4. Overview of Energy Storage Technologies Besides Batteries

Pumped hydroelectric storage (PHES) is one of the most common large-scale storage systems and uses the potential energy of water. In periods of surplus of electricity, water is pumped into a higher reservoir (upper basin). In demand times, this process is reversed, and the potential energy is transformed into electrical power by a generator within a short reaction time [20].

The energy density depends on the height difference between upper and lower reservoir and ranges between 70 and 600 meters, which corresponds to energy densities of 200–1600 Wh/m³ [21].

There are two different design principles: the tandem design and the use of pump turbines. In the tandem design, pumps and turbines are designed as independent units, whereas pump turbines can function both as pumps and turbines. Pumped storage power plants are characterized above all by high storage capacities and rapid operational readiness.

More than 96 % of installed storage capacity worldwide consists of pumped hydro storage systems. Table 4.1 shows the installed rated power and capacity of pumped hydro in the world since 1990.

With a storage duration ranging from a couple of hours up to several days and reaction times within seconds, pumped hydro storage systems are used for bulk energy services as well as ancillary services.

Of all energy storage systems, pumped hydro storage systems have the longest service life of 50–150 years [2]. Due to their design, they show neither a degradation of the high round-trip efficiency nor the capacity. Because of their low volumetric energy density, however, pumped storage power plants require large areas for storage lakes and a sufficiently high gradient between the upper and lower reservoirs. In practical terms, this means that PHES are primarily implemented in areas with natural height differences.

The main materials in the construction of PHES are concrete¹ and steel. Although these materials have significant CO₂ emissions during production, these are put into perspective by their long service life. The required materials are available worldwide, so they do not have any criticality worth mentioning. The operation and commissioning, which mainly consists of damming rivers or connecting existing lakes, can lead to many negative consequences for the local ecosystems. In particular, the constantly changing water levels can be negative for flora and fauna, but also for possible tourist use. Even if pumps, pipes, or turbines are hardly visible in modern designs such as cavern power plants, the intervention in nature is often viewed with skepticism or completely rejected by those affected. Therefore, “since the 1970s, especially in European countries, this has led to a continuously decreasing acceptance of these energy storage devices among the population” [1].

Decommissioning, dismantling, and recycling play a subordinate role with PHES because of the particularly long service life. At the same time, hardly any materials that are critical for disposal are installed here.

Compressed air energy storage is based on the compression of air and storage in geological underground voids (e.g., salt caverns) at pressures of around 100 bar. When discharging, the compressed air is released and expanded to drive a gas turbine to generate electricity. As air cools down during expansion, it has to be heated while releasing. Here, a distinction is made between diabatic and adiabatic compressed air energy storage.

When discharging a diabatic compressed air energy storage (D-CAES), the released air is heated via combustion using natural gas or fuel. Therefore, a D-CAES system is a hybrid system composed of a natural gas fired open cycle turbine and an electrical storage system.

In an adiabatic compressed air energy storage (A-CAES), the heat produced during the compression cycle is stored using thermal energy storage (TES). During discharging, the stored thermal energy is used to heat the released air. The compressor discharge temperature can reach more than 600 °C. The hot air is sent to a TES which is designed for the applied internal pressure and which is sufficiently insulated to minimize heat losses. The TES can be made of ceramic, concrete, or natural rock materials. In this setup, no additional combustion cycle is required.

Two large-scale D-CAES were installed in 1978 and 1991 in Huntorf, Germany, and McIntosh, USA, respectively. They were used for energy time-shift and spinning reserve for a generally conventional energy system. Since diabatic storage systems still depend on fossil fuels, research has been focusing on A-CAES since then. First commercial A-CAES have been commissioned in 2020 in China and the USA, to support the increased integration of renewable generation in these systems. Table 4.2 provides an overview of the worldwide installed rated power and capacity of compressed air energy storage systems.

CAES have the second highest service life of 30–50 years for the machines and even longer for the cavern. Also, they show neither degradation of the medium nor the capacity. Because of their low volumetric energy density, however, compressed air storage power plants require large, pressure-resistant, mostly underground volumes for storing the compressed air. These caverns are mostly salt caverns that are specifically created by leaching. Other types of storage volumes have not yet been implemented for cost reasons. Therefore, only areas where salt caverns exist or can be created have been considered for CAES.

The use of materials (steel, concrete) is significantly lower for CAES compared to PHES and focuses primarily on the machine house with the compressors and turbines.² In contrast to PHES, the CO₂ emissions in the production of the components for a CAES are rather low. However, it must be noted that when the salt cavern is drained, extremely high salt loads have to be disposed of by rinsing with warm water. This can cause further emissions during construction through long pipelines or affect flora and fauna through discharge into rivers.

