I'll raise you, and out bid your phone and get into the home business of making yogurt or drying paint or making candles with the extra heat.
And i will be using a spare Kw or so, via the combustion engine, on my way to work/at work/travelling etc.
Buy an established pizza restaurant and keep your oven filled with waste heat from mining. Problem is that the cost of that heat is a small portion of the operating cost and hassles of the business.
Stirling engines are very inefficient (compared to combustion engines) otherwise transportation might be a viable market. Adding unfocused heat to a combustion engine lowers the efficiency.
This is why smooth's efficiency arguments are irrelevant.
The Knowledge Age is not about physical constraints. It is about mental ones. Our ROI is coming from our mental efforts not incremental tangible efficiencies. I wouldn't waste an hour of my life on saving $10 a month. My opportunity cost is too great.