Basic Energy Engineering interview questions

150 questions with answers (ENE-001 to ENE-150). Try each one aloud before reading the answer.

Energy Fundamentals & Thermodynamics

ENE-001 · What is the difference between renewable and non-renewable energy?

Renewable sources are replenished naturally on human timescales, such as solar, wind, hydro and biomass. Non-renewable sources are finite stocks that take geological time to form, such as coal, oil, natural gas and uranium.

ENE-002 · What is the difference between kW and kWh?

kW is a unit of power, the rate of energy transfer. kWh is a unit of energy, equal to 1 kW sustained for one hour (3.6 MJ). A 2 kW heater running for 3 hours uses 6 kWh.

ENE-003 · What is energy efficiency?

The ratio of useful energy output to energy input, η = E_out ÷ E_in, usually expressed as a percentage. The rest of the input is lost, mostly as low-grade heat.

ENE-004 · What is capacity factor?

Actual energy produced over a period divided by the energy the plant would produce at rated power for the whole period: CF = E_actual ÷ (P_rated × t). It reflects resource availability, outages and curtailment.

ENE-005 · State the first law of thermodynamics.

Energy is conserved: it cannot be created or destroyed, only converted. For a closed system, ΔU = Q − W, where Q is heat added and W is work done by the system.

ENE-006 · State the second law of thermodynamics.

Heat cannot spontaneously flow from a colder to a hotter body, and no heat engine operating in a cycle can convert all heat input into work. The entropy of an isolated system never decreases.

ENE-007 · What is the difference between primary, secondary and final energy?

Primary energy is found in nature (crude oil, coal, sunlight). Secondary energy is a converted carrier (electricity, petrol, hydrogen). Final energy is what reaches the consumer's meter or tank before end-use conversion.

ENE-008 · What is Carnot efficiency?

The maximum possible efficiency of a heat engine between two reservoirs: η = 1 − T_c ÷ T_h, with temperatures in kelvin. It sets the upper bound for any real thermal power cycle.

ENE-009 · What is enthalpy?

A thermodynamic property H = U + pV, representing internal energy plus flow work. In steady-flow devices such as boilers and turbines, heat and work are calculated from enthalpy differences, in kJ/kg.

ENE-010 · What is entropy?

A state property measuring the dispersal of energy or the degree of disorder, defined by dS = δQ_rev ÷ T (kJ/kg·K). Real, irreversible processes generate entropy, which represents lost work potential.

ENE-011 · What is exergy?

The maximum useful work obtainable from a system as it comes to equilibrium with its surroundings (the dead state). Unlike energy, exergy is destroyed by irreversibilities, so it shows the quality of energy.

ENE-012 · What is the difference between HHV and LHV of a fuel?

Higher heating value includes the latent heat recovered by condensing water vapour in the flue gas. Lower heating value assumes the water leaves as vapour. HHV is always greater than LHV for hydrogen-containing fuels.

ENE-013 · What are common energy unit conversions?

1 kWh = 3.6 MJ, 1 BTU ≈ 1055 J, 1 kcal ≈ 4.187 kJ, 1 therm = 100,000 BTU, and 1 tonne of oil equivalent (toe) ≈ 41.87 GJ.

Thermal Power Plants

ENE-014 · What is a thermal power plant?

A plant that converts the chemical energy of fuel (coal, gas, oil, biomass) into heat, then into mechanical work through a steam or gas turbine, and finally into electricity through a generator.

ENE-015 · What is the Rankine cycle?

The ideal steam power cycle with four processes: isentropic pumping of water, constant-pressure heat addition in the boiler, isentropic expansion in the turbine, and constant-pressure heat rejection in the condenser.

ENE-016 · What is the function of a boiler?

It transfers heat from burning fuel to feedwater, converting it to high-pressure steam. It contains the furnace, water walls, drum (in drum-type units), superheaters, reheaters and economiser.

ENE-017 · Why is a condenser used in a steam power plant?

It condenses turbine exhaust steam at low pressure (vacuum), which increases the enthalpy drop across the turbine and efficiency, and recovers pure condensate for reuse as feedwater.

ENE-018 · What is a superheater?

