Latent Thermal Energy Storage System for Heat Recovery between 120 and 150 °C: Material Stability and Corrosion ^†

Abstract

Thermal energy represents more than half of the energy needs of European industry, but is still misspent in processes as waste heat, mostly between 100 and 200 °C. Waste heat recovery and reuse provide carbon-free heat and reduce production costs. The industrial sector is seeking affordable and rugged solutions that should adapt the heat recovery to heat demand. This study aims to identify suitable latent heat materials to reach that objective: the selected candidates should show good thermal performance that remains stable after aging and, in addition, be at a reasonable price. This paper details the selection process and aging results for two promising phase change materials (PCMs): adipic and sebacic acid. They showed, respectively, melting temperatures around 150 °C and 130 °C, degradation temperatures (mass lost higher than 1%) above 180 °C, and volumetric enthalpy of 95 and 75 kWh·m⁻³. They are both compatible with the stainless steel 316L while their operating temperature does not exceed 15 °C above the melting temperature, but they do not comply with the industrial recommendation for long-term use in contact with the steel P265GH (corrosion speed > 0.2 mm·year⁻¹).

1. Introduction

The continuous increase in the level of greenhouse gas emissions and the reduction in production costs are the main driving forces to encourage all stakeholders in the manufacturing sector to improve the energy efficiency of their production processes. In fact, at the present time, a large amount of energy is rejected at a low temperature level (between 0 and 150 °C). For instance, in France, the amount of industrial energy losses below 200 °C is estimated to be 83 TWh·year⁻¹, representing 75% of the total heat wasted by the French industry [1]. At the EU level, the waste heat potential is about 300 TWh·year⁻¹, with one-third corresponding to temperature levels below 200 °C [2]. According to the US Department of Energy (DOE), a barrier to massive heat recovery at low temperature levels is the lack of end uses that should be enlarged by introducing efficient heat storage solutions [3]. One option is to recover energy from the waste stream using thermal energy storage (TES) technology, including phase change material (PCM) [4]. This technology appears particularly attractive compared to sensible or thermochemical heat storage since it offers the best compromise between a sufficient Technology Readiness Level (TRL) and a high-energy storage density, facilitating the system’s integration in a complex industrial environment. Additionally, it provides an almost constant temperature, avoiding process disturbances or increasing the performances, for example, when the TES is combined with an Organic Rankine Cycle (ORC) [5,6].

These arguments led to the development of a storage system containing a PCM melting at a temperature included in the 120–150 °C range. This temperature range corresponds to the pre-heating of existing industrial processes. Many types of heat exchangers can be utilized [5]. The use of a shell and tube heat exchanger seems a relevant option because this technology is widely used in industry. Specifically, a multi-tubular heat exchanger is considered where the PCM is located in the shell, around the tubes (Figure 1). The shell acts as an envelope for the PCM, preventing its leakage, when it is in a liquid state.

Among the different energy storage technologies presented in Figure 2, the thermal one is considered in this study. The developed TES system involves PCMs whose storage capacity corresponds to their latent heat. The liquid-to-vapor transition shows the greater enthalpy, but is associated with high volumetric expansion, requiring consequent volumes and/or high-pressure tanks. This expansion is negligible in solid–solid transition, but the transition kinetic is generally too slow and associated with low enthalpies. The solid–liquid transition is preferred for both its intermediate latent heat and thermal expansion.

PCMs are classified into two distinct families: organic and inorganic materials [7]. This classification can be completed by incorporating eutectics, which can be a mixture of organic and inorganic compounds.

Organic materials [8] can be classified into two categories: paraffins and non-paraffins. Paraffins are by-products of petroleum distillation and belong to the alkane family. They are molecules with an empirical formula of C_nH_2n+2 and structural formula of CH₃-(CH₂)_n-CH₃. One of the main properties of these materials is that the longer the carbon chain is, the higher its melting temperature. Paraffins are non-toxic, chemically stable and inert below 500 °C. Their supercooling degree (the difference between the melting and crystallization temperatures) is generally low. However, it should be noted that they have a relatively low thermal conductivity (0.2 W·m⁻¹·K⁻¹) and a volume expansion during phase changes of up to 15%. A high thermal conductivity increases the heat transfers and provides high thermal storage\discharge power, and a low thermal expansion coefficient avoid mechanical stress on the PCM envelope. The non-paraffin subgroup is the largest group of PCMs in the literature, as it is composed of several subfamilies, including fatty acids and sugar alcohol polyols. Fatty acids, having a general formula of C_mH_nO₂, have been widely used because of their good thermo-physical properties: high thermal capacity, good thermal and chemical stability, non-toxic, non-corrosive, low degree of supercooling and low cost. However, they are flammable, have low thermal conductivity, have different levels of toxicity, and are unstable at high temperatures. Polyols are compounds with an empirical formula of (CHOH)_nH₂. These materials are increasingly studied because their melting temperature can be between −15 and 245 °C and their enthalpy change may be up to 413 J·g⁻¹, with a density often exceeding 1500 kg·m⁻³ [9]. Moreover, these PCMs are non-toxic and non-hazardous to humans and the environment. However, they have a high degree of supercooling [10] and a volume expansion at phase change of up to 15%.

