The PII Thermal Circuit is just an abstraction for us to use in the design of our solar power system (the Tenkiv Nexus). It provides a theoretical set of properties for us to try to implement in the Tenkiv Nexus and all of its components. The Tenkiv Nexus is not just a solar collector, it's a complete integrated solar power system (everything for collecting, moving, converting, and storing energy). So there is a lot of design work we had to do on many Tenkiv Nexus "components" and how they all fit together. Since we didn't have a baseline to iterate off of, we instead made a common set of theoretical structural properties that if perfectly implemented would yield the maximum possible efficiency (of course we can't perfectly implement the PII Thermal Circuit and we're not claiming the Tenkiv Nexus runs at the maximum theoretically attainable efficiency for a solar thermal power system). So to summarize the PII Thermal Circuit is basically just an abstract set of structural properties that we try to implement as closely as we can in all components of the Tenkiv Nexus. In hindsight it's possible we should have only used the PII Thermal Circuit abstraction internally as I think it makes the Tenkiv Nexus sound more complicated than it really is.
There's a section in our tech briefing on the PII Thermal Circuit which I think you may have already read.
I've been thinking about making a YouTube video where I just go through the properties of the PII Thermal Circuit one by one and talk about what they mean in practice. I think it might make things more clear if I do it well, let me know if that's something you'd like to see.