Intermediate Energy Engineering interview questions

150 questions with answers (ENE-151 to ENE-300). Try each one aloud before reading the answer.

Energy Fundamentals & Thermodynamics

ENE-151 · What is the Carnot efficiency of an engine working between 600 °C and 30 °C?

Convert to kelvin: T_h = 873 K, T_c = 303 K. η = 1 − 303 ÷ 873 ≈ 0.653, or about 65%. Real cycles at these temperatures achieve much less because of irreversibilities.

ENE-152 · What is the steady-flow energy equation?

For a control volume at steady state: Q̇ − Ẇ = ṁ[(h₂ − h₁) + (v₂² − v₁²) ÷ 2 + g(z₂ − z₁)]. It is used to analyse turbines, compressors, nozzles, boilers and heat exchangers.

ENE-153 · Why is second-law (exergy) efficiency more informative than first-law efficiency?

First-law efficiency treats all energy as equal. Exergy efficiency compares actual useful work with the maximum possible. An electric heater is about 100% first-law efficient but has low exergy efficiency, because it degrades high-quality electricity into low-temperature heat.

ENE-154 · What is the coefficient of performance (COP) of a heat pump?

COP_HP = Q_h ÷ W, the heat delivered per unit work input. It is related to the refrigerator COP by COP_HP = COP_R + 1. Typical air-source heat pumps achieve about 3–4.

ENE-155 · What is the Carnot COP of a heat pump delivering heat at 35 °C from a 0 °C source?

COP = T_h ÷ (T_h − T_c) = 308 ÷ (308 − 273) ≈ 8.8. Real heat pumps reach about 35–50% of the Carnot value.

ENE-156 · How much thermal energy is in one tonne of coal with a GCV of 4000 kcal/kg?

4000 kcal/kg × 4.187 kJ/kcal ≈ 16,750 kJ/kg. For 1000 kg that is about 16.75 GJ, or 16.75 × 10⁶ ÷ 3600 ≈ 4650 kWh of thermal energy.

ENE-157 · What is the difference between sensible and latent heat?

Sensible heat changes temperature without phase change, Q = m c ΔT. Latent heat changes phase at constant temperature, Q = m L. For water, L_vap ≈ 2257 kJ/kg at 100 °C and L_fus ≈ 334 kJ/kg.

ENE-158 · What are the three modes of heat transfer and their governing laws?

Conduction follows Fourier's law, q = −kA dT/dx. Convection follows Newton's law of cooling, q = hA ΔT. Radiation follows the Stefan–Boltzmann law, q = εσA(T₁⁴ − T₂⁴), with σ = 5.67 × 10⁻⁸ W/m²·K⁴.

ENE-159 · Why does raising the mean temperature of heat addition improve cycle efficiency?

Any cycle can be compared with a Carnot cycle operating between its mean temperature of heat addition and heat rejection. Raising that mean temperature, through superheating, reheating or higher pressure, raises the ideal efficiency η = 1 − T_L,mean ÷ T_H,mean.

ENE-160 · What is a Sankey diagram?

A flow diagram in which arrow widths are proportional to energy flows. It shows inputs, useful outputs and losses at each stage of a process, making major loss areas immediately visible.

ENE-161 · What is energy intensity?

Energy consumed per unit of economic output, such as MJ per unit of GDP, or per unit of product. Falling energy intensity indicates efficiency gains or structural change towards less energy-intensive activity.

ENE-162 · What are stoichiometric air and excess air?

Stoichiometric air is the theoretical minimum for complete combustion. Excess air is the additional air supplied to ensure complete burnout. Too little causes CO and unburnt fuel, and too much raises stack heat loss.

ENE-163 · How much energy is needed to heat 100 L of water from 20 °C to 60 °C?

Q = m c ΔT = 100 kg × 4.186 kJ/kg·K × 40 K ≈ 16,740 kJ, or about 4.65 kWh (ignoring losses).

Thermal Power Plants

ENE-164 · What is heat rate?

Fuel energy input per unit of electricity generated, in kJ/kWh or kcal/kWh. Lower is better. Heat rate = 3600 ÷ η kJ/kWh, so a 38% efficient plant has a heat rate of about 9470 kJ/kWh (about 2260 kcal/kWh).

ENE-165 · How does reheating improve the Rankine cycle?

Steam partly expanded in the HP turbine is returned to the boiler and reheated before the IP/LP turbines. This raises the mean temperature of heat addition and reduces moisture at the turbine exhaust.

ENE-166 · What is regenerative feedwater heating?

Steam bled from intermediate turbine stages preheats feedwater in open (deaerator) or closed feedwater heaters. This raises the mean temperature of heat addition and improves cycle efficiency, at the cost of slightly less turbine work per kg.

ENE-167 · How does condenser pressure affect plant performance?

