Indeed, in this energy transition process to a widespread use of renewable energy versus fossil fuels, thermal storage plays a key role. 

Not only it allows overcoming the obstacle of intermittence to which energy sources -such as wind or solar- are attached, or the gap between production and demand for energy. But also, because it is responsible for optimizing energy management in areas such as electricity production in solar concentration plants or the use of waste heat in industrial processes.

The main value of thermal storage lies in its capacity to store large amounts of energy at a relatively low cost and in sectors as diverse as heating and cooling of homes and buildings, in industrial heating and cooling processes, and even as a complement to the massive storage of energy for the electricity grid.

Types of thermal storage depending on the technology

Thermal Energy Storage (TES) systems can store heat or cold for later use under varying conditions such as temperature, location (when transported) or power. 

In these systems, energy is cycled based on energy charge, storage and discharge, and must meet several requirements such as high energy density, good thermal conductivity, chemical and mechanical stability, complete reversibility of cycles and low thermal losses during the storage period.

Thermal energy can be stored under three principles: through the use of the sensible heat of the bodies, through the latent heat when changing from one phase to another, or through the energy involved in a chemical reaction.

Sensible heat storage systems

So-called sensible heat storage systems are based on the variation of a material´s internal energy through a temperature variation. Heat is used to increase the temperature of a solid or fluid stored at maximum operating temperature until it enters the discharge phase.

Transfer fluids such as air, oil, and storage materials such as molten salt, rock, concrete, etc. are used. Properties such as material density, specific heat, or conductivity, among others, are taken into account in their selection.

One of the most commonly used liquids for the storage of sensible heat to medium-low temperatures is water, which can be used to produce domestic hot water (DHW), heating or air conditioning, using tanks for storage.

The storage of sensible heat at high temperatures is widespread in industrial and commercial applications due to its efficiency and simplicity. An excellent example of its maturity and expansion is the solar thermal power plants also known as concentrated solar plants (CSP), where double tank systems with molten salts are mainly used. 

These types of plants base their operation on the concentration of solar radiation using a range of mirrors that act as concentration systems and can be parabolic cylinder, central tower, linear Fresnel or parabolic dish. A heat transfer fluid (HTF) will be responsible for absorbing this heat and transporting the concentrated radiation to a storage system, to convert this heat into electricity later, when it is necessary.

Despite their proven effectiveness, concentrating solar power plants face the challenge of reducing their costs. In this sense, CSP developers are betting that the plants operate at higher temperatures than the current maximum (565 º C), which would improve efficiency in converting heat into electricity.

To this end, high-temperature materials are being researched that are also cost-effective as new-generation storage systems, such as ternary salts, supercritical CO2 or air as a heat transfer fluid.

Latent heat storage systems

As a second type of thermal energy storage, we can identify latent heat storage. In this case, heat is used to induce a phase change in the storage material: solidification, evaporation, condensation or sublimation.

The materials used in this type of storage are the so-called phase change materials or PCM. Although there are some as common as water (used as ice for cold storage since ancient times), very few materials or applications have reached the market. 

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, low thermal conductivity or a high cost.

Thermochemical storage systems

Unlike the other two types of heat storage, storage by chemical reaction is based on the storage material´s internal variation. In thermochemical storage, the heat produced from a reversible chemical reaction is absorbed, producing an endothermic reaction when it absorbs thermal energy and exothermic with the discharge.

Thermochemical storage systems have numerous advantages over other types of thermal storage: higher energy density (up to 10 times higher than sensible thermal storage), the possibility of storage heat even at room temperature, long-term storage, ease of transport...

However, this type of storage is still in a preliminary development stage due to its complex configuration, cost, and low heat transfer capacity. In fact, at present, researchers´ efforts are focused on identifying materials and systems that guarantee the reversibility and absence of energy capacity losses throughout the numerous charge and discharge cycles.

Today, only pilot projects (some of them with promising results) exist at laboratory scale or at a pre-industrial stage. This shows that this technology is still at a very low TRL (Technology Readiness Level), so we can consider that it will be many years before seeing thermochemical storage applied in industry.

Classification of thermal storage systems according to temperature level

As we have already advanced in the examples of sensible heat storage, the different applications of thermal storage systems can also be classified according to the temperature level: low, for applications up to 100-150 ºC; medium, up to around 300 ºC; and high-temperature thermal storage applications when they reach up to 1000 ºC.

One of the clearest examples of low-temperature thermal energy storage is underground storage or UTES, also known as surface geothermal. Due to the high heat capacity of the soil, as well as its properties as thermal insulation, this type of storage opens a wide range of possibilities in terms of energy savings or the application of renewable energies. 

An example that is already in the commercial phase is the storage of thermal energy in aquifers; a system responsible for pumping groundwater from hot wells in winter to heat a building or, on the contrary, pumps water from cold wells to cool a home in summer.

High-Temperature Thermal Storage Applications

The research we carry out at CIC energiGUNE is mainly (but not exclusively) focused on applications with temperatures above 300ºC. 

