jump to navigation

VIII.2. The Electric Farm Pt. 2

(originally published November 30, 2007)

In Part 1 of this post, I started to construct a scenario where a medium-sized farm would do most or all of its work using electric farm equipment. This model of a farm had 1 large tractor with 250kW(335 hp) maximum power output and 4 smaller tractors with 50kW (67hp) maximum power output (though working at 50% power on the peak energy day). I started to sketch out what would be the electric power demand on the peak day of the year and how many battery packs and battery pack exchanges (assuming that a quick battery exchange system was part of the tractor design) would be required to fulfill that demand within a workday of 10 to 14 hours. I have put to one side claims of companies that suggest that they have high energy density ultracapacitors that can charge and discharge very rapidly (EEStor for one). If such ultracapacitors were more than vaporware, a recharge tender vehicle rather than a battery exchange tender would be preferable as tons of batteries would not need to be changed in the field during the workday.

I figured that on this day all tractors would be required to do work for 10 hours and that the large tractor would work at 100% power while the 4 smaller tractors would work at 50% power. Road vehicles do not operate at anywhere near maximum power for most of the time they operate, so farm vehicles in certain tasks (deep plowing in particular or driving a heavy piece of machinery through its power take off drive shaft) are mass energy and power consumers. The diesel equivalent of the large tractor might consume 150-200 gallons of diesel fuel in a 10 hour day if it worked continuously at maximum power.

In actuality most farm tasks do not require maximum power but some do, therefore the variations in power among the tractors even on this peak energy use day. I assumed that half the weight of the tractor could be battery weight as the electric motor would be much smaller and lighter than the equivalent diesel motors it would replace. I assumed the total weight of these tractors would be the same weight as fully ballasted diesel tractors of a similar power rating. Using current lead-acid technology, there would need to be many battery exchanges with the high power tractor operating at full power all day (13 exchanges) but less with smaller tractors working at half-power (3-4 exchanges). Using current lithium-ion technology, there were much fewer if any battery exchanges per workday but of course much greater expense to purchase the battery packs using approximate pricing for 2007.

A table of the calculations for this scenario can be found here.

Charge Infrastructure

The type of battery makes a big difference in the amount of in-field battery exchange required and it also makes a difference in the size and power of the available charge infrastructure. We will assume that the tractors can plug in and recharge their onboard batteries at the same rate as battery packs on the external charger.

The charger and spare battery packs would be installed inside a vehicle-accessible shed or barn, especially in colder and wetter climates; in warmer, drier climates, a open shed with a roof may be sufficient. The battery tender or tractors would need to be able to dock on the recharge racks and have access to the batteries so that the battery enclosures or transport sleeves could interface without unnecessary lifting of the tonne or half-tonne battery packs. The battery packs might be stored on recharge racks with fittings that allow easy lock and release of the packs by horizontal sliding on rollers. A hydraulic mechanism might push and pull the packs in and out of the charge racks into either the battery tender or the tractors themselves.

The number and size of the farm’s battery inventory would depend in this example on the battery technology (lead-acid and lithium ion), its quick-charge potential, and the power of the charge unit. We will assume that all batteries, battery packs and vehicles have the power electronics and wiring to allow complete recharge within 60 minutes, which is within the claims of makers of the current generation of nanostructured batteries. Let’s say the charger operates on a 480V 220 amp circuit with a 106 kW power capacity. This means that the charger can recharge 4 of the lead-acid (25 kWh) packs per hour or takes a little over 70 minutes to recharge one of the 125 kWh lithium ion packs. In my calculations, I’ve allowed for there to be three times as many offboard as onboard lead acid batteries, with a one to one ratio for lithium ion packs as they carry more charge and therefore last much longer in the field than the lead acid packs.

Battery Inventory

The all-electric farm, in order to handle a total peak day energy use for the 5 tractors of 3.5 MWh (3.5 million watt hours) would use an electric tender vehicle (a specially designed truck with cranes or hydraulic battery pack tubes that remove and “inject” battery packs into the tractors or the charger stand) that also uses the same type of battery packs as an energy source to do the work of exchanging and carrying the heavy batteries to and from the charger to the tractors in the fields. I have assumed that the battery tender would need 25 kWh of energy to do one complete exchange whether for a smaller or a larger tractor (though in reality there would be somewhat less energy required to exchange 2.5 {small tractor} rather than 8 {large tractor} tonnes of batteries). The farm total, for all mobile uses, will have energy needs that vary then from 3.55 MWh for lithium ion to the much higher 4.225 MWh for lead acid, the difference being made up by the 25-29 battery exchanges performed by the battery tender with the lead acid batteries versus 2 exchanges with the lithium ion battery packs.

