The Viessmann ice energy storage system is an attractive heat source for brine/water heat pumps. This permit-free system combines energy from ambient air, solar radiation, and the ground. Its unique feature is that part of the thermal energy comes from the ice, or more precisely, from the so-called crystallization energy. This is the energy released when water freezes into ice. In an ice storage facility, this change in physical state occurs during the heating season, when the heat pump extracts heat.