Multi-scale experimental characterization and
management of the supercooling of isosorbide as phase
change material for thermal energy storage

Arnaud Bruch, Fabrice Bentivoglio, Baptiste Boit, Awa Sy

► To cite this version:

Arnaud Bruch, Fabrice Bentivoglio, Baptiste Boit, Awa Sy. Multi-scale experimental characterization and management of the supercooling of isosorbide as phase change material for thermal energy storage. Journal of Energy Storage, 2025, 134 B, pp.118032. 10.1016/j.est.2025.118032. cea-05527863

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Research papers

# Multi-scale experimental characterization and management of supercooling of isosorbide, a phase-change material for thermal energy storage☆

Arnaud Bruch<sup>a, *,</sup> , Fabrice Bentivoglio<sup>a</sup> , Baptiste Boit<sup>a</sup> , Awa Sy<sup>a</sup>

<sup>a</sup> Univ Grenoble Alpes, CEA, LITEN, DTCH, LCST, F-38000 Grenoble, France

ARTICLE INFO

Keywords:

Phase-change material

Supercooling

Isosorbide

Bubbling

Crystallization triggering

A B S T R A C T

Isosorbide, with a melting temperature about 60–65 °C, is an interesting bio-based material for use in Phase-Change Material (PCM) Thermal Energy Storage (TES) systems. It is suitable for low- to moderate-temperature applications, based on its safety characteristics and phase-change enthalpy. However, Isosorbide is known to exhibit moderate to significant overcooling, which may be an issue in TES applications. This study experimentally investigates the overcooling characteristics of Isosorbide and how to manage them. Tests were performed at three distinct scales: material-scale (about 10.3 mg sample), lab-scale (400 mL), and TES prototype-scale (37 kg). At all three scales, extensive supercooling is observed. At the material-scale, Isosorbide solidifies at temperatures below 5 °C, whereas melting occurs in the range 60–65 °C. At beaker-scale, an overcooling rate exceeding 30 °C is observed. Seeding and N2 bubbling trigger crystallization, with bubbling providing clearly higher efficiency. At TES-scale, overcooling remains stochastic, limiting experimental reproducibility. The addition of bubbling during discharge triggers crystallization, making it an effective means to manage supercooling. However, this approach also leads to temperature homogenization in the PCM volume. Tests at prototype-scale reveal that this homogenization is not an issue for TES performance as it allows energy to be recovered at higher temperatures during discharge.

# 1. Introduction

Global energy systems must be transformed to meet the conditions of the Paris Agreement. This implies rapid and significant changes in energy use. In particular, it is now widely accepted that we need to reduce carbon emissions into the atmosphere. To do so, we must deploy a higher share of renewable energy sources and promote highly energy-efficient systems [1]. In this context, Thermal Energy Storage (TES) is gaining attention, as witnessed by an exponentially growing number of publications [2].

TES solutions are typically categorized by the physical phenomenon used to store energy, using sensible, latent, or thermochemical heat. Comprehensive reviews of TES, covering recent advances, main technologies and applications have been published [3–5]. Sensible and thermochemical heat TES, respectively, represent the highest- and lowest-maturity level TES technologies. Due to its high storage density combined with a growing maturity level (close to industrial application), latent heat TES is attracting interest for applications in residential and urban heating networks, where space is at a premium [6–9].

Phase-Change Materials (PCM) are central to TES systems. They have been widely studied, with Sharma et al. [10] presenting a general classification of PCM, and Sharma et al. [11], Cardenas et al. [12] and Cabeza et al. [13] presenting the thermal properties of hundreds of solid-liquid PCM. Based on these studies, the most suitable PCM for the temperatures required in an urban heating network appear to be paraffin, hydrate salts, fatty acids, fatty alcohols and sugar alcohols. Isosorbide could also be a good candidate. This bio-based sugar alcohol is obtained from starch. It is non-toxic, non-corrosive, environmentally friendly and has an adequate melting temperature (63 °C) and a high enthalpy of fusion (>110 J/g). Isosorbide also has a high liquid density (1330 kg/m3), meaning that its volume storage density is high (>40 kW·h/m3). However, in parallel to these qualities, Isosorbide exhibits supercooling.

