Closed-loop systems circulate a working fluid through a sealed pipe or heat exchanger inside a borehole without that fluid mixing with groundwater, while open-loop systems draw groundwater directly through the surface equipment. Patent claims in the closed-loop space, which make up this 680-record dataset, tend to focus on sealed heat-exchanger geometry, borehole configuration and materials that withstand deep-well conditions. Open-loop claims more often concern water extraction, filtration and reinjection. Some records, such as US20180172318A1, blend the two by inducing groundwater flow across a sealed exchanger, which is why the boundary between the two claim types is not always sharp in practice.

The assignee ranking behind this dataset covers 100 companies, with the leading filer holding 149 of the 680 records in scope. The top 5 assignees combined account for 37.4% of all records, and the top 10 combined reach 46.0%, indicating meaningful concentration without a single company controlling the field outright. Beyond the leaders, filing count drops off quickly, leaving a long tail of entities with only a handful of filings each. This pattern is worth checking against the assignee table on this page rather than assumed, since it can shift as new families publish.

Filings peaked at 80 in 2019 and the reliable comparison window of 2021 to 2024 shows a 26% decline, from 62 down to 46 filings. This does not necessarily mean underlying R&D activity dropped by the same amount; publication lags actual filing by roughly 18 months, so the most recent years in any filing trend are always understated until later data catches up. A cleaner read is that filing activity cooled from its 2019 peak but has not collapsed, and the 2025-2026 figures should be treated as provisional rather than a continuation of a downward trend.

F24T, covering geothermal systems generally, is the largest class at 41.8% of the 680 records, followed by F03G-classified spring and motor mechanisms at 28.4% and E21B earth and rock drilling technology at 24.9%. Downstream classes such as F25B (refrigeration and heat pumps, 15.3%) and F28D (heat-exchange apparatus, 11.5%) capture the equipment that turns extracted heat into usable output. Because a single patent record can be tagged with multiple IPC codes, these percentages overlap rather than sum to 100%, and a thorough freedom-to-operate search should span at least the top three or four classes rather than relying on F24T alone.

Enforceability depends heavily on the specific combination of elements claimed rather than the general closed-loop concept, which is too broad and long-established to be independently patentable. A record like US20180172318A1 blocks a fairly specific combination — a sealed loop plus a pump-induced groundwater sweep across the same exchanger — and would not reach purely conductive closed-loop designs or unrelated induction mechanisms such as thermosiphon effects. Anyone assessing risk should read the independent claims of the specific patents in their target class, particularly within the densely filed F24T and E21B classes, rather than assume broad blocking coverage from the concept alone.