salt HYdrate Heat Pipes; a breakthrough in thermal energy transfer methods

Project Information

HYHP

Grant agreement ID: 101108814

DOI

10.3030/101108814

Project closed

EC signature date 13 April 2023

Start date 1 July 2023

End date 30 June 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 230 774,40

Coordinated by

DANMARKS TEKNISKE UNIVERSITET
Denmark

Periodic Reporting for period 1 - HYHP (salt HYdrate Heat Pipes; a breakthrough in thermal energy transfer methods)

Reporting period: 2023-07-01 to 2025-06-30

Summary of the context and overall objectives of the project

Heat pipes (HPs) are highly efficient passive devices employed for heat transfer between different regions. Due to their high thermal conductivity, HPs are capable of transferring heat through their evaporator and condenser sections without a significant temperature gradient. Recent advances in HP technology have demonstrated promising results across various applications, particularly in compact systems requiring high heat flux, such as aerospace and automotive industries, electronic device cooling (e.g. computers and mobile phones), fuel production, HVAC systems, and renewable energy technologies. Although current HPs provide acceptable levels of thermal efficiency, further improvements could significantly enhance their applicability, rendering them more effective in the fields above and beyond. This project evaluates a new generation of HPs through interdisciplinary research involving thermochemical salt hydrates (SHs). To achieve this objective, various categories of SHs were systematically investigated, with particular attention to their thermochemical and thermophysical properties. A comprehensive review of SHs used in renewable energy technologies across a range of temperature levels was conducted, addressing critical challenges such as toxicity, corrosiveness, and cost. Based on this review, the most promising SHs were identified in terms of chemical energy density and cost-effectiveness at different operating temperatures. Subsequently, the study outlined key challenges, highlighted existing research gaps, and proposed potential directions for future investigations. Among the candidate salts, the one exhibiting the greatest potential (MgSO4.7H2O) for HP applications based on criteria such as low operational temperature, high energy density, availability, and cyclic stability was selected for detailed experimental and computational fluid dynamics (CFD) analyses. Both powdered and aqueous forms of the salt hydrate are examined as working fluids. The results of parametric studies—including variations in concentration, filling ratio, and operational parameters—will be presented. In light of these efforts to overcome challenges, an additional salt (CaCl.6H2O) was assessed to further enhance system performance. Also, some competitive candidates for working fluids in HP were introduced. Furthermore, a geometric optimization through numerical simulations using COMSOL software will be employed to achieve optimal design and operational efficiency.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

A review on recent advancements in the utilization, storage, and integration of salt hydrates (SHs) in renewable energy (RE) systems has been done. Our preliminary assessments indicate that, among the mentioned salts, MgSO4·7H2O has the highest potential in HPs due to its relatively low working temperature, high energy density, availability, and cyclability. To examine this, extensive experimental and CFD analyses have been conducted to determine the phenomena occurring inside the thermosyphon HP. Powdered and aqueous salt hydrates as working fluids have been studied separately. The different concentrations, filling ratios, and parametric analyses under constant temperature conditions in the evaporator have been studied. Taking this into account, another salt hydrate, CaCl2·6H2O, with a lower melting point around 35°C, was evaluated. Our outcomes revealed that the type of salt in the used powder, in terms of energy density, working temperature, and placement, is the most important factors. In aquatic form, solubility and the dimensionless parameter of the Bubble Departure Number can be critical for choosing the type of salt. Under specific conditions, we could achieve up to a aroung 21% improvement in performance. To reduce size and cost while ensuring that performance remains uncompromised, an investigation into the thermal performance of novel frustum-shaped heat pipes through numerical simulations using COMSOL Multiphysics has been conducted. This study highlights the importance of shape optimization in heat pipe design to address diverse operational needs and thermal management challenges across engineering domains. To this end, shape optimization to enhance the thermal performance of a wicked heat pipe using COMSOL software has been performed, improving flow dynamics.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our preliminary assessments in aquatic MgSO4.7H2O indicate that crystal formation plays a crucial role in the final performance of the SH-HP, with HPs containing MgSO₄ at a concentration of 24.2%(9.8 wt%) improving thermal resistance by approximately 5% (33%) under constant temperature conditions in the evaporator. CaCl2·6H2O, another candidate, with a melting point around 30°C, demonstrated that the thermal performance of the thermosyphon using a 25.34 wt% calcium chloride hexahydrate solution was 21% higher than that of water at 90°C, and 17.6% greater at 100°C. Additionally, to reduce size and cost while ensuring uncompromised performance, an investigation into the thermal performance of novel frustum-shaped heat pipes through numerical simulations using COMSOL Multiphysics has been conducted. The Frustum-Cond structure may be suitable for super-long heat pipes, where a low-pressure drop is essential, as in geothermal systems. This study highlights the significance of shape optimization in heat pipe design to address diverse operational needs and thermal management challenges across engineering domains. To this end, shape optimization to enhance the thermal performance of a wicked heat pipe using COMSOL software has been performed, improving flow dynamics. The results indicated that a 21% improvement could be achieved by altering the thickness of the wick structure in the evaporator and condenser.

Graphical abstract