Geothermal energy offers the potential to provide continuous baseload renewable energy. Unlike conventional geothermal approaches, emerging closed-loop geothermal can employ specialized fluids that improve thermal energy recovery and expand the range of applications. However, the requirements for candidate fluids are challenging. The fluids must be physically and chemically stable under high-temperature, -pressure, and -shear conditions. Their effective heat capacity must be high and their viscosity must also be low, factors that motivated us to use a high-latent heat phase change material within a low-viscosity carrying fluid. Here, we prepare the microencapsulated phase change material-based slurries (PCSs) and perform a comprehensive thermal and hydrodynamic characterization to assess their suitability for use in a closed-loop geothermal operation. The thermophysical properties of the PCSs show promising results with minimal change in onset temperature (2.14 °C) and low supercooling (2.21 °C) during charging and discharging, respectively. A suitably low viscosity (in the range 0.01–0.05 Pa·s at 300 s–1) could be obtained with PCS concentrations in the range 20–30 wt %. Higher concentrations resulted in higher viscosities that would incur pumping energy costs exceeding the potential thermal storage benefits. At 30 wt % PCM, this fluid offers the potential for approximately 30% more stored energy than water alone for the 80 °C system. With respect to robustness under combined thermal and shear stresses, a PCS with 30 wt % phase change material showed no visible separation ratio and no shell rupturing or chemical changes when cycled 10× from 20 to 80 °C while being sheared at operation-relevant rates of 10–300 s–1. This study delineates and de-risks PCSs as geo-fluids, exploiting both sensible and latent heat storage with physical and chemical stability.

From physical property measurement to modelling pore-scale environments, the study of fluids at the microscale is key to understanding and optimizing fluids for large-scale energy applications. Silicon-glass microfluidics is now a proven technology for chemical effectiveness testing in the conventional oil and gas energy sector. We see potential to apply microfluidic fluid characterization technology to renewable sectors, such as geothermal and solar thermal energy recovery where fluid customization is central to performance. Key to unlocking performance gains in these renewable energy systems are phase change material slurries (PCSs)fluids that exhibit a high apparent specific heat capacity. However, testing PCS synthesis recipes is currently a slow and expensive process, given the challenges of dynamic testing at process-relevant temperatures and pressures. In this work, we develop and test a robust silicon microfluidic device and measure important PCS emulsion properties including (i) viscosity, (ii) shear stability, (iii) phase change temperature/hysteresis, and (iv) phase change stability under dynamic conditions where tests are performed quickly (<1 h) and require only minimal test fluid volumes.

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