  We investigate heat and charge transport in NN'IS tunnel junctions in the
diffusive limit. Here N and S are massive normal and superconducting electrodes
(reservoirs), N' is a normal metal strip, and I is an insulator. The flow of
electric current in such structures at subgap bias is accompanied by heat
transfer from the normal metal into the superconductor, which enables
refrigeration of electrons in the normal metal. We show that the two-particle
current due to Andreev reflection generates Joule heating, which is deposited
in the N electrode and dominates over the single-particle cooling at low enough
temperatures. This results in the existence of a limiting temperature for
refrigeration. We consider different geometries of the contact: one-dimensional
and planar, which is commonly used in the experiments. We also discuss the
applicability of our results to a double-barrier SINIS microcooler.
