General Aspects of Energy Management & Energy Audit — all 11 chapters. For every formula: what each symbol means and its unit, what the formula actually tells you, a worked example in the guidebook's style, and the exam trap. Plus 23 pictures of the graphs and diagrams the exam refers to, 73 short definitions, and 42 verified online videos (each link checked against the video's real title). 67 formulas in all. Values are the 2014 BEE Book-1's. ▶ Download the whole guide as one PDF · Paper-3 numericals drill
7–10 % of the paper. Mostly theory, but the two formulas below (toe conversion, energy intensity) come every year as objectives or a 5-marker.
Tonne of oil equivalent (toe)
1 toe = 10⁷ kcal = 11,630 kWh = 41,868 MJ · 1 MTOE = 10⁶ toe
What each symbol means · unit
toe
the heat released by burning one tonne of crude oil — the common yardstick for comparing coal, gas, electricity and oil
10⁷ kcal
kWh
one unit of electricity
1 kWh = 860 kcal
What it means
Different fuels are measured in different units (tonnes of coal, m³ of gas, kWh). To add them up or compare countries, every one is converted to the heat it contains, expressed as tonnes of oil. Divide any energy in kcal by 10⁷ to get toe.
Worked example. A plant uses 4,000 MWh of electricity a year. Energy = 4,000,000 kWh × 860 kcal/kWh = 3.44 × 10⁹ kcal ÷ 10⁷ = 344 toe.
Exam trap. Use 860 kcal per kWh (the electricity delivered), not the 2,500–3,000 kcal it took to generate it, unless the question says 'primary energy'.
Reserves-to-production ratio (R/P)
R/P (years) = Reserves remaining at end of year ÷ Production in that year
What each symbol means · unit
Reserves
proven reserves still in the ground
tonnes (or m³)
Production
amount produced in that one year
tonnes/year
What it means
How many years the known reserves would last if production stayed at this year's rate. Book values: coal ≈ 130 years (world), oil ≈ 45, gas ≈ 65; India coal ≈ 100+ years, oil ≈ 20.
Worked example. Reserves 892,000 million tonnes; production 7,800 million tonnes/year → R/P = 892,000 ÷ 7,800 ≈ 114 years.
Exam trap. It is a ratio of the same units, so the answer is in years — it is not a percentage.
Energy intensity
Energy intensity = Energy consumed ÷ GDP
What each symbol means · unit
Energy consumed
total (final or primary) energy used by the country in the year
toe
GDP
gross domestic product, at constant prices, or at purchasing-power parity (PPP)
million US$
What it means
Energy needed to produce one unit of economic output. A LOWER intensity means the economy gets more output from each toe. Comparing at PPP removes exchange-rate distortion, which is why India looks far better on the PPP basis than on the exchange-rate basis.
Worked example. Country A: 2,000 toe, GDP US$ 100 million → 20 toe per million US$. Country B: 2,500 toe, GDP 140 → 17.9. B uses energy more efficiently (2025 Paper-1, S-2).
Exam trap. Lower energy intensity does NOT automatically mean higher energy efficiency — it can come from a shift to services or a strong currency. This is the exact 2024 short question.
Per-capita energy consumption
Per-capita consumption = Total energy consumption ÷ Population
What each symbol means · unit
Total consumption
country's yearly energy use
toe or kgoe
Population
people
—
What it means
The book's indicator of living standard: India ≈ 0.5 toe per person against a world average ≈ 1.8 and USA ≈ 8 (2014 book figures).
Exam trap. India's figure is LOW because population is large, not because the country uses little energy in total.
The energy actually available to the economy in the year. Import dependence = Imports ÷ Gross inland consumption (India's oil import dependence ≈ 80 %).
Worked example. Production 500, imports 300, exports 50, stock drawn 10 → 760 MTOE.
Exam trap. Subtract exports; do not add them.
Energy intensity — why PPP changes the picture
Same energy, same physical output: valued at market exchange rate India's GDP looks small, so intensity looks high; at PPP the GDP is revalued upward and the intensity falls. The exam asks 'why is intensity expressed at PPP?' — this is the answer.
Definitions the exam asks
Primary vs secondary energy
Primary energy is found in nature (coal, crude oil, gas, sunlight, uranium); secondary energy is made from it (electricity, petrol, coke). Electricity is always secondary.
Commercial vs non-commercial
Commercial energy is bought and sold (coal, oil, gas, electricity); non-commercial is gathered free (firewood, dung, agricultural waste).
Renewable vs non-renewable
Renewable replenishes naturally in human time (solar, wind, hydro, biomass); non-renewable is finite (coal, oil, gas, nuclear fuel).
Energy security
Assured supply of energy at affordable prices, achieved by diversifying fuels and sources, strategic reserves, efficiency and domestic renewables.
Energy pricing in India
Coal, oil products and electricity are cross-subsidised: households and agriculture pay below cost, industry and commerce pay above it. Long-term marginal cost (LTMC) pricing is the book's recommendation.
Annual energy consumption ≥ threshold → notified as a Designated Consumer (DC)
What each symbol means · unit
30,000 MTOE
thermal power plants, fertiliser, cement, iron & steel, railways, pulp & paper
per year
12,000 MTOE
chlor-alkali
per year
7,500 MTOE
aluminium
per year
3,000 MTOE
textiles
per year
What it means
A DC must appoint a certified energy manager, get a mandatory audit by an accredited auditor, report energy data, and meet PAT targets.
Worked example. A textile mill using 4,200 MTOE/year is a DC (above 3,000).
Exam trap. MTOE here means metric tonne of oil equivalent (the Act's spelling), i.e. toe — not million toe.
PAT — Perform, Achieve and Trade
Target SEC for each DC → shortfall or excess (toe) → 1 ESCert = 1 toe of saving
What each symbol means · unit
SEC
specific energy consumption, energy per unit product
toe per tonne
ESCert
Energy Saving Certificate, issued for saving beyond target, tradable at power exchanges
1 toe each
What it means
A market mechanism under the 2010 amendment: over-achievers earn ESCerts, under-achievers buy them or pay a penalty. ESCerts can be bought, sold and banked for the next cycle, but not traded directly between DCs outside the exchange.
Worked example. Target 0.90 toe/t, achieved 0.85 toe/t on 100,000 t → 5,000 toe saved → 5,000 ESCerts.
Exam trap. 'ESCerts cannot be traded directly between DCs' — exchange-traded only (2015 and 2023 exams).
ECBC applicability
Commercial building with connected load ≥ 100 kW or contract demand ≥ 120 kVA
What each symbol means · unit
Connected load
sum of rated loads installed
kW
Contract demand
demand contracted with the utility
kVA
What it means
Energy Conservation Building Code sets minimum efficiency for envelope, HVAC, lighting, hot water and power in new large commercial buildings; states may adopt it and lower the thresholds.