Supercooling is the characteristic whereby a material remains liquid below its freezing temperature. Supercooling in PCM for TES has been notably reviewed by [35] and has been addressed in TES through two main considerations. Firstly, some authors use this property for long-term/seasonal heat storage where stable supercooling and a means to

<footnote>* This article is part of a Special issue entitled: 'Enerstock 2024' published in Journal of Energy Storage.</footnote>

<footnote>* Corresponding author.
E-mail address: arnaud.bruch@cea.fr (A. Bruch).</footnote>
<figure><img src="image_3.png" /><figcaption>Fig. 1. Isosorbide synthesis from starch.</figcaption></figure>

induce well-controlled supercooling are needed. Examples include PCM such as Erythritol [14], and SAT (Sodium Acetate trihydrate, [15]). Hua et al. reviewed the topic in 2023, and list other options. Secondly, for short- to mid-term applications, the main focus is on reducing and managing supercooling. Indeed, this phenomenon reduces the control- lability and predictability of the behavior of the TES system, which represents a clear drawback for system operators. In general, the ques- tion arises with these applications only due to a lack of choice of PCM and/or very attractive properties of the selected PCM. This is the case, for example, with the storage density of Xylitol [16]. Other studies have

focused on Erythritol [16,17] and SAT [18, 36].

Various strategies to reduce or even suppress supercooling have been developed and tested in all thermal storage durations. An extensive literature exists describing the modifications of the PCM based on the addition of nucleating agents or other additives including liquids such as paraffin and alcohols, or solid nanoparticles such as graphene or TiO₂ [19–21]. The addition of nanoparticles to suppress supercooling was recently reviewed by Gao et al. [22]. However, other techniques are also commonly used, including mechanical triggers [23], vibrational triggers [24], light-triggered systems [25], local cold [18], micro-encapsulation, stirring methods (mechanical and bubbling, [17]), seeding [17,26]. All these methods have proven their ability to trigger crystallization, but their use and implementation may face some drawbacks, as summarized in Pan et al. [14]. Thus, mechanical methods require precise control over parameters such as magnitude of force and point of application, without which mechanical failure and uneven crystallization may occur; light- triggering systems are energy intensive and may also provoke uneven crystallization; thermal methods are also energy intensive and suppose precise temperature management, and specific adaptation of the TES tank geometry; seeding, meanwhile, can result in the introduction of impurities.

The parameters affecting supercooling characteristics are numerous, including sample size [27], cooling rate [28,29], and the thermal cycling history of the material [30,31]. In addition, only a few experimental studies at relevant representative scale have been published. As a result, it can be complex to evaluate the efficiency of a given triggering method. A large body of literature presents studies at material-scale, using very small amounts of PCM [14,15, 32,33]. In contrast, very few studies at representative prototype/demonstrator scale exist. Piquard et al. [17] tested seeding and/or bubbling as methods to trigger supercooling at beaker-scale (400 mL) before integrating a bubbling system in a 42-kWhth TES prototype running on Xylitol. They concluded that a combination of bubbling and seeding allows fast and reproducible PCM crystallization, and that optimization of the seeding step can completely eliminate Xylitol supercooling. In another study, Wang et al. [18] successfully combined short and long-term storage of SAT, overcoming supercooling by the introduction of SAT crystals.

There are three main key lessons from the data presented in the literature. 1) Supercooling is a complex phenomenon displaying material-dependent behavior and intensity. 2) No exhaustive study of

<figure><img src="image_4.png" /><figcaption>Fig. 2. 1 °C/min heating/cooling/heating cycle measurement at material-scale (10.29 mg sample). For easier reading, the baseline measurements for the “cooling” and “heating 2” tests has been translated.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** 0.1 °C/min heating/cooling/heating cycle measurement at material-scale (18.32 mg). For easier reading, the baselines for measurement of the “cooling” and “heating 2” tests has been translated.</figcaption></figure>

supercooling behavior of a given PCM from material- to prototype-scale exists. 3) The “PCM/triggering system” couple should be specifically evaluated at relevant scales.

Here, we focus on the experimental characterization of the super-
cooling behavior of Isosorbide. Multi-scale experiments are conducted,
starting with a material characterization device and culminating with a
PCM TES prototype.