During operation, the CO₂ emissions that result from the additional firing of natural gas for D-CAES systems are dominant. It can be assumed that approx. 98 % of the total CO₂ emissions occur during operation [24]. Modern concepts with heat storage (A-CAES) do not require additional natural gas and are therefore significantly more attractive from an emissions point of view. Both types of construction require little space, as most of the system is underground. For this reason, CAES are hardly viewed with skepticism or rejected by those affected, except during in the leaching and construction phase.

Decommissioning, dismantling, and recycling play a subordinate role with CAES due to the particularly long service life. At the same time, hardly any materials that are critical for disposal are used.

Flywheels store electrical energy in the form of rotational energy. The flywheel is set in motion, or its speed is increased with the aid of an electric motor, thus storing energy. The amount of energy that can be stored depends on the rotational speed, since this is proportional to the mass moment of inertia and the square of the angular velocity. If required, the kinetic energy is converted back into electrical energy via a generator. Since the speed of the wheel changes both when energy is stored and when it is discharged, a frequency converter is required to adapt the voltage generated to the grid frequency [20, 26].

Under optimum operating conditions, flywheels can achieve an efficiency of up to 95 %. To keep the relatively high rest losses (approx. 20 % per hour), which are mainly caused by friction on the bearings and on the flywheel itself, as low as possible, the flywheels usually run in vacuum chambers. Furthermore, magnetic bearings with superconductors are used, which significantly reduce losses compared to roller bearings or plain bearings [11].

Flywheels show fast discharge times (within seconds) with high power densities of up to 10,000 W/kg [20].

Flywheel energy storage systems are mainly used for short-term storage application lasting from milliseconds up to minutes such as power quality services [1]. This can also be seen in Table 4.3, where the installed rated power of flywheel energy storage systems is significantly higher than the installed rated capacity.

Compared to battery storage systems, flywheel storage systems have a long service life of more than 20 years in most cases. Also, due to their design, they show neither a degradation in round-trip efficiency nor in capacity. However, self-discharge, which mainly results from air and bearing friction, must be considered in the emissions balance.

Since flywheels—in contrast to PHES and CAES—do not store the energy in the medium of water or air, but in a rotating mass, the use of materials is relevant. The materials used are primarily fiber-reinforced composite materials (e.g., CFRP) or steel. The materials are to be classified as not very critical in terms of procurement, but steel production brings increased CO₂ emissions into the balance, whereas recycling of composite materials is very difficult if not impossible. In terms of safety, flywheels must be secured against bursting, since the usually very fast rotating masses have a high-risk potential. This is often taken into account by building a containment for the flywheel or sinking the flywheels in the ground. Therefore, acceptance problems with this technology are not to be expected [1].

During operation, fundamental losses occur due to self-discharge. With slowly rotating steel flywheels, these tend to be higher than with composite flywheels, but even there they are not negligible compared to other storage technologies. Therefore, the mode of operation (average storage period) is of great relevance, i.e., the losses are higher with longer storage periods, so that in this case the CO₂ emissions—assuming a fossil energy mix—are much higher during operation than during production [24].

Decommissioning, dismantling, and recycling play less of a role with flywheels than with battery storage due to their long service life. At the same time, hardly any materials that are critical for disposal are installed. With composite flywheels, the materials still are a challenge to dispose of, and, unlike steel, they cannot yet be recycled.

Flow batteries are rechargeable batteries which use two different electrolytes—one with a positive charge and one with a negative charge—as storage medium. The most used electrolyte systems are vanadium-vanadium or the iron-chromium. One of the biggest advantages of this technology is the decoupling between power and energy ratings, as tank volume and stack size (active surface area) can be scaled independently.

The electrolytes are stored in external tanks and only pumped through the battery cell for charging and discharging in two separate hydraulic circuits. When operating, oxidation and reduction processes take place at the anode and cathode, which convert the electrical energy into chemical energy during charging and back into electrical energy during discharging. The two half-cells are separated by an ion-selective membrane. As the anolyte and catholyte are stored in separate tanks, the self-discharge rate of flow batteries is nearly zero. Additionally, as the battery electrodes do not actively participate in the chemical reactions, flow batteries are deep discharge proof [12].

Redox-flow batteries have been continually under development and have become more commonly used since 2010 as can be seen in Table 4.4. Applications range from small scale behind-the-meter applications in private households to providing bulk energy and ancillary services. The most common type of redox-flow battery is vanadium redox-flow batteries [1].

The advantage of redox-flow batteries in comparison with Li-Ion batteries is the separation of storage power and storage capacity, which can therefore be chosen individually to fit the application.