Lower condenser pressure lowers the saturation temperature of heat rejection, increasing turbine work and efficiency. A rise in back pressure, from fouling, air ingress or warm cooling water, raises heat rate.

ENE-168 · What is a supercritical boiler?

A once-through boiler operating above the critical point of water (22.06 MPa, 374 °C), where there is no distinct boiling and no drum is needed. Supercritical and ultra-supercritical units reach about 40–45% efficiency.

ENE-169 · What is the difference between plant load factor and availability factor?

Plant load factor (PLF) is actual generation divided by maximum possible generation at rated capacity. Availability factor is the fraction of time the plant is able to generate, whether dispatched or not.

ENE-170 · What are the direct and indirect methods of boiler efficiency testing?

The direct (input-output) method computes η = steam heat output ÷ fuel heat input. The indirect (heat-loss) method computes η = 100% − sum of losses, which identifies where the losses occur.

ENE-171 · What are the major heat losses in a coal-fired boiler?

Dry flue gas loss (usually the largest), loss due to moisture in fuel and air, loss due to hydrogen combustion forming water, unburnt carbon in ash, CO formation, and radiation and convection losses.

ENE-172 · Why is flue gas O₂ monitored in combustion control?

O₂ indicates excess air. Keeping it at the optimum minimises the sum of stack loss from too much air and unburnt-fuel loss from too little. Typical coal boilers target roughly 3–4% O₂ by volume.

ENE-173 · How is NOx formed and controlled in power plants?

NOx forms mainly through thermal (high flame temperature) and fuel-bound nitrogen routes. It is controlled by low-NOx burners, over-fire air, flue gas recirculation, and post-combustion SCR or SNCR using ammonia or urea.

ENE-174 · How does wet limestone flue gas desulphurisation work?

Flue gas is scrubbed with limestone slurry. SO₂ reacts with CaCO₃ to form calcium sulphite, which forced oxidation converts to gypsum (CaSO₄·2H₂O). Removal efficiencies above 90% are typical.

ENE-175 · How can gas turbine cycle efficiency be improved?

By raising turbine inlet temperature and pressure ratio, using regeneration (recuperation), intercooling during compression, reheat during expansion, or adding a bottoming steam cycle (combined cycle).

ENE-176 · Why does gas turbine output fall in hot weather?

Hot air is less dense, so the compressor's mass flow drops, and compressor work per kg rises. Output falls significantly and heat rate rises. Inlet air cooling (evaporative or chillers) is used to compensate.

Solar Photovoltaics

ENE-177 · What is MPPT?

Maximum power point tracking: a control algorithm in the charge controller or inverter that continuously adjusts the operating voltage or current so the array delivers maximum power as irradiance and temperature change.

ENE-178 · What is fill factor?

FF = P_max ÷ (V_oc × I_sc), a measure of how "square" the I-V curve is. Typical crystalline silicon cells have FF around 0.75–0.85. Series resistance and shunt leakage reduce it.

ENE-179 · How does temperature affect PV output?

Higher cell temperature slightly raises I_sc but significantly lowers V_oc, so power falls. For crystalline silicon, the power temperature coefficient is roughly −0.3 to −0.45%/°C.

ENE-180 · How does irradiance affect I_sc and V_oc?

I_sc is nearly proportional to irradiance. V_oc increases only logarithmically, V_oc ≈ (nkT ÷ q) ln(I_L ÷ I₀). Halving irradiance roughly halves current but reduces voltage only slightly.

ENE-181 · What is the single-diode model of a solar cell?

I = I_L − I₀[exp((V + IR_s) ÷ (nV_T)) − 1] − (V + IR_s) ÷ R_sh. Here I_L is photocurrent, I₀ is diode saturation current, n is the ideality factor, V_T = kT ÷ q, R_s is series resistance and R_sh is shunt resistance.

ENE-182 · How do series and shunt resistances affect the I-V curve?

High series resistance reduces the slope near V_oc and lowers fill factor. Low shunt resistance creates leakage paths, steepening the slope near I_sc. Both reduce maximum power.

ENE-183 · What is NOCT and how is cell temperature estimated?

Nominal operating cell temperature, measured at 800 W/m², 20 °C ambient and 1 m/s wind. Cell temperature is estimated as T_c ≈ T_a + (NOCT − 20) × G ÷ 800, where G is in W/m².

ENE-184 · What is performance ratio (PR)?

Actual final yield divided by reference yield, PR = (E_AC ÷ P_STC) ÷ (H_POA ÷ 1 kW/m²). It captures all system losses independent of irradiance. Well-designed plants achieve roughly 75–85%.

ENE-185 · What is specific yield?

Annual AC energy per kWp installed, in kWh/kWp/year. It combines site resource and system performance. For example, 1 MWp producing 1,500 MWh per year has a specific yield of 1500 kWh/kWp.

ENE-186 · What is the difference between central, string and microinverters?

