Earth Sheltered Caravan - suggestions please!

EarthVan - warm air ducted from polytunnel/conservatory to heat sinks beneath caravan

We are hoping to get our caravan out of the barn here soon onto a patch of land beside the orchard. Then we are planning to use some of Mike Reynold's Earthship ideas amongst others to keep warm - it's the EarthVan. The ground will have to be levelled anyway so we thought we could do a bit more digging and make heat sinks to store warm air harvested from a polytunnel/conservatory/greenhouse structure to the front of the caravan.

HEAT SINKS, HELP PLEASE...
How big should we make the heat sinks? And what's the best material to use for them?  Rubble? Bottles? If anyone has any experience of anything like this any tips would be welcome.

FANS & DUCTS
Also, we are planning to use fans to duct the air from the top of the polytunnel section down into the heat sinks, powered by a PV panel - any tips on what specifications for the fans, panel and ducts would work best would be greatly appreciated.

THANK YOU!

Treflach Polytunnel Renovation

Amongst many other jobs here at Treflach Farm, we've been really busy in the polytunnel. Back in January it was beginning to look as if "it had seen better times" as my folks would have said. We put on new doors and took out the old sleepers which gave off an over-powering smell of tar in any heat let alone any leaching. We're making the new beds so that they sit level on the tunnel's 1:10 slope in a kind of cascading terrace arrangement. Lots of stuff growing already...

"seen better days..."

New doors...

...and out with the old creosote-minging sleepers

Bye for now old creosote-minging sleepers - a new role awaits you in the pig field

The first new bed, much narrower for easy access

...and in a cascading terrace kind of arrangement on the 1:10 slope

It's always nice to get another use out of something - here an old drawer front fills up a gap

Lots of stuff growing already

with plenty more seedlings waiting for space

Wes and Ruth celebrating the first completed row

Orchard Renovation

 It's lovely to see the blossom coming out in the orchard and especially pleasing to see the tree in the middle of the picture below sprouting away happily as we gave it a radical prune in the winter and wondered if it would come away at all. Well done that tree!