1 Introduction

Scope of this Section J is to make a high-level techno-economic review of some advanced energy storage techniques, different from the ones already assessed in the previous sections of this report. These alternative techniques are becoming a realistic option in response to the challenges of the liberalized electricity market and the need to cover intermediate and peak load constraints, as well as to follow the daily and seasonal variation of the electricity demand. As a consequence, these energy storage technologies have potential for significantly reducing the need for operating power plants flexibly.

By introducing a power buffer storage for the electric grid, it is possible to store energy when production is higher than demand, while using it in the opposite situation.

Depending on the storage device, power and storage capacities and reaction time, several grid requests can be met, as also summarised in the following Table 1.1-1 and further discussed in this section.

Table 1.1-1: Energy storage applications


Load management

Two different aspects have to be considered for load management application, both significantly reducing the need for power plants to operate flexibly.

Load levelling consists in storing the electricity produced during off-peak hours and using it later, to meet peak demand. As a result, the overall power production requirements becomes flatter and thus cheaper base-load power production can be increased.

In load following application, the energy storage device acts as a sink when power required falls below production levels and acts as a source when power required is above production levels.

Spinning reserve

Energy storage devices used for spinning reserve usually require power ratings of 10 MW to 400 MW and are required between 20 to 50 times per year.

Depending on the response characteristics, the energy storage device can participate to the fast response spinning reserve, characterised by a quick response of the power capacity to network abnormalities, or the conventional spinning reserve if a slower response is required to the power capacity.

Back-up power

Energy storage devices can provide stabilization to the grid in case of electricity outage, until backup generation sources can be brought online, by absorbing or delivering power to generators when needed to keep them turning at the same speed. These faults induce phase, voltage and frequency irregularities that can be corrected by the storage device. This reduces the costs of electrical grid failure.

Fast response and high power ratings are required.

Transmission Upgrade Deferral

Transmission line upgrades are required to manage the generating expansions. Energy storage devices can be used instead of upgrading the transmission line until it becomes economical to do so.

Typically, transmission lines must be built to handle the maximum load required and hence it is only partially loaded for the majority of each day.

Therefore, by installing a storage device, the power across the transmission line can be maintained constant, even during periods of low demand. Then, when demand increases, the storage device is discharged preventing the need for extra capacity on the transmission line to supply the required power, and consequently avoiding upgrades in the transmission line capacities.

Peak Generation

Energy storage devices can be charged during off-peak hours and then used to provide electricity during short peak production periods.

Renewable Energy Integration

Energy storage technologies can also improve the availability of energy from renewable and intermittent sources, as the sun and the wind, characterized by a wide variation of the energy that they can provide. Electricity storage can smooth this variability, acting as a ‘renewable source back-up’ storing unused electricity to be dispatched at a later time.

A storage system used with renewable technology must have fast response times (less than a second), excellent cycling characteristics and a good lifespan (100 to 1,000 cycles per year).

End-Use Applications

The most common end-use application is power quality, which primarily consists of voltage and frequency control. These applications require short power durations and fast response times, in order to level fluctuations, prevent voltage irregularities and provide frequency regulation.

1.1 Energy storage technologies

There are currently several promising energy storage technologies, characterized by different power and storage capacities and reaction time, as shown in Figure 1.1-1:

  • Pumped hydropower and compressed air energy storage are characterised by large power and storage capacities;In Pumped-Hydropower Energy Storage (PHES) systems water is pumped into a storage reservoir at high elevation during times when electricity is inexpensive and in low demand. Stored water is then released and used to power hydroelectric turbines when demand for power is high. New developments in pumps and turbines, allowing for adjustable water flowrates have increased the flexibility and efficiency of pumped storage hydroelectric power. However, some limitations, such as suitable geographic location and facility size/capacity still exist.In Compressed Air Energy Storage (CAES) system, high efficiency compressors can be used to force air into underground reservoirs, such as mined caverns. When the commercial demand for power is high, the stored air is allowed to expand to atmospheric pressure through turbines connected to electric generators that provide power to the grid.
  • Battery Energy Storage (BES) devices are characterised by a wide range of power and storage capacity;Batteries can be used in a lot of energy storage applications due to their portability, ease of use and variable storage capacity. In particular, they can stabilize electrical systems by rapidly providing extra power and by leveling oscillation in voltage and frequency. Currently, numerous batteries including lead-acid, flow, sodium-sulfur, and lithium-ion all have commercial applications. However, many battery types have only limited market penetration, as they are expensive, or have short lifetimes.
  • Flywheels, superconducting magnetic energy storage (SMES) and electrochemical capacitors are characterised by small power and/or storage capacities.Flywheels store energy in a spinning disk on a metal shaft. Two generations of flywheels have raised storage capacity through increased disk mass (using steel) and increased rotation speeds (using light weight composite materials for the disk), but technical limitations are still present. New prototypes utilize magnetic levitation to increase speed and mass while minimizing previous technical issues. Wide commercial energy storage application of flywheels is primarily limited by materials properties and cost.Superconducting Magnetic Energy Storage devices are composed of superconducting windings that allow electric current to be stored indefinitely with little resistive energy losses. When the stored energy is needed, these devices can be discharged almost instantaneously with high power output over short time periods.Increasing the size of the windings can increase the amount of stored energy, but the increased magnetic field associated to the larger coils becomes difficult to be contained.In addition, as low temperature is needed to have superconducting property, expensive coolants are required.Electrochemical capacitors store energy in the form of two oppositely charged electrodes separated by an ionic solution. They are suitable for fast-response, short-duration applications, such as backup power during brief outages, and for stabilizing voltage and frequency. They have a temperature-independent response, low maintenance and long projected lifetimes (up to 20 years), but relatively high cost.

Power conversion systems (PCS), even if they do not represent a storage device explicitly, are essential for electricity storage applications, as they constitute the interface between the storage system and the electricity grid. A PCS is able to make the necessary conversions so that the stored energy can be taken from or returned to the grid in the correct phase, frequency and level of demand.

Figure 1.1-1: Capabilities of Existing Electricity Storage Technologies


Main characteristics of these technologies, which are further assessed in the following sections, and their applications are also summarised in Table 1.1-2.

Table 1.1-2: Energy storage technologies characteristics

Storage device Storage medium Power Capacity Storage Capacity Applications
Pumped-Hydroelectric Energy Storage Mechanical Large Large Load levelling, frequency regulation, peak generation…
Compressed Air Energy Storage Mechanical Large Large Load following, frequency regulation, voltage control
Lead-Acid Battery Chemical Medium Medium Back up power USP system
Nickel-Cadmium Battery Chemical Medium Medium storage for solar generation engine start-up
Sodium-Sulphur Battery Chemical Medium Medium Load management Power quality
Vanadium Redox Flow Battery Chemical Medium Medium Integration of renewable resources
Polysulphide Bromide Flow Battery Chemical Medium Medium frequency regulation voltage control
Zinc-Bromine Flow Battery Chemical Medium Medium Integration of renewable resources frequency regulation
Flywheels Mechanical Small Small USP system Integration of wind farms
Supercapacitor Energy Storage Electrical Small Small Power quality
Superconducting Magnetic Energy Storage Magnetic Small Small Integration of renewable resources Transmission upgrade deferral

Cost figures of the different storage technologies are shown in Figure 1.1-2. Cost ranges in this chart are referred to 2Q2001, so approximately 1.45 escalation factor should be considered for these data.

It is also noted that costs of these energy storage techniques might be changed, as a result of the normal technological development of last years.

Figure 1.1-2: Costs of Existing Electricity Storage Technologies