Central inverters serve large arrays with low cost per watt but a single point of failure. String inverters serve a few strings with multiple MPPTs. Microinverters attach to each module, maximising energy under shading at higher cost.

ENE-187 · What is a bifacial PV module?

A module that collects light on both front and rear faces, usually with a glass back. Rear-side gain from ground-reflected light depends on albedo and mounting height, adding a few percent to over 10% of energy.

ENE-188 · What is the Shockley–Queisser limit?

The theoretical maximum efficiency of a single-junction solar cell under unconcentrated sunlight, about 33% at a band gap near 1.34 eV. It arises from thermalisation, transmission and radiative recombination losses.

ENE-189 · What is DC/AC ratio (inverter loading ratio)?

The ratio of array DC nameplate to inverter AC rating. Ratios of about 1.2–1.4 raise inverter utilisation and energy yield at a small cost in clipping losses during peak irradiance.

Solar Thermal

ENE-190 · What is the Hottel–Whillier–Bliss equation?

Q_u = A_c F_R [S − U_L (T_i − T_a)], where Q_u is useful heat gain, F_R is heat removal factor, S is absorbed solar radiation per unit area, U_L is overall loss coefficient and T_i is inlet fluid temperature.

ENE-191 · How is flat-plate collector efficiency expressed?

η = Q_u ÷ (A_c G) = F_R(τα) − F_R U_L (T_i − T_a) ÷ G. Efficiency falls linearly as (T_i − T_a) ÷ G increases. The intercept is the optical efficiency and the slope is the loss coefficient.

ENE-192 · How does two-tank molten salt storage work in CSP?

"Solar salt" (about 60% NaNO₃ and 40% KNO₃) is heated in the receiver and stored in a hot tank at about 565 °C (tower), then passed through a steam generator back to a cold tank at about 290 °C. This allows generation after sunset.

ENE-193 · Why does CSP need direct normal irradiance and tracking?

Concentrating optics can only focus parallel rays coming from the sun's disc. Diffuse light cannot be concentrated. Tracking keeps the focus on the receiver, so CSP suits clear, arid sites with high DNI.

ENE-194 · What is solar fraction?

The fraction of a thermal load met by solar energy, SF = Q_solar ÷ Q_load. Raising it requires more collector area and storage, with diminishing returns, so economic designs often target a moderate value.

ENE-195 · What is the difference between single-axis and two-axis tracking in CSP?

Troughs and linear Fresnel systems track on one axis, typically north-south, with cosine losses. Towers (heliostats) and dishes use two-axis tracking, achieving higher concentration and temperature.

ENE-196 · What collector area is needed to supply 200 L/day of water heated by 40 °C?

Daily load = 200 × 4.186 × 40 ≈ 33.5 MJ ≈ 9.3 kWh. With 5 kWh/m²/day irradiation and 50% collector efficiency, each m² gives 2.5 kWh, so about 3.7 m² is needed.

ENE-197 · What is a solar pond?

A pool with a salinity gradient that suppresses natural convection. The dense bottom layer traps absorbed heat at about 70–90 °C, acting as both collector and long-term storage.

ENE-198 · What are solar air heaters used for?

Heating air for crop drying, space heating and industrial process air. Air has low heat transfer coefficients, so fins, roughened absorbers or packed beds are used to improve performance.

ENE-199 · What is stagnation temperature of a collector?

The maximum absorber temperature reached when there is no fluid flow and useful heat extraction is zero, so absorbed energy equals losses. It is a key design limit for materials and safety.

ENE-200 · What are common heat transfer fluids in CSP and their limits?

Synthetic thermal oils are limited to about 390–400 °C by decomposition. Nitrate molten salts reach about 565 °C but freeze at about 220–240 °C. Direct steam generation avoids an intermediate fluid but complicates storage.

ENE-201 · How can solar heat provide cooling?

Through thermally driven chillers. A single-effect LiBr–water absorption chiller uses hot water at roughly 80–90 °C from collectors to drive the generator. Adsorption and desiccant cooling are alternatives.

Wind Energy

ENE-202 · What is the Betz limit?

The maximum fraction of wind kinetic power that an ideal rotor can extract, 16/27 ≈ 59.3%. It occurs when the downstream velocity is one third of the upstream velocity.

ENE-203 · What is the formula for wind turbine power?

P = ½ ρ A v³ Cp, where ρ is air density (≈ 1.225 kg/m³ at sea level, 15 °C), A is swept area (πD²/4), v is wind speed and Cp is power coefficient. Power scales with the cube of wind speed.

ENE-204 · What is the wind power density at 10 m/s?

Power density = ½ ρ v³ = 0.5 × 1.225 × 10³ ≈ 612 W/m² of swept area. The turbine extracts only a fraction Cp of this.

ENE-205 · What is the Weibull distribution in wind resource analysis?

