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STIPINST-Stipendiatstillinger i instituttsektoren

Stipendiatstilling 2 SINTEF (2021-2023)

Awarded: NOK 4.2 mill.

Thermal energy storage enables disconnection of production and consumption of heat. This property is useful in energy supply networks relying on inherently unstable sources such as solar and wind. Consumption of heat from a storage when production output is low, referred to as load-shifting, or using stored heat to alleviate energy supply networks during peak consumption periods, called peak-shaving, are two important benefits of thermal energy storage. In other words, thermal energy storage is a key technology for ensuring stable heat supply in the future green energy sector. Thermal energy storage has been employed in various forms by humans throughout history. Consider the heat from hot stones placed in bonfires to be enjoyed through the hours of cold nights, however in both the modern domestic and industrial sector, the technology of thermal energy storage has not yet seen its full potential. Latent heat storage exploits the large amount of heat associated with solid-liquid phase change. Materials used for this purpose are commonly referred to as phase change materials (PCMs). One of the important properties of latent heat storage is the ability of storing and releasing heat at constant temperature. This is of particular interest in waste heat recovery from industrial processes, as even low temperature heat sources may be exploited. It all depends on the phase change temperature of the PCM. Currently, commercialized thermal energy systems are dominated by sensible heat storage, often in the form of hot water tanks. In such systems, energy is stored as sensible heat and the temperature increases by the amount of stored energy. A limitation exists, therefore, for the storage capacity before the temperature becomes unmanageable. Latent heat storage on the other hand, has the capability of storing more energy at lower temperature because of the constant temperature property of phase change. Unfortunately, latent heat storage with conventional heat exchangers suffers from low and uneven heat transfer rates due to build-up of crystallized PCM on heat transfer surfaces in heat exchangers. This problem may be mitigated in the less explored concept of direct contact latent heat storage (DCLHS). In this scheme, the PCM and heat transfer fluid (HTF) are in direct contact in the storage vessel. Being mutually immiscible fluids, HTF flows through the PCM in droplets, exchanging heat directly. The flow is governed by a difference in density between the fluids. Once heat has been exchanged, HTF is separated from the PCM at the outlet of the storage vessel and transported back to the thermal load. The challenge is to achieve fast and complete separation of the fluids. Two immiscible fluids tend to form an emulsion when mixed, and the separation process requires time. This imposes a limitation on the flow rate of HTF, hence also the heat transfer rate, and is recognized as the limiting factor for this technology. The goal of this project is to advance the DCLHS technology along the path towards commercialization. This will be achieved through identification and assessment of the limiting factors of the technology. Based on experience and pre-existing knowledge of fluid flow in immiscible multiphase systems, an experimental approach will be undertaken as an attempt to address the challenges. Specifically, the objectives are defined as: 1) Develop a facility needed to perform experiments at SINTEFs Multiphase Laboratories. This includes the construction and commissioning of a setup imitating a DCLHS with an appropriate heat source and sink. Together with a modular design with re-configurable components, the developed infrastructure should be able to act as a basis for future research on DCLHS 2) Investigate solutions to overcome the limiting factor of slow separation of PCM and HTF. This entails manipulation of the emulsion formed in the storage vessel. The goal is to ensure stable operation of the system at high power loads without degradation or fouling, while still maintaining high energy density in the storage.

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STIPINST-Stipendiatstillinger i instituttsektoren