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