1 Large-Scale Energy Storage
An Overview
Huamin Zhang
1.1 GENERAL INTRODUCTION
Throughout history, the development of energy technology has changed human life. Every innovation in the area of energy is accompanied by the rapid progress of socially productive forces, the huge improvement of human life quality, and great changes in human environment. In summary, the use of energy for human society has experienced four revolutions. The use of fire was the first human energy revolution; for example, using firewood for cooking, heating, and defense. The use of fossil fuels (e.g., coal and oil) was the second human energy revolution. From the early eighteenth century, the discovery and exploitation of coal, as well as the invention of coal-fired steam engine technology, brought human society into industrial civilization. In the mid-nineteenth century, the large-scale production and use of oil brought about the first industrial revolution. Internal combustion engines, cars, planes, and other new inventions changed the style of life and work. Faraday’s electricity inventions were a revolution in human energy use. In the 1940s, the invention of the controlled nuclear reactor was the prelude to the third energy revolution. Then, in 1973, the first oil crisis indicated that the oil age would end, and the depletion of fossil energy was expected, as well as global climate change. The appeal of clean, renewable energy utilization is growing, and its purpose is to gradually replace fossil energy sources with renewable sources of energy (including hydroelectric energy, biomass energy, solar energy, wind energy, geothermal energy, marine energy, hydrogen energy, etc.), to ensure the sustainable development of humanity. The fourth human energy revolution has begun.
Energy can be divided into primary and secondary energy. Primary energy refers to energy resources that exist in nature, including fossil fuels such as coal, petroleum, natural gas, oil shale, shale gas, and coalbed methane, and nonfossil energy such as water, wind, solar, geothermal, thermal, wave, and tidal energy. Secondary energy is generated from primary energy, including electricity, hydrogen, steam, gas, ethanol, liquefied petroleum gas, gasoline, kerosene, diesel oil, and heavy oil.
On the other hand, energy can be divided into two other categories, nonrenewable energy and renewable energy, based on the self-sustainable and recovery characteristics. Nonrenewable energy sources include fossil fuels (e.g., coal, gas, oil, etc.) and nuclear energy. Nonrenewable energy is difficult to reproduce in the short term. Renewable energy sources, including hydroelectric, wind, solar, geothermal, thermal, wave, and tidal energy, can be reproduced repeatedly.
Figure 1.1 forecasts the statistics and trends of global primary energy demand, which is mainly constituted by nonrenewable fossil fuels such as coal, oil, and natural gas [1]. However, limited nonrenewable energy sources cannot guarantee the sustainable development of humanity. According to statistics, in 2009, the global consumption of primary energy was 12,132 Mtoe, while it was only 7,219 Mtoe in 1980 [1]. It is predicted that the proven reserves of global oil and gas will be
FIGURE 1.1 Global demand and forecast of primary energy (until 2009). (From Fatih Biro, L., World Energy Outlook 2010 [J/OL], 2010 [1].)
exhausted in 40 years and 60 years, respectively, at current consumption rates. In addition, the use process of fossil energy emits large amounts of sulfides, nitrides, CO2 , and greenhouse gases, leading to the deterioration of the global climate and increasing the frequency of extreme weather events.
The consensus is that the large-scale application of renewable energy sources, mainly solar and wind power, is necessary to guarantee the sustainable development of human society. Although humans have used wind and solar energy for a long time (e.g., windmills and solar cooking), the large-scale use of wind power only began in the twentieth century. Currently, wind power is growing quickly and might reach 474 GW by 2020, according to a World Energy Council forecast [2]. Besides that, the utilization of solar energy has developed quickly in recent years, including photothermal conversion (solar cookers, solar heating systems, and solar thermal power), photovoltaic (PV) conversion, and photocatalytic conversion (e.g., artificial photosynthesis, PV hydrogen production). Photoelectric conversion has been successfully commercialized, with comprehensive environmental benefits. However, the intermittent nature of wind and solar energy make it difficult to directly transport and dispatch through the power grid. How to ensure power quality, under the premise of the safe and stable operation of the power grid, becomes the bottleneck in the further development of renewable energies.
