Enthalpy Recovery Research Articles

Hybrid membranes incorporating microencapsulated PCM for enhanced hygrothermal performance in enthalpy recovery ventilation systems

Advancements in building technologies are increasingly centered on improving energy efficiency, particularly within ventilation systems. Integrating Microencapsulated Phase Change Materials (MPCMs) into paper-based membranes enhances hygrothermal performance in enthalpy recovery ventilation systems. MPCMs, known for their latent heat storage capabilities, were incorporated at concentrations of 0 %, 5 %, 10 %, and 20 % by weight. The hybrid membranes exhibited a significant latent heat storage capacity, achieving up to 34.38 J/g at MPCM 20 wt% content. Although the latent heat decreased by approximately 10 % after 1000 cycles of extreme temperature fluctuations between 15 °C and 60 °C, the membranes still exhibited durability sufficient to endure a replacement cycle of over three years, maintaining their effectiveness in long-term applications. The addition of MPCMs also led to improvements in moisture resistance, with the moisture resistance coefficient increasing from 8.88 to 19.19 at MPCM 20 wt%, while still preserving sufficient moisture permeability for efficient enthalpy exchange. These improvements in thermal and hygrothermal properties underscore the potential of MPCM-integrated membranes to enhance energy efficiency and thermal comfort in HVAC systems, providing a sustainable solution for modern building designs. The findings suggest further exploration into optimizing MPCM content for varying climate conditions and assessing the long-term performance of these membranes in diverse practical applications.

Effect of cryogenic thermal cycling treatment on the mechanical properties of FINEMET amorphous nanocrystalline alloy

The brittleness of FINEMENT amorphous nanocrystalline alloys caused by annealing is an urgent problem to be solved because it limits the scope of their engineering application. We are herein to investigate the rejuvenation behavior of the as-annealed samples that not only are in a relaxed state but also the nanocrystals are precipitated in the amorphous matrix. The results show that the thermal temperature for cryogenic thermal cycling (CTC) treatment should not be larger than 483 °C in order to recover the enthalpy of relaxation. Further, the more suitable temperature for CTC treatment differs with the time that the as-annealed ribbon is employed. Meanwhile, the improvement of mechanical properties of the rejuvenated samples was examined with the help of hardness, flat plate bending, and nanoindentation experiments, and the results correspond well to the maximum of the relaxation enthalpy recovery. In addition, molecular dynamics simulations were used to visualize the changes in the local structure and found that the increased local five-fold symmetric clusters have increased the amorphousness of the amorphous nanocrystalline ribbon through CTC treatment.

Physical aging of a glassy polymer in cultural heritage conservation

AbstractArtworks, particularly easel paintings, are multi‐component materials intended to last indefinitely. The protective coatings applied to these artworks often consist of amorphous glass‐like polymers, which undergo slow physical aging and structural recovery below their glass transition temperature. This process alters key physical properties such as mechanical strength, optical clarity, and thermal stability over extended periods. This study investigates the time‐dependent evolution of these properties in Laropal A81, a widely used synthetic resin for cultural heritage conservation, particularly as a replacement for ancient varnishes. The investigation involves characterization of enthalpy recovery via differential scanning calorimetry, refractive index evolution via refractometry, and creep compliance evolution via rheological measurements. The aging behavior of Laropal A81 is further analyzed using the Kohlrausch–Williams–Watts function and the Tool–Narayanaswamy–Moynihan model, enabling the quantification of critical dynamic parameters such as activation energy, nonlinearity, partition coefficients, and non‐exponentiality. The gained insights into the long‐term behavior of this coating's properties can be valuable for improving preservation and restoration strategies for cultural heritage.

