• Energy Research
• 2016-2025close

Abstract The tube-in-tank is a compact configuration well suited for PCM thermal storage systems. However limited research has investigated the impact of the boundary condition applied to the PCM achieved through differing tube arrangements. In Part 1, using CFD and considering the discharging condition for CSP applications, it was determined that when the heat transfer fluid flow was in parallel, poor extraction of latent energy occurs, whereas in a counterflow arrangement maximum latent energy is extracted. In Part 2, the impact of mass flow rate and PCM thermal conductivity on the extraction of latent energy for these tube arrangements was investigated. It was discovered that the counterflow arrangement can experience poorer heat transfer which can be avoided through design. Furthermore, little investigation has considered the impact of the effectiveness of heat transfer with PCM systems with increased amounts of sensible energy, typical for CSP applications. It was determined that for latent dominant storage systems, the counterflow tube arrangement should be applied, while for sensible dominant PCM storage systems, parallel flow should be considered.

The rating of buildings using thermal models represents a contrasting regulatory approach to prescriptive measures to improve the energy efficiency of buildings. This paper investigates the relationship between measured household energy use for thermal comfort purposes and the modelled thermal energy calculated under the Nationwide House Energy Rating Scheme (NatHERS), which is used for the regulation of minimum energy performance standards for new housing in Australia. Two different sets of housing in Adelaide, South Australia which were built a decade apart and to significantly different energy performance standards represent the basis of this study. The results show that better insulated houses represented by higher stars under the NatHERS scheme do use less energy for heating and cooling.

In this work, melting of a high-temperature inorganic phase change material (PCM) eutectic (with a melting point of 569 °C) within a vertical cylindrical tank has been experimentally investigated. To promote the heat transfer rate, a periodic structure that is constructed by a commercial SS-304 mesh screen has been considered and immersed into the PCM tank. Thermal characteristics of the PCM-periodic structure tank under different initial temperatures (450, 490 and 546 °C) and wall temperatures (620, 640, 660, 680 and 700 °C), are then investigated and reported. The presented experimental data can facilitate practical engineers to find the best operating condition of similar PCM tanks; meanwhile, it can also be employed for the investigation of thermal response of transient heat conduction before melting starts.

Abstract Analysis of stainless steel 316 as a containment material in the presence of a phase change material (PCM) cycled at high temperature was carried out through a combination of X-ray photoelectron spectroscopy and X-ray diffraction methods. In this work, stainless steel tokens were half-immersed in a chloride carbonate-based PCM, which were then thermally cycled in an air or a nitrogen environment up to 500 times, allowing the PCM to repeatedly transform between solid and liquid phases. Spectroscopy and diffraction methods were applied on the tokens, as well as the cycled PCM, to investigate the extent and nature of corrosion in such steel alloys. With varying sputtering conditions, the oxidation state at different depths of token surfaces was quantified. From the outermost corroded layer through to the bulk, this study shows a gradual change in distribution in both Cr and oxidation of Fe, while Cr was specifically found to have depleted and migrated into the PCM. The oxidation and depletion have been found to increase with increasing exposure time to PCM while sigma-phase structure embrittled in the corrosion layer.

Abstract The formation and mechanism that drive corrosion in stainless steel as a containment material in the presence of phase change materials is of importance in solar thermal energy storage systems. In our work, half-immersed stainless steel 316 tokens in a carbonate-based phase change material (PCM 638) have been investigated. The samples were thermally cycled in air and Nitrogen environment up to 500 times within a high-temperature range, allowing the PCM to transform between solid and liquid states repeatedly. As a consequence of thermal cycling, severe thickness loss on steel token was observed, indicating a degree of oxidation occurring, which depleted the uncorroded steel. Cross-sectional microstructural analysis was carried out to determine the elemental distribution and structural morphology along the corrosion layers. This study shows that thermal cycling of SS 316 in PCM results in active Ni and Cr migration to the surface, leading to a significant depletion of Cr from steel moving into the PCM. Cr and Fe on the surface are found to oxidise with varying degrees, with respect to the exposure time. The depletion of Cr was found to be higher in SS 316 immersed in PCM, while also increasing with exposure time.

