[hayden55]

I see we are getting into masters of science in engineering problems.
I'd say a lot of the heat energy from car to house ideas would probably only be really feasible on RVs. ie: At warmest my garage is usually 20 degrees colder than my house in the winter. If I run a 2000w heater on full blast 24/7 I can pull it up to within 8 degrees.
For the most part a lot of the recoup energy storage ideas are very cool, but a lot aren't feasible because of initial investment vs roi.
Also, natural gas for heat appliances is just about the most efficient way to go.
I do think that the black water box on the roof idea would work. Then you could pipe it to a radiant heating pad in an insulated foundation slab and even a bypass for your water heater. Roofs get unbearably hot pretty much all year round. (radiant walls wouldn't be as efficient... less sun exposure)
White roofs are a way to go too. 90% reflection vs like 2% for black tar. I think its even mandated in nyc building code now. Drops inside temps by a couple of degrees C which is significant.
For the most part, if you were going to do energy saving projects the most bang for your buck would be to:
Reinsulate with High R-value materials, airtight to like 99% so no leaks, clean air management system, updated high-efficiency appliances, and all natural gas heating appliances (ofc tankless water heaters).
If you bought a place in need of a remodel/update and are not replacing good components it could pay for itself.
Then your energy bill would be so low it wouldn't matter. That's my goal. Do that as my first 90%, then my last 10% would be adding renewables/recap. The only problem with those is as stated: initial investment, ROI, doesn't add to value of the house... You would have to take it with you or anchor yourself. Also, here locally you're not allowed to be on the grid while you're generating solar.
Also for reference:
NGCC power plants are up to 62% efficient.
Honda Toyota are up to 41% efficient now.
LS cars are up to 36% efficient.
90's jap cars non lean burn were up to 32-36% efficient per bsfc charts.
Coal is actually up to 40% efficient.
Nuclear is up to 35% efficient.
A cool note in the spirit of the op:
My advisor has a patent on a device that captures the steam of the stack if the steam turbine at a power plant, and can recoup that water back into the system in areas where water is very scarce (ie: middle east, certain areas in far east).
Also, we had an ashrae meeting the other day and one of the firms in Little Rock is bidding the new Walmart Campus System in Bentonville. The project requirement: The entire campus is beyond net zero. It makes more power than it consumes and sends it to the grid.
Supposed to be big ****.
**Add step 2. Roof water capture system. Easy money right there.
Back to the fuel is fuel idea.
So imagine you have a power grid that is 32% coal, 32% natural gas, 32% nuclear, and ~4% renewable.
(0.32)(0.4)+(0.32)(0.62)+(0.32)(0.35)+0.04(100)= ~47.84% efficient
Then the power transfer efficiency at 10% loss -> 43.056% -> then ev charger to road ~87% = 0.37.5%
Disclaimer: exact specifics are a close ball park... not too exact. The main point to bring is that when comparing direct and indirect emission with the current power grid it really just depends on where you live if your EV is actually cleaner than a new efficient hybrid. But the $/mi cost of the EV is a little better than the hybrid.
I think the end goal will be all-electric end users so we can choose from many different sources of energy to avoid price hikes like gasoline.