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Engineering Science and Technology, an International Journal ■■ (2015) I
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Engineering Science and Technology, an International Journal
journal homepage: http://www.elsevier.com/locate/jestch
Full Length Article
An experimental investigation of shell and tube latent heat storage for solar dryer using paraffin wax as heat storage material Ashish Agarwal *, R.M. Sarviya
Department of Mechanical Engineering, Maulana Azad National Institute of Technology, Bhopal, MP, India
ARTICLE INFO
ABSTRACT
Article history: Received 3 June 2015 Received in revised form 5 September 2015 Accepted 29 September 2015 Available online
Keywords:
Latent heat storage
Melting and solidification process Paraffin wax
In the presented study the shell and tube type latent heat storage (LHS) has been designed for solar dryer and paraffin wax is used as heat storage material. In the first part of the study, the thermal and heat transfer characteristics of the latent heat storage system have been evaluated during charging and discharging process using air as heat transfer fluid (HTF). In the last section of the study the effectiveness of the use of an LHS for drying of food product and also on the drying kinetics of a food product has been determined. A series of experiments were conducted to study the effects of flow rate and temperature of HTF on the charging and discharging process of LHS. The temperature distribution along the radial and longitudinal directions was obtained at different time during charging process to analyze the heat transfer phenomenon in the LHS. Thermal performance of the system is evaluated in terms of cumulative energy charged and discharged, during the charging and discharging process of LHS, respectively. Experimental results show that the LHS is suitable to supply the hot air for drying of food product during non-sunshine hours or when the intensity of solar energy is very low. Temperature gain of air in the range of 17 °C to 5 °C for approximately 10 hrs duration was achieved during discharging of LHS.
Copyright © 2015, The Authors. Production and hosting by Elsevier B.V. on behalf of Karabuk University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
In the recent years, most of the developing countries around the world were facing the problem of energy crisis because of the large gap between demand and supply of energy. This problem can be minimized to some extent by utilizing renewable energy sources. Solar energy is the attractive form of the renewable energy. Solar energy is available abundantly in the world, but it is not continuous and its intensity also varies with time. Due to above reason the acceptability and reliability of solar based thermal system is lower than conventional systems. A properly designed heat storage system increases the reliability of solar thermal system by bridging the gap between the energy demand and availability. The thermal energy can be stored in the form of sensible heat, latent heat or thermo-chemical energy. Latent heat storage is a more attractive form because of high energy storage capacity per unit volume, absorbing and releasing heat at constant temperature, chemical stability, non-corrosiveness, low vapor pressure, small volume change during phase transformation etc. [1].
Abbreviations: DSC, Differential scanning calorimetry; HTF, Heat transfer fluid; LHS, Latent heat storage; PCM, Phase change material; PID, Proportional-integralderivative; wb, Wet basis.
* Corresponding author. Tel.: +91 9425680418, fax: +91 7552670562. E-mail address: er_ashishagarwal@yahoo.com (A. Agarwal).
A number of experimental and computational studies have been reported in the literature to evaluate the thermal performance of numerous latent heat storage systems. A detail review of phase change materials (PCMs) used in the thermal heat storage system has been given by Abhat [1], Lane [2], Zhou et al. [3], Garg et al. [4], Zhang et al. [5], Tyagi and Buddhi [6], Riffat et al. [7], Sethi and Sharma [8], Verma et al. [9], and Agrawal and Sarviya [10]. Many researchers have reported the thermal and heat transfer characteristics of latent heat storage systems with different geometrical configuration during charging and discharging. Khodadadi and Zhang [11] numerically studied the melting process of PCM in spherical container. The results of the numerical study show that the rate of melting is higher at top region of a sphere than at the bottom region. They investigate the effect of convection on the melting rate.
Medrano et al. [12] experimentally studied the heat transfer characteristics of five small heat exchangers working as latent heat thermal storage systems, during the charge and discharge processes. The results indicated that the double pipe heat exchanger with the PCM embedded in a graphite matrix had the highest values of heat storage. Liu et al. [13] experimentally investigated the thermal and heat transfer characteristics of stearic acid during the solidification processes in a vertical annulus energy storage system. They studied the movement of the solid-liquid interface in the radial direction, and the effects of Reynolds number on the heat transfer parameters. Also, the effects of the fin with various widths on the enhancement were reported. Their results indicated that the fin can
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enhance both the conduction and the natural convection heat transfer of the PCM.
Ezan et al. [14] experimentally investigated the thermal performance of shell and tube system during charging and discharging cycle using water as HTF. They evaluate the performance of the system under different operating conditions. They studied the effects of inlet temperature, flow rate, shell diameter and thermal conductivity of the tube material on the storage capacity of the system. The results show that the natural convection mode of heat transfer is dominant over conduction during melting and solidification. The conduction was dominant only during the beginning of melting and solidification processes. The effect of inlet temperature of HTF on rejected energy is more, compared to flow rate of HTF. Akgun et al. [15] experimentally evaluated the melting and solidification behaviors of PCM in a vertical shell and tube heat exchanger. The results of the study show that the melting time greatly decreased by increasing the temperature of HTF. Low value of mass flow rate of HTF leads to lower consumption of energy. Seeniraj et al. [16] conducted a numerical study of a finned tube latent heat thermal storage (LHTS) module. The shell side of the module is filled with PCM while the tubes carry the heat transfer fluid (HTF). The influence of various parameters viz. geometrical, thermophysical and various non-dimensional numbers on the performance of the unit is studied. They observed that some quantity of PCM at the end of the exit of the HTF tube have remained in the solid state in case of an unfinned tube. An appreciable enhancement in the energy storage process was observed with the addition of fins in the module.
Hosseini et al. [17] performed an experimental investigation to evaluate the melting behavior of PCM in horizontal shell and tube heat storage. They found that the high temperature region exists in the uppermost section due to buoyancy effects. They claimed that by increasing the temperature from 70 to 80 °C, the total melting time is reduced to 37%. Adine and El Qarnia [18] reported numerical study of melting of PCM in latent heat storage unit. The shell-and-tube type heat storage system was selected in this study. The effect of multiple PCMs on thermal performance was studied numerically. n-octadecane and P116 of melting temperatures of 50 °C and 27.7 °C was selected as a heat storage material and filled in shell side. Water was used as heat transfer fluid and flow through the inner tube. Water flow under forced convection and transfers the heat to PCM. The thermal performance of the latent heat thermal energy storage unit was numerically studied using multiple phase change materials and a phase change material during charging process (melting). The parametric studies were conducted to optimize the design and to evaluate the thermal performance of the system. The key parameters considered during the study were: the mass flow rate and temperature of the HTF and the proportion mass of Phase change materials.
Li and Kong [19] performed a numerical study for evaluating the thermal performance of a shell and tube heat storage unit using paraffin as PCM. The study was conducted using air and water as HTF. Parametric analysis has been conducted to evaluate the effect of HTF inlet velocity on the HTF outlet temperature, Nu, and melt fraction. Results indicate that the air inlet velocity has a great effect on the air outlet temperature and heat transfer rate, and the water inlet velocity has little effect on the water outlet temperature. Trp [20] conducted an experimental and numerical investigation of the shell-and-tube type latent thermal energy storage system during charging and discharging. The aim of the study is to provide guidelines for system performance and design optimization. The unsteady temperature distributions of the HTF, tube wall and the PCM have been obtained by a series of numerical calculations for various HTF working conditions and various geometric parameters. Zhang and Faghri [21] numerically studied freezing in an eccentric annulus using a temperature-transforming model. They investigated the effect of the eccentricity on the freezing process. Sari and Kaygusuz [22-24]
performed experimental investigations during melting and solidification of some acids including stearic acid, eutectic mixture of lauric and stearic acids and myristic acid. They claimed that the temperature and mass flow rate of HTF is more effective on the solidification behavior than the melting behavior of PCM.
Avci and Yazici [25] reported melting and solidification behaviors of paraffin in a horizontal tube-in-shell storage system. The effect of the inlet temperature on the melting and solidification time was determined. The focus of the study is to understand the physics of the process based on the temporal variation of temperature field inside the PCM. Wang et al. [26] numerically studied melting and solidification characteristics of a shell-and-tube phase change heat storage unit. Yusuf Yazici et al. [27] reported solidification characteristics of paraffin in a horizontal shell-and-tube type-storage system. In this study the effects of the eccentricity of the inner tube on the solidification were determined and discussed. The inner tube has been moved upward/downward according the center of the outer shell. Six different values of the eccentricity from the center of the outer shell are considered: e = -10, -20, -30,10, 20, 30 besides the concentric geometry (e = 0). The focus of the study was to understand the solidification behavior of PCM based on the transient temperature fields inside the PCM. Eccentricity is shown to affect the total solidification time considerably. The eccentricity, either upward or downward, was found to make the total solidification time longer. Pandiyarajan et al. [28] conducted an experimental study in order to evaluate the performance of heat recovery system for diesel engine exhaust. The finned shell and tube heat exchanger was used in this study. The reported heat recovery of the system was 10-15%. The effect of velocity and temperature of HTF, on the heat transfer was studied and reported.
Several investigations were carried out in the past for observing the dynamic characteristics of the latent heat storage system during charging and discharging [29-31]. Wu and Fang [29] developed numerical model to analyze the influence of mass flow rate and inlet temperature of HTF on the transient thermal characteristics of latent heat thermal storage system coupled with a solar heating collector, during the discharging process. The heat storage system consists of a cylindrical storage tank filled with spherical capsules containing a phase change material (PCM). Myristic acid was used as a PCM and water was used as heat transfer fluid (HTF). The mass flow rate and inlet temperature of HTF have strong influences on complete solidification time and the heat removal rate compared to the initial temperature of PCM. The heat release during solidification is divided into three stages, namely, liquid sensible heat release, solidifying latent heat release and solid sensible heat release. The temperature of PCM drops very rapidly at the liquid sensible heat release stage, then it stabilizes at the solidification temperature until the solidification process is completed.
Veerappan et al. [30] analytically studied the phase change behavior of different PCMs encapsulated in spherical enclosures to identify a suitable heat storage material. The developed model is useful for finding the interface positions and complete phase change time for solidification and melting of PCM in spherical enclosure. Results indicated that the solidification time and the solidified mass fraction was greatly influenced by the thermal conductivity of PCM. The results of the study show that the conduction is the dominant mode of heat transfer during initial stage of melting. After that, melting is greatly influenced by natural convection phenomena. Liu et al. [31] simulate the dynamic characteristics of the latent heat storage device with heat pipe during charging process. The dynamic charging process model has been developed to analyze the effects of initial temperature of the PCM and inlet temperature of heat pipe medium on the transient thermal performance of the system. Paraffin was used as phase change material (PCM) and water was used as heat transfer fluid (HTF). The thermal performance of the heat storage is measured in terms of total heat storage capacity, heat
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storage rate and outlet temperature of heat pipe medium. The result of the study indicates that the heat storage capacity and heat storage rate increases when the initial temperature of PCM decreases or the inlet temperature of the heat pipe medium increases.
The present study focused on the development of a shell and tube type heat storage system for solar dryer. Literature on solar dryer [32-34] reported that the drying air temperature in the dryer is not uniform and it fluctuates widely throughout the day and drying of food is not possible during late evening hours or non-sunshine hours. The reliability and working hours of conventional dryer could be improved with the help of heat storage system. Different types of materials such as rock, water, sand and granite, metal scrap, paraffin wax, mixture of aluminum power and paraffin wax have been used as storage material in the most of the previous research works [35-39]. In most of solar dryers designed by researchers, sensible heat storage materials such as rocks, oil etc. were used. Sensible heat storage materials have low energy storage capacity, thus a large volume of sensible heat storage material is required in designing of storage systems. Latent heat storage materials such as paraffin wax, salt hydrate have a high latent heat of fusion so that it requires less volume of storage material for the same amount of heat storage. Very limited information is available in the literature regarding the use of latent heat storage materials in the solar dryer. Some designs of solar dryer with a latent heat storage integrated with solar collector have been reported by researchers in the literature [40-42]. Some researchers suggested the different designs of solar air heater with a latent heat storage that include: solar air heaters with built-in PCM as the energy storage medium, solar air heaters connected to a heat exchanger with PCM. In solar air heaters with built-in energy storage medium [43,44], PCM is introduced in the form of capsules of different shapes and size below the absorber plate. If these kinds of heat storage system are used for solar dryers they require large modification in the design of conventional solar dryer which further increases its cost and complexity in the operation and maintenance of dryer.