The inorganic group is mainly composed of salts or hydrated salts and metals [11]. Metals have a melting point much higher than the temperature range required for this study. Hydrated salts contain water and are characterized by the general formula M_xN_y.nH₂O. Although hydrated salts have high volume enthalpy of phase change, good thermal conductivity and low volume expansion during phase transformation, their melting is very often incongruous (segregation). The transformation is thus incomplete and by difference in density, there can be a separation of several components. It is also possible to note a tendency of supercooling. Salts are pure bodies of ionic structure formed from anions and cations. These materials have properties similar to hydrated salts; however, the absence of the water molecule allows their use at higher temperatures.

In addition to these general characteristics, PCMs in the same family can also differ according to physical, chemical, and economic criteria that are important for industrial applications. The first part of this paper is dedicated to the screening and selection of the most suitable PCM for our application: waste heat recovery between 120 and 150 °C.

There is another important constraint to bear in mind. The selected PCM is meant to be used in a metallic heat exchanger, and should thus be compatible with long-term use within this envelope. Corrosion implies serious drawbacks, such as making the metal brittle and threatening a structure collapse. It could also modify the PCM’s properties, degrading its thermal performances or increasing its viscosity and thus the operational pumping costs [12]. Viscosity increase is not an issue in this work as the PCM is stacked in the heat exchanger and does not circulate. The corrosion issue has been studied over a wide range of temperatures, from cold applications [13,14,15] to high temperatures [16], especially for concentrated solar power applications and thus with molten salts used as PCMs [17,18,19], passing the building temperature level [20,21,22]. Fewer studies have focused on the medium temperature range that concerns us (120–150 °C) [23]: a recent paper investigated the compatibility between a commercial PCM named PlusIce S83 (mainly composed of magnesium nitrate) with aluminum, copper, and carbon steel [24]. Those previous works demonstrate the importance of such studies. The second part of this paper is dedicated to the stability and corrosion behavior of industrial-grade PCMs.

Previous works have already been carried out to select and characterize PCMs in the temperature range between 120 and 150 °C. However, not all the works have considered material stability and possible corrosion problems. In addition, the results obtained by the authors may differ from one publication to another. The aim of our work was to specify the properties of materials that have already been identified and studied in the literature, and also to select new ones potentially.

2. Phase Change Materials Screening and Selection

2.1. Selection Criteria

The first criterion to choose an appropriate PCM is its transition temperature: in the present study, the focus was on the 120–150 °C range. The selection also considers the following features [25,26]:

• Physical: besides the suitable transition temperature, the PCM should have:

  -

  High volumetric enthalpy (high latent heat and high density) to provide high-density storage. The value of 50 kWh·m⁻³ was taken as a reference. It corresponds to the variation enthalpy of water in a temperature range of approximately 0–43 °C;

  -

  High thermal conductivity to increase the heat transfer and provide high thermal storage\discharge power (may be optimized by the system geometry);

  -

  Low thermal expansion coefficient to avoid mechanical stress on PCM envelope;

  -

  Good phase equilibrium (no segregation during phase change), as phase segregation during transition disrupt heat transfer and can be partially irreversible;

  -

  No supercooling that delays crystallization.

• Chemical:

  -

  Long-term chemical stability to avoid performance degradation through thermal cycling;

  -

  Compatibility with enveloping materials to avoid corrosion;

  -

  No toxicity nor fire hazards.

• Economics:

  -

  PCM should be industrially available in large quantities;

  -

  For a cost-effective storage system, the PCM price should remain affordable. This is highly dependent on the targeted application: for a short-term storage (about 3 cycles per day), the International Renewable Energy Agency (IRENA) estimated a viable investment cost at 225 EUR·KWh⁻¹ for the whole system [27]. Accounting for approximately 45% of this investment for the phase change material [28], its cost should remain under 100 EUR·KWh⁻¹.

