Experimental characterisation of a novel phase change material heat storage unit for state-of-charge estimation

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Experimental characterisation of a novel phase change
material heat storage unit for state-of-charge estimation
J. S. Wilson*, R. Mills and R. J. Barthorpe
Department of Mechanical Engineering
The University of Sheffield, United Kingdom
Correspondence author. Email: james.s.wilson@sheffield.ac.uk
ABSTRACT
Energy storage methods will be a critical part of the future energy supply system, as demand is increased by electrification of
domestic heat and take-up of renewable generation increases the variability of supply. Domestic storage units are likely to be
key to this transition, and this paper will present investigations into a unit for storage of domestic heat. A phase change material
(PCM) heat storage unit has been developed that has the potential to provide effective heat storage facility in a domestic capacity.
Effective control and use of the storage units is dependent on accurate state-of-charge (SoC) estimation, tools for which are being
developed and supported by data presented in this paper. A novel test bed has been developed in order to acquire data for this
purpose. The rig is situated in a temperature-controlled chamber which enables charge and discharge tests at a range of ambient
temperatures. Automated control of the rig is also available, which allows for repeatable tests to be carried out to gather data for
model development.
Keywords: Characterisation, phase change material, thermal energy storage, state-of-charge
1. INTRODUCTION
Thermal energy storage (TES) will be key to the net-
zero energy transition. This paper presents initial out-
comes of a major project that aims to provide solutions
for advanced storage that will have significant impact
on electricity grid operation as part of the wider the
decarbonisation of domestic heat.
Domestic heating constitutes a major portion of
overall energy demand, making up 64.4% of energy
consumption in EU households in 2021 [1]; this demand
has traditionally been met by the use of gas boilers to
provide space heating and hot water. Decarbonisation of
this system will be critical to meeting climate change tar-
gets such as net-zero. A potential solution is to electrify
domestic heat by using heat pumps; this is an attractive
option given the high coefficients-of-performance avail-
able. However, widespread electrification will present a
major new demand on the supply of electricity, with
significant variations in the demand both in the short
and long term. A parallel challenge is that the ongoing
take-up for renewable energy generation means that the
supply of electricity itself is becoming more variable [2].
In light of these issues, energy storage will likely be
required in order to stabilise the demand for electricity
and to manage the peaks and troughs in its supply [3].
Access-level TES, where heat is stored at the point-
of-access in a domestic setting, offers a series of benefits.
In this system, TES units would allow users to store
heat generated by their domestic heat pump when supply
is greater than demand, and access the energy in the
stores when demand outstrips supply. This would enable
users to take advantage of variable-price tariffs without
compromising their access to heat when required, and
would reduce the overall strain on the electricity supply.
This paper presents a novel phase change material
(PCM) storage unit – developed at the University of
Loughborough – that could be incorporated as part of a
domestic TES system. Following a technical presenta-
tion of the device, a new experimental rig that has been
developed to test and characterise the technology will
be presented. Initial findings from experimental work
carried out on the rig will then be presented, followed
by discussions of the results and ideas for future work.
2. PCM STORAGE UNITS
Thermal energy can be stored by melting a solid
PCM, where energy can be stored in the latent heat of
melting by holding the material in its transitioned phase.
The energy can then be recovered by allowing the PCM
to re-solidify. PCM devices have a high energy storage
density (compared to sensible heat TES), and have the
benefit of releasing the majority of stored heat at a
consistent temperature (the crystallisation point of the
PCM). In addition, relatively large amounts of thermal
© The Author(s) 2024
P. Droege and L. Quint (eds.), Proceedings of the International Renewable Energy Storage and Systems Conference (IRES 2023),
Atlantis Highlights in Engineering 32,
https://doi.org/10.2991/978-94-6463-455-6_19

