Glossary

Every technical term used in the course, defined in one line, with a link back to the module that teaches it. Terms are added as modules publish — 160 so far.

A

Aerofoil: A wing-shaped cross-section that generates lift when air flows over it. Every turbine blade is one.Taught in Module 8: Wind Power
Alternating current (AC): Current that reverses direction rhythmically — 100 reversals a second on India's 50 Hz grid. What comes out of your wall socket.Taught in Module 2: Electricity From Scratch
Ampere (A): The unit of electric current — about 6.24 billion billion electrons passing a point every second.Taught in Module 2: Electricity From Scratch
Anode: The battery electrode that releases electrons to the external circuit while discharging.Taught in Module 11: Storage & EVs
AT&C losses: Aggregate Technical & Commercial losses: power lost as heat in wires and transformers, plus power delivered but never billed or paid for — theft, meter tampering, or simple non-payment.Taught in Module 12: Energy Economics & Policy

B

Bagasse: The fibrous pulp left after crushing sugarcane. Sugar mills burn it to power themselves and often export the surplus.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Band gap: The minimum energy a photon must carry to free an electron in a semiconductor. Silicon's is about 1.1 electronvolts.Taught in Module 7: Solar Power
Base load: The steady, round-the-clock floor of electricity demand, traditionally met by large plants that run continuously.Taught in Module 3: Coal & Thermal Plants
Battery degradation: The gradual permanent loss of a battery's capacity as its chemistry wears out over charge cycles and time.Taught in Module 11: Storage & EVs
Battery swapping: Exchanging a depleted battery for a charged one in minutes, instead of waiting to recharge.Taught in Module 11: Storage & EVs
BESS: Battery energy storage system — grid-scale batteries that charge on surplus and discharge at peak.Taught in Module 11: Storage & EVs
Betz limit: The proven ceiling on wind turbines: no rotor can extract more than 59.3% of the wind's kinetic energy.Taught in Module 8: Wind Power
Biomass: Plant or animal matter burned or digested for energy — crop residue, wood, bagasse, animal waste.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Black-start: The ability of a plant to start up with no electricity from the grid — essential for restarting a collapsed grid.Taught in Module 5: Hydroelectric Power
Blue hydrogen: Grey hydrogen whose CO₂ is captured and stored rather than released — cleaner in principle, dependent on capture actually working.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Boiler: The furnace-heated vessel where water is turned into high-pressure steam.Taught in Module 3: Coal & Thermal Plants
Brayton cycle: The compress–burn–expand cycle a gas turbine runs on, as distinct from the steam plant's Rankine cycle.Taught in Module 4: Gas, Diesel & Combined Cycle

