Next-generation ammonia adsorption heat pump cycles and technology 1=Energy 2=Energy Efficiency

in reducing the CO2 emissions associated with domestic heating. One ammonia - water absorption technology is commercialised, with good GUE (Gas Utilisation Efficiency, heat out/gross calorific value of gas in of c. 1.4) but high capital cost. An ammonia - carbon adsorption cycle is under development, offering reduced GUE of 1.2 but affordable capital cost.
Two possibilities exist to improve the adsorption cycle GUE without a major increase in capital cost. One is a development of the carbon ammonia technology employing either multiple adsorbers with improved heat recovery and the other using chemical adsorbents, generally halide salts embedded in a graphite matrix and used in resorption cycles. Both have been the subject of preliminary work at Warwick.
The research programme will first undertake sufficient analysis and modelling to decide which of either the active carbon or metal halide adsorbent types has the greater potential for eventual commercial adoption. The carbon adsorbent can certainly achieve a GUE of 1.4 at the expense of some extra complexity. The metal salt route is lower Technology Readiness Level but in theory could offer a GUE of 1.5 in a two-salt cycle and 2.0 in a three-salt cycle.
The challenges presented by the two technologies are somewhat different. The carbon adsorbent is well characterised and understood. The difficulties are in construction of a carbon ammonia adsorber with low thermal mass, good heat and mass transfer and low cost; this has already been the subject of many years' effort. A multiple bed with advanced heat recovery introduces further complexities in design and simulation.
The adsorbers in a metal salt system are very different in that the salts are contained within a conductive (non-adsorbing) graphite matrix which improves conductivity. The resorption cycles also have the benefits of fewer components. However, the chemical reactions that take place are much more problematic with reaction rates very difficult to predict. Where the active carbon in the carbon-ammonia machines is always in chemical equilibrium and operation is heat transfer limited, within metal salt - ammonia systems there is never equilibrium and operational performance depends on the poorly understood reaction rate dynamics.
Having made a choice between the two competing technologies (expected by Month 9) the research will enter a detailed design and simulation phase in which proposed adsorber designs are evaluated at 'unit-cell' level using the Large Temperature Jump (LTJ) technique already established at Warwick. With the validation of the adsorber design, construction of a proof of concept machine (3-10 kW output) will commence and simulation of control strategies begun. The POC machine will be tested first in the ThermExS laboratory, purpose-built under an EPSRC capital grant for the easy evaluation of novel thermodynamic systems. It consists of four computer-controlled thermal baths, valve and pump assemblies that act as heat sources and sinks from -10 to 180 C and powers from 7 to 30 kW.
This level of testing, electrically heated and within the laboratory is sufficient to prove the chosen cycle/adsorbents and result in new knowledge worthy of a PhD. However, it is quite probable that by this time there will be newly funded projects at Warwick that will enable the work to go further, perhaps integrating with a gas burner in a stand-alone system.