Tractors can easily run on batteries

Recently I saw a new book released by the Post Carbon Institute, entitled The Future is Rural. In that book, the author claims that everyone must relocate to the countryside because Peak Oil is near, and the tractors will stop running. According to the book, tractors cannot possibly run on anything other than liquid fossil fuels, so they will stop running fairly soon. We need to go back to farming by hand. Here is a quote from the book:

Farm equipment tends to operate near its horsepower capacity, whereas a car might only work near capacity when accelerating into traffic now and then. Hydrocarbon liquid fuels are the only known substances with enough energy density that can be carried easily onboard a tractor under typical working conditions and enable work to be performed continuously for many hours (pp 12).

Similar sentiments are echoed repeatedly within the energy decline movement. One of the core beliefs of that movement is that industrial agriculture will soon end, because of peak oil, and we'll need to revert to farming by hand.

It’s worth pointing out, right away, that tractors could easily run on alternative fuels like methane, ammonia, or hydrogen. One tractor manufacturer (New Holland) has already been preparing for several years to manufacture methane-powered tractors, and is releasing a methane-powered model for regular purchase later this year. Methane can easily be produced using renewable electricity and the Sabatier process, which has been in widespread use for more than a century. Ammonia and hydrogen are other chemical fuels which can be produced using renewable electricity and can be used to power tractors. A hydrogen-powered tractor is already in use (although it’s a prototype). The notion that tractors can only run on liquid fossil fuels is therefore clearly wrong.

However, decline theorists also argue that tractors cannot possibly run on batteries, either. The batteries would weigh too much for the tractor to carry. Tractors run all day continuously, and they run near the top rated horsepower for the entire time. As a result, they have enormous fuel consumption. Batteries do not have sufficient energy density to power tractors. The batteries needed to power a tractor all day (and near the top rated horsepower) would be too large and heavy to fit on the tractor. Most of the energy would be spent carrying the batteries themselves.

For example, I looked up a typical tractor here, and found it has a fuel tank of 135 gallons. Diesel weighs about 7 pounds per gallon, so the weight of the fuel is 945 pounds. A lithium-ion battery weighs about 100x more than diesel for the same energy (see here and here), however batteries are about 3x as efficient as small diesel engines, so the battery needed to replace 135 gallons of diesel fuel in that tractor would weigh 31,185 pounds (135*7*100/3). That is more than the weight of the tractor! As a result, tractors cannot run on batteries.

However, in this article, I will demonstrate that tractors can easily run on batteries. It can easily be accomplished using battery-swapping.

The convenient thing about tractors is they don’t travel in a straight line. Instead, they zig-zag across an agricultural field, like this:

--->--->--->--->
               |
<---<---<---<---
|
--->--->--->--->
               |
<---<---<---<---

(Apologies for the ASCII art).

You will notice that the tractor repeatedly returns almost to the same location, over and over again throughout the day. This allows us to use much smaller batteries on the tractor and swap the batteries on occasion. If this were done, then the energy density of batteries is more than sufficient to power a tractor all day.

I am proposing a new idea of battery swapping for tractors. I suggest that the battery for a tractor be divided into 32 smaller batteries. This is easy to do, because the batteries for EVs consist of many individually-packaged 2860 cells. So the battery for a tractor could be divided in to 32 smaller batteries which are packaged in removable battery trays. Those trays could be swapped using a forklift. The batteries would all recharge overnight. The forklift is also battery-powered. When the battery in the tractor is running fairly low and the tractor approaches the right-hand side of the diagram above, the forklift takes a new battery tray to the tractor and swaps out the old battery tray. The forklift ends up traveling only a fairly short distance throughout the day, because the tractor returns to the right-hand edge of the field repeatedly anyway, and the forklift meets it there.

Here is another ASCII art diagram:

--->--->--->--->
               | #
<---<---<---<--- |
|                |
--->--->--->---> |
               | *
<---<---<---<--- |
|                |
--->--->--->---> |
               | #
<---<---<---<---

The tractor follows the zig-zag pattern on the left, but the forklift only travels up and down the vertical bar on the right. The forklift meets the tractor at the hash marks. The batteries are stored in a small shed at the asterisk, which has a 480 volt recharger.

During each swap, the forklift will carry the depleted battery from the last swap (which had been left on the ground temporarily) to the recharging station, obtain a new battery, drive back to the tractor, remove the old battery from the tractor and set it on the ground, then install the new battery.

Of course, this does add a small additional energy expenditure. The forklift needs to travel back and forth along the right-hand edge of the diagram above, in order to accomplish the battery swapping.

