What is energy storage molecule made of?

6.1.2 Role of Chemical Energy Storage

As seen from Fig. 6.2, chemical energy storage technologies are mainly constituted by batteries (secondary and flow batteries) and renewable generated chemicals (hydrogen, fuel cell, SNG, and hydrocarbons). Batteries as electrochemical energy storage bring great promise in a range of small-scale to large-scale applications. The applications span from portable consumer electronics such as mobile phones, game pads, and tablet computers via mobile applications to electric vehicles, forklift trucks, boats, and emergency power in power plants at large-scale grid-based applications for stabilization and load balancing of intermittent, renewable sources like solar and wind energy. The battery technology can also be applied to aviation, distant transport of goods, and seasonal energy storage with further developments. Among batteries, the Li-ion chemistry can be used for low- and moderate-energy-density applications, such as in typical portable applications. The lithium-based batteries have potential large market share of batteries for transportation and mobility. As the usage of portable devices powered by battery increases and develops into new applications, a demand for new and specialized battery will be inevitable. For the sustainable energy infrastructure of the future, the greatest prospect for electrochemical storage is mostly on the stabilization of frequency and voltage in dealing with hourly and daily fluctuations. Battery technologies can be competitive for transport and stationary applications with substantial improvements in existing battery technologies and the development of novel high-performance batteries.

The chemicals have much greater energy density than the energy density of current battery technologies and have large discharge times since they can be stored for any time period. Hence, the applications of chemicals are varied, and specifically, they can be used for large-scale systems with applications for large duration. They can be used as raw materials for the chemical industry, for direct electricity production, and for the transportation sector as replacement fuel instead of fossil fuel. Synthetic fuels produced from sustainable energy can compliment or supplement batteries in the transportation sector. Examples are, hydrogen produced from solar energy, the forming of ammonia with nitrogen and methane, or methanol produced by electrochemical fixation of carbon dioxide. Thus, there will be substantial demand for chemical energy storage for the future renewable-resource-based energy sector.

In electric energy sector, excess electricity can be used to produce hydrogen with electrolysis to stabilize electric power to cope with demand changes. Hydrogen is an ideal molecule either to store itself as energy storage chemical or to process other storage molecules such as liquid hydrocarbons. Gasified biomass and carbon-containing waste fractions are other resources of renewable energy that can be used in the stabilization of fluctuating electricity production if produced in large capacity. Gasification is a well-known technology where the reaction of carbonaceous raw materials with steam at high temperature is carried out to produce SNG (mainly CO, CO2, and H2). SNG can be used to leverage the demand in electricity. During the period when electricity demands are met by renewables such as the sun and wind, then the SNG can be processed using catalytic processes into various fuels and chemicals such as methane, methanol, and synthetic gasoline and diesel. When there is more demand for electricity, the SNG can be directly used to produce electricity though combustion.

The syngas immediately produced in gasification has low hydrogen content than required for use as SNG; hence, it should be boosted with hydrogen, which can be produced from electrolysis of water. Similarly, many types of biomass have low energy density that can be upgraded by treatment with hydrogen, which in turn can be produced from water electrolysis. These technologies rely on catalysis and electrolysis that will play pivotal roles in storage routes. The chemical energy storage in the form of gaseous hydrogen or methane facilitate synthesis of SNG and hydrogen produced from electrolysis to liquid fuels such as dimethyl ether, methanol, and other liquid hydrocarbons to supply fuels to sectors such as aviation and heavy road transport. The production of liquid hydrocarbons fuels from electrolysis generated hydrogen and biomass would help reduce the dependence on fossil oil and to expand the use of surplus electric power into the aviation and heavy road transport segment.

