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“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
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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
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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.
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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
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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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