“EXPERMENTAL INVESTIGATION ON LAB-SCALE LATENT HEAT REPERTORY

PARADIGM CONSIDERING SUGAR ALCOHOL AS PHASE CHANGE MATERIAL”

Ayush Kumar1

M.Tech Scholar1

Department of Mechanical Engineering Rabindranath Tagore University, Bhopal (M.P.) India

___________________________________________________________________________Abstract : Demand of energy is increasing day by day in manufacturing,

transportation as well as interior area which has shown to a decrease in availability of

fossil fuel which in turn increases the price of fuel day by day. These fuels also cause

global warming and greenhouse effect. Solar energy one of the clean energy resources

but it is intermittent in nature, not continuous, depends on weather, season and climate

condition, that is the main drawback of solar energy, but problem is solved by storing

the energy with the help of latent heat storage system. Latent heat storage system is

gaining more attention in recent years with the increased emphasis on more renewable

energy sources. Energy storage is necessary whenever there is a greater amounts of

energy being produced than is required.

This thesis presents the performance tests carried out on a lab-scale latent heat

energy storage prototype during charging and discharging processes. The storage unit

is a shell-and- spiral tube type heat exchanger tubes, designed for an LHS capacity of

1.3 MJ. D-Mannitol as phase change material used. Thermion 55 is used as the heat

transfer fluid. Performance parameters viz., charging and discharging time and energy

storage and discharge rate were evaluated at different operating conditions. The

effects of HTF inlet temperature and flow rate on the storage characteristics of LHES

prototype were analyzed. It is observed that the temperature gradient in the

longitudinal direction of the LHES during melting process is significant. This is due

to the temperature gradient and natural convection heat transfer that occurred around

the liquid of PCM while charging. During the solidification process, the longitudinal

direction temperature variation is negligible as the solidification phenomenon is

controlled mainly by the conduction heat transfer. To overcome the effect of low

thermal conductivity of D-Mannitol, the effective surface area and overall thermal

conductivity for heat transfer is significantly improved by employing spiral shape

tube. Moreover, the vertical orientation of shell and spiral tubes supports natural

convection, which can assure rapid charging of D-Mannitol in LHS unit. Enthalpy

gradient is noticed between D-Mannitol at top, central and bottom positions in LHS

unit. it is concluded that volume flow rate of HTF has a relatively moderate influence

on thermal performance compression to increase in inlet temperature of HTF. It took

about 70 min/110 min for charging/discharging of the LHS prototype.

Keywords: Latent Heat Storage System; Thermal Energy Storage System; Heat

Exchanger; D-Mannitol; Sugar Alcohols; Phase Changing Materials; Natural

Convection; Renewable Energy; Heat Transfer.

__________________________________________________________________________________I. Introduction

Present days the demand of energy is increasing continuously because of high

consumption of energy in transport, domestic and industrial sectors. Fossil fuel was

only the source of energy that has fulfilled the needs of energy for long time. The high

consumption of fossil fuel leads to decrease in availability of reserves and its effect is

reflected on the price of fuel which is continuously increases and expected to continue

for upcoming years. Solar energy is one of the most important, renewable and clean

source of energy that can fulfil all the needs .

The amount of solar energy is not constant on the earth, its depend on day-night,

Energy storage system provided solution of mismatch between energy demand and

energy supply. Energy storage more important when energy is intermittent in nature,

dependent in nature, seasonal variation, demand of energy more than supply,

transportation of energy one place to another place. Nature themselves store energy in

the form of fuels, biomass, ocean thermal energy, high elevation of water in terms of

potential energy, plants material, mineral etc. Phase change materials store energy in

the form of sensible heating, latent heating and chemical heating. They store energy in

melting process and release energy in solidification process (changing from one phase

to another phase). When such material freezes it release large amount of energy in the

form of latent heat of fusion or/and energy of crystallization and melt it absorb energy

from immediate environment. Phase-change materials including organic paraffins,

metallic alloys and inorganic salts undergo reversible phase transformation.

Due to their isothermal behavior during the melting and solidification processes,

such materials can be used in such diversified applications as latent heat storage in

building or thermal control in electronic modules.

A latent heat storage system is preferable to sensible heat storage in applications

with a small temperature swing because of its nearly isothermal storing mechanism

and high storage density, based on the enthalpy of phase change (latent heat).

