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Sorption Energy Seeking to Commercialize Waste Heat-Driven Adsorption Heat Pump Technology for Vehicle Air Conditioning

A spinout from Warwick University (UK), Sorption Energy, is commercializing vehicle air conditioning systems based on waste heat-driven adsorption heat pump technology developed by Professor Robert Critoph and his team at University of Warwick School of Engineering.

Air-conditioning units in cars and light commercial vehicles burn more than 5% of the vehicle fuel consumed annually throughout the EU, the company notes. (A 2006 study by Lambert and Jones at San Diego State University on waste-heat powered mobile adsorption air conditioners put the power used by the mechanical air conditioning compressor of subcompact to mid-sized cars during commuting at 12-17%.) The UK alone emits about 3 million tonnes of CO2 each year simply from powering the air-conditioning in vehicles, Sorption Energy says. Sorption Energy’s system almost eliminates both this fuel consumption and accompanying CO2 emissions.

Although the basic adsoprtion heat pump process is discontinuous, operating two generators out of phase can result in continuous operation. Source: Sorption Energy. Click to enlarge.

The operation of adsorption heat pumps and refrigerators is based on the ability of porous solids (the adsorbent) to adsorb vapor (the adsorbate or refrigerant) when at low temperature and to desorb it when heated. Adsorption heat pumps thus use an adsorbent chemical rather than a mechanical compressor, and are driven by heat rather than mechanical work.

The systems often use natural refrigerants such as water, ammonia or methanol; Dr. Critoph and Sorption Energy prefer ammonia as a refrigerant and active carbons as the adsorbent.

Adsorption heat pumps are cyclic in operation and require a condenser, expansion valve and evaporator similar to those used in conventional compressor-driven systems. However, the compressor is replaced by the adsorption system.

The basic system consists of two linked containers, one of which contains the solid adsorbent (the generator). The other is the combined evaporator and condenser or receiver in which the refrigerant is evaporated and condensed.

At low temperature and pressure, the adsorbent contains a high concentration of refrigerant, while the receiver contains only refrigerant gas. When the generator is heated, the refrigerant is desorbed, raising the system pressure. Refrigerant condenses in the receiver, rejecting heat and producing a useful heat output if the system is to be used as a heat pump. Here, the evaporator is located outside the building and extracts energy from the surrounding air, transferring it back to the system and eventually to the condenser and generators for use in the dwelling.

Cooling the generator back down to its initial temperature completes the cycle and causes the adsorbent to readsorb the refrigerant. The system pressure is reduced and the liquid refrigerant in the receiver evaporates, absorbing heat. This produces the useful cooling effect if the system is to be used as a refrigerator. Here, the evaporator is used to extract heat from the space to be cooled. The heat from the condenser and the generator cooling is typically dumped outside.

Although the heating and cooling provided by a single generator is discontinuous, it can be made continuous by operating two or more generators out of phase.

While the basic concepts behind adsorption heat pumps are well known, the challenge has been in the required size of the systems; the basic adsorption cycle has low efficiency, which requires methods for recovery of heat between adsorbent beds, and the inherently low thermal conductivity of available adsorbent materials results in low power densities and machines of high capital cost.

The University of Warwick researchers developed an adsorption systems design that significantly shrinks these devices making them small and light enough for use in both domestic heating and automotive air conditioning.

In a paper published in January in the journal Heat Transfer Engineering, Critoph et al. describe a prototype compact sorption generator using an activated-carbon/ammonia pair based on a plate heat exchanger concept.

The heat exchanger uses nickel brazed shims and spacers to create adsorbent layers only 4 mm thick between pairs of liquid flow channels of very low thermal mass. The prototype sorption generator manufactured has been evaluated under the European Union (EU) car air-conditioning testing conditions.

While driven with waste heat from the engine coolant water (at 90 °C), a pair of the current prototype generators (loaded with about 1 kg carbon in each of two beds) has produced an average cooling power of 1.6 kW with 2-kW peaks.

Sorption Energy’s technology will work well in current vehicles with diesel or gasoline engines, and is expected to work with hybrid electric vehicles incorporating internal combustion engines.

This technology is expected to cost no more than the current systems which are driven by a compressor. Several components are common, but the compressor will be replaced, and the additional components provided by Sorption Energy can be manufactured at an acceptable cost.

(A hat-tip to John!)


  • Robert E. Critoph; Steven J. Metcalf; Zacharie Tamainot-Telto (2010) Proof of Concept Car Adsorption Air-Conditioning System Using a Compact Sorption Reactor. Heat Transfer Engineering, Volume 31, Issue 11 pages 950 - 956 doi: 10.1080/01457631003604459

  • M A Lambert, B J Jones (2006) Automotive Adsorption Air Conditioner Powered by Exhaust Heat. Part 1: Conceptual and Embodiment Design. Proc. IMechE, Part D: J. Automobile Engineering. Volume 220, Number 7 doi: 10.1243/09544070JAUTO221



This could be a smart way to effectively increase overall ICE efficiency by 5+% by using some of the wasted heat to regulate the cabin's temperature. From the information given, it could also eliminate the often troublesome and noisy compressor used in current vehicle AC system.

How long will the car industries take to incorporate it into their production line?

Will S

Only a matter of time before someone actually tried this basic concept. Good to see that it finally obtained traction.


So; twice the number of condensers and "evaporators".
Plus the complex control of air, "coolant" or exhaust heat to each of the 4.

Add to this the low efficiency of such units and it sounds worse than putting a windmill or 15’ x 30’ PV array on your car roof.

I am NOT ready to believe the U of W has shrunk these devices enough, and made the efficient enough, for use in EITHER domestic heating or automotive air conditioning.


Might be better off being plugged into the grid with the cars heat pump being used to preheat or cool the car. It could also heat the oil and cat up to the right temperature just before it was required.


Hmm. But what happens when you get into a car that's hot from being parked in the sunshine, but whose engine has cooled off? You'd have to wait for the engine to get hot before the A/C could work? I'm not very keen on that idea.


I think an electrically driven compressor, like the Prius has, is a much better way of reducing A/C fuel consumption while at the same time reducing complexity.


low efficiency? 1.6 kW it's equal to almost 5500 Btu/h...enough for a compact car.
And it's in an early development stage, so we still
don't know if it could work.
I study this in college once, it is a good idea, and completely possible but it needs development (geometry of heat exchanger(s), materials, heat source) to turn into a mass production technology.
For example, you can use heat storage technology, like the one used today in buildings air conditioning, where you use the system at full capacity when you can (typically at night) and save the "cool" by making ice in an isolated container.
You could do the same in a car after heated enough the catalytic converter, and in starts from cold using heat from exhaust.


This system stores refrigerant in a way that will allow it to work for a period of time while the engine is warming up. Adsorbers are 6" cubes. One condensor is used as well as one evaporator, and the adsorbers share them sequentially. Surface area in one of the cubes is said to be 3.5 ACRES. Methanol is the refrigerant. Think of the latent heat of a thin layer of methanol evaporating on a 3.5 acre surface.

Henry Gibson

If they can demonstrate it with engine block temperatures it can be demonstrated with exhaust gas temperatures. A system with water absorbtion and little hermetic electric pumps to push liquids around might be easier to build. Check up on the ICY BALL for a simple example of such refrigeration. The Platten-Munters system is used for refrigerators in many caravans and remote locations and also could be run off of engine heat. Large systems like to use Lithium Bromide and Lithium Iodide solutions to absorb water vapour. Another use of lithium in automobiles? Gas heat powered refrigerators should be required as an energy efficiency measure in homes in colder climates where the waste heat can be used for house heating. ..HG..

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