4 Alternative energy storage techniques

Some energy storage techniques, alternative to those discussed in the previous sections, 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. There are currently several promising energy storage technologies, characterized by different power and storage capacities and reaction times, as shown in Figure 4-1:

  • Pumped hydropower and compressed air energy storage, with large power and storage capacities;
  • Battery energy storage device, with a wide range of power and storage capacity;
  • Flywheels, superconducting magnetic energy storage (SMES), electrochemical capacitors, characterised by small power and/or storage capacities.

Figure 4-1: Capabilities of Existing Electricity Storage Technologies

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Pumped hydroelectric energy storage (PHES) is the most mature and largest storage technique available, providing about 3% of the world’s global generating capacity. PHES plants consists of two large reservoirs at different elevations and a number of pump and hydraulic turbine units. During off-peak electrical demand, water is pumped, using excess energy generated by other sources, from the lower reservoir to the upper reservoir, where it is stored. Once required, i.e. during high electricity demand period, the water in the upper reservoir is released through the turbines, producing electricity.

The main disadvantage of a PHES facility is the requirement of two large reservoirs with a sufficient amount of hydraulic head between them. A new concept that may potentially overcome this drawback is Underground Pumped-Hydroelectric Energy Storage (UPHES), as the upper reservoir is at ground level and the lower reservoir below earth’s surface.

PHES facilities are characterised by large power and storage capacities and fast reaction time, thus identifying load-levelling as the ideal application, though they can participate to peak load market and frequency control.

Compressed Air Energy Storage (CAES) cycle is essentially a variation of a standard gas turbine generation cycle, in which the air compression is separated from the combustion and steam generation cycle.

Air is compressed using off-peak electrical power, which is taken from the grid to drive a motor, and stored in large underground storage reservoirs. During peak demand period, the compressed air is released from the storage facility, heated and used to burn natural gas in the combustion chambers. The resulting combustion gas is then expanded in the turbine expander, generating electricity.

CAES is a very large scale storage technology with fast reaction time and then it is ideal for load following applications, ancillary services and renewable integration. Two CAES plants are in operation today: a 290 MWe plant in Huntorf, Germany, and a 110 MWe plant in McIntosh, Alabama.

Battery Energy Storage (BES) systems store electric energy in electrochemical form in the same way as conventional batteries, though on a large scale. Two electrodes are immersed in an electrolyte, while a chemical reaction generates a current when required. In Flow Battery Energy Storage (FBES) two charged electrolytes are pumped to the cell stack where a chemical reaction occurs, generating a current when required.

Using a battery energy storage device, a Power Conversion System (PCS) is required to convert from alternating current (AC) to direct current (DC) while the energy device is charged, and vice versa, when the device is discharged.

Main characteristics of these technologies and their applications are also summarised in Table 4-1. Cost figures of the different storage technologies are shown in Figure 4-2. Cost ranges in this chart are referred to 2Q2001, so approximately 1.45 escalation factor should be considered for these data. Costs of these energy storage techniques might be varied, as a result of the normal technological development of last years.

Table 4-1: Main Energy storage technologies characteristics

Storage device Storage medium Power Capacity Storage capacity Remarks
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. Life: 5 y, 250-1,000 cycles
Nickel-Cadmium Battery Chemical Medium Medium storage for solar gen., engine start. Life: 10-15 y, 1,000-3,500 cycles
Sodium-Sulphur Battery Chemical Medium Medium Load management, Power quality Life: > than others; 2,500 cycles
Vanadium Redox Flow Battery Chemical Medium Medium Integration of renewable resources. Life: 7-15 y, 10,000 cycles
Flywheels Mechanical Small Small USP system, Integ. 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

Figure 4-2: Costs of Existing Electricity Storage Technologies

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