Electrical Energy Storage Systems

2000/10/01 Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Energia berriztagarriak

Today it would be difficult for us to live without electricity. When used so much you can think that the technology is enormously developed and is, since long ago very efficient turbines are used to transform into electric energy. However, it still has some "problems", especially when it comes to transportation (where energy is lost) and storage (since it is not very easy to store electrical energy). In the article we will refer to the tools of storage of electric energy: the accumulators. The previous issue mentions the most recent leaf type accumulators, but this time the general characteristics of the three types of accumulators are presented.

In general, it is considered that electric power should be used at the same time it is produced. Or it is believed at the same time of use. Therefore, large power plants, of one type or another, must take into account the pointer use of electric energy throughout the day and from one day to another. However, as mentioned in the introduction, there are energy accumulators.

That of accumulators is a relatively wide field, perhaps not, but we could say that it is an expanding area, since as technologies advance, the efficiency of accumulators is increasing. There are already several models and types on the market (see chart 1). They are not used in places where a lot of energy is required, but they are very useful in some situations. For example, to store excess energy in isolated systems or to meet peak power demands. These energy storage systems or accumulators can be classified into three groups: 1) electromechanical accumulators (EMB) or Flywheel systems, 2) electrochemical accumulators (Pb-acid, nickel, Ni-Cd, NiMH, lithium...) and 3) laminar accumulators or ultracondensers.

Electromechanical accumulators

Also called flywheel systems. They use kinetic energy. They are small mass but high-speed systems (see figure 1). The energy obtained is expressed by the formula E = 1/2 J W2, where J is the inertia of the rotor (= m.R2) and W is the angular velocity. The components or parts of these accumulators are rotor, bearings, motor/generator, power and control electronics and vacuum generation structure. The function of rotors is to store kinetic energy. They can be single-ring or multi-ring, which in turn are made of steel or carbon fiber. The latter are stronger and lighter, and rotor failure reduces damage. They can reach speeds of 1,400-2,000 m/s and generate a specific energy exceeding 700 Wh/Kg. The motor transforms kinetic energy into electric or vice versa. The most used are those that generate a continuous magnetic field, among which are those of Halbach ordination. In this arrangement the magnets are alternately arranged in different directions to create a continuous magnetic field. The electronic device of the accumulator adapts the voltage and controls the operation of the rotor and the motor. The investor is the one that presents most of the difficulties, since it can have problems when it comes to increasing the speed a lot. The load of the rotor is supported by the bearings. They can be mechanical (steel, ceramic or hybrid), pneumatic or magnetic. Active magnetic bearings require a control system and are already marketed (60,000 rpm prototypes have been made). ). Liabilities, however, are in experimental phase, do not require control systems due to their natural stability and the materials are hyperconductive. The rotor is also empty inside a wrapper. This reduces the influence of friction and, in addition, the other components of the accumulator could be protected in case of deterioration or rupture of the rotor.

These mechanical accumulators are mainly used in vehicles (electric or mixed and locomotives), uninterrupted power supply systems (UPS), to introduce high quality power into the power grid, satellites and fast chargers.

Electrochemical accumulators

They are the most common accumulators. There are several types, but in general they can be grouped into four groups: lead acid (Pb-acid), nickel (Ni), lithium (Li) and metal-air. The essence of all is the same: inserted in an electrolyte that can be liquid or solid, but separated, there is a positive and negative electrode; when putting in contact both electrodes, the electrons pass from one side to another generating electrical current. Although not all of us will expose them, in the diagram of Ragonne you can see the specific energy and powers of most electrochemical accumulators.

The most used are those of pb-acid and the most developed technology. Of those who use nickel, the most developed technology is Ni-Cd, but now they are replacing NiMH with accumulators (nickel-metalhydrate), since cadmium is very polluting.

Those of metal air use as anode zinc (Zn), iron (Fe), aluminum (Al) and lithium (Li) and cathode is oxygen of the air, so only the recharge anode changes.

At the moment, as has already been indicated, it is the lead ones that are most used and the ones that are going to be used in the short term, and it is not known what will be the substitute in the future. It seems that NiMH, Li-ion and Li-polymer can be substitutes.

Ultracapacitors

They are also called page-type accumulators. They store energy by storing electrical charges, hundreds of times more than conventional capacitors. They can also give very large energy spikes.

There are two types: two layers and several. The two layers have two electrodes and electrolyte and the load accumulates on the surface between the electrode and the electrolyte. The energy stored depends on the electrode: those of metal oxides have a large specific capacity and a low surface, while those of carbon have a low specific capacity and a large surface. They are also testing the ultracapacitors that will have both mixed. Several layers have very thin layers and interspersed dielectric material. They are so fine accumulators that the control of the structure is atomic.

Ultra-capacitor technology is very new, but it is believed to be of great importance in the future. For more information on the September 2000 issue.