One of the thermocouple junctions is at room temperature $T_1 = 300 K$, and the other is in a thermally isolated vessel with ice at temperature $T_2 = 273 K$. The power developed by the thermocouple is released on the resistance of the heater, which is placed in another thermally insulated vessel containing water.
The mass of ice and water is the same. Specific heat of water $c = 4200$ J/(kg K) and latent heat of melting ice is $\lambda = 335000$ J/kg.