Abstract

This chapter discusses the state of the art in chemical energy storage, defined as the utilization of chemical species or materials from which energy can be extracted immediately or latently through the process of physical sorption, chemical sorption, intercalation, electrochemical, or chemical transformation. Storing electricity directly in batteries or capacitors from wind and solar at scale is challenging because even the most advanced electrochemical or charge storage devices, such as lithium ion batteries or ultracapacitors, have relatively low volumetric energy densities compared with liquid fuels like diesel, gasoline, or liquid methane. Hydrogen as an energy carrier is arguably one alternative to replacing petroleum products for transportation and stationary applications, if it can be produced in large quantities by clean and renewable means. Other chemical storage through sorption or chemical transformation provides advantages and viable alternatives to mechanical or thermal energy storage.

Chemical Energy Storage Systems—Power-to-X

Chemical energy storage in the form of biomass, coal, and gas is crucial for the current energy generation system. It will also be an essential component of the future renewable energy system. With each facility ranging in the terawatt-hours, chemical energy storage has by far the largest capacity. It is also the only option for seasonal energy storage using the charging technology power-to-gas in combination with the existing gas infrastructure for storing and converting gas into electricity.

Energy stored in the form of hydrogen or methane can be used by all three sectors—electricity, heating, and transport. There is already a large existing infrastructure for transporting, distributing, and exploiting methane in gas-fired boilers and CHP in the heating sector; in gas vehicles and gas ships (LNG) in the transport sector; and in gas turbines, combined cycle gas turbine plants, and CHP in the electricity sector. The infrastructure for hydrogen is still under development, and currently only available in chemistry parks. Surplus energy from renewable energy sources can be temporarily stored in the gas network or in gas storage facilities, and then supplied to other locations when demand is higher.

Only chemical energy storage can combine energy storage and energy transport at this scale. The transmission capacity of a large gas pipeline is about 10 times greater than that of a high-voltage transmission line. There is also significantly greater public support for expanding the gas network than for expanding the electricity network. But electricity transmission is more efficient than converting electricity into gas and then transporting the gas. Expansion costs and operations could be minimized by synchronizing the two networks. The strengths and weaknesses of electricity networks and gas networks complement each other quite well.

Another option with chemical energy storage is to convert electricity into basic chemical materials (methanol) or liquid fuels (power-to-liquid). These liquid fuels would be particularly useful in transport segments requiring high energy densities such as aviation (Fig. 11).

1.4 Thermochemical storage

Chemical energy storage systems utilize the enthalpy change of a reversible chemical reaction. The interest in these systems is motivated by the option to store energy at higher energy densities compared with other TES types [79–85]. Other attractive features of thermochemical storage are:

It is feasible to store reactants and products at ambient temperature to avoid thermal losses. This feature is attractive for long-term storage (e.g., seasonal).

Compared with sensible and latent heat systems, thermochemical storage can have different temperature levels for charging and discharging via pressure changes in the gas phase. Hence, it is feasible to upgrade and downgrade heat and supply heat at a suitable temperature level. Such systems are also known as heat transformers and chemical heat pumps [11].

It is feasible to discharge thermochemical systems (e.g., from room temperature) simply by bringing the reactants together (e.g., opening a valve for gas supply).

The energy is stored in the form of chemical compounds A and B created by an endothermic reaction and is recovered again by recombining the compounds in an exothermic reaction to compound AB [Eq. (1.5)]. At high enough temperatures, the products A and B are spatially separated.

(1.5)AB⇔ΔHrA+B

The heat stored and released is equivalent to the heat (enthalpy) of reaction ΔHr. ΔHr is often larger than the enthalpy of transition in latent heat storage or the sensible heat stored over a reasonable temperature span. Hence, the storage density, based on solid mass or volume, can be larger for thermochemical storage materials than for latent or sensible heat storage materials. Many thermochemical energy storage concepts are in an earlier stage of development compared with sensible and latent heat systems. In the low-temperature range (<150°C), thermochemical energy storage is commercially utilized in niche markets (e.g., sorption systems).

The potential of thermochemical storage was identified early during the evolution of CSP technology [86–88]. Many groups actively investigate solar driven chemical processes. These processes aim for production of solar fuel and useful chemical products. In a general sense, these approaches also form an energy storage system. In the scope of this chapter, solar driven chemical processes are not further considered.