2. Isosorbide

Isosorbide, a bicyclic diol, is derived from the sugar alcohol sorbitol obtained from biomass. As shown in Fig. 1, its production from starch

involves several transformation steps. This bio-based material is recog-
nized as safe, non-toxic (non-CMR, safe by ingestion), and non-
endocrine disrupting (no perturbation of hormonal systems). It is
approved for food contact. Isosorbide displays ready biodegradability
(OECD TEST GUIDELINE 301 B), with 65 % ThCO2 production within a
28 day-period. Combined with its high enthalpy of fusion, this makes
Isosorbide a good, safe bio-based candidate for PCM TES. For this study,
high grade POLYSORB® PA Isosorbate was provided by Roquette. This
formulation was chosen because it contains an additive (Roquette’s
proprietary formulation) to improve stability and reduce the risk of
thermal degradation.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Intermediate-scale device filled with 400 mL of supercooled isosorbide used to study overcooling management. The thermocouples are located at three different heights (1,2,3) and three positions: Exterior (E), Interior (I) and Rotational (R).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. Without seeding or bubbling, isosorbide has a latency time of around 1000 s but spontaneous solidification does occur. Representative results are shown.</figcaption></figure>

### 3. Experimental characterization of isosorbide

Isosorbide is experimentally characterized here in applications ranging from material-scale to prototype-scale.

The first analysis is a DSC analysis at material-scale (<20 mg). It is performed in a Q20 apparatus (TA Instruments) with temperature and calorimetric precision of ±0.05 °C and ± 0.1 %, respectively, and a sensitivity of 1 µW. The analysis involved heating/cooling/heating cycles, at 1 °C/min (10.29 mg sample, Fig. 2) and 0.1 °C/min (18.32 mg sample, Fig. 3) to evaluate the potential impact of the heating/cooling rate on overcooling characteristics. To improve the readability of Figs. 2 and 3, the “baseline measurement” (heat flow measurement far from any melting/cooling phenomenon, i.e. 0 W/g) was intentionally translated by shifting the blue and green curves up and down, respectively.

Starting from –60 °C (Fig. 2), the Isosorbide sample is heated up to about 100 °C, with a first melting occurring at around 42 °C (about 7 J/g) followed by the main melting between 60 and 65 °C (106 J/g). The corresponding total enthalpy change is 113 J/g, which is very close to the theoretical enthalpy of fusion. During cooling at 1 °C/min, crystallization starts at 3 °C (80 J/g), with a second phase of crystallization at –17 °C (3 J/g). It should be noted that the main crystallization peak is very sharp.

The second heating cycle is generally similar to the first one, with a small reduction in the melting temperatures (peak center at 63.2 °C for the first melting process and 62.7 °C for the second). However, the overall enthalpy changes are quite close, at about 113–114 J/g. The significantly lower enthalpy change during cooling (about 84 J/g) can be attributed to higher uncertainties due to the very sharp peak observed. When the thermal rate is reduced to 0.1 °C/min, very similar results are obtained, with the main difference being a minor change in the solidification temperature (4.8 °C instead of 2.6 °C). In both cases, the level of supercooling (> 50 °C) is clearly not acceptable for most thermal applications. Additional tests and measurements will be needed to assess the thermal stability of Isosorbide how the DSC results change with repeated thermal cycling.

Although the level of supercooling appears unpromising, this phenomenon is known to be enhanced in small volumes [27]. We therefore performed additional experiments at intermediate-scale (400 mL of Isosorbide) on a dedicated test bench (see Fig. 4). The main objectives of these experiments were to detect potential overcooling behavior and test methods to overcome this behavior. The test bench is composed of a

double-walled metallic cylindrical crystallizer filled with 400 mL of Isosorbide and connected to a heating bath containing silicone oil (SilOil M40) as heat-transfer fluid.

The system includes two thermocouples to measure the inlet and outlet temperatures of the heat-transfer fluid, and seven thermocouples (T-type 1 mm) are immersed in the PCM sample at different positions to monitor the thermal behavior (see [17] for a complete description). These nine thermocouples are named E1, E2, E3, R1, R2, R3 and I1, depending on their height and position, as illustrated in Fig. 4. Supercooling and triggering of crystallization are managed using two techniques: bubbling and seeding. Bubbling is induced using a needle that injects N2 at the bottom of the vessel. Seeding involves the introduction of a small quantity of solid Isosorbide to serve as nucleation germs. These two techniques were selected based on the literature review presented above, since their implementation is relatively simple and can be adapted for use with an existing TES tank and they are compatible with several PCM.

For bubbling tests, a flow sensor is used to maintain a constant gas flow rate (1 L/min). The needle is slightly inclined (a few degrees) to ensure a preferential path for bubble formation and (as far as possible) to induce reproducible agitation. The use of N2 prevents any potential chemical reactions between the PCM and air. To reduce the risks of unintentional seeding, a cover is placed at the top of the crystallizer, as recommended by [17]. For seeding tests, a small opening (seeding trap) is made in the cover, through which seeds are introduced into the crystallizer.