Another important utilization of large-scale energy technology is the construction of smart power grids. Due to the continuous improvement of people’s living standards, the demand for electricity has increased rapidly. The current global power generation capacity has reached about 15 TW. By the middle of this century, the installed capacity will be doubled. By the end of the century, the installed capacity will be three times that at the beginning of the century. The rapid growth of the power generation capacity needs to synchronize with the expansion of the transmission and distribution networks. However, the traditional power grid is usually designed for the maximum load, resulting in high grid infrastructure investment and
low system utilization, and it is difficult to accommodate grid expansion. Therefore, the smart grid as an important future energy infrastructure was proposed, with large-scale energy technology as a kernel component. In the power generation sector, it supports renewable energy and secure and efficient access to the grid; in the transmission and distribution sectors, it improves power utilization, delaying system upgrades; in electricity linking and load management, it increases user flexibility and the efficiency of energy use.
1.2 REQUIREMENTS OF LARGE-SCALE ENERGY STORAGE TECHNIQUES
Table 1.1 shows the advantages and disadvantages of various energy storage technologies. For large-scale energy storage technology used for renewable power systems such as wind energy and solar energy, the requirements for the system in the aspects of storage power and volume are enormous; therefore, for large-scale battery energy storage, the following issues need to be considered: safety, the cost performance of the life cycle, and the environmental effects of the life cycle.
Energy storage batteries used for the electric output generated by renewable energy, track schedule electric output, and intelligent control need to store power in huge volumes, unlike batteries used for mobile phones, computers, and electrical vehicles. Due to its large-scale storage power and volume, the safety of battery storage technology is one of the most important factors to be considered in practical applications. For example, in recent years, there have been several serious accidents
TABLE 1.1
Advantages and Disadvantages of Main Energy Storage Technologies
Energy Storage Technologies
Sodium–sulfur battery
Lithium-ion battery
Lead–acid battery
Supercapacitor
Pumped storage
Compressed air energy storage
Flywheel energy storage
Superconducting magnetic
energy storage
Flow battery
Advantages
High energy and power density
High coulombic efficiency, energy and power density
Mature technology and low cost
High power density and long cycle life
High capacity and mature technology
High capacity
High power density
High power density
Flexible design and safety
Disadvantages
High operating temperature and poor safety
Poor safety and short life cycle
Short cycle life and environmental pollution by lead
High cost and low energy density
Need appropriate geographical environment
Need appropriate geographical environment
Need vacuum and high requirements for flywheel material
Low-temperature operation and high cost
Low energy density
during the sample applications of sodium–sulfur batteries at the level of MW power. Meanwhile, large-scale battery energy storage technology needs to have long working times, easy maintenance, and high cost performance. With the wide application of large-scale energy storage, a large amount of batteries might fail to work, so the batteries need to be environmentally benign during their working life. The mechanisms of energy storage technologies, current developments, and challenges are introduced in the following sections.
1.3 TYPES AND CHARACTERISTICS OF LARGE ENERGY STORAGE TECHNIQUES
Over the last 100 years, various energy storage technologies have been researched and developed. Table 1.2 shows different energy storage technologies classified by the means of energy transformation. Basically, there are two types of energy storage technologies: physical storage and chemical storage. Physical energy storage mainly includes water-pumping technology, air compression technology, flywheel technology, superconducting technology, and supercapacitor technology. Chemical energy storage mainly includes lead–acid battery technology, flow battery technology, sodium–sulfur battery technology, lithium-ion battery technology, and so on.
As mentioned previously, water pumping, air compression, flow batteries, sodium–sulfur batteries, and lithium-ion batteries are considered to be suitable for power generation by renewable energy and large-scale energy storage in power systems. Water pumping, as one of the most proven technologies, is by far the most widely used for large-scale energy storage: 99% of the total power volume in energy storage systems worldwide was collected by water-pumping energy storage technology [1]. However, there are some problems related to the eco-environment and the location of factories that restrict the further development of water-pumping energy storage technology. According to current developments, advanced battery energy storage technology still cannot be used in large-scale commercial applications; however, some advanced battery energy storage technologies have met the requirements
TABLE 1.2
Energy Storage Technologies Classified by Conversion Forms
Category
Physical energy storage
Chemical energy storage
Conversion of Energy Form
Energy Storage Technology
Potential energy ⟺ Electric energy Hydropump
Compressed air
Kinetic energy ⟺ Electric energy Fly wheel
Electromagnetic energy ⟺ Electric energy
Electric energy ⟺ Electric energy
Superconductor
Supercapacitor
Chemical energy ⟺ Electric energy Flow battery
Sodium–sulfur battery
Lithium-ion battery
Lead–acid battery
for sample applications and show good potential for application to large-scale industrialization and commercialization.