In a latent heat thermal storage system, during phase change the solid-liquid

interface moves away from the heat transfer surface. In the case of solidification,

conduction is the sole transport mechanism, and in the case of melting, natural

convection occurs in the melt layer and this generally increases the heat transfer rate

compared to the solidification process. During the phase-change process, the surface

heat flux decreases due to the increased thermal resistance of the growing layer of the

molten or solidified medium. This thermal resistance is significant in most

applications and especially when the organic phase-change materials are used,

because the latter have rather low thermal conductivity.

II. LITERATURE REVIEW

Energy is the chief support of a country economic development. Due to rapid

growth in industrial and domestic energy demands, the dependency on fossil fuels to

meet required energy demands have further increased. However, the excessive usage

of fossil fuels has provoked global warming and climate change. Therefore, to limit

environmental pollutions and to meet energy demands, developments in technologies

are essential to utilize renewable energy sources. Solar energy is considered as a

crucial renewable energy source due to its clean, free of cost and worldwide

distribution of incident solar radiations. To overcome the inconsistent and

unpredictable nature of solar energy, TES system can provide a feasible solution. TES

system can be utilized to capture thermal energy at solar peak hours and release it at

solar off-peak hours or night times. To shorten the energy supply and demand gap,

LHS systems can be employed due to their higher thermal storage density, phase

change materials availability at wide range of temperatures, higher latent heat

capacity at almost isothermal condition and lower vapour pressure.

Analysis was conducted on four different boxes considering the waxes for use

in a latent heat storage unit.Benchmark experiments analyzing the cooling curves of

the waxes conducted.In addition,data was collected using a differential scanning

calorimeter to measure the latent heat of the phase change material as well as the

onset temperature of melting in the waxes.Based on the analysis.palm wax was

chosen as the phase change material to be used in a prototype latent heat storage.

In the search and development of better PCMs,sugar alcohol are recently

proposed as potential candidates.The high latent heat in sugar alcohol's liquid- solid

phase transition provides high storage density.The stable supercooled state of sugar

alcohol enables the storage of liquid of liquid at relatively low temperature with low

heat loss.Despite of the above advantages,there are still issues are resolved.

This project proposes a novel application of Phase Change Material (PCM)

based cooling system for supplementary cooling.One of the engineering challenges

that prevents the commercial application of latent heat energy storage system is the

lack of computationally efficient methods to model the transient nonlinear behaviour

of the system.In this dissertation, efficient modelling approaches for latent heat

energy storage systems are proposed at different scales for optimal design and

operational research.

In the current work,a shell and tube type heat exchanger with PCM on the

shell side and heat transfer on the tube side are considered.The effect of flow rate and

inlet temperature of heat transfer fluid on melting and solidification times are

investigated with single and double pass(counter and parallel) arrangements of Heat

Transfer Fluid (HTF).The major difficulty encountered in the melting of PCM is the

accumulation of solid(unmelted) part at the bottom during the charging process,while

the liquid part remains at the top during discharging process,which decrease the

efficiency of the system to quite a great extent.In this study,an attempt has made to

improve the efficiency of the system by considering two configuration of the shell and

tube heat exchanger and it is found that the latter case has better performance.

III. OBJECTIVE & PROBLEM FORMULATION

3.1 Objective

1. Time of Charging and Discharging are minimize:

We study the solidification and melting time of vertical Spiral

tube- type

fluid flow Latent heat thermal energy storage system.

2. Effect of inlet temperature and mass flow rate on Latent heat thermal

energy storage system prototype:

We study the performance of increase the inlet temperature and mass

flow rate of latent heat thermal energy storage system. To calculate

the variation in duration of charging and discharging at different

flow rates.

3.Rate of storage and recovering:

We study the rate of storage and recovering on Latent heat thermal

energy storage system mainly it depends on thermal physical

properties conductivity and thermal diffusivity,so we add high thermal conductivity

particle.

3.2 Problem Formulation

In this study we found that sugar alcohols are one of those PCMs which have

high latent heat storage capacity. D-Mannitol, which is a sugar alcohol, is studied and

experimental investigation using a latent heat energy storage system is shown in this

work.

