MOSFET relay as an alternative for TRIAC relay
It is definitely possible to control AC power with a pair of back-to-back MOSFETs, in series, sharing a common source terminal:







simulate this circuit &ndash; Schematic created using CircuitLab

The lower, DC part of the circuit above can easily be controlled by an opto-isolator. However, you will need to find a way of implementing a galvanically isolated DC power supply (pictured in the lower part of the schematic above) for driving the Gate of the MOSFETs. This could be in the form of a small, $5 DC-DC converter module.

Only two MOSFETs are required, and assuming you have galvanic isolation worked out, gate control is very simple, as the MOSFETs share a common source terminal and are both switched on together regardless of the phase of the AC cycle. Phase angle or zero-crossing detection is not necessary either, although you may want to do it for purposes of reducing device stresses and electrical noise during switching.

Unlike Triacs, MOSFETs can be easily paralleled for lower resistance, additional current handling, and lower heat dissipation.

It turns out that modern MOSFETs can also be significantly more efficient than Triacs:

A typical 650 V Triac has a relatively constant forward drop of 1.55 V. This equates to 15.5 W of heat dissipation under a 10 A load, requiring a substantial heat sink (or possibly active cooling). Triacs are not easily paralleled for better performance.

A nice 650 V MOSFET, such as ON Semiconductor's NTH027N65S3F, has a typical on-state resistance of 23 mΩ. Since two are required in series, that's 46 mΩ total, resulting in 4.3 W of heat dissipation under a 10 A load; less than 1/3 of the Triac. Passive cooling with a relatively small heatsink is much more practical.

The downside is that the cost of using MOSFETs is about 4x higher than a bare Triac. However, control is easier, the circuit design is quite simple, and the cost may be comparable to an all-in-one "solid-state relay module".

With any solid-state AC solution, I would protect the device with MOVs and definitely incorporate a separate fuse or breaker for safety purposes.