C

Capacity factor: The share of its theoretical maximum output a plant actually delivers over a year. Solar's is low because the sun sets.Taught in Module 7: Solar Power
Capital expenditure (capex): The upfront cost of building a power plant — land, equipment, construction — before it has generated a single unit.Taught in Module 12: Energy Economics & Policy
Carbon neutral: Releasing no net CO₂ over a full cycle — as when a plant absorbs while growing exactly what it releases when burned.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Carbon sink: Anything that absorbs more carbon dioxide than it releases — forests and soils being the main natural ones.Taught in Module 13: Climate & the Energy Transition
Carnot limit: The absolute ceiling on any heat engine's efficiency, set only by its hottest and coldest temperatures. No design can beat it.Taught in Module 3: Coal & Thermal Plants
Cascading failure: A chain reaction in which one component tripping overloads the next, collapsing an entire grid within seconds.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Cathode: The battery electrode that receives electrons from the external circuit while discharging.Taught in Module 11: Storage & EVs
Chain reaction: A self-sustaining sequence in which neutrons from one fission trigger the next.Taught in Module 6: Nuclear Power
Chemical energy: Energy stored in the bonds between atoms — in food, petrol, coal and batteries.Taught in Module 1: What Even Is Energy?
Circuit: A complete, unbroken loop for current to flow around. Break the loop anywhere and everything stops.Taught in Module 2: Electricity From Scratch
Cogeneration: Producing useful heat and electricity from the same fuel in one plant, instead of throwing the heat away.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Combined cycle: Running a gas turbine and a steam turbine together, using the first one's hot exhaust to boil water for the second.Taught in Module 4: Gas, Diesel & Combined Cycle
Compressed air storage: Storing energy by pumping air into a sealed cavern under pressure and releasing it through a turbine later.Taught in Module 11: Storage & EVs
Concentrated solar power (CSP): Using mirrors to concentrate sunlight into heat, then running a conventional steam turbine — solar that does spin a magnet.Taught in Module 7: Solar Power
Condenser: The heat exchanger that turns spent steam back into water, creating a vacuum that pulls more steam through the turbine.Taught in Module 3: Coal & Thermal Plants
Conductor: A material that lets current flow easily — metals, mainly, because they have loose electrons to spare.Taught in Module 2: Electricity From Scratch
Conservation of energy: The law that energy is never created or destroyed — only converted from one form to another.Taught in Module 1: What Even Is Energy?
Control rod: A neutron-absorbing rod pushed into the core to slow the chain reaction, or withdrawn to speed it up.Taught in Module 6: Nuclear Power
Cooling tower: The hyperbolic tower that dumps the condenser's waste heat to the air. Its plume is condensed water vapour, not smoke.Taught in Module 3: Coal & Thermal Plants
Criticality: The balance point where each fission causes exactly one more — a chain reaction that is steady rather than growing or dying.Taught in Module 6: Nuclear Power
Cross-subsidy: Charging one group of consumers more than their supply costs so another group can be charged less — industry overpaying so agriculture and homes underpay, in India's case.Taught in Module 12: Energy Economics & Policy
Cut-in speed: The wind speed at which a turbine first produces useful power — typically around 3 metres per second.Taught in Module 8: Wind Power

D

Day-ahead market: A power exchange segment where electricity for each hour of the next day is bought and sold today, in one auction.Taught in Module 12: Energy Economics & Policy
Diesel genset: A diesel engine coupled to a generator, used as backup power when the grid supply fails.Taught in Module 4: Gas, Diesel & Combined Cycle
Direct current (DC): Current that flows steadily in one direction — what batteries, solar panels and USB chargers deal in.Taught in Module 2: Electricity From Scratch
DISCOM: A distribution company — the utility that buys electricity, delivers it to your home and sends your bill.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Dispatch: The moment-by-moment instruction telling each plant how much to generate so supply matches demand.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Distribution: The final, lower-voltage network that delivers electricity from substations to homes and businesses.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Duck curve: The duck-shaped net-load curve created when solar floods the grid at midday and vanishes at the evening peak.Taught in Module 7: Solar Power

E

Efficiency: The fraction of input energy that comes out as useful output. The rest leaks away — almost always as heat.Taught in Module 1: What Even Is Energy?
Electric charge: The property of matter that electricity is made of. Electrons carry negative charge; pushing them along a wire is an electric current.Taught in Module 2: Electricity From Scratch
Electric current: The flow of electric charge, measured in amperes. In the water-pipe picture: how many litres per second are flowing.Taught in Module 2: Electricity From Scratch
Electrical energy: Energy carried by electric charges moving through wires — the most convenient form ever found.Taught in Module 1: What Even Is Energy?
Electrolysis: Splitting water into hydrogen and oxygen using electricity. The route to green hydrogen.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Electrolyte: The medium inside a cell that lets ions pass between electrodes while blocking electrons, forcing them through your device.Taught in Module 11: Storage & EVs
Electromagnetic induction: Faraday's discovery: a changing magnetic field near a conductor creates a voltage. The working principle of every generator.Taught in Module 2: Electricity From Scratch
Electron: The tiny negatively charged particle in every atom's outer shells. The moving crowd behind every electric current in a wire.Taught in Module 2: Electricity From Scratch
Electrostatic precipitator (ESP): A device that electrically charges ash particles in flue gas so they stick to collector plates instead of reaching the air.Taught in Module 3: Coal & Thermal Plants
Emissions intensity: Emissions per unit of economic output (GDP), distinct from total emissions — a country can cut intensity while total emissions still rise if the economy grows faster.Taught in Module 13: Climate & the Energy Transition
Energy: The capacity to make something happen — to move, heat, light or change things. Measured in joules.Taught in Module 1: What Even Is Energy?
Energy carrier: Something that stores and moves energy but is not a source of it — electricity and hydrogen are both carriers.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Energy density: How much energy is packed into each kilogram of a fuel or storage device.Taught in Module 1: What Even Is Energy?
Energy poverty: Lacking reliable, affordable access to modern energy — a broader and harder problem than simply having a wire reach your house.Taught in Module 12: Energy Economics & Policy

