Energy Resources: Renewable and Non-Renewable Energy Explained

Where Our Energy Comes From

Every light bulb, phone charger, generator and factory depends on energy taken from somewhere: oil and gas from the Niger Delta, water stored behind a dam, sunlight on a rooftop panel. Some of these sources will run out, and some cause pollution and climate change, while others are clean and limitless but harder to harness. Understanding energy resources helps you answer exam questions, and it also helps you understand one of the biggest challenges facing Nigeria and the world. This tutorial covers energy resources as tested in WAEC, NECO and JAMB physics: the conservation of energy, renewable and non-renewable sources, how fossil-fuel, nuclear, hydroelectric, solar, wind, biomass and geothermal energy work, their advantages and disadvantages, power calculations for hydro, solar and wind, efficiency, and saving energy.

Energy Conversion and Conservation

The principle of conservation of energy states that energy cannot be created or destroyed. It can only be changed from one form to another. Every "energy source" is really a store of energy that we convert into a useful form, usually electricity:

Fuels store chemical energy, released by burning.
Dams store gravitational potential energy in raised water.
Wind carries kinetic energy.
Sunlight carries radiant (light) energy.
Uranium stores nuclear energy.

In every conversion, some energy is wasted, mostly as heat. The efficiency of a device is:

Efficiency = useful energy output ÷ total energy input × 100%

The same formula works with power instead of energy.

Renewable and Non-Renewable Resources

Non-renewable resources are used up far faster than they can be replaced, so they will eventually run out: coal, crude oil, natural gas and nuclear fuels such as uranium.
Renewable resources are naturally replenished and will not run out: solar, wind, hydroelectric, biomass, geothermal, tidal and wave energy.

Most renewable sources come, directly or indirectly, from the Sun. Sunlight drives the winds and the water cycle that fills dams, and it grows the plants used as biomass. Fossil fuels are themselves ancient sunlight, stored by plants and animals millions of years ago. Geothermal, tidal and nuclear energy are the exceptions.

Fossil Fuels

Coal, crude oil and natural gas formed over millions of years from the remains of plants and sea creatures. In a thermal power station, the fuel is burned to boil water, the high-pressure steam spins a turbine, and the turbine drives a generator. A modern gas-fired station can use the hot exhaust gases a second time (a combined-cycle plant), raising its efficiency.

Advantages: reliable, available day and night, a large output from compact stations, and existing infrastructure. Nigeria has large reserves of oil and gas, and most of its grid electricity comes from gas-fired stations.
Disadvantages: non-renewable. Burning releases carbon dioxide, the main cause of global warming, and also sulfur dioxide (acid rain), smoke and soot. Extraction can cause oil spills and environmental damage, and gas flaring wastes energy and pollutes the air.

Nuclear Energy

A nuclear power station uses heat from the controlled fission of uranium-235 to make steam, as explained in the tutorial on nuclear energy.

Advantages: no carbon dioxide while operating, and an enormous energy output from a tiny mass of fuel.
Disadvantages: non-renewable fuel, radioactive waste that stays dangerous for thousands of years, very high construction and decommissioning costs, and the risk of serious accidents.

Hydroelectric Power

A dam stores water at a height. As the water flows down through pipes, its gravitational potential energy becomes kinetic energy, which spins turbines connected to generators. Nigeria's largest hydroelectric stations, including Kainji, Jebba and Shiroro, are on the Niger and Kaduna rivers.

The electrical power output is:

P = η × ρ × g × Q × h

Here η is the efficiency, ρ is the density of water (1000 kg/m³), Q is the volume flow rate in m³/s, and h is the height through which the water falls.

Worked example 1

Water flows through a dam's turbines at 100 m³/s, falling through 50 m. If the station is 85% efficient, find its power output. (g = 9.8 m/s²)

P = 0.85 × 1000 × 9.8 × 100 × 50 = 4.165 × 10⁷ W ≈ 41.7 MW.

Advantages: renewable, no fuel costs, no greenhouse gases while running, quick to start up, and the reservoir can supply water for irrigation.
Disadvantages: very expensive to build, it floods land and displaces communities, it disrupts river ecosystems and fish, and output falls in the dry season when water levels drop.

Solar Energy

Solar photovoltaic (PV) cells convert sunlight directly into electricity, using semiconductors. Many cells together form a solar panel.
Solar thermal collectors use black pipes or panels to absorb sunlight and heat water.

The power from a solar panel is:

P = efficiency × intensity of sunlight × area

Worked example 2

A solar panel of area 1.6 m² is 20% efficient. In bright sunshine, the intensity of sunlight is 1000 W/m². Find its power output, and the energy it produces in a day with the equivalent of 5 hours of full sunshine.

P = 0.20 × 1000 × 1.6 = 320 W. Energy = 320 W × 5 h = 1600 Wh = 1.6 kWh.