The thermochemical storage system can be classified into two major categories. Open-type systems exchange gases with the environment. During charging, gases are released in the environment. During discharging, a gas from the environment is utilized. Hence, these systems can operate without gas compression and storage, and this simplifies the system design. Gases of interest include oxygen, nitrogen, water vapor, and potentially carbon dioxide. Open systems may introduce unwanted substances from the environment. Impurities may be dust, sulfur dioxide, carbon dioxide, and organic compounds. Such substances may deteriorate the system performance. System designs with filters may avoid such difficulties. Storage of the unpressurized gas phase in a closed-type system is usually not feasible because of the unacceptable large gas volume. Commonly, closed-type systems compress or condense the gas. Subsequently, the pressurized gas or liquid can be conveniently stored. Alternatively, the gas may be reabsorbed by the second chemical reaction [81]. This discussion indicates that the overall system design rather than a single chemical reaction should be considered.

A sorption process can be considered a chemical reaction system based on weaker chemical bonds rather than covalent bonds encountered in other systems. In a sorption heat storage system, the sorbent is heated during charging, and vapor is desorbed from the sorbent. During discharging, vapor at a lower temperature is adsorbed (solid sorbent) or absorbed (liquid sorbent) and heat at a higher temperature can be released. In the low-temperature range, adsorption processes using water with materials such as silica gel and zeolites are utilized. Another low-temperature option is the absorption reaction. Typical absorption materials for water are MgSO4, LiCl, LiBr, CaCl2, MgCl2, KOH, and NaOH [10,89,90]. For high-temperature applications, sorption systems are typically not considered. Hence, sorption is not discussed further here.

Fig. 1.11 classifies the reversible reactions in three physical phases, namely solid–gas, liquid–gas, and gas–gas reactions. For the temperature range above 300°C, mainly solid–gas and to some extend liquid–gas reactions are relevant.

1.4.3 Storage integration and applications

For CSP, several high-temperature thermochemical storage systems have been researched. The dissociation of calcium hydroxide (Ca(OH)2↔CaO+H2O) has been investigated and reversibility of the reaction could be proven with a reaction enthalpy of 104 kJ/mol and an equilibrium temperature of 505°C at 1 bar [91]. Fixed bed and gravity-assisted moving bulk reactor on a scale of 10–100 kWh have been tested at relevant process parameter for integration in a CSP plant with Ranke power cycle [92].

Solid media sensible heat storage is the classical TES option for CSP central air receivers. As an alternative to solid media storage systems metal oxides thermochemical storage are examined. Several reactor designs were studied in lab on a small-prototype scale (e.g., rotary drum, fixed bed, gravity-assisted moving bulk reactor) with material systems such as manganese oxide or cobalt oxide [93–97].

Applications in the areas of storage and transportation could utilize gas–gas reactions. The reaction products are transported in a pipeline from the charging source to consumer with a discharging sink. This system is called chemical heat pipeline or chemical heat pipe (Fig. 1.12).

Figure 1.12. Scheme of the simplified principle of a chemical heat pipeline.

For the CSP-catalyzed gas phase reactions for ammonia, sulfur trioxide and methane reforming are of interest (Table 1.2). The methane reforming cycle of up to 10 MW was developed for nuclear application and is known as the EVA/ADAM process [20]. On-sun prototype reactors with a power level of 10 kW to several 100 kW were tested [97,99]. The main drawback of the approach of heterogeneously catalyzed gas phase reactions in terms of storage is the requirement to separate, compress, and store gases thus adding to the cost and complexity of the process.

This disadvantage can be overcome by additional reaction steps as followed for the sulfur based cycle with multiple reactions (Table 1.2) [100–102]. The cycle consists of on-sun sulfuric acid evaporation and decomposition, sulfur dioxide disproportionation, and efficient sulfur combustion [102–105]. Several prototype reactors for the different reaction steps have been tested [84]. A large-scale experience of many components of this cycle exists from sulfuric acid plants. The storage capacity of this cycle originates from sulfuric acid and sulfur storage volumes. In addition, other cycles for hydrogen production, such as the sulfur–iodine cycle [106] and the Westinghouse cycle [107], are examined.