1.4 DEVELOPMENT OF DIFFERENT ENERGY STORAGE TECHNIQUES
1.4.1 hydro -PumPing energy Storage technology
Since the first hydro-pumping energy storage station was built in Zurich, Switzerland, in 1882, the development of hydro-pumping energy storage technology has had a long history, and it is also very mature. The advantages of hydro-pumping energy storage are that it has a huge storage volume, a long working life, and low costs, and it occupies the vast majority of the installed capacity in the energy storage market. Hydro-pumping energy storage technology can deal with peak regulation, amplitude modulation, frequency modulation, and peak-load shifting, and it is also the most effective way to solve issues related to wind curtailment.
1.4.1.1 Mechanism of Hydro-Pumping Energy Storage Technology
Hydro-pumping energy storage stations are usually installed with a hydro-pumping and power generator set (Figure 1.2). During high-demand periods, the discharge of water from the reservoir will provide extra electric power to alleviate the shortage of electricity. During low-demand periods, water will be pumped up to the reservoir to store the extra electricity, which means it will be transformed as the potential energy of water in the reservoir. Therefore, this hydro-pumping and power generator set could help to transform electricity and potential energy to deal with the shortage of electricity in the peak period and store surplus electricity in the off-peak period. The supplementing of electricity can last from hours to a few days, and total energy efficiency can be about 75% [2]. Due to high electric prices in peak periods and low electric prices in off-peak periods, the economic benefits of hydro-pumping energy
Pumped-storage plant Visitors' center
FIGURE 1.2 Illustration of a hydro-pumping station. (From
storage stations can be improved. The purpose of a hydro-pumping energy storage station is to regulate peak-load shifting, amplitude modulation, and frequency modulation, deal with emergency accidents, and act as a standby station.
Hydro-pumping energy storage stations can be classified by different situations. For example, they can be classified into two categories by considering the issues related to natural water supplements.
1. Pure hydro-pumping energy storage stations: There is only a small amount of natural water flowing into the reservoir to supplement the evaporation and leaking of water. The total volume of water in the reservoir stays constant and acts as an energy carrier, which can be used up and down to generate and store energy in one working cycle. This kind of hydro-pumping energy storage station can be used for the peak shift of electrical demand and meet an emergency or standby electrical requirement, but it cannot be used for general power generation.
2. Mixed hydro-pumping energy storage stations: There is a constant natural water influx into the reservoir, and the volume of influx water can be used for generating electricity to meet the general power load. For mixed hydro -pumping energy storage stations, a hydro-pumping and power generator set is installed and the power is generated from the natural influx of water and the water pumped from a lower position. Not only can mixed stations be used for the peak shift of electrical demand and to meet an emergency or standby electrical requirement, they can also be used for general power generation.
Hydro-pumping energy storage stations can also be classified into three categories by their regulation cycles.
1. Daily regulation hydro-pumping energy storage stations: These will pump water to store energy in the off-peak period and generate power in the peak period. They have a daily cycle, which means that the reservoir will empty its water volume every day to generate electricity in the peak period and water will be pumped up to the reservoir to store energy in the off-peak period. Most pure hydro-pumping energy storage stations are regulated daily.
2. Weekly regulation hydro-pumping energy storage stations: Similar to daily regulation stations, these will pump water to store energy in the off-peak period and generate power in the peak period; however, the volume of generated power is larger than the storage energy. These are the working conditions five days a week, and the water will be pumped up to the reservoir during the other two days due to the low-power loads at weekends.
3. Seasonal regulation hydro-pumping energy storage stations: Every flood season, hydro-pumping energy storage stations will take advantage of the power from hydropower stations to pump water up to the reservoir to store the energy and ensure the generation of sufficient power in the dry season, when there is insufficient natural water. These are usually mixed stations.
According to the geographical location of power stations, hydro-pumping energy storage stations can be classified into conventional stations with high-position river dams and low-position reservoirs, and new seawater/underground storage stations. The former are restricted by geographical conditions, topography, and environment factors, while the latter is not restricted by the environment or topography and the construction costs are low; it is thus being paid more and more attention. Power stations built in rivers require high-position reservoirs and low-position reservoirs. However, the low-position reservoir of seawater power stations is the sea, which means that all they need to do is to pump the seawater up to the high-position reservoir. Compared with freshwater power stations, seawater power stations do not require a low-position reservoir, which saves on construction costs; however, the material used for the pipes, pumping equipment, and generator needs to be anticorrosive and resistant to marine organism adhesion.
