solar water heater for ground heat????

So, I've read a bit about putting daytime heat into the ground and this seems to usually be using air as a medium.

Thinking this seems quite inefficient so you need a big surface area, big expensive pipes in one example, and because I have some leftover domestic underfloor heating system pipe (somewhere in the region of half inch diameter of a type usually put in a floor screed), and because I have looked into solar thermal for domestic shower water heating, I was wondering about burying lots of loops under my about to be built polytunnel beds, connected to a header tank, with black painted copper pipe of a relatively small size because it's quite bendy and can follow tunnel curves and back and forth to give a big area in the sun up top.

I'm wondering if this would even need a pump.

So underfloor heating pipe under the beds, connected to the black pipes in the sun and those just dumping into a header tank. The bottom of the header tank is the start of the underfloor pipes. There could be several unconnected circuits because all would merge at the tank.

Probably using a non toxic type of antifreeze.

A water pipe in dry soil probably won't share heat, but I imagine the bottom of an in use veg bed would be quite moist?

I have a solar panel I couldn't quite fit on my campervan roof which could directly power a waterpump when the sun shines if more circulation is needed.

This all seems very sensible and will definitely work in my head. Any input appreciated

Lotta, lotta hype about solar.  Not sure what zone you are in, but mostly it doesn't work daily unless you are in a non-freeze part of the world that gets sunny days 300+ days of the year, like San Diego, or Arizona, USA.  Storms can easily go for a week, so even if some heat comes through the clouds, winter air temps still affect a building.   If you do get solar heat, it's only when the sun is up and directly on the collection surface, so from between 10:00 AM and 3:00 PM, aproximately, on average throughout the year.  Fewer hours in winter, more hours in summer, with the afternoon hours coming in windows that face the sinking sun, in addition to the main south-facing windows (in the Northern Hemisphere.)

Small pipe won't be able to do much for you as far as hot air.  Because the soil below the freezing line is a consistent 50F degrees, passive non-freezing air in large pipes under a house can be fed from a deeper pipe sink.  If done right,  that might keep the interior of a very well-insulated house that is attached to the ground,  in the zone of 50F.  That would make it easier for the traditional source of heat to start at a higher temp at warming rooms, rather than heating a room entirely.

I think in general this might be....might be... an adjunct to a traditional heating system, whether it's heating water or air.  The irony of solar heat is that it's very available on days you don't need it.  When it's hot it works well for heating water for showers.  Hot water climbs, so you can play with that aspect of it.  

If you can get an old hot water heater tank that's insulated, and store heated water from black poly hose that is in the sun, that helps.  But any water left in the tank will go to air temp before long, and will take a lot of very hot water to get it up to comfortable shower temp.   There is a math equation about how much water is in 100 feet of 5/8th inch black poly hose wound in a tight circle on a board in the sun.  It's not much, and 200 feet works a little better for real amounts of water.  But that poly pipe needs protection from freezing because it's not easy to drain.

Different times of year get very different temps/results.

Check out this article on the Solar Thermal Fuel that should be available in a few years.

The renewable, emissions free energy device is made up of a concave reflector with a pipe in the centre, which tracks the sun like a sort-of satellite dish, ands the system works in a circular manner. Pumping through transparent tubes, the fluid is heated up by the sunlight, turning the molecule Norbornadiene into its heat-trapping isomer, Quadricyclane. The fluid is then stored at room temperature with minimal energy loss.

When the energy is needed, the fluid is filtered through a special catalyst that converts the molecules back to their original form, warming the liquid by 63 degrees Celsius (113 degrees Fahrenheit). After a series of rapid developments, the researchers claim their fluid can now hold 250 watt-hours of energy per kilogram, which is double the the energy capacity of Tesla’s Powerwall batteries. But there’s still plenty of room for improvement. With the right manipulations, the researchers think they can get even more heat out of this system, at least another 110 degrees Celsius (230 degrees Fahrenheit).

A few bullet points:

1. The molecule Norbornadiene into its heat-trapping isomer, Quadricyclane when exposed to sunlight (UV & blue spectrum so far).
2. The molecule suspended in a liquid can be stored for up to 18 years.
3. When exposed to its catalyst it converts back to its original form and can be reused at least 125 times.
4. Currently the molecule increases in temperature 63° c (113° f) and “at least another“ 110° f (230° f). So liquid stored at 60° f will heat to 173° w/ current tech and eventually over 400° f.
5. The process to manufacture the molecule and catalyst are related to the manufacture of acetylene and uses minimal.

My understanding is that the process has been well vetted and the test results confirmed by peer review. Apparently the group had received development money from investors and are moving towards creating a product. Other uses of the technology are being developed including building panels that can absorb heat and release it on demand.

I'm not sure about the size of your collector.
I've been told that small diameter pipe is not good for thermal siphoning.
How much of it are you planing on using?
Too little won't do much,  too much will block the light to the plants, and be expensive.
An insulated  and glazed collector that is separate from the green house might make sense.
The ground loop you describe could be used with multiple heating arrangements, and you have the materials so if you are up for the digging, it seems like a good idea.

Thank you for the replies. I hadn't realised I hadn't put my location up, changed now.

A domestic DIY thermal solar set up uses small diameter pipe (10mm copper is often used DIY it seems) and seems to be a worthwhile addition to a domestic heating system anywhere you otherwise pay to heat your water. I would do that but I would need a new mains pressure domestic water tank with a coil available to run the solar hot water through. £1000 :(

A previous house had an air source heat pump which was professionally installed and a huge waste of money. But the installer allowed for a whole roll of the underfloor pipe per installation, so every one created quite a bit of excess he didn't want to use so he gave me quite a few of the part rolls from previous installs. I need a way to use it!

My poly tunnel is a pile of parts, some bits somewhere in the post, and an area of cleared topsoil so at the moment I don't have much digging to put the underground pipe in!

I thought I might use something like 50m/yds of 10mm copper pipe painted black held up on the sunny side somehow.