Sustainability & Environment (Agri-Food Systems) · Postgraduate & Professional Advanced · also Energy and Environment, Renewable Energy and Sustainability, Energy Transitions
Energy systems and their environmental footprints, and the transition in agriculture: fossil and renewable sources, energy-environment nexus, energy efficiency and auditing, and agri-energy options. Solar, wind and bioenergy engineering is in the Agricultural Engineering track (renewable-energy-sources, bioenergy-systems); this subject covers the environmental assessment and policy.
In the exams: GATE ES Section 9 (energy and environment); ICAR AICE subject 71 Unit 6 (whole unit); UGC NET Unit 5; BEE energy auditor exam
Energy is the largest source of greenhouse gas emissions and a major cause of air pollution, water stress, land change and waste, and it is also the input that powers irrigation, fertiliser manufacture, machinery, processing and cold chains. This subject studies energy systems through their environmental footprints and the transition now under way: the world's energy resources, the impacts of fossil, renewable, nuclear and other sources across their life cycles, the water–energy–food nexus, the energy embodied in agriculture and food, energy efficiency and auditing, the competition and synergies between solar power and farmland, the sustainability of bioenergy, decarbonisation pathways for farms, and the social justice of the transition.
The engineering of solar, wind and bioenergy systems is taught in the Agricultural Engineering track (renewable-energy-sources and bioenergy-systems), and this manual refers to those subjects rather than repeating device design. Finance for the transition, carbon pricing and transition risk are covered in Climate Finance, Climate Risk & Sustainable Investment and in Carbon Markets & Offsets; energy scenarios such as the IEA's are discussed in Climate Models, Scenarios & Downscaling. Here the focus is environmental assessment, efficiency practice and policy.
Examiners treat the subject at several levels. GATE Environmental Science Section 9 and UGC NET Environmental Sciences Unit 5 ask about energy resources, conventional and non-conventional sources and their impacts, and simple energy calculations. ICAR AICE Environmental Sciences tests energy and environment as a whole unit, including bioenergy and agricultural energy use. India's Bureau of Energy Efficiency (BEE) National Certification Examination for Energy Managers and Energy Auditors tests energy management, auditing, and the performance of thermal and electrical equipment. Numerical work in this manual (capacity factors, emission intensities, pump power, payback, energy ratios) is shown step by step, and country-specific programmes are given with India as the worked example.
Energy is measured in joules (J), with large quantities in exajoules (1 EJ = 10¹⁸ J); electricity in kilowatt-hours (1 kWh = 3.6 MJ); and fuels often in tonnes of oil equivalent (1 toe = 41.868 GJ by the IEA convention). Power is the rate of energy flow, in watts. Confusing capacity (power, kW or MW) with generation (energy, kWh or MWh) is the most common error in energy questions. Primary energy is energy as found in nature: crude oil, natural gas, coal, uranium, biomass, falling water, wind and sunlight. It is converted to secondary forms such as electricity, refined fuels and heat, and delivered as final energy to users, who obtain useful energy as motion, light or heat. Losses occur at every step; a thermal power station typically converts only about a third to a little under half of its fuel energy to electricity, depending on technology. Geologists distinguish resources, the total amount of a mineral or fuel thought to exist, from reserves, the part that is known and economically and technically recoverable today. The McKelvey diagram classifies deposits by geological certainty (identified to undiscovered) and economic feasibility. Proved reserves (often called 1P) have a high probability of recovery; probable and possible reserves add less certain volumes. Reserves change as prices, technology and exploration change, which is why estimates of 'years left' have repeatedly been revised upward. The reserves-to-production ratio (R/P), reserves divided by annual production, gives the number of years reserves would last at current production; it is a snapshot, not a forecast of exhaustion.