A first series of experimental tests is completed to study primary nucleation without seeding or bubbling. The scenario is the following:

* The heat-transfer fluid flow rate is set to its maximum value, about 100 kg/h;

* The heating bath temperature is set to 75 °C until the Isosorbide has completely melted. When all the thermocouple inside the PCM reaches the set temperature (75 °C) and maximum flow rate is achieved, the bath is maintained at this temperature for a few hours to ensure complete melting of all the Isosorbide in the crystallizer;

* At the beginning of the crystallization tests (time = 0 s in the figures below), the heating bath temperature is rapidly decreased, reaching a value below the melting temperature of Isosorbide (i.e. 61 °C). Different bath temperatures are studied;
<figure><img src="image_8.png" /><figcaption>Fig. 6. With seeding, isosorbide has a latency time of <300 s.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. With bubbling and without seeding, isosorbide has a latency time of <150 s.</figcaption></figure>

* Each test (with or without triggering system) is repeated 3 to 5 times, to verify the reproducibility of the results.

The heat-transfer fluid temperature during the cooling phase is regulated at 30 °C at the inlet of the crystallizer (see representative results in Fig. 5). The supercooling rate is experimentally evaluated as the temperature shift between the solidification temperature and the temperature at which crystallization effectively starts. The latency time is the corresponding duration, i.e. the period during which the PCM temperature is below its solidification temperature without crystallization occurring. These concepts are both illustrated on the experimental curves shown in Figs. 5, 6 and 7. The first observation from the experimental results is that, in the absence of a triggering system, supercooling is extensive – with Isosorbide remaining liquid at temperatures significantly below the crystallization temperature – but that it is naturally overcome at some point (Fig. 5). Thus, homogeneous primary

nucleation occurs in liquid Isosorbide.

However, in these conditions, crystallization only takes place after a long latency time (> 1000 s) and with a supercooling rate of more than 30 °C. Solidification is triggered simultaneously throughout the volume of Isosorbide, but at different overcooling rates depending on the position inside the crystallizer. Once crystallization is initiated, it occurs throughout the entire beaker, and the Isosorbide temperature rises to its solidification temperature. This behavior is common to all supercooled materials [15]. The latency time is about 1000 s at all TC positions (with a slight reduction between the bottom and the top of the crystallizer). As a result of this position effect, the overcooling rate at the bottom and the top of the crystallizer are about 18 °C and 30 °C, respectively. Similar behavior was previously reported with Xylitol as PCM at different scales [16,17]. Following the initiation of crystallization, the temperature inside the beaker reaches a plateau before decreasing. The plateau is maintained until solidification of the PCM is complete. The temperature
<table><caption>**Table 1**
Results of the tests without seeding or bubbling.</caption><tr><td>Bath temperature (°C)</td><td>30</td><td>30</td><td>40</td><td>40</td><td>55</td><td>55</td></tr><tr><td>Latency time (s)</td><td>477</td><td>1014</td><td>850</td><td>740</td><td>1730</td><td>2177</td></tr><tr><td>Crystallization triggering temperature (°C)</td><td>38.8</td><td>32.6</td><td>42.3</td><td>42.9</td><td>55.1</td><td>55</td></tr></table>

<table><caption>**Table 2**
Results of the tests with seeding, bubbling and seeding + bubbling.</caption><thead><tr><th></th><th>Bath temperature (°C)</th><th>30</th><th>30</th><th>30</th><th>55</th><th>55</th><th>55</th></tr></thead><tbody><tr><td rowspan="5">Latency time (s)</td><td>Tests with seeding (without bubbling)</td><td>632</td><td>610</td><td>303</td><td>810</td><td>820</td><td>870</td></tr><tr><td>Tests with bubbling 1 L/min (without seeding)</td><td>133</td><td>139</td><td>135</td><td>250</td><td>242</td><td>225</td></tr><tr><td>Tests with seeding and bubbling</td><td>147</td><td>141</td><td>137</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

at which it is reached depends on the position of the TC inside the beaker, it therefore depends on the effective thermal exchanges. After this plateau, the slopes of decreasing temperatures are lower than those observed at the beginning of the test due to lower thermal conductivity in the solid phase than in the liquid phase and the absence of convective movements.

