Energy Storage Systems for Power Networks

Applications of Storage Technologies in Power Systems

Storage technologies can provide various services in electrical networks.

Why We Need Storage

The power system needs to be flexible because conventional power plants are out of merit-order in the electrical markets.
Revenues for generators: Electricity price x Generation
Power plants with lower variable costs gain additional revenues to cover investment costs or gain additional profit.
Generators access the day-ahead electrical market based on their variable operational costs, with the cheapest ones selling their generation first.
The mix of renewables and flexible generation affects the electricity price.
Flexibility options for the power system:
- Grid reinforcement.
- Demand-side response.
- Flexible generating plants.
- RES curtailment.
- Energy storage.
Flexibility (power balancing) at different time scales (seconds, hours, days, seasons).
Grid ancillary services: power reserves.

Review of Applications

General overview:
- Services for the final user.
- Ancillary services for the operation of transmission and distribution systems.
- Support to transmission and distribution infrastructures.
- Services for the balance of the power sector at large scale.

Services for the Balance at Large Scale of the Power Sector

These services involve using energy storage technologies with very large power and energy storage capacity.
Large storages can be exploited for a competitive or private user, according to the indications of the network operators.
Storage systems can store surplus renewables during valley hours, avoiding curtailment.
The potential for energy storages to provide these services increases with increased renewables integration into the network.
Managing stored energy can help optimize the electrical network techno-economically.
Stored energy could be used in peak hours, avoiding the operation of fuel-based generators, thus reducing the environmental footprint.
Large energy storage capacity can also help reduce the commission of new generation capacity in the network.

Support to Transmission and Distribution Infrastructures

Energy storage systems can defer in time the increase of generation capacity of the network and updates in transmission and distribution networks.
Managing large amounts of power during peak hours can shave peak loads flowing through high and medium voltage cables that are not prepared for that.
Such management translates into an extended life of existing infrastructure and better power quality for consumers in general.

Ancillary Services for the Operation of Transmission and Distribution Networks

These services are mainly addressed to the operators of the transmission and distribution networks and address aspects related to the planning of such infrastructures in a mesoscale level.
Large and medium-scale energy storages can be operated, addressing the intraday power balance in the network; the services are named network ancillary services.
The catalog of ancillary services associated with energy storages is large and responds to the need to manage the complexity of an electrical network, where a continuous balance between generation and demand is required.
A balanced electrical network translates into constant frequency and voltage levels throughout.
In case of imbalances between generation and demand, the system should manage power reserves, usually provided by conventional generation plants (e.g., fuel or gas-based ones).
Energy storage technologies could also provide such reserves.
These technologies could contribute to the voltage control of the network as well and help to the black start of diverse types of generators, such as wind power driven ones.
The provision of ancillary services from generation plants is a regulated activity.
Some services, such as voltage control or black start capability, are even considered requirements (not remunerated) for the grid connection of renewable-based plants (with installed power above a certain threshold level).
Power reserves for frequency regulation are usually remunerated services that controllable power plants, such as fuel or gas-based ones, provide, applying to the corresponding markets.
Resolution of the Spanish ministry of industry, energy, and tourism (February 9th, 2016) modifies the royal decree 413/2014, opening the door for renewable generators to providing network ancillary services (but not yet to energy storages).

Services for the Final User

The operator/proprietary of the energy storage systems providing the previous services can be the network operator (or a third private party); for the applications included in this last category, the operator of the energy storage system can be the final user, from both the domestic and tertiary sectors.
Address storage systems rated at 10 MW in power at most.
From a technical point of view, the energy storage applied here can be used to:
- Improve power quality for final users (e.g., filtering of harmonics, protection under short-circuits, supply failures, voltage control).
- Enable active management of consumption and demand, including the consumption of buildings and electric vehicles, based on exogenous signals such as energy price and other technical considerations.
One clear goal is to reduce dependency on the external grid or, in other words, to promote self-consumption.

