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High Performance Seasonal Solar Energy Latent Heat Thermal Storage Using Low Grade, Low Melting Temperature Metallic Alloys

Periodic Reporting for period 1 - THERMOSTALL (High Performance Seasonal Solar Energy Latent Heat Thermal Storage Using Low Grade, Low Melting Temperature Metallic Alloys)

Reporting period: 2016-11-01 to 2018-10-31

Energy storage technologies are valuable components in most energy systems and could be an important tool in achieving a low-carbon future as is defined as the objective in the European Union’s “20-20-20” targets and in the European Commission’s Energy Roadmap 2050. It is envisaged that the share of Renewable energy sources in EU gross final energy consumption will achieve 55% in 2050. Thermal energy storage technologies can increase resource use efficiency and to support increasing use of variable renewable energy supply resources.

The focus of this project has been on a Latent Heat Thermal Energy Storage System (LHTESS) which is currently a key international priority and relates to a phase transformation of the phase change materials (PCMs), typically changing from solid phase to liquid and vice versa. The major barrier for currently used PCMs (organic and hydrated salts) is their very low heat conduction coefficient, low density, chemical instability and tendency to overcooling.

The aim of the project, performed at the Faculty of Engineering & Environment of Northumbria University, has been to deploy low grade, eutectic low melting temperature metallic alloys (ELMTAs) with low purity as a PCM. The ELMTAs are currently produced for application in other areas and have not been considered in detail for the thermal energy accumulation, except in some very limited number of studies. These alloys are stable, and their thermal heat conduction is significantly greater than that of conventional PCMs and this feature can be used to reduce significantly the complexity of the design of a LHTESS and costs of manufacturing.
As highlighted above, the main objective has been to study and develop a high capacity LHTESS based on ELMTAs for application in individual dwelling and small business buildings.
A number of prospective ELMTAs with melting points in the range between 70 and 227 °C were identified for potential application in LHTES systems and detailed characterisation of their thermophysical properties was performed. The commercial available ELMATS are have very high level of purity. For that reason, proposed ELMTAS have been specially sintered in the laboratory conditions using metals with the initial low purity.

Differential Scanning Calorimetry analysis of ELMTAs was performed to determine precise values of the latent and specific heats of the prepared sample alloys in argon gas environment using a Setaram EVO131 DSC apparatus. Their thermal conduction coefficients were accurately measured. Additionally, thermal stability tests were run on the samples, selected as prospective candidates for further applications. The tests results demonstrated that thermophysical properties of selected ELMTAs are stable and these can be used for long time service thermal storage systems. The database of the thermophysical properties of the selected ELMTAs was created using obtained data.

The small-scale laboratory prototype of the LHTESS has been manufactured and tested to calibrate the developed CFD modelling approach. The developed 3-D CFD model has been created with utilisation of data obtained on the thermo-physical properties of the ELMTAs. The CFD modelling approach was further improved using obtained experimental information from the tests on the laboratory prototype of the LHTESS.

As the outcome of analysis of flow and heat transfer results, obtained in experimental and numerical investigations, a dimensionless correlation was proposed to relate the evolution of the liquid phase of the PCM as a function of Fourier and Stefan numbers, which in turn determined by heat transfer processes in the thermal storage. This correlation can be used for designing of the LHTESS to provide a certain thermal storage capacity and period of time for thermal energy charging and discharging.

The improved CFD model was then used to propose practical recommendations for the industrial partner on the design of the pre-commercial prototype of ELMTA LHTESS, which is currently being manufactured for field tests.
A number of prospective ELMTAs with melting points in the range between 70 and 227 °C were identified for potential application in LHTES systems;

The database was created for the selected ELMTAs, which contain their thermophysical properties, measured with high level of precision using state-of-the-art equipment.

The multi-dimension CFD approach was developed for modelling the flow and heat transfer processes in the LHTESS with ELMTAs. The CFD model was experimentally calibrated and improved with the refined mushy zone parameter.

Dimensionless correlation was derived for relating the evolution of the liquid phase of PCM to flow and heat transfer processes inside the thermal storage. This correlation can be used for sizing practical LHTESS with ELMTAs to provide certain thermal storage capacity and period of time for charging and discharging thermal energy.

The project was completed in close cooperation with the industrial partner, namely Aavid Thermacore Europe. As a result of the cooperation, the company has developed a new product, the thermal storage system, which can use ELMTAs as PCMs along with other organic and inorganic phase change materials. Depending on the demand, the company will consider to commercialise this product. Creation of the new product strengthen the company competitiveness on the market.

The new product enhances further utilisation of Renewable Energy Sources and waste heat and is directly related to the issue of reduction of negative impact of greenhouse gases, reduction of carbon footprint and energy savings. The project results have positive environmental, economic and societal impact. These results are fully aligned with EU objectives and strategy on Environment and Climate Protection.
A small Latent Heat Thermal Storage System with Low Melting Temperature Alloy as