Energy Resources and Demand
Where Does Our Energy Come From?
Every time you switch on a light or charge a phone, you are drawing on an energy source somewhere in the national supply chain. The main energy sources available to us fall into two categories:
• Non-renewable sources will eventually run out. They include fossil fuels (coal, oil and natural gas, formed over millions of years from dead organic material) and nuclear fuel (uranium and plutonium). Fossil fuels currently provide the majority of global energy, but their reserves are finite and their combustion releases carbon dioxide, contributing to climate change.
• Renewable sources are naturally replenished. They include solar, wind, hydroelectric, tidal, geothermal, wave and biomass.
Non-renewable: fossil fuels (coal, oil, gas) and nuclear. Renewable: solar, wind, hydroelectric, tidal, geothermal, wave, biomass. Nuclear is non-renewable (uses uranium ore) but produces no direct CO₂ emissions during operation.
Energy Demand and How It Changes
Energy demand is not constant. It varies over daily, seasonal and historical timescales:
• Daily variation: demand is highest in the morning (when people wake and use appliances) and in the early evening (cooking, lighting). It falls overnight. Power grid operators must match supply to demand in real time.
• Seasonal variation: demand is higher in winter because of heating and lighting needs.
• Long-term trends: energy demand has grown historically as populations increase and living standards rise. However, improvements in energy efficiency (better insulation, LED lighting, efficient appliances) are beginning to reduce the energy needed per unit of economic output in many countries.
The concept of baseload is important: some power stations (typically nuclear and large gas plants) run continuously to provide a steady minimum supply. Peak demand capacity is additional supply brought in when demand surges above this minimum.
Environmental Impacts
Every energy source has environmental trade-offs. The choice of energy mix involves balancing cost, reliability, environmental impact and carbon emissions:
Solar: no CO₂ during operation, low running costs. Disadvantages: only generates in daylight and sunshine; large land area needed for solar farms; high upfront cost.
Wind: no CO₂ during operation, abundant resource. Disadvantages: intermittent — only generates when wind blows; visual and noise impact; can harm birds.
Hydroelectric: very reliable, can respond instantly to demand changes. Disadvantages: requires damming rivers, flooding valleys, displacing communities and disrupting ecosystems.
Tidal: highly predictable (tides are regular). Disadvantages: very expensive to build; limited suitable locations; can affect marine ecosystems.
Geothermal: reliable, low emissions, continuous output. Disadvantages: only available where hot rocks are near the surface (volcanic regions); high drilling costs.
Biomass: considered broadly carbon-neutral if managed sustainably (CO₂ released equals CO₂ absorbed during crop growth). Disadvantages: requires large areas of land; produces air pollution when burned; carbon neutrality is disputed if land use change is factored in.
Fossil fuels release CO₂ and other pollutants when burned. Nuclear power produces no CO₂ during generation but creates radioactive waste that must be stored safely for very long periods. On a full lifecycle analysis (which includes the energy and emissions from manufacturing, construction and decommissioning), wind, solar and nuclear all have much lower carbon footprints than coal or gas.
No energy source is without trade-offs. Renewables produce little CO₂ during operation but many are intermittent or require specific geography. Fossil fuels are reliable and energy-dense but emit greenhouse gases. Nuclear is reliable and low-carbon but produces long-lived radioactive waste.
Sankey Diagrams and Energy Transfer
A Sankey diagram shows how energy flows through a system. The total width of the arrow on the left (input energy) equals the total of all arrows on the right (useful output + wasted energy). This reflects the principle of conservation of energy — energy cannot be created or destroyed, only transferred.
A coal power station takes in 100 J of chemical energy in fuel. Of this, 35 J is transferred as useful electrical energy and 65 J is wasted as heat (to the cooling towers and surroundings). On a Sankey diagram, the input arrow is 100 units wide. The useful output arrow pointing right is 35 units wide. The wasted heat arrow pointing down is 65 units wide. The total width of outputs (35 + 65) equals the input (100).
The efficiency of this transfer is:
$$\text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100\%$$
$$= \frac{35}{100} \times 100\% = 35\%$$
The remaining 65% is dissipated as thermal energy to the surroundings, where it is no longer in a useful form. This is not energy being ‘lost’ — it still exists — but it has been transferred to a form that is too spread out to be useful.
The National Grid
The National Grid is the network of high-voltage cables and transformers that distributes electricity across the country. Electricity is transmitted at high voltage to minimise energy losses during transmission. Because power loss in cables is proportional to the square of the current (\(P = I^2 R\)), reducing the current by using a higher voltage substantially reduces wasted heat in the wires.
Step-up transformers at power stations increase the voltage (to hundreds of thousands of volts) for efficient long-distance transmission. Step-down transformers near homes and businesses reduce the voltage to safe levels for use (230 V in UK homes). Balancing supply and demand on the grid in real time requires careful management as more intermittent renewable sources connect to the system.