Some of you might remember the 9 pages on this topic that I began in February. I gleaned a few ideas from that and will be placing them into a journal that I'll place on my site as this build progresses (I'll also update that over the next three years to see how some of the technologies have faired).

Anyway on to today's question. I'm creating the underground storage areas ala Don Stephens (greenershelter.org) and was wondering what opinions any of you might have on the pipe used to transfer the heat into the ground. I was thinking of using MDPE mainly because it's cheap Under the extension it'll twist around 10m2 of space so not huge for getting rid of the energy from 4m2 of solar panel or 30m2 of roof. Which brings me to the second question. We're going to make some solar panels and I was wondering if there was any piping that you'd recommend. In particular if there was any piping that didn't require much work e.g. welding. Or if there was a ready made grid of pipework that'd collect heat, I thought of the backs of fridges which wouldn't be good from a thermo siphoning aspect. I did see that floorstore.org were using HDPE in their roof collector but from what I've read it doesn't transfer heat well which is essentially why I'm asking these pipe related questions.

It needs to be able to pick up heat well from the roof (and/or in the solar panels) but not transfer it into the surrounding air until it gets underground where it can transfer it into insulated gravel pits. I will insulate the pipework as it travels through the room on its way underground.

The biscuit boy

Locally I am having these discussions too, either solar panels with high temperature/low volume fluid (smaller collector area) verses floorstore type collector low temperature/high volume (use all roof area because of inefficiencies)?

To keep it simple I went for HDPE pipework under dark coloured roof steel roof because of the controlability, smaller header tank and slower speed (of fluid).
A good use of reflective foils is under HDPE pipework to improve performance and essentually making the whole roof a solar collector (the bit facing the sun any way).

The other sinerio was using oversized solar panels for domestic hot water and a large DHW tank and as soon as DHW was up to temperature dirvert hot fluid to interseasonal store with additional pump.

Michael, I agree with your methodology and to get better perfoming solar collector (high temp, low flow) but what are you using the fuild for? If you are using large tanks of water then ok.
An interseasonal store can be many different substances, I am using some concrete, compacted gravel and natural dry ground for my store. I am aiming for a large volume of low grade heat (max 40 degC), lower temps, lower losses!

This is the system I am using with low energy houses on top.. http://www.floorstore.org/