I’d like to see the cooling system. I would suspect it requires more than 70W to function.
Actually, it sounds mostly passive -- almost kinda like high-temp heat pipes:
Specifically, the IBM team used a very thin layer of a liquid metal made of a gallium and indium compound that they applied between the chip and a cooling block. Such layers, called thermal interface layers, transfer the heat from the chip to the cooling block so that the chip temperature can be kept low. The IBM liquid metal solution offers the best thermal performance available today, at low costs, and the technology was successfully developed by IBM to cool high power computer chips earlier.
The big technical problem is removing heat from the solar cell. They do that by moving it to a big heat sink ... and it's no big deal to cool one of those.