Fossil fuels have supplied around 80 per cent of the world's primary energy for decades, although renewables have grown rapidly since the 2010s. Oil dominates transport; coal remains the largest single source of electricity globally and dominates electricity in several Asian countries, including India and China; natural gas supplies electricity, heat, industry and fertiliser feedstock. Global R/P ratios are of the order of decades for oil and gas and more than a century for coal, though they vary by region, and the climate constraint is now regarded as more binding than resource depletion: as the climate system science subject explains, burning even proved reserves would exceed the carbon budgets for the Paris goals. Hubbert's peak theory, developed by M. King Hubbert in the 1950s, predicts that production from a region follows a bell-shaped curve, peaking when roughly half of the recoverable resource has been extracted. It predicted the peak of conventional oil production in the lower 48 United States around 1970; later, unconventional production from shale changed the picture. Energy return on investment (EROI), the ratio of energy delivered to energy invested in obtaining it, has declined for many fossil sources as easier deposits are depleted. Renewable resources are flows rather than stocks: the solar energy reaching the Earth exceeds human energy use by several thousand times, and technical potentials for wind and solar are many times current demand. Their constraints are cost, variability, land and materials, grid integration and storage, not resource size. Critical minerals for the transition, such as lithium, cobalt, nickel, copper and rare earth elements, raise new questions of supply concentration, mining impacts and recycling.
Every energy source has environmental impacts across its life cycle: extraction, processing, transport, construction, operation and decommissioning. Fossil fuels cause the largest impacts. Combustion releases carbon dioxide, the main driver of climate change; coal is the most carbon-intensive fuel per unit of energy, followed by oil and natural gas. Combustion also releases sulphur dioxide and nitrogen oxides, which cause acid deposition and form secondary particles and ozone; fine particulate matter (PM2.5), the leading environmental cause of premature death worldwide; and mercury and other trace metals from coal. Coal mining disturbs land, causes subsidence and fires and contaminates water through acid mine drainage, and coal combustion produces large volumes of fly ash and bottom ash; India's coal, with high ash content, makes fly ash management and use in cement, bricks and road fill a major issue. Oil and gas production causes spills, gas flaring and leaks of methane, a potent greenhouse gas; methane leakage can erode much of the climate advantage of gas over coal. Pollution control technologies reduce some of these impacts: electrostatic precipitators and fabric filters capture particles, flue gas desulphurisation (wet limestone scrubbing) removes sulphur dioxide, low-NOx burners and selective catalytic reduction reduce nitrogen oxides, and carbon capture and storage (CCS) can capture carbon dioxide at a cost and with an energy penalty. These are covered in the air pollution subject; here the point is that they add cost and do not address all impacts.
Renewable sources have low operational emissions but are not impact-free. Large hydropower floods land, displaces people, traps sediment, blocks fish migration and alters river flows; reservoirs in the tropics can emit significant methane from decomposing organic matter; the World Commission on Dams (2000) set out a framework for assessing such projects. Wind turbines cause bird and bat collisions at some sites, noise and visual impacts, and need materials including steel, concrete and, for some designs, rare earth magnets. Solar photovoltaics require land for utility-scale plants, energy-intensive silicon purification, and materials such as silver; panels reach end of life after about 25–30 years, creating a growing waste stream that needs recycling; panel cleaning uses water in dusty regions. Bioenergy's impacts are examined in a later chapter. Life-cycle assessment allows fair comparison. The IPCC Fifth Assessment Report (Working Group III, Annex III) compiled median life-cycle greenhouse gas emissions per kilowatt-hour of electricity: about 820 g CO2-e for coal, 490 g for natural gas combined cycle, 230 g for dedicated biomass, 48 g for utility-scale solar PV, 41 g for rooftop PV, 38 g for geothermal, 27 g for concentrated solar power, 24 g for hydropower, 12 g for nuclear and offshore wind, and 11 g for onshore wind. These are medians with wide ranges depending on technology, location and methods, and values for solar have fallen since as manufacturing has become more efficient. Annual generation from a plant is capacity × capacity factor × 8760 hours. Capacity factors differ widely: solar PV typically achieves about 15–25 per cent, onshore wind about 25–40 per cent, and nuclear or baseload coal often 60–90 per cent, depending on location and operation. A 10 MW solar plant with a 20 per cent capacity factor produces about 17,520 MWh a year. Variability of wind and solar requires flexibility from storage, demand response, grid interconnection and dispatchable generation.