Atomistic origin of structural relaxation in lead metasilicate and lithium disilicate glasses

AbstractA combination of density measurement, differential scanning calorimetry, refractometry, and Raman and 29Si nuclear magnetic resonance (NMR) spectroscopy is employed to characterize the evolution of the structure and physical properties of lithium disilicate (Li2Si2O5) and lead metasilicate (PbSiO3) glasses during physical aging. Both glasses display marked increase in density, enthalpy recovery, and refractive index following a temperature down‐jump near their glass transition regions. The collective analysis of Raman and NMR spectra reveals that the aging‐induced structural relaxation in these silicate networks involves a significant disproportionation of Q‐species as well as their spatial clustering.

Correlations between shadow glass transition, enthalpy recovery and medium range order in a Pd40Ni40P20 bulk metallic glass

Two distinct enthalpic states were induced in a Pd40Ni40P20 bulk metallic glass through cyclic annealing at 473 K (0.81Tg) and 558 K (0.96Tg), targeting individual β or α relaxation modes, here referred to as the so-called shadow glass transition (SGT) and enthalpy recovery, respectively. Fluctuation electron microscopy (FEM) was employed to systematically investigate the medium-range order (MRO) within the amorphous structure in the post-annealed state and subsequent quenching after the SGT. Comparing the results with literature data from the same glass quenched from SCL samples, a significant increase in MRO was observed after annealing at 0.81Tg. Conversely, samples quenched directly after the SGT exhibit a reduced MRO, similar to those quenched from the supercooled liquid (SCL). These findings suggest a correlation between the SGT and a decrease in MRO within the glass, a phenomenon discussed in the context of underlying structural rearrangements associated with β relaxation, encompassing the SGT. Additionally, prolonged annealing at 0.96Tg led to glass separation in the material, with remixing observed as an endothermic transformation in the SCL. The origin of this transformation is discussed as either a potential liquid–liquid or glass–liquid transition and their connection to modifications in medium-range order.

The glass transition and enthalpy recovery of polystyrene nanorods using Flash differential scanning calorimetry.

The glass transition (Tg) behavior and enthalpy recovery of polystyrene nanorods within an anodic aluminum oxide (AAO) template (supported nanorods) and after removal from AAO (unsupported nanorods) is studied using Flash differential scanning calorimetry. Tg is found to be depressed relative to the bulk by 20 ± 2K for 20 nm-diameter unsupported polystyrene (PS) nanorods at the slowest cooling rate and by 9 ± 1K for 55 nm-diameter rods. On the other hand, bulk-like behavior is observed in the case of unsupported 350 nm-diameter nanorods and for all supported rods in AAO. The size-dependent Tg behavior of the PS unsupported nanorods compares well with results for ultrathin films when scaled using the volume/surface ratio. Enthalpy recovery was also studied for the 20 and 350nm unsupported nanorods with evolution toward equilibrium found to be linear with logarithmic time. The rate of enthalpy recovery for the 350nm rods was similar to that for the bulk, whereas the rate of recovery was enhanced for the 20nm rods for down-jump sizes larger than 17K. A relaxation map summarizes the behavior of the nanorods relative to the bulk and relative to that for the 20 nm-thick ultrathin film. Interestingly, the fragility of the 20 nm-diameter nanorod and the 20nm ultrathin film are identical within the error of measurements, and when plotted vs departure from Tg (i.e., T - Tg), the relaxation maps of the two samples are identical in spite of the fact that the Tg is depressed 8K more in the nanorod sample.

Harvesting CO2 reaction enthalpy from amine scrubbing

Amine-based CO2 capture technology suffers high energy consumption, of which the exergy loss of CO2 absorption enthalpy is a significant contributor to the process irreversibility. Recovery and utilisation of CO2 absorption enthalpy is therefore of paramount significance to advance amine technology towards low capture cost. The present study proposed a reaction enthalpy harvesting amine process (REHA) with the purpose to directly utilise the CO2 absorption enthalpy for building heating. In the REHA process, we redefine the operational conditions of CO2 capture process with dual targets of high capture performance as well as high enthalpy recovery. It is revealed that using the benchmark mono-ethanolamine (MEA) absorbent 1.62 GJ/tonne CO2 could be harvested though process modification and multiple heat extraction, enabling a high energy recovery efficiency of 82%, whilst maintaining 90% absorption efficiency and low heat requirement of 2.6 GJ/tonne CO2. Integration of REHA with CO2 capture process enables an appreciable cost of CO2 avoided decreased from $63.5/tonne to $54/tonne CO2 (16% reduction) at a heat price of $7/GJ and 5-months heat supply. The economic viability of REHA process is anticipated to be further promoted with the rise of heating price and increasing demand of space heating.

