Energy sources - uses and management practice questions.

Twenty years after a large hydroelectric dam was built, farmers on the delta downstream report falling soil fertility and the coastline is retreating. The most likely cause is that the dam has

• raised evaporation losses from the reservoir surface
• increased the salinity of the river water upstream of the wall
• trapped the sediment load behind the wall
• cut the total annual discharge of the river to zero

Answer: trapped the sediment load behind the wall

A reservoir turns a fast river into still water, so the suspended sediment settles out behind the wall. The water leaving the dam is sediment-starved, which removes the annual nutrient input to the delta and lets coastal erosion outpace deposition; the Nile delta below the Aswan High Dam is the standard example.

Two energy sources are compared using the ratio of the energy a plant delivers over its lifetime to the energy needed to build, fuel and run it. On this measure a source can score poorly even though its fuel costs nothing at the point of use. The measure being used is

• energy return on investment
• capacity factor
• the energy density of the fuel
• the carbon intensity of the electricity generated

Answer: energy return on investment

Energy return on investment divides the energy a source delivers by the energy invested to obtain it, so a plant with free fuel but a very large construction energy cost can still score badly. Capacity factor measures how much of the rated output is actually achieved and says nothing about the energy spent building the plant.

Which environmental impact is most directly associated with hydraulic fracturing for shale gas?

• acidification of the oceans caused by dissolved carbon dioxide
• contamination of groundwater by fracturing fluids
• depletion of the stratospheric ozone layer above the well site
• eutrophication of nearby lakes by nitrate-rich run-off

Answer: contamination of groundwater by fracturing fluids

Fracturing fluid is injected at high pressure into the rock, and poorly sealed wells or surface spills can move it into aquifers — the impact tied to the technique itself. The other options are real environmental problems, but they are driven by combustion, CFCs and fertiliser use, not by fracturing.

A 40 MW onshore wind farm is built on a ridge directly above a small upland village. The turbines are seen and heard from the village, the electricity is fed into the national grid, and the owner of the ridge receives an annual rent for the site. Which group carries a cost of the scheme while receiving none of its direct benefits?

• Households in the village, whose outlook and quiet are altered while the electricity and the rent leave the area
• The landowner, who gives up free use of the ridge in return for the annual rent
• Electricity consumers elsewhere in the country, who receive the output of the wind farm
• The grid operator, which must reinforce the transmission network before the output of the new wind farm can be carried away

Answer: Households in the village, whose outlook and quiet are altered while the electricity and the rent leave the area

Energy schemes concentrate their local impacts and disperse their benefits: the landowner is paid, the grid and its consumers gain the output, and the neighbours receive neither. Naming who gains and who loses is more useful than listing impacts without an affected group.

A geothermal field has supplied electricity for 25 years. Steam pressure at the wellheads has fallen steadily and replacement wells must be drilled ever deeper into the same reservoir. Which statement describes this situation?

• Geothermal energy is non-renewable, because the heat stored in the interior of the Earth is finite
• Heat is being taken from the field faster than it is replenished, so the source is being used beyond its renewable rate
• The reservoir rock has cooled to the temperature of the surrounding crust, which ends geothermal generation at that site permanently
• A renewable source cannot be depleted, so the falling pressure must come from wear in the wells rather than from extraction

Answer: Heat is being taken from the field faster than it is replenished, so the source is being used beyond its renewable rate

Renewable does not mean unlimited: a source stays renewable only while extraction remains at or below the rate of natural replenishment. Reinjecting water and spacing the wells can keep a geothermal field within that rate.

A government reports that renewable sources supplied 45 % of the country's electricity in 2024, while the same report puts renewables at 18 % of total primary energy in that year. Which reason accounts for the gap between the two figures?

• Primary energy is measured after transmission losses, so the same generation appears smaller in that figure
• Hydroelectricity counts as renewable in the electricity figure but is excluded from the primary energy figure
• Electricity is only one part of final energy use; the country's heating, industry and transport still run mainly on fossil fuels
• The primary energy figure leaves out the electricity exported to neighbouring countries, and that is where most of the renewable output goes

Answer: Electricity is only one part of final energy use; the country's heating, industry and transport still run mainly on fossil fuels

A renewable share of electricity flatters the energy transition, because electricity is typically only about a fifth to a third of final energy demand. The wider indicator shows how much of the whole energy system has actually changed.