Agricultural Engineering · Farm Machinery & Power · also Sources of Power on the Farm, Agricultural Power
Where the energy for farm work comes from, how much is available, and how mechanisation is measured.
In the exams: GATE AG Section 3 Sources of Power; ICAR AIEEA-PG Unit V; PRC ABE power and machinery (32%)
Farm power is the rate at which energy is delivered to do farm work: pulling a plough, lifting water, threshing grain, drying a crop, milking a herd. Every farm, from a one-hectare rice holding worked by hand to a 5,000-hectare grain enterprise, runs on some mix of human muscle, draught animals, internal combustion engines, electric motors and, increasingly, wind, sun and biomass. This subject asks three questions of each source: how much power can it deliver and for how long, what does it cost in energy and money, and what limits it.
The subject matters in practice because the amount and kind of power a farm can command decides what it can do and when. Timeliness of sowing and harvest is driven by power; drudgery and the health of farm workers depend on how much of the load is taken off human bodies; the energy bill and the carbon footprint of a crop depend on which source does the work. At the level of a district or a country, planners measure mechanisation by power availability per hectare and by mechanisation indices, and use these figures to set targets for subsidies, custom hiring and rural electrification.
Worldwide the subject is examined as the opening block of farm power and machinery. In India it sits under 'Sources of Power' in GATE Agricultural Engineering, under Unit V of the ICAR AIEEA-PG paper and inside the farm power portions of the PhD entrance and state engineering-service syllabi. In the Philippines it is part of the power, energy and machinery subject of the Agricultural and Biosystems Engineers licensure examination. Engineering registration syllabi in Nepal, Bangladesh and elsewhere include it under farm power and rural energy. Questions are mostly short calculations (power from force and speed, motor input and output, wind and solar power, biogas yield, kilowatts per hectare) plus recall of the characteristics of each source. The skill examiners look for is careful handling of units: newtons against kilogram-force, kilometres per hour against metres per second, metric against imperial horsepower, kilowatts against kilowatt-hours.
This manual treats each source in turn, then the two ways of summarising a farm's or a region's power position: power availability in kW/ha and mechanisation indices, including the energy-audit approach used in research. Engine thermodynamics, tractor transmissions, traction mechanics and the detailed design of solar and wind systems have their own subjects; here they appear only as far as needed to compare sources.
Work is done when a force moves its point of application: work (joules, J) equals force (newtons, N) multiplied by the distance moved in the direction of the force (metres, m). Energy is the capacity to do work and has the same unit. Power is the rate of doing work or of delivering energy: one watt (W) is one joule per second. For a force F moving at a steady speed v in its own direction, power P = F × v. For a rotating shaft, power P = 2πNT/60, where N is speed in revolutions per minute and T is torque in newton-metres. For lifting a mass m through a height h in time t, useful power P = m g h / t, with g = 9.81 m/s². For pumping water, hydraulic (water) power P = ρ g Q H, where ρ is density (1000 kg/m³ for water), Q is flow in m³/s and H is total head in metres. These four expressions cover most farm power calculations. A clear distinction must be kept between power (kW) and energy (kWh or MJ). A tractor rated at 40 kW that works for 5 hours at full load delivers 200 kWh of work. A source may be powerful but short-lived, as a person sprinting is, or weak but sustained, as a bullock pair working through a morning is. For farm planning the sustained power, and the number of hours per day and days per year it can be sustained, matter more than the peak.
Farm power literature mixes unit systems, and most examination errors come from this. The key conversions are: 1 kilogram-force (kgf) = 9.81 N; 1 kilowatt-hour (kWh) = 3.6 megajoules (MJ); 1 kilocalorie (kcal) = 4.1868 kJ; 1 km/h = 1/3.6 m/s, so 3.6 km/h = 1 m/s. Two horsepowers are in use. The imperial (mechanical) horsepower, defined by James Watt as 550 foot-pound-force per second, equals about 745.7 W. The metric horsepower (PS, ch or cv in European usage) is defined as 75 kgf·m/s and equals about 735.5 W. Many older Indian and European textbooks use the metric horsepower, so a draught of so many kgf at so many m/s is divided by 75 to give horsepower. Tractor and engine power is now normally stated in kilowatts, but horsepower survives in trade and in farmers' speech, and a problem must say which one it means. When a question gives draught in kgf, convert to newtons before multiplying by speed to get watts, or divide kgf·m/s by 75 to get metric horsepower; mixing the two is the classic slip. Speeds of animals and tractors are usually quoted in km/h and must be converted to m/s for power in watts.
