[mcnugget]

The tubes are called earth tubes or ground-earth heat exchangers. If you look for geothermal heating you'll end up getting the wrong information, mostly for ground source heat pumps. The earth tubes aren't strictly speaking geothermal though, they're massive thermal reservoirs that you're taping into. You extract heat and then putting it back later either from day-night cycles or seasonal cycles.

[tylorr]

How is that different than what heat pumps are doing?

[jnsaff2]

Well it does not have the heat pump. So you will only ever get the temperature the ground is at. In his case the temperature of the ground is 11C. Which I guess is fine for the greenhouse with the additional sun gain.

This is nowhere near enough for human habitation comfort let alone hot water.