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!)

Resources

• 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