Using Waste Engine Heat to Cool Your Car

GCC
Sorption Energy is a UK company aiming to commercialise vehicle air conditioning systems based upon adsorption heat pump technology. The Sorption units will operate on the waste heat from normal engine operation, rather than parasitising mechanical power from the actual engine output. Sorption aims to save 10% or more of fuel costs during warm weather by this method.

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. _GCC

Sorption is a spinoff from University of Warwick, UK, and utilises some significant breakthroughs in adsorption heat pump technology developed at Warwick which improve the efficiency of the system appreciably, and makes the actual devices much smaller and more manageable for small vehicles such as cars. The Sorption devices are based upon an activated carbon / ammonia pairing using a highly compact plate heat exchanger using nickel brazed shims and spacers.

Here is an analysis from USC / CSULB looking at the use of adsorption heat pumps in mass transit vehicles in the Los Angeles area.

This type of heat pump has been used on board satellites, and on shipping containers. They can be driven by any type of heat source, including solar heat. For automobiles, the use of waste engine heat to drive the heat pump is reasonable, and could result in significant savings during summer months. The heat pump could be used for heating the vehicle during cold weather, of course, but conventional automobile heating systems already utilise waste engine heat to keep car occupants warm.

The possibility of using this technology for domestic home and apartment heating purposes is being considered as well -- contingent upon finding an economical and reliable heat source to drive the system, and absolute proof that any system using ammonia as a refrigerant would be safe for household use. Methanol is an alternative coolant.

Cross-posted to Al Fin