Farm power sources are conventionally grouped as: human (manual) power; animal (draught) power; mechanical power from internal combustion engines, used as stationary engines, in power tillers and in tractors and self-propelled machines; electrical power, mainly through electric motors fed from a grid or a local generator; and renewable sources, namely wind, solar and biomass-derived energy, which may drive machines directly (a wind pump, a solar dryer) or through conversion to electricity or fuel. Human and animal power are called animate sources; engines and motors are called mechanical and electrical, and together they are often called inanimate or mechanical sources. A second useful division is by mobility. Tractive work in the field, such as tillage and haulage, needs a mobile source: people, animals, tractors, power tillers and self-propelled machines. Stationary work at the farmstead or well, such as pumping, threshing, grinding, chaff cutting and processing, can use fixed engines, electric motors, wind and solar power, which are often cheaper per unit of energy. A third division is by fuel: food and fodder for animate power; petroleum fuels (diesel, petrol, kerosene), biofuels and gases for engines; and the grid, sun and wind for the rest. Choosing a source means matching its power level, mobility, reliability and energy cost to the job.
The human body converts chemical energy in food into mechanical work through muscle, with a gross mechanical efficiency usually quoted at around 20 to 25 percent for favourable tasks such as pedalling, and lower for awkward tasks. The rest of the metabolic energy appears as heat, which the body must lose through sweating, breathing and skin blood flow. The mechanical power a healthy adult can sustain over a working day is small. Standard agricultural engineering textbooks commonly take it as about 0.1 horsepower, that is roughly 75 W, for continuous work; many planning studies use a lower figure, around 0.05 kW, averaged over the working day. For a few seconds a fit person can develop several hundred watts, even over 1 kW in a sprint or jump, but output falls steeply as the duration increases. The figures vary with body size, fitness, nutrition, age, sex, climate and the task, so they are planning conventions, not constants. In energy terms a worker delivering 75 W for 8 hours does 0.6 kWh of mechanical work, which is the output of a small electric motor for a fraction of an hour. This is why manual labour, although flexible and skilled, is the most expensive source of farm power per unit of mechanical energy wherever wages are more than subsistence.
The rate at which a person can sustain work is limited by the rate at which the heart and lungs can deliver oxygen to working muscles. Metabolic energy expenditure is therefore estimated from oxygen consumption: each litre of oxygen consumed releases roughly 20 to 21 kJ, depending on the mix of fuels being burned. Mechanical output can then be estimated as metabolic power multiplied by mechanical efficiency. Heart rate rises roughly in proportion to oxygen uptake over the normal working range, and because it is easy to measure it is widely used to grade the heaviness of farm tasks; ergonomic studies classify work from light to extremely heavy by heart rate or energy expenditure bands, though the exact band limits differ between authors. Work that can be continued all day must stay well below a person's maximum aerobic capacity; a common rule in ergonomics is that sustained work over a shift should use no more than about a third to a half of that capacity, and rest pauses must be built in above that. Heat is the second limit: in hot, humid field conditions the body cannot shed the heat produced by hard work, core temperature rises, and output must fall or the worker risks heat illness. Posture is the third: stooping and squatting, as in transplanting rice or hand weeding, reduce the power a person can apply and cause musculoskeletal injury over time.
Because the leg muscles are the largest in the body, people deliver more sustained power by pedalling or treading than by arm cranking or pushing. Pedal-driven threshers, treadle pumps and cycle-type weeders exploit this. Tools should let the worker stand upright where possible, keep loads close to the body and use rhythmic movements at a comfortable rate. Weight and force limits for manual lifting, pushing and pulling are set by ergonomic guidelines and national occupational-safety rules, which differ between countries and between men and women, so a designer must use the rule that applies where the equipment will be used. The practical lessons are these. Human power is best used for tasks needing skill, judgement and dexterity, such as transplanting, fruit picking and selective harvesting, rather than raw energy. Tasks that are heavy and repetitive, such as primary tillage, lifting water and threshing, are the first to be transferred to animals or engines as wages rise. Drudgery reduction, especially for women who do much of the weeding, transplanting and post-harvest work in many countries, is a legitimate design goal in its own right, separate from saving money.
Draught animals remain an important source of farm power in parts of South and South-East Asia, sub-Saharan Africa and Latin America, and on small farms elsewhere. The main species are cattle (bullocks, meaning castrated males, and in some places cows), water buffaloes, horses, mules, donkeys and camels, with yaks and others locally. The pull an animal can sustain for several hours is commonly taken in textbooks as about one tenth of its body weight, with a range of roughly 10 to 14 percent for well-fed, trained animals; horses and mules can exert a larger fraction for short periods. Working speeds are slow: bullocks typically walk at about 2.5 to 3.5 km/h under load, horses somewhat faster. From force and speed, an average bullock develops of the order of a few hundred watts, and a pair of bullocks roughly 0.5 to 1 kW, which is why a pair is often compared with about one horsepower. All these figures vary with breed, body weight, condition, training, feeding, soil and climate. An animal can work only a limited number of hours a day, commonly four to six, and fewer in hot weather, and cannot work at all when sick, heavily pregnant or in poor condition. It needs feed, water and care every day of the year, whether or not it works, and that cost must be set against its output.
