by: Michael Hoexter
Urbanites and suburbanites tend to forget that our civilization is based on agriculture, an agriculture that is heavily mechanized and dependent on fossil fuel.
The US has the most highly mechanized farm economy; one estimate has put the number of people supported by one farmer’s food output currently at 100 for the US. If we try to account for farm family members, farm laborers and illegal immigrant farm laborers (added together 6 million) as well as adjust for the fact that the US is a net exporter of food products, the number of people outside agriculture supported by a single worker in agriculture is something like 40-50 in the US, and in all probability fewer in Europe and still fewer in developing countries. The degree to which human social groups can develop into civilizations and economies with highly specialized roles is entirely dependent upon this ratio.
Tad Patzek, in his stinging critique of biofuels from an energetic perspective, mentions in passing how the enhancement of human labor by fossil fuels turns ordinary workers into “supermen”. But energy balance is not just an issue in raising fuel crops. In fact it is more important for determining what kind of food agriculture (i.e. the original kind), and therefore society, is ultimately sustainable. A ratio of 20, 30 or 40 non-agricultural workers for every ag worker is unthinkable without energy subsidy from a source other than human muscle, what students of agricultural energetics call exosomatic (outside the body) energy as opposed to the endosomatic (inside the body) energy of food. Exosomatic energy flow is what we usually think of when we discuss “energy”, the energy of fossil fuels, renewables and nuclear.
The word “sustainable agriculture” brings up many definitions and potential variables, some so complex that it takes committees of farming specialists, ecologists, and economists to sketch the dimensions of the problem. A list of sustainability issues in agriculture includes water balance, soil preservation and restoration, carbon balance, energy balance, farm economics, use of toxins, balance of ecological relationships between key species, and food economics. Organic or biologic agriculture standards have tackled one portion of what sustainability might mean in food production but have excluded energy subsidy in the form of fossil fuel energy as a consideration (other than limiting fertilizer inputs which happen to be made from fossil sources). A combined view of the energetics and material balance of agriculture has yet to take on a common form recognized by the institutional forces that shape contemporary agriculture: we better get going on creating a science-based roadmap to sustainability in agriculture, given our dependence on unsustainable fossil fuels to help feed us.
One place to start making modern farming more sustainable is to ensure that all exosomatic energy in agriculture will come from renewable sources in a way that would be affordable for agriculturalists themselves or the complex of business and government entities that work together to bring food to market. In the short term, petroleum products are the most affordable options given the farm equipment available. Long-term affordability will mean sourcing exosomatic energy in ways that have sustainable effects on the local and regional ecosystems on which the farms and, by extension, our society depend. In addition, agricultural scientists, inventors and farmers might work together to further reduce energy demand on farms by utilizing natural energy flows directly or more targeted use of exosomatic energy.
In the US, energy use on farms, including “indirect” fossil energy used to produce fertilizers, peaked in the 1978 at 2.4 quadrillion BTU (~2.4 exajoules) or 2.96% of total U.S. energy use for that year but through a series of measures has decreased 26% for direct use (powering devices on farms) and 31% for fertilizer production. Much of the reduction has come from the use of more efficient diesel equipment, use of low-tillage and no-tillage plowing techniques, and more efficient targeted energy use in irrigation and crop drying. In 2002, agricultural energy use, including both direct and indirect energy was 1.7 quadrillion BTU (-1.7 exajoules) or 1.7% of total energy use. All in all, this is not a huge contribution to US energy use but within agriculture, an absolutely critical sector of the economy, fossil energy is an absolutely critical input.
Managing greenhouse gases from farming is more complicated than simply limiting fossil fuel use, as methane emissions by rice paddies, cattle and manure are an additional area of concern. In the US, agriculture accounts for about 8-9% of CO2 equivalent emissions because of methane’s strength as a greenhouse gas.
While agriculture may not in itself contribute as much to global warming as building or transport energy use, it is very vulnerable to rises in fuel prices and availability. Financially farming is a very slim margin business and energy costs can be as much as 30% of total production costs for farms depending on the crop; insulating farmers from the vagaries of petroleum has environmental, economic, and food security benefits.
Rudolf Diesel’s Dream Deferred
More productive and potentially carbon neutral biofuels may be on the way but these are still in the indeterminate future. A return to spark ignition engines (once dominant on farms replaced by diesel in the last decades of the 20th Century) would allow farmers to use ethanol in farm equipment, though ethanol from corn is not particularly productive. The fuel needs of farmers vary but it appears that a farmer would need from 3 to 10% of their cultivated land to grow biofuel crops for their own needs and then process the crops into biofuel or pay to have their fuel crops processed. Current biofuel conversion processes capture around 0.15% of solar energy that is then only converted into around 0.05% of work capacity by your typical diesel engine. The idling that tractors engage in during the work day waste further energy. A combined solar array (15% efficient), storing electricity in a battery (85%) with an electric motor (90% efficient) could use around 11.5% of the sun’s energy to do work, a 228 x more efficient use of the sun’s energy
Batteries, Battery Exchange and Battery Tenders
Batteries for the foreseeable future will have energy densities (energy per unit weight or volume) somewhere in the area of 20 to 300 Watt-hours/kg (Wh/kg). For a larger tractor this means that a metric tonne (2200 lbs) of batteries will be able to hold from 20 kWh to 300 kWh depending on the battery type used and the future development of batteries into the higher end of this range. At the current upper end, the Tesla Roadster’s lithium ion Energy Storage System including all of its battery management systems has a energy density of approximately 124 Wh/kg so a metric-tonne sized version of it would contain 124 kWh of energy. The Tesla’s battery system is quite expensive, probably costing something on the order of $25000, so a pack twice its size would cost upwards of $50K. On the more economical end, a metric tonne of lead acid batteries might have 25 kWh and cost $3K or $15K (5 tonnes) for the equivalent charge of the lithium ion pack above.
