Fuel is Fuel.... An alternative view to economy

[Bicycle Bob]

I've been planning to store engine heat for my micro-RV, and had spent decades waiting for Glauber's salt to hit the shelves. That's not going to happen in the foreseeable future, but paraffin has nearly the same heat of fusion and its temperature can be readily increased with stearic acid. I've been thinking of using a water muffler to recover the heat and steam from the exhaust, with a pump to circulate it to a tank containing mostly paraffin in thin plastic tubes. The coolant could also circulate to a similar tank. The wax would slowly melt, storing heat near the boiling point for water. To use that heat in your house, you'd only have to hook up two hoses, and circulate water to a radiator.

[servant74]

If we would put a mini-split in the garage and the house, we could use the garage as a heat source in the winter to get heat from for the house. -- This really means we need smarter HVAC/mini-split systems to allow using a garage or other area as a secondary heat source/sink rather than just an inside vs outside perspective. Even stealing heat from another section of a building and moving it to another is preferable (with a lower delta T) vs inside vs outside fairly often. Especially with some minisplits getting high heat transfer ratings.

[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.

[Bicycle Bob]

Heat from an in-vehicle storage tank can be recovered from a cold garage. An insulated hose would help more to a vehicle outside.
My roof gets a nice blanket of snow for several months, and chipping ice from the colder eaves to prevent pools of meltwater getting under the shingles is a springtime chore.
Burning natural gas should be done with a condensing vent, not a heat-operated chimney at a minimum. Realistically, high-grade energy should always be used to run a heat pump to multiply the yield for low-grade heat.

[stonebreaker]

Storing the energy as heat is inefficient. Use the heat from the exhaust to boil water and generate steam to turn a generator and charge batteries. When the batteries are full, turn the gasoline motor off and just run on battery power.

[Bicycle Bob]

Originally Posted by stonebreaker

Storing the energy as heat is inefficient. Use the heat from the exhaust to boil water and generate steam to turn a generator and charge batteries. When the batteries are full, turn the gasoline motor off and just run on battery power.

If heat is needed, storing it makes sense. The secondary steam engine has been an obvious option for a century, but steam engines are only efficient with a much higher temperature difference, so they would dissipate most of any remaining heat.

[Piotrsko]

How about a sterling engine powering the generator?

[Bicycle Bob]

Originally Posted by Piotrsko

How about a sterling engine powering the generator?

Even with high-grade heat, sterlings only get 20%. In heat engines, efficiency is totally dependent on temperature difference. That's why jet engines use such expensive high-temperature materials, and diesels still beat them by letting ordinary metal cool off between thermal peaks. https://en.wikipedia.org/wiki/Carnot...thermodynamics)

[stonebreaker]

Originally Posted by Bicycle Bob

If heat is needed, storing it makes sense. The secondary steam engine has been an obvious option for a century, but steam engines are only efficient with a much higher temperature difference, so they would dissipate most of any remaining heat.

No, not necessarily. While modern batteries have poor energy density compared to gasoline, they're still way lighter and more compact than hauling a water heater tank around in your Prius. The other thing to think about is, in a hybrid car, you ALREADY have a storage mechanism - the batteries - built in. All you have to do is build the steam generator and add a heat exchanger to condense the steam back into water.

As for the efficiency of the steam generator, so what if it's not as efficient as a modern steam plant? You're recovering waste heat anyway, so anything you get back is a win. I would think that the reason they don't already have this technology on hybrid cars has more to do with weight, complexity, packaging and safety than thermal efficiency.

[stonebreaker]

Originally Posted by Piotrsko

How about a sterling engine powering the generator?

That's a good idea. It would probably be more compact and lighter than a steam turbine.