IV. EQUATION

• SENSIBLE HEAT STORAGE

Thermal energy is stored by raising the temperature of a solid or a liquid medium by using its heat capacity. Sensible heat, Q, is stored in a material of mass m and specific heat Cp by raising the temperature of the storage material from T1 to T2 and is expressed by

Q = ∫T1𝑇2𝑚Cpdt ...................(1)

Q = ∫T1𝑇2 𝑚Cpdt..................(2)• LATENT HEAT STORAGE

Latent heat storage uses the latent heat of the material to store thermal energy.

Latent heat is the amount of heat absorbed or released during the change of the

material from one phase to another phase.Thermal energy may be stored as latent heat

if a material undergoes phase transition at temperature that is useful for the

application. If a material with a phase change temperature of Tm is heated from T1 to

T2 such that T1 < Tm < T2, the thermal energy Q stored in a mass m is

Q = ∫T1𝑇𝑚 𝑚Cp𝑑𝑡 + 𝑚𝜆 + ∫Tm𝑇2 𝑚 C pdt ........(3)

• ENERGY STORED

ES, C = Sensible Heat = m × CPS(TM-Tini) + m × CPL(TT-TM)…… (4)

EL, C = Latent Heat = m × L × θ......................................(5)

ET, C = Total Energy = Sensible Heat + Sensible Heat ...........…. (6)

• ENERGY DISCHARGED.

Similar to the charging phenomenon, energy in the forms of sensible and

latent heat gets discharged from the PCM during the discharging process. Sensible,

latent and total heat discharge d from the PCM during the discharging process can be

calculated. The derivation of EL, C and EL, D is given in the publication of the author’s

Niyas et al.

ES, D = Sensible Heat = m × CPL(TM-Tini) + m × CPS(TT-TM) …… (2) EL, D = Latent Heat = m × L × θ ……. (3)

ET, D = Total Energy = Sensible Heat + Sensible Heat.................…. (4)

VI. METHODOLOGY

Most of the PCM, melt over a finite temperature range. This temperature range

wherein, the melt exists in both solid and liquid phases is generally called mushy

zone. The highest temperature at which, the material is completely in the solid state is

called solidus temperature (TS = TM -ΔTM) and the lowest temperature at which, the

material is completely in the liquid state is called liquidus temperature (TL = TM +

ΔTM). Melt fraction is a non-dimensional parameter, which quantifies the percentage

of liquid phase in the mushy region. Melt fraction of the PCM can be calculated based

on the lever rule applied between the solidus and liquidus temperatures. The

temperature (T) in Eq. (1), is the temperature recorded by the thermocouple in the

PCM region.

Mass Fraction(θ) =(T-TS)/(TM-TS) …………………………. (1)

{0 for T < TS

{0-1 for TS < T < TL

{1 for T > TL

V. RESULTS AND DISCUSSION

Various results obtained during the performance testing of the lab-scale LHS

prototype in the charging and discharging processes are presented. The ambient

temperature during the performance tests (charging and discharging processes) was

about 30 °C.

5.1 CHARGING

5.1.1. TEMPERATURE VARIATION IN CHARGING

Fig. 5.1 and 5.2 shows the local and average temperature variations of the

LHS prototype during the charging process. Initially, the LHS prototype is at 150 °C.

When the HTF at 210 °C is passed through the HTF tubes, heat transfer takes place

from the high-temperature HTF to the comparatively low-temperature PCM through

the HTF tubes. It can be noted from Fig. 5.1 that the increase of temperature is faster

in Top and Middle part than Bottom part. This is due to the fact that there exists a

higher heat transfer potential due to more temperature difference in the entry portion

than the exit portion of the prototype. It can be seen from Fig. 5.1 that the increase in

temperature of the PCM in the top region of the prototype is faster than that of middle

and bottom regions in all the portion. This is due to the buoyancy-driven natural

convection during which the high temperature and less dense particles present in the

bottom portion of the prototype move to the top portion of the prototype. Also, it can

be seen from Fig. 5.1 that there is a steady increase in temperature up to a certain

point, after that the slope of the curve decreases and finally after a certain interval, the

temperature has increased sharply. This transition is due to the melting phenomenon

during which a large amount of latent heat gets stored in the PCM. This is due to the

convective movement of the higher temperature particles in the top portion, by which

the temperature has increased sharply. Hence, the distribution of the temperature in

the LHS prototype during the charging process is in line with the expected and the

trends are similar to the results reported in the literatures Hosseini et al. [27]. Average

temperature shown in Fig. 5.2 is the arithmetic mean of the local temperatures

measured at nations in the prototype. The average temperature of PCM reaches about

200 °C in 70 min, which is 10 °C less than the HTF inlet temperature. The

temperature of the PCM in the LHS prototype varies in axial, radial and angular

directions. Hence, to find the exact volumetric average temperature of the prototype

with better accuracy, an experimentally validated numerical model is required. In the

present work, certain temperature data are recorded in the LHS prototype that are

meant for validating a developed numerical model developed by the author’s Niyas et

al. team. The results of the developed numerical model is fully volumetric averaged

and are published in an article Niyas et al. [52].