Bullocks of zebu breeds tolerate heat well and can work on low-quality roughage, which is why they dominate draught work in tropical Asia and Africa. Water buffaloes are heavier and can exert a larger pull, and they work well in puddled paddy fields, but they are slower, have fewer sweat glands and suffer in hot sun, so they need shade or wallowing and are worked in the cooler hours. Horses are faster and more powerful for their weight, work well in collar harness and suit temperate climates, but need better feed. Mules combine hardiness with strength. Donkeys are small, cheap to keep and useful for carting and light tillage, especially where women farm, but their pull is limited. Camels are used in arid areas for ploughing, carting and water lifting. The right animal is the one that suits the local climate, feed supply, soil, tasks and ownership pattern; there is no universal best.
Draught is the horizontal force required to pull an implement. A spring or electronic dynamometer placed in the line of pull measures the pull; if the chain or beam is inclined at an angle θ above the horizontal, the horizontal component (the useful draught) is pull × cos θ and the vertical component, pull × sin θ, lifts the implement and partly unloads it. Power is draught multiplied by speed: for example, a pull of 1 kN at 0.8 m/s is 0.8 kW. In kgf and m/s units, draught × speed divided by 75 gives metric horsepower. Field capacity follows from implement width, speed and field efficiency: theoretical field capacity in ha/h equals width in metres × speed in km/h ÷ 10, and effective field capacity is that multiplied by field efficiency, which allows for turning and other time losses. Signs of fatigue that tell the operator to rest the animals include rising respiration rate, rising body temperature and heart rate, frothing at the mouth, reluctance to move and lying down; performance tests of draught animals record these along with pull and speed.
A harness transfers the animal's pull to the implement without injuring it. The main types are the yoke, the collar and the breast-band. The neck (withers) yoke, a wooden beam resting on the necks of a pair of cattle just in front of the hump, is the usual system for zebu cattle in South Asia and much of Africa; the hump takes the thrust. The head or horn yoke, strapped to the horns or forehead, is used in parts of Europe, Latin America and Africa and suits cattle without a pronounced hump. Single-animal yokes and padded collar-type harnesses for cattle have been developed so that one animal can work alone, for example on light weeding, and so that the pull is spread over a larger area. Horses, mules and donkeys are harnessed with a collar that bears on the shoulders, or with a breast-band (breast strap) across the chest; the collar allows a horse to exert its full pull without pressing on the windpipe. Traces (chains or ropes) run from the harness to a swingletree (whippletree) and then to the implement. When two or more animals work together an evener, a pivoted bar, divides the load between them so that the stronger animal does not do all the work and the implement is not pulled sideways.
Poorly fitted harness is a major cause of low output in draught animals. Hard, narrow or rough yokes cause sores, swelling and pain, the animal pulls less and loses working days, and in the long term it may be ruined. Contact surfaces should be broad, smooth and padded, fit the animal and keep the pull evenly distributed. The line of draught should be straight from the point of attachment on the animal through the hitch to the centre of resistance of the implement; if the angle is too steep much of the pull is wasted lifting the implement, and if too shallow the animal may be pulled down. The yoke should keep the pair at a width that suits the row spacing or furrow. Animals of a pair should be matched in size and speed. Harness and yokes should be kept clean and checked daily for wear. Improved harness designs from research stations in several countries have shown gains in pull and comfort, but adoption depends on cost, local materials and whether farmers can make and repair them.
An animal-drawn implement must be sized so that its draught at the required depth and speed is within what the animals can sustain. Draught of a tillage tool is often estimated from specific draught (the force per unit cross-sectional area of the furrow, in N/cm²), which varies widely with soil type, moisture and tool shape. For a mouldboard plough cutting a furrow 20 cm wide and 12 cm deep in a soil with specific draught 4 N/cm², the draught is 20 × 12 × 4 = 960 N. If that exceeds the sustained pull of one pair, the farmer must reduce width or depth, work when soil moisture is favourable, or use two pairs. Typical animal-drawn implements include wooden and steel ploughs, harrows, levellers, seed drills and planters, inter-row weeders, puddlers for paddy, carts and water-lifting devices such as the Persian wheel. Multipurpose tool carriers, wheeled frames carrying interchangeable tools, were promoted in several countries but were often too costly and heavy for smallholders. For stationary work such as water lifting and crushing, the animal walks a circular path turning a capstan or sweep; the power obtained is the pull on the sweep arm multiplied by the walking speed.