The farm’s battery inventory will vary by number, weight and cost of batteries, depending on battery chemistry, given that we are planning for 3 times the number of offboard charging batteries for the lead acid chemistry while just a one-to-one ratio for the lithium ion batteries. The battery packs in this scenario are either one metric tonne (2200 lbs) or a half tonne, for either chemistry. The large 250kW tractor would carry 8 metric tonnes while the smaller 50 kW tractors would carry 2.5 metric tonnes of batteries (we will assume that the larger tonne-sized packs are exchanged on the smaller tractors rather than a single half tonne pack).

As the lithium ion batteries on the four smaller tractors do not need exchanging on our peak day, we will only allow for a single tonne-sized battery pack to remain in reserve for these tractors. The 250kW tractor is working at peak power all day in this peak energy day scenario (on other days, the smaller tractors might work harder and therefore use extra batteries that on the peak day are used by the larger tractor) so requires reserves for all 8 of its battery packs. We are then looking at 12 metric tonne-sized battery packs off the tractors on the chargers with 18 metric tonnes of battery packs on the tractors. The tender requires one metric tonne sized battery pack to do its work (more than enough for 2 battery exchanges). We are then looking at 31 metric tonnes worth of lithium ion batteries for the entire farm distributed in 29 tonne-sized packs and 4 half-tonne packs with a total capacity of 3.87 MWh. Assuming an energy density of 125 Wh/kg (the energy density of the Tesla motors ESS) and a 2007 cost of $.48 per Wh, the 2007 battery cost for the lithium-ion based farm would be $1.857 million dollars.

The battery inventory picture is somewhat different for the lead acid battery-based system. More exchanges, more offboard battery reserve, and lower energy per unit weight means greater total battery inventory tonnage to supply the tractors and a much harder working battery tender. The onboard battery capacity for the 5 tractors will be 450 kWh and offboard will be 1.35 MWh. The battery tender will need to carry two tonnes of batteries for its own use in order to be able have the energy to do 25kWh battery exchanges and exchange its own batteries after pretty much every exchange (the capacity of the lead acid pack is 25 kWh). The battery tender would need to work at a frenetic pace all day to be able to effect 25-29 exchanges. Offboard, then there would need to be 60 tonnes of batteries on the charger and onboard 20 tonnes on the 5 tractors and the battery tender for a total of 80 tonnes of batteries. Despite the increased tonnage of batteries needed, the 2007 cost of the lead acid battery inventory is relative to the cost of the lithium ion battery inventory, favorable: with lead acid batteries at $.12 per Wh and 25 Wh/kg, the 80 tonnes of batteries with a capacity of 2.00 MWh would cost just $240,000. The lower capacity requirement and therefore much lower than 1/4th the total cost of the lead acid batteries is due to the more complete usage of the lead-acid capacity through more battery exchanges; in the lithium ion scenario there is excess battery charge both on the battery charger and onboard the smaller tractors.

Farm Energy Requirements and Renewable Energy

One of the major advantages of using electricity is that it is a highly flexible energy carrier that is particularly suited to using renewable energy as its primary energy source. In addition electric motors are about 3 times as efficient as internal combustion engines, and have high torque at low rpm, perfect for farm work. On a given farm there may the opportunity to use wind, solar, biogas, or waste biomass to generate electricity, beyond the use of the latter two to generate process heat for crop drying and barn-heating. Before we design any renewable energy systems for the electric farm, we first need to determine what are its overall energy requirements. As this is an idealized scenario we are free to make assumptions that would need in the future to be modified by more accurate statistics and real-world prototyping of a renewable and sustainable farm energy system. We will assume that non-mobile energy use on a farm is one-third of mobile farm energy use, which is slightly higher than the 20% of farm energy that was delivered in the form of electricity in 2002.

We have found that a lead-acid based battery-exchange system will require more battery exchanges which on our peak day scenario leads to a total mobile energy usage for our electric farm of 4.225 MWh while for the lithium-ion based scenario the usage is 3.55 MWh, barely over the energy needed by the tractors themselves. For the sake of simplicity, we will take a value in the midpoint between these two numbers and divide it by 3 to arrive at an average daily mobile energy use on a farm, which we will say is 1.4 MWh. We will assume that tractors are used 200 days/year, so annual mobile energy use for the farm will be 280 MWh. If non-mobile use of the energy on the farm is 1/3 of that of mobile usage, then we come up with 450 kWh/day, and assuming that stationary systems must operate 300 days/year we come up with annual non-mobile energy usage of 135 MWh. The proportions and total amounts of these figures will vary greatly depending upon the type of agriculture that is being practiced on the farm, the amount of on-farm processing that goes on, the climatic zone, and the types of crops. Using this scenario we come up with a total yearly energy requirement of 415 MWh.

