In short, the ECAT produces the energy through The Rossi Effect which is a newly discovered LENR process involving Lithium and Hydrogen to which Leonardo corporation holds the IP rights, see patents. This LENR process is exothermic, which means that it produces excess energy. The mass energy of the rest product, Helium, is less than the mass energy of Lithium and Hydrogen alone and this excess energy is released as heat energy as part of The Rossi Effect.
The current design of the reactor is a small cylindrical rod with length 20cm and diameter 5 cm (external dimensions of the inner reactor core). The fuel, Lithium Aluminium Hydride + the catalyst Nickel of very fine granularity, is placed in a thin layer at the center of the reactor core. The reactor is located at the bottom of a box (the outer hull of the E-Cat is currently at a size that can easily fit on a small table) and directly attached to the reactor is cooling elements in the form of wings. These wings are crucial as they transmit the heat generated in the reactor to the surrounding water.
There is a cold water inlet into the outer box and a steam outlet. As the cold water flows past the reactor, the cooling elements will heat it up and the flux of the water (e.g. liters/h) dictates the difference between in and out temperature. In this way hot water or steam can be produced.