recuperator

Sorry, to be clear, the point of a recuperator is to exchange indoor air with outdoor air, reducing indoor CO2 without losing as much heat as opening a window.

They were using simple black-body radiators and the efficiency was pretty poor, even with a recuperator pre-heating the combustion air.

Lastly, it's entirely possible to slap a recuperator, intercooler, and reheater on the engine if things will get too desperate.

Is the homebrew recuperator cheaper and/or better somehow?

The recuperator in Allam Cycle CO2 turbines.

Recuperator-type heat exchangers need high-strength materials because both the strength of a wall and its thermal resistance are proportional to its thickness.

"single room recuperator" from aliexpress is in every flat after renovation in Lithuania.

The ideal space-filling "core" of a rete mirabile recuperator is (I argue) a checkerboard, with black and white squares flowing in the opposite direction.† This makes all adjacent channels opposite polarity (unlike a honeycomb) for maximum heat transfer, and also reduces internal drag and manufacturing complexity (unlike triangles).

"single room recuperator" from aliexpress is in every flat after renovation in Lithuania.

The ideal space-filling "core" of a rete mirabile recuperator is (I argue) a checkerboard, with black and white squares flowing in the opposite direction.† This makes all adjacent channels opposite polarity (unlike a honeycomb) for maximum heat transfer, and also reduces internal drag and manufacturing complexity (unlike triangles).

Recuperator-type heat exchangers need high-strength materials because both the strength of a wall and its thermal resistance are proportional to its thickness.