In terms of ecological assessment, redox-flow batteries differ from conventional batteries in many respects. The crucial point here is that the energy is usually stored in two liquids, which are stored in conventional containers, mostly made of plastic. The decisive advantage in the design is that the capacity can be precisely adapted to the application, regardless of the output, so that there is no unnecessary material consumption due to oversizing in output or capacity.

It is advantageous that the materials mostly used (plastic, aluminum) have no or rather low criticality (vanadium), since there are many new exploration projects at an advanced stage worldwide [9, 28, 33]. Nevertheless, it must be considered that the storage medium—in contrast to, for example, pumped storage power plants—is an expensive recyclable material and that vanadium, the electrolyte most used, requires safe containment, since it is a heavy metal that must not be released into the environment. At the same time, however, these mostly double-walled storage containers offer safety advantages compared to some lithium-ion batteries, which have risky materials (e.g., cobalt in NMC cells) finely distributed in each cell due to their design. Likewise, the mostly water-based electrolytes do not pose any fire hazards, in contrast to lithium-ion batteries.

During operation—if neither storage nor withdrawal occurs—only a small amount of self-discharge occurs. When storing and withdrawing, however, the pumps that pump the anolyte and catholyte through the cells of the stack are the main consumers of energy, which is equivalent to self-discharging. As a result, the round-trip efficiency of vanadium redox-flow batteries at around 70 % is also significantly lower than that of other battery types. Therefore, the mode of operation is of great relevance, i. e., the losses are lower with longer storage periods, but significantly higher with frequent charging and discharging. In this case, the CO₂ emissions—assuming a fossil energy mix—are much higher during operation than with more efficient storage systems. This aspect does not apply to a primarily renewable energy mix.

From an ecological point of view, the greatest advantage is that the storage fluids are usually of a single type, uncontaminated, and in liquid form. They can be easily regenerated during operation and simply pumped out at the end of their life and recycled with almost no material loss (related to vanadium). From a purely economic point of view, the value of the electrolyte is an advantage here since recycling is more worthwhile [3]. At the same time, recycling has a positive effect on the criticality of vanadium since the vanadium can be recovered and used in new VRFBs. The aging of the storage media and the associated degradation are also significantly lower than with conventional batteries. A service life of up to 20 years is assumed, which puts the production emissions into perspective [15].

Power-to-heat applications use electric power to generate or redistribute heat. Consequently, they serve to couple these two sectors. The heat is generated, for example, in electrode boilers, in electric boilers, or by using heat pumps.

In electrode boilers, the operating principle of direct resistance heating [16] comes into effect, where the flow of the electric current through the medium to be heated leads to heat generation. The electric boiler works according to the operating principle of electrical resistance heating, in which a current-conducting heating element heats up and then transfers this heat to the surrounding medium that can be stored. Both boilers are also suitable for steam generation. Both processes are established technologies and are structurally simple and low maintenance storage solutions that can be implemented in different scales. They are therefore suitable for household, commercial, or industrial applications. Electrode boilers in large-scale applications are mainly used in combination with heat networks or to produce superheated steam [18].

In heat pumps, electricity is used to transfer heat from a heat reservoir to a location to be heated using a carrier medium. The transfer occurs in a circular process in which the carrier medium is compressed, liquefied, expanded, and evaporated. During evaporation, it absorbs the heat energy, which it then releases to the location or medium to be heated during condensation. Overall, the transported heat is raised to a higher temperature level in the circular process [27]. Heat pumps make use of ambient heat. The amount of electrical energy required for the circular process is thus less than the amount of energy yielded by the process. The thermal efficiency of heat pumps is therefore very high but varies greatly depending on the type of system and ambient conditions. There is a large number of different types of heat pumps, which differ in terms of process (compression/evaporation or use of other physical principles) and heat sources (e.g., groundwater, ambient air, waste heat) [6].

If the heat pump is combined with a heat storage system, a higher-value utilization concept is created for the energy transition: by storing the heat from power-to-heat processes, the technologies contribute both to meeting the heat-side demands and to integrating renewable electricity into the energy system in the best possible way and providing required flexibilities. In this way, they couple the electricity sector with the heating sector in a beneficial way to the energy transition.

The electricity-based generation of cold by refrigerators (power-to-cold) also belongs to the field of thermal technologies and is also combined with suitable storage solutions [32]. However, electricity-based thermal utilization without intermediate storage is also conceivable, for example, for the provision of space heating by resistance heaters or process heat in industry. Such technologies fulfill a “power-to” purpose if they contribute to the electrification of processes that were previously operated differently.

