for #9 strathmeyer:

100,000 sq ft roof (typical big box roof top)

2 inch rainfall

100,000 sq ft x 1/6 ft = 16,667 cubic ft of water

Assume the building is 20 feet tall and we empty it all in 1 hour, according to the formula given in post #5:

Head[ft] X Flow[cuft/sec] / 12 = KW

20 ft x 16,667/(3600 sec)/12 = 7.7 kW or 7.7kwh delivered over an hour. Enough to run 128 60-watt lightbulbs for 1 hour

That's some electricity but really, how often do you get a 2 inch rainfall? Not as often as you think, I bet.

More on the human waste.
Think of anaerobic digestors as giant stomaches. Put bad stuff in your stomach and you get out bad gas...Humans eat all the wrong foods. Mix in the T.P., dish soap, laundry soap, industrial waste and everything else that goes into a metro sewer system and it is hard to keep the bacteria happy. Then you get soloxanes from make-up and some fast foods and it turns to glass in the pistons of your gas fired generator.

Cows are perfect for this, though. Their stomaches are nature's best methane generators but they only use about 15% of the energy from the foods they eat. Put that in a digestor and everything is balanced pretty close to perfect for the microbes and then you can mix in some other waste from a food plant or similar and you get a pretty good methane supply. This gas can be used for electric production, heat or cleaned up and put back in the natural gas pipeline.

JGS: The reason we don't is because it would increase the cost of the fridge from around $800-$1k up to the cost of most central A/C units ($3k - $4k). The units would have to be much beefier to handle the low condensing temperatures of winter, too. If you read your owner's manual, it usually warns against using in a space below 50F.

Using cold outside air in coolers/freezers does not work. First, the unit would have to be against an outside wall. Then think of what is outside that wall...diesel/gasoline exhaust? Dumpsters? Mold? Back alley smells? And you want to bring that directly into a place where you store food? No, you do not.

You have to filter that air. Now the fan motor has to be sized large enough to push through all that filter, which uses more energy than the refrigeration system does because of the cool outside air, provided it is a built up system that rejects heat to the outdoors (think supermarket) instead of the indoor space (think domestic refrigerators).

If you really want "free cooling", go with Thinkerer's garage idea.

As for the other ideas, all heat (or cold) can be recovered. Its just a matter of what capital cost you are willing to pay to do it.