While touring the laundry rooms and closets of Olympia, Washington after the Passive House NW conference, I saw a few of these Sanden heat pump water heaters. These first appeared on the TreeHugger radar when BuildingGreen declared it one of its top green products of 2016; I also tried to explain how CO2 heat pumps work in this post on a big installation in Alaska, complete with really bad drawings.
Most people are now familiar with split system heat pumps for heating and cooling air, which are filled with a hydrofluorocarbon refrigerant with a big global warming potential (GWP) as much as 1700 times that of CO2. The Sanden is a split system with CO2, which has a GWP of precisely 1. But the phase change takes place under high pressure and at a much higher temperature, so it is no good for cooling.
However it is terrific for heating, and in temperate climates where air conditioning is not a big deal, it works far more efficiently than a conventional water heater, with a COP (coefficient of performance, or multiple of efficiency over conventional resistance heating) of up to 5 when it is hot outside.
In a really well insulated house the unit can be used to heat both the domestic hot water and a hydronic heating system, as is shown in the Vogel Haus photo at top. This seems like a bit of a contradiction, because the colder it gets outside (and the more you need that hydronic heat) the less efficient it gets. But Albert Rooks explains on his Small Planet Supply site that it can be adapted:
For homes with a design temp of 23°F or higher, and a heat load of 8kbtu/hr or less, this can be the entire DHW and space conditioning system. Additional on demand hot water heater systems can be added into a design for homes with larger heat loads or used back up systems to provide added capacity for extreme weather events or large homes where all but the coldest days can be supplied by the default Sanden system.
These are not cheap units; on Green Building Advisor, Martin Holladay quotes one early adopter who thought the whole installation would cost about $ 5,000 after rebates. But it does do double duty, and Albert Rooks of Small Planet Supply, who sells them (and has one installed in his cute little house) says it should last a long time.
Anyone seriously considering green building today really should be thinking about getting off fossil fuels and getting rid of hydrofluorocarbons. The Sanden system certainly is an interesting approach.