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Refereed/Peer-reviewed Thermal energy storage is increasingly needed in a sustainable world because of its potential of capturing waste heat and being incorporated in solar power plants. For power generation, in particular, as turbine technology advances, a demand for higher temperature thermal energy storage materials also grows. For this purpose, latent thermal energy storage fits in well since it uses phase change materials (PCMs) which generally have a higher energy density compared to their sensible heat counterparts. In the present study, a eutectic Na2CO3(41.69%)-(33.1%)KCl-(25.21%)NaCl phase change material (PCM) with a melting temperature of 569 ° C was chosen as the storage material to experimentally assess the performance benefit of using a readily available stainless steel (ss304) wire mesh (as the periodic structure) to enhance heat transfer within the domain. In addition, for discharging, a numerical model was developed and compared with the experimental results. Furthermore, for discharging, a numerical investigation of the influence of the heat transfer fluid (HTF) flow-rate to the rate of heat transfer was performed. Overall, it was experimentally observed that the charging time for the composite case was shortened by about 35%, compared to the pure PCM case. For discharging, in the axial direction, the composite solidification time when compared to the pure PCM case was on average 10% shorter. Regarding the radial discharging performance of the composite, there was only about 5% improvement compared to the pure PCM case, which was expected due to the thermal contact resistance in the radial direction. Discharging experimental results were used to validate a discharging numerical model. Discharging results from the model showed that increasing the flow rate of the heat transfer fluid (HTF) reduced the time for solidification. It was observed that for the HTF flow rate of 5 L/min, 10 L/min, 20 L/min and 30 L/min, the discharge time was shortened by 23%, 30%, 33% and 35%, respectively.

Refereed/Peer-reviewed Demand for high temperature storage is on a high rise, particularly with the advancement of circular economy as a solution to reduce global warming effects. Thermal energy storage can be used in concentrated solar power plants, waste heat recovery and conventional power plants to improve the thermal efficiency. Latent thermal energy storage systems using phase change materials are highly thought for such applications due to their high energy density as compared to their sensible heat counterparts. This review, therefore, gives a summary of major factors that need to be assessed before an integration of the latent thermal energy system is undertaken. In addition, challenges faced when constructing and experimenting with the storage systems are mentioned. Finally, an insight on the cost analysis and the general performance metrics of the latent thermal energy storage systems is provided before conclusions are drawn.

The use of metallic periodic structures was considered for melting rate enhancement of a phase change material (PCM) contained in a rectangular enclosure isothermally heated from the side. The critical (optimized) cell size, or pore size, of a periodic structure with fixed porosity, realising the shortest melting time by maximizing the convection and conduction heat transfer rate into the PCM, was studied. Furthermore, the effects of material properties (copper, aluminium, nickel, and stainless steel), enclosure length, wall-melting temperature difference and porosity were numerically investigated. It was observed that increasing porosity and/or reducing thermal conductivity enlarged the critical cell size (i.e. the optimal cell size that minimizes the melting time). The critical PPIs (pores per inch) of copper and aluminium periodic structures for all studied porosities were 10; for nickel, the critical values were 10 PPIs for porosity values of 0.75, 0.8 and 0.85 while it reduces to 5 PPI for the highest porosity considered here being 0.95. Interestingly, showing a different trend, the critical PPI of stainless-steel structures was 5 for the lowest porosity (0.75) and reduced to 3 for higher porosities. The results clearly demonstrated localised melting which was observed in all periodic structures except for the 10 PPI stainless-steel case. Scattered melting islands are observed as opposed to a moving interface when ϕ=(dp/L)αligament/αPCM>1. For such cases, localized melting occurs and the PCM is melted at the ligaments away from the heated wall before the melt front reaches those ligaments.

