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Making POW useful requires nothing more a than changing the mindset. There are many situations where the heat produced has more value than the electricity consumed. Ever used electricity to produce heat? If the objective is to use electricity to produce heat, then POW mining of crypto currency becomes simply a way to reduce costs.
Electric heating elements are, however, substantially more economical.
How? I am talking about obsolete mining equipment, which in theory has zero value, particularly if one is talking about ASICS.
I didn't realize you were so specifically referring to
obsolete Bitcoin mining equipment. Regardless, I still contend that electricity, if it is to be utilized in heating, is more efficiently made to produce heat with equipment designed specifically for the task.