Mass Flow Rate: 1.8 L/minInlet temperature of HTF- 210°C

Fig.5.1 Local Temperature versus time during melting of PCM

Mass Flow Rate: 1.8 L/minInlet temperature of HTF- 210°C

Fig. 5.2. Average temperature variation during melting of PCM

5.1.2. AXIAL TEMPERATURE VARIATION

Fig. 5.3 shows the axial temperature variation of the LHS prototype during the

charging process. Temperature measurements made at the middle portion of each of

the three parts are compared. It can be noted from Fig. 5.3 that PCM temperature in

the Top part in the entry portion of the LHS prototype is increased faster than the

bottom part. This is mainly due to the existence of a higher temperature difference

between the PCM and HTF at the Top part of the prototype. During the flow of HTF,

it exchanges heat with the PCM thereby reducing its temperature. Because of this, the

temperature difference between the PCM and HTF is low near the bottom part of the

prototype. But, once the PCM near the entrance region (Top part) gets heated up to a

higher temperature, the temperature difference between the HTF and PCM in the

entrance region gets reduced. Due to this, the heat transfer rate between the HTF and

PCM becomes lesser in the entrance region and the temperature reduction of HTF

becomes slower. Hence, a relatively higher temperature HTF will be available near

the Bottom region of the LHS prototype. This is due to the fact that during that time,

PCM in the first two Top and Middle region are completely melted and high

temperature HTF with minor temperature drop is available for charging the Bottom

region.

Mass Flow Rate: 1.8 L/minInlet temperature of HTF- 210°C

Fig.5.3. Axial temperature versus time during melting of PCM.

5.1.3. ANGULAR TEMPERATURE VARIATION

Fig. 5.4 shows the angular temperature variation of the LHS proto type during

the charging process. Temperature measurements made at the left, middle and right

positions of the middle region are compared. It can be noted from Fig. 5.4 that the

PCM temperature at all the three positions was almost same with negligible difference

until about 160 °C. After that, the temperature variation profile for the three positions

started deviating. This is due to the natural convection, which happens after the

formation of liquid PCM. The high temperature and less dense PCM started moving

from the middle and bottom positions to the top position, leaving the cold and dense

PCM in the middle and bottom positions of the LHS prototype. This steep increase in

PCM temperature in the top position continues until the temperature difference

between HTF and PCM in the top position is quite enough. Once this temperature

difference becomes marginal, the PCM temperature in the top position also increases

slowly. During this period, the PCM temperature of the middle position increased

steeply. This is due to the natural convection and presence of reasonable temperature

difference between the HTF and PCM in the middle position of the LHS prototype. A

similar trend with a lesser magnitude is also observed in the PCM temperature of the

bottom position.

Mass Flow Rate: 1.8 L/minInlet temperature of HTF- 210°C

Fig.5.4 Angular temperature versus time during melting of PCM

5.1.4. EFFECT OF HTF INLET TEMPERATURE AND FLOW RATE

The charging performance of the LHS prototype in the charging process can

be controlled by two major parameters:

(1) HTF inlet temperature and

(2) HTF flow rate.

Hence, a detailed parametric study is vital for the better understanding of the

LHS system. Fig. 5.5 shows the influence of HTF inlet temperature and Fig. 5.6 flow

rate on the charging rate of the LHS prototype and their value at respective table 5.1

and 5.2. Experiments were conducted with the same initial PCM temperature of T =

150 °C for three different HTF inlet temperatures viz. 170 °C, 190 °C and 210 °C, at

two different HTF flow rates, viz. 1.8 l/min and 2.1 l/min. Two observations can be

directly inferred from Fig. 5.5.

(1) The charging time is lower for the higher HTF inlet temperature case. This

is due to the huge temperature difference that exists between the HTF and PCM.