Internal combustion engines convert the chemical energy of fuel into shaft work. Spark-ignition (petrol) engines are light and cheap and are used on small machines such as brush cutters, sprayers and small pumps. Compression-ignition (diesel) engines have higher compression ratios, higher thermal efficiency and good torque at low speed, and they power nearly all tractors, power tillers, combines and larger stationary pump sets. Brake thermal efficiency of farm diesel engines at good load is commonly around 30 to 40 percent, falling at part load; the rest of the fuel energy leaves as heat in the exhaust and cooling system. Brake specific fuel consumption, fuel mass per unit of brake work (kg/kWh or g/kWh), is the practical measure of how economically an engine converts fuel. Engine power falls at high altitude and high air temperature because the air is less dense and less oxygen enters each cycle, which is why engines are derated for such conditions. Stationary single-cylinder diesel engines of a few kilowatts are still common for pumping and threshing where grid electricity is absent or unreliable. Engine cycles, combustion, fuel systems and performance testing are treated in the engines subject; here the engine is considered as one of several ways of supplying power.
A power tiller, also called a two-wheel or walking tractor, is a single-axle machine steered by handles and usually fitted with a rotary tiller; it can also pull a trailer, drive a pump or thresher and work in puddled paddy fields where four-wheel tractors struggle. Power tillers are common in East and South-East Asia and parts of South Asia, and the engine is usually a small diesel. The four-wheel tractor is the principal mobile power unit of mechanised agriculture. It delivers power in four ways: as drawbar pull for trailed implements, through the power take-off (PTO) shaft for driven implements, through the hydraulic system for lifting and for hydraulic motors, and as belt or pulley power on older designs. Tractors are classed by power, by drive (two-wheel drive, four-wheel drive with smaller or equal-size front wheels, tracked) and by use (general purpose, row-crop, orchard, garden). Tractor power is quoted at several points. Engine (flywheel) power is highest; PTO power is lower because of transmission losses; drawbar power is lower still because of slip, rolling resistance and the power needed to move the tractor itself. The ratio of drawbar to PTO power depends strongly on the tractor type and the soil surface, being best on firm surfaces and worst in loose or soft soil; standard data sets such as ASABE D497 give typical ratios for planning.
The advantages of engine power are the large power available from one unit, the ability to work long hours at a steady rate, speed and timeliness, and the ability to drive high-power operations such as deep tillage, combine harvesting and high-capacity pumping that animals and people cannot do. The disadvantages are capital cost, dependence on fuel supply and spare parts, the need for skilled operation and repair, soil compaction by heavy machines, noise and exhaust emissions, and safety risks, notably tractor overturns. For a small farm a tractor may be under-used, which raises its cost per hour; custom hiring, cooperative ownership and machinery rings address this by spreading fixed costs over more hours. For comparison purposes, the drawbar power of a 30 kW tractor on firm soil corresponds to the output of dozens of bullock pairs, and a tractor can work far more hours per day. The number of hours a tractor or engine actually works, and its average load factor (the fraction of rated power used on average), decide how much energy it delivers in a year: annual work in kWh equals rated power × hours × load factor.
Electricity reaches farms mainly through rural distribution networks, and also through diesel or biogas generator sets and solar photovoltaic systems. Its main farm uses are driving electric motors for irrigation pumps, which in many countries are the largest single farm electricity load, and for threshers, chaff cutters, feed grinders, milking machines, milk coolers, ventilation fans, grain dryers and conveyors; heating, as in brooding, incubation and crop drying; lighting; and refrigeration and processing. Rural electrification changed irrigation in South Asia and elsewhere by making tubewell pumping cheap and convenient. It also created problems: where agricultural electricity is sold at flat rates based on connected load rather than on metered consumption, the marginal cost of pumping is nil, which encourages over-pumping of groundwater and wastes energy. Supply quality in rural networks can be poor, with low and fluctuating voltage and limited hours of supply, and motors must be protected and selected with that in mind. Tariff policy differs sharply between countries and states, so the numbers must be taken from the local utility.
The workhorse of farm electrification is the three-phase squirrel-cage induction motor: simple, robust, cheap and nearly maintenance-free. Where only single-phase supply is available, single-phase induction motors of the capacitor-start or capacitor-run types are used for smaller loads. A motor's nameplate rating is its output shaft power at rated conditions. Electrical input power for a three-phase motor is P = √3 × V_L × I_L × cos φ, where V_L and I_L are line voltage and current and cos φ is the power factor; for a single-phase motor it is P = V × I × cos φ. Motor efficiency is output divided by input. Efficiency is higher for larger motors and falls when a motor runs well below its rated load, as does power factor; an oversized motor wastes energy. Typical full-load efficiencies range from the low eighties in percent for small motors to above ninety for large ones, and national efficiency classes set minimum values. Synchronous speed is Ns = 120 f / p, where f is supply frequency and p the number of poles; a four-pole motor on a 50 Hz supply has a synchronous speed of 1500 rpm. The rotor runs slightly slower; slip s = (Ns − N)/Ns, commonly a few percent at full load. On 60 Hz supplies, as in the Americas and parts of Asia, speeds are 20 percent higher for the same pole number.