What size of renewable energy system would deliver this amount of energy per year to the farm?

While in reality, to supply this energy, farms will have access to some combination of wind, solar and biomass energy, as well as grid electricity, we will design this model electric farm using solar arrays for comparative purposes to show how much farm land would be needed to generate the energy needed. While often, solar systems tied to the electric grid are sized to cover the energy costs rather than the actual site energy needs in kilowatt hours, here I will size the system to produce enough energy to cover all on-farm energy needs, assuming that the utility will credit the farmer for excess production. In net metering schemes, the electric utility credits a customer who uses a solar array to generate electricity at the daytime rate that is higher than average per kilowatt-hour costs. So solar arrays are sized to zero-out the bill, even though often this means that in net energy terms, the customer is using more than they are producing. Net metering does not allow system owners to make money from their installation, only to cover costs. The goal here is zero net energy.

A small innovation in the area of switches might take advantage of the PV solar array’s DC output, which is the same current type that batteries need. Sending current through an inverter to transform it to AC can lose from 4 to 8% of the energy. Ideally, a solar array on a farm with high battery recharging requirements would have a smart switch that directs current to the charger or to the grid depending upon the state of charge of the battery packs. For the purposes of this simplified model we are assuming negligible inverter losses in outputting AC to the grid.

To generate 415 MWh per year, the size of solar array depends on where the farm is located and what type of solar array is chosen. In agricultural settings, a typical larger array is mounted on the ground if there is not sufficient space on top of farm buildings. Arrays are usually mounted at a fixed angle to maximize production while minimizing the land footprint, while larger arrays sometimes use a motorized tracking system to follow the sun from east to west every day. Rob Erlichman of Sunlight Electric counsels agricultural customers to use a 10 percent fixed tilt or less to cut down on valuable land usage (higher tilt angles require substantially more spacing between them. Alternatively, arrays mounted on livestock-proof racks at sufficient height would allow grazing and sun shelter for animals below the solar array. In California’s Central Valley, an array rated at 200-250 kW could generate 415 MWh of energy per year. An array such as this would occupy anywhere from 1 to 3 acres depending on the spacing and the angle of installation of the units. In the Northeast an array of around 275-325 kW would be required to generate the same amount of power.

Photovoltaic arrays are very handy sources of power but remain expensive because of a shortage of crystalline silicon. On the other hand, the purchase or long-term lease of a solar array (or wind turbine) locks in energy prices (payments on the purchase of the array as the sunlight is free) for a period of at least 20 to 30 years, when fossil energy and grid electric prices will no doubt rise. Current costs for crystalline silicon PV arrays are somewhere in the area of $8/watt with the additional costs of $1 to $2 per watt being incurred for more distant connections to the grid. Taking a middle course of $9/watt for 2007, the array in California would cost before rebates $1.8 to $2.25 million and assuming a rebate of $2.5/watt, $1.3 to $1.6 million net. In the Northeast, the larger array required would cost from $2.48 to $ 2.92 million at current prices. We would expect pricing in 2009 to be significantly less, when more silicon production capacity is online. Cost projections by manufacturers of new thin film materials, like Nanosolar’s Powersheet, are hoping to reduce the cost of panels to $1/watt or less that may yield installed costs of less than $3/watt. Installations with these less expensive materials may occupy a larger footprint depending on their efficiency which is sometimes half that of the more expensive crystalline silicon arrays.

A farm with these energy and equipment requirements will probably occupy several hundred acres depending of course on the type of agriculture and crops. The solar array, depending on where it is installed and whether the land it occupies can be used for other uses, will use less than 1 or maybe 2% of the farm’s land. Generating electricity on the farm may not be the most efficient use of the footprint of the farm, especially if the farm contains no marginal or unproductive land. On the other hand, using farm buildings or pasture for energy production will duplex energy use upon the primary use, reducing the energy generation footprint to near zero.

Summary and Evaluation of the Electric Farm Concept

Farming will remain dependent on petroleum derivatives or biofuels with questionable environmental effects and efficiency until farm equipment manufacturers apply contemporary and near future electric vehicle technology to tractors, harvesters and other powered farming implements. Somewhat ambitiously, I have taken on a moderately difficult farming task, a peak energy use day on a middle sized farm, to see whether currently available electrical power systems might be able to handle the task requirements with no revolutionary technology breakthroughs.

Working through this scenario has yielded a number of crucial results.