In conclusion, hydro-pumping energy storage stations can store extra power and effectively regulate the balance between production, supply, and usage in the power system. They can store high volumes of electricity and help to provide safe, stable, and economic output. In addition, they have the functionality of peak shifting with electrical demand; the energy can be saved in the power system by cooperating with fossil power generation units. Usually, fossil power generation units will reach the maximum efficiency under the rated power. With the help of hydro-pumping energy storage technology, on the one hand, fossil power generation units will reduce the peak load dispatching operation times and stabilize the output power. Meanwhile, they will increase the load rate with high efficiency and reduce fuel consumption. On the other hand, off-peak electricity can be generated by low-cost generators. By reducing the peak load dispatching operation times and decreasing fuel consumption, emissions of sulfide, nitric oxide, dust, and carbon monoxide can be reduced, which helps to realize the goal of energy conservation and emission reduction.
With the widespread application of wind energy, solar energy, and other renewable energies, the appropriate scale of energy storage stations can be built into the power system, which can improve the capability of power generation absorption in wind energy and solar energy and reduce the phenomenon of wind curtailment. When the supply of electricity is surplus or insufficient, hydro-pumping energy storage technology can be used to stabilize the electric output in the power system. Meanwhile, energy storage stations can not only balance the unstable electric output by renewable energy but also regulate the amplitude and frequency modulation and reduce the impact of the electric output of renewable energy on the power system, which solves the practical problems related to electricity generation by wind energy and solar energy and its connection to the power system.
Hydro-pumping energy storage stations occupy the vast majority of the installed capacity in the energy storage market, and it is an effective solution to deal with peak regulation, amplitude modulation, frequency modulation, peak-load shifting, and wind curtailment. In northern China, the wind power resources are rich but water resources are insufficient, which means these areas do not meet the demand to build large-scale hydro-pumping energy storage stations. And the construction of PV power stations also faces the same issues. In recent years, most large-scale centralized PV power stations have been built in the Gobi Desert, but it is unpractical
to build hydro-pumping energy storage stations in such a dry environment. It has been a hotspot to take advantage of seawater and offshore natural conditions to build large-scale hydro-pumping energy storage stations, which can help to solve the issues of developing wind resources.
The location of a hydro-pumping energy storage station is restricted by topography and the ecological environment. For example, the station location should have a short horizontal distance and a long vertical distance between the high-position and low-position reservoirs. In addition, rock strength, antiseepage capability, rich water resources, reservoir volumes, water quality, climate change, and soil salinization are also significant factors to be considered.
1.4.1.2 Current Situation of Hydro-Pumping Energy Storage Technology
The first hydro-pumping energy storage station was built in Zurich, Switzerland, in 1882. Since the 1960s, it has been developing rapidly. Industrialized developed countries such as the United States and Japan drive the large-scale development of hydro-pumping energy storage stations. In Japan, when one nuclear power station is built, another peak load–shifting power station must be built at the same time.
According to reports from the World Wind Energy Association [4], local economy and policy play key roles in determining the electricity volume of wind energy in power systems, and the difficulties in technology and operational issues are not the real problems. Western coastal countries have done research into the issues related to wind energy being integrated into the power grid and the adjustment of power systems. The conclusion is that it is possible to integrate over 20% of volume of wind energy into the power grid, and the collaborative operation of wind power and storage stations could improve the quality of electric energy and promote the development of wind power.
In EU countries, wind power holds a large proportion of the power system. In Spain, wind power is 20% of the total installed power equipment, 20% of the total volume of power comes from wind power, and 10% is hydro-pumping energy. In Germany, the percentage of installed wind power equipment is 17%. Wind energy produces 7% of total power, and a small proportion of power comes from hydropower. In order to take advantage of wind power, it has a strong connection to the European power grid, and hydro-pumping energy storage is 10% of the total installed power equipment. In Denmark, the percentage of installed wind power equipment is 25% and the volume of wind power is 16%, and by 2025, 50% of power will come from wind energy. The Danish national power grid will connect with the Northern European power grid. The efficiency of wind power in Denmark will increase by taking advantage of hydro-pumping energy storage, which can effectively conduct peak regulation [5].