An induction motor started direct on line draws a starting current several times its full-load current, which causes voltage dips on weak rural lines. Small motors are started direct on line; larger ones use star-delta starters, which reduce starting line current and torque to one third of the direct-on-line values, or autotransformer or soft starters and variable-frequency drives. Motors must be protected by fuses or circuit breakers against short circuit and by overload relays against sustained overcurrent; single-phasing (loss of one phase) and prolonged low voltage cause overheating, because a loaded motor draws more current when voltage falls. Earthing and residual-current protection are essential around water and livestock. Power factor is the ratio of real power to apparent power; a low power factor means more current for the same real power, higher line losses and larger cables and transformers. Induction motors run at a lagging power factor, worse at light load, and shunt capacitors are fitted at the motor or at the distribution point to improve it. Compared with a diesel engine of the same output, an electric motor is cheaper, more efficient at the point of use, quieter, cleaner at the farm, and starts instantly; its drawbacks are that it is tied to a supply point and to the reliability and price of grid electricity.
Wind energy is the kinetic energy of moving air. The power carried by wind through an area A swept by a rotor is P = ½ ρ A V³, where ρ is air density (about 1.225 kg/m³ at sea level and 15 °C) and V is wind speed. Three consequences follow. Power rises with the cube of wind speed, so doubling the speed gives eight times the power and a site with slightly higher average wind is far more valuable. Power rises with swept area, that is with the square of rotor diameter. Power is proportional to air density, which falls with altitude and rising temperature. No rotor can extract all of this power, because the air must keep moving to leave the rotor. The Betz limit shows that the maximum fraction a rotor can extract in ideal conditions is 16/27, about 59.3 percent. Real rotors achieve less, and the power coefficient of the whole system, including gearbox, generator or pump, is lower again. Because power depends on V³, the average power of a variable wind cannot be found from the average speed: the mean of the cubes is larger than the cube of the mean, and energy estimates use the frequency distribution of wind speed, often represented by a Weibull or Rayleigh distribution.
Wind speed increases with height above the ground because surface friction slows the air near the ground. A common engineering approximation is the power law V2/V1 = (h2/h1)^α, with the exponent α about 1/7 for open, flat terrain and larger over rough terrain with trees and buildings. Raising a rotor from 10 m to 30 m in open country therefore increases speed by about 17 percent and power by about 60 percent. Sites are assessed by measuring wind speed at or near hub height for at least a year where possible; farm-scale estimates often rely on nearby meteorological data, adjusted for height and terrain. Obstacles such as trees and buildings create turbulence downwind for a distance of many times their height, and rotors should be placed clear of them. A turbine's power curve shows its output against wind speed: output starts at the cut-in speed, rises to rated power at the rated speed, stays at rated power up to the cut-out speed, and the machine is then stopped to protect it. Capacity factor, actual annual energy divided by rated power × 8760 hours, summarises how well a turbine suits a site.
The classic farm windmill is a multi-blade, high-solidity rotor on a lattice tower, driving a reciprocating piston pump through a crank and pump rod. High solidity (the blades cover much of the swept area) gives high starting torque at low wind speed and low rotational speed, which suits a piston pump that must start under load; the tip speed ratio, blade-tip speed divided by wind speed, is low, around one. Such windmills are used for stock watering and domestic water in dry grazing lands in many countries, where they run unattended and store water in tanks rather than energy in batteries. Their overall efficiency from wind to water is low, often a few percent to around ten percent at the design wind speed. Wind-electric generators use two or three slender blades, low solidity and high tip speed ratios, commonly around five to eight, which suits fast-turning generators. Small wind turbines can supply farm electricity, charge batteries or drive electric pumps; larger turbines on farmland are often owned by developers who lease the land. The water-pumping output of a windmill follows from hydraulic power: ρ g Q H equals the wind power multiplied by the overall efficiency.
The sun delivers radiation to the top of the atmosphere at an average intensity, the solar constant, of about 1361 to 1367 W/m² depending on the measurement series used. At the ground, after absorption and scattering, the direct plus diffuse radiation on a surface facing the sun reaches about 1000 W/m² on a clear day around noon, which is why that value is used as the standard test irradiance. Daily radiation on a horizontal surface varies with latitude, season and cloud, and is often expressed as peak sun hours: the number of hours at 1000 W/m² that would deliver the same daily energy. For a fixed collector or panel, tilting it towards the equator at an angle close to the site latitude maximises annual collection; a steeper tilt favours winter and a flatter one favours summer. Solar energy is used on farms in two ways: as heat, through solar dryers, water heaters, air heaters, cookers, stills and greenhouses; and as electricity, through photovoltaic (PV) cells, which power pumps, fences, lighting, milk chilling and other loads, on or off the grid.