Numerical study of inward and outward melting of a high temperature PCM in cylindrical enclosures were performed, using FLUENT 15. For validation purposes, numerical modeling of inward melting of a low temperature PCM was initially conducted and the predicted results were compared with the experimental data from the literature. The validated model for the low temperature PCM was used for two high temperature cases; inward melting of a high temperature PCM in a cylindrical enclosure and outward melting in a cylindrical case with higher aspect ratio. The results of this study show that the numerical model developed is capable of capturing the details of melting process with buoyancy driven convection for Ra<108, i.e. laminar flow, for a high temperature PCM and can be used for the design and optimization of a latent heat thermal storage unit.

Abstract Thermal energy storage systems provide a means to store energy for use in heating and cooling applications at a later time. The storage of thermal energy allows renewable sources of energy to be stored if the time of demand does not coincide with the time of production. It also enables access to off-peak electricity tariffs offered during times of low electricity demand. Storage systems can be charged during the low-cost tariff period and provide heating or cooling later when required. This benefits consumers with lower electricity costs and power generators with demand levelling. Thermal energy storage systems predominantly store heat as sensible heat in a substance. However, during a phase change heat energy can be stored as latent heat. Phase change material (PCM) thermal storage systems can store a greater amount of thermal energy per unit volume than sensible heat storage systems. Historically a drawback of using PCMs as a storage medium has been the low rates of heat transfer. Heat transfer enhancement techniques studied have included the use of additional metallic material and increasing heat transfer surface area such as fins to improve heat transfer rates of the PCM. Although these techniques are effective, they add significant cost and reduce the compactness factor of the thermal energy storage system. Recent research has been conducted on heat transfer enhancement that makes use of moving or transporting the PCM. This method is not only effective for increasing the heat transfer; it is less expensive and maintains a high compactness factor for the thermal energy storage system. This review paper presents the different heat transfer enhancement techniques reported in the literature. It also summarises the research conducted on phase change storage systems where the PCM is moved in the storage system.

Abstract This paper presents a numerical investigation of a thermal energy storage tank consisting of phase change material (PCM) encapsulated in a sphere, using the effectiveness-number of transfer unit (e-NTU) method. An experimental validated simulation model was used to validate the e-NTU model. Three configurations of PCM container were investigated; a plain sphere, a sphere with conducting pins, a copper-plated sphere with conducting pins. The results showed that the local effectiveness values were increased by an average of 65% for the employment of 32 embedded pins. The average effectiveness was further improved by almost 256% when using copper plated sphere. A good comparable results from the experimental and simulation work indicates that the e-NTU method is applicable to the formulation of heat transfer in the PCM encapsulated sphere despite the change in geometry. Furthermore, the method of defining thermal resistance was found to be applicable to the modified configurations. The values of p factor (isothermal–parallel path ratio) for the PCM sphere with pinned sphere and copper plated sphere with conducting pins were found to be 0.73 and 0.198, respectively. The results suggested that the heat flow through the pinned sphere predominately followed a parallel path, while through the copper-plated sphere was predominately isothermal.

Considerable effort has been devoted to the characterization of thermal properties of the different types of materials that can be used as thermal energy storage (TES) media, but scarce literature exists concerning the materials to manufacture the tanks that can be used to contain these storage media. One of the main concerns when selecting the most suitable material for these tanks is its resistance to corrosion due to molten salts that constitute the TES system. Dynamic gravimetric analysis is a newly proposed method for the study of corrosion on metals, which optimizes the standard procedure described by ASTM G1-03. The new technique avoids the direct handling of samples, so more accurate values can be obtained. In this work, the resistance to corrosion of AISI 316 stainless steel samples in contact with commercial grade molten salts of the Li2CO3-Na2CO3-K2CO3 system, at 600 °C for different exposure times, has been determined by using this new methodology. The results show that the initial corrosion rate is lower at higher amounts of lithium carbonate present in the molten salts mixture.