There is a need for development of simple and low cost latent heat storage unit for solar dryer, which can easily connect with the existing solar dryer without major modifications in the design of the dryer. The objective of the present experimental study is to develop the latent heat storage for solar dryer and evaluate its performance during charging and discharging. During daytime, hot air from a solar collector flow through the latent heat storage to transfer the excess solar energy to the latent heat storage system. Stored energy in the latent heat storage is utilized during non-sunshine hours to heat ambient air by forcing the ambient air through the latent heat storage system. Heated air is used to dry the food product.
In the first part of the present experimental study, the melting and solidification behaviors of paraffin wax in shell and tube heat storage unit have been determined to study the thermal and heat transfer characteristics during the charging and discharging process. The air is used as heat transfer fluid (HTF) for the present study. A series of experiments have been conducted to study the effect of the flow rate and temperature of HTF on the melting and solidification of paraffin wax and the energy stored. The time-wise variations of temperature distribution along the radial and longitudinal directions of the latent heat storage unit have been obtained to analyze the heat transfer phenomenon in heat storage unit. In the last part of the study, the effect of the energy released from the latent heat storage on the drying kinetics of potato slices, which is used as modal food product, has been determined and discussed.
2. Experimental setup and procedure
2.1. Experimental setup
An experimental set-up constructed for the present study is shown in Figs. 1 and 2. Fig. 1 shows the experimental set-up without attaching drying chamber to latent heat storage (LHS). This set-up is used for evaluating the heat transfer characteristics during charging and discharging of LHS. Fig. 2 shows the experimental set-up with drying chamber for evaluating the effectiveness of LHS for drying of food product. The experimental set-up includes heat exchanger, K-type thermocouples, data logger, dual core PC, air heater with proportional-integral-derivative (PID) temperature controller and rotameter. A PID temperature controller is required to supply air at a constant temperature to the LHS. In the present study Selec Make DTC324 PID controller is used to supply the hot air to LHS at a steady temperature from air heater.
The shell and tube heat exchanger which works as a heat storage consists of two concentric cylindrical tubes. The external cylindrical tube made of galvanized iron is 1000 mm long with an inner diameter of 127 mm while the 25 mm outer diameter copper tube located centrally in the external cylindrical tube was the heat transfer tube through which air flows. The phase change material is filled in the shell side of the heat exchanger. The outside surface of the heat exchanger is well insulated with glass wool of 8 mm thickness to reduce the heat losses to the surroundings. The system consists of an air-flow loop that can be used to charge and discharge the LHS. During charging heat is transferred from the hot air to PCM, while during discharging heat is transferred from the PCM to air. The temperature at different axial and radial locations of heat storage was determined by recording temperature by
Computer
Fig. 1. Schematic diagram of experimental set-up for evaluating heat transfer characteristics during charging and discharging of a LHS.
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Fig. 2. Schematic diagram of experimental set-up with drying chamber.
different thermocouples. Thirty six thermocouples were positioned in the LHS to measure temperature variations. The locations of thermocouples in the PCM along the axial directions of LHS are denoted by A, B, C and D, as shown in Fig. 3. The nine thermocouples were located at each axial location at an angular direction of 0°, 90° and 180° (Fig. 4). Three thermocouples were located at each angular direction at radial distance of 15 mm, 30 mm and 45 mm from the center as shown in Fig. 4. The drying chamber is fabricated which is attached to LHS during the discharging process to check the effectiveness of LHS for drying of food products (Fig. 2). The drying chamber is vertical and cylindrical in shape and is made of galvanized iron sheet and insulated with glass wool of 7 cm thickness. The inner diameter and height of drying chamber is 180 mm and 750 mm, respectively. A data acquisition system is used to scan and record all output values from the thermocouples after a definite interval of time. This system comprised of a microprocessor based temperature scanner with inbuilt memory to record and store the data. RS232 cable is used to connect the data acquisition system to the computer. Computer software is used to communicate with data loggers and to analyze and display data on the computer. Flow of air through the heat transfer pipe is measured by the rotameter. An air heating unit is fabricated for supplying hot air at constant
temperature to LHS. An insulated enclosure of 610 mm length and 76.2 mm diameter made of galvanized iron is used as an air heating unit. Finned electric resistance heating element is used in air heater. A constant air temperature at the inlet of the LHS is maintained during the charging and discharging process using a proportionalintegral-derivative (PID) controller, which controls the operation of the heater, based on the signal received from thermocouple positioned at the inlet of the HTF tube. A flow control valve is used to supply air at the required flow rate during the experiment.
Uncertainty analysis is necessary to prove the accuracy of the experimental results. In the present work the Kline and McClintock method [45] has been used for measurement of uncertainty in calculating the heat transfer rate. The heat transfer rate (Q) is calculated by measurement of mass flow rate of air (m) and temperature of air at the inlet (Tj) and outlet (To) of LHS. The uncertainty in heat transfer rate (UQ) is dependent on the uncertainty associated with measurement of air flow rate and temperature. Kline and McClintock [45] suggested the root-sum-square uncertainty method for calculating the uncertainty in single sample experiments using independent uncertainties. According to this method the uncertainty in the heat transfer rate for the present work is calculated by the following equation
Fig. 3. Dimensions of PCM, HTF tubes and location of thermocouples at positions A, B, C and D in the LHS.
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Location 0°
Fig. 4. Location of different thermocouples in LHS at positions A, B, C and D.
I )2 )'+(! )2 U )2 +(* )2 U )2
where Um and UT are the uncertainty in measurement of mass flow rate and temperature of air, respectively. In the present experimental work, the uncertainty in measurement of mass flow rate and temperature of air is ±0.02 kg/sec and ±0.01 °C respectively. The measured uncertainty in heat transfer rate is 2.5%.
2.2. Experimental procedure
Preceding the experiments, the heat exchanger system is placed horizontally and liquid PCM is filled in the heat exchanger. A few test runs are required in order to check the leakage of PCM and to calibrate the system. Charging experiment is performed when PCM comes in the solid state. The initial condition for charging is established when all thermocouples inside the paraffin shows the same temperature.
