Waste heat recovery

Chilled water

Absorption chilling is a technology that converts low-temperature waste heat to chiller capacity. Absorption chillers are attractive investments in plants with high chilling needs and large amounts of available low-grade heat. Especially if electricity prices are high, since the power consumption of an absorption chiller is practically negligible compared to that of a conventional chiller.

Working principle

Absorption chillers provide cooling by using heat instead of a compressor. A mixture of ammonia and water is one of two common cooling media (the other one being a mix of lithium bromide and water). The following shows the working principle of an absorption chiller using this refrigerant:

  1. The generator contains the ammonia and water mixture. When heated by the waste heat, the ammonia evaporates and the vapour is led to the condenser.
  2. The ammonia vapour is condensed using cooling water and is released through an expansion valve into the low-pressure vessel, the evaporator, causing a drop in temperature in the ammonia.
  3. In the evaporator, the ammonia is vaporized at low temperature by the media to be chilled.
  4. The cold ammonia vapour is led into the absorber vessel where it is absorbed into the ammonia/water solution and pumped back to the generator. By circulating the water/ammonia mixture in the generator and absorber, the concentration is kept at a constant level in the generator and absorber.

The efficiency of absorption chillers depends on the temperature of the waste heat and the cooling water; the larger the temperature difference the better the efficiency. 

Operating principle of an absorption chiller


In 2008, Alfa Laval supervised a thesis from Lund University studying waste-heat powered absorption chilling. Assuming there is a high need for chiller capacity and good supply of low-temperature waste heat, absorption chillers proved to be a profitable investment, both when replacing existing chillers and in new plants. 



All graphs are based on the master’s thesis: “Evaluation of different alternatives for recovering waste heat from the sulphuric acid process”, D. Iwarsson (2008), Master’s Thesis, department of chemical engineering,
Lund University.