Low-grade heat

Utilities or municipal power systems supplying electric power and low-grade heat (e.g., 422 K [300°F]) for local distric t heating systems... 

Haldane, T. G. N. (1930). The Heat Pump An Economical Method of Producing Low-Grade Heat from Electricity. Journal of the Institution of Electrical Engineers 68 666—675. 

The absorption system can be used to advantage where there is a cheap source of low-grade heat or where there are severe limits to the electrical power available. A modified system of the ammonia-water absorption cycle has been developed for small domestic refrigerators. 

A heat pump is a device for raising low grade heat to a temperature at which the heat can be utilised. It pumps the heat from a low temperature source to the higher temperature sink, using a small amount of energy relative to the heat energy recovered. 

Heat pumps are increasingly finding applications in the process industries. A typical application is the use of the low grade heat from the condenser of a distillation column to provide heat for the reboiler see Barnwell and Morris (1982) and Meili (1990). Heat pumps are also used with dryers, heat being abstracted from the exhaust air and used to preheat the incoming air. The use of a heat pump with an evaporator is described in Volume 2, Chapter 14. 

FIGURE 2.1 Distribution of deposited energy among different degrees of freedom as a function of time, represented by pt = -log t (sec). Note that, for exothermic reactions, low-grade heat can terminate above the level of absorbed energy. Luminescence can only alter the picture in a minor way. Reproduced from Mozumder (1969a), by permission of John Wiley Sons, Inc. ... 

The traditional way to free water of dissolved solids is to distil it, either at atmospheric pressure or by multistage flash evaporation at reduced pressure. Distillation removes virtually all solutes but is wasteful of energy unless the low grade heat can be economically recovered from the condensers. Flash evaporation is attractive in countries such as Saudi Arabia where energy is inexpensive and the only plentiful source of water is the sea, but problems usually arise with deposition of CaC03, Mg(OH)2, and CaS04 scales. 

The overhead from the second stage is heated by an exchange with hot solvent. The fired heater further raises the temperature of the solvent/demetallized oil mixture to a point above the critical temperature of the solvent. This causes the demetallized oil to separate. It is then flashed and steam-stripped to remove all traces of solvent. The vapor streams from the demetallized oil and asphalt strippers are condensed, dewatered, and pumped up to process pressure for recycle. The bulk of the solvent goes overhead in the supercritical separator. This hot solvent stream is then effectively used for process heat exchange. The subcritical solvent recovery techniques, including multiple effect systems, allow much less heat recovery. Most of the low grade heat in the solvent vapors from the subcritical flash vaporization must be released to the atmosphere requiring additional heat input to the process. 

The reformed gas leaves the furnace at a high temperature where high grade heat is recovered successively to a reformed gas boiler, steam superheater process feedstock heater and boiler, feedwater heater. The reformed gas then passes to the distillation area where low grade heat is efficiently recovered via column reboilers and a demineralized water heater. 

Absorption refrigeration utilizes low grade heat and may be employed to chill compressor suction and hence reduce compressor horsepower. Similarly it may be used to chill the loop circulating gas and increase the conversion per pass. This would lead to a reduction in the circulator power of up to 10%. 

Power recovery turbines, which let down suitable refrigerants under pressure, are also being developed (2). In conjunction with absorption refrigeration, these could be used to recover power from compressor coolers and other low grade heat. 

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