A Phenomenological Model for Enthalpy Recovery in Polystyrene Using Dynamic Mechanical Spectra.

This paper studies the effects of annealing time on the specific heat enthalpy of polystyrene above the glass transition temperature. We extend the Tool-Narayanaswamy-Moynihan (TNM) model to describe the endothermic overshoot peaks through the dynamic mechanical spectra. In this work, we accept the viewpoint that the enthalpy recovery behavior of glassy polystyrene (PS) has a common structural relaxation mode with linear viscoelastic behavior. As a consequence, the retardation spectrum evaluated from the dynamic mechanical spectra around the primary Tg peak is used as the recovery function of the endothermic overshoot of specific heat. In addition, the sub-Tg shoulder peak around the Tg peak is found to be related to the structural relaxation estimated from light scattering measurements. The enthalpy recovery of annealed PS is quantitatively described using retardation spectra of the primary Tg, as well as the kinetic process of the sub-Tg relaxation process.

Optimization of a rotary desiccant wheel for enthalpy recovery of air-conditioning in a humid hospitality environment

Excessive condensation on cooling coils can be costly energy-wise due to humid process air streams. An innovative desiccant cooling system is introduced into an existing hotel in Taoyuan, Taiwan, where the ambient is humid year-round. Rejected heat from the existing cooling system is used as the energy source to heat the regeneration airstream for dehumidification of the process airstream via a rotary desiccant wheel. The paper develops and validates a numerical model for the heat and mass (moisture) transfer of the rotary desiccant wheel. The validated numerical model is used to simulate the expected performance of the rotary wheel under various conditions and operating strategies. The desiccant wheel’s performance includes the steady-state moisture removal capacity (MRC) and the transient response time to reach steady-state. The examined factors include the ambient temperature and humidity, air flow rate, rotational speed of the wheel, wheel-split, and regeneration airstream temperature. From the simulation results, the paper offers the optimized control strategies for the operation of the rotary desiccant wheel system in the hotel.

Kinetics of enthalpy recovery studied by temperature-modulated differential scanning calorimetry

An analyzing method using periodic temperature modulation is applied to the kinetics of enthalpy recovery that occurs when an amorphous material in the glassy state devitrifies during heating. In particular, this study demonstrates that an analytical expression of the kinetic response can be formulated by assuming temperature modulation with a periodic temperature jump and isothermal holding. This analysis provides a reasonable interpretation for the well-known fact that the magnitude of the dynamic heat capacity of complex quantity obtained by temperature modulation does not exhibit a large peak corresponding to the enthalpy recovery peak. Furthermore, it is demonstrated that the phase angle peak of the dynamic heat capacity was affected by the kinetics. Its quantitative evaluation provides useful information on the relaxation time during glass transition. Numerical simulations and experimental results of atactic polystyrene examined by conventional differential scanning calorimetry and fast scanning calorimetry confirm these findings.

Adsorption-desorption effect on physical aging in PMMA-silica nanocomposites

Poly(methyl methacrylate) (PMMA)-silica nanocomposite was used to study the adsorption-desorption of polymer chains on the surface of silica nanoparticles. We demonstrate that the adsorption-desorption is a dynamic process with respect to temperature and time. The adsorption condition could be changed after annealing at Tg + 16 °C. Then, the physical aging under confinement was investigated under different adsorption conditions. At relatively high aging temperatures, we reveal a one-step equilibration with reduced enthalpy recovery of nanocomposite compared with neat PMMA. In contrast, the aging rate of composite is decelerated and then accelerated after a long period of aging at a relatively low temperature due to the emergence of two-step equilibration. Furthermore, the material system with a higher adsorption fraction exhibits a stronger confinement effect on aging. We speculate that the transition from one-step to two-step equilibration corresponds to the activation of segmental α′ relaxation in the interfacial region. The adsorbed chains result in restrained segmental α′ and β′ relaxation and low enthalpy recovery at relatively high aging temperatures. At a relatively low temperature, the interfacial α′ relaxation with high released enthalpy comes to the surface after a long aging time, inducing the elevated aging rate.