Exam trap. It is 100 kW OR 120 kVA — either one makes the code applicable.
Standards & Labelling (S&L)
Star rating 1★ (least efficient) → 5★ (most efficient), against the BEE test standard
pass/fail mark that the product meets a minimum standard
—
MEPS
minimum energy performance standard below which sale is not allowed
—
What it means
Mandatory for some appliances (frost-free refrigerators, room ACs, TFLs, distribution transformers…), voluntary for others. Manufacturer declares, BEE verifies by sample testing.
Exam trap. More stars = LESS energy; the star bands are revised upward every few years.
The EC Act at one glance
Who does what: the Act empowers the Centre and States, BEE implements, designated consumers comply through energy managers, audits, PAT and reporting.
Definitions the exam asks
BEE
Bureau of Energy Efficiency, set up 1 March 2002 under the Act, under the Ministry of Power; it recommends norms, certifies energy managers/auditors, runs S&L, PAT, ECBC and awareness programmes.
Powers of Central Government
Notify designated consumers, norms of energy consumption, energy managers, mandatory audits, S&L for appliances, ECBC, and set up the Central Energy Conservation Fund.
Powers of State Government
Adopt/amend ECBC, enforce audits and reporting in the state, set up the State Energy Conservation Fund, appoint the State Designated Agency (SDA).
Penalty (2010 amendment)
Up to ₹10 lakh for each failure, plus up to ₹10,000 per day of continuing failure; PAT non-compliance is charged at the price of the shortfall in energy.
Energy Manager vs Energy Auditor
Both certified by BEE exam; a DC must appoint or designate an energy manager; the mandatory audit must be by an accredited energy auditor (Paper-4 qualified).
The biggest chapter: 18–22 % of the paper. Units, electricity, heat, steam and fuels. Almost every objective numerical comes from here.
Ohm's law and electrical power
V = I × R · P = V × I = I² × R = V² ÷ R
What each symbol means · unit
V
voltage (electrical pressure)
volt, V
I
current (flow of charge)
ampere, A
R
resistance
ohm, Ω
P
power
watt, W
What it means
Power is how fast energy is used. Doubling voltage across the same resistance quadruples power (V²/R) — this is why a 250 V heater at 230 V gives less heat.
Worked example. Kettle of 25 Ω on 250 V: I = 250/25 = 10 A; P = 250 × 10 = 2,500 W (2023 long, part 1).
Exam trap. I²R loss: halving current cuts loss to one quarter — this is the whole reason for high-voltage transmission and PF correction.
AC power — single and three phase
P₁ph = V × I × cos φ · P₃ph = √3 × V_L × I_L × cos φ
What each symbol means · unit
V_L
line-to-line voltage (415 V in India LT)
V
I_L
line current
A
cos φ
power factor
0–1
√3
1.732
—
What it means
Only the part of current in phase with the voltage does work; cos φ is that fraction. Divide by 1000 for kW.
Worked example. 415 V, 12 A, PF 0.9: P = 1.732 × 415 × 12 × 0.9 ÷ 1000 = 7.76 kW.
Exam trap. Use the LINE voltage with √3; never assume PF = 1 unless told.
Power triangle and power factor
kVA² = kW² + kVAr² · PF = cos φ = kW ÷ kVA · kVAr = kVA × sin φ
What each symbol means · unit
kW
real (active) power — does the work
kW
kVAr
reactive power — builds the magnetic field, does no work
kVAr
kVA
apparent power — what the cables and transformer must carry
kVA
φ
angle between kW and kVA
degrees
What it means
Think of a right-angled triangle: kW along the bottom, kVAr up the side, kVA the diagonal. Adding capacitors shortens the kVAr side, so kVA falls and PF (= kW/kVA) rises. Utilities bill and penalise on kVA and PF.
Worked example. kW 812, PF 0.70 → kVA = 812/0.70 = 1,160; kVAr = √(1160² − 812²) = 828.
Exam trap. Improving PF does not reduce kW; it reduces kVA, current and I²R losses.
Motor kVA from HP
kVA = (HP × 0.746) ÷ (η × PF)
What each symbol means · unit
HP
motor rating in horsepower
hp
0.746
kW per HP
kW/hp
η
motor efficiency
0–1
PF
power factor
0–1
What it means
Converts a nameplate HP into the apparent power the supply must provide, allowing for the motor's own losses and its lagging PF.
Worked example. 50 HP, η 0.9, PF 0.8: kVA = 50 × 0.746 ÷ (0.9 × 0.8) = 51.8 kVA.
Exam trap. Divide by both η and PF, not just one.
Motor loading
% loading = Input kW ÷ (Rated kW ÷ Rated η) × 100
What each symbol means · unit
Input kW
measured √3·V·I·cos φ
kW
Rated kW
nameplate output
kW
Rated η
nameplate full-load efficiency
0–1
What it means
Compares what the motor is drawing now with what it would draw at full load. Motors below ~40 % load run at poor efficiency and PF.
Worked example. 10 HP (7.46 kW) motor, η 0.9, drawing 4.5 kW → rated input 8.29 kW → loading 54 % (2021/2024 short).
Exam trap. Never estimate loading from the current ratio alone — current does not fall in proportion at low loads.
Synchronous speed and slip
Nₛ = 120 f ÷ P · Slip % = (Nₛ − N) ÷ Nₛ × 100
What each symbol means · unit
f
supply frequency (50 Hz)
Hz
P
number of poles
—
N
actual rotor speed
rpm
What it means
A 4-pole motor on 50 Hz has a synchronous speed of 1,500 rpm; it runs slightly slower (e.g. 1,470 rpm = 2 % slip). Speed can only be varied by changing frequency (VFD) or poles.
A unit of electricity is one kilowatt for one hour. 860 kcal is the heat you get from a unit in a resistance heater (100 % conversion).
Worked example. 5 kW heater for 8 h = 40 kWh = 34,400 kcal; at ₹7/kWh = ₹280.
Exam trap. Cooling 2,000 kg of water by 10 °C removes 2,000 × 1 × 10 = 20,000 kcal — the 2025 objective; do not divide by 860 unless kWh is asked.
Sensible heat
Q = m × Cp × ΔT
What each symbol means · unit
Q
heat added or removed
kcal
m
mass
kg
Cp
specific heat (water = 1 kcal/kg °C; air ≈ 0.24)
kcal/kg °C
ΔT
temperature change
°C
What it means
Heat that changes temperature without changing state. Water has the highest common specific heat, so it stores the most heat per °C — the reason it is the universal coolant.
Worked example. Heat 25,000 kg/h water from 28 to 80 °C: 25,000 × 1 × 52 = 1,300,000 kcal/h.