The same series of tests was performed at different heating bath temperatures (between 30 °C and 55 °C), with two replicates for each temperature. The results are presented in Table 1. The stochastic behavior is remarkable, with considerable differences between the pairs of tests performed at the same temperature. The influence of temperature is also remarkable, with a shorter latency time when a lower heating bath temperature is used. The results of this second test clearly illustrate a specificity of crystallization by homogeneous primary nucleation following supercooling: it is a stochastic phenomenon, depending on the probability of creation of the first nuclei. This probability in turn depends on the PCM temperature. This temperature-dependent stochastic effect is well known and has been discussed in D. M. Herlach et al. [34].

A second series of experiments is conducted with seeding. The seeds (Isosorbide powder ~1 mg) are introduced into the vessel when a first thermocouple in Isosorbide reads 58 °C.

Adding Isosorbide seeds into the beaker significantly reduces the latency time and increases the triggering temperature for crystallization (Fig. 6). However, both parameters still depend on the axial position in

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Prototype PCM TES, the figure highlights the tube bundle, the tube fins, and the TC instrumentation.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 9. Without bubbling, TES inlet temperatures were relatively reproducible. Outlet temperatures showed some stochastic behavior. During tests, water flow rate and TES inlet temperature were set to 100 kg/h and 22 °C, respectively.</figcaption></figure>

the crystallizer. Near the Isosorbide/air interface, i.e. close to the location of seeding (elevation 3 in the figures), overcooling is completely eliminated. Compared to the test without seeding (Table 1), the results are more repeatable (except in one case at 30 °C) and the latency time is shorter (see Table 2). Consequently, we conclude that seeding improves the supercooling behavior of Isosorbide.

A third set of experiments is done with N2 bubbling, without seeding (Fig. 7). Bubbling is activated from the beginning of the discharge, when the bath temperature first starts to decrease. The supercooling temperature and the latency time are strongly reduced (from 30 °C and 1000 s without bubbling to 5 °C and less than 150 s with bubbling), and the results are more repeatable (Table 2). Bubbling is therefore a more efficient way to mitigate Isosorbide supercooling than seeding.

The data shown in Fig. 7 also reveal an effect of the bubbling on the thermal homogenization of the beaker. This result contrasts with those from the tests without bubbling: in the cooling and crystallization phases, the nine thermocouples indicate the same temperature, whereas in the Fig. 5 (no bubbling, no seeding) and Fig. 6 (seeding, no bubbling), the thermocouple temperatures depend on their position in the vessel.

A final set of experiments were conducted at beaker-scale combining seeding and bubbling. The behavior obtained with bubbling alone was not improved, as shown in Table 2.

These results at beaker-scale are generally in good agreement with published results, notably those of Piquard et al. [17] and Duquesne et al. [26] using Xylitol in similar configurations. We thus conclude that bubbling and seeding can be used to efficiently trigger supercooling, with an associated reduction in stochastic behavior and overall thermal homogenization (for bubbling).

This specific behavior and its influence on overall thermal behavior merited evaluation at TES-scale.

These results at intermediate-scale suggest that it may be possible to manage the overcooling rate of Isosorbide at the scale of a TES. To test this possibility, a prototype PCM-filled TES based on tube and shell technology (Fig. 8) was used. The setup consists in a 2.15 m high heat exchanger placed inside a 0.22 m diameter cylindrical shell. The heat-transfer fluid (water) circulates inside the tubes and exchanges heat with the PCM contained in the shell. The heat exchanger corresponds to a bundle of seven finned tubes (20 mm internal diameter tube, 0.4 mm thick aluminum fins, 10 fins par inch, external fin diameter 57.4 mm), as illustrated in Fig. 8. Hydraulic inserts are added inside the tubes to

create an annular space to increase the water velocity and related heat exchange. For more details on the system, see the similar configuration presented in Bentivoglio et al. [6].

The prototype is filled with 53.5 kg of Isosorbide, corresponding to a liquid Isosorbide volume of 40 dm3 (liquid density = 1325 kg/m3). Between 25 °C and 85 °C, the total energy that can be stored in the prototype is 7.2 kWh.

