Experimental demonstration of an ammonia-salt resorption cycle heat pump
The design, manufacture, test and analysis of a kW-scale proof-of-concept resorption heat pump is presented. The adsorbent reactors use sodium bromide and manganese chloride salts with ammonia refrigerant. The reactor design was informed by Large Temperature Jump (LTJ) testing using a representative ‘unit-cell’ reactor. Extensive LTJ tests were conducted on ammonium chloride and sodium bromide, the latter being chosen as a suitable low temperature salt. Manganese chloride was selected as the high temperature salt. The salt selection methodology and analytical tools used in the LTJ analysis have been advanced compared with previous works, providing greater confidence in the identification of suitable salts and the working ‘equilibrium’ lines. The design of the heat pump is summarised, detailing component selection, calibration, and measurement and control methodologies. Compact adsorption reactors and a supporting test rig to measure and assess reactor performance are described. Automatic resorption cycling control has been achieved, enabling over 100 resorption cycles to be completed with continually connected reactors. Ammonia mass flow measurements and reactor heat transfer fluid heat balances show consistent, repeatable cycling and from which, dynamically measured COPs and power outputs were calculated taking into account all thermal masses. The contact resistance between the composite salt and the tube is limiting, resulting in a lower than designed power output. Swelling of the salt composite has also been observed, however, there is no evidence of performance degradation. COPs in the range 1.04 up to 1.28 have been achieved with average power outputs in the range 750 W to 3 kW ( 490 and 690 W/kg salt or 60 to 85 W/litre). Analysis of the dynamic performance and optimum control strategies are discussed. As designed, the resorption system is for domestic heating, but more widely the results show promise for use of ammoniasalt reactions for use in recoverable (waste) heat upgrading.