Tractors on larger farms can be called upon to work as many as 3000 hours a year, and perhaps in increments as long as 12 to 14 hours during the peak season. Harvesters need to work around the clock at points during the year when it is critical that the crops be brought in on time. The large but not largest equipment usually runs on 250-300 hp diesel engines so we would use a 250kW electric motor with similar maximum output characteristics. Though the two configurations may not be exactly equivalent, I am inventing this configuration to arrive at an hourly energy usage, in this case 250 kWh at maximum power. Motors modeled on locomotive traction systems (either all-electric or diesel-electric hybrids) might be appropriate as these supply high continuous power output. In any case, this energy usage rate would require 2 metric tonnes of the lithium ion and 10 tonnes of the lead acid batteries at full charge for an hour of work
If the more energy intensive farm tasks were to be entirely electrified, a modular battery exchange system would need to be developed that allows tractors and harvesters to stop at the end of a row for a mobile quick charge or a battery exchange. Mobile quick charge displaces the same problem of energy density and then adds the problem of extreme high voltages and amperes (quick exchange of charge) to a recharge truck so it would appear that battery exchange might be one solution that would work at some point in the not too distant future.
To do battery exchange at the field’s edge, there would need to be a central charge station and a battery tender vehicle that would be able to transport and install multiple 500kg (1100 lbs) or 1000kg (2200 lbs) battery packs on farm equipment anywhere within a 5 to 10 mile (8 to 16 km) radius of the charge station. Battery tender vehicles might vary in size depending upon the size of the farm but would probably have some features of a flatbed truck with a hoist and a series of drawers or sliding tables to handle half-tonne and tonne sized battery packs.
The farm’s inventory of battery packs would need to exceed the farm’s total daily energy usage for mobile equipment on the highest demand day of the year, its peak energy usage. If we extend the example above and invent an example farm’s energy demand we will assume the larger tractor’s 250kw demand for 10 hours and 4 vehicles averaging 25kw for 10 hours. The larger tractor would consume around 2.5 MWh of battery charge in an intense workday while the 4 vehicles at lower power levels would each consume 250 kWh for a total of 1 MWh. At this usage rate, the 250kw tractor would require 20 metric tonnes of lithium ion batteries or 100 metric tonnes of the lead acid batteries while the tractors operating at 25 kW power would each require 2 tonnes of the lithium and 10 tonnes of the lead acid per day.
To fill out this scenario, we need to arrive at a battery carrying capacity for each of the tractors, the 250 kW and the 50kW models (operated in the scenario at 25 kW). Conventional tractors at the 250hp level weigh with ballast somewhere around 15 metric tonnes (33500 lbs) while conventional 80 hp tractors weigh around 5 metric tonnes (11100 lbs.). If we figure that around 50% of the weight of electric tractors can be devoted to batteries, the larger tractor could theoretically carry 8 metric tonnes of batteries while the 50kW models could carry 2.5 metric tonnes of batteries. The larger tractor then could with lithium ion battery packs carry 1 MWh of charge and 200 kWh of charge with lead acid. The 50 kW tractor could carry 312 kWh of lithium ion batteries and 62.5 kWh of lead acid batteries. As tractors can operate in a wide range of power levels depending on the task at hand which an electric tractor can tap into more efficiently, some days these tractors may not require battery exchange while other days they might require hourly exchange of batteries.
Using our sample workday above, there are two different scenarios for how many battery exchanges will be required per day depending on the energy density of the batteries used. I will assume a 100% discharge of batteries though in reality it may be necessary to exchange batteries that still contain charge in them to preserve battery life. Starting with the lithium example, the 250kW tractor carrying 1MWh of charge would require 2 exchanges of batteries per day and the 50kW tractors operating at 25kW power carrying 312kWh of charge would not need a battery exchange. With lead acid, the frequency of battery exchange goes up substantially: the 250kW tractor would require 13 exchanges to work 10 continuous hours and the 50kW tractor working at 25kW would require 3 to 4 exchanges.
To facilitate in-field battery exchange, battery packs might be carried on the vehicle in such a way as to promote easy access for the battery tender vehicle and also to provide ballast to the tractor. They might be situated underneath the cab or in slots in a space between the wheels. A battery powered tractor might have a more elongated design than conventional tractors to allow battery exchange in a space between the wheels and underneath the cab. If a motorized system of battery pack exchange and automatic connection and disconnection has been worked out, these exchanges probably would require from 10 to 20 minutes.
As this post is already long enough, I will continue working out the Electric Farm concept in my next post and evaluate it from the perspective of its current and nearnear-future feasibility. Also, the main rationale for the Electric Farm concept will be discussed, i.e. using on-farm or near-farm renewable electricity generation to fuel contemporary agriculture.
Original Post: http://terraverde.wordpress.com/2007/11/12/the-renewable-electron-economy-part-viii1-the-electric-farm/
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