Heat recovery for cooling

It sounds counterintuitive, but heat can be converted into cooling or chilling duty with absorption chillers.


Benefits

Using waste heat for an existing cooling demand can displace whatever is currently being used to create the chilling. In many cases, it can displace a refrigeration or air conditioning unit that requires electricity to operate. As a result, the primary benefits are:

  • Significant savings through a reduction in operating costs
  • Reduction in capital spend (avoidance of a air conditioner purchase)
  • Potential to monetize the heat by selling it to nearby facilities with a cooling load
  • Reduction or elimination of electricity or fuel
  • Reduced CO2 and other emissions
  • Less price volatility

Examples of using heat to cool

Absorption cooling is most frequently used to air condition large commercial buildings. Absorption chillers can be teamed with electric chillers in "hybrid" central plants to provide cooling at the lowest energy costs. In this case, the absorption chillers are used during the summer to avoid high electric demand charges, and the electric chillers are used during the winter when they are more economical. Because absorption chillers can make use of waste heat, they can essentially provide free cooling in certain facilities. (source: US DOE)


How absorption chillers work

Absorption chillers work on the same principles as traditional, mechanical chillers.  The only difference is, instead of a mechanical compressor, absorption chillers use heat to create pressure and drive the cycle.  In practice the most common absorbents are Lithium Bromide and Ammonia.

There are some great resources online to dive deeper into the process of an absorption chiller:


Technologies

Single effect chillers

These chillers have a single generator/condenser stage, which boils off the refrigerant (usually water) from the absorbent (LiBr or Ammonia). They typically operate with a lower temperature water or lower pressure steam heat source.  The lower temperature means that these chillers will have a reduced efficiency compared to double effect chillers.

Double effect chillers

These chillers have two stages of generator/condenser to increase the amount of refrigerant produced, increasing efficiency compared to a single effect chiller. The high temperature stage uses the heat source boil off refrigerant while the second stage uses the vaporized refrigerant from the first stage to boil additional refrigerant.  This operation requires higher temperature water or higher pressure steam.


Considerations

Absorption chillers are not for every application, but when they do fit, they can provide significant operating improvements.  Some things to keep in mind as you consider an absorption chiller.

  • The cooling demand needs to occur at the same time the heat is present.
  • The temperature of the heat source will dictate the chiller options.
  • Absorption chillers require some electricity to operate small pumps. Take this into account when running the numbers.