Energy, water and food systems are tightly linked, and the water–energy–food nexus approach, popularised at the 2011 Bonn conference on the nexus, studies them together. Energy needs water: thermal power plants, whether coal, gas or nuclear, need cooling water, and once-through cooling withdraws large volumes while recirculating (cooling tower) systems withdraw less but consume more by evaporation; dry cooling uses little water but reduces efficiency. Hydropower depends on river flows that climate change and irrigation alter, and droughts have forced cuts in hydro and thermal generation in several countries. Fuel extraction, including coal washing and hydraulic fracturing, uses and contaminates water, and biofuel crops need irrigation in many regions. Water needs energy: pumping groundwater, lifting canal water, desalination and wastewater treatment use large amounts of electricity. In parts of South Asia, subsidised or free electricity for agricultural pumps has encouraged over-extraction of groundwater, lowering water tables, raising pumping energy further, and straining utility finances, a vicious circle that links energy policy to aquifer depletion. Feeder separation, which supplies farm pumps through separate rural feeders with rationed hours, and solar pumps with appropriate safeguards are among the responses. Food needs both: irrigation, fertiliser, machinery, processing and refrigeration use energy, and agriculture uses about 70 per cent of global freshwater withdrawals. Decisions in one sector shift burdens to others: biofuel mandates raise demand for land and water; cheap power for pumps depletes aquifers; hydropower dams alter downstream fisheries and floodplain agriculture.
Energy is the main source of air pollution as well as of greenhouse gases. Household burning of solid fuels, wood, crop residues, dung and coal, for cooking and heating causes severe indoor air pollution, with the World Health Organization attributing millions of premature deaths each year to household air pollution, mostly among women and children in low- and middle-income countries. Outdoor air pollution from power plants, vehicles, industry and agricultural residue burning causes further deaths and crop damage, as the impacts subject notes for ozone. Land use differs greatly among energy sources. Measured as land occupied per unit of energy delivered, the life-cycle land footprint is smallest for nuclear and natural gas, larger for coal (including mining), larger again for ground-mounted solar and wind (although wind farms can share land with farming), and largest for bioenergy crops and hydropower reservoirs. Rooftop solar, floating solar on reservoirs and canal-top solar avoid new land use. The nexus approach seeks such synergies: agrivoltaics that produce food and power from the same land, solar pumps that reduce diesel use, and biogas from manure that provides fuel and fertiliser while reducing methane. Climate change also feeds back on energy systems: higher temperatures raise cooling demand, reduce the efficiency of thermal plants and solar panels, and warm cooling water; droughts cut hydropower; extreme events damage grids. Energy planning therefore needs climate projections, as in the climate models subject, and adaptation as well as mitigation.
Agriculture uses energy directly and indirectly. Direct energy includes diesel for tractors, harvesters and pumps, electricity for irrigation pumps, processing and lighting, and fuels for drying grain, heating greenhouses and livestock buildings. Indirect energy is embodied in inputs manufactured off the farm: above all nitrogen fertiliser, whose synthesis by the Haber–Bosch process uses large amounts of natural gas (or coal) as both feedstock and fuel and accounts for roughly 1–2 per cent of global energy use; and also phosphorus and potassium fertilisers, pesticides, machinery, steel and plastics. In intensive cereal systems, nitrogen fertiliser is often the largest single energy input. Beyond the farm, the whole food system, including processing, packaging, transport, storage, retail, cooking and waste, uses far more energy. FAO's 'Energy-Smart Food for People and Climate' (2011) estimated that the food sector accounts for about 30 per cent of the world's energy consumption, with the share used after the farm gate largest in high-income countries and cooking a large share in low-income countries. Food loss and waste therefore also waste the energy embodied in it. Energy use in farming differs widely between systems. Mechanised, irrigated, high-input systems use much more energy per hectare but may use less per tonne of output than low-yield systems. Manual and animal-powered farming in many smallholder systems uses little fossil energy but relies on human labour, much of it women's, and has low labour productivity; mechanisation and electrification can relieve drudgery if appropriately designed.
Energy budgeting of crops counts energy inputs by multiplying the quantity of each input by an energy equivalent, such as megajoules per litre of diesel, per kilogram of nitrogen, per hour of human labour or per kilowatt-hour of electricity, and compares the total with the energy content of the output. Energy equivalents vary between studies, depending on whether manufacture, transport and machinery are included, so comparisons are valid only when the same coefficients are used. Common indicators are the energy use efficiency or energy ratio (output energy ÷ input energy), energy productivity (output in kg per MJ of input), specific energy (MJ of input per kg of output) and net energy (output minus input). For example, if a wheat crop uses 15,000 MJ ha⁻¹ of inputs and yields grain and straw with an energy content of 120,000 MJ ha⁻¹, its energy ratio is 8.0. Grain crops typically have energy ratios well above 1, while greenhouse vegetables and livestock products, especially those fed with grain, usually have ratios below 1, because animals convert feed energy to food energy inefficiently. The main ways to reduce agricultural energy use include improving nitrogen use efficiency (so less fertiliser is needed), using legumes and manures, conservation tillage that cuts tractor passes and diesel, efficient irrigation pumps and water-saving irrigation, solar pumps and dryers, efficient cold chains, and reducing food loss. The energy embodied in fertiliser is one reason that fertiliser prices rise sharply when natural gas prices spike, transmitting energy shocks to food prices, as seen in 2021–2022.