Exam trap. Equal heat to equal masses: the metal with the LOWER specific heat gets hotter (iron 0.11 vs copper 0.09 → copper rises more).
Heat absorbed while a substance changes state at constant temperature. Melting ice or boiling water takes far more heat than warming it: raising water 1 °C costs 1 kcal/kg, boiling it costs 540.
Worked example. 2,000 kJ to 500 kg ice at 0 °C melts 2,000 ÷ 335 = 5.97 kg (2013/2017/2022 objective).
Exam trap. Latent heat is per kg of the substance CHANGING state, not the total mass present.
Steam: enthalpy and dryness fraction
h_wet = h_f + x × h_fg · x = mass of dry steam ÷ total mass
What each symbol means · unit
h_f
enthalpy of saturated water (sensible heat)
kcal/kg
h_fg
latent heat of evaporation at that pressure
kcal/kg
x
dryness fraction, 1.0 = dry saturated
0–1
What it means
Wet steam carries less heat than dry steam because part of it is still water. Superheated steam has extra sensible heat above saturation. Latent heat FALLS as pressure rises (540 kcal/kg at 1 atm, ~460 at 10 kg/cm²).
Worked example. At 3 bar: h_f ≈ 133, h_fg ≈ 517 kcal/kg; x = 0.9 → h = 133 + 0.9 × 517 = 598 kcal/kg.
Exam trap. Higher pressure steam has LESS latent heat per kg — good for temperature, bad for heat delivered per kg.
Fuel calorific value — GCV and NCV
NCV = GCV − latent heat of the water vapour formed ≈ GCV − 9 × H × 587 ÷ 100 (kcal/kg)
What each symbol means · unit
GCV
gross (higher) calorific value: all heat, with the water vapour condensed
kcal/kg
NCV
net (lower) value: vapour leaves as vapour, its latent heat is lost
kcal/kg
H
hydrogen in the fuel
% by mass
What it means
Burning hydrogen makes water vapour; if it goes up the stack you never get its latent heat back. Efficiency on GCV basis is always LOWER than on NCV basis for the same boiler.
Worked example. GCV 10,000, H 12 %: NCV ≈ 10,000 − 9 × 12 × 587/100 ≈ 9,366 kcal/kg.
Exam trap. India reports boiler efficiency on GCV; Europe on NCV — the same boiler shows a higher number on NCV.
Fuel energy and TOE from fuel
Heat = mass × GCV · toe = Heat (kcal) ÷ 10⁷
What each symbol means · unit
mass
fuel burnt
kg or tonnes
GCV
calorific value
kcal/kg
What it means
Turns a fuel quantity into energy so it can be added to electricity and other fuels (in toe or MTOE).
Worked example. 125 t coal × 4,000 kcal/kg = 5 × 10⁸ kcal = 50 toe.
Exam trap. Watch units: 125 tonnes = 125,000 kg.
Common conversions
1 kcal = 4.187 kJ · 1 kWh = 3,600 kJ = 860 kcal · 1 HP = 746 W · 1 BTU = 252 cal = 1.055 kJ · 1 atm = 1.013 bar = 760 mm Hg = 10.33 m water = 1.033 kg/cm² · K = °C + 273
What each symbol means · unit
1 kg/cm²
≈ 10 m of water head ≈ 0.98 bar
—
Specific gravity
density ÷ density of water (no unit)
—
What it means
Exam numericals live or die on these. Watt = joule per second; kW is power, kWh is energy.
Worked example. 1 MJ = 278 Wh; 1 kcal/h = 1.163 W.
Exam trap. 'Rate of energy transfer' is power (watt), not kcal.
Psychrometry basics
RH % = (actual water vapour in air ÷ maximum it can hold at that temperature) × 100
What each symbol means · unit
DBT
dry-bulb temperature — ordinary thermometer
°C
WBT
wet-bulb temperature — wetted thermometer; lower because of evaporative cooling
°C
Dew point
temperature at which air becomes saturated on cooling
°C
What it means
To find RH you need DBT and WBT (a sling psychrometer). When DBT = WBT = dew point the air is saturated (RH 100 %) and no water evaporates from a wet surface. Heating air without adding moisture LOWERS its RH.
Worked example. If a wet substance stops losing water, the air's RH is 100 %.
Exam trap. Sensible heat content of moist air is shown by the DRY-bulb temperature; total heat (enthalpy) by the wet-bulb.
Power triangle — what a capacitor does
kW stays fixed; the capacitor supplies the magnetising kVAr locally, the triangle gets shorter, kVA falls and cos φ rises. Utilities charge on kVA, so the bill falls.
Heating curve of water — sensible and latent heat
Temperature rises with sensible heat (sloping lines) but stays flat during melting and boiling while latent heat is absorbed. The flat boiling step (540 kcal/kg) is 5× longer than heating from 0 to 100 °C.
GCV vs NCV — where the difference goes
Both start from the same fuel; the latent heat carried away by water vapour in the flue gas is the gap between gross and net.
Definitions the exam asks
Energy vs power
Energy is the capacity to do work (kWh, kcal, joule); power is the rate of using it (kW, W). A 100 W bulb for 10 h uses 1 kWh.
Sensible vs latent heat
Sensible heat changes temperature (you can sense it); latent heat changes state at constant temperature (hidden).
Heat capacity
Heat needed to raise the whole body by 1 °C = m × Cp (kcal/°C).
Superheated steam
Steam heated above its saturation temperature at that pressure; used in turbines, not for heating (poor heat transfer).
Frequency
Cycles per second of AC, 50 Hz in India; the speed of AC motors depends on it.
Kilovolt-ampere hour (kVAh) billing
Some utilities bill on apparent energy (vector sum of kWh and kVArh) — so poor PF costs money directly.
7–9 %: audit types and steps, instruments and their parameters, benchmarking, plant energy performance and payback. The PEP formula and the instrument table are asked almost every year.
Specific energy consumption (SEC)
SEC = Energy used ÷ Units of production
What each symbol means · unit
Energy used
electricity, fuel or total in the period
kWh, kcal, toe
Units of production
output in the same period
tonnes, pieces, m²
What it means
The basic benchmark: energy per unit of product (kWh/tonne of cement, kcal/kg of steel). Compare with the previous year, with sister plants (internal benchmarking) or with the best in the industry (external benchmarking).
Worked example. 12,000 MWh for 40,000 t → 300 kWh/t.
Exam trap. SEC rises when production falls, because fixed energy (lighting, idling) is spread over fewer units — that is not a real efficiency loss.
Production factor
Production factor = Current year production ÷ Reference year production
What each symbol means · unit
Reference year
the base year against which improvement is judged
—
What it means
Adjusts for the plant making more or less product than in the base year, so energy can be compared fairly.
