Review of the Use of Entropy to Understand the Thermodynamics of Pure-Substance PCMs

Advanced 3D-printed phase change materials

High-chain fatty acid esters of 1-octadecanol as novel organic phase change materials and mathematical correlations for estimating the thermal properties of higher fatty acid esters' homologous series

The shape-stabilized phase change materials composed of polyethylene glycol and graphitic carbon nitride matrices

Effect of phase change and ambient temperatures on the thermal performance of a solid-liquid phase change material based heat sinks

Flexible engineering of advanced phase change materials

In order to optimize the phase-change energy storage materials for asphalt pavement and analyze the feasibility and applicability of phase-change energy storage materials for asphalt pavement, the experimental methods of thermogravimetric analysis (TG) and Fourier infrared spectroscopy (FT-IR) were adopted. The thermal stability, chemical stability and chemical compatibility of various phase change energy storage materials were tested and analyzed. The results show that the fatty acids, stearic acids and fatty alcohols showed significant weight loss at 200°C. PEG and DTC phase change materials showed no mass loss at 200°C and had good thermal stability. PEG2000 and DTC have no chemical changes after high temperature treatment, and PEG2000, DTC and SBS modified asphalt are physical blend, and the chemical compatibility with asphalt is good. Crystalline hydrated salts, paraffin, fatty acid, stearic acid, fatty alcohol phase change materials are not suitable for asphalt pavement, PEG organic phase change materials and DTC composite phase change materials can be used for intelligent temperature control asphalt pavement research.

To achieve affordable housing in a carbon-neutral society, new buildings require a dual-purpose approach that comprises efficient construction and a green energy supply. Energy screw piles [1, 7] meet this demand as they combine the agility of screw pile drilling with the capability of extracting clean shallow geothermal energy. Moreover, the screw piles can be filled with phase change materials (PCM) to provide latent thermal storage. Studies regarding the use of PCMs as borehole backfill in a ground source heat pump system (GSHP) conclude that PCM implementation can improve GSHP performance. However, most commercially available PCMs have low thermal conductivities (λ) which undermine heat exchange rates [5, 10]. Besides, using PCMs as a composition of a concrete energy pile can reduce the pile structural performance since PCMs usually have a low mechanical capacity [2, 8]. The authors recently introduced PCM-sand mixtures as a core in the central hollow part of an energy screw pile, which does not impact the pile structural capacity [6]. The mixtures with a higher PCM content benefit the pile heat exchange through its latent heat, but only when the PCM does not reduce the mixture’s λ. To overcome this problem, this work tests a new underground heat exchange system where instead of mixing the PCM in the energy screw pile filling material, regular screw piles (i.e., without the heat exchange tubes) are filled with pure PCM, acting as thermal storage piles. A numerical model built via COMSOL [4] is used to evaluate how this combination of screw piles performs thermally when supplying/rejecting heat for a GSHP system operating for a whole year.
 This work uses a validated numerical model [3, 9] to simulate a grid of evenly distributed screw piles, where Energy Piles (EP) and Thermal Storage Piles (TSP) are positioned interspersed, evenly spaced 0.7 m apart. Inside the EPs, an U-loop pipe is inserted in the pile steel case and the remaining is filled with grout. In contrast, the steel case of the TSPs is filled with only PCM [11]. All other material properties and the screw pile geometry are based on [1]. The thermal load is based on the design of a building located in Melbourne, Australia (Figure 1(a)). The model considers the hourly operation of a GSHP for one year, calculating the GSHP Coefficient of performance (COP) at every time step and adjusting the ground thermal load. For comparison, two simulations (one where the TSPs are filled with PCM and a reference where they are filled with only air) are undertaken. Besides the energy stored, the COPs of both cases are compared as a relative increase/decrease percentage (COPchange = (COPPCM / COPReference) – 1).
 Figure 1(b) presents the energy stored by the PCM and its corresponding percentage in a solid state over the simulation. As the EPs reject heat underground due to GSHP summer operation for cooling, the TSPs temperature rise and store sensible heat, until they reach their phase change temperature (Tpc), when the PCM melts to store significantly more heat energy in a shorter time window due to latent heat. Conversely, the PCM solidifies when EPs extract more heat than reject due to GSHP winter operation (heating) for a certain period. The PCM melts again on the following summer, restarting the whole process. The peak energy stored in the TSPs is 190.3 MJ/m3. Even though the thermal load varies hourly, the PCM phase change process happens without immediate responses to the load variation. Figure 1(c) presents the resulting change in the hourly COP considering cooling (CCOP) and heating (HCOP) from implementing PCM in the TSPs, compared to the reference scenario (empty TSPs). The extra heat stored by the PCM lowers the circulating fluid temperature while the latent heat is engaged (months 2 to 7 and 12), which results in a higher CCOP and a lower HCOP. By plotting a monthly average of the COPchange (grey markers), it is clear that the impact of the PCM on the COP is positive when cooling is dominant and slightly negative when heating becomes dominant while the latent heat is engaged. When the PCM is implemented in the EP [6, 10], the heat exchanged by the EP drops once the phase change process is over due to the low λ of the PCM, but by positioning it outside the EP the higher CCOP is sustained even after the phase change ends. However, lowering the fluid temperature increases CCOP while lowering HCOP at the same time, which can harm thermal performance if not designed properly. The results underlie the thermal storage potential available not only in screw piles, but also in any hollow pile foundation, by simply implementing PCM in the hollow case.