In China, hydro-pumping energy storage stations were not built until the 1960s: two small hydro-pumping energy storage stations were built in Gang Nan and Mi Yun in 1968 and 1973, respectively [6]. At the beginning of the 1990s, the development of hydro-pumping energy storage stations was boosted due to the rapid national economic development. Ten hydro-pumping energy storage stations were built at the end of 2004, and the installed capacity reached 5.7 GW. In 1968, a hydro-pumping energy storage station with an 11 MW capacity was built to collaborate with a hydropower station in
Gang Nan, He Bei Province, in 1968. In 1992, a mixed hydro-pumping energy storage station with a 0.27 GW capacity was built in Panjiakou, He Bei Province. In 1997, a hydro-pumping energy storage station with a 0.8 GW capacity was built in Beijing. Two hydro-pumping energy storage stations were built in Guangzhou in 1994 and 2000, respectively, and the total capacity is 2.4 GW. In 1997, a hydro-pumping energy storage station with a 90,000 kW capacity was built in Yang Zhuo Yong Hu. In 1998, a hydro-pumping energy storage station with an 80,000 kW capacity was built in Xi Kou, Zhejiang Province. In 2000, a hydro-pumping energy storage station was built with a 1.8 GW capacity in Tian Huang Ping, Zhejiang Province, and another with an 80 MW capacity in Xiang Hong Dian, An Hui Province. In 2002, a hydro-pumping energy storage station with a 0.1 million kW capacity was built in Sha He, Jiang Su Province, and another with a 70 MW capacity was built in Tian Tang, Hu Bei Province [7].
By the end of 2005, the capacity of hydro-pumping energy storage stations was 6.245 GW, which is 1.2% of the total national installed capacity in China. By the end of 2009, it was still quite low: only 1.66% of total installed energy storage capacity. However, for developed countries, the percentage usually ranges between 3% and 10% [8]. By the end of 2010, the installed capacity of hydro-pumping energy storage stations was 16.345 GW, which ranks third in the world. However, the percentage of installed energy storage capacity is still rather low.
With economic development and the improvement of the quality of people’s lives, the demand for the volume and quality of electric power is gradually increasing. In order to meet these demands, the construction of hydro-pumping energy storage stations is necessary; it can boost the large-scale development of wind energy, solar energy, and other renewable energy resources, and it can also help to realize the intelligent regulation of power systems.
1.4.1.3 Developmental Tendencies of Hydro-Pumping Energy Storage Technology
In modern hydro-pumping energy storage stations, the previous four-engine or threeengine units have been replaced by pump turbine and hydro-generator mixed units, which reduce civil work and equipment investigation. And a series of advanced technologies have been used in civil construction, such as impermeable bitumen concrete, high-strength steel structures, inclined-tunnel whole-section heading machines, highposition reservoirs, underground powerhouse informational construction, and so on. In order to further enhance the entire economy, the new direction is to develop power units with high heads, high speeds, and high volumes. Currently, the manufacture limitation of the single-pump turbine and air-cooled generator has been obtained. Therefore,more attention will be paid to research on vibration, cavitation erosion, deformation, magnetic properties, and the reliability and stability of operation. More attention will be paid to research on vibration, cavitation erosion, deformation, magnetic properties, and the reliability and stability of operation. Small changes in amplitude and continuous speed regulation units with a high quality of electric power will also be emphasized to realize the control of automatic frequency. In addition, improving the level of reliability and automation, building up a unified management mode to promote centralized monitoring and automatic operation, and research into the core technologies of seawater and underground water-pumping energy storage stations from other countries need to be
emphasized in the future [9]. New hydro-pumping energy storage stations, such as those using seawater and underground water, will effectively supplement conventional hydropumping energy storage stations; however, in a short period of time, restricted by the maturity of technology, the scope of application, and the economic costs, new energy storage stations can only slightly adjust and supplement the whole energy system [10].
1.4.2 comPreSSed air energy Storage technology
It has been 30 years since compressed air energy storage (CAES), a kind of energy storage technology based on air turbines, was invented. The mechanism is to separate the compressor and the turbine. Energy can be stored by compressing air into the air container, and the power is discharged by injecting the high-pressure air into the combustion chamber to drive the turbine and generate power.
1.4.2.1 Mechanism of Compressed Air Energy Storage Technology
The Huntorf plant was commissioned in 1978 to become the world’s first CAES plant [3]. As shown in Figure 1.3, a CAES station is comprised of a compressor, a steam turbine, an electric generator set, a heat exchanger, and high-pressure air storage equipment. There are many essential technologies that will influence CAES. These mainly include effective compressor technology, steam turbine technology, combustion chamber technology, energy storage technology, air storage technology, and control technology. The compressor and turbine are the core units in a CAES system, and their capabilities play a key role in the performance of the entire system.