2.23.5.4.2 Chemical energy storage systems

Regarding chemical energy storage, batteries are considered as the most common and representative technology. They are the most widely adopted storage technique used in many RES-based applications. Different battery types exist; each one has its own special characteristics, over a wide range of applications. The most mature battery types are the lead-acid (PbSO4) and the nickel-cadmium (Ni-Cd) batteries. Lead-acid batteries are characterized by their considerable self-discharge rate, low maintenance requirements, low energy-density, limited service period, low depth of discharge and considerable environmental impacts. Nickel-cadmium batteries are characterized by their higher energy density and self-discharge rate, deep discharge rate, longer service period, high capital cost, low efficiency rates and quite severe environmental impacts. More advanced battery technologies includes sodium-sulfur (Na-S), metal-air and lithium-ion (Li-ion) batteries. For sodium-sulfur batteries, an operating temperature of 300 °C is required, meaning that heat supply is necessary. On the other hand, sodium-sulfur batteries have no self-discharge, and efficiency and depth of discharge are quite high. Lithium-ion batteries have high energy density, a considerable number of charge-discharge cycles, and deep discharge rates. On the other hand, the main drawback of the technology is the high capital cost and the required protection circuits to maintain voltage and current within safety limits. Finally, metal-air batteries are characterized by high energy density, low system performance, short service period, low self-discharge rate, and very low system cost.

Flow batteries store energy by means of a reversible chemical reaction. The energy is stored in two liquid electrolyte solutions. The energy capacity and the rated power of the system are independent of one another. Energy capacity depends on the quantity of electrolytes used. Flow batteries are used in a number of large-scale and stand-alone RES installations. Different technologies of flow batteries exist (vanadium redox, polysulfide bromine, zinc-bromine) which are characterized by the different electrolytes used. The efficiency of flow batteries ranges between 60% and 80%, with future prospects ensuring high cycling capacity and deep discharge rates. Finally, the significant environmental impacts should also be considered as a drawback of the technology.

Production of hydrogen is one of the ideal methods for the absorption of intermittent/stochastic RES such as wind energy. In FC-HS systems, the renewable energy is converted to fuel (hydrogen), which is stored in appropriate storage tanks. The storage capacity depends only on the amount of the hydrogen that need to be stored and is theoretically independent of the fuel cell׳s nominal power. During the discharging procedure, hydrogen is released from the storage tank and is fed to the fuel cell unit, which then generates electricity. The main drawback of the FC-HS systems is the low charge-discharge cycle efficiency estimated to be between 30% and 40%, including the losses during both the electrolysis to produce hydrogen and the storage stage. The advantages of the technology, on the other hand, include the low energy cost, the high energy density, and the negligible self-discharge rate.

Battery Storage

Electro-chemical energy storage has been in use for many years in the form of lead-acid batteries. The “New Technology” battery systems may eventually replace the lead-acid type, although development times of 5 to 10 years are frequently quoted. The concensus within utilities and the battery industry is that lead-acid batteries will be the only viable option for the next five years for systems of 1 MWh and above. Even then the advanced lead-acid battery may compete favorably with the new technology batteries, depending on the particular application.

The response time of a battery system is of the order of tens of milliseconds, and the capital costs of installation are low, compared to the flywheel for example. Drawbacks include the maintenance requirements and the relatively low number of charge/discharge cycles before cell replacement is required (2000 to 3000 cycles). Also, long charging periods are required.

The estimated installed capacity costs for the Advanced Battery systems change frequently in the published literature. The ranges of costs for Lead Acid and Advanced batteries are given in Table 1.

7.2.1.5 Batteries

Even though chemical energy storage using PtG or PtL is specifically reviewed in Chapter 5, hybrid energy storage system can significantly enhance the storage characteristics. For example, fuel cells or electrolyzers usually have relatively slow response to electrical power input (second-level). The peak power of the fuel cells or electrolyzers is also limited for longer lifetime. Battery storage is a typical representative of electrical energy storage with fast response and high peak power. Consequently, batteries show high complementary with fuel storage based on PtG or PtL.