A heat exchanger that raises steam temperature above saturation at constant pressure. Superheated steam increases cycle efficiency and reduces moisture and blade erosion in the last turbine stages.

ENE-019 · What is an economiser?

A heat exchanger in the flue gas path that preheats boiler feedwater using exhaust heat, improving boiler efficiency and reducing thermal stress on the drum.

ENE-020 · What is an air preheater?

A heat exchanger that uses low-temperature flue gas to warm combustion air, improving combustion and boiler efficiency. Common types are regenerative (Ljungström) and tubular.

ENE-021 · What is the role of a cooling tower?

It rejects condenser heat to the atmosphere, mainly by evaporating a small part of the circulating water. Types include natural-draught (hyperbolic) and mechanical-draught towers.

ENE-022 · What is the difference between impulse and reaction turbines?

In an impulse turbine the entire pressure drop occurs in fixed nozzles, and the blades only change the jet's direction. In a reaction turbine the pressure drops across both fixed and moving blades, which produce thrust as steam accelerates.

ENE-023 · What is the Brayton cycle?

The ideal gas turbine cycle: isentropic compression, constant-pressure heat addition in the combustor, isentropic expansion in the turbine, and constant-pressure heat rejection. Its efficiency rises with pressure ratio.

ENE-024 · What is a combined cycle power plant?

A plant in which hot gas turbine exhaust (about 500–650 °C) raises steam in a heat recovery steam generator to drive a steam turbine. Modern CCGTs reach about 55–62% efficiency (LHV).

ENE-025 · What is fly ash and how is it captured?

Fine mineral residue from pulverised coal combustion carried in the flue gas. It is captured by electrostatic precipitators or fabric (bag) filters and reused in cement and bricks.

ENE-026 · What is cogeneration (CHP)?

Simultaneous production of electricity and useful heat from one fuel source. Using the rejected heat raises total fuel utilisation to roughly 70–90%, compared with 35–45% for power-only plants.

Solar Photovoltaics

ENE-027 · How does a solar PV cell work?

Through the photovoltaic effect: photons with energy above the band gap create electron–hole pairs, and the built-in electric field of the PN junction separates them, driving current through an external load.

ENE-028 · What is the difference between monocrystalline, polycrystalline and thin-film modules?

Monocrystalline cells are cut from a single crystal and offer the highest efficiency. Polycrystalline cells are cast from multiple crystals and are cheaper but less efficient. Thin-film types (CdTe, CIGS, a-Si) use thin deposited layers, cost less per area and are less efficient.

ENE-029 · What are Voc and Isc?

Voc is the open-circuit voltage, the maximum voltage at zero current. Isc is the short-circuit current, the maximum current at zero voltage. Isc is roughly proportional to irradiance.

ENE-030 · What are Standard Test Conditions (STC) for PV modules?

Irradiance of 1000 W/m², cell temperature of 25 °C and an air mass 1.5 spectrum. Module nameplate ratings (Wp) are measured under STC.

ENE-031 · What is the difference between a cell, module, string and array?

A cell is a single PV device. A module is many cells encapsulated together. A string is modules connected in series to build voltage. An array is several strings in parallel to build current and power.

ENE-032 · What does an inverter do in a PV system?

It converts the DC output of the PV array into AC at grid voltage and frequency. Grid-tied inverters also perform MPPT, synchronisation and anti-islanding protection.

ENE-033 · What is a bypass diode?

A diode connected in antiparallel across a group of cells. When those cells are shaded, it lets the string current bypass them, limiting power loss and preventing hot spots.

ENE-034 · What is the band gap of a semiconductor and its value for silicon?

The energy difference between the valence and conduction bands that a photon must supply to create a free electron. Crystalline silicon has a band gap of about 1.12 eV at room temperature.

ENE-035 · What is the difference between grid-tied, off-grid and hybrid PV systems?

Grid-tied systems feed the utility grid and usually have no battery. Off-grid systems rely on batteries and serve isolated loads. Hybrid systems combine grid connection, batteries and sometimes a generator.

ENE-036 · What is the difference between irradiance and irradiation?

