The core classes are F01K (steam and thermal power plants), F02G (hot-gas and external-combustion engines) and F28D (heat-exchange apparatus), which together account for the large majority of filings in this space. Turbine design (F01D), refrigeration and heat pump cycles (F25B), exhaust treatment (F01N), boiler design (F22B) and certain motor mechanisms (F03G) form a secondary layer of adjacent claim territory. A thorough search should cover all of these, since a single waste heat recovery system often touches several subclasses at once, for example an ORC system with an exhaust-gas recuperator sitting across F01K, F02G and F01N.

Filing activity concentrates around a small set of established thermal and turbomachinery specialists with long histories in power generation and engine technology, rather than being evenly spread across the industry. Several of the historically most active assignees show no filings in the most recent tracked year, which may reflect a genuine slowdown, a shift toward trade secrecy, or simply the reporting lag between filing and publication. Co-assignee relationships in this space are narrow, typically linking one company to a specific named inventor across multiple filings rather than reflecting broad industry alliances.

Annual filings peaked at 139 in 2017, held roughly steady through the early 2020s, and have since trended downward toward the most recent partial year. This pattern is consistent with a technology whose core architectures — ORC cycles, steam-based heat recovery, standard recuperator layouts — were substantially claimed during the mid-2010s buildout. It does not necessarily mean the field is exhausted; publication lags filing by roughly 18 months, so recent years will look thinner than they eventually turn out to be, and a slowdown in filing volume can also reflect consolidation among fewer, more selective filers.

Relative to the dense core around steam-cycle and ORC architectures, subclasses covering refrigeration/heat pump hybrid cycles, exhaust-treatment integration and low-grade (sub-100°C) heat conversion carry noticeably fewer filings. That thinner density suggests more room for differentiated claims around working-fluid selection for low-temperature loops, hybrid heat-pump-assisted ORC cycles, and modular or skid-mounted waste heat units. Any white-space claim should still be checked against the full IPC set, since low-density subclasses can still contain a small number of broad, blocking claims.

China accounts for 724 filings by receiving office, close to double the 392 filed in the United States, with Europe (EPO), WIPO/PCT filings and South Korea forming a smaller second tier. This shift means a large share of recent technical disclosure in this field is only fully visible through Chinese-language patent literature. Anyone doing competitive tracking or freedom-to-operate work in this space should treat a China-only or China-inclusive search as the baseline, not an optional extra.