Diagrams show the difference between a Sunbox system and one using an industry standard Solar Thermal panel (or evacuated tube array), and the middle one being a hybrid (that doesn't exist) but I might use on a future occasion.
• Sunboxes are plumbed in as part of the GSHP’s ground loop. Driven by the heat pump, they work at night and in cloudy conditions if there is enough warmth in the air, capture heat from the build up of air temperature - and capture a lot more from the sunshine when it is available. Control is by one smart thermostat, diverter valve and a small circulating pump. Our 4 sqm will capture about 3,000 kWh/yr. They work best vertically on the wall, because they are ‘solariums’, and are optimised to gather heat in equinox and winter when most heat is needed. Even in freezing conditions, the temperature inside the boxes are warmer if there is any brightness in the sky.
If the black panels are un-enclosed, they would work in freezing temperatures, even tolerating ice formation on the black collectors. But I don't see that happening when they are enclosed, and they didn't freeze in the December of 2010.
• Solar thermal panels require sunlight or very bright conditions, a large insulated dual coil water tank, they have their own loop, pumps and thermostats. According to the SAP calculator, 4 sqm will only put about 1,100 kWh/yr into the tank. They mostly need to be on the roof, I haven't seen a house with vertical ones.
I guess the reason that figure is so low is that it realistically takes into account that in periods of summer, the system is in stasis for much of the time, and people aren't using a huge volume of hot water. That is a very good reason for using the middle diagram, and trying to share some of that heat for the space heating.