Commissioning and reconstruction of a H2O/LiBr falling-film absorber for the analysis of the effect of additives on the heat and mass transfer

The global energy demand for refrigeration and air conditioning (RAC) systems has surged by over 40% in the last decade, driven by population growth, urbanization, and rising global temperatures. Currently, RAC systems account for nearly one-fifth of global electricity consumption, with emissions from both energy use and refrigerant leaks contributing significantly to global warming. In Europe, cooling demands have increased rapidly due to more frequent heatwaves, with RAC systems comprising 12-15% of the region’s total electricity consumption. Conventional vapor-compression refrigeration (VCR) systems, which dominate the market, rely on electricity-intensive compressors and synthetic refrigerants such as hydrofluorocarbons (HFCs), which have high global warming potential (GWP). Lithium bromide-water (LiBr-H₂O) absorption chillers offer a sustainable alternative by utilizing low-grade waste heat (70-120°C) or renewable thermal energy instead of electricity. These systems replace mechanical compression with thermally driven sorption cycles, eliminating synthetic refrigerants. Water is employed as the refrigerant, featuring zero ozone depletion potential and negligible global warming potential, while lithium bromide serves as a non-toxic absorbent. Over the past few decades, extensive research has been conducted to improve the efficiency of absorption chillers. Key advancements include optimization of absorber tube configurations, flow regime analysis, and the use of surfactant additives to enhance heat and mass transfer performance. Despite these environmental benefits, absorption chillers remain underutilized, constituting less than 5% of the global cooling market. Their widespread adoption is hindered by lower efficiency under steady-state conditions compared to VCRs, higher initial investment costs, and sensitivity to heat source fluctuations. However, their ability to utilize industrial waste heat and solar energy makes them a crucial solution for decarbonizing cooling, particularly in energy-intensive industries and hot climates. This study focuses on the commissioning and reconstruction of an existing experimental setup for a falling-film horizontal tube bundle absorber at the Department of Technical Thermodynamics, University of Kassel. The setup is designed to analyse the effects of surfactant additives on heat and mass transfer performance. Key operational challenges, including pressure sealing issues, crystallization, and rust formation within the absorber test rig, were systematically investigated and addressed. The experimental results indicate that the absorber test rig's efficiency has declined over time, with mass transfer experiencing a more significant reduction compared to heat transfer. This suggests that operational factors such as sealing issues, crystallization, and rust formation have progressively impacted the absorber's performance, necessitating further investigation and optimization.