Other sources of dynamic electricity

Chemical sources

There are several variations of this method: the most common are those of the primary and secondary cell. These are discussed in detail in Chapter 3.

A less common method is the combining of elements such as hydrogen, carbon and oxygen to form com­pounds such as methane, water or carbon dioxide. Chemical bonding involves ions and the transfer of elec­trons from one atom to another, energy being either gen­erated or absorbed in the process.

Adaptation of this method leads to a variation of a fuel cell. One type of fuel cell consists of two chambers with two porous elec­trodes separated by an electrolyte. Hydrogen and oxygen are supplied to the two chambers and, in the presence of a catalyst, react to provide ions and free electrons. A by-product is water. Fuel cells provide high efficiencies and under ideal conditions can last for many years.

A typical operating figure is 70%. The fuel cell proper has no moving parts, produces no noxious fumes and some fuel cells produce potable water. This is an important reason for their use in space travel. Some fuel­ cell systems require extensive (and expensive) auxiliary equipment and their use is accordingly restricted.

Solar cells

The basis of the solar cell is a thin junction of two semi­conductors. When light is shone on the junction, light energy is converted direct to electrical energy. Theoret­ically of unlimited life, they are subject to some deteri­oration of the materials and a consequent falling off in efficiency. Solar cell panels are available for battery charging and are finding a rapidly increasing use in remote areas. A similar. type of cell is called a photo­voltaic cell, the modern type being of similar material to the solar cell. The light falling on the cell generates a voltage which is read against a scale. Many modern automatic cameras use the cell to assist in setting the camera adjustments.

Typical operating efficiencies are in the region of 5-7%. Figure 2.8 shows one use for solar cells-charg­ing batteries in remote areas.

Electrostatic sources

“There are many effects of electrostatic charges. An elec­tric shock from a car door-handle is one. The modern electrical meter case is usually of transparent plastic and rubbing it clean with a piece of cloth often causes the pointer to move across the scale without the meter being connected. Two common ways of producing substantial electrical charges are by the Wimshurst machine or the van de Graaff generator.

The Wimshurst machine consists of two parallel insulating plates rotating in opposite directions, the charge being conducted away by two sliding contacts (see Fig. 2.9).

The Van de Graaff generator has a motor-driven rubber belt rubbing lightly against a comb. At the other end of the belt another comb conducts the charge to a hollow ball. This type of generator has its main use in the study of high-voltage effects.

The uses for static electricity in industry are steadily growing. Typical uses are dust precipitation and spray painting. The dut particles are first charged negatively by passing them through a grid and then through a pos­itively charged collector. The charged particles are attracted to the collector and are contained in a hopper for later disposal, rather than being dis 1harged directly into the atmosphere. In the case of spray painting, the object to be painted is made negative with respect to earth and the paint particles are attracted to the object. The result is a more complete coat of paint with less waste. It is important to remember that very high volt­ages are used (typically 50 k’,’) and precautions have to be taken when working on electrostatic units.

Thermoelectric sources

The thermoelectric method of voltage generation is mostly used a:, a means of temperature measurement. The thermocouple itself consists of two dissimilar metals joined at a point where the heat is applied (hot junction). A potential difference is created and appears at the other two ends (cold junction), where it is measured with a meter usually calibrated as a thermometer (see Fig. 2.10).

The potential differences produced are very small and the whole circuit, including the indicator, must have a very low opposition to the flow of electrons. Different conductor materials are used for various temperature ranges, a typical thermocouple being made of copper and constantan (an alloy). For a temperature rise of 200°C the potential difference created would be of the order of 0.009 volt. This would occur when the cold junction was at a temperature of 0°C. If the cold junction temperature was 32°C with the hot junction heated to 2006C, the output voltage would be approximately 0.004 volt. For greater voltages, thermocouples are sometimes connected as a group, so their voltages add up to a higher value. The unit is then called a thermopile.

Piezoelectric sources

When certain materials are subjected to mechanical stress, a voltage is generated across the faces of the material. This is called the piezoelectric effect. Materials most commonly used are naturally occurring crystals such as quartz and Rochelle salts. These are cut into chips (also called crystals), with their faces basically par­allel to one or other of the three axes in the main crystal, and then mounted in holders before being connected into circuits.

 

When a voltage is apl lied to the opposite faces of the chip, it distorts and the movement can be measured. When the chip (crystal) is distorted, a voltage appears at the opposite faces. These two effects occur at a natural rate of resonance, depending mainly on the angle of cut and the physical dimensions of the crystals. As such, their most common use is in radio-transmitter circuits, although they can also be found as the initiating element in echo sounders, ultrasound diagnostic machines in hos­pitals, and for the generation of very-high-frequency sound waves in ultrasound cleaning machines.

In recent times it has been found that ceramic mate­rials can be used to produce the same effect while having the advantages of being hard, chemically inert and not affected by humidity. Because of their ceramic nature they can be moulded into shape during manufacture, while slight variations in composition allow emphasis on specific properties required for a special application. For example, a high-voltage generator, as used in cigarette lighters, oxy-acetylene lighters and fuse ignition for explosives, would have different requirements to those of a transducer used for ultrasonic cleaning and degreas­ing. The first example requires a comparatively long length with small cross-zection while the second requires a large cross-sectional area and a very short length. Figure 2.11 shows an elementary striker ignition system and the accompanying graph shows the voltage which can be generated. A light weight hitting the element at a comparatively high speed is ample to produce the nec­essary voltage. The striker mechanism is usually oper­ated by stressing a spring which is released by a pawl and this accelerates the weight towards the unit.

Early power distribution systems

At first, distribution systems consisted of supplying con­sumers with direct-current power. This consisted of two conductors with the current flowing outwards from the generator in one direction and returning via the other conductor.

In an endeavour to minimise the size and length of conductors in street lighting, the circuits initially had banks of lamps connected in series. Carbon-arc lights were used and the overall voltage of a unit circuit was quite high.

As distribution systems grew in size and the number of customers increased, cable sizes became unwieldy. There were side effects such as return currents flowing through the earth. These caused such problems as early deterioration of metal pipes buried in the ground and unwanted voltages such as between iron fences and metal pipes entering the ground.

Modern power distribution systems

Almost without exception, electrical energy is distrib­uted to consumers with a three-phase four-wire system. It is generated in power stations at voltages ranging from 7000 to 22 000 volts. It is then transmitted to a switching yard where it is connected to various switches and trans­formers. These transform the voltages to suitable values for the next transmission stage.

Long-distance transmission of power usually means that the voltage is converted to very high voltages. In Australia, voltages of 132 kV, 330 kV and 500 kV are used. In Europe and some parts of the USA the voltages are even higher. Voltages in excess of 500 kV can be encountered in large European systems.

The idea of using high voltages is to reduce power losses to a manageable level. Electrical power is a function of both the voltage and the current consumed in the load. If the current can be reduced to a lower value, the losses in the transmission lines can be reduced. The same amount of power can still be trans­mitted if the voltage is increased to a higher value but a lesser current flow.

At the consumer end of the transmission line the conductors lead into a substation where the voltages are reduced to lower levels. A result of this method is that few power stations are needed, but there is, however, an increased need for substations to convert voltages to suitable levels. Substations are often cross-linked to other substations so that alternative routes for supplying those substations are available.

Figure 2.12 shows a generalised layout for a trans­mission system. The power station generates at I 1 000 volts and in its switching yard changes it to 132 000 volts. It is then conducted to substations as a three-phase system.

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