Exercises: Energy and mineral resources

Classify these energy resources into renewable and non-renewable: coal, solar energy, natural gas, wind energy, uranium and hydroelectric energy.

The diagram shows an anticline petroleum trap, with three numbered geological elements (1–3) and fluids separated by density. Name the three elements (source rock, reservoir rock and cap rock), explain why oil and gas are trapped in this position, and summarise how petroleum is formed.

What is biomass as an energy resource, and why is it only considered renewable if managed sustainably?

What is a fossil fuel and what are its three main forms? What do their origin and their atmospheric impact have in common?

Which physical process does a nuclear power plant harness and how does it differ from coal combustion in terms of atmospheric emissions and waste generated?

What is the difference between primary energy, such as coal, uranium or the sun, and final energy, such as electricity or petrol? Why are there losses between the two?

What is the calorific value of a fuel and why does anthracite have a higher one than peat? Relate it to the rank of coalification.

What is a pumped-storage hydroelectric plant and which storage function does it fulfil in an electricity system with a lot of solar and wind energy?

Coal originates from ancient buried and transformed forests. Order the series of increasing rank: bituminous coal, peat, anthracite and lignite, and explain how carbon content and moisture vary along the series.

A hydroelectric power station and a wind farm are renewable facilities, but not harmless. Compare their environmental impacts (land, wildlife, landscape) and their ability to respond to electricity demand.

Open-cast coal mining leaves large pits and spoil tips. Describe its impact on landforms, water bodies and soil, and explain what a mine restoration plan should include.

Oil forms from buried organic matter. Explain the role of kerogen and of the "oil window" (the temperature interval of formation), and what happens if burial exceeds those temperatures.

Compare natural gas and oil: common origin, state and composition, transport (gas pipelines and liquefied gas versus oil pipelines) and main uses.

Hydraulic fracturing (fracking) extracts gas from shale rocks. Explain what the technique consists of and analyse two advantages (new resources, energy independence) and two risks (induced seismicity, water pollution).

Which energies do tidal and wave power plants harness and how do they differ? Assess their current development compared with solar and wind energy.

What is the capacity factor of a renewable source? Explain why the intermittency of solar and wind energy requires backup systems (pumped storage, batteries, interconnections) and how they are managed.

Imagine that a report published in the press states: "Given that Spain already generates more than 50% of its electricity with renewable sources (wind and solar), its foreign energy dependence has been drastically reduced and lacking sedimentary basins rich in hydrocarbons has ceased to condition its economy."

1. a) Technically refute this statement by distinguishing between the electricity generation mix and the primary energy mix in Spain, justifying why peninsular geology continues to determine strategic vulnerability and the foreign energy bill.
2. b) Design two sectoral proposals to decouple heavy transport and thermal industry from imported fossil fuels, evaluating the geological or critical materials bottlenecks associated with each.

A company is considering two sites in Spain to install a binary cycle or steam geothermal power plant that requires at least 150 °C at less than 2 km depth to be economically viable:

• Borehole A (Tertiary sedimentary basin in stable continental crust): average surface temperature of 14 °C and average geothermal gradient of 3,0 °C per 100 m.

• Borehole B (volcanic terrain in the Canary Islands on a shallow thermal anomaly): average surface temperature of 20 °C and local geothermal gradient of 8,5 °C per 100 m.

1. a) Calculate the exact depth at which 150 °C is reached in each borehole and justify the technical and economic feasibility of drilling in both cases.
2. b) Describe the four indispensable elements of a high-enthalpy geothermal reservoir (heat source, permeable aquifer, cap rock, and recharge mechanism) and predict what hydrogeological or induced seismicity problems could arise if the reservoir of the viable borehole is overexploited without fluid reinjection.

A Carbon Capture and Geological Storage (CCS) project is being evaluated to confine 1,5 million tonnes of $\text{CO}_2$ per year from a steel mill. Two formations are available:

• Formation A: highly permeable sandstone at 450 m depth, brine-saturated, sealed by 60 m of massive, unfractured siltstones.

• Formation B: porous sandstone at 1100 m depth, brine-saturated, capped by 150 m of impermeable anhydrites and clays, crosscut at one end by a fault inactive since the Miocene.

1. a) Based on the average geothermal gradient (3,0 °C/100 m, $T_{\text{sup}} = 15\ \text{°C}$), the normal hydrostatic gradient (~1,0 MPa per 100 m), and knowing that the critical point of $\text{CO}_2$ is at $T_{\text{c}} = 31,1\ \text{°C}$ and $P_{\text{c}} = 7,38\ \text{MPa}$, determine in which of the two formations the $\text{CO}_2$ will remain in a supercritical state, and justify why this density condition is essential for the volumetric feasibility of the storage.
2. b) Evaluate the risks of leakage and induced seismicity in both formations, and design a monitoring programme (reflection seismics and geochemical piezometry) to detect early leaks into overlying drinking-water aquifers.

