Absorption chillers: when waste heat can drive cooling

How does an absorption chiller make cold without a compressor?

In a lithium bromide chiller the refrigerant is water and the absorbent is a lithium bromide salt solution. Heat in the generator boils water out of the solution, the vapour condenses, then evaporates at very low pressure in the evaporator and chills the building's water before the solution absorbs it again. CIBSE Journal describes the cycle as chemical rather than mechanical, with the compressor replaced by an absorption process, a generator and a pump.

That makes it a heat-operated cycle, and CIBSE says the only electrical input is for circulating pumps and control valves. Because the whole system runs well below atmospheric pressure, a leak draws air in, which reduces capacity or can stop the machine altogether.

This guide treats absorption as one option in chiller selection. For how it sits beside the compressor-driven types, read the types of chillers guide, and for the wider picture see the commercial chillers page.

Which heat sources can drive one?

CIBSE gives the dividing line between the two common designs: single-stage chillers need heat at a minimum of 80 °C, while double-stage machines are more efficient but need higher-grade heat, generally above 140 °C. Its list of sources runs from hot water at 80 to 100 °C through superheated water, steam and exhaust gas to thermal oil, and newer designs can work from hot water as low as 70 °C.

One manufacturer's UK range shows the spread. Johnson Controls lists a single-stage hot-water model for 105 to 6,153 kW of cooling, with single-effect hot water up to 160 °C, and a double-lift model that uses waste heat as low as 55 °C. These are one maker's published figures, not limits of the technology.

CIBSE's guidance on when absorption cooling is worth considering works as a screen. Look for:

• waste heat, such as exhaust steam, that would otherwise be dissipated;
• a low-cost heat source such as landfill gas or geothermal heat;
• an existing site with an electrical load limit that would be expensive to upgrade;
• a site that is sensitive to noise or vibration; or
• solar thermal energy that can be harnessed.

Heat recovered from a combined heat and power engine is one more possible source, and the Energy Technology List's absorption category is limited to chillers driven by a CHPQA-certified scheme.

What COP can an absorption chiller reach?

COP is cooling delivered divided by heat put in. CIBSE gives typical figures of 0.65 to 0.84 for single-stage commercial units and 1.00 to 1.50 for double-stage, and an earlier CIBSE module puts a typical absorption cycle at about 0.7 against about 3.5 for a vapour compression chiller. The two numbers measure different inputs, heat for one and electricity for the other, so they are not a like-for-like efficiency comparison. The COP stays practically constant down to about half of the design cooling load.

Here is illustrative arithmetic, not a quotation. The highest single-stage COP CIBSE reports is about 0.84, so take 0.83. Delivering 500 kW of cooling then needs about 602 kW of heat (500 divided by 0.83). A vapour compression chiller at the typical COP of 3.5 would draw roughly 143 kW of electricity for the same 500 kW. The absorption machine is attractive only if 602 kW of usable heat is available at a cost, or with a capacity limit, that beats 143 kW of electricity.

CIBSE's 2009 module gave one very general estimate for an 800 kW plant: capital cost about 30% higher and annual running costs 10 to 15% lower than vapour compression. It is old and depends wholly on the price of the heat, so treat it as a pointer to the shape of the trade-off, not a forecast.

How much heat rejection and cooling water does it need?

An absorption chiller must reject both the heat it removed from the building and the heat that drove it. An earlier CIBSE module puts the absorber duty at about 2.5 times the cooling capacity for air conditioning, which means larger heat rejection equipment. The table shows the same point as illustrative arithmetic.

Chiller type	Energy input	Typical COP (CIBSE)	Heat rejected per kW of cooling (illustrative)
Vapour compression	Electricity to a compressor	About 3.5	About 1.3 kW
Single-stage absorption	Heat of 80 °C or more	About 0.7	About 2.4 kW
Double-stage absorption	Heat generally above 140 °C	About 1.2	About 1.8 kW

The last column is 1 plus 1 divided by the COP: the cooling delivered plus the energy that drove the cycle. It ignores pump power and part-load effects.