During charging (melting of paraffin wax), hot air from proportional-integral-derivative controlled air heater at a required temperature, over the melting range of PCM, started to circulate and
data collection begins. Temperature data for all thermocouple is collected in every 10 minutes. The charging experiment is finished when the reading of all thermocouples is above the melting temperature range. The discharging (solidification of paraffin wax) experiment is then started with established conditions. Cold air at a constant flow rate and constant temperature started to circulate. Reading from thermocouples is measured and logged at the 10 min time interval as in the charging experiment. The mass flow rate of air during solidification was varied in the range of 0.0015 kg/sec to 0.003 kg/ sec. Several experiments were conducted during charging for different inlet temperatures of air (80 °C, 85 °C, 90 °C). Time wise variations of PCM temperature during melting, in axial and radial directions of shell are determined and discussed.
During discharging mode the drying chamber was attached to the LHS vessel in order to study the effectiveness of the use of an LHS for drying of food product and also on the drying kinetics of a food product. The schematic diagram of the experimental setup with drying chamber is shown in Fig. 2.
2.3. Selection and characterization of PCM
The aim of this work is to develop low cost heat storage for solar dryer. To achieve the objective low cost commercial grade paraffin wax was selected as a phase change material instead of pure paraffin wax. Commercial grade paraffin waxes are a combination of different, mainly straight chain hydrocarbons with more than 15 carbon atoms and are obtained from petroleum distillation [46,47]. Due to the combination of different hydrocarbons, its melting temperature range is large. Commercial grade paraffin wax manufactured by Indian oil company was used as a phase change material for the present study because it is cheap and easily available in different melting temperature ranges in Indian market.
For calculating the performance of the system the characterization study of PCM is necessary. Therefore, DSC (Differential Scanning Calorimetry) and thermal conductivity analysis of paraffin wax was conducted. DSC analysis of paraffin wax sample has been conducted for determining its specific heat, latent heat and melting range. DSC analysis was performed on a Thermal Analyzer Instrument (Pyris DSC 6000, TA) in heating and cooling cycle. Samples with a mass of 10 mg were sealed in an aluminum pan; an empty pan was used as reference. The analyses were performed in the temperature range of -10.00 °C to 170.00 °C with a heating and cooling rate of 10.00 °C/min and under a constant stream of nitrogen at a constant volume flow rate. The result of DSC analysis is shown in Fig. 5 and it shows large melting range. The selected paraffin wax for the present study is a combination of different straight chain
? E 10
D o "O U
« n> -20
-60.86
S --- \
_ I 1
•he— H
Peak = 3 3.03 °C T \ A ea = -1588.301 n iJ
Area = -17 5.486 mJ \ Li elta H = -158.830 3eak = 54.90 X i J/g
Delta H = 17.5486 J/g \
Temperature C'C)
Fig. 5. DSC curve of paraffin wax sample.
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Table 1
Thermo-physical properties of paraffin wax.
Property Value
Melting temperature range (°C) 41-55
Latent heat capacity (kJ/kg) 176
Specific heat (kJ/kg-K) 2.8
Density (kg/m3) 835
Thermal Conductivity (W/m-K) 0.21
hydrocarbons, so that its melting temperature range is large. Transient Plane Source (TPS) method was used for the measurement of Thermal conductivity of paraffin wax, with a means of Hot Disk Thermal Constants Analyzer TPS-2500. A simple experimental test using the classical Archimedes principle was used for measurement of density of paraffin wax. The measured thermo-physical properties of paraffin wax are shown in Table 1. The measured thermophysical properties of paraffin wax are agreed well with the available literature values [47-49].
3. Experimental results and discussion
Using the apparatus and procedures described above, several experiments were conducted to study the transient thermal behavior of the LHS unit during its charging (melting) and discharging (solidification) processes using air as heat transfer fluid (HTF). The selection of maximum charge time and the appropriate HTF temperature and mass flow rate for charging LHS was determined by solar availability time, and operational requirements of drying of food, i.e. air must be dry, hot and moving at a required velocity over the food. For majority of food product the temperature of air for drying should be in the range of 40-60 °C [10]. Daily solar availability in India for most of the season is nearly equal to 8 h, prompting the need to charge the heat storage system in this duration. The selection of inlet HTF temperature in the range of 8090 °C was also appropriate because this temperature range can easily be achieved by using an evacuated tube solar air heater [50-52]. The thermal performance of LHS is evaluated in terms of the temperature distribution of paraffin wax, time for charging and discharging, cumulative energy charged and discharged for different values of inlet temperature and mass flow rate of HTF. The effect of varying the inlet HTF temperature on the energy charged and discharged during charging and discharging processes is assessed at inlet HTF temperatures of 90 °C, 85 °C and 80 °C.
3.1. Transient variations of PCM temperature during charging ofLHS
Transient variations of PCM temperature in different angular and axial directions of LHS during charging indicate the variations of temperature distribution and heat transfer in different directions of LHS at different instants of time. The transient temperature distribution helps to determine the heat transfer rate and melted region in the LHS. The transient variations of PCM temperature are presented for HTF inlet temperature of 90 °C. The mass flow rate of HTF through LHS is maintained at a constant level of 0.003 kg/sec.
3.1.1. Transient variations of PCM temperature at different axial locations during charging ofLHS
Temperature from all thermocouples was recorded in angular and axial directions of LHS during charging. The angular (0°, 90° and 180°) and axial locations (A, B, C and D) in the LHS are shown in Figs. 3 and 4. In the angular directions, the average temperature were calculated by thermocouple readings, situated at position 0°, 90° and 180° as shown in Figs. 3 and 4. Fig. 6 shows the temperature profiles of PCM in the angular direction in the system at position A, B, C and D during charging at HTF inlet temperature of 90 °C. The
differences in temperature along the angular directions indicate that the temperature is not uniform in all directions. The high temperature was observed at location 180° compared to 0° and 90° locations after some duration of time during charging.