1. Excess or off-peak power is used to compress air
4. e electricity produced is delivered back onto the gird
2. Air is pumped underground and stored for later use
When electricity is needed, the stored
FIGURE 1.3 Schematic of an adiabatic CAES system.
Conventional CAES systems need to have a large volume of compressed air, which is usually sealed in an underground salt mine, hard-rock grotto, or multihole grotto. For small-scale CAES systems, a high-pressure gas storage container placed aboveground can be used to replace the air grotto underground. Unlike a normal gas turbine station, the turbine and compressor in a CAES station are set at each end of an electric generator and connected by a clutch, which means they can operate separately.
The mechanism of CAES technology is that extra electric power in the off-peak period is used to compress air into the high-pressure sealed grotto (or abandoned mine, etc.), which is conducted by connecting the generator and compressor, and the facilities are used to transform electric power into a pressure potential of compressed air. While in the peak period, the generator and gas turbine will be connected and take advantage of the gas turbine, combustion chamber heater, and so on in the subpower system. The combustion of mixed oil or natural gas with compressed air in the air container will drive the gas turbine to generate electric power.
An abandoned mineral well, grotto, expired gas well, gas storage well, or gas storage tank under the sea can be used as a gas storage station. One of the most ideal modes is to seal gas with water under constant pressure, which can output constant-pressure gas and stabilize the operation of the gas turbine. The ideal depth to store compressed air is 150–900 m [11]; beyond this depth, there is no obvious daily change in temperature and pressure. The requirements for an air storage container are that it does not leak and that it is stable. In addition, high-pressure air can be stored in the artificial cave or take advantage of natural loose aquifers among rocks.
So far, there are only two large-scale CAES stations outputting over 100 MW in the world, and they are located in Germany and the United States [12]. Usually, the output power of a normal gas turbine is one-third of the shaft power of the turbine, and the remaining two-thirds of the power is used for driving the compressor. However, when a CAES station is operated for peak-load regulation, all the shaft power will be used to generate electricity. Therefore, the efficiency of CAES stations is three times that of normal gas turbine power stations.
CAES stations have a lot of advantages:
1. Large volumes of energy storage, long cycle periods of energy storage, and little investment.
2. CAES stations can improve the load rate in the electric grid due to the regulation of peak load, which can help steam power plants maintain efficient operation, reduce output fluctuations, and increase economic benefit and reliability.
3. When a CAES station is used for peak-load regulation, the compressor will be driven by extra electric power in the off-peak period to store energy. During the peak electricity-consuming period, compressed air will be released to drive the air generator. Therefore, when peak-load regulation is needed, the output power from an air turbine driven by compressed air is three times that of a gas turbine.
4. Compressed air can be used instantly, and it only takes three minutes to reach the rated output power. This is suitable for spinning reserve power stations.
A conventional CAES system is not an independent technology, it must work with a gas or oil turbine power station; however, it may not be suitable for all the countries, for example, in China, the electric power is mainly generated by coal-fired power stations, and conventional CAES systems rely on fossil fuel to provide a heat source, which is not suitable for the Chinese energy strategy. They also have to face the increasing price of fossil fuel and restrictions on emission pollutants. Meanwhile, similar to hydro-pumping energy storage stations, conventional CAES systems also have special requirements for geological conditions to build large air containers, such as a rock cavity, grotto, abandoned mine, and so on.
The disadvantage of CAES technology is low energy efficiency; for example, the energy efficiency of the CAES station in Huntorf, Germany, is only 42% [13]. The reason for the low efficiency is that the temperature will rise when air is compressed, and air expansion will lower the temperature. Current CAES systems will lose energy in terms of heat in the process of compressing air and reheating the air before air expansion (with natural gas as the heat source), which leads to the low energy efficiency and the increasing emission of greenhouse gases.
A stable geological structure is necessary to build a CAES station, which is one of the most significant factors restricting the development of CAES technology. In addition, CAES stations usually work with a natural gas– or oil-fired peak load–regulation power station. However, these have low economic efficiency due to the high price of natural gas and petroleum.
