It consists of accumulating energy by increasing the internal energy of a body by adding heat to it. The amount of energy stored by a body of mass m and specific heat Cp, which is heated from a temperature Ti to a temperature Tf responds to the following equation:
This equation shows that the stored energy density is directly proportional to the amount of material, to its specific heat and to the thermal jump that this material undergoes. The higher these are, the greater the storage capacity.
In addition to the level of storage temperature, which may require the use of different materials and components, these systems can be classified according to the storage time, with a distinction being made between short- and long-term storage systems.
Short-term storage is defined as storage in which the storage period (time between loading and unloading) is only a few hours and is therefore loaded and unloaded daily. For low temperatures (<200°C), sensible heat storage technologies using water as storage material are the most widely used. The storage system basically consists of a thermally insulated water tank, which may or may not be pressurized. This thermal insulation is very effective and prevents significant energy losses during the storage period. Although the energy density achievable with these technologies is relatively low – ranging from 30 kWh/m3 to 60 kWh/m3 – the investment cost is limited, since it is below 20 €/kWh. In higher temperature ranges (>250°C) and with larger storage capacities, the use of the double tank system with molten salts associated with concentrating solar power plants is widespread. These salts, which are a mixture of nitrates, store energy in the temperature range from 290°C to 365°C, with an energy density of 150 kWh/m3 and a cost that can reach 60 €/kWh.
As an alternative solution to salts and seeking to reduce the cost of these systems, in recent decades the use of fixed bed technology has been proposed, where heat is stored in a single tank filled with particles of solid material, ranging from mineral rocks to industrial waste. Some demonstrators have already been carried out and point to a significant cost reduction, which is around 20-30 €/kWh.
Finally, another industrial application for short-term storage using a similar technology is regenerative accumulators, which are widely used in industries such as glass or steel. In this case, the energy extracted from the fumes of the melting furnaces is stored in refractory solids that reach temperatures close to 1300°C, with the aim of preheating the combustion air for the burners of the next melting cycle. This storage is of very short duration, given the fact that these are continuous casts, but it substantially improves the thermal efficiency of these furnaces.
On the other hand, the development of long-term storage solutions (months) has focused on capturing summer solar energy for heating buildings in winter, hence it is also called seasonal storage. These are again systems that primarily store hot water, although fixed-bed systems can also be found, and are distinguished not only by the long duration of the storage period, but also by the large amount of energy they can store and, consequently, by their large size (2,000 – 100,000 m3).
To avoid thermal losses, these large reservoirs are usually buried, limited by demanding civil works or natural cavity arrangements, and for this reason, they are more common in northern European countries, which can afford the high investment costs due to higher heat demands.
The main function of energy storage systems is the ability to decouple energy generation from demand over time. In this way, it makes it possible to optimize energy generation, transport and distribution infrastructures, but, above all, it makes it possible to achieve a greater penetration of renewable energy, since it eliminates the dependence on the existence of renewable resources at the time of demand.
Once the energy is stored, different applications are enabled, such as arbitration, peak demand reduction, frequency or voltage regulation, or capacity reserves to meet potential incidents.
Due to the characteristics of energy storage in the form of sensible heat, most applications are currently associated with the storage of renewable energy, so that the availability of this energy can be shifted to periods of higher demand. In this way it is possible to reduce the energy bill and at the same time the emissions associated with generation using fossil fuels.
At low temperature (< 100 °C), the most commonly used thermal storage material is water. Apart from a high specific heat value, water is free (or almost free) and does not present any safety, toxicity or environmental problems. Although of more ad hoc use, industrial solutions using pressurized water for temperatures >100°C can also be found.
The most commonly used liquid materials at medium-high temperatures (up to approx. 600°C) are molten salts. As solid materials, special concretes are commonly used in applications up to 500 °C and ceramic materials at temperatures up to 1500 °C.
The following table gives values for application temperature ranges, specific heat and volumetric heat storage capacity by sensible heat of these media.
In high-temperature applications (>600°C), very low-cost solid materials (natural rocks and industrial by-products) are being studied, which could replace concrete and ceramic materials.
The application par excellence for temperatures up to 600°C is concentrated solar power plants, where it has already been demonstrated that by making use of molten salt thermal storage they are capable of supplying electricity 24 hours a day without interruption.
Above 600°C there are applications in the industrial sector with a lot of potential still to be exploited. These are industrial waste heat and its ability to substantially improve process efficiency.
Most of the energy used in sectors such as food, paper, steel and chemicals, among many others, is in the form of heat and accounts for more than 50% of global heat consumption. The problem is that once this thermal energy is used, between 20% and 50% is discarded in the form of waste heat. The main reason is that no means have been developed to capture this energy and reintroduce it back into the process, avoiding the energy consumption associated with new heat generation. In this sense, thermal storage can play an important role, since there are technologies that are capable of storing heat up to temperatures even above 1300°C, at a reasonable cost, for use on demand. Regenerative accumulators are a good example of this type of application, as mentioned above.
I have no doubt that thermal storage will be one of the protagonists of decarbonization, because it has a differential value with respect to other storage technologies, and that is that it is a low-cost technology, with still a lot of room for improvement.
Whether for large mass grid storage needs or small quantities for domestic use, its main advantage is that it is a simple technology with very low investment ratios and reliable and long-lasting performance. In a future grid consisting mainly of renewable energy sources, it is very likely that multiple storage technologies will coexist, and thermal storage will undoubtedly be one of them.
CIC energiGUNE is one of the institutions that has invested the most in research on thermal storage technologies, to the point that it represents a reference center at European level. The CIC energiGUNE research team has published more than 200 scientific publications in the field and has led the start-up of several industrial demonstrators.
Focusing on the development of thermal storage solutions for industry and being aware that the cost of the technology was an obstacle to its implementation, CIC energiGUNE has been one of the pioneering centers in the construction of thermal storage systems that use solid waste as storage material, thus also contributing to the circular economy.
Today, with the rising price of fossil fuels, renewable energies and energy efficiency are, by necessity, going to become industrial priorities and CIC energiGUNE’s thermal storage solutions are ready to take up the challenge.
In fact, as the world is changing at breakneck speed, CIC energiGUNE is already working on second-generation storage systems, which use phase change materials, and even third-generation storage systems, where heat is stored in the form of reversible chemical reactions, both with storage capacities far superior to sensible heat.
