Chapter 6 - The Second Law of Thermodynamics - Problems - Page 322: 6-125

$\eta=12.7\%$

$\eta=1-\frac{T_L}{T_H}$ $T_L=35°C,\ T_H=80°C$ $\eta=0.127=12.7\%$ There must be a difference in temperature for heat transfer to occur, so the temperature interval is lower making the real efficiency smaller.