A probability distribution of wind speed defined by shape factor k and scale factor c. It is used to estimate energy yield from the power curve. When k = 2 it is the Rayleigh distribution.

ENE-206 · Why is mean wind speed insufficient to estimate wind energy?

Power varies with v³, and the mean of v³ exceeds the cube of the mean speed. Using the frequency distribution captures the large contribution of high-speed hours. The ratio is called the energy pattern factor.

ENE-207 · What is the difference between fixed-speed and variable-speed wind turbines?

Fixed-speed turbines use induction generators directly connected to the grid, so rotor speed is nearly constant. Variable-speed turbines use power converters so the rotor runs at optimum tip speed ratio, capturing more energy and reducing loads.

ENE-208 · What are Type 1–4 wind turbine generators?

Type 1 is a fixed-speed squirrel-cage induction generator. Type 2 is a wound-rotor induction generator with variable rotor resistance. Type 3 is a doubly fed induction generator (DFIG). Type 4 is a full-converter machine, often permanent-magnet synchronous.

ENE-209 · Why is a DFIG converter only partially rated?

The converter handles only rotor slip power, about 25–30% of rated power for a ±30% speed range, while the stator connects directly to the grid. This lowers converter cost compared with full-converter designs.

ENE-210 · What is the difference between stall and pitch regulation?

Stall regulation uses fixed blades designed so airflow separates and lift drops at high wind speeds, passively limiting power. Pitch regulation actively rotates blades to limit power more precisely and allows aerodynamic braking.

ENE-211 · What is the annual energy output of a 2 MW turbine at 35% capacity factor?

AEP = P × 8760 h × CF = 2 MW × 8760 × 0.35 ≈ 6132 MWh per year.

ENE-212 · How do altitude and temperature affect wind power?

Power is proportional to air density, ρ = p ÷ (R T). Higher altitude lowers pressure, and higher temperature lowers density, both reducing output at a given wind speed. Density drops roughly 10% per 1000 m of altitude.

ENE-213 · How does rotor diameter affect power?

Swept area A = πD²/4, so power scales with D². Doubling rotor diameter quadruples power at the same wind speed, which is why rotors have grown rapidly.

ENE-214 · How is a wind resource assessed?

Using met masts with anemometers and wind vanes at several heights, or ground-based LiDAR/SoDAR, ideally for at least one year. The data is correlated with long-term reference data and modelled for wake and terrain effects.

Hydropower

ENE-215 · What is the output of a hydro plant with 10 m³/s flow, 50 m net head and 90% efficiency?

P = ρ g Q H η = 1000 × 9.81 × 10 × 50 × 0.9 ≈ 4.41 × 10⁶ W, or about 4.4 MW.

ENE-216 · What is specific speed and how does it guide turbine selection?

N_s = N√P ÷ H^(5/4), the speed of a geometrically similar turbine producing unit power under unit head. Low N_s suits Pelton (high head), medium suits Francis and high suits Kaplan or propeller (low head).

ENE-217 · What is cavitation in hydraulic turbines?

Formation and collapse of vapour bubbles where local pressure drops below vapour pressure, typically at the runner exit. It causes pitting, vibration and efficiency loss. It is controlled through turbine setting relative to tailwater, using Thoma's cavitation factor σ.

ENE-218 · What is water hammer and how is the pressure rise estimated?

A pressure surge caused by sudden valve closure in a penstock. For rapid closure, the Joukowsky equation gives Δp = ρ c Δv, where c is the pressure wave speed. Surge tanks and slow closure reduce it.

ENE-219 · What is a flow duration curve?

A plot of river flow against the percentage of time that flow is equalled or exceeded. It is used to select design flow and estimate firm and average energy for run-of-river schemes.

ENE-220 · What is the difference between firm power and secondary power?

Firm power is the output available with high reliability even in dry periods, based on minimum flow. Secondary power is the additional output available only when flow exceeds that minimum.

ENE-221 · What is the role of the governor in a hydro plant?

It controls guide vane or nozzle opening to keep turbine speed (and grid frequency) constant as load changes. Water inertia and water hammer must be considered in governor tuning.

ENE-222 · What limits pumped storage round-trip efficiency to about 70–80%?

Losses occur twice: pump and motor efficiency during charging, and turbine and generator efficiency during generation, plus penstock friction and transformer losses in both directions.

ENE-223 · What is a reversible pump-turbine?

A single Francis-type machine that operates as a pump in one direction and as a turbine in the other, coupled to a motor-generator. It reduces cost compared with separate pump and turbine units.

ENE-224 · What are the environmental impacts of large hydro dams?

Displacement of communities, loss of land and habitat, blocked fish migration, altered downstream flow and sediment transport, reservoir sedimentation, and methane emissions from flooded vegetation in some reservoirs.

ENE-225 · Which turbines suit small hydro?

