Absorber Designs in Heat Pumps: A Review

What is it about?

This review explores the absorber—the heart of absorption heat pump systems that use waste heat or solar energy for cooling or heating buildings instead of electricity from fossil fuels. In these eco-friendly cycles, the absorber mixes vapor (like ammonia or water) with a liquid absorbent (often salt solutions) to release or capture heat, but its design affects the whole system's size and efficiency. Cooling systems need absorbers that prioritize fast vapor absorption at room temps to shrink the hot generator part, while heat transformers focus on grabbing exothermic heat from absorption for upgrading low-grade waste (e.g., 80°C factory water to 120°C steam). The paper surveys 50+ experimental studies since the 1970s, testing designs like falling films (thin liquid sheets for quick mixing), bubble absorbers (gas bubbled through liquid for better contact), and packed beds (porous fillers to increase surface area). Key findings: Falling films achieve high mass transfer (up to 0.01 m/s) but uneven wetting; bubbles boost rates 20-50% via turbulence but risk foaming; sprays offer uniform distribution but higher pressure drops. Fluids matter—ammonia-water excels in cooling (COP 0.7), LiBr-water in heat pumps (up to 0.5). Real tests show 10-30% efficiency gains with enhancements like ultrasonic waves or nanoparticles. Challenges: Corrosion and scaling limit longevity. This guide helps engineers pick absorbers for apps like air conditioning (reducing grid strain) or industrial drying (recycling 20-50% wasted heat), promoting sustainable energy amid fossil fuel shortages.

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Photo by Glen Carrie on Unsplash

Photo by Glen Carrie on Unsplash

Why is it important?

This review compiles experimental data across 40+ absorber types/fluids, quantifying trade-offs (e.g., bubble vs. film transfer rates) absent in theoretical overviews, spotlighting untapped enhancers like vibration (30% boosts). Timely as heat pumps surge (projected 50% HVAC growth by 2030), yet absorbers bottleneck efficiency—addressing Kyoto-era waste heat (abundant but underused, 20-50% industrial loss). Impact: Informs compact designs slashing system sizes 20-40%, cutting energy use 30% in cooling/heating, and enabling solar/waste integration for off-grid sites. It could save billions in fuel costs, reduce CO2 by millions of tons yearly, and accelerate adoption in developing regions facing energy poverty.

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