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Anastasovski, A.; Andreucci, M.B.; Kádár, J.; Delli Paoli, M. Energy Storage in Urban Areas. Encyclopedia. Available online: (accessed on 16 April 2024).
Anastasovski A, Andreucci MB, Kádár J, Delli Paoli M. Energy Storage in Urban Areas. Encyclopedia. Available at: Accessed April 16, 2024.
Anastasovski, Aleksandar, Maria Beatrice Andreucci, József Kádár, Marco Delli Paoli. "Energy Storage in Urban Areas" Encyclopedia, (accessed April 16, 2024).
Anastasovski, A., Andreucci, M.B., Kádár, J., & Delli Paoli, M. (2024, March 05). Energy Storage in Urban Areas. In Encyclopedia.
Anastasovski, Aleksandar, et al. "Energy Storage in Urban Areas." Encyclopedia. Web. 05 March, 2024.
Energy Storage in Urban Areas

Positive Energy Districts can be defined as connected urban areas, or energy-efficient and flexible buildings, which emit zero greenhouse gases and manage surpluses of renewable energy production. Energy storage is crucial for providing flexibility and supporting renewable energy integration into the energy system. It can balance centralized and distributed energy generation, while contributing to energy security. Energy storage can respond to supplement demand, provide flexible generation, and complement grid development. Photovoltaics and wind turbines together with solar thermal systems and biomass are widely used to generate electricity and heating, respectively, coupled with energy system storage facilities for electricity (i.e., batteries) or heat storage using latent or sensible heat. Energy storage technologies are crucial in modern grids and able to avoid peak charges by ensuring the reliability and efficiency of energy supply, while supporting a growing transition to nondepletable power sources.

PED energy transition energy storage electricity heat chemical energy

1. Introduction

Energy Storage Systems (ESSs) have become a critical issue in energy generation from Renewable Energy Sources (RES). Rotondo et al. [1] report on energy storage as one of the key points to ensure ecological transitions in urban areas. There are a few separate energy storage methods currently available for the storage of heat and electricity as the primary forms of energy that can be generated directly from RES. Dehghani-Sanij et al. [2] collected and summarized data on a SWOT (Strengths, Weaknesses, Opportunities, and Threats) analysis of CAES, PHS, BES, and hydrogen storage systems. According to that research, CAES and PHS have strengths in terms of low-cost and high capacity, but they need large spaces to host and operate the technology.
On the contrary, high-cost ESSs, such as BES and hydrogen storage, need less space and offer a high response to the distribution of energy requirements. Xylia et al. [3] illustrated the key factors considered for selecting energy storage. These factors are energy density (how much energy can be stored per mass unit), power density (how fast that energy can be realized), storage duration (how long energy should be stored), and costs (how much it costs compared with other solutions).

2. Thermal Energy Storage

Thermal energy storage types do not require significant investments. They are prevalent for storing solar heat energy. This is especially true for sensible heat storage demonstrating the lowest energy capacity (heat storage effectiveness). The heat that can be stored is limited to the capacity of equipment, i.e., the amount (volume) of the heat storage material. In general, the primary sensible heat storage material is water. It is a multipurpose material. It can be used as hot water for household needs and heat storage for other purposes. According to EUROSTAT in 2019 [4], the primary energy consumption in EU households was 64.1% for space heating, 14.8% for water heating, 14.4% for lighting and appliances, 5.6% for cooking, 0.3% for space cooling, and the remaining 0.9% for other purposes.
Sensible heat storage is the cheapest energy storage. It uses the different abilities of materials to absorb heat during the heating process [5]. The volume of storage material is proportional to the energy that can be stored. In contrast, the latent heat storage (HS) includes materials that change phase (PCM—Phase Change Material) during heat storage. The process is based on the latent heat characteristics of materials. The phase change is at a certain temperature [6]. Therefore, there must be selected appropriate materials for the required process parameters. In many cases, some expensive materials need to be selected. Furthermore, absorption and adsorption systems show remarkably high energy storage density. Their energy density is up to 1000 MJ/m3. Alva et al. [7] connected HS in buildings with the use of new building composite materials. This is shown in Figure 1, where different ways of HS are determined for use in buildings.
Figure 1. Thermal energy storage in buildings (adapted from [7]).
They divided HS into active and passive storage systems. PCM and sensible HS systems are determined as passive energy storage. Their use is intermittent. On the other hand, active HS is continuously used by HVAC systems in buildings, or these systems are part of the building structure or the neighborhood. Dynamic storage systems are only sometimes possible due to the space and construction that they need. Aquifers, pits, rocks, and brick thermal storage systems exist in urban areas. Denmark uses pits for community HS systems. Similarly, aquifers are used in the Netherlands [8].

