Unit 6: Energy sciences

Energy science studies how chemical, nuclear and electronic systems store and release usable energy. This unit hangs on one governing idea: energy is released when high-energy bonds or nuclei convert to lower-energy states, and engineering measures, stores and delivers that release efficiently.

• Fuel: a substance that liberates large, controllable amounts of heat on combustion or reaction, mainly through C–H and C–C oxidation.
• Fuels and its types: classified by state and origin.
  • Solid: wood, coal, coke, charcoal — cheap, easy to store, but high ash and smoke.
  • Liquid: petrol, diesel, kerosene, fuel oil — high calorific value, clean handling, volatile.
  • Gaseous: LPG, CNG, water gas, producer gas, natural gas — clean burning, easy control, high storage risk.
  • Primary vs secondary: natural (coal, petroleum) versus derived (coke, petrol, coal gas).
• Basis for all comparison: the energy released per unit mass or volume, i.e. calorific value.

II. Calorific Value and Fuel Analysis

Quantifying fuel quality by heat content and composition.

A. Calorific value of fuels

The heat liberated by complete combustion of unit quantity of fuel.

• Definition: heat released when 1 kg (solid/liquid) or 1 m³ (gas) is completely burnt.
• Units: kcal/kg, kJ/kg, or kcal/m³, kJ/m³.
• Gross (Higher) Calorific Value (GCV/HCV): heat released when combustion products are cooled to room temperature and water vapour condenses, recovering its latent heat.
• Net (Lower) Calorific Value (NCV/LCV): heat available when water escapes as vapour.

NCV = GCV − (mass of H2O formed × latent heat of steam)
NCV = GCV − 0.09 × H × 587   (kcal/kg)

• H: percentage of hydrogen in fuel; 0.09H is water formed (9 g water per g H); 587 kcal/kg is latent heat of steam.

B. Determination of calorific value

Measured experimentally by burning a known mass and tracking heat gained by water.

• Bomb calorimeter: solid/liquid fuel burnt in a steel bomb charged with O₂ at ~25 atm, immersed in a known mass of water.
• Principle: heat lost by fuel = heat gained by water and calorimeter.

GCV = [(W + w) × (t2 − t1)] / m     (kcal/kg)

• W: mass of water; w: water equivalent of calorimeter; t₂−t₁: temperature rise; m: mass of fuel.
  • Boys/Junkers calorimeter: used for gaseous fuels at constant pressure.

C. Analysis of coal

Determines the commercial quality of coal through two tests.

1. Proximate analysis — determines four fractions by heating.
   • Moisture: heat 105–110 °C; high moisture lowers effective calorific value.
   • Volatile matter: heat 950 °C in absence of air; governs flame length and smoke.
   • Ash: residue after burning; inert, lowers value and causes clinker.
   • Fixed carbon: by difference; higher fixed carbon means higher calorific value.
2. Ultimate analysis — determines elemental composition (C, H, N, S, O) for exact NCV and pollution assessment.
   • C and H: by combustion, weighed as CO₂ and H₂O.
   • S: oxidised to sulphate, precipitated as BaSO₄; predicts SO₂ emission.

III. Primary Cells

Non-rechargeable cells; reaction is irreversible and cell is discarded after use.

A. Primary cells

Convert chemical energy to electrical energy once, with no practical recharge.

• Dry cell (Leclanché): Zn anode, graphite/MnO₂ cathode, NH₄Cl–ZnCl₂ paste electrolyte; ~1.5 V.
  • Anode: Zn → Zn²⁺ + 2e⁻
  • Cathode: 2MnO₂ + 2NH₄⁺ + 2e⁻ → Mn₂O₃ + 2NH₃ + H₂O
• Uses: torches, clocks, remotes — low current, intermittent duty.
• Limitation: voltage drops during discharge; cannot be reused.

IV. Secondary Batteries

Rechargeable cells; electrode reactions are reversible, so electrical energy stored as chemical energy and recovered repeatedly.

A. Lead storage battery

Reversible Pb/PbO₂ acid cell used in vehicles; ~2 V per cell, six cells give 12 V.

• Anode (discharge): Pb + SO₄²⁻ → PbSO₄ + 2e⁻
• Cathode (discharge): PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO₄ + 2H₂O
• Electrolyte: ~38% H₂SO₄; specific gravity drops on discharge, so density measures charge.
• Charging: reverses both reactions, regenerating Pb and PbO₂.
• Merits/limits: high surge current and cheap, but heavy and lead is toxic.

B. Ni-Cd

Nickel–cadmium alkaline rechargeable cell; ~1.2 V.

• Anode: Cd + 2OH⁻ → Cd(OH)₂ + 2e⁻
• Cathode: 2NiO(OH) + 2H₂O + 2e⁻ → 2Ni(OH)₂ + 2OH⁻
• Electrolyte: KOH.
• Features: long cycle life, tolerant of abuse; suffers "memory effect" and Cd toxicity.

C. Ni-metal hydride battery

Replaces cadmium with a hydrogen-absorbing alloy (MH), giving higher capacity and lower toxicity; ~1.2 V.

• Anode: MH + OH⁻ → M + H₂O + e⁻
• Cathode: NiO(OH) + H₂O + e⁻ → Ni(OH)₂ + OH⁻
• M: intermetallic alloy (e.g. LaNi₅) that stores hydrogen atoms.
• Uses: hybrid vehicles, camera packs; higher energy density than Ni-Cd but self-discharges faster.

D. Li-ion battery

Lithium ions shuttle between intercalation electrodes; high energy density, ~3.6 V.

• Anode: graphite (LiC₆); Cathode: LiCoO₂.
• Discharge: LiC₆ → C₆ + Li⁺ + e⁻ (anode); Li⁺ + e⁻ + CoO₂ → LiCoO₂ (cathode).
• Electrolyte: lithium salt (LiPF₆) in organic solvent.
• Merits: light, high voltage, no memory effect; risk of thermal runaway.

E. Li-air battery

Uses atmospheric O₂ as cathode reactant, giving very high theoretical energy density.

• Anode: Li → Li⁺ + e⁻
• Cathode: 2Li⁺ + O₂ + 2e⁻ → Li₂O₂
• Significance: energy density approaches gasoline because O₂ is not stored on board.
• Limitation: poor cycle life, clogging by discharge products, and moisture sensitivity keep it experimental.

V. Fuel Cells

Devices that convert chemical energy of a continuously supplied fuel directly into electricity, without combustion.

A. Principles

Electricity is generated by electrochemical oxidation of fuel and reduction of oxidant at separated electrodes.

• H₂–O₂ cell: fuel and oxidant fed continuously; no charging needed.
  • Anode: H₂ + 2OH⁻ → 2H₂O + 2e⁻
  • Cathode: ½O₂ + H₂O + 2e⁻ → 2OH⁻
  • Overall: H₂ + ½O₂ → H₂O, EMF ≈ 1.23 V.
• Efficiency: not limited by Carnot cycle, so 40–70% conversion, higher with heat recovery.

B. Applications

• Space missions: power and drinking water in Apollo/Gemini craft.
• Transport: fuel-cell electric vehicles and buses.
• Stationary power: backup and combined heat-and-power units.

C. Advantages/disadvantages

1. Advantages: high efficiency, quiet, water is the only emission, no moving parts.
2. Disadvantages: costly Pt catalysts, hydrogen storage difficulty, and slow start-up limit spread.

VI. Hydrogen Energy

Hydrogen as a storable, clean energy carrier with the highest energy per unit mass (~142 kJ/g).

A. Production

• Steam reforming: CH₄ + H₂O → CO + 3H₂ — cheap but CO₂-emitting ("grey").
• Electrolysis of water: 2H₂O → 2H₂ + O₂ — clean ("green") when powered by renewables.
• Others: coal gasification, photo-biological and thermochemical splitting.

B. Storage

• Compressed gas: 350–700 bar cylinders; simple but bulky.
• Liquid hydrogen: cryogenic at −253 °C; dense but needs insulation and suffers boil-off.
• Metal hydrides: absorbed in alloys (LaNi₅H₆); safe, reversible, heavier.

C. Safety aspects

• Wide flammability: ignites over 4–75% in air, so leaks are hazardous.
• Small molecule: diffuses fast and embrittles metals; needs special seals.
• Mitigation: flame detectors, ventilation, and odourless-gas sensors.

D. Significance as a clean and sustainable energy carrier

• Zero-carbon use: combustion or fuel-cell use yields only water.
• Energy storage: stores surplus solar/wind energy as chemical fuel.
• Versatility: links power, transport and industry into one clean cycle.

VII. Nuclear Energy

Energy released from changes in the atomic nucleus, millions of times denser than chemical fuels.

A. Principles of nuclear energy

• Mass–energy equivalence: E = mc²; small mass defect releases huge energy.
• Fission: heavy nucleus splits; ²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n + energy.
• Chain reaction: released neutrons sustain further fission, controlled by moderators (graphite/water) and control rods (Cd, B).
• Fusion: light nuclei merge; ²H + ³H → ⁴He + n, powering stars, still hard to contain.

B. Sustainable energy production

• High density, low carbon: one uranium pellet ≈ one tonne of coal, with no CO₂.
• Reliability: base-load power independent of weather.
• Sustainability challenges: finite uranium, radioactive waste, and disposal; breeder reactors and fusion aim to extend fuel supply.

VIII. Spintronics

Electronics that exploit the electron's spin, not just its charge, for information storage and logic.

A. Spintronics and its engineering applications

Uses the two spin states (up/down) as an extra degree of freedom to cut power and raise density.

• Principle: spin-polarised currents change resistance of magnetic layers.
• Giant Magnetoresistance (GMR): resistance depends on relative magnetisation of layers; basis of read heads.
• Applications:
  • Data storage: GMR/TMR read heads in hard disks; MRAM non-volatile memory.
  • Sensors: magnetic-field and position sensors in automotive systems.
  • Computing: spin transistors and logic promising low-power, fast, non-volatile devices.
• Advantage: retains data without power and switches with little energy, aiding sustainable computing.