Irradiance is instantaneous solar power per unit area (W/m²). Irradiation, or insolation, is energy per unit area over time (kWh/m²/day). Daily irradiation in kWh/m² equals "peak sun hours".

ENE-037 · What is net metering?

A billing arrangement in which a bidirectional meter records both import and export, and the consumer pays only for net consumption. Exported solar energy offsets imported energy at an agreed tariff.

ENE-038 · What tilt angle is usually chosen for fixed PV panels?

For maximum annual yield, a tilt roughly equal to the site latitude, facing the equator. A lower tilt favours summer output and a steeper tilt favours winter output.

ENE-039 · What is air mass (AM) in solar energy?

The relative path length of sunlight through the atmosphere compared with the overhead path, AM ≈ 1 ÷ cos θz. AM0 is outside the atmosphere, AM1 is the sun at zenith, and AM1.5 (θz ≈ 48°) is the standard test spectrum.

Solar Thermal

ENE-040 · What is solar thermal energy?

Use of solar radiation to produce heat, either directly for water heating, drying and space heating, or at high temperatures through concentrators to generate steam for electricity.

ENE-041 · What are the main parts of a flat-plate collector?

A dark absorber plate with attached fluid tubes, a transparent glazing cover, back and side insulation, and a casing. The glazing creates a greenhouse effect that reduces heat loss.

ENE-042 · What is an evacuated tube collector?

A collector in which each absorber sits inside a glass tube with a vacuum between walls. The vacuum nearly eliminates conduction and convection losses, so it performs better in cold or windy conditions.

ENE-043 · How does a thermosiphon solar water heater work?

Water heated in the collector becomes less dense and rises naturally into a storage tank mounted above it, while cooler water sinks back to the collector. No pump is needed.

ENE-044 · What is concentrated solar power (CSP)?

Technology that uses mirrors or lenses to concentrate direct sunlight onto a receiver, heating a fluid to high temperature to drive a steam or gas turbine. It can include thermal storage.

ENE-045 · What is a parabolic trough collector?

A long, curved mirror that focuses sunlight onto a receiver tube along its focal line. A heat-transfer fluid, typically synthetic oil, is heated to about 390 °C. The trough tracks the sun on one axis.

ENE-046 · What is a solar power tower?

A central receiver system where a field of sun-tracking flat mirrors (heliostats) reflects sunlight onto a receiver at the top of a tower. Molten salt or steam reaches about 565 °C.

ENE-047 · What is a parabolic dish system?

A dish-shaped concentrator that tracks the sun on two axes and focuses light onto a receiver at its focal point, often driving a Stirling engine. It achieves very high concentration ratios.

ENE-048 · What is a linear Fresnel reflector?

A CSP system using long rows of flat or slightly curved mirrors that focus sunlight onto a fixed elevated receiver tube. It is cheaper and simpler than a trough but less optically efficient.

ENE-049 · What is concentration ratio?

The ratio of collector aperture area to receiver area, or of flux at the receiver to incident flux. Troughs reach roughly 70–100 suns, towers and dishes several hundred to over 1000.

ENE-050 · What is a selective surface coating?

An absorber coating with high absorptance (above about 0.9) for short-wave solar radiation and low emittance for long-wave infrared, so it absorbs sunlight while radiating little heat.

ENE-051 · What is the difference between DNI, DHI and GHI?

DNI is direct normal irradiance from the sun's disc. DHI is diffuse horizontal irradiance from the sky. GHI is the total on a horizontal surface: GHI = DNI × cos θz + DHI. CSP uses only DNI.

Wind Energy

ENE-052 · How does a wind turbine generate electricity?

Wind exerts aerodynamic lift on the blades, turning the rotor. The rotor drives a generator, directly or through a gearbox, converting the wind's kinetic energy into electricity.

ENE-053 · What is the difference between HAWT and VAWT?

A horizontal-axis turbine has its rotor shaft parallel to the wind, needs yaw control and is the dominant utility design. A vertical-axis turbine (Darrieus, Savonius) accepts wind from any direction but has lower efficiency.

ENE-054 · What are the main components of a HAWT?

Blades and hub (rotor), main shaft, gearbox (if geared), generator, nacelle, yaw drive, pitch system, tower, foundation, power converter and controller.