Figure 1 The latest-generation PCM unit
energy can be stored at relatively low temperatures,
which reduces the loss of energy in a standing charged
unit. A performance analysis on a previous version of
the unit presented in this paper can be found in [4].
The key parameters controlling the state-of-charge
(SoC) of the unit presented in this paper are the inlet
and outlet water temperatures, and the flow rate through
the heat exchanger. If it is assumed that energy losses
are small relative to these heat transfer rates, these
parameters will determine the rate at which energy can
be transferred to the PCM via the heat exchanger.
The PCM itself is a commercial material called
CrodaThermTM 53; it has a melting temperature of
53oC, a latent heat of melting of 226kJ/kg and a latent
heat of crystallisation of −225kJ/kg. The test units are
each fitted with six thermocouples; three within the
PCM, labelled T1, T2, T3, and three on the surface of
the inner wall (an outer insulating case comprising a
Polyisocyanurate layer is also used to reduce heat loss),
labelled T4, T5 and T6. T5 was fixed to the upper side
of the internal wall of the cell, T4 and T6 were hung
loose on each side of the cell in the air gap between the
external and internal walls. The overall dimensions of
the unit are 900x600x320mm. The units can be operated
singly or in groups (these can be set for use in series or in
parallel). The nominal capacity of each unit is 4.5kWh.
3. EXPERIMENTAL RIG
The LVV is a state-of-the-art facility at the University
of Sheffield for verification and validation testing. It con-
tains three environmentally-controlled chambers, which
are ideal for developing a controlled testing environment
for the testing of TES technology.
The rig was developed with the aim of providing
a basis for testing the PCM units in a fully controlled
environment. This would then allow for specific tests to
be designed targeting the investigation of key behaviours
of the units, such as their ability to hold thermal energy
over a standing period.
Two storage tanks are used to store hot and cold
water respectively, with heating and cooling energy
provided by a Huber Unistat 510. Flow control valves
were installed to ensure that either the hot or cold tank
was in operation depending on whether the rig was in
heating or cooling mode. Valving was also designed to
ensure that, while the water flowed in the same direction
on the outer side of the rig, it could be reversed through
the PCM unit. Hot water was provided to the top of
the heat exchanger when charging the unit, and cold
water was supplied to the bottom of the exchanger
when discharging the unit; this was to avoid pressure
build-ups due to expansion or contraction in melting or
solidification of the PCM. A schematic illustrating the
operation of the LVV rig is given in Figure 4.
The key benefit of this rig is that it can be used to
control the main conditions for charging and discharging
the PCM units. The inlet temperature is controlled by the
Huber, the flow rate is actively controlled by feeding
back meter data to the pump, and the ambient tem-
perature can be set in the environmental chamber. Fur-
thermore, the rig will enable controlled and automated
testing that replicates the conditions seen in real-world
environments, including part-charging, part-discharging
and varying ambient temperatures. As a result, the
performance of the storage device could be characterised
in a far more detailed way than had previously been
possible, supporting the development of advanced SoC
methods.
4. EXPERIMENTAL TESTING AND
RESULTS
An experimental programme is presented in this
paper to demonstrate the capabilities of the rig at the
LVV. This represents an initial test phase, focusing on
standing losses in the units, with further tests planned
to investigate part-charging and -discharging and the
effect of ambient temperature on performance. The
data recorded in this initial phase will allow for the
development of models for the standing charge loss of
the units.
The recorded data for each test comprised the time
stamps, the flow rate through the rig and a series of
temperature measurements. These included the environ-
mental chamber temperature, the hot and cold water tank
188 J. S. Wilson et al.

Figure 2 A sketch of the PCM assembly, including locations of internal thermocouples
temperatures, the inlet and outlet temperatures at the
Huber heat exchanger, the inlet and outlet temperatures
to the PCM unit both at the unit and on the rig side,
and the unit thermocouples described in Section 2.
The measurements were recorded at a sampling rate of
0.5Hz.
Experiments were designed to quantify the standing
loss in time for a single PCM unit. The unit was fully
charged, and then disconnected from the heat supply.
The unit was then left to stand for a period of time
before it was fully discharged. The difference between
the energy stored in the unit and the energy recovered in
the discharge phase constituted the standing loss, which
could then be estimated as a function of standing period
by carrying out multiple experiments over a range of
standing periods.
For these experiments, the unit was considered
‘fully charged’ when all temperature measurements had
reached a steady value around the charging temperature.
Conversely, the unit was considered ‘fully discharged’
when all temperature measurements had reached a
steady value around the discharging temperature. For all
tests, the unit was charged to 70oC and discharged to
20oC. The experiments carried out are summarised in
Table 1.
Upgrades were carried out on the test rig throughout
the gathering of data for this paper. From Test 3 onwards,
active flow control was added to the rig. During these
tests, issues were identified with maintaining a steady
temperature in charging and discharging. An immersion
heater was added to the hot water tank for Test 4 on-
wards in order to help maintain the supply of hot water
in charging cycles. From Test 7 onwards, additional
heat metering was added to the water circuit and a
mixing circuit was added to the cold water tank; these
upgrades were also intended to help maintain steady
supply temperatures in charging and discharging cycles
(the charging time was reduced for Test 8 onwards as a
result of these upgrades).
The charge and discharge cycles for Test 4 are illus-
trated in Figures 5 and 6 respectively. These figures show
the temporary failure of steady heat supply at around
45 minutes; this issue was resolved as discussed above
by modifications to the rig. It can be seen in Figure 5
that a steady state for all measured temperatures could
be reached in around 3–3.5 hours on the charge cycle;
this time was reduced following upgrades to the rig to
around 2.5–3 hours. On the discharge cycle, the steady
state could not be reached for all measured temperatures
within working hours, as can be seen in Figure 6. At
Experimental characterisation of a novel phase change material 189

