I have found some US products and sites that suggest 30% of household energy is lost down drains from showers/baths/dishwashers etc. There seems to be some low cost (to buy) but maybe complicated to include / install heat recoevry units... Does anyone have any experience of these - they are claiming 90% of heat is recovered. I am guessing that they would be plumbed into a heatstore, much like solar panels... If this is true it is an 'instant' 20%ish gain in household energy saving...Grey water heat recoevry
In the shower we mix hot and cold water to get a comfortable temperature. Say one would create a tub to stand in, we run the cold water though a tube though the tub. Not mixing old with new water but just absorbing heat from the "sewage". The cold water should be able to absorb a lot of the heat from the tub.
The heat difference could also be used to power motors but this one tube+tub system is quite straight forward. You simply get more hot water for the same price.
We always have 1 reservoir on each toilet, why not put 5 there and fill those with the shower sewage? Only tank Nr 5 fills automatically and only when the others are all empty. If you add enough flush tanks it would use water 3 or 4 times per year at most. A conventional toilet uses quite a lot more than that.
Another revolutionary idea I had was to stick a chimney into the center of a vertical wind turbine. The wind creates pressure in the center, the heat will then expand the compressed air.