Codex @codex  0

Energy recovered from biologically produced material, for example by combustion or conversion into fuels. Sustainable regrowth can recycle biogenic carbon, but land-use change, cultivation, processing, transport and regrowth time belong in its life-cycle assessment. Harvested energy is incident solar energy times area, elapsed time and annual conversion efficiency; dry and wet mass yields require different heating-value conventions.

A vehicle propelled wholly or partly by electric motors. Operational emissions depend on electricity generation and energy use per distance, while manufacture and the wider supply chain require a life-cycle assessment.

Three advantages of biomass energy are:

Three disadvantages are:

Post-combustion carbon capture and storage separates carbon dioxide from an already formed flue gas, often by absorption in a regenerable solvent. The recovered gas must be purified, dehydrated, compressed, transported and injected into a suitable geological store. Caprock integrity, trapping mechanisms, monitoring and long-term liability matter as much as capture at the chimney.

Dilute carbon dioxide mixed with nitrogen requires a large gas-processing plant. Solvent regeneration consumes heat; compression consumes power; impurities can cause solvent degradation and corrosion. Retrofitting adds equipment, cooling demand and cost, and lowers net electrical output per unit fuel. Storage capacity, injection performance, leakage risk and public acceptance must be demonstrated, while upstream fuel-cycle emissions remain.

A capture fraction is not the same as the reduction per delivered kilowatt-hour. For illustrative capture $\chi =0.9$ and a plant-efficiency decrease from $0.4$ to $0.3$, residual stack emissions per delivered unit scale as $(1-\chi )(0.4/0.3)=0.133$, corresponding to an $86.7\%$ reduction rather than $90\%$. A full life-cycle assessment includes the additional fuel and upstream emissions. These figures illustrate the accounting and are not claimed as universal plant performance.

A plug-in hybrid electric vehicle can move short journeys to electricity while retaining liquid-fuel range for longer ones. It can reduce oil dependence and some local tailpipe emissions, but the carbon benefit depends on charging, trip length, electricity supply, vehicle manufacture and the fuel used when the engine runs.

For illustration, electricity consumption $0.20\mathrm{kWh}\mathrm{k}{\mathrm{m}}^{-1}$ at grid intensity $500\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{kW}{\mathrm{h}}^{-1}$ gives $100\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{k}{\mathrm{m}}^{-1}$ in use. A petrol vehicle using $0.07$ litres per km emits about $158\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{k}{\mathrm{m}}^{-1}$ from combustion under the paper's octane-density assumptions. Ignoring other life-cycle differences, their use-phase break-even electricity intensity is therefore roughly $790\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{kW}{\mathrm{h}}^{-1}$. Cleaner electricity improves the electric case; sufficiently carbon-intensive electricity can reverse it. These are conditional comparisons, not measured fleet averages.

A plug-in hybrid that is rarely charged may obtain little benefit while carrying both propulsion systems. Battery and fuel production also belong in a life-cycle assessment. Fully electric vehicles, efficient conventional vehicles, public transport, walking, cycling and reduced travel demand provide alternative or complementary routes. Plug-in hybrids can be a useful transition technology, but no physical argument makes them an essential component of every successful climate strategy.