Introduction
Environment protection has become a big challenge in today’s world. Extensive studies
have been done to protect the environment by using energy generated from renewable sources.
Among those, solar energy is one. Solar energy is alternative energy resource produced from the
sun. It is an ultimate source of renewable energy. Energy storage is important in solar energy
utilization because supply is intermittent and the demand often occurs at a different time. There
are two commonly used thermal energy storage methods that are sensible heat storage (SHS) and
latent heat storage (LHS). SHS requires large storage capacity. It has poor heat transfer rates
during heat storage and recovery processes so insufficient means of thermal energy is stored. For
the enhancement of high heat storage capacity new technology had been developed which is
LHS system. LHS system of thermal storage requires small unit size and has become famous on
last two decades. Phase change materials (PCM) used in LHS system undergoes isothermal
behavior during charging and discharging and has high energy storage density over a small
temperature range, so that promising results can be yield. PCM can be either organic or
inorganic. However inorganic PCM has the greater phase-change enthalpy. During the night or
cloudy day whenever the temperature decreases PCM undergoes phase change releasing heat
which helps to heat the air and water in the absent of solar radiation too.
Solar energy can be stored in various forms other than thermal storage. It can be stored as
kinetic energy, chemical solutions, magnetic energy and other novel approaches. In this paper
different process done in past to store energy has been discussed, however the emphasis is given
to the thermal storage using PCM.
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Literature Review
Solar energy storage method depends on the capacity and discharge time. Life cycle
assessment (LCA) is an important tool to evaluate energy consumption and environmental
impact of renewable energy processes. Thermal energy can be stored in a thermal reservoir either
directly or via thermal physical reaction or by chemical reaction creating new chemical species.
Electrical energy harvested from solar energy can be stored either directly in devices such as
capacitors or superconducting magnetic devices and so on. For any kind of storage method the
materials that enters the system and leaves the system should be checked whether it has any
environmental impact or not [1].
Cruickshank and Harrison [2] conducted the thermal response of multi-task thermal
storage test using natural convection heat exchangers which were connected either in series or in
parallel flow configuration. Energy storage rates and tank temperature profiles were
experimentally measured during charge periods of two clear days or combination of clear and
unclear day. The experiment showed that the series connection was sequential to performing
stratification in the component tanks and to distributing energy to a different temperature level.
As a result of this when charge temperature was high, temperature stratification level was high in
storage tank and desertification was limited when charge temperature was low. This effect was
not observed in the parallel configuration. However at the high flow rate, the temperature
distributions were similar in parallel and series charging. Disadvantage of both configurations
was that falling charge loop temperatures tend to mix and destroy the storage tank.
Solar energy can be stored in photo galvanic cells. Based on the mixed dye system photo
potential, photocurrents etc. are recorded for photo galvanic cell. To optimize the performance of
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the cell the Rose Bengal (as photo sensitizer) was used with d-Xylose and sodium lauryl sulphate
(NalS) as a reductant and surfactant respectively [3]. As a result of experiments the value of
photo generation of photo potential was found to be 885.0mV and photocurrent was
460.0microA and the maximum power of the cell was 407.10microW. This led to the storage
capacity of the photogalvanic cell to be 87.87%.
Chakrabarti et al. [4] used Ruthenium based redox flow battery system to perform solar
energy storage experiment. Acetonitrile was used as solvent along with porous graphite as
electrodes and ruthenium acetylacetonates (0.02M and 0.1M) as the electrolyte. On the basis of
this, different operating conditions were determined. As a result of higher mass transport, current
densities and power output were higher with an increase in the concentration of ruthenium.
Furthermore voltage efficiency was better for full battery rather than empty battery at the same
concentration of ruthenium. As the concentration was increased voltage efficiency increased too.
This shows some hope for future system of solar energy storage.