(2) The charging time is lower for the higher HTF flow rate case. This is due

to the higher convection heat transfer coefficient in the HTF side, which enhances the

overall heat transfer rate of the LHS system.

Though the charging time is lower for the higher HTF inlet temperatures and

higher HTF flow rates, there exists a threshold in both the parameters beyond which

the decrease in the charging time is too minimal. Because, there is a limit in the heat

storing rate of the PCM, which is mainly due to its low thermal conductivity. This

phenomenon can be noted well in the cases of different HTF inlet temperatures for a

specific HTF flow rate. There exists a huge reduction in the charging time between

the cases with HTF inlet temperatures of 170 °C and 190 °C, then the cases with HTF

inlet temperatures of 190 °C and 210 °C. Similar effect is also noticeable to a lesser

extent in the cases of different HTF flow rates for a specific HTF inlet temperature.

Hence, varying the HTF inlet temperature has a greater impact on the charging time

when compared with HTF flow rate. Still, the effect of HTF flow rate is more

pronounced at lower HTF inlet temperature. Because, at lower HTF inlet temperature,

the temperature difference between the HTF and PCM is less and hence, the heat

transfer is dominated by the convective action of the HTF flow rather than the

temperature difference between the HTF and PCM.

Table 5.1 Time taken by charging at different Temperature

Inlet

Temperature

of HTF

Flow rate Charging time

(Approximate)

Reduction in charging time

by increasing the HTF

temperature170°C to 190°C,

210°C in percentage

170°C 1.8 l/min 100 min 0

190°C 1.8 l/min 90 min 10

210°C 1.8 l/min 75 min 25

Fig. 5.5 Inlet temperature of heat transfer fluid at mass

flow rate 1.8 l/min

Table 5.2 Time taken by charging at different flow rate

Inlet Temperature of

HTF

Flow rate Charging time

(Approximate)

210°C 2.l/min 68 min

210°C 1.8 l/min 75 min

5.1.5 PHASE TRANSITION PERFORMANCE OF PCM

Fig. 5.6 shows the pictorial depiction of phase transition performance of PCM in LHS

unit. It can be noticed that due to higher temperature difference between inlet

temperature of HTF and PCM, the phase transition rate at top region is significantly

higher as compared to otherzones. After 40 min of heat transfer, it can be noticed that

top region is already either in mushy phase or liquid phase, whereas the rest of the

zones are still in solid phase. Likewise, it can be observed that after 60 min of

charging cycle, the top and bottom region is in complete liquid phase, bottom zone are

in mushy phase or still in solid phase. Thus, it leads to rapid melting at top position

whereas, central and bottom positions are still in mushy zone. After 70min. it can be

seen that the all zones are in liquid phase. This is due to the fact that thermal energy is

extracted from HTF at top region and by the time HTF reaches bottom region; the

available thermal energy is lesser to generate higher temperature gradient. The effect

of natural convection is evident in all zones. Liquid PCM at central and bottom

positions rise above to top position due to density gradient and temperature

difference. Thus, it is expected that PCM temperature at top position will always be

higher as compared to central and bottom positions. melted.

5.2. DISCHARGING

5.2.1. TEMPERATURE VARIATION IN DISCHARGING

Temperature variation Fig. 5.7 and Fig. 5.8 shows the local and average

temperature variations of the LHS prototype during the discharging process. Initially,

the LHS prototype is at 190 °C. When the HTF at 100 °C is passed through the HTF

tubes, heat transfer takes place from the high temperature PCM to the comparatively

low-temperature HTF through the tubes. Due to the conduction heat transfer that takes

place in the PCM throughout the solidification process, there exists an even

temperature distribution in the radial direction of the prototype. It can be noted from

Fig. 5.7 that the decrease in temperature is faster in Top region than middle and

bottom. This is due to the existence of higher temperature difference between the

PCM and the HTF in the entrance region than the exit region of the LHS prototype.

Unlike charging process, a sharp plateau is not noticeable in the discharging process.

This is due to the fact that the heat source input during the discharging process is the

LHS prototype itself, the potential of which gets reduced while exchanging heat with

the HTF. Also, during the discharging process, a thin solid layer o PCM gets formed

around the HTF tube. Over a period of time, the thin layer becomes thicker.

This increases the thermal resistance and hence, the solidification rate reduces.

