C1. Absorption Basics and Absorption Versus Mechanical Chillers

Absorption cycles are similar to mechanical-chiller cycles in utilizing a condenser, evaporator and expansion device (see the schematic in Figure 2.18). The difference lies in how the low-pressure vapor that exits the evaporator is converted into the high-pressure vapor that enters the condenser. Instead of the work-driven compressor of a mechanical chiller, thermal power is the driving force. The heat is usually delivered in the form of hot water or steam, and is commonly derived from the combustion of natural gas, industrial waste heat, geothermal sources, or solar energy collection.

Figure 2.18: Schematic of an absorption chiller cycle. This particular illustration is for a single-stage steam-fired unit with a non-volatile solute as in the LiBr-water pair. The same schematic applies equally well to a hot water-fired device. Also, the heat pump mode involves extracting the useful effect as heating at the condenser/absorber, as opposed to the cooling mode where the useful effect is heat removal at the evaporator.

A vapor-compression chiller produces its cooling at an evaporator (a heat pump produces its heating at a condenser). The corresponding absorption system includes two additional heat reservoirs: a generator and an absorber. A volatile working fluid (refrigerant) is partially separated from the carrier solution by the heat input at the generator. The refrigerant and solution are subsequently recombined in an exothermic process at the absorber. The absorber functions as a heat rejection unit (in addition to the condenser). Were the absorber...