Energy Resources and Efficiency
Renewable and Non-Renewable Energy Sources
Every power station or device that generates electricity must draw on an energy source. These sources split into two broad categories. Non-renewable sources — coal, oil, natural gas and nuclear fuel — will eventually run out. Fossil fuels formed over millions of years from ancient organic matter; once burned they are gone. Nuclear fuel (uranium) is extracted from finite ore deposits. Renewable sources are naturally replenished: solar, wind, hydroelectric, tidal, wave, geothermal and biomass.
Non-renewable sources (fossil fuels and nuclear) will eventually run out. Renewable sources (solar, wind, hydro, tidal, wave, geothermal, biomass) are replenished naturally. Nuclear does not produce CO2 during generation but creates radioactive waste.
Comparing Energy Sources
Each source has distinct advantages and disadvantages that must be weighed against each other:
Fossil fuels release carbon dioxide when burned, contributing to the greenhouse effect and climate change. They also release sulfur dioxide (which causes acid rain) and particulates. Despite this, fossil fuels remain significant because they are reliable, energy-dense and can generate electricity on demand.
Nuclear power produces no CO2 during generation and can run continuously as a baseload source. The drawback is radioactive waste, which remains hazardous for very long periods and requires secure storage. Nuclear stations also carry a small but serious risk of accidents.
Renewable sources produce little or no CO2 during operation, making them essential for reducing carbon emissions. Their main challenges are intermittency (wind and solar depend on weather) and, for some, limited suitable locations (tidal, geothermal).
Solar: no pollution during operation, negligible running costs. Disadvantages: depends on sunlight, large area needed for solar farms, panels are costly to manufacture.
Wind: no fuel cost, no direct emissions. Disadvantages: intermittent, visual impact, noise, potential harm to wildlife.
Hydroelectric: reliable, can respond quickly to changes in demand. Disadvantages: damming rivers floods valleys, displaces communities and disrupts habitats.
Tidal: highly predictable because tides are regular. Disadvantages: expensive infrastructure, limited suitable estuaries, risk to marine ecosystems.
Geothermal: reliable, low emissions, compact land use. Disadvantages: only viable where hot rocks are near the surface, high initial drilling costs.
Biomass: theoretically carbon-neutral (CO2 absorbed during plant growth equals CO2 released during burning). Disadvantages: still produces pollutants, and land used for fuel crops competes with food production.
Calculating Efficiency
No energy transfer is perfectly efficient. Some energy is always wasted — usually as heat or sound. Efficiency tells us how much of the energy input is usefully transferred. The equation is:
$$\text{efficiency} = \frac{\text{useful output energy transfer (J)}}{\text{total input energy transfer (J)}}$$
Efficiency has no units and will always be between 0 and 1 (or 0% and 100%). To express as a percentage, multiply the decimal answer by 100. For example, if a motor receives 500 J of electrical energy and transfers 350 J as useful kinetic energy, its efficiency = \(\frac{350}{500} = 0.70\), or 70%. The remaining 150 J is wasted, mostly as heat in the motor windings.
A wind turbine receives 2400 J of energy from the wind in one second. It transfers 960 J as useful electrical energy. Calculate its efficiency and identify the wasted energy.
Step 1: Write the equation. \(\text{efficiency} = \frac{\text{useful output}}{\text{total input}}\)
Step 2: Substitute values. \(\text{efficiency} = \frac{960}{2400} = 0.40\)
Step 3: Convert to percentage: \(0.40 \times 100 = 40\%\)
Step 4: Wasted energy = 2400 − 960 = 1440 J, mostly lost as heat due to friction in the bearings and turbulence in the air flow.
Efficiency = useful output energy ÷ total input energy. The answer is a decimal between 0 and 1 (multiply by 100 for a percentage). Efficiency can never exceed 1 (100%) because energy cannot be created.
Reducing Wasted Energy
Increasing efficiency means reducing the energy lost to unwanted pathways. Common techniques include:
Lubrication: oil or grease between moving parts reduces friction, so less energy is wasted as heat. Insulation: wrapping pipes, boilers and buildings reduces energy lost as heat to the surroundings. Streamlining: shaping vehicles and turbine blades to reduce air resistance reduces energy lost to turbulence. Better materials: using low-resistance conductors in electric motors reduces resistive heating losses.
Sankey diagrams display energy flows visually. The width of each arrow is proportional to the energy it represents. The main arrow splits into a useful output arrow and one or more wasted energy arrows. A more efficient device has a thicker useful output arrow relative to the total input.
The National Grid transmits electricity at very high voltage and low current. This matters because energy lost in cables depends on the current squared multiplied by resistance (\(P_{\text{lost}} = I^2 R\)). Keeping current low dramatically reduces heating losses along thousands of kilometres of cable.
Step-up transformers at power stations raise voltage to hundreds of thousands of volts for efficient long-distance transmission. Step-down transformers near homes and businesses reduce voltage to safe levels for appliances. Transmission efficiency is therefore very high compared with lower-voltage alternatives.