Table 1. Description of tests carried out
Test no. Charge time (hrs) Standing period (hrs) Discharge time (hrs) Target flow rate (l/min)
1a01:00:00 14:48:00 04:24:08 No control applied
2a01:00:00 00:03:00 01:58:00 No control applied
3 03:30:00 18:37:00 07:58:00 12
4 03:18:00 19:10:00 06:30:00 5
5 03:17:00 116:26:00 04:39:00 5
6b03:30:00 163:55:42 05:22:14 5
7 02:43:00 18:53:00 07:07:00 5
8 02:45:00 69:27:00 04:53:00 5
9 03:00:00 44:00:00 04:00:00 5
10 02:46:00 91:58:00 04:00:00 5
11 02:40:00 00:30:00 03:30:00 5
aFlow ‘wrong’ way through PCM (hot water supplied to the bottom of the unit, cold water supplied to the top)
bRig upgrades carried out during standing period. Brief initial discharge wrong flow direction
Figure 3 The experimental rig at the LVV
the end of the recorded cycle, T5 and T6 were still
discharging; however, it can be seen that at this point the
water circuit had reached a stable point, so no further
useful heat was being extracted.
The charge profiles for the tests are plotted in Figure
7. Similar initial profiles can be observed for Tests 1
and 2, with a peak charging power of around 14kW.
However, a sawtooth profile can be observed in the data
from Test 1 – this was due to the internal temperature of
the Huber reaching a safety limit. This limit was raised
so that it did not impact on following tests. From Test 3
onwards, the flow was reversed to the correct direction
through the PCM unit resulting in a new charging profile
– this allowed for a higher peak charging power of
around 17kW to be reached. For Tests 2 onwards, it can
be seen that the heat supply temporarily fails at around
45 minutes – this was due to the supply of preheated
water from the hot water tank failing (possibly due to a
short circuit through the tank) and the Huber not being
able to meet the heat demand quickly enough. This issue
was resolved by the upgrades implemented from Test 7
onwards.
Figure 7 shows the discharge power for each test.
This shows significant variation between tests, which is
not surprising given the varying charging periods and
standing periods between the tests. Tests 3 and 4 have
similar discharge profiles to each other – this makes
sense given that the test conditions were similar for these
tests (see Table 1). As with the charge profiles, in Tests
3 and 4 there is a period in which the cold supply fails
before the Huber could begin supplying cold water to the
PCM unit. This issue was also resolved by the upgrades
implemented from Test 7 onwards.
The energy stored in the PCM unit during a charge
cycle was calculated using the flow rate and temperature
difference across the unit; the same process was followed
for the energy recovered during discharging. A minimum
power of 0.5kW was set when calculating these values;
this meant that the charge and discharge power curves
200 J. S. Wilson et al.

Figure 4 A schematic diagram of the rig at the LVV
Figure 5 The measured temperatures during the charge
cycle for Test 4
Figure 6 The measured temperatures during the dis-
charge cycle for Test 4
Figure 7 The charging power over time for each test
Figure 8 The discharging power over time for each test
Experimental characterisation of a novel phase change material 191

