\(\eta  = \left( {1 - \frac{{{T_2}}}{{{T_1}}}} \right)\)

Freezing point of water \( = {0^ \circ }C = 273\,K\)

Boiling point of water\( = {100^ \circ }C = \left( {100 + 273} \right)K\)

\( = 373\,K\)

\(T_2\) Sink temperature\(=273 K\)

\(T_1\) Source temperature \(=373 K\)

\(\% \eta  = \left( {1 - \frac{{{T_2}}}{{{T_1}}}} \right) \times 100 = \left( {1 - \frac{{273}}{{373}}} \right) \times 100\)

\( = \left( {\frac{{100}}{{373}}} \right) \times 100 = 26.8\% \)