2.2. Materials Screening

The literature review selected the organic PCMs whose melting temperature is included in the targeted range (120–150 °C), as referenced in Table 1 [29]. This table also gives their density ρ, their latent heat ∆Hm, and their volume enthalpy variation ∆Hm,vol. A price estimation range is given based on ∆Hm,vol: <50 EUR·KWh⁻¹ (+ + +), <100 EUR·KWh⁻¹ (+ +), <150 EUR·KWh⁻¹ (+), <500 EUR·KWh⁻¹ (-), < 1000 EUR·KWh⁻¹ (- -), and >1000 EUR·KWh⁻¹ (- - -). This is a rough criterion that does not include the sensible part of the storage (would increase ∆Hm,vol) nor the system envelope and porosity (would decrease ∆Hm,vol).

Three PCM classes were ruled out in this study:

• Metals, for their high costs. Two PCMs fit in the temperature range [30]: Indalloys 255 (55.5%mBi + 44.5%mPb, toxic) and 281 (58%mBi + 42%mSn), with the respective melting temperatures of 125 °C and 138 °C. They show a similar volume enthalpy variation to organic PCMs (respectively, 58 and 107 kWh·m⁻³), but their thermal conductivity is ~100 higher, supporting higher power.
• Eutectics, for their complex industrial production which hinders a precise and stable eutectic. Five PCMs could have been selected for their melting temperature [17,31]: 30%mLiNO₃ + 18%mNaNO₃ + 52%mKNO₃ (120 °C), 33%mLiNO₃ + 67%mKNO₃ (125 °C), 38.2%mLiNO₃ + 61.8%mCO(NH₂)₂ (125 °C), 44%mCa(NO₃)₂ + 44%mNaNO₃ + 12%mKNO₃ (140 °C), and 40%mNaNO₂ + 7%mNaNO₃ + 53%mKNO₃ (142 °C).
• Inorganics, whose candidates in the focused range [31,32] were industrially unavailable (ammonium zinc sulfate hexahydrate and magnesium nitrate dihydrate, T_m = 125 °C and 130 °C, respectively), or suffered early damage (sodium acetate trihydrate, T_m = 137 °C).

Among the 32 remaining candidates, several were removed for the following reasons [31,32,33,34]:

• High cost (- -): mandelic acid, valporic acid, suberic acid, methyl-4′-acetanilide [30], chlorobenzoic acid, and xylose-L;
• Early damage (decomposition temperature closer than 20 °C to the melting temperature): urea, malonic acid, maleic acid, DL-malic acid, trans cinamic acid, fructose-D, and glucose-D;
• Enthalpy variation lower than 50 kWh·m⁻³: HDPE [35], stilbene, and trans-1,4-polybutadiene [6];
• Hazardousness (flammability, toxicity): picric acid, benzamide, phenacetine, and anthranilic acid;
• Significant supercooling (more than 30 °C): erythritol (despite a very attractive latent heat) and tromethanol (in addition to a high cost) [36].

Some polyols could be of interest: maltitol, isomalt [37], xylose-D, and lactitol. However, the first two decomposed during their first thermal cycling and the last two were not industrially available when this study started.

Finally, six PCMs were selected for thorough examination: succinic anhydre acid, benzoic acid, phtalic anhydre [38], sebacic acid [39], dimethyl terephtalate, and adipic acid.

Table 1. PCM candidates for latent heat storage in the range 120–150 °C, the 6 selected are in bold. * [31,32,33,34] <50 EUR·KWh⁻¹ (+ + +), <100 EUR·KWh⁻¹ (+ +), <150 EUR·KWh⁻¹ (+), <500 EUR·KWh⁻¹ (-), < 1000 EUR·KWh⁻¹ (- -), and >1000 EUR·KWh⁻¹ (- - -). Reprinted with permission from Y. Lalau, D. Haillot, S. Rigal, J.-P. Bedecarrats (2020), copyright 2020 Société Française de Thermique [29].