Technical Requirements for Applications

Thermal energy storage is a highly promising technology for urban areas. Its principle involves the storage of thermal energy through the heating or cooling of a storage medium. This stored energy can be utilized later for various heating and cooling applications and power generation [9]. There have been innovative advancements in district heating, such as solar thermal district heating systems, large-scale heat pumps, and the integration of geothermal and waste heat. These developments are the most effective when operating at low temperatures [10]. The Giga_TES project aims to create large-scale thermal energy storage concepts for Austria and Central European urban districts [11]. Its ultimate objective is to achieve a 100% renewable energy heat supply for cities [11]. This can be accomplished only with the use of large underground hot water tanks and pits, which will serve as multifunctional energy hubs for future district heating systems. Expansive thermal energy storage technologies, like these, will facilitate the seasonal and short-term storage of a wide range of volatile energy sources, thereby significantly increasing the proportion of renewable energies in the urban energy mix.
The heat energy storage in urban areas is subject to various legal and regulatory challenges, which can limit its widespread adoption. Some of these challenges include building codes and safety regulations [12], as heat ESSs can pose safety risks such as respiratory problems or eye irritation due to some PCMs. Regulations can add complexity and costs to the design and installation of heat ESSs in urban areas.
The relevant regulations have not yet been determined. The installation of heat ESSs in urban areas often requires various permits and regulatory approvals. This process can involve navigating a complex landscape regulation, which can be time-consuming and costly [13]. Furthermore, local authorities may have conflicting requirements and varying levels of expertise to assess the safety and impact of heat ESSs. Integrating heat ESSs into existing energy systems in urban areas can also pose challenges. For example, the HS system may need to be interconnected with the local electrical or heating grid, which may require regulatory approval and negotiation of tariffs and service agreements with utility companies.
Safety must be ensured by the companies that install the equipment by using nontoxic materials for storage, and protecting living organisms from the high-pressure containers that hold materials at relatively high temperatures. Technical requirements refer to meeting the regulations for high-pressure and mid-range temperatures in equipment, and achieving the connection, measurements, and control in the distribution [14]. Furthermore, high-volume units are required for storage. Underground applications can have an impact on the environment. There is rare use of toxic materials. The main challenges are increasing HS capacities and their integration into the energy system with cogeneration (use of the ORC or other types of heat pumps). Moreover, there is a need to provide heat for district heating systems connected to polygeneration systems.

3. Electricity Storage (Electrical and Electrochemical Energy Storage)

In 2019, the International Energy Agency (IEA) [3] reported that only 3–4% of electricity generated from RES globally is stored. To avoid a temperature rise of more than 2 °C, energy storage must increase by approximately 280% from 160 GW (2021) to 450 GW by 2050. Batteries have a few strengths that make them a favorable mode of energy storage. They are good for distributed storage, and present excellent configurability, high response time, and high energy efficiency and density [2].

Technical Requirements for Applications

The legal regulations for installing and operating electricity storage systems are widely analyzed due to the many potential problems that these units can cause to the building, neighborhood, and environment when using batteries. Lithium ion, lead acid, lithium iron or other battery technologies can cause acid leakage, toxic material emissions, and explosions. Compressed air represents an alternative electricity storage technology where electricity is used to compress air and store it, often in underground caverns. When electricity demand is high, the pressurized air is released to generate electricity through an expansion turbine generator. Moreover, electricity can be used to accelerate a flywheel through which the energy is conserved as kinetic rotational energy. When the energy is needed, the spinning force of the flywheel is used to turn a generator, providing an additional storage means (United States Environmental Protection Agency, 2023).
The EU Parliament accepted the directive of the EU Council related to common rules in the internal market for electricity (Directive 2012/27/EU). Article 36 is related to distribution system operators’ ownership of storage units [15], according to which, distribution system operators cannot own, develop, manage, or operate storage facilities. There are exceptions where storage is fully integrated into the network, storage facilities are an essential part of the distribution system, or the regulation authorities determine it as a necessity. The EU Commission has only published, in 2019, a few regulations on electricity storage and its importance. They are related to constructing a globally integrated, sustainable, and competitive industrial base for batteries in the EU (COM/2019/176) [16][17].
Furthermore, the EU prepared the Batteries Europe platform based on the previous Strategic Energy Technology Plan (SET Plan). According to these plans, the focus is on creating sustainable batteries, including by researchers and stakeholders [18]. The year 2021 was the switch point for energy storage. That was the year when the energy crisis started. That was a reason for finding better solutions for energy storage and regulating those systems.
The implementation of projects for energy storage in urban areas in North America faces legal barriers [19]. Any energy storage installation within a local government area must be permitted, inspected, and approved. For example, indoor or outdoor installations must meet local requirements. Storage system host sites may also impose restrictions on available, permutable space for energy storage that does not meet local jurisdictional requirements. Local governments are still learning about energy storage and developing relevant regulations like fire and safety procedures. As a result, the energy storage installation may take longer as local government procedures catch up with technology adoption. Storage policy integration is a versatile choice that can provide multiple revenues to building owners and developers. However, in some cases, intentionally or unintentionally, policies and programs exclude ESSs. The operating model for ISO-New England wholesale market programs was not designed to accommodate smaller distributed storage resources because it was designed to work with PHS.
Furthermore, some programs have requirements for behind-the-meter (BTM) storage projects [20]. The Federal Energy Regulatory Commission [21] has mandated that independent system operators review and update their policies to accommodate advanced energy storage. As a result, policies in the United States are expected to evolve in the coming years.
The connection of electricity storage systems to the local electricity grid can also pose challenges, as grid operators may have different requirements for connection and operation. For example, some grid operators may require the installation of specialized equipment, such as inverters, to ensure that stored electricity can be safely and efficiently integrated into the grid [22]. In some cases, electricity storage systems may also be subject to net metering policies, which regulate the flow of electricity between the storage system and the local grid. These policies can vary between states and countries and can impact the economy of electricity storage systems, making it more difficult for urban residents to adopt these systems. The operation of electricity storage systems can also be subject to environmental regulations, such as regulations on the disposal of batteries and other components at the end of their useful life. These regulations can add complexity and cost to the operation of electricity storage systems in urban areas.
Electricity prices vary at separate times of the day. Therefore, it is sensible for systems to charge batteries with cheap electricity and discharge them when the price is high. The situation is different in the Netherlands, where the feed-in tariff for electricity is identical to the electricity price.
