Renewable Energy — Practice · Grade 12 Science

Chapter: Energy, Water & Material Resources

What this practice covers

By the end of Renewable Energy you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: renewable energy, photovoltaic (PV) cell, intermittency, capacity factor.

Problem 1

Which device converts sunlight directly into electricity?
  1. A wind turbine
  2. A photovoltaic (solar) cell
  3. A steam turbine
  4. A geothermal well
Answer and explanation

A photovoltaic (solar) cell

A photovoltaic cell turns sunlight straight into electricity with no moving parts.

Problem 2

Which renewable energy source uses the kinetic energy of moving air?
  1. Hydroelectric
  2. Geothermal
  3. Wind
  4. Biomass
Answer and explanation

Wind

Wind turbines capture the kinetic energy of moving air and convert it into electricity.

Problem 3

A solar farm has a nameplate capacity of 80 MW but averages only 20 MW of output over the year. What is its capacity factor, as a percent?
Answer and explanation

25 %

Capacity factor = average output / maximum output = 20 / 80 = 0.25 = 25%.

Problem 4

Which of these is a genuine drawback of large hydroelectric dams?
  1. They produce no electricity at all during the day
  2. They cannot be turned on or off to match demand
  3. They release more CO2 than a coal plant
  4. They can flood large areas of land and block fish migration
Answer and explanation

They can flood large areas of land and block fish migration

Dams flood land behind them and block migrating fish, and they trap sediment and reshape rivers — real ecological trade-offs even though the power itself is clean.

Problem 5

'Intermittency' means that a power source...
  1. only produces power when its natural input, like sun or wind, is available
  2. produces exactly the same output every hour of the day
  3. can never be stored in any form
  4. releases greenhouse gases as it runs
Answer and explanation

only produces power when its natural input, like sun or wind, is available

Intermittent sources such as solar and wind generate power only when the sun shines or the wind blows, not on demand.

Problem 6

Biomass is sometimes described as 'carbon-neutral.' Explain the reasoning behind that claim and one important condition that must be met for it to actually hold true.
Answer and explanation

Burning biomass releases CO2, but the plants pulled that same CO2 out of the air through photosynthesis while they were growing, so in principle the net addition to the atmosphere can be close to zero. For this to truly hold, the plants must actually be regrown at the same rate they are burned, so the released carbon is reabsorbed. If forests are cleared faster than they regrow (or if lots of fossil fuel is used to grow, harvest, and transport the biomass), it is not really carbon-neutral.

Problem 7

A wind generator supplies average 2 kW over 10 h. Find energy. Show the reasoning, not just the conclusion.
Hints

Rated power and actual time-integrated energy differ.

Answer and explanation

20 kWh.

Problem 8

Why might renewable deployment still need storage, transmission or demand management?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Supply varies in time and location; useful service requires matching production to demand and grid constraints.

Problem 9

Energy timing and storage losses can change adequacyEnergy timing and storage losses can change adequacyHour blockDemandGenerationStorage change beforelossesDaylight10 kWh16 kWh+6 kWhNight8 kWh0 kWh−8 kWh neededDaily total18 kWh16 kWhDeficit before lossesA daily energy total does not guarantee hourly adequacy.
Illustrative two-block supply/demand account; it is not an optimized real grid design.
Interpret the supplied visual. Can loss-free storage alone eliminate the displayed net daily 2 kWh deficit indefinitely?
Answer and explanation

No. It can shift timing but cannot replace a persistent energy deficit without another input.

A source can produce substantial daily energy while failing to meet demand at certain hours. Storage shifts energy through time but has capacity, power and round-trip-loss limits; it does not create the missing energy in a daily deficit. The displayed 16 kWh generation is already below 18 kWh demand before losses. Compare time-resolved supply and demand, initial storage state and allowable shortfalls rather than assuming an annual energy total guarantees continuous service.

No. It can shift timing but cannot replace a persistent energy deficit without another input.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

A 5 kW system has 30% capacity factor for 2000 h. Find generated energy. If storage returns 80% of an allocated 500 kWh, how much useful stored energy returns?
Answer and explanation

Generation=5×0.30×2000=3000 kWh. The allocated storage input returns 400 kWh and loses/transfers 100 kWh under the stated boundary; do not apply the storage loss to energy that bypasses it.

Generation=5×0.30×2000=3000 kWh.

The allocated storage input returns 400 kWh and loses/transfers 100 kWh under the stated boundary; do not apply the storage loss to energy that bypasses it.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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