Modern energy storage systems are a key technology for the successful energy transition – especially in the energy-intensive industrial sector, which is still largely dependent on fossil fuels. We discuss what types of energy storage systems are available on the market and for which applications they are suitable. Further, we take a look at the advantages and disadvantages of the different technologies.
Storage technologies of the future: Why energy storage matters
The need to limit CO2 emissions and thus drive decarbonization is undisputed. To achieve this, fossil fuels such as gas, coal and oil must be replaced by energy deriving from renewable sources. However, in view of the weather-, day- and season-related fluctuations in renewable energies, as well as the increasing demand for electricity due to advancing electrification, the electricity system must become more flexible in order to be able to guarantee system and supply security in the future. Energy storage systems are indispensable for a modern energy supply, as they decouple power generation and consumption over time, thus enabling flexible use. This benefits not only consumers, but also improves the stability of the grids.
Different types of energy storage solutions
There are various technologies for storing energy, which differ both in their operating principle and in the form of the energy they store. While some energy storage systems have been used successfully for many years, others are still new to the market.
1. Mechanical energy storage
Mechanical energy storage systems are based on classical Newtonian mechanics. The energy is stored in kinetic or potential form and as pressure energy. The best-known mechanical energy storage systems include pumped storage power plants, compressed air storage systems and flywheels.
1.1 Pumped storage power plants: the power of water
Pumped storage power plants are particularly suitable for storing electrical energy on a large scale. Water is pumped from a lower basin to a higher basin (upper basin) using surplus energy. With this principle, the electrical energy is stored in the potential energy (position energy) of the water. When this is required, the water is released into the lower basin via a turbine and converted back into electrical energy with the aid of a generator. The efficiency of pumped storage power plants amounts to approximately 75 to 80 percent. Due to geographical conditions, this form of energy storage is only suitable for large-scale use to a limited extent in many countries.
1.2 Energy storage via compressed air accumulators
Compressed air storage systems store energy by compressing air. The air is enclosed under high pressure in suitable containers or underground tanks. The stored mechanical energy can be released as needed by discharging the compressed air via turbines, which are used to drive generators. Although their efficiency is comparatively low, compressed air storage systems are suitable, for example, for storing electricity generated from renewable sources such as wind power.
1.3 Energy storage via flywheels
In the context of the use of flywheels or flywheel mass storage devices, excess electrical energy is stored in the form of kinetic energy. For this purpose, a flywheel is set in motion with the aid of an electrically driven motor. When the stored energy is to be extracted, the rotating mass is stopped and the rotational energy is recovered by a generator. Due to the high friction loss, this type of storage is generally used as short-term storage, for example to cushion peak loads.
2. Electrochemical energy storage
Electrochemical energy storage devices store energy in the form of chemical energy. During the discharging process, the latter is converted back into electrical energy. Electrochemical energy storage systems include both batteries and accumulators. Particularly in the area of small storage capacities – for example in car batteries – electrochemical storage has been popular for a long time.
It is impossible to imagine industry and private households without batteries. Depending on their purpose, different materials are used in their construction. Disposable batteries, for example, are generally more compact and have a higher energy density than rechargeable batteries. Accordingly, the former are used in medical implants or wristwatches, for example. Accumulators are used in the industrial sector.
Accumulators are a widely used form of electrochemical energy storage and are available in various sizes. Their key advantage is the fact that the processes that take place when the battery is charged are largely reversible. Thus, the energy loss that occurs is very low and usually amounts to only a few percent. The extent of self-discharge varies depending on the battery type. However, it is generally considered to be comparatively moderate. The energy density of a battery is significantly higher than that of other capacitors. However, it is very low compared with the energy density of fuels – this makes it comparatively difficult, for example, to design electric cars with a long reach.
3. Thermal energy storage
Particularly with regard to the necessary heat transition, thermal energy storage solutions are increasingly represented in the public discussion. There’s great potential, especially in the industrial sector. The most widely used thermal heat storage systems include sensible heat storage, latent heat storage and thermochemical heat storage. The different operating principles vary according to storage duration, temperature and the principle of storage.
3.1 Sensitive heat storage
In sensitive energy storage systems, thermal energy is stored by raising the temperature of a material. The storage materials used include water, thermal oil, concrete, sandstone, bricks or molten salts. Depending on which material is used for storage, the storage duration changes. Sensitive thermal storage, such as it occurs in ENERGYNEST’s ThermalBattery™, is considered the most established and cost-effective method of thermal energy storage.
3.2 Latent heat storage
In latent heat storage systems, which are still rarely used in an industrial context, thermal energy is stored with the aid of a phase changing material – including salt or kerosene. During the change of phases, for example from solid to liquid, the material absorbs energy, which then remains bound in the material as so-called latent energy. Latent heat storage systems can store energy without major losses over a longer period of time.
3.3 Thermochemical storage
Reversible gas-solid reactions are also used in thermochemical heat storage systems. In this context, the energy is stored as part of an endothermic reaction instead of an increase in temperature. The key advantage is a relatively low storage leak.
4. Electrical energy storage
Electrical energy from renewable energy sources is of fundamental importance in the context of the energy and heat transition. Various technologies make it possible to store this energy directly in electrical energy storage systems without conversion. However, this is not always economically viable in practice. In the context of electrical energy storage, a distinction is made between capacitors and superconducting electrical energy storage systems.
4.1 Capacitors: energy storage in an electric field
Capacitors generally make use of electrostatic forces. Energy storage in a capacitor is based on maintaining an electric field in which the energy is stored. “The key advantage is the fact that with these types of storage, electrical energy does not have to be converted into other forms of energy and in this way high conversion losses can be avoided. However, this benefit is relativized by the disadvantage of extremely low energy densities – both in terms of volume and weight – as well as high costs. For this reason, their application is currently found more in niche areas,” writes the Research Center for Energy Networks and Energy Storage (FENES).
4.2 Superconducting magnetic energy storage devices
In another type of electrical energy storage, so-called superconducting magnetic energy storage, a direct current from a rectifier flows through a coil made of superconducting material such as compounds of iron, phosphorus, lanthanum or oxygen. This creates a magnetic field in which the energy is stored.
After the accumulator is loaded, the power supply is interrupted and a switch made of likewise superconducting material is actuated. This switch is responsible for disconnecting the coil from the inverter. The circuit is then reconnected to the inverter to discharge the stored energy. In this way, alternating current is generated from the direct current.
The efficiency of this type of energy storage system for generating direct current is around 97 percent. However, considerable cooling requirements need to be taken into account, which often stand in the way of the technology’s economic industrial use.