Crossflow (Banki–Michell) turbines are simple and handle a wide flow range. Turgo turbines suit medium-high head. Pelton turbines suit high head, and propeller or Kaplan types suit low head.

ENE-226 · Why are desilting chambers used in hydro plants?

They slow the water so suspended sediment settles before reaching the turbines. Abrasive silt, common in Himalayan rivers, erodes runners, guide vanes and seals, reducing efficiency and raising maintenance costs.

Biomass & Bioenergy

ENE-227 · Why is biomass often described as carbon-neutral, and when is that not true?

CO₂ released on combustion was recently absorbed during growth, so the cycle can be balanced if harvested biomass is regrown. It is not neutral when land-use change, deforestation, fossil inputs for cultivation and transport, or long regrowth times are significant.

ENE-228 · What factors affect biogas yield in a digester?

Temperature (mesophilic about 30–40 °C, thermophilic about 50–60 °C), pH (about 6.8–7.4), C/N ratio (about 20–30), hydraulic retention time, organic loading rate, mixing, and the absence of toxic inhibitors such as ammonia.

ENE-229 · How is biogas upgraded to biomethane?

CO₂, H₂S, moisture and siloxanes are removed by water scrubbing, pressure swing adsorption, membrane separation, amine scrubbing or cryogenic separation. This raises methane to about 95–98% for grid injection or vehicle fuel (CBG).

ENE-230 · What is the difference between updraft and downdraft gasifiers?

In an updraft gasifier, air enters at the bottom and gas exits at the top, giving high efficiency but tar-rich gas. In a downdraft gasifier, gas passes through the hot combustion zone before exiting, which cracks tars and makes it better for engines.

ENE-231 · What is equivalence ratio in biomass gasification?

The ratio of actual air supplied to stoichiometric air for complete combustion. Gasifiers typically operate at about 0.2–0.4. Lower values increase tar and char, and higher values move towards combustion and lower gas heating value.

ENE-232 · What is the transesterification reaction for biodiesel?

Triglyceride + 3 methanol → 3 fatty acid methyl esters (biodiesel) + glycerol, usually with an alkaline catalyst (NaOH or KOH). High free fatty acid feedstocks need acid pre-esterification to avoid soap formation.

ENE-233 · Why does moisture content matter for biomass fuels?

Water adds mass without energy and absorbs latent heat on evaporation, lowering the net heating value, flame temperature and boiler efficiency. Moisture can be reported on a wet or dry basis, and the basis must be stated.

ENE-234 · What are the steps in producing ethanol from lignocellulosic biomass?

Pretreatment to break the lignin–hemicellulose structure, enzymatic or acid hydrolysis of cellulose and hemicellulose to sugars, fermentation of those sugars to ethanol, then distillation and dehydration.

ENE-235 · How does bagasse cogeneration work in sugar mills?

Bagasse, the fibrous residue after crushing cane, fires high-pressure boilers. The steam drives turbines for electricity, and extraction or back-pressure steam supplies process heat for juice heating and evaporation. Surplus power is exported to the grid.

ENE-236 · What is the approximate calorific value of biogas?

Methane has an LHV of about 35.8 MJ/m³ at standard conditions, so biogas with 60% CH₄ has about 21–22 MJ/m³, roughly 6 kWh/m³.

ENE-237 · Why is tar a problem in biomass gasification?

Tars are condensable hydrocarbons that deposit in pipes, filters and engine intakes as gas cools, causing fouling and failures. They are reduced by gasifier design, catalytic or thermal cracking, and gas cleaning.

ENE-238 · What are the advantages of biomass pelletisation?

Pelleting increases bulk density and energy density, gives uniform size and low moisture, reduces transport and storage costs, and enables automated feeding and consistent combustion.

Nuclear Energy

ENE-239 · What is the four-factor formula?

k∞ = η f p ε, where η is neutrons produced per absorption in fuel, f is the thermal utilisation factor, p is resonance escape probability and ε is the fast fission factor. Including fast and thermal non-leakage gives the six-factor formula for k_eff.

ENE-240 · What is reactivity?

ρ = (k − 1) ÷ k, the fractional departure from criticality. Zero means critical, positive means supercritical and power rising, negative means subcritical. It is often expressed in pcm or in dollars (units of β).

ENE-241 · Why are delayed neutrons important for reactor control?

About 0.65% of U-235 fission neutrons are emitted seconds to minutes later from fission product decay. They lengthen the effective neutron generation time, so power changes slowly enough for mechanical control rods to manage.

ENE-242 · What is xenon poisoning?

Xe-135, formed mainly from the decay of I-135, has a huge thermal neutron absorption cross-section. After a shutdown or power reduction, xenon builds up and can prevent restart for many hours (the xenon pit). It also causes spatial power oscillations.

ENE-243 · What is fuel burnup?