ENE-055 · What are cut-in, rated and cut-out wind speeds?

Cut-in speed (typically about 3 m/s) is where generation starts. Rated speed (about 11–13 m/s) is where rated power is first reached. Cut-out speed (about 25 m/s) is where the turbine shuts down for safety.

ENE-056 · What is a wind turbine power curve?

A plot of electrical output against hub-height wind speed. It shows zero output below cut-in, a cubic-like rise to rated power, a flat region to cut-out, then zero.

ENE-057 · What is a yaw system?

A drive and bearing assembly that rotates the nacelle so the rotor faces into the wind, using a wind vane signal. It maximises energy capture and reduces asymmetric loads.

ENE-058 · What is blade pitch control?

Rotating the blades about their long axis to change the angle of attack. It limits power above rated speed, aids start-up, and feathers the blades for braking during high winds or faults.

ENE-059 · What is tip speed ratio?

The ratio of blade tip speed to free-stream wind speed: λ = ωR ÷ v. Each rotor has an optimum λ for maximum Cp, typically about 6–8 for modern three-bladed turbines.

ENE-060 · What is the difference between onshore and offshore wind?

Onshore wind is cheaper to build and maintain. Offshore wind has stronger, steadier winds, allows larger turbines and has fewer land constraints, but costs more for foundations, cables and maintenance.

ENE-061 · What is the wake effect in a wind farm?

Downstream turbines see slower, more turbulent air in the wake of upstream turbines, reducing their output and raising fatigue loads. Turbines are spaced several rotor diameters apart to limit it.

ENE-062 · Why do most large wind turbines have three blades?

Three blades balance aerodynamic efficiency, cost and dynamic stability. They give smoother torque and lower yaw-induced vibration than two blades, and more blades add cost with little gain in Cp.

ENE-063 · What is wind shear?

The increase in wind speed with height above ground. It is often estimated with the power law v₂ = v₁ (h₂ ÷ h₁)^α, where α ≈ 1/7 for open terrain.

ENE-064 · What is the power coefficient Cp?

The fraction of the wind's kinetic power that the rotor extracts: Cp = P_rotor ÷ (½ρAv³). Its theoretical maximum is the Betz limit of 0.593. Modern rotors reach about 0.45–0.50.

Hydropower

ENE-065 · How does a hydropower plant work?

Water stored at height flows through a penstock to a turbine, converting potential energy into kinetic energy and then shaft work. The turbine drives a generator before the water discharges through the tailrace.

ENE-066 · What is the formula for hydropower output?

P = ρ g Q H η, where ρ is water density (1000 kg/m³), g is 9.81 m/s², Q is flow (m³/s), H is net head (m) and η is overall efficiency. A useful shortcut is P (kW) ≈ 9.81 Q H η.

ENE-067 · How are hydro plants classified by head?

As high-head plants (above about 300 m), medium-head plants (about 30–300 m) and low-head plants (below about 30 m). The boundaries vary by source. Head largely determines turbine type.

ENE-068 · What is the difference between storage, run-of-river and pumped storage hydro?

Storage (impoundment) plants use a dam and reservoir to regulate flow. Run-of-river plants use natural flow with little storage. Pumped storage plants pump water uphill when power is cheap and generate during peaks.

ENE-069 · What is a Pelton turbine?

An impulse turbine with spoon-shaped buckets on a wheel, struck by high-velocity jets from nozzles. It suits high head and low flow. Flow is regulated by spear valves, and deflectors control load rejection.

ENE-070 · What is a Francis turbine?

A mixed-flow reaction turbine in which water enters radially through guide vanes and leaves axially. It suits medium heads and is the most widely used hydro turbine.

ENE-071 · What is a Kaplan turbine?

An axial-flow reaction turbine with adjustable runner blades and guide vanes. Double regulation keeps efficiency high over a wide flow range, which suits low head and high flow.

ENE-072 · What is a penstock?

A large pipe or tunnel that carries water under pressure from the forebay or reservoir to the turbine. It is designed for static head, water hammer pressure and minimal friction loss.

ENE-073 · What is the purpose of a surge tank?

