alt Hacker News> In most places on Earth it's somewhere around 40 mW/m2 (i.e. accounting for conversion losses you need to capture heat from ~500 m2 to renewably power one LED light bulb!)
Ground-source heat pumps extract about 1000 times more power from ground loops, where does the difference come from?
A number of sources. Often the air above - ground source relies on the ground being the average temperature of the year round air once you get deep. They also tend to run in heating mode half the year, and cooling mode the other half.