Energy efficiency means delivering the same service with less energy. It is often the cheapest way to cut emissions and costs, and the IEA calls it the 'first fuel'. Energy management is the systematic practice of monitoring and improving energy performance. ISO 50001 (first published in 2011 and revised in 2018) specifies requirements for an energy management system based on the plan–do–check–act cycle: an energy policy, an energy review identifying significant energy uses, baselines and energy performance indicators, objectives and action plans, operational control, monitoring and measurement, and management review. An energy audit is the verification, monitoring and analysis of energy use, with recommendations for improvement and an action plan. A preliminary or walk-through audit uses existing data and a site visit to identify obvious savings and to decide whether a detailed audit is needed. A detailed (comprehensive) audit measures energy use by equipment and process, establishes an energy balance, quantifies savings and costs for each measure, and prioritises them. Audits use portable instruments such as power analysers, flow meters, flue gas analysers, thermal imagers, lux meters and tachometers. A central indicator is specific energy consumption (SEC), energy used per unit of output, such as kWh per tonne of product or per cubic metre of water pumped. Benchmarking compares SEC with similar facilities or best practice. Measures are appraised by simple payback period (investment ÷ annual savings), net present value and internal rate of return; a measure costing 300,000 that saves 100,000 a year has a simple payback of three years. In India, the Energy Conservation Act 2001 created the Bureau of Energy Efficiency (BEE), which runs the Standards and Labelling (star rating) programme for appliances, the Energy Conservation Building Code, and the Perform, Achieve and Trade (PAT) scheme, launched in 2012, which sets SEC reduction targets for designated energy-intensive industries and lets those who exceed targets sell energy saving certificates. The Energy Conservation (Amendment) Act 2022 enabled a national carbon credit trading scheme. BEE certifies energy managers and auditors through its national examination.
Electric motors drive pumps, fans, compressors and machines and account for a large share of electricity use. Motor efficiency classes are defined by the IEC (IE1 standard, IE2 high, IE3 premium, IE4 super premium efficiency). Oversized motors running at low load are inefficient; variable speed drives match motor speed to demand and save large amounts of energy on pumps and fans, because power varies roughly with the cube of speed for centrifugal loads (the affinity laws). Power factor correction with capacitors reduces reactive power, apparent power (kVA) and line losses: apparent power = real power ÷ power factor, so a 90 kW load at a power factor of 0.75 draws 120 kVA, falling to 100 kVA at 0.90. Irrigation pumps are a major efficiency opportunity in agriculture. The hydraulic power delivered by a pump is ρgQH, where ρ is water density (1000 kg m⁻³), g is 9.81 m s⁻², Q is flow (m³ s⁻¹) and H is total head (m). Lifting 0.02 m³ s⁻¹ through 30 m requires 5.9 kW of hydraulic power; with a pump efficiency of 50 per cent and a motor efficiency of 85 per cent, electrical input is about 13.9 kW. Many farm pumpsets in South Asia operate at low efficiency because of poor selection, worn impellers, undersized pipes, foot valves with high losses and low-quality motors; agricultural demand-side management programmes replace them with star-rated pumps. Reducing friction losses in pipes and avoiding throttling also save energy. In thermal systems, such as boilers, dryers and furnaces, efficiency is improved by optimising excess air (measured with flue gas analysis), recovering waste heat, insulating pipes and vessels, maintaining steam traps and returning condensate. Cold storage efficiency depends on insulation, refrigerant choice, compressor efficiency and door management. Efficiency gains can be partly offset by the rebound effect, in which lower costs lead to greater use; estimates vary by sector, but efficiency usually still reduces total energy use.