Energy extracted per unit mass of initial heavy metal, in MWd/kgU (or GWd/tHM). Higher burnup uses fuel better but increases cladding stress and fission gas release.

ENE-244 · What is the Doppler (fuel temperature) coefficient?

As fuel temperature rises, U-238 resonance absorption peaks broaden, increasing neutron capture and reducing reactivity. This prompt negative feedback is a key inherent safety feature of most reactors.

ENE-245 · How much energy does complete fission of 1 g of U-235 release?

1 g contains about 2.56 × 10²¹ nuclei. At 200 MeV (3.2 × 10⁻¹¹ J) each, that is about 8.2 × 10¹⁰ J, roughly 22,800 kWh of heat or about 0.95 MWd.

ENE-246 · What is binding energy and why do fission and fusion both release energy?

Binding energy is the energy needed to separate a nucleus into its nucleons, from the mass defect via E = Δm c². Binding energy per nucleon peaks near iron-56, so splitting heavy nuclei and fusing light nuclei both move towards higher binding and release energy.

ENE-247 · Why is light-water reactor thermal efficiency only about 33%?

The coolant pressure limit keeps steam conditions low. A PWR produces saturated steam at roughly 6–7 MPa and 280–290 °C, far below fossil plant superheat conditions, so Rankine efficiency is lower.

ENE-248 · What is defence in depth in nuclear safety?

Multiple independent layers of protection: fuel pellet, cladding, reactor coolant pressure boundary and containment as physical barriers, plus prevention, control systems, engineered safety features and emergency planning.

ENE-249 · What is an emergency core cooling system (ECCS)?

Safety systems that inject water into the reactor after a loss-of-coolant accident to remove decay heat and prevent fuel damage. They include high- and low-pressure injection, accumulators, and in newer designs, passive gravity-driven systems.

ENE-250 · What are the stages of the nuclear fuel cycle?

Mining and milling, conversion to UF₆, enrichment, fuel fabrication, irradiation in the reactor, interim storage of spent fuel, and either reprocessing (open versus closed cycle) or final geological disposal.

Energy Storage & Batteries

ENE-251 · What battery capacity is needed to run a 2 kW load for 4 hours at 80% DoD and 90% inverter efficiency?

Energy delivered = 2 × 4 = 8 kWh. Required nameplate = 8 ÷ (0.8 × 0.9) ≈ 11.1 kWh. At 48 V, that is about 231 Ah.

ENE-252 · What is the difference between calendar aging and cycle aging?

Calendar aging is capacity loss over time even at rest, accelerated by high temperature and high SoC. Cycle aging is degradation from charge–discharge cycling, driven by DoD, C-rate and temperature.

ENE-253 · What is the SEI layer in lithium-ion cells?

The solid electrolyte interphase, a passivating film formed on the graphite anode from electrolyte decomposition during initial cycles. It is needed for stability, but its continued growth consumes lithium and causes capacity fade.

ENE-254 · What is thermal runaway?

A self-accelerating process in which rising cell temperature triggers exothermic reactions (SEI breakdown, electrolyte decomposition, cathode oxygen release), producing more heat, gas, fire and propagation to neighbouring cells.

ENE-255 · What is the difference between passive and active cell balancing?

Passive balancing bleeds excess charge from higher-voltage cells through resistors as heat. It is simple and cheap. Active balancing transfers charge between cells using inductors, capacitors or converters, which is more efficient but more complex.

ENE-256 · What is the discharge reaction of a lead-acid battery?

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O, with a nominal cell voltage of about 2.0 V. Electrolyte specific gravity falls as the cell discharges, giving a simple SoC indication.

ENE-257 · What is the difference between energy density and power density?

Energy density (Wh/kg or Wh/L) is how much energy is stored. Power density (W/kg) is how fast it can be delivered. A Ragone plot compares technologies, with supercapacitors high in power and batteries high in energy.

ENE-258 · What is compressed air energy storage (CAES)?

Electricity compresses air into underground caverns. To generate, the air is released through turbines. Diabatic CAES burns fuel to reheat the air, while adiabatic CAES stores compression heat to reuse, improving efficiency.

ENE-259 · How does a flywheel store energy?

As rotational kinetic energy, E = ½ I ω². Because energy rises with ω², high-speed composite rotors in vacuum enclosures with magnetic bearings are used. Flywheels give high power and cycle life for short durations.

ENE-260 · What is state of health (SoH)?

A measure of a battery's condition relative to new, typically current capacity ÷ rated capacity, or a resistance-based measure. EV batteries are often considered at end of first life near 70–80% SoH.

ENE-261 · What is Peukert's law?

An empirical relation for lead-acid batteries, t = H (C ÷ (I H))^k, showing that available capacity falls at higher discharge currents. The Peukert exponent k is about 1.1–1.3 for lead-acid and close to 1 for Li-ion.

