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Miscibility Gap Alloys: Commercialising A ‘Missing Link’ For Renewable Energy Source: MGA Thermal Industry
Large scale (>200 MWh) gridconnected energy storage is arguably the greatest challenge standing in the way of a complete transition to renewable energy. Lack of adequate storage is preventing full penetration of renewables into the market, and the continued burning of fossil fuels is adding to the problem. There are very long lead times for the implementation of new gridscale storage technologies such as photosynthetic production of hydrogen, efficient ammonia production, Na ion batteries, flow batteries, as well as for older large-scale technologies such as pumped hydro. What is missing is a transitional technology that builds off existing infrastructure to the maximum extent. New macroscopically solid Thermal Energy Storage (TES) materials, known as Miscibility Gap Alloys (MGA), may pave the way for this transition. What is an MGA? Miscibility Gap Alloys (MGA) are a class of thermal energy storage materials comprising a high thermal conductivity matrix enhanced by an internal Phase Change Material (PCM) dispersed as fine particles. The particles, which are typically present at 50% by volume, are completely encapsulated and separated by the matrix phase into a non-percolating microstructure. The PCM embedded within the MGA melts and stores energy as latent heat, while the material remains macroscopically solid and stores sensible heat. MGA were originally devised from pairs of essentially immiscible metals. However, traditional methods of production, such and melting and casting, lead to the unsuitable ‘natural microstructure’, in which the lower melting point metal forms the matrix around the particulate high melting point metal. Processing adaptations and design are required to produce the desired ‘inverse microstructure’, in which the metal acting as a PCM is trapped within the conducting matrix. 40 | SEPTEMBER 2020
Natural microstructure (Sn white, Al dark).
Competing TES materials Thermal energy storage can be accomplished by simply raising the temperature of a solid or liquid (sensible heat), accessing the enthalpy associated with a phase change (latent heat) or moving a compound through a dissociation and re-combination cycle (thermochemical storage). Each has its strengths and weaknesses. Thermochemical storage promises very high energy density, however in order to separate and then recombine reactants. generally requires a particulate system. Particulate systems have extremely poor thermal transport properties and so large scale implementation has proven elusive. Latent heat storage also has the allure of high energy density, however containment, erosion, thermal expansion and thermal contact problems have made it similarly elusive. Sensible heat storage often relies on large quantities of very low cost materials (concrete, rocks, sand and various salts) which also generally have very poor thermal transport properties. This can be overcome with liquid sensible heat storage, by vigorous circulation of hot liquids using pumps, and extensive heat exchange infrastructure leading to the state-ofthe-art TES technology, two-tank molten (nitrate) salt storage. Graphite overcomes the thermal transport problem in a solid material, but at increased material cost.
Inverse microstructure (Sn white, Al dark).
and so by choosing a different PCM component, an MGA material can be made to match a particular end use such as a steam Rankine cycle operating around 600 C. This gives a consistent quality heat such as is required by many thermodynamic systems. Latent heat also means that the energy density is high. If we include 100 C of sensible heat around the phase change, MGA systems store and deliver between 0.65 and 2.2 MJ/L. High thermal conductivity means that thermal energy is accepted and delivered, from storage, by conduction. This allows the materials to be agnostic to the source of heat, adapting equally well to heating via a heat transfer fluid, electrical resistance or direct absorption of concentrated sunlight. An additional benefit is derived because the materials are macroscopically solid with no secondary heat transfer fluids. Compared with two-tank molten salt storage, there is greater system simplicity for thermal energy storage systems, and no parasitic energy consumption for trace heating, which can be as high as 24% in those
Enter MGA By accessing the latent heat of fusion of the included metal particles, MGA offer a number of advantages over other TES media. Latent heat is accepted and delivered at near constant temperature, BACK TO CONTENTS
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