Utility-scale solar photovoltaics need land: ground-mounted plants occupy of the order of one to two hectares per megawatt, depending on module efficiency, spacing, tracking and latitude. Flat, sunny, accessible land near grids is often good farmland, so rapid solar expansion can compete with agriculture, raise land rents and provoke local opposition. Planning frameworks therefore steer solar towards rooftops, degraded or barren land, reservoirs (floating solar), canal tops and land with low agricultural value, and towards combined uses. Agrivoltaics (agrophotovoltaics or dual-use solar) combines crop or livestock production with solar generation on the same land. Designs include elevated panels high enough for machinery, widely spaced vertical bifacial panels between crop strips, panels over greenhouses or orchards, and conventional plants grazed by sheep. Panels shade the crops below, which reduces yields of light-demanding crops but can benefit shade-tolerant or heat-sensitive crops such as some leafy vegetables, berries and fruits, especially in hot, dry climates, by lowering leaf temperature and evaporation. Panels can also protect crops from hail and heavy rain and allow rainwater harvesting from the panel surfaces; crops in turn cool the panels slightly, improving their efficiency. The land equivalent ratio (LER), borrowed from intercropping research, measures land-use efficiency: LER = (crop yield under agrivoltaics ÷ crop yield on separate land) + (electricity from agrivoltaics ÷ electricity from a conventional solar plant on the same area). For example, if the crop reaches 80 per cent of its open-field yield and the system produces 70 per cent of the electricity of a conventional plant, LER = 0.80 + 0.70 = 1.5, meaning that separate systems would need 50 per cent more land. Field studies have reported LER values above 1 for many combinations, but results depend strongly on crop, climate and design.
Countries have begun to define agrivoltaics in regulation, setting minimum agricultural yields or panel heights so that solar developers cannot claim 'dual use' while farming only token crops. Japan, France, Germany, Italy and parts of the United States have such provisions, with differing requirements. In India, the PM-KUSUM scheme, launched in 2019, supports solar energy in agriculture through three components: Component A for small decentralised grid-connected solar plants on barren or fallow land, including farmers' land; Component B for standalone off-grid solar pumps replacing diesel pumps; and Component C for solarisation of existing grid-connected agricultural pumps, with farmers able to sell surplus power to the grid. Solar pumps cut diesel costs and emissions and can provide reliable daytime power. However, because the marginal cost of pumping falls almost to zero, they can encourage more groundwater extraction unless combined with incentives to sell surplus power, metering, water budgets or micro-irrigation, which is why grid-connected solarisation with buy-back of surplus power is often preferred where aquifers are stressed. Trade-offs in solar–agriculture interactions include higher capital costs for elevated agrivoltaic structures, constraints on machinery, the distribution of benefits between landowners, tenants and agricultural labourers, effects on landscape and biodiversity (solar parks can support pollinator habitat if managed for it), and soil compaction during construction. Agrivoltaics is not a universal solution; it fits best where water and heat stress limit crops and where crops tolerate partial shade. Assessments should compare it with alternatives such as rooftop solar and siting on degraded land, using criteria of food production, energy output, farmer income, water and biodiversity.
Bioenergy is energy from biomass: wood and residues burned for heat and power, biogas from anaerobic digestion of manure, residues and wastes, and liquid biofuels for transport. First-generation biofuels are made from food crops: ethanol from sugarcane, maize or sugar beet, and biodiesel from vegetable oils such as rapeseed, soybean and palm. Second-generation (advanced) biofuels use lignocellulosic biomass, such as crop residues, wood waste and dedicated energy grasses, through biochemical or thermochemical routes. Third-generation biofuels from algae remain largely experimental. Biogas and compressed biomethane from agricultural wastes are often the most sustainable options because they use waste and reduce methane from manure. The engineering of these conversion systems is covered in the bioenergy-systems subject of the Agricultural Engineering track. Traditional biomass, the burning of wood, charcoal, dung and residues in inefficient stoves, still supplies energy for cooking to well over two billion people; it is renewable only where harvesting is sustainable, and it causes severe household air pollution. Modern bioenergy can reduce emissions if it displaces fossil fuels without causing land-use change, but its benefits depend on feedstock, supply chain and counterfactual.
