Electrical energy from thermal conversion

Figure 1.2 The relationship between chemical energy and other forms of energy, with examples. Note that a reaction can have energy transferred to or from several types of energy, as in the conversion of chemical energy into electrical energy and thermal energy.

Cogeneration - The generation of electricity or shaft power by an energy conversion system and the concurrent use of rejected thermal energy from the conversion system as an auxiliary energy source. 

Figure 2.11 Comparison of the primary energy usage for ethanol/water separation using traditionai distillation/adsorption process (Fig. 2.9) and hybrid membrane-assisted vapor stripping (MAVS Fig. 2.10) process. Minimum energy (from minimum work calculation) shown as reference. Assumptions 37% and 85% efficient conversion of primary energy to electrical energy and thermal energy, respectively, 0.02 wt% ethanol in stripping column bottoms, and 99.5 wt% ethanol product (0.5 wt% water).

The Seebeck effect corresponds to the electricity production from a difference of temperature. This effect can be reversible and is the inverse of the Peltier effect, which is the phenomenon of conversion of electric energy into thermal energy (heat). These effects can be superimposed onto the dissipative processes of transport by conduction of electric charges (Joule effect) and to the transport of heat (Fourier equation) which are both irreversible processes. 

Electrical heating is accomplished with resistance bauds or ribbons which must be electrically insulated from the machine body but in good thermal contact with it. The heaters must be carefully spaced to avoid a succession of hot and cold areas. Sometimes they are mounted in aluminum blocks shaped to conform to the container walls. Their effective temperature range is 150 to 500°C (about 300 to 930°F). Temperature control is precise, maintenance and supervision costs are low, and conversion of electrical energy to useful heat is almost 100 percent. The cost of electrical energy is usually large, however, and may be prohibitive. 

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. 

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