Worked example. 2010: 45,000 t; 2009: 50,000 t → 0.9.
Exam trap. It is a plain ratio, no percentage.
Reference-year equivalent (RYE)
RYE = Reference year energy use × Production factor
What each symbol means · unit
Reference year energy
energy used in the base year
kcal or kWh
What it means
The energy the plant WOULD have used this year at base-year efficiency, given this year's production.
Worked example. 12 million kcal × 0.9 = 10.8 million kcal.
Exam trap. Multiply by the production factor, do not divide.
Plant energy performance (PEP)
PEP % = (RYE − Current year energy) ÷ RYE × 100
What each symbol means · unit
Current year energy
actual energy used this year
same unit as RYE
What it means
Percentage improvement (positive) or deterioration (negative) in efficiency after removing the effect of production change. Monthly PEP tracks progress; a rolling 12-month PEP smooths seasonality.
Worked example. RYE 10.8, current 11 million kcal → PEP = (10.8 − 11)/10.8 × 100 = −1.85 % (worse).
Exam trap. Capacity utilisation is NOT needed for PEP — a recurring objective (2009, 2015, 2024).
Simple payback (audit report format)
Payback (months) = Investment ÷ Net savings per year × 12 · Net savings = Annual savings − Annual operating cost
What each symbol means · unit
Investment
one-time cost
₹
Net savings
yearly benefit after running costs
₹/year
What it means
Quick screening measure used in the audit report for each energy conservation opportunity; details are in Chapter 7.
Worked example. ₹12 lakh investment, ₹4 lakh/yr net savings → 36 months.
Exam trap. Use NET savings (benefits minus costs).
Heat rate and energy cost of output
Heat rate = kcal of fuel ÷ kWh generated · Energy cost per unit product = Σ(fuel × price) ÷ production
What each symbol means · unit
Heat rate
fuel heat needed per unit of electricity; a benchmark for power plants
kcal/kWh
What it means
A benchmark parameter must be energy per unit output (kcal/kWh, kWh/kg, kWh/m²). 'kg/°C' is not one — the 2017/2021 objective.
Worked example. 2,500 kcal/kWh means 34.4 % efficiency (860/2,500).
Exam trap. Lower heat rate = better plant.
Detailed energy audit — the three phases
Pre-audit (plan, walk-through, brief), audit (data, measurements, trials, analysis) and post-audit (report, implement, follow-up). Ten steps sit inside these three phases.
Plant energy performance — what the formula compares
The bar for 'what we should have used' (RYE) is scaled from the reference year by the production factor; the gap to actual use is the PEP.
Definitions the exam asks
Energy audit (EC Act definition)
Verification, monitoring and analysis of energy use, including a technical report with recommendations for improving efficiency with cost-benefit analysis and an action plan.
Preliminary vs detailed audit
Preliminary (walk-through): 1–3 days, readily available data, identifies major opportunities and the need for a detailed study. Detailed: full measurement, mass and energy balances, economics of every opportunity, 3 phases (pre-audit, audit, post-audit), 10 steps.
Ten-step methodology
1 Plan & organise · 2 Walk-through · 3 Primary data & process flow · 4 Conduct survey & monitoring · 5 Trials/experiments · 6 Analyse energy use · 7 Identify opportunities (ENCONs) · 8 Cost-benefit analysis · 9 Report & recommendations · 10 Implementation & follow-up.
Avoid oversized equipment and 'just in case' running; supply energy at the lowest grade that does the job (e.g. do not use steam where hot water will do).
Fuel substitution
Switching to a cheaper or cleaner fuel for the same duty (e.g. LDO → LSHS, coal → natural gas) after checking availability, cost and equipment changes.
Energy Management System (EnMS) / ISO 50001
A systematic Plan-Do-Check-Act framework for continually improving energy performance; adopted to manage energy systematically, not only to cut cost.
15–18 %, almost all numerical: drying, evaporation, mixing, dust collection, humidity, boiler blowdown. Every question is the same two equations applied carefully.
The mass-balance law
Mass in = Mass out + Mass stored (steady state: Mass in = Mass out)
What each symbol means · unit
Mass in
every stream entering the box
kg/h
Mass out
products, wastes, losses leaving
kg/h
Stored
accumulation inside (zero at steady state)
kg
What it means
Draw a box around the process; whatever enters must leave or stay. Apply it to the TOTAL mass and separately to each component (solids, water, fat, ash). Two unknowns need two balances.
Worked example. Milk 5,833 kg/h = cream Y + skim Z (total balance) and 0.04 × 5,833 = 0.45 Y + 0.0045 Z (fat balance) → solve Y and Z.
Exam trap. Pick the component that does NOT change (solids in drying, salt in evaporation) — its balance gives the answer in one line.
Solids balance for drying and evaporation
Dry solids in = Dry solids out → W₁ × (1 − m₁) = W₂ × (1 − m₂) · Water removed = W₁ − W₂
What each symbol means · unit
W₁, W₂
wet mass before and after
kg
m₁, m₂
moisture fraction (wet basis) before and after
0–1
What it means
The dry solid passes through unchanged; only water leaves. This single line solves every 'moisture reduced from x % to y %' question.
Worked example. 200 kg at 60 % moisture → dry solid 80 kg; at 30 % moisture final mass = 80 ÷ 0.7 = 114.3 kg; water evaporated 85.7 kg (2015/2016 objective).
Exam trap. Moisture % is on WET basis unless stated: 60 % moisture means 40 kg solid per 100 kg wet material.
Gas mixtures are handled in moles because equal volumes hold equal moles. Combustion balances (C + O₂ → CO₂) work in moles too.
Worked example. 25 kg salt in 100 kg water: 25/125 = 20 % w/w. 90 kg water = 5,000 moles (90,000/18). 7.5 g H₂ (3.75 mol), 3.25 g O₂ (0.10), 5.55 g N₂ (0.20) → mole fraction O₂ = 0.10/4.05 = 0.025.
Exam trap. Salt in water: divide by the TOTAL (salt + water), not by the water alone.
Energy balance
Energy in = Energy out + Energy stored · Heat duty Q = m × Cp × ΔT · Steam heat = m_steam × latent heat · Electrical heat = kWh × 860
What each symbol means · unit
Q
heat gained or lost by a stream
kcal/h
Cp of water
1 kcal/kg °C
—
Latent heat
of the steam at its pressure (≈ 540 kcal/kg at 1 atm)
kcal/kg
What it means
Heat given up by the hot stream equals heat taken by the cold stream plus losses. Used for heat exchangers, evaporators, dryers and boilers.
Exam trap. Include the sensible heat to bring the feed up to boiling before the latent heat of evaporation.