ENE-262 · What are sodium-ion batteries and their advantages?

Batteries that use sodium ions instead of lithium. Sodium is abundant and cheap, the cells avoid lithium and cobalt, perform well at low temperatures, and can be stored at 0 V. Energy density is lower than Li-ion.

ENE-263 · What are the types of thermal energy storage?

Sensible storage changes material temperature (water, molten salt, rock). Latent storage uses phase change materials at near-constant temperature. Thermochemical storage uses reversible reactions or sorption, with the highest energy density.

Hydrogen & Fuel Cells

ENE-264 · How much electricity is needed to produce 1 kg of hydrogen by electrolysis?

The theoretical minimum is about 39.4 kWh/kg (HHV, thermoneutral). Practical systems need about 50–55 kWh/kg including balance of plant, which is roughly 70–80% efficiency on an HHV basis.

ENE-265 · How much water is needed per kg of hydrogen?

Stoichiometrically about 9 kg of water per kg of H₂ (18 g H₂O per 2 g H₂). Real consumption is higher due to purification rejects and cooling.

ENE-266 · What is the theoretical maximum efficiency of a hydrogen fuel cell?

η = ΔG ÷ ΔH = 237.1 ÷ 285.8 ≈ 83% (HHV basis, 25 °C, liquid water). Unlike heat engines, it is not limited by Carnot efficiency.

ENE-267 · What does a fuel cell polarisation curve show?

Cell voltage versus current density. Voltage drops from activation losses at low current, ohmic losses (linear) at moderate current, and mass transport (concentration) losses at high current.

ENE-268 · How does a solid oxide fuel cell work?

Oxygen ions (O²⁻) conduct through a ceramic electrolyte, typically yttria-stabilised zirconia, at about 600–1000 °C, and react with fuel at the anode. The high temperature allows internal reforming of methane and high-grade waste heat.

ENE-269 · Why do PEM fuel cells need platinum and high-purity hydrogen?

Low operating temperature makes reaction kinetics slow, so platinum catalysts are needed. Platinum is poisoned by CO even at ppm levels, and by sulphur, so hydrogen must be very pure.

ENE-270 · What is hydrogen embrittlement?

Absorption of atomic hydrogen into metals, especially high-strength steels, which reduces ductility and fracture toughness and causes cracking under stress. It drives material selection for pipelines, tanks and valves.

ENE-271 · Why is ammonia considered a hydrogen carrier?

NH₃ contains about 17.6 wt% hydrogen, liquefies at about −33 °C at atmospheric pressure or at about 10 bar at ambient temperature, and has established production and shipping infrastructure. It can be cracked back to H₂ or used directly as fuel.

ENE-272 · What are the main hydrogen safety hazards?

A wide flammability range in air (about 4–75%), very low ignition energy, a nearly invisible flame, high diffusivity and leak tendency, and embrittlement. It is very buoyant, which helps it disperse outdoors.

ENE-273 · How is hydrogen production related to electrolyser current?

By Faraday's law: ṅ_H₂ = I ÷ (2F), where F = 96,485 C/mol. One ampere for one hour (3600 C) produces about 0.0187 mol, or about 0.0376 g H₂, per cell.

ENE-274 · What are typical practical fuel cell efficiencies?

Electrical efficiencies of about 40–60% (LHV), depending on type and load. SOFC and MCFC with heat recovery can exceed 80% total CHP efficiency. Efficiency is often higher at part load, unlike combustion engines.

ENE-275 · What is the effect of blending hydrogen into natural gas pipelines?

Hydrogen has about one third of methane's volumetric energy (LHV about 10.8 vs 35.8 MJ/m³), so a given volume blend delivers less energy. Higher blends require checks on embrittlement, leakage, metering and appliance compatibility.

Grid Integration & Smart Grids

ENE-276 · Why does grid frequency change when generation and load are unbalanced?

A deficit in generation is supplied from the kinetic energy of rotating synchronous machines, which slow down and lower frequency, and a surplus speeds them up. The rate of change depends on system inertia.

ENE-277 · What are primary, secondary and tertiary frequency control?

Primary control is automatic governor droop response within seconds. Secondary control (AGC) restores nominal frequency and scheduled interchange within minutes. Tertiary control redispatches reserves over longer periods.

ENE-278 · What is speed droop?

The percentage frequency change that causes a generator to move from no load to full load, R = (Δf ÷ f₀) ÷ (ΔP ÷ P_rated). Typical values of 4–5% let generators share load changes in proportion to their ratings.

ENE-279 · How is voltage controlled in a power system?

By managing reactive power through generator excitation, transformer on-load tap changers, capacitor banks, reactors, STATCOMs, SVCs and inverter reactive power control. Voltage is a local quantity, closely linked to reactive power.

ENE-280 · Why do inverter-based renewables reduce system inertia?

