Energy and Environment II: Hydropower, Tidal Energy, Ocean Thermal Energy Conversion, Wind, Geothermal and Solar Energy (Collectors, Photovoltaics and Solar Ponds), Bioenergy, and the Environmental Implications of Energy Use — Energy Patterns, CO₂ Emissions, Radiative Forcing and the Impacts of Large-Scale Energy Exploitation

The power in the wind passing through a rotor of swept area A is ½ρAv³, where ρ is the air density (about 1.2 kg/m³) and v the wind speed. Two consequences dominate wind engineering. First, power rises with the cube of wind speed, so doubling the speed multiplies the power by eight, and siting on windy ridges, coasts and offshore, and building taller towers to reach faster winds, matter enormously. Second, a turbine can extract only a fraction of this power, the power coefficient Cp, which cannot exceed the Betz limit of 16/27, about 59.3%, because the air must keep moving to leave the rotor; modern turbines reach about 45 to 50%. For an 80 m rotor (A ≈ 5,027 m²) in a 10 m/s wind the power in the wind is about 0.5 × 1.2 × 5,027 × 1,000 ≈ 3.0 MW, of which a turbine with Cp = 0.4 would extract about 1.2 MW. In India, Tamil Nadu and Gujarat lead in wind capacity, and offshore sites off Gujarat and Tamil Nadu are being explored.

Geothermal energy is heat from the Earth's interior, from radioactive decay and primordial heat, concentrated near plate boundaries and hot spots. Dry-steam plants use steam directly from the reservoir; flash plants lower the pressure of very hot water so that part flashes to steam; and binary-cycle plants pass moderately hot water through a heat exchanger to boil a low-boiling working fluid, extending use to lower temperatures. Ground-source heat pumps use the steady temperature of shallow ground for heating and cooling almost anywhere. Geothermal power is available round the clock, but it can release hydrogen sulphide, carbon dioxide and dissolved salts, arsenic and boron, and fluid extraction can cause subsidence and minor earthquakes. India's identified geothermal provinces, such as the Puga valley in Ladakh, Manikaran in Himachal Pradesh and Tattapani in Chhattisgarh, remain largely untapped.

Solar thermal collectors convert sunlight to heat. Flat-plate collectors, a blackened absorber plate under glazing with insulation behind, heat water or air to below about 100 °C for domestic and industrial use; evacuated-tube collectors lose less heat and reach higher temperatures. Concentrating collectors focus direct sunlight: parabolic troughs and linear Fresnel reflectors onto a pipe, parabolic dishes onto a point, and fields of heliostats onto a central receiver on a tower, reaching several hundred degrees to raise steam in concentrating solar power plants, which can store heat in molten salt for generation after sunset. Concentrators use only the direct beam and so need clear skies and tracking.

A photovoltaic cell converts light directly to electricity. It is a p-n junction of a semiconductor, usually silicon, whose band gap of about 1.1 eV means that photons with enough energy free electrons, which the junction's electric field drives through an external circuit as direct current. Cells are connected into modules and modules into arrays, and an inverter converts the output to alternating current. Crystalline silicon, monocrystalline or polycrystalline, dominates the market, with commercial module efficiencies of about 20%; thin-film technologies use cadmium telluride, copper indium gallium selenide or amorphous silicon, and perovskites are an emerging material. A single-junction cell cannot exceed the Shockley-Queisser limit of about 33%, because photons with less energy than the band gap are not absorbed and the excess energy of more energetic photons is lost as heat; multi-junction cells stack band gaps to exceed it. Output falls as cells get hotter and as dust accumulates.

A solar pond is a large, shallow body of water that collects and stores solar heat by preventing convection. In a salt-gradient pond, salinity increases with depth in three zones: a thin upper convective zone of nearly fresh water; a non-convective (gradient) zone in which salinity rises steadily with depth, so that water warmed below remains denser than the water above it and cannot rise; and a lower convective zone of concentrated brine that acts as the heat store and can reach about 80 to 90 °C. Heat is drawn off from the bottom for process heat, desalination or low-temperature power generation. India's best-known example is the solar pond at Bhuj in Gujarat, which supplied process heat to a dairy.

Biomass is renewable and, if regrown, close to carbon neutral, but only if the carbon released is reabsorbed by new growth and if land-use change and fossil inputs are counted. Burning biomass in open stoves remains one of the world's largest causes of indoor air pollution, which is why cleaner cookstoves, biogas and LPG matter for health. India's National Policy on Biofuels (2018), amended in 2022, brought forward the target of blending 20% ethanol in petrol to 2025–26, and the SATAT scheme (2018) promotes compressed biogas from waste.

Fossil fuels still supply about four-fifths of the world's primary energy, with oil the largest single source, followed by coal and natural gas; renewables are the fastest-growing source of electricity. India's energy system is more coal-dependent than the world's: coal supplies more than half of its primary energy and the majority of its electricity, oil is largely imported, and per-capita energy use is well below the world average. India's updated nationally determined contribution (2022) commits it to reducing the emissions intensity of its GDP by 45% from the 2005 level by 2030 and to about 50% of installed electric power capacity from non-fossil sources by 2030, and it has pledged net-zero emissions by 2070.

Emissions differ sharply between developed and developing countries, and it matters which measure is used. China has been the largest annual emitter of carbon dioxide since the mid-2000s, the United States is second and India third. Per person, however, India emits under half the world average, far below the United States and the European Union; and cumulatively, the measure that matters for warming because carbon dioxide persists for centuries, the United States has emitted the largest share since the industrial revolution, followed by the European Union and China. This is the scientific basis of the principle of common but differentiated responsibilities.

Radiative forcing is the change in the net energy flux at the tropopause, in W/m², caused by a change in a driver of climate, positive forcing warming the system. For carbon dioxide it is approximately ΔF = 5.35 ln(C ÷ C₀): doubling carbon dioxide gives 5.35 × ln 2 ≈ 3.7 W/m², and the rise from about 280 ppm to about 420 ppm gives 5.35 × ln 1.5 ≈ 2.2 W/m². The logarithm means each additional ppm adds a little less forcing than the one before. Global warming potentials compare other gases with carbon dioxide over 100 years: about 27 to 30 for methane and 273 for nitrous oxide in the IPCC's Sixth Assessment Report, and thousands for many fluorinated gases. That report found global surface temperature in 2011–2020 about 1.1 °C above 1850–1900 and gave a best estimate of 3 °C for the equilibrium warming from a doubling of carbon dioxide. Aerosols, mostly from the same fossil combustion, exert a negative forcing that masks part of the warming.