Energy conversion efficiency and losses
Efficiency % = Useful energy out ÷ Energy in × 100 · Losses = Energy in − Useful out
What each symbol means · unit
Energy in
fuel (kg × GCV) or electricity (kWh × 860)
kcal
What it means
A Sankey diagram draws the energy balance: one wide input arrow splitting into useful output and each loss, arrow width proportional to energy. The book's DG-set Sankey: fuel 100 % → electricity ≈ 35 %, exhaust ≈ 30 %, cooling water ≈ 25 %, radiation ≈ 10 %.
Exam trap. w is per kg of DRY air, so multiply by the dry-air mass flow.
Dust / particulate balance
Dust in = Dust collected + Dust in clean gas · Collection efficiency % = collected ÷ inlet × 100
What each symbol means · unit
Dust loading
concentration in gas
g/Nm³ or mg/Nm³
Gas flow
volume at normal conditions
Nm³/h
What it means
Bag-house, cyclone and ESP questions: mass of dust per hour = flow × concentration; the balance across the collector gives what escapes.
Worked example. 50,000 Nm³/h at 10 g/Nm³ in, 0.1 g/Nm³ out → 500 kg/h in, 5 kg/h out, 495 collected, 99 %.
Exam trap. Keep concentration and flow in the same normal-condition basis.
The balance box
Everything crossing the boundary is listed; at steady state the arrows in equal the arrows out for total mass AND for each component.
Drying — the solid is the tie component
The 80 kg of solid is the same on both sides; only the water leaves, so the final wet mass is solid ÷ (1 − final moisture).
Sankey diagram of a diesel generator
Fuel enters at 100 %; the arrow splits into ≈ 35 % electricity and three loss streams — exhaust gas is the biggest, which is why waste-heat recovery targets it.
Definitions the exam asks
System boundary
The imaginary box you draw; choose it so the unknown crosses it and the fewest streams do. A whole plant, one unit, or one heat exchanger can each be a system.
Basis of calculation
Fix a basis first (100 kg feed, 1 hour, 1 kmol) and carry it through; results scale linearly.
Tie component
A component that passes through unchanged (dry solid in drying, ash in combustion, salt in evaporation) — balance on it first.
Recycle, by-pass and purge
Recycle returns part of the output to the inlet (raises conversion); by-pass sends part of the feed around the unit; purge bleeds off inerts that would build up in a recycle loop.
Process flow diagram (PFD)
Boxes for units, arrows for streams with flow, composition and temperature; the starting point of every balance.
Sankey diagram
Energy-flow picture with arrow widths proportional to energy; shows at a glance where the losses go.
Only 2–3 %, pure theory: energy policy, the action-plan steps, force-field analysis, duties of the energy manager. Learn the lists; there are no numericals.
Energy conservation target (as a formula)
Saving % = (Baseline energy − Target energy) ÷ Baseline energy × 100
What each symbol means · unit
Baseline
energy use in the base year, normalised for production
kWh or toe
Target
the level management commits to
same unit
What it means
Targets in an energy policy must be specific, measurable and time-bound (e.g. 'reduce SEC by 5 % in 2 years'). Progress is tracked by M&T (Chapter 9).
Worked example. Baseline 300 kWh/t, target 285 → 5 %.
Exam trap. A target with no baseline, date or responsibility is not a target.
Force-field analysis
Driving forces push the change, restraining forces hold it back; the change happens when the drivers outweigh the restrainers.
Definitions the exam asks
Energy policy
Top management's written commitment: a declaration of intent, objectives, targets, responsibilities, a review cycle and a link to the corporate objectives; short (one page), signed, communicated to all staff.
Action-plan steps (book order)
1 Top-management commitment → 2 Appoint energy manager & form the energy committee → 3 Energy policy → 4 Energy audit / baseline → 5 Set targets and priorities → 6 Action plan (who, what, when, cost) → 7 Implement → 8 Monitor & report → 9 Review and revise.
Force-field analysis
Lewin's tool: list the DRIVING forces for a change (rising costs, management support, new technology) against the RESTRAINING forces (lack of funds, resistance to change, poor awareness); strengthen drivers and weaken restrainers. It considers BOTH positive and negative forces.
Energy manager — duties (Act)
Prepare the annual activity plan, set up an energy-efficiency team, report to the SDA, implement audit recommendations, monitor consumption and submit data, create awareness, and be the link with BEE.
Responsibilities
Report to management, coordinate the audit, establish benchmarks, follow the action plan, prepare the budget and information system.
Motivation & training
Awareness campaigns, suggestion schemes, recognition and rewards, and training at every level are the book's soft measures for sustaining savings.
5S / Kaizen / TPM
Japanese workplace tools the book mentions: 5S (sort, set in order, shine, standardise, sustain), Kaizen (continuous small improvements), Total Productive Maintenance — all reduce hidden energy waste.
ISO 50001
Energy management system standard: Plan-Do-Check-Act; requires an energy baseline, energy performance indicators (EnPIs), objectives, targets and action plans, and management review.
13–16 %, numerical: payback, ROI, NPV, IRR appear EVERY year (NPV or IRR is a 10-marker). Master the discount-factor table and the interpolation for IRR.
Simple payback period
Payback (years) = Investment ÷ Annual net savings · Net savings = Annual savings − Annual operating cost
What each symbol means · unit
Investment
first cost (capital)
₹
Annual net savings
yearly cash benefit after running costs
₹/year
What it means
How long until the project pays for itself. Simple, so widely used to screen, but it ignores the time value of money and everything after the payback point.
Worked example. ₹2 lakh VFD saving ₹45,000/yr with ₹5,000/yr maintenance → 2,00,000 ÷ 40,000 = 5 years.
Exam trap. Payback 'does not consider cash flow after the payback period' and 'does not take into account time value of money' — both appear as exam statements.
Return on investment (ROI)
ROI % = Annual net savings ÷ Investment × 100 (= 1 ÷ payback × 100)
What each symbol means · unit
Annual net savings
₹/year
—
Investment
₹
—
What it means
The yearly return the project earns on its cost; compare it with the company's cost of capital or bank interest.
Worked example. Saves ₹45,000 on ₹2,00,000 → 22.5 % (2017/2024 objective).
Exam trap. The question may want the figure after operating costs — read carefully.
Money today is worth more than the same money later because it can earn interest. To compare cash flows in different years, discount each one back to today.
Worked example. ₹1,00,000 in 3 years at 10 %: PV = 1,00,000 ÷ 1.331 = ₹75,131. Discount factors at 10 %: yr1 0.909, yr2 0.826, yr3 0.751, yr4 0.683, yr5 0.621.
Exam trap. Investment at the START of year 1 is at year 0 — factor 1.0, not 0.909.
include the resale value in the final year's cash flow
₹
What it means
Sum of all discounted future cash flows minus what you pay now. NPV > 0 → the project earns more than the cost of capital → accept. Choosing between projects: the higher NPV wins.