1.4.2.2 Development of Compressed Air Energy Storage Technology
It has been 30 years since CAES technology was invented. Nowadays, there are only two large-scale CAES stations outputting over 100 MW for commercial use. The first commercial station was established in Huntorf, Germany, in 1978 [14]. Its compression power is 60 MW and the output power is 290 MW, and the compressed air is stored in an abandoned mine 600 meters underground. The total volume of the mine is 3.1 × 105 m 3, and the pressure of the compressed air can reach as high as 10 MPa. The facility can compress air for eight hours, and the output of electric power can last two hours. From 1979 to 1991, the station operated 5000 times to connect with the power grid, and the launch reliability is as high as 97.6%. Exemplary operation results demonstrate that this station has great utilization and reliability. The second commercial CAES power station, called McIntosh, was built in Alabama in 1991 [14]. The air is stored 450 m underground. The total volume is 5.6 × 105 m 3 and the pressure of compressed air is 7.5 MPa. The compressor power is 50 MW, and the output power is 110 MW. The station can continuously compress air for 41 hours and output electric power for 26 hours. So far, these two stations are still in operation.
In 1998, the Hokkaido Electric Power Company began to build a CAES power station in Kamisunagawa, Sorachi District, Hokkaido, which started to operate in 2001. Its output power is 2 MW [15]. It can output power for four hours and takes 10 hours to compress air into an abandoned coal mine 450 m underground. The air pressure is 4.0–8.0 MPa.
The third 100 MW-grade large-scale CAES power station was planned to be built in Iowa [16]. The investment amount was $0.4 billion. The planners had hoped to take advantage of the rich wind sources in Iowa state. However, it was aborted because of
a geological evaluation that showed that the location was not suitable for building a large-scale CAES power station. This project would have begun operation in 2015 [17].
The development of CAES technology started relatively late. With the rapidly increasing demand for energy storage, more attention was paid to CAES technology by some universities and academic institutes. The Chinese Academy of Sciences Institute of Engineering Thermophysics, Tsing Hua University, North China Electric Power University, Xi’an Jiaotong University, Huazhong University of Science, and so on do research into the thermal properties and economic benefits of CAES power stations. However, most of them are on the theoretical side or operate in laboratories. None of them is available as a commercial power station.
1.4.2.3 Developmental Tendencies of Compressed Air Energy Storage Technology
There are two problems for the construction of CAES power stations. One of them is the stable geological structure of the grotto, which is a key factor restricting the development of CAES power stations. In addition, CAES power stations usually work with gas- or oil-fired peak load–regulation power stations. However, the price of natural gas and petroleum is rather high, and the economic benefits of gas- or oil-fired peak load–regulation power stations are unsatisfactory. In order to solve these two problems, people have recently been paying more attention to new CAES technologies: for example, CAES systems with heat storage devices, microminiature CAES technology, liquefied air energy storage technology, supercritical CAES technology, and CAES technology coupled with renewable energy sources [18].
CAES technology with heat storage devices is also called heat insulation CAES technology, as the process of air compression involves heat insulation and storing a large amount of compressed heat energy in the storage device. The stored heat energy can then be used to heat the compressed air before outputting the electric power. Compared with conventional CAES technology, by burning fossil fuels to heat the compressed air, the efficiency of the energy storage system increases, but the cost of investment is higher.
Small-scale CAES technology usually refers to the 10 MW grade. It uses a highpressure container to store compressed air, which solves the problem of dependency on an air storage grotto [19]. Liquefied air and supercritical CAES technology have been put forward in recent years [19]. Liquefied air CAES technology does not need a large air container because the density of liquefied air is much greater than normal air. However, liquefied air costs more energy, which means the efficiency is rather low. In order to improve efficiency, supercritical CAES technology has been put forward. This technology combines the properties of the supercritical state of air and the advantages of normal and liquefied air CAES technology. This new technology has a lot of advantages, such as large-scale energy storage, high efficiency, low investment cost, high energy density, and so on, and there is no need for a large storage device.
1.4.3 lithium-ion batterieS
The elemental metal lithium has the lowest molar mass (relative atomic mass of 6.94, density of 0.53g/cm 3) and the lowest electrode potential (–3.04 V vs. a standard
hydrogen electrode) in the natural world. Lithium batteries have a lot of advantages, such as high operating voltages, specific capacities, and high energy density. In the 1970s, M.S. Whittingham from Exxon developed the lithium-ion battery [20]. TiS3 was used as the cathode material, and metal lithium was used as anode material. However, metal lithium-ion batteries have some disadvantages. On the one hand, the inhomogeneous deposition of lithium ions on the electrode surfaces will tend to be highly dendritic, and these lithium dendrites can result in the penetration of the membrane separator and contact with the cathode, leading to an internal short circuit or even battery burning and explosion. On the other hand, when lithium dendrites fall from the anode, they will not react in the charge/discharge cycles, causing a remarkable, irreversible capacity loss. Therefore, people lost interest in the secondary lithium-ion battery.