1,12-dodecanediol among similar fatty alcohols as a phase change material for thermal energy storage

ABSTRACTA binary mixture of Pentaerythritol (PE) and Tris (hydroxymethyl) aminomethane (TAM) as Solid-Solid Phase Change Materials (SS-PCMs) has a tunable temperature (lower than that of individuals) suitable for Thermal Energy Storage Materials (TESMs) applications. Challenges of phase change materials include low thermal conductivity, a liquid state when changing phases, and high phase change temperatures. This study determined the thermal properties and performance of a coal-PE/TAM on a parabolic solar cooker. The best sample selected was only tested in a parabolic solar cooker as enhanced box solar cookers rarely reach a temperature of about 150 since 144.8 was recorded as the lowest phase change temperature.Thermal conductivity and specific heat capacity were determined experimentally where 17.23 % and 92.57 % increases of these quantities, respectively, were found. It was shown that the thermal conductivity of the pure substance does not depend solely on the amount of the substance in the binary mixture but decreases with increasing temperature. However, specific heat capacity did not show dependence on the added material but increased with the temperature increases. The Phase change Temperatures (PCTs) and enthalpies of the samples were determined using a Differential Scanning Calorimeter (DSC). The binary mixture of has a peak phase change temperature of 152.3 and a combined phase change enthalpy of 98.84 J/g, which is higher than other composites. The cooking designed with Phase Change Materials heats 0.5 kg of water from room temperature to for 55 minutes with a standardized cooking power of. Keywords: Thermal conductivity; Specific heat capacity; Differential Scanning Calorimeter (DSC); Graphene; Pentaerythritol (PE); Tris(hydroxymethyl)aminomethane (TAM); Parabolic and Box solar cookers.

Adjustable temperature and high thermal conductivity composite phase change material based on sodium acetate trihydrate–alanine/expanded graphite for thermal management of electronic devices