An open standpipe or chamber between the headrace tunnel and penstock that absorbs pressure surges (water hammer) when the load suddenly changes, and supplies water during sudden load increases.

ENE-074 · What is a draft tube?

A gradually expanding tube at the exit of a reaction turbine. It lets the runner sit above tailwater level and recovers kinetic energy by converting it into pressure, increasing net head use.

ENE-075 · What are micro, mini and small hydro plants?

Size categories for hydro plants. Typical usage is micro below about 100 kW, mini from about 100 kW to 1–2 MW, and small up to about 10–25 MW. Exact limits depend on the country's policy definitions.

ENE-076 · What is the difference between gross head and net head?

Gross head is the elevation difference between headwater and tailwater levels. Net head is gross head minus hydraulic losses in the intake, penstock and fittings, and it is the head actually available to the turbine.

Biomass & Bioenergy

ENE-077 · What is biomass energy?

Energy derived from recently living organic matter such as wood, crop residues, animal manure and organic waste. It stores solar energy captured by photosynthesis and can be burned or converted into gaseous or liquid fuels.

ENE-078 · What are the main biomass conversion routes?

Thermochemical routes are combustion, gasification, pyrolysis and torrefaction. Biochemical routes are anaerobic digestion and fermentation. Chemical routes include transesterification to make biodiesel.

ENE-079 · What is biogas?

A gas produced by anaerobic digestion of organic matter. It is typically about 50–70% methane and 30–50% CO₂, with traces of H₂S and moisture.

ENE-080 · What is anaerobic digestion?

Microbial breakdown of organic matter without oxygen in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. It produces biogas and a nutrient-rich digestate.

ENE-081 · What is gasification?

Partial oxidation of biomass at high temperature (about 700–1000 °C) with limited air, oxygen or steam, producing producer gas or syngas, which is mainly CO, H₂, CH₄, CO₂ and N₂ (when air is used).

ENE-082 · What is pyrolysis?

Thermal decomposition of biomass in the absence of oxygen, yielding bio-oil, char and gas. Fast pyrolysis (around 500 °C, short residence) maximises bio-oil. Slow pyrolysis maximises biochar.

ENE-083 · What is the difference between biodiesel and bioethanol?

Biodiesel is made by transesterifying vegetable oils or fats with methanol, giving fatty acid methyl esters as a diesel substitute. Bioethanol is made by fermenting sugars or starch and is blended with petrol.

ENE-084 · What are first-, second- and third-generation biofuels?

First-generation fuels come from food crops (sugarcane, corn, vegetable oil). Second-generation fuels come from non-food lignocellulosic residues and wastes. Third-generation fuels come from algae.

ENE-085 · What is torrefaction?

Mild pyrolysis at about 200–300 °C without oxygen that removes moisture and volatiles. It produces a dry, hydrophobic, grindable fuel with higher energy density, suitable for co-firing with coal.

ENE-086 · What is co-firing?

Burning biomass together with coal in an existing boiler, usually at a modest energy share. It cuts fossil CO₂ at low capital cost but needs attention to fuel handling, slagging and fouling.

ENE-087 · What is proximate analysis of a fuel?

Determination of moisture, volatile matter, fixed carbon and ash content by mass. It indicates combustion behaviour and ash handling needs.

ENE-088 · What is ultimate analysis of a fuel?

Determination of the elemental composition (C, H, N, S and O) plus ash and moisture. It is used to calculate air requirement, flue gas composition and heating value.

Nuclear Energy

ENE-089 · What is nuclear fission?

The splitting of a heavy nucleus such as U-235, after neutron absorption, into two lighter fragments plus 2–3 neutrons and about 200 MeV of energy, mostly as fragment kinetic energy.

ENE-090 · What is nuclear fusion?

The joining of light nuclei, such as deuterium and tritium, into a heavier nucleus with release of energy. D–T fusion gives 17.6 MeV per reaction. It requires very high temperatures to overcome Coulomb repulsion.

ENE-091 · What is a chain reaction?

A self-sustaining sequence in which neutrons released by fission cause further fissions. A reactor is critical when, on average, exactly one neutron per fission causes another fission (k = 1).

ENE-092 · What is the role of a moderator?

