I started a thread over in Solar Heating
http://ecorenovator.org/forum/solar-...-insulate.html
a couple of websites are linked, that combine Solar, HP, underground heat storage battery (large uninsulated tank (Austria 6600 gal.))
The website from Austria has a DHW tank with internal HX kept @ 42-49C, which can be heated at a fairly high COP by Heat Pump, without fear of Legionella.