Name	Formula	ρ [kg·m−3]	Tm [°C]	∆Hm [kJ·kg−1]	ΔHm, vol [kWh·m−3]	N° CAS	Price [EUR kWh−1]	Comments	Ref
Erythritol	C4H10O4	1480	118–120	340	140	149-32-6	+ + +	Supercooling > 80 °C	[36]
Succinic anhydride acid	C4H4O3	1560	118–121	206	89	108-30-5	+ +		*
Mandelic acid	C6H5CH(OH)CO2H	1300	118–121	161	58	90-64-2	- -	High cost	[30]
Valporic acid	C₈H₁₆O₂	904	120	203	51	99-66-1	- -	High cost	[30]
Benzoic acid	C₇H₆O₂	1266	121.7	143	50	65-85-0	+ +	Supercooling: 22 °C	*
Picric acid	C6H3N3O7	1760	122	75	37	88-89-1		Flash point 150 °C, explosive	*
HDPE	(C₂H₄)n	940	125	167	44	9002-88-4		Enthalpy < 50 kWh·m−3	[35]
Stilbene	C₁₄H₁₂	970	126	167	45	103-30-0		Enthalpy < 50 kWh·m−3	*
Benzamid	C₉H₇NO	1341	127.2	169	63	55-21-0		Toxic	*
Tromethamine/Tromethanol	C4H11NO3	1353	131	285	107	77-86-1	-	Supercooling: 66 °C, High cost	[36]
Anydride phthalic	C₈H₄O₃	1530	131	159	68	85-44-9	+ + +	Supercooling: 23 °C	[38]
Sebacic acid	C₁₀H₁₈O₄	1209	131–133	243	82	111-20-6	+ +		[39]
Maleic acid	C₄H₄O₄	1590	131–140	235	104	110-16-7		Decomposition ~145 °C	*
DL- malic acid	C₄H₆O₅	1601	131–140	225	100	6915-15-7		Decomposition ~150 °C	*
Urea	CO(NH₂)₂	1323	132	251	92	57-13-6		Low stability	*
Malonic acid	C₃H₄O₄	1620	132–136			141-82-2		Decomposition ~135 °C	*
Trans cinnamic acid	C₉H₈O₂	1250	133	153	53	140-10-3		Decomposition ~146 °C	*
Phenacetin	C₁₀H₁₃NO₂	1240	134	175	60	62-44-2		Toxic	*
Chrolobenzoic acid	C7H5ClO2	1540	140	164	70	118-91-2	- -	High cost	*
Suberic acid	C₈H₁₄O₄	1020	141–144	245	69	505-48-6	- -	High cost	[30]
Dimethyl terephtalate	C₁₀H₁₀O₄	1290	142	170	61	120-61-6	+ +		*
Fructose-D	C₆H₁₂O₆	1690	144–145	145	68	57-48-7		Early degradation	*
Isomalt	C12H24O11	1040	145	170	71	64519-82-0	-		[37]
Trans-1,4-polybutadiene (TPB)	C4H6	1010	145	144	40	25038-44-2		Enthalpy < 50 kWh·m−3	[6]
Maltitol	C₁₂H₂₄O₁₁	1620	145–152	173	78	585-88-6	+ + +		*
Methyl-4′-acetanilide	C9H11NO	1370	146–151	180	69	103-89-9	- - -	High cost	[30]
Lactitol	C₁₂H₂₄O₁₁	1690	146–152	135–149	70	585-86-4	+ + +	Industrial availability	*
Anthranilic acid	C6H4(NH2)COOH	1410	147	148	58	118-92-3		Flash point 150 °C	*
Xylose-D	C₅H₁₀O₅	1525	147–151	216–280	118	58-86-6	+ + +		*
Xylose -L	C₅H₁₀O₅	1525	147–151	213	90	609-06-3	- - -	High cost	*
Glucose -D	C₆H₁₂O₆	1540	149–152	180	82	50-99-7		Low stability	*
Adipic acid	C₆H₁₀O₄	1360	151–155	260	98	124-04-9	+ + +		*

Table 1. PCM candidates for latent heat storage in the range 120–150 °C, the 6 selected are in bold. * [31,32,33,34] <50 EUR·KWh⁻¹ (+ + +), <100 EUR·KWh⁻¹ (+ +), <150 EUR·KWh⁻¹ (+), <500 EUR·KWh⁻¹ (-), < 1000 EUR·KWh⁻¹ (- -), and >1000 EUR·KWh⁻¹ (- - -). Reprinted with permission from Y. Lalau, D. Haillot, S. Rigal, J.-P. Bedecarrats (2020), copyright 2020 Société Française de Thermique [29].