It slows fast fission neutrons to thermal energies, where U-235 has a much higher fission cross-section. Common moderators are light water, heavy water and graphite.

ENE-093 · What do control rods do?

They contain strong neutron absorbers such as boron, cadmium or hafnium. Inserting them reduces reactivity, and withdrawing them increases it. Rapid full insertion (scram) shuts down the reactor.

ENE-094 · What is the difference between a PWR and a BWR?

In a PWR, the primary water is kept at high pressure (about 15.5 MPa) so it does not boil, and steam is raised in separate steam generators. In a BWR, water boils in the core and the steam goes directly to the turbine.

ENE-095 · What is enriched uranium?

Uranium in which the U-235 fraction is raised above its natural 0.7%. Light-water reactors typically use about 3–5% U-235 (low-enriched uranium).

ENE-096 · What is a PHWR?

A pressurised heavy water reactor, such as CANDU, that uses heavy water as moderator and coolant. Its good neutron economy allows natural uranium fuel and on-power refuelling.

ENE-097 · What is a fast breeder reactor?

A reactor that uses fast, unmoderated neutrons and produces more fissile material (for example Pu-239 from U-238) than it consumes. It typically uses liquid sodium as coolant.

ENE-098 · What is half-life?

The time for half of the nuclei in a radioactive sample to decay. Activity falls as A = A₀ (½)^(t ÷ T½). For example, Cs-137 has a half-life of about 30 years.

ENE-099 · What is spent nuclear fuel?

Fuel removed from a reactor after use. It contains fission products, transuranic elements and remaining uranium, and it is highly radioactive and heat-generating. It is cooled in pools, then stored in dry casks or reprocessed.

ENE-100 · What is decay heat?

Heat produced by the radioactive decay of fission products after shutdown. It is about 6–7% of full power immediately after shutdown and falls over hours to days, so cooling must continue.

Energy Storage & Batteries

ENE-101 · Why is energy storage needed?

To balance supply and demand over time, firm variable renewables, shave peaks, provide frequency and voltage support, give backup power, and defer network upgrades.

ENE-102 · What are the main categories of energy storage?

Electrochemical (batteries, flow batteries), mechanical (pumped hydro, compressed air, flywheels), thermal (molten salt, ice, sensible and latent heat), chemical (hydrogen, synthetic fuels) and electrical (supercapacitors, SMES).

ENE-103 · What is the difference between energy capacity and power rating of storage?

Power rating (kW or MW) is the maximum charge or discharge rate. Energy capacity (kWh or MWh) is the total energy stored. Their ratio gives duration, for example 100 MW / 400 MWh is 4 hours.

ENE-104 · What is state of charge (SoC)?

The available charge as a percentage of current full capacity. 100% is full and 0% is empty. It is estimated by coulomb counting, voltage-based methods or model-based observers.

ENE-105 · What is depth of discharge (DoD)?

The fraction of capacity that has been discharged, DoD = 100% − SoC. Deeper cycling generally shortens cycle life, especially for lead-acid batteries.

ENE-106 · What is C-rate?

Charge or discharge current relative to capacity. 1C fully discharges the battery in 1 hour, so for a 100 Ah cell, 1C is 100 A and 0.5C is 50 A.

ENE-107 · What is round-trip efficiency?

Energy delivered on discharge divided by energy consumed during charging. Typical values are about 85–95% for Li-ion, 70–85% for pumped hydro and 70–80% for lead-acid.

ENE-108 · How does a lithium-ion battery work?

Lithium ions shuttle between a graphite anode and a metal-oxide or phosphate cathode through an electrolyte, while electrons flow through the external circuit. On charge, ions move into the anode; on discharge, they return.

ENE-109 · What is the difference between NMC and LFP chemistries?

NMC (nickel-manganese-cobalt) offers higher energy density and is common in EVs. LFP (lithium iron phosphate) has lower energy density but better thermal stability, longer cycle life and no cobalt, so it suits stationary storage.

ENE-110 · What is a flow battery?

A battery that stores energy in liquid electrolytes held in external tanks and pumped through a cell stack. Power depends on stack size and energy on tank volume. The vanadium redox flow battery is the common example.

