Heat and Mass Exchanger Design for Inter-seasonal Liquid Absorption Heat Storage

Abstract

Sorption heat storage gives perspective for renewable space heating in buildings. Benefits put forth are, loss free storage over time and increased energy density exceeding that of hot water storage. Critical performance parameters in this application are: gross temperature lift, volumetric power density and volumetric energy density. Heat and mass transport in light of these parameters is a challenge holding back this technology from commercialisation.
In this thesis, an effective heat and mass exchanger design for liquid absorption heat storage with aqueous sodium hydroxide for compact inter-seasonal heat storage is presented. In this work, global sorption heat storage performance parameters and operating temperatures were defined, followed by a categorisation of sorption heat storage process types and analysis of performance potential. The closed transported process with single pass and true counterflow was found
favourable. Mass transport was recognised as the dominant boundary in this process, and detailed analysis from milligram to kilogram scale was undertaken. Still film absorption measurements showed the need for extended exposure time for sufficient absorbent exchange (energy capacity). Hereto a novel vertically installed spiral finned tube heat and mass exchanger was developed. Highly temporally and spatially resolved analysis of the mass diffusion in thin absorbent film as present on the spiral finned tube was undertaken with Raman spectroscopy. Mass diffusion in the film was found to be the limiting parameter, indicating that there is potential for improved mass flux through mixing. Visual evaluation of absorption and mass transport was carried out with neutron imaging. In this approach, strong concentration stratification due to buoyancy force was detected. A method towards engaging this buoyancy movement for increased absorption kinetics was found by flooded spiral finned heat and mass exchanger. The functionality of this advanced approach was extensively examined on the lab test bench and good performance was found.

Date of Award	May 2020
Original language	English
Sponsors	Swiss Innovation Agency Innosuisse grant Nr. 1155002545 & Swiss Federal Office of Energy SFOE grant Nr. SI/ 501605-01
Supervisor	Philip Griffiths (Supervisor) & Neil Hewitt (Supervisor)