Exam trap. Depreciation is not a cash outflow, but the tax it saves is a cash INflow — 'if depreciation is considered net operating cash inflow is HIGHER' (2018/2024 objective).
Internal rate of return (IRR)
IRR = the discount rate i at which NPV = 0 · IRR ≈ i₁ + NPV₁ ÷ (NPV₁ − NPV₂) × (i₂ − i₁)
What each symbol means · unit
i₁
a rate giving a positive NPV₁
%
i₂
a higher rate giving a negative NPV₂
%
What it means
The project's own earning rate. Accept if IRR > cost of capital. Found by trial: compute NPV at two rates that bracket zero, then interpolate.
Worked example. NPV at 15 % = +2,000, at 20 % = −1,500 → IRR ≈ 15 + 2,000/3,500 × 5 = 17.9 %.
Exam trap. IRR is the rate where NPV is ZERO (not positive, not negative) — 2017/2024 objective. For a single equal annual saving A over n years, IRR solves Investment ÷ A = annuity factor.
Annuity (uniform series) present value
PV of A per year for n years = A × [ (1 + i)ⁿ − 1 ] ÷ [ i (1 + i)ⁿ ] · Capital recovery factor = the inverse
What each symbol means · unit
A
equal yearly cash flow
₹/year
n
years
—
What it means
Shortcut when every year's saving is the same. The bracket is the annuity factor (at 10 %, 5 years = 3.791). Capital recovery factor turns a loan into equal yearly instalments.
Worked example. ESCO project: ₹10 lakh/yr for 7 years at 30 % → factor 2.802 → maximum investment ₹28.0 lakh (2018 short).
Exam trap. Use the annuity factor only when the yearly amounts are equal.
Depreciation — straight line
Annual depreciation = (Cost − Salvage value) ÷ Useful life
What each symbol means · unit
Salvage
resale value at the end of life
₹
Useful life
years
—
What it means
Spreads the capital cost over the years for accounting and tax. Reduces taxable profit, so it raises after-tax cash flow via the tax shield.
Worked example. ₹12 lakh, salvage ₹2 lakh, 5 years → ₹2 lakh/year.
Exam trap. Depreciation itself is not cash — do not subtract it from cash flow in NPV; add back the tax saving.
Sensitivity and risk analysis
Recompute NPV/IRR after changing one input (energy price, savings, tax rate, cost of capital) by ± x %
What each symbol means · unit
Macro factors
tax rates, interest rates, energy prices, inflation
—
Micro factors
savings estimate, capital cost, operating hours
—
What it means
Shows which assumption the decision is most sensitive to, because 'all the above situations' can occur — the result is uncertain. Change in TAX RATE is the book's example of a macro factor.
Worked example. If savings fall 20 % and NPV stays positive, the project is robust.
Exam trap. Sensitivity analysis is applied because of uncertainty in all the inputs — 'all of the above' (2017/2019 objective).
Cash-flow timeline and discounting
Each year's cash flow is shrunk by its discount factor before adding; the investment at year 0 is not discounted.
NPV falls as the discount rate rises — IRR is where it crosses zero
At low rates NPV is positive; at the cost of capital you read the project NPV; where the curve crosses zero is the IRR. Interpolate between two bracketing rates.
Definitions the exam asks
Cash flow
Actual money in and out each year: savings, operating costs, taxes, salvage. Profit is not cash flow.
Cost of capital / discount rate
The rate the company must earn to satisfy lenders and shareholders; used as i in NPV and as the hurdle for IRR.
Debt vs equity financing
Debt: borrow (loan or bond) and repay with interest, company keeps ownership. Equity: sell shares (or use retained earnings), investors share ownership and profit.
Leasing
Lessee pays rent to use equipment owned by the lessor — no large capital outlay; operating lease vs finance lease.
ESCO / performance contracting
An Energy Service Company funds and implements the project and is paid from the savings: shared-savings (ESCO takes a % of savings) or guaranteed-savings (client borrows, ESCO guarantees the saving).
Life-cycle cost
All costs over the life — capital, energy, maintenance, disposal — discounted to present value; the right basis for comparing an efficient motor with a cheap one.
10–12 %: a CPM/PERT network is nearly always a 10-mark long question, plus objectives on float, expected time and Gantt charts.
CPM — forward pass
ES = largest EF of all predecessors · EF = ES + t
What each symbol means · unit
ES
earliest start
weeks/days
EF
earliest finish
—
t
activity duration
—
What it means
Go left to right: an activity can start only when ALL activities before it have finished. The largest EF at the end is the project duration.
Worked example. A (4 wk) starts at 0 → EF 4; B (5 wk) after A: ES 4, EF 9.
Exam trap. Where two arrows enter a node, take the LARGER EF.
CPM — backward pass
LF = smallest LS of all successors (last activity: LF = project duration) · LS = LF − t
What each symbol means · unit
LF
latest finish without delaying the project
—
LS
latest start
—
What it means
Go right to left from the project end: the latest an activity may finish is the earliest any following activity must start.
Worked example. Project 20 wk; last activity E (3 wk): LF 20, LS 17.
Exam trap. Where two arrows leave a node, take the SMALLER LS.
Float (slack) and the critical path
Total float = LS − ES = LF − EF · Critical path = the longest path = activities with zero float
What each symbol means · unit
Float
time an activity can slip without delaying the project
—
What it means
Critical activities have no slack: a one-day delay delays the whole project. Non-critical ones can be delayed by their float. The critical path is the LONGEST route through the network.
Worked example. LS 8, LF 12, duration 1 wk → ES could be as late as 8 and EF 9... float = LF − EF = 12 − 9 = 3 weeks (2018/2024 objective).
Exam trap. The critical path is the longest path, and 'minimum project duration' equals its length — never the shortest path.
PERT — expected time and variance
tₑ = (a + 4m + b) ÷ 6 · σ = (b − a) ÷ 6 · Variance = σ²
What each symbol means · unit
a
optimistic time
—
m
most likely time
—
b
pessimistic time
—
σ
standard deviation of the activity
—
What it means
PERT uses three estimates weighted 1-4-1 (a beta distribution). Project variance = sum of variances along the critical path; project σ = √(sum).
Worked example. a 15, m 18, b 60 days: tₑ = (15 + 72 + 60)/6 = 24.5 days; σ = 7.5. (Real exam: 15/18/21 → 18 days.)
Exam trap. Add VARIANCES (σ²), never standard deviations, along the critical path.
Probability of finishing by a date
Z = (T − Tₑ) ÷ σ_project → probability from the normal table
What each symbol means · unit
T
target completion time
—
Tₑ
expected project duration (critical path)
—
σ_project
√Σ variances on the critical path
—
What it means
Z = 0 → 50 % chance; Z = +1 → 84 %; Z = +2 → 97.7 %.