The world first solar-driven ammonia-based thermo chemical energy storage system
operates at a nominal power level of 1kW and uses a solar reactor design [5]. A closed-loop
thermo chemical energy storage system was used which passes reactant (ammonia and 3:1
hydrogen/nitrogen gas mixture) between energy storing and releasing reactor. To maintain the
temperature storage with less thermal loss, counter flow heat exchanger is used to transfer heat
between ingoing and outgoing reactant. The solar storage and heat recovery reactors operate at
constant pressure. Energy recovery was carried out using ammonia synthesis reactor. To keep the
reactor at a high operating temperature throughout the process the ammonia mass flow was
varied and solar reactor was operated in cloud cover. By the end of the experiment pressure was
increased and the ammonia liquid level was reduced which shows storage of energy. This
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experiment shows that the ammonia dissociation reactors are suitable for operation through the
solar transients. Also the heat recovery reactors are also capable for high quality superheated
stem production. These suggest that this technology could be one of the most cost-effective
routes to the provision of continuous 24-h solar electricity.
Vikram et al. [6] performed an experiment in the solar water heating system using phase
change materials. The system consisted of two heat observing units, one was a solar water heater
and other was a heat storage unit consisting of phase change materials (PCM) (paraffin). The
experimental setup was made up of a cylindrical tank which holds PCM, solar flat plate
collector, flow meter and a circulating pump. Water was used as heat transfer fluid (HTF) which
was circulated continuously through the tank and the solar collector. This HTF absorbs solar
energy from the flat plate collector and exchanges heat with the PCM in the storage tank. The
PCM undergoes a phase change by absorbing latent heat and supplies hot water during the night.
The heat was recovered from the unit by passing water at room temp through it. Fresh water
entered the unit after the water was used from the storage tower. This water was heated by the
PCM to maintain equilibrium. The storage tower was completely insulated to prevent loss of
heat. The efficiency of this system was examined for the solar conditions. The storage tank was
made of stainless steel which had the capacity to supply water for a family of four.
Ali et al. [7] conducted a numerical analysis of the thermal behavior of a rectangular
storage system using a phase change material (PCM). Acetamide and silicon oil were used as
PCM and Heat Transfer Fluid (HTF) respectively in the Solar Energy Storage system. Parabolic
collector was used to collect solar energy. The system was operated from 9am to 5pm by solar
energy provided by the collector and was operated from 5pm to 9pm by the storage unit. Solar
radiation was collected from parabolic plate and was transformed to the storage unit and main
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system. The HTF was circulated between the storage system, main unit and the collector. Stepper
motor was used to spin the collector hourly. Numerical method was applied to determine the
mathematical model to the system to calculate the design parameters. Those parameters helped to
design a solar energy storage system which could support apartments in Tehran.
Lehigh University research team developed a phase change thermal storage systems to
store thermal energy when sunlight isn’t available [8]. They used encapsulated phase change
materials (EPCMs) to store large quantities of thermal energy. EPCMs can be designed to have
high melting points with constant temperature during a phase change. The heat transfer fluid
circulating from collector to storage was heated by solar collector to 400-450 0C. The zinc as a
phase change material was contained within a small spherical or cylindrical capsule made of
appropriate encapsulating material to maintain its integrity and a shield. These preserve the
PCM’s heat transfer and phase changing qualities. The team performed theoretical analyses to
model the heat transfer and phase change processes within individual capsules during melting
and solidification, using cylindrical capsules with diameters ranging from 25 to 75 mm. the
researchers are optimistic about their encapsulated phase change materials approach to provide a
very good solution for current issue of storing solar thermal energy during night.
Ahmet Sari and Kamil Kaygusuz [9] performed an experiment to study the thermal
performance and phase change stability of stearic acid as a latent heat energy storing material.
The experimental setup was consisting of the heat storage container, a high and low temperature
bath, circulating pumps, piping systems and thermocouple. The heat storage container was
consisting of two concentric cylindrical copper pipes. The PCM (stearic acid) was filled in the
annulus of two concentric cylindrical tubes. The heat storage container was isolated by glass
wool. The hot water was circulated through the inlet at the high temperature to melt the PCM at
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different flow rate. The phase change behavior of the stearic acid was determined separately at
horizontal and vertical position of container. The high temperature difference between the PCM
and inlet water temperature was maintained which increased the conduction heat transfer rate. It
was concluded that stearic acid is a good PCM energy storage for domestic solar water heating.