Bioenergy competes for land, water and nutrients. Rapid expansion of crop-based biofuels in the United States, the EU and Brazil in the 2000s contributed, with other factors such as oil prices, weather and export restrictions, to the food price spikes of 2007–2008, though the size of the biofuel contribution remains debated. This 'food versus fuel' concern led to caps and reforms. Carbon debt arises when natural ecosystems are cleared to grow bioenergy feedstock. Fargione and colleagues (2008) estimated that converting rainforests, peatlands, savannas or grasslands to produce food-crop biofuels in Brazil, Southeast Asia and the United States releases 17 to 420 times more carbon dioxide than the annual greenhouse gas reductions the biofuels provide by displacing fossil fuels; peatland conversion for palm oil was the worst case. The carbon payback time is the carbon debt divided by the annual savings: clearing land that releases 600 t CO2 ha⁻¹ for a biofuel that saves 6 t CO2 ha⁻¹ yr⁻¹ takes 100 years to repay. Indirect land-use change (ILUC) occurs when bioenergy crops displace food production onto other land, which is then cleared elsewhere; Searchinger and colleagues (2008) argued that including ILUC could make some maize ethanol worse than petrol for decades. ILUC cannot be measured directly and is modelled, with large uncertainty. Policies respond with sustainability criteria. The EU Renewable Energy Directive sets minimum greenhouse gas savings for biofuels relative to fossil fuels, excludes feedstock from land with high carbon stock or high biodiversity, caps the contribution of food and feed crops, and phases out high-ILUC-risk feedstock. India's National Policy on Biofuels (2018, amended 2022) brought forward the target of 20 per cent ethanol blending in petrol to 2025–26 and promotes ethanol from sugarcane juice, molasses, damaged and surplus grains and residues, and compressed biogas. Large-scale bioenergy with carbon capture and storage (BECCS), relied on in many mitigation pathways as the climate models subject notes, raises the same land, water and biodiversity concerns at even greater scale.
Nuclear power plants generate heat by fission of uranium-235 (or plutonium) in a reactor, usually moderated and cooled by water, to raise steam for turbines. Their life-cycle greenhouse gas emissions are among the lowest of any source, and they produce firm, high-capacity-factor power on a small land area, which is why many decarbonisation scenarios include them. India's three-stage nuclear programme, conceived by Homi Bhabha, envisages pressurised heavy water reactors using natural uranium, then fast breeder reactors, and finally thorium-based reactors, reflecting India's large thorium reserves. The environmental and social issues are distinctive. Uranium mining and milling produce tailings with radioactive and toxic residues. Spent fuel and other high-level radioactive waste remain hazardous for thousands of years; most is stored in cooling pools and dry casks at reactor sites, and deep geological disposal is the international consensus solution, with Finland's Onkalo repository the first under construction. Severe accidents, notably Chernobyl (1986) and Fukushima Daiichi (2011), released radioactivity, forced large evacuations and caused long-term contamination and social disruption; the International Nuclear and Radiological Event Scale rates both at its maximum level 7. Other issues are thermal pollution of cooling water, high capital costs and long construction times, decommissioning, and the risk of proliferation of nuclear weapons materials, which the International Atomic Energy Agency safeguards system addresses. Public acceptance varies greatly by country. Small modular reactors are being developed to reduce costs and construction times, but their economics are not yet proven at scale.
Geothermal energy uses heat from the Earth's interior, accessed where hot rock or water lies near the surface, typically in volcanic and tectonically active regions such as Iceland, New Zealand, Kenya, Indonesia, the Philippines and the western United States. High-temperature resources generate electricity, and lower-temperature resources provide direct heat for buildings, greenhouses, aquaculture and crop drying; ground-source heat pumps, which use shallow ground as a heat source or sink, work almost anywhere. Geothermal power has low life-cycle emissions and high capacity factors, but some fields release carbon dioxide and hydrogen sulphide (with its rotten-egg smell and toxicity) from geothermal fluids; brines can contain arsenic, boron and mercury and must be reinjected or treated; extraction and reinjection can cause land subsidence and induced seismicity, a particular concern for enhanced geothermal systems that fracture hot dry rock. Over-exploitation can deplete a reservoir's heat or pressure. Tidal energy exploits the rise and fall of tides or tidal currents. Tidal barrage schemes dam an estuary and generate power as water flows through turbines; La Rance in France, operating since 1966 with 240 MW, and Sihwa Lake in South Korea, at 254 MW the world's largest, are the main examples. Barrages alter estuarine hydrology, sediment transport, water quality and the intertidal habitats used by migratory birds and fish, which is why few have been built. Tidal stream turbines, placed in fast currents like underwater wind turbines, have smaller footprints, but their effects on marine mammals and fish, through collision and noise, are still being studied. Wave energy is at an earlier stage. These sources are predictable but site-limited and, so far, expensive. As with all sources, environmental impact assessment, discussed in its own subject, is the main tool for evaluating and mitigating these impacts at project level, and strategic assessment helps choose among options at the planning level.
