At the ESRF, the power absorbed by the first optical element will vary between a hundred watts on undulator beamlines, and several kilowatts on wiggler beamlines. One consequence is a distortion of the optical element, leading to a degradation of the performances of the beamline optics. Cooling certain materials (Si, Ge, InSb) to low temperature (typically around 125 K for Si) was proposed as a possibility to reduce the thermal deformation. In this paper, we report on the development of liquid nitrogen cooling of silicon crystal monochromators for ESRF beamlines. First, a prototype of a closed liquid nitrogen loop is presented., Then, hydraulic and thermal measurements are given, showing the influence of the flow rate, the pressure, and of the absorbed power.