Sanzhar Sharapov, Head of Prospective Development and Projects Technical Support Department Hevel EG

To integrate and increase the share of renewable energy sources, such as solar and wind power, it is necessary to ensure the flexibility of energy system, while maintaining reliability and supporting the efficient use of renewable resources. Use of energy storage systems is seen as the main solution that allows increasing the flexibility of the power system due to the possibility of rapid consumption of electricity, storage for a given period of time and further generation of electricity.

Main directions of the ESS

Currently, it is possible to identify several directions in terms of the use of ESS in the electric power industry:

·            use of ESS in provision of services to ensure system reliability (Installation in-front of the meter (ETM));

·            installation of ESS at the consumer in order to optimize the cost of electricity supply (Installation behind-the-meter (BTM));

·            application of ESS as an element of the power grid infrastructure in order to ensure the reliability of electricity supply and delay investment in modernization/new grid construction;

·            use of ESS as part of distributed energy facilities, especially in isolated power systems.

Compared to customary systems, energy storage systems at the power system level, such as hydro-accumulating power plants (HAPP), ESS based on stationary battery packs (electric-chemical generators) have a number of advantages, in terms of location independence, scalability of the solution, and possibility of placement in the immediate vicinity of any type of power plant. In turn, the HAPP requires at least special geological conditions.

Stationary ESS systems have a wide range of energy capacities, ranging from a few MWh to hundreds of MWh. Currently, there are a number of technologies that are used for grid stationary ESS. Since 2013, the balance has begun to be dominated by projects based on lithium-ion batteries, which by 2017 already accounted for more than 90% of the market.

Solution for integrating the energy storage system with the ESS allows ensuring the reliability and quality of power supply, ensuring the redundancy of consumers, providing system services with a high degree of maneuverability and realize the full potential of a solar power plant. The solar power plant will meet the energy needs of the power plant during the day, and the excess solar energy will accumulate in the energy storage system and redistribute the power of ESS over time (participation in the evening load peaks), as well as provide a reserve of power in the event of an emergency shutdown of the line.

ESS functions in electric power system

Large grid electricity storage systems will play a key role in facilitating the next phase of the energy transition by increasing the share of RES in the electric power system. It should be noted that for system operators (operators of the power system and grids), the integration of ESS will allow conducting a number of system services, such as participation in the general primary frequency control (GPFC), providing a "hot" reserve of capacity in the system, regulating the rate of change in loads and capacities in the electric power system. Use of ESS allows reducing the cost of peak generating capacity, modernization of the grid infrastructure. As we said before, integration of large renewable energy sources into the energy system allows increasing the share of renewable energy sources that have a sharply variable and hard-to-predict power output profile, which in turn allows increasing the reliability of electric power system. Integration of RES+ ESS allows redistributing excess electricity overtime, thereby allowing using renewable sources to the maximum. Moreover, the complexes based on RES and ESS already allow providing cheaper electricity to hard-to-reach regions and power systems isolated from centralized grids, the supply of which is provided by diesel generation with high fuel delivery costs.

A key distinguishing feature of ESS is the ability to participate in several tasks or perform a number of functions within different segments of the electric power complex.

Types of system services

Participation in frequency control

For normal operation, the system must maintain a balance between the power generated and the power consumed, respectively, the resulting imbalance entails a decrease or increase in frequency in the electric power system, which is unacceptable. Traditionally, when regulating the frequency in the system, thermal power plants (CPP, GTPP, CCGT) and hydroelectric power plants (depending on the type of regulation) are involved. However, provision of these functions can negatively affect the efficiency of power plants. It should also be noted that the response time of the above types of plants can range from a few seconds to minutes. ESS, in turn, can provide similar frequency control functions, with a response time of milliseconds.

ESS are actually an alternative to peak generation, and their economy is determined by the ability to compete with large "system" power plants.