Worked example. Tₑ 40 wk, σ 2, target 44 → Z = 2 → 97.7 %.
Exam trap. Use the project σ (from the critical path), not one activity's σ.
Crashing — cost slope
Cost slope = (Crash cost − Normal cost) ÷ (Normal time − Crash time)
What each symbol means · unit
Crash
the shortest possible duration with extra resources
—
What it means
Extra cost per unit of time saved; crash the critical activity with the lowest slope first, and re-check which path is critical after each step.
Worked example. Normal 10 days ₹1 lakh, crash 7 days ₹1.6 lakh → ₹20,000/day.
Exam trap. Crashing a non-critical activity saves nothing.
CPM network with forward and backward pass
ES/EF above each activity, LS/LF below; the thick path has zero float and is the critical path (longest route).
Gantt chart of the same project
Bars show when each activity runs; the critical activities are shaded — the chart cannot show WHY they are critical, which is the Gantt limitation.
PERT three-time estimate
A beta curve: most likely time near the peak, optimistic and pessimistic at the tails; the mean is weighted 1-4-1.
Definitions the exam asks
Project
A temporary, unique endeavour with a defined start and end, scope, budget and quality — e.g. installing a cogeneration plant.
8–10 %: the energy-vs-production line (base load), CUSUM (a recurring 10-marker), specific energy consumption and target setting.
Energy–production relationship (regression line)
E = m × P + c
What each symbol means · unit
E
energy in the period
kWh, toe
P
production in the period
tonnes
m
slope — variable energy per unit product
kWh/t
c
intercept — fixed (base) energy used even at zero production
kWh
What it means
Plot monthly energy against production and fit a straight line (linear regression). The intercept is the base load (lighting, idling, standby losses); the slope is the true variable SEC. The line is the 'standard' against which targets are set.
Worked example. E = 0.4 P + 180 toe/month: at 500 t → 380 toe; the 180 is fixed energy.
Exam trap. Linear regression is the statistical technique that quantifies the relationship (2013/2017 objective); CUSUM is not.
Specific energy consumption trend
SEC = E ÷ P = m + c ÷ P
What each symbol means · unit
c ÷ P
the fixed energy spread over production
kWh/t
What it means
Because of the fixed term, SEC falls as production rises even with no efficiency change — so SEC alone can mislead; use the regression line or CUSUM.
Worked example. At 200 t: 0.4 + 180/200 = 1.3 toe/t; at 500 t: 0.76.
Exam trap. A rise in SEC in a low-production month is not proof of waste.
CUSUM — cumulative sum of differences
Difference = E_actual − E_predicted (from E = mP + c) · CUSUM = running total of the differences
What each symbol means · unit
E_predicted
energy the standard line says the month should have used
same unit
CUSUM
cumulative deviation from the standard
same unit
What it means
Read the slope, not the value: horizontal = performing as the standard; sloping DOWN = saving energy (using less than predicted); sloping UP = waste. A change of slope marks the month something changed. Savings over a period = CUSUM at end − CUSUM at start.
Worked example. Months 1–6 differences −20, −25, −18, −30, −22, −15 toe → CUSUM after 6 months = −130 toe = saving of 130 toe.
Exam trap. A negative CUSUM is GOOD (less energy than predicted). The 2022 long question asks for savings from a CUSUM table.
Target setting
Target = best historical performance line (or standard line − agreed % improvement)
What each symbol means · unit
Standard performance
the regression line from past data
—
Target
a tighter line management commits to
—
What it means
M&T needs a target to compare against; targets are based on historical best, benchmarks, or engineering estimates, and are revised as improvements stick.
Worked example. Standard 0.4P + 180 → target 0.38P + 160.
Exam trap. Monitoring without targeting shows nothing to act on.
Energy vs production — the standard line
Each dot is a month; the fitted line's intercept c is the fixed energy, the slope m the variable energy per tonne.
Reading a CUSUM chart
Flat = on standard; falling = saving; rising = waste; a kink shows when performance changed. Savings between two dates = the vertical drop.
Definitions the exam asks
Monitoring & targeting (M&T)
A management technique using energy information to eliminate waste, reduce cost and improve performance: measure, compare with a standard/target, report, act. Typical savings 5–10 %.
Energy account centres (EACs)
Departments or processes to which energy use is allocated and for which a manager is accountable — the unit of M&T.
Elements of M&T
Recording (meters), analysing (relate energy to drivers), comparing (against targets), setting targets, monitoring, reporting, controlling.
Data and information
Raw meter readings become information when related to production, weather (degree-days) and time; an EMIS (energy management information system) automates this.
Degree-days
Driver for heating/cooling energy: sum of daily (base temperature − average temperature) over the period.
Standard vs target energy performance
Standard = what past behaviour predicts (regression); target = the improved level to aim for.
Puts all greenhouse gases on one scale. SF₆ has the highest GWP; perfluorocarbons (PFC) have the LONGEST atmospheric lifetime (thousands of years).
Worked example. 10 t CH₄ = 210 t CO₂e.
Exam trap. Highest GWP = SF₆; longest lifetime = PFC — two different questions.
The greenhouse effect
Incoming short-wave radiation passes in; outgoing long-wave radiation is partly trapped by greenhouse gases and re-emitted back to the surface.
Carbon to CO₂ — the 44/12 rule
12 units of carbon pick up 32 units of oxygen to make 44 units of CO₂ — the mass grows 3.67 times.
Definitions the exam asks
Greenhouse effect
Short-wave sunlight passes through the atmosphere, warms the earth, which re-radiates long-wave infrared; greenhouse gases absorb and re-emit it downward, keeping the surface ≈ 33 °C warmer than it would be. Extra gases from human activity strengthen it — global warming.
Six Kyoto gases
CO₂, CH₄, N₂O, HFCs, PFCs, SF₆ (water vapour is a GHG but not counted; CFCs are covered by Montreal, not Kyoto).
Ozone depletion
CFCs (and HCFCs, halons) release chlorine in the stratosphere that destroys ozone; the Montreal Protocol (1987) phases them out. Ozone depletion and global warming are DIFFERENT problems — global warming does not enlarge the ozone hole.
UNFCCC and IPCC
UNFCCC (Rio 1992) — the treaty; IPCC (1988) — the scientific body assessing climate change; COP — the yearly conference of parties.
Kyoto Protocol (1997, in force 2005)
Annex-I (developed) countries took binding reduction targets (≈ 5 % below 1990 by 2008–12); India is non-Annex-I with no binding target.
Flexible mechanisms
Emission Trading (between Annex-I), Joint Implementation (Annex-I project in another Annex-I country → ERUs), Clean Development Mechanism (Annex-I finances a project in a developing country → CERs; 1 CER = 1 t CO₂e). India's DNA = Ministry of Environment & Forests.
