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1.4.Energy Storage

1.4.1.Introduction

The use of electricity generated from renewable energy sources, especially electricity generated by wind turbines, as these sources fluctuate independently on the demand requires energy storage systems to regulate supply, to achieve the equilibrium between supply and demand.

The energy storage has many technical and economical advantages. The storage of conventional generated energy can compensate the interrupting in energy generation of a power unit and ensures energy supply according to commercial obligation. The stored energy produced from renewable energy sources can make predictable this type of energy. Power demand of end users has large variation during the day, thus the ratio between peak and average power levels can reach a value of 10 [1]. This variation leads to over-size of generation and transport equipments, as they are designed to operate at peak demand level not at average daily demand. By reducing the load fluctuations the supply becomes more predictable and it is not necessary to extend the existing transportation and distribution facilities. By levelling the end user consumption the installed power can be reduced and the existing power network should not be oversized. Despite the load levelling, the power demand varies considerably during the day or from season to season and the use of energy storage becomes important.

The flexibility of energy storage and retrieval systems can provide instant response to demand, leading to load levelling and more flexible network [1]. Network fluctuations can be caused by temporary production deficit or production failures. In this case the energy storage system should ensure a cost effective double conversion electricity-storable energy-electricity.

1.4.2.Classification of energy storage technologies

Energy can be stored in many forms: mechanical, thermal, chemical and electrical energy. There are many energy storage technologies which meet specific technical and economic criteria of different applications and needs.

The storage techniques can be divided into four categories, according to their applications [1]:

  • low-power application in isolated areas, essentially to feed transducers and emergency terminals;

  • medium-power application in isolated areas (individual electrical systems, town supply);

  • network connection application with peak levelling;

  • power-quality control applications.

The energy storage techniques can be also classified according to the following criteria [1]:

- type of application: permanent or portable;

- storage duration: short or long term;

- type of production: maximum power needed.

In Table XVI, the energy storage technologies are grouped into five categories: direct electric, electrochemical, mechanical, direct thermal, and thermochemical [2]. In this table, the principal applications of energy storage technologies are:

- utility shaping (the use of very large capacity storage devices to fit electric demand, when a renewable resource is not producing sufficient generation);

- power quality (the use of very responsive storage devices, capable of large changes in output over very short timescales, to smooth power delivery during switching events, short outages, or plant run-up). Power-quality applications can be implemented at central generators, at switchgear locations, and at commercial and industrial customers’ facilities;

- distributed grid technologies enable energy generation and storage at customer locations, rather than at a central (utility) facility. The distributed grid is an important, enabling concept for photovoltaic technologies that are effective at a small scale and can be installed on private homes and commercial buildings;

- automotive applications include battery-electric vehicles, hybrid gasoline–electric vehicles, plug-in hybrid electric vehicles, and other applications that require mobile batteries larger than those used in today’s internal combustion engine cars.

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