The use of energy storage technologies to increase the reliability of energy system based on renewable energy sources



Over the last decade, there has been a significant increase in the share of renewable electric power (hereafter RES) in the total amount of generating capacity. However, the introduction of an increasing number of RES poses new challenges to the energy system. The intermittent nature of RES generation, as well as the gradual transition from a centralized power supply system to a distributed one, leads to a decrease in the stability and reliability of the power system. One of the recognized options for solving this problem is the use of electric energy storage (hereafter EES) systems. The article provides a comparative analysis of modern RE technologies and identifies the best options for use at the level of distributed generation with the participation of renewable energy sources.

За последнее десятилетие произошло значительное увеличение доли возобновляемой электроэнергии (далее ВИЭ) в общем объеме генерирующих мощностей. Однако внедрение все большего количества ВИЭ создает новые проблемы для энергетической системы. Прерывистый характер генерации ВИЭ, а также постепенный переход от централизованной системы энергоснабжения к распределенной, приводит к снижению стабильности и надежности энергосистемы. Одним из признанных вариантов решения этой проблемы является использование систем хранения электроэнергии (далее СХЭ). В статье приведен сравнительный анализ современных технологий ВИЭ и определены оптимальные варианты для использования на уровне распределенной генерации с участием возобновляемых источников энергии.

Introduction

According to the forecasts of the world expert community and leading research institutes in the field of electric power, one of the key trends determining the further development of the industry will be a gradual increase in the share of renewable electric power sources (RES) in the total volume of generating facilities.

Thus, according to the statistical service of the European Union «Eurostat» the total share of renewable energy in the total generation capacity of the 28 countries of the European Union from 2014 to 2021 has doubled. According to reports of the world organization Renewable Energy Policy Network for the 21st Century (REN21), the increase in generation from the most developing types of RES in 2014, wind and solar energy was 15.9 % and 29.2 %, respectively. And investment in research and development in renewable energy grew more from 2014 to 2021.

However, the ever-increasing role of renewables, which are characterized by instability over time, leads to a decrease in stability and, consequently, in the reliability of distribution networks. One way to combat this problem is the use of electric energy storage (EES). The purpose of this article is to analyze the existing RE technologies and assess the possibility of their application in power grids with a high level of RES implementation.

Classification of electric energy storage devices

EES allow to transform electric energy into other types of energy, suitable for storage in certain time intervals, with further possibility of reverse transformation into electric energy. Based on the above, it is possible to divide all EESs by the type of energy in which storage takes place (see Figure 1).

Fig. 2. Classification of electric energy storage devices

The main parameters of the studied EES are summarized in the table 1 and 2.

I. Mechanical energy accumulators

1. Pumped storage power plant (PSPP)

Hydroelectric pumped storage power plant is the technology of renewable energy sources with a long history. The first pumped storage power plants appeared at the end of XIX century.

The first pumped-storage power plants were built at the end of the 19th century. Renewable power plants account for 99 % of the world's electrical energy.

A hydroelectric power plant consists of a set of generators and pumps, or reversible hydroelectric generators. During the hours of nighttime minimum electricity consumption, a hydroelectric power plant uses cheap electricity to pump water into the upstream reservoir. During morning and evening peak hours, the pumped storage power plant generates expensive electricity by discharging water into the tailrace basin.

Installed capacity of existing pumped storage plants varies from 1 to 3,000 MW, with an efficiency of about 70–85 % and an operating life of up to 40 years.

2. Compressed air energy storage technology (CAES)

Energy storage in the form of compressed air is carried out by means of an electric compressor that pumps air at high pressure into naturally occurring underground cavities or special reservoirs. Pumping takes place at night, during the hours of minimum power consumption, and during the hours of maximum power consumption, the accumulated compressed air is used to run the turbine generator. CAES technologies can be used both for storing large amounts of energy (similar to a hydroelectric power plant), with air pumped into natural reservoirs, and for local use, with air pumped into artificial reservoirs.

The main barrier to the application of CAES technology is to find a suitable geographical location of the storage and lower efficiency compared to a hydroelectric power plant. A development of CAES technology is AA-CAES, which integrates thermal storage of electrical energy.

3. Super flywheel (FES).

Modern super flywheel design typically includes the following components: flywheel, bearings, electric motor/generator, vacuum cage. Electrical energy is stored and released by accelerating or decelerating the flywheel. The amount of stored energy in the super flywheel depends on the rotation speed of the latter. All super flywheels can be divided into 2 categories: low (6000 rpm) and high (up to 100000 rpm) speed. Super flywheels have a high efficiency, relatively high energy density.In 2011, Beacon Power commissioned a super flywheel-based storage system with a total installed power of 20MW. The purpose of this system is to quickly regulate the frequency of the mains voltage .

II. Chemical storage of electrical energy

1. Batteries

Batteries are one of the most widely used RE technologies in both industry and at home. The principle of battery operation is based on the reversibility of chemical reactions. The most common types of commercially available batteries are: lead-acid, lithium-ion, nickel-cadmium.

Lead-acid batteries: The reagents in the lead-acid batteries are lead dioxide (Pb02) and lead (Pb), and the electrolyte is a solution of sulphuric acid solution. In terms of application lead-acid batteries are divided into the following groups: starter (for starting internal combustion engines), stationary (as a backup power source), traction (electric transport) and portable (power for tools, instruments).