F

Fast breeder reactor: A reactor that creates more fissile fuel than it consumes, and the crucial second stage of India's thorium plan.Taught in Module 6: Nuclear Power
Fission: Splitting a heavy nucleus into lighter fragments, releasing heat and spare neutrons.Taught in Module 6: Nuclear Power
Flue gas: The hot exhaust from burning fuel — mostly nitrogen and CO₂, plus pollutants — that leaves via the chimney.Taught in Module 3: Coal & Thermal Plants
Flue-gas desulphurisation (FGD): Scrubbing equipment that removes sulphur dioxide from flue gas, usually by spraying it with limestone slurry.Taught in Module 3: Coal & Thermal Plants
Fly ash: The fine mineral ash carried up with flue gas; captured by precipitators and used to make cement and bricks.Taught in Module 3: Coal & Thermal Plants
Flywheel storage: Storing energy in a heavy spinning rotor — very fast to respond, but only for seconds to minutes.Taught in Module 11: Storage & EVs
Francis turbine: The most common hydro turbine: water spirals inward through fixed and moving blades. Suits medium head.Taught in Module 5: Hydroelectric Power
Frequency (Hz): How many times per second the alternating current completes a full back-and-forth cycle. India's grid runs at 50 hertz.Taught in Module 2: Electricity From Scratch
Fuel cell: A device that combines hydrogen and oxygen to produce electricity and water directly, without combustion.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Fuse: A deliberately thin wire that melts and breaks the circuit when too much current flows — sacrificing itself before the wiring catches fire.Taught in Module 2: Electricity From Scratch
Fusion: Joining light nuclei to release energy — the Sun's method, and still an unsolved engineering problem on Earth.Taught in Module 6: Nuclear Power

G

Gas turbine: An engine that compresses air, burns fuel in it, and lets the hot gas blast through a turbine — a jet engine bolted to the ground.Taught in Module 4: Gas, Diesel & Combined Cycle
Generator: A machine that converts spinning motion into electricity by moving magnets past coils of wire (or coils past magnets).Taught in Module 2: Electricity From Scratch
Geothermal power: Using the Earth's internal heat to make steam and spin a turbine. Abundant only where hot rock lies close to the surface.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Gravity storage: Lifting a heavy mass to store energy and lowering it to recover it — pumped hydro's idea, using solids instead of water.Taught in Module 11: Storage & EVs
Green hydrogen: Hydrogen made by electrolysing water using renewable electricity, so its production emits essentially no CO₂.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Greenhouse effect: The process by which certain gases let sunlight through but trap the heat it creates, warming the planet's surface.Taught in Module 13: Climate & the Energy Transition
Greenhouse gas: A gas that absorbs and re-emits infrared heat, trapping it near Earth's surface — chiefly CO₂, methane and water vapour.Taught in Module 13: Climate & the Energy Transition
Grey hydrogen: Hydrogen made from natural gas by steam reforming, releasing CO₂. Almost all hydrogen used today is grey or similar.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Grid inertia: The stabilising effect of heavy spinning generators, whose stored rotational energy resists sudden frequency change.Taught in Module 10: The Grid: Civilisation's Biggest Machine