Comment on Comment on “Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven‐based experiments”*

AbstractCangialosi, Alegría, and Colmenero have made a comment on a paper of ours [Polym. Eng. Sci. 2022:1–13], in which we discussed the concern that the enthalpy recovery data reported by Cangialosi and co‐workers [Phys. Rev. Lett. 2013;111(9):095701] for polystyrene aged up to 15 K below glass transition temperature was anomalous and contradicted existing experimental results from the literature over a similar range of aging conditions. Their response shifts the focus away from the raised questions about their experimental results and attempts to invalidate the data that we cited in support of our argument. Here we respond to the comment and add additional analysis that suggests the structural recovery response of glassy materials exhibits smooth behavior over the full range of measurements, up to 7 or 8 logarithmic decades. We do this by referring to the work on the intrinsic isotherm down‐jump and memory responses of poly(vinyl acetate) between 40°C and 15°C over six logarithmic decades by Kovacs [Fortsch. Hochpolym. Fo. 1963;3(1/2):394–508], the small‐strain tensile creep of poly(vinyl chloride) quenched from 90°C to 40°C (approximately 40°C below Tg) over seven logarithmic decades from Struik [Polym Eng. Sci., 1977;17:165–173], and the volume recovery behavior for aging times up to 3 months in a temperature range between 95°C and −50°C by Greiner and Schwarzl [Rheol. Acta. 1984;23(4):378–395]. We also add discussion that an isothermal aging procedure using a vacuum oven is highly vulnerable to temperature errors due to the problem of good temperature control when the heat transfer mechanism is primarily radiative.

Energy performance enhancement in air-source heat pump with a direct evaporative cooler-applied condenser

The energy consumption of the air-source heat pump operated in cooling mode is affected by the outdoor air condition in summer. It is important to not only install the evaporative cooler in front of the condenser inlet side but also consider the operation strategy of the evaporative cooler under the hot and humid condition. In this study, the energy saving potential of the air-source heat pump with an evaporative cooler was analyzed according to the operation strategy. A three-story office building measuring 100 m2, using an enthalpy recovery ventilator, was used as a model building conditioned by the proposed system for detailed energy simulation. The building thermal load and energy consumption were estimated using simulation tools. The results show that the proposed system reduces 8.87% of energy consumption compared with the conventional air-source heat pump when the evaporative cooler operated under the 72% of relative humidity condition.

Enthalpy relaxation of unconstrained and constrained amorphous phase for low isotacticity polypropylene

The influence of crystallization on the amorphous phase of low isotacticity polypropylene (LT-PP) was studied by analyzing the enthalpy relaxation behavior using fast scanning calorimetry (FSC) and temperature modulated FSC. The high temperature shift of the enthalpy recovery peak and the glass transition temperature during crystallization was interpreted as a transformation of the unconstrained amorphous (UCA) to constrained amorphous (CA) phase. Enthalpy relaxation approach had the same result as the conventional heat capacity analysis of mobile amorphous (MA) phase as proposed by Wunderlich. Our study revealed that successive glass transition of rigid amorphous (RA) phase in a three-phase model and the gradual increase in the heat capacity of the MA phase in a two-phase model are experimentally indistinguishable above the crystallization temperature.

