More on battery-powered tractors

In this article, I intend to continue my prior article and drill down on some of the details regarding battery-powered tractors. I'll fill in some missing details. It will be detailed and boring.

I had a few additional ideas to refine my last article. These ideas would maximize the number of useful recharge cycles and minimize battery size, keeping battery costs down and earning additional money. Also, there are some ideas to minimize the distance driven by the tractor. And there are some other ideas. If all these things are implemented, I believe this idea could be cheaper than operating tractors on diesel, at current prices.

In order to keep battery costs down, it is necessary to use the batteries as many cycles as possible before they expire due to calendar life. Batteries expire after a certain time period (like 10 years). We must maximize the number of useful cycles during that time period, because the upfront purchase price is a sunk cost and we must get as much usage as we can. This would be done in several ways.

1. The same battery trays could be used for the combine harvester also, and for other agricultural machinery. The same algorithm would be used to power the harvester across the field. Different pieces of agricultural machinery are used at different times (for example, the harvester is used in the Fall). As a result, the same batteries could be swapped between different pieces of equipment, using the same forklift.

2. The batteries could fast recharge during the day, while the tractor is operating, and thereby be recharged and used twice in one day. This halves the number of batteries we would need.

3. The area of the field in the diagrams above must be increased by a factor of 21. The tractor will then take 21 days (a typical planting season) to complete the entire field. The tractor would stop at night and the batteries would recharge. The battery swapping idea will still work as long as the tractor returns to the right-hand edge of the field more than once during a single day.

4. The same batteries could feed electricity back into the grid and be used for peak shaving the rest of the year. The recharging station on the farm would need an inverter to feed power back to the grid. This could cycle the batteries even more and earn the farmer additional money, thereby paying for the batteries.

Electricity peaks are much higher during the hot months of June, July, and August. However, the planting and harvesting seasons are in Spring and Fall. Thus, they do not overlap. This would allow the batteries to be used for planting during April or May, peak shaving in June, July, and August, and harvesting later, for example.

As a result, each battery would recharge twice per day for 21 days each of planting, fertilizing, and harvesting. I'll assume the batteries are used to shave peaks 40 times during the summer and are discharged almost completely during those 40 times. Thus, each battery would undergo 1,660 recharge cycles over 10 years. If we assume a battery cost of $100/kwh, then the price of batteries per kwh delivered is 6 cents (100/1660). That price is way below the cost of peak generators, and that price plus the price of electricity is below the price of diesel. As a result, this idea is cost effective and would be modestly cheaper than what is done now.

The price of the battery trays would be on the order of $150,000 every 10 years. This compares with a total price for tractor, harvesters, and so on of $600,000 or so, which also last about 10 years (the engines run continuously near the top rated horsepower for 4000 hours). Thus, the batteries would add 25% more cost than just buying a tractor and harvester. However, that additional cost would be offset by the farmer being paid for peak shaving, and also savings on fuel costs.

Overall, this idea could be slightly cheaper than using diesel at current prices.

A few additional ideas

Instead of the forklift going back and forth to the recharge station every swap, the forklift could bring 4 battery trays at once to the general area where 4 swaps would take place. There could be a covered raised shed for swaps so the batteries don't need to be left on the ground. Each shed could have a stack for fresh battery trays and another for depleted. This would minimize forklift driving. For example, there could be 8 sheds along the right side of the field in diagram above, and the forklift removes 4 spent batteries from the shed when necessary, takes them all at once to the recharging station, and then takes 4 fresh batteries from the recharging station to the next shed. The individual swaps then involve only driving to the nearby shed and back. The farmer would drive 4 fresh batteries to a shed, operate the tractor, stop periodically on the right hand side of the diagram, remove the depleted tray, place it in the nearest shed, take a fresh battery from that shed back to the tractor, and so on. When the shed has nothing but depleted batteries, the farmer drives all 4 trays back to the recharging station and drives 4 fresh trays from the recharging station to the NEXT shed.

The electrodes to recharge the battery trays can be way up some round holes in the bottom of the trays. The batteries are recharged using poles which stick up the holes on the bottoms of the trays and have electrodes on the ends. The holes close automatically using spring loaded plastic doors. In that manner, the electrodes are not exposed to rain or debris.

Of course it would be possible to optimize the size of the batteries, the number of battery trays, the number of swaps, the number of sheds, and so on.

Similar battery swapping ideas could be used for mining dump trucks, which also return to the same location over and over again. Similar ideas could also be used for ferries, shuttle buses, and so on.

Minor update 2023-01-18: Of course it would be possible to have only two small batteries, and to swap back and forth between them. This might be the most economical thing. If the farmer needs to run an electrical line to his field anyway, it would cost very little extra to run a higher voltage line. This implies that the tractor couldn't recharge overnight.

Minor update 2023-12-16: The forklift which delivers the batteries to the tractor could be automated and could be a robot with no human operator. Since the task involves incredibly simple and repettitive movements, it seems like the most obvious thing is for the forklift to be automated and have no human operator.

I think the optimal solution is to use an automated forklift and only two batteries. When one battery is being used, the other is being recharged. This reduces to the total battery cost. The energy density of the batteries is less important since the batteries are frequently swapped. As a result, it would be possible to use cheaper batteries (such as sodium-ion batteries) with lower energy density.

I think the optimal solution might be to use only two cheaper sodium-ion batteries with an automated forklift.

This solution would almost certainly be cheaper than diesel tractors are now, but for one impediment. I don't know how expensive it would be to run an 960v electrical line to a shed at the edge of the field. This would require wooden poles, an aluminum cable, and a transformer for where the cable attaches to the power grid.