It sounds counter intuitive but heat can be used to power air-conditioning and refrigeration.
The technology used to do this is adsorption or absorption chillers. The basic principle is the same as a heat pump in that both use a cycle of evaporation, compression, condensation and expansion as explained in an earlier post.
The difference is that electric chillers use a compressor driven by an electric motor to raise the pressure of the refrigerant gas while adsorption/ absorption chillers use heat. The thermal compressor that consists of an absorber/adsorber, a generator, a pump, and a throttling device.
Adsorption/ absorption chillers have a much lower coefficient of performance (COP) than electric chillers around 0.7 compared to 3 for electric chiller or heat pumps. The advantage is that the input energy is heat that would otherwise typically go to waste such as the rejected heat from gas fired electric generation equipment. A common use of adsorption chillers is to use them in conjunction with co-generation plants (CHP) to form what is often called a tri-generation plant. EMC has also looked at using the jacket water and exhaust gases from diesel generators as a heat source for adsorption chilling.
Understanding the difference between adsorption and absorption is where things get a bit more technical. The primary difference is that absorption uses a liquid absorbent and adsorption uses a solid adsorbent.
The basic absorption cycle employs two fluids, the refrigerant and the absorbent. The refrigerant vapour from the evaporator is absorbed into the absorbent releasing heat. The liquid refrigerant/absorbent solution is pumped to a high pressure generator. This pumping uses a lot less energy that the compressor used in a heat pump. The heat from the waste heat source causes the refrigerant to desorb from the absorbent. The refrigerant-depleted solution then returns to the absorber via a throttling device (labelled as weak in the diagram). The two most common refrigerant/ absorbent mixtures used in absorption chillers are water/lithium bromide and ammonia/water. Kerosene and LPG power refrigerators used in caravans are a common example and use an ammonia/water mix.
The refrigerant vapours flow to a condenser and condense to a high-pressure liquid. The liquid is then throttled though an expansion valve to the lower pressure in the evaporator where it evaporates and takes up heat which produces the cooling effect. The low-pressure refrigerant vapours return to the absorber where they are combined with the absorbent so the cycle can be repeated.
The process is very similar in adsorption chillers, the difference is that adsorption used a solid instead of a liquid and the refrigerant is bound to the surface of the adsorbent rather than absorbed into a liquid in the case of absorption.
The solid in the case of the emissionless adsorbent chiller shown below is a proprietary permanent silica gel. The silica gel in its neutral state has adsorbed the refrigerant which in this case is water. When heated, the solid desorbs (releases) refrigerant vapour which subsequently is cooled and liquefied. This liquid refrigerant then provides its cooling effect at the evaporator, by absorbing external heat and turning back into a vapour. In the final stage the refrigerant vapour is (re)adsorbed into the solid.
Adsorption chillers are simpler and require less maintenance primarily because they avoid the use of lithium bromide which is toxic and can crystallise in the system.
The technology is also being used in conjunction with solar thermal systems for solar powered air-conditioning. An example is the system installed at the Echuca Hospital.
While less efficient that heat pumps adsorbent and absorbent chillers can deliver significant cost savings because they derive their energy (except for a small amount of electricity for pumping) from waste heat.