Refereed/Peer-reviewed Phase change materials (PCMs) are extensively used in thermal energy storage (TES) applications but most of them own poor thermal conductivities. Melting of a PCM can be thermally enhanced by means of inserting high-conducting fin and/or foam structures, nevertheless, relevant design guideline for a PCM system with these thermal enhancers is yet absent partly because our interpretation of such systems is limited to particular experiments or simulations that usually valid only over a certain (and narrow) range of parameter values. In order to develop such guideline for practicing engineers, one must gain a comprehensive understanding of melting characteristics under different conditions. Hence, this review concerns only on theoretical works of melting with/without fin and foam structures. Isothermal melting without inserts inside a rectangular box under lateral, basal and inclined heating conditions will be firstly reviewed, afterwards, heat transport dimensions of melting in horizontal cylinder and sphere are clarified. Of a PCM system fin structures can be classified into straight and non-straight fins, whereas the former can be further subdivided into “short” lumped fins and “long” semi-infinite fins. Comparatively, only a few works of non-straight fins and porous foams are perceived due mainly to their irregular and complicated structures. Furthermore, different criteria for evaluating the performance of melting rate enhancement are summarized. Accordingly, the research gaps of theoretical works are remarked and several recommendations are introduced.

The increasing drive to save the planet from accumulating greenhouse gas emissions has resulted to increased search for new technologies in sustainable energy. Renewable energy technology, particularly concentrated solar power plants are mature but still unreliable especially during the periods of no irradiance from the sun. However, incorporation of thermal energy storage systems make them reliable. Latent thermal energy storage systems using phase change materials are highly thought for this application due to their high energy density as compared to their sensible heat counterparts. They, however, suffer from low thermal conductivity and thus take long to charge/discharge. In this study, a cheap but relatively efficient metal insert was used to enhance heat transfer as compared to the expensive commercially available metal foams/graphite foams. Results obtained in ANSYS Fluent 19.2® numerical solver show about 30% improvement in heat transfer when a 70% porous metal insert is used in melting. In solidification, on the other hand, for the same porosity, over 80% improvement was noticed as compared to a non enhanced case (pure phase change material).

In this study, a numerical analysis is used to compare the effects of uniform and cascaded graphite foams on the charge–discharge cycle performance of thermal energy storage systems where a phase change material (PCM) with a phase change temperature of 855 K is used as the storage media. Firstly, for a constant PCM mass, the insertion of single graphite foams with respective porosities of 80%, 85%, 90% & 95% in the storage tank resulted to an exponential increase in the charge–discharge time. It was therefore concluded that a graphite foam with a porosity higher than 90% is less beneficial because the gradient of the porosity versus charge–discharge time graph becomes steeper. Secondly, for a constant PCM mass, graphite foams of porosity 80%, 85% and 90% were combined to obtain a cascaded porosity structure. The three foam structures were cascaded in such a way that the average porosity was either 85% or 89%. Overall, results obtained for cascaded porosity structures showed that combinations having a low porosity graphite foam near the cooling/heating wall performed better than their counterparts having a higher porosity foam near the heating/cooling wall. Particularly, the best combination at an average porosity of 85% and 89% accelerated the charge–discharge time by 5% and 4%, respectively, as compared to their respective single graphite foam cases. Refereed/Peer-reviewed

The aim of this paper is to shorten the melting/solidification time of a high-temperature phase change material (PCM) using graphite foam inserts. Specifically, the phase change behaviour of a two-dimensional rectangular thermal storage tank, containing PCM with graphite foam insert, fully or partially filling the tank, was studied. The composite enclosure was designed assuming it was heated or cooled from the left side wall for charging or discharging, respectively, while the other three walls were perfectly insulated. First, the effect of foam porosity (0.8, 0.85, 0.9, and 0.95), under fully-filled scenarios, was numerically investigated. Then the phase change behaviour of four partially-filled scenarios, with averaged 0.9 porosity, was carried out. The 0.9 porosity foam case showed an excellent cycle performance. With this case, it only takes 68.2 and 65.1 min for entire melting and solidification, respectively. For a tank with no insert, it will take 164/856 min, respectively, to entirely melt/solidify the same mass of PCM as that of the 0.9-porosity-case. As expected, lower porosity values lead to higher heat transfer through conduction. However, our results show that with a fixed mass of foam, it is preferred to increase the foam porosity to fully fill the tank as opposed to a design with a lower porosity foam that only partly fills the tank. Finally, given the high graphite to PCM thermal conductivity ratio, the heat transfer through the foam is mainly due to conduction. Based on this assumption, a theoretical model is presented in parallel to numerical results. Our analysis for a foam-saturated PCM storage tank shows that the dimensionless time taken for completely melting the PCM, expressed as Fourier number, scales with Fo ρ PCM , s ρ PCM , l k eff k PCM , l e Ste .

