By Priya Raman · Updated August 3, 2026 · 6 min read
Short answer
An absorption chiller makes chilled water using heat instead of a mechanical compressor. In the common lithium bromide-water type, water is the refrigerant and LiBr is the absorbent; ammonia-water units use ammonia as the refrigerant. Single-effect machines run on low-grade heat, double-effect on hotter steam or gas. They make sense where waste heat or cheap fuel is available, but are bulkier and less efficient than electric chillers.
An absorption chiller produces cooling from heat. Instead of an electric compressor, it uses a pair of fluids, a refrigerant and an absorbent, and a heat source such as steam, hot water, exhaust gas or direct gas firing. The most common design uses water as the refrigerant and lithium bromide (LiBr) as the absorbent. A second design uses ammonia as the refrigerant and water as the absorbent. These machines are not a general replacement for electric chillers, but they fit well where heat is plentiful and cheap.
How does the absorption cycle work?
The cycle has four main stages in a LiBr-water machine, running in a near-vacuum shell:
- Evaporator. Water refrigerant boils at a very low pressure, which allows it to boil at a low temperature, and it chills the water circulating through the tubes. This is the useful cooling.
- Absorber. The water vapor is absorbed by a strong LiBr solution. This keeps the pressure low and pulls more vapor from the evaporator. Absorbing releases heat, which a cooling water loop removes.
- Generator. The weak solution is pumped to the generator, where the heat source boils off the water vapor, leaving a strong LiBr solution to return to the absorber.
- Condenser. The water vapor condenses and returns to the evaporator through a restriction, ready to boil again.
Compared with the vapor-compression cycle in how a chiller works, the absorber and generator replace the compressor. The only major moving parts are small solution and refrigerant pumps.
What is the difference between LiBr-water and ammonia-water?
| Feature | LiBr-water | Ammonia-water |
|---|---|---|
| Refrigerant | Water | Ammonia |
| Absorbent | Lithium bromide | Water |
| Lowest chilled temperature | Above freezing, typically for water at about 40°F and up | Below freezing is possible |
| Pressure | Vacuum | Above atmospheric |
| Typical use | Comfort and process chilled water | Low-temperature and industrial cooling |
| Safety notes | Crystallization risk, corrosion control | Ammonia toxicity, see safety rules |
LiBr machines cannot make water below freezing, because the refrigerant is water. Ammonia-water units can reach lower temperatures, but they involve ammonia hazards. For the safety and system side, see ammonia refrigeration system basics. OSHA publishes process safety requirements relevant to larger ammonia systems.
What do single effect and double effect mean?
An “effect” is a generator stage.
- Single effect. One generator. It works with relatively low-temperature heat, such as hot water in the range of roughly 190°F or low-pressure steam. The COP is modest, often around 0.7.
- Double effect. Two generators arranged so the heat from the first drives the second. It needs higher-grade heat, such as medium-pressure steam or direct gas firing, and reaches a COP typically in the range of about 1.0 to 1.4.
- Triple effect designs exist but are less common.
COP here is cooling delivered divided by heat input, so it is not directly comparable with the electric COP of a compressor chiller. For that comparison, see chiller efficiency: kW per ton, COP and IPLV.
When does an absorption chiller make sense?
- Waste heat is available. Exhaust from engines or turbines, process heat, or steam that would otherwise be discarded.
- Combined heat and power. A cogeneration plant that has summer heat with no use.
- Gas is cheap relative to electricity, or electric demand charges are high.
- Electric capacity is limited. Where the grid connection cannot support a large electric chiller plant.
- Low noise and vibration are priorities.
- District or campus plants with an existing steam or hot water network.
It is a poor fit if the heat would have to be generated purely to run the chiller at low efficiency, or if space and cooling tower capacity are tight.
What are the limits?
- Larger and heavier than an equivalent electric chiller.
- Higher heat rejection. The machine rejects both the cooling load and the heat input, so the cooling tower must be larger.
- Higher first cost per ton in most cases.
- Corrosion and crystallization control. LiBr solution is corrosive, and the machine needs good vacuum and water treatment. See chilled-water system water treatment basics.
- Slower response to load changes than electric machines.
- Low-temperature limits for LiBr units.
Many plants pair absorption machines with electric chillers, using heat when it is available and electricity otherwise. The comparison with other types is in types of chillers.
Frequently asked questions
Do absorption chillers use refrigerants like R-134a?
Not the common ones. LiBr machines use water, and ammonia-water machines use ammonia. That reduces exposure to the HFC phasedown, though other rules, such as those for ammonia safety, apply.
Are absorption chillers more efficient than electric chillers?
Not in terms of energy input. Their value comes from using heat that is cheap or free, or from reducing electric demand. When the heat is a purchased fuel, an electric chiller is usually more efficient.
What is crystallization in a LiBr chiller?
If the solution becomes too concentrated or too cold, the LiBr can form solid crystals that block flow. Modern controls are designed to prevent it, and units have dilution cycles at shutdown.
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