Change Latent Heat Research Articles

Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications

The purpose of this study is to determine the thermal reliability of stearic acid, palmitic acid, myristic acid and lauric acid as latent heat energy storage materials with respect to various numbers of thermal cycles. The fatty acids, as phase change materials (PCMs), of industrial grade (purity between 90% and 97%) were subjected to accelerated thermal cycle tests. The differential scanning calorimetry (DSC) analysis technique was applied to the PCMs after 0, 120, 560, 850 and 1200 melt/freeze cycles in order to measure the melting temperatures and the latent heats of fusion of the PCMs. The DSC results indicated that the change in melting temperature for the PCMs was in the range of 0.07–7.87 °C, and the change in latent heat of fusion was −1.0% to −27.7%, except for stearic acid between 560 and 1200 melt/freeze cycles. However, the decrease in the latent heats of fusion for all the PCMs was not regular with increasing thermal cycles. The experimental results also show that the investigated fatty acids as latent heat energy storage materials have a good thermal reliability in view of the latent heat of fusion and melting temperature with respect to thermal cycling for thermal energy storage applications in the long term.

A Diagnostic Study of the Role of Remote Forcing in Tropical Atlantic Variability

This observational study focuses on remote forcing of the dominant pattern of north tropical Atlantic sea surface temperature (SST) anomalies by ENSO and NAO. Based on a spring SST index of the north tropical Atlantic (NTA) SST (5°–25°N), it is shown that almost all NTA–SST extreme events from 1950 to the present can be related to either ENSO or NAO. Since the SST NTA events lag NAO and ENSO events, NTA variability is interpreted as being largely a response to remote NAO or ENSO forcing. The local response of the tropical Atlantic to these external sources—whether it be ENSO or the NAO—is observed to be rather similar: changes in surface winds induce changes in latent heating that, in turn, generate SST anomalies. Once generated, the latter are damped through local air–sea interaction, at a rate estimated to be 10 W m−2 K−1. Experiments with simple models, but driven by observations, strongly suggests that variability on interannual to interdecadal timescales—both time series and spectral signatures—can be largely explained as a result of direct atmospheric forcing, without the need to invoke a significant role for local unstable air–sea interactions or ocean circulation.

Accelerated thermal cycle test of acetamide, stearic acid and paraffin wax for solar thermal latent heat storage applications

1500 accelerated thermal cycle tests have been conducted to study the changes in latent heat of fusion and melting temperature of commercial grade acetamide, stearic acid and paraffin wax. It has been noticed that the stearic acid melts over a wide range of temperature, has shown two melting points and has large variations in latent heat of fusion. Paraffin wax and acetamide have shown reasonably good thermal stability for melting temperature and variations in latent heat of fusion during the cycling process.

Phase diagram of aqueous solution at high pressure and low temperature

This paper presents a constant mass high pressure isothermal calorimeter and describes the range of applications that this type of equipment can deal with. As an example, the thermophysical properties of an aqueous solution of MgSO4 at low temperature and at high pressure have been investigated. The main results showed that the phase change heat of the solution decreased with increasing pressure and the phase change temperature of the eutectic concentration was depressed under high pressure. Those changes could be related to the MgSO4 solubility and to water latent heat changes.

Accelerated thermal cycle test of latent heat-storage materials

Accelerated laboratory experiments have been conducted to study the change in latent heat of fusion, melting temperature and specific heat of commercial-grade stearic acid, acetamide and paraffin wax subjected to repeated 300 melt/freeze cycles. The latent heat of fusion, melting temperature and specific heat of the samples were measured after 0, 20, 50, 70, 100, 150, 200, 250 and 300 thermal cycles using a differential scanning calorimeter. Acetamide and paraffin wax were found to be more stable phase change materials (PCMs). However, acetamide absorbs moisture from the surrounding atmosphere.

Crystal structure of poly[bis(3,4-dimethylphenoxy)phosphazene

The crystal structure and chain conformation of poly[bis(3,4-dimethylphenoxy)phosphazene] — PB (dMe) PP — have been studied by X-ray diffraction techniques. The unit cell of this polymer shows an orthorhombic form with the crystallographic parameters a=2.05, b=1.49, c (chain axis)=0.998 nm. Its space group is determined as Pbcn-D 2h 14 where the molecular chains are located at the centre and each corner of the unit cell, which contains eight monomeric units; these molecular chains possibly have a — (trans 3 cis)2-conformation. The R factor calculated from the final crystal structure was estimated as 16.9%. As with other types of polyorganophosphazenes, the thermotropic transformation in PB (dMe) PP takes place from the three-dimensionally ordered state to its two-dimensional psuedohexagonal form (a h = 1.49 nm), accompanied by a latent heat change at 96 °C.

