Analysis of a protective RL circuit (freewheeling diode)

A relay is modeled by a coil (R=20 ohm, L=2 H) powered under E=24 V. The supply switch is abruptly opened without a protective diode. Explain, drawing on the concepts of magnetic energy and current continuity, why a voltage surge appears. Estimate the energy initially stored in the coil just before opening, and propose a technical solution to protect the circuit. Justify your answer.