I have been thinking and dreaming….

I know that a heap pump gives a better COP if the “output” temperature is as close to possible to the temperature of the water it is taking heat from. A better COP leads to a lot better government payment. I also understand that a big issue with water based solar inter seasonal store is the insulation and that having lot of luke warm water is not of much value.

So why not combine the two?

Use the inter seasonal store for UFH while it is still warm enough.
Then use the inter seasonal store as the “input” to the heat pump while it is still warmer then the ground
Then use the ground loop.

However the few heat pumps I have looked at in passing having a max input temperature of 15c that is too low.

Inter-seasonal storage in water just isn't viable on a single dwelling level. The installation costs and standing losses are just too high at that scale.

Paul in Montreal has previously posted links to a district based system in Canada which from memory performs on the margins of viability and there are currently two EU funded research projects at district level, one in Poland and one in Spain part of the objectives of which is to develop a suitable heat pump system for extracting higher grade energy from the store.

The EU systems are funded by the "EinStein project" but it has only been running for a little over two years now and has yet to report.

The system in Canada that Crispy is probably referencing is Drake Landing: http://www.dlsc.ca/

There's another district store in Europe (closer to him is southern Sweden) on a Danish island: http://www.solarge.org/index.php?id=1235&

Still, I really hope he's wrong about single domestic interseasonal storage: http://edavies.me.uk/2014/07/energy-budget/

Posted By: Chris P BaconI don't see from your link details of your store Ed?

It'll be 10 tonnes or so of water. Long narrow tanks along the north side of the house within the insulated envelope so all “losses” are just heating. I'll see how it goes but there might not be any need for additional radiators - just leakage from the store ought to be enough.

A small increase in the size of your wind turbine should ensure that your store only has to act as a buffer for calm days rather than as any type of inter-seasonal store.

Indeed, it's a fine line between interseasonal and “just” a buffer for dull/calm days. Still, I think you'll wind up over-engineering things if you design for the run of short days lasting forever.

Getting back to ringi's original point: a small heat pump for, e.g., DHW would allow use of a wider range of temperatures in the heat store and would allow the solar collectors to operate more efficiently because they'd be a lower temperature which is important even for evacuated tube collectors when there's not a lot of sunshine; 1 hour at 200 W/m² is much the same as 2 hours at 100 W/m² for PV (not quite, but almost) whereas the 100 W/m² is worth a lot less with solar thermal. As tony said it's getting close to so-called thermodynamic systems, just with a lot more buffering between the heat pump and the panels.

The question, it seems to me, is whether the cost and extra complexity of such a heat pump is worthwhile. It depends a lot on circumstances but basically you've got to weigh up the cost of the heat pump vs just having more PV and then heating the output directly with resistance heating.