Can “second Life” Ev Batteries Work as Grid-Scale Energy Storage?

A typical house in Midwest needs around 22,000kWh (7.913×10^10 J) over the winter (75 million BTU - https://www.eia.gov/todayinenergy/detail.php?id=57321 ).

If we assume the delta of 550 degrees (600 down to 50), you'll need: 7.913×10^10 J / (550K * 1000Jkg^-1K^-1) = 143,872,727 kg of material in your pile. This is a ridiculously stupid number. And I don't see any obvious mistakes?

Having said that: a good design for sand batteries would use insulated silos, pushing/dropping sand into a fluidized bed heat exchanger where some heat transfer gas is intimately mixed with it. This is the NREL concept that Babcock and Wilcox was (still is?) exploring for grid storage, with a round trip efficiency back to electricity of 54% (estimated) using a gas turbine. Having a separate heat exchanger means the silos don't have to be plumbed for the heat exchange fluid or have to contain its pressure.

Getting the sand back to the top (where it will be heated and dropping into silos) is a problem that could be solved with Olds Elevators, which were only recently invented (amazingly).

https://www.youtube.com/watch?v=-fu03F-Iah8

Better idea: put 9 2.7-meter wires in parallel on each of the three circuits, so each wire can have 9×0.173Ω = 1.56 Ω = 0.58Ω/m. That's 32-gauge copper magnet wire, 0.2mm diameter, 0.54Ω/m; or its thicker equivalent in other metals. Iron's resistivity is 5.7 times copper's, so you need a 5.7 times thicker wire: 0.5mm, 24-gauge. Nichrome is 11 times the resistivity of iron, so you'd need 1.6-mm-diameter nichrome.

I don't know, I think the copper would probably melt faster than the sand could conduct the heat away from it, and the nichrome would definitely be fine, but too expensive. But you can extrapolate from this how to solve the problem: by shortening the distance along the heating wires to low-resistance busbars (possibly made of rebar or leftover angle iron) and thus increasing the number of parallel paths, you allow the use of higher-resistance-per-unit-length and thus cheaper and more workable heating elements; the limit of this lightweighting is that the wires' surface area in contact with the sand must cool them enough to prevent melting. By this method you can use a small amount of a conductor of any resistivity at all, limited mainly by the temperature.

All these metals are fine at 700°, or for that matter 1000°. Copper will have less of a tendency to oxidize than iron, which would require a reducing atmosphere, and nichrome will oxidize but remain protected by its oxidation. (A reducing atmosphere will destroy nichrome.) But, at a lower temperature still, like 600°, you could use 10μm thick household aluminum foil, which is much easier to work with than any kind of 20μm wire, but has a similar ratio of surface area to volume. It has 54% more resistivity than copper, so a 10μm × 1mm strip is 2.7 ohms per meter. Our previous objective of 0.58Ω/m is a 4.6mm-wide-strip, which transfers heat to the sand along its 9.2mm perimeter, like a 10-gauge wire. 75m × 4.6mm is the size of about 5 or 6 pages of A4 paper cut into strips.

But the heat pump doesn't save you 10kW over resistive heating when it's running full-tilt. It saves you 10-3 = 7kW. So it costs 39¢ per watt of saved energy, which is 6 times as much as the solar panels.

In some simplified theoretical sense, if you decide you need another 10kW of heating for your house, you could spend US$2700 on this heat pump, and also buy 3000 Wp of solar panels to power it, costing US$194, for a total cost of US$2894. Or you could buy 10000 Wp of solar panels, costing US$645, and a resistive wire, costing US$10, for a total cost of US$655. US$655 is almost five times cheaper than US$2894. (4.4 times cheaper.)

There are a lot of factors that this simplified cost estimate overlooks; for example:

• Maybe you need to run the heater 16 hours a day but you only get sunlight 7 hours a day, either because it's winter in Norway, or because there are tall pine trees that shade your property most of the day, and you can't put the panels up on the trees. So maybe in some sense one watt of peak heater output is worth 2.3 watts of peak solar panel output. Or maybe it's the other way around, where your house only needs active heating during a few hours at night, so one watt of peak heater output is only worth 0.43 watts of peak solar panel output.