Molten salt is regarded as one of most promising candidates for solar energy storage due to possessing stable properties and large energy storage densities. However, the intrinsically low thermal conductivity of molten salt has become a bottleneck for rapid heat storage and transport. The addition of nanoparticles is generally considered to be a most effective way to improve the thermal conductivity of molten salt phase change materials (PCMs), while the phase change enthalpies of the nanocomposite phase change materials usually show two opposite trends of enhancement or decrement. Furthermore, the reason for the abnormal change of phase change enthalpy has not been clear in the literature so far, so the mechanism of change needs to be further explored. In this work, graphene nanosheets (GNS)@NaNO<sub>3</sub> (sodium nitrate) nanocomposite phase change materials are prepared by the hydration ultrasonic method. The materials are characterized by scanning electron microscope, and the phase change characteristics are measured using differential scanning calorimeter. Molecular dynamics simulation is carried out to explain the mechanism for the formation of the NaNO<sub>3</sub> dense layer and the non-collateral decrease of the enthalpy from the microscopic level. With the increase of GNS mass fraction, the melting point of the GNS@NaNO<sub>3</sub> composite phase change material decreases slightly while the phase change enthalpy decreases significantly with a non-colligative trend. A 13.81% decrease of the theoretical phase change enthalpy is observed with a GNS doping ratio of 1.5%. The NaNO<sub>3</sub> clusters observed on the surface of GNS are considered to have not melted, thereby resulting in a reduction in the phase change enthalpy. The mechanism is further investigated by molecular dynamics simulation, showing that the strong van der Waals attraction between GNS and NaNO<sub>3</sub> leads the 2–4 Å-thick NaNO<sub>3</sub> dense layer to form in the vicinity of GNS. With the increase of GNS mass fraction, the centroid equivalent distance between the dense layer and GNS gradually increases, which leads their mutual attraction to first increase and then weaken. When GNS mass fraction is 1.5%, the centroid equivalent distance reaches the position closest to the potential well, leading to a strongest mutual attraction. In other words, the phase change enthalpy decreases most obviously at this mass fraction. Thus, some conclusions can be drawn as follows. The type of interaction between molten salt and nano-enhancers and the position of the potential well are the fundamental reasons for the thickness of molten salt dense layer and the reduction of phase change enthalpy. The calculation of the interaction energy can be used to guide the selection of the mass fraction of the nano-enhancers, so as to avoid the loss of core material cluster and phase change enthalpy caused by the introduction of the nano-enhancers to a greatest extent. The preparation cost of the composite phase change material can also be reduced to a certain extent.

In the paper, four kinds of fatty acid binary mixtures and liquid paraffin and lauric acid binary mixture were prepared. Fatty acid binary mixtures were made of capric acid and myristic acid, lauric acid, palmitic acid and stearic acid. High-density polyethylene was used as supporting material to prepare the shape-stabilized phase change materials. The 25 kinds of shape stabilized phase change materiasl samples were prepared by adding fatty acid mixtures to high density polyethylene according to the mass addition ratios of 50%, 60%, 70%, 80% and 90%.The phase change temperatures, the phase change latent heats, the uniformity and stability of the 25 kinds of shape-stabilized phase change materials were studied experimentally by using differential scanning calorimeter(DSC), the aim was to find suitable phase change materials for phase change wall energy storage. The results showed that the phase change temperatures and the phase change latent heats were related to the components and contents of fatty acids. The optimum content of fatty acid in the shape stabilized phase change materials was 70%. The homogeneity and stability of the shape stabilized fatty acid phase change materials were better, and they are suitable for use in phase change wall. The results in this paper can provide references and basis for the application of fatty acid phase change materials in the wall.

Fundamental structure-function relationships in vegetable oil based phase change materials: A critical review

The application range of existing real scale mobile thermal storage units with phase change materials (PCM) is restricted by the low phase change temperature of 58 for sodium acetate trihydrate, which is a commonly used storage material. Therefore, only low temperature heat sinks like swimming pools or greenhouses can be supplied. With increasing phase change temperatures, more applications like domestic heating or industrial process heat could be operated. The aim of this study is to find alternative PCM with phase change temperatures between 90 and 150 . Temperature dependent thermophysical properties like phase change temperatures and enthalpies, densities and thermal diffusivities are measured for the technical grade purity materials xylitol (CHO), erythritol (CHO) and magnesiumchloride hexahydrate (MCHH, MgCl6HO). The sugar alcohols xylitol and erythritol indicate a large supercooling and different melting regimes. The salt hydrate MgCl6HO seems to be a suitable candidate for practical applications. It has a melting temperature of 115.1 ± 0.1 and a phase change enthalpy of 166.9 ± 1.2 with only 2.8 supercooling at sample sizes of 100 . The PCM is stable over 500 repeated melting and solidification cycles at differential scanning calorimeter (DSC) scale with only small changes of the melting enthalpy and temperature.

High-chain fatty acid esters of 1-hexadecanol for low temperature thermal energy storage with phase change materials