By Cliò E. Agrapidis, PhD
In 2024, solar power accounted for 11% (304 TWh) of EU electricity generation, overtaking coal (10%, 269 TWh) for the first time. This marked a milestone in the EU’s solar energy strategy, which aims to reach over 320 GW of solar photovoltaic capacity by this year and nearly 600 GW by 2030.
Large solar energy plants in the EU can produce hundreds of megawatts to over a gigawatt of power. However, this power is often not used at the exact moment it is produced, highlighting the need for energy storage. A common solution for large solar plants is Thermal Energy Storage (TES) tanks, where excess solar energy is stored as heat.
One of the technologies used in TES systems is latent heat thermal energy storage using phase change materials (PCM). The heat transfer process in this system follows two steps: charging and discharging. The charging process begins when the material absorbs heat and its temperature increases without changing phase. Once the transition temperature is reached, the phase transition occurs, and the material melts, maintaining a nearly constant temperature. Finally, after complete melting, the liquid PCM continues to absorb heat and increase in temperature.
The latent heat energy is then released during the discharging process. Starting with the liquid PCM, a cooling process takes place. As the surrounding temperature drops, the liquid PCM releases heat and decreases in temperature. Once the solidification temperature is reached, the material begins to change back to its solid form while releasing latent heat and maintaining a nearly constant temperature. Finally, the solidified PCM continues to release heat as it cools further.
While this approach offers a high energy storage density, requiring less volume for the same amount of stored power, one challenge with PCMs is their low thermal conductivity, which slows down the charging and discharging process. The SUSHEAT project addresses this by developing a new, bio-inspired TES tank designed to speed up heat transfer. This new design and its characteristics, developed by researchers from the University of Lleida—a SUSHEAT partner, were presented in a recent publication in Applied Sciences.
The inspiration for this design comes from the “branched vein pattern observed in plants and animals,” such as the blood vessels in the human body or phloem in leaves. These branching structures, similar to those in leaves and blood vessels, enhance heat distribution, helping to overcome the thermal conductivity limitations of PCMs. To create this branching structure with the utmost precision, researchers at the University of Lleida employed 3D printing techniques. They first created a digital design, which was then sent to the 3D printer to build the object layer by layer.
In order to test this new design for the SUSHEAT system, researchers created a custom-built experimental setup. They proceeded by taking several measurements under different conditions and compared the results with those from the same setup using a more traditional shell-and-tube tank. The two TES systems performed similarly in terms of the time needed to reach the transition temperature of the PCM and the charging rate. On the other hand, “the discharging rate was substantially higher for the bio-inspired TES tank,” with the time required to discharge the heat stored being 52% less for the new TES design compared to the traditional one.
An analysis of the factors affecting the thermal performance of the TES tank revealed that the bio-inspired TES tank is influenced by the thermal flow inlet temperature (the temperature at the entrance of the tank) and the mass flow rate (the amount of heating/cooling fluid passing through the tank in a given time).
This new bio-inspired TES tank, designed within the SUSHEAT project, demonstrates how “the integration of a biomimicry technique into the design of a TES tank serves as proof of concept for improved thermal performance.” By improving the discharge rate, this new design allows for faster energy release, enhancing the system’s ability to deliver stored energy when needed and potentially reducing energy losses. Moreover, a higher discharge rate provides greater flexibility in meeting varying energy demands and is particularly important for applications where energy needs can fluctuate rapidly. Lastly, effective discharge rates are essential for TES systems to complement variable renewable energy sources, helping to balance supply and demand in the power grid.
In addition to a new approach to shape, this new TES system draws more inspiration from the natural world and plans to include evolutionary algorithms, integrating AI solutions as a novel approach in TES. These algorithms are inspired by biological evolution, with their strength lying in solving complex optimization problems. In this new TES tank, evolutionary algorithms will help, among other things, in developing optimal control strategies for charging and discharging TES systems, maximizing energy efficiency and cost savings.
Innovations like this bio-inspired TES tank highlight the potential of advanced energy storage to support Europe’s transition to renewables. As solar power capacity expands, investing in efficient storage solutions will be critical for ensuring a stable and resilient power grid.
Beyond grid applications, this TES system is a key component of a new heat pump designed for industrial processes. Industrial heating accounts for a significant share of global energy demand, much of which still relies on fossil fuels. By integrating high-performance TES with heat pumps, industries can store and reuse heat more effectively, reducing both energy costs and carbon emissions. This development marks a step forward in making industrial heating more sustainable, aligning with the EU’s broader goals for energy efficiency and decarbonization.