I was wondering if someone knows a way to convert a ESP8266 to non-WiFi. I.e. have it connected through Ethernet, preferably with PoE possibility.
Reason for asking: My plan is to have sensors in a locker which is not a wifi-friendly environment. I want to monitor humidity and temperature in particular. I have a lot of ESP8266 units and like the firmware ESPeasy, thus wanting to stick to that platform.
The simplest solution is to add an external ethernet interface driven over SPI, as used in this (removed broken link).
It's also possible to bit-bang the ethernet interface directly, as decsribed in this article, Ethernet Controller Discovered in the ESP8266. Performance might not be as good as with a dedicated interface though.
Neither of these include Power over Ethernet, and the latter will probably not leave you with very much spare compute resource (although your application many not care).
If you're interested specifically in PoE, this Teensy forum thread discusses building a shield for a board. Although the base board is in production, it doesn't look like the ethernet shield is yet.
The ESP8266 was not designed with an Ethernet MAC, but this should not stop you. However, as Sean has said, it imposes a set of pretty severe restrictions on you.
You say that you wish to stay with the ESP8266 platform, but if your project cannot deal with the measly data rates provided by using an ENC28J60-style chip, or bit-banging Ethernet, then there is an alternative. The ESP32 has a 10/100 Mb/s Ethernet MAC that only requires a PHY, magnetics, and an RJ45 connector, and the ESP32 modules are just as cheap (if not cheaper) than the ESP8266 ones.
The unfortunate downside to this approach is that it does not appear that many ESP32 shields have made it to market yet.