Solar power suits Nigeria well, because the country receives plenty of sunshine all year round, especially in the north. Panels with batteries and an inverter are increasingly used in homes, shops, clinics and schools as a quiet, fuel-free alternative to petrol generators.

Advantages: renewable, silent, no fuel or emissions while running, and ideal for remote places far from the grid.
Disadvantages: no output at night and less on cloudy days, so batteries are needed for storage. Panels need a large area for a big output, and the initial cost is high.

Wind Energy

Wind turns the blades of a wind turbine, which drives a generator. The power available from the wind is:

P = ½ ρ A v³ × efficiency

Here ρ is the density of air (about 1.2 kg/m³), A is the area swept by the blades (πr²) and v is the wind speed. Because of the v³, doubling the wind speed gives 8 times the power, so turbines are placed where winds are strong and steady.

Worked example 3

A wind turbine has blades 20 m long. The wind speed is 8 m/s, and the turbine is 40% efficient. Find its power output.

A = π × 20² ≈ 1257 m². Power in the wind = ½ × 1.2 × 1257 × 8³ ≈ 386,000 W. Output = 0.40 × 386,000 ≈ 154 kW.

Advantages: renewable, no fuel or emissions, and the land beneath can still be farmed.
Disadvantages: the output varies with the wind, many turbines are needed for a large output, they can be noisy, and some people consider them unsightly.

Other Renewable Sources

Biomass: wood, crop waste and animal dung are burned, or fermented to make biogas (mainly methane) and biofuels such as ethanol. It is renewable if replanted, and it uses waste, but burning releases smoke, and large-scale cutting of firewood causes deforestation and desertification.
Geothermal: heat from hot rocks deep underground produces steam to drive turbines. It is reliable and clean, but only practical where hot rocks are near the surface, as in Iceland or Kenya.
Tidal and wave energy: the rise and fall of the tides, or the motion of waves, drives turbines. Tides are predictable, but suitable sites are few and construction is expensive.

Choosing Between Energy Resources

No single source is perfect. Planners balance:

Reliability: can it supply power whenever it is needed?
Cost: of building, fuel, maintenance and eventual decommissioning.
Environmental impact: greenhouse gases, pollution and effects on land and wildlife.
Availability: the local sunshine, wind, rivers or fuel reserves.

A sensible energy mix combines reliable sources with clean renewables, plus storage such as batteries.

Saving energy

The cleanest energy is energy that is never wasted. Switch off appliances when they are not in use, use LED bulbs (which use about a fifth of the power of filament bulbs for the same light), choose energy-efficient appliances, insulate buildings and service generators and vehicles regularly.

Common Mistakes with Energy Resources

Calling nuclear power renewable. Uranium is a non-renewable resource, even though nuclear power produces little carbon dioxide.
Saying energy is "used up" or "lost". It is converted into other forms, mostly heat that spreads out and becomes less useful.
Forgetting to cube the speed in wind-power calculations.
Forgetting the efficiency, or entering it as 85 instead of 0.85.
Claiming renewables have no disadvantages. Each has drawbacks, such as variable output, high initial cost or environmental impact.

Energy Resources: Practice Questions

Classify the following as renewable or non-renewable: coal, wind, uranium, biomass, natural gas, sunlight.
State the energy conversions in a hydroelectric power station.
Water falls through 80 m at 50 m³/s in a dam that is 90% efficient. Find the power output. (g = 10 m/s²)
A solar panel of area 2.0 m² and efficiency 18% receives sunlight of intensity 800 W/m². Find its output.
If the wind speed falls from 10 m/s to 5 m/s, by what factor does a turbine's power output change?
Give two environmental problems caused by burning fossil fuels.

Answers: 1. Renewable: wind, biomass and sunlight. Non-renewable: coal, uranium and natural gas. 2. Gravitational potential energy to kinetic energy (of the water and the turbine) to electrical energy. 3. 0.9 × 1000 × 10 × 50 × 80 = 3.6 × 10⁷ W = 36 MW. 4. 0.18 × 800 × 2.0 = 288 W. 5. The speed halves, so the power falls to (½)³ = ⅛ of its original value. 6. Any two of: global warming from carbon dioxide, acid rain from sulfur dioxide, air pollution from smoke and soot, and oil spills.

Conclusion

Energy cannot be created or destroyed, only converted, and every conversion wastes some energy as heat, so efficiency matters. Non-renewable resources (coal, oil, gas and uranium) are reliable but will run out, and fossil fuels cause pollution and global warming. Renewable resources (solar, wind, hydroelectric, biomass, geothermal and tidal) will not run out and are cleaner, but they often vary with the weather and cost more to build. Hydro power is P = ηρgQh, solar power is efficiency × intensity × area, and wind power is ½ρAv³ × efficiency, which depends strongly on the wind speed.