Thermophysical Property Characterization of Commercial PCMs : A Real Thermal Energy Storage Application in KTH Live-in-Lab

Sammanfattning

This thesis presents the experimental thermophysical characterization of two commercial bio-based phase change materials (PCMs), RT57HC and RT60HC, using n-eicosane as a reference. The study supports the design and optimization of latent heat thermal energy storage (LHTES) systems, particularly in building energy applications, within the context of the Horizon HYSTORE project and a PCM-TES integration at KTH Live-in-Lab. The work aims to provide high-quality input data for simulation and system modeling through detailed property measurements and uncertainty analysis. Key thermophysical properties: density, specific heat capacity (𝑐𝑝), enthalpy of fusion, thermal conductivity, and dynamic viscosity. From the literature, PCMs are recognized for their favorable melting ranges, high latent heat values, and chemical stability, with high enthalpy of fusion and low thermal conductivity. However, available data often lack uncertainty quantification and temperature dependency. This thesis addresses those gaps for RT57HC and RT60HC using methods such as DSC (for enthalpy and heat capacity), Hot Disk (for thermal conductivity), and rotational viscometer (for dynamic viscosity). Liquid and solid phase densities were determined over a 40–80°C range. RT57HC and RT60HC exhibited higher liquid densities than n-eicosane, with solid-phase densities of 0.88 g/cm³ and 0.87 g/cm³, respectively, contributing to 18% and 7% volume savings, respectively. Linear fits of density versus temperature produced R² values greater than 0.99, presenting a linear variation of the liquid densities. For RT57HC, peak melting enthalpy values ranged from 239.3 to 263.1 J/g depending on scanning rate (from 0.025 K/min to 0.25 K/min), while RT60HC showed values from 214.3 to 264.9 J/g. RT57HC exhibited 𝑐𝑝 values increasing from 1.73 J/(g·K) at 10°C to 2.75 J/(g·K) at 75°C, with an expanded uncertainty as ± 0.07 J/(g·K) (with 95% confidence). RT60HC displayed 𝑐𝑝 values ranging from 1.86 J/(g·K) at 10°C to 2.43 J/(g·K) at 75°C, with higher uncertainty (± 0.10 J/(g·K) with 95 confidence). Thermal conductivity was assessed with a Hot Disk TPS 2500S instrument. In solid phase, RT57HC ranged from 0.25 to 0.27 W/(m·K) and in liquid phase the value was 0.18 W/(m·K). RT60HC demonstrated solid-phase values of 0.23 to 0.26 W/(m·K) and liquid-phase values of 0.17 W/(m·K). These results, are comparable with organic PCMs, support their feasibility in controlled thermal environments. The dynamic viscosity measurements using a Brookfield viscometer indicated decreasing viscosity with rising temperature, within the temperature range 62-65 °C. RT57HC viscosity ranged from 10.1 mPa·sat 61.5 °C to 5.78 mPa·s at 80 °C. RT60HC, though exhibiting slightly higher values at corresponding temperatures, remained within a narrow and stable range of approximately 6–10 mPa·s, demonstratingconsistent flow behavior. From a sustainability standpoint, RT57HC and RT60HC can reduce peak energy demand, improve thermal efficiency, and support climate action goals. However, long-term use requires addressing limitations such as high initial costs and material degradation. The findings of this thesis provide validated data that improve simulation accuracy and support the design of efficient, compact, and sustainable TES systems for smart buildings. Key future work includes Integration into full-scale systems and environmental and economic assessments.