CIBSE says lithium bromide machines need cooling water cooled by equipment such as cooling towers, dry or adiabatic coolers, and that the cooling water must not fall below 20 °C or the solution can crystallise. Each choice brings its own water and Legionella considerations, set out in the comparison of cooling towers, dry coolers and adiabatic coolers.

Where do absorption chillers not fit?

Chilled water temperature is the first limit. Because the refrigerant is pure water, CIBSE says the minimum chilled water temperature a lithium bromide machine can reach is 4 °C. Below 0 °C the most common pairing is ammonia as the refrigerant with water as the absorbent, which is a different machine. Sub-zero loops such as glycol circuits are covered in the guide to process and glycol chillers.

Efficiency rules are the second. Both ecodesign regulations that cover process chillers exclude absorption chillers, which the first defines as process chillers in which refrigeration is effected by an absorption process using heat as the energy source. The legal efficiency floors written for electric process chillers therefore do not apply to them, so compare a maker's declared performance with a vapour compression alternative instead.

Plant detail is the third. CIBSE lists the advantages as use of otherwise wasted heat, no moving parts apart from two small circulation pumps, no vibration and very low electrical power consumption. The trade is vacuum-tight construction, larger heat rejection plant and the need for steady heat whenever cooling is wanted.

How do you decide between heat-driven and electric cooling?

1. Establish the heat: how many kilowatts, at what temperature, and in which hours it is actually available.
2. Establish the cooling: the duty, the chilled water temperature and when the load occurs.
3. Check the electrical position: whether the site has spare capacity or faces an expensive upgrade.
4. Size the heat rejection route and the space it needs, including water treatment duties.
5. Compare whole-life cost and carbon against a vapour compression option, using the real price of the heat rather than a headline COP.

CIBSE calls the carbon comparison fairly complex and says each application needs to be considered with all the relevant data. If you are weighing the options for a building, you can send an enquiry through the site's form, and the commercial chillers page explains the wider selection process.

Sources

Regulations and guidance are revised, so confirm the current text before relying on a figure.

• Module 91: High-efficiency waste heat-powered lithium bromide absorption chillers, CIBSE Journal, posted March 2016; checked 22 October 2026.
• Module 10: Absorption refrigeration, CIBSE Journal, posted November 2009; checked 22 October 2026.
• Absorption Chillers (product range), Johnson Controls UK, manufacturer's own page; checked 22 October 2026.
• Absorption & Other Heat Driven Cooling & Heating Equipment, Energy Technology List, UK Government; checked 22 October 2026.
• Commission Regulation (EU) 2015/1095, article 1, legislation.gov.uk, revised 09/01/2017; checked 22 October 2026.
• Commission Regulation (EU) 2016/2281, article 1, legislation.gov.uk, revised 09/01/2017; checked 22 October 2026.
• Commission Regulation (EU) 2015/1095, article 2, legislation.gov.uk, revised 09/01/2017; checked 22 October 2026.

Common questions

Is an absorption chiller more efficient than an electric chiller?

Not in terms of COP. CIBSE puts single-stage units at 0.65 to 0.84 and a typical electric chiller at about 3.5, but one counts heat and the other electricity. An absorption chiller wins only when its heat is surplus or cheap, or when electrical capacity is the limit.

Can an absorption chiller be fired directly by gas?

Direct-fired models exist; Johnson Controls lists a double-effect direct-fired chiller-heater in its UK range. Whether gas-fired absorption beats electric cooling on carbon depends on the site, so it needs its own assessment.

Can an absorption chiller produce chilled water below 4 °C?

A lithium bromide and water machine cannot, because the refrigerant would freeze. CIBSE says that below 0 °C the usual pairing is ammonia as the refrigerant with water as the absorbent. Those machines serve a different range of duties.

Does an absorption chiller fall under the F-gas rules?

The F-gas rules attach to equipment containing fluorinated gases. CIBSE notes that absorption units avoid HCFC and HFC refrigerants, and a lithium bromide machine uses water. Any HFC-charged plant on the same site, such as a back-up electric chiller, keeps its own leak-check duties.

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