In all four axial locations (A, B, C and D) of heat storage the temperature profiles are similar as shown in Fig. 6. The temperature is nearly the same during the initial stage of charging and as time advances, the temperature increases at locations A and B are somewhat higher than those in locations C and D. As shown in Fig. 6a-d the increase in temperature at location 180° is more compared to other locations (0°, 90°) of heat storage. As shown in Fig. 6a at time instant of 80 min, the difference of temperature between angular locations of 90° and 180° is 5 °C at position A; this difference increases with time. At time instance of 110 min, the temperature difference of about 8 °C exists between the angular location of 0° and 180° of heat storage. Melting of PCM first occurred at an angular location of 180° near the pipe wall and melting front moves in the upward direction, so that the temperature of the PCM increases rapidly at angular location of 180°. As time advances the small temperature gradient is developed at the angular location of 180° and natural convective flow of melt occurred in the upward direction. The molten PCM moves towards the location of 90° due to gravity and low density. As the molten PCM comes into the contact with solid PCM it transfers heat to solid PCM. Thus, temperature increases more rapidly at angular location of 90° compared to 0°. Due to conductive dominated heat transfer at location 0° and low thermal conductivity of PCM, the temperature increase at location 0° is less compared to those at 90° and 180° angular locations of heat storage. At 480 min the melting of PCM is completed at locations 180° and 90° of heat storage. Small amount of PCM is left at 0° location of heat storage in the solid state.
Similar trends of transient variation in temperature are observed at location B, C and D. In all four axial locations of LHS the temperature at location 180° increases very rapidly compared to those at 0° and 90° locations of heat storage. The difference in temperature exists between the 180° and 0° location of heat storage at the end of the charging process. Three regions exist in the LHS during the charging process: solid region, mushy region and liquid region. The conduction phenomenon of heat transfer is responsible for the heat transfer inside the solid PCM. Solid PCM receives the heat from the liquid region by convection. The convective flow of melt inside the molten region is responsible for heat transfer. The convective flow of melt is developed due to density gradients in the molten region as a result of the temperature differences. The convective flow of melt enhanced the heat transfer within the melted PCM region.
Fig. 7 shows the time wise variation of PCM average temperature at different axial positions of LHS during charging at Tin = 90 °C and m = 0.003 kg/sec. Time wise variation of temperature at different axial positions during charging were calculated by taking the average of thermocouples readings at locations A, B, C and D. The position of A, B, C and D in LHS is shown in Fig. 3. The average temperature of PCM at these positions was calculated from the readings of all thermocouples situated in the respective position. The temperature of PCM at positions A and B increases more rapidly compared to that in position C and D. The high heat transfer occurs at positions A and B due to large temperature difference between PCM and HTF at the beginning of LHS. The temperature gradient exists between the inlet and outlet of HTF tube. The average temperature difference of HTF between the inlet and outlet of LHS is approximately 15 °C. The lowest temperature of PCM is observed at position D, which is located at the end of LHS due to low temperature difference between PCM and HTF. At the end of the charging process the temperature of PCM is nearly same for different locations of LHS. The average temperature of PCM at the end of charging process is 60 °C. The melting temperature of PCM used in the present analysis is 55 °C which shows that the PCM is melted in most of the region of LHS in the desired time duration.
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Tln = 90oC m= 0.003 kg/sec
i'1 i I
Melting range
35 ■ L ^
► Angular position 180° ■ Angular position 90° i Angular position 0°
100 200 300 400
Time (min)
(c) (d)
Fig. 6. (a-d) Time-wise variations of PCM angular temperature profiles at positions a, b, c and d during charging of LHS at Tin = 90 °C and m = 0.003 kg/sec (Error bars represent standard errors of the mean).
Fig. 7. Transient variations of PCM average temperature at positions A, B, C and D during charging of LHS at Tin = 90 °C and m = 0.003 kg/sec. (Error bars represent standard errors of the mean).
3.1.2. Transient variations of PCM temperature in the angular direction during charging ofLHS
For calculating the temperature distribution in different angular directions of LHS, the thermocouples are embedded at different angular locations of LHS. The detailed locations of thermocouple at different angular directions of LHS are shown in Fig. 4. The temperature profiles in the angular direction were calculated from thermocouples readings at different angular locations, namely 0°, 90° and 180°. The average temperature readings in the angular direction were calculated using time average readings of thermocouple at same angular position at locations A, B, C and D. For example, an average of all thermocouple readings at a particular time instant, at angular position 0° at position A, B, C and D give the average reading for the 0° angular location. In the same manner average temperature is measured at angular location 90° and 180°. The temperature profiles at three angular locations are shown in Fig. 8.
It is clearly shown in Fig. 8 that the temperature at 180° angular position is higher than angular positions of 0° and 90° at different instants of time during charging of LHS. During the initial stage of melting the heat is transferred by conduction to the PCM which is nearer to the heat transfer tube. As time advances more melting occurs around the heat transfer tube, due to density differences between warmer and cooler PCM the natural convection forms and molten PCM moves in the upward direction. Due to natural
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55 ----------------
m = 0.003 kg/sec T,„ = 90oC
«- 11
ii! ti
Melting range
W_____
► Angular position 0° ■ Angular position 90° » Angular position 180°
0 100 200 300 400 500 600 Time (min)
Fig. 10. The effect of HTF inlet temperature on charging time.
Fig. 8. Transient variations in PCM average temperature profile at different angular locations of the annulus during charging cycle of LHS at Tin = 90 °C and m = 0.003 kg/ sec. (Error bars represent standard errors of the mean).
convection, heat is transferred more rapidly at location 180°, located at the uppermost portion of heat storage. The low heat transfer is observed in other angular locations (90°, 0°) as shown in Fig. 8, due to low thermal conductivity of PCM and conductive dominated heat transfer. The temperature at the location (90°) is higher than the location (0°) of heat storage due to closeness to high temperature PCM between locations 90° and 180°.
3.2. Effect of heat transfer fluid temperature on the charging ofLHS
In some climatic conditions the air at temperature of 90 °C could not be obtained by evacuated tube solar air heater. To examine the effect of HTF (air) inlet temperature on the charging time and heat transfer rate, the charging study is also conducted for HTF inlet temperatures of 85 °C and 80 °C. Charging time is defined as the time required during charging process when the temperature of thermocouples at all positions of a LHS reached to melting temperature (55 °C). Fig. 9 shows the transient variations of PCM average temperature during charging cycle of LHS for HTF inlet temperatures of 90 °C, 85 °C and 80 °C and at a mass flow rate of 0.003 kg/sec for complete charging. Average temperature profile of PCM in LHS for different HTF inlet temperatures is measured using time average
Time (min)
Fig. 9. Average temperature profile of the PCM at various HTF inlet temperatures during charging process at HTF mass flow rate of 0.003 kg/sec.
readings from all thermocouples at locations A, B, C and D, the positions of thermocouples are determined in Figs. 3 and 4. The temperature profiles show the different state of heating i.e. sensible heating, phase change heating and liquid heating.
The temperature profiles illustrate that the average temperature of the system is higher when the temperature of HTF was 90 °C. Sharp rise in temperature takes place during the temperature range of 35-45 °C, due to sensible heating of PCM. At each axial location, increasing the inlet temperatures of the HTF leads to smaller melting times and gives more enthalpy flow from the HTF into the PCM. The temperature difference between PCM and HTF is the main driving power for the heat transfer process. As the temperature of HTF increases, the rate of heat transfer increases proportionally. The high temperature of PCM is observed when HTF inlet temperature is 90 °C, compared to HTF inlet temperatures of 85 °C and 80 °C. During the early stage of charging heat is transferred in the form of sensible heat until the temperature of PCM reaches to melting range of PCM. During the early stage of charging, conduction controls the heat transfer in the PCM. As more and more PCM get melt, the effect of buoyancy driven convection heat transfer resulted to higher melting rate.
The effect of HTF inlet temperature on charging (melting time) is shown in Fig. 10. In this figure the time required to complete melting is shown in terms of HTF inlet temperature. Charging time is defined as the time required during charging process when the temperature of thermocouples at all positions of a LHS reached to melting temperature (55 °C). It is clearly shown in Fig. 10 that by increasing the HTF inlet temperature, the time required for melting decreases. The total melting time decreases up to 9% and 16% when the inlet HTF temperature increases from 80 °C to 85 °C and from 80 °C to 90 °C, respectively. Complete melting was achieved within 8 hrs 40 min duration of time for the HTF temperature of 90 °C, whereas 9 hrs and 20 mins and 10 hrs 20 mins durations of time were required for HTF inlet temperature of 85 °C and 80 °C respectively. Major portion of PCM melted in the time limit of 8 hrs when the HTF inlet temperature was 90 °C. HTF should be supplied at 90 °C or above temperature to achieve complete melting within 8 hrs duration of time.
3.3. Transient variations of PCM temperature during discharging of LHS
Fig. 11 shows the transient variation of PCM temperature during discharging at four axial locations. Air (which used as HTF) at a
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Time (min)
Time (min)
Fig. 11. Transient variations in PCM average temperature at locations A, B, C and D of LHS during discharging of LHS at Tin = 30 °C and m = 0.003 kg/sec. (Error bars represent standard errors of the mean).
Fig. 12. Average temperature profiles of PCM during discharge cycle for different mass flow rates of HTF. (Error bars represent standard errors of the mean).
constant mass flow rate of 0.003 kg/sec and at a constant temperature of 30 °C is passed through the heat storage system to extract the heat from the system during discharging of LHS. It is found that the average temperature of PCM drops from 60 °C to 40 °C in 890 min as shown in Fig. 11. The temperature drops suddenly during the initial period of discharging due to release of sensible heat. The heat releases rapidly during the initial period due to high temperature difference between HTF and PCM, which is the main driving power for heat transfer. As soon as the temperature of the system drops below the liquidus temperature (55 °C) of PCM the temperature decreases very slowly.
At the start of discharge process the temperature of the paraffin wax is high, and the paraffin wax is in the liquid state. At this state liquid sensible heat was discharged from the system and its temperature drops from 60 °C to 50 °C. At this time instant the con-vective driven heat transfer occurs in the molten PCM. The curve indicates that heat is extracted in the form of liquid sensible heat for first 100 min followed by phase change heat. As PCM near the heat transfer tube reaches the solidification point, the solidification process starts and proceeds into the phase change controlled period. Then the paraffin wax adjacent to the PCM tube begins to freeze and discharge its latent heat. The frozen layer constitutes the main heat resistance for heat transfer from the inner paraffin wax to outside. Hence, we find that the PCM releases its sensible heat very rapidly, and then a longer time is needed to transfer the latent heat during the phase change. The major portion of heat dissipated from the PCM is its latent heat. The temperature drops from 50 °C to 40 °C in approximately 790 min. In this duration, latent heat is released from the system to HTF. The average solidification time recorded during the sensible and phase change recovery was 890 min in the temperature range of 60-40 °C. In the first 100 min heat was released from the system in the form of sensible heat and high temperature drop occurred in the system. The time required to heat recovery in the form of latent heat is approximately 8 times as compared to heat recovery in the form of sensible heat.
3.4. Effect of heat transfer fluid flow rate on the discharging of the LHS
The effect of the HTF flow rate on the discharging (solidification) is shown in Fig. 12. Discharging process was performed for HTF mass flow rates of 0.0015,0.0022 and 0.003 kg/sec and the temperature of HTF at inlet is maintained at constant temperature of 30 °C for all experiments. The temperature profiles during discharg-
ing shows that the average temperature of the PCM is higher for mass flow rate of 0.0015 kg/sec compared to mass flow rates of 0.0022, 0.003 kg/sec. At each axial location, decreasing the flow rate leads to low heat transfer from the PCM in to HTF which further increases the total time for discharging. As the flow rate of HTF increases, the rate of heat transfer increases proportionally.
The effect of HTF mass flow rate on discharging time is shown in Fig. 13. Experimental results showed that the discharging time increased by decreasing the mass flow rate of HTF and more time is taken by heat storage system for discharging. Discharging time was 890,1010 and 1100 min for HTF mass flow rate of 0.003, 0.0022 and 0.0015 kg/sec, respectively. The discharging time increased by 13% and 23% when the mass flow rate of HTF decreased from 0.003 kg/sec to 0.0022 kg/sec and from 0.003 kg/sec to 0.0015 kg/sec, respectively.
3.5. Energy charged and discharged during charging and discharging of LHS
The thermal performance of the LHS is measured in terms of cumulative energy charged and discharged, during the charging and discharging process of LHS, respectively. The cumulative energy charged/discharged is evaluated by integrating the instantaneous heat transfer over the entire charging/discharging process.
Discharging time = -1E+07m2 - 75714m + 1245.7
Tin = 30o C
0.001 0.0015 0.002 0.0025 0.003 0.0035
Mass flow rate (m) (kg/sec)
Fig. 13. The effect of mass flow rate of HTF (m) on discharging time.
