Absorption Cooling
Absorption Cooling
The mature, commercially dominant heat-driven cooling cycle — and the closest cousin of Adsorption Cooling. Where adsorption uses a solid sorbent in a batch cycle, absorption uses a liquid absorbent continuously circulated by a small solution pump. That one difference buys higher COP and continuous operation, at the cost of a liquid that can corrode, crystallize, or (for ammonia systems) need rectification. Absorption is the benchmark adsorption is usually measured against.
How it differs from adsorption
Same job — move heat from cold to hot using thermal energy instead of a compressor — but:
| Absorption | Adsorption | |
|---|---|---|
| Sorbent | Liquid (LiBr or water/ammonia solution) | Solid (silica gel, zeolite, salt composite) |
| Operation | Continuous (solution pumped generator↔absorber) | Batch / multi-bed switching |
| COP (single-effect) | ~0.7 (double-effect ~1.2–1.4) | ~0.4–0.6 |
| Drive temp | 70–200 °C | 60–95 °C (lower floor) |
| Moving parts | One small solution pump | None (valves only) |
| Failure modes | Corrosion (LiBr), crystallization, NCG, rectification (NH₃) | Lower SCP, large footprint |
Adsorption’s edge is the lower regeneration temperature and no moving/corrosive parts; absorption’s edge is higher COP and maturity. Many designs now combine them.
The two working pairs
- Lithium bromide / water (LiBr = absorbent, water = refrigerant): the air-conditioning workhorse. Water is the refrigerant, so it only cools above 0 °C, runs under deep vacuum, and risks LiBr crystallization at high concentration / low cooling-water temperature. Single- and double-effect machines dominate large commercial chilled-water plants.
- Water / ammonia (water = absorbent, ammonia = refrigerant): reaches below 0 °C for refrigeration and ice-making, runs at higher pressure, and needs a rectifier to purify the ammonia vapor of water. Favored for industrial refrigeration and off-grid solar ice makers.
Commercial machines
- Yazaki water-fired LiBr/water chillers run on hot water at 70–95 °C — a notably low drive temperature, so more waste heat is usable per ton. Water-fired 5–100 tons; direct-fired (gas) 30–200 tons. Their CCHP case: a CHP plant delivers 80 useful energy units per 100 fuel vs 56 for separate power + boiler, by routing waste heat to the chiller. Maintenance is light (chemicals every 8000 h, a single hermetically-sealed solution pump). Off-grid economics drive adoption where peak grid power exceeds 30 ¢/kWh or transmission costs up to $1M/km.
- Thermax two-stage LiBr/water units span 200–1640 TR (700–5765 kW) at COP ~0.81 — the high-COP commercial benchmark cited throughout Performance & Numbers.
These sit alongside the adsorption vendors in Commercial Adsorption Chillers.
Solar & off-grid experiments
Solar ammonia absorption refrigerators are a recurring DIY/appropriate-technology thread: Vanek & Vanek’s intermittent parabolic-trough solar ice maker produced ~10 lb of ice per cycle for ~$510; Buehn et al.’s senior-design vaccine cold-chain refrigerator (2–8 °C) used an ammonia pair at 8–14 bar for a ~$251 build. These mirror the intermittent solar adsorption ice makers in Solar Adsorption Cooling — same goal, liquid sorbent instead of solid.
Combined absorption–adsorption
Because the two cycles share the same evaporator/condenser hardware and the same waste-heat input, hybrids are being studied: Hassan et al. modeled a combined LiBr/water absorption + silica-gel/water adsorption system sharing the evaporator and condenser — in series at 85 °C it delivered 0.344 kW at COP 0.623, the absorption stage lifting the adsorption stage’s output. Román et al. simulated solar absorption and adsorption chillers managed by a modulating tempering valve. The combination aims to capture absorption’s COP and adsorption’s low-temperature reach in one machine.
See Also
- Adsorption Cooling — the solid-sorbent sibling cycle
- Commercial Adsorption Chillers — the thermally-driven chiller market, absorption included
- Solar Adsorption Cooling — the solar/intermittent parallel
- Alternative Cooling Technologies — where absorption sits among non-vapor-compression options
- Cooling Technologies Overview — the map of what drives the pump
Sources
- Yazaki — off-grid cooling / absorption chillers — water-fired LiBr/water specs, CCHP, off-grid economics
- Vanek & Vanek — solar ammonia absorption ice maker — intermittent parabolic-trough ice maker
- Buehn et al. — solar ammonia absorption refrigerator — vaccine cold-chain design
- Hassan et al. — combined absorption–adsorption cooling — shared evaporator/condenser hybrid, COP 0.623
- Román et al. — absorption & adsorption chillers with modulating tempering valve — TRNSYS control study