A flat-plate collector consists of a dark absorber plate with attached tubes or ducts, a transparent cover to reduce heat loss, and insulation behind. Its instantaneous efficiency is the useful heat gained by the fluid divided by the solar radiation falling on the collector: η = m cp ΔT / (A G), where m is mass flow rate, cp specific heat, ΔT temperature rise, A collector area and G irradiance on the collector plane. Efficiency falls as the operating temperature rises above ambient, because losses increase. Concentrating collectors reach higher temperatures but need direct radiation and usually tracking. Solar dryers for crops range from simple cabinet dryers, where air is heated under a transparent cover and rises through the product by natural convection, to tunnel and greenhouse-type dryers with fans. Compared with open sun drying, they protect the crop from dust, insects, birds and rain, reach higher temperatures and dry faster, which improves quality; their limits are capital cost, dependence on weather and limited capacity. Solar water heaters in dairies and solar air heaters for grain drying reduce fuel use.
A PV cell converts light directly into direct-current electricity. Its current-voltage curve is characterised by the open-circuit voltage Voc, the short-circuit current Isc, and the maximum power point at voltage Vmp and current Imp. The fill factor FF = (Vmp × Imp)/(Voc × Isc) measures how square the curve is; values around 0.7 to 0.8 are usual for good crystalline silicon cells. Module efficiency is maximum power divided by irradiance × module area. Ratings are given at Standard Test Conditions: 1000 W/m², cell temperature 25 °C and air mass 1.5 spectrum. Output falls as cell temperature rises, for crystalline silicon by roughly 0.3 to 0.5 percent per degree Celsius, and cells in sun run well above air temperature, so real output is below the rating; a performance ratio or derating factor covering temperature, dust, wiring and inverter losses is applied in sizing. Daily energy is about array rating × peak sun hours × performance ratio. Solar pumps use a PV array with a DC or AC motor-pump set, a controller that tracks the maximum power point, and usually water storage rather than batteries; they are sized from the daily water need and head: hydraulic energy per day = ρ g V H, divided by the pump subsystem efficiency to give the PV energy needed, and then by peak sun hours and derating to give the array size. Batteries, where used, must not be discharged below their permitted depth of discharge. Agrivoltaics, growing crops or grazing under raised or widely spaced PV arrays, shares land between food and power.
Biomass is organic matter of recent biological origin: crop residues such as straw, husk, stalks and cobs; animal dung; wood and prunings; agro-processing wastes; and energy crops. It can supply heat by direct combustion, gaseous fuel by anaerobic digestion (biogas) or thermochemical gasification (producer gas), and liquid fuel by fermentation (ethanol) or by chemical conversion of oils (biodiesel). Air-dry crop residues and wood commonly have calorific values in the range of about 14 to 19 MJ/kg, lower when wet and lower still for high-ash residues such as rice husk; the value must be taken for the material and moisture content concerned. Direct combustion in improved stoves, furnaces and boilers is the simplest route, used for crop drying, jaggery making, parboiling and steam raising. Densification into briquettes and pellets improves handling. Burning residues in the field wastes this energy and causes serious air pollution; using residues for energy, soil incorporation or fodder are the alternatives, but removing too much residue can deplete soil organic matter.
Biogas is produced when organic matter such as cattle dung, pig and poultry manure, crop residues and food waste is digested by bacteria in the absence of air. It consists mainly of methane, commonly 50 to 70 percent, and carbon dioxide, commonly 30 to 45 percent, with small amounts of hydrogen sulphide, water vapour and other gases. Its calorific value, which depends on the methane content, is roughly 20 to 25 MJ/m³. Digestion proceeds through hydrolysis, acid formation and methane formation; methane-forming bacteria are sensitive to temperature, pH and toxic substances. Most farm digesters operate in the mesophilic range of about 30 to 40 °C, and gas production falls sharply in cold weather; thermophilic digestion around 50 to 60 °C is faster but harder to control. Family-size digesters in South Asia are of two main types: floating-drum digesters, in which a steel gas holder rises and falls and delivers gas at nearly constant pressure, and fixed-dome digesters of masonry, with no moving parts and lower cost but varying gas pressure; plug-flow and bag digesters are used elsewhere. Digester sizing uses the daily feed and the hydraulic retention time (HRT): digester volume = daily slurry volume × HRT. Fresh cattle dung is usually mixed with about an equal quantity of water; in Indian design practice gas yield is commonly taken as roughly 0.035 to 0.04 m³ per kg of fresh dung and HRT as 30 to 55 days depending on climate, but both vary and should be checked locally. Biogas is used for cooking and lighting, and in engines: in a diesel engine run on dual fuel, a pilot injection of diesel ignites the gas; spark-ignition engines can run on biogas alone. The digested slurry is a valuable manure.