Anomalous structural recovery in the near glass transition range in a polymer glass: Data revisited in light of temperature variability in vacuum oven‐based experiments*

AbstractThere is considerable interest in data reported by Cangialosi et al. [Cangialosi, D., Boucher, V. M., Alegría, A., & Colmenero, J. (2013). Physical Review Letters, 111(9), 095701] that purportedly showed unusual irregular responses in the isothermal structural recovery behavior of glasses because the observation challenges the general view of the dynamics being smooth functions of time as observed in extensive dilatometric experiments by Kovacs [Kovacs, A. J. (1964). In Fortschritte der Hochpolymeren‐Forschung, 3, 394–507. Springer, Berlin, Heidelberg.] in the 1960s. The reported data show an apparent two mechanism structural (enthalpy) recovery at aging temperatures ranging from 6 to 17 K below which is also controversial as it could not be reproduced in work from Koh and Simon [Koh, Y. P., & Simon, S. L. (2013). Macromolecules, 46(14), 5815–5821.] where a second plateau observed in the enthalpy loss curve was not reproduced in nearly 1‐year‐long experiments at 15 K below . Therefore, it is important to determine what might be possible reasons for the apparent non‐smooth relaxations. Here, we examine the possibility that the poor temperature control of a typical vacuum oven could explain the anomalous results. We used the Tool–Narayanaswamy–Moynihan (TNM) model of structural recovery to calculate the effect of typical vacuum oven temperature variation on the structural recovery of polystyrene and find that we can reproduce the reported experimental results. The issue of temperature control using a vacuum oven is discussed, and data from thermocouple measurements of temperature at various locations inside a vacuum oven are shown.

An indirect approach to measure glass transition temperature in metallic glasses

Abstract Glass transition behavior of metallic glasses under some extraordinary conditions (such as under high pressures) remains unexplored. Conventional measurements of glass transition temperature, T g, are very difficult to perform under these extraordinary circumstances. In the present paper, we introduce an indirect approach to characterize glass transition, using enthalpy recovery experiments. With annealing deeply relaxed glassy samples and subsequent DSC measurements, a variation of enthalpy change upon heating with annealing temperature can be obtained. The variation of enthalpy change, a signature of glass transition, was found to correlate well with the directly measured DSC curves for the glass transition. This unique method was successfully applied in determining T g of several metallic glasses under hydrostatic high pressures and compression stresses.

Physical aging of hydroxypropyl methylcellulose acetate succinate via enthalpy recovery.

Amorphous solid dispersions (ASDs) utilize the kinetic stability of the amorphous state to stabilize drug molecules within a glassy polymer matrix. Therefore, understanding the glassy-state stability of the polymer excipient is critical to ASD design and performance. Here, we investigated the physical aging of hydroxypropyl methylcellulose acetate succinate (HPMCAS), a commonly used polymer in ASD formulations. We found that HPMCAS exhibited conventional physical aging behavior when annealed near the glass transition temperature (Tg). In this scenario, structural recovery was facilitated by α-relaxation dynamics. However, when annealed well below Tg, a sub-α-relaxation process facilitated low-temperature physical aging in HPMCAS. Nevertheless, the physical aging rate exhibited no significant change up to 40 K below Tg, below which it exhibited a near monotonic decrease with decreasing temperature. Finally, infrared spectroscopy was employed to assess any effect of physical aging on the chemical structure of HPMCAS, which is known to be susceptible to degradation at temperatures 30 K above its Tg. Our results provide critical insights necessary to understand better the link between the stability of ASDs and physical aging of the glassy polymer matrix.

Optimization of a Rotary Desiccant Wheel for Enthalpy Recovery of Air-Conditioning in a Humid Hospitality Environment

Optimization of a Rotary Desiccant Wheel for Enthalpy Recovery of Air-Conditioning in a Humid Hospitality Environment

Optimization of a Rotary Desiccant Wheel for Enthalpy Recovery of Air-Conditioning in a Humid Hospitality Environment

Optimization of a Rotary Desiccant Wheel for Enthalpy Recovery of Air-Conditioning in a Humid Hospitality Environment

Shape memory effect in metallic glasses

Shape memory effect in metallic glasses