The entropy of black holes via Noether's theorem

We generalise some properties of Noether charges that have been related recently to the entropy of black holes. We define the variation of the entropy of black holes starting from suitable variations of superpotentials. Superpotentials are associated to Noether currents and are properly defined in any gauge-natural field theory. In this covariant framework the entropy is defined in such a way that it automatically satisfies the first principle of thermodynamics. We consider the BTZ black hole solution as a test model. Entropy is explicitly computed both by our method and by the prescription given by Wald in order to compare results.