Carbon capture and sequestration (CCS)
Capturing CO₂ from point sources and storing it (geological, ocean, mineral) or enhancing natural sinks (forests). 'Carbon sequestration' = the storing step.
Impacts of warming
Melting ice caps, sea-level rise, unpredictable climate, crop and water stress — not 'a bigger ozone hole'.
Energy efficiency as mitigation
The cheapest CO₂ reduction is energy not used: every kWh saved avoids the grid's emission factor.
10–12 %: solar (PV and thermal), wind (P ∝ v³), hydro, biomass and gasification, fuel cells, geothermal, tidal. Many objectives plus one long question with a calculation.
Solar constant and insolation
Solar constant ≈ 1,368 W/m² (top of atmosphere) · average insolation at the surface ≈ ¼ of it ≈ 342 W/m²
What each symbol means · unit
Insolation
solar energy received per unit area per day
kWh/m²/day (India ≈ 4–7)
What it means
Only part of the top-of-atmosphere energy reaches the ground, after reflection and absorption; India gets about 300 sunny days a year.
Exam trap. The constant is at the TOP of the atmosphere, not at the ground.
rated (peak) power at standard test conditions, 1,000 W/m²
W
Panel area
m²
—
What it means
Typical crystalline cells 12–15 %; the book's example: 175 W panel of 0.75 × 1.5 m → 175 ÷ (1,000 × 1.125) = 15.6 %. Efficiency depends on the cell material, area and peak power — NOT on the inverter.
Exam trap. 'The energy conversion efficiency of a solar cell does not depend on the inverter' (2017/2024 objective).
Solar PV sizing
Energy per day (kWh) = Panel kWp × Peak-sun hours × System efficiency · Number of panels = Required kWp ÷ kWp per panel · Roof area = panels × area each
Net-metering questions (2021 short: 1,200 m² roof) ask how many kW fit, how many units per year, and the payback from the tariff.
Worked example. 10 kWp × 5 h × 0.8 = 40 kWh/day ≈ 14,600 kWh/yr.
Exam trap. 1 kWp of crystalline panels needs roughly 10 m² of roof (book-era panels).
Wind power
P = ½ × ρ × A × v³ · A = π D² ÷ 4 · Power ∝ (wind speed)³
What each symbol means · unit
ρ
air density ≈ 1.2 kg/m³
kg/m³
A
swept area of the rotor
m²
v
wind speed
m/s
Cp
power coefficient — the fraction actually captured, max 0.59 (Betz), practical ≈ 0.35–0.45
—
What it means
Because of the cube law, doubling wind speed gives 8× the power and tripling gives 27×; siting for wind speed matters more than anything else. Doubling rotor diameter gives 4× (area).
Worked example. ρ 1.2, D 40 m (A = 1,257 m²), v 8 m/s → ½ × 1.2 × 1,257 × 512 = 386 kW available; × Cp 0.4 = 154 kW.
Exam trap. Wind speed TRIPLES → energy output ×27, not ×3 (this was a corrected answer in the bank).
Hydro power
P (kW) = ρ × g × Q × H × η ÷ 1000 = 9.81 × Q × H × η
What each symbol means · unit
Q
water flow
m³/s
H
net head
m
η
turbine-generator efficiency (≈ 0.75–0.9)
—
ρ g
1000 × 9.81
—
What it means
Power comes from mass flow × height. Classification by size (book): micro up to 100 kW, mini 101 kW – 2 MW, small 2 – 25 MW, large above 25 MW.
Worked example. 3 m³/s, 10 m head, η 0.77 → 9.81 × 3 × 10 × 0.77 = 226.6 kW (2013/2017 objective). 20 L/s, 12 m, 60 % → 9.81 × 0.02 × 12 × 0.6 = 1.41 kW.
Exam trap. Convert litres/second to m³/s (÷1000) before using the formula.
Biomass gasifier conversion efficiency
η = (Gas produced × CV of gas) ÷ (Biomass fed × CV of biomass) × 100
What each symbol means · unit
Gas produced
producer gas volume
m³
CV of gas
≈ 1,000–1,200 kcal/m³ (low)
kcal/m³
CV of wood
≈ 3,200–4,000 kcal/kg
kcal/kg
What it means
Gasification converts solid biomass into producer gas (mainly CO + H₂ + CH₄ with N₂) by partial combustion; cold-gas efficiency is typically 60–75 %.
Worked example. 20 kg wood × 3,200 = 64,000 kcal; 46 m³ gas × 1,000 = 46,000 → 71.9 % (2014 book / 2012 & 2018 exam).
Exam trap. Producer gas = CO, H₂ and CH₄ (2021/2025 objective) — it is NOT mainly methane.
Input: hydrogen (fuel) + oxygen (air) → electricity + water + heat, by electrochemical reaction — no combustion
What each symbol means · unit
Efficiency
40–60 % electrical, higher with heat recovery
—
What it means
Like a battery that is refuelled instead of recharged; types PEM, PAFC, MCFC, SOFC by electrolyte. The INPUT is hydrogen (2015/2018 objective).
Exam trap. It is not a combustion device and its input is not electricity.
Wind power rises with the cube of speed
Doubling the wind speed gives eight times the power — the curve bends upward steeply.
Hydro — head and flow
Power is set by how far the water falls (head) and how much falls per second (flow); the turbine converts most of it.
Solar PV system
Panels make DC; the inverter makes AC for the building; surplus goes to the grid through the net meter.
Definitions the exam asks
Solar thermal
Flat-plate collectors (water heating, ≤ 100 °C), evacuated tubes, concentrating collectors (parabolic trough/dish for process heat and power); solar cookers, dryers, ponds.
Solar PV
Photovoltaic cells (silicon: mono, poly, amorphous) convert light directly to DC; inverter makes AC; grid-tied (net metering) or off-grid with batteries.
Wind
Horizontal-axis turbines dominate; sites need annual mean speed > 5–6 m/s; India's potential is concentrated in Tamil Nadu, Gujarat, Maharashtra, Karnataka, Rajasthan.
Biomass routes
Direct combustion, gasification (producer gas), anaerobic digestion (biogas), biofuels (ethanol from sugar/starch, biodiesel from oil seeds — jatropha).
Geothermal, tidal, wave, OTEC
Heat from the earth's crust; tidal range/stream; wave motion; ocean thermal energy conversion using the surface–deep temperature difference.
Availability-based tariff (ABT)
Grid tariff with fixed (capacity), variable (energy) and UI (unscheduled interchange) components to discipline frequency — appears in the 2023 long.
Net metering
Rooftop solar exports surplus to the grid; the meter runs backward, so the consumer pays only for net units.