Why a heat engine with 100% efficiency can never be realised in practise ?

The efficiency of heat engine $\mathrm{\eta }=1-\frac{{\mathrm{T}}_{\mathrm{L}}}{{\mathrm{T}}_{\mathrm{H}}}$
For 100% efficiency, $\mathrm{\eta }=1\text{ if }{\mathrm{T}}_L=0\mathrm{k}$,
which is not possible.
So 100% efficiency can never be realised in practice.