The thermal performance of a latent heat energy storage system utilising tightly fitted, thermally bonded and non-bonded metal inserts as a heat transfer enhancement material has been investigated. Two methods were used for this purpose. Firstly, a numerical study using volume averaging method was performed and secondly, an analytical study using scale analysis to obtain the theoretical prediction of the performance of the phase change material. Both models agreed with the experimental data in the literature. A further numeric-parametric study was performed to determine the effect of increasing the thermal resistance gap on the total melting/solidification time. It was observed that the storage performance (complete charge/discharge) asymptotically decreased as the gap grows in size and that convection was negligible when periodic structures are used. Generally, it can be concluded that having tightly inserted metal inserts in the storage improves the performance by about three times compared to a case with a 1 mm thermal resistance gap. Numerical results show that when a thermally adhesive glue is used to make ligaments touch the inner wall (thus eliminating the gap), an improvement of about 10% of heat transfer enhancement in charge/discharge is realised as compared to the case of a 1 mm thermal resistance gap Refereed/Peer-reviewed

Application of different heat transfer augmentation techniques, including the use of fins or foams, were investigated to enhance the melting rate of a solid phase change material within an annulus where the inner and outer pipes were subjected to constant wall temperature. The carbon fibre fins as well as three commonly-used foams (made of three different materials: nickel, aluminium and copper) were simulated. Firstly, keeping the total fin volume constant, the fin number density effect on the melting rate was investigated. After an optimal fin number density was obtained, three possible strategies (unequal length, uneven intervals and tree-shaped fins) were explored aimed at a more comprehensive understanding of the induced heat transfer enhancement. It was observed that with a fixed fin thickness and volume, the melting time is not a monotonic function of the fin number density and can be optimized. Comparing pure PCM melting, the use of optimized fin number reduced over 60% of melting time, while additional 8% and 4% further time reduction could be achieved by appropriately increasing lengths and decreasing intervals of bottom fins, respectively. The use of tree-like fins resulted in a longer melting time, comparing to that of longitudinal straight fins, which indicates it is not always a good option. Finally, the results, primarily the melting rates, were compared with those obtained through the use of metal foams with different metals. It was observed that the melting time of optimized strategy-1 is rather less than those of Cu and Al foams, and approximately 2200s shorter than that of Ni foams. These results indicate that the fins, if designed properly, can be as efficient as foams. Refereed/Peer-reviewed

Abu Dhabi is predicting a huge growth in population over the next 20 years (Plan Abu Dhabi 2030); further, it seeks to become an international destination for tourists, businesses, and investment while protecting its cultural heritage. A crucial aspect of achieving this goal is the development of large integrated transportation system, underground, and above ground, to ensure Abu Dhabi becomes a sustainable city on a global scale. The presence of gypsum rocks that occurs within Abu Dhabi’s bedrock is a major threat to underground construction and understanding the phenomena is of paramount importance. They are persistent quasi-horizontal bands, at different levels (top level between 10 and 15 m and bottom level between 15 and 25 m), prone to volume change by dissolution or swelling, due to changes in the stress regime and water chemistry and flow. The dissolution of gypsum is also a cause for cavities that can be found within this formation in greater Abu Dhabi. In this paper, the description and results of an experimental study aimed at obtaining a better understanding of the gypsum dissolution process, as well determine factors affecting it, are presented. Tests on the dissolution process of gypsum rock were performed using artificially created intact and fractured gypsum samples which are a representative of the collected in situ fractured gypsum rock samples obtained from Abu Dhabi. The samples are subjected to flow-through tests. Results obtained show that for an initially saturated gypsum specimen, there is a sharp decline in concentration with time (Stage I), followed by a constant concentration (Stage II) before a slight gradual increase is observed (Stage III) with time. This is a fundamental study—part of a larger set of experiments studying the gypsum dissolution process in Abu Dhabi. Using the data collected from the field and the experiments mentioned above, gypsum geohazard risk-related maps which reflect subsidence, swelling, cavity collapse, and cavity flooding associated with Gypsum Karst shall be developed using Geographic Information Systems.