In 1980, Armand first put forward the concept of the rocking chair lithium-ion battery [21]. In this kind of battery, lithium exists in the form of ion, not solid metal, and in the process of electrochemical reaction, lithium will not be deposited or resolved on the surface of the anode, which means that this concept will resolve the issue of lithium dendrites. And in order to obtain higher working voltages, cathode material with a high potential for lithium ions to escape and anode material with a low potential for lithium ions to embed are needed to improve bacterial performance. Goodenough et al. first found out that transition metal oxide LiMO2 (M = Co, Ni, Mn) can be used as cathode material, which shows advantages in the aspect of capacity and potential [22]. However, there are still no suitable anode materials for rocking chair batteries, which has restricted the development of the lithium-ion battery.
Until 1990, Japanese company Sony used petroleum tar (graphite) as anode material for lithium batteries, which significantly improved the working battery voltage. Then, the company developed a type of commercialized battery that uses LiCoO2 as cathode material. The working voltage of this battery is over 3.6 V, and the energy density per mass ranges from 120 to 150 Wh/kg. Since then, the lithium-ion battery has rapidly occupied the battery market, and it is still the major power source for portable electronic products.
After that, more and more attention was paid to the development of the lithiumion battery, and large amounts of cathode and anode material were reported. In 1996, Padhi and Goodenough found that phosphates with olivine structures, such as LiFePO4, could be used as cathode material [23]. Compared with conventional LiCoO2 material, LiFePO 4 has a lot of advantages, such as safety, high temperature resistance, and overcharge resistance. By far, LiFePO 4 has been one of the most popular cathode materials for lithium-ion batteries suitable for discharging heavy currents.
Since 2000, lithium titanate has been developed and used as a new anode material [24]. Compared with conventional graphite anode material, lithium titanate has some advantages, such as no volume changes, stable discharging voltages, high lithium diffusion coefficients, resistance to lithium dendrites, and so on, all of which meet the requirements in the field of electrical vehicles. Battery working voltages usually range from 2.4 to 3.0 V, and charging currents reach 2 A.
However, secondary lithium-ion batteries were recognized as having lower energy density per mass (< 150 Wh/kg), which makes it hard to meet the demand
for electrical vehicles. Lithium–sulfur and lithium–air batteries are gradually being researched and developed. For lithium–sulfur batteries, metal lithium and sulfur are used as anode and cathode material, respectively, and their theoretical capacity is 1675 mAh/g [25]. Lithium and oxygen are considered anode and cathode material, respectively, in lithium–air batteries, and their theoretical energy density is 11,140 Wh/kg (oxygen not included) [26]. However, these two kinds of batteries have a lot of problems to be solved, and they still need to be developed in the laboratory.
1.4.3.1 Mechanism of Lithium-Ion Batteries
A lithium-ion battery (rocking chair battery) is a kind of rechargeable battery. The charge/discharge cycle happens when lithium ions are transferred between the cathode and the anode. During the process of lithium ions escaping and embedding, an equivalent volume of electrons accompany the transfer of ions. As shown in Figure 1.4, when the battery is discharged, lithium ions from the anode embed into the cathode through electrolytes. Anode carbon material usually has a lamellar and microporous structure. Lithium ions from the cathode will embed into these micropores in the charging process, resulting in a large number of lithium ions gathering in the anode. When the battery discharges, embedded lithium ions will escape from the anode and transfer to the cathode. The discharging capacity depends on the number of lithium ions coming back from the anode.
For lithium–sulfur batteries, sulfur is the active component in the cathode and lithium is the active component in the anode, which can be seen in Figure 1.5. When the battery discharges, lithium from the anode loses electrons, creating lithium ions. The lithium ions transfer to the cathode and react with sulfur to form polysulfide, with the help of electrons from an external circuit. When the battery charges, with the help of an external voltage, opposite reactions happen on the anode and cathode, respectively [27].
For lithium–air batteries, metal lithium acts as the anode-active component and oxygen in the air acts as the cathode-active component, which can be seen in Figure 1.6. When the battery discharges, metal lithium in the anode is oxidized to release electron and lithium ions. The lithium ions pass through electrolyte material and react with oxygen to form Li2O and Li2O2 , with the help of electrons from an
FIGURE 1.4 Illustration of the Li-ion battery principle.
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