Gasification converts solid biomass into a combustible gas by partial combustion with a limited air supply. Producer gas from air-blown gasifiers contains carbon monoxide and hydrogen as the main fuels, with some methane, and a large fraction of nitrogen and carbon dioxide, so its calorific value is low, roughly 4 to 6 MJ/m³. Updraft gasifiers produce gas rich in tar and suit direct heating; downdraft gasifiers pass the gas through the hot zone, crack much of the tar and are preferred for engines; the gas must still be cooled and cleaned. In compression-ignition engines producer gas is used in dual-fuel mode with a pilot diesel injection, which saves a large part of the diesel but not all of it; spark-ignition engines can run on gas alone with some power loss. Ethanol is made by fermenting sugars from sugarcane juice or molasses, or starch from grains after hydrolysis, and distilling; it is blended with petrol, with blends named by ethanol percentage by volume, such as E10 and E20. Biodiesel is made by transesterification: vegetable oils or animal fats react with an alcohol, usually methanol, in the presence of a catalyst, commonly an alkali, to give fatty acid methyl esters (biodiesel) and glycerol as a by-product. Compared with diesel, biodiesel has a somewhat lower heating value, a higher cetane number, higher flash point and better lubricity, but poorer cold-flow properties; blends are named by biodiesel percentage, such as B20. Straight vegetable oil is too viscous for most unmodified diesel engines. Whether a biofuel saves energy and emissions depends on its whole production chain, which is why energy-balance and life-cycle studies are central to biofuel research.
Farm power availability expresses the total power available for farm work in a farm, district, state or country per unit of cultivated land, in kW/ha. It is calculated by counting the units of each source, multiplying by a power figure for each, summing, and dividing by the area. For tractors, power tillers, diesel engines and electric motors the rated power of each unit is used, usually from registration or livestock and machinery census data. For people and animals, conventional power factors are used, because their output cannot be read from a nameplate; Indian studies have commonly counted an agricultural worker at about 0.05 kW and a draught animal at about 0.38 kW, but other studies use different factors, and the choice changes the result. The area is usually the net sown or net cultivated area; some studies use gross cropped area, which gives a lower figure where land is cropped more than once a year. For example, a block with 400 ha net sown area, 25 tractors of 30 kW, 10 power tillers of 9 kW, 150 electric pump motors of 5 kW, 60 diesel engines of 4 kW, 300 draught animals and 900 workers has 750 + 90 + 750 + 240 + 114 + 45 = 1989 kW, or about 5 kW/ha. Always state the factors and the area basis used.
Farm power availability is used to compare regions, track mechanisation over time and set policy targets. It correlates positively with land productivity across regions, since power allows timely operations, better tillage and irrigation, and higher cropping intensity, but correlation is not proof that power alone causes higher yield: irrigated, fertile regions both earn more and buy more machines. In India, published estimates show power availability rising from well under half a kilowatt per hectare around 1960 to around two kilowatts per hectare in the 2010s, with the share of animate power falling and that of tractors, power tillers, engines and electric motors rising; policy documents set higher targets. The latest figure should be taken from the current official source, because estimates depend on the method and data year. Many industrialised countries report much higher values, but again methods differ. The index has clear limits. It counts rated power, not power actually used; a tractor that works 300 hours a year counts the same as one that works 1500. It does not show whether power is available at the right time, in the right place, to the smallholder who needs it, or whether machines are old and unreliable. It mixes mobile and stationary power, so a region with many irrigation pumps may appear well mechanised while its tillage and harvesting are still manual.
For planning at farm level, the question is not the average power per hectare but whether the power available can complete each operation within its time window. The required number of power units follows from the area, the work rate of each unit, the number of days available and the working hours per day, reduced by the proportion of days lost to weather. For example, if ploughing 60 ha needs 3 tractor-hours per hectare and there are 10 working days of 8 hours, the requirement is 180 tractor-hours against 80 hours per tractor, so 2.25 tractors, meaning three are needed or some area must be ploughed late. Planners then look at the peak operation, usually seedbed preparation or harvest, since that sets the power needed; other operations use spare capacity. Custom hiring centres and machinery-sharing arrangements raise the power effectively available to small farms without each farmer owning a machine. Power per worker, kW per agricultural worker, is a complementary measure: it indicates labour productivity and drudgery, and rises as workers leave agriculture.
