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Question

Does anyone know if it would be possible to convert an Aluminum smelter into a battery for storing excess energy from a renewable energy source?

I imagine, if so, the cycle efficiency might not be too good due to the necessity to keep the battery at a high temperature but this might be somewhat improved by using a super insulator such as the re-entry tiles on the Shuttle.  If sufficiently insulated, the current flowing through the battery might be enough to keep the material molten as is the case with liquid metal batteries.

Answers

Hi William : I am not sure of the thrust of the question. But I will offer some thoughts and you can let me know if I did not get to what you are looking for.

An aluminum smelter facility is designed to use high volumes of electricity to reduce aluminum ore into pure aluminum. Typically, these facilities have been located near massive hydro (dams) to supply the needed electricity at the lowest price possible.

Liquid aluminum metal is not a good energy storage media as it's heat content  is too high, making it very difficult to store in liquid form. Instead, the liquid metal is usually poured directly into forms/sheets/bars for transport to aluminum product production facilities.

If one has a significant amount of energy available that needs to be stored, there are a variety of storage media available that can store the heat at a temperature beyond the boiling point of water over the long term, and be tapped to release their energy on demand.  Beyond liquid storage options, safer storage can be found using bricks and rocks in a thermal system designed to function as a giant battery.  Another way to store electricity is just to use a utility scale battery system such as flow battery, or lithium ion batteries.

Hope this helps your thinking.

Larry Eisenberg

 

William Hughes-Games's picture
William Hughes-Games on Jul 20, 2020 8:34 pm GMT

Hi Larry

Many thanks for your reply.  I wasn't thinking of the heat as a store of energy but rather, wondered if the electro-chemical process is reversable.  Roger has explained that it is not a reversable process so I guess that is a non starter.  Lithium batteries seem to be getting better and better all the time so it will be a hard 'uphill' for other systems to compete but for static applications, liquid metal, redox and ZnBr plating batteries look promising. 

For all practical. purposes, the answer is no, it can't. The commercial Hall-Heroult process for producing aluminum from a solution of aluminum oxide dissolved in molten cryolite is only partially electrolytic and not reversible. It would be more aptly labeled an "electrochemically facilitated carbothermic reduction". Aluminum plates out at the cell anode, but the cathode is consumable carbon. The gas evolved at the cathode is not oxygen but rather carbon dioxide. The reaction is carried out at conditions far from equilibrium. That's why it's not reversible.

It is possible to use pure aluminum as an energy source. Under controlled conditions, it can react with water to produce hydrogen and aluminum oxide. The hydrogen can then generate power in a hydrogen fuel cell. It's not efficient and it's not economical, but it has been considered--I think it may even have been demonstrated--as a way to produce a lot of electrical energy from a low mass of "fuel". Recycling the water from the hydrogen fuel cell would effectively create a high energy density aluminum-air battery. Such batteries could support relatively long distance electrically powered air transport. However, the overall energy efficiency would suck, and "fuel" (i.e., aluminum) would be too expensive for that application.

Bob Meinetz's picture
Bob Meinetz on Jul 19, 2020 2:19 pm GMT

"It is possible to use pure aluminum as an energy source."

It's possible to use pure hydrogen as an energy source, too. But because neither exist in nature we have to borrow chemical energy from other sources to make them, and the most economically-feasible sources from which to borrow are fossil fuels.

"However, the overall energy efficiency would suck.."

Seems entropy, like an unwanted houseguest, is always raiding the renewables 'fridge.

Roger Arnold's picture
Roger Arnold on Jul 19, 2020 7:21 pm GMT

It occured to me that my answer above is too narrowly focused. I gave a technically correct answer to the question as it was asked, but didn't answer the question as it could have been asked.

If the "question behind the question" is whether it's possible to run aluminum smelting operations in a way that can deliver power to the grid when needed, the answer is a definite yes! It cannot be done by running the aluminum production pots in reverse as batteries, but it can be done another way. The method is even efficient and cost-effective.

Aluminum smelting lines can be configured to serve as "virtual batteries" for regulating power to the grid. The line is operated as a variable load, to match variable supply. When supply is insufficient to serve other loads, aluminum production can be throttled back. That makes the power it would have consumed available for other loads. This has in fact been done in Germany.

A "virtual battery" can only reduce the power it draws from the grid. On its own, it can't supply net power to the grid. However, a virtual battery can easily be paired with a real battery. The combination is synergistic, delivering more capacity and functionality than either alone would be able to. It makes for efficient utilization of capital resources.

Bob Meinetz's picture
Bob Meinetz on Jul 20, 2020 3:00 pm GMT

Roger, what you describe is the industrial equivalent of demand-response, and assumes subtracting demand is equivalent to adding supply.

Lost in the equation is responsibility. Who pays the price when an aluminum smelter can't meet a production contract because the weather is hot?

No small irony, that making consumers responsible for an adequate supply of electricity guarantees an inadequate supply of electricity.

Roger Arnold's picture
Roger Arnold on Jul 22, 2020 11:40 pm GMT

Aluminum ingots are an eminently storable commodity. There's no question of missing production contracts. Production can easily be scheduled over a long enough window to account for daily or even seasonal variations in electricity supply. Storage costs for aluminum ingots are negligible, even for an entire year's worth of production.

It's true that variability reduces the specific productivity of capital, in terms of tons of aluminum per year per dollar of plant. But net return on capital investment is higher, because of the lower average price of the electricity used. The German executives at TriMet, who implemented the scheme, are not idiots.

Incidentally, the same scheme would make equal sense if Germany were supplied entirely by nuclear power. There would still be variation in other demand over the course of a day, and it would still make sense to throttle aluminum production according to available supply. That's completely aside from whether NPPs are capable of load following. Even if they can vary their output, it makes sense for NPPs to keep producing at full power by selling what would otherwise be surplus to aluminum smelters at bargain prices.

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