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A detailed heat transfer model of a storage tank was developed to assess the performance of a district heating system with de-centralized storage solutions. The performance of two cases, with water and a low-cost phase change material emulsion as storage medium was analysed for 40 residential buildings in Zaragoza (Spain). The chosen configuration is a typical cylindrical tank with internal coils through which the district heating water flows for the charge and discharge. This decentralized thermal energy storage unit reduced peak demand and mass flow in the network, allowing additional buildings to connect to a saturated grid. Four configurations were examined: (1) A 180 m3 water tank reduced district heating power demand by 36.6 %. (2) The same 180 m3 tank using the emulsion required fewer coils, lowering costs, stabilizing TES temperature, and increasing water supply temperature, thus extending system lifespan. (3) A 130 m³ tank with the emulsion offered operational characteristics similar to the water tank, with economic and structural benefits from reduced size. (4) An 180 m³ optimised tank with the emulsion achieved a 38.8 % reduction in district heating power demand. These configurations demonstrate the efficiency and cost-effectiveness of using phase change material emulsion in thermal energy storage systems. • Cost-effective PCM emulsion as a decentralized TES integrated into DH system. • Development of a simple mathematical model to simulate the integrated system. • Reduction in peak heating demand and mass flow facilitated by the PCM emulsion. • Enablement of additional building connections in saturated DH grids. [ABSTRACT FROM AUTHOR]

PCM emulsions have attracted considerable interest as the media for thermal energy storage (TES) owing to their high thermal storage capacity, desirable fluidity and thermal conductivity. However, the direct transportation of PCM emulsions in TES systems for both charging and discharging is rarely reported. In this work, a PCM-in-water nano-emulsion was prepared with n-hexadecane and suitable surfactants for cooling energy storage at a charging and discharging temperature range of 20–5 °C and 5–15 °C, respectively. It was applied to a tubular heat exchanger system to evaluate its TES performance for a cooling panel of 0.2 m2 total surface area. The thermal storage performance was notably increased with the flow rate of emulsion through the exchanger tube. The volumetric thermal storage capacity of charging was 50% higher than that of water. The cooling energy could be rapidly released in the discharging process, 79% of the stored energy at averaging 25 W during most of the discharging period. The emulsion remained stable throughout the test period. Overall, the results demonstrated that the PCM nano-emulsion has the unique characteristics of high static stability, and high energy releasing efficiency and the promising potential for air-conditioning application in buildings.

Abstract Phase change material emulsions (PCMEs) were largely investigated as potential working fluids for various applications as in HVAC systems, solar thermal storage, and heat transfer. They basically contain water and phase change materials, possess much larger energy storage capacity than currently used chilled water based systems and exploit higher thermal conductivity than bulk PCM reservoirs. Main barriers to their application are the difficulty in maintaining emulsion stability and the undercooling effect. In this work, a new solvent-assisted route was developed to produce nano-emulsions of commercial paraffin waxes in water. Concentrations from 2 to 10 wt% were obtained starting from two commercial paraffin waxes, with nominal melting temperatures around 55 °C and 70 °C. The droplet dimensions were verified and resulted below 100 nm for lower concentrations and around 180–220 nm for 10 wt%. The stability of emulsions was verified by long-term tests. An undercooling effect was verified and strongly reduced by testing some nucleating agents (a paraffin melting at 70 °C in the paraffin melting at 55 °C or a carbon nanostructure, single wall carbon nanohorns). The heat of melting was lower than expected, probably due to PCM molecules on the surface of the nanoparticles anchored to the surfactant. However, calculating the thermal capacity of PCMEs with respect to pure water, a gain up to 40% for operating temperatures close to PCM melting temperature was estimated. Optical properties of single wall carbon nanohorns/paraffin composites were also measured. Highlights • We developed a new method to produce emulsions of nano-encapsulated paraffin in water. • The stability was verified with time and temperature up to 10 wt% of PCM. • A strategy to reduce undercooling to 2 °C was found. • An increase up to 40% of thermal capacity of water was estimated. [ABSTRACT FROM AUTHOR]