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= h H H
g K •■S
1600 1400 1200 1000 800 600 400
100 200 300 400
Time (min)
Fig. 14. Time wise variations of cumulative energy given by HTF during charging of LHS at various inlet temperatures of HTF (Tin = 80 °C, 85 °C, 90 °C) (mass flow rate of HTF m = 0.003 kg/sec).
3.5.1. Energy charged during charging ofLHS
Instantaneous energy (qch) given by HTF during charging can be computed by the following relation:
Qch - m cp,HTF (Tin - Tout )
where m is the mass flow rate of HTF, Tin = Temperature of HTF at the inlet of LHS, Tout = Temperature of HTF at the outlet of LHS, cp,HTF = Specific heat of HTF.
Cumulative energy (Qcum,ch) given by HTF during charging can be computed by following relation:
Qcum,ch - ["ohi dt + Cqch2 dt +.......+ ï'" qcKn
J0 Jti Jt"_i
where tn and tn-1 are the time instant between which heat transfer is measured.
The effect of HTF inlet temperature on cumulative energy given by HTF is shown in Fig. 14. The flow rate of HTF is maintained at 0.003 kg/sec through the LHS during the experiment. The cumulative heat given by HTF is shown for total melting time of 520 min for HTF inlet temperature of 90, 85 and 80 °C. It is seen from Fig. 14 that the cumulative heat given by HTF is nearly same at the initial stage of melting, but as the time advances the slope of the curve varies depending on the HTF inlet temperature. It is due to fact that as time advances the temperature difference between HTF and PCM decreases with time. At the time instant of 520 min the total cumulative energy given by HTF was approximately 1088,1314,1517 kJ for HTF inlet temperature of 80, 85 and 90 °C respectively.
3.5.2. Energy discharged during discharging ofLHS
Instantaneous energy (qdis) gained by HTF during discharging can be computed by following relation:
Qdis - m cp,HTF (Tout - Tin )
where m is the mass flow rate of HTF, Tin = Temperature of HTF at the inlet of LHS, Tout = Temperature of HTF at the outlet of LHS, cp,HTF = Specific heat of HTF.
Cumulative energy (Qcum) gained by HTF during discharging can be computed by following relation:
fti j*t 2 çt"
Qcum,dis - L qdisi dt + L qdis2 dt +.......+ [ qdis,n
J0 Jti Jtn_l
H H « M
400 600
Time (min)
Fig. 15. Time wise variations of cumulative energy gained by HTF during discharging of LHS at different flow rates of HTF and at a constant HTF inlet temperature (Tin) of 30 °C.
where tn and tn-1 are the time instant between which heat transfer is measured.
Fig. 15 shows the variation of cumulative energy gained by HTF during discharging of LHS at HTF mass flow rate of 0.0015, 0.0022 and 0.003 kg/sec. The rate of cumulative energy gain is higher during the initial state of discharging but as the time advances the slope of the curve varies depending on the flow rate of HTF at the inlet of LHS. The convection thermal resistance on the HTF side plays a larger role in the overall heat transfer process between PCM and HTF; as the HTF velocity decreases the convection thermal resistance increases, hence the instantaneous heat transfer decreases. At higher flow rate of HTF, as the instantaneous energy gain rate is higher the cumulative energy gain rate is also higher. At the time instant of 890 min the total cumulative energy gain by HTF are approximately 1168 kJ, 1074 kJ and 821 kJ for HTF flow rate of 0.003 kg/ sec, 0.0022 kg/sec and 0.0015 kg/sec, respectively. The cumulative heat gain for a HTF flow rate of 0.003 kg/sec is 8.7% higher compared to the HTF flow rate of 0.0022 kg/sec. The difference in cumulative heat gain rate is smaller during the initial state of discharging but as the time advances the difference in cumulative heat gain rate increases till the end of discharging process. The effect of the HTF flow rate on the cumulative energy gain is more pronounced at low flow rate of HTF. It is clear from Fig. 15 that the decrease in cumulative heat transfer is high for a HTF flow rate of 0.0015 kg/sec compared to 0.0022 kg/sec. It is observed that by increasing the flow rate of HTF there is a decrease in outlet temperature of HTF, since the HTF has a shorter time in contact with the PCM. The higher outlet temperature of HTF is observed for low flow rate of HTF, but due to low flow rate of HTF the instantaneous heat transfer is lower. The instantaneous heat transfer is higher for high flow rate of HTF. It is inferred from the above results that the high flow rate of 0.003 kg/sec is able to provide high cumulative energy transfer for the present system.
3.6. Temperature gain of HTF during discharging ofLHS
Temperature gain of HTF (air) during discharging of LHS at HTF velocity of 8 m/sec is shown in Fig. 16. The air velocity of 8 m/sec corresponds to mass flow rate of 0.003 kg/sec. The temperature of air at the inlet of LHS was maintained at 30 °C. Temperature gain of air, in the range of 17 °C to 5 °C for approximately 10 hrs duration was achieved during discharging of LHS. The high temperature gain is obtained during the initial period of discharging due to large temperature difference between air and PCM. The outlet
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18 16 14 12 10 8 6 4 2
400 600
Time (min)
Fig. 16. Temperature gain of HTF during discharging of LHS at inlet air velocity of 8 m/sec through LHS.
temperature of air through LHS was sufficient for drying agricultural food product.
The performance of the system is comparable with other configurations (packed bed, plate heat exchanger) of heat storage reported in the literature [43,44,53-56]. For example, output air temperature in the range of 42-30 °C was reported by Alkilani et al. [53] during discharging cycle of PCM storage unit for air flow rate of 0.05 kg/s. PCM storage unit consists of inline single row of cylinders contain a compound of paraffin wax. The freezing time of the PCM cylinders was approximately 8 hrs. Similarly, Fath [43] mathematically evaluated the performance of solar air solar air heater with a built-in thermal energy storage system. The heater absorber consists of a corrugated set of tubes filled with a paraffin wax as a latent heat thermal energy storage material. The temperature of air at the outlet was approximately 5 °C above the ambient temperature for about 16 h time duration due to the integration of heat storage. Esakkimuthu et al. [54] conducted an experimental study on PCM based thermal storage for solar air heater. The PCM was encapsulated in the form of spherical balls to form a packed bed. The temperature of air at the exit of storage tank was in the range of 65-50 °C and the air is supplied at ambient temperature of 30 °C at the inlet of storage. The discharging time is approximately 4 hours for the mass flow rate of 200 kg/hr. A significant improvement in the heat transfer rate is obtained by encapsulation of PCM in the different geometries to form a packed bed. However the high initial cost and high pressure drop are the major drawback of such units. The main advantage of shell and tube configurations of heat storage over packed bed is simple in design, low in cost and very less pressure drop. The shell and tube latent heat storage system does not require large modification in the design of conventional solar dryer.
