| Date | May 2018 | Marks available | 1 | Reference code | 18M.3.sl.TZ2.12 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Calculate | Question number | 12 | Adapted from | N/A |
Question
The process of converting heat to electricity is limited by its thermal (Carnot) efficiency.
\({\text{Thermal efficiency}} = \frac{{{\text{temp. of steam at source (K) }}-{\text{ temp. heat sink (K)}}}}{{{\text{temp. of steam at source (K)}}}} \times 100\)
Calculate the thermal efficiency of a steam turbine supplied with steam at 540°C and using a river as the choice of sink at 23 °C.
Power plants generating electricity by burning coal to boil water operate at approximately 35% efficiency.
State what this means and suggest why it is lower than the thermal efficiency.
Markscheme
«\(\frac{{813{\text{K}} - 296{\text{K}}}}{{813{\text{K}}}}\) × 100» = 64 «%»
[1 mark]
35% of chemical/potential energy available in coal is transformed to electricity/electrical energy
not all chemical energy from burning fuel transferred into heating water
OR
energy dispersed elsewhere/energy lost due to friction of moving parts
OR
heat loss to the surroundings
Accept “stored energy” for “potential energy”.
[2 marks]