Name	Formula	ρ [kg·m−3]	Tm [°C]	∆Hm [kJ·kg−1]	ΔHm, vol [kWh·m−3]	N° CAS	Price [EUR kWh−1]	Comments	Ref
Erythritol	C4H10O4	1480	118–120	340	140	149-32-6	+ + +	Supercooling > 80 °C	[36]
Succinic anhydride acid	C4H4O3	1560	118–121	206	89	108-30-5	+ +		*
Mandelic acid	C6H5CH(OH)CO2H	1300	118–121	161	58	90-64-2	- -	High cost	[30]
Valporic acid	C₈H₁₆O₂	904	120	203	51	99-66-1	- -	High cost	[30]
Benzoic acid	C₇H₆O₂	1266	121.7	143	50	65-85-0	+ +	Supercooling: 22 °C	*
Picric acid	C6H3N3O7	1760	122	75	37	88-89-1		Flash point 150 °C, explosive	*
HDPE	(C₂H₄)n	940	125	167	44	9002-88-4		Enthalpy < 50 kWh·m−3	[35]
Stilbene	C₁₄H₁₂	970	126	167	45	103-30-0		Enthalpy < 50 kWh·m−3	*
Benzamid	C₉H₇NO	1341	127.2	169	63	55-21-0		Toxic	*
Tromethamine/Tromethanol	C4H11NO3	1353	131	285	107	77-86-1	-	Supercooling: 66 °C, High cost	[36]
Anydride phthalic	C₈H₄O₃	1530	131	159	68	85-44-9	+ + +	Supercooling: 23 °C	[38]
Sebacic acid	C₁₀H₁₈O₄	1209	131–133	243	82	111-20-6	+ +		[39]
Maleic acid	C₄H₄O₄	1590	131–140	235	104	110-16-7		Decomposition ~145 °C	*
DL- malic acid	C₄H₆O₅	1601	131–140	225	100	6915-15-7		Decomposition ~150 °C	*
Urea	CO(NH₂)₂	1323	132	251	92	57-13-6		Low stability	*
Malonic acid	C₃H₄O₄	1620	132–136			141-82-2		Decomposition ~135 °C	*
Trans cinnamic acid	C₉H₈O₂	1250	133	153	53	140-10-3		Decomposition ~146 °C	*
Phenacetin	C₁₀H₁₃NO₂	1240	134	175	60	62-44-2		Toxic	*
Chrolobenzoic acid	C7H5ClO2	1540	140	164	70	118-91-2	- -	High cost	*
Suberic acid	C₈H₁₄O₄	1020	141–144	245	69	505-48-6	- -	High cost	[30]
Dimethyl terephtalate	C₁₀H₁₀O₄	1290	142	170	61	120-61-6	+ +		*
Fructose-D	C₆H₁₂O₆	1690	144–145	145	68	57-48-7		Early degradation	*
Isomalt	C12H24O11	1040	145	170	71	64519-82-0	-		[37]
Trans-1,4-polybutadiene (TPB)	C4H6	1010	145	144	40	25038-44-2		Enthalpy < 50 kWh·m−3	[6]
Maltitol	C₁₂H₂₄O₁₁	1620	145–152	173	78	585-88-6	+ + +		*
Methyl-4′-acetanilide	C9H11NO	1370	146–151	180	69	103-89-9	- - -	High cost	[30]
Lactitol	C₁₂H₂₄O₁₁	1690	146–152	135–149	70	585-86-4	+ + +	Industrial availability	*
Anthranilic acid	C6H4(NH2)COOH	1410	147	148	58	118-92-3		Flash point 150 °C	*
Xylose-D	C₅H₁₀O₅	1525	147–151	216–280	118	58-86-6	+ + +		*
Xylose -L	C₅H₁₀O₅	1525	147–151	213	90	609-06-3	- - -	High cost	*
Glucose -D	C₆H₁₂O₆	1540	149–152	180	82	50-99-7		Low stability	*
Adipic acid	C₆H₁₀O₄	1360	151–155	260	98	124-04-9	+ + +		*

2.3. Calorimetric Analysis

Calorimetric analysis performed by differential scanning calorimetry (DSC) and the T-history method [40] were the main methods used to obtain the storage capacity of PCMs. DSC was used because it is fast and available in the laboratory. These measures were coupled with thermogravimetry. The results are presented after the description of the operating method.

2.3.1. Material & Method

Differential Scanning Calorimetry (DSC)

The calorimetry measurements were performed in hermetic (21 bars) 60 µL cells thanks to a power-compensation calorimeter (Perkin Elmer Pyris Diamond). Once filled with PCM (~20 mg), the cells were sealed with a press. They were weighted by a Mettler Toledo XP26 scale with a 0.002 mg precision, resulting in an uncertainty of measurement of 0.004 mg (empty and filled cells).

Those measurements allow both the melting temperature and phase change enthalpy variations to be obtained, as well as information regarding the thermal cycling behavior of the selected materials. In order to reduce the characterization duration, 20 °C·min⁻¹ cooling and heating rates were applied between 30 °C under and above the melting temperature, as presented in Figure 3. The 38th and 75th cycles were compared to the 2nd, which was taken as the reference value.

