The energy efficiency of absorption can be improved by recovering some of the heat normally rejected to the cooling tower circuit. A two-stage or two-effect ARS accomplishes this by taking vapors driven off by heating the first stage concentrator (or generator) to drive off more water in a second stage. Many ARS manufacturers offer this higher efficiency alternative.
The double-effect ARS takes absorption to the next level. The easiest way to picture a double-effect cycle is to think of two single-effect cycles stacked on top of each other (as shown in Figure 3.50). Note that two separate shells are used. The smaller is the first stage concentrator. The second shell is essentially the single effect ARS from before, containing the concentrator, condenser, evaporator, and ARS. The temperatures, pressures, and solution concentrations within the larger shell are similar to the single-effect ARS as well. The cycle on top is driven either directly by a natural gas or oil burner, or indirectly by steam. Heat is added to the generator of the topping cycle (primary generator), which generates refrigerant vapor at a relatively higher temperature and pressure. The vapor is then condensed at this higher temperature and pressure and the heat of condensation is used to drive the generator of the bottoming cycle (secondary generator), which is at a lower temperature. If the heat added to the generator is thought to be equivalent to the heat of condensation of the refrigerant, it becomes clear where the efficiency improvement comes from.
For every unit of heat into the primary generator, two masses of refrigerant are boiled out of solution, or generated: one in the primary generator and one in the secondary generator. In a single-effect cycle only one mass is generated. Therefore, in a double-effect system, twice the mass flow of refrigerant is sent through the refrigerant loop per unit of heat input, so twice the cooling is delivered per unit of heat input. Using this approach a double-effect system has a COP that is roughly twice that of a single-effect cycle. However, this simplifying assumption does not account for cycle inefficiencies and losses. In actuality, a single-effect system has a COP of about 0.65, and a double-effect system has a COP of about 1.0. Note that the reuse of the vapors from the first stage generator makes this machine more efficient than single stage absorption chillers, typically by about 30%.