Topic: Thermodynamics
A solar energy based heat engine which receives 80 kJ of heat at $100^{\circ}C$ and rejects 70 kJ of heat to the ambient at $30^{\circ}C$ is to be designed. The thermal efficiency of the heat engine is
- A. 70%
- B. 1.88%
- C. 12.5%
- D. None of the above
Answer: C. 12.5%
Explanation: The heat supplied to the engine is $Q_S = 80$ kJ and the heat rejected is $Q_R = 70$ kJ. The net work output is $W_{net} = Q_S - Q_R = 80 - 70 = 10$ kJ. The actual thermal efficiency is defined as $\eta_{th} = W_{net} / Q_S = 10 / 80 = 0.125$, which is $12.5\%$.