The main characteristics of batteries are connected with their lifetime and the number of charging/discharging cycles. The negative side of batteries is their short lifetime and the negative influence of low temperatures on the functioning of batteries (cold-climate regions). BES is already included in transportation systems (public and private electric or hybrid vehicles), and its use is significant in the RES demanded by cities. The selection of the battery type can be exceedingly difficult due to rapid and fast changes in the technologies used for electricity storage.
The charge/discharge cycles are limited. Guney and Tepe [23] determined the storage system size and the discharge time. They divided them into three categories: uninterruptible power supply (UPS), T&D grid support, and bulk power management. In the category of UPS power, there are ”located systems” up to 100 kW: high-energy supercapacitors, high-power supercapacitors, high-power flywheels, Ni-MH, Ni-Cd, lead-acid batteries, Li-ion, and flow batteries (Zn-Cl, Zn-Br, vanadium redox). For the second category, T&D grid support includes high-capacity batteries from the first group (high-power supercapacitors, high-power flywheels, Ni-MH, Ni-Cd, Lead-acid batteries, Li-ion, and flow batteries (Zn-Cl, Zn-Br, vanadium redox)), NaNiCl2, advanced lead-acid batteries, and NaS batteries. The third category, “bulk power management”, includes only PHS and CAES. The last group and some flow batteries have discharge times measured in hours. In the second group, in general, modules discharge within minutes. The lowest discharge time has high-power flywheels and capacitors, measured in seconds. The capacity is limited.
The storage of electricity produced from RES in urban areas with rechargeable batteries can pose various risks that must be considered to ensure safe and reliable operation. Some critical risks are associated with using rechargeable batteries for energy storage in urban areas. These include battery degradation, fire risk, environmental impact, and grid connection [24]. Rechargeable batteries can degrade over time, reducing their capacity to store and discharge electricity. This can lead to reduced system performance and efficiency and may also pose a risk to the system’s safety. Rechargeable batteries can pose a fire risk if not handled, stored, and transported correctly. For example, batteries can overheat or catch fire if damaged, exposed to high temperatures, or subjected to overcharging or over-discharging. The production and disposal of rechargeable batteries can significantly impact the environment, including releasing toxic substances and using finite natural resources. The connection of rechargeable battery storage systems to the local electricity grid can also pose challenges, including the need for specialized equipment, such as inverters, to ensure that the stored electricity can be safely and efficiently integrated into the grid. Rechargeable batteries must be equipped with the appropriate battery management systems to ensure that batteries are safely and efficiently charged, discharged, and monitored [25]. Battery management systems must also be able to detect and respond to any potential safety incidents, such as overcharging or over-discharging, to prevent damage to the batteries or other components of the ESS. Rechargeable battery storage systems must be equipped with appropriate energy management systems to ensure that stored electricity can be effectively integrated into the local grid. Energy management systems must dynamically balance the supply and demand of electricity, considering the availability of RES, such as wind and solar power, and the urban district’s electrical load.
Better City and the Cadmus Company [19] presented the advantages and disadvantages of several types of batteries. Lead-acid batteries are a type of electrochemical battery storage that uses a chemical reaction to store and release energy. The most common types are sealed, flooded, valve-regulated, absorbent glass mat, and gel. The structure and used additives provide a variation in lifetime and performances as benefits. Examples of applications include resilience, limited grid support, peak load management, renewable energy stabilization, and UPS. With an estimated 150–300 $/kWh cost, the life expectancy is 5–10 years. Benefits include being well-known, reliable technology, which can withstand deep discharges, but with a shorter life expectancy and a lower cost. The disadvantages include a reduced life expectancy due to fewer useful cycles. They have a lower energy density, meaning that more space is needed to store the same amount of energy as other technologies.