Decarbonising farm energy follows the general pattern of energy transitions: use less energy, electrify what can be electrified, supply the electricity and heat from low-carbon sources, and use low-carbon fuels where electrification is not feasible. On farms, efficiency comes first: efficient pumps and motors, variable speed drives, pipe and valve improvements, micro-irrigation that applies less water, insulation and efficient lighting in livestock buildings and greenhouses, and conservation tillage that cuts diesel use. Electrification options include solar or grid-electric irrigation pumps replacing diesel pumps, electric and hybrid tractors and small machines (an emerging technology, viable first for light tasks and in orchards and greenhouses), heat pumps for greenhouses, dairy and buildings, and electric milking, chilling and processing equipment. Low-carbon supply options include on-farm solar and wind, biogas from manure for heat, power or vehicle fuel, and biomass boilers using residues. Indirect energy emissions, chiefly from fertiliser manufacture, can be cut by improving nitrogen use efficiency, using legumes and organic sources, and in future by fertiliser made with green ammonia, produced by electrolytic hydrogen from renewable electricity instead of natural gas. Non-energy emissions from agriculture, such as methane from livestock and rice and nitrous oxide from soils, are larger than energy emissions in most farming systems and are covered in the agricultural greenhouse gas mitigation subject; a complete farm decarbonisation plan addresses both. The emission savings from switching a diesel pump to solar can be estimated from fuel saved times the diesel emission factor (about 2.68 kg CO2 per litre combusted, the IPCC default range for diesel oil): a pump that used 1000 litres of diesel a year saves about 2.7 t CO2 a year in operation. Replacing grid electricity requires the grid emission factor, which varies by country and year and is published by national authorities, such as India's Central Electricity Authority CO2 baseline database.
Farm and sector decarbonisation pathways are built with marginal abatement cost curves (MACCs), which rank mitigation measures by cost per tonne of CO2-e avoided and show their abatement potential. Measures with negative cost, such as many efficiency improvements, save money over their life; others need carbon prices, subsidies or regulation. MACCs are useful screening tools but hide barriers such as upfront capital, information, split incentives between landowners and tenants, and behavioural factors, and they treat measures as independent when they interact. Barriers to electrification on farms include weak or unreliable rural grids, high upfront costs of solar systems and electric machinery, limited service networks, and the seasonal, peaky nature of farm loads. Enablers include feeder separation and grid upgrades, time-of-day tariffs that reward daytime solar-matched use, finance and leasing models, pay-as-you-go solar, cooperatives and service providers who rent equipment, and productive-use-of-energy programmes that link electricity with income-generating activities such as cold storage, milling and processing. Decentralised renewable energy for productive uses, such as solar cold rooms, solar dryers, solar milling and solar-powered irrigation, can reduce post-harvest losses and raise incomes in off-grid areas while avoiding diesel. Their sustainability depends on maintenance, local ownership, market links and fitting the technology to the crop and season. National pathways for agriculture, such as those in nationally determined contributions and long-term strategies, increasingly include farm energy, but energy is usually a small part of agricultural emissions, so policies must not overlook the larger non-energy sources.
A just transition means moving to a low-carbon economy in a way that is fair and inclusive, creating decent work and leaving no one behind. The idea originated in the labour movement and is now part of international climate policy: the Paris Agreement's preamble refers to the imperatives of a just transition of the workforce and the creation of decent work and quality jobs, and the International Labour Organization's Guidelines for a Just Transition (2015) set out principles including social dialogue among governments, employers and workers, respect for labour rights, social protection, skills development and active labour market policies, and policy coherence. Justice in transitions has several dimensions. Distributional justice concerns who bears the costs and receives the benefits: workers and communities dependent on coal mining or fossil power; consumers facing higher energy prices; and those who gain new jobs and cheaper renewable power. Procedural justice concerns who participates in decisions, including affected workers, Indigenous peoples and local communities. Recognition justice concerns whose identities, knowledge and needs are acknowledged. Restorative justice concerns repairing past harms, such as mine land rehabilitation. Coal regions face the sharpest challenges. In India, coal mining is concentrated in states such as Jharkhand, Chhattisgarh and Odisha, where coal supports employment, public revenue and the railways, and where many livelihoods depend informally on the coal economy. Just Energy Transition Partnerships (JETPs), launched with South Africa at COP26 in 2021 and later agreed with Indonesia and Vietnam, pair international public and private finance with national plans to retire coal and support affected workers and regions.