Using DSC, the melting temperature T_m was graphically determined by the onset method, as illustrated in Figure 4 [41], and then named T_onset. A crystallization temperature was also defined by this method, allowing us define the criteria ΔT_mean, which is the difference between the melting and the crystallization temperature.

This high-speed heating rate allowed us to reduce the characterization duration to verify the materials’ properties compared to the literature and to roughly determine their thermal stability through a high number of cycles. The finest characterization of the ultimately selected materials (Section 3) was performed at a reduced heating rate of 2 °C·min⁻¹.

Thermogravimetry Analysis (TGA)

The thermogravimetry analysis gives the PCMs’ mass loss when submitted to an unusual temperature increase in order to quantify their maximum usable temperature within the TES system.

The selected PCMs were placed in an alumina crucible, weighted with the same scale used for DSC tests, and tested under air as gas purge in 170 µL in the Setsys Cryostat from Setaram (Caluire-et-Cuire, France).

Each sample (~100 mg) was submitted to the following temperature profile:

• A 10 min isothermal step at 35 °C below the melting temperature;
• A 2 °C·min⁻¹ slope up to 100 °C above the melting temperature.

The irreversible damage threshold was defined at the temperature T1% where the PCM loses 1% of its initial mass. To be used safely in industrial conditions, the difference between the melting temperature and this degradation temperature T1% should be sufficient (greater than 20 °C as order of magnitude).

2.3.2. Results and Analysis

The DSC heating and cooling curves are provided for two of the studied PCMs in Figure 5a adipic acid and Figure 5b sebacic acid. For both of them, a small evolution appeared between the reference cycle (2nd cycle) and the 38th cycle, but the sample behavior seems have stabilized between the 38th and 75th cycle.

The mass loss profiles from thermogravimetric analysis are illustrated in Figure 6a for adipic acid and Figure 6b sebacic acid. For both of them, their mass remains stable up to approximately 180 °C, reaching a 1% mass loss at, respectively, 186.1 °C and 181 °C. This threshold is 35 °C and 51 °C above their respective melting temperatures.

The measurement results on selected materials are synthetized in

Table 2. Thermal characterization synthesis of PCM candidates (grey boxes enlighten values below the targeted performances).

	DSC (2nd, 38th, and 75th Cycles)							TGA
	Tonset [°C]			ΔHm, vol [kWh·m−3]			ΔTmean [°C]	T1% [°C]
Precision	±0.4 °C			±1.3%			±0.4 °C	-
Succinic anhydride	115.3	115.7	115.3	77.6	74.5	71.9	60.8	115.7
Benzoïque acid	121.5	120.5	120.7	49.6	47.1	46.4	25.0	122.4
Phtalic anhydride	129.1	128.9	129	65.9	60.4	58.7	27.3	124.0
Sebacic acid	131.5	130.3	130.8	75	70	71	11.7	181.0
Dimethyl terephtalate	140.2	139.9	140.4	58.4	54.8	53.4	9.7	128.7
Adipic acid	150.0	149.1	149.6	95.2	92.6	92.2	10.0	186.1

.

According to the DSC analysis, the melting temperatures are consistent with the literature and remain stable along the thermal cycles, except for succinic anhydride, which melted at 115 °C instead of 118–121 °C, excluding this candidate from the focused temperature range, i.e., 120–150 °C.

Even if DSC gives information on supercooling PCM behavior, a specific value cannot be determined, as the supercooling phenomenon is mainly dependent on the sample volume [36], which is not representative of industrial system volumes. However, although the supercooling decreased with the increase in the volume, the DSC test revealed a potential issue with the succinic anhydride (ΔT_mean = 60.8 °C) and to a lesser extent with the phtalic anhydre (ΔT_mean = 27.3 °C) and the benzoic acid (ΔT_mean = 25 °C).

A slight decrease (generally less than 5%) was observed between the literature and the measured values of latent melting heat [31,32,33,34,38,39], with a more noteworthy difference for sebacic acid (8.5%) and succinic anhydride (12.5%). During the thermal cycling, this latent heat showed a decrease above 5% for succinic anhydride, phtalic anhydride, dimethyl terephthalate, and benzoic acid, and up to 11% between the 2nd and 75th phtalic anhydride cycles. This performance degradation could be detrimental to industrial uses involving melting/crystallization repeated phases and should be considered in the system design.

This critical stability was evaluated by thermogravimetry analysis: the four PCMs lost more than 1% in mass at a temperature close to, or even below, the melting temperature, leading to a complete sample evaporation at the end of the test. This degradation being incompatible with the industrial use, those materials are removed from the study.

