Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 663

• Energy storage capacity is the amount of electrical energy the device can store,

usually measured in kilowatt-hours (kWh) or megawatt-hours (MWh).

• Effi ciency indicates the quantity of electricity that can be recovered as a

percentage of the electricity used to charge the device.

• Response time is the length of time it takes the storage device to start releasing

power.

• Round-trip effi ciency indicates the quantity of electricity which can be recovered

as a percentage of the electricity used to charge and discharge the device.

In addition to the above, the following parameters are also used in describing
rechargeable batteries and fl ow batteries:

• Charge-to-discharge ratio is the ratio of the time it takes to charge the device

relative to the time it takes to discharge the device. For example, if a device
takes 5 times longer to charge than to discharge, it has a charge-to-discharge
ratio of 5:1.

• Depth of discharge ( DoD ) is the percentage of the battery capacity that is discharged

during a cycle.

• Memory effect . If certain batteries are never fully discharged they 'remember'

this and lose some of their capacity.

3 Energy storage plant components

Every energy storage facility is comprised of three primary components: storage
medium, power conversion system (PCS), and balance of plant (BOP).

3.1 Storage medium

The storage medium is the 'energy reservoir' that retains the potential energy
within a storage device. Storage media range from mechanical (PHES), chemical
(BES) and electrical (SMES) potential energy.

3.2 Power conversion system

It is necessary to convert from alternating current (AC) to direct current (DC) and
vice versa, for all storage devices except mechanical storage devices, e.g. PHES
and CAES [3]. Consequently, a PCS is required that acts as a rectifi er while the
energy device is charged (AC to DC) and as an inverter when the device is dis-
charged (DC to AC). The PCS also conditions the power during conversion to
ensure that no damage is done to the storage device.
The customization of the PCS for individual storage systems has been identifi ed
as one of the primary sources of improvement for energy storage facilities, as each
storage device operates differently during charging, standing and discharging [3].