ENE-111 · What is pumped hydro storage?

Water is pumped from a lower to an upper reservoir using surplus electricity and released through turbines when power is needed. It is the largest installed form of grid storage worldwide.

ENE-112 · What is a battery management system (BMS)?

Electronics that monitor cell voltage, current and temperature, estimate SoC and state of health, balance cells and protect against overcharge, over-discharge, overcurrent and over-temperature.

ENE-113 · What is a supercapacitor?

An electrochemical double-layer capacitor that stores charge electrostatically at a high-surface-area electrode. It has very high power density and cycle life but low energy density, so it suits short bursts.

Hydrogen & Fuel Cells

ENE-114 · Why is hydrogen called an energy carrier, not an energy source?

Free hydrogen does not occur naturally in useful quantities. It must be produced using another energy source, such as gas reforming or electrolysis, and then stores and delivers that energy.

ENE-115 · What is electrolysis of water?

Splitting water into hydrogen and oxygen using electricity: 2H₂O → 2H₂ + O₂. The theoretical minimum cell voltage is 1.23 V (thermoneutral 1.48 V). Practical cells run at about 1.8–2.0 V.

ENE-116 · What are the main electrolyser types?

Alkaline (AEL) uses KOH electrolyte and is mature and low cost. PEM uses a proton-exchange membrane and responds fast. Solid oxide (SOEC) runs at high temperature with high electrical efficiency. AEM is an emerging fourth type.

ENE-117 · What is steam methane reforming (SMR)?

The main industrial hydrogen route: CH₄ + H₂O → CO + 3H₂ over a nickel catalyst at about 700–1000 °C, followed by water-gas shift, CO + H₂O → CO₂ + H₂.

ENE-118 · What is a fuel cell?

An electrochemical device that converts the chemical energy of a fuel (usually hydrogen) and an oxidant (oxygen) directly into electricity, with water and heat as by-products. There is no combustion.

ENE-119 · How does a PEM fuel cell work?

At the anode, H₂ splits into protons and electrons. Protons cross the polymer membrane, electrons flow through the external circuit, and at the cathode they combine with O₂ to form water. It operates at about 60–80 °C.

ENE-120 · What are the main fuel cell types?

PEMFC, alkaline (AFC), phosphoric acid (PAFC), molten carbonate (MCFC) and solid oxide (SOFC). They differ in electrolyte, operating temperature, fuel flexibility and application.

ENE-121 · What is the energy content of hydrogen?

About 120 MJ/kg (LHV), or 142 MJ/kg (HHV), which is about 33.3 kWh/kg LHV. That is roughly three times petrol per kilogram, but its volumetric energy density is very low as a gas.

ENE-122 · How is hydrogen stored?

As compressed gas (typically 350 or 700 bar), as cryogenic liquid at −253 °C, in metal hydrides, or in chemical carriers such as ammonia and liquid organic hydrogen carriers (LOHC).

ENE-123 · What is the theoretical open-circuit voltage of a hydrogen fuel cell?

About 1.23 V at 25 °C, from E = −ΔG ÷ (nF), with ΔG = −237 kJ/mol and n = 2. Real cells show about 0.6–0.8 V under load due to losses.

ENE-124 · What is a fuel cell stack?

Many individual cells connected in series, separated by bipolar plates that distribute gases and conduct current, to reach a useful voltage and power output.

ENE-125 · What are the main applications of hydrogen?

Oil refining, ammonia and methanol production, steelmaking (direct reduction), fuel cell vehicles, long-duration storage, high-temperature industrial heat, and synthetic fuels for aviation and shipping.

Grid Integration & Smart Grids

ENE-126 · What are the main parts of a power system?

Generation, transmission (high voltage, long distance), distribution (medium and low voltage), and consumption (loads). They are linked by substations and transformers.

ENE-127 · What is a smart grid?

An electricity network with digital sensing, communication and control that supports two-way power and data flow, enabling demand response, distributed generation, automation and better reliability.

ENE-128 · What is distributed generation?

Small generators connected near loads, usually on the distribution network, such as rooftop PV, small wind, CHP and diesel gensets, rather than large central plants.

ENE-129 · What is a microgrid?

