Jeroen,

A triac can only be used for switching AC. Why? Because there is no way of actively switching it off. A triac, like an SCR, only switches off when the current through it goes to 0 (or at least below some threshold value). With an AC signal this happens every at zero-crossing, but with DC .... ?

There are solid state relays available that use MOSFETs and can switch either AC or DC signals. The LH1540 from Vishay, for instance, would be a pin-for-pin replacement for your MOC3020.

Regards,
Rob.