Figure 9 The energy stored for each test compared to
the discharge energy and standing losses
Figure 10 The round trip efficiency for the PCM unit
with second-order polynomial line of best fit added
were only counted towards the stored or discharged
energy totals when they were above this threshold.
The standing loss could then be calculated from the
difference between the two; these results are plotted in
Figure 9.
It can be seen that from Test 3 onwards, for which the
unit was fully charged, the stored energy was around 4–
4.5kWh (the nominal capacity of the unit). It is also clear
that for tests with a short standing period (Tests 1–4), a
high proportion of the stored energy could be recovered
in discharging. For longer-duration tests (Tests 5 and 6),
the standing losses were much more significant.
The round trip efficiency can be easily calculated
by dividing the discharged energy by the stored energy.
Round trip efficiency would be expected to have a
maximum value near 100% in the absence of any charge
or discharge inefficiencies, and would be expected to
decline as the standing period increases and the standing
losses increase. The relationship between round trip
efficiency and standing period is plotted in Figure 10. It
can be seen that, as expected, the round trip efficiency
steady decreases with standing period. The maximum
efficiency appears to be around 85–90%; the relationship
between efficiency and standing period closely fits a
second-order polynomial.
5. DISCUSSIONS AND FUTURE WORK
The results of the tests carried out in this research
both validate the design specifications of the PCM unit,
and provide promise as to its suitability for domestic
TES. The unit can be readily charged to its nominal
storage capacity of 4.5kWh in a relatively short time
period of a few hours. The apparent round trip efficiency
is promising, with a maximum value of 85–90% and
good performance over several hours.
A few limitations were identified in the rig dur-
ing these tests: initially it was difficult to achieve the
charging temperature of 70oC. Further to this, main-
taining a steady charge and discharge temperature was
also difficult in the first few tests. These issues were
overcome by modifying the use of the Huber in the
rig. An enduring challenge in utilising the rig is that
fast switching between charge and discharge modes
is limited by the time taken to change the internal
temperature of the Huber, allowing it to provide water
to the unit at the correct temperature.
The data presented in this paper provide a good
starting point for the development of an SoC estimation
tool. A regression model fitted to the data in Figure 10
would allow for a controller to estimate the SoC of the
unit given a known amount of stored energy. However,
significantly more data would be required to traina
more comrehensive model than is currently available;
still more data would then be required to validate its
predictions.
Future tests will be key to developing a compre-
hensive SoC estimation tool. A series of part charging
tests will be required to enable a controller to estimate
the available stored energy following a given charging
period. Furthermore, the impact of ambient temperature
on both the stored energy during charging and the
standing loss energy should be quantified. The ambient
temperature was not controlled during the tests presented
here, but could be set to a range of temperatures in
future. Finally, the PCM units are intended to be de-
ployed in a modular layout to enable storage capacity
to be scaled as appropriate. Therefore, the performance
of the units should be assessed in groups, as well as in
isolation.
6. CONCLUSIONS
The aims of this paper were to demonstrate a novel
PCM TES unit for domestic use. An experimental rig
was constructed to test and characterise the unit, and the
test results were set out in this paper. The unit contained
45kg of PCM with a nominal storage capacity of 4.5kWh
– this was demonstrated to be an accurate estimate
of available storage. The unit was instrumented with
six thermocouples, and further measurements on the
experimental rig allowed for accurate measurement of
charging and discharging power, from which the stored
and recovered energy could be calculated.
192 J. S. Wilson et al.

The tests carried out allowed for estimates of the
round trip efficiency to be made given a particular
standing period. The maximum efficiency, for an imme-
diate charge and discharge, was around 85–90%. The
efficiency reduced (approximately) quadratically with
standing period, but remained high for durations of up to
24 hours, at which point the round trip efficiency would
be expected to be around 70% – this indicates that the
unit would be highly effective for medium-term storage
applications. Furthermore, higher efficiencies could be
achieved by focusing operations of the store around the
melting point of the PCM; this would reduce sensible
heat losses from the unit.
Further work is planned to expand on these results,
in particular investigating the behaviour of the storage
devices during part-charge and part-discharge tests and
the effect of ambient temperature on the performance of
the units. These tests would then allow for development
of an effective SoC estimation tool.
ACKNOWLEDGEMENTS
This research was funded by the UK Department
for Energy Security and Net Zero (DESNZ) through
the Advanced Distributed Storage for grid Benefit (AD-
SorB) project. The research made use of The Labora-
tory for Verification and Validation (LVV) which was
funded by the EPSRC (grant numbers EP/R006768/1
and EP/N010884/1), the European Regional Develop-
ment Fund (ERDF) and the University of Sheffield.
The PCM store presented in this research was devel-
oped by the team at the Centre for Renewable Energy
Systems Technology at the University of Loughborough,
who also supported the practical work.
REFERENCES
[1] Eurostat, Energy consumption in households,
ec.europa.eu/eurostat/statistics-
explained/index.php?title=Energy consumption in households,
accessed: 03/08/2023, ISSN 2443–8219.
[2] Congressional Research Service, Variable
renewable energy: An introduction,
crsreports.congress.gov/product/pdf/IF/IF11257,
accessed: 03/08/2023, 2019.
[3] David J. C. MacKay, “Sustainable energy-
Without the hot air,” 1, UIT Cambridge. United
Kingdom, 2009.
[4] Mohamed Fadl, Daniel Mahon, and Philip C.
Eames, Thermal performance analysis of compact
thermal energy storage unit- An experimental
study, “International Journal of Heat and Mass
Transfer,” volume 173, pp.121262, March 2021.
Experimental characterisation of a novel phase change material 193

194 J. S. Wilson et al.
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