Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2012 / S4 / 3

Variability reduces firm capacity and creates balancing costs, but does not impose a universal tiny upper limit on the energy fraction from wind power. As an illustrative scale calculation, $10\mathrm{GW}$ installed at capacity factor $0.30$ produces $26.3\mathrm{TWh}$ annually, about $8.8\%$ of an assumed $300\mathrm{TWh}$ annual demand. Larger installations can supply larger fractions if the rest of the system is adapted.

Geographical spread, forecasting, interconnection, flexible generation, demand response and energy storage manage fluctuations on different timescales. They are not interchangeable: a battery that balances seconds does not necessarily cover a week of low wind. Correlated weather, curtailment, network constraints and reserve requirements become more important at high penetration. Backup capacity need not burn fuel continuously at the wind plant's nameplate output; energy and capacity should be accounted for separately. The defensible conclusion is that significant penetration requires system planning and has costs, not that variability alone confines wind to a negligible contribution.^[ref]

The judgement depends on the objectives and the replacement system, rather than following from a single technological fact. In the 2012 context, Germany had decided after Fukushima to accelerate its planned nuclear exit, with a timetable then extending to 2022. The contemporary official statement presented this as a reassessment of residual risk and a transition to efficiency and renewable supply.^[ref]

There is a serious climate opportunity cost if low-operational-carbon nuclear generation is replaced by unabated fossil generation. For example, replacing an illustrative $100\mathrm{TWh}\mathrm{yea}{\mathrm{r}}^{-1}$ with generation emitting $400$ or $900\mathrm{g}\mathrm{C}{\mathrm{O}}_2\mathrm{kW}{\mathrm{h}}^{-1}$ adds $40$ or $90$ million tonnes of carbon dioxide annually. Replacing that energy by wind at capacity factor $0.30$ would require about $38\mathrm{GW}$ of nameplate capacity, plus an adequate reliability plan; equal annual energy is not equal firm capacity.

Against continued operation must be weighed accident consequences, waste management, ageing, public preferences and the costs and feasibility of alternatives. Germany's specific risk assessment cannot be inferred solely from the different geography of Fukushima. A fast exit without credible low-carbon replacement can conflict with emissions goals; an exit supported by efficiency, infrastructure and low-carbon alternatives can pursue other legitimate objectives. Calling the decision intrinsically rational or irrational without those comparisons is not a technical conclusion. This answer evaluates the choice as posed in 2012, rather than importing later events as foreknowledge.

Tidal power offers predictable astronomical timing, while wave power can exploit an energetic marine resource. Neither is constant: tidal streams have slack-water periods, and wave energy varies with weather and season. Diversity can help a renewable portfolio, but resource size does not by itself establish economical extractable output.

For scale, a mean marine output of $5\mathrm{GW}$ would produce $43.8\mathrm{TWh}$ per year, about $15\%$ of an assumed $300\mathrm{TWh}$ annual electricity demand. Achieving that mean at capacity factor $0.30$ would require about $17\mathrm{GW}$ installed. This is an illustrative deployment requirement, not a measured UK resource estimate or a 2012 forecast. Tidal-stream kinetic power density scales as $\rho v^3/2$, making energetic sites valuable, but channel-wide extraction also changes the flow, so isolated-rotor power densities cannot simply be multiplied by sea area.

Harsh marine conditions, access for maintenance, survivability in storms, cable connections, navigation and ecological effects constrain deployment. Barrages additionally change estuaries and sediment transport. Research and demonstration may be worthwhile for energy diversity and industrial capability, but “important to investigate” is weaker than “indispensable at any cost”. Marine investment should be compared with efficiency and other low-carbon options using delivered energy, system value and environmental impacts.

The MENA solar resource could provide a useful supplement, but geographical potential does not make imports logically essential to European decarbonisation. With an illustrative annual mean insolation $200\mathrm{W}{\mathrm{m}}^{-2}$, photovoltaic conversion $15\%$ and system factor $0.8$, mean delivered-at-plant power is $24\mathrm{W}{\mathrm{m}}^{-2}$. A collector area of $10^4\mathrm{k}{\mathrm{m}}^2$ would then give $240\mathrm{GW}$ mean or about $2100\mathrm{TWh}$ annually before export losses. This is a scale calculation with stipulated assumptions, not a land-availability survey.

Long transmission corridors, converter losses, night-time supply, storage, dust, cooling or cleaning water, financing and political dependence must also be included. Solar thermal storage can shift some output but does not remove every seasonal or geopolitical risk. Projects should benefit host populations and avoid treating nominal desert area as empty infrastructure-ready land.

Europe also has domestic wind power, solar power, hydroelectricity, potential nuclear generation, demand reduction and other mitigation options. A technically credible combination of those is a counterexample to the word “essential”. MENA imports could reduce costs or improve diversity within a broader low-carbon system; whether they are preferable requires a complete system comparison rather than solar resource density alone.

A common minimum carbon price can make the external cost of emissions visible in investment and dispatch decisions, encourage reductions where they are cheapest, and reduce incentives to relocate emissions-intensive production to unpriced jurisdictions. A credible rising or durable price signal can be more useful for long-lived investment than a volatile short-term spot price.

For an illustrative price of $40$ currency units per tonne of carbon dioxide, generation emitting $0.9$ tonnes per MWh incurs $36$ units per MWh, while generation at $0.4$ tonnes per MWh incurs $16$. Those changes are large enough to affect some operating choices, but the actual switching point depends on fuel prices, efficiency and capacity constraints.

A price is not sufficient by itself. Measurement, enforcement, distributional effects and revenue use matter; infrastructure, innovation spillovers, information failures and landlord-tenant incentives can justify complementary policies. Nor is a uniform international minimum a logical prerequisite for any effective reduction: standards, clean-power deployment and efficiency policies can reduce emissions even before every country agrees. The strongest defensible claim is that broad, credible pricing is a valuable coordination instrument, not a uniquely necessary or automatically sufficient solution.^[ref]

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.