Regulation of the discharge rate and load surge/RES power

An increase in share of RES in the electric power system reduces the burden on other plants in the electric power system, but the unstable and sharply changing nature of the RES power profile can cause certain power fluctuations, which in the future will require keeping a "hot" reserve of power at maneuverable power plants. In order to integrate a larger volume of RES and ensure reliable and efficient operation of the system, ESS allows smoothing (regulating) the amount of power changes both directly at the RES facilities and when connected to other nodes of the system. With a sharp decrease (disconnection) of the load in the system, ESS is also able to provide a smooth decrease in the load. Thus, ESS is able to provide static and dynamic stability.

Reserve capacity source/Backup power source In case of local damages and outages, temporary repairs of the grid infrastructure, ESS is able to provide local power supply with the function of uninterruptible power supply, which increases the reliability and quality of power supply to consumers.

Reactive power compensation and voltage regulation

Integration of ESS is carried out through the use of power converter technology, which allows using the ESS as a consumer or a source of reactive power and, accordingly, allows for local voltage regulation.

Reducing grid infrastructure costs

ESS installed in distribution grids can decrease the burden on power centers of distribution grids; provide additional reliability in the mode of an uninterruptible power supply (UPS) in the event of disconnection of high-voltage grid elements, as well as in case of short-term interruptions of power supply; and also improve the quality of electricity, stabilize the voltage. Due to the possibility of using ESS as an element of "peak" generation, it is possible to reduce the costs of the grid infrastructure, eliminating the reconstruction of the grid complex or substations.

Use of ESS directly at the consumer, have even wider functionality. The above functions also include:

·            ensuring uninterrupted operation directly at the consumer in case of grid accidents;

·            ensuring the quality of electricity to power the consumer's equipment, depending on its sensitivity to the continuity of technological processes;

·            providing additional peak power without the need to apply for technological connection to the grid company.

Integration of ESS with renewable energy sources

The main problem of RES-based power plants is the lack of possibility of delivering declared capacity due to the stochastic nature of renewable resources and the complexity of their forecasting. Moreover, depending on the climatic conditions, the output power of power plants based on renewable energy sources can be fluctuating in nature, due to a sharp change in the weather, for example, sharply variably cloudy leads to a sharp change in the power of a solar power plant, churlish blasts affect the production of wind power plants, etc.

Based on the above, we can distinguish two areas of integration of ESS and RES:

·            ESS directly within the RES power plant (including distributed generation at the consumer)

·            ESS and RES within the local section of the power system/ power node or isolated power system.

Use of ESS within RES power plant allows ensuring the minimum declared power of the power plant, ensuring the predictability of the plant's output in the daily range and improving the accuracy of the planned schedule of power output. ESS will allow redistributing the generation profile during the day, accumulating excess electricity depending on the scheme of integration of ESS at the power plant, and reducing the fluctuating nature of renewable energy generation. In case of use of ESS and RES at the consumer allows minimizing/eliminating flows to the external grid and optimizing operating modes of own equipment.

In case of use of ESS within the local section of the electric power system with a large volume of RES, they can provide an additional volume of RES generation and solve problems with the quality of electricity supplied to the grid. Peak of renewable energy production does not coincide with the peak of consumption, forming surpluses in some periods. ESS allow eliminating the daily imbalance between the production of renewable energy and demand for electricity, and, thereby, further increasing the share of renewable energy in the balance of electric power system.

Also, as mentioned earlier, ESS will allow reducing the cost of grid infrastructure, ensuring the redistribution of power from renewable energy sources during the day, eliminating the flow of power from energy-deficient areas (during certain operating hours), eliminating the cost of reconstruction of tie-stations and preserving the volume of environmentally friendly electricity in the region.

It is necessary to separately consider the use of energy complexes based on RES and ESS for power supply of isolated electric power systems (objects distant from centralized power supply grids). Impracticability of building long power lines to supply relatively low-capacity facilities makes the decision to use energy complexes based on renewable energy sources or a combination of RES and ESS a competitive solution today. ESS acts as a guaranteed source of power and voltage, and in combination with an automated control system allows optimal using the RES to ensure reliable power supply. Integration of ESS and RES systems for parallel operation with diesel generator plants allows saving expensive imported diesel fuel, reducing operating costs by switching off diesel generation at certain hours of time or optimizing the load schedule for diesel generator plants.