A wind farm with 20 wind turbines is planned on a limestone mountain ridge at an altitude of 1300 m, with average winds of 7,5 m/s. The crest features karst sinkholes (dolines) and limestone pavements (karren) that feed the water supply springs of two valleys, and constitutes the daily flight corridor for a colony of griffon vultures (Gyps fulvus) and Egyptian vultures (Neophron percnopterus).

1. a) Analyze the specific geological and hydrogeological impacts resulting from the opening of access tracks and the excavation of footings in a karst massif, predicting how they may alter infiltration and water turbidity in the springs.
2. b) Design a tiered plan of preventive and corrective measures to reconcile the facility with conservation: determine what technical conditions you would require for turbine placement, what automated bird detection and shutdown systems you would implement, and how you would solve the electrical evacuation to minimize landscape fragmentation.

Following the definitive shutdown of fission reactors, the dilemma of managing high-level radioactive waste (spent fuel) arises: storing it in surface-level Individualized Interim Storage Facilities (ATI) for 80 years or confining it in a Deep Geological Repository (DGR) at a depth greater than 500 m in a compact clay or granite formation.

1. a) Geologically justify the superiority of a DGR as a passive multi-barrier solution over a timescale of thousands of years compared to a surface ATI, comparing its resilience against climate crises, earthquakes, and military conflicts.
2. b) Evaluate the petrophysical advantages and disadvantages of siting the DGR in a deep clay formation versus a granitic batholith (consider permeability, plasticity/self-sealing, and hydraulic fracturing).
3. c) Refute the proposal to replace nuclear energy by increasing coal or gas combustion to "avoid the waste problem", contrasting the temporal toxicity, volume per GWh generated, and mortality attributable to atmospheric emissions of both cycles.

An average electric vehicle requires multiplying by six the mass of critical minerals (lithium, cobalt, nickel, manganese, and rare earths) compared to an internal combustion vehicle. 70% of cobalt comes from the Democratic Republic of the Congo and over 65% of lithium and rare earth refining is geographically concentrated in China. In Spain, hard-rock lithium mining projects (Cáceres) and rare earth projects (Ciudad Real) face strong social and environmental opposition.\n\na) Analyze the territorial and environmental inconsistency involved in adopting massive electrification policies while externalizing the ecological and social footprint of extractive mining to countries with lax environmental or labor frameworks.\nb) Design a set of three strategic measures (technical and regulatory) for the European Union to reduce supply vulnerability for these metals without deteriorating local ecosystems, critically assessing to what extent closed-loop recycling can replace primary mining over the next 15 years considering the growth of the battery fleet.

Observe the following typical values of Energy Return on Investment (EROI = Energy obtained / Energy invested):\n- Conventional oil extracted in 1930: ~100:1\n- Current conventional oil: ~15:1\n- Tar sands and shale oil (fracking): ~4:1\n- Modern onshore wind: ~18:1\n- Solar PV with chemical battery storage: ~4:1 (compared to ~10:1 without batteries)\n\na) Interpret the historical decline in the EROI of conventional oil based on the law of diminishing returns and the geological constraints of extraction.\nb) Several bioeconomists argue that a society with public healthcare, education, and advanced research requires an average system EROI of at least 10:1. Critically evaluate the risk that renewable electrification could trigger an "energy cliff" if electrochemical storage is overbuilt to address intermittency.\nc) Predict what structural adaptations (in energy demand, per capita consumption, and spatial patterns) a country will need to undertake if the net EROI of its mix falls below 5:1.

An industrial project plans to supply a steel plant with green hydrogen produced via water electrolysis powered by a solar photovoltaic array (electrolyzer efficiency: 65%). Since solar radiation is seasonal, the plan is to store the hydrogen under pressure underground and re-electrify a fraction using fuel cells (fuel cell efficiency: 55%; compression and transport losses: 10%).\n\na) Calculate the overall energy efficiency of the entire PV-hydrogen-electricity chain and justify why hydrogen is classified as an energy carrier and not as a primary source. Based on the results, technically refute the use of green hydrogen for domestic heating compared to geothermal/aerothermal heat pumps.\nb) Justify why surface metal tanks are ruled out for massive seasonal storage and salt caverns dissolved in bedded salt formations (deep halite) are chosen instead, comparing the tightness and reactivity of salt against storage in depleted gas fields in porous sandstones (evaluate porosity, molecular leakage via diffusion, and activity of methanogenic or sulfate-reducing bacteria).