Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S4 / 3

“Never” cannot be established by one present price comparison. A useful comparison is levelized cost with common financing, capacity and system boundaries. For illustration, solar capital cost 1000 currency units/kW, annual capital-plus-maintenance charge 8%, and capacity factor 0.15 imply about 61 units/MWh. A coal plant with fuel, maintenance and capital costs of the same order is not separated by an immutable physical barrier. These are illustrative inputs, not current price quotations. Carbon costs and air pollution can worsen coal's comparison; intermittency, land and balancing costs can worsen solar's. Compare delivered services as well as panel-level energy. The defensible conclusion is contingent competition, not a universal impossibility.

Electricity is a strong early decarbonization target because large stationary sources are comparatively accessible and a low-carbon grid enables cleaner vehicles, heat pumps and industry. For example a heat pump with coefficient of performance three divides electricity-related heat emissions by three per unit useful heat. But grid decarbonization does not automatically outrank every efficiency, methane, land-use or industrial intervention. Marginal abatement cost, cumulative emissions, construction time and available technology determine priorities. A very coal-intensive grid makes electrification alone less effective; a clean grid does not remove aviation or process emissions. The statement is a plausible strategic emphasis, not a deduction valid for every country and timescale.

For a generous illustrative roof area 100 m${}^2$, annual rain depth one metre and five-metre usable fall, maximum potential energy is $\rho A{h}_{\mathrm{rain}}gH\simeq 4.9$ MJ, only 1.36 kWh per year before conversion losses. Even 25 million roofs give only about 34 GWh annually, around $10^{-4}$ of a 300 TWh electricity system. Tiny intermittent flows and installation embodied energy make the practical result worse. Household roof-rain generation is therefore a very small carbon-saving resource under realistic domestic heights; the small gravitational drop and water flux explain why, without needing to assume an inefficient turbine.

A constant renewable contribution can come from geothermal resources, some hydro schemes and sustainably supplied biomass; variable resources can also contribute to firm supply through geographic diversity, storage, interconnection and demand management. These examples refute the absolute “never.” They do not show that every renewable mix can economically supply an unchanged constant demand without additional resources. Baseload is a load/system service rather than an intrinsic virtue of one generator. Long low-wind periods and seasonal storage require energy-capacity accounting, while dispatchable clean systems may make an inflexible minimum-output plant less desirable.

At fixed electrical output, improving efficiency from ${\eta }_0$ to ${\eta }_1$ reduces fuel and associated combustion emissions by $1-{\eta }_0/{\eta }_1$. A change from 0.33 to 0.45 saves about 27%. If an illustrative large coal fleet emits 3.6 Gt annually, that is nearly 1 Gt, exceeding a hypothetical 0.5 Gt national total and far exceeding a 0.1 Gt electricity-sector total. Thus the comparison can be true on reasonable scale assumptions. It is not established by the words “state of the art”: fleet sizes, existing efficiencies, feasible retrofit fractions and whether UK electricity or all UK energy is meant must be specified. Continued growth and rebound can offset technical savings, and the options are not mutually exclusive.

Storage has multiple performance measures: energy and power density, efficiency, cycle/calendar life, duration, cost, materials, safety and siting. Lithium-ion batteries offer high round-trip efficiency, fast response and compactness, making them useful for vehicles and short-duration grid services. That does not make them optimal for every duty. Pumped hydro can suit large stationary energy stores where geography permits; thermal stores can serve heat directly; chemical fuels or other long-duration systems can trade efficiency for cheap energy capacity. A technology that is excellent per kilogram may be poor for seasonal storage. “Best” requires a specified service and life-cycle comparison.

Fission power combines dense fuel and low direct carbon emissions with high capital commitments, long build schedules, financing risk and expensive decommissioning. Environmental assessment must include mining, enrichment and fabrication, cooling-water demands, normal releases, severe-accident risk and comparison with avoided fossil pollution. None of these is captured by operational carbon emissions alone.

Waste requires containment and isolation appropriate to activity, heat production and decay times. Short-lived operational waste and spent fuel have different handling needs, while geological disposal depends on engineered and natural barriers plus credible institutions. Recycling changes inventories and resource use but does not remove all waste or proliferation concerns. Public consent, transparent regulation, security and intergenerational responsibility are part of the challenge. Quantitative comparisons should specify reactor type, financing and waste policy rather than treating either zero risk or inevitable catastrophe as a physical law.