Justice also concerns energy access. Despite progress, hundreds of millions of people still lack electricity, mostly in sub-Saharan Africa, and over two billion lack clean cooking, relying on polluting fuels; Sustainable Development Goal 7 aims for universal access to affordable, reliable, sustainable and modern energy by 2030. Programmes such as India's Pradhan Mantri Ujjwala Yojana (2016), which provides LPG connections to poor households, increased access to clean cooking fuel, though sustained use depends on refill affordability. Transition policies can be regressive if not designed carefully. Carbon taxes and fossil fuel subsidy removal raise prices for poor households and farmers who spend a larger share of income on energy; revenue recycling, targeted cash transfers and protecting essential uses can offset this. Subsidies for farm electricity and diesel, while socially popular, often benefit larger farmers most and encourage waste and groundwater depletion; reforming them requires alternatives such as direct benefit transfers, solar pumps with buy-back, and support for efficient equipment. For rural areas, the transition brings both risks and opportunities: land demand for solar, wind and bioenergy can raise rents and displace tenants and pastoralists, or it can provide new income through leasing, community ownership and agrivoltaics; decentralised renewables can power productive uses and reduce costs; and new jobs in installation and maintenance can arise if skills programmes reach rural youth and women. Free, prior and informed consent and benefit-sharing are essential where projects affect Indigenous and local communities. The social dimensions of transitions, including those for farmers and rural workers more broadly, are covered in the sustainability transitions and governance subject.
A student uses this subject to master the energy units and the numerical core (capacity factor, life-cycle emission intensities, R/P ratios, energy ratios of crops, pump power and payback), the impacts of each energy source, the arguments about bioenergy carbon debt and ILUC, and India's energy-efficiency institutions and schemes. Exams frequently confuse capacity with generation, reserves with resources, and first- with second-generation biofuels, so these distinctions repay careful study.
A five-year practitioner, such as an energy auditor, a sustainability officer in an agri-food company, an EIA consultant or a programme officer for solar irrigation, uses the subject to audit farm and factory energy use, compute specific energy consumption and savings, appraise efficiency and solar projects, assess the sustainability of bioenergy feedstocks against criteria such as the EU RED, design agrivoltaic or solar-pump schemes that do not deplete groundwater, and estimate emission savings with appropriate emission factors.
A senior professional with 25 years' experience, such as a utility planner, a state energy secretary, an agri-business head of operations or a development-bank energy lead, uses the subject to set energy and biofuel policies and targets, reform farm power subsidies, plan coal-region transitions with workers and communities, decide where to site solar without harming agriculture, and integrate energy, water and food planning. Such a person weighs the cost, reliability, environmental and social dimensions of each option.
A researcher with a PhD and 25 years' experience works on life-cycle assessment of energy systems, land and water footprints of renewables, agrivoltaic system design and crop responses, bioenergy sustainability and ILUC modelling, the water–energy–food nexus, energy access and justice, and pathways for decarbonising food systems. Such a researcher contributes to national energy plans and international assessments and trains interdisciplinary teams.
Energy statistics, costs and capacity factors change every year; this manual gives ranges and points to the Energy Institute Statistical Review and IEA reports for current figures, and gives no national energy balances. Life-cycle emission values are AR5 medians, which have since fallen for solar and wind. Device-level engineering of solar, wind, biogas and biofuel systems is left to the Agricultural Engineering track, and finance and carbon pricing to their own subjects. Energy equivalents for crop energy budgeting are not tabulated because published coefficients differ widely. National programme details (PM-KUSUM components and subsidies, biofuel blending targets, PAT cycles) change with policy revisions and should be checked against current guidelines. Hydrogen, storage technologies and grid integration are mentioned only briefly.
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.
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