Anne Mallow

Bio: Anne Mallow is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Latent heat & Phase-change material. The author has an hindex of 5, co-authored 12 publications receiving 109 citations. Previous affiliations of Anne Mallow include Sandia National Laboratories & Oak Ridge National Laboratory.

Investigation of the stability of paraffin–exfoliated graphite nanoplatelet composites for latent heat thermal storage systems

Abstract: Organic materials, such as paraffin wax, are sought as stable and environmentally friendly phase change materials (PCM) for thermal energy storage, but they suffer from low thermal conductivity which limits the rate at which thermal energy flows into and out of the material. A common method to improve the PCM thermal behavior is through loading with high thermal conductivity particulate fillers. However, the stability of these composites in the molten state is a concern as settling of the fillers will change the effective thermal conductivity. In this work, we investigate the stability of wax loaded with exfoliated graphite nanoplatelets either of 1 μm (xGnP-1) or 15 μm (xGnP-15) diameter. The effect of dispersants, oxidation of the wax, viscosity of the wax, mixing time, and hydrocarbon chain length on stability is reported. It was found that the addition of octadecylphosphonic acid (ODPA) is an effective dispersant for xGnP in paraffin and microcrystalline wax. In addition, mixing time, viscosity, and oxidation of the wax influence stability in the molten state. Overall, it was found that a mixing time of 24 hours for xGnP-15 along with ODPA mixed in a high viscosity, oxidized microcrystalline wax results in composite PCM systems with the greatest stability determined at 80 °C in the molten state.

Enhancing the thermosiphon-driven discharge of a latent heat thermal storage system used in a personal cooling device

Abstract: Personal cooling devices reduce energy loads by allowing buildings to operate with elevated setpoint temperatures, without compromising on the occupant comfort. One such novel technology called the Roving Comforter (RoCo) uses a compact R134a based vapor compression system for cooling. Following its cooling operation, during which waste heat from the condensing refrigerant is stored in a phase change material (PCM), a two-phase loop thermosiphon is used to discharge (solidify) the PCM to enable its next operation. The transient operation of this thermosiphon is the focus of the present article. Use of a PCM as the storage medium provides high energy density due to the ability to store thermal energy as latent heat during the phase transition; however, the discharge rate is limited by the low thermal conductivity of the PCM. Insertion of a graphite foam within the PCM can increase the rate of discharge and decrease the downtime of the cooling device. Since graphite enhancement involves a tradeoff between improving the discharge time at the expense of PCM volumetric latent heat, the impact of graphite foam density on the PCM discharge rate is investigated by using a Modelica-based transient model of the thermosiphon. The semi-empirical model, which uses relevant heat transfer coefficient and pressure drop correlations for both refrigerant and airside heat transfer, captures the complex phenomena involving simultaneous phase change of the refrigerant and the PCM. The graphite enhanced PCM selected from this analysis results in a 51% reduction in the discharge time with addition of only 5% to the thermal storage weight, without compromising the required cooling time.

Review of Inorganic Salt Hydrates with Phase Change Temperature in Range of 5 to 60°C and Material Cost Comparison with Common Waxes

Abstract: Phase change materials (PCMs) with desirable phase change temperatures can be used to provide a constant temperature thermal source or sink for diverse applications. As such, incorporating PCMs into building materials, equipment, or appliances can shift and/or reduce the energy load. The motivation of this work is to identify low-cost inorganic salt hydrate PCMs that can complement current building systems and designs, and compare them with common paraffins. In this work, we analyzed inorganic salt hydrates with phase change temperatures in the range of 5-60°C, to target both space heating and cooling applications. The properties of the salt hydrates were compared with paraffins over the same temperature range. The results showed that PCMs with a melting temperature above 20°C, salt hydrates have advantages over paraffins including higher thermal energy density (45-120 kWh/m for salt hydrates; 45-60 kWh/m for paraffins) and generally lower material energy cost (1-20 $/kWh for salt hydrates; 20-30 $/kWh for comparable paraffins). For PCMs with a melting temperature less than 20°C, the material cost is higher for both salt hydrates and paraffins (30-110 $/kWh for both classes of materials) and salt hydrates retain their advantage of greater thermal energy density (50-120 kWh/m for salt hydrates; 45-60 kWh/m for paraffins). In all cases, factors including thermal cyclability, stability, congruency, corrosion, and supercooling must be considered when comparing paraffins and salt hydrates for a particular application. Finally, we give an overview of enhancement techniques for salt hydrate PCMs and find that limited efforts have been pursued to tune salt hydrate phase change temperatures, with a wider range of studies investigating stabilization and minimization of supercooling. This analysis shows the potential of developing salt hydrate PCMs for low-cost heating and cooling thermal energy storage systems for a range of applications.

Nanoconfined phase change materials for thermal energy applications

Abstract: Phase change materials (PCMs) have been extensively characterized as constant temperature latent heat thermal energy storage (TES) materials. Nevertheless, the widespread utilization of PCMs is limited due to the flow of liquid PCMs during melting, phase separation, supercooling and low heat transfer rate. In order to overcome these inherent problems and to improve thermo-physical properties, the confinement of PCMs at the nanoscale has been identified as a versatile strategy, which ensures the encapsulation of PCMs in much smaller nano-containers. Such strategies including core–shell, longitudinal, interfacial and porous confinement have been widely presented in recent years to efficiently encapsulate PCMs in nanospaces and are presenting attractive ways to enhance thermal performance. This review summarizes the recent advancement and critical issues of nanoconfinement technologies of PCMs from the point of view of material design. In addition, the potential applications of nanoconfined PCMs in diverse fields, including energy conversion and storage, thermal rectification and temperature controlled drug delivery systems, are presented in detail. Finally, the major drawbacks associated with nanoconfined PCMs and their prospective solutions are also provided.

Enhanced thermal conductivity of PEG/diatomite shape-stabilized phase change materials with Ag nanoparticles for thermal energy storage

Abstract: Ag nanoparticles (AgNPs) are a promising additive because they can enhance the thermal conductivity of organic phase change materials. In this paper, a series of high thermally-conductive shape-stabilized phase change materials (ss-PCMs) were tailored by blending PEG with AgNP-decorated diatomite. In order to enlarge its pore size and specific surface area and make it a suitable PEG carrier, the effect of alkali leaching on the microstructure of diatomite was studied. While PEG melted during phase transformation, the maximum load of PEG could reach 63 wt%, which was 31% higher than that of the raw diatomite. Spherical-shaped crystalline AgNPs with a diameter range of 3–10 nm were uniformly decorated onto diatomite. The XPS results for this material proved that the valence state of silver in the PEG/diatomite PCM was mainly zero. The phase change enthalpy of the PEG/diatomite/Ag PCM reached 111.3 J g−1, and the thermal conductivity of the PEG/diatomite PCM containing 7.2 wt% Ag was 0.82 W m−1 K−1, which was 127% higher than that of the PEG/diatomite composite. The reduced melting and freezing periods indirectly proved that heat transfer in the composite material during the heat storage and release process was enhanced through the thermal conductivity improvement. The composite PCM was thermally and chemically stable even after 200 cycles of melting and freezing. This indicated that the resulting composite PCMs were promising candidate materials for building applications due to their large latent heat, suitable phase change temperature, excellent chemical compatibility, improved supercooling extent, and high thermal stability.