• The prices are in different countries. Solar panels are more expensive in the US, even wholesale.

• US$2700 is the retail price of the heat pump, including installation and warranty, and 6.5¢/Wp is the wholesale price of low-cost solar panels with no warranty ("Minderleistungs-Solarmodule, B-Ware, Insolvenzware, Gebrauchtmodule, PV-Module mit eingeschränkter oder ohne Garantie, die in der Regel auch keine Bankability besitzen.") Even in Europe the retail price of solar panels is three or four times this.

• Driving a resistive heating element from solar panels is considerably easier than driving a heat pump from solar panels; adapting a heating element to run on lower voltage is just a matter of connecting more wires to the middle of it, while adapting a heat pump to run on lower voltage may involve redesigning the whole power supply board or even rewinding the motor. Which is in a hermetically sealed refrigerant circuit, by the way, which you'd have to reseal. In practice, you'd just buy an inverter, but a 3000-watt inverter is expensive.

• As you said, for sensible-heat thermal storage, the heat pump craps out at about 50° or 60°, while any garden-variety resistive heating element (plus a lot of crappy improvised ones) will be just fine at 600° or 700°. That means you need ten times as much thermal mass for the same amount of storage. Sand is dirt cheap, but once you get into the tens of tonnes, even dirt isn't really cheap.

Despite such complications, I still think that pair of numbers is a useful summary of the situation: the heat pump costs 39¢ per watt saved, while the solar panel costs 6.5¢ per watt produced.

It's under Weights/Capacities but you have to expand the section yourself, no way to link directly to it.

The problem is not just the mean capacity factor, but the capacity factor in _winter_. It's terrible for China, less than 15%. And more importantly, you can have _weeks_ with essentially zero solar power when you need it most.

We might ask why regulations are so putatively damaging to nuclear, when they aren't to civil aviation. One possibility is that aircraft are simply easier to retrofit when design flaws are found. If there's a problem with welding in a nuclear plant (for example) it's extremely difficult to repair. Witness the fiasco of Flamanville 3 in France, the EPR plant that went many times over budget.

What would this imply for fusion? Nothing good. A fusion reactor is very complex, and any design flaw in the hot part will be extremely difficult to fix, as no hands on access will be allowed after the thing has started operation, due to induced radioactivity. This includes design or manufacturing flaws that cause mere operations problems, like leaks in cooling channels, not just flaws that might present public safety risks (if any could exist.) The operator will view a smaller problem that renders their plant unusable nearly as bad as a larger problem that also threatens the public.

I was struck by a recent analysis of deterioration of the tritium breeding blanket that just went ahead and assumed there were no initial cracks in the welded structure more than a certain very small size. Guaranteeing quality of all the welds in a very large complex fusion reactor, an order of magnitude or more larger than a fission reactor of the same power output, sounds like a recipe for extreme cost.

One doesn't need super high quality welding and concrete pours becuase of regulations as much as the basic desire to have a properly engineered solution that lasts long enough to avoid costly repairs.

Take for example this recent analysis on how to make the AP1000 competitive:

https://gain.inl.gov/content/uploads/4/2024/11/DOE-Advanced-...

There are no regulatory changes proposed because nobody has thought of a way that regulations are the cost drivers. Yet there's still a path to competitive energy costs by focusing hard on construction costs.

Similarly, reactors under completely different regimes such as the EPR are still facing exactly the same construction cost overruns as in the rest of the developed world.

If regulations are a cost driver, let's hear how to change them in a way that drives down build cost, and by how much. Let's say we get rid of ALARA and jack up acceptable radiation levels to the earliest ones established. What would that do the cost? I have a feeling not much at all, but would like to see a serious proposal.