Battery-powered equipment and tractors
The full electric drive of agricultural machinery and equipment is increasing, although current energy density of the batteries has not yet reached a sufficient level to properly spread this solution, which is undoubtedly more respectful of the operator and the environment. A further problem is the lack of satisfactory battery life, which in the agricultural field often exceeds, sometimes by far, the classic 8 hours per day
Precisely because it is new, every revolution initially provides, historically speaking, a more or less long period that is rather confusing, during which an unambiguous orientation of change does not emerge, but is clarified instead with the passage of time. Given that it has now been widely established that sooner or later fossil fuels will run out (or it will no longer be appropriate to exploit them, due to the high cost of extraction, but above all to preserve the environment), the current era is necessarily moving towards the transition to electric vehicles. Especially as far as agricultural ones are concerned, first and foremost the tractor, the revolution is only in its infancy, and much progress has yet to be achieved or consolidated, especially in terms of autonomy. Indeed, batteries are the main current problem, considering the current heavy "gap" regarding the so-called "energy density" compared to fossil fuels (see box).
Several models of full-electric tractors are already available on the market, but their diffusion is still extremely limited, both because of their high costs and, above all, because of the considerable limitations in terms of available power and autonomy, which basically relegate these models to working with medium-light loads. We can mention, in no particular order, the Californian Soletrac with a 25 kWh battery pack, the Swiss Rigitrac with a 50 kW motor, the Indian Farmtrac 25 with 15 kW and many others. Leading brands have also moved in this direction: already in 2016 John Deere presented the SESAM (Sustainable Energy Supply for Agricultural Machinery) with two engines of an impressive 150 kW each, powered by 130 kWh lithium batteries, but without a doubt Fendt's e100 Vario was the recent example most similar to traditional models: equipped with a 50 kW motor and a 100 kWh Li-ion battery pack at 650 V, it provides, according to the manufacturer, a range under real-world conditions of 5 hours. The tractor is equipped with a standard power take-off and a traditional hydraulic circuit, which allows it to be coupled with the normal machines available on the farm, but of course it is ready to conveniently operate all electric equipment, with a "boost" power up to 150 kW, for short periods.
However, if the full-electric "tractor fleet" also includes lawn tractors, the landscape becomes more crowded, because the amount of energy that can be harnessed compared to the limited power typically required allows for a reasonably long range. Still in Italy, Del Morino of Caprese Michelangelo (AR) has for some time now been producing Rino, a compact mower tractor with a mass of 640 kg, also equipped with a 300 kg capacity front 3-point linkage and management with position control and floating, to which numerous attachments can be coupled. It mounts two electric motors of 24 HP overall, powered by a 48 V Li-ion battery pack with 18 kWh capacity: the first is dedicated to traction, while the second is used to drive the connected machines. Recharging the batteries takes just over 8 hours with a power of 1.2 kW, but the time can be halved if the available power doubles. In addition to typical processing in the maintenance of green areas (such as cutting and mulching grass, with blades or helical mulchers, bio-crushing, suction of leaves and transport of materials) and road maintenance (for small excavations and movements with bucket and front shovel and snow removal, also with turbine), the manufacturer offers a series of tools for some typical open field operations, such as a hoe (also in the stone burier version), a small cultivator and a power harrow, possibly completed with a seeder. The maximum speed of the vehicle is 16 km/h, and it can tackle slopes up to 15%.
In addition to a one-kilowatt on-board power socket for attaching external tools, there is also a "shelter" equipped with photovoltaic panels and an inverter with accumulator, to make the recharging of the battery pack independent (and free of charge). "Similarly, Gianni Ferrari of Reggiolo (RE) has two models of full-electric lawn tractors in production, one with a ventral cutting deck (GTM+) and the other with a front deck for mulching (GSR+). In particular, the latter has three 112 cm working width blades, which rotate at approximately 3000 rpm. The machine, with a mass of 395 kg, has two forward speeds: in the highest one (transfer speed) it reaches 10 km/h. The machine is powered by eight 54 V lithium ion battery packs with a total capacity of approximately 11 kWh. The manufacturer claims a range of 6-8 hours, with a recharge time of 5 hours. In combination with the tractor, it offers an optional 8 kW power-bank for quick, albeit partial, recharging, for example during the lunch break".
The small equipment
The limitation of the energy density of the modern batteries currently available is not a problem where the power required is limited, i.e. for a lot of equipment for manual use, where the market already has a considerable range on offer, such as chainsaws, blowers, brushcutters, hedge trimmers and pruning shears for professional use, but also wheelbarrows and lawn mowers, but at the moment these are mainly intended for hobby use.