PV and full-converter wind connect through power electronics that decouple any rotating mass from grid frequency, so they provide no natural inertial response. High RoCoF results unless synthetic inertia or fast frequency response is added.

ENE-281 · What is the difference between grid-following and grid-forming inverters?

Grid-following inverters use a PLL to track existing grid voltage and inject controlled current. Grid-forming inverters set their own voltage and frequency, behaving like a voltage source, so they can support weak grids and black start.

ENE-282 · What is low-voltage ride-through (LVRT)?

A grid code requirement that generators stay connected during short voltage dips from faults instead of tripping, often injecting reactive current to support voltage recovery.

ENE-283 · What problems does reverse power flow cause in distribution networks?

Voltage rise at the feeder end, protection maloperation or blinding, overloading of transformers or lines designed for one-way flow, and incorrect operation of voltage regulators and tap changers.

ENE-284 · What is economic dispatch and merit order?

Economic dispatch allocates load among online generators to minimise total cost, ideally equalising incremental costs. Merit order ranks plants by marginal cost. Renewables with near-zero marginal cost are dispatched first.

ENE-285 · What is unit commitment?

Deciding which generating units to start up and shut down over a time horizon to meet forecast demand and reserves at minimum cost, considering start-up costs, minimum up and down times and ramp limits.

ENE-286 · Why is renewable forecasting important?

Accurate day-ahead and intraday forecasts of wind and solar output reduce reserve requirements, deviation penalties and curtailment, and improve unit commitment and market bidding.

ENE-287 · What is hosting capacity?

The maximum distributed generation a feeder can accept without violating voltage, thermal, protection or power quality limits. It depends on feeder strength, load profile and the location of the generation.

ENE-288 · What is vehicle-to-grid (V2G)?

Bidirectional charging that lets parked EVs discharge to the grid for peak shaving, frequency regulation or backup. It requires bidirectional chargers, communication and a way to manage battery degradation.

Energy Audit, Conservation, Economics & Policy

ENE-289 · What are the main steps of a detailed energy audit?

Pre-audit data collection, baseline energy and cost analysis, site measurements, energy and mass balances, identification of savings measures, techno-economic evaluation, a report with prioritised recommendations, and follow-up with M&V.

ENE-290 · What instruments are used in an energy audit?

Power and harmonic analysers, clamp meters, flue gas analysers, ultrasonic flow meters, infrared thermometers and thermal cameras, lux meters, anemometers, tachometers, and pressure and humidity meters.

ENE-291 · What are the affinity laws for pumps and fans?

For the same impeller, Q ∝ N, H ∝ N² and P ∝ N³. Small speed reductions therefore give large power savings, which is why variable-frequency drives beat throttling or damper control.

ENE-292 · How much power does a fan save if speed is reduced to 80% with a VFD?

P₂ ÷ P₁ = (0.8)³ = 0.512, so power falls by about 49% in theory. Actual savings are somewhat lower due to drive losses and static head or pressure in the system.

ENE-293 · What are IE efficiency classes for electric motors?

IEC 60034-30-1 defines classes IE1 (standard), IE2 (high), IE3 (premium) and IE4 (super premium). Higher classes have lower losses and are cost-effective for long running hours.

ENE-294 · How is compressed air leakage estimated?

By running the compressor with all end users off and timing load and unload cycles: leakage % = T_load ÷ (T_load + T_unload) × 100. Leaks above about 10% indicate a poor system.

ENE-295 · What is luminous efficacy?

Light output per unit electrical input, in lumens per watt. Incandescent lamps give about 10–15 lm/W, CFLs about 50–70 lm/W and good LEDs more than 100 lm/W. It is the main metric in lighting retrofits.

ENE-296 · What is net present value (NPV)?

NPV = Σ [CF_t ÷ (1 + r)^t] − C₀, the discounted value of future net cash flows minus the initial investment. A positive NPV means the project earns more than the discount rate.

ENE-297 · What is internal rate of return (IRR)?

The discount rate at which NPV equals zero. A project is attractive if IRR exceeds the cost of capital or hurdle rate. It is usually solved iteratively.

ENE-298 · What is an ESCO and energy performance contracting?

An energy service company designs, finances and implements efficiency projects and is paid from measured savings. In shared-savings models it shares savings, and in guaranteed-savings models the client finances and the ESCO guarantees savings.

ENE-299 · What is the Perform, Achieve and Trade (PAT) scheme?

An Indian BEE market mechanism that sets specific energy consumption reduction targets for designated consumers. Those that beat targets earn tradeable energy saving certificates (ESCerts), and those that fall short must buy them or pay penalties.

ENE-300 · How can maximum demand charges be reduced?

By load scheduling and staggering, demand controllers that shed non-critical loads, shifting loads to off-peak periods, power factor correction (when billing is on kVA), and using storage or captive generation for peaks.

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