A necessity to store energy in purpose to use it later in time arises in different occasions. For example, in uninterruptible power supplies systems examined in Chapter 10, stored energy is required when a drop off of the electrical network system happens. In systems used to obtain energy from renewable sources examined in Chapter 9, the necessity to store energy also arises. This is explained by the fact that energy is not always possible to be consumed at the moment of its generation and of the same quantity. For example, the periods of strong sun lightening or strong wind may not coincide with the periods of maximum consumption.
TopOverview
A necessity to store energy in purpose to use it later in time arises in different occasions. For example, in uninterruptible power supplies systems examined in Chapter 10, stored energy is required when a drop off of the electrical network system happens. In systems used to obtain energy from renewable sources examined in Chapter 9, the necessity to store energy also arises. This is explained by the fact that energy is not always possible to be consumed at the moment of its generation and of the same quantity. For example, the periods of strong sun lightening or strong wind may not coincide with the periods of maximum consumption.
Nowadays, the following methods to store energy are developing and using:
- •
Storage of energy as electrochemical energy
- •
Storage of energy as electromagnetic energy
- •
Storage of energy as electrostatic energy
- •
Storage of energy as mechanical energy
As a consequence of all the above mentioned energy types, electrical energy, which is used by consumers, can be obtained.
Basic indicators of the processes of storage and giving energy are efficiency coefficient and long-lasting time – number of cycles charge/discharge. Figure 1 displays a comparison among the different methods of energy storage.
Figure 1. Comparison among different methods of energy storage
The figure includes: accumulator batteries –lead-acid, nickel-cadmium (NiCd), sodium NaS, lithium-ion (LiIon); Fuel Cells and Flow Batteries; Supercapacitors (Supercaps); Flywheel batteries; storage in magnetic field under super conductivity (SMES).
It appears in Figure 1 that supercapacitors and Flywheel batteries have the best indicators considering long-lasting time and efficiency coefficient.
Besides the efficiency coefficient and number of cycles, other important indicators are specific power and specific energy. Comparative data about different methods for energy storage also exist for the last mentioned indicators (Ter - Gazarian, 1994).
TopStorage Of Energy As Electrochemical Energy
The most widely spread method for energy storage is its storage as electrochemical energy. Accumulators are devices used to realize that storage. The first Pl-acid accumulator was made by Gaston Plante in 1859. Basic principle of operation and also chemical processes of Pl-acid accumulator have remained during the time. The researches in Pl-acid accumulators are orientated towards their seal hermetically and changes in their construction.
To choose accumulator, its specific energy (Wh/kg), its specific power (W/kg) and their change dependent on the duration of energy giving at discharge are used as general indicators. Also, choosing an accumulator, the following parameters are taken in consideration – efficiency coefficient, expiring period, necessity of accumulator serve, technical safeness, price, etc.
Figure 2 illustrates the change of the specific energy and specific power of different accumulators at different discharge time – form 0.1 to 100 hours. Also, data for fuel cell is presented. An idea about the possibilities of the different elements may be obtained using this data.
Figure 2. Data about specific power and specific energy of different types of accumulators and fuel cells
Curves 1, 2 and 3 are for primary electrochemical elements (so-called unchargable batteries). Curve 4 is for Pl-acid accumulators, curve 5 –NiCd accumulators, curve 6 - Fe-Ni accumulators, curve 7 – silver- zinc accumulators, curve 9 – NaS accumulators, curve 10 – Al accumulators, curve 11 – Li accumulators and curve 8 – fuel cells. Accumulator efficiency decreases at lower discharge times, i.e. higher discharge currents. Figure 2 shows the advantage of the fuel cells regarding the specific energy, as well as this of NiCd accumulators regarding the specific power.