Lithium-ion batteries [3] are a type of electrochemical battery storage that stores and releases energy through a chemical reaction. There are numerous variations, but all contain lithium, cobalt, nickel, manganese, and aluminum. Similarly, they are used as lead batteries for resiliency, grid support, peak load shifting, renewable energy firming, and UPS. The expected lifetime is 10–15 years, costing 250–1500 $/kWh. The advantages are high energy density that allows for high-power applications, deep discharges, and a long cycle life. These allow more intensive use and a longer life. The disadvantages are higher price than traditional ESSs, requiring a sophisticated control system to mitigate fire risk, materials are not readily recyclable, and toxic waste is generated.
Flow batteries [26] represent another type of electrochemical storage that charges and discharges electricity by using a system of tanks, pumps, dissolved chemicals, and chemical reactions. This technology is still in its initial development stage and is waiting commercialization. Its usage benefits are related to resiliency, grid support, peak load shifting, renewable energy firming, UPS, and bulk power management. It costs approximately 680 to 2000 $/kWh with a lifetime of 10 to 20 years. Benefits include being safe, easy to scale up, well suited for higher capacity (duration) uses, and having a long useful life. On the other hand, the disadvantages are relatively excessive costs, low efficiency (less than 70%), low energy density and thus taking up more space, and high maintenance due to pumps, which are currently in the initial stages of commercialization.
Batteries have a high environmental impact. This is especially significant in the case of battery disposal and recycling. Moreover, their production relates to environmental pollution and landscape modification: mining, manufacturing, use, collection, transportation, and storage. Batteries are not dangerous during their usage [2]. The most common primary types of batteries used for portable devices are not rechargeable. This means that they have no charge/discharge cycles; they are used only once. The metals used in the production of batteries include lead (Pb), lithium (Li), nickel (Ni), cobalt (Co), zinc (Zn), manganese (Mn), magnesium (Mg), mercury (Hg), silver (Ag), cadmium (Cd), vanadium (V), potassium (K), titanium (Ti), chromium (Cr), sodium (Na), tin (Sn), aluminum (Al), iron (Fe), copper (Cu), indium (In), silicon (Si), antimony (Sb), lanthanum (La), and cerium (Ce). The non-metals used include carbon or graphite (C), fluorine (F), chlorine (Cl), bromine (Br), sulfur (S), and germanium (Ge). Because of the geographical location of metal sources (often in unstable or controlled economies) and the depletion of the most accessible sources first, increased battery manufacturing impacts natural resource access and economics. Furthermore, some of these materials are valuable (Ag) and are used as currency, while others are expensive (In and Hg), or rare (La and Ce). Additional quantities of minerals from existing sources and discoveries must be generated to meet the increased demand for metals such as lead, zinc, lithium, aluminum, copper, and so on. The mining industry presents significant environmental and social issues, particularly in less developed countries with lax or corrupt regulatory oversight, and these may worsen if demand forces prices upward. All these factors and issues are indirectly related to battery usage. Therefore, batteries seem to be environmentally friendly, but they are highly polluting materials.
Batteries can be damaged during their usage, and leakage of their contents can cause damage to living organisms. Alkaline batteries contain corrosive electrolytes. Pb-acid batteries contain corrosive acids and very toxic lead. Similarly, very toxic materials are present in Ni-Cd batteries. Ni-MH can self-combust in the case of damage. Lithium batteries have a substantial risk of fire and explosion. GHG emissions per kg of battery are slightly higher than direct CO2 emissions, and Pb-A emits the least CO2. The average emissions for each battery are less than 30 g/kg of battery, excluding SOx emissions from Ni-MH and Ni-Cd batteries. Furthermore, the relative average change among batteries for each emission is the same. In general, Pb-A batteries emit the fewest contaminant emissions of any battery.
Raugei et al. [27] analyzed the impact of using electricity storage on PV with Lithium-ion batteries on the LC global warming potential within their storage duration scenarios. They negatively impacted the environment and increased the warming potential between 7 and 30% compared to PV systems with no electricity storage system. Energy storage in batteries is approximately 400 $/kWh (in 2016), rapidly increasing in the forthcoming years. Due to these facts, adding lithium-ion battery storage to photovoltaics does not impact their overall sustainability.