The TES prototype is equipped with thermocouples on both the water and PCM sides (see Fig. 8 for the positions of the TC inside the PCM). Nineteen 1-mm T-type thermocouples are installed in the PCM. Ten 1-mm T-type thermocouples are installed in the water. This embedded instrumentation allows us to closely follow the thermal behavior of the PCM in this TES by measuring the temperature in the PCM near the wall of the central tube (label “C”), in the PCM near the wall of an external tube (label “G”), in the PCM near the end of the fins on an external tube (label “D”), in the water in the central tube and in a peripheral tube (not shown in Fig. 8).

Based on the results obtained at beaker-scale, it was decided to only test bubbling as a means to manage overcooling. Like at beaker-scale, bubbling is triggered using a N2 injection device introduced at the bottom of the prototype, with bubbles flowing up through the PCM.

All the tests presented here were performed according to the same protocol: initially the system is uniformly heated to about 82 °C, the TES is then cooled with a 100 kg/h flow of water at a temperature varying between 22 °C and 30 °C, depending on the test. The tests with bubbling are performed with a N2 flow rate of 5 NL/min.

The first tests were performed without bubbling. Results are presented based on the inlet and outlet temperatures (Fig. 9), based on the local temperature at a given position inside the PCM (positions “C” in Fig. 8), and based on the discharged power and energy (Fig. 11). Discharged power and energy are calculated by applying Eqs. (1) and (2) (where $\dot{m}$ is the mass flow rate, $T_{out}$ is the outlet temperature, $T_{in}$ is the inlet temperature, and $C_p$ is the mean heat capacity):

$$Power_{hf} = \dot{m} \times C_p_{average} (T_{out} - T_{in}) \qquad (1)$$

$$Energy = \int_{0}^{t} Power_{hf} dt \qquad (2)$$

The results of four distinct experimental tests, performed in the same conditions, are presented.
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Temperature at position C5 confirms lack of reproducibility of PCM crystallization in tests performed without bubbling. For these tests, water flow rate and TES inlet temperature were set to 100 kg/h and 22 °C, respectively.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Discharged power and energy in tests without bubbling shows power to closely follow temperature (compare to curves to those in Fig. 9) whereas the curves for energy release are smoothed. Differences between tests nevertheless remain visible (notably for Test 2). Water flow rate and TES inlet temperature were set to 100 kg/h and 22 °C, respectively.</figcaption></figure>

For very similar inlet conditions, the TES outlet temperatures and the local PCM temperatures vary significantly between tests. This difference is due to the stochastic nature of overcooling, with crystallization being randomly triggered, as observed in Fig. 10. Thus, depending on the test, the crystallization at positions “C” starts at different times (from 1800 s to 3950 s) and different temperatures (from 55 °C to 42 °C). Overall, the later the crystallization, the lower the temperature peak. This relation is clearly visible in Fig. 10. This local triggering of solidification induces an increase in the outlet temperature (Fig. 9), which becomes less and less marked as the latency time increases. For Test 1, characterized by the longest latency time, this increase of the outlet temperature is barely observable on the outlet temperature.

Similar behaviors are observed on the discharged power, with peaks

emerging when overcooling is ruptured (Fig. 11). These peaks become less marked when solidification starts later. Interestingly, these differences are considerably smoothed when examining the energy discharged. At the beginning (before 2200 s), the discharged energy is identical for all tests. Later, Test 5 presents a slightly higher discharged energy compared to other tests. After 6000/7000 s, when crystallization has occurred for all tests and the TES is almost completely discharged, the cumulated discharged energy is independent of the test conditions, as expected. The results on crystallization observed at smaller scale, with poor reproducibility due to the stochastic induction of crystallization, are confirmed at TES prototype-scale.

Typical comparison of results obtained with and without bubbling are presented in Fig. 12 (water temperatures) and Fig. 13 (PCM
<figure><img src="image_14.png" /><figcaption>Fig. 12. Water temperature is more reproducible with bubbling (dashed curves) than without bubbling (solid curves).</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 13. PCM temperature is more reproducible with bubbling (dashed curves) than without bubbling (solid curves).</figcaption></figure>

temperatures at positions “G”). Without bubbling, a solidification front is observed that moves from the bottom (inlet in discharge) to the top (outlet in discharge) of the storage tank. There is no temperature plateau as would be expected for PCM solidification, but a temperature drop is followed by a temperature peak, corresponding to overcooling and its rupture. This temperature fluctuation is observed on both PCM and water sides.

