About those solar heat collectors: I can confirm it works because I’ve built one. It’s not as efficient as collectors that use coolant and a compressor system, but it’s very reliable and cheap.

I will second the opinion about aluminum recycling. Making aluminum from alumina is very energy expensive. Melting down cans is efficient compared to that.

P.S.

Notes about cans: if one wishes to make a model engine (e.g. compressed air engine, Stirling engine, lightweight models that work but cannot produce practical amounts of energy), some soda cans fit inside each other with extremely tight clearances (on the order of micrometers) and there is no seal, they slide on a layer of air. Of course, they have to be cut (with scizzors) and have to be kept clean, and despite keeping clean, there is abrasion (they wear down).

It’s very hard to machine a part to those tolerances. If a piston-cylinder system does not have to withstand detonation (not an internal combustion engine) and doesn’t have to do heavy work or last long, cans are a neat way to quickly get matching pistons and cylinders for experimenting.

• example 1: piston 355 ml “Red Bull”, cylinder 330 ml generic can
• example 2: piston “Devil’s Bit” 500 ml, cylinder “Battery” 500 ml can

(examples cannot be relied on as manufacturers have different production lines and production batches)