Experimental performance analysis of an ammonia-water diffusion absorption refrigeration cycle

Abstract

Absorption-refrigeration systems are emerging as an important technology for the utilisation of low- and mediumtemperature thermal-energy, such as renewable- and waste-heat sources, as a means of addressing the quickly accelerating global demand for cooling. In particular, systems based on the ammonia-water diffusion absorption refrigeration (DAR) cycle are of particular interest for cooling in off-grid regions and in developing countries due to their low capital costs, low maintenance requirements and unique design in which the requirement for an electrically driven pump is omitted. In this work, a detailed experimental evaluation is undertaken of a DAR unit with a nominal cooling capacity of 120 W. Electrical cartridge heaters are used to provide the thermal input which can be varied in the range 300-700 W at generator temperatures of 150-200 °C. Heating is also supplied to the air inside an insulated cold-box constructed around the evaporator which, by counteracting the cooling effect, allows the cooling output to be measured indirectly. Tests are performed with the DAR system charged to default factory settings (23.8 bar and an ammonia concentration of 30% by wt.). The cooling output obtained is in the range 79-104 W, corresponding to a coefficient of performance (COP) in the range 0.11-0.26. Results are compared to performance calculations from a steady-state system model, showing acceptable agreement for a model of this fidelity. Temperature measurements at various points in the system are used to provide further verification of the model assumptions. In particular, the temperatures measured in the condenser are used to evaluate the degree of sub-cooling and the effectiveness of the rectification process; while the air temperature and flow regime in contact with the evaporator are found to have a non-negligible effect on the predictions of the cooling output and the COP of the system.