Investigation of a hybrid compression absorption heat pump system for high temperatures

Abstract

The industrial sector is a large consumer of high temperature heat. Usually this heat is provided by resources that consume primary energy. At the same time there is a vast amount of low-grade surplus heat available that is rejected instead of utilised in the processes. In this regard, the use of a heat pump to upgrade the heat to a higher temperature level can save primary energy. The compression absorption heat pump, or as popular called the hybrid heat pump, can be utilised for this purpose.

A continuing work of Wersland (2016) is conducted, where two industrial cases are investigated to evaluate the possibility of utilising the compression absorption heat pump to upgrade surplus heat. Simulation models in Dymola are created and the systems are optimised by changing the circulation ratio. The first case is located at a dairy plant, where a heat source of 30℃ is available and process water at 115℃ is needed. A model of the system utilising a two-stage piston compressor with heat recovery and water cooling at the intermediate compressor stage obtained a COP of 2.04 when the circulation ratio, intermediate pressure and compressor cooling was optimised. A system utilising a screw compressor was found to have lower performance. For the second case a large amount of condensation heat at 24℃ is available from a refrigeration system as a heat source, whereas water should be heated from 17℃ to 67℃. A system utilising a one-stage piston compressor obtained a COP of 4.33 for an optimised circulation ratio. In contrast, a system using a screw compressor was again found to have a lower performance due the volume ratio of the screw compressor utilised was too low compared to the pressure ratio of the system. A cascade heat recovery solution between the refrigeration and heat pump system was used with a co-current arrangement in the heat exchanger.

A simulation model of an existing facility was made to validate the hybrid heat pump model. Several of the simulation results agreed very well with the measured data from the facility, and are within the deviation range 0%-2.3%. The larger deviations that occurred can be explained by an overestimation of heat transfer coefficients in the absorbers, poor isentropic and volumetric efficiency correlations and that the input value of the volumetric flow of the heat source was from another data set. If the volumetric flow of the pump is increased in the validation model the COP was found to increase. In comparison with the findings of Jensen (2015) a more optimal combination of the circulation ratio and rich ammonia mass fraction was achieved. A possible solution of direct heat recovery of the condensation heat from an ammonia refrigeration system for the validation model have also been evaluated. A simplified calculation indicated that the COP for the total system could increase by 2.5%.