Earth Tube Layout - Grid VS Spiral

Hello

Researching in ground heat banking for winter I notice the absence of spiral tubing layouts. It seems that everything is grid based.

A grid fits nicely into corners and side by side leaving no spaces but it would seem to me that the tubes closest to the air intake would get the most heat and the further down the manifold you get the cooler the air gets along with less air flow.

A spiral would leave triangles in corners and next to one another unheated but the heat and air are in a single length to tube. Sure as the air progresses further in or out of the spiral, depending on air direction, the temperature bleeds off but it seems a much more simple design. Less connections and less potential leaks.

Can anyone explain why there seems to be no use of spirals? Are there that many disadvantages that they simply arent used?

If a spiral works in a compost pile to extract heat why not in the ground to bank heat?

Thanks

Trench diggers don't like curves?