Add a hot gas bypass to the evaporator. Regulate the amount of hot gas so the evap warms to say -50c for several minutes every 30 to 60 minutes or as determined by testing. you can use a solenoid valve and a PID timer. Or you can design a evap with electric heating cartridges (like load testers use) then put the electric circuit on a timer or flip a simple toggle switch to defrost............... Or as you said you can just shut it down.