  The observed flux of ultra-high energy (UHE) cosmic rays (CRs) guarantees the
presence of high-energy cosmogenic neutrinos that are produced via
photo-hadronic interactions of CRs propagating through intergalactic space.
This flux of neutrinos doesn't share the many uncertainties associated with the
environment of the yet unknown CR sources. Cosmogenic neutrinos have
nevertheless a strong model dependence associated with the chemical
composition, source distribution or evolution and maximal injection energy of
UHE CRs. We discuss a lower limit on the cosmogenic neutrino spectrum which
depends on the observed UHE CR spectrum and composition and relates directly to
experimentally observable and model-independent quantities. We show explicit
limits for conservative assumptions about the source evolution.