Batteries are also a complex electrochemical device. Similarly, a physical battery model should consider charge transfer, mass transfer, heat transfer, as well as electrochemical reactions. A detailed physical model is used in our previously built dynamic system model [2]. This model reveals high accuracy and is capable of predicting the battery characteristics at various temperature and charge-discharging current in dynamic operation. In Ref. [2], we calibrated this model and validated it with a LiMn2O4 battery with a capacity of 11.5 Ah. The comparison between model prediction and experimental results is shown in Fig. 7.3. The battery voltage with time fitted well with experimental data at different charging current and temperature.

Another common battery model is the equivalent circuit model. Compared with the physical model, the equivalent circuit model can significantly simplify the model and reduce the calculation quantity. A well-validated equivalent circuit model can predict the battery dynamic response well, further is applicable for system design, integration, and optimization. The model also should be applicable for multiple battery suppliers and types with easy achieved model parameters. Fig. 7.4 gives a typical equivalent circuit of a battery model. The state-of-charge (SOC) is represented as a function of initial capacity, current history, time, and temperature. Then, the OCV can be calculated by look up table from the SOC value and temperature. The transient response of battery will consider the effects of double layer capacitance and diffusion capacitance (pseudo capacitance). This model takes the advantages of high accuracy, clear physical interpretation and straightforward derivation of model parameters from experimental tests and all of the elements in the circuit are responsible for certain physical-chemical phenomenon. But the self-discharge process was neglected, thus, the prediction for the long-term operation is less accurate.

Similarly to the stack-scale submodel of the fuel cells or electrolyzers, the battery model can be connected in parallel or series to scale up in the premise of the uniform assumption.

3.6.6 Battery

Battery is a chemical ES form and comes in many types, mainly lead-acid battery, nickel-cadmium battery, lithium-ion battery, sodium sulfur battery, and vanadium redox flow battery.

Lead-acid battery: Its lifetime will be reduced when working at a high temperature. Similar to a nickel-cadmium battery, it has a low specific energy and specific power, but is advantageous because of its low price and cost, high reliability, and mature technology and has been widely used in electric power systems. However, it has a short lifetime and causes environmental pollution during manufacture. It is mainly used as the power source for closing of circuit breakers during system operation, and an independent power source for relay protection, driver motor, communication, and emergency lighting in the event of failure of power plants or substations.

Nickel-cadmium battery: It has a high efficiency and long lifetime, but the capacity decreases as time goes by, and the charge retention needs to be enhanced. Furthermore, it has been restricted by the EU due to heavy metal pollution. It is rarely used in electric power systems.

Lithium-ion battery: It has a high specific energy and specific power, little self-discharge, and causes no pollution. However, due to the influence of the process and difference in ambient temperature, the system indices are more often worse than those of a cell, and in particular, the lifetime is several times or even more than 10 times shorter than that of a cell. What is more, integration of a high capacity is very difficult and the cost for manufacture and maintenance very high. In spite of this, the lithium-ion battery is expected to be widely used in DG and the microgrid thanks to advancements of technologies and reduction of costs.

Sodium sulfur battery: Owing to its high energy density, its size is just 1/5 of a lead-acid battery while the efficiency is up to 80%, contributing to convenient modular design, transportation, and installation. It can be installed by stage according to the intended purpose and capacity, and suits urban substations and special loads. It is a promising ES technology for DG and the microgrid in improving the system stability, shifting loads, and maintaining power supply in an emergency.

Flow battery: Flow battery features slight electrochemical polarization, 100% discharge, long lifetime, and rated power independent of rated capacity. The capacity can be increased by adding electrolyte or increasing the concentration of the electrolyte. The storage form and pattern can be designed according to the location. It is a promising ES technology for DG and the microgrid in improving the system stability, shifting loads, and maintaining power supply in an emergency.