Only two candidates were ultimately selected: sebacic acid and adipic acid. To confirm their suitability, their property stability had to be studied. In order to analyze their real performances, the following stability and corrosion studies were conducted in industrial grade materials provided by Altichem^® (Saint-Ouen-l’Aumône, France).

3. Stability and Corrosion Study

3.1. Materials and Method

3.1.1. Stability Study: PCM Thermal Aging

The stability study aimed to observe PCM behavior when exposed to higher temperatures than the standard conditions. This could happen during monitoring failures, and should not deteriorate the PCM’s thermophysical properties. It is also a way to ensure that the properties of the material remain stable over time despite the multiple cycles of temperature rise and fall.

The PCMs were exposed to two-month thermal aging in three stoves at a specific temperature. Each stove contained as many hermetically sealed bottles as samples to characterize by DSC at the given frequency: 1 day, 2 days, 4 days, 8 days, 16 days, 30 days, and 60 days. The stove temperatures were set to 15 °C, 25 °C, and 35 °C above the PCM melting temperature (respectively, T1, T2, and T3). The DSC measurements were performed by the same apparatus as presented in the previous paragraph, but at a reduced speed (2 °C·min⁻¹), and only three cycles were performed to obtain a mean value of T_onset and ΔH_m.

3.1.2. Corrosion Study: PCM and Envelope Thermal Aging

In the corrosion study, a piece of the materials used for the storage system envelope was added to the sealed bottles, and the same thermal aging protocol as for the stability study was followed. The DSC measurement controls the influence of the envelope material on the PCM properties, and an observation coupled with the weight of the envelope material gives information on its corrosion by the PCM.

This study took place in an industrial research project where the TES consisted of a shell and tube heat exchanger (HX) filled with the PCM. The intended HX was made of 361L stainless steel (tubes) and P265GH steel (shell). These material samples were submerged in the liquid PCM, and had the following size: a 30 × 12 mm² surface for both and thicknesses of 0.5 mm and 4 mm, respectively, for 316L and P265GH. The steel sample mass differences before and after aging were measured to calculate the corrosion speed v_corr according to ASTM G01 norms [42].

$$v_{corr}=\frac{K\times \left(m_i-m_f\right)}{S\times ∆t\times \rho }$$

(1)

where K = 87,600, a unit converter coefficient (from cm·h⁻¹ to mm·year⁻¹), S is the total surface sample in cm², Δt is the experiment duration in h, $\rho$ is the metal density in g·cm⁻³ and m_i, and is m_f the initial and final sample mass in g. According to

Table 3. Industrial guide for corrosion mass loss acceptance [14,43].

Corrosion Speed [mm·Year−1]	Recommendations
>2	Completely ruined in a few days
0.2–2	Not recommended for use longer than one month
0.1–0.2	Not recommended for use longer than one year
0.02–0.1	Use carefully depending on conditions
<0.02	Long-term use

, the corrosion speed should not exceed 0.02 mm·year⁻¹ to be economically viable for industrial use.

3.2. PCM Thermal Aging

The T_onset evolution is illustrated in Figure 7 and Figure 8, respectively, for adipic and sebacic acid, while Figure 9 and Figure 10 describe the latent melting heat evolution for the same materials. The initial values were T_onset = 152 °C and ΔH_m = 240 kJ·Kg⁻¹ for adipic acid and T_onset = 131 °C and ΔH_m = 224 kJ·Kg⁻¹ for sebacic acid. The temperatures T1, T2, and T3 were, respectively, 15, 25, and 35 °C above the PCM onset temperature. The small variations (~1% for T_onset and ~5% for ΔH_m) did not reveal a downward tendency over the two months studied, neither for adipic nor sebacic acid; the materials should therefore be suitable for long-term use as thermal storage materials in the targeted temperature range. Thus, the compatibility study with the storage envelope was relevant for both materials.

3.3. PCM and Envelope Thermal Aging

The same measurements of T_onset and ΔH_m were conducted on sebacic and adipic acids in contact with stainless steel 316L or steel P265GH, respectively, shown in Figure 11, Figure 12, Figure 13 and Figure 14 with steel corrosion speed depicted in Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19 with steel corrosion speed depicted in Figure 20. In each graph, T1, T2, and T3 are the above-mentioned aging temperatures. The steel samples were immersed from 1 to 60 days in the melted PCM.