A local group of loads, generation and storage with defined boundaries that can operate connected to the main grid or disconnect and run as an island.

ENE-130 · What is demand response?

Changing consumer electricity use in response to price signals or utility requests, by shifting or reducing load during peaks to help balance the grid.

ENE-131 · What is advanced metering infrastructure (AMI)?

A system of smart meters, communication networks and data management that provides two-way interval metering, remote reading, outage detection and time-of-use billing.

ENE-132 · What is base load, intermediate load and peak load?

Base load is the minimum continuous demand. Intermediate load follows daily variation. Peak load is the highest short-term demand, met by fast, flexible plants or storage.

ENE-133 · What is a load curve?

A plot of demand versus time, usually over a day. Its area gives energy consumed, and its shape guides generation scheduling and plant selection.

ENE-134 · What is load factor?

Average load divided by peak load over a period. A high load factor means better use of installed capacity and lower cost per kWh.

ENE-135 · What is grid frequency and its standard values?

The rate at which AC voltage alternates, set by synchronous generator speed. Standard values are 50 Hz (India, Europe and most of the world) and 60 Hz (the Americas and parts of Asia).

ENE-136 · What is islanding?

A condition where a distributed generator keeps energising part of the network after it is disconnected from the main grid. Unintentional islanding is a safety hazard, so inverters must detect it and trip.

ENE-137 · What is curtailment?

Deliberately reducing renewable output below what is available, due to grid congestion, low demand, or stability limits. It represents lost zero-marginal-cost energy.

ENE-138 · What is the duck curve?

The net load shape (demand minus solar) in high-PV systems, which dips at midday and ramps steeply in the evening. It requires flexible resources to meet the evening ramp.

Energy Audit, Conservation, Economics & Policy

ENE-139 · What is an energy audit?

A systematic inspection and analysis of energy use in a facility to identify where energy is consumed and wasted, and to recommend cost-effective savings measures.

ENE-140 · What is the difference between a preliminary and a detailed energy audit?

A preliminary (walk-through) audit uses existing data and a site visit to find obvious savings quickly. A detailed audit uses measurements, energy balances and cost-benefit analysis of each measure.

ENE-141 · What is energy conservation?

Reducing energy use by avoiding waste or changing behaviour, such as switching off idle equipment. Energy efficiency means getting the same service with less energy through better technology.

ENE-142 · What is specific energy consumption (SEC)?

Energy used per unit of output, such as kWh per tonne of cement or GJ per tonne of steel. It is the main benchmark for comparing plant efficiency over time.

ENE-143 · What is power factor and why does it matter?

The ratio of real power to apparent power, PF = P ÷ S = cos φ for sinusoidal loads. Low PF raises current, losses and demand charges. It is corrected with capacitors.

ENE-144 · What is simple payback period?

Initial investment divided by annual net savings. For example, ₹5 lakh spent to save ₹2 lakh per year pays back in 2.5 years. It ignores the time value of money.

ENE-145 · What is the Energy Conservation Act, 2001 in India?

The Indian law that established the Bureau of Energy Efficiency (BEE) and provides for designated consumers, mandatory energy audits, energy managers, appliance labelling and building energy codes.

ENE-146 · What is the BEE star labelling programme?

A scheme in India that rates appliances from 1 to 5 stars by energy efficiency, with more stars being more efficient. It is mandatory for some appliances and voluntary for others.

ENE-147 · What is a carbon footprint?

Total greenhouse gas emissions caused directly and indirectly by an activity, product or organisation, expressed in tonnes of CO₂-equivalent.

ENE-148 · What is a time-of-day (time-of-use) tariff?

A tariff that charges different energy rates at different times, higher at peak and lower off-peak, to encourage consumers to shift load.

ENE-149 · What is a renewable purchase obligation (RPO)?

A regulatory mandate requiring distribution companies and certain large consumers to source a minimum share of their electricity from renewable sources, met by direct purchase or certificates.

ENE-150 · What are Scope 1, 2 and 3 emissions?

Scope 1 is direct emissions from owned sources. Scope 2 is indirect emissions from purchased electricity, heat or steam. Scope 3 is all other indirect emissions across the value chain.

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