Thermal energy storage plays a key role in the energy transition process. It makes possible to overcome the intermittence’s obstacle inherent to traditional renewable power generation sources (such as wind or solar), or the mismatch between available energy production and consumer’s demand, solved nowadays by inefficient management of resources, with paid plants on standby and reducing loads –and efficiency- of working plants. Until today, thermal energy storage has been associated with electric power generation technologies such as concentrating solar plant (CSP) technology. However, thermal energy storage is a key element in the decarbonization of industrial thermal processes, where more than 80% of the energy consumed in the world is involved. In this application, reliable, low-cost storage that adjusts to the thermal profile of each process is demanded.
If we focus our scope on high temperature applications, sensible molten salts are the most widely used material for sensible storage
Thermal energy can be stored under three principles: through the use of the sensible heat, through the latent heat when changing from one phase to another, or through the energy involved in a chemical reaction.
Sensible heat storage systems are based on the variation of a material´s internal energy when its temperature varies. Heat is used to increase the temperature of a solid or fluid, and then stored at its maximum operating temperature until the energy is required, entering then into the discharge phase to recuperate this energy. Multiple materials can be used for storage, such as molten salts, rocks, concrete, etc., and intermediate fluids can be used to transfer the energy from one source to another, such as air, oil water or steam. In order to select the optimum materials, properties such as material density, specific heat or conductivity, among others, are weighted and evaluated for each case.
In particular, the “solar salt” binary mixture (NaNO3/KNO3, 60/40%w) is typically used in CSP applications (up to 565 °C), where the ternary mixture known as Hitec® containing sodium/potassium nitrates (NaNO3/KNO3/NaNO2, 7/53/40%w) is also present due to its characteristic lower freezing temperatures (solar salts freeze at relatively high temperatures, up to 240ºC). The advantages of these fluids are their high volumetric heat capacity, high boiling point, high temperature stability, and their vapor pressure being close to zero. Additionally, they are relatively cheap, readily available, neither toxic nor flammable, and can act as an intermediate heat transfer fluid (HTF) as well as a storage material. An excellent example of its maturity and expansion is the use of these salts in the CSP, where double tank systems using molten salts are widely used. More than 21 GWh are currently installed around the World.
Aside of this use, in Build-to Zero we consider that this molten salt thermal energy storage systems have great potential for applications in large industrial processes, or even in our day-to-day lives in residential or administrative buildings
… in this line, we can use molten salt storage systems with electric heaters to directly heat up the molten salts, storing in this way heat when there is a surplus of electricity, and having it available for whenever needed.
If a power block is attached, this stored heat can be transformed back into electricity, which could particularly play an important role in the reconversion of decommissioned coal plants where the steam cycle can still be operative. In this way, we can use the surplus of the renewable energy to charge the thermal storage system in the industry, allowing the decoupling of the heat source from its supply, achieving an energy use on-demand, and allowing its reuse by regulating factors such as temperature and/or power. In order to achieve this objective, the design of the electric heater must be carefully considered, aiming to guarantee the efficiency and durability of this system in the harsh environment of high temperature and high corrosive molten salts. In this sense, all related elements must be considered: the integrity of the storage media, the container material and the heating element, guaranteeing its performance and the global efficiency.
We are currently working on the design of this component for large-scale applications, aiming to improve the heat management of the system in the applications known as Power to Heat (P2H) and Power to Heat to Power (P2H2P). These applications use the excess electricity in the grid provided by uncontrollable renewable sources to generate heat (which is easier to store) and make it available on demand as heat (P2H) or electricity (P2H2P).
The need of steam as a working medium for process heat applications gives rise to a growing demand for latent heat storage units.
Even though heat is easier to store than electricity, the space required for its storage is relevant. This may not be a problem when this storage is implemented in a new facility installed in the countryside, such as the CSP plants where the available area is not an issue, but it becomes a challenge when installed in traditional industries with the aim to use this stored heat as process heat. Also, the variation of the temperature of the storage media during the discharge (the molten salts are getting colder in order to heat up the industrial process) make this system not ideal, even more when most of the industries use steam at a certain pressure as their main heat transfer fluid.
Both of these challenges are tackled by using latent heat storage instead of sensible heat storage. In latent heat storage, heat is used to induce a phase change in the storage material: solidification, evaporation, condensation, or sublimation. The energy required to produce these phenomena is very high, which make these systems much more compact than the equivalent storage systems based on sensible heat, and since a material does not change its temperature during these processes, all the energy provided during this phase change is kept at the same temperature level. This ability to provide energy at constant temperature is key to obtain a high efficiency in its operation, especially for steam generation.
The materials used in this type of storage are the so-called Phase Change Materials or PCM. Although latent heat storage has a higher energy density than sensible heat storage, this type of storage has some limitations in terms of phase separation, corrosion, long-term stability, or low thermal conductivity which in the end tends to lead to a high cost.
All these challenges are being addressed during our studies with promising results, and the higher efficiency obtained can cope with the high cost of the PCM storage systems, making them a viable option for decarbonizing the heat industry, a key and necessary step in order to finally reach the decarbonization of the European industry.
 : IRENA (2020), Innovation Outlook: Thermal Energy Storage, International Renewable Energy Agency, Abu Dhabi.