Bubble Absorbers in Plate Exchangers: Testing Ammonia Mixtures for Better Cooling

What is it about?

This study explores the improvement of absorption refrigeration systems—eco-friendly coolers that utilize heat instead of electricity from fossil fuels—by testing a plate heat exchanger (PHE) as a bubble absorber with three ammonia-based mixtures: NH3-H2O (ammonia-water, the standard), NH3-LiNO3 (ammonia-lithium nitrate), and NH3-NaSCN (ammonia-sodium thiocyanate). Absorption systems are ideal for utilizing waste heat or solar energy, but require improved components, such as absorbers, which enable the mixing of vapor bubbles into liquid to release heat and facilitate cooling. Traditional absorbers are bulky and inefficient, so researchers simulated a PHE (compact, wavy plates for improved flow) under real refrigeration conditions: low generator temperatures (70-90°C) and an evaporator temperature of 5°C for 1 kW of cooling. They built a mathematical model that balances heat/mass transfer, viscosity effects, and flow rates, running scenarios such as minimum generator temperature (to mimic solar limits), similar solution flows, or matched generator temperatures. Results: NH3-H2O and NH3-NaSCN absorbed more vapor (up to 0.005 kg/s) and handled higher heat loads (0.5-1 kW) than NH3-LiNO3, which struggled due to high viscosity (slowing mixing, 20-50% lower rates). However, in full-cycle simulations, NH3-LiNO3 achieved the highest efficiency (COP 0.48), while NH3-NaSCN (COP 0.45) outperformed NH3-H2O (0.43), thereby avoiding the need for rectifiers required for water mixtures. PHEs proved compact and practical, potentially reducing system sizes by 30-50%. This could make absorption cooling cheaper and greener for air conditioning or food storage, as well as for recycling industrial waste heat (20-50% lost), amid fuel shortages. NH3-NaSCN is emerging as a promising alternative for low-temperature operations, such as solar-powered fridges.

Featured Image

Photo by Denes Kozma on Unsplash

Photo by Denes Kozma on Unsplash

Why is it important?

This work simulates a Plate Heat Exchanger bubble absorber with alternative ammonia salts, quantifying the drag on viscosity (with 20-50% drops for LiNO3) and ranking NaSCN over H2O for rectifier-free efficiency—gaps in prior studies on H2O/LiBr. The timely post-2010s renewable boom, as absorption lags compression due to size/cost, spotlights low-temperature fluids for solar/waste heat amid 20-50% industrial losses and Kyoto goals. Impact: Enables compact systems boosting COP 10-20%, cutting energy 30-60% in cooling (e.g., 1 kW units for off-grid), with 2-5 year payback; could distill water or integrate with fuel cells, slashing CO2 tons/site and expanding to developing regions for sustainable HVAC/food preservation.

Perspectives