International Conference on Breakthrough in Engineering, Science & Technology– 2016 (INC-BEST’16)
High Temperature Thermal Energy Storage System
using D-Mannitol as Phase Change Material.
1
1
Subash S, 2Srikanth Salyan, 2Udaya Kumar G, 2Suresh S
Saranathan College of Engineering, Tiruchirapalli, Tamilnadu, India [email protected]
2
National Institute of Technology, Tiruchirapalli, Tamilnadu, India [email protected]
Abstract
Energy storage in some form is the need
of the hour to even out the mismatch between
energy supply and demand. Latent heat TES
(Thermal Energy Storage System) system
employing a phase change material (PCM) has
been widely considered as an effective way to
store and retrieve energy due to its high heat
storage capacity at almost constant temperature
during the phase change. In this work, an energy
storage system is designed to study the heat
transfer characteristics of D-Mannitol in a
dedicated heat storage system. In order to
analyze the melting and solidification
characteristics of PCM at various stages, the
storage system was fabricated using high
temperature borosilicate glass. Throughout the
experiment, around 2 Kg of D-Mannitol was
filled in the storage container 80 mm diameter
and 300 mm length. The heat transfer fluid used
was therminol as it is the most frequently used
heat transfer fluid in present day high
temperature
applications.
Experimental
investigations were done for the amount of heat
stored, temperature distribution in PCM during
phase change process, and the effect of
Reynolds number. Series of experiments were
performed for different mass flow rates and
different constant inlet temperature of heat
transfer fluid (HTF). The amount of heat stored
was 1421.8 kJ/Kg, the effect of Reynolds
number had a great impact on the melting
process.
Keywords: Phase change material (PCM),
TESS (Thermal Energy Storage System), Heat
Transfer Fluid (HTF).
Introduction:
Energy storage system plays important
roles among conservation of available energy
and improving its utilization, since many energy
sources are intermittent in nature. Short term
storage of only a few hours is essential in most
applications; however, long term storage of a
few months may be required in some
applications. Solar energy is available only
during the day, and hence, its application
requires efficient thermal energy storage so that
the excess heat collected during sunshine hours
may be stored for later use during the night.
Similar problems arise in heat recovery systems
where the waste heat availability and utilization
periods are different, requiring some thermal
energy storage. Also, electrical energy
consumption varies significantly during the day
and night, especially in extremely cold and hot
climate countries where the major part of the
variation is due to domestic space heating and
air conditioning. Such variation leads to an off
peak period, usually after midnight until early
morning. Accordingly, power stations have to be
designed for capacities sufficient to meet the
peak load. Otherwise, very efficient power
distribution would be required. Better power
generation management can be achieved if some
of the peak load could be shifted to the off peak
load period, which can be achieved by thermal
storage of heat or coolness. Hence, the
successful application of load shifting and solar
energy depends to a large extent on the method
of energy storage used.
Materials to be used for phase change
thermal energy storage must have high latent
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International Conference on Breakthrough in Engineering, Science & Technology– 2016 (INC-BEST’16)
heat of fusion and high thermal conductivity.
They should also have a melting temperature
lying in a practical range of operation, melt
uniformly, and be chemically inert, low cost,
non toxic and non corrosive.
D-mannitol is used as PCM for thermal
energy storage in this work. The heat transfer
characteristics of D-mannitol for various inlet
temperatures and mass flow rate are studied and
also the performance of the system is to be
examined here. The present study from the
previous works is that, it compares the influence
of HTF operating conditions during melting and
solidification process, the effect of Reynolds
number on the HTF and also to study the
distribution of temperature along the axial
direction from the inlet and radially by
measurements during melting and solidification
process for horizontal double pipe latent heat
storage system.
Experimental Setup:
The block diagram of the experimental
setup is shown in fig. no 1. The setup comprises
of a heater, hot fluid tank, cooling unit,
connecting
tubes,
thermocouples,
and
temperature indicator. The heat transfer fluid
(HTF) used is silicone oil that is passed through
a copper pipe of 10mm inner diameter and
12.5mm outer diameter. The insulation for the
outer surface is done using glass wool to reduce
the heat loss. Two ways fluid flow process is
conducted in the system; during melting process
the heat is transferred from the heat transfer
fluid pipe (HTFP) to the PCM, during the
solidification cycle the energy from the PCM is
transferred to the (HTF). Thermocouples were
located at various distances from the inlet along
the axial direction. The thermocouples are used
to measure the temperature distributions and
they are in contact with the PCM, two
thermocouples were placed at the inlet and outlet
of the (HTFP) to measure the inlet and outlet
temperatures of the HTF. A heating mantle of
power 1000W was provided to maintain a
constant inlet temperature of the HTF.
The hot fluid from the hot container is
circulated through the copper rod using a 1HP
pump. The flow rate of the hot temperature fluid
is measured using an oil rotameter (1-20 LPM).