It can also be noted tha the PCM temperature in the bottom portion decreases slightly

faster than the middle and top portions. This is due to the negligible natural

convection that exists during the solidification process. Hence, the temperature

distribution of the LHS prototype during the discharging process is correct as

expected and the trends are similar to the previous results reported Hosseini et al [53].

Average temperature shown in Fig. 5.8 is the arithmetic mean of the local

temperatures measured at ten locations in the prototype. The average temperature of

PCM reaches about 118 °C in 110 min.

Fig. 5.7. Local Temperature versus time during melting of PC

Mass Flow Rate: 1.8 L/min

Inlet temperature of HTF – 100 °C

Fig. 5.8 Average Temperature versus time during the melting of PCM

• Effect of HTF inlet temperature and flow rate

Similar to the charging process, the discharging performance of the LHS

prototype in the discharging process can be controlled by two major parameters:

• HTF inlet temperature and

• HTF flow rate.

Table 5.3 & Table 5.4 shows the influence of the HTF inlet temperature and

flow rate on the discharging rate of the LHS prototype. Experiments were conducted

with the same initial PCM temperature of T=190 °C for three different inlet

temperatures viz. 100°C, 110 °C and 120 °C, at two different flow rates, viz. 0.18

l/min, 2.1l/min. Two observations can be directly inferred from Table 5.3.

• The discharging time is lower for the lower HTF inlet temperature case. This

is due to the huge temperature difference that exists between the HTF and

PCM.

• The discharging time is lower for the higher HTF flow rate case. This is due

to the higher convection heat transfer coefficient in the HTF side, which

enhances the overall heat transfer rate of the LHS system.

Although the discharging time is lower for the lower HTF inlet temperatures and

higher HTF flow rates, there exists a threshold in both the parameters beyond which

the decrease in the discharging time is too minimal. Because, there is a limit in the

heat releasing rate of the PCM, which is mainly due to its low thermal conductivity.

This phenomenon can be noted well in the cases of different HTF inlet temperatures

for a specific HTF flow rate. Similar effect is also noticeable to a lesser extent in the

cases of different HTF flow rates for a specific HTF inlet temperature. Hence, varying

the HTF inlet temperature has a greater impact on the discharging time when

compared with HTF flow rate. Still, the effect of HTF flow rate is more prominent at

higher HTF inlet temperature. Because, at higher HTF inlet temperature, the

temperature difference between the HTF and PCM is less. Hence, the heat transfer is

dominated by the convective action of the HTF flow rather than the temperature

difference between the HTF and PCM.

Table 5.3 Time taken by charging at different Temperature

Inlet Temperature of Flow rate Discharging time

HTF (Approximate)

100°C 1.8 l/min 110min

110°C 1.8 l/min 140min

120°C 1.8 l/min 125min

Table 5.4 Time taken by discharging at different flow rate

Inlet Temperature of

HTF

Flow rate Charging time

(Approximate)

110°C 1.8 l/min 110min

110°C 2.1 l/min 98min

Conclusions from the Results

• Natural convection plays a crucial role during charging process. It does not

play a major role during the discharging process and heat transfer is

conduction-dominated during solidification.

• The charging process is faster than discharging process due to the additional

natural convection, which takes place after the phase change temperature. It

took about 70 min/110 min for charging/discharging of the LHS prototype.

• Varying HTF inlet temperature has a greater effect on charging/discharging

time when compared with HTF flow rate.

• Effect of HTF flow rate is more prominent at lower/higher HTF inlet

temperature during charging/discharging process. An increase in volume flow

rate has a relatively moderate influence on thermal performance. The reason

behind this; as the natural convection starts dominating the heat transfer in

LHS unit, an increase in volume flow rate of HTF in tubes enhances the forced

convection coefficient in tubes and thus heat transfer improves. For

• It is noticed that due to vertical orientation of shell-and-spiral tube heat

exchanger in LHS unit, the PCM at top position melts quickly as compared to

central and bottom position. The reason behind this, is the natural convection

and upward rise of high temperature liquid PCM molecules. Initially, the heat

transfer is dominated by conduction. However, as the liquid fraction increases,

natural convection is dominant mode of heat transfer.

• Besides enthalpy gradient at the top and bottom positions, it is noticed that

significant quantity of thermal energy is extracted from HTF at the earlier

zones in LHS unit; therefore, the later zones has smaller temperature gradient,

which results in a weaker phase transition rate.

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