Power-to-heat systems must be considered separately ecologically for energy conversion unit and thermal energy storage. The thermal storage tanks, which are mostly designed as simple hot water tanks with insulation, have a very long service life and contain no risk materials. The service life of heat pumps is in the range of 10–15 years.

When looking at the materials used, the focus is primarily on heat pumps, as they often have critical refrigerants—the other materials are to be regarded uncritical (standard metals). The refrigerants primarily used to date are fluorinated or partially halogenated refrigerants, the use of which is restricted by the EU regulation (EU 517/2014, F-Gas Regulation) [1]. At the same time, these PFAS (perfluorinated alkyl substances) are very much part of the ecological debate because they are considered persistent and potentially toxic. Therefore, the development goes in the direction of natural refrigerants such as propane. Here, however, the potential flammability and explosiveness of the refrigerant must be considered. This is probably the only aspect that can lead to a slightly reduced acceptance of the power-to-heat technology.

During operation, there are practically no losses. The thermal storage—depending on the storage period—has a low level of self-discharge through heat losses to the outside. Due to their high efficiency, heat pumps have significantly reduced CO₂ emissions compared to natural gas and even electric boilers. Also, no degradation in conversion efficiency or storage efficiency or capacity is to be expected.

Decommissioning, dismantling, and recycling play only a minor role in power-to-heat systems, especially in the storage tanks, due to their long service life. Only the refrigerants of the heat pumps must be disposed of safely.

Power-to-gas technologies are associated with material conversion processes. Various chemical reactions can be triggered using electric current. Gaseous products are obtained, for example, from various electrolysis processes, from the electric arc process, microwave plasma activation, or single-stage electrosynthesis using CO₂ [10]. Via the electrolysis of water, hydrogen and oxygen are obtained as products. The associated electrolysis processes differ in flexibility and degree of efficiency and in their technology readiness levels. Most developed and market-ready is the alkaline electrolysis (AEL), followed by the proton exchange membrane electrolysis (PEM) and—with a larger distance—the electrolysis using a solid oxide electrolysis cell (SOEC) operating at high temperatures. In an electrolyser, water is split by means of the electric current applied to electrodes which are surrounded by an electrolyte and separated by a semipermeable diaphragma into the anode and the cathode compartment, so that hydrogen and oxygen can be collected separately. AEL uses an alkaline electrolyte and an ionic conductive membrane as diaphragma, while PEM uses solid electrode membrane assemblies to realize the necessary separation. The SOEC also operates with a solid electrolyte as diaphragma. Here, water steam is used instead of liquid water due to the high operating temperatures [29].

Via co-electrolysis, syngas (hydrogen, carbon monoxide) can be produced from the reactants water and carbon dioxide [4]. The electric arc process produces ethyne and hydrogen from methane [17]. Microwave plasma activation decomposes carbon dioxide to carbon monoxide and oxygen [25]. Electrosynthesis can be used to produce alkenes, especially ethene, from carbon dioxide and water [34]. In addition to these direct power-to-gas products, further indirectly electricity-based gaseous products can be synthesized. Via the hydrogen path, methane is, for example, obtained as an indirect power-to-gas product by means of chemical or biological methanation [13]. The electric arc process and the chemical methanation operate at high technology readiness level (TRL), the co-electrolysis and the biological methanation at medium TRL and the electrosynthesis and the microwave plasma activation at low TRL.

The gases produced by power-to-gas processes contribute to sector coupling, since they can be used in a material or energetic way in different sectors, and their storability enables sector-specific requirements to be decoupled from electricity production over time. The power-to-gas products hydrogen and methane are particularly important for the energy storage system, as they can be converted back into electricity in combined heat and power (CHP) systems or fuel cells and thus used energetically.

Up to now, the service life of electrolysers has been in the single-digit range, depending on the type of electrolysers (AEL or PEM) and operating parameters. And—comparable to battery storage—there is a degradation in efficiency.

The materials used (membranes, catalysts) are expensive and some (catalysts) can be recovered, but membranes cannot. Since some catalyst materials are particularly rare (e. g., iridium), the development of a recycling industry parallel to the development of electrolysers production is necessary [19].

In electrolysis mode, the CO₂ emissions can be reduced by around 90 %—compared to the natural gas reforming that is common today [31]. Since the electrolysis process has so far only had an efficiency of approx. 60–70 %, it is obvious that a high proportion of fossil energies in electrolysis would lead to more CO₂ being emitted in extreme cases than if hydrogen would be from natural gas. In addition, it must be considered that—if hydrogen is stored under high pressure (300–700 bar)—additional energy expenditure is required for the compression.

Decommissioning, dismantling, and recycling play an important role in electrolysers because of the rare and expensive catalysts, but also because of the shorter service life [1].