Let’s calculate how far the forklift needs to travel, and how much energy is consumed by doing so. We’ll assume a hypothetical farm which has a square plot of land. The tractor must zig-zag throughout the entire plot of land during a single day. If it takes more than one day, then the tractor could recharge overnight, so we’ll assume that one day is the maximum energy expenditure between recharging. We’ll assume that the width of the agricultural machinery dragged by the tractor is 1/200th the width of the plot of land (this number is realistic), so the tractor must zig-zag 200 times in a day to cover the entire plot of land. As a result, the maximum distance travelled by a tractor in a single day is 201 times the width of the land (going back and forth 200 times, and also going all the way down the length of the land once). In contrast, the forklift must travel 16 times the width of the land in a day. The forklift travels only along the edge, and only 32 times, because 32 is the number of battery swaps. Furthermore, the average distance travelled by the forklift for each battery swap is only half the width of the land (sometimes, the tractor happens to be right near the middle of the field anyway, near the recharging station, where the fresh batteries already were). As a result, the distance travelled by the forklift is 8% of the distance travelled by the tractor (16/201 = 0.08). It is also worth noting that the forklift could weigh less than 10% of what the tractor weighs. If we assume that energy consumption is proportional to weight, then the forklift will use 0.8% of the total energy that the tractor uses. This is still a massive overestimate, because most of the energy used by the tractor is spent on dragging plows through the Earth, not just carrying the weight of its battery, whereas the forklift needs only to carry the weight of the tractor’s battery. Suffice it to say that the energy consumed by the forklift would be far less than 0.8% of the energy used by the tractor. As a result, the battery-swapping scheme imposes negligible additional energy costs.

Now that we have divided the battery into 32 sub-batteries, we can calculate the weight of those sub-batteries and see how much they would affect the weight of the tractor. Lithium ion batteries weigh about 100x more than diesel for the same amount of energy (as described above). However, I will assume (as a rough estimate) that batteries have 3x the energy efficiency of a small diesel engine (this is realistic; small internal combustion engines waste more than 70% of their energy as waste heat). Furthermore, we have divided the battery into 32 smaller batteries so we can swap them. Conveniently, the weight of each swappable battery works out to be approximately the same as the diesel fuel it replaces (1*100/3/32 = 1). Thus, our battery swapping scheme would not increase the weight of the tractor at all. In fact, it would slightly reduce the weight of the tractor, because the diesel engine and transmission could be removed, and electric motors are lighter.

As a result, we can easily power tractors with batteries. Energy decline theorists assumed it could never be done, but they wrongly assumed that a single large battery must be used. If we divide the battery, and use battery swapping, then it becomes entirely feasible to use batteries for tractors.

Of course, it’s worth pointing out that small diesel engines in tractors lose more than 70% of their energy as waste heat, whereas batteries and electric motors lose only about 15% as waste heat. Thus, the battery-swapping scheme I described above is actually far more energy-efficient than the diesel tractors we use now. Although battery swapping imposes a 0.8% energy loss due to forklift usage, diesel engines impose a 70% energy loss.

Postscript

Originally, I intended to write this post only as a hypothetical example. I wanted to show that there are many alternatives to diesel for tractors, and even batteries would work. I certainly don't expect that this will be used in practice. I admit to knowing very little about farming.

However, after considering the idea further, I think it’s actually plausible and could be used in practice. In fact, this idea might be preferable to alternative fuels. Alternative fuels (such as hydrogen, ammonia, and synthetic methane) would impose large efficiency losses and are much more expensive than diesel fuel. Battery swapping, however, could be slightly cheaper than diesel fuel, even including the cost of replacing worn-out batteries after 12 years.

The big drawback of this idea is that it requires additional labor. The person who operates the tractor would need to stop every 5 or so passes across his field, get out, walk to where the forklift was last parked (the last battery swap, which would be about 50 feet away), pick up the last depleted battery from the ground, drive the forklift to the recharging station at the center of the right edge of the field, drop off the last depleted battery tray, fetch a new battery tray, drive back to the tractor, remove the recently depleted battery tray from the tractor and leave it on the ground, install the fresh battery, get back in the tractor, and keep driving. Since he would have to do this 32 times per day, I would guess it would add at least two hours of labor. He would end up driving a forklift for 6 miles if we assume a plot of land that’s 2000 feet on a side, so just driving the forklift that distance in a day would take more than half an hour. However, it would save at least $350 each day for an additional two hours of labor, compared to using synthetic fuels (I assume synthetic fuels would cost $6/gallon which is $2 more than diesel, and a typical tractor uses 175 gallons in a day). My labor is worth less than $350 for two hours. Of course, the farmer would have to buy a small forklift too, which looks like it would cost about $4,000. However, that cost would be recovered in less than 12 days of usage (4000/350 = ~12). As a result, this looks like it could actually be the best alternative to diesel fuel for tractors.