Model of Chainsaws
The availability of battery-powered electric motor models is now widespread for 25-35 cm cutting bars. The batteries, with a capacity of 200/400 Wh and a mass of about one kilogramme, are installed directly in the body of the machine; for greater autonomy and therefore higher weights, you can use independent kits that are attached to the belt, connected to the chainsaw by cable, or battery packs placed in a backpack, in this case for masses up to 3-5 kg and a capacity of one kilowatt hour. With this solution, handling the tool is certainly optimized, but the presence of the cable can hinder the operation in particular settings (such as tree climbing). In general, it is quite difficult to estimate the duration of work on a single charge, considering that the chainsaw is typically subject to highly variable and discontinuous peaks of energy absorption. In intensive use, the autonomy with standard batteries can drop to a few tens of minutes, while an average use of the high-performance backpack battery pack can last even a full day of work.
The case of blowers
Due to the particular operation, which typically involves working at maximum power almost continuously over time, the same batteries applied to chainsaws significantly reduce the autonomy of blowers. A limited-capacity model lasts no more than 10 minutes, while the backpack types last 2-3 hours, which is more than enough time to recharge a second battery pack.
Brushcutters power system
These machines are also quite energy intensive, because like blowers they operate for long periods with power commitments close to maximum. Keep in mind that battery-powered models can not yet compete in terms of performance with those equipped with traditional internal combustion engines, but are suitable to work in many situations, apart from heavy interventions, such as cutting in brambles, reeds, etc. With 200-400 Wh batteries you can work for 1-2 hours, while you can reach a full day's autonomy with the backpack kit.
Run time of hedge trimmers
These machines are on average less demanding, both than blowers and chainsaws, so with the batteries attached to the machine, you can work even a couple of hours, while with the batteries carried on the shoulder you can complete the normal working day.
Pruning shears
Although relatively high power is required to drive the cutting blade, the operation of this tool is very discontinuous in time, so even batteries of limited volume inserted in the machine body (for example, with a capacity of only 80 Wh) can ensure a satisfactory autonomy. With elements of less than 500 g you can work for 2-3 hours, the time needed to recharge the second battery. The 200/400 Wh models worn on the belt keep the shears light and manageable and can be used for half a day or more, while the backpack models do not need to be recharged every day.
Economic considerations
As you can easily guess, the purchase price of a battery-powered agricultural machine is always higher (sometimes by a lot) than the equivalent model equipped with an endothermic engine. Moreover, the operating cost, essentially due to fuel consumption, means that the higher investment is then recovered fairly quickly. This is even more true for small equipment for manual use: a small brushcutter with 1 kW of engine power typically consumes 0.75 l/h of mixture, for a cost of about 1.70 euro/h; alternatively, in the same time an equivalent electric battery model will consume just over 1 kWh of electricity, for a cost of 0.20 euro. The savings are as much as 1.5 euros per hour, but can be significantly higher with other more energy-intensive equipment, such as blowers. It is also necessary to take into account the maintenance costs, which for battery-powered models are lower, not having to include, for example, engine lubricant and filters for oil and combustion air.
The energy density of batteries
This is a fundamental characteristic of batteries, and refers to the amount of energy that can be stored per unit mass (or volume).
The main reason that still hinders a wide diffusion of electric vehicles is precisely this peculiarity, which still "pays the price" compared to fossil fuels.
For example, when it is subjected to combustion in diesel engines (i.e. its lower calorific value is taken into account), one kilogramme of diesel develops almost 12 kWh of energy, compared to 0.3-0.4 kWh of energy stored in the same mass of the best current battery.
In other words, in principle the energy density of diesel fuel is a good 30-40 times higher than that of the most advanced batteries available today.
For a more realistic comparison, however, it is also necessary to take into account the actual use of the two solutions applied to vehicles, i.e. to consider the important energy losses of the endothermic engine due to its normal operation (substantially in the form of heat, generated by internal friction and contained in the exhaust fumes), much more intense than the electric engine.
In addition, the latter’s typical performance means that the overall performance of the vehicle is significantly better than when operating with the endothermic powertrain.
Ultimately, the advantage of the fossil fuel supply is reduced to 12-15 times, which however remains a huge "gap" for the current batteries, especially if we think of agricultural applications, where an optimal autonomy of the machine, i.e. the tractor, should provide up to 10-12 hours of heavy work per day (for example to perform a deep plowing).
While fossil fuel powered engine technology is now largely mature, battery powered electric motor technology is evolving and shows great promise in terms of increased efficiency.
Many studies are underway for new types (in some cases even revolutionary) of battery, able to greatly increase the energy density.
For example, solid electrolyte lithium/ceramic batteries and even more so zinc/air or other metals/air batteries bode well for their development, not only because of the increase in energy density, but also because of other multiple characteristics that must necessarily be considered, such as recharge times, service life, performance at low and high temperatures, heat dissipation due to the Joule effect, etc.