4. Chemical Energy Storage

Chemical energy storage involves using chemical compounds and the chemical reactions between them for storing any other form of energy. Some compounds can be very reactive and some of them very explosive. Therefore, this kind of energy storage requires caution. The use of chemical storage systems, such as propane, butane, ethanol, biodiesel, and hydrogen in urban areas is subject to various legal and regulatory challenges, which can limit their widespread adoption. This is related to current regulations and standards regarding storage and the transport of these components.
The most common fuels used in urban areas are coal, natural gas, diesel, gasoline, liquefied petroleum gas (LPG), propane, butane, ethanol, biodiesel, and hydrogen [28]. All these fuels are sources of chemical energy. Only ethanol, biodiesel, hydrogen, and some hydrocarbons can be produced from other materials as renewables. Therefore, they are popular as clean or sustainable fuels for urban areas. Regarding chemical energy storage, hydrogen and Synthetic Natural Gas are the most suitable chemical energy storage materials [29]. Recent research has developed mechanisms that use reversible thermochemical reactions. Specific reactants are ammonia on one side and hydrogen with nitrogen on the other side of the reaction. The reversibility of chemical reactions in both directions comprises the charging/discharging cycles [30].
Hydrogen as a product of energy used in chemical or electrochemical processes improves environmental performances with minor negative effects [31]. The efficiency is low due to the low energy density (low enthalpy of oxidation). In contrast to hydrocarbons, hydrogen has the highest enthalpy at 142 kJ/kg. Hydrogen can exist in three different forms due to the production process [10]. Hydrogen produced from fossil fuels is grey hydrogen. Blue hydrogen is produced from fossil fuels where the carbon emissions are captured and reused. Finally, green hydrogen is produced from RES without carbon emissions. According to the Det Norske Veritas group (DNV) report [32], the total hydrogen production is 5% of the estimated global energy demand for 2050. That is one-third of the hydrogen needed for net zero emissions. Hydrogen has a fast penetration in the market. The main advantages of hydrogen energy storage are that it can be stored for a long time with low losses, and the combustion product is pure water. [32]. Hydrogen storage uses hydrogen pressurization and the absorption of metal hydrides, adsorption of hydrogen on carbon nanofibers, and hydrogen liquefaction. Pressurized hydrogen can be stored at 200–250 bar (sometimes 350 bar). The efficiency of this storage system depends on its high storage pressure. In the case of 700 bar storage, extremely high energy is used [33]. Similarly, metal hydrides depend on the ability of their components to adsorb hydrogen. Hydrogen stored in metal hydrides and pressurized hydrogen require cooling and heating during charging and discharging in storage tanks. This is a large part of the energy lost, or it can be used for other purposes. The concept of Power-to-X gives a product X that is efficiently distributed on a large scale of energy storage for extended periods [34]. Fuel cells are quite interesting devices for converting hydrogen into electricity. This is a clean and efficient way of producing electricity, except when hydrocarbons are used as fuel. In the case of hydrocarbons, carbon dioxide is produced as an emission from processes in the fuel cell. Fuel cells are excellent for long-term energy storage, but their price is extremely high because of the components used, such as platinum [35]. The key issues are the cost of storage, storage period, and repetition of charge/discharge cycles. Regarding energy storage, fuel cells absorb stored chemical energy and convert it directly into electricity by the use of a specific fuel type. [36]. Fuels can be hydrogen used with oxygen as fluids that pass through different electrodes (anode and cathode), form water, and generate electricity with some amount of heat [37][38].