Lithium-ion batteries: The negative electrode is a carbon material into which lithium ions are reversibly introduced. The active material of the positive electrode is usually cobalt oxide into which lithium ions are reversibly introduced. The electrolyte is a solution of lithium salt in a non-aqueous aprotonic solvent. Batteries have a high energy density, high service life and can operate at low temperatures. The disadvantages are high cost, sloping discharge curve and relatively high self-discharge. Due to high specific energy their production in recent years has increased dramatically.

Nickel-cadmium batteries: The reagents are nickel hydroxide and cadmium, the electrolyte is a KOH solution, and therefore they are also called alkaline batteries. The main advantage of this type of batteries is a long service life. They are used to power portable equipment.

2. Fuel cells

A fuel cell is similar to a battery in principle, but differs in that the substances involved in the electrochemical reaction are supplied externally. In hydrogen fuel cells, the chemical energy of hydrogen is converted into electrical energy, bypassing the combustion process. Fuel cells have high efficiency and can be used on a par with the battery for buffer storage of energy from renewable energy sources. [4]

III. Electrical storage devices

1. Conventional capacitors

Traditional electrolytic capacitors, in the simplest case, are a device for energy storage of an electric field, consisting of two plate-shaped electrodes separated by a dielectric. Capacitors are used to store small amounts of electrical energy and are characterized by high energy density and short charge/discharge times.

2. Supercapacitors

Supercapacitors are devices in which electrical energy is stored by charging a double electric layer. This layer is formed by the surface of the conductor and a layer of adjacent electrolyte ions. The double electric layer can be viewed as a capacitor with two covers, the capacity of which is proportional to the area of the covers and inversely proportional to the distance between them. Due to the fact that the distance between the charged surface of the conductor, from which the electrodes are made, and the layer of ions is very small (measured in angstroms), and the value of the conductor surface (eg, activated carbon) is up to 1500... 2000 m2/g, the capacitance of a carbon electrode with a mass of 1 g can be 100... 500 F.

According to the main parameters, supercapacitors occupy an intermediate position between chemical sources of electrical energy and conventional capacitors. Together with the battery can act as a hybrid storage of electrical energy, leveling the disadvantages of both elements. [5]

3. Superconducting Magnetic Energy Storage (SMES)

This type of NE stores magnetic field energy created by current flowing through a solenoid of superconducting material cooled to below the critical temperature of superconductivity. The SMES is a highly efficient NEE with an efficiency of over 95 % and has a short time delay between charge and discharge processes. Currently, SMES drives are mainly used for quality control electrical energy. [6]

Table 1

Main parameters of EES

Technology	Energy Density 10 3 Wh/m 3	Capacity Density 10 3 Wh/m 3	Specific energy, Wt/h/kg	Unit energy W/kg	Nominal energy MW
Pumped storage power plant (PSPP)	0.5–1.5	0.5–1.5	0.5–1.5	-	100–5000
Industrial CAES units	3–6	0.5–2	30–60	-	Up to 300
Super Flywheel	20–80	1000–2000	10–30	400–1500	Up to 0.25
Lead-acid batteries	50–80	10–400	30–50	75–300	Up to 20
Lithium ion batteries	200–500	1500–10000	75–200	150–315	Up to 0.1
Nickel cadmium batteries (NiCd)	60–150	80–600	50–75	150–300	Up to 40
Fuel cell	500–3000	500	800–10000	500	Up to 50
Capacitor	2–10	100000	0.05–5	100000	Up to 0.05
Supercapacitor	10–30	100000	2.5–15	500–5000	Up to 0.3
SMES	0.2–2.5	1000–4000	0.5–5	500–2000	Up to 10

Table 2

Additional parameters of EES

Technology	Rated capacity, MWh	Self-discharge per day, %	Life-time	Number of charge-discharge cycles	Charge/discharge cycle efficiency, %
Pumped storage power plant (PSPP)	500–8000	0	40–60	10000–30000	70–85
Industrial CAES units	Up to 1000	0	20–40	8000–12000	42
Super Flywheel	Up to 5	More than 20 % per/h	15	More than 20000	90–95
Lead-acid batteries	Up to 40	0.1–0.3	5–15	500–1000	70–80
Lithium ion batteries	0.024	0.1–0.3	5–15	1000–10000	90–97
Nickel cadmium batteries (NiCd)	6.75	0.2–0.6	1–20	2000–2500	60–70
Fuel cell	0.321	0	5–15	1000	20–50
Capacitor	-	40	5	50000	60–70
Supercapacitor	0.0005	20–40	10–30	100000	90–97
SMES	0.0008	10–15	20	100000	95–97

Conclusion

As a result of the analysis of various EES technologies, it was found that:

1. The growth of implementation of RES, leading to an increase in the variability of generated power in the energy system and reducing its reliability, requires the search for effective solutions for the accumulation of electricity at all levels of the energy system.
2. Mechanical energy accumulators are characterized by high installed capacity and inertial nature. At the moment the main area of application of such accumulators are large power systems, in which a large share of generation are powerful thermal and nuclear power plants, incapable of instantaneous changes in the amount of generated electrical energy. However, mechanical storage can also be used together with renewables and in relatively small networks remote from the centralized power system.

Combination of chemical and electric RE is efficient, which can be used both in systems of distributed generation based on RES, and in centralized energy system with RES. Combination of these types of renewable energy sources allows combining fast performance of electric renewable energy sources with high energy density of chemical renewable energy sources.

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