, Malaysia. References
4. Conclusion In this study, a shell and tube thermal energy storage for solar water heater system has been examined experimentally. As a first objective, energy and exergy efficiencies of the TES system were determined for different flow rates of water. Subsequently, the energy and exergy cost for the TES system was calculated. Finally, total life cycle cost was also determined for different flow rates of the HTF. Results obtained for the TES unit with different flow rates of HTF are summarized here. ▪ With the increase of flow rate of HTF i.e. water from 0.033 kg/min to 0.167 kg/min, energy efficiency increases from 63.88% to 77.41%. ▪ With the increase of flow rate of HTF from 0.033 kg/min to 0.167 kg/min, exergy efficiency decreases from 9.58% to 6.02%. ▪ Highest exergy efficiency at the time of discharging is found to be about 13.98%. ▪ With flow rate of 0.167 kg/min, total price of lost energy and exergy was found to be around $ 79.38 and $ 57.38, respectively. ▪ With 0.033 kg/min flow rate of HTF, the total life cycle cost has been estimated $ 654.61. However, the total life cycle cost can be reduced to $ 609.22 by increasing the flow rate to 0.167 kg/min.
660
650
Total cost, $
640
630
620
610
600 0
0.034
0.068
0.102
0.136
Flow rate, kg/min. Fig. 9. Total life cycle cost for different flow rates.
0.17
0.204
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