H

Head: The vertical height water falls through in a hydro plant. More head means more energy per litre.Taught in Module 5: Hydroelectric Power
Heat recovery steam generator (HRSG): The boiler that captures a gas turbine's exhaust heat to raise steam for a second turbine.Taught in Module 4: Gas, Diesel & Combined Cycle
Heavy water: Water made with deuterium instead of ordinary hydrogen. It slows neutrons while absorbing very few, letting reactors run on natural uranium.Taught in Module 6: Nuclear Power

I

Insulator: A material that blocks current — rubber, plastic, glass, dry wood. The reason wires have plastic skins.Taught in Module 2: Electricity From Scratch
Inverter: The box that converts solar panels' direct current into the alternating current the grid uses.Taught in Module 7: Solar Power
Isotope: Atoms of the same element with different numbers of neutrons — like uranium-235 and uranium-238.Taught in Module 6: Nuclear Power

J

Joule (J): The SI unit of energy. Lifting a small apple up by one metre takes about one joule.Taught in Module 1: What Even Is Energy?
Just transition: Shifting away from fossil fuels in a way that protects the workers and communities whose livelihoods depend on them.Taught in Module 13: Climate & the Energy Transition

K

Kaplan turbine: A propeller-like turbine with adjustable blades, built for low head and very large flows.Taught in Module 5: Hydroelectric Power
Kilowatt-hour (kWh): The energy a 1,000-watt appliance uses in one hour — the "unit" on Indian electricity bills. Equal to 3.6 million joules.Taught in Module 1: What Even Is Energy?
Kinetic energy: The energy of motion — anything that moves has it, from a cricket ball to the wind.Taught in Module 1: What Even Is Energy?

L

Learning curve: The consistent pattern where a technology's cost falls by a roughly fixed percentage every time cumulative production doubles.Taught in Module 12: Energy Economics & Policy
Levelised cost of electricity (LCOE): The total lifetime cost of building and running a power plant, divided by the total electricity it produces — the standard way to compare very different technologies on one scale.Taught in Module 12: Energy Economics & Policy
Lifecycle emissions: The total greenhouse gas emissions of a power source across its whole life — building it, running it and decommissioning it, not just burning fuel.Taught in Module 13: Climate & the Energy Transition
Lift: The sideways force produced by air flowing over an aerofoil — what pulls a turbine blade round, rather than the wind pushing it.Taught in Module 8: Wind Power
Lithium-ion battery: The rechargeable cell behind phones, EVs and grid storage, working by shuttling lithium ions between two electrodes.Taught in Module 11: Storage & EVs
Load curve: A graph of electricity demand across a day or year, showing when people actually use power.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Load shedding: Deliberately disconnecting some consumers to save the whole system when supply cannot meet demand.Taught in Module 10: The Grid: Civilisation's Biggest Machine

M

Market clearing price: The single price at which all matched buy and sell bids in an auction settle, found where supply and demand curves cross.Taught in Module 12: Energy Economics & Policy
Moderator: A substance that slows fast neutrons down so they can cause further fissions. India's reactors use heavy water.Taught in Module 6: Nuclear Power
Must-run status: A rule requiring certain plants (often renewables) to keep generating whenever available, rather than being told to switch off.Taught in Module 12: Energy Economics & Policy

N

Nationally Determined Contribution (NDC): A country's formally submitted climate pledge under the Paris Agreement, stating its emissions targets.Taught in Module 13: Climate & the Energy Transition
Negative electricity price: A price below zero, where generators pay buyers to take their power — happens when shutting down costs more than giving electricity away.Taught in Module 12: Energy Economics & Policy
Net metering: Billing that lets a rooftop solar owner export surplus power to the grid and be credited for it.Taught in Module 7: Solar Power
Net zero: A state where any remaining greenhouse gas emissions are balanced by an equal amount removed from the atmosphere, so the net addition is nil.Taught in Module 13: Climate & the Energy Transition
Nuclear energy: Energy stored inside the nucleus of an atom, released by splitting heavy nuclei (fission) or joining light ones (fusion).Taught in Module 1: What Even Is Energy?
Nucleus: The dense core of an atom, made of protons and neutrons, where nuclear energy is stored.Taught in Module 6: Nuclear Power