On the PCM side, the rupture of overcooling leads to temperature increases until values close to the melting temperature are reached (55–60 °C for a melting temperature of about 60–63 °C). When looking at Fig. 13, it seems that the overcooling rate (the temperature difference between the melting temperature and the temperature at which the

rupture of overcooling occurs) slightly decreases during the discharge, i. e. slightly decreases from the bottom to the top of the TES tank (position G1 to G5). However, further tests and analysis are needed to confirm these observations.

Bubbling induces a completely different behavior: there is no longer a solidification front that progresses through the PCM and, except on the bottom of the TES (“Tube inlet” in Fig. 12 and “G1” in Fig. 13), the temperature changes simultaneously throughout the height of the TES. A closer examination of the data reveals that, with bubbling, solidification occurs between 1300 s and 1800 s (compared to between 1000 s and 4000 s without bubbling), with an overcooling rate of 11 °C on position G1 and 5 °C to 8 °C elsewhere (compared to 15 °C on G1 and
<figure><img src="image_16.png" /><figcaption>Fig. 14. Temporal evolution of the discharged power and energy, with and without bubbling.</figcaption></figure>

12 °C elsewhere without bubbling). Thus, the homogenization of tem-
perature over the whole tank height is incomplete. As a consequence, a
solidification front is still present, but its duration is limited (1300s to
1800 s). It should also be noted that bubbling induces a much longer
temperature peak after solidification is triggered. This peak therefore
looks more like a plateau. This local behavior of the PCM has conse-
quences on the outlet temperature (Fig. 12): at the beginning of the test,
the outlet temperature is the same in both conditions. With bubbling, the
water outlet temperature increases during the second part of discharge
of the storage medium. This parameter is of prime importance for certain
TES applications, such as when producing domestic hot water.

These results agree well with experiments described in the literature
on a 42-kWh PCM TES container filled with Xylitol [16]. Piquard et al.
[16] tested seeding, bubbling, and seeding + bubbling configurations,
we only tested bubbling at this scale. As Piquard et al. observed,
bubbling triggers supercooling, inducing an associated vertical homog-
enization of the temperature.

The power and energy discharged over time (Fig. 14) reflect the
behaviors mentioned above. The two conditions evolve in the same way
until solidification starts. At 2200 s, the discharge energy with bubbling
becomes higher, due to the increase in outlet temperature. This

difference is reduced when solidification without bubbling occurs and
the outlet water temperature starts to increase, i.e. after 4000 s. After
7000 s, there is no longer a significant difference in power and energy
output between the two conditions, since the PCM is fully solid in both
cases.

Reproducibility studies were performed with bubbling (see Figs. 15, 16 and 17 monitoring inlet/outlet temperature, PCM temperatures and power/energy, respectively). In strictly identical boundary conditions, the results with bubbling appear more reproducible than without, but further assessment is needed. The storage outlet temperature (Fig. 15), the PCM temperatures (Fig. 16) and the power (Fig. 17) show two distinct reproducible behaviors, i.e. tests 2, 3 and 4 on one hand and tests 1 and 5 on the other hand. Tests 2, 3 and 4 exhibit higher performances in term of outlet temperature and power than tests 1 and 5. These two groups of results are quite unusual and we cannot explain the two distinct thermal behaviors. However, they are consistent with recent reports in the literature concerning sugar alcohols, where, with Xylitol, it was shown that bubbling alone is not sufficient to ensure repeatability [16]. As we observed no significant enhancement with the bubbling/seeding combination at beaker-scale, we did not consider seeding at TES-scale. Thus, we are unable to experimentally evaluate the potential added benefit of seeding in combination with bubbling when seeking to trigger supercooling. The presence of some residual nucleating sites could explain the behavior observed. However, this explanation does not seem fully relevant since same protocol was followed in all tests, and the curves grouped as follows: tests 2/4 and tests 1/3/5. This behavior will need to be explored further to allow a better understanding with the aim of achieving better crystallization control. Once again, it should be remembered that these differences tend to be extensively smoothed when looking at the energy discharged.

From an implementation point of view for effective applications, the
main questions focus on scalability and cost. These aspects have not
been deeply studied, but some elements can be extrapolated from our
study. The system used here to help trigger supercooling (i.e., bubbling
through a needle placed in the bottom of the TES tank) is mechanically
simple, cost-effective and easy to integrate into the TES tank setup.
Nevertheless, it does need to be considered early, during the definition
and engineering study. The N2 circuit could be an open or closed circuit,
the differences between the two were not addressed here. But whatever
the nature of the N2 circuit, it is simple and may also be cost-effective.