This article presents the results of heat transfer coefficient and volumetric energy density measurements in an agitated tank containing a low-cost phase change material emulsion, heated by water flowing in a coil. For the stirring a three-stage impeller is placed in the central axis of a 46 l commercial tank. By measuring the temperature dependency on time and solving the transient enthalpy balance, the heat transfer coefficient between the helical coil and the agitated phase change material emulsion is determined, based on the impeller Reynolds number. The thermal energy storage efficiency has also been analysed. This phase change material emulsion shows a phase change temperature range between 30 and 50 °C. Its solid content is about 60% with an average size of 1 μm. The results have shown that the overall heat transfer coefficient is around 3.5–5.5 times higher when a stirring rate of 290–600 rpm is used. Furthermore, even at the lowest stirring rate, the thermal energy storage efficiency improves from 76-77%–100%, without detriment to the energy consumption of the stirrer. [ABSTRACT FROM AUTHOR]

In this paper, a novel phase change material emulsion (PCE-10) consisting of an organic PCM (RT10) and water has been developed. Its thermophysical properties such as heat of fusion, viscosity and sub-cooling temperature have been established. The chemical stability during both storage and discharge periods have also been evaluated. The results indicate low sub-cooling temperature and relatively long period of stability without any sign of segregation but the viscosity was found to be much higher than that of water. Further improvement and experimental studies into its flow characteristics are therefore being encouraged. [ABSTRACT FROM AUTHOR]

Phase change material emulsions (PCMEs) are multifunctional fluids consisting of phase change materials (PCMs) and carrier fluids. PCMEs could be potential candidates as heat transfer media in heating, ventilation and air conditioning (HVAC) systems. This is mainly because PCME could take advantage of its high heat capacity to reduce flow rate and thus saving pumping power whilst delivering the same amount of cooling effect. PCME can also simultaneously act as cold storage to shift peak-load to off-peak time and improve the COP of systems. However, the optimum design of integrated system requires a good understanding of flow behaviour and heat transfer characteristics of PCMEs. In this paper, comprehensive reviews of their thermo-physical properties and potential applications as thermal energy storage and as alternative heat transfer fluids in air conditioning systems have been carried out to establish their limitations for future research. [ABSTRACT FROM AUTHOR]

Abstract: Phase change materials (PCM) in the form of slurries have had an increasingly important role as heat transfer fluids and as thermal energy storage media. Although it is a recent technology in the field of thermal energy storage with phase change materials, the volume of literature begins to be significant. This investigation carries out a compilation of information on two latent thermal fluids: PCM emulsions and microencapsulated PCM slurries (mPCM slurries). This review presents tables containing information on the different PCM emulsions and mPCM slurries studied by different researchers, as well as commercially available products. Thermophysical and rheological properties are analyzed, making a special effort to analyze heat transfer phenomena, concluding with the enumeration of application examples available in literature. [Copyright &y& Elsevier]

Phase change material nano-emulsions are investigated as potential working fluids for various applications including heat transfer and solar applications. Main hindrances to their application are the difficulty in maintaining emulsion stability and the subcooling effect. In this study, new phase change material nano-emulsions (PCMEs) were designed and characterized as possible heat transfer media to be used at 21 and 55 °C, by investigating two commercial phase change materials melting at these temperatures. In particular, the influence of graphene oxide (GO) and reduced graphene oxide (rGO) on emulsion stability and on thermal and optical properties of PCMEs has been investigated. The stability of emulsions was verified and no phase separation or significant growth of PCM droplets were observed through storage and after freeze–thaw cycles. GO was less successful in case of paraffin melting at 21 °C, while it was effective in reducing subcooling to only 1 °C for nanoemulsion with paraffin melting at 55 °C and strongly influenced heat transfer capability. This resulted in a fluid thermal capacity increase of more than 20% within 10 °C from PCM melting temperature. The partial reduction of GO did not result in significantly changing the thermal properties of nanoemulsions and slightly increased the optical absorption, due to higher optical absorption of rGO with respect to GO. [Display omitted] [ABSTRACT FROM AUTHOR]