O

Offshore wind: Turbines built in the sea, where wind is stronger and steadier but construction and maintenance cost far more.Taught in Module 8: Wind Power
Ohm (Ω): The unit of resistance. One volt pushes one ampere through one ohm.Taught in Module 2: Electricity From Scratch

P

p–n junction: The boundary between two differently doped layers of silicon, whose built-in electric field forces freed electrons to travel one way.Taught in Module 7: Solar Power
Peak load: The highest demand of the day, typically in the Indian evening, which the whole system must be built to meet.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Peaker plant: A plant that runs only during demand peaks — expensive per unit, but able to start in minutes.Taught in Module 4: Gas, Diesel & Combined Cycle
Pelton turbine: A wheel of spoon-shaped buckets struck by high-speed water jets — the choice for very high head and low flow.Taught in Module 5: Hydroelectric Power
Penstock: The large steep pipe that carries water from the reservoir down to the turbine.Taught in Module 5: Hydroelectric Power
Perovskite: A family of cheap, easily made crystal materials promising very high solar efficiency, held back so far by durability.Taught in Module 7: Solar Power
Photovoltaic effect: Light knocking electrons loose in a semiconductor so that they flow as current — electricity with no moving parts.Taught in Module 7: Solar Power
PHWR: Pressurised Heavy Water Reactor — India's indigenous reactor design and the backbone of its nuclear fleet.Taught in Module 6: Nuclear Power
Plant load factor (PLF): How much a plant actually generated over a period, as a share of what it could have generated running flat out.Taught in Module 4: Gas, Diesel & Combined Cycle
Potential energy: Stored energy due to position or state — like water held high behind a dam, waiting to fall.Taught in Module 1: What Even Is Energy?
Power: How fast energy is used or produced. Measured in watts: 1 watt = 1 joule per second.Taught in Module 1: What Even Is Energy?
Power exchange: A marketplace where electricity is bought and sold for delivery in the near future, with prices set by supply and demand every few minutes.Taught in Module 12: Energy Economics & Policy
Power purchase agreement (PPA): A long-term contract locking in a price between a generator and a buyer, usually for 15–25 years — how most Indian electricity is actually sold.Taught in Module 12: Energy Economics & Policy
Pulverised coal: Coal ground to a talcum-fine powder so it burns almost instantly when blown into the furnace.Taught in Module 3: Coal & Thermal Plants
Pumped storage: Pumping water uphill when power is cheap and releasing it through turbines when power is dear. The world's dominant grid battery.Taught in Module 5: Hydroelectric Power

R

Radiant energy: Energy travelling as light and other electromagnetic waves. Sunlight is Earth's great delivery of it.Taught in Module 1: What Even Is Energy?
Ramp rate: How fast a plant can change its output, in megawatts per minute. The measure of how flexible it is.Taught in Module 4: Gas, Diesel & Combined Cycle
Rankine cycle: The boil–expand–condense–pump loop that every steam power plant runs on, named after engineer William Rankine.Taught in Module 3: Coal & Thermal Plants
Reservoir: The artificial lake behind a dam — simultaneously a water store and an energy store.Taught in Module 5: Hydroelectric Power
Resistance: How much a material fights the flow of current, measured in ohms. The narrowness of the pipe.Taught in Module 2: Electricity From Scratch
RoCoF: Rate of change of frequency — how fast the grid's frequency is moving. High RoCoF means little inertia and little time to react.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Round-trip efficiency: The share of stored energy you get back out again. Every storage technology loses some on the round trip.Taught in Module 11: Storage & EVs
Run-of-river: A hydro plant that uses a river's natural flow with little or no storage reservoir.Taught in Module 5: Hydroelectric Power