No single index of mechanisation is universally agreed, so every index must be quoted with its definition. Four families are common. The first is power-based: the share of mechanical and electrical power in total farm power, or the power availability in kW/ha described in the previous chapter. The second is operation-based: for each operation (tillage, sowing, plant protection, irrigation, harvesting, threshing) the degree of mechanisation is the percentage of the area, or of the crop, on which that operation is done by machine, and an overall level is obtained by weighting the operations, for example by their share of labour or energy. A region may be highly mechanised in tillage and threshing but not in transplanting and harvesting, and the operation-wise figures show where effort is needed. The third is energy-based: a mechanisation index defined as the mechanical energy input divided by the total of human, animal and mechanical energy input to a crop, expressed as a percentage. The fourth is stock-based: tractors per thousand hectares or per thousand workers, and machinery value per hectare. Each answers a different question, and they can move in different directions.
Energy analysis assigns an energy equivalent to every input and output of a crop system so that they can be compared on one scale. Direct energy comes from human labour, animals, diesel and electricity; indirect energy is embodied in seed, fertilisers, pesticides and machinery. Energy equivalents are taken from published tables; widely used Indian coefficients, for example, count adult male labour at about 1.96 MJ per hour and female labour at about 1.57 MJ per hour, and a litre of diesel at about 56 MJ, which is more than its heating value because it includes the energy used to produce and deliver the fuel. Other tables give different coefficients, and results should be reported with the table used. From the audit, the energy ratio (output energy ÷ input energy), energy productivity (kg of product per MJ input), specific energy (MJ per kg of product) and net energy (output minus input) are calculated. Operation-wise breakdowns usually show fertiliser, especially nitrogen, and irrigation pumping or diesel as the largest inputs in intensive systems.
Indices are tools for comparison and policy, not goals in themselves. A rising mechanisation index generally reflects labour leaving agriculture, rising wages and better access to machines; it can rise even while total energy use falls, if manual and animal work is replaced by more efficient machine work. A high energy ratio can mean an efficient system or a low-input, low-yield one. When comparing farms or regions, keep the same definitions, coefficients and area basis, and report the data year. At the policy level, operation-wise mechanisation indices identify gaps, such as rice transplanting or cotton picking, where research and subsidy can have most effect; farm power availability and tractor density show the overall stock; and energy analysis shows the environmental and resource cost of the power used. At the research level, open questions include how to value machine services obtained by hire rather than ownership, how to account for renewable energy produced on the farm, and how to link mechanisation measures with yield, employment and emissions without confusing correlation with cause.
Learn the four power formulas (force × speed, 2πNT/60, mgh/t, ρgQH) and the unit conversions until they are automatic, because most questions on this subject are one-step calculations that go wrong on units. For each source, be able to state its typical power level, its sustained hours, its main advantages and limits, and one calculation. Practise converting kgf to newtons, km/h to m/s and metric to imperial horsepower, and keep kW and kWh separate. Work the biogas sizing, wind power, PV fill factor and motor input-output problems until you can do them in under two minutes each.
A practising engineer uses this subject to choose the power source for a job: whether an irrigation pump should be electric, diesel or solar given supply hours, tariff, water need and head; how large a motor to fit so that it runs near rated load; whether a biogas plant can run an engine or only cooking; how many tractors or hired hours are needed to finish sowing on time. Typical tasks are pump-motor sizing from flow and head, solar pump sizing from daily water need, advising on star-delta starters and capacitors on weak rural lines, and checking harness and implement size for draught-animal farmers. Always use local data for tariffs, fuel prices and animal performance.
At senior level the decisions are about fleets, programmes and policy: planning custom hiring centres so that machine power reaches small farms at the peak of the season, setting targets for farm power availability and judging whether they measure what matters, deciding between grid extension and solar pumping for a district, and weighing the groundwater and fiscal cost of flat-rate agricultural electricity. A senior engineer reads mechanisation indices critically, asking what basis was used and whether rated power reflects use, and uses operation-wise mechanisation data to find bottlenecks such as transplanting or harvesting where investment will pay.
Research questions include better measures of mechanisation that count utilised rather than rated power and hired machine services; causal links between farm power, productivity, employment and emissions using panel data rather than cross-sectional correlation; energy and life-cycle analysis of crop systems with harmonised coefficients; the role of renewable sources, such as solar pumping and biogas, in reducing diesel dependence and their groundwater consequences; and the physiology and ergonomics of human and animal work under rising heat stress. A senior researcher designs studies with explicit definitions, uncertainty on coefficients, and comparisons across countries that respect differences in method.
This manual does not give current national figures for farm power availability or tractor density beyond the broad Indian trend, because estimates change with method and data year; take them from the latest official source. It gives no electricity tariffs, fuel prices or subsidy rates, which differ by country and change often. Physiological band limits for grading human workload, animal-specific draught tables by breed, and detailed ASABE D497 power ratios are described but not tabulated. Energy-audit coefficients are quoted only as examples from Indian tables. Engine thermodynamics, tractor transmissions and traction, and detailed solar, wind and biogas system design are left to their own subjects.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.