Abstract
Statistically low-grade heat below about 150 °C, which is the temperature range of waste heat emitted by industry, has so far not usually been recovered and utilized. A large amount of such unused heat is either exhausted to the atmosphere, discarded to cooling water or otherwise lost. When thermal engines such as the Rankine and Kalina cycles are used to generate power from low-grade heat, the second law of thermodynamics binds the conversion efficiency of low temperature heat into work or electricity. The Kalina cycle could achieve higher efficiency in producing acceptable power at the given process conditions than the Rankine cycle because it used a high concentration ammonia-water mixture as a secondary fluid and could be fitted to the falling temperature of a heat source with a finite heat capacity. A low heat power generation (LHPG) system based on the concept of the Kalina cycle has been successfully developed and implemented in Japan. This paper studied that the principle of thermal engine, which was able to generate power by using low-grade heat, and the efficiency of both Kalina and Rankine cycles at around the temperature (120 °C) of the overhead vapor from the fractionator in a refinery. As the heat source of the overhead vapor was heavily fluctuated, it was necessary to develop the countermeasure.
