An earlier posting referred to the idea of a Woodburning stove as another method of heating, in partnership with a solar tank. WBS may seem laughable, as the heat is only in one room, and maybe they get too hot for that room, and they are emitting smoke to the atmosphere. Also, it needs plenty of ventilation, which goes against the PassivHaus idea, which includes airtight houses.
But Woodburning stoves are considered carbon neutral, as they are burning carbon that was grown in your own lifetime, and it is low grade carbon, not the highly concentrated carbon from fossil fuels. Using a Backboiler model, you can convey heat away from the stove to another place to store for later. And efficient stoves can be managed by slow burning, they don't have to burn all at once, they can stay in overnight.
Stoves on Line have a proposal for just this - this is their illustration. This proposal is using a large accumulator tank getting heat from the woodburning stove for winter storage and also from a solar panel for summer. This backs up the domestic hot water, and supports space heating through underfloor heating or low temperature radiators to draw heat from this tank to give the rest of the house. See also BoilerStoves webpage for more detail.
We can't do this to our house, as it would be most difficult to insert a WBS and we don't need an accumulator tank because we have a heatpump and our accumulator is the borehole. But this seems an ideal combination for a newbuild house, or for upgrading an existing house.