GATE GG key terms

Heat released when a fuel burns completely and the water in the products stays as vapour, so the latent heat of vaporisation is not recovered. Used for combustion calculations where exhaust leaves above dew point.

Total heat released on complete combustion with products cooled so that water vapour condenses, recovering latent heat. Always greater than LHV for hydrogen-bearing fuels; the gap grows with fuel hydrogen content.

Routine coal test reporting moisture, volatile matter, fixed carbon and ash as mass percentages. Fixed carbon is obtained by difference. Used for combustion stoichiometry and boiler design rather than for precise chemical composition.

Elemental composition of a fuel, giving mass percentages of carbon, hydrogen, nitrogen, sulphur, oxygen and ash. Needed to compute stoichiometric oxygen demand and flue-gas composition precisely.

Minimum mass of air supplying exactly enough oxygen for complete combustion of unit mass of fuel, with no excess oxygen or unburnt fuel remaining. Computed from the ultimate analysis of the fuel.

Actual fuel-to-air ratio divided by the stoichiometric fuel-to-air ratio. Values below one mean lean (oxygen-rich) mixtures; above one mean rich mixtures. Its reciprocal is often called the excess air ratio.

Percentage by which actual air supply exceeds the stoichiometric requirement. Raises oxygen in flue gas, lowers flame temperature, and trades off reduced unburnt losses against higher stack heat loss.

Temperature attained by products of complete combustion when no heat is lost to surroundings. Depends on fuel heating value, reactant preheat and excess air; dissociation limits the real value at high temperature.

Ideal vapour power cycle with four processes: isentropic pump compression, constant-pressure heat addition in a boiler, isentropic expansion in a turbine, and constant-pressure heat rejection in a condenser. Basis for steam power plants.

Modification of the Rankine cycle in which steam expands partway, returns to the boiler for reheating at constant pressure, then expands again. Raises cycle efficiency and keeps exhaust moisture within turbine limits.

Bleeding part of the steam from a turbine to preheat boiler feedwater in closed or open heaters. Reduces irreversible heat addition at low temperature, lifting cycle efficiency at the cost of some turbine work.

Maximum efficiency of any heat engine operating between a hot source at temperature TH and a cold sink at TL, expressed as 1 minus TL/TH with absolute temperatures. A theoretical ceiling, not achievable in practice.

Statement that entropy of an isolated system never decreases, so heat cannot be fully converted to work in a cyclic device. Underpins limits on power plant and refrigeration performance.

Maximum useful work obtainable as a system reversibly reaches equilibrium with its environment. Unlike energy, exergy is destroyed by irreversibilities such as friction, mixing and finite-temperature heat transfer.

Ratio of actual work output to ideal work for a turbine, or ideal to actual work for a compressor, between the same inlet state and exit pressure. Measures how far a device deviates from reversible adiabatic operation.

Ideal gas turbine cycle with isentropic compression, constant-pressure heat addition, isentropic expansion and constant-pressure heat rejection. Its efficiency depends mainly on pressure ratio and peak temperature.

In a gas turbine, the fraction of the available exhaust-to-compressor-discharge temperature difference actually recovered by the heat exchanger. Higher values improve thermal efficiency but add pressure loss.

Power plant pairing a gas turbine (Brayton) with a steam turbine (Rankine) whose steam is raised by the gas turbine exhaust. Achieves thermal efficiencies well above either cycle alone.

Ratio of useful cooling or heating delivered to the work input of a refrigerator or heat pump. For a heat pump, COP equals the cooling COP plus one for the same operating temperatures.

Refrigeration cycle using a compressor, condenser, expansion valve and evaporator, exploiting latent heat of a refrigerant. The expansion valve is a throttling device, so the cycle is irreversible compared with Carnot.

Generation of electron-hole pairs in a semiconductor when photons of sufficient energy are absorbed, producing a voltage across a junction. Photons below bandgap energy pass through without contributing current.

Ratio of maximum power deliverable by a photovoltaic cell to the product of its open-circuit voltage and short-circuit current. A quality indicator; typical crystalline silicon values lie between 0.75 and 0.85.

Voltage across a photovoltaic cell under illumination with no external load, so no current flows. Falls with rising cell temperature and rises roughly logarithmically with irradiance.

Reference conditions for rating photovoltaic modules: irradiance of 1000 W/m2, cell temperature of 25 degrees Celsius, and an air mass 1.5 reference spectrum. Needed to compare nameplate ratings fairly.

Power available per unit swept rotor area, equal to one-half the air density times the cube of wind speed. The cubic dependence explains why site wind speed dominates turbine energy yield.

Maximum fraction, about 0.593, of the kinetic energy in an undisturbed wind stream that any turbine can extract. Arises from the need to slow the flow while maintaining mass continuity through the rotor disc.

Ratio of the linear speed of a rotor blade tip to the free-stream wind speed. Each turbine design has an optimum value for peak power coefficient; high-speed rotors use fewer blades.

Microbial breakdown of organic matter in the absence of oxygen, producing biogas (mainly methane and carbon dioxide) plus a nutrient-rich digestate. Used for energy recovery from waste and manure.

Partial oxidation of a carbonaceous feedstock at high temperature to yield a combustible synthesis gas of carbon monoxide and hydrogen. Distinct from combustion, which aims at full oxidation.

Energy released per unit volume or mass of biogas on combustion, strongly dependent on its methane fraction. Carbon dioxide dilution lowers the value, so upgrading is needed for pipeline injection.

Remaining charge in a cell or pack expressed as a percentage of rated capacity. Estimated from voltage, current integration or models, since it cannot be measured directly.

Fraction of a battery's rated capacity removed in a given cycle, complementing state of charge. Repeated deep cycling shortens life for many lithium-ion and lead-acid chemistries.

Ratio of charge extracted from a cell during discharge to charge supplied during the preceding charge. Values near unity indicate low parasitic side reactions and slow capacity fade.

Cell voltage corresponding to the Gibbs free energy change of the overall reaction divided by nF. It sets the thermodynamic ceiling; actual voltage is lower because of activation, ohmic and concentration losses.

Relation describing activation overpotential as a linear function of the logarithm of current density, with slope proportional to temperature and inversely to charge-transfer coefficient. Used to extract kinetic parameters from polarisation data.

Current density at which forward and reverse electrode reaction rates balance at equilibrium potential. A high value means fast kinetics and small activation loss in a fuel cell or electrolyser.

Storing heat or cold in a sensible, latent or thermochemical medium for later use. Sensible storage depends on specific heat and temperature swing; latent storage exploits phase-change enthalpy at near-constant temperature.

Substance chosen for its high latent heat of fusion over a narrow temperature range, used to store or buffer thermal energy. Common examples include paraffins and salt hydrates for building and solar applications.