The PCM filled in the PCM container around the
(HTFP) with a heat storage capacity of 1421.8
kJ (latent heat only). In order to investigate the
heat transfer effect on the storage medium with
inlet temperature of (HTF) and flow rate,
various operating condition are employed.
Fig. 1. Schematic representation of experimental setup.
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International Conference on Breakthrough in Engineering, Science & Technology– 2016 (INC-BEST’16)
Experimental procedure:
Charging process:
The temperature distributions of HTF
and the PCM in the PCM tank for various mass
flow rates are recorded during charging
processes. The amount of the heat that could be
stored and the efficiency of system are studied in
detail during the melting process. Experiment
was conducted with flow rates 5, 7, 8, 10 litre
per minute (LPM) and the inlet temperature of
the hot water was kept at 180-210 °C. In the
process of melting the HTF is allowed to pass
through the copper rod in the PCM container
continuously. Initially temperature of PCM is 32
0
C and as the HTF temperature increases the
heat energy is transferred to PCM, the PCM is
allowed to heat until it reaches the melting
temperature (storing the energy as sensible heat),
once the PCM melts and becomes liquid the heat
is stored as latent heat. At an interval of 5
minutes temperature of the PCM and HTF are
recorded. The charging process is continued
until the PCM temperature reaches the inlet
temperature of the HTF. The temperatures of the
HTF at inlet and outlet are recorded .Also the
temperatures of the PCM at nine locations are
recorded.
q Latent = mpcm × L
(2)
here the L represents the Latent heat of fusion of
D-mannitol.
Results and discussion:
Temperature
profiles
Charging process:
during
the
The temperature profiles in the axial
directions of the PCM during the melting
process are shown in Fig. 2 when the Reynolds
number was 16297.77 (for 8 L/min). Similar
trends are found in other HTF temperatures. As
seen from the melting curves, the initial energy
transferred to the D-mannitol will raise its
temperature from the initial temperature to the
onset temperature. This sensible heat of the
PCM is transferred from the wall of the HTFP to
the D-mannitol by pure conduction which
increases the temperature of D-mannitol
gradually to its melting point. This change in
sensible heat is a fast process because of the
large initial temperature gradient between the
HTFP wall and the solid D-mannitol.
Mass flow rate: (8 L/min)
Inlet temperature: (1800 C )
Data reduction:
Energy storage capacity of the LHSS
The thermal energy storage capacity
consists of sensible heat and latent heat. The
sensible heat capacity can be calculated using,
q Sensible = mpcm cp ΔT
(1)
Where
mpcm = mass of D-mannitol, the c p
represents the Specific heat of D-mannitol and
ΔT represents the
Temperature difference
between the initial temperature and final
temperature of D-mannitol,
The latent heat capacity is given by,
Fig. 2. Temperature versus time during
melting of D-mannitol in axial direction.
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International Conference on Breakthrough in Engineering, Science & Technology– 2016 (INC-BEST’16)
After
this rapid increase, the
temperature remains constant during the melting
period. It is well understood from the literature
that once the PCM melts, as melting proceeds a
thin layer of liquid is formed between the wall
and solid phase, and natural convection starts to
take place. The melting time was estimated from
the time elapsed between the onset of transition
until the completion of transition, which is the
phase change temperature range. The melting
ends when all the thermocouple reaches liquidus
temperature of 165-1720C. The time was
determined as 50mins.
Effect of the inlet heat transfer fluid
temperature:
The effect of the inlet water temperature
on the time wise variation of temperature of
PCM temperature at axial locations during
melting process is illustrated in Figs. 3. For the
sake of comparison, the time taken to reach
maximum temperature corresponding to the inlet
temperature of water by one the thermocouples
for different inlet temperatures water is
compared.
by increasing the inlet water temperature from
1800C to 2000C, hence increasing inlet
temperature, in case of melting results in high
temperature difference between HTF and PCM.
This obviously enhances the heat transfer rate
and thus, the phase transition time gets reduced.
This may not be possible with sensible storage
mediums.
Conclusion:
The melting and solidification processes
of D-mannitol in horizontal double pipe latent
heat
storage
unit
are
investigated
experimentally. The heat transfer characteristics
of D-mannitol are explored for various HTF
inlet temperatures and various mass flow rates.
 Based on this experimental work, the
following conclusions are drawn:
 D-mannitol with phase transition
temperature range of 160-1680C is a
suitable candidate for storing thermal
energy.
 Charging time can be considerably
reduced by increasing inlet temperature
of HTF, due to enhanced heat transfer
rate.
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Fig. 3. Temperature distribution during melting of
D-mannitol at same water flow (7L/min) and
various water inlet temperatures.
It can be deduced from Fig. 3 that the
time for complete melting at T3 gets decreased
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