Technical Requirements for Applications

According to the EU Commission, hydrogen as fuel and an energy storage medium is high on the list of plans in the EU energy system. The first regulation regarding hydrogen for energy storage was announced in 2020 as the “EU strategy on Hydrogen” (COM/2020/301) [39]. The main concern is the source of hydrogen (it must be from RES). The EU Commission proposed the establishment of a global European hydrogen facility (EU hydrogen Bank) to create investment security and business opportunities for European and global renewable hydrogen production (COM (2023)156) [39]. This shows the importance in EU policy of hydrogen as a fuel and energy storage material. There are also some initiatives such as “Clean Hydrogen Partnership”, “European Clean Hydrogen Alliance”, and “Hydrogen Public Funding Compass” involved in future research and implementation of the use of hydrogen in the energy system [39].
The storage capacity when using hydrogen is limited. Its volume can take up a significant part of facilities. Fuel cells, especially hydrogen-based fuel cells, have energy efficiencies of approx. 60%. When formed waste heat is used, its overall efficiency is up to 90% [40]. The capacity of fuel cells starts at 100 kilowatts and can be extended flexibly to a range of thousands of watts. Thanks to their plug-and-play design, fuel cells have a fast and simple installation process. They are suitable for flexible integration into different systems. Hydrogen fuel cells have a minimal impact on the environment and are suitable for urban environments. They are good as an energy resource for clean, pollution-free, highly efficient, flexible, and promising microgrid applications. They provide continuous operation and do not require recharging [41]. This makes them an essential facilitator for energy transition, especially in urban areas, buildings, industrial facilities, and data centers. However, it should be noted that while fuel cells have many benefits, they also have challenges to overcome, including fuel cell costs and hydrogen storage and distribution [42].
Another technology uses solar radiation for direct generation of hydrogen. Ru(bpy)2+ can split water into hydrogen and oxygen via a photoreaction upon exposure to the Sun [31]. Formic acid in the presence of Ruthenium forms hydrogen and carbon dioxide. Carbohydrates can be transformed via a chemical reaction to produce hydrogen. The storage density in carbohydrates is the best for hydrogen chemical storage. A similar reaction can occur for semiconductors. Hydrogen can be used in two forms for energy storage, namely gas or liquefied, and as a solid in the form of metal hydride. The liquification requires unique cryogenic methods and high-pressure equipment. Due to this, the infrastructure cost for storage and distribution is extremely high. It also includes electronic devices for control and safety. Energy stored in hydrogen can be used directly as hydrogen in fuel cells and special combustion engines for cars. Furthermore, hydrogen can be used for heat generation in later processes (Rankine cycles).
Hydrogen storage depends on the form in which it needs to be stored. This can be as pure hydrogen in the form of pressurized and liquefied hydrogen but also connected within a molecule of chemical components such as metal hydrides, ammonia, formic acid, carbohydrates, liquid organic hydrogen carriers, physisorption, carbon materials (fullerenes, nanotubes, grapheme), zeolites (metal–organic frameworks, covalent organic frameworks, microporous metal coordination materials, clathrate), glass capillary arrays, glass microspheres, and organo-transition metal complexes. Physisorption is a method for hydrogen storage in which different porous materials absorb hydrogen on the surface. The best results have been obtained with nanotubes and zeolites. Compressed hydrogen is a simple form of storage using compression but is inefficiently related to volumetricity and gravimetry. However, cryogenic liquification can reduce the volume by more than half. Therefore, the second form of hydrogen storage is more efficient. Chemical reactions, such as ammonia (the reversible reaction of hydrogen and nitrogen gases), are also used for hydrogen storage. Another reaction is the formation of a metal hydride. These products can release hydrogen upon heating. They are safe for storage, but some are not safe for humans. They are toxic or irritants. Moreover, they are aggressive in the presence of moist air. The storage efficiency is 2% of the total mass.
According to the study by Niaz et al. [31] the cheapest hydrogen storage is liquefied hydrogen storage (approximately 6 $/kWh). More expensive is storage as metal hydride (approximately 8–16 $/kWh).

5. Other Types of Storage

Other Types of Storage Technologies Used in Urban Areas

Mechanical energy storage is another type of energy storage, but it requires specific conditions and configuration of the environment. CAES uses compressed air for energy storage via the process of compression. As previously mentioned, this type of energy storage requires underground space (caves) [43]. PHS can be considered an environmentally friendly technology but with some concerns in terms of biodiversity. These systems are exceptionally reliable and have a long lifetime. The storage efficiency is the highest compared to the new energy storage technologies. PHS has options for wind turbine parks for the storage of generated electricity by using small-sized reservoirs [43].
Nevertheless, PHS has become less popular due to the impacts on nature. The largest energy storage capacities by their type are given in Figure 2 (percentage of stored energy by type of storage facility built in European countries) and Figure 3 (percentage by the number of plants for energy storage based on the type of storage system in European countries). Mechanical energy storage is the most efficient and used form of energy storage in general, but it is primarily suitable for industrial-sized facilities [44]
Figure 2. Chart of the percentage of stored energy by type in storage facilities built in European countries [44].
Figure 3. Chart of the percentage by number of plants for energy storage based on the type of storage system in European countries [44].
It must be mentioned that all these energy storage plants are not related to urban areas. They are in general use. Furthermore, some of them can be regarded as “virtual” ESSs.
According to the analyzed data, all the mentioned ESSs are compared on the basis of differences in energy density, power density, storage duration, costs, regulations, technical requirements, environmental impact, safety, and risks. The main aspects of these comparisons are given in Table 1.
Table 1. Summarized comparison of data for different energy storage systems.


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