3.7. Drying kinetics of food product during discharging of LHS
To check the effectiveness of LHS for drying of food product, the fabricated drying unit is attached with the LHS during discharging. For drying of food the temperature of the air at the outlet of LHS should not be high (above 60 °C) otherwise it affects the quality of dried food product. The potato was used as a model food product for the present study. The oven-drying Method (AOAC method 934.01, procedure 4.1.03) [57] was used for determining the initial moisture content of potato slice. In oven-drying method known mass of the sample is placed in the oven and heated under specified temperature condition; the weight of the sample is recorded at different time intervals. The loss in weight of the sample is equal to the
100 90 80 70 60
60 80 Time (min)
Fig. 17. Drying kinetics of potato slices during drying under different velocities of air.
moisture removed from the sample. The drying of the sample is continued until its weight does not change with time. The difference of the weight of the sample at the initial and at the end of drying process represents the moisture content in the sample. The initial moisture content of potato slice was 88% (wet basis). Before drying, potatoes were cut into slices with a thickness of 2 mm. Drying was performed using air, which firstly passed (and extracted energy from) the LHS. The drying was continued until the moisture of potato slices fell to the safe moisture level.
During discharging of LHS, drying kinetics of potato slices was monitored to check the effectiveness of LHS for drying. The weight of dried potato slices is calculated at different time intervals during the drying process to calculate the moisture content of potato slices at different instants of time. The moisture content is the relation of the mass of the water and the total mass of fresh food. The moisture content of potato slices on wet basis is calculated by following equation:
Jms - m0 ) x 100
where Mw = Moisture content on wet basis (%), mg = mass of the potato slices before drying, and mo = mass of the potato slices after drying.
The drying rate of the food product shows the rate of moisture removal from the food product per unit time. The drying rate is calculated by the following equation:
Г-\ ■ 4. Mw t + dt — Mw t
Drying rate = —-—
where Mw,t+dt and Mw,t are the moisture present in the food at time t + dt and t, respectively.
Drying kinetics of potato slices were studied for different flow rates of air through LHS. The velocities of air corresponding to flow rate of 0.003 kg/sec and 0.0015 kg/sec were 8 m/sec and 4 m/sec, respectively, through the heat transfer tube. The drying curves of potato slices undergoing drying at different inlet air velocities are shown in Fig. 17. The time required to dry potato samples from initial moisture content of 88% (wb) to the final moisture content of 4.3% (wb) was 105 and 130 min for a HTF velocity of 4 and 8 m/sec, respectively. Air velocity through LHS had an important effect on drying time. The results of the experimental study revealed that the mass
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Fig. 18. Variations of drying rate of potato slices for different velocities of air.
transfer from the sample is more rapid for low velocity of HTF. This is due to the fact that air at low velocity extracted more energy from the LHS due to the longer time it exchange heat from the PCM so that its temperature at the outlet of LHS was higher. It is also observed from Fig. 17 that the drying occurred at very low rates after 80 and 100 min when using inlet air velocity of 4 and 8 m/sec, respectively.
Fig. 18 shows the variations of drying rate of potato slices under different velocities of air through LHS. The high drying rate is observed in the initial period of drying due to rapid release of mass transfer from the samples of food product. The same trend is observed for different velocities of air i.e. 4 and 8 m/sec. The high drying rate is followed by a subsequent decrease in drying rate. Similar trends are observed by other investigators in the literature [58,59]. The influence of air velocities can be observed in the entire range of the sample moisture content. The high drying rate is observed for air velocity of 4 m/sec at the beginning of drying process and as the time advances the difference between the drying rate for air velocities of 4 and 8 m/sec decreases. The initial drying rate is more influenced by the velocity of air.
4. Conclusions
Based on the results of the present experimental study on the charging and discharging of LHS, the following conclusions have been drawn:
• Heat transfer during charging is largely influenced by natural convection. Melting of PCM first occurred at the uppermost section of the system, and then it moves downward.
• Due to the horizontal position of heat exchanger the temperature gradient in axial direction is very small.
• The experimental results show that melting rate is faster at the uppermost section due to buoyant effect.
• The experimental studies show that by decreasing the temperature of HTF from 90 °C to 80 °C, the charging time is increased by 20%.
• Heat transfer during discharging is mainly governed by conduction and due to low thermal conductivity of PCM, the rate of heat transfer during discharging is comparatively low.
• Time required for discharging of LHS is longer compared to time required for charging of LHS due to low heat transfer rate between PCM and HTF during discharging.
• The discharging time increased by 23% by decreasing the HTF flow rate from 0.003 kg/sec to 0.0015 kg/sec.
• The highest cumulative heat transfer was observed for high flow rate of HTF during discharging of LHS for a given duration of time.
• High heat transfer is observed during initial period of discharging due to the release of the sensible heat of PCM and due to high temperature difference between PCM and HTF.
• A temperature gain of air in the range of 19 °C to 5 °C for approximately 10 hrs duration was achieved during discharging of LHS.
• Experimental results show that LHS is suitable to supply the hot air for drying of food product during non-sunshine hours or when the intensity of solar energy is very low.
Nomenclature
Tin HTF inlet temperature, °C
Tout HTF outlet temperature, °C
m Mass flow rate of HTF, kg sec-1
qch Instantaneous energy during charging, kJ
Qcum,ch Cumulative energy given by HTF during charging, kJ
qdis Instantaneous energy during discharging, kJ
Qcum,dis Cumulative energy gained by HTF during discharging, kJ
cp,HTF Specific heat of HTF, kJ kg-K-1
t Time, sec
Vin Velocity of HTF at the inlet of LHS, m sec-1
Mw Moisture content, % wet basis
mg mass of the sample before drying, kg
mo mass of the sample after drying, kg
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