Energy Paths to a Sustainable Agriculture

The often-overlooked area of agricultural energy may yield a key area for developing large-scale battery powered or grid-optional work vehicles. In the early 20th Century, over a period of decades, agriculture in industrialized nations became dependent upon fossil fuels; it will not be easy to wean agriculture and by extension our civilization off its dependence on these fuels to produce food. While current arguments about agriculture focus on the size of the farm, its use of toxics, diversity and regional appropriateness of food species and its proximity to its market, the Electric Farm concept is applicable to almost every size of farm, from the market gardener to the largest agribusiness, no matter what their cultivation practices and proximity to markets. Despite the focus on energy here, I hope that all food businesses will continue to move towards sustainability in the use of inputs other than energy.

Analysis of the Electric Farm concept has highlighted some key areas where agriculture can move to greater sustainability and minimize energy and climate risk.

Reduction of Farm Energy Requirements

The movement of the last few decades towards low- and no-tillage farming is a bright sign pointing towards the future of farm energy use. Plowing/tilling the soil has been historically one of the most energy and power-intensive farm activities, for which first animal power was used and in the last century fossil fuel powered tractors. No-till and low-till farming was originally paired with increased use of chemical inputs to control weeds and pest formerly controlled by turning the soil. There are now efforts underway to develop organic no- and low-till techniques; previously organic agriculture has substituted physical and therefore energy intensive methods for chemicals.

Agricultural scientists and farmers might be able to work together to further reduce energy use by developing agricultural machines and implements that use energy more efficiently. Electric farm implements will have greater flexibility than fossil fuel driven implements as electric motors are much easier to scale to the appropriate size and power for a task. Furthermore, automated and robotic farm equipment may facilitate the development of new, more energy effective farm tasks that are less disruptive to the farm ecosystem.

Plug-In, Battery-Exchange Serial Hybrids

The all-electric farm may be a vision for a time in the future when batteries or ultracapacitors increase in energy density and down in price: to bridge the gap, it may be necessary to reduce but not eliminate dependence upon fossil fuels by using flexible fuel electric vehicles. The ultimate flex-fuel farm equipment that would minimize fossil fuel inputs would be a serial, battery-exchange, biofuel-capable hybrid tractor or harvester. Such a machine would be able to use grid electricity, charged batteries, biofuels and petroleum to do farm work. An internal combustion engine or a fuel cell would generate electricity for the electric traction and implement drive motors when the substantial batteries, charged from the grid or from local farm generators, are exhausted. The amount of onboard batteries would be limited by weight considerations and cost but these batteries could also be exchanged as in the Electric Farm scenario.

The addition of an electric generator to the battery-exchange electric tractor outlined in the scenario allows for a significant reduction or elimination of off-board battery inventory as tasks that require more energy can supplement battery energy with biofuel or fossil energy. Lead-acid or lithium ion battery capacity could be sized for all-electric use for 60-80% of farm tasks, reserving fossil or biofuel to cover peak energy use. The cost of the generator, fuel tank and its mount would be small in comparison to the overall cost of the tractor or several battery packs.

Commercialization of Electric Farm Equipment

A market for electric farm equipment will emerge in ethically motivated agricultural concerns that have lower daily power and energy requirements for one or more of their tractors. In addition, air quality regulators in intensive agricultural areas like California’s Central Valley may offer incentives to develop zero or near-zero emissions agricultural equipment.

Initial models of electric farm equipment will probably be serial plug-in hybrids with lead acid batteries at a size that will offer most of the functionality of a 40 to 80 hp diesel tractor. Because of cost concerns and the lesser emphasis on weight reduction in tractors than in road vehicles, lead-acid batteries will be an early choice. Generators for this equipment may be either gasoline or diesel engines capable of using biofuels.

Machine designers may start by mimicking the functions of internal combustion tractors but soon realize that electric drive offers additional flexibility that will lead to a new “no-compromise” farm vehicle that can do more than the equivalent traditional tractor. Another path to commercialization may be for equipment manufacturers to start by building small multi-use vehicles like electric ATVs and garden tractors like the Elec-Trak and gradually build up power and functionality as demand arises. With substantial decreases in cost for lithium ion or other high energy-density mobile electric storage, electric farm equipment will gain greater applicability as most farm tasks will be able to be achieved without emissions, especially when paired with renewable energy.

To speed commercialization efforts, a zero-emissions agriculture consortium could be formed by equipment manufacturers, farmers, farm advocates, inventors, engineers, and agricultural scientists which might help develop a research and development program. This program will help locate the most promising niches for growth in this area and best address issues of farm productivity, energy efficiency, and ecological sustainability as regards farm machinery and mechanization.


No comments yet — be the first.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: