Sounds like an external heat exchanger.
External heat exchangers can be good in that they are a separate unit from the thermal storage tank.
The downside is that they usually have less surface area, and often require a second pump.
If you can find and repurpose equipment from other industries, which will work for a project, great! However, I have found that anything with with the word "boat" or "marine" in the title always means it costs more!