For over two decades, the EU has been at the forefront of global renewable energy deployment. The adoption of long-term targets and supporting policy measures has resulted in strong growth, from a 9 % share in gross final energy consumption in 2005 to approximately 20 % by 2020. However, renewable energy deployment must be combined with some form of energy storage to handle fluctuations in the energy output. Thermal energy storage can balance energy demand between daytime and night-time, storing summer heat for winter heating, or winter cold for summer air conditioning. Thermal energy storage concept appears to be simple at first glance, but further exploration reveals a more complex picture. Issues with commercial phase-change materials (PCMs) “Thermal energy storage techniques that exploit latent heat have been garnering tremendous attention lately because their readiness level is close to commercial use for solar energy utilisation systems,” notes Prof. Khamid Mahkamov, who has been in charge of the EU-funded project THERMOSTALL, a Marie Skłodowska-Curie Individual Fellowship grant initiative. There are several challenges to be met prior to exploiting PCMs. Such materials accumulate or discharge tremendous amounts of energy during their phase transformation by absorbing and releasing thermal energy during melting and freezing, respectively. Attractive features include high latent heat of fusion, high thermal conductivity, high specific heat and density, and durability during repeated cycling. “Organic and hydrated salts are some of the best known PCMs used in solar energy systems. However, their low heat conduction coefficient, low density and, depending on the case, their chemical instability and the tendency to overcooling poses a barrier to their widespread use in solar energy utilisation systems,” explains Prof. Mahkamov. Owing to the inferior thermophysical properties of these PCMs, energy storage systems have large dimensions and lack high charge/discharge efficiency. Eutectic alloys make their mark on thermal energy storage THERMOSTALL found a possible remedy for these problems – eutectic metal alloys. In other words, researchers used a low-purity alloy mixture consisting of low-melting metals. The new PCM formulations were targeted for use in seasonal thermal energy storage systems in dwellings and small business buildings. “The favourable properties of low-melting metallic alloys make them a perfect match in diverse areas especially the electronics field. Yet, this is the first time they have been introduced in thermal energy accumulation,” notes Prof. Mahkamov. Eutectic materials have several attractive features: they are stable, heat conduction is two times greater than conventional PCMs, whereas their thermal storage capacity is three times greater. Commercial eutectic alloys typically have high purity and are therefore relatively expensive. To minimise costs, researchers focused on low-purity alloys and their samples were sintered in the laboratory. They experimented with a wide range of metals with melting points ranging between 70 and 230 degrees Celsius to identify mixtures with the most suitable thermophysical properties. Using differential scanning calorimetry, the team determined the latent and specific heat values of the prepared alloys. “Overall, eutectic alloys can reduce significantly design complexity and manufacturing costs of thermal energy storage systems,” adds Prof. Mahkamov. Project work led to the design of a pre-commercial prototype for seasonal thermal energy storage for domestic and small business residential buildings.
THERMOSTALL, phase-change material (PCM), eutectic alloys, seasonal thermal energy storage, latent heat, low-melting, prototype