3.3.1. Sebacic Acid

The sebacic acid T_onset remained stable either when the PCM was in contact with 316L (Figure 11) or P265GH (Figure 13). The latent heat of fusion seemed to be more affected: at T3 = 166 °C (35 °C above the PCM onset temperature), small drops of 5% and 10% were observed, respectively, for 316L (Figure 12) and P265GH (Figure 14). However, the stability considering ΔH_m evolution was better at lower temperatures, and the industrial use of sebacic acid should be validated under T1 = 146 °C (15 °C above the PCM onset temperature) without restrictions for 316L and with vigilance for P265GH.

The corrosion speed (Equation (1)) also worsened when the aging temperature increased (Figure 15). A constant value was reached after ~400 h, except for the stainless steel sample immersed in the PCM at T3 that required more than 1400 h to stabilize. Concerning the stainless steel samples, a satisfying value below 0.02 mm·year⁻¹ was attained at all temperatures, satisfying the recommendations from Table 3 for long-term use. Conversely, the P265GH steel samples showed corrosion speeds above 0.1 mm·year⁻¹ even at the lower temperatures, impeding its long-term use in the targeted environment.

3.3.2. Adipic Acid

The investigated thermal properties of adipic acid maintained a stable value when it was in contact either with 316L or P265GH at the lower temperature T1 = 166 °C (15 °C above the PCM onset temperature). However, the temperature increase had a noticeably damaging effect on those properties. The T_onset fell up to 4 °C at T2 = 176 °C and T3 = 186 °C for both steels (Figure 16 and Figure 18). This ~2.5% variation could be acceptable if it tended to stabilize, which could be established by longer experiments. The latent heat of fusion appears to start a decrease which limits cannot be estimated after 2 months of thermal aging (Figure 17 and Figure 19). Between the initial measurement and the 30th day value, this property lost at T2 and T3, respectively, 7% and 11% of its value for the pair PCM/316L and 24% and 33% of its value for the pair PCM/P265GH. Thus, the adipic acid should be used preferably below T1 = 166 °C in the targeted exchanger.

As depicted in Figure 20, the corrosion speed of 316L is far below the recommendations at T1 = 166 °C, and suitable for careful use at 176 °C and 186 °C. The corrosion speed plateau was not reached for these last two temperatures, and the final value should be even closer to the recommended rank. As for the steel P265GH, the corrosion speed is likely to stabilize around 0.2 mm·year⁻¹ for all three temperatures, meaning this material is not appropriate in long-term use for this application.

4. Conclusions

This work aimed to identify suitable phase change materials (PCMs) for waste heat recovery and reuse between 120 °C and 150 °C, the temperature range with an underexploited high-volume potential. Thirty-two PCMs in this temperature range were evaluated on criteria from the literature: thermal stability, non-toxicity, minimal enthalpy variation (more than 50 kWh·m⁻³), cost (ideally less than 100 EUR·KWh⁻¹), and industrial availability. This reduced the list to six candidates: succinic anhydre acid, benzoic acid, phtalic acid, sebacic acid, dimethyl terephthalate, and adipic acid.

These materials’ cycling behavior and thermal damage were studied by calorimetry and thermogravimetry. The succinic anhydre acid, the benzoic acid, the phtalic acid, and the dimethyl terephthalate showed a mass loss higher than 1% at temperatures close to, or even below, the melting temperature (less than 2 °C difference). They consequently were considered ill fitted for industrial use. The two remaining candidates’ (sebacic and adipic acid) ability to conserve their properties when submitted to punctual temperature increases were tested by a thermal aging study: the PCMs were exposed to temperatures above 15 °C, 25 °C, and 35 °C to their melting temperatures during 1 to 60 days. No noticeable degradation was revealed by the regular thermo-physical property characterization (T_onset and ΔH_m) when they were aged alone. Then, their compatibility with the envelope materials (stainless steel 316L and steel P265GH) were evaluated through the same PCM thermal aging campaign, now with enveloping samples immersed in the liquid. The steel P265GH caused a degradation of the PCM latent heat of fusion, especially as to adipic acid, and corroded at a speed not suitable for long-term use according to industrial recommendations. This steel should then be replaced from the exchanger to ensure satisfying performances of the thermal storage system through its lifetime. The use of the stainless steel 316L fit with the corrosion recommendations and the conservation of thermal properties for both PCMs, but the operating conditions should not exceed a temperature of 15 °C above the melting temperature.

Now that the MCP has been selected, it needs to be integrated into the storage system (heat exchanger). The combination of this material with the selected heat exchanger design must be tested to ensure that it is suitable for real applications. Essentially, this will involve checking whether the storage system delivers the expected power.