Absorption chillers have the largest market share amongst thermal cooling systems. The thermal compression is produced by a combined cooling and solvent cycle. If temperatures above 5°C are required, usually water is used as the refrigerant and lithium bromide (LiBr = salt) as the solvent. This pair of compounds is most frequently used in absorption chillers. For temperatures under 0°C, the refrigerant/solvent pair of ammonia (NH3)/water is used. The heat used to operate the system must be at least 70°C.
The chilling process is primarily based on the low pressure (e.g. 2mbar) created by the absorption of the refrigerant and the solvent (e.g. water and lithium bromide). This allows the evaporation of the refrigerant at low temperatures (e.g. 6°C) and simultaneously cools the chilled water circuit (e.g. from 12°C to 6°C). To ensure the absorption, the highly concentrated LiBr solution must be injected using jets into the absorber. In this process, parts of the water vapor are absorbed. Absorption and evaporation occur in one tank (absorber, evaporator). The cold water that will be chilled is introduced into the evaporator section of the tank via a coiled pipe.
In a second tank (ejector, compressor and generator), evaporation (ejection) and condensation occur side by side. Heat is transferred to the watery LiBr solution using coiled pipes. The water evaporates and a highly concentrated LiBr solution collects in the lower portion of the tank. This resulting solution is then introduced into the absorber portion of the first tank via jets. Between the tanks, there is a heat exchanger that cools the highly concentrated solution from the absorber/evaporator tank by means of the watery LiBr solution. The heat released by absorption is removed by a cooling coil. The coolant is subsequently fed into the ejector, where the ejected water is cooled and condensed on the side of the tank using another coiled pipe. The heat of condensation is discharged along with the coolant. The condensed water is conveyed into the absorber/evaporator to be evaporated.
The following diagram shows the absorption cycle:
Figure 3: Absorption Cycle;
Source: Carrier GmbH & Co. KG;
Animated graphic with additional component labels
The Energy Efficiency Ratio (EER) for single-stage water/LiBr absorption chillers is in the range of approx. 0.55 – 0.7.[3] In single-stage absorption processes, the term single effect (SE) is also used.
Combined absorption chiller and heat pump
More and more manufacturers are offering absorption chillers that, in principle, can be run in reverse as heat pumps. This increases the capacity utilization of the system’s technology. It is a sensible option, for example, if the chiller is being used for air conditioning. If no cooling is required during transitional or winter seasons, an absorption system can efficiently provide heat.
Multi-effect absorption chillers
Absorption chillers can use heat even more efficiently, if multiple-stage processes are employed. The terms ‘effect’ and ‘lift’ describe this layering of the absorption processes in the system [5].
[3]: Henning Hans-Martin, Urbaneck Thorsten, Morgenstern Alexander, Nunez Thomas, Wiemken, Edo, Thümmler, Egbert, Uhlig, Ulf (2015) Kühlen und Klimatisieren mit Wärme. Bine-Fachbuch.
[5]: Schleith Mario. (2013). Anwendungsmöglichkeiten für Absorptionskältemaschinen und Wärmepumpen. Artikel auf cci-Dialog GmbH Internetseite, Fa. Thermax Europe Ltd.
An animated illustration of an absorption chiller can be viewed on the website of Carrier GmbH & Co.KG, one of the project partners: