Current Sources

devices that convert various forms of energy into electric power. They have been arbitrarily divided into chemical and physical sources according to the type of energy converted. Information on the first chemical current sources (electrochemical cells and storage batteries) dates to the 19th century (for example, the voltaic and Leclanché cells). However, until the 1940s no more than five types of voltaic couples had been developed and produced. Beginning in the mid-1940’s, as a result of the development of radio electronics and the wide use of self-contained current sources, about 25 additional types of voltaic couples were created. Theoretically, the free energy of the chemical reaction between virtually any oxidizer and reducer can be realized in a current source; therefore, several thousand types of voltaic couples are possible.

The principles of operation of most physical current sources were known as early as the 19th century. Subsequently, as a result of rapid development and improvement, turbogenerators and hydrogenerators became the principal industrial sources of electric power. Physical current sources based on other principles were developed industrially only during the 1950’s and 1960’s because of the growing and rather specific requirements of modern technology. During the 1960’s the technically developed countries had industrial models of thermogenerators, thermal emission generators (the USSR, the Federal Republic of Germany, and the USA), and atomic batteries (France, the USA, and the USSR).

Technical progress and the advent of electrical engineering and electronics in transportation, daily life, and medicine stimulated the development of self-contained electric power sources, among which chemical current sources were prominent in terms of numbers, since they had become a consumer item. Portable lights, tape recorders, radio receivers, television sets, and medical apparatus, as well as railroad rolling stock, motor vehicles, tractors, airplanes, artificial satellites, spacecraft, and communications facilities, are equipped with small current sources.

The theory of current sources involves the study of all stages in the process of generation of electric current, using as a basis modern concepts regarding the physics of solids, liquids, and gases; charge transfer processes; and electrochemical reactions. The theory also studies the problems of optimization, which include both the choice of initial parameters that will provide optimum output characteristics of the current source and the development of methods for predicting the characteristics of future current sources. Among the most important characteristics are efficiency, energy capacity (or specific energy capacity), power (or specific power with respect to unit weight or volume), service life, and the nature of the electric power generated (frequency, voltage, overload capability, cost, and reliability).

Chemical current sources. Devices that produce an electric current as a result of an oxidation-reduction reaction between chemical reagents are considered to be chemical current sources. Depending on the operating circuit and the ability to deliver power to an electric circuit, chemical current sources are divided into primary, secondary, and reserve types, as well as electrochemical generators. As a rule, the primary types (electrochemical cells and batteries) permit only a single use of the energy of the chemical reagents. Some designs of electrochemical cells and batteries permit short-term repeated use of the energy of the reagents after recharging. The positive and negative electrodes (the cathode and anode, respectively) are separated by a fluid or paste electrolyte or by a porous separator membrane in which the electrolyte is absorbed; they are connected electrically (conductive coupling) throughout the service life of the current source.

Secondary current sources (certain storage cells and batteries) make possible repeated use of the energy of the constituent chemical reagents (hundreds and thousands of charge-discharge cycles). During the service life of storage batteries, the electrodes and the electrolyte are in conductive coupling with one another. To increase the life of such batteries under certain specific conditions, a method of dry-charged storage has been developed. Such batteries are filled with electrolyte before being used.

Reserve current sources permit the energy of the chemical reagents to be used only once. Unlike electrochemical and storage cells, the electrolyte in reserve cells is never conductively coupled with the electrodes during storage. It is stored as a liquid (in glass, plastic, or metal ampuls) or as a solid (which is nonconducting) in the space between the electrodes. To prepare reserve sources for operation, the ampuls are broken by compressed air or an explosion, and the crystals of a solid electrolyte are melted by electric or pyrotechnical heating. Such sources are used to power electrical apparatus that may be (or may be required to be) in reserve for a long period. The storage life of modern reserve current sources is 10–15 years.

Electrochemical generators (fuel cells) are a type of chemical current source. They can generate an electric current continuously for an extended period by conversion of the energy of gaseous or liquid chemical reagents that are supplied to the generator from without.

By 1970 industrial models of electrochemical generators had been developed in the USA and the USSR. Intensive work is in progress to create generators for space vehicles, electrically driven vehicles, and stationary installations. A variety of generators are being developed (high-, medium-, and low-temperature types with gaseous, fluid, and solid reagents), the most promising of which are those that convert the energy of a natural fuel directly into electricity.

Physical current sources. Physical current sources are devices that convert thermal, mechanical, and electromagnetic energy, and also the energy of radiation and nuclear decay, into electrical energy. According to the most commonly used classification, the physical current sources include dynamoelectric generators, thermoelectric generators, thermal emission generators, mag-netohydrodynamic generators, and generators that convert the energy of solar radiation and atomic decay.

Dynamoelectric generators, which convert mechanical energy into electric power, are the most common type of electric power source, the basis of modern power engineering. They may be classified according to their power (from fractions of a watt to hundreds of megawatts), their purpose and operational features (stationary, transportation, reserve, and so on), the type of prime mover (diesel generators, turbogenerators, and hydrogenera-tors), and the kind of actuating medium (steam, water, or gas). Because of the long period for theoretical, design, and technological improvement, the characteristics of this type of current source have reached values close to the theoretical.

The operation of a thermoelectric generator is based on the use of the Seebeck effect. The actuating material in a thermoelectric generator may be any one of various semiconductor compounds of silicon and germanium (usually solid solutions). The efficiency is 3–15 percent over the temperature range from 100° to 1000°C. Studies on thermoelectric generators are being conducted in the USSR, the USA, and France. They have potential for use as self-contained power sources (in transportation, communications facilities, and medicine) and anticorrosion protection (on pipelines).

The principle of operation of a thermal emission converter is based on the use of the thermal emission effect (the emission of electrons from the heated surface of a metal). The thermal emission electron flow is chiefly a function of temperature and the properties of the material’s surface. The efficiency of some laboratory models of thermal emission converters is as high as 30 percent, and operational power installations reach 15 percent (the electric power obtained per unit of the cathode surface is equal to 30 watts per sq cm). The most promising application of thermal emission converters is for self-contained high-power sources (up to 100 kW). Work on such devices is being conducted in the USSR, the USA, the Federal Republic of Germany, and France.

The principle of operation of converters for solar radiant energy is based on the use of the internal photoelectric effect. Such a generator (solar battery) consists of a set of photovoltaic cells that convert solar radiant energy into electricity. The virtually direct conversion of solar radiant energy became possible only after the development in 1953 of a highly efficient photocell from monocrystalline silicon. The best silicon photoelectric cells have an efficiency of about 15 percent; their service life is virtually unlimited. Solar batteries are used mainly in space technology, where they occupy a dominant position as a power source on artificial earth satellites, orbital space stations, and spacecraft; they are also used to supply electric power in areas far from electrical transmission lines that have a large number of sunny days per year, such as the Turkmen SSR, India, and Pakistan.

Current sources that convert the energy of atomic decay (atomic batteries) use the kinetic energy of the electrons that are formed during beta decay. These sources were in the development stage by 1971, and their practical use requires the solution of many design and technological problems. Their efficiency is low (about 1 percent), and their field of application can only be determined after sufficient experience in their use has been accumulated.