Session: K6-07: Thermal Energy Storage Systems - II

Paper Number: 190901

190901 - Multi-Stage Boiling Concept for Heat Extraction of Metallic Phase Change Thermal Energy Storage Systems 

Abstract: 

The utilization of metallic phase change materials (mPCMs) represents a promising approach for high-temperature thermal energy storage, particularly for improving the thermal management of electric vehicles operating in cold environments. Offering high energy density and thermal conductivity, mPCMs enable compact storage concepts capable of delivering substantial heat during vehicle operation. However, the wide operating temperature range of these systems poses challenges for thermal discharge when transferring heat to the vehicle cabin, especially when a two-phase intermediate fluid is involved. Large temperature differences between the storage material and the heat transfer fluid can result in boiling crises involving film and transition boiling, which limits performance and practical applicability. Currently, there are few engineering solutions, experiments, and simulations addressing the heat extraction process of such storage systems.

To address these challenges, the present study introduces a novel multi-stage boiling heat extraction concept designed to enable robust and efficient thermal discharge from high-temperature mPCM storage materials. The proposed system employs a two-stage, closed-loop boiling and condensing cycle using water as the working fluid. By dividing the overall heat extraction process into two thermally coupled stages, the concept mitigates excessive local heat fluxes and improves the boiling process over a wide temperature range.

Experimental investigations were conducted using a laboratory-scale prototype that incorporated the aluminum-silicon alloy AlSi₁₂ as the storage material. Experiments were performed across a broad operating temperature range, from 650 °C down to 100 °C. The results demonstrate high specific discharge rates at various storage temperatures, confirming the suitability of the concept for rapid and effective heat extraction. Additionally, the system successfully utilizes the latent and sensible heat of the storage material, resulting in enhanced effective storage density.

Based on these results, a heat transfer calculation model was derived for the multi-stage concept. The correlation-based model was adapted to the experimental data to accurately estimate the heat flow. The simulation model results show that coupled, controlled operation of the two stages enables constant heat output across a wide operating temperature range, demonstrating the practical functionality of the novel storage concept. This is achieved by actively adjusting the mass flow rate in each stage with dependence on the changing storage temperatures.

Overall, the presented multi-stage boiling heat extraction concept effectively overcomes the challenges associated with transferring high-temperature storage heat to a vehicle system. The results highlight the practical feasibility of the novel concept for mPCM-based thermal energy storage in electric vehicles and provide a promising technical solution for future applications.

Presenting Author: Frank Nees German Aerospace Center (DLR)

Presenting Author Biography: Frank Nees studied aerospace engineering at the University of Stuttgart, KTH Royal Institute of Technology in Stockholm, and the University of Tokyo. After completing his studies, he worked in industry for more than eight years as an engineer at Linde, where he was involved in the design of heat exchangers. Since 2020, he has been a researcher at the German Aerospace Center (DLR) in Stuttgart.

Authors:

Frank Nees German Aerospace Center (DLR)