In fact, we have a prototyping and thermal testing laboratory that allows us to reproduce the real conditions of this type of application in a controlled and safe environment. We are talking about an air and oil loop (in addition to a steam loop that is under construction) that can withstand temperatures from 400ºC to 800ºC, and that allows the validation, at a relevant scale, of new products, systems or components. 

In addition to the solar thermal or solar concentration plants (CSP) that we have already mentioned, in this temperature range, there are other systems such as energy storage through compressed air (known as CAES). This system takes advantage of the excess energy coming from the grid to generate energy using an air compression phase and its subsequent expansion through a turbine. If, in addition, a thermal storage system is incorporated to store the heat generated during the process, the whole system´s efficiency will be improved.

However, this is not a very exploited application at an industrial level, given its reduced economic viability.

This is not the case of what is considered one of the most powerful tools in terms of energy efficiency in the industrial sector: industrial waste heat recovery systems (IWHR).

If we talk about energy transition and the need to bet on renewable energies, it is as vital that this production is sustainable as that the management of it is adequate. Even more when we are talking about one of the largest consumers of energy, as is the case of industry, with almost 35% of total consumption.

Industries such as iron, steel, cement, glass, forge or foundry industry, all of which are large consumers of energy at high temperatures, are the perfect candidates for waste heat recovery systems, since it is estimated that in most of their processes, they lose between 20 and 50% of energy as waste heat in terms of exhaust gases, chemical combustion processes, water cooling, etc.

But if, in addition to recovering this excess energy as heat, a storage system is implemented to reuse it, a huge reduction in primary energy supply can be obtained, along with the obvious reduction of greenhouse gas emissions.

In fact, a thermal storage system in the industry allows the decoupling of the heat source (which is usually intermittent) from its supply, achieving an energy use on-demand, and allowing its reuse by regulating factors such as temperature and/or power. 

This shows the potential that thermal storage systems contribute to the industry´s energy efficiency, and the European Union has taken this into account by supporting numerous projects and programs.

Research projects in thermal storage

One of them is ReSlag. This European project led by CIC energiGUNE and framed within the Horizon 2020 initiative, had as one of its objectives to investigate the applicability of steel slag (in particular the electric arc furnace) as a storage material for heat recovery applications within the steelworks. The project concluded with constructing a 1/10th scale prototype demonstrator for the ArcelorMittal facilities in Sestao (Bizkaia). 

Project collaborators such as DLR and ENEA also studied the slag´s behavior to be used as a material for thermal storage in thermo-solar plants, both in the new generation ones (working with air) and in the traditional ones (with molten salt).

On the other hand, the ECOSLAG project aims to research solutions that allow the heat of the slag itself to be recovered in processes where it is currently wasted. 

The different collaborators in the project are working with steelworks in Germany, Italy and Sweden. At the same time, at CIC energiGUNE, we have developed a new concept of heat recovery from molten slag using an exchanger embedded in the Sidenor slag heap floor in Basauri (Bizkaia).

Despite the advances that have been made in recent years in industrial heat recovery systems, there are still considerable margins for improvement, which is why at CIC energiGUNE, we continue to research both the development of new materials and new technological solutions that allow us to propose viable scenarios for energy use.

Research in thermal storage in progress

Moreover, at CIC energiGUNE, in addition to researching applications for industrial processes, we work on thermal storage solutions for large-scale applications and for improving heat management.

To this end, as far as solar concentration plants are concerned (included in the section on large-scale applications), we have the advice and collaboration of our board member SENER (the world market leader). 

One of CIC energiGUNE´s milestones related to CSP is a novel storage system based on magnetite packed beds that we have implemented in a demonstration plant in Ben Guerir (Morocco) as an alternative to the molten salt double tank technology.

On the other hand, together with Iberdrola, the leader in energy production, distribution and marketing, we explore the applications known as Power to Heat to Power. Systems that, starting from the excess of electrical energy in the network, produce heat to later generate electricity again according to the energy demand of the moment.

In the heat management section, Thermlab stands out; a recent initiative by CIC energiGUNE that offers advanced heat management solutions, which help to detect anomalies and develop ad-hoc technologies for industries such as power electronics, manufacturers of electrical equipment goods, renewable energies and the automotive industry, among others.

Sectors such as the construction industry, on the other hand, benefit from research such as the one that we are carrying out with the NRG-Storage project, which aims to replace the current insulating materials used in buildings with a new foam, which in addition to efficiently insulating, allows active heat storage.

But we cannot forget the project that allows us to join the areas of electrochemistry and thermal of CIC energiGUNE.

This is the joint work between Bcare, CIC energiGUNE´s spin-off specialized in the integral assessment of the health status of batteries and condensers, and the thermal energy storage team, through which they are investigating the optimization of the heat management in batteries.

Thermal energy storage: a great candidate for decarbonization

It is clear that, in the context of a society with an increasing energy dependency, where there is also a growing concern for environmental protection, it is necessary to make optimal management of available energy resources, as well as to continue the search for alternative energy technologies.

This is where thermal storage stands out, as it has demonstrated its capacity to store large amounts of energy at a reasonable cost, improving system efficiency and energy management.

However, there is still a long way to go. A path along which researchers in energy storage are advancing step by step and which will allow us to reach the longed-for goal of total decarbonization of the economy of European citizens.

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