Phase Change Material of Copper–Germanium Alloy as Solar Latent Heat Storage at High Temperatures

Nobuyuki Gokon,Chew Shun Jie,Yuya Nakano,Shogo Okazaki,Selvan Bellan,Tatsuya Kodama,Tsuyoshi Hatamachi

doi:10.3389/fenrg.2021.696213

Abstract

A copper–germanium alloy (Cu–Ge alloy) was examined as a phase change material, at temperatures exceeding 600°C, for latent heat storage in solar thermal applications. First, the thermo-physical properties of the Cu–Ge alloy were examined using differential scanning calorimetry, thermomechanical analysis, and laser flash analysis. Second, to evaluate the thermal response and reliability of the Cu–Ge alloy, the cyclic properties of thermal charge/discharge were examined under various thermal conditions. The alloys obtained after the tests were examined for their chemical compatibility with the stainless steel container using an electron probe micro analyzer. The elemental distribution of each Cu–Ge alloy was evaluated using cyclic performance tests. Finally, the chemical compatibility of the Cu–Ge alloy was evaluated using a high-temperature test with candidate materials of a phase change material container vessel [stainless steel (SUS310S), Inconel625, silicon carbide (SiC), and alumina (Al2O3)]. The Cu–Ge alloy exhibited significant potential as a latent heat storage material in next-generation solar thermal power plants because it demonstrates various advantages, including a superior storage capacity at a temperature of 644°C, temperature coherence to the phase diagram, a quick thermal response, satisfactory cyclic behavior of charge/discharge modes, a thermodynamically stable metallographic structure, and non-reactivity with container ceramic materials (SiC and Al2O3).

Highlights

Renewable energy sources are attractive alternatives to fossil fuels because of their promising social, environmental, and economic benefits
Eutectic and hypereutectic Cu–Ge alloys were studied as promising metallic PCMs, with liquefaction temperatures to 800°C, for next-generation CSP applications at temperatures exceeding 600°C
The thermophysical properties of the Cu–Ge alloy, including the melting temperature, specific heat capacity, volume change, density, thermal diffusivity/conductivity, and latent heat were examined to evaluate the potential of the Cu–Ge alloy as a metallic PCM for thermal energy storage (TES)

Summary

Introduction

Renewable energy sources are attractive alternatives to fossil fuels because of their promising social, environmental, and economic benefits. Solar energy is one of the most environmentally friendly energy sources, and the milestones for solar energy exploitation are energy capture, energy conversion, and energy storage. Solar energy can be harnessed in two different ways, namely, photovoltaic cells and thermal conversion systems (Goswami, 2015). Concentrating solar power technologies (CSPs) convert sunlight into thermal power, which is traditionally used as a heat source for power generation by thermodynamic cycles (Islam et al, 2018). CSP technology has the advantage of higher utilization efficiency of solar energy, extension of the energy operating period from day to night, or from sunny to cloudy weather due to the capability to store energy in the thermal storage system and use it when required (Cohen, 2008; Skumanich, 2010; Stoffel et al, 2010). In accordance with relevant physicochemical mechanisms, the working principles of thermal energy storage (TES) are typically classified into three types: sensible heat storage, latent heat storage, and thermochemical heat storage (Gil et al, 2010; Pelay et al, 2017)

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