Here’s another example of thinking through a whole system, rather than trying to optimize one product or process in isolation. Greenhouses are usually energy sinks; this system re-uses the waste from one process to power or supply the next, in a way that puts almost all the bright local sunshine to work.
Mind you, this style of thinking is not entirely unique. The City of Vancouver has a nifty arrangement to burn the gases produced by decomposition in its landfill, both to generate renewable power and to supply heat to some nearby greenhouses—and not just a little heat, but 10 TJ/annum (something like 30,000 MWh if my sums are correct). (More technical details here.)
Similarly, their Olympic Village development uses a combination of heat recovery from a bakery, rooftop solar, and ground-source heat pumps recovering heat from a sewage line, to supply heat to a whole neighbourhood. Those projects are baby steps toward the sort of systems thinking cities are going to need in the future.
If I were renovating a flat to suit my tastes and priorities, heat recovery would be a central concern. Every time I take a shower, I wonder why carefully purified potable water, heated steaming hot, spends one second washing my body, then disappears down a hole unregarded. There has got to be a better way.