Hevel EG has already implemented a number of projects in Russia (see Figure 3) with use of ESS, both at the system level (modernization of existing 5 MW ESS with use of 580 kWh ESS, construction of 10 MW ESS and 4 MW/8 MW*h ESS), and at the autonomous level (for isolated regions) of hybrid SPP complexes+ ESS for fuel economy at diesel power plants.

ESS technologies

Currently, the lithium-ion technology-based ESS are a proven solution for problems of integrating RES into the electric power system, performing system services (frequency, voltage regulation, etc.). The main battery technologies that are used in projects can be divided into the following groups.

1) Lead-acid and alkaline batteries

Lead-acid, nickel-iron and nickel-cadmium batteries are traditional technologies of electrochemical batteries, the technology of which has been known since the beginning of the last century.

There are modifications of technologies that can bring the number of cycles of battery pack to 5 and 10 thousand, respectively.

2)       Sodium-sulfur batteries (NaS)

It is also a fairly old proven technology that has a number of drawbacks that have led to their displacement largely from the market by lithium-ion technology. In the working order, the electrolyte must be heated to 300-350°C, which leads to a relatively high energy consumption to maintain the battery in operation, and also takes time to bring it into working condition. In addition, the high temperature of the electrolyte makes the battery a fire hazard, given the high corrosivity of the expanded sodium anode. Currently, there are new developments in the field of low-temperature batteries based on sodium salts.

3)       Lithium-ion batteries

Lithium-ion technology drives are the dominant technology today. Currently, the projects include the following main types of Li-Ion-based technologies:

·            Lithium Titanate (LTD)

·            lithium-iron-phosphate (LFP)

·            lithium-cobalt oxide (LCD)

·            lithium-nickel-cobalt-aluminum (NCA)

·            lithium-manganese-cobalt (NIVIC)

·            lithium-magnesium oxide (LIVID)

4) Flow batteries

This is a relatively new technology with a growing market share. The first projects began to appear starting in 2012, which is two years later than the first relatively large projects based on lithium-ion technologies. Due to the development of renewable energy technologies, which are characterized not only by daily but also seasonal generation irregularities, interest is aroused by flow-through redox-vanadium batteries and systems based on the hydrogen cycle.

5)       Super capacitors

Storage devices that have a huge (up to several hundred thousand cycles) cycling resource, but a small capacity and a high cost per kWh. Super capacitors are designed to respond quickly and deliver power in short periods of time, which is relevant in the case of a large share of renewable energy sources.

6)       Electromechanical storage devices

Like super capacitors, they are a storage system with a very small storage capacity. Flywheels are a niche solution that can be used for consumers who value quality, or in combination with diesel - as a backup source.

7)       Compressed air systems

Such systems are capable of providing power for a longtime. Compressed air is a technology used, usually, in the wholesale market for quite large groups of consumers.

To sum up what has been said

Use of ESS allows increasing the fl exibility of the power system, increasing the share of renewable energy integration and providing a number of functions for system operator in order to improve the reliability and quality of electricity. This affects the rapid development of ESS technologies around the world. At the moment, the most widely used and developed technology is storage systems based on lithium-ion technologies.

However, the integration of storage systems also requires an increase in the number of projects, their application requires study and development at all stages, such as:

·            development of technical solutions and development of technologies that allow effi cient and economically justifi ed using the ESS, depending on the purpose in the electric power system;

·            elaboration of regulatory documentation and removal of administrative barriers by all participants of energy industry (system operator, grid companies and power supply companies, power generators, operators of the electricity and capacity market);

·            elaboration of transparent market mechanisms for stimulating and enabling the implementation of ESS projects.