General Characteristics for the Application of Storages

Services for the balance at large scale of the power sector
- Application: Generation capacity, seasonal storage; Integration of surplus of renewables in valley hours
  - Capacity requirements: Up to 100x MW; Between 10 MW and 100x MW
  - Time response: Minutes
  - Cycles / year: Between 5 and 100; Between 300 and 500
  - Eligible technologies:
    - Mainly CAES and pumped hydro.
    - Also hydrogen and batteries (especially those with easy scalability, such as flow batteries).
    - For systems rated at 10x MW, batteries based on lithium, lead, or sodium are commercially available alternatives.
Support to transmission and distribution infrastructures
- Application: Shaving of peak loads and update deferral for infrastructures
  - Capacity requirements: Between 10 MW and 200 MW.
  - Time response: Seconds
  - Cycles / year: Between 300 and 500
  - Eligible technologies:
    - To achieve 100x MW, suitable options are CAES and pumped hydro.
    - For systems up to 10x MW, batteries based on lithium, lead, or sodium are commercially available alternatives.
Ancillary services for the operation of transmission and distribution networks
- Application: Tertiary power reserves, Secondary power reserves, Primary power reserves
  - Capacity requirements: Between 1 MW and 100 MW
  - Time response: Seconds
  - Cycles / year: Between 200 and 400
  - Eligible technologies:
    - To achieve 100x MW, suitable options are CAES and pumped hydro.
    - For systems up to 10x MW, batteries based on lithium, lead, or sodium are commercially available alternatives.
- Application: Inertial response
  - Capacity requirements: Between 1 MW and 100 MW.
  - Time response: Milliseconds.
  - Cycles / year: Between 200 and 400.
  - Eligible technologies: Batteries and secondary batteries are suitable. Lithium-ion batteries are especially suitable. Also flywheels can easily reach tens of MW in power, and present higher cyclability and shorter time response than batteries.
- Application: Black start
  - Capacity requirements: Between 5 MW and 50 MW.
  - Time response: Seconds.
  - Cycles / year: Up to 50.
  - Eligible technologies: Batteries and secondary batteries are suitable. Lithium-ion batteries are especially suitable. Also flywheels can easily reach tens of MW in power, and present higher cyclability and shorter time response than batteries.
- Application: Ramping limitation and voltage control
  - Capacity requirements: Between 1 MW and 100 MW.
  - Time response: Milliseconds
  - Cycles / year: Between 1000 and 5000.
  - Eligible technologies: The high requirements for cyclability yield flywheels and supercapacitors as especially suitable canditates for the provision of this service. Among batteries, the cyclability and short time response of lithium-ion batteries is also remarkable for this service.
Services for the final user
- Application: Electric vehicle integration / self-consumption
  - Capacity requirements: Between few kW up to 10 MW.
  - Time response: Seconds
  - Cycles / year: Between 300 and 500.
  - Eligible technologies: Batteries. For stationary systems for domestic installations, lead acid batteries are the preferable option. However, lithium-ion based ones and flow batteries are becoming remarkable competitors.
- Application: Power quality
  - Capacity requirements: Between few kW up to few MW.
  - Time response: Milliseconds
  - Cycles / year: Between 1000 and 5000
  - Eligible technologies: Flywheels and supercapacitors. The market is dominated by diverse technologies including lead-acid batteries, such as UPS systems and active filters. These systems are actually mature and competitive.

General Overview of Projects Worldwide

The Department of Energy (DOE) of the United States maintains a public database (DOE Global Energy Storage Database) with a collection of projects around the application of energy storage systems in electrical systems.
- Around 1636 projects worldwide.
- Total installed power 193 GW (183 GW in pumped hydro).
- The other 10 GW are distributed in:
  - 3.3 GW for electrochemical systems.
  - 3.6 GW in thermal energy storage.
  - 2.6 GW in CAES and flywheels.
  - 18 MW in hydrogen.
According to the database of the DOE, lithium-ion batteries are attracting much interest nowadays in I+D (636 out of 1636 projects are around this technology).
- Such a volume of activity should favor a dramatic cost reduction for these batteries in a short and mid horizon.
The electromechanical system most explored so far is pumped hydro.
- The technological maturity and the associated low costs of energy are two of the main advantages for these systems.
- The search for new concepts so as to minimize environmental impact is one of the principal research vectors.
Sodium-based and flow batteries are of increasing interest.
- The easy scalability and reduced cost of flow batteries depicts them as suitable candidates for multi-megawatt solutions.
- Sodium-based batteries (specially low-temperature ones) are even considered future competitors to lithium-ion based ones.
In terms of the applications provided by the storage systems, it is worth noting that the utilization of storages so as to accommodate surplus of renewables is one of the principal aims explored.
In domestic and tertiary sectors, demand management and self-consumption are applications of growing importance, in parallel with the deployment of smart grids.

Example: Smoothing the Output of a Wind Power Plant with Batteries (Japan)

In Futumata (Japan), a battery energy storage system of 34 MW is connected to the output of a 51 MW wind power plant.

Example: Power Smoothing for Wind Power Plants

Fast wind power fluctuations can cause fast voltage variations affecting power quality levels.
High flicker levels can be noted due to cyclic perturbations to the rotational torque (rotating sampling effect) and other factors.
A flywheel can be used so as to smooth out such fast power fluctuations of the wind turbine, thus enhancing its grid integration.
An energy management system should be designed so as to combine the operation of the flywheel with the turbine.

Example: Flywheels for Primary Frequency Support

In Stephentown (New York), 20 MW @ 15 min of power and energy storage capacity are provided by 200 flywheels 100 kW each connected to the 69 kV distribution grid since 2011.
The plant is providing the service of frequency regulation to the operator NYSO.
Flywheels are charged and discharged between 3000 – 5000 times yearly.
Round trip efficiency: 95%.

Example: Vehicles-to-Buildings

Idea: to explore the potential of electric vehicles (EVs) to optimize the energy management in buildings.
To enable a smart energy exchange between EVs and the building, these are interfaced through bidirectional chargers.
A mixed integer linear programming problem (MILP) is formulated, which minimizes the energy bill for the building and for the EV users.
The methodology is tested with a study case concerning the inclusion of bidirectional chargers in a data center.
Results confirm that the proposed strategy is effective in reducing the grid-contracted power by the data center while also benefiting the EV users.
- Without EVs:
  - Power contracted (data center): $1000 kW$
  - Energy bill (data center): $362850 €$
  - Energy bill (EV users): $2863 €$
  - Total energy bill: $365713 €$
    *
- With EVs:
  - Power contracted (data center): $907 kW$
  - Energy bill (data center): $355612 €$
  - Energy bill (EV users): $0 €$
  - Total energy bill: $355612 €$
- Variation:
  - Power contracted (data center): -9.3 %
  - Energy bill (data center): -2.0 %
  - Energy bill (EV users): -100.0 %
  - Total energy bill: -2.8 %