<figure><img src="image_17.png" /><figcaption>Fig. 15. Reproducibility of tests with bubbling, TES inlet and outlet temperatures. Water flow rate and TES inlet temperature were set to 100 kg/h and 22 °C, respectively.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Fig. 16. Reproducibility of tests with bubbling, PCM temperature at positions C5. Water flow rate and TES inlet temperature were set to 100 kg/h and 22 °C, respectively.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 17. Reproducibility of tests with bubbling, discharged power and energy as a function of time. Water flow rate and TES inlet temperature were set to 100 kg/h and 22 °C, respectively.</figcaption></figure>

The main remaining question concerns the scalability, and notably the number of needles needed in a large TES tank. Although this point has not yet been addressed, we do not consider it a potential issue.

## 4. Conclusion

Due to its thermal, safety and environmental credentials, Isosorbide is a potentially good candidate as a storage material in PCM TES systems. However, like many PCM, Isosorbide exhibits supercooling, which may limit its usefulness.

The thermal behavior of Isosorbide, focusing on overcooling and its management, was investigated at three different scales, from material to thermal storage scales. The following conclusions can be formulated (summary is presented in Table 3):

* At material-scale (few mg in a DSC apparatus), crystallization was triggered, indicating that primary nucleation occurs, but with an overcooling rate exceeding 50 °C. This is quite high for TES applications.

* At beaker-scale (400 mL), extensive overcooling was also recorded, with an overcooling rate of or exceeding 15 °C. In addition, the results for both overcooling rate and latency time were not very reproducible, with the two parameters depending on temperature. These results bring to light the stochastic behavior of overcooling and its temperature-dependency. Various strategies to trigger solidification were tested: seeding, bubbling, and simultaneous seeding and bubbling. Seeding was shown to effectively reduce overcooling (both overcooling rate and latency time) while having only a limited effect on reproducibility. The addition of N2 bubbling strongly
<table><caption>**Table 3**
Summary of main behaviors observed at the different scales studied.</caption><thead><tr><th></th><th>Material-scale (mg)</th><th>Beaker-scale (hundreds of g)</th><th>TES-scale (kg)</th></tr></thead><tbody><tr><td>Primary nucleation</td><td>YES</td><td></td><td></td></tr><tr><td>Without supercooling triggering system</td><td>Rate: &gt;50 °C</td><td>Rate: up to 30 °C<br>No reproducibility on (rate, latency)</td><td></td></tr><tr><td>With supercooling triggering system</td><td>N.A.</td><td>Seeding/bubbling: (rate, latency) ↗<br>Seeding: Reproducibility ↔<br>Bubbling:<ul><li>Reproducibility ↗</li><li>Global mixing</li></ul></td><td>Bubbling:<ul><li>(Rate, latency) ↗</li><li>Reproducibility ↗</li><li>Global mixing</li><li>P<sub>discharge</sub> ↗ at beginning of discharge</li></ul></td></tr></tbody></table>

reduced the latency time and the overcooling rate, also leading to homogenization of the overall temperature inside the crystallizer. The combination of bubbling and seeding did not improve the results obtained with bubbling alone.

* At TES-scale (53.5 kg of Isosorbide), the effect of overcooling remained evident, both on local PCM temperatures and on the water outlet temperature. N2 bubbling significantly changed the thermal behavior of the TES, by reducing the latency time and the overcooling rate. In addition, it clearly improved reproducibility. It also led to homogenization of the temperatures at storage scale, with the consequence that crystallization was triggered simultaneously for all the thermocouples in the crystallizer. Finally, the behavior observed at TES-scale was very similar to that observed at beaker-scale. From a process point of view, N2 bubbling increases the discharged power and outlet water temperature in the first moments of discharge, which can clearly be an advantage. However, from energy point of view, the differences tend to be smoothed in both conditions tested.

Next steps will include performing further experimental tests and evaluating the benefits of triggering crystallization, depending on process constraints and limitations.

### CRediT authorship contribution statement

**Arnaud Bruch:** Project administration, Methodology, Investigation, Conceptualization, Writing – original draft. **Fabrice Bentivoglio:** Methodology, Investigation, Data curation, Conceptualization. **Baptiste Boit:** Methodology, Investigation, Conceptualization. **Awa Sy:** Investigation, Data curation.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgment

This work is supported by the EU Horizon 2020 project "Compact bio-based thermal energy storage for buildings" (ComBioTES) (No. 864496).

### Data availability

Data will be made available on request.

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