Heat, electrical energy converted

The magnitude of the energy conversion can be calculated as follows. The amount of electrical energy converted to heat per unit time is given by the product of voltage V and current i. This heat production is spread over the volume of the conducting medium LA, where L is length and A the cross-sectional area of the medium. Thus the rate of heat input H per unit volume is... 

L. S. Socha, D. F. Thompson, and P. S. Weber. Reduced Energy and Power Consumption for Electrically Heated Extruded Metal Converters, SAE 930383, Society of Automotive Engineers, Warrendale, Pa., 1993. 

Cogeneration is an energy conversion process wherein heat from a fuel is simultaneously converted to useful thermal energy (e.g., process steam) and electric energy. The need for either form can be the primary incentive for cogeneration, but there must be opportunity for economic captive use or sale of the other. In a chemical plant the need for process and other heating steam is hkely to be the primaiy in a pubhc utility plant, electricity is the usual primary produc t. 

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

The oxidation of hydrogen to water (Hj -t- i Oj -> HjO) is thermodynamically spontaneous and the energy released as a result of the chemical reaction appears as heat energy, but the decomposition of water into its elements is a non-spontaneous process and can be achieved only by supplying energy from an external source, e.g. a source of e.m.f. that decomposes the water electrolytically. Furthermore, although the heat produced by the spontaneous reaction could be converted into electrical energy, the electrical... 

In working Example 18.8, we have in effect assumed that the electrolyses were 100% efficient in converting electrical energy into chemical energy. In practice, this is almost never the case. Some electrical energy is wasted in side reactions at the electrodes and in the form of heat This means that the actual yield of products is less than the theoretical yield. 

K = — 64800 , i.e., the cell converts only 76 per cent, of its chemical into electrical energy, and emits the rest as heat. 

The total inapplicability of the Thomson rule to this case is at once apparent none of the electrical energy comes from chemical change, but the cell functions as a heat engine, converting the heat of its environment into electrical work. 

A sketch of the process helps to identify what takes place. The electrical heater converts electrical energy into heat that flows into the calorimeter and raises the temperature of the water bath. 

One possibility to avoid this limitation is the conversion of heat into another kind of energy like mechanical or electrical energy. In this case (see Figure 231) the converter is producing entropy free work, which can be stored without theoretical limitations. Examples are pump storages, where water is pumped to a higher level, or flywheels, where kinetic energy can be stored. 

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