The optimal design of integrated evaporation systems

The heat integration of evaporators with overall processes leads to optimal designs very different from optimal stand-alone designs. The grand composite curve has been previously suggested for assessing alternative evaporator integration schemes. This paper demonstrates that if the effect of the evaporator integration on the overall capital-energy trade-off is ignored then misleading conclusions can be obtained. A procedure is suggested which takes account of this capital-energy trade-off. Design complications such as boiling point rise and latent heat changes which have often been ignored in the past in the grand composite approach are also considered.

An overview of interfacial tension as a component of freeze stress in winter cereal plants

Consideration of what might happen at liquid interfaces between ice and hydrophilic plant substances has led to the definition of an adhesive constraint. Equilibration of freezing by a shift in the intensity of melting would result from a shift in activation energy with a corresponding change in latent heat. Measured calorimetrically, the latent heat of freezing water associated with cellulose at −10 ° C was found to equal the heat of hydration, and these were calculated to be less than the latent heat of freezing of free water. This decrease in latent heat is an energy of adhesion; full development requires absence of solutes. Adhesive interactions predominate as freezing begins in hardy winter cereal plants, and these help to control the initial growth of ice. As the temperature decreases and freezing progresses, hydrolysis of fructan provides periplasmic sugar, which changes freeze stress from adhesive to osmotic.

Temperature gradient snow metamorphosis

This note shows how the rate of ice crystal growth during temperature gradient metamorphosis can be calculated using irreversible thermodynamics. The method describes the transfer of mass as a consequence of heat transfer. The main heat transfer is the latent heat change by condensing and freezing. Local equilibrium and no convection is assumed. The transport equation, with the temperature gradient as the force of transport, predicts mass transfer measurements.

Temperature gradient snow metamorphosis

This note shows how the rate of ice crystal growth during temperature gradient metamorphosis can be calculated using irreversible thermodynamics. The method describes the transfer of mass as a consequence of heat transfer. The main heat transfer is the latent heat change by condensing and freezing. Local equilibrium and no convection is assumed. The transport equation, with the temperature gradient as the force of transport, predicts mass transfer measurements.

Thermal Analysis of Lignocellulosic Materials. Part II. Modified Materials

Abstract Thermal analysis is a group of techniques in which a physical property of a substance is measured as a function of temperature while the substance is subjected to a controlled temperature program. The output of thermal analysis represents data predominantly in the form of curves characteristic for a particular chemical substance or a mixture of substances. Changes in latent heat in a chemical substance are detectable by differential thermal analysis (DTA) or by differential scanning calorimetry (DSC). Progressive weight loss is detectable by thermogravimetry (TG) or differential thermogravimetry (DTG). Progressive changes in certain mechanical properties are detectable by thermomechanical analysis (TMA). Many other thermoanalytical techniques are available, but are used appreciably less often.

Latent Heat of Vaporization, Surface Tension, and Temperature.

A semitheoretical equation for latent heat of vaporization has been derived and tested. The average error in predicting the value at the normal boiling point in the case of about 90 compounds, which includes polar and nonpolar liquids, is about 1.8%. A relation between latent heat of vaporization and surface tension is also derived and is shown to lead to Watson's empirical relation which gives the change of latent heat of vaporization with temperature. This gives a physico-chemical justification for Watson's empirical relation and provides a rapid method of determining latent heats by measuring surface tension.

Change of Latent Heat of Vaporization with Temperature.

Change of Latent Heat of Vaporization with Temperature.

Hot Ice

I VENTURE, in referring to Dr. Lodge's letter of this week, to put before your readers the meaning of the remarks made on Dr. Carnelley's experiment at the Chemical Society by Prof. Ayrton, who is now away from England. I understood him to say that as Dr. Carnelley's hot ice is obviously in a condition which cannot be represented within the as yet known fundamental water surfaces, it is necessary to produce these surfaces beyond the places at which, hitherto, abrupt changes have been supposed to take place in them. He took as an instance the ice-water surface which has hitherto been assumed to stop at Prof. James Thomson's “triple point,” and showed that although Sir Wm. Thomson's experiments have proved that it is nearly plane for the stable state of water and ice, yet in the imaginary district beyond the triple point a change of latent heat might give such a change of curvature as to bring this surface into the hot-ice region.