It is now a fact of life for everyone that the energy transition is a global priority whereby the transport, industry and buildings sectors will have to use more renewable energy. The International Renewable Energy Agency (IRENA) gives the keys to this transition in its annual report World Energy Outlook (WEO) 2018: the dominance of renewable energy and energy efficiency.

In this sense, the electricity sector is increasing its share of renewables to be the primary energy vector in this transition. Apart from the transport sector, which has been focused for some years on the vehicle fleet´s electrification, the industrial and residential sectors are key points to carry out this energy transition. The presence of renewables as a source of energy in both sectors is still below the necessary levels, so it is vital to promote their electrification. The direct use of renewable energies such as solar thermal and increased energy efficiency would be ideal for reaching where electrification cannot.  

Whether for the electrification of these sectors, the direct use of solar thermal energy or the increase of energy efficiency, thermal storage appears as an essential pillar. Thermal storage makes it possible, among other things, to improve the flexibility of the electricity grid, overcome the intermittency of renewable energies, such as solar thermal, or take advantage of waste heat in industrial processes.

One of the biggest challenges to boost the market penetration of thermal storage lies in developing versatile systems, with a considerably higher thermal storage density than current ones, especially in applications where space is limited, and all this at a contained cost. For this purpose, thermal storage systems based on phase change materials (PCMs) are an interesting alternative.

PCMs make it possible to store a large amount of thermal energy (in the form of latent heat) thanks to their phase changes (solid-liquid or solid-solid) while maintaining a constant temperature during the transition process. These systems offer a much higher storage density with a narrower temperature range between storage and heat release than those based on sensible heat, such as water tanks, prevalent in many of today´s building applications. Because of these properties, PCMs can be employed either as thermoregulating materials or as heat storage for narrow temperature ranges.

PCMs in buildings

Of the total energy consumption in Spain´s residential sector, 42% is for heating and 27% for air conditioning, while in the European Union these consumptions represent 64% and 15% respectively. Thus, the use of PCMs in this sector contributes significantly to the reduction of total energy consumption. Concerning their application in buildings, thermal storage systems based on PCMs can be classified as passive systems or active systems.

In passive systems, PCMs are usually integrated into the building elements themselves, such as gypsum panels, polyurethane foams, concrete or cement. These elements primarily have a thermoregulatory function, preventing rooms from overheating during the day in the warm months or reducing the need for heating at night in the winter. Passive systems can be combined with active ones to increase their effectiveness, thus favoring the complete charging and discharging of PCMs.

These types of solutions are present in CIC energiGUNE, as part of the NRG-Storage project, which aims to replace the current materials used in building envelopes with a novel, ultralight, non-flammable cementitious foam that allows active and passive energy storage.  This is a highly innovative "green" option that combines the use of non-flammable and ultra-lightweight material with bio-based materials to create a multifunctional cementitious foam, called NRG foam.

CIC energiGUNE is mainly involved in the characterization of all the composite ingredients of this innovative material (cementitious paste doped with graphene nanoparticles and microencapsulated phase change material) to achieve the best fit between thermal insulation properties and heat storage capacity. The following image shows a conceptual scheme of the material developed in the framework of this project, as well as the microcapsules used in the doping of the foam.

Unlike passive systems, in active systems, PCMs are integrated into heat recovery systems, underfloor heating systems or heat pumps. The latter is of particular interest in this sector´s electrification, being today the most efficient means of converting electricity into heat. Heat pumps are the next big family of heating and cooling technologies expected to become massively widespread in the sector. Their combination with thermal storage systems is a cost-effective way to improve the flexibility of the power grid.

In these active methods, we can find many applications with temperatures ranging from 30ºC to 90ºC, each of which requires a specific PCM with a specific phase change temperature. Thus, each system or application requires an individualized optimization process to select the appropriate PCM. Currently, most of the PCMs used in these temperature ranges are paraffins, fatty acids or hydrated salts with solid-liquid phase changes, and it is necessary to search for the specific compound that best suits each application. Therefore, the development of more versatile PCMs would facilitate the market incorporation of latent heat-based thermal storage systems.

To drive this market integration, at CIC energiGUNE, we develop efficient and cost-effective ultra-compact thermal energy storage solutions at the level of materials, prototypes and systems. An example of these systems is based on the so-called Plastic Crystals, a particular type of PCM with solid-solid phase transitions. Plastic Crystals have a storage temperature range from 30 °C to 190 °C, with unusually high latent heat. Their binary and ternary mixtures give rise to new solid PCMs with "on-demand" energy storage properties, allowing them to tune the system´s phase transition to the desired range.

The following schematic shows how these developments work. In addition to being able to adjust their working temperature, these PCMs can be combined with other solid-liquid PCMs to increase their thermal storage capacity.

Images and microstructure of the materials under development: these materials are doped with different conductive particles that increase their charge/discharge power.

Compared to the current water tanks used in the domestic sector, these developments could reduce the tank´s size by up to 2-3 times. The possibility of adapting the working temperature in a simple way confers to these composites the versatility requested by the market, and likewise, their solid transitions allow the use of these composites in direct contact with the heat transfer fluid, dispensing with the heat exchanger inside the storage tank, and thus drastically reducing the costs of the device.

PCMs in industry

The wide working temperature range of these materials (<190ºC) also allows their use to be extended to the industrial sector. Given that low and medium temperature heat represents 45% of the total heat use in industrial processes, thermal energy storage alone, or its combination with heat pumps or ORC (Organic Rankine Cycle) technology has a significant potential to contribute to the decarbonization of the sector.

At CIC energiGUNE, we work on the development of innovative and cost-effective thermal energy storage solutions that meet the demanding industrial requirements. Thus, the materials developed based on Plastic Crystals would allow not only to have more compact devices but also with a fast response and high charge/discharge power to meet peak heat demands and process start-up. In addition, the development of coatings to allow direct contact of these materials with the main heat transfer fluids would also allow their use in steam generation applications, an auxiliary service in great demand in the industrial sector.

In short, the Phase Change Materials (PCM) developed at CIC energiGUNE not only have a positive impact on the final consumer with more compact and economical devices that favor a significant reduction of the energy bill in the residential sector but also in the industry where they satisfy the demanding heat-to-power processes in an innovative and profitable way.