So while constructing the solar energy storage system we could use stearic acid as a phase
change materials.
Phase change materials have high storage capacity and isothermal operation. Mazman et
al. [10] perform an experiment to demonstrate the thermal performance of the PCM in solar
domestic hot water (SDHW) tank. The team used paraffin and mixture of different fatty acid to
demonstrate the result. Graphite was added to the PCM mixture to increase heat transfer in the
modules. The PCM was heated to above 200c above its melting temperature before it was put in
modules. The experiment was performed with and without PCM for both cooling and heating
process and temperature measurement were done at 10s interval. For the cooling process
temperature was decreased in 36 hours when PCM was used and in 24 hours when PCM was not
used. This shows that PCM helped to keep temperature range for 12 hour more. During the
reheating experiment it was observed that 3kg of PCM could increase the temperature of 14-36L
of water at the upper part of the SDHW tank by 3-4 0C. PCM helps to increase the energy
storage density so more energy will be available to meet the heat demand. When the cold water
is entering the system the PCM will freeze and release its latent heat, so the water will remain
hot for longer duration even in absent of solar radiation.
Shuangmao Wu and Guiyin Fang [11] performed an experiment to study the dynamic
performance of solar heat storage system using myristic acid as a phase change materials. The
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system was consisting of solar heat collector, indoor unit and storage tank. Storage tank was
filled with spherical capsule containing PCM. Author believed that spherical capsule filled in
packed bed is convenient and effective method of encapsulation. Water was used as heat
transferring fluid. The numerical simulations were conducted at various inlet temperatures,
various mass flow rates and various initial temperature of packed bed. Water was circulated
along the collector and storage tank. During the discharging process, water flows over the packed
bed from bottom to the top to solidify the PCM. The result showed that the inlet temperature and
mass flow rate had the strong influence for heat release rate and complete solidification time.
The latent efficiency was higher for the higher HTF temperature; it lowers the heat release rate
and increase time for complete solidification. They concluded that higher initial temperature was
not suitable for their storage system. The inlet temperature of 50 0C, the flow rate of HTF to be
10kg/min and initial temperature of packed bed to be 66 0C were chosen to study the thermal
character in solar energy storage system.
Suat Canbazoglu et al. [12] performed an experiment during November in Malatya,
Turkey for the enhancement of solar thermal energy storage performance using sodium
thiosulfate penthydrate as phase change materials. The storage system was consisting of flat
parallel aluminum solar collector, hot and cold water tank, and measurement and data logger
system. Electronic measurement system was installed to measure the temperature and to
accumulate data. Water temperature was measured in upper, middle and lower part of storage
tank; at inlet of collector and at the outlet of heat storage tank. Although the experiment was
performed in November it was observed that thermal effectiveness of the solar radiation was
increased. The result showed that the temperature difference at the midpoint of the storage tank
and outlet of the collector for heat storage tank with PCM was greater than the system without
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PCM. A large part of solar energy can be stored during the daytime by choosing the suitable
phase change materials and this stored energy can be used to heat water and air for the night
time. It was concluded that the solar thermal storage system combined with PCM could be able
to use efficiently and widely in practical application with minimum cost.
Conclusion
Solar energy is the demand of the modern world to prevent the environmental hazards
and fulfill the universal need of alternating energy resources. Many technologies have been
developed so far for the storage of solar energy. Among the various methods of energy storage,
the latent heat thermal energy system using PCM is quite appealing, because of its high energy
storage density and its ability to provide heat at a constant temperature. It is believed that PCM
can store 5-14 times more solar thermal energy by volume than traditional thermal energy stores,
such as rock, water and masonry [13]. Solar radiation collected from the collector could be
either directly used during daytime to heat the water and air in the building or stored in storage
systems which contain PCM. PCM stores the energy in the latent heat which helps to heat the
water and air during night or cloudy day.
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References
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[8] Lehigh Energy Update, 2010, “High Temperature Storage of Solar Energy Using Phase
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