S

Semiconductor: A material such as silicon that conducts electricity only under the right conditions — the basis of all solar cells and chips.Taught in Module 7: Solar Power
Sievert (Sv): The unit of radiation dose as it affects human tissue. Everyday doses are measured in millisieverts or microsieverts.Taught in Module 6: Nuclear Power
Smart meter: A meter that records consumption in intervals and communicates it automatically, enabling time-varying tariffs.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Sodium-ion battery: A cheaper battery chemistry using abundant sodium instead of lithium, at the cost of storing less per kilogram.Taught in Module 11: Storage & EVs
Spent fuel: Used reactor fuel, intensely radioactive at first, stored under water then in dry casks.Taught in Module 6: Nuclear Power
Steam turbine: A shaft lined with bladed wheels that spins as steam expands through it, converting heat energy into rotation.Taught in Module 3: Coal & Thermal Plants
Substation: A node where voltage is transformed and power is switched and routed between parts of the network.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Supercritical boiler: A boiler running above water's critical pressure, where liquid and steam stop being distinct — hotter steam, higher efficiency.Taught in Module 3: Coal & Thermal Plants
Superheated steam: Steam heated well past its boiling point, so it carries more energy and stays dry through the turbine.Taught in Module 3: Coal & Thermal Plants
Swanson's law: The observation that solar module prices have fallen by about 20% for every doubling of cumulative global production since the 1970s.Taught in Module 12: Energy Economics & Policy
Swept area: The circle a turbine's blades trace out. Doubling blade length quadruples it, and so quadruples the power captured.Taught in Module 8: Wind Power
Synchronous generator: A conventional spinning generator locked in step with the grid's frequency — the source of grid inertia.Taught in Module 10: The Grid: Civilisation's Biggest Machine

T

Thermal efficiency: The share of a fuel's heat energy that a plant actually converts into electricity.Taught in Module 3: Coal & Thermal Plants
Thermal energy: The energy of atoms and molecules jiggling about randomly — what we feel as heat.Taught in Module 1: What Even Is Energy?
Thermal runaway: The self-feeding overheating that makes a damaged lithium-ion cell catch fire: heat drives reactions that make more heat.Taught in Module 11: Storage & EVs
Three-phase power: Electricity delivered as three synchronised alternating currents, offset by a third of a cycle each — smooth, efficient, and why transmission towers carry wires in threes.Taught in Module 2: Electricity From Scratch
Tidal power: Generating electricity from the rise and fall or flow of tides — utterly predictable, but brutally expensive to build.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Time-of-day tariff: Charging different prices at different hours, to nudge consumption away from the expensive evening peak.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Transformer: A device that uses induction to raise or lower AC voltage almost losslessly. The reason the grid can span a continent.Taught in Module 2: Electricity From Scratch
Transmission: Moving bulk electricity long distances at very high voltage, on the tall towers between regions.Taught in Module 10: The Grid: Civilisation's Biggest Machine
Transmission loss: Energy lost as heat while electricity travels through wires — minimised by using very high voltage.Taught in Module 10: The Grid: Civilisation's Biggest Machine

U

UDAY (Ujwal DISCOM Assurance Yojana): A 2015 scheme where state governments took over most DISCOM debt in exchange for reform commitments — it eased the debt but did not fix the underlying tariff gap.Taught in Module 12: Energy Economics & Policy

V

Vehicle-to-grid (V2G): Letting a parked electric vehicle feed power back into the grid, turning a car park into a battery.Taught in Module 11: Storage & EVs
Volt (V): The unit of voltage. A torch cell provides 1.5 V; an Indian wall socket about 230 V.Taught in Module 2: Electricity From Scratch
Voltage: The electrical "pressure" that pushes current around a circuit, measured in volts. No voltage difference, no flow.Taught in Module 2: Electricity From Scratch

W

Waste-to-energy: Burning municipal solid waste to generate electricity, reducing landfill volume in the process.Taught in Module 9: Biomass, Geothermal, Tidal & Hydrogen
Watt (W): The SI unit of power: a flow of one joule of energy per second.Taught in Module 1: What Even Is Energy?
Well-to-wheel: Counting a vehicle's emissions across the whole chain — from producing the fuel or electricity to turning the wheels.Taught in Module 11: Storage & EVs