The two dominant absorbent-refrigerant pairs in commercial absorption chillers are lithium bromide solution with water as the refrigerant, and an ammonia-water pair where ammonia is the refrigerant and water is the absorbent. Lithium bromide systems are more common in building-scale cooling because water is non-toxic and the cycle runs efficiently above 0°C, while ammonia-water systems are used where sub-zero refrigeration is needed since ammonia has a much lower boiling point. This patent corpus reflects that split, with the largest single chemistry cluster (C09K, 175 records) covering working-fluid compositions built around these two pairs.

Aqueous lithium bromide solution is alkaline and, in the presence of oxygen, corrosive to the carbon steel and copper alloys typically used in chiller shells and tubing. The corrosion cluster in this dataset (C23F, 42 records) traces a decades-long effort to control this through inhibitor chemistry and alkalinity management, starting with early approaches like the carbon dioxide dosing method in US3968045A. Because the underlying corrosion mechanism has not changed, filers keep returning to inhibitor formulation and dosing-control claims rather than solving the problem once.

The strongest historical filers in this dataset include Carrier, DuPont, Honeywell and BorgWarner, alongside research bodies such as Gas Research Institute and university programmes. Notably, every one of these tracked assignees shows zero filings in the most recent year, which is unusual for a field with this filing history and likely reflects a combination of publication lag and a genuine slowdown in new system-level patenting. The strongest co-assignee links in the corpus sit inside DuPont's internal inventor network rather than across companies.

Filing volume peaked at 11 in 2017 and has declined since, reaching 4 at the 2022 midpoint and 2 in the most recent tracked year — a flat-to-declining trend rather than a growing one. Because publication lags filing by roughly 18 months, the last one or two years understate true activity, so the field is not necessarily dormant, just harder to read at the most recent edge. The more reliable signal is the IPC split: C09K materials claims and C23F corrosion claims remain active relative to the much larger F25B system-hardware base.

The clearest gaps sit below the dense F25B system-architecture layer, in narrower chemistry and component claims: corrosion-inhibited lithium bromide formulations, low-global-warming-potential ammonia-water pairs, falling-film absorber heat-exchanger geometries, and crystallization-prevention methods in the separation stage. These sub-areas show materially lower filing density than core refrigeration-cycle claims, meaning a well-drafted composition or method claim there is less likely to run into blocking prior art than a system-level claim would.