11.4.1 Reversible chemical reactions

The basic concept of chemical energy storage is to absorb excess heat in an endothermic reaction. The reaction products are stored separately. During the discharge process, the reaction products are recombined exothermically and the heat of reaction can be used. The concept is illustrated schematically in Fig. 11.19.

Catalytic gas-gas reactions represent one group of reactions considered for energy storage. An example for this group is the CH4 reforming-methanation reaction, which originates from activities aiming at the storage of heat generated from nuclear energy:

CH4gas+H2Ogas↔COgas+3H2Ogas

The endothermic reforming reaction is carried out in the solar receiver at temperatures between 800°C and 1200°C. The products are cooled to ambient temperature and stored at high pressure. The reaction is reversed in the methanator system providing heat in the temperature range of 350–700°C. There is a large body of work addressing the feasibility of this concept, culminating in a demonstration system using a volumetric receiver installed on a solar central receiver reaching a power level of 480 kW. The storage system shows a storage density of about 45 kWhth/m3. The group at CSIRO in Australia are continuing to investigate the solar-driven endothermic half of the system for ‘open loop’ solar value-adding to natural gas (Stein, Edwards, Hinkley, & Sattler, 2009).

Another example is the dissociation of ammonia

NH3↔1/2N2+3/2H2

which has been extensively investigated by the Solar Thermal Group at the Australian National University (ANU) (Lovegrove, Luzzi, Soldiani, & Kreetz, 2004). A receiver was operated at a power level of 15 kW with solar energy provided from a dish system. To enhance reaction rates in the exothermic ammonia synthesis reaction, system pressures up to 30 MPa are proposed. At 10 MPa, the volumetric storage capacity is in the range of 40 kWhth/m3. One of the major advantages of the ammonia-based system is that the heat recovering exothermic reaction is the well-known Haber Bosch process, employed on a major scale around the world for fertilizer and explosives production. Hence, there is a large scale proven reactor technology already commercially available.

Thermal dissociation of solids and liquids can also be applied for energy storage. By the addition of solar heat to a liquid or solid a gas is released. During the discharge process, the synthesis of the dissociation products provides energy. One example of this kind of reaction is the dehydration/hydration cycle:

CaOH2↔CaO+H2O

The storage capacity of this system is in the range of 300 kWh/m3 (Schaube, Woerner, & Tamme, 2010). Lab-scale experiments and options to integrate this storage technology into a CSP plant are described by Schmidt and Linder (2017).

1.2.4 Chemical energy storage materials

There is crossover between the TES and chemical energy storage. Abovementioned chemical adsorption/absorption materials and chemical reaction materials without sorption can also be regarded as chemical energy storage materials. Moreover, pure or mixed gas fuels are commonly used as energy storage materials, which are considered as chemical energy storage materials. The key factors for such kinds of chemical energy storage materials are as follows:

High calorific value;

Large density;

Easy to store and transport;

Compatible to the existing infrastructure;

Easy to produce and high round-trip efficiency;

Environment friendly.

Different chemical energy storage materials are listed as follows.

Hydrogen. Hydrogen is the most important alternative fuel to fossil fuels because it is clean and affordable. Hydrogen can be produced by electrolyzing water. The production process is normally called power to hydrogen. The produced hydrogen can be stored in the forms of high pressure gas, adsorbed gas by solids of large surface area, metal hydrides, alanates, and other light hydrides [17].

Methane. Power can be converted to methane through the reaction between hydrogen and CO2. The storage of methane can use existing infrastructure; the volumetric energy storage density of methane is nearly four times as large as that of hydrogen [18]; the power to methane process is also accompanied with reduction of CO2 emission.

Ammonia. Ammonia has been recently evoked as an alternative fuel source as well as chemical energy storage material. Ammonia has been massively produced in agriculture sector; the conventional manufacturing process releases large quantities of CO2. However, it can also be produced through renewable ways, like using hydrogen produced by water electrolysis and nitrogen from air. Ammonia can be converted back to power through fuel cell or combustion-based technology [19].