A student collects data on three types of power station. Each power station converts energy from a fuel or source into electrical energy. The table shows th...

Marking Scheme and Explanation:

(a) Efficiency of each power station [3]

Efficiency is calculated using: \( \text{efficiency} = \frac{\text{useful output}}{\text{total input}} \)

• Coal-fired: \( \frac{320}{800} = 0.40 \text{ (40\%)} \) [1]
• Gas turbine: \( \frac{300}{600} = 0.50 \text{ (50\%)} \) [1]
• Nuclear: \( \frac{525}{1500} = 0.35 \text{ (35\%)} \) [1]

Each station wastes a significant fraction of the input energy, mostly as thermal energy in cooling water and exhaust gases.

(b) Most efficient power station [1]

The gas turbine station is the most efficient at 50%. Modern gas turbine (combined-cycle) plants can exceed 50% because they recover energy from hot exhaust gases using a secondary steam cycle.

(c) Power wasted by the coal-fired station [1]

\( P_{\text{wasted}} = P_{\text{input}} - P_{\text{output}} = 800 - 320 = 480 \text{ MW} \)

This is 60% of the input power.

(d) Two ways energy is wasted in a coal-fired power station [2]

1. Thermal energy (heat) is lost in the exhaust gases that go up the chimney. These gases carry away energy that was not transferred to the steam. [1]
2. Thermal energy is lost in the cooling water discharged from the condenser. The steam must be cooled back to water after passing through the turbine, and this rejected heat is carried away by cooling water to a river, lake, or cooling tower. [1]