Depending on location, ground temperature tends to stabilize once you get a few feet down. Here in the northeast United States, it's about 60 F year-round once you get 15 feet below the surface.
I worked for a company that was making direct-exchange geothermal heat pumps for homes - think about a big refrigerator whose coils are buried in the soil.
(About 15,5 °C)
Is that cold enough? My fridge normally about 1-4 degrees Celsius.
Depends on the beer.
Makes for a great lukewarm british ale :)
15 feet sounds pretty deep to dig to install something like this. Any idea what the temperature is like 2-4 feet?
That's going to vary a lot more by location, and 2-4 feet puts you at much higher risk for your beers freezing (although if you fully load this with beer, you'll have some beers at 2-4 feet anyway). Where I live, for instance, beers will probably freeze 2 feet below ground in winter. But my well is down below 15 feet and is always liquid.
I don't think this is supposed to be used at winter, and during summers I don't think most people need to worry about beers freezing.
Here in Estonia (very close to Nordics) we put water pipes underground at least 80 cm to PREVENT them from freezing during the winter.
It'd be good if this worked in warmer places like Australia, where you actually want to drink beer.